Anal. Chem. 1902, 5 4 , 447-450
modifier as expressed by the retention of the tricyclic antidepressants was in the following order: ammonia (longest retention) > urea > butylamine > dicyclohexylurea. The type of amine made a large difference in retention which was more than would be expected from only a reduction in ionic attraction to the ion-exchange site. This is additional evidence that the organic amine is adsorbed and changes the surface character.
ACKNOWLEDGMENT The authors thank I,. Jaros and J. Balderson for help in the preparation of this manuscript, B. Murphy for technical assistance, and U. N e w and P. McDonald for helpful discussions. LITERATURE CITED (1) Bldllngmeyer, B. A. J . Chromatogr. Scl. 1980. 18, 525. (2) Waters Associates, Bulletln No. N80. 1981. (3) Bldlingmeyer, 6. A.; Demlng, S. N.; Price, W. P., Jr.; Sachok, 6.; Petrusek, M. J . Chromafogr. 1979, 188, 419. (4) Sokolowski, A.; Wahlund, K. G. J . Chromatogr. 1980, 189, 299. (5) Twitchett, P. S.; Moffat, A. C. ./. Chromatogr. 1975, 111, 149. (6) Wahlund, K. G.; Sokolowski, A. J . Chromafogr. 1978, 151, 299. (7) Johansson, I.M.; Wahlund, K. G.; Schlll, G. J . Chromatogr. 1978, 149, 281. 18) . , Proelas. H. F.: Lohmann. H. J.: Miles. D. G. Clln. Chem. (Winston-&/em, N k . ) , 1978, 2 4 , ‘1948. (9) Van Deu Maeden, F. 6. P.; Van Rens, P. T.; Buytenhuys, F. A.; Buurman, E. J . Chromafogr. 1977, 142, 715.
(10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31)
447
Baker, J. K. Anal. Chem. 1979, 51, 1693. Knox, J. H.; Jurand, J. J . Chromatogr. 1977, 142, 651. Kikta, E. J.; Grushka, E. Anal. Chem. 1978, 4 8 , 1098. Kirkland, J. J. Chromatographla 1975, 8, 661. Wehrll, A.; Hildenbrand, J. C.; Keller, H. P.; Stampfli, R. J . Chromafogr. 1978, 149, 199. Kraak, J. C.; Bljster, P. J . Chromatogr. 1977, 143, 499. Rabel, F. M. J . Chromatogr. Scl. 1980, 18, 394. Cooke, N. H.; Olsen, K. J . Chromatogr. Sci. 1981, 18, 512. Berendsen, 0. E.; Pikaart, K. A,; DeGalan, L. J . Liq. Chromatogr. 1980, 3 , 1437. Thurman, E. M. J . Chromatogr. 1979, 185, 625. Snyder, L. R. “Prlnciples of Adsorption Chromatography”; Marcel Dekker: New York, 1968; pp 155-182. Iler, R. K. “The Chemlstry of Sllica”; Wiley: New York, 1978; p 660. Unger, K. “Porous Silica”; Elsevier: Amsterdam, 1979; pp 130-141. Nahum, A.; Horvath, C. J . Chromatogr. 1981, 203, 53. BIj, K. E.; Horvath, C.; Melander, W. R.; Nahum, A. J . Chromatogr. 1981, 203, 85. Scogglns, B. A.; Maguire, K. P.; Norman, T. R.; Burrows, G. D. Clin. Chem. (Wlnston-Salem, N . C . ) 1980, 2 6 , 5. Brodie, R. R.; Chasseaud, L. F.; Hawklns, D. R. J . Chromatogr. 1977, 143, 535. Proelas, H. F.; Lohmann, H. J.; Miles, D. G. Clln. Chem. (Winston-Sa/em, N . C . ) 1878, 2 4 , 1948. Watson, 1. D.; Stewart, M. J. J . Chromafogr. 1977, 132. 155. Vandermark, F. L.; Adams, R. F.; Schmidt, 0. J. Clln. Chem. (Wlnston-Salem, N . C . ) 1978, 2 4 , 87. Kuss, Von H. J.; Nathman, M. Drug Res. 1978, 2 8 , 1301. Waters Assoclates, Technical Bulletin J50, 1979.
for review
20, lg81*Accepted
20,
1981.
Liquid-Phase Photoionization Detector for Liquid Chromatography David C. Locke,” Bhim S. Dhingra, and A. D. Baker Department of Chemlstty, City University of New York, Queens College, Flushing, New York 11367
A direct liquid-phase photoionlratlon detector with picogram sensitivity to polycyclic aromatic hydrocarbons eluted with hexaneI2-propanol from slllca columns Is described. Llght from a mlcrowave-excited continuum xenon source Irradiates the HPLC effluent In a 10-pL flow cell. The amplified photocurrents are llnear with concentratlon over at least 6 orders of magnltude. No signal is observed for phenols or chlorinated compounds, although nanogram quantltles of various 0- and N-containing compounds are detectable.
Although a multitude of detectors has been developed for HPLC (I),there is yet to be invented a universal and sensitive detector. In attempting t o develop such a detector, we have investigated techniques based on photoionization. In a previous publication (2))a photoionization detector (PID) was described in which resonance radiation from xenon or krypton discharges, or Lyman a radiation from a hydrogen/helium lamp, was used to irradiate the flash-evaporated effluent from a HPLC column. To circumvent the problem of effluent evaporation, we now report on a PID based on the direct irradiation of the liquid e€fluent stream. Measurement of the resulting solute photocurrent allows detection of picogram quantities of certain polycyclic aromatic hydrocarbons and various other substances eluted from normal-phase (silica) HPLC columns with hexane/2-propanol mobile phases. Photoionization in liquids is a more complex phenomenon than that in gases. In the gas phase at low pressures, organic 0003-2700/82/0354-0447$01.25/0
molecules with a-electron systems typically have ionization potentials (IP)of the order of 9 eV and show a sharp rise in photocurrent for photon energies slightly larger than the IP. In condensed phases, however, a steep threshold for photoionization does not usually exist (3))and because of stabilization of the positive ion and photoelectron formed, by solvation, r-electron photocurrents are observed for incident photon energies typically 2-4 eV less than the vacuum IP value (4).Molecules with electrons only, such as alkanes, have vacuum IP values 1-2 eV higher than those for a-electrons and a smaller lowering by solvation of the liquid-phase ionization threshold. Typical HPLC eluents have vacuum IP values reported (2) as follows: tetrahydrofuran, 9.45 eV; 2-propanol, 10.09 eV; ethyl acetate, 10.15 eV; n-hexane, 10.18 eV; methanol, 10.85 eV; chloroform, 11.42 eV; acetonitrile, 12.20 eV; and water, 12.59 eV. Resonance lines from low pressure (0.1 torr) rare gas discharge lamps are characteristic of the filling gas: Xe emits a resonance line of wavelength 1469.6 A, equivalent to photon energy of 8.44 eV; Kr, 1235.8 %.,10.03 eV; Ar, 1048.2 A, 11.83 eV; Ne, 735.9 A, 16.85 eV; and He, 584.3 A, 21.22 eV (5, 6). Thus in the liquid phase, to avoid eluent ionization, resonance lines cannot generally be used as the source of ionizing radiation. At higher gas pressures, however, microwave- and RF-excited discharges in rare gases emit continua, which for example in the case of xenon at 200 torr has a fairly sharp low wavelength cutoff at 1600 A, corresponding to 7.81 eV (7). This is insufficient to ionize liquid n-hexane, for which the photoionization threshold is observed at 9.59 eV (8))but 0 1982 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 ~
.-. - -. -. -. . -.
-- - - - - - .
Cross-sectional diagram of detector cell (top view): (E) in. 0.d. electrodes; (I) '/le in. 0.d. stainless steel inlet from HPLC; (0) 'Is in. 0.d. stainless steel drain to outlet; (dotted circle in center) in. hole for UV llght to enter; lamp face is 1 in. diameter; cell is 1 in. diameter X 2 In. long. Flgure 2.
C
-
__ -
Table I. Characteristics of the PID with Pyrene as Test Solute in Hexane/2% 2-Propanola MDQ
Schematic diagram of PID cell and electrical circuit: (A) microwave generator; (B) collector electrode power supply; (C) Teflon PID cell; (D) Kelthley 610C electrometer ampllfier; (E) gold-plated brass electrodes; (F) recorder; (0)Ophthos Xe lamp; (H) sheet aluminum cage; (I) mlcrowave cavity: and (L) microwave cable. Flgure 1.
is quite capable of photoionizing pyrene, which has an ionization threshold in hexane solution of 5.5 eV (9). Indeed, our detector has been shown to be quite sensitive to polycyclic aromatic hydrocarbons eluted with hexane or hexane/2propanol mixtures. In associated liquids, stabilization of the ionization products by solvation can lower the ionization energy requirements by extraordinary amounts. The threshold for photoionization of liquid water, for example, has been reported to be 6.05 eV (IO),less than half the vacuum IP value. Associated liquids tend however to photodissociate as well as to photoionize; in methanol the yield of solvated electrons to photodissociatively formed H2 is 1/30; in water this ratio is 1/7 (10). Similarly, solutes dissolved in polar solvents are ionized in greater extent than when dissolved in nonpolar solvents (11). A second basic difference between condensed-phase and gas-phase photoionization involves processes occurring after the initial photoionization step. In liquids, it is possible to have geminate recombination, formation of radical anions, electron capture, molecular rearrangement or fragmentation, or some combination thereof, in addition to simple solvation of product ions. Recombination of cation and electron before solvation is most likely to occur in nonpolar solvents (12). Electron capture (13) and radical anion formation (14) also reduce observed photocurrents, presumably because of the lower mobility of organic anions than solvated electrons. That we detected no photocurrents for halogenated compounds or phenols, which are known (11)to be photoionized, is probably explicable in terms of such mechanisms. The present PID operates on a different principle than the recently announced photoconductivity detector of Tracor Instruments (15). In the latter, the organic compounds containing halogen, nitrogen, or sulfur atoms irradiated with 254-nm UV light photodecompose to an acid and other products. In aqueous solution the acid dissolves and dissociates to ordinary hydrated ions. The conductivity of the irradiated aqueous solution is measured with a conductivity bridge. This detector is quite sensitive and selective for certain types of compounds and might be considered complementary to the present PID. As noted above, water is ionized at low photon energies. In addition, even pure water has a relatively high intrinsic conductivity. Both of these conditions result in a high background current in the present design. Since most HPLC is conducted in the reversed-phase mode with water/methanol and water/acetonitrile eluents, we are investigating alternative experimental designs to make our detector amenable to aqueous systems.
MDC
LDR effective IE backgd current noise level a
2 x 10-I' g 2 x lo-" g/cm3 > l o 6
8 X lo-'
1 x 10-13 A 1x 1 0 4 4 A
See text for acronyms and definitions.
EXPERIMENTAL SECTION The detector cell, illustrated in Figures 1 and 2, is constructed from a 1in. diameter rod of Teflon. Figure 1is a side view; Figure 2 is a top view of a horizontal cross section cut through the middle of the electrodes E. The UV light is provided by microwave excitation of a 200 torr pressure xenon lamp in a Type N Evenson cavity, both supplied by Ophthos Instrument Co. This lamp emits a continuum of radiation with a short wavelength cutoff at 1600 A. Microwave power is supplied by a Raytheon Model KV-104 NB medical unit which provides up to 125 W of power at 2450 MHz. The optimum signal-to-noise ratio is achieved when the microwave power is restricbd to 30-40% of maximum. Light from the xenon lamp passes into the detector cell through a '/I6 in. hole in the center of the cavity, which is ll/sin. diameter X 1/4 in. deep, milled out of the side of the Teflon rod (C in Figure 1; the large dotted circle in Figure 2). The ll/sin. end of the lamp fits snugly into this cavity. Effluent from the HPLC enters the cell through a in. 0.d. stainless steel inlet tube and is drained through a 1/8 in. 0.d. stainless steel outlet tube. The internal volume of the cell is 10 pL. Two electrodes (E) monitor the current flow and are polarized up to 3000 V dc vs. ground using a Power Designs Pacific HV-1545 power supply. The electrodes, constructed of brass or in more recent experiments gold-plated brass, are 1/8 in. 0.d. and are inserted into the cell through 1/8 in. holes drilled into the Teflon rod. A Keithley Instruments 610C electrometer is used to amplify the photocurrents. In operation the entire cell is shielded by placing it inside a large grounded cylinder constructed a sheet aluminum. All electrical connections are made with Belden shielded cable to reduce noise pickup. The output from the electrometer drives a 3-V Honeywell Electronik strip chart recorder. In the experiments described here, the new PID was used with a Micromeritics Model 7500 HPLC system. Whatman Partisil PXS columns packed with 10-pm silica gel were 25 cm X 4.6 mm i.d. All chemicals were of HPLC quality. Solvents were degassed by bubbling with helium. RESULTS AND DISCUSSION General Comments. The characteristics of the PID were studied by using pyrene as a test solute with hexane/2% 2-propanol as eluent. Some of the important properties of the detector in its present form are summarized in Table I. Acronyms in the table are defined below. A typical chromatogram is shown in Figure 3. Effect of Microwave Power. Increasing microwave power from 10% to 30% of maximum approximately doubles the current signal for pyrene. Increasing the power above 30% produces smaller increases in response but contributes to premature aging of the lamp. Thus microwave powers be-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
449
Table 11. MDQs for Various Compounds acenaphthene 70 Pg benz[a] anthracene 50 benz[a Ipyrene 10 50 diphenylene dibenzanthracene 5 fluorene 350 4 perylene phenanthrene 100 pyrene 20 triphenylene 250 benzene 30 ng xylene 22 ng
Figure Chromatogram of poiycycllc aromatic hydrocarbon mi: ire: (A) acenaphthene, 6 ng; (B) pyrene, 12 ng; (C) benz[a]anthracene, 20 ng; (D) benz[a]pyrene, 4.7 ng; (E) perylene, 1.4 ng. Chromatographic conditions: 25 cm X 4.6 mm i.d. Whatman Partisii PXS 10 pm; 0.66 mL/min hexane/2% 2-propanol: temperature, ambient; sample size injected, 1 pL; electrometer, 0.3 X lo-" A f.s.; collector voltage, -3000 V; microwave power, 30% maximum.
tween 30% and 40% of maximum were generally used. Effect of Electrode Spacing. In the present cell, electrode spacing distances exceeding 0.4 cm lead to a decrease in sensitivity, presumably because ions are lost by recombination or other processes before they can diffuse to the electrodes. Closer spacing of the electrodes, however, allows sufficient reflected and scattered light to impinge on the electrodes to produce increased background currents from the photoelectric effect (16). In practice the electrodes are manually positioned to maximize signal-to-noise, which in the present design corresponds to an optimum spacing of 0.4 cm. Effect of Collector Voltage. Higher collection voltages lead to larger observed currents. Thus for pyrene in hexA/V ane/2% 2-propanol, current increases at about 4 x from 0 to 1600 V, and a t about a %fold higher rate for 1600-3000 V. Collector plotentials in excess of 3200 V in this solvent cause electrical breakdown and arcing. To maximize current but avoid sparking, we used collector voltages of 3000 V. Collection of Positive or Negative Ions. For the polycyclic aromatic hydrocarbon solutes studied, currents collected using positive potentials on the collector electrode were generally approximately equal to those obtained when positive ions were collected a t a negative potential of the same magnitude. This suggests that the negative species is a radical anion rather than a solvated electron. The mobility of the negatively charged species under the present conditions is calculated by using the method of Gary et al. (17)to be of the order of cm2/(V 8 ) . This agrees exactly with the measurements of Minday et al. (18), who found positive and negative photoions in hexane solution to have the same mobilities. The positive species resulting from photoionization may be the parent ion or the product of subsequent reactions, although the exact nature of the negative or positive species has not yet been determined. Further work on the mechanism of response should provide a more accurate description of the ions collected. Background Current. At the optimum electrode spacing and collector voltage, background currents of the order of 10-14 A were observed for hexane, which remained unchanged when the UV source was extinguished. Background current increases with increasing amounts of 2-propanol in hexane, being about A for 2% 2-propanol and about A for 15% of the alcohol, presumably because of the intrinsic conductivity of 2-propanol. As could be expected the background current increases with decreasing electrode spacing and increasing collector voltage. The background current is backed off using
acetic anhydride 1 2 ng acetone 9 azobenzene 110 cinnainaldehy de 30 diethyl ether 10 ethyl propionate 26 heptene-1 20 isoph orone 8 1 inorpholine nitrobenzene 150 nitromethane 17
the electrometer amplifier controls. Minimum Detectable Quantity. The detection limit can be expressed in various ways, for example, the minimum detectable quantity (MDQ), defined as that quantity of solute introduced into the detector which produces a peak current twice the noise level. The MDQ of pyrene was determined by injection of measured volumes of known concentrations of pyrene and hexane/2% 2-propanol. Data were plotted as peak current vs. solute weight injected, and the MDQ was determined by extrapolating to the peak current twice the A, with negligible drift noise level. Noise was about 1 X once the detector temperature stabilized. The MDQ of pyrene g. MDQs for the compounds listed in Table I1 is 2 X were determined by comparison of peak heights of known concentrations of pyrene and the other compounds injected into the LC. We are currently measuring MDQs for a wide variety of compounds in a systematic fashion. Figure 3 shows a typical chromatogram of several polycyclic aromatic hydrocarbons a t a low concentration. Minimum Detectable Concentration. An analogous way to express sensitivity is the minimum detectable concentration (MDC). Despite its popularity, the MDQ as determined suffers the obvious limitation that it depends not only on the det,ector sensitivity but on the chromatographic efficiency. The MDC takes into account the fact that although the peak current is measured, the solute is distributed in a Gaussian fashion over the peak. The relationship between the weight of solute introduced, w ,and the peak solute concentration, C, is the familiar C, = w / ( 2 ~ ) ' /uv, ~ where uv is the peak standard deviation (in volume units), defined as half the peak width measured at a fraction 0.607 of the peak height. In the present case for pyrene, uv = 0.4 cm3 and the MDC works out to be 2 X 10-l' g/cm3, or 0.1 pM. Linear Dynamic Range. The linear dynamic range (LDR) can be defined as the range of sample weights from the MDQ to the weight injected at which an arbitrary deviation (often 5% or 10%) from linearity is observed in the plot with unit slope of peak current vs. solute weight on log-log paper. This was determined as described above, and for the present system, linearity was maintained up to the highest weight injected, 2 X g of pyrene, i.e., over at least a 106-foldrange. In addition, the same response was observed by injecting 1 FL of pyrene solution of one concentration as by injecting 10 pL of that solution diluted 10-fold. Effective Ionization Efficiency. The ionization efficiency (IE) is defined as the ratio of the number of ions formed to the number of molecules of ionizable solute present. However, the absolute IE should be distinguished from the effective or chromatographic IE. The latter is the ratio of the number of ions collected under the experimental conditions used to the number of solute molecules injected. These effective IEs were determined by injecting known weights of solute into the HPLC, converting the resulting peak areas (peak height X width at half-height) to Coulombs from the absolute settings of the amplifier and recorder, and using Faraday's law. For
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
pyrene the effective IE is 8 X Similar values were obtained for other polycyclic aromatic hydrocarbons. There seem to be no published data on photoionization in liquids which can be used for comparison. The small effective IEs may reflect a low absolute IE or the failure of the ions formed to reach the collector electrode because of recombination, ionradical reactions, etc. Comparison of the Present Liquid Phase PID and the Vapor-Phase PID. Some comparisons can be made between the present detector and the earlier PID (2), in which the effluent was evaporated and the vapors passed into a gas-phase PID.No substantial work on the latter was carried out beyond that discussed in ref 2. The test solute studied previously was methylaniline in pentane. Assuming the characteristics in Table I can be compared legitimately with the corresponding values reported for the vapor-phase detector (2), we note the following. The liquid-phase detector MDQ, LDR, and drift are comparable. The background current is about 1%that of the earlier detector. The effective ionization efficiency is 10 times less, but the noise is 4 times less, which combination produces comparable MDQs. This comparison is an operational one, because the fundamental differences between gas-phase and liquid-phase photoionization preclude any theoretical comparison. We attempted earlier to use the first design as a direct liquid-phase PID; these trials were unsuccessful because, as noted above, the higher photon energies of the resonance lines cause extensive solvent ionization, i.e., large background currents and high noise levels. The use of the lower energy light source is the crucial element in the present design. The basic problem with the earlier detector is the requirement of evaporating the total column effluent. This limits application of the detector to low volatility solutes and to eluants containing salts; rapid pyrolysis of the effluent might overcome the volatility problem. Nonetheless the earlier version of the PID is compatible with aqueous mobile phases. Its fullest application will probably be made as a detector used with microbore HPLC columns, where flow rates are of the order of 10 pL/min. The liquid-phase PID has obvious limitations. Detectable solutes must be photoionizable to charged produds sufficiently long-lived to be collected. Solvents must neither give appreciable photocurrents nor have high intrinsic conductivity; ionic species must not be dissolved in the eluant. The more
widely used reversed-phase mobile phases do not meet these criteria; specifically, in aqueous eluents high background currents are observed because of the conductivity of water and hydroxylic solvents, photoionization of the solvent, and electrolysis of water at collecting voltages exceeding about 0.25 V. To attempt to circumvent these three problems, we are investigating alternative detector designs. We are also studying the mechanism of response of the PID in the photocurrent mode, in order to determine why no photocurrents are observed in some cases and to account for its response to other compounds.
ACKNOWLEDGMENT The authors thank Elie Hayon for helpful discussions. LITERATURE CITED Ettre, L. S. J. Chromatogr. Sci. 1978, 16, 396. Schmermund, J. T.; Locke, D. C. Anal. Left. 1975, 8, 611. Bullot, J.; Gauthler, M. Can. J. Chem. 1977, 5 5 , 1821. Lesclaux, R.; Joussot-Dubien, J. I n "Organlc Molecular Photophysics"; Blrks, J. B., Ed.; Wlley: New York, 1973; p 457. Gorden, R.; Rebbert, R. E.; Ausloos, P. NBS Tech. Note ( U S . ) , 1969, No. 496. Samson, J. A. R. "Techniques of Vacuum UV Spectroscopy"; Wiley: New York, 1967; p 131. Sampson, J. A. R. "Technlques of Vacuum UV Spectroscopy"; Wiley: New York, 1967; p 105. Casanovas, J.; Grob, R.; Sabattler, R.; Guelfucci, J. P.; Blanc, D.Rad&. Phys. Chem. 1980, 15, 293. Plciulo, P. L.; Thomas, J. K. J. Chem. Phys. 1978, 6 8 , 3280. Anbar, M.; St. John, G. A.; Gloria, H. R.; Reinsch, A. I. I n "Water Structure at the Water-Polymer Interface"; Jellinek, H. H. G., Ed.; Plenum: New York, 1972. Grossweiner, L. I.; Joschek, H A . A&. Chem. Ser. 1985, No. 5 0 , 279. Holroyd, R. A.; Allen, M. J. chem. Phys. 1971, 5 4 , 5014. Holroyd, R. A.; McCreary, R. D.; Bakale, G. J. Phys. Chem. 1979, 83, 435. Hayon, E. J. Chem. Phys. 1970, 5 3 , 2353. Popovlch, D. J.; Dixon, J. 8.; Ehrllch, B. J, J. Chromatogr. Sci. 1979, 17, 643. Gordon, J. G.; Finklea, H. 0. J. Phys. Chem. 1979, 8 3 , 1834. Gary, L. P.; de Groot, K.; Jarnigan, R. C. J. Chern. Phys. 1988, 49, 1577. Mlnday, R . M.; Schmidt, L. D.; Davls, H. T. J. Chem. Phys. 1989. 5 0 , 1473.
RECEIVED for review July 21,1981. Accepted November 12, 1981. This work was supported in part by a grant from the National Science Foundation, and in part by Grant No. 12287 from the PSC-CUNY Research Award Program of the City University of New York.
