Lasers and Analytical Chemistry A wide spectrum of techniques was represented at the 32nd Annual Summer Symposium on Analytical Chemistry, called "Lasers and Analytical Chemistry", held at P u r d u e University J u n e 27-29, 1979. But as T o m a s Hirschfeld of Lawrence Livermore Laboratory noted, there were also some important omissions. For instance, there were no papers on plain old laser Raman spectroscopy, currently the most widely-used application of lasers to analytical chemistry, according to Hirschfeld. Laser microprobes were not on the program, and neither were scattering techniques, used in the analysis of particulates. "So this meeting is strongly futureoriented," Hirschfeld remarked. "Perhaps we should have called it 'Sexy Lasers and Analytical Chemistry.' " But future-orientation is perhaps excusable in this case, since lasers have only recently arrived on the analytical scene. There can be little doubt t h a t we've come a long way since what Fred Lytle of P u r d u e University believes to be the first ANALYTICAL CHEMISTRY paper in which a laser was used: "Remote Opening of Metal Containers with a Laser Beam", page 1782 of the December 1965 issue. Lytle considers 1971 to have been a turning point in the application of lasers to analytical chemistry, as a variety of laser papers began to appear at t h a t time on subjects such as atomic fluorescence and intracavity absorption. And the wide spectrum of techniques discussed at the conference suggests lasers will play an important role in analytical chemistry for quite a time to come. Topics discussed at the symposium included matrix isolation fluorimetry, laser induced fluorescence, laser multiphoton ionization mass spectrometry, laser magnetic resonance, thermal blooming, optoacoustical spectroscopy, intracavity absorption, nonlinear Raman spectroscopy, supersonic molecular beams, and others. T h e talk on laser multiphoton ionization mass spectrometry was presented by Daniel Lichtin on behalf of fellow investigators L. Zandee and R. B. Bernstein at Columbia University. In their experiments, multiple photon ionization spectra were obtained by irradiation of molecules in the ion source of a mass spectrometer. T h e investigators showed that a molecule such as benzene will absorb two pho-
tons of laser light to attain an excited electronic state, from which the molecule is easily ionized by further absorption, a process known as resonance enhancement. A mass spectrum can be recorded by setting a pulsed, tunable laser at one of the resonance absorption wavelengths and scanning the mass analyzer. Alternatively, a vibronic resonance spectrum can be generated by detecting the ion current at an a b u n d a n t mass spectral peak while the laser is tuned through a range of wavelengths. Two-dimensional vibronic/multiple photon ionization spectra are highly specific: every molecule, and each of its isotopes, has a unique spectrum. As Lichtin puts it, "It's a better fingerprint, because you have more information there than just a standard fragmentation pattern. There's a more definite possibility for identification of the molecule."
LMR Detection of free radicals by laser magnetic resonance was discussed by Carleton J. Howard of the National Oceanic and Atmospheric Administration. Laser magnetic resonance (LMR) was originally developed about ten years ago as a spectroscopic technique, but Howard explained its present application as an analytical tool. L M R is very similar to other magnetic resonance methods such as ESR and NMR. While N M R uses radiofrequency radiation to produce transitions between nuclear spin levels, and ESR uses microwave radiation to produce transitions between electron spin levels, L M R uses radiation in the far infrared to produce transitions between rotational levels in paramagnetic molecules. Absorption is accomplished by tuning the molecule into resonance with a far infrared laser by means of a magnetic field. T h e attenuation of the laser radiation by absorption is generally a small fraction of the o u t p u t power of the laser. Therefore, it is necessary to modulate the magnetic field and use phase sensitive detection of the laser signal. Howard reported at the symposium on the use of L M R to elucidate the rates of free radical reactions in the atmosphere. His research has resulted in rate constant determinations which have altered predictions of the significance of various atmospheric processes. For instance, previous esti-
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Tomas Hirschfeld: "There is always a need for doing analysis at a place you are not. For example, when your sample happens to be a chemical warfare agent, local analysis methods would tell you, 'You are dead.' "
mates predicted the destruction of 7% of the ozone layer by fluorocarbons at steady state, but Howard's present estimate is up by a factor of three. T h e estimated effects of SST exhaust have changed over the years, but Howard's findings are rather dramatic because he has changed the sign. "Whereas formerly it was thought that S S T exhaust would destroy ozone, the present estimates are that it will actually make a small amount of ozone," he explained. " B u t we're not yet advocating additional S S T flights to cancel out the fluorocarbons." Hirschfeld of Lawrence Livermore discussed remote detection and flow cytometry. His rationale for remote detection was quite convincing: " T h e r e is always a need for doing analysis at a place you are not. For example, when your sample happens to be a chemical warfare agent, local analysis methods would tell you, 'You are dead.' " Hirschfeld used laser Raman spectroscopy to determine 2 ppm kerosene in the atmosphere at a distance of 220 m from his instrument. }ie explained t h a t the success of the analysis was
