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Anal. Chem. 1984, 56, 1674-1677
smaller contributions at other values of m / e . This is similar to a 70-eV electron bombardment spectrum. Supplementary gas-phase experiments which measure the fragmentation patterns and relative cross sections of closed electronic shell and specially prepared radical molecular species will be of immense value in understanding future SALI experiments. Knowledge of a molecule’s electronic spectroscopy also can be an important aid in predicting both the fragmentation pattern and the order of the MPI process. Nonresonant MPI is currently capable of identifying many molecular species and chemical functional groups and can therefore yield sensitive measurements of relative changes in concentrations of molecular surfce species as a function of varying experimental conditions. General quantitative analysis of relative amounts of different molecules is a significant research goal. Another obvious extension of the capabilities of the SALI technique allows measurements of the kinetic energy distributions of neutral atoms and molecules desorbing as a result of electron, photon, or ion impact. By using a very short duration probe beam for temporal resolution and by varying the time delay between the probe and ionizing beam pulses, one can obtain an ion signal that is determined by the distribution of flight times from the surface to the laser zone. This yields a velocity or kinetic energy distribution for the desorbing neutral atom or molecule. Preliminary measurements have been made, for example, on the kinetic energy distributions of Cuz sputtered from Cu metal by Ar+ at 3 keV. The technique is also clearly capable of measuring angular distributions of desorbed species. In this case, well-defined geometries of the two beams are needed. By moving a focused probe beam’s position on the surface relative to a fixed laser ionization zone, perhaps simply by changing the voltage on a deflection plate, one can obtain the angular distribution for all removed masses simultaneously. Planned studies include detection and montoring of adsorbates relevant in selected heterogeneous catalytic systems and mass spectrometry of nonvolatile and thermally labile organic molecules.
ACKNOWLEDGMENT The authors thank S. E. Buttrill, Jr., formerly of SRI, now of Charles Evans and Associates, for many stimulating discussions and some valuable assistance in the laboratory. The authors also thank W. K. Bischel of SRI for helpful advice on optics. Preliminary experiments used a Nd:YAG laser borrowed from the San Francisco Laser Center. LITERATURE CITED Honlg, R. E. J . Appl. Phys. 1958, 2 9 , 549. Woodyard, J. R.; Cooper, C. B. J . Appl. Phys. 1964, 35, 1107. Oechsner, H.;Gerhard, W. Surf. Scl. 1974, 4 4 , 480. Winograd, N.; Baxter, J. P.; Kimock, F. M. Chem. Phys. Lett. 1982, 88, 581. Benninphoven, A. Surf. Sci. 1975, 53, 596. Colton, R. J. J . Vac. Scl. Techno/. 1981, 78, 737. Williams, P. “Applied Atomic Collision Physlcs”; Datz, S., Ed.; Academic: Orlando, FL, 1983; Vol. 4, Chapter 7, p 327. Kovalev, I. D.; Maksimov, G. A.; Suchkov, A. I.; Larin, N. V. Int. J . Mass Spectom. Ion Phys 1978. 27, 10 1. Conzemius, R. J.; Capellen, J. M. Int. J . Mass Spectrom. Ion Phys. 1980, 34, 197. Hurst, 0. S.; Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. Phys. 1979, 57, 767. Payne, M. G.; Chen, C. H.; Hurst, 0. S.; Foltz, G. W. Adv. At. Mol. Phys. 1981, 77, 229. Johnson, P. M.; Otis, C. E. Ann. Rev. Phys. Chem. 1981, 32, 139. Kimock, F. M.; Baxter, J. P.; Winograd, N. Surf. Sci. 1983, 724, L41. Lambropouios. P. Adv. At. Mol. Phys. 1978, 72, 87. Morellec, J.; Normand, D.; Petite, G. Adv. At. Mol. Phys. 1982, 78,
.
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i’Huiller, A.; Lompr6, L. A.; Mainfray, G.; Manus, C. J . Phys. 6 1983, 78, 1363. James, D.; McKee, T. J.; Skrlac, W. I€€€ J . Quantum Electron 1979,
QE-75. 335. -~
MamyrkB. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Sov. Phys .-JETP (Engl. Transl.) 1973, 37, 45. Boesl, U.; Neusser, H. J.; Weinkauf, R.; Schiag, E. W. J . Phys. Chem. 1982. 86. 4857. Delone. G. A.; Delone, N. B.; Plskova, G. K. Sov. Phys .-JETP (Engl. Transl.) 1972, 35, 672. Cervenan, M. R.; Chan, R. H. C.; Isenor, N. R. Can. J . Phys. 1975, 53, 1573.
RECEIVED for review February 13, 1984. Accepted April 9, 1984. Internal research and development funding from SRI International is gratefully acknowledged.
Effect of Laser Beam Shape Parameters on Photothermal Deflection Densitometers Tsuey Ing Chen and Michael D. Morris* Department of Chemistry, University of Michigan, A n n Arbor, Michigan 48109
The effect of beam shape on photothermal deflectlon densitometry is Investigated. It is shown that a 2-fold expansion of the probe laser beam in the plane parallel to the TLC plate increases sensitivity by 10-fold and improves measurement precision to about f1.5% RSD. Line focus of the heating laser beam degrades the signal to noise ratlo. Both results are due to local irreguiarltles on the TLC plates employed. Optimum performance is obtained when pump beam dlameter matches probe beam expanded wldth. A mismatch of 25 YO degrades signal to noise ratio by only 10%.
Photothermal deflection spectroscopy is emerging as a powerful tool for probing surface absorption spectra and for generating depth-resolved absorption spectra of solids (1-4). 0003-2700/84/0356-1674$01.50/0
Studies of thin films (5-7), semiconductors (8),and structural defects in inorganic compounds (9-11) have been the most frequent applications of the technique. Recently, photothermal deflection has been applied to problems of general interest to analytical chemists: detection of Fourier transform infrared signals from solids (12) and thin-layer chromatography densitometry (13). Our own group has shown that photothermal deflection densitometry provides detection limits almost 3 orders of magnitude lower than those obtainable by conventional diffuse reflectance densitometers. Most of the reported applications have been to samples either where a very localized measurement is desired or where it is assumed that the surface is homogeneous. For this reason, neither theory nor parametric studies have addressed the problem of measuring the absorbance of a sample whose lateral extent is somewhat greater than the conventional probe beam 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
diameter and whose average properties only are desired. This situation is encountered in thin-layer chromatography. Here, typical developed spot diameters are 1-2 mm. Probing such a sample with a 1-2-mW He-Ne laser, with a beam diameter of about 0.6 mm, can cause systematic errors. In particular, alignment of the system over exactly the same relative position of successive samples is necessary. Moreover, the highly localized measurement emphasizes the effects of small irregularities of the surface coating and sample distribution. Both problems can cause a decrease in the accuracy and precision of photothermal deflection measurements of absorbance of chromatographic plates. Jackson, h e r , Boccara, and Fournier (2)have investigated the dependence of photothermal deflection signals on the probe beam-surface distance. They have shown that signal decreases exponentially as the probe beam is offset from the surface. To obtain good sensitivity, it is necessary to keep the center of the probe beam as close as possible to the surface. For a chromatographic instrument, therefore, neither a simple telescope probe beam expansion nor focusing with a spherical lens is appropriate. Probe beam expansion results in a corresponding displacement of the probe from the surface and a dramatic fall-off in sensitivity. Probe beam focusing gives a localized signal, rather than an average over the chromatographic spots. Previous theoretical studies have assumed that the diameters of the heating laser and the probe are identical or that the heating laser beam has a much larger diameter than the probe beam. These conditions, while easy enough to satisfy, are not necessarily optimal. We have, therefore, studied the effect of pump/probe beam diameter ratio on chromatographic signals. It is also well-understood that the ideal TLC densitometer probes an infinitely thin slice of the chromatographic spot (14).We have therefore investigated the effect on sensitivity of the aspect ratio of the pump beam. EXPERIMENTAL SECTION The photothermal displacement densitometer described in our previous publication was used for these studies. The mechanical system and detector were unmodified. A one-dimensionalbeam expander was placed directly in front of the probe He-Ne laser. The expander was a 2X Galilean telescope constructed from -19 mm and +40 mm focal length cylinder lenses oriented with their curved surfaces perpendicular to the plane of the TLC plate. Provision was made to translate the 40-mm lens to adjust the collimation of the beam. For some experiments the 125-mm focusing lens in the argon ion laser heating beam was replaced with a 125-mm cylinder lens. This lens was oriented so that the aspect ratio of the pump beam be varied by translating the lens. This system allowed the pump to simulate a slit shape. As before, argon laser power was adjusted to about 20 mW and was reduced to about 15 mW at the plate surface by reflection losses in the optical train. For some studies the 488-nm line was employed, and for others the 458-nm line was used, as noted. The helium-neon beam dimensions were measured directly with a fiiely divided scale. Argon ion beam dimensions were measured directly at the position of the focusing lens and at several points between the lens and the TLC plate. The dmenaions of the beam at the plate were then extrapolated from these measurements. Chromatogram peak areas were measured by two-term Romberg integration, after subtraction of any base line. Signal/noise ratios were calculated as the ratio of the peak height to one-fifth of the peak-peak fluctuations on the base line. The chomatqgaphic procedures previously reported were used here. Whatman 4807-400 high-performance plates were used throughout this study. All samples were applied to the plate as 50-nL spots. The three quinones used in our earlier report, 1,2-naphthoquinone,a-ionone, and phenanthrenequinone,were used here. RESULTS AND DISCUSSION In our previous work, absorbance was measured at 488 nm, the wavelength of one of the strongest argon ion laser emis-
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Flgure 1. Effect of argon ion/heiium-neon beam diameter ratio on
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Flgure 2. Effect of aspect ratio of argon ion laser on photothermal signal: (A) aspect ratlo 1:l (round beam); (B) aspect ratio 2:3 (longItudinaktransverse); (C) aspect ratlo 1:2 (iongitudinai:transverse).
sions. However, a t 458 nm, 1,2-naphthoquinone sensitivity is about ten times greater than that at 488 nm. Phenanthrenequinone and a-ionone sensitivities are the same at either wavelength. Most recent work has been carried out at 458 nm to take advantage of the improved naphthoquinone sensitivity. Figure 1shows the effect of mismatch of probe and pump beam diameters. The data were taken with an unexpanded He-Ne beam. As expected, the signal/noise ratio is maximized when the He-Ne laser is probing the same width of sample that the Ar+ beam is heating. The signal is degraded if the beam diameters are not approximately matched so that there is complete overlap between the probe beam and the heated region of the plate. How much mismatch is acceptable is a in beam diameters decreases signal,/noise ratio only about 10%. It is unlikely that signal/noise ratios are more constant than this from sample in any event. Translation of the pump laser focusing lens to make the beam diameters equal by visual inspection is adequate to optimize this parameter. Figure 2 shows the effect of changing the aspect ratio of the argon ion heating laser. We fiid that heating an extended region of the spot gives better results than heating a slitshaped region. This effect might seem surprising, since the amount of heat deposited in the sample is independent of
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
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Flgure 3. Dependence of slgnaihoise ratio on fraction of probe power passed by razor blade, wlth 2 X transverse telescope in place.
Distance
Flgure 5. Transverse scan across through a-ionone spots from several chromatograms: (left to right) 93 pg, 62 pg, 62 pg, and 6.2 pg.
Table I. Detection Limits
Art 488.0 nm Art 457.9 nm Art 457.9 nm test compound 1,2-naphthoquinone phenanthrenequinone a-ionone
Distance
Flgure 4. Chromatogram of 3.8 ng of 1,2-naphthoqulnone,46 ng of phenthrenequlnone,and 75 pg of a-ionone.
whether the beam is expanded or focused in one dimension. Consequently, one would expect the integrated deflection to vary weakly with aspect ratio. In these experiments the argon ion laser power density is increased by a factor of 2 or less. It is improbable photodecomposition contributes appreciably to the reduction in signal/noise ratio. However, focusing in one dimension emphasizes local irregularities in plate coating or solvent flow, and, therefore, results in a noisy trace. We have examined the dependence of signal to noise ratio on the fraction of the probe beam power eclipsed by the knife edge. The signal/noise ratio maximizes over a wide range, about 1.5% to 30%,as shown in Figure 3. Placement of the knife edge is not critical. Over a wide working range, only the absolute signal is affected by knife edge position. Similar behavior is observed with an unexpanded probe beam, where signal/noise ratio maximizes over the 6-45% range (13). Expanding the He-Ne beam in the plane parallel to the TLC plate improves the precision of the experiment, decreases detection limits, and eliminates artifacts caused by probing a small region of the chromatographic spot. These beneficial effects are illustrated in Figures 4 and 5. Figure 4 shows a typical chromatogram taken with the beam expander system. Phenanthrenequinone is present at a level which would be barely detectable in the absence of beam expansion, while naphthoquinone would give a noisy, but readily quantitated trace without beam expansion.
unexpanded He-Ne
unexpanded He-Ne
expanded He-Ne
2.3 ng 31 ng
300 Pg 31 ng
25 Pg 1.3 ng
7.5 Pg
7.5 Pg
6.2 Pg
Figure 5 shows the a-ionone bands taken laterally across a set of chromatograms on the same plate. Although the 62-pg samples were spotted too close to be completely resolved in this measurement, their areas differed by less than lo%, despite the uncertainties of base line extrapolation. More significantly, the multiple peaks observed in our earlier experiments are not present in this chromatogram. We have measured the reproducibility of peak areas in a series of 15 chromatograms of a mixture of 2.3 ng of 1,2naphthoquinone and 93 pg of a-ionone. For both compounds the relative standard deviation of the peak area is about 1.5%. The precision of our present measurements compares favorably with the precision of conventional densitometer measurements. The limiting factor is the precision with which we can introduce spots onto the plate with the available apparatus. In our earlier experiments, the standard deviation of chromatogram area measurements was about f2.5% (13). In our earlier report, we demonstrated that a-ionone peaks broke up into multiple peaks at small sample loadings (
