Laser Spectroscopy in Analytical Chemistry

The first aspect of laser applications in analytical chemistry is the sensitive detection of small concentrations of impurity atoms or molecules. With the laser-spectroscopic techniques discussed in Chap. 1, detection limits down into the parts-per-billion (ppb) range can be achieved for molecules, which corresponds to a relative concentration of 10 . Atomic species and some favorable molecules can even be traced in concentrations within the parts-per-trillion (ppt) (=10 ) range. Recently single molecule detection in solids, solutions, and gases has become possible. 

A very sensitive detection scheme is the photoacoustic method in combination with a multipass optical resonator, illustrated in Fig. 10.1. With this apparatus absorption coefficients down to o = 10 cm can be measured. 

Example 10.1 With a diode laser spectrometer and a multipass absorption cell (Fig. 10.1) NO2 concentrations down to the 50-ppt level in air were detected on a vibrational-rotational transition at 1900 cm NO concentrations down to 300 ppt, while for SO2 at 1335 cm a sensitivity limit of 1 ppb was reached [1371]. 

If the spectroscopic detection of atoms can be performed on transitions that represent a true two-level system (Sect. 9.1.5), atoms with the radiative lifetime r may undergo up to T/2r absorption-emission cycles during their transit time T through the laser beam (photon burst). If the atoms are detected in carrier gases at higher pressures, the mean free path A becomes small A d) and T is only limited by the diffusion time. Although quenching collisions may decrease the fluorescence 

Example 10.2 For gases at low pressures where the mean free path A is larger than the diameter d of the laser beam, we obtain the typical value T = d/v=lO ps for = 5 mm and u = 5 x 10 m/s. For an upper-state lifetime of T = 10 ns the atom emits 500 fluorescence photons (photon burst), allowing the detection of single atoms. With noble gas pressures of 1 mbar the mean-free path is 0.03 mm and the diffusion time through the laser beam may become 100 times longer. Although the lifetime is quenched to 5 ns, which means a fluorescence quantum yield of 0.5, this increases the photon burst to 5 X 10 photons. 

For gases at low pressures where the mean free path A is larger than the diameter d of the laser beam, we obtain the typical value T = d/v = 10 xs 

Another very sensitive detection scheme is based on resonant two- or three-photon ionization of atoms and molecules in the gas phase (Sect. 6.3). With this technique even liquid or solid samples can be monitored if they can be vaporized in a furnace or on a hot wire. If, for instance, a heated wire or plate in a vacuum system is covered by the sample, the atoms or molecules are evaporated during the pulsed heating period and fly through the superimposed laser beams L1+L2 (+L3) in front of the heated surface (Fig. 15.2). The laser LI is tuned to the resonance transition / - k) of the wanted atom or molecule while L2 further excites the transition k) f). Ions are formed if Ef is above the ionization potential IP. The ions are accelerated toward an ion multiplier. If L2 has sufficient intensity, all excited particles in the level / ) can be ionized and all atoms in the level [/ flying through the laser beam during the laser pulse can be detected single-atom detection) [15.10-15.12]. If 

The first aspect of laser applications in analytical chemistry is the sensitive detection of small concentrations of impurity atoms or molecules. With the laser-spectroscopic techniques discussed in Chap. 6, detection limits down 

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Vol. 34. Neutron Activation Analysis. By D. De Soete, R. Gijbels, and J. Hoste Vol. 35 Laser Raman Spectroscopy. By Marvin C. Tobin Vol. 36 Emission Spectrochemical Analysis. By Morris Slavin Vol. 37 Analytical Chemistry of Phosphorus Compounds. Edited by M. Halmann Vol. 38 Luminescence Spectrometry in Analytical Chemistry. By J. D.Winefordner, S. G. Schulman, and T. C. O Haver... 

Sampling in surface-enhanced Raman and infrared spectroscopy is intimately linked to the optical enhancement induced by arrays and fractals of hot metal particles, primarily of silver and gold. The key to both techniques is preparation of the metal particles either in a suspension or as architectures on the surface of substrates. We will therefore detail the preparation and self-assembly methods used to obtain films, sols, and arrayed architectures coupled with the methods of adsorbing the species of interest on them to obtain optimal enhancement of the Raman and infrared signatures. Surface-enhanced Raman spectroscopy (SERS) has been more widely used and studied because of the relative ease of the sampling process and the ready availability of lasers in the visible range of the optical spectrum. Surface-enhanced infrared spectroscopy (SEIRA) using attenuated total reflection coupled to Fourier transform infrared spectroscopy, on the other hand, is an attractive alternative to SERS but has yet to be widely applied in analytical chemistry. 

Zybin A, Schnilrer-Patschan C, Bolshov MA, and Niemax K (1998) Elemental analysis by diode laser spectroscopy. Trends in Analytical Chemistry 17 513-520. 

Future Trends. Methods of laser cooling and trapping are emerging as of the mid-1990s that have potential new analytical uses. Many of the analytical laser spectroscopies discussed herein were first employed for precise physical measurements in basic research. AppHcations to analytical chemistry occurred as secondary developments from 10 to 15 years later. 

Fluorescence spectroscopy and its applications to the physical and life sciences have evolved rapidly during the past decade. The increased interest in fluorescence appears to be due to advances in time resolution, methods of data analysis and improved instrumentation. With these advances, it is now practical to perform time-resolved measurements with enough resolution to compare the results with the structural and dynamic features of macromolecules, to probe the structures of proteins, membranes, and nucleic acids, and to acquire two-dimensional microscopic images of chemical or protein distributions in cell cultures. Advances in laser and detector technology have also resulted in renewed interest in fluorescence for clinical and analytical chemistry. 

Mass-spectrometry principles and techniques have been employed in other kinds of surface studies in which sample atoms are sputtered by interaction with a laser beam or by RF glow discharges. These approaches are more highly specialized, but it should be clear that mass spectrometry is an important tool in surface chemistry. The student should compare SIMS and ISS with other surface analytical techniques such as ESCA, Auger spectroscopy, electron microprobe, and low-energy electron diffraction (see Chaps. 14 and 15). 

We have mentioned earlier that the brightness of laser light provides the ideal conditions for non-linear spectroscopy in atomic and molecular physics and analytical chemistry, but it can also lead to blood-free and sterile surgery in medicine and other application in modern biomedicine. 

Without going into any further detail, we will mention one other type of interferometer that is encountered in molecular spectroscopy experiments, namely the scanning Michelson interferometer (for detailed descriptions refer to standard textbooks on optics or laser spectroscopy). These are used in so-called Fourier transform spectrometers for high-resolution molecular spectroscopy in the IR such instruments are commonly known as FTIR spectrometers. While rather popular in analytical molecular spectroscopy of IR wavelengths, namely to record, identify and quantify molecular vibrations, they are less suitable in laser chemistry experiments because of the rather long acquisition times required to record... 

At V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry a laser IR spectrometer has been developed in co-operation with the International Laser Center of Moscow State University. It is designed for the remote sensing of various atmospheric pollutants by the long path IR-spectroscopy in a 3 mcm atmospheric window [8,9]. the functional scheme of this device is presented on Fig.4. 

Reid GD, Wynne K (eds) (2000) Ultrafast laser technology and spectroscopy in encyclopedia of analytical chemistry Meyers RA. Wiley, Chichester, pp 13644—13670... 

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