Method and apparatus for preventing the destruction of an alkali source of a nitrogen-phosphorous detector

A method and apparatus for preventing the destruction of an alkali source in a nitrogen-phosphorus detector by certain reactive and derivatizing reagents encountered in gas chromatography is disclosed. Protection is afforded by lowering the temperature of an electrically heated alkali source during the period in which offensive substances in a gas under analysis are eluted, with the temperature of the source being restored after passage of the offensive substances. The temperature of the alkali source is changed by altering the current through an electrical resistance heater with a temperature control circuit that utilizes a wheatstone bridge.

BACKGROUND OF THE INVENTION 
This invention relates to alkali source ionization detectors used in gas 
chromatography. More specifically, this invention relates to the 
protection of an alkali source ionization detector from adverse effects 
resulting by reason of contact with contaminants or reactive agents 
passing through the detector. 
In gas chromatography, a multicomponent sample is separated in the gas 
chromatographic column and eluted with a carrier gas in order to isolate 
the components of the original sample as mixture with the carrier gas. 
This eluate is conducted to any one of a number of detectors, such as 
flame ionization detectors and the like. One type of ionization detector 
utilizes an alkali source in the ionization chamber to contact the 
effluent of the column as it is eluted. Certain compounds, for example 
nitrogen and phosphorus compounds, produce ionic emissions when contacted 
with a heated alkali source. The collection of these ionic emissions and 
the measurement of the resulting ionic current can provide a quantitative 
indication of these specific compounds. 
Recent advances have been made in alkali sensitized detectors for gas 
chromatography. One significant improvement has been the utilization of 
electrically heated alkali sources to stabilize the response 
characteristics of alkali sensitized detectors. Previously, alkali source 
detectors employed a small hydrogen/air flame for volatilization of an 
alkali salt and initiation of the reaction mechanism that produced the 
detector signal. However, since the sensitivity of such early detectors 
was dependent upon the temperature of the alkali material, these detectors 
exhibited instabilities due to fluctuating flame temperatures. 
An electrically heated alkali source provides for more accurate control of 
the temperature of an alkali source, resulting in greater stability of the 
detector. In electrically heated alkali source detectors, a mixture of 
alkali salts in a silica gel matrix is fused to a heating element, the 
temperature of which is maintained by a regulated power supply. 
At elevated temperatures of 700.degree. C. or higher, alkali sources are 
easily damaged by certain solvents and derivatizing reagents that are 
commonly used in gas chromatography. Contact with such solvents can result 
in damage to the heating element and/or the alkali source. For example, 
the alkali of the source can be unduly depleted due to the formation of 
volatile alkali halides when a halogen containing solvent is passed 
through the cell. Alkali sources can also be destroyed by derivatizing 
reagents used to prepare the sample such as 
N,O-bis-(trimethylsilyl)-acetamid (known as "BSA") that are decomposed by 
the hot source and covered with decomposition products. See Gehrke, et al, 
U.S. Pat. No. 3,415,864. Therefore, it is desirable to protect the alkali 
source during the period of passage of such potentially damaging 
materials. 
Typically, halogen solvent and derivatizing agents have a low retention 
time in a chromatograph column. Accordingly, these materials are among the 
first materials to exit the column following injection of the sample, and 
their passage period is predictable. 
At present, if potentially damaging materials are to be used, a four-port 
valve is employed to divert the carrier gas stream away from the detector 
during the elution of the potentially offensive materials. However in many 
instances, utilization of a valve is not an ideal solution since it is 
expensive, introduces dead volume, introduces additionally reactive 
surfaces which must be protected, and increases the number of components 
that require attention. Jahnsen et al, U.S. Pat. No. 3,859,209 teaches the 
use of two multi-port valves to divert organic chemical compounds. 
In Giuffrida, U.S. Pat. No. 3,372,994, the use of an alkali-metal salt 
fused to an electrode and heated by a hydrogen flame is disclosed. The 
alkali coating allows the detector to selectively emphasize 
phosphorus-containing organic compounds in mixtures. 
Kolb et al, U.S. Pat. No. 3,852,037, addresses the deterioration problem 
attendant the use of an alkali glass bead which is maintained in a heated, 
softened state by a hydrogen flame during operation of the detector. Kolb 
teaches the use of a sensing electrode located above the alkali glass 
bead, with the electrical conductivity between the bead and the electrode 
being measured to indicate deterioration of the glass bead. A continuous 
supply of alkali is made available as the surface area of the glass bead 
gradually deteriorates. 
SUMMARY OF THE INVENTION 
In accordance with this invention, an electrically heated alkali source is 
reduced in temperature during the period of elution of potentially 
damaging materials. The temperature is reduced to a level below that which 
promotes destructive reactions on the source, but is maintained 
sufficiently high to preclude the condensation of non-volatile materials 
on the source. The reduction in temperature of the electrical heater is 
referred to as "desensitization" of the source. 
In accordance with a more particular aspect of the present invention, the 
source temperature is varied by regulating the current through an 
electrical resistance heater on which an alkali source is mounted. One 
means for regulating current through the resistance heating element 
comprises a wheatstone bridge having the alkali source and electrical 
resistance heater arranged as one leg of the bridge. A fixed resistance 
value is connected in each of the remaining three legs of the bridge. 
Additional means is included to selectively change the voltage divider 
ratio of one side of the bridge to reduce the voltage drop across the 
resistance heater, reducing the current therethrough and lowering the 
source temperature from the first elevated temperature to the second lower 
temperature. The means for changing the voltage divider ratio of one side 
of the bridge can be an external resistance which is connectable in 
parallel with a leg of the bridge that does not include the source. When 
offensive substances are about to pass the alkali source, a control 
element coupled to the external resistance is actuated manually or by 
timed means to connect the external resistance and desensitize the alkali 
source. 
A resistance heating element having a resistance proportional to its 
temperature is preferably utilized. Accordingly, the means for regulating 
the current through the electrical heater element may function in response 
to the resistance value of the heater element. To so operate, the means 
for regulating current flow through the heater element may further 
comprise a differential amplifier or comparator connected across the 
output terminals of the bridge. The differential amplifier or comparator 
detects and amplifies a potential appearing across the bridge output 
terminals due to an unbalanced condition. The output of the amplifier is 
applied to a series regulator supplying power to the bridge. 
When the control element is actuated connecting the external resistance in 
parallel with a leg of the bridge, the bridge becomes unbalanced, and a 
voltage difference results across the output terminals. To balance the 
bridge, the resistance of the electrical heater must be varied. A 
variation in resistance of the electrical heater needed to balance the 
bridge is achieved by varying the voltage applied across the bridge by the 
series regulator. For example, a reduction in bridge voltage causes a 
corresponding reduction in the voltage across and the current through the 
source heater. As a result, the electrical heating effect is reduced and 
the temperature of the source decreases. As the source temperature 
decreases, so does its resistance, which continues until the bridge is 
once again in balance. The alkali source temperature is lowered and is, 
therefore, desensitized from the offensive substances that would otherwise 
damage it.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The FIGURE is a simplified schematic diagram of circuitry suitable for use 
in gas chromatograph detector apparatus that utilizes an electrically 
heated alkali source to implement the present invention. 
The alkali source 10 is preferably a mixture of alkali salts in a silica 
gel matrix fused to an electrical resistance heater element. Alkali source 
10 is heated by current flow through a resistance heating element. 
In normal operation of an alkali sensitized gas chromatograph detector used 
for the determination of trace quantities of nitrogen and phosphorous 
containing compounds, the source temperature is maintained at 
approximately 700.degree. C. To prevent damage to the source and/or heater 
element by certain solvents and derivating agents used in gas 
chromatography, means is provided in accordance with the present invention 
for selectively varying the temperature between the normal operating 
temperature and a lower temperature that is below the temperature at which 
destructive reactions occur, but above the temperature at which 
non-volatile materials would condense on the source. For the more commonly 
encountered destructive reactants, a reduction of source temperature to 
about 400.degree. C. has been found satisfactory. 
Suitable means for achieving the desired result of source desensitization 
can take on many forms. However, since source temperature control is 
typically provided for accurately controlling the temperature of the 
source, means for varying the source temperature will be most conveniently 
incorporated therein. Further, inasmuch as source temperature is 
controlled by varying the current through the heater element, means for 
varying the source temperature between the aforementioned levels will be 
most conveniently achieved by regulating the voltage applied across the 
heater element. 
To provide accurate monitoring of source temperature, the heater element 
desirably is one, such as platinum wire, having a resistance value that 
varies substantially in proportion to heater element temperature. With 
such characteristics, the heater element can effectively be used as a 
feedback device directly indicating source temperature. The circuitry 
shown in the FIGURE is for use in gas chromatograph detector apparatus 
having an alkali source that is heated by a heating element with such 
temperature-resistance characteristic. 
The particular means shown in the FIGURE for varying the source 10 
temperature between the prescribed levels establishes a prescribed flow of 
electrical current through the electrical resistance heater in response to 
its resistance value to effect temperature level control. The preferred 
embodiment shown utilizes a wheatstone bridge circuit 12 to detect 
variations in the resistance of heater/source 10 and provide an output 
signal indicative thereof. 
As shown in the schematic diagram, the electrically heated source 10 is 
connected in leg A of wheatstone bridge 12. Leg B of bridge 12 is a 
resistance comprising resistors 14 and 16 arranged in parallel. Leg C 
comprises a resistance 18 and a trim pot 20. Leg D comprises a resistance 
22. Legs C and D are interconnected by a variable voltage divider 24, the 
wiper contact of which serves as one output terminal of bridge 12. 
Power to bridge 12 is applied from power supply 26 via a controlled series 
regulator 28, such as a darlington pair. Current through series regulator 
28 is applied to bridge 12 at node 30, which is the interconnection of leg 
B to leg C. The voltage across bridge 12 established by regulator 28 is 
measurable between node 30 and node 32 at the interconnection of legs A 
and D. 
The output voltage from bridge 12 is available via output terminals 34 and 
36. Terminal 34 is the wiper contact of variable voltage divider 24, and 
terminal 36 is a connection to node 38 at the interconnection of legs A 
and B. Output terminals 34 and 36 are connected to differential amplifier 
or comparator 40, the output of which is applied as the controlling signal 
for series regulator 28. 
Any imbalance of bridge 12 will produce a voltage differential between 
output terminals 34 and 36 that is detected by amplifier 40. During an 
unbalanced condition, amplifier 40 outputs a voltage level that causes 
regulator 28 to modify the voltage across the bridge between nodes 30 and 
32 in an attempt to balance the bridge. 
An unbalanced condition will occur when the resistance in any leg of the 
bridge is changed. With the exception of heater/source 10 resistance all 
resistances in the bridge, once trim pot 20 and variable voltage divider 
24 are set, are fixed in value. Preferably, the resistances in legs B, C 
and D have a low coefficient of thermal resistance and, therefore, vary 
negligibly with temperature. 
The mathematical relationship of a wheatstone bridge at balance is defined 
by the following expression with respect to the FIGURE: 
EQU R(leg B).times.R(leg D)=R(leg A).times.R(leg C) 
The effective resistance of parallel resistors 14 and 16 in leg B is 
equated to the appropriate resistance of electrical heater and source 10 
in leg A, when the temperature of the source is at the normal operating 
temperature. Accordingly, to balance bridge 12 the resistance of leg C 
must equal the resistance of leg D, thereby providing the same voltage 
divider ratio as that established by legs A and B. 
Since the resistance in each of the legs of the bridge, except the 
electrical heater/source 10 in leg A, is a fixed resistance value 
following an initial calibration of trim pot 20 and variable voltage 
divider 24, an imbalance of bridge 12 can result only from a variation in 
the resistance value of the heater/source 10. Since the resistance value 
of the heater/source 10 is proportional to the temperature of the source, 
a change in resistance from the desired value, which would balance the 
bridge and establish the source at the proper operating temperature, will 
cause a voltage to appear across the output terminals of bridge 12. 
Accordingly, if the temperature of source 10 increases above the desired 
operating temperature, causing the resistance in leg A of the bridge to 
increase, the bridge will become unbalanced and a voltage differential 
will be detected by comparator 40. 
The output voltage of comparator 40 will, in response to a bridge output 
voltage indicative of an increase in source temperature, change in a 
manner so as to drive series regulator 28 to reduce the voltage applied 
across the bridge between nodes 30 and 32. A reduction in voltage across 
the bridge lowers the current through each side of the bridge. A reduction 
in the current flowing through leg A reduces the temperature of the heater 
element and source 10, which correspondingly causes a reduction in 
resistance value. And as the resistance value in leg A of the bridge 
decreases, so does the differential voltage appearing across the bridge 
output terminals. The reduction in bridge output voltage is detected by 
comparator 40 and the output control signal to the series regulator is 
accordingly adjusted until the bridge is again balanced and a stabilized 
source temperature condition is established at the desired operating 
point. 
In order to accomplish the desired desensitization of source 10 while 
reactive substances are passing through the detector apparatus, and to 
subsequently restore the source to the elevated temperature, the 
particular means shown in the FIGURE for varying the temperature of source 
10 further comprises means to selectively unbalance the bridge. Such means 
in the preferred embodiment is one that changes the voltage divider ratio 
of the side of the bridge not containing the heater element and source. 
Since it is desired to reduce the temperature of the source, rather than 
further increase the temperature, the voltage divider ratio established by 
the resistance values in legs C and D of the bridge must be changed to 
reduce the effective resistance value of leg D. Accordingly, the means to 
unbalance the bridge by changing the bridge voltage divider ratio can 
suitably be an electrical resistance element 42 which is selectively 
connectable in parallel with the resistance already in leg D of the 
bridge. Further, the means preferably includes a control element for 
connecting and disconnecting the electrical resistance element. 
In the particular embodiment illustrated in the FIGURE, the control element 
is an opto-electric coupler 44 comprising a light emitting diode and 
photosensitive resistor. The photosensitive resistor portion of coupler 44 
is connected in series with an external resistor 46, with the series 
combination being connected between the wiper of variable voltage divider 
24 and node 32 of the bridge. The light emitting diode portion of coupler 
44 is connected in series with a current limiting resistor 48 that is 
further connected to the power supply for the circuitry. The cathode of 
the light emitting diode is connectable to ground through either a 
manually actuable switch 50, or by timer means comprising a transistor 
driver 52 to be discussed more completely herein. 
Desensitization of source 10 is accomplished by the circuitry in the FIGURE 
when resistance 46 is connected in parallel with leg D of the bridge, 
which reduces the effective resistance thereof. The voltage divider ratio 
formed by legs C and D of the bridge is altered in a manner that produces 
a voltage differential across the bridge output terminals that simulate 
the condition of excess temperature of source 10. Accordingly, comparator 
40 drives series regulator 28 to decrease the voltage applied across the 
bridge, causing a corresponding decrease in current through leg A. As 
outlined previously, a decrease in current in the electrical heat 
resistance heater in leg A of the bridge reduces the temperature of source 
10 with a reduction in heater element resistance following it. When the 
source temperature falls to the desired level, as indicated by a 
resistance value for the heater element and source that corresponds to 
such temperature, the bridge will be balanced. 
Upon disconnecting parallel resistance 46, the voltage across the 
resistance in leg D will immediately increase, causing an imbalance of the 
bridge. Comparator 40 and series regulator 28 react to the imbalance, 
which is an indication of low source temperature, to increase the voltage 
across the bridge and correspondingly increase current through leg A. The 
increase in current through leg A causes the source temperature to 
increase until the normal operating temperature is restored. 
When the light emitting diode portion of coupler 44 is turned off, 
photosensitive resistor portion has a high resistance which keeps the 
parallel circuit comprising external resistor 46 open. When the light 
emitting diode is energized, the resistance of the photosensitive resistor 
falls to a low, negligible value such that external resistor 46 is 
connected in parallel with leg D of the bridge. 
In addition to the means for selectively varying the alkali source 
temperature, means can also be provided for controlling the source 
temperature varying means to selectively reduce the temperature of the 
alkali source from the higher temperature to the lower temperature during 
the time that alkali reactive substances are passing through the detector 
and subsequently restore the source temperature to the higher level. Such 
control means can be timed means that causes the source temperature 
varying means to reduce the source temperature for a set period of time. 
In the schematic diagram of the FIGURE, timer 54 provides a control voltage 
to pulse generator 56 via manual switch 58. A spring button switch 60 on 
pulse generator 56 initiates the operation thereof. The output of pulse 
generator 56 is applied to switching transistor 52 via base resistor 62. A 
resistor 64 and diode 66 are connected to the collector lead of transistor 
52. Both timer 54 and pulse generator 56 may comprise a Signetics NE555 
integrated circuit device. 
Either a manual or timed mode is selectable by way of switch 58. In the 
manual mode, pulse generator 56 turns on transistor switch 52 upon being 
initiated by push-button 60. The anode of the light emitting diode in 
electro-optical coupler 44 is taken to ground potential, thereby forward 
biasing the light emitting diode and closing the parallel resistance 
circuit. In the timed mode, pulse generator 56 is controlled by timer 54 
to turn on transistor 52 for the prescribed period of time. The timed 
period begins when push button 60 is depressed initiating preparation of 
pulse generator 56. When the timed period expires, timer 54 turns off 
pulse generator 56. Timed periods are typically from 5 to 120 seconds. 
The foregoing description has been directed to a particular preferred 
embodiment of the present invention for purposes of explanation and 
illustration. It will be apparent, however, to those skilled in this art 
that the invention admits to embodiment in many forms. For example, the 
means for selectively varying the temperature of the alkali source may 
comprise other types of circuitry for regulating the flow of electrical 
current through an electrical resistance source heater other than the 
wheatstone bridge shown. Further, feedback sensing for establishing the 
elevated operating temperature and the reduced, non-reactive temperature 
of the source can be other than in response to a change in resistance of 
the electrical heater; for example, a thermocouple can be utilized to 
provide a feedback signal indicative of source temperature. These and 
other embodiments of the invention will be apparent to those skilled in 
this art. It is the intention that the following claims cover all such 
equivalent modifications and variations as fall within the scope of the 
invention.