amino acids with the exception of tryptophan and arginine. Tryptophan can probably be analyzed by establishing a response factor for each of the two derivatives obtained, but the chemistry of arginine will need to be investigated in greater detail. LITERATURE CITED
(1) Bayer, E., Reuther, K.-H., Born, F.,
Angew. Chem. 69, 640 (1957). (2) Bier, M., Teitelbaum, P., Ann. N . Y . Acad. Sci. 72, 641 (1959). (3) Blsu, K., Darbre, A,, Bwchem. J . 88, 8p (1963). (4) Cruickshank, P. A,, Sheehan, J. C., ANAL.CHEM.36, 1191 (1964). (5) Ettre, I>.S., J . Gas Chromatog. 1, S o . 10, 15 (1963). (6) Graff, J., Wein, J. P., Winitz, XI., Federation Proc. 22, 244 (1963). (7) Hagen, P., Black, W., Ibid., 23, 371 (1964). (8) Hunter, I. R., Dimick, K. P., Corse, J. W., Chem. Ind. (London)1956, p. 294.
(9) Johnson, D. E., Scott, S. J., Meister, A,, Abstracts of Papers, p. 48C, Am. Chem. Soc., Chicago, Ill., 1961.
(10) Johnson, L). E., Scott, S.J., Meister, A., ANAL. CHEY. 33, 669 (1961). (11! Landowne, R. A , , Lipsky, S. R., ,$ature 199, 141 (1963). ( 1 2 ) Liberti, A,, “Gas Chromatography,” D. H. Desty, ed., p. 341, Butterworths Scientific Publications, London, 1958. (13) Melamed, N., Renard, M., J . Chromatoa. 4. 339 (1960). 14) Nlchoils, C. H., illakisumi, S., Saroff, H. A,, Ibid., 11, 327 (1963). 15) Pisano. J. J.. S‘anden Heuvel. W. J. A:, Horning, E. C., Biochem: Biophys. Res. Comm. 7, 82 (1962). 16) Ruhlmann, K., Giesecke, W., Angew. Chem. 73, 113 (1961). 17) Saroff, H. A., Karmen, A., Anal. Bzochem. 1, 344 (1960). 18) Saroff, H. A,, Karmen, A,, Healy, J. A,, J . Chromatog. 9, 122 (1962). (19) Stalling, D. L., Gehrke, C. W., Shahrokhi, F., 16th Annual Midwest Chemistry Conference, Xov. 20, 1964.
(20) Wagner, J., Rausch, G., 2. Anal. Chem. 194, 350 (1963). (21) Wagner, J., Winkler, G., Ibid., 183, 1 (19611. (22)‘Weygand, F., Kolb, B., Prox, A., Tilak, M. A., Tomida, I., 2. Physiol. Chem. 322, 38 (1960). (23) Youngs, C. G., ANAL. CHEM.31, 1019 (1959). (24) Zlatkis. A,, Orb, J. F., Kimhall. A. 32, 162 (1960).
RECEIVED for review Sovember 20, 1964. Accepted January 6, 1965. Division of Biological Chemistry, 148th Meeting, ACS, Chicago, Ill., September 1964. Taken in part from the Ph.1). dissertation of William M. Lamkin, University of Missouri, Columbia, Mo., 1964. Contribution from the Missouri Agricultural Experiment Station. Journal Series Xo. 2804. This research was supported in part by grants from the National Science Foundation (G-18722 and GB-1426).
Study of the Photoionization Detector for Gas Chromatography DAVID C. LOCKE and CLIFTON E. MELOAN Department of Chemistry, Kansas State University, Manhattan, Kan.
b A study has been made of the properties of a photoionization detector for gas chromatography. The source of ionization is a beam of high energy photons generated in a glow discharge in an inert atmosphere at reduced pressures. The ultraviolet radiation i s of such energy that photoionization of the sample molecules occurs without any accompanying ionization of the carrier gas. The main advantage of this detector over other types of ionization detectors is its inherent selectivity based on differences in ionization potentials among the chemical types in the sample. The detector was constructed of a cell made of Teflon and Pt tubing and electrical leads. In general, as ionization potential decreases, response increases. N o response was or found for the fixed gases COZ, HzO. The halogenated compounds disturb the discharge. The apparent ionization efficiency of the device, which has a noise level of lo-’* amp. and a background current of 5 X amp., is about loe4. The linear dynamic range is lo5. The minimum detectable quantity of propane i s 2 X lo-’’ gram.
Son,
T
of the photoionization detector is based on the specific ionization of some ionizable gas or vapor in the presence of an inert carrier gas by irradiation of the mixH E OPERATION
ture with photons of the appropriate energy. A reduced pressure glow discharge in one of the rare gases, N2,or H2, can be used to supply ultraviolet radiation of energy greater than the ionization potential of most organic compounds, but less than t h a t of the common carrier gases. The application of such a device seems to have been contemplated first by Robinson and Brubaker ( I d ) , but a practical version of the detector was first described by Lovelock ( 7 ) . Recently, a preliminary study of the characteristics of a photoionization detector has been discussed by Roesler ( I S ) . Spectrometry in the vacuum ultraviolet has become a field of considerable interest to molecular and atomic spectroscopists. Much valuable information about the highly excited states and electronic structures of molecules is available from this region of the spectrum (9). I t has been shown by Watanabe (16) that photoionization measurements in the vacuum ultraviolet provide a powerful technique for the determination of ionization potentials ( 8 ) . Subsequent work has been carried out using a mass spectrometer in conjunction with a vacuum ultraviolet monochromator-ion source for the determination of photoionization efficiences and cross sections. These are of interest in studying the upper energy states of molecules (14). Photoionization occurs when a n ir-
radiated molecule absorbs a quantum of the appropriate energy and undergoes an electronic transition to an ionized state. The ion current resulting from photoionization sets in very sharply as the energy of the incident photons is increased. For compounds of distinct absorption spectra in the vacuum ultraviolet and favorable transition probabilities, even minimum energy quanta-Le., those of the threshhold energy-give rise to a n ion pair. For example, for aromatic hydrocarbons, as the energy of incident photons is increased, the ionization yield shows a sharp leveling off at the ionization potential (adiabatic ionization potentials are obtained with photoionization). With paraffins, however, the break in the curve is less sharp, which is consistent with the diffuse nature of their absorption spectra in the vacuum ultraviolet (10). Photoionization-mass spectrometric studies on polyatomic organic compounds have shown that the ionization and absorption cross sections for the parent ion increase sharply a t the ionization threshhold, pass through a masimum, and fall off as the incident photon energy is increased. However, fragment ions often are formed which have higher appearance potentials than the parent ion and cross sections estending to higher energies. Thus an ionization current may be obtained over a wide energy span. VOL. 37, NO. 3, MARCH 1965
389
r
-0
Figure 1 . paratus 1 2
3 4 5
6 7
a
9 10 11 12
Schematic diagram of apPhotoionization detector cell Capillary restrictorr Dry ice-acetone traps Hg manometer Vacuum pump Six-way gas sampling valve Microbubbler Needle valve Sample source Carrier and diluent gas source To vent Molecular sieve column
The simple photoionization detector, in which the total output of the glow discharge ultraviolet source irradiates the sample end of the cell, will thus produce an ionization current for nearly all ionizable compounds, even though the incident photons may have energy considerably above the ionization potential of the sample molecules. With this simple detector, where no attempt is made to separate the photon energies produced, it is desirable to have a simple emission spectrum. This is accomplished by using a D.C. discharge of a few milliamperes, operated in Ar, He, Kr, Nz, or Hz, a t a few millimeters H g pressure. It is expected that only the most intense lines will be available for interaction with the sample entering the detector. Impurities in the supply gases in addition to those introduced through small leaks in the apparatus will absorb or filter out the most energetic and the low intensity lines. All these lines arise from the neutral atoms, except the continuum in H e which originates from the He2molecule formed in the discharge (15). Comparison of the ionization potentials of these gases with the energy supplied to the discharge indicates that the highest multiple charged ions that will be obtained in the photoionization detector glow discharge are Ar(II), Kr(III), and X(I1). This is consistent with the mass spectrometric results of Knewstubb and Tickner (3-5), who studied ions in glow discharges set up under conditions comparable to those used in this work. The presence of lines of these energies places definite limitations on the response of the detector. The use of a Kr discharge should make the detector insensitive t o compounds of ionization potential greater than 10.64 e.v.-Le., some inorganic gases, C1-Ca saturated 390
ANALYTICAL CHEMISTRY
X = Distance cathode protrudes All dimensions given are in cm.
hydrocarbons, etc. On the other hand, photons generated in He discharges are able in principle to ionize all compounds, assuming small energy and intensity losses in tranversing the detector tube and favorable ionization cross sections and transition probabilities. EXPERIMENTAL
Construction and Design. Several preliminary experiments with trial models of the photoionization detector were carried out. These results indicated t h a t detectors constructed of soft glass with Pyroceram cement were mechanically unsatisfactory; in one experiment, the heat produced by a n accidental arcing of the discharge irreparably melted the end of the device. Further, movement and positioning of the electrodes was difficult, although the Pyroceram cement is soluble in hot 1 : l "Os. The use of Teflon TFE-fluorocarbon resin (DuPont), however, simplified the problem of making and adjusting electrical and gas lead connections to the device. One-eighth-inch pipe '/Isinch tubing Swagelok unions are well adapted to this purpose. Further, Teflon has the advantage of filtering out ultraviolet radiation which is harmful to the operator's eyes over extended periods of time. However, it is not possible to see the shape and configuration of the glow discharge except in a darkened room. It was further found that the greater the distance from the discharge to the collector, the lower is the background signal in the absence of ionizable vapor. This signal was reduced from lo-* amp. to 10-10 amp. by lengthening this distance from 4 to 7 em. I n addition to maintaining maximum distance between the discharge and the collector, a grounded shield containing two fine-mesh Pt screens supported in
a small brass cylinder was necessary to prevent interactions between the discharge and the ion collector. Without the screens, pronounced space charge effects were obtained a t high collecting voltages (25 volts). These effects were evidenced by ill-shaped peaks for large samples, excessive noise, and unusally high background signals. Further, removal of the ground lead from the cylinder anode had no effect under these conditions. The two grounded grids, however, entirely eliminated these problems. Experiments such as these led to the final version of the detector studied here. Figure 1 gives a schematic diagram. As shown in Figure 2, entering at the extremes of the cell are the leads for the discharge and sample gases. These leads were 0.14-cm. 0.d. Pt tubing, connected to the apparatus using l/ls-inch Swagelok fittings with silicone rubber O-ring compression seals. These leads also served as electrodes for the discharge and ion collector, respectively. The collector anode was a brass cylinder, 1.23 cm., 0.52-cm. i.d. which fit snugly in the cell and which was grounded through the vacuum connection. The 500-volt D.C. power supply was a Microtek Instruments Model HCP500 thryatron tube current-regulated hollow cathode power supply. The positive terminal was grounded. Ionization currents were measured with a Keithley Model 610A electrometer amplifier used in the negative feedback (fast) mode of operation. I n experiments requiring a recorder, the 1-ma. fullscale signal from the electrometer was fed into a Texas Instruments ServoRiter recorder across a 1.02-ohm resistor. A.C. line power was voltagestabilized. The discharge was set up between two 0.14-cm. o.d. Pt rods, separated by a 0.12-em. gap. Although the discharge
""I
480-
-
460-
-
v,
-
440-
4.00 420
-
3801
I
I
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I
I
2
3
4
5
Figure 3. Discharge current as function of discharge voltage in He
was usually self-initiating, a Tesla coil was occasionally required to strike the discharge, especially a t higher pressures. When the lead supplying the gas to the discharge was grounded, the discharge could be moved with the aid of the Tesla coil to cover the whole end of the cell. The lead was located 1.65 cm. from the discharge electrodes and was perpendicular to the midpoint of the gap. This resulted in a more stable discharge and a lower background current. Power was supplied to the electrodes from the power supply through a 2watt, 2.5-megohm potentiometer which allowed continuous adjustment of the discharge voltage up to 500 volts. After extended periods of operation (several hundred hours) under conditions of high discharge currents, sputtering of P t black from the discharge cathode necessitated removal of the electrodes and thorough cleaning with a small wire brush. Faster cleaning was accomplished in a solution of concentrated "03 in a sonic cleaner. The collector electrode was polarized negatively with respect to ground by the use of five Eveready 7.5-volt industrial "C" batteries in series. The batteries were connected in a guard ring circuit and shielded by wrapping them individually with the following materials: first, aluminum foil, connected to the positive terminal of the battery; second, sheeting of Teflon to cover completely the guard ring; and third, aluminum foil, connected to ground. Without this shielding, current leakage from the surface of the batteries and A.C. pickup produced signals of the order of lo-' amp.; after shielding in this manner, the current from the batteries was reduced to less than amp., with negligible A.C. pickup even a t the highest sensitivity ranges of the electrometer. Belden 8240 RG58/1; shielded cable was used for all electrical connections.
Since the photoionization current collector electrode also served as the sample inlet tube, connection to the gas sampling valve was made through a piece of Teflon. Care was taken to keep the electrical leads as short as possible, to avoid ground loops, and to shield the apparatus from sources of A.C. pickup and static electricity. Calibration and Flow System. A method is required for the reproducible introduction of small, accurately known amounts of sample vapor. The several dynamic methods available for calibration (6) are not readily adapted to reduced pressure operation. A microbubbler system similar to that described by Riley (11) and a gas sampling valve was used in this work. A 200-ml., 3-neck, round-bottom flask containing 75 ml. of concentrated H&O( was used. Nondetectable diluent gas and sample vapor (usually propane) were bubbled into the HzS04 from opposite sides of the flask through fine glass jets. The gases mixed in the chamber and were flushed out the middle neck into one side of a 6-way Perkin-Elmer gas sampling valve. Concentrated HzS04was used because none of the common carrier gases or simple saturated hydrocarbons are appreciably soluble in it. Further, traces of water vapor and olefins are removed from the gas streams. To determine the quantity of propane in the diluent gas, the volume, V , per bubble of propane was determined by counting several times the number of bubbles required to displace 50.00 ml. of water from a 50-ml. buret. With the jet used here, V = 0.045 ml./bubble a t S.T.P. The Golume of the gas sampling loop on the gas sampljng valve, which consisted of a short piece of capillary tubing containing a volume-reducing coaxial P t wire, was 20 pl. This value was determined from the weight of Hg
required to fill it under actual conditions; good agreement was obtained with the volume calculated from the dimensions of the loop. In practice, it is most convenient to measure the time, tzl required for 10 bubbles to emerge from the jet. Further, the diluent gas plus propane flow rate is most easily measured with a soap bubble flow meter where tl is the time required for a bubble to rise 10 ml. After correcting the measured diluent gas plus propane flow rate to S.T.P., calculating the fraction of propane in the mixture, and applying the ideal gas equation, it is readily calculated that the weight of propane introduced to the ionization chamber in tl/h gram. tJt, 20 pl. is 1.75 X ratios smaller than 0.01 are easily achieved by proper variations of the flow rates. This is entirely adequate for the sensitivity of this detector. This quant,ity could be reduced further by reducing the sample volume. For samples soluble in HzS04, a microsyringe was used to inject vapor samples. The flow system supplying gases for the discharge, for the bubbler, and for carrier gases to the detector, is shown schematically in Figure 1. He, Ar, and Nz were passed through a dry ice-acetone trap and a molecular sieve column to remove water and other impurities. The organic gases used were Matheson Instrument grade and were dispensed from lecture bottles through two needle valves. RESULTS AND DISCUSSION
Discharge Parameters. As noted above, the glow discharge was set up in He, Ar, Nz across two Pt electrodes. A typical plot of discharge current, iD, as a function of discharge voltage, V D , is presented in Figure 3. I n general, much lower currents and higher voltages were required in He to produce and sustain the discharge than in Ar or Nz; the largest currents were necessary in Ar. The plot is shaped like a backwards N. A high voltage is required to strike the discharge. Vpon starting, an arc may be formed which can be turned off by varying the potentiometer, reducing the V D . As the voltage is increased again, the current rises nearly linearly until the arc is initiated. Arcing occurred in He at about 5 ma.: approximately 20 ma. were required in Ar. Arcing produced much sputtering of the electrodes, background currents of the order of lo-' amp., and rapidly burned out the potentiometer in the circuit. Cell pressure had a definite effect on the voltage required to initiate the discharge. Figure 4 reveals a minimum in this curve with increasing pressure in millimeters Hg of Kz. Similar behavior is reported in Goldstein's review ( 2 ) of gaseous electronics. Ideally, it is desired to have a pure source of photons. With the simple photoionization detector, however, the VOL. 37, NO. 3, MARCH 1965
391
"D I
2oo
of about 10. As seen below (Figure 5), minimizing the exposed area of the collector electrode decreases the photoelectric current. By setting up the discharge perpendicular to the axis of the detector tube, directional properties of electrons in the discharge are eliminated which reduces the current resulting from electron impact. Response of Photoionization Detector to Propane. EFFECTOF DIS-
t
CHARGE C U R R E N T A N D C E L L P R E S S U R E . The effect of discharge current,
iD,and Ar cell pressure on response of I
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LOG P Pressure dependence of voltage required to initiate discharge in
Figure 4.
,
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Units of pressure are mm. Hg
proximity of the glow discharge to the collector electrode produces some interaction. The background current is produced from several sources: (1) photionization of traces of impurities present in the chamber; (2) photoelectric current from the collector electrode; and (3) ionization of impurities by bombardment with high
r I40b
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energy electrons from the discharge. Cleanup of the gases used and prevention of leaks will reduce the current arising from (1) and (2); inclusion of the dry ice-acetone trap reduced the background current by a factor of about 1.5. The use of fine-mesh Pt screens in the collector anode, as noted above, reduced this current by a factor
3rnm
the detectorto 3.00 X gram of propane is displayed in Figure 6. An important feature of these curves is the sudden increase in response a t some low io. This is apparently due to the application of sufficient energy of discharge to ionize Ar+ to Ar+2. The second ionization potential of Ar is 27.62 e.v. At a cell pressure of 3 mm., a n i~ = 1.2 ma. corresponds to a discharge voltage of 230 volts, which for an electrode gap of 0.12. cm. is equivalent to 27.6 e.v., which coincides very well with the second ionization potential, Arcing occurs before Ar+3 can be formed. For He discharges, no such jump in the curve is noted; in N2, one jump is found at a n i~ corresponding to a n energy of about 30 e.v. (the second ionization potential of N is 29.593 e.v.). Response decreases with increasing cell pressure. This can be accounted for in two ways: first, higher pressures will allow less radiation to reach the sample; second, recombination of ions is expected to occur to a larger extent as the pressure is increased (1). EFFECTOF COLLECTOR VOLTAGE.A normal current-voltage relationship is obtained with the photoionization detector, as shown in Figure 7 . At the lowest cell pressure studied, 3 mm. Hg, any probe potential greater than about 5 volts may be used. At higher pressures, the linear region is attained a t higher voltages, probably due to recombination effects. I n general, 17 volts was used in this work. C o L L E c T o n CATHODE POSITION. The extension of the collector cathode had a marked effect on the response of the photoionization detector. Figure 5 shows the effect of cathode position, 2, a t two different pressures and a t several different probe potentials. The center of the vacuum hole (i.d. = 0.37 cm.) was located 0.33 cm. from the end of the tube (z = 0). It appears that as z increases to about 0.1 cm., sufficient time
I Figure 5. Effect of collector position on response of photoionization detector Ar discharge; io = 1.0 ma.; V , = 17 volts; sample weight = 1.00 X 1 0 - 6 gram propane X i s defined by Figure 2 Dark circles are for background current a t V , = 17 volts Conditions:
392
ANALYTICAL CHEMISTRY
3ol 26
0
4
d
1.2
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2.0
1.6
i, Figure 6.
2.2
p z 3mm
2.8
3.2
(ma)
Effect of discharge current on response of photoionization detector
Ar discharge; collecting voltage = 17 volts; cathode protrudes 0.33 cm.; sample weight = 3.00 X gram propane Dark circles a r e for background current a t 3 mm. and 121 mm. Hg
Conditions:
loT7
This indicates that some ionization is occurring inside the entrance tube, which was located coaxially with respect to the cell. EFFECT OF FLOWRATE TO DISCHARGE. Only a small flow of gas is required to sustain the discharge. For this reason, a long capillary flow restrictor was used to supply gas to the discharge. Because the gas is pulled into a vacuum, its flow rate is not easily measured under operating conditions. The effect of .4r tank pressure, which is proportional to flow rate, was briefly studied at a constant cell pressure of 3 mm. Hg. The results are presented in Figure 8. As the flow increases, the discharge current rises slightly; the background current and response to 4.6
is allowed for the sample to interact with the photons before it is removed by the vacuum. Between this distance and about 0.7 cm., however, most of the sample is gone before it can be measured. Beyond this, more efficient ionization and ion collection is realized. The increased collector surface area serves mainly to increase the background current and noise level. This behavior was reproducible. Had the vacuum line been attached closer to the discharge end of the detector, a response curve similar to that shown by the dotted line would probably result. It is interesting to note that as the collector is withdrawn into the Teflon body, ionization continues to occur.
X gram of propane increases sharply, passing through a fairly broad maximum. The flow rate measured under atmospheric conditions a t the maximum was 0.5 cc per minute. This flow rate was used in most experiments. EFFECT OF AMBIENT TEMPERATURE CHANGES.This study was conducted a t room temperature, nominally 27" C. The stability, sensitivity, and noise level were unaffected by small changes in ambient temperature and pressure. Operation of the detector a t high discharge currents did produce significant local heating. Under normal operating conditions (roughly 2 ma. in Ar, 1.5 in K2, and 0.5 in He), no excessive heating was noted. The outside of the detector remained a t ambient temperature: the metal fitting through which the discharge electrode passed was only warm (about 40" C). LINEARITY.Figure 9 shows the response of the detector to different quantities of propane from photons generated in Ar, He, and K2 discharges. The diluent gas in each case was the same as the discharge gas. As seen in the figure, response is highest for the He discharge and lowest for the X2 discharge. I t is apparent that ionization cross sections and transition probabilities of propane are highest for the photons generated in the He discharge, and lowest for those of IT2. As could be anticipated, and as will be seen below, this does not necessarily apply to all compounds. The linear portion of the curve extends to about 4 X low5gram. Larger
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20
30
40
50
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4
6
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12
14
16
VP Figure 7. Conditlons:
Current-voltage relationship for photoionization detector Ar discharge; io = 1.8 ma.; P = 3 mm. Hg.; cathode protrudes 0.1 cm.; sample weight = 1.00 X 100 gram propane
18
Figure 8. Effect of discharge gas flow rate on discharge current, background current, and response of detector Conditions:
Ar discharge; cathode protrudes 0.57 cm.; V , = 17 volts; one screen in anode; in = 1.8 ma.; sample weight = 4.59 X lo-' gram propane
VOL. 37, NO. 3, MARCH 1965
393
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Conditions:
tion efficiency (6) of the detector under optimum conditions is about 10-4, midway between that of the hydrogen flame ionization detector and the argon detector (6). RESPONSE OF PHOTOIONIZATION DETECTOR TO DIFFERENT COMPOUNDS. The operating parameters of the photoionization detector were studied using propane as the test sample, as recIn ommended by Lovelock (6). addition, the response to several other compounds was determined. As noted above, compounds of ionization potential greater than the available photon energy cannot be detected. I n general, this conclusion was verified. The
Response of the Photoionization Detector to Various Molecules
Compound Illethane Ethane Propane n-butane isobutane Cyclopropane Neopentane Cyclopentane n-pentane Benzene cis-Butene-2 Acetylene Acetone Dimethyl ether Diethyl et,her hIethvl chloride
394
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10.0
POTENTIAL
9.6
9.2
(ev.)
Ar discharge
V , = 17 volts; P = 3 mm.; ;D = 0.5 ma. (He), 1.8 ma. (Ar), 1.5 ma. (N:); cathode protrudes 0.05 cm.
quantites of sample altered t h e properties of the discharge by diffusing into it. This produced spurious results, accompanied by an increase in the discharge current, and effects similar to those caused by space charges. An external ultraviolet source would eliminate this problem. The random noise level of the detector was of the order of 10-'2 amp. Based on the ionization efficiency of the device (see Figure 8), that quantity of propane producting a signal twice the noise level is about 2 X gram. This provides a linear dynamic range (6) of about IO5, which is comparable to the Argon detector. The apparent ioniza-
Table 1.
10.8
Figure 10. Effect of sample ionization potential on relative response (propane = 1 .OO)
10-3
Figure 9. Efficiency of photoionization detector using He, Ar, and NEdischarges Conditione
11.2
IONIZATION
Ionization potential
ANALYTICAL CHEMISTRY
12 11 11 10 10 11 10 10 10 9 9 11 9 10 9 11 11 11 11 10 10 12 13 12
98 65 07 63 56 06 37 52 34 25 13 41 69 00 53 28 35 42 45 15 08 59 79 34
Discharge medium and carrier gas Ar He NZ 0 0.26 1.00 1.74 1.78 1.50 1.20 2.02 3.37 4.80 3.57 ...
2.78 1.11 1.91 1.13 2.72? 2.32? 1 ,go? 01 4.47 0 0 0
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...
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maximum energy available from Ar is 11.83 e.v.; from He, 21.22 e.v.; from Nz, 11.30 e.v. The response to different compounds relative t o propane = 1.00 is listed in Table I. A plot of some of these relative responses is presented in Figure 10. I n general, as ionization potential decreases, response of the detector increases. With some notable exceptions, as the incident energy increases, response increases. As expected, no response is obtained for methane using Ar or Nzdischarges; a low response is obtained for He. Walker and Back (15) attribute He-produced photionization of CH4 to the resonance He 584 A. line, to the Hez 600-900 A. continuum, and to ionization by collisions with He metastable atoms produced by the resonance line. They were unable t o separate the three effects. Although in principle the photons from the He discharge are capable of ionizing any molecule or atom, no response was noted for HzO, SOZ, COZ, air, Hz, etc. This anomaly shows the need for further study of the response of the detector. Strongly electron-attaching compounds produced spurious results by disturbing the discharge. With the NZ and He discharges, the chloromethanes (except CH&l), Freon-12, hexafluoroacetone, and, to some extent, HzS, CSz, and MezO, caused either a large increase in the discharge current, or a large increase immediately followed by extinguishment of the discharge. As the sample was pumped out of the chamber, the current either dropped back, or restarted itself and rose, to its equilibrium value. Although water-saturated air is reported to be entirely satisfactory for a
discharge medium ( 7 ) , the admission of small amounts of water vapor to the detector produced results similar to those encountered with electronattaching compounds. The lack of response noted for NH8 may not be due t o insensitivity of the detector to it, but rather t o its never reaching the detector because of reaction with the brass fittings and union t h a t were used in the gas line carrying the sample vapors. The response to paraffins falls on a curve which rises sharply from butane to pentane. I t is expected that the curve will level out for higher members of the series. Qualitatively, the generalizations about ionization potentials noted above apply to the response of the photionization detector. Isobutane has a slightly higher response than n-butane. The detector’s sensitivity is neopentane < cyclopentane < n-pentane. Butene-2 produces a much larger response than the saturated butanes. Comparison of the results with the three discharge gases reveals certain anomalies. I n He, abnormally large response is found for acetone and butene -2. I n Nz, neopentane gives a n unusually large signal. Further study of the response of the detector should account for or correct these results.
CONCLUSIONS
The photoionization detector is a quite versatile device, with good sensitivity and linearity, and with potential for extreme selectivity. The operating and constructional parameters are not critical. Future studies should be concerned with investigation of a n external source and vacuum monochromator, utilizing LiF or CaFz windows. Several advantages to this arrangement are anticipated. First, the problems associated with the use of a glow discharge in the detector would be eliminatedthe sample could not interact with the photon source, the source-generated background signal would be eliminated, and a more intense energy could be applied t o the sample. Second, the potential extreme selectivity of the device could be realized by using the monochromator. Third, the detector would also be of use as a n instrument for making precise physical measurements. Fourth, by the inclusion of a photocell in the chamber, the detector could be used as a nondestructive far ultraviolet monitor for aromatic hydrocarbons. LITERATURE CITED
ics and Electron Physics,” Vol. VII, p. 399, L. Marton, ed., Academic Press, New York, 1955. (3) Knewstubb, P. F., Tickner, A. W., J . Chem. Phys., 37, 2941 (1963). (4) Knewstubb, P. F., Tickner, A. W., Zbzd., 36, 674 (1962). (5) Zbid., p. 684. (6) Lovelock, J. E., ANAL.CHEM.33, 162 (1961). ( 7 ) Lovelock, J. E., K’ature 188, 401 (1960). (8) Nicholson, A. J. C., J . Chem. Phys. 39, 454 (1963). (9) Price, W. C., “Advances in Spectroscopy,” Vol. I, p. 56, H. W. Thompson, ed., Interscience, New York, 1959. (10) Price, W. C., Bralsford, R., Harris, P. V., Ridley, R . G., Spectrochim. Acta 14, 45 (1959). (11) Riley, B., “Gas Chromatography,” p. 81, R . P. W. Scott, ed., Butterworths, London, 1960. (12) Robinson, C. F., Brubaker, W. M., U. S. Patent 2,959,677, November 8, 1960 (filed May 2, 1957). (13) Roesler, J. F., ANAL.CHEM.36, 1900 (1964). (14) Steiner, B., Giese, C. F., Inghram, M. G., J . Chem. Phys. 34, 189 (1961). (15j Walker, D. C., Back, R. A., Zbid., 37, 2348 (1962). (16) Watanabe, K., Nakayama, T., Mottl, J., “Final Report on Ionization Potential of Molecules by a Photoionization Method,” Dept. of Army Project No. 5B 99-01-004, 1959, Department of Physics, University of Hawaii.
(1j Cobine, J. D., “Gaseous Conductors,”
Dover, Xew York, 1958.
(2) Goldstein, L., “Advances in Electron-
RECEIVED for review June 8, 1964. Accepted November 5, 1964.
Semiquantitative Determination of Impurities in Bisphenol A by Circular Paper Chromatography N. H. REINKING and A. E. BARNABEO Plastics Division, Research a d Development Department, Union Carbide Corp., Bound Brook, N. 1.
b A semiquantitative method for the determination of the principal impurities in commercial bisphenol A utilizes reversed-phase circular paper chromatography and is especially useful in analyzing samples containing minor amounts of impurities. As little as 0.03% of an individual impurity can b e detected in bisphenol A by this method. The components are separated on circular filter paper impregnated with tricresyl phosphate, using an aqueous solution of trisodium phosphate as the eluent. The chromatogram is sprayed with a diazonium salt solution to develop color. The concentration of each impurity in an unknown can b e calculated by determining its respective extinction point and using the previously established sensitivity value.
T
HE principal impurities normally associated with commercial bisphenol A have been reported ( f , 4 ) as Dianin’s compound (4-p-hydroxyphenyl - 2,2,4 - trimethylchroman), the 2,4‘ isomer [2-(o-hydroxyphenyl)2 - ( p - hydroxyphenyl)propane], and a trisphenol [2,4-bis(a,a-dimethy1-4hydroxybenzy1)phenol. Work in this laboratory has confirmed these findings and, additionally, has revealed the presence in trace amounts of several other impurities. I n certain reactions, the presenc,e of any impurity in bisphenol A is objectionable; in other cases, the amounts of individual impurities present are of concern. Of the methods generally available for identification and measurement of impurities, the most useful, in this case, have been found to be gas
liquid and paper chromatography. Tominaga has reported on the direct analysis of bisphenol A using gas liquid chromatographic analysis (6). Gill has also reported on a quantitative gas liquid chromatographic method after acetylation of all reactive hydroxyl groups in the bisphenol d sample ( 3 ) . Paper chromatographic methods have been reported by Anderson, Carter, and Landua ( I ) , who used a dual chromatographic scheme to separate and measure impurities and by Challa and Hermans ( 2 ) , who employed a single one-dimensional chromatographic method. Whereas Anderson, Carter, and Landua (1) and Challa and Hermans ( 2 ) require preconcentration of impurities when present in low concentration, work in this laboratory has resulted in a simple analytical procedure using reversedVOL. 37, N O . 3, MARCH 1965
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