Sample injection apparatus and method

An apparatus for sample concentration for gas chromatography and mass spectrometry for analysis for volatile organic compounds, including a hollow differential pressure switch to switch sample flow non-mechanically, and so that the gas chromatograph and apparatus may be used with different sample levels of volatile organic compounds by changing the timing and temperature parameters by changing the flow non-mechanically. The differential pressure switch preferably has a central helium port, a sample flow port, a sweep flow port, a vent end, and an assist/column flow end; an expansion volume chamber having an inlet tube, which is inserted through the assist/column flow end and extends to halfway between the helium port and sample port, a coiled piece of tubing, and an outlet tube; a primary collection trap coiled about a rod having an entry port for a cryogenic substance, an inlet end, a heater at the inlet end, and an outlet, wherein the expansion volume chamber outlet tube connects to the primary collection trap inlet end; and an assist/column interface having an inlet end connected to the outlet of the primary collection trap, an arm connected to a mass flow controller valve and vacuum system; and an outlet end connected to the analytical column. The method of the invention utilizes this apparatus to concentrate a sample suspected of containing VOC's, by adjusting the temperature, flow rates, and whether particular gas flows are turned on or off.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to systems for performing gas chromatography and 
mass spectrometry on environmental samples, and in particular, pertains to 
an apparatus and method for sample injection for analysis of air samples 
for volatile organic compounds. 
2. Description of the Related Art 
Many volatile organic compounds (VOCs) are considered to be pollutants and 
a danger to health even when they are at such low levels in an indoor or 
outdoor environment that they are only detectable by sophisticated gas 
chromatography (GC) and mass spectrometry (MS). The sources of undesirable 
VOCs include such diverse sources as microbial emissions from decay or 
other biological processes, chemical spills, chemical release from 
synthetic products, and airborne emissions from industrial processes. VOCs 
include a wide variety of chemical compounds such as sulfur-containing 
compounds, aldehydes, alcohols, ketones, chlorinated hydrocarbons, 
aromatic hydrocarbons, amines, and terpenes. 
The very low levels at which many VOCs are toxic or otherwise harmful, in 
comparison with the higher levels of other compounds present in an average 
gas sample, means that gas analysis techniques must be very sensitive to 
the VOCs being analyzed in the presence of higher levels of these other 
compounds. Techniques such as differential temperature treatment, 
cryogenic trapping, and use of solid sorbents, or combinations thereof, 
have been developed in an attempt to remove compounds not being measured 
and selectively enrich for the VOCs being measured. 
It is therefore an object of this invention to provide a method and 
apparatus which allows detection and measurement of very low levels of 
VOCs even in samples containing water vapor and/or other components at 
relatively high levels. The invention thus allows determination of VOCs 
which may be hazardous to health and the quality of life. 
Other objects and advantages will be more fully apparent from the following 
disclosure and appended claims. 
SUMMARY OF THE INVENTION 
The invention herein provides an apparatus, for connection to an analytical 
column of a gas chromatograph for analyzing samples for volatile organic 
compounds, comprising a hollow differential pressure switch so that sample 
flow may be switched without mechanical means, and so that the gas 
chromatograph and apparatus may be used with samples having anywhere from 
very high to very low levels of volatile organic compounds by merely 
changing the timing and temperature parameters in the computer control 
program for the gas chromatographic analysis, and simply changing the flow 
non-mechanically, without changing anything mechanically. 
The apparatus of the invention in its preferred embodiment comprises a 
hollow differential pressure switch, having a central helium port, a 
sample flow port on a first side of said helium port, a sweep flow port on 
a second side of said helium port, a vent end, and an assist/column flow 
end; an expansion volume chamber having an inlet tube, which is inserted 
through said assist/column flow end and extends to halfway between said 
helium port and said sample port, a piece of tubing having a plurality of 
coils, and an outlet tube; a primary collection (cryogenic) trap 
comprising a piece of tubing coiled about a rod, said rod having an entry 
port for a cryogenic substance, an inlet end, a heater at said inlet end, 
and an outlet, wherein said expansion volume chamber outlet tube connects 
to the primary collection trap inlet end; and an assist/column interface 
having an inlet end connected to the outlet of the primary cryogenic trap, 
an arm connected to a mass flow controller valve and vacuum system; and an 
outlet end connected to the analytical column. Each pathway that the 
sample or other gases travel in the apparatus is treated so that the 
interior surface is inert, such as being coated with fused silica or other 
surface deactivation process producing an inert surface. 
The method of the invention utilizes this apparatus to concentrate a sample 
suspected of containing VOC's, by adjusting the temperature, flow rates, 
and whether particular gas flows are turned on or off. 
Other aspects and features of the invention will be more fully apparent 
from the following disclosure and appended claims.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF 
The present invention provides an apparatus and method for processing of 
samples so that the sample is actually introduced into a GC/MS without 
encountering any surface in the apparatus other than an inert surface 
which has been treated with a surface deactivation process, preferably a 
coating with fused silica, a deactivated fused silica surface, or a 
specially coated fused silica surface, or with other surface deactivation 
processes which produce an inert surface such as electro-polishing, or 
other glass-vapor deposition processes. As used herein, the term "inert" 
means that the material forming the surface, coating or treatment has no 
surface chemical activity with organic compounds, no absorption of organic 
compounds by the material and no re-emanation by the material of organic 
compounds. 
In the preferred embodiment, a fused silica coating is applied to the 
interior of tubing and other structures with any procedure known in the 
art. For example, Restek Corporation (Bellefonte, Pa.) utilizes a 
proprietary process in which high temperatures and pressures are used when 
applying the coating. When two pieces of coated tubing are joined 
together, a SWAGELOK.TM. tube fitting (Swagelok Co., Solon, Ohio) which 
has also been interiorly coated with fused silica is used to join the two 
pieces of tubing. Coated fittings and tubing are available from Restek 
Corporation. 
The apparatus of the invention allows VOCs to be measured at the very low 
levels which are often present in ambient air, in the range of, for 
example, 0.2 ppbv (parts per billion volume), as well as at the higher 
levels which may be encountered in industrial, chemical or waste sites, up 
to saturation vapor pressures, and all levels in between these extremes. 
This diversity of measurements may be made with only changes in timing 
sequence and temperature set points in the apparatus and method, and 
without hardware changes to the apparatus of the invention. 
An overall schematic view of the major components of the injection 
apparatus of the invention is shown in FIG. 8. As discussed in more detail 
below, the apparatus of the invention comprises four system subassemblies: 
a differential pressure switch, a primary collection trap and expansion 
volume chamber, an assist flow, a sample assist/column interface, an 
external secondary cryogenic focusing assembly, a vacuum source and 
ballast, and valving and pressure and flow regulation with associated 
plumbing. 
Structure of the Apparatus of the Invention 
Cabinet enclosure 20 for the invention shown in FIG. 4 and by the outer 
dotted lines in FIG. 8 is preferably made of aluminum to minimize 
cost-effectiveness, but other materials may be used without adversely 
affecting the invention. Cabinet enclosure 20 consists of a base 22, a 
three sided cover 24, a rear panel 26, and a rear subpanel 28. Cabinet 
enclosure 20 is mounted on top of an elevation bracket 30 that positions 
the injection system over the external refocusing unit for example, a 
cryofocus unit Model 354A manufactured by Graseby Nutech (Graseby 
Anderson, Inc., Durham, N.C.) may be used. This unit may be incorporated 
into cabinet enclosure 20 eliminating the requirement for a bracket. 
Cabinet enclosure 20 is typically mounted directly to the top of a gas 
chromatograph. 
Heated oven enclosure 32 (FIG. 5) comprises a base 34 and a cover 36, each 
of which is covered externally with pieces 38 of 1/2-inch thick rigid 
insulation (1/2" Ceraboard 100 made by R. R. Horne & Co., Inc. Stone 
Mountain, Ga.). The enclosed volume is heated by a 125-watt strip heater 
40 (No. S1J5U1, manufactured by Watlow Manufacturing Company, St. Louis, 
Mo.) which is mounted to oven base 34. A type K thermocouple sensor 
positioned inside the oven is used for temperature feedback. Base 34 of 
heated oven enclosure 32 also contains mounting hardware for a 
differential pressure switch bracket 42, an expansion volume bracket 44, a 
union elbow bracket 46, and a cryotrap assembly 48. Heated oven enclosure 
32 mounts to cabinet base 22 by one-inch standoffs 50 preferably made of 
stainless steel or other material. The lower the thermal conductivity of 
the material, the less heat is transferred to the base of the cabinet 
enclosure. A 0.272 inch diameter hole 52 in base 34 of heated oven 
enclosure 32 provides an entrance for the analytical column to enter the 
enclosure and attach to the assist/column interface. Assist/column 
interface ("T" fitting) 96 (see FIG. 6) is mounted to the outlet of the 
primary collection trap, providing its only support. 
The apparatus of the invention comprises a differential pressure switch 54 
(FIG. 1) which allows gas to be sampled and analyzed by gas chromatography 
without encountering any surface in the apparatus except an inert surface 
or a surface which has been deactivated so that it is inert. Rather than 
use a conventional physical valve that reroutes flow mechanically, and as 
such, contains metal parts and deformable materials for gaskets, and the 
like, the system inlet comprises a linear volume with five specially 
placed gas flow ports that allow the sample flow and the helium carrier 
gas flow to be alternately introduced into the system through differential 
pressure control. There are no moving parts, metal surfaces or deformable 
materials in or surrounding the gas being sampled or transported in the 
apparatus. 
Sample flow is established through some external means, for example, a 0.25 
mm (I.D.) fused silica tube 18 inches long, which connects the sample 
container to the injection system, as is known in the art, in the range of 
approximately 50 to 250 ml/min. The apparatus and method of the invention 
do not require a specific flow rate, but only that there is excess flow 
detected at the "vent" port when the helium valve is in the "off" state. 
The upper limit may be empirically determined in a particular system by 
detection of sample infiltration into the helium carrier gas, or by a 
calculation of the approximate flow rate at which flow becomes 
non-laminar. For the geometry of the switch (0.75 mm I.D.), the 
theoretical limit as determined by a Reynolds number of 2000 is about 1100 
ml/min. It is known in the art that above this Reynolds number, flow in a 
circular cross-section tube becomes turbulent. For a safety factor, to be 
sure that turbulent flow is avoided, a flow rate of 250 ml/min. is 
preferably chosen as the upper limit. 
There are two competing factors which determine the preferable arrangement 
of differential pressure switch 54 for optimal operation. First, the 
distance between sample port 56 and helium port 58 should be as large as 
possible to avoid diffusion or turbulent mixing. Second, the overall dead 
volume should be as small as possible to assure precise and rapid flow 
switching. As discussed in more detail below, with the preferred injection 
tubing having an internal diameter of 0.75 mm, the adjustment is made in 
the length of differential pressure switch 54, particularly between the 
sample and helium ports. A minimum distance of ten internal diameters 
spacing has been empirically determined to be a reasonable compromise 
between these two factors. The preferred spacing for the preferred 
injection tubing is about 1 cm between the sample and helium ports. A 
greater spacing does not cause differential pressure switch 54 to cease 
working, but rather, increases the dead volume and thus affects switch 
precision without appreciable benefit in discrimination against 
sample/helium mixing. Reducing this spacing is beneficial for precise 
injection control especially for high concentration samples; however, the 
reduced spacing requires greater helium flow to keep the sample from 
infiltrating into the system and thus increase overall helium consumption. 
Differential pressure switch 54 is held in place on heated oven base 34 by 
a machined aluminum bracket 42. The construction of differential pressure 
switch 54 (FIG. 1) consists of a piece of 0.3125 inch diameter T316 
stainless steel rod 60 (commercially available) cut to 1.164 inch in 
length as the body of the switch. A hole is drilled through the center of 
the rod to give an inside diameter of 0.125 inch. Three holes with a 0.063 
inch diameter are drilled perpendicular to the axis of the rod from the 
outer diameter to the inner diameter. The middle of these three holes is 
located in the center of the 1.164 length with each of the two other holes 
offset on each side of the middle hole by 0.394 inch (1 cm). Three pieces 
of identical 1/16-inch chromatography grade stainless steel tubing 62 
(Model No. T316 SMLS, 0.0625 inch (O.D.).times.0.030 inch (I.D.) tubing 
made by Handy & Harmon Co., Philadelphia, Pa.) twelve inches in length are 
each inserted into one of the three holes in the rod at a depth flush with 
the inside diameter of the rod. Tubes 62 are attached to rod 60 by silver 
solder. Two 1/16-inch stainless steel caps 64 (Part No. SS-100-C, Swagelok 
Co, Solon, Ohio) as shown in FIG. 1, drilled through in the center with a 
0.063-inch diameter hole are attached to each end of rod 60 by a heliarc 
weld. These caps provide exits from the switch for the vent flow and 
assist/column flow, as well as providing 1/16-inch SWAGELOK.TM. fitting 
threads for interface connection purposes. The internal surface area of 
the rod assembly is preferably coated with deactivated fused silica 
(Restek Corporation) to provide an inert inner surface. 
The helium flow path enters the cabinet enclosure through a 1/16" bulkhead 
fitting 65 (Part No. SS-100-61, Swagelok Co., Solon, Ohio) mounted on the 
cabinet rear panel. The bulkhead fitting connects to the inlet of a 
two-way solenoid valve 111 (Part No. 51ZOO520KM, Peter Paul Electronics 
Co., Inc., New Britain, Conn.) which is normally open. The valve outlet 
connects to the center tube inlet 58 of differential pressure switch 54, 
which is formed with a 90-degree angle to remain inside the heated oven 
(FIG. 6). 
Preferably the invention also includes a sweep flow port 66. Sweep flow 
port 66 allows cleaner, more rapid switch operation which is particularly 
important for the analysis of very high concentration samples. In 
addition, the use of sweep flow port 66 reduces the effective dead volume 
of differential pressure switch 54, which makes the overall system less 
susceptible to error induced by the mixing zone at the sample/helium flow 
interface for both high and low concentrations of analytes. Finally, for 
applications involving collection of a sample from the surrounding air or 
from a chamber at atmospheric pressure, the sweep flow provides the 
mechanism for delivering sample to the system. In such cases, vent port 68 
must be blocked, for example, by using a SWAGELOK.TM. plug fitting (Model 
No. B-100 for brass or SS-100-P for stainless steel of Swagelok Co., 
Solon, Ohio, or other 1/16 O.D. plug) which is attached to the vent 
bulkhead fitting on the rear panel. Since there is no pressure 
differential between the sample and the vent line which is at atmospheric 
pressure, vacuum must be used to pull the sample into the switch. The vent 
must be plugged so that the sample will not be diluted with flow being 
pulled in through the vent line. The assist flow pulls the sample through 
the system and through assist valve 112, and the sweep flow provides the 
assist flow plus excess flow to eliminate the switch dead volume. 
The sweep flow exits differential pressure switch 54 through the rear tube 
and connects to the inlet of a 1/16" fine metering valve 70 (Part No. 
SS-SS1-A, Nupro Co., Willoughby, Ohio) mounted on cabinet rear panel 26. 
The outlet of the valve connects to a 1/16 bulkhead-tube fitting (not 
shown)(Part No. SS-100-61, Swagelok Co, Solon, Ohio) also mounted on rear 
panel 26. The bulkhead fitting connects to vacuum ballast 74 shown in FIG. 
8 (for example, Part No. 8548.80V1000, Graseby Nutech). 
The sample flow enters the switch through the front tube or sample flow 
port 56. This tube protrudes through the top of the oven and cabinet 
allowing for the interface to the sample source. 
The vent flow exits differential pressure switch 54 through the forward 
horizontal tube fitting (vent port 68) and connects to a 1/16 bulkhead 
union 76 (Part No. SS-100-61, Swagelok Co.) mounted on cabinet rear panel 
26. 
Primary collection (cryogenic) trap 78 and expansion volume 80 (FIG. 6) are 
composed of 0.75 mm I.D. tubing internally coated with deactivated fused 
silica. These two components may either be constructed in one piece or be 
separately constructed and jointed with a similarly coated junction to 
allow easy replacement of the trap for maintenance. Expansion volume 80 
provides a fixed volume into which the gas may flow. 
A first purpose of expansion volume chamber 80 is to serve as a heated 
inert transfer line of sample to primary collection trap 78, and to 
isolate the trap temperature from differential pressure switch 54, 
particularly when the trap is at the cold set-point. A second purpose of 
expansion volume 80 is to serve as a linear reservoir for the sample when 
condensed water, CO.sub.2, or other major components of the sample expand 
more rapidly than the column flow into the analytical system can 
accommodate. Expansion volume 80 remains at the internal temperature of 
the differential pressure switch 54, while primary collection (cryogenic) 
trap 78 (FIGS. 3A and 8) is cooled for sample concentration, and then 
heated. Having two separate pieces allows the trap to be changed 
independent of the expansion volume. 
The geometry of expansion volume 80 is the same as that of the trap 78 to 
minimize dilution with infiltrating helium from the switch. The volume is 
chosen to accommodate about 0.5 ml of back flow during desorption which 
occurs when the primary collection trap is heated to vaporize the 
condensate. This configuration takes into consideration the removal rate 
of condensate from the trap, the temperature ramp rate during desorption, 
and the thermodynamics of the volatilization of the primary trapped 
component of most samples, water. A longer (larger volume) expansion 
volume would serve the same purpose but would be more cumbersome, and a 
shorter (smaller volume) would be more likely to lose sample through the 
switch vent. 
Expansion volume 80 and primary collection trap 78 of the preferred 
embodiment of the invention are coiled as shown in FIGS. 2 and 3A, 
respectively, to conserve space and simplify temperature control. 
Sample expansion volume chamber 80 is held in place by expansion volume 
bracket .44 mounted to oven base 34. Expansion volume chamber 80 consists 
of a 96-inch length of 0.0625-inch outside diameter (O.D.), 0.030-inch 
(I.D.) T316 chromatography grade stainless steel tubing (Handy and Harmon 
T316 SMLS 0.0625.times.030" tubing). The tube is coiled into 19 coils 
having the orientation shown in FIG. 2. An inner coil diameter of the 
expansion volume of about 15/8 inch with approximately 19 coils provides 
sufficient volume for most analyses. The internal surface of the tube is 
coated with deactivated fused silica for inertness (Restek Corporation). 
This size was determined based on the requirements discussed herein in the 
discussion of the primary collection trap and expansion volume. 
Enclosure pieces 94a,b for primary collection trap (FIGS. 3A-3D) are 
designed to evenly cool the trap tubing through use of a specialized 
distributive flow pattern of the metered liquid/gaseous nitrogen cryogen. 
The specific design is not critical to the overall operation of the 
apparatus and method of the invention; however, the compact nature and 
precise performance of this design is a great advantage over other 
conventional hardware. The heating pattern during desorption is 
particularly important. The trap tubing in the preferred embodiment is in 
thermal contact with a specifically formed aluminum rod 88 that contains 
the heater assembly in such a way that the upstream portion of the trap is 
heated more quickly than the downstream portion (FIG. 3A). This helps to 
maintain the bandwidth of the injection volume at a minimum. Though not 
critical to the operation of the system, this feature helps improve 
overall performance of the invention. 
Primary collection trap 78 consists of a 13-inch length of 0.0625 inch 
O.D., 0.030 inch I.D. T316 chromatography grade stainless steel tubing 
(Handy and Harmon T316 SMLS 0.0625.times.030" tubing). The tube is coiled 
three complete revolutions with a 1-inch coil inside diameter. An inner 
coil diameter of about 1 inch is sufficient for adequate cryogenic 
concentration of most samples. The equal lengths of excess tubing on each 
side of the coils are bent with a 0.500 radius r perpendicular to the trap 
coils (FIG. 3A). Side top 86 of the outlet tube from primary collection 
trap 78 is beveled (FIG. 3A) to allow increased "assist" flow rates from 
the outlet tube into the assist flow "T" fitting 96. The internal surface 
area of primary collection trap 78 is coated with deactivated fused silica 
for inertness. Coiled primary collection trap 78 is mounted on the outer 
surface of a machined aluminum rod 88 which contains heater 90 (150 watt 
1/4-inch O.D. cartridge heater, Watlow No. E1A53) and type K thermocouple 
sensor. Rod 88 also contains a 0.125 inch diameter hole (not shown) that 
allows liquid nitrogen (LN2) to enter the trap assembly for cooling. 
Heater 90 is positioned at the sample inlet end of rod 88. 
Primary collection trap 78 is enclosed by a two-piece TEFLON.TM. housing 
94a,b that provides heat zone isolation from the heated enclosure. Housing 
94a,b forms a leak-tight seal for the LN2 gas used for subambient cooling. 
Housing 94 is machined from commercially available 2-inch diameter 
TEFLON.TM. rod (FIGS. 3A-3D). Each housing piece 94a,b has a hole 93 for 
the tubing to go through. Additional holes 92 are placed in one of housing 
pieces 94a for liquid nitrogen to cool primary collection trap 78. The two 
pieces of the enclosure are held together by two stainless steel brackets 
124 (FIG. 6) connected by three 0.250-inch O.D. stainless steel tubes 126 
(FIG. 6). 
The assist flow of the apparatus of the invention allows a sample 
collection flow that is greater than the column flow that is always 
sweeping the trap, while maintaining the overall design criterion of 
avoidance of valves and valve surfaces. The assist flow connection is made 
at the outlet of the primary collection trap 78 in a "T" fitting 96 (FIG. 
6) that is internally coated with deactivated fused silica. This flow is 
precisely set through a mass flow controller 98 (for example, Model 
201-AFASVBAA, Porter Instrument Company, Hatfield, Pa.) and is turned on 
and off through a computer controlled valve 112 (see FIGS. 6 and 8). 
Although the connection of valve 112 to controller 98 is shown as a direct 
connection for schematic purposes in FIG. 6, the preferred connection 
utilizes a fitting mounted on rear panel 26. The assist flow is set to an 
appropriate value to maximize sample consistency throughout yet avoid trap 
breakthrough of the very volatile VOCs. For the preferred configuration, 
an assist flow of 20-30 ml/min is appropriate. 
An important parameter of the assist flow is the on/off timing. During the 
desorption of primary collection trap 78, the assist flow is in the off 
state to assure that all of the sample is transferred to the analytical 
system. After analytes are transferred, the assist flow is reestablished. 
This sweeps away any remaining water vapor and unwanted heavy organic 
compounds that may have become entrained in the system. For most of the 
applications of this system, such as those discussed herein, the assist 
flow is a necessary feature. Helium flow from a helium source (not shown) 
is activated by helium valve 111. 
Assist/column connection 102 (FIG. 6) is the interface of differential 
pressure switch 54 to expansion volume chamber 80. Expansion volume inlet 
101 is inserted through assist/column flow tube fitting 102 until the end 
of expansion volume tube 80 is equidistant between the helium and the 
sample inlets of differential pressure switch 54 (FIG. 7). Expansion 
volume chamber 80 connects to the inlet of primary collection trap 78 by a 
union elbow tube fitting 84 (Part No. SS-100-9, Swagelok Co.) which is 
internally coated with deactivated fused silica. 
Assist/column interface 104 connects to the outlet of primary collection 
trap 78. Interface 104 consists of a union "T" 96 derived from a 1/8-inch 
GC connector kit (Part No. 14183, Alltech Associates, Deerfield, Ill.) in 
which the stainless steel stem is removed. The wide bore side 106 of the 
straight through path connects to the outlet of primary collection trap 
78. The assist flow path splits from the column path through the 90-degree 
arm 108 of "T" 96 and connects to a 1/16-inch bulkhead tube fitting 110 
(Part No. SS-100-61, Swagelok Co.) mounted on cabinet rear panel 26. 
Bulkhead fitting 110 connects to the inlet of a 2-way normally closed 
solenoid valve 112 (Peter Paul No. 52H8DGB). The outlet of valve 112 
connects to a 100-cc/min. mass flow controller 98 (Porter Model 
201-AFASVBAA)(FIG. 6). The outlet of mass flow controller 98 connects to a 
1/4-inch "T" tube fitting 114 which connects to vacuum ballast 74 which 
connects to vacuum pump 116 for a stable vacuum source (FIG. 8). 
Third arm 118 of assist/column interface 104 connects the injection system 
to the secondary refocusing trap 122, which is mounted below the injection 
system. 
Secondary refocusing trap 122 (for example, Part No. 354A, Graseby 
Nutech)(FIGS. 9-11) is used as a secondary trap to refocus the sample into 
a very small volume on column 120 or a precolumn. Secondary refocusing 
trap 122 is mounted inside a rectangular housing 124 (FIG. 9) and 
comprises an inner tube 126 and an outer tube 128 (FIG. 11). A coiled 
heater 130 is coiled about the inner tube 126 without touching it, inside 
the outer tube 128. During the cooling cycle, liquid nitrogen (LN2) enters 
an entry tube 132 (see FIG. 10) at the top of secondary refocusing trap 
122, and goes into the space between the inner and outer tubes. The LN2 
exits by means of an exit tube 134 which carries the LN2 from the lower 
area between the inner tube and the outer tube away from secondary 
refocusing trap 122. Secondary refocusing trap 122 is used in the 
invention to avoid some common capillary column chromatographic problems 
often encountered in the presence of excess water vapor. Such problems 
include multiple peaks, peak broadening, and peak tailing. Secondary 
refocusing trap 122 allows achievement of sharp, well-shaped GC peaks of 
the very volatile compounds. For small sample volumes, or samples with 
very high concentration compounds, secondary refocusing trap 122 may be 
redundant with primary collection trap 78. For most applications, however, 
secondary refocusing trap 122 is necessary to achieve reliable 
chromatography and quantitation. In any case, having secondary refocusing 
trap 122 as part of the apparatus, allows the operator to analyze a 
variety of samples whether or not secondary refocusing trap 122 is 
necessary for a particular sample. 
Secondary refocusing trap 122 receives analytes from primary collection 
trap 78 in a helium matrix with the CO.sub.2 and a portion of the water 
vapor already removed. The tubing material of secondary refocusing trap 
122 may thus be smaller than that of primary collection trap 78, for 
example, 0.53 mm I.D. Secondary refocusing trap 122 is rapidly heated to 
inject a tight bandwidth of sample to the gas chromatograph. Secondary 
refocusing trap 122 uses LN2 to reach subambient temperatures. A 
0.040-inch I.D. tube travels through the center of the unit and contains 
the column. This provides a shield for the column from potential damage 
caused by the pressurized LN2. External secondary refocusing trap 122 
contains a nickel 200 tube (0.0625 inch O.D..times.0.040 inch I.D.) that 
begins at the top of secondary refocusing trap 122, runs through the 
center of trap 122, and out the bottom of trap 122, extending an 
additional length (typically 6 inches) out the bottom of trap 122. The 
nickel tube is contained in another tube (preferably with a 3/8-inch O.D) 
which is sealed at its outer diameter at the top and the bottom of 
secondary refocusing trap 122. This allows liquid nitrogen to flow around 
the outside of the tube, but not touch the inside. The trap is typically 
mounted to the top of the gas chromatograph by a mounting bracket with the 
bottom of the trap close enough to the top of the gas chromatograph that 
the tube extends through the top of the gas chromatograph and into the gas 
chromatograph oven. The gas chromatograph analytical column is fed up into 
the tube of the secondary refocusing trap before the column enters the 
secondary refocusing trap. 
Gas Flow Parameters 
As discussed above, there are six gas flow parameters in the preferred 
embodiment of the invention: the sweep flow, assist flow, helium flow, 
column flow, sample flow and vent flow. The sweep flow, which is the flow 
of helium and sample when both are flowing from the differential pressure 
switch to a metering valve. The sample and helium are pulled through the 
switch with a vacuum to prevent back diffusion of the sample. The sweep 
flow is set between 10 and 30 ml/min, while there is excess helium purging 
the switch. This may be fine-tuned later based upon need as discussed 
herein. The assist flow, which is the flow of helium or the sample, 
depending on the stage of the method of the invention, from the 
differential pressure switch through the expansion volume and trap, and 
out the assist "T" of the assist/column interface is set to achieve the 
required sample volume in a reasonable sample trapping time (t). For this 
configuration, 10-35 ml/min is appropriate. The total volume trapped in 
time t is the sum of the column flow and the assist flow times t. The 
helium flow, activated by helium valve 111, is set so that there is 
between 20 and 100 ml/min excess flow measured at the vent under the 
conditions when sample port 56 is capped (no sample is entering the port 
at this point), the assist flow is on, and the sweep flow is on. The exact 
helium flow is adjusted in conjunction with all the other settings. For 
example, with assist flow at 20 ml/min, sweep flow at 29 ml/min, column 
flow at 1 ml/min and a vent flow of 50 ml/min, the helium flow would be 
100 ml/min. 
The column flow is generated by the MS vacuum system through the direct 
column GC/MS interface. The exact value is determined by the column 
configuration. A typical 30 meter by 0.32 mm I.D. column will have a 
column flow of about 2 ml/min helium at 5.degree. C. The exact column flow 
can be determined by calculation using the volume of the column and the 
elution time of an air peak. The sample flow is set external to the 
system. As discussed herein, the system is designed for a range of sample 
flow of 50-250 ml/min. Sample flow must be greater than the combined total 
of the assist flow plus sweep flow plus column flow so that only the 
sample is trapped when the helium switch is off. The maximum supply flow 
can be determined empirically by increasing sample flow and monitoring air 
infiltration in the switch as a mass spectrometer (MS) response. When 
nitrogen and oxygen appear in excess above background in the MS, sample 
flow is too high. 
The vent flow is monitored periodically in two configurations: (a) helium 
on, sample on, assist on, sweep on; or (b) helium off, sample on, assist 
on, sweep on. The vent flow should always be positive (flowing out of the 
system) unless the sweep is being used to pull an ambient pressure sample 
from the air or a chamber. In such a case, the vent port is capped. 
In operation of the apparatus of the invention when concentration is 
required for low level samples, primary collection trap 78 is cooled 
cryogenically to a well controlled set-point and serves as the analyte 
concentration zone as in prior cryogenic concentrators, where nitrogen and 
oxygen are removed from the sample, and CO.sub.2 is partially removed from 
the sample. The internal diameter (0.75 mm) determines the overall 
geometry of the whole injection system from a chromatographic standpoint 
and from construction considerations. Other inside diameters are possible 
and have been tested with good results. Experimental analysis reveals, 
however, that an I.D. of 0.32 mm is essentially too narrow to accommodate 
typical air samples (100 ml to 500 ml volume) and still avoid blockage due 
to ice formation during the collection phase. An I.D. of greater than 0.75 
mm is likely to have breakthrough of the very volatile analytes as the 
surface to volume ratio decreases (proportionately to 1/radius) for the 
same length of tubing. This decreases the probability for capturing a 
particular molecule. 
The computer system used to control the various valves and switches of the 
invention and to provide the operator with information on the status of 
the system may be any as are known in the art, for example, the Model 2000 
controller (Graseby Nutech) having a serial interface to a compatible 
computer. 
The apparatus and method of the invention allow high level processing for 
VOCs (i.e., for concentrated samples having high levels of VOCs) in 
analysis of wastes, product head space, soil gas, etc. Based on the 
inherent inertness of the system, which avoids analyte adsorption, 
chemical reaction, or permeation into the surfaces, there may be a rapid 
switching of the helium valve from off to on and back off, for example, to 
determine the injection bandwidth. The injection bandwidth is determined 
by the amount of time that the helium is not flowing, i.e., when the 
helium is flowing the sample is pushed out the vent and is not analyzed by 
the gas chromatograph. Critical parameters to allow accurate and complete 
analysis in these circumstances include the dead-volume of the switch, 
which determines the minimum injection bandwidth, run to run helium flow 
stability, inside diameter of the expansion volume and the initial trap, 
sweep flow stability, and assist flow rate and stability. 
The apparatus and method of the invention may also be used for trace-level 
and ultra trace-level VOCs measurement for a wide variety of VOCs and 
polar VOCs. This includes applications where the Environmental Protection 
Agency (EPA) has guidance criteria or standards and minimum levels for 
particular compounds such as air toxics, ozone precursors (excluding C2 
hydrocarbons) and odors analysis. This type of measurement is also 
appropriate for other air matrices such as in exposure chambers for 
materials offgas testing. Critical parameters in adjusting the apparatus 
of the invention for this use include system cleanliness, thermal zone 
stability to allow precise concentration and injection of large air 
volumes. The "assist flow" feature of the apparatus and its on/off control 
are important in allowing rapid trapping without desorption losses. 
A third major application of the apparatus and method of the invention is 
the area of exhaled breath analysis for mid-level and trace-level VOCs, 
with special emphasis on polar VOCs. High levels of CO.sub.2 and H.sub.2 O 
in breath, as well as the mixture of relatively high acetone, ethanol, and 
isoprene levels, together with trace-level compounds of interest, make 
this a difficult matrix for analysis. Critical parameters include the 
overall inertness of the system to allow difficult (reactive, adsorptive) 
compounds to be transferred, insensitivity of chromatographic injection to 
water disruption (as provided by a refocusing trap) and overall capacity 
to focus both high and low level concentration simultaneously without 
loses (as provided by the trap/expansion volume design). 
The features and advantages of the present invention will be more clearly 
understood by reference to the following examples, which are not to be 
construed as limiting the invention. 
EXAMPLES 
EXAMPLE I. Concentration of a Low Level Sample 
Upon the start of a sample concentration sequence, the injection system 
performs an initialization. The oven enclosure, primary collection trap 
(Cryotrap), and secondary refocusing unit are all at their elevated 
temperature set-points (each at 100.degree. C. for example) The helium 
valve is set at "off" which means that the valve is allowing helium to 
flow, since it is a normally open solenoid valve. The assist valve 112 is 
on, which allows assist flow of helium to flow through the assist valve, 
which is a normally closed solenoid valve. This assist flow is 
flow-regulated by the mass flow controller positioned between the assist 
valve and the vacuum ballast (see FIG. 8). When this initialization is 
complete, as determined by the solenoid valves set to the proper state, 
and the temperature zones are at setpoint, the system prompts the operator 
to begin the concentration process when ready. 
Once initiated by the operator, the system performs a helium purge of the 
Cryotrap at the current elevated temperature to flush anything remaining 
in the Cryotrap out of the system. 
The Cryotrap is cooled to a subambient temperature for the concentration 
process (typically -150.degree. C.). When the Cryotrap reaches the desired 
set-point and is stable at that temperature, the operator is prompted by 
the status line on the computer display to open the sample container in 
which the sample was originally collected. The operator opens the sample 
container and waits 30 seconds for the sample to displace all air in the 
sample line. This results in both the helium flow and sample flow entering 
the differential pressure switch. The concentrator is in a wait state in 
the concentration process while the operator performs these manual 
operations. When the operation resumes the concentration process, the 
helium valve is turned "on" thus shutting off the helium flow into the 
switch. 
The assist flow pulls the sample through the cool Cryotrap until the 
desired volume has been trapped based on the integrated flow from the mass 
flow controller controlling the assist flow. Once the concentration is 
complete, the helium valve is turned off allowing the helium flow into the 
differential pressure switch, the Cryofocus unit begins cooling, and the 
operator is prompted to close the sample container, removing sample flow 
from entering the differential pressure switch, and preserving the 
remaining sample in the sample container. The helium flow flushes any 
balance gas remaining in the Cryotrap out of the assist flow for 60 
seconds. This also gives the Cryofocus time to reach its sub-ambient 
set-point. Next the assist valve is turned off, stopping the assist flow 
and diverting 100 percent flow to the column. The Cryotrap is heated to an 
elevated temperature which transfers the concentrated sample to the 
Cryofocus unit. Once this transfer has taken place, the Cryofocus is 
heated to an elevated temperature (typically 150.degree. C.), and the GC 
analysis is started. The assist valve is turned on to clean up any 
remaining material in the Cryotrap. This completes the concentration and 
injection process and the system waits for the time necessary for sample 
analysis, before it initializes for the next injection. 
EXAMPLE II. Injection of a High Level Sample 
Upon the start of a high level sample injection sequence, the injection 
system performs an initialization, setting the appropriate valve states, 
temperature zone setpoints, and flow control setting as follows. The 
helium valve is set to "off" which allows helium flow through this valve, 
and the assist valve is set to "off", shutting off the assist flow through 
this valve. The sweep flow is always on. 
The temperature zones, oven enclosure, primary collection trap, and 
secondary refocusing trap are typically set to 100.degree. C. The flow 
controller that controls the assist flow is set to 0-flow (off). After 
these initialization steps are completed the system prompts the operator 
to begin the injection process by displaying a message on the computer 
display. 
Unlike the low level sample example in which the primary collection trap is 
cooled at this point, this trap remains hot at all times. The secondary 
refocusing trap is not required for this sequence, but use of the 
secondary refocusing trap is encouraged to significantly improve the 
injection system performance. The secondary refocusing trap is cooled at 
this time with a typical setpoint of -190.degree. C. 
The operator opens the sample container and waits typically 30 seconds for 
the sample to displace all air in the sample line. At this point, both 
helium and sample are entering the differential pressure switch. Because 
of the switch structure, only helium makes up the column assist flow and 
sweep flow. All of the sample flow and the balance of the helium flow exit 
the differential pressure switch out of the vent port. When the operator 
resumes the sequence, the helium valve is turned "on", shutting off helium 
from entering the differential pressure switch. The "on" time specified by 
the operator is a variable set before the start of the operation and 
depends on the injection volume desired. When the helium flow is stopped 
from entering the differential pressure switch, only the sample flow 
remains, thus accounting for the makeup of the sweep and column flows at 
this point. The injection volume is determined by the product of column 
flow rate and the time duration that the helium valve is "on". While the 
helium flow is shut off, the sample is being concentrated on the secondary 
refocusing trap. After the time delay is complete, the helium valve is 
turned "off" allowing flow to the switch. The helium flow flushes the 
sample that has not yet reached the secondary refocusing trap and also 
purges the column of the balance gas of the sample obtained during the 
injection. After a time delay for this purging to occur (about 2 minutes), 
the secondary refocusing trap is heated (for example, 150.degree. C.) and 
the gas chromatographic analysis is started. This completes the injection 
process and the injection system waits for the time necessary for the 
sample analysis, before it initializes for the next injection. 
While the invention has been described with reference to specific 
embodiments thereof, it will be appreciated that numerous variations, 
modifications, and embodiments are possible, and accordingly, all such 
variations, modifications, and embodiments are to be regarded as being 
within the spirit and scope of the invention.