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Anal. Chem. 1986, 58, 1294-1298
+200-V bias applied to the secondary surface most sputtered positive silver ions are repelled, thus allowing only neutral and negatively charged silver species to reach the secondary surface. There is no difference in SERS behavior of samples prepared in this manner. However, the behavior was found to be sensitive to the proximity of the secondary to the primary surface. This point is under further investigation. In addition to being stable in air for several weeks, samples prepared by sputter deposition are reusable. When a sample is removed following a SERS experiment, it can be reimmersed soon thereafter in the same solution without loss of activity. Typically a fresh growth in the SERS signals occurs, as with a virgin surface. The ability to reuse samples is not confined exclusively to studies of the same adsorbate. For example, it is possible to obtain a SER spectrum of thiocyanate in solution, remove the sample, rinse it with water, and obtain an intense spectrum of adsorbed pyridine in a different solution, the thiocyanate SERS bands having disappeared entirely. These sputter-deposited silver surfaces can also be employed to detect SERS for adsorbates at silver-gas interfaces. For example, a strong SERS band at 255 cm-l is obtained for adsorbed chlorine formed by exposing the surface to gaseous dichlorethane (15). Adsorbed atomic oxygen has also been detected in this manner (15). The SERS activity persisted for at least 2 days in the gas-phase environment. Most importantly, these substrates provide robust SERS active silver surfaces that can be stored in air for extensive periods, and then activated in either electrochemical or gasphase environments simply by laser irradiation. Although the mechanism controlling their formation is not clear at present, a number of practical applications of these materials-for example, in situ monitoring of heterogeneous catalytic systems-can be envisioned. It will also be of interest to ascertain if materials displaying similarly robust SERS behavior can be prepared by sputter deposition of gold and copper. The former is of particular interest in view of the remarkably stable and intense SERS recently obtained for
a variety of adsorbates at electrochemically roughened gold electrodes (16).
ACKNOWLEDGMENT Experimental Raman expertise was provided by Mary Patterson and Dave Gosztola. The scanning electron micrographs were obtained by L. D. McCabe. Registry No. Ag, 7440-22-4; NH,SCN, 1762-95-4; SCN-, 302-04-5.
LITERATURE CITED (1) Jeanmaire, D. L.; Van Duyne, R. P. J . Nectroanal. Chem. 1977,8 4 ,
1. (2) Rowe, J. E.; Shark, C. V.; Zwemer, D. A.; Murray, C. A. Phys. Rev. Lett. 1~80, 4 4 , 1770. (3) Liao, P. F.; Bergman, J. G.; Chemia, D. S.; Wokaun, A.; Melngailis, J.; Hawryluk, A. M.; Economou, N. P. Chem. Phys. Lett. 1981,8 2 , 355. (4) LaPack, M. A.; Pachuta, S.J.; Busch, K. L.; Cooks, R. G. Int. J . M s s Spectrom. Ion Phys. 1983,53, 323. (5) Tadayyoni, M. A.; Farquharson, S.;Li, T. T-T.; Weaver, M. J. J . Phys. Chem. 1984. 88. 4701. (6) Weaver, M. J.; Ban, F.; Gordon, J. G., 11; Phiipott, M. R. Surf. Sci. 1983. 125. 409. (7) Surfice-Ehanced Raman Scattering ; Chang, Richard K., Furtak, Thomas E., Eds.; Plenum: New York and London, 1982; pp 297, 365. (8) Chen, T. T.; Van Raben, K. V.; Owen, J. F.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. lS82, 9 1 , 494. (9) Brandt, E. S.Anal. Chem. 1985,5 7 , 1276. (10) Barz, F.; Gordon, J. G., 11; Philpott, M. R.; Weaver, M. J. Chem. Phys. Lett. 1982,9 1 , 291. (11) Macomber, S. H.; Furtak, T. E.; Devine, T. M. Chem. Phys. Lett. 1982, 9 0 , 439. (12) Chang, R. K.; Laube, B. L. Crlt. Rev. Solid State Mater. Sci. 1984, 12, 1. (13) McCracken, G. M. Rep. Prog. Phys. 1975,38, 241. (14) Hupp, J. T.; Larkin, D.; Weaver, M. J. Surf. Sci. 1983, 725, 429. (15) Nowobilski, P. J.; Patterson, M. L.; Davies, J. P., unpublished results, Purdue University, June 1985. (16) Gao, P.; Patterson, M. L.; Tadayyoni, M. A,; Weaver, M. J. Langmuir 1985, 1 , 173.
RECEIVED for review September 3,1985. Accepted January 27, 1986. This work was supported by the NSF Materials Research Laboratory at Purdue University (R.G.C., M.J.W.) and the Air Force Office of Scientific Research (M.J.W.). S.J.P. received support from the Dow Chemical Co. as part of the Purdue Industrial Associates Program.
Ambient Pressure Laser Mass Spectrometry of Organophosphorus Pesticides on Plant Tissues John J. Morelli and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Thin Formvar films were used to Isolate plant tissues from the vacuum of a LAMMA-1000 laser mass spectrometer. This technique allowed mass spectra to be obtained from pestlclde residues on plant surfaces. The spectra contained structural Information relevant to the specles of Interest. The Formvar film and leaf matrlx gave relatively clean background spectra, minlmlzing possible interferences from matrixgenerated ions. Spectra of chlorpyrifos obtalned under normal vacuum conditions were compared to spectra obtained under near ambient conditions uslng thln polymer films. Differences observed in the spectra were prlmarlly due to ions resutting from the additlon of water molecules in the latter.
Obtaining mass spectra of thermally labile and/or vacuum-sensitive materials is a difficult problem. Attempting laser microprobe analysis of such materials in matrices containing
high-vapor-pressure components presents additional difficulties. Although high-vapor-pressurespecies can be removed by freeze-drying, such operations may change the spatial distribution of components or remove important contaminants having moderate volatility. Until recently, atmospheric pressure ionization (1) and liquid ion evaporation (2) have been the only mass spectrometric techniques that allowed analysis of samples at atmospheric pressure. These techniques, however, require gaseous samples or samples in solution. Indeed, there has not been any method capable of obtaining mass spectra of condensed, thermally labile samples at atmospheric pressure. Several commercially available mass spectrometers have the ability to acquire mass spectral information from solids without prevaporization (3-8); of these techniques, the LAMMA-1000 laser mass spectrometer has shown enormous potential for microprobe analysis of bulk materials (9-12). Recently, a thin polymer film (13)was used as a vacuum seal
0003-2700/86/0356-1294$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58,
between the LAMMA-500 laser microprobe and the sample, permitting the ionizing laser to penetrate the film; thus, mass spectra were consistently obtained from condensed, thermally labile samples at atmospheric pressure. The recent development of thin polymer seals by Holm et al. presents laser mass spectrometry (LMS) as one of the newest techniques capable of atmospheric mass spectral analysis, comparable to the established atmospheric pressure ionization and liquid ion evaporation techniques. The geometric configuration of the LAMMA-500 laser, sample, ion optic axis is -90°/900with respect to the sample plane (14);the laser contacts one face of the sample, and ions pass from the opposite face to the extraction lens of the time-of-flight mass spectrometer. This geometry allows a thin Formvar film to act as a barrier between the sample (remaining on the atmospheric side of the seal) and the operating vacuum of the LAMMA-500 instrument. Its geometry also limits the LAMMA-500 to studies of thin sections. The configuration of the LAMMA-1000 (45O/9Oo laser, sample, ion optic axis) allows bulk analysis. Here, implementing a thin film as a seal cannot be done in the manner prescribed for the LAMMA-BOO -90° /90° geometry because the entire sample must be placed within the vacuum chamber of the LAMMA-1000. Instead, a "balloonlike" seal is necessary, where the greater pressure (atmospheric) is contained by the integrity of a polymer film. The present work adapted the use of thin films for analyses with the LAMMA-1000. A plant section was sandwiched between a Formvar film and Zn foil. This effectively sealed the tissue from the vacuum, allowing the laser to penetrate the film and successfully ionize the sample, essentially under atmospheric conditions.
EXPERIMENTAL SECTION Laser mass spectra were obtained with a Leybold-Heraeus laser microprobe mass analyzer, LAMMA-1000, connected to a Hewlett-Packard HP-1000 data system for data manipulation. The ionization source of the LAMMA-1000 is a Nd-Yag laser having the fundamental wavelength (1060 nm) frequency quadrupled to 265 nm. The resulting beam is Q-switchedto yield a laser pulse of 15 ns half-width. The beam is focused on the sample via a series of lenses to a spot -5 pm in diameter having a power density ranging from lo5to l@W/cm2. The 1.8-m time-of-flight analyzer is followed by a 17-dynode secondary electron multiplier for ion detection. The signal is amplified and fed to a Biomation 8100 transient recorder. The LAMMA-1000 has been described in detail elsewhere (15). The geometry of the LAMMA-1000 requires that the entire sample be placed in a chamber that must reach a base pressure below 2.0 X 10" torr before the instrument will operate (15). Studying volatile matrices using the LAMMA-1000 requires isolation of these matrices; such isolation would allow the sample to be maintained under atmospheric conditions,while the system remains at moderate vacuum. The method of preparing the samples used was as follows: a 0.4% w/w Formvar solution was prepared by dissolving 400 mg of poly(vinylforma1)in 100 mL of 1,l-dichloroethane. One side of a standard glass slide employed in optical microscopy was then coated by dipping the slide into about 20 mL of this solution. The slide was allowed to dry, resulting in the formation of a thin, transparent film of less than 1pm thickness (13). The film was floated on deionized water; a razor was necessary to aid in releasing the Formvar film from the glass slide. The method for preparing these films is common in electron microscopy (16).For this work, the size of the films typically were 1 X 2 cm2. Film thickness was not directly measured; however, the estimate (less than 1pm) was based on previous work (13) and microscopic observation. Samples were placed on the floating film with the doped surface face down against the Formvar. Zinc foil was used to lift the sample and film out of the water. A completed preparation has the sample sandwiched between the Formvar film and the zinc foil, as shown in Figure 1. A proper seal was consistently made by allowing the film to overlap the edges of the foil. All LMS
-
450
-4
NO. 7, JUNE 1986
1295
VACUUM
ions tlnc loll leaf .actLon formrar film atmospberlc pressure
VACU
Figure 1. Schematic diagram of leaf section during laser ionization.
were obtained in regions where the Formvar film remained smooth relative to the leaf surface; this condition was determined via microscopic observation at 250X. Some space between the film and the leaf may exist, but due to the viewing perspective (15) and the depth of field of the microscope, no actual determination of this gap has been made. Leaf sections were doped with CsI or azinphos-methyl (1)by placing a drop of the dopant dissolved in MeOH onto the leaf. CsI and azinphos-methyl solutions were prepared by dissolving approximately 40 mg of the solid in 1 mL of methanol. Five milligrams of NaI was added to the azinphos-methyl solution to increase the production of [M + Na]+ and [M + 11- quasi-molecular species.
1
2
Grass samples were treated with a commercial formulation of Dursban. The undiluted formulation contains 13.5% w/w chlorpyrifos (2) in xylenes. Directions supplied on the bottle label were followed in applying the pesticide to grass; this involved diluting 1 mL of Dursban to -250 mL with tap water. The solution was then deposited on grass using an ordinary household plant mister. The Formvar film employed for analysis was predoped with a drop of 5 mg/mL NaI in methanol to increase the production of quasi-molecular [M + Na]' and [M + I]- ions. Azinphos-methyl was obtained from Chem Service. The reported purity of 98% was confirmed by using 300-MHz proton NMR. Leaf and grass samples were obtained from local sites; these were cut to
.-+
-
314
I
ZE 'CJ@ CI
fragment
mJz
169 198 204 260 314 350 372 378 278 368 386 404 422 458
[M -Z]+
[ZO + 2Hl+ [Z Na + HI+ [M + H - 2Et0]+ [M-C1]+ [ M + H]+ [M + Na]+ [M + CzHc]+ [M + H - 2Et0 + Hzol+ [M + H + H,O]+ [M + H + 2HzO]+ [M + H + 3HzO]+ [M + H + 4HzO]+ [M + H + 6HzOI'
+
CI
Woter Addition Ions
chlorpyrifos chlorpyrifos chlorpyrifos on Zn on leap on Znc with HzOd 82
200
96 100 56 18 38
84 80 51
68 100 37
15 99
37
13 7 5 8
20 23 18
*
LMS obtained a NaI added to increase production of [M + Na]+. under atmospheric conditions using thin Formvar films, see Figure 5a. LMS obtained under normal vacuum conditions, see Figure 5b. Base peak m/z 64 [Zn]+. dLMS obtained under atmospheric conditions using thin Formvar films. see Figure 5c.
expected for ions containing three chlorine and one sulfur atoms. The fragment ions at mf z 260 and mlz 278 also show this distribution. Table I summarizes the fragment ions observed in Figure 5. The fragments are listed with their corresponding m f z ratios and relative intensities. The first set of ions presented corresponds to fragmentation not involving water addition. The latter ions correspond to those involving
300 m/z
- -
400
500
- -
Flgure 5. Positive ion LMS spectrum of Dursban: (a) on grass blades 0.7 pJ, bulk coverage 4 X with Formvar, laser energy g/cm2; (b) on Zn foil, laser energy 5.0 pJ, bulk coverage 2X g/cm2; (c) on Zn foil with Formvar film and water, laser energy M. 5.0 pJ, solution concentration during analysis - 2 X
-
water addition. These latter ions only occurred in samples containing a significant amount of water and analyzed under atmospheric conditions. Water additions have not been reported previously in LMS. Regarding Figure 5a, there is one last important point to be made; while preliminary results indicate that there is no spatial dependence on the LMS obtained from the leaf, no time-dependency studies were made to determine botanical uptake differences at a cellular scale. Plant tissues were analyzed immediately after treatment; therefore, spatial distributions would be on a macro scale (depending on the droplet size of the plant mister used to apply the pesticide) relative to the areas analyzed in this work (-20 pm). Furthermore, the microareas analyzed were not chosen in a systematic way based on cellular structure. A more rigorous study is needed before any definite conclusion can be made on the microprobe aspects of this experiment. Figure 5b shows the positive ion LMS of the pure analyte, chlorpyrifos, prepared by dissolving 40 mg of the analyte in
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
1 mL of methanol, adding approximately 5 mg of NaI and applying the solution to a Zn foil. The major difference between the spectrum of the pure analyte obtained under vacuum conditions and the spectrum of the leaf sample taken at ambient conditions is the presence of peaks corresponding to the HzO additions in the latter. Furthermore, the grass sample did not exhibit an [M + CzH,]+ ion (mlz 378) a t the laser energies employed. Closer inspection of Figure 5a,b shows one other difference; the peak at m/z 204, which occurs in Figure 5a, is absent from Figure 5b. In place of this there is a peak at mlz 198; this ion has been observed in standard E1 spectra of chlopyrifos obtained from the mass spectral data base at Cornel1 University (17) and is probably due to [ZO + 2H]+. While approximately the same amount of NaI was added to each sample, the presence of [ZH + Na]+ in Figure 5a may be due to an increase in Na+ caused by the Na' content in the Formvar film or in the leaf matrix. Figure 5c shows the positive LMS of chlorpyrifos taken under near-atmospheric pressure on Zn foil covered with Formvar film and with a drop of water added. This spectrum also shows the presence of water addition peaks observed in the chlorpyrifos doped leaf spectrum, Figure 5a. The water addition peaks extend to mlz 458, corresponding to addition of up to six water molecules. Of special interest is the absence of odd water molecule addition ions; however, the absence of these ions in Figure 5c may be misleading because the absolute intensity of this spectrum was significantly lower than the intensity of the spectrum shown in Figure 5a. The base peak of Figure 5a corresponded to about 100 mV, while the base peak of Figure 5c was observed to be only 40 mV. Notice in Figure 5a the [M + 3H20]+is of lower intensity than the even water molecule addition [M 4H20]+. Figure 5c also shows a significant decrease in fragmentation in favor of the formation of the quasi-molecular ion [M + H]+ a t m/z 350 and the water addition peaks. This can be attributed to the increased availability of protons and water molecules from the excess water present during laser ionization. Lastly, in Figure 5c the presence of a [M + Na]+ ion is seen. It is clear that water addition reactions in LMS is a fruitful topic for future investigations. In summary, by using a thin polymer film as an effective seal, laser mass spectra were obtained from a sample under atmospheric conditions. This technique allowed spectra t o be obtained from pesticide residues on a plant leaf. These spectra contain ions relevant to structural features of the analyte of interest. The Formvar film did not significantly contribute to the background of the spectrum, thus minimizing possible interferences.
+
Ionization of chlorpyrifos under atmospheric conditions resulted in a significantly different spectrum when compared to that obtained under normal vacuum conditions. The differences were due primarily to the presence of fragments corresponding to water molecule additions. This preliminary study indicates that samples analyzed under near-atmospheric conditions may yield spectra significantly different from standard spectra obtained in vacuum. Finally, these results indicate that the use of thin films as a vacuum seal for obtaining LMS of species in solution may be applied to enhance quasi-molecular [M + H]+ species while decreasing fragmentation, as seen in the spectra obtained from chlorpyrifos. In effect, this technique expands the range of potential chemical ionization sources for LMS (18). It would, therefore, prove interesting to investigate the LMS of a variety of aqueous and nonaqueous solutions. Registry No. 1, 86-50-0; 2, 2921-88-2.
LITERATURE CITED Dzidic, I.; Caroil, 1.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1975, 47. 1308. Thomson, B. A.; Irlbne, J. V.: Dziedzic, P. J. Anal. Chem. 1982, 5 4 , 2219-2224. Hercules, D. M.; Balasanmugam, K.; Dang, T. A,; Li, C. P. Anal. Chem. 1982, 5 4 , 280A-305A. Heinen, H. J. I n t . J . Mass Spectrom. I o n . Phys 1981, 38, 309-322. Day, R. J.; Unger, S. E.; Cooks, R. G. Anal. Chem. 1980, 5 2 , 557A572A. Bennlnghoven, A.; Sichtermann, W. Org. Mass Spectrom. 1977, 12, 595-597. Barber, M.; Bordoli, R. S.;Sedgewick, R. D.: Tetler, L. W. Org. Mass Spectrom. 1981, 16, 256-260. Surman, D. J.; van den Berg, J. A.; Vlckerman, J. C. S I A , Surf. I n terface Anal. 1982, 4, 160-167. Novak, F. P.; Baiasanmugam, K.; Viswanadham, K.; Parker, C. D.; Wilk, Z. A.; Mattern, D.; Hercules, D. M. Int. J. Mass Spectrom. I o n PhyS. 1983. 5 3 , 135-149. Lyons, P. C.; Hercules, D. M.; Morelli, J. J., University of Pittsburgh, unpublished results, 1984. Hercules, D. M. Pure Appl. Chem. 1983, 55, 1869-1885. Novak, F. P.; Wllk, Z. A.; Hercules, D. M. J. Trace Microprobe Tech. 1985, 3(3), 149-163. Holm, R.; Kampf, G.; Kirchner, D.; Heinen, H. J.; Meier, S. Anal. Chem. 1MA. 690-692. ..., 56(4). .. - - --Vogt, H.: Helnen, H. J.; Meier, S.; Wechsung, R. 2. Anal. Chem. 1981. 308(3\. 195-200. Heinen, H.'J.: Meier, S.; Vogt, H. I n t . J. Mass Spectrom. I o n Phys. 1983, 47, 19-22. Cosslett, V. E. Practical Nectron Microscopy; Academic Press: New York, 1951; Chapter 8. WlleyINBS Mass Spectral Data Base on Magnetic Tape, Cornell University, Ithaca, NY. Balasanmugam, K.; Vlswanadham, S. K.; Hercules, D. M. Anal. Chem. 1983, 55(14), 2424-2426. \
I .
RECEIVED for review July 3,1985. Accepted January 13,1986. This work was supported, in part, by the National Science Foundation under Grant CHE-8541141.
