Anal. Chem. 1082, 54, 1546-1551
ACKNOWLEDGMENT We thank B. A. Bidlingmeyer,B. Sachok, and W. P. Price, Jr., for helpful discussions. LITERATURE CITED
TINE
Flgure 14. Simulated chromatograms showlng details of the second chromatogram from the top In Flgure 13: (solid line) component I; (dotted line) component J.
the same as that exhibited in the real chromatograms in Figure 12. Figure 14 shows a composite chromatogram of the concentrations of aniline and IIR for the simulation in Figure 13 with a 20 tube delay. The tall dotted peak in Figure 14 corresponds to the elution of IIR, and the two smaller peaks on either side correspond to a split aniline peak. The early eluting portion of aniline has not been overtaken by the IIR, while the later eluting portion of the aniline sample has been strongly retarded by the IIR. This type of splitting occurs only when the sample and IIR coelute. Thus, it can be avoided if conditions are adjusted so that the postinjected component is completely eluted from the column before the sample components of interest elute (8).
(1) Berek, D.; Bleha, T.; PevnB, Z. J . Chromatogr. Sci. 1978, 14, 560-563. (2) Rellley, Charles N.; Hlldebrand, Gary P.; Ashley, J. W. Anal. Chem. 1982, 34, 1198-1213. (3) Scott, R. P. W.; Scott, C. G.; Kucera, P. Anal. Chem. 1972, 4 4 , 100-1 04. (4) Slais, K.; Krejci, M. J . Chromatogr. 1974, 9 1 , 161-166. (5) Hendrlx, D. L.; Lee, R. E., Jr.; Baust, J. G. J. Chromatogr. 1981, 210. 45-53. (6) Keller, Roy A.; Glddings, J. Calvin J . Chromatogr. 1980, 3 , 205-220. (7) Tseng, Paul K.; Rogers, L. B. J . Chromatogr. Sci. 1978, 16, 436-438. (8) Stranahan, John J.; Deming, Stanley N.; Sachok, Bohdan J. Chromatwf. 1980, 202, 233-237. (9) Campos, A.; Borque, L.; Flgueruelo, J. E. J . Chromatogr. 1977, 740, 219-227. (10) Katlme, I.; Campos, A,; Rlvera, J. M. T. Eur. Polym. J . 1979, 75, 291-293. (11) Bldllngmeyer, 8. A.; Demlng, S. N.; Price, W. P., Jr.; Sachok, B.; Petrusek, M. J . Chromatogr. 1979, 186, 419-434. (12) McCormlck, R. M.; Karger, B. L. J . Chromatogr. 1980, 199, 259-273. (13) Denkert, M.; Hackzell, L.; Schlll, 0.;Sjijgren, E. J . Chromatogr. 1981, 218, 31-43. (14) Parrls, N. A. “Instrumental Liquid Chromatography”; Elsevler: Amsterdam, 1976; Chapter 12. . Ramette, Richard W. “Chemical Equlllbrium and Analysis”; AddisonWesley: Readlng, MA, 1981; Chapter 4. Rosen. Milton, J. “Surfactants and Interfacial Phenomena”; Wiley: New York, 1978; Chapter 2. Parrls, N. J. Liq. Chromatogr. 1980, 3 , 1743-1751. Su, S. Y.; Jurgenson, A,; Boiton, D.;Winefordner, J. D. Anal. Lett. 1981, 74, 1-6. Knox, J. H.; Hartwick, R. A. J . Chromatogr. 1981, 204, 3-21. Stranahan. John J.; Deming, Stanley N., unpublished work, Unlverslty of Houston, 1981.
RECE~VED for review March 10,1982. Accepted April 12,1982. This work was supported in part by a grant from Chevron Research Company.
Plasma Chromatography with Laser-Produced Ions David M. Lubman* and Me1 N. Kronlck Quanta-Ray, Inc., 1250 Charleston Road, Mountain Vlew, California 94043
Laser multiphoton lonlzation (MPI) is used to produce ions In an ion moblllty spectrometer. The Ions created are then separated by gaseous electrophoresis, Le., according to their moblllty In a drtft gas under the Influence of an applied electric field. The MPI process allows direct lonlzatlon of organic compounds with production of only one peak whlch is elther the molecular Ion or MH’. The problem of multiple peaks occurrlng due to the Ion-molecule process In plasma chromatography Is thus slgniflcantiy reduced. This technlque can provlde great sensitivity, Le., at least down to 1 ppb In the case of benzene. I n addltion, the laser wavelength can provlde an additional means of dlscrlmlnation of molecules In an Ion moblllty spectrometer.
We introduce a unique method for producing ions for plasma chromatography (PC). This technique involves using laser multiphoton ionization to ionize molecules directly under atmospheric conditions in a commercial ion mobility spectrometer. 0003-2700/82/0354-1546$01.25/0
There are many reviews of the theory of plasma chromatography (1-15). In conventional plasma chromatography, ions are initially produced in a carrier gas by a 63Ni@-decay source. These ions initiate a sequence of ion-molecule reactions that eventually yield a few molecular ions which then undergo ion-molecule reactions with the trace compound to form the ions measured in a drift tube. In the drift region the ions are moved through a column of gas by an electric field and separated according to their mobilities. Nitrogen is usually used as the drift gas to prevent further ion-molecule reactions. Eventually the ions diffuse to a detector where either positive or negative ions are detected. The output of the detector is in the form of current as a function of time. An ion mobility spectrum is thus obtained which fingerprints molecules in a manner similar to gas chromatography. Since the drift time of the ions is in the millisecond regime, the separation and detection can be performed in real time. In addition, N2 can serve as a universal column thus alleviating the problem of column choice inherent in gas chromatography. The initial creation of ions through the use of the ionmolecule reaction technique often produces data difficult to 0 1982 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
HI VOLTAGE
c
1547
.GUARD RINGS
////k///LIONIZATION
+ L
L
CONTINUUM
_i_
L
__
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__ la1
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I I
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Flgure 1. Energy level diagram showing MPI transitions: (a) nonresonant MPI, (b) one photon riesonant REMPI, (c) two photon resonant REMPI.
interpret. The ion-molecule reaction often creates several different ion-molecule combinations with the trace compounds. Plasma chromatography with laser-produced ions (PCLI) minimizes the problem of nonspecific ionization. The ionization source for PClLI in our experiments is ultraviolet laser light from a Nd:YAG pumped pulsed laser system. The laser radiation allows direct ionization of molecules, thus circumventing limitations of the ion-molecule method. Laser photoionization provides extremely efficient ionization of large molecules with only the molecular ion appearing when the laser operates a t sufficiently low intensity (16-22). The ionization process can be accomplished directly in air (23,241. The laser ionization process occurs when ionization follows the absorption of several photons by a molecule in the presence of an intense visible or UV light source, Le., it is a multiphoton process (Figure 1). When the laser source is tuned to an allowed n-photon transiition, the process is greatly enhanced. This is referred to as resonance enhanced multiphoton ionization, Le., REMPI. In the case of REMPI, ionization occurs via a real intermediate state. Since the density of states above the lowest energy state populated is usually quite high, subsequent absorptions are resonant or nearly resonant. Excitation from the intermediate state to the ionization continuum is quite rapid and may involve the absorption of as many as six additional photons (25). The transition to the lowest intermediate real state is generally the rate-limiting step. When the laser is not tuned to a real state, the probability for multiphoton ionization (MPI) is nearly negligible. Most large aromatic and aliphatic compounds have two salient features: strong absorptions in the UV and low ionization potentials, typically between 7 and 13 eV. The UV absorption spectra of these molecules is rather broad at room temperature due to the large number of rovibronic states populated. Most larger molecules can thus be ionized with the fourth harmonic of ithe NdYAG laser (266 nm) with the absorption of one photon to resonance and, subsequently, one to three more photons t o ionization. The process is thus extremely efficient. Since REMPI is based upon the resonance absorption of molecules, there are several other advantages which we gain from this technique. Laser radiation at wavelengths greater than 200 nm will not ionize air at the power levels required to ionize organic compounds. Molecules such as COz,02,and Nz, etc. from the enviroriment will thus not interfere with the data. In addition, by using the laser ionization technique the unique absorptions can be used to spectrally separate the different compounds in the environment. Klimcak and Wessel have demonstrated the spectral separation of two isomers,
u%E Flgure 2. Plasma chromatograph modified for use with laser ionization.
anthracene and phenanthrene, a t atmospheric pressure in a proportional counter using a tunable dye laser (23). They in fact suggested in the same reference that time-resolved ion mobility measurements might actually provide additional discrimination in those cases where molecules have similar one-photon absorptions or where ionization potentials are nearly equivalent. EXPERIMENTAL SECTION The experimental setup is shown in Figure 2. The ion mobility spectrometer is a modified version of a commercial plasma chromatograph obtained from PCP, Inc., West Palm Beach, FL. The present setup can be operated as a conventional plasma chromatograph with a Ni-@source or a laser chromatograph by changing the bias circuit and introducing a laser beam through the cell window. When used as a conventional plasma chromatograph, ion control grid 1 must allow ions to pass. The ionmolecule reactions occur between the @ source and grid 2. The ions are injected into the drift region by pulsing grid 2. The drift distance from grid 2 to the ion collector is 8.0 cm. The injection pulse can vary from 0.05 to 0.5 ms. A smaller puke provides greater resolution but also leas sensitivity. Typically, a pulse length of 0.2 ms was used to monitor the presence of compounds using PC. In the case of PCLI, grid 1 should be biased to block the ionization from the radioactive source. Grid 1 then provides the bias for ions created by MPI in the interaction region. The drift distance from the center line of the laser beam to the ion collector is 12.5 cm. Suprasil quartz windows (2.54 cm diameter) were used in order to transmit UV laser radiation. The apparatus was operated at above 200 OC at all times in order to keep the apparatus free from contamination. The quartz inlet was operated at 275 "C. A countercurrent flow of dry N2was filtered with a molecular sieve trap before it entered the PC in order to remove water vapor or other contaminants. The laser source consisted of a Quanta-Ray DCR-1A Nd:YAG laser used alone at its fourth harmonic (266 nm) or used at its second harmonic (532 nm) to pump various dyes in a Quanta-Ray PDL-1 dye laser. In order to produce tunable UV light, the output from the dye laser was frequency doubled in a phase-matched KD*P crystal. This was performed for the various dyes using the Quanta-Ray WEX-1 wavelength extender device which can produce scannable W radiation over the frequencydoubled range of each dye. The dye laser wavelength was scanned by use of a stepping-motor controlled by a Quanta-Ray CDM-1 control display module. The signal from the ion mobility spectrometer was digitized into 800 points over a 40-ms interval using a DEC ADV-11 A/D interface to our DEC PDP-11 computer. The digitized signal was then signal averaged over 500 laser pulses. The resulting ion mobility spectrum was then displayed on an x-y recorder. The MPI spectra as a function of wavelength for a particular compound were taken by a PAR Model 160 boxcar averager with the inte-
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
gration window placed over the peak of interest in the ion mobility spectrum. The samples used in the various experiments were prepared in several ways. For our initial studies, benzene and toluene samples were prepared for us commerciallyas mixtures in concentrations of 1.0 ppm and 2.0 ppm in nitrogen respectively (Scott Specialty Gases, San Bernardino, CA). When the sample gas is flowed very slowly, i.e., 1-5 cm3/min, then the counterflow of N2 gas at approximately 600 cm3/min dilutes the sample gas in the interaction region approximately according to the ratio of flow rates (26). This method does not provide an accurate concentration but does provide an estimate of the concentration,probably within 50%, that is sufficient for characterizing the physical properties of this technique. For more accurate measurement of the limits of sensitivity of PCLI the exponential dilution flask method was used (27). The apparatus consists of a 250-mL flask containing a magnetic stirrer. The magneticstirrer used had extension f i s to increase the mixing between the benzene and N2 gas. The benzene was diluted by injecting 1mL of benzene in a 100-mL volumetric flask and then diluting with methanol. One milliliter of this solution was further diluted in a 500-mL volumetric flask. When 1ILLof this solution was injected into the 250-mL flask a concentration of approximately 32 ppb was obtained. The flask was heated to 150 O C and kept at a constant temperature using a temperature proportional controller. All gas sample inlet lines were kept at above 200 " C in order to prevent benzene from absorbing on the surfaces. The N2 carrier flow rate was typically approximately 100 cm3/min. In order to check for completemixing, we used a stop-flow method. A sample was examined at a given time. The flow was stopped and mixing allowed to continue. A sample was then taken at a later time. The signal was the same for each of these samples thus indicating that mixing was complete. The dilution flask method was repeated ten times and the results were averaged. In the case of the solid samples, azulene and naphthalene, the diffusion tube method was used (28). Diffusion tubes (Vici Metronics, Santa Clara, CA) with apertures of either 0.5 or 0.2 cm and a length of 7.62 cm were used to create samples in concentrations on the order of 10 ppm. Provided the carrier N2gas was flowing slowly the sample was further diluted by the drift gas to concentrations on the order of 10-20 ppb. The diffusion tubes were used at room temperature.
RESULTS AND DISCUSSION We have found that resonant multiphoton ionization can be used to ionize softly various hydrocarbons a t atmospheric pressure in an ion mobility spectrometer. The MPI process directly ionizes the compound under study but does not ionize background gases. The ion mobility spectrum subsequently allows identification of each component. Figure 3 illustrates laser MPI mobility spectra of benzene and toluene produced by 266-nm laser light. Only one mobility peak is obtained from the ionization of each compound. We identify this peak as either the molecular ion or MH+ ions based upon data obtained from /3 source ionization as well as the work of Griffin e t al. (11). In Figure 3 other unidentified peaks are present at longer mobility time even with no sample present. These are due to background'contamination present in the device. If the beam was tightly focused a spectrum with several peaks was obtained as in the case of toluene (Figure 4). Under vacuum conditions we know that fragmentation of the molecules occurs a t high power densities as identified by a mass spectrometer. It thus is probable that fragmentation is occurring in the interaction region of this device. There were no ions present in the negative ion mobility spectrum, although there was a large peak due to electrons produced as a result of the MPI process. The laser ionization technique appears to be very efficient. We have been able to detect easily mobility spectra for toluene and benzene a t concentrations below 10 ppb in nitrogen at 266 nm with no more than approximately 2 mJ of input laser energy per pulse. We used the rate equations derived by Reilly and Kompa (29) to estimate the number of ions which could
DRIFT TIME
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1
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Figure 3. (a) Laser-induced mobility spectrum of benzene and toluene. The peak at 15.4 m s corresponds to benzene and that at 16.6 ms, to toluene. The conditions under which this spectrum was recorded are as follows: X = 266 nm, T = 200 "C,input laser energy = 2 mJ, laser beam diameter = 6 mm, electric field = 171 V k m , benzene concentration = 30 ppb, toluene concentration = background. (b) Laser induced toluene mobility spectrum at a concentration of 15 ppb. The amplifier sensitivity and gain settings are the same as in Figure 3a.
bl
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Figure 4. (a)Laser induced mobili spectrum of toluene, parent peak:
X = 266 nm, T = 210 "C,laser Input energy = 2 mJ, laser beam diameter = 6 mm, toluene concentration = background, electric field = 171 V k m . (b) Laser induced mobility spectrum of toluene, fragmentation pattern: X = 266 nm, T = 210 'C, laser input energy = 8 mJ, laser beam focused to a point using a 10-cm focal length lens, electric field = 171 V/cm, toluene concentration = background. The amplifier sensitivity and gain settings are the same as in Figure 4a.
in theory be produced a t a given input intensity. According to these authors the number of molecular benzene ions produced by the 2-photon ionization process is given by
where I is the laser intensity in photons cm-2 8,7 is the intermediate state lifetime, A,, is the ground state concentration, T i s the duration of the laser pulse, aiare cross sections for the respective stimulated transitions
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
a2 = q l ( a J
+ 1/r)
(4)
We have performed our experiments with the fourth harmonic of the Nd:YAG laser (266 nm). The beam was an annulus with a 6 mm outer diameter and 2 mm inner diameter. The laser input intensity was approximately 2 mJ. We calphotons cm-2 s-l. Literature data for culate I N 1.08 X benzene (30,31)indicate that, at 266 nm, u1 = 5 X cm2. The value of u2 (266 nm) was taken to be u2 (249 nm) = 3.4 X 1O-l’ cm2 (29). This approximation can be made since for absorption into a largely structureless continuum, the cross section for one-photon iclnization of the intermediate excited states of benzene should be independent of wavelength above threshold (32). The value of 7266 is taken to be 100 ns (33). Using these values we obtain a value of C6H6+= (2.8 X 10-3)A1,. At 220 OC, 10 ppb is equhalent to 1.36 X 10l1 molecules/cm3 of neutral benzene molecules. If the interaction region is 0.28 cm3, then the laser can interact with 3.8 X 1O1O molecules. If, as is usually the case, Alg number of neutrals c! 3.8 X 1O’O molecules (34, then the above calculation would predict that approximately 1 X 108ions are produced per laser pulse. This is true since typically a small fraction of the neutral molecules ( < l o % ) is excited from Alg to the first excited state, and only EL fraction of these will ionize due to competing processes. At 10 ppb benzene we obtain a signal of 1.5 X A. The pulse width is approximately 650 ps C. This fwhm so that the total charge produced is 9.7 X is equivalent to 6.1 X lo5 ions actually detected. Therefore (6.1 X 106)/(1X lo8) 0.6% is a lower bound estimate of the efficiency of detection of the PCLI technique after ionization. Of course the efficiency of ionization itself can be increased by increasing the laser power. In fact, the signal has been found to increase linearly with laser power at 266 nm for toluene and benzene. However, if too many ions are created, space charge effects begiin to appear and thus resolution decreases as the signal width starts to broaden. In order to determine the experimental limits of detection of PCLI, we used the exponential decay flask method. Benzene was chosen for atudy since absorption of benzene on the cell walls was less of EL problem than in the case of toluene, naphthalene, or azulene. In this method the gas concentration, C, decays with time according to the relationship (27)
-
where V is the volume of the vessel, U the gas flow rate, C, the initial gas concentration, and t the time elapsed after introducing the test gas. If the signal amplitude from the detector is plotted against elapsed time, a perfect detector would indicate a log-linear decay of gas concentration with a slope of 1/r = U / V . The response can be followed down to the background noise :levelof the device and any departure from linearity can be detected immediately. We were able to detect at least 1ppb of benzene in nitrogen by using this technique. The lower limit of detection was determined by residual benzene from the cell walls and not by signal to noise limitations. The value of‘ r was typically on the order of 3.5 min. As mentioned above, Ispace charge effects can influence resolution and sensitivi1;Jr. Provided we consider the space charge effect produced mainly by widely separated ions, then the criterion for negligible space charge distortion of an applied field Eo is then
n
EO
