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1 , -
I
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P.4RTICLE DIAMETER, d ( p m )
Flgure 10. Selectivity vs. particle diameter for flow FFF. Parameters P. are w = 0.254 mm, T = 298 K, and 7 =
steric FFF is clearly illustrated. The figure also shows the greater range over which high selectivity values extend for the higher field strength. Figure 10 shows corresponding linear selectivity plots for flow FFF. Notable is the fact that the limiting selectivity for normal flow FFF is the same as for steric FFF. The wider range over which high selectivity values are maintained relative to sedimentation FFF is apparent.
here some limitations in our results which stem from uncertainties in the steric branch of FFF. While steric effects are described by fairly simple theoretical equations, such as the first term on the right of both eq 14 and 15, factor y conceals some fairly complicated hydrodynamic phenomena which are not fully quantified (9). For example, y has been observed to increase with flow velocity, which means that the right hand side of the foregoing curves would all shift slightly with velocity. There is probably a slight size dependence to y as well. Furthermore, the magnitude of y is not known for various kinds of nonspherical particles. Despite the above limitations, we believe that the above model provides a simple and effective treatment of FFF over a very broad range. We expect the variations in y to be quite small relative to the enormous range of the other parameters covered. Therefore the model and the subsequent results are expected to provide a useful treatment of FFF, not only adequate for most present applications but also an effective guide for future refinements.
LITERATURE CITED Giddlngs, J. C. Anal. Chem. 1981, 5 3 , 1170. Giddlngs, J. C.; Yoon, Y. H.; Myers, M. N. Anal. Chem. 1975, 47, 126. Giddings, J. C. Pure Appl. Chem. 1979, 5 1 , 1459. Giddings, J. C.; Myers, M. N.; Caldwell, K. D.;Fisher, S. R. "Methods of Biochemical Analysis": D. Giick, D., Ed.; Wiley: New York, 1960; p 79. Giddings, J. C.; Myers, M. N. S e p . Sci. Technol. 1978, 13, 637. Glddings, J. C. S e p . Sci. Technol. 1978, 13, 241. Martin, M.; Jauimes, A. Sep. Sci. Techno/. 1981, 16, 691. Glddings, J. C. S e p . Sci. 1973, 8 , 567. Caidweil, K. D.;Nguyen, T. T.; Myers, M. N.: Giddings, J. C. Sep. Scl. Technol. 1979, 14, 935.
CONCLUSIONS The foregoing equations and plots provide fundamental information on the steric inversion and trends extant on either side of the inversion point. The study should serve as a useful background source for experimentalists and others dealing with conditions near those of the steric transition. This work is clearly required as one of the important bases for any complete optimization studies in FFF. The results of this paper are expected to be fairly accurate over the range dominated by normal FFF. However, we note
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RECEIVED for review July 6,1982. Accepted August 9, 1982. This material is based upon work supported by the National Science Foundation under Grant No. CHE 79-19879.
Discrimination of Isomers of Xylene by Resonance Enhanced Two-Photon Ionization David M. Lubman* and Me1 N. Kronick Quanta-Ray, h e . , 1.250 Charleston Road, Mountain View, California 94043
Resonant two-photon lonlratlon Is shown to be a sensitive technlque capable of dlscriminatlng isomers of xylene in air at atmospheric pressure. I n addltlon, this technlque can be combined with an lon-mobility spectrometer (IMS) In whlch the abillty to monlitor xylene at a concentration on the order of several parts per billion can be demonstrated. The Isomers of xylene cannot be dlscrlmlnated on the basis of their spectroscopy at the normal elevated temperature of an IMS. However, It Is shown that by proper selectlon of the temperature the characterlstic lonlration spectra of the isomers may be observed.
Multiphoton ionization (MPI) has been shown to be valuable as a sensitive and selective means of detection for chemical analysis. High sensitivity is achieved through ef-
ficient ionization (1-4) and the ability to detect ions with high efficiency. Spectral selectivity is provided by the use of a tunable dye laser (5-13). If the laser source is tuned to an allowed n-photon transition in a MPI process, ionization is greatly enhanced. This is referred to as REMPI, i.e., resonance enhanced multiphoton ionization. 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. In resonant two-photon ionization (R2PI) the first photon is absorbed by a real state while the second photon ionizes the molecule. The spectrum that results reflects the one-photon absorption spectrum of that real state and can often be used to identify a molecule uniquely. Resonant two-photon ionization has been demonstrated as a technique capable of monitoring polyaromatic molecules at atmospheric pressure in real time (2,14). Other methods such
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as laser-induced fluorescence are available but are usually limited by small fluorescence quantum yields at atmospheric pressure. R2PI can be applied to molecules that have low fluorescence quantum yields or normally do not fluoresce since ionization can be induced more rapidly than competing processes. In addition, the problem of scattered light background which is a major problem in fluorescence measurements does not exist in R2PI. Two-photon ionization not only can provide spectral selectivity but also can be used with either a TOF mass spectrometer (15) or an ion-mobility spectrometer at atmospheric pressure (16) to provide mass selectivity and hence even more definitive chemical information. In this report, we demonstrate the ability to detect and spectrally discriminate the isomers of xylene based upon their R2PI spectra at room temperature a t atmospheric pressure. In addition, we demonstrate the ability to monitor the isomers of xylene in an ion-mobility spectrometer at atmospheric pressure a t concentrations on the order of several parts per billion using laser RBPI.
EXPERIMENTAL SECTION The atmospheric pressure ionization experiments were performed in Pyrex ionization cells with quartz windows. The cells contained a pair of stainless steel electrodes -1 cm apart. One of the plates was biased at -150 V, and charged species were collected. The current was integrated by the Quanta-Ray DGA-1 dual-gated amplifier and displayed by a strip-chart recorder. The laser source consisted of a Quanta-Ray DCR-1A NdYAG laser used at its third harmonic (355 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 is frequency doubled in a phase-matched KD*P crystal. This was performed with the Quanta-Ray WEX-1 wavelength extension device which can produce scannable W radiation over the frequency-doubledrange of the dye. The dye laser was scanned with a stepping motor controlled by a Quanta-Ray CDM-1 control display module. The laser beam was typically 2 mm in diameter and the energy kept constant at -0.2 mJ/pulse. The ion-mobiiity experiments were performed as previously described (16) in a modified commercial plasma chromatograph (PCP, Inc., West Palm Beach, FL) in which laser ports with quartz windows were added to allow passage of the light beam through the device. The output signal, Le., the mobility spectrum was signal averaged by using a DEC ADV-11 A/D interface to our DEC PDP-11 computer. The R2PI spectra taken as a function of wavelength were obtained by using a gated integrator (Evans Associates, Berkeley, CA) with the gate placed over the appropriate mobility peak. The limits of sensitivity experiment was performed in an ion-mobility spectrometer using the expontential decay flask method (17) as described in our previous work (16). The apparatus consists of a 250-mL flask containing a magnetic stirrer to increase mixing. The o-xylene was diluted by injecting 1 mL of xylene 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 5 mL of this solution was injected into the 250-mL flask, a concentration of approximately 160 ppb was obtained. The flask was heated to 160 "C and kept at a constant temperature with a temperature proportional controller (Love Controls Corp., Model 495). All sample inlet lines were kept at above 200 "C in order to prevent xylene from sticking to the surfaces. Introduction of xylene at constant low levels was accomplished by use of diffusion tubes (VICI Metronics,Santa Clara, CA).
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RESULTS AND DISCUSSION Figure 1 shows the R2PI spectra of 0-,m-, and p-xylene as performed in an ionization cell in air at 1 atm at a concentration of l:104. The spectra are uncorrected for the variation of the average laser output power over the tuning range of the Coumarin 500 dye being used. The power, in fact, was constant to within 10% over the range scanned. The spectra shown were taken in the region between 271 and 267.5 nm
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Figure 1. (a) Ionization spectrum of -1:104 p-xylene in air: laser energy Input ( E ) = 0.2 mJ, beam diameter ( D ) = 3 mm, bias voltage ( B ) = -150 V, temperature ( T ) = 25 O C . Vertical scale is ionization signal in arbitrary units. (b) Ionization spectrum of 1:104 m-xylene in air. Same parameters as in (a). Vertical scale is ionization signal In arbitrary units. (c) Ionization spectrum of 1:104 o-xylene in air. Same parameters as in (a). Vertical scale is ionization signal in arbitrary units.
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based upon the absorption spectrum observed in a UV spectrophotometer. Spectra were also obtained for the region 267.5-264 nm but the difference in the spectra were not as striking as in the other region examined. The wavelength dependence of the ion yield is due to that of the lB2 lAl resonant absorption system (18) and follows the basic dependence of the one-photon ultraviolet spectrum. The spectra obtained for the three isomers are completely different from one another. Thus, R2PI provides the ability to detect and uniquely discriminate these isomers at atmospheric pressure. In addition, other compounds that absorb light in this wavelength region but whose ionization potential is greater than the present two-photon energy will not be ionized and thus will be discriminated against. The ionization spectra obtained under vacuum conditions are substantially the same +-
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tached groups maintains its sharp spectral features even at 220 o c . An experiment was performed to determine potential detection limits for R2PI of xylene. This experiment was performed in an IMS where the ionization signal from xylene could be discriminated from other low-level amounts of background at other masses. The IMS device is maintained at 220 “C to minimize contamination from the walls. Accurate low concentrations can be produced by the exponential decay flask method (17). In this method the gas concentration, C,, decays with time according to the relationship
e = e, q ( - ; t ) Flgure 2. I o n mobility spectrum of o-xylene. o-Xylene peak present a t 18.9 ms. The peak at 17.7 m s is background toluene present a t
