Background free-detection

Hirose et al. [26] proposed a homodyne scheme to achieve the background-free detection of the fourth-order field. With pump irradiation in a transient grating configuration, the fourth-order field propagates in a direction different from that of the second-order field because of different phase match conditions. The fourth-order field is homodyned to make ffourth(td. 2 D) and spatially filtered from the second-order response hecond td, 2 D). 

Lisuride dihydrogen maleate (h/Jj 30-35), methysergide maleate (h/ij 40-45), dihydroergotamine mesylate (hRf 45-50) and ergotamine tartrate (hRj 70-75) appeared as blue violet chromatogram zones on a yellow background. The detection limits — calculated for free base — were 3-4 ng substance per chromatogram zone. 

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

The sensitivity of RBS for the detection of trace impurities in bulk samples depends strongly on the sample composition and the experimental conditions. For impurities heavier than the substrate, the signal of which lies in the background-free region, it is possible to measure infinitesimal amounts of impurities simply by increasing the integrated charge. The near surface depth resolution... 

During the detection stage ensure that there is no free detection fluid in the film cassette as this will cause high backgrounds. The film must be kept dry. 

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