Abstract
In biological applications of resonance Raman spectroscopy it is frequently desirable to reduce the instantaneous and long-term power density of the focused laser beam in order to preserve the sample. Both flowing sample methods and laser beam defocusing techniques have been used successfully by various groups to minimize damage to photolabile samples. The use of flowing sample cells under conditions in which sample recycling is practical has the additional advantage that long acquisition times can be achieved with fairly minimal sample consumption. Nonetheless, the flowing cells described to date are most useful when both fairly large volumes of sample are available and recirculation is feasible. Recirculation is often not possible if the sample of interest is an unstable reaction intermediate, and such an application requires rapid mixing of the precursor compounds. A further consideration with Raman flow cells involves the sample containment technique in the scattering volume. Quartz capillaries are often used, but with these, quartz scattering is severe in the low-frequency region, where it overlaps vibrational modes of the sample and makes their detection difficult. Several groups have avoided this problem by arranging their flowing cells so that the sample forms a free jet in air in the scattering volume. A notable example of this is the microdroplet mixing technique developed by Kincaid and co-workers that uses continuous wave excitation and both rapid mixing and Raman scattering in air.
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