Stray light effect

Ultraviolet spectrophotometers cont.), single-beam, 225 standardisation, 226 Ultraviolet spectrophotometry, 221-232 absorption cells, 226 colorimetry, 228 derivative, 230 difference method, 229 dual-wavelength, 229 identification by, 231 influence of pH, 224 influence of solvent, 224 laws of absorption, 222 quantitative applications, 227 stray-light effects, 224 Ultraviolet-visible detector, 202 multiwavelength, 211 Unicontin, 1011 Unidiarea, 474 Unidone, 356 Uniflu, 557, 893 Unilobin, 709 Unimycin, 846 Uniphyllin, 1011 Uniprofen, 677 Unisom, 576... 

Photomultiplier tubes have also been developed with response limited to the ultraviolet region (160 to 320 nm), the so-called solar-blind photomultipliers. They are helpful in reducing stray light effects from visible radiation and are useful as UV detectors in nondispersive systems. 

Figure 2.11 Absorbance at 280 nm as a function of concentration for a typical protein solution, showing linear behaviour at low concentrations (leff) but deviations from Beer-Lambert behaviour due to stray light effects at higher absorbances...

At low concentrations the graph of A versus c is quite linear, and obeys the Beer Lambert law. However, at higher concentrations, when the absorbance approaches 2 or more, we begin to see values lower than we should expect, eventually reaching a plateau above which the absorbance apparently remains constant regardless of concentration. This is an example of the stray light effect in spectrophotometers and is a major instrumental limitation. 

Fig. 4.5.1. Stray light effects in the low U.V. region, obtained using 0.099 M aqueous KCl solution, a water reference, and a nitrogen purged spectrophotometer. Cell path lengths of 10 mm...

Third-harmonic generation (THG), in which a fundamental beam at frequency 0) is converted to frequency 3a), is a technique frequently used to measure to, w, cS) of organic molecules or polymers. This technique is rather accurate and sensitive even to small y (-3a> o>, w) values. Only electronic effects contribute to THG. Therefore, thermal or stray-light effects, which can be troublesome in measurements based on the intensity-dependent refractive index, do not contribute to o), to, w). However, important slower contributions to... 

Negligible stray light. Because of the way in which the FT-NIR spectrometer modulates each source wavelength, there is no direct equivalent of the stray light effects found in dispersive spectrometers. 

Other common error sources that are included are technician error in sample preparation, temperature differences in standards or instrument while taking data, cahbration standard instability, instrument noise and drift, changes in instrument wavelength sethng, nonlinearity, stray light effects, particle size differences, color differences with concentrahon, solvent interaction differences with change in concentrahon, and the reference method does not measure the same component as a spectroscopic method. An example of the last problem would typically be found if measuring the percent... 

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