Anal. Chem. i986,58,487-490
limit for the effectiveness of this general strategy. Based on these considerations, the 12-carbon aldehyde is currently considered to represent the optimum reagent for this procedure. It is also important to note that, when primary ion currents exceed about 2 X 10l2 particles cm-2 s-l, the critical first monolayer may be eroded faster than it is renewed. Very high primary ion currents result in an increase in the sampling of bulk solution with the result that advantages which accrue from enhanced surface activities are diminished (2). The observations of McEwen and Hass (8) regarding the use of high intensity guns are very much in agreement with this interpretation. It may be concluded, therefore, that this method provides a simple and rapid strategy for improving the secondary ion yield (sensitivity) of relatively hydrophilic species that contain a primary amine and a negative charge but lack structural components that can confer surface activity. As noted in the case of leucylleucine, relatively hydrophobic peptides (and probably most large peptides) may not benefit from this derivatization. This work also suggests that surface activity should be included among the many factors to be considered
487
in the preparation of samples for this technique. Registry No. Dodecanal, 112-54-9;glycylglycine, 556-50-3; alanylalanine, 1948-31-8;valylvaline, 3918-94-3; phenylalanylalanine, 3918-87-4;leucylleucine,3303-31-9;methionylmethionine, 7349-78-2. LITERATURE CITED (1) Barber, M.; Bordoli, R. S.; Elliot, G. J.; Sedgwlck, R. D.; Tyler, A. N. Anal. Chem. lQ82, 5 4 , 645A. (2) Ligon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Proc. 1984, 5 7 , 75-90. (3) DePauw, E.; Pelzer. 0.; Viet, D. D.; Marlen, J. Biochem. Biophys. Res. Commun. lQ84, 123, 27. (4) Ligon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Proc. lQ84, 6 1 , 113. (5) Llgon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Proc. 1985, 63,315. (6) Ligon, W. V.; Dorn, S. 6. Anal, Chem. 1985, 5 7 , 1993-1995. (7) Rlnehart, K. L. Sclence lQ82, 278, 254. (8) McEwen, C. E.; Hass, J. N. Anal. Chem. lQ85, 5 7 , 890-892.
Woodfin V. Ligon, Jr. General Electric Company Corporate Research and Development Schenectady, New York 12301 RECEIVEDfor review July 15,1985. Accepted October 7,1985.
Supercritical Carbon Dioxide Injection in Supersonic Beam Mass Spectrometry Sir: The ultracold spectroscopy afforded by supersonic expansions in combination with multiphoton ionization has provided a means of selective ionization for mass spectrometry (1-7). The wavelength selectivity of this method is particularly useful for problems that may be difficult to solve with conventional techniques alone; i.e., one needs to discriminate a small concentration of impurity in a large bath of a similar molecule. Such problems may arise in pharmaceutical synthesis where even trace amounts (300 'C. Strong cluster formation is not observed in the mass spectra although this might be expected since cluster formation is proportional to Po2D. Although clusters of PNAH with COz are observed, the signal of any observed cluster peak is more than an order of magnitude smaller than any PNAH peak and the size of the clusters remains small. The small number of clusters formed may be due to minor shock waves in the orifice that may be sufficient to destroy weakly bound van der Waals complexes but may not be severe enough to destroy the supersonic flow. No clusters of the solute-solute type were observed in our experiments as expected at the fairly low seed concentration used. Thus, supercritical fluid injection of C02molecular beams can provide sufficient cooling to allow reasonably sharp spectral features for identification in chemical analysis. The attractive feature of this method is that low temperatures can be used to dissolve nonvolatile or thermally labile molecules into a supersonic jet. Although the spectra obtained in expansions of high-pressure C 0 2 are not as sharp as with Ar carrier gas, the cooling is sufficient to cool out the broad structureless rotational contours observed at room temperature. The use of the mass spectrometer allows identification of each compound by mass analysis in conjunction with the spectroscopic identification achieved by laser spectroscopy.
ACKNOWLEDGMENT We thank Werner Gerber of Ciba-Geigy, Basel, Switzerland, for helpful suggestions during the course of this work. Registry No. COz, 124-38-9;acenaphthene, 83-32-9;phenanthrene, 85-01-8; carbazole, 86-74-8; pyrene, 129-00-0; triphenylene, 217-59-4;tetracene, 92-24-0. LITERATURE CITED Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 5 4 , 660-665. Sin, C. H.; Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 5 6 , 2776-2781. Zandee, L.; Bernstein, R. 8.J . Chem. Phys. 1979, 77, 1359-1371. Dletz, T. G.; Duncan, M. A.; Liverman, M. G.; Smalley, R. E. J . Chem. Phys. 1980, 73, 4816-4821. Sack, T. M.; McCrery, D. A.; Gross, M. L. Anal. Chem. 1985, 5 7 , 1290-1295. Johnston, M. V. Trends Anal. Chem. 1984, 3,58. Rhodes, G.;Opsal, R. 8.;Meek, J. T.; Reilly, J. P. Anal. Chem. 1983, 55, 280-286. Glddlngs, J. C.; Myers, M. N.; McLaren, L.; Keller, R. A. Science 1988, 162, 67-73. Jentoft, R. E.; Gouw, T. H. Anal. Chem. 1972, 4 4 , 681-686. Fjelsted, J. C.; Lee, M. L. Anal. Chem. 1984, 56, 619A-626A. Smith, R. D.; Udseth, H. R. Anal. Chem. 1984, 55, 2266-2272. Smith, R. D.; Udseth, H. R.; Kaiinoski, H. T. Anal. Chem. 1984, 5 6 , 2973-2974. Randall, L. G.; Wahrhaftig, A. L. Anal. Chem. 1978, 5 0 , 1703-1705. Otis, C. E.; Johnson, P. M. Rev. Sci. Instrum. 1980, 51, 1128-1129. McClelland, G. M.;Saenger, K. L.; Valentini, J. J.; Herschbach, D. R. J . Fhys. Chern. 1979, 83, 947-959. Lubman, D. M.; Rettner, C. T.; Zare, R . N. J . Phys. Chem. 1982, 86, 1129-1135. Smalley, R. E.; Wharton, L.; Levy, D. H. A c c . Chem. Res. 1977, IO, 139-145.
Chung Hang Sin Ho Ming Pang David M. Lubman* Department of Chemistry The University of Michigan Ann Arbor, Michigan 48109
Jens Zorn Department of Physics The University of Michigan Ann Arbor, Michigan 48109 RECEIVED for review July 29,1985. Accepted October 7,1985. We gratefully acknowledge financial support from a Cottrell Research Grant and the donors of the Petroleum Research Fund, administered by the American Chemical Society. We acknowledge support of this work under NSF Grant CHE 83-19383 and partial support from the Army Research Office under Grant DAAG 29-85-K-1005.
Photoacoustic Spectroscopy in Supercritical Fluids
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Sir: Laser-inducedcalorimetric techniques of spectroscopic detection are based on the direct measurement of the optical energy absorbed rather than that transmitted by the sample. In absence of photochemistry, optical excitation of sample molecules leads to either fluorescence or nonradiative decay of excited states that deposit, respectively, some or all of the excitation energy into the surrounding solvent as heat. Methods of detecting this thermalized energy fall into two classes where the evolved heat is measured either as a pressure change in the case of photoacoustic spectroscopy (1-3) or as a refractive index change in the case of thermooptical absorption techniques (4-6). The sensitivity of these calorimetric methods depends not only on the incident optical power used for excitation but also on the thermophysical properties of the solvent in which the measurement is made. For most solvents, heating the volume of sample liquid results in a 0003-2700/86/0358-0490$0 1.50/0
decrease in density due to the positive volume thermal coefficient of expansion, 0 (7, 8). Supercritical fluids or dense gases are a class of solvents that are generating considerable interest and applications in analytical chemistry, as both chromatographic (9-12) and spectroscopic solvents (13-15). Recent studies using supercritical fluids as solvents for thermoopticalmeasurements have shown a more than 100-fold sensitivity improvement, relative to thermal lens and photothermal deflection measurements in carbon tetrachloride (14,15). These techniques share with photoacoustic spectroscopy a dependence of signal on the thermal expansion coefficient, that is, the thermally induced change in volume or density of the solvent. The large volume expansion coefficient of supercritical fluids responsible for the outstanding improvements in sensitivity for thermal lens spectroscopy, therefore, holds the promise for similar increases 0 1986 American Chemical Society
