A sequence of these isotropic and anisotropic etches is used to form the
substrate GC wafer. Though described separately below, many of these
etches for the valve, capillary column, and detector gas channel can be
performed simultaneously, reducing the total number of necessary photo-
lithographies and etches. The detector chip is removable from the sub-
strate wafer and is batch fabricated on separate wafers using similar
etching techniques.

2.3 MINIATURE GC FABRICATION
2.3.1 Capillary Column

The miniature capillary column is formed by etching a long, narrow spiral
groove into the surface of a silicon wafer and then hermetically sealing
the wafer to a Pyrex glass cover plate, The capillary colum is the
prominent feature in Figure 2.4, a photograph of the GC wafer. Most of
the miniature GC's produced have used capillary columns 1.5-m long, about
200-ym wide, and 30-ym to 55-ym deep. Holes etched through the wafer at
the ends of the column serve as entrance and exit ports for the carrier
gas.

Figure 2.4. PHOTOGRAPH OF THE GC WAFER. The view is
through the 5-cm square Pyrex cover glass.
(A) Carrier gas input feedthrough hole.
(B) Sample gas input and exit feedthrough
holes. (C) Sample injection valve, back-
side of wafer. (D) 1.5-m long separating
colum. (E) Detector gas channel, back-
side. (F) Vent to atmosphere.

9


These surface hydroxyl groups will react with organosilane compounds of
the form RSiX3 or R3SiX (where R is an alkyl group and X is either a
chloride or an alkoxide group) to form a Si-O-Si linkage between the sili-
con and the silane as

ao

Si-OH + R,Six + Si-O-Si-R + HX

pe

We have employed this chemistry to react trimethylchlorosilane (CH3)3 SiCL
with the column surface. The reaction is carried out by forcing the un-
diluted silane through the column for about 10 min at 70°C. The capillary
is then washed with water and methanol and blown dry. This procedure
serves to convert the OH groups which populate the surface of the column
and to provide a monomolecular layer to which the stationary phase can
adhere,

Dynamic Column Lining--

The liquid stationary phase can be applied to the miniature capillary walls
by means of a conventional dynamic coating procedure (3). The OV-101
liquid is dissolved in a very volatile solvent such as chloroform, and a
plug of the mixture is forced through the column. If the speed of the
moving plug is kept constant, a uniformly thick layer of OV-101 is left on
the column walls. The approximate thickness (t) of the layer is given by

© = (35)(3) (2

where C is the concentration of the solution (%v/v), r is the radius of
curvature of the capillary tube, Lt is the average velocity of the plug of
lining solution, n is the solution's viscosity, and y is its surface ten-
sion.

Although this lining procedure is common practice with conventional capil-
lary GC columns, it is less than ideal for the miniature capillaries since
they have extremely smooth surfaces and nonconstant radii of curvature.
The resulting distribution of stationary phase appears to be nonuniform
both across the width of the capillary channel and along its length. The
lining distribution is unstable with time and the OV-101 bleeds out of the
column.

The distribution and performance of the column lining can be significantly
improved with the addition of a small percentage of 7-10 & $i02 particles
into the OV-101 liquid phase. These particles tend to increase the
stability of the thin liquid layer and result in greater surface area of
liquid phase per unit length of column, an attribute which increases the
column's ability to separate gases.

All of the miniature capillary columns which are fabricated now include
Si0, particles in the liquid stationary phase. A column lined with OV-101

30


100

10/--

PENTANE IN,

PERCENT STANDARD DEVIATION

1 L pou l
1 10 100
NOMINAL GAS CONCENTRATION (ppm)

Figure 4.2, SYSTEM REPRODUCIBILITY VS SAMPLE SIZE.

of error fall roughly into four categories, GC system errors, detector
noise and nonlinearities, quantization errors in the A/D converter, and
errors in the computer algorithms used to compute the peak areas. Each
of these sources of error has received some study, and the performance in
each area can undoubtedly be improved with additional work.

The GC process itself probably contributes the least amount of uncertainty
to the system reproducibility. If identical samples of constant gas con-
centration are injected at the head of the column, then identical peaks
are expected at the output if identical temperature and flow conditions
are maintained. Possible sources of error are in the sample collection
and injection system, however. Some of these errors, such as slightly
different sample injection volumes are taken into account in the final
concentration calculation. There are some dead volumes in the sample in-
let region which result in a finite rise time of about 4 sampling intervals
to reach 954 of a step change in concentration. In the reproducibility
test it is assumed that the GC system is basically not responsible for any
of the system error.

The GC detector, as discussed in Section 4.3.2, is slightly nonlinear and
has some low level noise. In this prototype the detector noise and the
analog electronics noise is about 2-yV peak-to-peak, referred to the

26


Lom
w
als

The plate height of an open-tubular capillary column with rectangular
cross-section has been related to the column parameters by Golay (5) as

B
He 7 + (Cy + cu (4)
where
B= 2D
2 22
Cc, = ———>—_ (5)
*  3¢1 + &)°K’D,
2 2
4( + 9k + 51k")/2]z)
iran ira (6)
105(1 + k)"D,
and

Z, = 1/2 of the etched column depth

u = carrier gas velocity

D = diffusion coefficient of solute vapor in carrier gas
D, = diffusion coefficient of solute vapor in liquid stationary
phase

The partition ratio k used in the above equations is a function of the
column height and width. It is related to a,more fundamental parameter,
the partition coefficient K, by the equation

* the partition coefficient is defined as the concentration of solute vapor

in the stationary phase divided by the concentration in the mobile phase,
It is only a function of the thermodynamic properties of the solute vapor
and the stationary phase. Its exponential dependence on temperature makes
it the most temperature-dependent parameter, and thus facilitates the con-
trol of column performance with simple temperature adjustments.

35


Figure 1.1. PHOTOGRAPH OF THE PROTOTYPE GAS ANALYSIS SYSTEM. The package
measures about 20 cm x 22 cm x 10 cm and weighs 2.8 kg.


GAS PARAMETERS

Parameter Number

Definition

0 Select/deselect flag

Retention time "A parameter"

1
2 Retention time "B parameter"
3

Overall concentration calibration
multiplier

S

Overall concentration correction

with temperature

Corn nau

Concentration

Time weighted average concentration
Maximum concentration detected
Alarm level

Reserved

SYSTEM PARAMETERS

Parameter Number

Can nurwnero

Figure 4.24.

keyboard.
stores the appropriate concentrations
turns off power to itself.

Definition

Maximum chromatogram length
Gas detection threshold
Carrier pressure

Valve injection pressure
Thermistor constant Vo
Thermistor constant B
Thermistor constant 1/T¢
Automatic mode wait timer
Swept time td

Swept time 1/a

GAS AND SYSTEM PARAMETER LISTING.

For example, when the computer finishes a chromatogram, it

in nonvolatile CMOS memory and then

When a concentration is requested from the

keyboard, the D button is pushed, which produces a signal which turns on

power to the microprocessor.

The computer recognizes the button, goes to
the display part of the program and then turns power off again.

The

entire button pushing sequence takes about 1 ms, which is the amount of

time the microprocessor draws power.

the machine continues to immediately power down.

63

As each additional button is pressed,
After the desired


and detectivity. The most important parameter for the miniature system
in environmental monitoring applications is detectivity. Assuming that
the detector noise remains constant, the electronic system with the
highest sensitivity will be optimum for the miniature system. Figure
4.12 shows a graph of calculated detector sensitivities as a function of
temperature for the bridge, constant current, and constant temperature
configurations. The choice is basically between constant current and
temperature. The maximum temperature limit for these detectors is about
300°C above ambient, but they are usually operated below this point. The
constant current mode results in the highest sensitivities at higher tem-
peratures due to a thermal feedback effect because of the positive TCR of
the nickel resistors. At a given operating point a sample peak causes
the thermal conductivity of the gas to decrease. This causes an increase

1000

CONSTANT
CURRENT

100
CONSTANT
TEMP

BRIDGE

10

RELATIVE DETECTOR RESPONSE

1 1 1 L 1 1 n
° 30 60 90 120 150 180 210

DETECTOR TEMP. ABOVE AMBIENT, °C

Figure 4.12. RELATIVE DETECTOR RESPONSE FOR THE THREE DETECTOR DRIVING
SYSTEMS.

45


in a burner, the hydrocarbons in the output gas stream are burned, and

a small percentage of the combustion products become ionized. These ions
are collected by electrodes and the current between the electrodes is
integrated to determine the mass of sample in an output peak. The ability
of a flame ionization detector to measure small concentrations is due to
its low background current and the ability of electronic systems to mea-
sure small currents.

The most important property of a detector for many applications is its
detectivity. Since ionization detectors are basically mass sensitive
instruments, their response is generally given in coulombs/gram and mini-
mum detectable quantities in grams. Thermal conductivity detectors are
concentration sensitive devices where the minimum detectable sample is
usually given in parts-per-million. The miniature gas chromatograph has
some unusual properties due to its small size and small gas flows. It
places stringent requirements on the detector. One aspect of these re~
quirements is the small sample that the detector system will be required
to measure. In order not to overload the capillary column, the injected
sample must be kept very small, in most cases about 10 nl of gas. For
many substances, it is desired to measure concentrations below the 5 ppm
level. For gases with molecular weights around pentane, 5 ppm of 10 nl
of injected gas represents only about 150 x 10-1. grams (150 fg). Because
of the gas flow rate of about 4-1/s out of the column and the narrow peak
widths of less than 100 ms, the absolute maximum detector volume cannot
exceed 20 nl, without serious peak broadening due to dilution in the
detectgr volume. Thus for a column with chromatographic efficiency simi-
lar to that of the prototype, the maximum detector dimensions are 100 ym
x 200 Um x 1000 um. If a shorter column is used or if more efficient
linings are developed, the maximum allowable detector volume would be re-
duced by a corresponding factor.

For purposes of comparison, the detectivity of a high quality flame ioni-
zation detector is 18 pg (6) of, in this case, propane, some two orders of
magnitude larger than needed for the integrated gas chromatograph. Even a
photoionization detector, which is reported (7) to have significantly better
detectivity than the flame, can see no less than 6 pg of benzene, again
almost two orders of magnitude larger than is needed. These figures were
found by injecting quite large volume samples into short, packed columns.
In the photoionization case, 5 ml of 0.3 ppb benzene was used. Even
though the concentration of this sample is very low, the total mass of the
sample is still quite high, and the volume of injected sample is equal to
the total carrier gas flow for the miniature system for 25 minutes, These
detectivity problems, coupled with the difficulties associated with trying
to fabricate an ionization detector in a region a few hundred micrometers
across, have limited the feasibility of making a miniature ionization de~
tector.

Conventional thermal conductivity detectors have not been known for their
good detectivities. A Varian 3700 TCD can achieve sample detectivities
of 1 ng [8]. about four orders of magnitude larger than needed for the
miniature system. The Varian system also has a minimum detectable peak
concentration of about 200 ppb at the detector. The important property of
thermal conductivity detectors is that they are concentration sensitive

43


10,000,

1,000}

TT
4
a

100}—

NITROGEN PEAK VOLTAGE (mv)

@ EXPERIMENT|
- 4 THEORY

@ 1=6ma

' {o} 20 40 60 80 100 120 140
DETECTOR OPERATING POINT TEMP ABOVE AMBIENT, °C

Figure 4.14. DETECTOR OPERATING POINT TEMPERATURE AND NITROGEN PEAK VOLT-
AGE AS A FUNCTION OF DETECTOR CURRENT.

in the G, term, but if the magnitude of the gas flow changes, the detector
temperature and hence the output signal will change. The flow can change
due to either changes in carrier gas supply pressure or at the introduction
of higher pressure sample gas at the injection valve. The injection arti-
fact can be a rather large signal, at high gain, but is over by the time
the air peak reaches the detector. An example of such a signal can just
be seen, immediately after injection, in Figure 4.4. The change in signal
with carrier gas pressure is noticeable in the prototype instrument, but
does not degrade performance. The change in carrier pressure is due to
the on/off valve and surge tank arrangement in the miniature carrier gas
supply system. The pressure in the surge tank drops about 7 x 102 Pa
(0.1 psi) during a chromatogram. That drop introduces a drift in the

49


where Oj, is the standard deviation of the input gaussian signal and Of is
the standard deviation of the filter in the time domain.

In order to simplify the determination of the location of the signal maxima
and points of inflection, a differentiator was included in the filtering
algorithm. Differentiation at this stage is tolerable since much of the
noise has been removed by the optimum filter.

The overall impulse response of the optimum filter and the differentiator
is given by

= 2
a(t) = —, t exp [- > (a2)
2 oY 20.

This filtering function is graphed as a function of time in Figure 4.17.
When the original gaussian peak is operated on by this filtering function,
a different output is obtained which is given by the convolutional integral

t
Y(t) = f x(T) h (t - T) dt (13)

Using Equations (10), (12), and (13) the filter output signal Y(t), is
given by

~h(t)

———
TIME

Figure 4.17. IMPULSE RESPONSE, h(t), OF THE OPTIMUM FILTER.

53


The temperature dependence of retention follows directly from the ex-
ponential relation of equilbria in phase transformations such as vapori-
zation to which the partitioning of a solute between gas and liquid
phases is closely related. The equilibrium partition coefficient K is

given by
(=)
K = C exp Rr

where
C is a constant
AH, is the heat of solution
R is the gas constant
T is the absolute temperature

The solution process is usually exothermic (AH, is negative), thus increas—
ing T decreases K.

Over the limited temperature range at which the column operates, AH, is
assumed to be constant and the partition ratio k is found from Equation

(7) to be of the form
AH,
k=HA — 5s
wa RT

The constants A and AH, can be determined experimentally for each gas on

a miniature capillary column by performing separations over the tempera—
ture range and fitting an exponential curve to the data. Figure 4.10 shows
the results of such a procedure for chloroform and methylene chloride on
an OV-101 capillary column.

To predict the retention time of a given gas, the computer only has to
measure the column temperature, insert the appropriate values of A and AHg,
and determine the partition ratio k. Then using the air peak retention
time (ty) from the previous chromatogram, the gas retention time is calcu-
lated as

thet, Gtk

4.3 DETECTOR PERFORMANCE

A metal film thermal conductivity detector has been chosen for use in the
integrated gas chromatograph. This device was chosen for its compatibility

41


co
thick, (100) silicon. About 1000 A of SiO) is thermally grown on the wafer
followed by the sputter deposition of about 1.5-um of Pyrex. Large holes,
about 300-Um by 700-um are etched from the back of the wafers to remove
the thermally conductive silicon from the detector region. About 1000 &
of nickel is evaporated on the Pyrex membrane, defined photolithographi-
cally, and etched to produce the detector resistor. The chips are then
sawn apart and lead wires are bonded to the resistor to complete the
fabrication process.

13


and Si02 particles was operated with constantly flowing carrier gas for
more than 6 months. No attempt was made to condition the column, and
numerous temperature cycles were performed, but the relative retention
times of the peaks changed less than 10% over the length of the test, and
no detectable amount of column lining was blown into the detector channel.

Static Column Lining--

With the miniature capillary colum as well as with conventional colums,
the static coating procedure (4] is replacing the dynamic method because
it yields more reproducible and uniform stationary phase distributions.
The OV-101 liquid is dissolved in chloroform along with Si02 particles and
the capillary is totally filled with the mixture. The output end of the
capillary is then sealed off and the miniature GC column is put in a vacuum
chamber at less than 1 Torr pressure. The chloroform in the capillary
evaporates slowly from the head of the column leaving the OV-101/Si02
residue coating the capillary walls. The evaporation rate is slow and
constant and all of the OV-101 remains in the column. This procedure
gives very uniform lining distributions in which the exact weight of
liquid phase per unit length of column is known.

4.2.2 GC Separations

One of the important features of the miniature gas chromatograph is its
very, rapid separation of the gases of interest. As a simple example of
its capabilities, an oscilloscope photograph of the amplified detector
output, or chromatogram, is shown in Figure 4.3. The sample gas consisted
of a mixture of nitrogen, pentane, and hexane vapors and was injected at
time zero. The nitrogen has no affinity for the OV-101 column lining so

Fifugre 4.3. OSCILLOSCOPE PHOTOGRAPH OF THE AMPLIFIED DETECTOR OUTPUT OR
CHROMATOGRAM. The sample was injected at the beginning of
the sweep. The peaks are (1) nitrogen, (2) n-pentane, and
(3) n-hexane.

31


COMMAND FUNCTION DESCRIPTION

A Survey mode The machine performs one sample analysis
and updates concentrations of programmed
gases.

B System param— To display a system parameter the param-
eter display/ eter is entered. After the parameter is
change displayed, pressing 3 will allow the user

to change that parameter.

Cc Gas parameter To change a gas parameter, the gas # and
change parameter # are entered, followed by the

new value for that parameter.

D Gas parameter Entering the gas # followed by parameter
display # displays the value of that gas parameter.

E Sample calibra- In this mode the machine determines the
tion overall concentration multiplier and re-

tention time "A parameter" of the gas
under calibration. After choosing a # for
the gas, the known concentration of the
calibration gas is entered. The machine
then performs one analysis and updates

the necessary parameters.

Automatic Mode: The machine samples the atmosphere and updates maximum
concentration and TWAs of programmed gases with the
sampling interval specified by system parameter 7.

Figure 4.23. KEYBOARD COMMAND DESCRIPTION.

sequence F,0,0,0,2,9,F is used, the first F is interpreted as a decimal
point and the last F is an enter button to tell the machine that the num-
ber is complete. The D button is similar to the B, but for display of
the gas parameters. The sequence D15, for example, displays the concen-
tration of gas 1. The last button, E, activates the calibration routine;
the sequence E 4 1000 calibrates gas 4 at 1000 ppm. A complete list of
the function keys and gas and system parameters is listed in Figure 4.24,
Mistakes are cleared by simply pressing another function key instead of
the "enter" button. The 4-digit display can show up to a 10-digit number
by shift keys which move the display one place to the right or left.
Since many of the displayed numbers are betwen 9999 and 0.001, this shift
is not always required.

Power Down--

The present digital electronics system uses a Z-80A microprocessor, six
EPROM's, four RAM's, plus a number of support chips. During operation

the device consumes about 300 mA at 5V, or 1.5 W. In order to save power
a novel power down system for the digital electronics is used. The general
scheme is to power the microprocessor only during those times that is
actually computing, and to power down whenever it is waiting for a key-
board entry. This system also works in between key strokes from the

62




5. CONCLUSION

A prototype miniature gas analysis system has been designed and built
using a combination of new technologies. The techniques of integrated
circuit processing have been utilized to miniaturize the components of a
gas chromatography system to a size which is compatible with a portable
package. The electronics system used in the gas analyzer is made from
recently introduced, state-of-the-art microprocessor, memory, interface,
and analog circuits. The combination of devices results in a portable
instrument with unique capabilities.

Some of the significant capabilities of the prototype instrument are:
(1) Automatic atmospheric analysis for 8 hours.

(2) Measurement of the concentration of 10 different toxic
gases simultaneously.

(3) Concentration information within 30 seconds of sampling.

(4) Concentration measurements from the sub-10 ppm to the more
than 1000 ppm level.

(5) Reproducibility within +5% at the 95% confidence level for
several substances between 100 and 1000 ppm.

(6) Calculate and store the time-weighted-average and peak
concentration for each of the gases.

(7) Convenient display of any concentration or system calibra-
tion factor.

(8) Sound an alarm when any gas concentration exceeds a preset
maximum value.

(9) Provide automatic calibration for any gas within the instru-
ments measurement range.

(10) Relatively small size and weight for convenient portable
operation.

The miniature gas chromatograph performs the actual separation and detec-
tion of the constituents of the gaseous sample. It consists of a 1.5-m
long separating column, a sample injection valve, and a thermal conduc-
tivity detector all of which are integrated on a 5-cm diameter silicon

65


concentration is displayed, the computer again powers down; the number is
retained in the display driver and continues to be displayed. This power
down system has allowed the battery size to be significantly reduced with-
out sacrificing instrument performance.

Future Improvements--

The computer system incorporated in this portable instrument represents
the most advanced possible system given the present state of electronic
development. A number of improvements are foreseen using components which
are either announced for future delivery, or can be reasonably expected
to be available in the next year or two. One such product is a low power
Z-80A. A product essentially equivalent and software compatible with the
present Z-80A but requiring only 20% of the power should be available in
1981. Along with lower power memory and support circuits also soon to be
available, the battery required to drive the digital system can be further
reduced without reducing performance,

New digital signal processing devices are just being introduced. These
devices are basically special purpose microprocessors, optimized for the
multiplications and divisions of high precision numbers which is most
important for signal processing applications. A device such as these
could be used to prefilter the signal, freeing the microprocessor from
this job. The microprocessor could then use a more elaborate peak detec-
tion program, and since it would not have to perform the filtering, the
sampling rate of the detector signal could be increased which would in-
crease accuracy.

There are some changes and additions to the signal processing algorithms
which should be included in future versions of the instrument. The base-
line drift problem needs additional study to more reliably locate peaks
with long retention times. In this prototype the problem of overlapped
peaks has not been addressed. The filter used to reduce noise has been
designed to be optimum in the presence of white noise, It appears that
the actual power spectral density of the noise decreases with frequency
proportionally to the reciprocal of the frequency, "1/f noise". This
will require some changes in the filtering function to obtain an optimum
filter.

There are also improvements likely in the keyboard, display, and interface
circuitry which will increase the operator convenience and allow the in-
strument to automatically output the concentration data taken during a

day to a larger computer for further processing, storage, or hard copy
display. A larger computer could also periodically monitor the performance
of the instrument, pointing out long-term changes in the calibration, or
troubleshooting the device in cases where repairs are needed.

64


10.

REFERENCES

Terry, S.C., J.H. Jerman, "A Feasibility Study of a Pocket-Sized Gas
Chromatographic Air Analyzer," Stanford Electronics Laboratories,
TR No. 77-027, Stanford University, Stanford, Calif., 1977.

Terry, S.C., "A Gas Chromatography System Fabricated on a Silicon
Wafer Using Integrated Circuit Technology," Ph.D. dissertation,
Stanford Electronics Laboratories, TR No. 4603-1, Stanford Univer-
sity, Stanford, Calif., 1975.

Concus, P., "On the Liquid Film Remaining in a Draining Circular
Cylindrical Vessel," J. Physical Chemistry, Vol. 74, 1970, p. 1818.

Bouche, J., M.J. Verzele, Gas Chromatography, Vol. 6, 1968, p. 501.

Golay, M.J.E., "Theory of Chromatography in Open and Coated Tubular
Columns with Round and Rectangular Cross Sections," in Gas Chromato-
graphy (Desty, ed.), Butterworths, London, 1958, p. 36.

Dick, R., C.H. Hartman, "Critical Evaluation of the Response Charac-
teristics of the Hydrogen Flame Detector," Varian Aerography Tech-
nical Bulletin, pp. 133-167.

Driscoll, J.N., et al., "Applications of a Gas Chromatograph Employ-
ing an Integrated Photoionization Detector," American Laboratory,
Jan 1980, pp. 84-93.

Kern, H., M. Elser, "Trace Gas Analysis by a New High Sensitivity
Thermal Conductivity Detector," Mikrochimica ACTA, Vol. I, 3-4,
1978, pp. 319-328.

Tsederberg, N.V., Thermal Conductivity of Gases and Liquids, MIT
Press, Cambridge, Mass., 1965.

Papoulis, A., Signal Analysis, McGraw-Hill, New York, 1977, pp. 324-
327.

67


Table 4.1. Results of reproducibility test.

Run Pentane Hexane Air Run Pentane Hexane Air
# (ppm) (ppm) (mV) # (ppm) (ppm) (av)
1 51.6 66.1 1962 26 50.7 67.0 1900

51.6 70.1 1924 50.0 67.4 1924
46.3 65.9 1902 51.7 69.5 1902
48.6 66.7 1926 48.5 63.5 1918
5 51.3 61.7 1926 30 49.5 68.7 1942
48.3 66.3 1932 51.8 66.3 1908
53.1 68.3 1950 49.8 66.9 1930
50.7 67.8 1918 48.1 69.1 1924
49.6 70.0 1918 51.2 73.4 1910
10 48.0 66.8 1912 35 51.1 63.1 1904
51.8 72.7 1930 48.9 65.4 1912
46.1 72.0 1922 49.4 65.9 1910
50.5 68.9 1896 53.0 74.9 1916
51.6 71.9 1934 49.8 67.0 1916
15 52.1 71.4 1930 40 51.6 67.6 1902
47.7 65.7 1914 51.2 70.1 1902
50.6 67.2 1898 50.9 64.3 1908
50.1 70.7 1918 50.6 64.3 1892
46.5 67.1 1916 46.3 66.8 1914
20 50.2 67.8 1930 45 47.2 66.1 1914
47.1 71.6 1926 50.2 62.6 1908
47.5 62.7 1944 52.7 68.5 1888
46.2 69.0 1924 48.8 60.3 1908
48.1 71.5 1932 48.4 65.6 1894
25 51.5 73.9 1910 50 49.7 65.0 1898
Pentane mean: 49.75 ppm Pentane standard deviation: 3.85%
Hexane mean: 67.86 ppm Hexane standard deviation: 4.89%
Retention times: Column temperature 27°C
Air 2.7 s OV-101 stationary phase
Pentane 3.36 s Helium carrier gas
Hexane 4.55 s

This use of the percent standard deviation is a valid measure of the re~
producibility of the instrument for single measurements, that is, any
single reading can be expected to be within +2 standard deviations of the
mean with a 95% confidence level. When the instrument is used to repeat-
edly sample the atmosphere or a gas sample, the overall reproducibility of
the final time weighted average (TWA) figure will have substantially
smaller percentage error, assuming all other conditions are constant. The
percentage error will theoretically decrease by a factor of vn, where n
is the total number of samples. During an 8-hour day, the instrument can

24


The capillary column and gas feedthrough ports are fabricated in two
separate photolithographic and etching steps. The starting material is

a 200-um thick, 5-cm diameter silicon wafer of (100) surface orientation.
The capillary colum is first formed by transferring the 1.5-m long
folded spiral into a 1-ym thick layer of Si02. Since the spiral does

not follow any particular crystallographic direction, an isotropic HF-
nitric acid etchant is then used to etch the spiral groove into the
silicon. The remaining Si0g is then removed, and more Si09 is grown as
the etch mask for the feedthrough hole etch, The holes are etched through
the wafer using an anisotropic etchant in order to minimize their volume.

The wafer is again stripped of the remaining oxide and is placed in con-
tact with a 5-cm square piece of Pyrex glass. A relatively low tempera-—
ture anodic bonding process is used to hermetically seal the wafer to the
glass. The silicon and glass are heated to about 400°C and a potential
of 600 V is applied to the structure. The resulting electrostatic field
pulls the two materials together, and a bond is quickly formed. The seal
between the glass and silicon is hermetic and irreversible, thus forming
the etched spiral groove into an enclosed capillary tube. This unlined
capillary column must be coated with a column lining before any chromato-
graphic separations can be obtained. The methods of column lining will
be discussed in Section 4.5.1.

2.3.2 Sample Injection Valve

The purpose of the sample injection valve is to introduce a very brief
pulse of sample gas into the carrier gas stream at the head of the capil-
lary colum. For the miniature GC, the volume of injected sample is
about 10 nl, which generally requires that the valve remain open for about
5 ms. This volume of injected sample is about three orders of magnitude
smaller than what can be obtained with the smallest commercial valves

and connecting tubing, so a custom, integrated one-valve injection system
has been developed for use with the GC colum.

This injection system uses a miniature, normally closed, solenoid~actuated
diaphragm valve with an etched silicon valve seat, as shown in Figure 2.5.
The valve seat is fabricated using two isotropic silicon etches on the
opposite side of the wafer from the capillary colum. The details of the
etched valve seat can be seen in the SEM photograph, Figure 2.6. The
nickel diaphragm is held against the valve sealing ring by the body of
the solenoid. An adjustable compression spring normally forces the sole~
noid plunger and diaphragm against the valve seat. When the solenoid is
energized, the plunger is pulled back into the body of the solenoid, al-
lowing the diaphragm to relax. The gas on the input side of the valve is
held at a slightly higher pressure than the output, so that the gas is
forced over the valve seat, injecting a pulse of gas into the capillary
column. When the solenoid is turned off, the plunger again forces the
diaphragm against the valve seat, stopping the flow of gas. Since the
valve is fabricated directly on top of the capillary colum, there is
little dead volume associated with the feedthrough hole connecting the
valve seat and the column. The volume of the feedthrough hole on the out-
put of the valve is only about 4 nl, and this volume is swept quickly by
the 2 to 5 Ul/s carrier gas flow. Experiments with this valve design

10


PRESSURE!
HIGH SENSOR COLUMN

RESISTANCE RESISTANCE
Ne WN >
ON/OFF EXHAUST
VALVE
HIGH PRESSURE SURGE
GAS CYLINDER TANK

Figure 3.2. SCHEMATIC OF THE CARRIER GAS SUPPLY SYSTEM.

a wide range of helium source pressures and provides a nearly constant
carrier gas supply to the GC. With a 15-cm3 surge tank and the nominal
3 ul/s carrier gas flow, the carrier supply pressure will drop about

7 x 102 Pa (0.1 psi) during the course of an analysis.

3.2 SAMPLE INJECTION SYSTEM

The sample injection system chosen for the GC is the simple one-valve
system shown in Figure 3.3. An atmospheric sample is drawn through a
check valve and through the input side of the injection valve by a servo-
driven piston pump. After about 0.6 cm3 of sample has been gathered, the
pump reverses, pressurizing the gas sample. When the pressure of the
sample reaches a predetermined value, the computer opens the injection
valve for a few milliseconds, causing a few nanoliters of sample to be
injected as a brief plug of gas at the head of the separating column.

The remainder of the sample is bled off from the pump before the next
sample is taken. The pump is capable of reaching pressures as high as
3.8 x 105 Pa (55 psig) although typical injection pressures are around
1.7 x 105 Pa (25 psig).

3,3 ANALOG ELECTRONICS

The primary purpose of the analog electronics section is to condition the
outputs of the various sensors in the system, including the GC detector,
for use by the microprocessor. The main portion of the analog electronics
is involved in driving and amplifying the detector. A block diagram of
the detector electronics is shown in Figure 3.4. A constant current
source is used to heat the nickel film resistor. Another current source
drives a reference resistor to reduce the effect of power supply variations
on the output. The difference between the reference voltage output and
the detector output is amplified by a gain of 5 and then fed into a high
gain, auto-zeroing amplifier. The auto-zeroing feature is important for
the high gain amplifier since the ambient temperature of the GC colum

is not controlled. At a total gain of about 5000, the amplified detector
output would drift beyond the dynamic range of the amplifier due to small
changes in ambient temperature. The high gain amplifier is zeroed before
each chromatogram to keep its output within the range of the analog to

16


2.3.3 Detector Configuration and Fabrication

After an extensive review of GC detector technology, a thermal conduc-
tivity detector (TCD) was chosen for use in the integrated gas chromato-
graph. The advantages of the TCD include fast and general response, ease
in fabrication and adaptation to the miniature system, ruggedness, and
simplicity of support electronics. The detector was designed to be re-
movable from the substrate GC wafer so that it could be cleaned or re-
placed in the event of detector contamination or failure.

The detector structure is shown in cross section in Figure 2.7. The gas
flowing through the capillary column exists through a feedthrough hole to
the detector gas channel. The separate detector chips are mechanically
clamped to the backside of the GC wafer, forming one side of the detector
channel. The sensing element is a thin-film nickel resistor which is
supported by a thin Pyrex glass membrane. This glass membrane supports
the resistor and forms one wall of the gas channel. The resistor is
heated by an external current source, and its temperature is monitored
by measuring the voltage dropped across the resistor. When a sample peak
passes by the detector, the thermal conductivity of the carrier gas is
decreased, which causes a corresponding increase in the temperature of
the detector. This temperature increase causes the detector voltage to
increase which is a measure of the concentration of the sample gas. The
operation of the detector is covered in more detail in Section 4.3.

PYREX GLASS

Si GC SUBSTRATE
ETCHED GAS CHANNEL

ae
1.5 PYREX

1000 & NICKEL FILM GLASS
DETECTOR Si
ETCHED DETECTOR CAVITY SUBSTRATE

Figure 2.7. CROSS-SECTIONAL VIEW OF DETECTOR STRUCTURE. Vertical scale
exaggerated for clarity.

The detector chips are separately batch fabricated using integrated circuit
processing techniques and silicon etches. The starting wafers are 200-um

12


Figure 4.1. TYPICAL CHROMATOGRAM FROM THE REPRODUCIBILITY TESTS. Peaks
are (A) air, (B) pentane, (C) dichloromethane, (D) hexane,
(E) benzene, and (F) trichloroethane. Sample compounds are
approximately 50 ppm concentrations.

take more than 400 samples, so that the final TWA figures can be expected
to be considerably better than the single reading percent standard devia-
tions. For example, for the pentane results shown in Table 4.1, the mean
for each column of 25 analyses are found to be 49.468 ppm and 50.044 ppm.
These two results have a standard deviation of only 0.82%, which is quite
nearly a factor of /25 better than the single measurement error of 3.84%
standard deviation.

The results of a series of tests with pentane, hexane, and carbon tetra-
chloride with concentrations from 10 to 1000 ppm are shown in Figure 4.2.
The reproducibility of pentane is better overall than hexane, for example,
because the retention time of pentane is shorter than that of hexane,
which means that for equal concentrations, the peak height of pentane will
be significantly higher than that of hexane. The increased peak height
improves the signal to noise ratio, which improves reproducibility, es-
pecially at small concentrations. In these tests at room temperature, the
respective retention times for air, pentane, hexane, and CCl4 were about
2.7, 3.3, 4.6, and 6.0 seconds. Since the pentane peak is only about 500
ms from the air peak, pentane peaks less than 20 ppm are lost in the tail
of the air peak by the present computer program.

The reproducibility of the instrument at larger concentrations of about
2.5% standard deviation is quite acceptable. It is important to identify
the sources of error at the different concentration levels in order to
determine possible future improvements in system performance. The sources

25


PHOTOLITHOGRAPHY PROCESS

Silicon eS

sicads
l~Silicon

(a)

Photoresi St pee

Mask (glass)

Glass
a

(e)

eg——— ea
<<< ca

(f)

(e)

Figure 2.2. THE STEPS IN THE PHOTOLITHOGRAPHIC PROCESS ARE: (a) a thin
layer of Si02 is thermally grown on the silicon surface;
(b) a layer of photosensitive material is spun onto the oxide
layer; (c) a glass mask is placed in contact with the wafer
and exposed to UV light; (d) the glass mask has a negative
image of the desired pattern; (e) the unexposed photoresist
is developed away; (f) the oxide layer is etched in buffered
HF; and (g) the photoresist is chemically removed.

of the silicon, the type of etchant used, and the conditions under which
the etch is performed. An anisotropic etchant such as potassium hydroxide
(KOH) in water will produce V-shaped grooves in (100) oriented silicon,

as shown in Figure 2.3a. This etchant will not attack the (111) crystal
planes in silicon, which form the V-walls. For a relatively narrow groove,
where the V-walls meet at the bottom, the depth of the grooves can be


wafer. The column is formed by etching a long spiral groove on one sur-
face of the silicon wafer and the groove is converted to a tube by hermeti-
cally sealing a Pyrex glass cover plate to the silicon surface. The inte-
grated sample injection valve consists of a solenoid actuated diaphragm
valve in conjunction with an etched silicon valve seat and orifice.

The integrated thermal conductivity detector consists of a miniature nickel
film resistor supported in the output gas stream by a glass and silicon
structure. All of the components of the chromatograph are fabricated using
novel "micromachining" techniques.

A small servo-driven syringe pump serves to draw in the air samples from
the environment and pressurize them for injection into the column. The
helium carrier gas for the chromatograph is supplied in replaceable 15-
cm”, high pressure gas cartridges. The gas is delivered by a miniature
pressure regulator consisting of an integrated silicon valve, resistive
gas path, pressure sensor, and 15-cm? surge tank. Both the sample pump
and regulator are under the direct control of the computer.

An advanced computer system controls all aspects of the operation of the
instrument. The system consists of a powerful microprocessor chip, pro-
gram memory, data memory, an analog-to-digital converter, a keyboard,

and display. The computer controls the automatic sequence of events which
cause the instrument to periodically inject atmospheric samples to the GC.
As the separated sample peaks evolve from the chromatograph, the computer
filters the data to remove noise, measures their retention times for pur-
poses of identification, and determines the concentrations of the gases
in the atmospheric sample. This data is used to update the peak and TWA
concentrations of the 10 selected contaminant gases.

The instrument is capable of operation in two different modes. The device
can be used as a survey instrument, for rapid and accurate sampling under
command of an industrial hygienist. In this mode the instrument is in-
structed to take an atmospheric sample. After about 30 seconds the analysis
is complete, and concentration information is immediately available for
any of the 10 selected contaminants, This rapid analysis should prove
useful in determining the location and extent of potentially harmful
conditions around a work place. The instrument is also designed to per-
form automatic analyses at about 1.5 minute intervals. The device can
then be used to record the TWA exposure and peak concentrations encountered
by a mobile worker during an 8-hour shift. In this mode an alarm is
activated whenever the concentration of a gas exceeds a preset value,
warning the worker immediately of potentially dangerous situations.

Realization of the miniature GC gas analysis system is another example of
the benefits made possible by the constructive application of silicon inte-
grated circuit processing techniques to problems in diverse fields. The
instrument should greatly expand the ability to monitor workers' exposures
to toxic gases and therefore aid in the reduction of such exposures, and
provide better epidemological data in future health studies. In such an
application, the device should prove to be a useful analytical tool and
should have significant impact on the fields of industrial hygiene and
occupational safety.

66


detector signal approximately equivalent to the drift introduced by am-
bient temperature variations. A filtering algorithm in the computer
program is used to compensate for these drifts.

The flow of gas past the detector also causes some degree of noise. This
noise is thought to be due to extremely slight flow instabilities in the
detector gas channel. The flow itself is highly laminar, due to the small
dimensions and low gas velocity, but these slight instabilities can be
detected as noise. In the prototype instrument electronics, this detector
noise is about the 0.5-uV peak-to-peak level, somewhat under the 2-1V
peak-to-peak noise of the current source and input noise of the detector
amplifier. At high detector currents, which produces higher detector
sensitivity, the flow noise becomes the dominant noise source of the sys-
tem.

4.4 COMPUTER SYSTEM

The miniature gas chromatograph described in this report is capable of
significant analytical performance. The unique capabilities of the device
are realized only when the device is controlled by, and the results ana-—
lyzed by, a sophisticated computer system. Since the chromatograph can
separate and detect up to 10 vapors in less than 10 seconds, the data ana-
lysis system must operate quickly and be compact enough for inclusion in
a portable package. In addition, in order to make a truly useful instru-
ment, the operation, calibration, data storage and display functions must
be convenient and complete. The computer hardware system and its asso-
ciated programhave been designed to fulfill these requirements as com-
pletely as possible given the present state of the art in miniature, low
power electronics.

The computer system was designed to be as flexible as possible, giving the
computer access via the A/D converter to the pressure and temperature
sensors and the amplifier outputs which describe the state of the analyti-
cal system. The computer has been programmed to use this information to
run the system and has a complex data analysis section to process the
detector output signal. In addition, a significant portion of the com-
puter program is dedicated to interfacing the electronics system to the
user via a keyboard and display. The algorithms and programs necessary
to run the system are described in the remainder of this section.

4.4.1 Data Analysis

The most difficult part of the computer program is associated with analyz-
ing the detector output signal. A typical chromatogram is a rather complex
signal, especially for a modest computer system. There is often a pres~
sure artifact after injection, a very large air peak, followed by any num-
ber of potentially very small gas peaks. The uncontrolled GC temperature
introduces considerable drift to the amplifier output along with both
flow noise and noise from the electronics. In order to measure the con-
centration of an output peak the area of the output peak must be measured
and compared to the area of the air peak. The major analytical problem
is thus reduced to finding the area of a small output peak in the presence
of noise and drift.

51


4.1

4.2

TABLES

Results of reproducibility test. ......-e ee eee

Analog electronics performance .... 1... esse ee

ix


Figure 3.8. PHOTOGRAPH OF THE INTERIOR OF THE PROTOTYPE INSTRUMENT. The
GC is in the lower right and the digital computer board oc-
cupies most of the remaining top layer.

and the electronics, batteries, and carrier gas supply could be carried
on the belt. After the battery and electronics systems are reduced in
size and weight, the total system will be much easier to wear for an 8-
hour shift.

22


258.

3 U4
Sed
g
258. a@L___1 1 L L 1 1 1 1 1 1 L L L
2.88 1.88 2. 88 3. 88 4.88
TIME (€ ee0 >
258. Bi
a
3
8 8.20
258. 21 L L i] L n oe | n 4 | 44 1
8.82 1,88 2. 88 3. 88 4.68
Figure 4.20. COMPUTER SIMULATION OF GAUSSIAN PEAK WITH NOISE. (a) Before

filtering; and (b) after filtering.

58


a wide peak would not be matched to the characteristics of the filter and
the benefits of the filtering would be substantially reduced. It is not
practical to change the filter characteristics during the course of the
chromatogram, so "time" is changed instead. The sampling period is
linearly increased during the chromatogram so that each peak is a constant
number of sampled points wide. This then guarantees effective filtering
for all of the peaks at the slight disadvantage of having to convert from
sample point units to real time for all retention time calculations. This
process of changing the sampling rate has been called "swept time". The
match between standard deviation increase and linearly increasing period
is quite close, so that this swept time algorithm is well suited to the
needs of the portable chromatograph.

4.4.2 Operating Program Flow

The peak finding section is a part of a larger program which operates the
system, As shown in diagram form in Figure 4.22, the program proceeds as
follows. Upon a signal to start an analysis the computer first takes a

reading of the column temperature. This is used mainly to calculate the
expected retention times of the sample peaks of interest for that chroma-
togram. Time windows are then set up so that the computer knows where to
look for the zero crossing of a peak, The high gain amplifier is zeroed
to take care of any amplifier zero drift since the last analysis and the
high pressure valve in the carrier gas supply is opened to charge up the

BEFORE DURING AFTER
INJECTION SEPARATION CHROMATOGRAM
POWER UP FILTER 1X OUTPUT CALC. PEAK AREAS
MEASURE TEMP. DETECT AIR PEAK DETERMINE CONC.
CALC. RET. TIMES INTEGRATE PEAK CHECK FOR ALARM
SET He PRESSURE STORE AIR DATA UPDATE TWA DATA
ZERO AMPLIFIER FILTER S@@@X OUTPUT DISPLAY?
ACTIVATE PUMP DETECT GAS PEAKS POWER DOWN
CHECK PRESSURE INTEGRATE PEAKS
PULSE VALVE STORE GAS DATA

Figure 4.22. SIMPLIFIED FLOW CHART OF THE MAIN PROGRAM.

60


pressure regulator surge tank to the correct pressure. The computer
monitors the pressure sensor on the surge tank until the desired pressure
is reached, when it closes the high pressure valve. The computer then
signals the sample pump to withdraw, pulling in an atmospheric sample
through the sample input system. By monitoring the pressure drawn by the
sample pump, the computer determines when the pump has finished drawing
in the sample, The pump is then reversed, pressurizing the sample. When
the desired sample pressure is reached, the computer fires the injection
valve, which introduces the sample to the head of the separating column.
The computer then starts sampling the detector output through the A/D
converter. Just before the air peak is expected, the swept time circuitry
is activated, which begins to lengthen the signal sampling period. After
the air peak is detected the computer switches to the high gain amplifier
output and starts searching for peaks. When a timer expires signaling the
end of the chromatogram, the computer computes the area of the peaks,
calculates the respective concentrations, updates the appropriate maximum
and TWA concentration registers, and then shuts itself off, awaiting a
command from the keyboard or the automatic timer.

Calibration Routine--

It is possible for this machine to perform an automatic calibration cycle.
It has been programmed to automatically find and measure the retention time
of a gas peak and store the retention time and amplitude calibration factors
in memory. To calibrate the machine for a particular gas, that one gas is
connected to the input of the instrument and the calibration routine is
selected. The temperature coefficient for that gas must be previously
entered, along with the known concentration of the calibration gas. The
computer then performs the analysis and updates all pertinent calibration
coefficients,

This simple routine should reduce the amount of periodic service of the
device for calibration purposes and allows for on-site calibration to in-
sure the accuracy of the portable instrument.

Keyboard and Display Program--

For the prototypes, a simple keyboard and display were desired. The amount
of front panel space is limited so a large keyboard is not practical. A
16-switch keyboard was chosen along with a 4-digit display. When coupled
to the computer these limited devices are capable of a number of functions.

In the prototype instrument a large number of the internal parameters are
capable of being changed or updated by the user. There are 10 system
parameters plus 9 gas parameters for each of 10 different gases which can
be displayed or changed by the user. In addition there is a calibration
mode, normal sampling mode, and automatic sampling mode which can be
selected. A simplified chart of the keyboard routine is shown in Figure
4.23. When the machine is in its quiescent state, it is waiting for a
function key, either A, B, C, D, or E. If A is pushed the machine begins
a new analysis. B is used for displaying and changing the system param-
eters. For example, if B and then 1 are pushed, system parameter 1, the
gas peak threshold, is displayed. C is used to change the value of a gas
parameter. A sequence here might be to change the temperature coefficient
of gas number 5. The button sequence would be C,5, and then 3, the button
3 for the temperature parameter. The display would then zero, waiting for
the new value to be entered. To input .00029, for example, the button

61


The remainder of the analog electronics allows the computer to drive the
sample pump, injection valve and pressure regulator valve. In addition
there are voltage references and regulators to stabilize the battery
voltages and provide drive signals for the pressure and temperature
sensors.

3.4 DIGITAL ELECTRONICS
The digital electronics package consists of a microprocessor chip, its

associated program and data memories, a keyboard interface, and a liquid
crystal display, as shown in Figure 3.6.

COMPUTER SYSTEM

2-80 ara
POWER Hd
| DOWN J ¢ fi (}e0gRES 1]
SWEPT R BUS
TIME ig j i U i! I
R
LOK e cmos} [A] |éoaro] | ois
IS-
EPROM cON- | |inveRe
ALARM § RAM | \VERTER| | Face PLAY
5
0 l [ I I J
R CONTROL LINES

Figure 3.6. DIGITAL ELECTRONICS BLOCK DIAGRAM.

The microprocessor chosen for the system is a Zilog 2-80a. It isa
relatively high speed, 8-bit device with an advanced instruction set.
Since the power dissipation of this device is relatively high (1 Watt),

a unique powerdown system is used to minimize the time during which power
is applied to the high power consuming portions of the system. It is
anticipated that functionally equivalent Z-80 devices will be available
in 1981 which will dissipate about 20% of the present power. This advance
will be a significant help in extending the battery life of the system or
reducing the present battery size.

The computer's memory is divided into two sections, permanent program
memory and volatile random access memory. The program for the computer

is held in 10 kilo-bytes of electrically programmable read-only memory,
EPROM. Since there is no reason to change the program during its use,

the EPROM is programmed once and will store the program almost indefinitely.
The volatile memory is used by the computer as a scratch pad for calcula-
tions and to store system parameters, gas parameters, and data on the

19


DEPARTMENT OF HEALTH AND HUMAN SERVICES
PUBLIC HEALTH SERVICE
CENTERS FOR DISEASE CONTROL
NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH
ROBERT A. TAFT LABORATORIES
4676 COLUMBIA PARKWAY, CINCINNATI, OHIO 45226

OFFICIAL BUSINESS
PENALTY FOR PRIVATE USE. $300

Third Class Mail

POSTAGE AND FEES PAID
U.S, DEPARTMENT OF HHS
HHS 396

DHHS (NIOSH) Publication No. 81-115


4.4 Computer System. ......

4.4.1 Data Analysis.

4.4.2 Operating Program Flow

5.

REFERENCES .

GLOSSARY .

CONCLUSION . .

vi

51
51
60

65
67

69


CONTENTS

Abstract 26 6 ew we ew ee ee ee ee ee eee ee we we we ew Abd

1, INTRODUCTION. 2... 2 ewe ee we we

1,1 Background. . 2 6 ee ee ee ee ee ee

my

2. THE MINIATURE GAS CHROMATOGRAPH. . . 2... 2 ee ee wee ewe 5
2.1 Introduction to Gas Chromatography. .......s. ee ee 5
2.2 Silicon Etching Technology. ... 2... 2 eee eee eee 6
2.2.1 Etched Geometries... 2... ee eee ee eee ee 6
2.3 Miniature GC Fabrication. . 1. 1 1 ee ee ee ee ee 9
2.3.1 Capillary Colum... 1. eee ee ee ee ewe 9
2.3.2 Sample Injection Valve. .....+..2++.2e+e+e-.e. 10
2.3.3 Detector Configuration and Fabrication. ...... 12

3. INSTRUMENT SYSTEM
3.1 Carrier Gas Supply. ... 2... 2. ee eee eee ee ew ee 15
3.2 Sample Injection System... . 2... 2. eee ee ee eee 16
3.3 Analog Electronics. . . 2... 2 eee eee eee ee eee 16
3.4 Digital Electronics... 1 6 ee ee ee ee we ee ee ee 19
Bey 23t-( 2-1-0) ONO)ONOO)O OOOO Oo OOo ovo) G4)

4. PERFORMANCE
4.1 Overall Instrument Performance. . .... 2-24 ee ee ee 23
4.1.1 Amalog Electronics Performance ........2..+.. 28
4.2 GC Column Performance... . 1.2. e ee ee eee eee ee 29
4.2.1 Column Linigs. ... 1... 2. ee eee ee ee ee 29
4.2.2 GC Separations... 2. 1 eee ee ee ee ee ee 8
4.2.3 Optimization of the GC Column. ........... 34

4.2.4 Temperature Dependence... .....+ 52+ sss 40

4.3 Detector Performance. . . 1... ee ee ee eee ee eee 4D
4.3.1 Comparison with Other Detector ........... 42
4.3.2 Detector Operation... 1... ee ee eee ee ee 44

v


Sou SiO, MASK

36°:
SILICON

(b)

(c)
Figure 2.3. REPRESENTATIVE ETCHED CROSS SECTIONS IN Si.
(a) Grooves etched with KOH and water in (100)

Si. (b) Grooves etched with KOH and water in

(110) Si. (c) Grooves etched with HF-nitric
solutions in Si.

precisely determined by the initial width of the etch mask. In (110)
oriented silicon and along certain directions in (100) silicon, a KOH
etch will produce grooves with precisely vertical walls as shown in
Figure 2.3b. The choice of geometries is limited with these etches
because of their dependence upon the crystal structure of the silicon,

so that some desired features, such as round holes with vertical walls,
cannot be realized.

A mixture of hydrofluoric and nitric acids can be used as an isotropic
etchant to produce grooves as shown in Figure 2.3c. This etchant is
used in forming many of the features of the miniature GC system since it

produces nearly rectangular grooves, oriented in any direction of the
wafer.


4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

Golay plot of miniature chromatographic column with OV-101
column lining. . 2. 6 ee ee ee te ee eee

Optimum plate height and separation factor as functions of

Zt ee ee

Optimum carrier gas velocity as a function of Zo wee

Number of effective plates as a function of maximum allowable

carrier gas pressure and etched depth. .... 2... eee

Partition ratio as a function of 1/T for chloroform and
methylene chloride... 1. ee ee eee eee

Detector cross section... se ee ee eee ew eee

Relative detector response for the three detector driving
systems. 2 6 eee ee ee ee ee ee te ee

Graphical solution to detector operating points for dif-
ferent detector currents... 6 ee ee eee ee eee

Detector operating point temperature and nitrogen peak
voltage as a function of detector current... ....e+

Thermal conductivity of Ar/He gas mixtures as a function of
the percentage of Ar in the mixture... 1... 2 ee eee

Linearity of detector sensitivity as a function of argon
mole fraction in the helium output stream... ......

Impulse response, h(t), of the optimum filter. ......

The resultant waveform after filtering a gaussian input
Signal . 6 6 6 we ew we ee we ewe ee ee te eee

Graphical representation of the filtering process. ....
Computer simulation of gaussian peak with noise. .....
Flow chart for peak finding and area calculations. ....
Simplified flow chart of the main program. ......+-e.
Keyboard command description... . + ee ee eee eee

Gas and system parameter listing... . +. ++ see eee

viii

37

38
38

39

42
44

45

47

49

50

50

53

54
56
58
59
60
62

63




United States Public Health Service
Department of Health Centers for Disease Control
and Human Services National Institute for Occupational Safety and Health

EPA ene SEE come
A Prototype Gas
Analysis System
Using a Miniature
Gas Chromatograph

Interagency

Energy/Environment
R&D Program
~ Report



A Golay plot of H as a function of carrier velocity can be generated
experimentally, and a typical plot for a miniature column is shown in
Figure 4.6. The values of B, Cys and Cg as well as the optimum carrier
velocity can be derived from this plot. Unfortunately, almost all of
the terms of Equation (4) are functions of column geometry and partition
ratio parameters; however by assuming typical column operating conditions
and plotting Equation (4) as a function of a specific column parameter,
some insight can be gained into that parameter's effect on the operation
of the column.

0.5

0.4

0.3

PLATE HEIGHT, H (mm)

1 1 1 1
fe) 0.2 0.4 0.6 0.8 1.0
CARRIER GAS VELOCITY (msec)

Figure 4.6. GOLAY PLOT OF MINIATURE CHROMATOGRAPHIC COLUMN WITH OV-101
COLUMN LINING.

The most signficant geometrical feature of the miniature capillary column
is its roughly rectangular cross section with very shallow depth (Z). The
calculated values of H,,; and Nop¢ are plotted in Figure 4.7 as a function
of column depth. The carrier gas velocity required to achieve these opti-
mum values is plotted in Figure 4.8 as a function of column depth. It is
obvious from Figure 4.7 that the best column performance (largest number
of effective plates, N) is realized as the etched column depth is decreased.
However, the price to be paid for increasing the column efficiency by re-
ducing its etched depth is the much higher pressure drop across the column
which is required to achieve the optimum carrier gas velocity. Given a
maximum operating pressure, Equation (4) can once again be used to show
how N varies with column depth. Such curves are plotted in Figure 4.9 for
several values of maximum carrier pressure. Clearly there is an optimum
column depth for each value of carrier pressure.

37


1. INTRODUCTION

1.1 BACKGROUND

The miniature gas chromatographic air analyzer is a unique instrument con-
bining a gas chromatograph fabricated on a silicon wafer and a sophisti-
cated microcomputer system for analyzing the data and controlling the
device. The development of the miniature system has depended principally
on advances in the field of integrated circuit processing. The reduction
in size of the gas chromatograph is due to the application of silicon
etching technology to the miniaturization of the separating column, injec-
tion valve, and detector. The sophisticated electronics system needed to
control the system has been made possible through the use of recently
introduced one-chip microprocessors and high density, low power memory
chips. The combination of these technologies results in an instrument
with unique analytical and field capabilities.

There is a widespread need in the industrial hygiene field for the rapid
and accurate monitoring of potentially hazardous concentrations of many
gases in the air. Currently there are several types of portable, direct-
reading instruments for the monitoring of single contaminants. These
instruments lack the versatility needed to adequately monitor the work
place. A variety of small, passive monitors are becoming available, but
these either require expensive and time-consuming analysis in the labora-
tory or are designed for only a single contaminant. A gas chromatograph
(GC), which is capable of separating, identifying, and measuring the con-
stituents of a gaseous sample, is an ideal instrument for performing the
analysis of an industrial environment. A conventional GC, however, is
much to bulky to be used either for a portable survey instrument or as an
exposure monitor for a mobile worker.

The reduction in size of the components of a GC realized by using silicon
etching technology have made a portable instrument possible. The feasi-
bility of such a portable instrument_was studied at Stanford University
under NIOSH contract #210-76-0140 ( and a prototype, which is the subject
of this report, has been fabricated. The prototype is capable of automatic,
real-time analysis of atmospheric gas samples for up to 10 different vapors
at concentrations down to a few parts-per-million. These instruments
promise to significantly aid in the investigation of potentially harmful
environments.

The work on the miniature GC started at Stanford University in 1971 in an
attempt to develop a miniature instrument for NASA to fly on the Viking

landers to Mars. A separating column was rapidly developed, but there was
insufficient time to develop the integrated valve and detector structures

1




THERMAL CONDUCTIVITY

0.12

o 0:08
-/,
c]
2/3
€
0.04
re) 1 i 1 rl L 1 1 L 1 1
ie) 20 40 60 80 100
PERCENT ARGON IN HELIUM
Figure 4.15, THERMAL CONDUCTIVITY OF Ar/He GAS MIXTURES AS A FUNCTION OF
THE PERCENTAGE OF Ar IN THE MIXTURE.
16
2 1,5-
°Q
=
2 14h
a
u
WwW 1.3
qj
£
Fah
w
a
oa)
”
i
fo}
10° 10° 10° 109 10? 16! 1
ARGON MOLE FRACTION
Figure 4.16. LINEARITY OF DETECTOR SENSITIVITY AS A FUNCTION OF ARGON

MOLE FRACTION IN THE HELIUM OUTPUT STREAM.
50


The sketch of an idealized chromatogram shown in Figure 4.5 defines the
parameters which characterize an output peak. The retention times from
sample injection to the maximum of each peak for the unretained air peak
and the sample peak are ty and tp respectively. An important measure of
the operation of the column is the length of time that the sample is
retained by the stationary phase. This is the adjusted retention time
tk, given by th = tp - ty. For a gaussian peak with standard deviation o,
the peak basewidth is defined as W = 40.

w
$
a
cs iv
e\2 a
rd
i |
Lb ty—- + 1, TIME
{ tr
w
é
an
wie
c
© g
Ble |
a
1.5,
5{}-_t
iM re TIME
—tre

Figure 4.5, IDEALIZED CHROMATOGRAM SHOWING RETENTION TIMES AND PEAK WIDTHS.

One practical measure of column efficiency is the effective number of
theoretical plates (N) in the colum,

2
i
N= 16 () qa)

It is desirable for N to be large, indicating a well retained yet very
narrow peak, The number of effective plates calculated for the peaks in
Figure 4.4 ranges from 385 for peak #2 to 2300 for peak #8. For 30-m long
and efficient standard capillary columns, the number of effective plates
typically ranges between 10,000 and 100,000. The improvement in perfor-
mance of the conventional column is due to its greater length and the
superiority of its stationary phase distribution.

33


DIGITAL ELECTRONICS
AND COMPUTER BOARD

ANALOG CIRCUITRY

TURI
MINIATURE, co E
se HOUSING

TOP VIEW: MIDDLE LAYER TOP VIEW: TOP LAYER

fe

20cm:

6V BATTERY

4

+ SYRING!
6V BATTERY Bone

6V BATTERY [
—————— PUMP
a SERVO
HELIUM TANK,

ae
i
a

k 20 cm +
Pressure} >
SENSOR: pam
MINIATURE DIGITAL BOARD LEVEL
ULAR SC COLUMN
= WAFER Aa HOUSING
em | ANALOG BOARD LEVEL enn
SURGE TANK VALVE:
x 3) L_ SYRINGE
‘GC HOUSING SURGE TANK PUMP.
HIGH, Sc \ aoa
PRESSURE
VALVE WAFER ‘SUPPORT
SOLENOID
TOP VIEW: BOTTOM LAYER INTERNAL VIEW FROM FRONT

Figure 3.7. LAYOUT OF THE PROTOTYPE MINIATURE GAS ANALYSIS SYSTEM.

21


10 0.1

% ar 70.08
SF 4 =
5 §
FE 6r 70.06
g x
m4 4 g
Fa 2
—E 4 0.04 wi
q E
& Sopt J fs)
& Hopt

® oL 40.02

1 1 1 1 1 1 1 r 1
ite) 20 30 40 50
Zq (microns)

Figure 4.7. OPTIMUM PLATE HEIGHT AND SEPARATION FACTOR AS FUNCTIONS
OF COLUMN DEPTH (Z)).

100;

80
s
g 60
€
&
Baot
>

20-

A L A L 1 L A 1 1
10 20 30 40 50
Zo (microns)

Figure 4.8. OPTIMUM CARRIER GAS VELOCITY AS A FUNCTION OF 2:

38


concentrations which it has determined. These results are held in 2
kilo-bytes of extremely low power CMOS memory which is powered continu-
ously, even when the machine is "turned off". The keyboard allows the
user to completely control the operation of the instrument by displaying
and by modifying the parameters which are used by the computer to control
the instrument.

In addition to the interfaces with the analog electronics, the digital
system includes a 16 function keyboard and a 4-digit liquid crystal dis-
play. The keyboard offers multiple levels of control depending upon key
sequence. It can be used to retrieve the sample gas concentrations, in-
put calibration data to the computer, control the operation of the machine,
and retrieve other important parameters such as carrier gas pressure,
temperature, and peak retention times. The display is used to output all
computer data and to check the accuracy of inputted calibration parameters.

3.5 PACKAGE

The complete instrument is housed in a package about 20 cm x 22 cm x 10 cm
(8" x 9" x 4") which weighs 2.8 kg (6.2 lbs). The details of the physical
layout of the package are shown in Figure 3.7 and in the photograph,
Figure 3.8. A majority of the volume and weight of the present package is
taken up by batteries and electronics boards. It is anticipated that the
present battery weight of 765 gm, which is 27% of the total, will be re-
duced by a factor of about 2 when the lower power digital components
become available. The volume associated with the electronics can be re-
duced by at least a factor of 2 by using multilayer printed circuit boards.
Since the digital system design is still subject to modification as im-
proved components become available, wire-wrap techniques, which are more
easily changed than printed circuit boards, are presently used in the
prototype instruments. It is further anticipated that new interface and
memory components will become available which could be used to further
reduce the size of the electronics boards.

The computer controls the entire system, so there is no need for user
adjustable flow controls, zero controls, or meters to read. The front
panel only contains a keyboard to interface with the computer, a mode
switch to select automatic or survey mode, and the digital display. The
only maintenance tasks are replacing the high pressure helium cartridges
and recharging the batteries. The high pressure helium cartridges are
changed by removing the empty cartridge from a holder on the bottom of the
unit and placing a full cartridge in the holder. A screw is then rotated,
forcing the cartridge against a puncturing needle and a sealing gasket.
The entire operation takes less than 1 minute, and the helium cartridges
are available from a commercial source for about $1.75 in quantities of
1000. Since the cartridge lasts for at least 4 days of operation, the
helium operating costs are quite modest.

The complete GC subsystem occupies about 15% of the present instrument
package and a similar percentage of the weight. The GC system could be
separated from the electronics, if a worker-wearable version of the system
is desired. The GC could then be worn by the worker near the breathing zone,

20


Errors-—

In the miniature system the main sources of error in the peak area calcu-
lations are due to the finite sampling rate, which limits the precision
in finding Sys if Oy is calculated directly from t,;, and ty,,. Since
there are usually only about 30 sample points between tyi, and tmax, an
uncertainty of one sample point is about a 3% error. In the presence of
noise, the uncertainty is increased due to nonideal waveforms. There are
basically three ways to improve this performance. Conceptually the eariest
is to sample more frequently, which would increase the number of points
between tmin and tmax, and thus improve the resolution of determining oy
In order to achieve large increases in performance, however, the compu-
tational speed would have to be proportionally increased. The micro-
processors necessary to perform these calculations are not presently
available, so this solution is not now feasible. The other two methods
use interpolating filters or curve fitting to achieve effectively greater
time resolution than the sampling interval. The curve fitting procedure
was found to be very helpful, but is very time consuming, taking many
seconds per peak area calculation. This is not desirable for the portable
instrument. It is also possible to use a class of interpolating filters
to increase the effective time resolution. This procedure seems to be
the most promising to increase the accuracy of the oy determination and
thus the area measurement, and it will be investigated at the earliest
opportunity.

An additional source of error is due to the action of the differentiator
in the filtering algorithm on the input signal, In the presence of drift,
the differentiator will change the drift into an offset. This offset
shifts the position of the zero crossing of the filter output signal. For
peaks with large retention times and low amplitude, the baseline offset
signal may be so large that no zero crossing is detected. The computer
program will be modified in the future to compensate for this baseline
offset to increase ability to detect the later peaks.

Filter Simulation--

The effectiveness of the filtering system can be seen in the computer
simulation results shown in Figure 4.20. The top output shows a gaussian
peak plus 1/f noise before the filtering,and the lower output is after the
filtering algorithm. A computer simulation is useful in analyzing the
performance of the filter since the amplitudes of the peak and the noise
can be easily adjusted and the "retention time" of the gaussian peak is
exactly known. The filter parameters and standard deviations can also
be changed to characterize the filter over a wide range of conditions. A
systematic characterization of various filtering and area algorithms will
be the subject of a forthcoming report.

Peak Finding--

The filtering scheme mentioned is designed to increase the signal to noise
ratio of the detector output signal and thus make it easier for the com-
puter to locate and measure the peaks. The filtering process is a part of
a section of the program dealing with finding and measuring the peaks. A
simplified flow chart of this program is shown in Figure 4.21. The main
criteria for locating a peak is the steep zero crossing in the filtered
data. If a positive maximum above a threshold is found in a time window,
followed by a zero crossing, a peak is said to occur. It is not until the
zero crossing, however, that the computer has determined that a peak exists,

57


1.1
2.1

2.2

3.5
3.6
3.7

3.8

4.2

4.3

4.4
4.5

FIGURES

Photograph of the prototype gas analysis system. ....
Schematic of a simple gas chromatography system. ....
The steps in the photolithographic process .......
Representative etched cross sections in Si.......
Photograph of the GC wafer... 1... eee eee ene
Cross-sectional view of sample injection valve.....
SEM photograph of an etched silicon valve seat. ....
Cross-sectional view of detector structure .......
Block diagram of prototype gas analysis system. ....
Schematic of the carrier gas supply system. ......

Sample injection system... 2... 2 ee ee eee ae

Schematic of detector current source and detector amplifiers

A/D converter and multiplexer system. ........004

Digital electronics block diagram. ........-2.24.4-.
Layout of the prototype miniature gas analysis system. .
Photograph of the interior of the prototype instrument .
Typical chromatogram from the reproducibility tests. . .
System reproducibility vs sample size. .........

Oscilloscope photograph of the amplified detector output
or chromatogram. «6 ee ee ee ee ee ee ee we

Miniature GC chromatogram using OV-101 stationary phase.

Idealized chromatogram showing retention times and peak
widths 6 6 ee ee et eee we te eee eee ws

vii

11
11
12
15
16
17
17
18
19
21
22
25

26

31
32

33


SOLENOID.

SOLENOID PLUNGER

6-32 —.
THREADED NUT

=
\Cseauine
SILICON RING
SUBSTRATE,

ORIFICE INPUT
r 3mm i

Figure 2.5. CROSS-SECTIONAL VIEW OF SAMPLE INJECTION VALVE. Vertical
scale exaggerated for clarity.

on very short columns have not shown any tailing of the injected pulse
due to valve dead volumes. Several valves have been put through a million
operations with no detectable change in performance.

Figure 2.6. SEM PHOTOGRAPH OF AN ETCHED SILICON VALVE SEAT. (A) Feed-
through hole to capillary column, (B) Valve seating ring.
(C) Feedthrough holes to sample input channels. (D) Valve
sealing ring.

11


gas peaks, but for air peaks the voltage from the detector can double,
which can introduce considerable nonlinearities. These nonlinearities
in response to large peaks can be computer corrected if a form for the
response is known.

In order to find the output voltage of the detector for a given peak con-
centration, Equation (9) is solved twice, once for the operating tempera—
ture Ty, and again for the peak temperature T' when the thermal conductance
is reduced by a gas peak to G§. The output voltage is then

ve = IR(T’) - IR(T,)

The output voltages for a given change in Gy have been found for a set of
detector currents I and the results are shown in Figure 4.14. Here current
is varied as a parameter and the operating point temperature and the out-
put peak voltage are both found. The experimental points are determined
by measuring the peak voltage for the same set of currents, and measuring
the temperature of the detector by measuring its resistance. The agree~
ment between the points is excellent over a wide range of operating con-
ditions. A wide variety of curves like this can be determined for in-
jected peaks of different sizes.

To determine the linearity of the detector, the output voltage must be
found as a function of changes in thermal conductivity, for a fixed de-
tector current. In order to convert these results to output voltage as
a function of mole fraction of the sample gas in the carrier gas, the
thermal conductance of the gas mixtures must be known. Theoretical re-
sults are not sufficiently accurate for most gas mixtures, so experimental
results of thermal conductivities of gas mixtures as compiled by Tseden-
berg |9J were used. The experimental points for argon in helium were
curve fitted as.shown in Figure 4.15. This result was used to find the
thermal conductivity change for a given argon concentration, which was
then used to find the output voltage at a given detector current. The
output voltage is then divided by the argon concentration to produce the
output detector sensitivity. A graph of theoretical detector sensitivity
vs argon mole fraction is shown in Figure 4.16 for a detector current of
12 mA and an operating point temperature of 50°C above ambient. The
linearity is good over the range from a part-per-million, which is near
the lower limit of detectivity of the detector, to a few percent argon in
helium, which is near the maximum sample concentration at the detector
during the passage of the airpeak.

The mounting of the detector in the output gas stream introduces sources
of noise to the system such as mass flow artifacts and flow noise. The
detector operates by dissipating heat by conduction through the gas stream.
Since the gas is moving past the detector, there is an additional com-
ponent in the detector heat balance equation given approximately by
CyVTqg/2, where Cy is the specific heat of the gas, V is the column flow
rate, and Ty is the detector temperature above room temperature. At a
constant carrier gas flow this heat loss is incorporated in Equation (9)

48


so that the original peak area, Ay, can be recovered by performing the
calculation.

Ay = KS oy (16)

where K is a constant which is the same for all input gaussians. Thus by
measuring the area S and tmay and tyi,, the gaussian area can be found.
This method is theoretically more reproducible than measuring the area of
the original gaussian peak, since much of the noise which could interfere
with the integration has been removed by the filtering process.

Filter Realization--

In a digital system, the filtered output signal, Y(t), cannot be deter-
mined continuously, but must be calculated at discrete time intervals.
This is achieved by first sampling the input waveform by an A/D converter
to obtain a number proportional to the input voltage. As the input is
repeatedly sampled, a set of numbers is obtained as a function of time.
This set is multiplied by a set of filter coefficients which are found by
sampling the impulse response of the filter, h(t) at the same rate. Each
filtered point is the sum of a large number of sampled points multiplied
by the filter coefficients. The larger the number of multiplications
performed, the closer the resulting sum will be to the actual value of
Y(t).

In a microcomputer realization of the digital filter, the speed at which
the multiplications can be performed is the limiting factor. A compromise
must be found in the chosen sampling rate of the analog signal, the pre~
cision of the numerical representation of the signal, the precision of
the filter coefficients, the number of multiplications performed for each
sample point, and the way the data points are stored in the computer's
memory.

In order to perform the filtering function, the convolutional integral,
Equation (13), must be approximated in discrete form as

m
ie 2s xh (17)

where

% are the filtered sample points

Xy are the sampled input data points

hy are the filter coefficients
The computer program to perform this filtering function is described below.
The digital representation of the detector amplifier signal is stored in

the computer memory along with the last 63 such sample points. After each
sample is taken and stored, the accumulator is zeroed, every other point

55


4.1.1 Analog Electronics Performance

The computer system, since it is digital, is immune to temperature and
battery voltage changes, as long as they remain within the operating
range of the devices. The analog electronics, however, is subject to
temperature and power supply variations. Most of these variations can
be minimized by appropriate circuit design, and the use of resistors and
capacitors with inherently low temperature coefficients. The primary
reference element in the analog electronics is a voltage source which is
stable to within 0.08% over the operating temperature range of 0 to 45°C,

The reduced results of the analog electronics test are shown in Table 4.2.
This test measured the changes over a temperature range of from 25 to 50°C
and over the maximum voltage range of the two batteries of +5 to +7 V and
-14 to -10 V. The two individual effects are listed along with the total
extreme over both temperature and voltage. The portions of the circuit

measured are the most important ones for the analytical performance of the

Table 4.2. Analog electronics performance.

Circuit Drift over Temp Drift over Supply Total Extreme

25°C to 50°C -14,7 to -10,5 T and V

Thermistor amp 0.064°C 0.034°C 0.10°C
equiv. drift

Detector amp 2.2 mV 0.34 mV 4.6 nV
zero drift

Pressure output 0.58 psi 0.07 psi 0.70 psi
zero drift

Pressure output 0.17% 0.37% 0.37%
30 psi

Swept time number 0.63% 0.72% 1.6%
10 seconds

Detector current 1.08% 0.65% 1.65%

instrument. The thermistor amplifier error is the output voltage error due
to the reference and amplifier converted to an equivalent temperature
measurement error. The detector amplifier zero drift is the total output
voltage change of the xl amplifier with a metal film resistor in place of
the detector. This 4.6-mV total change is less than 1 A/D level and in-
cludes the effects of current source changes on the input bridge, the
temperature change of the detector reference resistor, and the xl amplifier
drift. Even though this change is small, after an additional gain of 5000
the drift would be considerable. The high gain amplifier is zeroed before
each chromatogram to reduce the effect of these drifts on the high gain
output. If the instrument is subjected to a 20°C step change in tempera~
ture, the resulting high gain output drift is about 1 V during a chromato-
gram. The pressure errors include the temperature effects of the pressure

28




sensors along with the bridge driving voltage and the transducer ampli-
fiers. The swept time error is the change in the number of clock pulses
generated over a 10 second period by the swept time circuits. The 1.6%
total error corresponds to less than a standard deviation of one of the
output peaks. The relatively large (1.65%) total error in detector cur-
rent is of little importance, since. the detector sensitivity is changed
little by this difference, and the concentration calculation performed by
the computer is concerned only with the relative sizes of the air and
sample peaks, so sensitivity changes cancel when the ratio is taken.

4.2 GC COLUMN PERFORMANCE

The performance of any GC column is determined by a myriad of parameters,
including the thermodynamic properties of gases and liquids, and the
geometry of the column and its effects on gas flow. In general, the most
important factors are the stationary phase used and the geometry of the
GC column. Since the start of gas chromatography in the 1950's, there
have been numerous attempts to provide a complete theoretical description
of the gas chromatography process. The geometry of the miniature chroma-
tograph is sufficiently similar to that of conventional instruments that
the GC theory can be used to provide clues to help optimize the analytical
performance of the instrument. Unfortunately, the mathematical descrip-
tions of the actions of the GC linings do not indicate how to actually
line a column to produce maximally efficient linings. The lining tech-
niques must be separately developed for the modified geometry of the mini-
ature GC, and the theory can then be used to optimize the size of the
columns to perform specific separations.

4.2.1 Column Linings

The stationary phases used thus far in the miniature capillary columns
have been the same substances as those used in conventional gas chromato-
graphy. Modified lining techniques have been developed which achieve
reproducible, uniform distributions of the stationary phases on the column
walls. Most of the separations performed with the miniature capillary
columns were achieved with a silicone oil, OV-101, as the stationary phase.
The following sections describe the column treatment and lining procedure
for the OV-101 liquid phase.

Column Pretreatment~-

The construction of the integrated capillary column is such that the cap-
illary bottom and side walls are silicon while the top plate is Pyrex
glass; however, the silicon surface is coated with a layer of silicon
dioxide (Si0)) several tens of Angstroms thick which is formed during the
anodic bonding process. The SiO) and glass surfaces are naturally covered
with a high density of silyl ether and hydroxyl groups. Treatment of
these surfaces with an aqueous solution of a strong acid (e.g. concentrated
HC1) converts many of the surface ethers into hydroxyl groups.

Si. Si-O
So +20

si Z $i-OH

29


detectors, that is, the output signal is proportional to the concentration
of sample gas in the output gas stream, If the dimensions of the detector
are reduced, the sensitivity of the detector is not greatly affected.

This allows a thermal conductivity detector to be made on the size scale
required for the integrated GC without a penalty for the small dimensions
involved. The detector retains the concentration performance of the larger
detectors, and since the detector volume is so much smaller, the mass
detectivity is reduced to very small levels. The integrated TCD described
below is capable of detecting below 150 fg of injected pentane and has a
detector volume of less than 1 nl.

4.3.2 Detector Operation

The nickel film integrated TCD used with the miniature gas chromatograph
is shown again in Figure 4.11. A nickel film was chosen as the sensing
element because of its relatively high temperature coefficient of resis-
tivity (TCR), moderately high melting point, ease of deposition and
selective removal, and relative inertness. Most larger TCD's use tungsten
or tungsten alloys for the sensing element which have about the same TCR,
a higher melting point, but more difficult deposition than nickel. The
structure and fabrication sequence used for the detectors can accommodate

a wide variety of metals as sensors; nickel has performed adequately in
the initial tests.

There are a number of ways to heat the sensor. In a self-heated system
the choices are among the electrical circuit configurations used to drive
the detector. The three most common are the bridge, constant current, or
constant temperature. Each different drive configuration produces dif-
ferent sensitivity, thermal time constants, temperature extremes, linearity

PYREX GLASS

Si GC SUBSTRATE
ETCHED GAS CHANNEL

—_
1.5 PYREX
1000 & NICKEL FILM GLASS
DETECTOR Si
ETCHED DETECTOR CAVITY SUBSTRATE.

Figure 4.11. DETECTOR CROSS SECTION.

44


SAMPLE IN

CHECK
VALVE
SERVO PISTON PRESSURE
MOTOR PUMP SENSOR
ON/OFF
INJECTION
VALVE
CARRIER
WA
INPUT RESISTIVE ean SE
GAS PATH

Figure 3.3. SAMPLE INJECTION SYSTEM.

DETECTOR ELECTRONICS

© x1 AMP
LOW GAIN
OUTPUT TO A/D
DETECTOR / Seer x1000 AMP.
CURRENT (|
SOURCES

La

HIGH GAIN
OUTPUT TO A/D

AUTO-ZERO
SWITCH

OFFSET
INTEGRATOR

Figure 3.4. SCHEMATIC OF DETECTOR CURRENT SOURCE AND DETECTOR AMPLIFIERS.

17


0.01 -—

TT

DETECTOR POWER, WATTS

TTT

0.001

pot toy 4 1 Ll rn 1
O 20 40 60 80100 140 180 220 260

DETECTOR TEMP. ABOVE AMBIENT, °C

Figure 4.13. GRAPHICAL SOLUTION TO DETECTOR OPERATING POINTS FOR DIF-
FERENT DETECTOR CURRENTS. RS = 200 2, Gc, = 0.002 °C/Watt.

Ry = 200 2 and G, = 2 x 10-3 W/°C, the detector goes into thermal runaway
before 12 mA is reached. This first order theory is sufficient for analyses
of operation below about 200°C above ambient. In fact, the device does

not run away at 12 mA in the above sample because the TCR of the nickel
resistor decreases enough at high temperature to remain in a stable state,

Two of the more important characteristics of the detector are sensitivity
and linearity. The small signal sensitivities are determined by differ-
entiating equations such as (9) to get dV/dC, the change in output voltage
for a small change in sample gas concentration. This is usually done as-~
suming that dG/dC, the change in thermal conductance for a given change in
concentration, is constant. These assumptions are generally correct for

47


is retreived from memory, multiplied by the correct filter coefficient for
that point, and the result is added to the accumulator. After 31 such
multiplications the number in the accumulator is the filtered result for
that set of 64 data points and this result is stored. About 3 milliseconds
later another sample command is given, the 64th data point is erased

from memory, all previous data points are shifted down one place, and the
new data point is stored to begin the process again. This process is shown
in graphical form in Figure 4.19. Here the filter coefficients are shown;
these are the multipliers of the delayed sample points mentioned above.
When the gaussian peak is processed by the filter, the result is shown
below. The filter is seen to produce the signal, Y(t), which is nearly
the derivative of the input gaussian. The retention time of the peak is
the zero crossing point of the output signal, and twice the standard
deviation, dy, is the positive peak to negative peak time. In order to
obtain Ay, the area of the original gaussian, the area S$ must be found.
This is done by a simple point by point numerical integration of the
filtered signal.

x(t)

0 .
TIME a we
= aoc? [reey GAUSSIAN
| INPUT
POINT BY POINT; “7 ¥ 7"" *
MULTIPLICATION a
h(t) ore
Onn FILTER
irre . COEFFICIENTS
7 .
& See J
SUMMATION

x OUTPUT
DATA

«
a oeTM POINTS,
«

x x*
x y¢x%
PREVIOUS POINTS += FUTURE POINTS

Figure 4.19, GRAPHICAL REPRESENTATION OF THE FILTERING PROCESS. Each
output point represents the sum of the point by point multi-
plication of x(t) and h(t).

This particular filter is optimized for maximum signal to noise ratio at
the zero crossing of the signal, so that this point is used by the com-
puter can look at the data and determine the area of the peak at that re-
tention time.

56


ABSTRACT

The techniques of integrated circuit processing have been utilized to
miniaturize the main components of a gas chromatography system. A 1,5-m
long separating column, sample injection valve, and output thermal con-
ductivity detector are all fabricated on a single 5-cm diameter silicon
wafer, These components are combined with a sophisticated microcomputer
system, consisting of a microprocessor, solid state memory, analog-to-
digital converter, keyboard, and display, to form an advanced, portable
gas analysis and data logging system. The system can automatically
sample the air, measure the concentration of up to 10 contaminant vapors,
calculate and store contaminant concentrations, and estimate average
worker exposures. The computer system allows exceptional flexibility in
the retrieval of concentration information, ease in operation, and sim-
plicity of calibration. The instrument can be used as an exposure moni-
tor to sample the air at 1.5 minute intervals for an 8-hour shift, or
it can be used as a survey instrument to take and analyze samples and
provide concentration information within 30 seconds of sampling. The
small size, advanced capabilities, and ease of operation of the system
should have a significant impact on the fields of industrial hygiene and
occupational safety and health. The instrument will greatly expand the
ability to monitor workers’ exposures to toxic gases and therefore aid
in the reduction of such exposures, and provide better epidemological
data in future health studies.

iii


_ d(2Z) + Y)

k = ——— K (7)
Zoy

where

a
UF

thickness of the layer of liquid phase lining the colum

Kt
W

column width

The B/u term in Equation (4) accounts for peak spreading due to longitu-
dinal diffusion of vapor molecules (diffusion parallel to the colum axis)
Longitudinal diffusion occurs only while the vapor molecules are in the
mobile phase and can be characterized by a diffusion coefficient Dg for
the specific vapor in a specific carrier gas.

The C, and Cp terms can be regarded as the "resistance to mass transfer"
of thé solute vapor in the mobile and stationary phases, respectively.
They represent the departure from the ideal condition of instantaneous
equilibrium between the vapor in the gas and liquid phases. C, is asso-
ciated specifically with the parabolic carrier gas profile across the
column's vertical dimensions. The faster moving vapor molecules in the
center of the gas stream take longer to diffuse to the stationary phase
than do their slower counterparts near the column walls. C, can be re-
duced by reducing the column depth (thus shortening the diffusion path
for any vapor molecule) or by increasing Dg (speeding up the rate of dif-
fusion).

The Cg term accounts for the diffusion of vapor molecules into the liquid
stationary phase. Once a vapor molecule impinges upon the liquid surface,
it tends to migrate into the liquid phase as the vapor concentration in-
creases locally in the mobile phase. As the vapor peak passes by and the
mobile phase concentration decreases, the process is reversed, and the
vapor molecules in the liquid diffuse back to the surface and transfer to
the gas stream. The Cg term can be reduced by increasing the vapor dif-
fusion rate through the liquid phase (increasing Dg) or by reducing the
liquid layer thickness d.

Since the Equation (4) for plate height contains the carrier gas velocity

in both the numerator and denominator, it can be minimized with respect to
u. Such an optimization yields

36


The utility of the number of effective plates lies in its ability to
predict if a colum is efficient enough to resolve two peaks whose ad-

justed retention times tk y and tk) are very close. The standard measure
of resolution of two peaks is defined as

0 ope
po 7 tri

Ro Gi, FH)/2 @)

A resolution value of R = 1.5 results in peaks separated as shown in Figure
4,5B. The number of effective plates, N, needed for a colum to separate
two peaks with a resolution of R = 1.5 can be determined from

w= 32(>55) (3)

where asa

As an example, consider peak #4 of Figure 4.4. This peak is really two
unresolved peaks of chloroform and hexane for which @ = 1.05, Equation (3)
predicts that the column would need N = 14,122 plates to just separate
those two vapors. Since N was only 1067 plates for this particular column,
those two vapors were not separated and thus appeared as only a single
peak.

The ability of the miniature capillary colum to separate peaks is deter-
mined by the stationary phase, the column geometry, and the column opera-
ting conditions. The current columns 1.5-m long by 40-um by 175-um wide
and lined with OV-101, are only about 20% efficient. That is, the columns
exhibit only 20% of the maximum number of theoretical plates possible for
a column with those dimensions. Thus, improvements in the stationary phase
composition and lining techniques should result in a significant improve-
ment in column performance. Once the stationary phases have been improved,
the dimensions of the capillary columns can be optimized for given appli-
cations and specific separations.

4.2.3 Optimization of the GC Colum

The number of effective theoretical plates that a column possesses for any
gas is related to another parameter, the plate height (H) by

where L is the column length and k is the partition ratio for a gas of
interest and is given by

34


PEAK ANAYLSIS FLOW

SAMPLE DATA
AND FILTER
NO
YES
NO YES
GET RET. TIME INTEGRATE PTS &
OF NEXT PEAK DETERMINE SIGMA

AREA CALCULATIONS
AND DISPLAY

Figure 4.21. FLOW CHART FOR PEAK FINDING AND AREA CALCULATIONS.

and at that point half of the filtered peak data has already been calcu-
lated. The computer thus needs to continuously store the filtered output
points so that the data is available when a legitimate zero crossing is
found. After the zero crossing, the second half of the peak data is also
stored, and the complete peak data is later analyzed by the peak area calcu-
lation program.

Swept Time--

In order to achieve maximum benefit from the filter, the standard deviation
of the filter should be very close to that of the input gaussian peak. In
a chromatogram this is very rarely the case. The air peak is quite narrow,
with a standard deviation of about 20 ms. For a peak with a long retention
time, the standard deviation can increase by a factor of five or more. Such

59




in the detector temperature which causes an increase in resistance and
subsequently an increase in power, 12R(T), dissipated in the device.

This increase in power also increases the detector temperature which again
increases sensitivity. This feedback effect is normally stable, unless
the operating point temperature is set too high. If the current source
can put out unlimited power, then the detector can enter so-called thermal
runaway and destroy itself. Normally the characteristics of the current
source are designed to limit the maximum power available to drive the
detector so this condition is averted. This thermal feedback can cause
nonlinear response for very large peaks as described below.

The constant temperature mode can also be used, where the detector voltage
is constantly adjusted to keep the detector at a particular temperature.
When a gas peak goes past the detector, the voltage drive is reduced,
reducing the power dissipated by the detector. This mode is not as sensi-
tive as constant current at moderate (100-200°C) tempeatures, but does
prevent thermal runaway.

Basic thermal and electrical theory can be used to calculate the operating
point, temperature, temperature excursions, sensitivity, and linearity of
the integrated T@. It is most convenient to assume that the structure
is always in thermal equilibrium. The thermal time constant of the device
is about 1 ms, much smaller than any peak width, so that the input power
to the device always matches the power dissipated by the detector, which
is mostly conducted through the carrier gas. In the constant current case
the dissipated power is simply I2R(T). The power dissipated by the detec-
tor is approximately G)T(1 + BT/2) where G, is the thermal conductance of
the combination of carrier gas and detector support and 8 is the tempera-
ture coefficient. of thermal conduction of helim. To solve for the detec-
tor temperature a form of R(T) must be found. For the nickel films used
in these detectors the resistance is quite closely given by

R(T) = RB, exp (aT) (8)

where R, is the detector resistance at ambient temperature, and T is the
temperature above ambient. For nickel films the TCR, & = 0.00426 ocr],
is appropriate to use. To solve for the operating point temperature the
equation

Gyr ( + m). ve exp (AT) (9)

must be solved. This is usually done numerically, but a graphical solution
may be employed for purposes of discussion. The graph shown in Figure

4.13 shows the power dissipated by four constant currents through the
detector for a range of detector temperatures to 280°C above ambient.

Also plotted is the power dissipated by the detector for those same tempera—
tures. The points of intersection, A, B, and C, are the operating point
temperatures for 6, 8, and 10 mA. For the conditions used in this graph,

46


There are a wide variety of methods to detect the presence of small signals
in noise. Given some knowledge of the properties of the signal and the
noise, it is usually possible to select a system which enhances the ability
to measure the desired attribute of the signal and to minimize the effects
of the noise on that measurement. In the case of the gas chromatograph,
the signal is the detector voltage, which is proportional to the concen-
tration of gas in the output gas stream, and the noise consists basically
of random fluctuations in detector output, electrical noise introduced by
the amplifier, and a variety of drift components. Due to the chromato-—
graphic process, the gas concentration and hence the detector signal,

x(t), is quite nearly gaussian shaped with respect to time

A ( (t - so") (
x(t) = —*— exp [- —-— 10)
v2n o. 207
x x

This gaussian peak can be described as having three distinguishing charac-
teristics, a retention time, to, a peak area, Ax, and a standard devia-
tion, Ox, and all three must be measured by the computer system.

Filtering Algorithm--

In order to perform these measurements, the analytical problem is broken
into two parts, a filtering procedure to maximize the signal to noise (S/N)
ratio, and an analysis of that filtered data. The filtering procedure was
chosen to maximize the ability to determine the existence of, and measure
the retention time of, the gaussian peak. Filtering theory is then used
to find the optimum way of finding the retention times, to. The transfer
function H(U), of an optimum filter is found to be {io

*
H(v) = ae exp (-j2mt,) qu)
N

where
x) is the complex conjugate of the input Fourier transform
Yy©) is the power spectral density of the noise
K is a constant

In order to solve for the transfer function of an optimum filter, some form
of the power spectral density of the noise must be assumed. For this pro-
totype the noise was assumed to be white, that is YyQ) is a constant. For
white noise the optimum filter will have the same shape as the input signal
so that in the case of a gaussian input signal the output of the filter
would also be a guassian. The standard deviation of the output signal
(out) would be given by

>



specific time. The column's output is thus a series of vapor peaks sep-
arated by regions of pure carrier gas. To detect these peaks, the output
gas stream from the column is passed over a detector which measures a
particular property of the gas, such as thermal conductivity, which can
be related to the concentration of sample vapor in the carrier gas. The
detector produces a signal which is amplified and recorded as a function
of time. Chromatograms obtained in this manner are used in the quantita-
tive analysis of the sample mixture since the identity and quantity of
each vapor in the mixture can be determined from its retention time in
the column and the area under its output peak.

2.2 SILICON ETCHING TECHNOLOGY

Miniaturization of the components of the GC requires the ability to fabri-
cate small three-dimensional structures with tightly controlled geometries,
Photolithographic techniques have been developed by the integrated circuit
industry for patterning silicon with complex two-dimensional circuits with
features less than 5-um wide. The combination of these photolithographic
techniques with different silicon etches has resulted in the ability to
form the valves, columns, and detectors required for a miniature GC.

In larger gas chromatographs, the common practice is to fabricate the
individual elements of the system as separate units, and then assemble
the components into a system using connecting tubing. In the integrated
GC, however, the volumes of the individual components (column, valve, and
detector) are so small that any discrete tubes and connectors would con-
tribute unacceptably large dead volumes which would excessively dilute
the sample peaks. The miniature GC system must be connected with tubes
of diameter nearly equal to the column diameter, which necessitates the
integration of many of the system components on the same substrate. With
silicon etching technology, the connecting tubes can be fabricated at the
same time and with similar dimensions as the system elements, so no extra
processing steps are required.

2.2.1 Etched Geometries

A wide variety of different features can be etched in silicon depending
upon the shape of the mask and the type of etchant used. Fabrication of
these features involves a series of oxidation, photolithography, and etch-
ing steps similar to those of standard integrated circuit processing. The
basic photolithography process is depicted in Figure 2.2. A layer of
silicon dioxide (Si02) is first grown on the wafer in a high temperature
furnace. This oxide layer will serve as the etch mask in the subsequent
silicon etching step. The desired pattern must then be transferred to the
oxide. A thin layer of photosensitive material is spun onto the wafer.

A glass photographic plate with an emulsion image of the desired pattern
is placed in contact with the wafer, and the photoresist is exposed with
UV light. The photoresist on the wafer is then developed, leaving the
oxide protected by the resist in the desired etch mask pattern. The
excess Si02 is etched away, leaving bare silicon where the etched feature
is to be located.

Several cross-sectional profiles of grooves are available, depending upon
the orientation of the groove with respect to the crystallographic planes

6


DISCLAIMER

Mention of company name or product does not
constitute endorsement by the National Institute
for Occupational Safety and Health.

DHHS (NIOSH) Publication No. 81-115

Dr. Laurence J. Doemeny
NIOSH Project Officer

di


detector, at nominal operating conditions over a bandwidth of 0.1 to 30
Hz. This detector noise is thought to be due to extremely small flow in-
stabilities in the detector cavity, which causes minute changes in
detector operating temperature. This detector noise only becomes impor-
tant at the lower peak concentrations. The unit is generally operated so
that the output peak height, referred to the detector, is about 0.5 to
1.0 UV/ppm for peaks such as hexane and chloroform. In the reproduci-
bility tests, the injection pressure was set so that sufficient sample
was injected to obtain about a 2000-mV air peak and a sensitivity of about
0.7 UV/ppm for hexane.

The predominant source of error in the system at lower concentrations is
caused by the A/D converter. The present 10-bit A/D converter has a
minimum resolution of 5 mV. Under the conditions of the reproducibility
test, a 20 ppm hexane peak is 70 mV, which is about 14 A/D levels in
amplitude. This lack of amplitude resolution introduces significant error
in the peak area calculation.

When measuring small gas concentrations, it is possible to improve repro-
ducibility by a factor of about 2 in the sub-10 to 50 ppm range by in-
creasing the size of the peaks by injecting a sample at higher injection
pressure. This increases the amplitude of the peaks, which improves the
signal to noise (S/N) ratio. The only disadvantage of this procedure is
that a 1000 ppm sample of, for example, pentane would be so large as to
be out of the dynamic range of the A/D converter. Thus, at higher injec-
tion pressures it would not be possible to simultaneously measure a 1000
ppm hexane peak and a 10 ppm CCl, peak.

Peaks in the 20 ppm range are still well above the detector and electronics
noise level so the reproducibility at these levels could be improved by
increasing the gain of the amplifier or by increasing the resolution of
the A/D converter. The amplifier gain has been selected such that a 1000
ppm pentane peak is just in the maximum range of the converter. It would
be possible to change the gain of the amplifier during the chromatograph,
but this would introduce additional complications in the computer program,
which would probably unacceptably slow down the program, and would further
reduce the sampling rate. A 12-bit A/D converter is presently being tested
and is most likely the quickest way to achieve better performance, and when
faster, low power computers become available, the program can be modified
to handle amplifier gain changes.

It appears that the main source of inaccuracy in the computer system is
the finite sampling rate. The computer has only enough time to take about
30 samples during the time equivalent to two standard deviations of the
roughly gaussian output peaks. This causes imprecision in the determination
of the peak standard deviation which is used to calculate the peak area,
which is further used to determine the peak concentration. There are two
ways to reduce this error. One is to sample faster, which would require
faster and more sophisticated computers, and the other is to use an addi-
tional interpolating filter to achieve higher precision than the sampling
frequency allows. Both approaches may be used in more advanced versions
of the instrument.

27


GLOSSARY
A/D converter - analog to digital converter - a device capable of con-
verting an analog signal into an equivalent numerical value
algorithm - a numerical procedure used for solving a mathematical problem

anisotropic - an etch which has a directional dependence, usually caused
by the crystalline nature of the substrate

CMOS - complementary metal oxide semiconductor - a semiconductor process
used to produce devices with very low power dissipation

column - the long narrow tube in gas chromatography used to separate a
gaseous sample

digital filtering - a procedure used to remove unwanted signals from a
signal by numerical manipulation of the data

EPROM - electrically programmable read only memory - a program storage
device

effective plates (N) - a measure of efficiency of a GC colum

GC - gas chromatography - a process used to separate and measure the
constituents of a gaseous sample

microcomputer - a small scale electronic programmable system for the
manipulation of data

microprocessor ~ a single computational device which is the main arith-
metic and logical element in a microcomputer system

nl - nanoliter - 1x 107? liter - a volume equal to a cube 100 Um on a
side

OV-101 - a trademark for a silicone oil GC capillary column lining
material

partition coefficient, K - the ratio of solute vapor to stationary phase
concentration

partition ratio, k - the ratio of adjusted peak retention time to air
peak time

69


3. INSTRUMENT SYSTEM

The gas chromatographic air analyzer is a system built around two key
components, the miniature gas chromatograph and a powerful microcomputer.
The chromatograph separates and detects the components of a gaseous sample
and the computer analyzes and displays the output of the chromatograph
and controls the operation of the entire system.

A block diagram of the system is shown in Figure 3.1. The chromatographic
section includes the integrated GC described in Chapter 2 plus the sample
pump and injection system, a 15-cm3 high pressure helium supply and a
pressure regulator. The electronics section includes analog electronics
to amplify the detector output, other pressure and temperature sensors,
and digital electronics to process those signals, display the results,
and sequence and control the GC system.

t v t | DISPLAY

GAS SUPPLY SAMPLE SEPARATING] | DETECTOR
AND [> INJECTION | >} >} AND
REGULATOR SYSTEM COLUMN AMPLIFIER

t f ALARM

SAMPI
aia KEYBOARD

}

ATMOSPHERIC
SAMPLE

[> COMPUTER

Figure 3.1. BLOCK DIAGRAM OF PROTOTYPE GAS ANALYSIS SYSTEM.

3.1 CARRIER GAS SUPPLY

The carrier gas supply is shown in the block digram of Figure 3.2. The
high pressure helium is passed through a resistive gas path to a high
pressure valve. The pressure in a surge tank downstream from the valve
is monitored by a pressure transducer. As carrier gas flows from the
surge tank through the column, the pressure in the surge tank falls very
slowly. Before the injection of a sample pulse in the GC, the high pres-
sure valve is activated until the pressure in the tank reaches a preset
valve. The computer then closes the high pressure valve and begins the
sample injection process. This pressure regulator system works over a

15


A PROTOTYPE GAS ANALYSIS SYSTEM USING

A MINIATURE GAS CHROMATOGRAPH

Professor James B. Angell, Principal Investigator
John H. Jerman
Stephen C. Terry, Ph.D.

Soheil Saadat

Integrated Circuits Laboratory

Stanford Electronics Laboratory

Stanford University
Stanford, California 94305

Contract No. 210-77-0159

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control
National Institute for Occupational Safety and Health
Division of Physical Sciences and Engineering
Cincinnati, Ohio 45226

April 1981


20)

a
T

INPUT PRESSURE P; (psig)
b VARIED AS PARAMETER

fe)
T

COLUMN LENGTH=100 cm

NUMBER OF EFFECTIVE PLATES (x 103)

oo

z (microns)

Figure 4.9. NUMBER OF EFFECTIVE PLATES AS A FUNCTION OF MAXIMUM ALLOWABLE
CARRIER GAS PRESSURE AND ETCHED DEPTH.

The preceding example illustrates how one of the many parameters which con-
trol the operation of the miniature capillary column can be optimized to
increase the efficiency of the column. When such analyses are performed
for the other column parameters, the following guidelines are suggested for
optimizing the design of the miniature column with a specific stationary
phase for the separation of a known group of sample vapors.

(1) The maximum allowable carrier gas pressure should be used.
As shown in Figure 4.9, the peak value of the number of ef-
fective plates increases with input pressure.

(2) The column should be optimized for the separation of the
vapors with the smallest retention times or for the most
difficult separation.

(3) The column length should be as long as possible if the maxi-
mum separation is desired.

39


4. PERFORMANCE

The performance of a complicated instrument such as the miniature gas
chromatograph is dependent on both the individual performance of the
separate elements and the manner in which they are interconnected to form
the system. The following section will concentrate on the system perfor-
mance, with the succeeding sections devoted to the performance of the
individual chromatographic, detector and computer sections.

4.1 OVERALL INSTRUMENT PERFORMANCE

The basic measure of performance used for this instrument is the overall
system reproducibility, defined here as the ability of the instrument to
measure and display the concentration of a standard gaseous sample. Most
of the reproducibility tests have used dilutions from commercially avail-
able calibrated gas samples of pentane and hexane at original concentra-
tions of 1000 ppm. A large number of analyses are performed on a sample
gas, and the mean and standard deviation of the distribution of results
are calculated. Since the analyses are performed in less than one minute,
a rather large number of essentially independent measurements can be made.
The results of a typical test consisting of 50 analyses showed the instru-
ment's measurement of a sample containing both pentane and hexane to have
a percent standard deviation of 2.5% for pentane and for hexane, to give
a reproducibility at the 95% confidence level of plus or minus 5% at

1000 ppm concentrations. In such a test at approximately room temperature,
the pentane retention time is about 3.3 seconds, closely following the
air peak, while the hexane at 4.6 seconds is roughly in the middle range
of retention times (4 to 8 s) for this instrument.

A series of tests of reproducibility were performed. on the prototype instru-
ment. Each test involved taking 50 samples of a gas mixture, and the mean
and standard deviation of the concentration results were determined. The
gas samples were prepared by successive dilutions of a 1000 ppm standard
gas, and due to uncertainties in the dilution process, the dilute gas
samples are of unknown absolute accuracy. From the more than 3000 analyses
which were performed on the prototype instrument a representative set of
50 consecutive analyses of diluted samples of pentane and hexane at about
50 ppm has been selected and the individual results are shown in Table 4.1.
This table lists the computer calculated concentrations of pentane and
hexane in ppm and the measured air peak heights in mV from an external
oscilloscope. A typical chromatogram from this series is also shown in
Figure 4.1. The calculated percent standard deviation for pentane is
3.85% with a mean of 49.7 ppm, and for hexane the mean is 67.9 ppm with a
4.89% standard deviation. The maximum and minimum calculated concentra-
tions are within +2 standard deviations of the mean.

23


it emerged from the column after 2.6 seconds, a time determined solely by
the helium carrier gas velocity. The finite base width of the nitrogen
peak ("air peak") is caused by gaseous diffusion of the nitrogen in the
carrier gas, since the injected peak was only 5.0-ms wide and there are
no appreciable mixing or detector time constants. The next peak at about
3.3 seconds is the pentane, followed at 4.6 seconds by the hexane. The
increased retention times are due to the times spent by the pentane and
hexane adsorbed on the column lining, and the increased base widths of
these peaks are due both to gaseous diffusion and diffusion of the species
in the column lining. It is preferable to keep the peak spreading to a
minimum, since this will facilitate the separation of more components of
a mixture and will maximize the peak height which increases the signal to
noise ratio of the detector. The complete separation was performed in
less than 5 seconds, which is significantly less than the several minutes
which a conventional GC system might take to give the same separation.

Figure 4.4 shows another output chromatogram using OV-101 as the stationary
phase. This example shows the ability of the device to separate very
similar branched and straight chain hydrocarbons as well as chlorinated
hydrocarbons. Again, the relatively short length of the miniature capil-
lary column (1.5 meters) enabled the separation to be performed in less
than 10 seconds. The analysis time is shorter than a typical convention-
al GC, but the separation of these closely related chemicals which was
achieved by the miniature GC is not as good as could be achieved with a
standard column. By applying capillary GC theory the separating power of
the miniature GC can be characterized.

Figure 4.4. MINIATURE GC CHROMATOGRAM USING OV-101 STATIONARY PHASE.
Peak (1) nitrogen, (2) n-pentane, (3) 3-methylpentane, (4)
n-hexane and chloroform, (5) 2,4-dimethylpentane, (6) 111-
trichloroethane, (7) cyclohexane, and (8) n-heptane.

32


digital (A/D) converter. A low gain amplifier is also connected directly
to the detector to monitor the 1-volt air peak signals, which saturate
the high gain amplifier.

The A/D converter is the primary way that the computer obtains information
from the analog electronics. It is switched among the various pressure
sensor amplifier outputs and other signals as shown in Figure 3.5. The
pressure sensor outputs are used to control the operation of the carrier
gas supply and the sample injection system. A thermistor is used to
measure the column temperature in order to calculate the expected reten-
tion time of the sample peaks.

CARRIER
GAS —
San AMPLIFIER
SENSOR
SAMPLE
INJECTION =
IE CHIONE AMPLIFIER
SENSOR
M
THERMISTOR U
COLUMN CURRENT t
TEMPERATURE SOURCE
SENSOR | ND
p |? |converter>>
L
E
DETECTOR row oan Ey B
R
LJ} HIGH GAIN |__»I
x 5000 AMP!
BATTERY a |
CONTROL

Figure 3.5. A/D CONVERTER AND MULTIPLEXER SYSTEM.

18


before the launch. The first integrated GC, which consists of a long
separating column, a sample injection valve, and a thermal conductivity
detector, all fabricated on a single silicon wafer, was developed in
1974 (2]. It operates by continually passing a carrier gas through the
separating column. This column is lined with a material capable of ad-
sorbing and desorbing the constituents of a gaseous sample. A brief
pulse of sample gas is injected at the head of the separating column.

As the constituents are swept along the column they are each retained in
the lining for characteristically different total times, They thus exit
from the tube as separate peaks, which are detected by the changes in
thermal conductivity of the output gas stream.

The components of the integrated GC have been developed and refined in

the last few years and the performance of the device has steadily improved.
There have been concurrent rapid advances in the semiconductor industry

in the field of miniaturized digital computers. A significant effort has
been expended to apply those miniature computers to the GC system to form
a portable analytical system with advanced capabilities. The computer
controls the sequence of operation of the GC and implements sophisticated
digital filtering, peak detection, peak area calculations, data storage,
and display functions which would have been impossible in a portable
package the size of the prototype before 1979.

Work has also been done to miniaturize the other parts of a total instru-
ment, such as the carrier gas supply and sample injection system. Since
the miniature GC is so much different from conventional instruments, most
available components are not suitable for use in this system. The GC,
support components, and electronics system have been combined to form a
portable instrument, shown in Figure 1.1, about 20 cm x 22 cm x 10 cm

(8" x 9" x 4"), which weighs 2,8 kg (6.2 lbs). It is capable of sampling
the air automatically at specified time intervals, or on command from the
operator. The instrument is programmed to look for up to 10 different
constituents of the gas sample, calculate their respective concentrations,
calculate and store time-weighted-average concentrations, display the
concentration of any desired compound on demand, and sound an alarm if
any concentration exceeds a preset level. It contains sufficient carrier
gas in convenient, replaceable cartridges to last for at least four days
of continuous operation and has a rechargeable battery system for up to

8 hours of operation between charges. Since the computer handles all
aspects of the instrument operation, the unit is very simple to use.


2

(t-t )

aon (t_-t) exp-| 2
20 oe ° 206

Y(t) =

N

where

oO. = ¥20.
y x

Ay is the area of the input gaussian

Ay is a constant from Equation (12)

oO, is the input gaussian standard deviation
This function is shown in Figure 4.18. This is the filtered gaussian wave-
form which must be measured to determine the concentration of the gas peak,
In order to do that, the original standard deviation, ox, and the original
amplitude must be reconstructed from measuring this waveform. The stan-
dard deviation can be determined from tyay and tmin, since it can be shown
from Equation (12) that tay, - tmax = 2oy.
Furthermore, the area under the positive portion of Y(t) is

t
s-f °xe ae = (as)

27 0
y

Y(t)

to tin

—_
tmax TIME

Figure 4.18. THE RESULTANT WAVEFORM AFTER FILTERING A GAUSSIAN INPUT
SIGNAL.

54


plate height (HETP) - a measure of efficiency of a column; see Section
4.2.3

program - the set of stored instructions for a computer system

RAM ~ random access memory - the temporary data storage elements in a
computer system

SEM - scanning electron microscope

TWA - time weighted average

70

U.S. GOVERNMENT PRINTING OFFICE:1961--787-074/1074


(4) Once the values of the partition coefficient, column length,
and carrier pressure are known, a plot of Equation (4) will
indicate the column depth which maximizes the separating
power of the column.

Such an optimization of the miniature capillary column must be tailored

to fit many other requirements imposed by the rest of the instrument sys~
tems, For example, the column dimensions and the carrier gas velocity
determine the rate of consumption of carrier gas and thus influence the
size of the helium supply tank. Long columns are desirable to perform dif-
ficult separations, yet the resulting analysis time may be prohibitively
long. Very short columns for rapid separations are too fast for the com-
puter to keep up with in real time, and thin column linings require very
small sample injections which may limit system detectivity. Many such
trade-offs exist, making system optimizations a difficult task.

The miniature GC columns which are currently in use are 1.5-m in length
with 40-Um x 175-"m cross sections. There has been no comprehensive ef-
fort to optimize the geometry of these columns since the stationary phase
distribution and performance are not under good control. The choice of
some of the column dimensions and operating parameters has been dictated
by reliable valve operating pressures, desired anslysis times, and by
computer speed.

The column length of 1.5-m was arbitrarily chosen at approximately half of
the maximum length that we can currently fabricate. The 40-um column depth
gives air peak retention times of 1 to 3 seconds over carrier pressures

of 30 to 10 psig. The Golay plot shown in Figure 4.6 indicates that car—
rier gas velocities between 61 and 87 cm/s yield the minimum plate height
and hence the best column performance for the stationary phase distribution
which is currently achieved. The velocity of 61 cm/s, which corresponds
to a helium carrier gas pressure of 19 psig, was chosen so that the re-
sulting air peaks and little-retained gas peaks would have base widths of
at least 130 ms. Such peaks are of the minimum width which can be ade-
quately sampled by the microcomputer.

The nonpolar columns which are currently used are only partially optimized
as described above, but the optimization process will continue as better
stationary phases, detectors, and computer hardware and software systems
are developed.

4.2.4 Temperature Dependence

The retention time of a sample vapor in a GC column is a strong function of
the column temperature. For this reason, conventional chromatographs
typically maintain an elevated column temperature constant to within 19°C
or better, or they utilize temperature programming to facilitate certain
separations. In the portable instrument there is insufficient battery
power to run a controlled-temperature oven, so the capillary colum oper-
ates at approximately room temperature. Since room temperature varies
roughly between 0 and 45°C, the computer must be able to predict the re-
tention times of all peaks of interest over this temperature range.

40


0.8b
CHLOROFORM
0.6+
“=
og 0.4F
=
<
ina
z
6 +f
Ee METHYLENE CHLORIDE
&
é
0.2
rn 1 1 L 4 4
3.21 3.23 3.25 3.27 3.29 3.31 3.33 3.35
(39°C) (26°C)

ABSOLUTE TEMPERATURE”! « 1073

Figure 4.10. PARTITION RATIO AS A FUNCTION OF 1/T FOR CHLOROFORM AND
METHYLENE CHLORIDE.

with the miniature system, ease in fabrication, generality of response,
and ruggedness. In the many months of testing it has been shown to be a
consistent performer.

4.3.1 Comparison with Other Detectors

There are a number of different types of detectors used in gas chromato-
graphy. They measure one of a wide variety of physical parameters of the
output gas stream. Of the detectors with relatively general response, the
most commonly used are flame ionization detectors and thermal conductivity
detectors. Lately photoionization detectors have been shown to be a
general purpose detector with good analytical performance. The ionization
detectors are typically known for their high sensitivity to hydrocarbons,
and the thermal conductivity detectors are used for applications where
their generality of response to nonhydrocarbons is important.

The flame ionization detector works by introducing hydrogen in the output
gas stream of the chromatograph, The resulting gas mixture is then ignited

42


2, THE MINIATURE GAS CHROMATOGRAPH

The miniature gas chromatograph is the heart of the portable gas analysis
system. It separates the gaseous sample into its component vapors and
detects and measures the concentration of the separated constituents.
The miniature gas chromatograph is similar in function to larger labora-
tory chromatographs, but the method of fabrication of the components is
quite different.

2.1 INTRODUCTION TO GAS CHROMATOGRAPHY

Gas chromatography is a technique widely used in analytical chemistry for
the separation and analysis of gaseous samples. A typical gas chromato-
graph (GC) consists of a carrier gas supply, a sample injection system,

a separating column, and an output detector, as shown in the block diagram
(Figure 2.1). Separation of the sample vapors is achieved via their dif-
ferential migration through a capillary column, The sample vapors are
injected at the input of the column by a valve and are swept through it
by an inert carrier gas. The column is lined with a liquid stationary

| SAMPLE OOSOSOSOSC~SSSS 7
| GAS
|
CARRIER GAS FS | JL (~>\ [Aur |
Onan 3
REGULATOR} SAMPLE DATA
INJECTION CAPILLARY THERMAL |
| COLUMN CONDUCTIVITY PROCESSING
; VALVE DETECTOR | UNIT
bo

FABRICATED ON SINGLE
SILICON WAFER

Figure 2.1. SCHEMATIC OF A SIMPLE GAS CHROMATOGRAPHY SYSTEM.

phase, a substance capable of absorbing and desorbing each of the com-
ponent vapors. The migration rate of each vapor along the column depends
on the carrier gas velocity and the degree to which the vapor is absorbed
by the stationary phase. The mixture of sample vapors, injected into the
capillary as a single pulse, is separated as it travels through the column,
with each component vapor traveling at a different rate and emerging at a

5