Preliminary Study of Characteristics of Photoionization Detector for Gas Chromatography JOSEPH F. ROESLER Division of Air Pollution, Robert A. Taft Sanitary Engineering Center, U . S. Public Health Service, Cincinnati, Ohio
b A photoionization detector was constructed utilizing Kovar metal seals. The glow current was regulated achieving a noise level on the order of 70 ppa. The effects of carrier gas and argon flow rate were observed. Nitrogen and hydrogen were the primary carrier gases used. An extra electrode was incorporated into the detector in an effort to control the standing current. Polarities with respect to the glow discharge were important, affecting the sensitivity and linearity. Thermal effects of the glow discharge were also investigated. A 4.9-cc. sample of 2.8 p.p.m. propane in nitrogen gave a maximum response of 660 ppa.
I
tors and power supplies were built to optimize sensitivity and to determine the factors affecting sensitivity for potential use in air pollution analyses. EXPERIMENTAL
For this study three detectors were constructed of Corning 7052 glass and Kovar metal. The first detector consisted of three electrodes. The glow discharge was generated between the first and center electrodes which were spaced 1 mm. apart. The first and third electrodes, made of Kovar metal tubes sealed to the glass, served as entrances for the effluent gases from the chromat'ographic column and the gases used to generate the glow, respectively. The center electrode was constructed of stainless steel supported inside the glass by springs. This electrode was electrically common or ground. The third electrode was connected t,o the electrometer. The second and third detectors were similarly constructed (Figure 1) except that the second detector did not have the extra B1electrode. These detectors lvere built with concentric glow electrodes. Of the several power supplies used with t,he detec.tor units, a lambda power supply was the most effective, allowing operation at 10 ma. and 260 volts. All of the power supplies were voltage regulated. h current regulator was designed that incorporated a Philbrick operational amplifier. This circuit (Figure 2) regulated the current to less than 0,00470 for a 10% variation in the input voltage. The symbols K,P, K,X and K2B are standard Philbrick amplifiers. The external power could be provided by any voltage-regulated power supply. The Philbrick circuit emliloys 1007, feedback and is choimer-stabilized. The chopper increaqed ;he gain of the
several detectors based on the ionization principle have proved reliable and extremely sensitive for use in gas chromatography. These detectors] however, are limited in application to air pollution work, which requires response to all compounds except water and the elemental and inert gases. Among the several types of ionization detectors that have not yet been fully explored for such applications, those based on photoionization appear promising. Ion generation by photoionization has been demonstrated by Lossing ( 2 ) . Lovelock (3-5) observed that his detector based on photoionization did not respond significantly to watersaturated air. For air pollution analyses such a detector must respond to gases a t very low concentrations, comparable to the concentrations of contaminant gases in the atmosphere. This paper describes a preliminary study in which several typeb of photoionization detecN RECENT YEARS
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regulator and yielded a more stable current. Some decrease in noise was also discernible, and the range of current selection was increased. The electrometer was a Gyra Model E-302. h 6-liter flask was placed in the vacuum line to level out the pulsation of the vacuum pump. Argon flow rates were measured by a calibrated orifice which was a stainless steel porous filter. ;ill lines mere fabricated of stainless steel except where Teflon was required to maintain electrical insulation. The detector was housed in a shielded] grounded metal case. A11 gases used were prepurified grade. The gas chromatographic column was a 9-foot long silica gel column conditioned by passing carrier gas through it for a 24-hour period prior to use. RESULTS
Detector No. 1 was used in conjunction with the Lambda power supply and at 1 ma. glow current showed good operating characteristics in the range of 1% hydrocarbon or more. Detection of trace amounts of hydrocarbon necessitated higher glow currents, 10 ma., which resulted in high noise levels. In trace determinations, the noise level correlated well with the variations in the brightness of discharge and glow current. The discharge also followed the argon flowv,and discharges occurred by the supporting springs mostly near the vacuum line. The second detector was designed SO that the discharge was self-contained. The current-voltage characteristics of this discharge are sholqn in Figure 3. The detector was operated with the Philbrick circuit. Electrode' D was made pobitive in polarity a i t h respect to c.
7052 GLASS I l B l N WKOVAR TUBING
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STAINLESS STEEL TUBE 1/4 IN.
TO VACUUM I OK
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HOOKUP POINTS A.B.C.D.
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PHOTOIONIZATION DETECTOR N0.3
Figure 1 .
1900
Photoionization detector No. 3
ANALYTICAL CHEMISTRY
Figure 2.
Philbrick current regulator
1
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L 25 0
Figure
1 i
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300 350 VOLTAGE ACROSS DISCHARGE, VOLTS
3. Current-voltage curve for detector No. 2
The detertor was yery responsive to 700 p.i).ni. propane; however, this response wab not linear with change of the feedback resistance of the electrometcr (Figure 4). As the current range selector switch, which determines the feedback resistance, is varied, the output of the electrometer peaks and then falls to a constant value. This phenomenon is de1)endent on glon. current; for 10 ma. glow current, the response is linear for currents larger than 0.1 pa, A s the argon flow mte was decreased (Figure 5 ) , there was a point a t which the drtector sen,sitivity ( 6 ) , (expressed in t,he more correct unit pa mg.-' second, mirroanii)ere seconds per milligram) was zero and further reduction caused a reversal of polarity or a positive response. The liolarity of the glow electrodes was changed so that C \vas positive and, D negative. The effect was strikinga 4.9-cc. sample of 2 3 11.p.m. propane gave a res1)onse of 660 ,ups., as compared to an estimated 2.0 p p a . with the original hookup. The electrodes gave a linear resyionse to propane a:, the electrometer or wa:. changed. The noise level was about 710 ppa. The carrier gas floir rate had little effect on sensitivity over the flow ranges
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1
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C U R R E N T R A N GE (AMPERES)
Figure 4.
Nonlinear response to 700 p.p.m. propane
of 36 to 64 cc. per minut'e. The argon flow rate had a marked effect. At high sensitivities it was difficult to reproduce the response exact'ly. I n general, higher argon flow rates cause higher sensitivity and greater noise. Argon flow rates of 60 to 135 cc. per minute are recommended for best operation. The response at various glow currents for 135 cc. per minute argon flow is displayed graphically in Figure 6. At flow rat'es of less than 60 cc. per minute, the sensitivity decreased and t'he noise increased again. ?io inversion of polarity was experienced. High glow currents (8 ma. or more) usually gave higher noise levels, which appeared intermittently. The background current, or the standing current that exists with no sample input, varies with the flow rate of argon. Figure 7 illustrates these relations for hydrogen and nitrogen as carriers. For nitrogen, higher background currents were produced, which leveled off and became independent of the glow discharge current b e b e e n 5 and 10 ma. Sitrogen was used as carrier for most of the remaining experiments because this plateau gave better base lines:. These background currenk were about 10 times the signal response. Brief investigations with air as a carrier gas showed that 2.8 p.p.m. propane was not, detectable. Three other peaks previously unseen became very prominent, appearing 30 seconds after single injection. The background current increased to 0.4 pa. The effect,s of glow temperature on background current, noise, and sensitivity were also investigated. Cooling springs were placed over the glow portion of the detector. The springs were cooled with an air blower and the temperature was measured by a thermocouple on the glass surface. Higher temperatures were obtained by insulating the glow portion with glass wool. Teniperaturcs as high as 168" C. were obtained with a background current of 0.16 pa. Operation at 32" C. gave a background current of 5500 ppa. At the higher temperatures, the noise level increased and the sensitivity decreased. At 168" C. no sensitivity to propane was observed. I t was thought that the background current could be reduced by appropriate bias voltages or that the signal could be enhanced by appropriat~e accelerating voltages. For a better understanding of the potential relations in the detector, all the electrode potentials were measured, as recorded in Figure 2. The potential on B electrode varied from -0.2 to -0.34 volt. The lower potential was observed at 190 volts positive bias on .4. With no bias the potential was -0.34 volt. A positive accelerating voltage or bias of 50 volts at 1: in series with the
0
2'3 40
60 80 I00 120 140 160 180 2W 220240 ARGON FLOW RATE c c h i n
Figure 5. Responses of detector No. 2 with D electrode positive
electrometer proved sufficient to null a background current of 1800 p p a . Potentials as high as 190 volts are required for 5000 ppa. background current. Higher potentials were not used because of the possibility that discharge might occur. Sensitivity did not depend on the amount of positive bias on A . Negative bias, however, decreased sensitivity and increased the background current. Detector To. 3 incorporated an extra grid or B1 electrode. The electrode B1 reduced the background current to zero or beyond to positive values with small potentials. Sensitivity varied with the background current (Figure 8). Maximum sensitivity was attained a t -1.05 volts, which was also maximum background current. Accelerating potentials connected between the electrometer and the collecting electrode, B , did not control sensitivity but did increase background current. The effect of glow discharge gas was also briefly investigated. I n theory only gases capable of forming metastable states can be used as glow gases. -4rgon has a metastable state a t 11.55 electron volts (e.v.) and emits light with a wavelength ( 1 ) of 1250 -\. The metastable state of helium is a t 20.96 ev. The higher enetgy should increase the back-
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Sensitivity vs. glow current) detector No. 3
IO. SEPTEMBER 1964
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Figure 7. Background current vs. carrier flow
ground current. With pure helium, background currents of 54,000 ppa. were observed. The response to 2.8 p.1i.m. propane &as only 1000 ppa. T h e n helium was contaminated with 2.2 p.p.m. propane, the background current dropped to 520 ppa. but no sensitivity to propane in the carrier wa5 evident. Argon contaminated with 10yo methane quickly shorted the glow electrodes with carbon particles, which were easily seen on the electrodes. DISCUSSION A N D C O N C L U S I O N S
In simple theory a glow discharge is developed between electrodes C and L). Photons are radiated in the direvtion of electrodes :I and B. K i t h argon as glow gas these photons should have an of about 10e.v. ( 1 ) . I%ecw~se the ion potential of the carrier gases (N2, He, H2) is greater than 10 e.\'., no ionization of carrier gas is ex1)ccted. R h e n measurements a t concentrations of 1 1i.p.m. or lower electron multiplication higher glow dischargr c quired; these in t,urn cause higher background currents and noise. For operation a t high glo~v currents: well regulatrd miver sui )lies and concmt ric glow el(~~ti.odcs are essential to cliininat c noise. The background m r r t n t , is generally ten times the sensitivity. T h k caused difficulties in zeroing the clrrtromrtcr for oi)tiinuni sensitivity. l'hus, thc
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ANALYTICAL CHEMISTRY
Figure 8. Sensitivity vs. bias on grid of detector No. 3
limiting factor for sensitivity is the background current, which is probably a function of several parameters. Because the 10 e.v. radiated by the glow is in excess of the work functions of the metals usctl as electrodes, the photoelectric effwt must contribute to the background current. Considerable contribution from thermal electrons may also be assumed because g l o ~discharges generate considerable heat. Finally, there is also a contribution from ionization of impurities in the carrier gas. The use of biasing and accelcmting potentials to control and regulate background current and sensitivity has been tried with limited success. 'I'he bias electrode B 1 demonstrated a proportional relationship between background current and sensitivity. Accelerating potentials applied between e1cctrodt.s .-1 and B had no control over sensitivity but did deterniine to soiiir extent the background current. This SU that most of the measurable ionization i)roccw is occurring near electrode B so that, the accelerating potential has limited cffert. IClec-trons generated by ionization prowsses further removed are al)parently qucnchrd hy ilositivc ions Iirfore ai,rival at B . The addition of ncgativc iiotc.ntial on R 1 distort:: the shape of the electric firld near B by acwlrrating the positive ions t o w d B 1 and consequrntly alloiving the collrction of more electrons. Positive ions may have causrd t h r
decixwe in s:cbnsitivityFvitli the olcc*(i~odr 1) positivv. 'I'hus, 1)ositivc ions grnrratcd by thv glow tlischarge were proi)elled by tlic argon flow into thc remainder of t hr detect or and interfered with sensitivity and cwllection efficiency as (Ic~monstratctl1)y a noriliiitw electrometer rcsi)onse. .\t low argon flow rates i\.iili c ~ l c c t i ~ ) t i cI> 1 imitivc, the collrct ion diel i ~ v 1d~a r i~t y .s ~ 'I'hc. skin t ( w i i ~ w t i i r cof~ thc tletector \ v w also cariticd. Cont iiiiwiis air cooling resultcd iii iiiii)ro\.uI dctrction tlecause of lo\vrr I)ackgroiunti cwrrent,s. Apparrntly, t ~ h r ~ ~ i dwti,ons nal froin the gloiv diwharge were interf(bring. The use of hcliuiii as a plow rliwharge gas deliionstrated that 1iiglic.r Ilhoton energies tentled t o inc*rease I)wkground currents. Glon. tlisc*hargc. gab containinatcd with liy(1rocarI)oiis a])parently qucnrhtd s:ecoritlai,y c l ~ ~ t r o n s and decreascd nuich of the 1)ackground rurrcnt and scnhitivity. The sensitivity of thc dctector to i)rol)an' is d(,i)ciidvnt 0 1 1 th(. matrix. I.'or air I)ollrition \vork the scnsitivity to hydrocarlions is too lo\v when air is used as a matrix. T h e al)i)ear:incr of thcb three unuwal ~)eakssrlpge sensitivity to coinimunds with short rrtention timw. l'llerrforc~,tht, swsitivity of t h ( b clvttxc'tor t o oxides ot ca~.l)or~, of nitrogen. and 1)erhaps of sulfur sliould 1)r investig:tted for futuw allplication to air ~~ollution analysw. 'I'he high background currents also sllggwt t ut urc invrstigution into thv elcc't 1 ~ 0 1 1 -
capturing effect$:. For this work the sample should perhaps be interjected between electrodes B and C to take full advantage of secondary electrons. This study has demcinstrated that the I,osition, glo,v gas go,,., I,olarity, and potential on each eleci-rode are critical, affecting not only sensitivity but also linearity and noise. II-ell rexulated poivc>rsu1)lilies and (soncentric electrodes are r e c o m n l e n ~ e tfor ~ a
dissipate its enormous heat from the discharge.
Filla’lyj the be shielded froin thermal drafts, yet be able to
( 2 ) Lossing, F. P., Tmaka, Ikuzo, J . Chem. Phys. 2 5 , 1031 (1955).
ACKNOWLEDGMENT
The author thanks Maynard D. Lay for his technical assistance.
( 3 ) Lovelock, J. E., ASAI,. CHEW 33,
162 (1961). (4) Lovelock, J . E., S u t u r e 188, 401 (1960). ( 5 ) Lovelock, J . E., Research and Deuelopment, October, 1961. (6) Ongkiehong, L., “Gas Chrornatography, 1960,” R. P. W. Scott, Ed.,
p. 11, Butterworths, London, 1960.
LITERATURE CITED
(1) Loeb, L., “Basic Processes of Gaseous Electronics,” I’niversity of Cuhfornia
Press, Berkeley, 1961.
RECEIVEDfor review 1Iay 6, 1064. Accepted June 23, 1964. Presented March 2, 1964, at The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa.
Determinaition of Optimum Solvent Systems for Countercurrent Distribution from Paper Chromatographic Data Binary Organic Solvent/Formamide
Systems
EDWARD SOCZEWINSKI, ANDRZEJ WAKSMUNDZKI, and WIESLAWA MACIEJEWICZ Department o f Inorganic Chemistry, Medical Academy, lublin, Staszica 6, Poland
periments confirm the possibility of approximate estimation of optimum extraction systems from chromatographic data obtained with papers impregnated with formamide.
M O ~ I’M E
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of 1)ubIic;itions ( I , 2, 5-7, 9.IO), attenilits have been made to estirnatc suital)lc rstiwtion system< from 1iali(~. c,hromrttoRi,:~iihic data. The basic cmtlition for sucah a method is a liartition nicrhariisni o:! the chrornatoI’ai,tic-ularly successful results h a w 1jee.n otitairwd for a number of alkaloids c.hmiiatogral)hed by the nioist tiuffered 1ial)er method ( 8 ) . F o r ~ i a n i i d r - i r n ~ ~ r ~ ~ gpapers i ~ a t e ~are ~ often u s d in the c.hroriiatogial,h. of in the method of g i v m by \T’aldi viciv of the analogy bet\wen c~h~oinatogra~~liir and c a v a d e 11roc~~sscs which lici~niit~. thr estimation of ,stiitalile ,solvent mc~thods, nonaqum containing forrrianiitle as t h r Iiolar pha,- and c~oriiitc~i~c~~iricrli dist,iiliution is piwented in 1~igur.c1. .In optimum system for paper liartition chromatogral)hy shotiltl also optirnum for Craig’s method provitlvtl that identical rorniio+itionsof phase; aye used in both methods at identicad ratio of volumes of tht, two phases. J3y neglecting any of these cwiditioiis, changes in the I)ai‘tition are introtlticwi which may affect the efficiency of srparations. .I rhxnxr of the volunic~ ratio ( T ) may l i t rorrcrtcd a siiital)l(, change of pH of the water phase, or of the coml)osit,ion of the niiscd Iihasc. AI theoretical treatment of the calculations involved is given in the first p:ipei,s of the series cited ( 6 , 7 ) .
rf,, =
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L’OL. 36, NO. IO, SEPTEMBER 1964
1903
