Anal. Chem. 1984, 56. 019-821
819
vantageous method for the direct analysis of solids by mass spectrometry. ACKNOWLEDGMENT We are grateful to the Department of Energy, Division of Chemical Sciences, for research support. Registry No. 56Fe+,51377-81-2; 64Fe, 13982-24-6; 66Fe, 14093-02-8; 57Fe,14762-69-7; Ar, 7440-37-1. LITERATURE CITED
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mle Flgure 4. Mass spectrum of an iron cathode in an argon glow discharge showing the iron isotopes at masses 54, 56, and 57. Peak at mass 40 may result from imperfect background subtraction of ‘OAr*.
compared to the same interval from the glow discharge. Copper ionization is an interesting example of an evidently non-RIMS process. The two major ground-state originating lines for copper are at 324.7 and 327.4 nm. A wavelength ionization spectrum taken over this region shows strong copper ion formation a t each of these two lines plus several other weaker transitions. A mass spectrum at 324.7 nm reveals only the copper isotopes at masses 63 and 65. However, the absorption of two photons of this energy will not promote the electron into the ionization continuum. A two-photon absorption would bring the electron only very close to ionization, requiring supplemental energy from the plasma to then cause ionization. Alternatively, the mechanism may be similar to that proposed for laser-enhanced ionization in flames (8) wherein a single photon is absorbed, followed by collisional energy transfer to effect ionization. We are presently seeking additional information in other similar experiments. Whether by direct resonant ionization or laser enhanced ionization, the result is a selective mass spectrum which features only isotopes of the selected element. The glow discharge offers possibilities for both RIMS and LEI with its various internal excitation possibilities. The combination of a tunable dye laser with the glow discharge can yield an ad-
(1) Young, J. P.; Hurst, G. S.; Kramer, S. D.; Payne, M. G. Anal. Chem. 1070, 5 1 , 1050A. (2) Donahue, D. L.; Young, J. P.; Smith, D. H. Inf. J. Mass Specfrom. Ion Phys. 1982, 43,293. (3) Donahue, D. L.; Young, J. P. Anal. Chem. 1983, 55, 378. (4) King, D. S.; Schenck, P. K.; Smyth, K. C.; Travis, J. C. Appl. Opt. 1077, 10, 2617. (5) Green, R. B.; Keller, R. A.; Luther, G. G.; Schenck, P. K.; Travis, J. C. Appl. fhys. Lett. 1078, 2 9 , 727. (6) Green, R. B.; Keiier, R. A.; Schenck, P. K.; Travis, J. C.; Luther, G. G. J . Am. Chem. SOC. 1078, 9 8 , 8517. (7) Turk, G. C.;DeVoe, J. R.; Travis, J. C.Anal. Chem. 1982, 5 4 , 643. (8) Travis, J. C.; Turk, G. C.; Green, R. E. Anal. Chem. 1982, 5 4 , 1006A. (9) Gonchakov, A. S.; Zorov, N. B.; Kuyzyakov, Y. Y.; Matveev, I. 0. J. Anal. Chem. USSR (Eng. Trans/.) 1980,34, 1792. (10) Winograd, N.; Baxter, J. P.; Kimock, K. M. Chem. fhys. Left. 1082, 8 8 , 581. (11) Harrison, W. W.; Magee, C. W. Anal. Chem. 1974, 48, 461. (12) Daughtrey, E. H.; Donahue, D. L.; Sievin, P. J.; Harrison, W. W. Anal. Chem. 1075, 47, 683. (13) Bruhn, C.G.; Harrison, W. W. Anal. Chem. 1978, 5 0 , 16. (14) Bentz, B. L.; Bruhn, C. G.; Harrison, W. W. Int. J. Mass Specfrom. Ion Phys. 1078, 2 8 , 409. (15) Mattson, W. A.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1978, 4 8 , 489. (16) Harrison, W. W.; Bentz, E. L. Anal. Chem. 1070, 5 1 , 1855. (17) Loving, T. J.; Harrison, W. W. Anal. Chem. 1983, 54, 1523. (18) Gerhard, W.; Oechsner, H. Z . Phys. 8 1075, 2 2 , 41. (19) Waish, A. Spectrochim. Acta, Part 8 1980, 358,639. (20) Perkin-Elmer Corporation Manual: “Analytical Methods for Atomic Absorption Spectrophotometry”; Perkin-Elmer Corp.: Norwaik, CT, 1982.
P. J. Savickas K. R. Hess R. K. Marcus W. W. Harrison* Department of Chemistry University of Virginia Charlottesville, Virginia 22901 RECEIVED for review November 28, 1983. Accepted January 16, 1984.
Zeroth and Second Derivative Fluorescence and Phosphorescence Analysis of Mixtures of Hydroxyl Aromatics Adsorbed on Filter Paper Sir: Recent research activity in the area of luminescence from compounds adsorbed on solid supports has centered around the development of room-temperature phosphorescence (RTP) as an analytical technique. Much of this work is summarized in recent reviews (1-5). Filter paper has been the most widely used adsorbent for inducing RTP from aromatic molecules. In addition, considerable work has been reported on room-temperature fluorescence (RTF) analysis of organic compounds adsorbed on solid surfaces (I). However, there has been only one report of the combined use of solid-surface RTF and R T P for qualitative analysis (6). One limitation in the use of solid surface RTP and RTF for qualitative analysis arises from the fact that the spectra usually have broad bands. For this reason, the identification of individual compounds by examination of the luminescence
spectra from a mixture can sometimes be difficult. This is especially true for compounds whose spectra overlap extensively. In this study, the combined information of RTP excitation and emission and RTF emission spectra was used for qualitative analysis. Both zeroth and second derivative RTP and RTF spectrometry were used in the work. EXPERIMENTAL SECTION Apparatus. Luminescence excitation and emission spectra were obtained with a Farrand MK-2 spectrofluorimeter, fitted with a phosphorescence rotary chopper. Source radiation was provided by a 150-W Xe lamp and the detector was a R928 photomultiplier tube (Hamamatsu Corp., Middlesex, NJ). For RTP measurements, metal slits giving a bandwidth of 10 nm were used at the entrance and exit positions of both the excitation and emission monochromators. For fluorescence measurements, 10-nm 4
0003-2700/84/0356-0819$01.50/0
0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984 RTP
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slits were used in the excitation monochromator and 2.5-nm slits in the emission monochromator. Filter paper samples were held in a sample holder which was described previously (7). A Bascom-Turner Model 4120 data handling system with floppy disk data storage capabilities was connected to the Farrand MK-2 for electronically calculating second derivative luminescence spectra, Reagents. Water and absolute ethanol were purified by distillation. Sodium chloride was reagent grade and filter paper was Whatman No 1. Both were used as received. 1-Naphthol, 2naphthol, 1,7-dihydroxynaphthalene,and 4-phenylphenol were purchased from Aldrich Chemical Co. (Milwaukee,WI) and were recrystallized from absolute ethanol. Procedures. Luminescence Measurements. Luminescent compounds were adsorbed onto 3/16 in. filter paper circles from solutions composed of 50:50 ethanol-water and containing 60 mg/mL NaC1. Adsorbing NaCl along with luminescent compounds onto filter paper has been shown to enhance RTP intensity from the compounds (8). A total of 1.0-1.5 WLwas spotted in each case, containing between 100 and 300 ng of each compound and 60-100 l g of NaCI. The smaller amounts of compound were used when mixtures were being examined. The filter paper with sample was dried in an oven at 90 "C for 15 min before luminescence measurements. Luminescence spectra obtained with the Farrand MK-2 were stored in the Bascom-Turner recorder and second derivatives were then calculated and plotted. Second Derivative Spectra. The quality of the second derivative spectrum was found to be quite dependent on the magnitude of the x axis length over which the zeroth derivative spectrum was recorded. This has been discussed by Vo-Dinh and Gammage (9). The various luminescence spectra were originally recorded over 100% of the x axis scale of the Bascom-Turner X-Y chart paper, which resulted in quite broad bands. Second derivative spectra calculated over the entire x axis scale were not satisfactory since the second derivative mode does not enhance broad bands as much as narrower bands (9). By compressing the zeroth derivative luminescence spectra to approximately 20% of the Bascom-Turner x axis scale for each 100 nm scanned and then calculating and plotting the second derivative spectra, we obtained the most satisfactory spectra. All RTP excitation spectra were scanned from 250 to 370 nm with the emission wavelength set at 500 nm. The Bascom-Turner sampling rate for RTP excitation spectra was 290 ms/point, for 500 points. All RTF emission spectra were scanned from 310 to 410 nm with the excitation wavelength set at 290 nm. The Bascom-Turner sampling rate for RTF emission spectra was 120 ms/point, for 500 points. All RTP emission spectra were scanned from 425 to 625 nm with the excitation wavelength set at 300 nm. The Bascom-Turner sampling rate for RTP emission spectra was 240 ms/point, for 500 points. Scan rates were 100 nm/min for emission spectra and 50 nm/min for excitation spectra. Luminescence spectra were smoothed twice before calculating the second derivatives. The spectra were smoothed by averaging over a moving group of five points. Each Y value was replaced by the weighted average of itself and the two preceding and two following data points, as described in the Bascom-Turner 4120 manual.
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Figure 2. Correlation of the Wavelength positions of second derivative luminescence bands for a l-naphthol/2-naphthoI/1,7-dihydroxynaphthalene mixture with the wavelength positions of 1-naphthol (1NAP), 2-naphthol (2-NAP), and 1,7dihydroxynaphthalene(1,7-DHNAP) standards: ( 0 )negative bands; (0')positive bands.
RESULTS AND DISCUSSION Zeroth and second derivative RTP excitation, RTF emission, and RTP emission spectra for a mixture containing 100 ng each of 1-naphthol, 2-naphthol, and l,7-dihydroxynaphthalene adsorbed on filter paper are shown in Figure 1. The various spectra for the components in the mixture overlapped extensively. By definition, a maximum in a zeroth derivative spectrum corresponds to a minimum extreme value, or negative band, in the second derivative spectrum (IO). Second derivative spectra also contain positive bands. In this work, both negative and positive bands in the second derivative spectra were used for identification purposes because of the greater amount of information obtained. Figure 2 correlates the positions of the second derivative luminescence bands of the l-naphthol/2-naphthol/l,7-dihydroxynaphthalene mixture spectra with the second derivative bands of the individual components. An identification of a band was made if a mixture second derivative band was within h2.5 nm of a band of an individual component and if the band was properly oriented up or down. The reproducibility of the band positions of the second derivative luminescence spectra was found experimentally to be less than f2.0 nm. The correlation limit of h2.5 nm was chosen empirically by taking into account the reproducibility of the second derivative bands and the fact that shifts in the positions of second derivative bands can occur. Griffiths et al. (11)have discussed the occurrence of shifts of even-order derivative bands caused by overlap of adjacent bands. The amount of the shift of the derivative bands cannot be generally predicted since each combination of bands and the extent of their overlap is unique (11). For three components at a 1:1:1 ratio the approximate limit of detection for each component would be 1 ng. It can be seen in Figure 2 (reading from left to right) that the mixture second derivative RTP excitation spectrum contains five wavelength positions that correspond only to 2-naphthol, clearly indicating its presence. Second derivative
Anal. Chem. 1084, 56,821-823
wavelength positions corresponding to the other two components are also seen in the excitation spectrum, with one wavelength each for 1,7-dihydroxynaphthaleneand 1-naphthol (Figure 2). The mixture second derivative RTF emission spectrum indicated the presence of 1,7-dihydroxynaphthalene, with three bands corresponding to that compound. However, one wavelength position corresponded to 1-naphthol, and there were no correlations for 2-naphthol. The mixture second derivative RTP emission spectrum is characteristic of the most intense RTP emitter of the three, 1-naphthol, with five bands corresponding to that compound. Also, three bands corresponding to 1,7-dihydroxynaphthalenewere indicated in the short wavelength region of the spectrum of Figure 2, but none was indicated for 2-naphthol. With this three-component mixture, the advantage of the second derivative technique for providing additional spectral information compared to conventional luminescence spectrometry is quite apparent because considerably more data for identification is available. In Figure 2, five of the mixture second derivative RTP excitation and six of the RTF emission bands corresponded to bands of two or three of the individual standards. This overlap of second derivative bands of the mixture with several of the standard bands is the result of very similar features in the zeroth derivative spectra of the individual components. Also, a few uncorrelated bands appear for the various mixture second derivative spectra (Figure 2). There can be several reasons for the appearance of uncorrelated bands in second derivative mixture spectra. The appearance of wings or combinations of wings can account for some of the uncorrelated bands. Griffiths et al. (11)have noted that the absence of a second derivative band can be caused by the overlap of a positive wing of one band with an adjacent negative band. Another possible cause for uncorrelated bands occurs with the combination of two or more overlapping zeroth derivative luminescence bands yielding a new zeroth derivative band which is between the positions of the original bands. The second derivative band would then indicate the position of this new band. Because of the appearance of a few uncorrelated bands, there is some ambiguity about the presence of only three components, but there is no ambiguity about the identification of the three components. Two other three-component mixtures containing 1naphthol/2-naphthol/4-phenylphenoland 2-naphthol/l,7dihydroxynaphthalene/4-phenylphenolwere also examined by the zeroth and second derivative luminescence technique. In these cases, each component was identified by the various derivative luminescence spectra. Several two-component mixtures were examined by the zeroth and second derivative luminescence technique, with 150 ng of each component adsorbed can filter paper. The two-component mixtures examined included l-naphthol/2naphthol, l-naphthol/4-phenylphenol,l-naphthol/l,7-dihydroxynaphthalene, 2-naphtholf4-phenylpheno1, 2naphthol/ 1,7-dihydroxynaphthalene,and 4-phenylphenolf 1,7-dihydroxynaphthalene. Two two-component mixtures containing 4-phenylphenol and 1,7-dihydroxynaphthalene,
021
where one component was in 10-fold excess (300 ng and 30 ng), were also examined. For all of the two-component mixtures examined, each component in the mixture showed characteristic second derivative bands which clearly indicated its presence. A four-component mixture containing 100 ng each of 1naphthol, 2-naphthol, 1,7-dihydroxynaphthalene,and 4phenylphenol adsorbed on filter paper was examined by the zeroth and second derivative technique. Several characteristic bands in the derivative spectra appeared for 1-naphthol, 2naphthol, and 1,7-dihydroxynaphthalene,which indicated their presence. The bands that appeared for 4-phenylphenol, however, overlapped other component bands in each case. The presence of 4-phenylphenol was therefore not directly indicated. This study has shown that by using RTP excitation and emission and RTF emission zeroth and second derivative spectra from compounds with very similar spectral characteristics, identification of several components in various mixtures can be accomplished. The additional spectral information provided by using both phosphorescence and fluorescence spectra facilitated the identification of mixtures. The advantages of solid surface luminescence analysis, namely, good sensitivity and selectivity, small sample size, and ease of sample handling, coupled with the second derivative technique, make it a valuable method for trace organic analysis of simple mixtures.
ACKNOWLEDGMENT Financial support for this project was provided by the Department of Energy, Division of Basic Energy Sciences, Contract No. DE-AC02-80ER10624. Registry No. 1-Naphthol, 90-15-3; 2-naphthol, 135-19-3; 1,7-dihydroxynaphthalene,575-38-2; 4-phenylphenol, 92-69-3. LITERATURE CITED (1) Hurtubise, R. J. "Solld Surface Luminescence Analysis: Theory, Instrumentatlon, Applications"; Marcel Dekker: New York, 1981. (2) Parker, R. T.; Freedlander. R. S.; Dunlap, R. B. Anal. Chim. Acta 1980, 120, 1. ( 3 ) Parker, R. T.; Freedlander, R. S.; Dunlap, R. B. Anal. Chim. Acta 1980, 719, 189. (4) Ward, J. L.; Walden, G. L.; Wlnefordner, J. D. Talanta 1981, 2 8 , 201. (5) Vo-Dinh, T.; Winefordner, J. D. Appl. Specfrosc. Rev. 1977, 13, 261. (6) Ford, C. D.; Hurtublse, R. J. Anal. Left. 1080, 13 (A6), 485. ( 7 ) Dalterio, R. A.; Hurtublse, R. J. Anal. Chem., in press. (8) Dalterio, R. A.; Hurtubise, R. J. Anal. Chem. 1982, 5 4 , 224. (9) Vo-Dinh, T.; Gammage, R. B. Anal. Chim. Acta 1979, 707, 281. (IO) Talsky, G.; Mayring, L.; Kreuzer, H. Angew. Chem., I n t . Ed. Engl. 1978, 17, 785. (11) Griffiths, T. R.; King, K.; St. A. Hubbard, H. V.; Schwing-Welll, M. J.; Meullemeestre, J. Anal. Chim. Acta 1982, 743, 163.
R. A. Dalterio R. J. Hurtubise* Chemistry Department The University of Wyoming Laramie, Wyoming 82071
RECEIVED for review October 13,1983. Accepted December 23, 1983.
Photoacoustic Spectroscopy of Matrix-Isolated Polycyclic Aromatic Compounds Sir: Polycyclic aromatic compounds (PACs) are pervasive in our environment. Many are known or suspected carcinogens, or precursors to carcinogenic metabolites ( I ) . The principal anthropogenic source of environmental PACs is the
incomplete combustion of organic matter, primarily fossil fuels (2). The detection and quantification of these toxic compounds in environmental matrices have become high-priority analytical problems. Because many PACs fluoresce with high
0003-2700/84/0356-0821$01.50/00 1984 American Chemical Society
