Anal. Chem. 1986,58,2335-2337
2335
s-l must have been sufficient over an 0.1-s lifetime in the ion source to permit ion/molecule charge-transfer reactions. The parallel behavior for BEN and NAP curves suggested a 1:l exchange ratio. In contrast to behavior described above, when the BEN concentration was fixed at 17.2 ppb and NAP was metered into the ion source, increased concentrations of NAP lead to decreased intensity of M+ for BEN. Since charge will reside on the compound with lower IP (NAP), curves in Figure 4b were consistent with a competitive collisional acquisition of charge. The creation of M+ for BEN through charge transfer from NAP should not occur and was not observed. Results from binary mixture studies with PI-IMS for BEN/ANT are shown in Figure 4C,D and behavior paralleled results from the BEN/NAP study. At fixed concentration of 4.26 ppb for ANT (IP = 7.43 eV), increased concentration of BEN caused an enhanced intensity in M+ for ANT. While peak heights for ANT should have remained constant at 2 pA with no charge transfer the actual measured values were 5 , 18, and 23 pA at BEN concentrations of 10, 500, and 1000 ppb, respectively. The difference in peak current for BEN from 500 to 1000 ppb should have been 16 pA but was only 10 PA. However, peak heights for benzene were 13 pA at 500 ppb and 18 pA at 1000 ppb rather than expected values of 23 and 39 PA, respectively. Conservation of charge was generally obeyed. Results from BEN/NAP and BEN/ANT studies differed in that the distance between curves for individual product ions in BEN/ANT was larger than that for BEN/NAP. This difference may be related to AIP values of 1.88 for BEN/ANT but only 1.15 for BEN/NAP. As observed with NAP/BEN studies, increased concentration of ANT caused nearly complete elimination of BEN at fixed concentration. Clearly, molecular properties in the form of ionization potentials govern the final observed IMS response for binary mixtures even when compounds are photoionized. Similar results should also be expected with mixtures of BEN and other aromatic hydrocarbons including PYR (IP = 7.43 eV) as well as with combinations of NAP/ANT, PYR/NAP, and PYR/ANT. Other combinations of PAH were studied and such investigations would be better suited for IMS/mass spectrometry.
especially when the IP for the analyte of interest is greater than IP values for interferences. While photoionization does add a measure of class selectivity in ionization, ion-molecule chemistry will still govern quantitative response within a chemical class. Preseparation of these compounds on a capillary GC (22)may afford fewer complications than are found with direct analysis of a mixture. Alternatively, extensive ionization of all components to M+ ions in a mixture may eliminate complications from change-exchange selectivity; however, this would negate the inherent advantages of selective ionizations. Registry No. Benzene, 71-43-2; naphthalene, 91-20-3; anthracene, 120-12-7;pyrene, 129-00-0.
CONCLUSION The importance of these results is that analytical utility of photoionization as a means of tunable Selectivity in analysis by IMS can be severely complicated by charge-transfer reactions. Generally, this means that the analyst is again at a disadvantage when working with samples of unknown matrix,
Department of Chemistry New Mexico State University Las Cruces, New Mexico 88003
LITERATURE CITED Karasek, F. W. I n f . J. Envlron. Anal. Chem. 1972, 2, 157. Karasek, F. W.; Cohen, M. J.; Carroll, D. L. J. Chromafogr. Sci. 1971,
9. Karasek, F. W. ResJDev. 1970, 21(3), 34. Vandiver, V. J.; Leasure, C. S.; Eiceman, G. A. I n f . J. Mass Spectrom. Ion Processes, in press. Vandiver, V. J.; Eiceman, G. A., submitted for publication in J. Chem. Phys . Vandiver, V. J.; Eiceman, G. A., in preparation. Keller, R. A.; Metro, M. M. Sep. Purlf. Methods 1974, 3, 207. Karasek, F. W.; Spangier, G. E. J. Chromafogr. Lib. 1981, 20, 377. Carroll, D. I.; Dzidic, I.; Stillwell, R. N.; Horning, M. G.; Hornlng, E. C. Anal. Chem. 1974, 46, 6. Proctor, C. J.; Todd, J. F. J. Anal. Chem. 1984, 56, 1794. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54, 1546. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1983, 55, 867. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1983, 55, 1486. Eiceman. G. A., Anderson, G.; Leasure. C. S.; Vandiver, V. J.; Tiee, J.; Danen, W. Anal. Chem. 1986, 58, 1690. Mattern, D. E.; Lin, F.;Hercules, D. M. Anal. Chem. 1984, 56, 2762. Tabet, J.; Cotter, R. J. Anal. Chem. 1984, 56, 1662. Subra Rao, S. C.; Fenselau, C. Anal. Chem. 1973, 50, 51 1. Slmonsick, W. J.; Hltes, R. A. Anal. Chem. 1984, 56, 2749. Lane, D.; Sakuma, T.; Kuan, E. Polynuclear Aromatic Hydrocarbons; International Symposium on Chemical and Biological Effects, 4th ed.; Batelle Press: Columbus, OH, 1980, p 199. Horning, E. C.; Carroll, D. I.; Dzidic, I.; Haegaie, K. D.; Horning, M. G.; Stillwell, R. N. Adv. Mass Specfrom. Biochem. Med. 1978, 7 , 1. Eiceman, G. A.; Leasure, C. S.; Vandiver, V. J.; Rico, G. Anal. Chim. Acta, in press. Baim, M. A.; Eatherton, R. L.; Hill, H. H. Anal. Chem. 1983, 55, 1761. Lubman, D. Anal. Chem. 1984, 56, 1298. Poole, C. S.; Zlatkis, A. J. Chromafogr. Lib. 1981, 20, 23. Driscoll, J. N. J. Chromatogr. 1977, 134, 49.
G . A. Eiceman* V. J. Vandiver
RECEIVED for review November 21,1985. Accepted April 22, 1986.
Excited Triplet State Infrared Spectrometry as an Analytical Technique Sir: Fourier transform infrared spectrometry (FTIR) has been used extensively in the analytical laboratory for the identification of compounds. FTIR, as well as dispersive infrared, has been limited when identification of a single compound from a complex mixture of compounds is required, due to the numerous vibrational bands present. Although FTIR has the capability to perform intricate subtraction routines, it would be helpful to simplify IR spectra for some analytical applications. For a new IR method to be advantageous, it would have to limit the number of absorption bands 0003-2700/66/0358-2335$0 1.50/0
to a handful. A new method, which we have labeled as excited triplet state infrared spectrometry (ETSIR), simplifies complex IR spectra so that only a few bands appear, allowing for unequivocal identification of components within a mixture. The method relies on creating a steady-state population of molecules in their triplet state. Upon continuous wave (CW) excitation of a frozen, dilute solution, intramolecular photophysical processes (in which the excitation rate and the triplet decay rate constitute the critical dynamics) establish a substantial fraction of excited triplet molecules, relative to those 0 1986 American Chemical Society
2336
ANALYTICAL CHEMISTRY, VOL. 58, NO. 11, SEPTEMBER 1986
Optical Bench
I
Sample Compartment
I Y N -
u
2-
U V Lamp
Figure 1. Schematic dlagram of the optical bench for the ETSIR experiment. Abbreviations are as follows: M, reflective mlrror; K, potasslum bromide optlcai wlndows; S,sample cell; SH, shutter; QLA, quartz lens assembly; N, nitrogen inlet.
in the ground state. The vibrational potential energy when excited may be different from that in the ground state, leading to possible shifts in vibrational energy absorptions. To the best of our knowledge, ETSIR has not been used as an analytical tool. Investigators have identified new infrared bands in the excited triplet state for a number of molecules: naphthalene (I),naphthalene-d8 (2),phenazine (3),acridine (3), and triphenylene ( 4 ) . Only one of these ( 4 ) has put to use the advantages of FTIR. In this paper we report utilization of the increased sensitivity and capacity for data massaging provided by FTIR to identify a mixture of naphthalene and its deuterated analogue. The resultant ETSIR spectrum contains only one paired set (a positive and negative) of vibrational bands per compound. This spectrum is obtained by ratioing the ground state spectrum of the naphthalene mixture against the excited triplet state spectrum for the same.
EXPERIMENTAL SECTION Naphthalene and naphthalenedBwere obtained from Aldrich Chemicals. The deuterated sample (>98% deuteration) was used without further purification. Naphthalene was subjected to more than 200 passes through a zone refiner to remove an impurity detected via the emission spectrum (phosphorescencespectra were used as a final check for purity). Nujol (Aldrich)was used without further purification. The naphthalene-h8-d8 mixture was a combination of two separate 0.1 M solutions in Nujol. The sample solution was pipetted into a KBr infrared cell with a path length of 0.2 mm provided by a lead spacer. This assembly was then attached to a cold finger of a conduction type cryostat and cooled down to -90 K with liquid nitrogen. The sample cell was positioned in the infrared beam of a Nicolet 6000 series Fourier transform infrared spectrophotometer. Ultraviolet (UV) irradiation was provided by an air-cooled 200-W J3g arc lamp (Oriel Model 6143). The UV radiation was filtered with a CoS04 solution (10 cm path length) and controlled by an electromechanical shutter (ILEX No. 5 Synchro electronicshutter). The UV lamp was positioned perpendicular to the IR beam; UV radiation was then focused, by reflection from a mirror, onto the sample cell. Multiple scans (256) were taken at 0.5 cm-' resolution with the UV source on, as well as shuttered. The sample compartment in the spectrophotometer was constantly purged with nitrogen gas, as well as isolated, in order to minimize frosting of the KBr windows. A quartz lens assembly was attached t o the Nicolet optical bench to focus the ultraviolet radiation and contain the nitrogen atmosphere. (See Figure 1.) RESULTS AND DISCUSSION In order for ETSIR to be utilized in the analytical laboratory, an appreciable steady-state population in the triplet
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Figure 2. Infrared spectrum of a naphthalene/naphthal8 mixture
in a KBr pellet at 1.0-cm-' resolution.
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Figure 3. ETSIR spectrum of a naphthalenelnaphthalene-d, mixture in a Nujol solution at 0.5-cm-' resolution. The 740-715 cm-' region is blanked because of over absorption of Nujoi.
state must exist for the particular molecule of interest. This population determines the absorption value of the new vibrational band; therefore an efficient pumping system is necessary to maximize this value. The triplet state lifetime also indirectly affects the ETSIR signal; in our case, naphthalene and naphthalene-& exhibited fairly long lifetimes of 2.2 and 18.0 s, respectively (5). For reference purposes, the 1800-400 cm-' region for a mixture of naphthalene and naphthalene& in a KBr pellet is shown in Figure 2. Due to the numerous bands, it is difficult to identify one specificcompound from this spectrum, even with subtraction and library search routines. Groundstate bands at 958, 782, and 481 cm-' have been assigned to naphthalene (6). Prominent naphthalenedBbands are present at 877, 790, and 630 cm-' (6, 7). Figure 3 displays the triplet state spectrum ratioed against the ground state spectrum of the mixture in a Nujol solution. The 740-715 cm-' region of the spectrum has been omitted because upon ratioing, the over-absorption of Nujol at -720 cm-' results in an inconsistent line shape. The upward peaks are due to the appearance of new vibrations when molecules are excited into their triplet state. The modes whose frequencies have shifted show downward peaks since a small fraction of the overall collection of molecules has been pro-
Anal. Chem. 1986, 58, 2337-2340
moted out of the ground state. The peak at 681 cm-' represents a new vibration produced by the triplet state of naphthalene and corresponds to Nishikida's results (I). The upward peak at 535 cm-' arises from the triplet state of naphthalene-de and correlates with Clarke's work (2). The downward peaks at 782 and 630 cm-' can be assigned to the bSuout-of-plane mode for naphthalene and naphthalene-d8, respectively (6, 3, and reflect the origins of the newly created absorptions at 681 and 535 cm-'. The ratioed spectrum then leads to the unambiguous assignment that naphthalene and naphthalene-de are present in the mixture. Clearly this simplified spectrum presents an advantage over that shown in Figure 2. The absorbance of the exicted triplet state bands depends upon the steady-state population of the molecules in the triplet state. To obtain a spectrum, usually files are ratioed against a background and then converted to an absorbance spectrum. However, due to the capabilities of our FTIR, we may obtain a final absorption spectrum by ratioing the triplet state file against the ground state file. This produces an absorbance spectrum with upward and downward peaks as seen in Figure 3. The ratioing and subtraction methods produce the same absorbance values for specific vibrational peaks (assuming the cell path length and molar absorptivity are constant). This is as expected since Subtraction:
A = [-log 10-(T1)]- [-log 10-(sO)]
(1)
Ratioing :
A = -log ~O(TI/SO) = [-log 10-(T1)]- [-log 1O-(s0)] (2) While the absorbance values are the same, the ratioing technique is preferred. This method eliminates ratioing data files against a background file, and the accompanying timeconsuming FT processing. A single beam instrument, such as the Nicolet 6000 FTIR, lends itself better to this form of spectrometry. For dual beam instruments, precision matched cells and dual cryogenic apparatus would be necessary for properly attained ETSIR spectra. The relative concentrations of molecules in the triplet state with our particular excitation conditions may be calculated from the ETSIR and ground state spectra using the following equation:
2337
% triplet concn = (area TIband)/(area So band) X 100 (3) with the ground-statepeak area being measured when the UV radiation is shuttered. Since the ground state and triplet state peaks differ in line width (Av = 2.0 cm-' for So;Av = 4.5 cm-' for Tl), the integrated intensities are used for this calculation instead of the absorbance at the peak maximum. With our results, this leads to a relative triplet state concentration of approximately 2% for both the de and he naphthalene isomers. In summary, once triplet-accessible aromatic systems have been assigned triplet state infrared bands, ETSIR may be used to identify these specific molecules from mixtures of unknown composition with increased simplicity. The method lends itself particularly well to single-beam FTIR instruments because computer capabilities allow for ratioing of data files instead of subtraction, leading to shorter experimental time. Future work involves assignment of triplet state peaks for additional organic molecules and quantum mechanical calculations to correlate shifts of the triplet state infrared bands. Registry No. Naphthalene,91-20-3;naphthalene-de,1146-65-2. LITERATURE CITED (1) Nishkida, K.; Kamura, Y.; Seki, K.: Iwasaki, N.; Kinoshita, M. Mol. phvs. 1983, 49, 1505-1507. (2) Clarke, R. H.;Kosen, P. A,; Lowe, M. A.; Mann, R. ti.: Mushlin, R. J . Chem. Soc., Chem. Commun. 1973, 528-529. (3) Mitchell, M. B.; Smlth, 0. R.; Gulllory, W. A. J . Chem. phvs. 1981, 75, 44-48. (4) Baiardo. J.: Mukherjee. R.: Vala, M. J . Mol. Stroct. 1982, 8 0 , 109- 112. ( 5 ) de Groot, M. S.; van der Waals, J. H. Mol. phvs. 1961, 4 , 189-190. (6) Scully, D. B.; Whiffen, D. H. Spectrochim. Acta 1960, 16, 1409-1415. (7) Wee, A.; Kydd, R. A. Spectrochim. Acta, part A 1989, 2 6 A , 1791-1803.
David E. Bugay Willem R. Leenstra* Department of Chemistry University of Vermont Burlington, Vermont 05405 RECEIVED for review November 22,1985. Accepted April 28, 1986. This work was supported by grants from the Research Corporation (9326), the donors of the Petroleum Research Fund, administered by the American Chemical Society (13295-G6), and the University of Vermont (UVM PS-11).
Indirect Electrochemical Detection in Liquid Chromatography Sir: Considerable interest has recently been given to the development of so-called indirect detection approaches for use in liquid chromatography (1-5). In these systems, a suitable concentration of a species that can be readily monitored by a conventional detection scheme such as W-visible absorption or fluorescence is intentionally added to the mobile phase to generate a constant background signal. If the elution of sample components is accompanied by a displacement of the additive from the mobile phase as observed at the detector, a transitory decrease in the background level is thereby produced. The analyte species is thus monitored indirectly as a negative peak or trough in the steady-state absorbance or fluorescence signal. The primary advantage of this approach lies in the capability that it affords for the detection and quantitation of species which themselves possess no 0003-2700/86/0358-2337$01.50/0
strongly absorbing chromophore or other group readily lending itself to direct monitoring by one of the usual detection modes. Refractive index detection, utilizing measurement of the change in refractive index of the eluent plus analyte compared to that of the eluent alone, represents a familiar example of the indirect detection approach. Recently, schemes utilizing absorbance ( I ) , polarimetry (2,3), and fluorescence (4) have also been successfullydemonstrated. However, the possibility of indirect amperometric detection of nonelectroactive analin an eluent containing an easily oxidizable additive has not yet been seriously considered for liquid chromatography. (Very recently, the use of such an approach for the determination of volatile hydrocarbons following gas chromatography (6)has been described.) Although direct electrochemical detection of analytes following liquid chromatography (LCEC) 0 1986 Amerlcan Chemlcal Soclety
