Microspectroscopies

In the laser Raman microprobe (LRM), a continuous laser beam is micro-focussed on the sample.  The photons interact with the molecules of the sample by the phenomenon of Raman scattering, in which the largely vibrational modes of the molecules are detected by characteristic energy transfers to and from the photon.  The optical spectrum of the scattered photons, called the (Stokes-) Raman spectrum, is highly diagnostic of the compound(s) present in the sample region analyzed.  The analytical sample can be a bulk solid or a single particle of micrometer size.  Both organic and inorganic compounds can be detected and identified by their characteristic spectra.  Detection limits for most scattering molecules are typically of the order of 1-3 wt% in a non-interfering matrix.  Raman microprobe spectroscopy can be made quantitative when analysis is carried out by means of working curves determined from standards similar in makeup and concentration.  As a molecular microprobe, the Raman microprobe greatly complements both the electron- and ion-beam microprobes that furnish elemental microanalysis data.  Applications of Raman microspectroscopy include the characterization of environmental particles, a broad range of high-technology materials (e.g., ceramic coatings and films, superconductors, synthetic diamond), and biological/pathological microanalysis of  thin sections of tissue. Recently, the Surface and Microanalysis Science Division has been collaborating with other divisions in the Chemical Science and Technology Laboratory to develop luminescent glass standards for the calibration of Raman spectral intensity.

Fourier-transform infrared (FT-IR) microspectroscopy is a second molecular microanalysis technique that furnishes a unique vibrational spectrum whose information content allows the characterization and often unequivocal identification of the sample, or microscopic sampling region, under study.  Thus, in the FT-IR microscope, an apertured sample region is typically subjected to a transmission measurement by the collimated infrared beam which, upon subtraction of the background spectrum, leads to the  “fingerprint” sample spectrum.  For most samples, the minimum sample size is approx. 10 micrometers for analytical quality spectra to be obtained, and here also, successful quantitation can be obtained through the use of standards. In cases where a transmission measurement of the sample is not possible, such as for thin films supported by an interfering substrate, the FT-IR microscope can be operated in the absorption-reflection mode to yield a good IR spectrum.  Extensive infrared spectral libraries exist, typically computer-based, to permit the rapid identification of organics and polymers.  These library reference spectra generally cover the mid-infrared range where inorganic compounds and materials usually do not show much of an infrared spectrum.  Applications of infrared microspectroscopy include the study of polymeric fiber samples, the elucidation of conformational structure of biomolecules, and the spectral characterization of high explosives and that of closely related chemical agents.

Near-Field Microspectroscopy circumvents the diffraction limit to far-field optical microspectroscopy by scattering the near-field components of light confined by optimal wave-guide and antenna structures. The Division has significant efforts in developing both vibrational spectroscopy and dielectric spectroscopy as contrast mechanisms for near-field optical microscopes.

Information provided by: http://www.cstl.nist.gov