He1ium Photoionization Detector UtiIizing a Microwave Discharge Source Robert R. Freeman’ and W. E. Wentworth Department of Chemistry, University of Houston, Houston, Texas 77004 A helium photoionization detector for use in gas chromatography systems has been developed. A microwave discharge in purified helium produces an intense 21 electron volt ionization source whose stability and reliability meet the requirements of analytical instrumentation. Use of the helium resonance line enables “universal” detection which is uniform for most compounds. The detector has a sensitivity of 1.3 x lo-” g/ml (argon) and a linear dynamic range of lo4. The design of the detector is such that the background current, noise level, and response are independent of temperature ( r 2 280 “C).
RECENTDEVELOPMENT of reliable and extremely sensitive ionization detectors has done much to enhance the widespread use of gas chromatography. Of these new detectors perhaps one of the most promising is the nonradioactive ionization detector which utilizes the principle of photoionization. One of the first photoionization (PI) detectors was built by Lovelock ( I ) and employed an argon glow discharge as a source of ultraviolet radiation. The capabilities of photoionization detection demonstrated by this detector encouraged others to initiate studies in the field (2-9). Subsequent PI detectors can be divided into two categories based on the type of discharge used for ionization. Price (2), Roesler (3), and Karmen ( 4 ) continued to use argon while others (5-9) used helium. Since the PI detector is theoretically able to detect any sample which has an ionization potential less than that of the impinging ultraviolet radiation, both the argon and helium detectors have inherent advantages and disadvantages. The argon PI detector will not respond to any sample with an ionization potential greater than 11.7 eV; consequently, the detector will respond to most organics while at the same time give no response to many inorganic gases; thus a low background current and a stable base line are characteristic of the argon detector. The great advantage of using a helium discharge is that detection of all compounds is possible. Although “universal” detection is possible, use of helium PI detector has been generally confined to inorganic gas analysis. One apparent disadvantage of using helium is that considerable effort must be made to ensure the purity of the helium if optimum sensitivity and “universal” detection are to be realized. Work by Goldbaum (9), however, indicates Present address, Texas Research Institute of Mental Sciences, Houston, Texas 77025 (1) J. E. Lovelock, Nature, 188,401 (1960). (2) J. G. W. Price, D. C. Fenimore, P. G. Simmonds, and A. Zlatkis, ANAL.CHEM., 40, 541 (1968). (3) J. F. Roesler, ibid., 36, 1900(1964). (4) A. Karmen and R. L. Bowman, Narure, 196, 62 (1962). (5) A. Karmen, L. Giuffrida, and R. L. Bowman, ibid., 191, 906 (1961). (6) M. Yamane, J. Chromatogr., 11, 158 (1963). (7) Zbid., 14, 355 (1964). (8) R. Villalobos, Z.S.A. Trans., 7, 38 (1968). (1961). (9) L. R. Goldbaum, T. J. Domanski, and E. L. Schloegel, J. Gas Chromatogr., 6 , 394 (1968).
that at extremely high helium flow rates (ca. 300 ml/min) the proper environment in the detector to ionize atmospheric gases can be obtained using commercial helium. The required ultraviolet radiation can be produced using several different methods; however, current PI detectors commonly utilize a glow discharge. This almost exclusive use of electrical discharges requires careful regulation of several operational parameters such as electrode separation, acceleration voltage, and discharge current, and has hindered the actual use of PI detection. Not only do electrode contamination, irreversible surface reactions, and other “aging” effects limit the use of the detector but, just as importantly, the thermal instability of the glow discharge imposes an upper operating temperature limit of 170 “C on the detector. Elevated temperatures substantially reduce the efficiency of the detector. Price (2) reports a drop in the response of the detector from 2.6 X lO-’A (@ 25 “C) to 1.7 X A (@ 150 “C). This temperature effect was dramatically illustrated by Roesler (3) who observed no sensitivity at all to propane at 168 “C. It is primarily because of this thermal instability that use of currently available PI detectors has been restricted to inorganic gas analysis. Work in the past eight years in the area of photoelectron spectrometry provided the technology and insight necessary for the design and construction of the photoionization detector presented at this time. Investigators (10) in the field of photoelectron spectrometry have long utilized microwave excitation as a means of obtaining the 21-electron volt helium resonance line. It seemed obvious to us that the incorporation of this technique into the design of a PI detector would have certain advantages over conventional glow discharge detectors. By using purified helium at reduced pressures (ea. 1 mm Hg) a 21-electron volt ionization source can be generated using microwaves. The use of microwaves eliminates most of the operational problems encountered in the use of an electrical discharge, while at the same time, the microwave discharge is quite stable and extremely intense. The detector presented at this time incorporates a microwave discharge in purified helium and is designed to have a large operating temperature range of at least the equivalent of Lovelock‘s (11) high temperature eaNi electron capture detector. EXPERIMENTAL
A schematic diagram of the photoionization detector and the supporting gas lines and electronic components are given in Figures 1 and 2, respectively. The detector was constructed of stainless steel type 304 and gold plated (0.001 in.) in order to minimize adsorption onto the desorption from the walls. The electrodes are in a parallel plate configuration and are electrically isolated by a ceramic (Aremco Products (10) D. C. Frost, C. A. McDowell, and D. A. Vroom, Proc. Roy. SOC.,Ser. A , 296, 566 (1967). (11) P. G. Simmonds, D. C. Fenimore, B. C. Pettitt, J. E. Lovelock, and A. Zlatkis, ANAL.CHEM., 39, 1428 (1967).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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nn
Figure 1. Schematic diagram of the photoionization detector : ( A )discharge tube; ( B )sample inlet; ( C ) sample removal; (D)helium removal; ( E ) electrodes
NO. 502-1100) which has good insulating properties at high temperatures. The detector was encased in a brass block which was equipped with two 165-watt cartridge heaters controlled by a variable transformer. Detector temperature was monitored by a ChromaLConstantan thermocouple which was embedded in the detector wall in. from the ionization region. The ultraviolet radiation of the discharge is collimated by a length of capillary which was aligned with the discharge tube (3/s-in. heavy wall borosilicate glass). The intense heat of the discharge requires that the discharge tube be air-cooled. The detector current was amplified by a Cary electrometer (Model 31) and traced on a Leeds & Northrup Speedomax G recorder. The repeller voltage was applied using a regulated dc (0-120 volt) power supply, and the microwave generator was a Baird-Atomic Hg 198 excitor. Both the helium removal pump and the sample removal pump are Cenco Hyvac 14‘s. Helium (Matheson Gas, high purity 99.999%) for the discharge was passed through a Molecular Sieve 5A trap and an activated charcoal trap (at 77 OK) prior to use. All sample gases were from Matheson Gas and at least of 99% purity. A gas sampling valve was used to “inject” the sample into a l/s-in. X 3-ft poropak Q column which was used throughout the investigation. In every instance, helium was used as the carrier gas.
uu Figure 2. Experimental arrangement: ( A ) helium supply; ( B ) regulating valves; (C)porapak Q column; (D)detector; ( E ) 5A Molecular Sieve drying column; (F) activated charcoal trap; (GI discharge tube; (H)electrometer; ( I ) recorder; ( J ) dc power supply; ( K ) vacuum pump; ( L )microwave generator
50
t
I
-I
0
I
I
LOG P, MM HG
2
3
Figure 3. Dependence of background current (m) and sample response ( 0 )on detector pressure Temperature = 208 “C; repeller voltage 0.014 ml of oxygen
=
-110 V.; sample
=
DISCUSSION
It is of some importance to determine if the low temperature activated charcoal trap is necessary in order to obtain the best possible detector performance, or if unpurified commercial grade helium can be utilized without adversely affecting the detector performance levels. This involved the measurement of the background current, noise level, and sample response using first purified helium and then commercial grade helium. The results of this study appear in Table I. As indicated, 18 % more microwave power is required in order to obtain a constant response for a given argon sample. For a given power level, the intensity of the discharge in unpurified helium is lower and the discharge is noticeably red shifted. The use of purified helium as the discharge gas reduces the background current by 2 0 x and the noise level by 67%. Hence, the detector operates more efficiently with greater sensitivity when purified helium is used as the discharge gas. There are two operational parameters which must be properly regulated in order to obtain optimum detector performance. These are detector pressure and repeller voltage. Figure 3 illustrates how the background current and 1988
Table I. Effects of Discharge Gas Purity on Detector Performance Purified helium (activated charcoal at 77 “K) Unpurified helium Microwave power required for discharge 4% 52 Background current, amperes 5 x 10-9 6.26 X l l F 2 x 10-11 6 x Noise, amperes Response to argon, amperes 2 . 9 x lo-* 3 x 10-8
z
sample response vary as a function of detector pressure. The background current is due principally to impurities in the carrier gas stream and degassing of the column. The observed increase in the background can be attributed to a concentration effect-as the pressure increases, the concentration of impurities increases. The response to a given sample
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
-5 0
-200
-100 -150 V O L T A G E , VOLTS
Figure 4. Effects of repeller voltage on sample response Temperature = 208 " C ; pressure = 1 mm Hg; sample
=
0.014 ml
of oxygen
(oxygen) remains constant at detector pressures less than 2 mm Hg. The increase in the response from this point is due to the concentration of sample. The dramatic decrease in response at pressures over 25 mm Hg is due to either of two effects, The increased pressure reduces the mean free path of each electron so that for a given repeller voltage, the number of electrons collected could be reduced, thus resulting in a decrease in the response of the detector. Another possibility is the increased absorption of the ionizing radiation before it enters the cell, thus decreasing the radiation density and subsequent ionization that can occur in the cell. Either of these possibilities would account for the decrease in detector response at P > 25 mm Hg. The dependence of the response on repeller voltage appears in Figure 4. As the repeller voltage is increased, the response increases until a plateau is reached somewhere between -110 and -200 volts. These data are easily understood once one realizes that the photoelectrons are ejected at all angles. The positive intercept in Figure 3 represents those I
I
x-x- .x-
I6t
I4t
=- t g
U L
12
IO
I
x-
I
Table 11. Response of Photoionization Detector to Various Types of Sample Materials Response Ionization x 102, Mass, Response potential, A-in./mole Sample grams x 103/gram eV 0.33 15.75 40 8 . 3 Argon 0.24 32 7.5 12.06 Oxygen Carbon 14.0 0.28 monoxide 28 10.0 Carbon 44 4.1 13.7 0.18 dioxide 0.28 28 10.0 Nitrogen 15.58 0.23 Nitric oxide 30 7.7 9.25 0.37 17 21.7 Ammonia 10.24 Sulfur 0.05 64 78.1 12.34 dioxide Sulfur hexa1.0 0.15 fluoride 146 Boron tri15.5 67 1.6 fluoride 0.11 1.35 15.45 2 675.0 Hydrogen 16 0.67 12.6 Methane 41.8 0.75 30 11.65 Ethane 25.0 0.38 Propane 44 8.6 9.98 0.52 Ethylene 10.5 28 18.5 Dimethyl 9.98 ether 0.41 46 8.9 31 3.0 0.09 8.97 Methylamine 3.0 8.86 46 0.14 Ethylamine
electrons which are ejected toward the collection electrode. As the negative repeller voltage is increased, an ever-increasing number of electrons are collected until the point is reached where the repelling voltage is able to reverse the direction of those electrons ejected directly toward the repelling electrode. At this time the number of electrons which can be affected by the repelling voltage are collected, resulting in the observed plateau. The temperature of the detector was varied over the range 25 to 275 "C, and the background current, noise level, and sample response were monitored. The results of this study can be found in Figure 5. The temperature independent nature of these three parameters is due primarily to the design of the detector which permits the temperature of the ionization and discharge regions to be independently regulated. The discharge tube is air-cooled and maintained at room temperature. Since the noise is due primarily to fluctuations in the discharge and the background current and sample response depend principally on the discharge intensity, it is not surprising that these parameters are temperature independent. A determination of the upper operating temperature limit has not been attempted; however, the detector operates efficiently at temperatures greater than 275 "C. The linearity and sensitivity data were obtained at 208 "C which is 35" above the maximum operating temperature reported for any other PI detector. Table I1 presents the response of the photoionization detector to various types of sample material. The response is expressed in terms of the area of the peak in the units of amperes times inches. The response per mole and per gram are both given in Table 11. A constant flow rate was used for all samples and the comparison of the relative responses can be made regardless of whether the detector is concentration or mass dependent. The universal nature of the detector is clearly evident, for the sample compounds in Table I1 range from argon and nitric oxide to ethylene and dimethyl ether. Apparently the response/mole for a given sample is fairly con-
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Table 111. Performance Characteristics of Currently Available Photoionization Detectors Lovelock ( I ) Price (2) Yamane (7) Apparent ionization o.ooo1 0.001 ... efficiency Background current, 5 x 10-10 2 x 10-10 10-9-1 0-10 ampere Carrier gas Ar Ar, HP,NZ He Linear dynamic 104 103 105 rangea Minimum detectable ... 1 x 10-12 2.8 X 10-" quantity and test oxygen propane substance, g/ml 6.8 x lo-" propane Noise level, ampere 1.6 X 10-11 ... 10-13 Maximum operating ... 175 temperature, "C Type of discharge Glow (argon) Glow (helium) Glow (argon)
Substance detectable
5
This work 0.003 6 X 10-0
He 104
4
x 10-11
oxygen 1.3 X Argon 4 x 10-11 2274 Microwave (helium) Universal
Most organics Universal Most organics and some and some inorganics inorganics These are computed ranges with the exception of Price's lo6 value, which was measured using an exponential decay chamber.
stant, although factors such as molecular complexity, volume, and ionization potential do cause some deviations. The linearity of response was investigated using argon, oxygen, nitrogen, and methane as sample gases. A repeller voltage of - 110 volts was employed and the detector temperature was 208 "C. A plot of peak current/gram of sample us. the log gram of sample appears in Figure 6. When such a plot is used, a linear response is represented by a zero slope line. The responses for all samples are linear up to 1.6 X 10-5 gram of sample. Generally, this amount of sample gives a signal of around 2 X lo-' ampere. The noise level is 4 x 10-11 ampere; consequently, the computed linear dynamic range is lo4. The performance characteristics of the helium photoionization detector developed in this work are compared with those of the argon photoionization detector of Lovelock ( I ) and Price (2) and the helium photoionization detector of Yamane (7) in Table 111. Note that the ionization efficiency is greater using helium with a microwave discharge compared to argon with a glow discharge. This probably results from the greater intensity with the microwave source. In fact, a larger cross sectional area of the ionizing radiation along with an improved geometry of the collector electrode could increase this further. Despite this higher ionization efficiency, the sensitivity of the argon PI detector is an order of magnitude greater than the helium PI detector as a result of the higher noise level in the helium PI detector. The source of this noise in the helium PI detector probably arises from fluctuations in the microwave discharge. Possibly a better regulated power supply would improve the stability of the light source. The linear dynamic range of lo4is on the order of the other PI detectors. The higher background current using He is significantly higher than with Ar. Presumably this results from the sensitivity of the helium PI detector to high ionization potential impurities from the carrier gas and column. This is probably the most serious limitation to using helium rather than argon in the PI detector. CONCLUSIONS
In this study we have shown how an electrodeless microwave discharge can be used as an ionizing radiation source in a helium PI detector. The use of such a source in a PI detector has the advantages of simplicity of design and operation since no electrodes are required for the discharge; of intense radiation source with a high ionization efficiency; and of no 1990
0
7
0
2
X I@
LOG (GM. ;AMPLE)
Figure 6. Peak current/gram of sample us. log gram of sample: (o),argon; (.),nitrogen; ( f ) oxygen: ( X ) methane Repeller voltage
= -110 V ;
pressure
=
1 mm Hg; temperature =
208 ' C
temperature limitation since the light source is external to the ionization region. In addition to these advantages, use of a microwave discharge in a PI detector should lead to greater long range stability since any contamination or erosion of metal electrodes would be eliminated. This has not been proved experimentally in this initial study. Although this study has been restricted to helium as the discharge gas, there is no reason why the same principles and design could not be used with any other inert gas such as argon. The above advantages should apply equally well to discharge in either gas. The use of pure helium (7) at low pressures gives a resonance line at = 21 eV which is capable of ionizing any atomic or molecular species and is therefore a universal detector. On the other hand, since the helium PI detector is sensitive to all atomic and molecular species, it also has a greater background current which tends to lower its sensitivity relative to an argon PI detector. FUTURE WORK
In this initial study, we have shown how a microwave discharge can be used in a PI detector and have pointed out
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
some of the advantages in its use. However, there are other possible advantages for using a microwave discharge in this geometrical arrangement, which are not obvious, and these will be investigated in future developments of the detector. As shown in Figure 1, the radiation from the microwave discharge passes through a glass capillary and is therefore collimated in a small circular beam passing through the sample gas. If a cylindrical electrode were used, no potential would be required to collect the ionized electrons and the collection should be more complete, This should improve the signal to noise ratio. However, even more importantly, the ejected electrons are given off with different kinetic energies and the distribution of such energies is characteristic of the species being ionized. By applying a cylindrical retarding grid, as first used by Turner in the development of photoelectron spectrometry (12), the dis(12) M. I. AI-Joboury and D. W. Turner, J. Chern. Soc., 1963,5141.
tribution of electron energies could be determined and used for a qualitative identijication of the sample. Thus the detector could be used for qualitative identification of the chromatographic peaks, as is conventionally done with a mass spectrometer, in addition to quantitative analysis as with the PI detector. Furthermore, a judicious choice of retarding potential could be used to eliminate any low energy electrons arising from impurities providing the impurity had a higher ionization potential than the sample gas. This should markedly decrease the background current and, consequently, increase the range and the signal to noise ratio. With these developments this detector could be one of the most versatile, yet sensitive, detectors available. RECEIVED for review July 15, 1971. Accepted August 19, 1971. The authors acknowledge the financial support of the Robert A. Welch Foundation for this research.
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