Scattering stray radiation

In atomic absorption the background continuum is usually negligible and the resonance line intense. To give the maximum discrimination against stray radiation, and hence the lowest detection limit, the slit width should be small. In atomic emission and fluorescence the analytical signal is smaller and the background due to scattered light and con-... 

In turbid or cloudy solutions, in which absorbance is affected by scattering of the sample, two absorbance measurements are made one at and the other at a different wavelength where the analyte does not absorb. Then, the difference being proportional to concentration of the analyte, the analyte concentration can be calculated. Difference spectrophotometry is used in the determination of constituents in tablets, complex pharmaceutical preparations, plant extracts, syrups, biological matrices such as blood and serum, injectable oil preparations, and the like. For this technique, in addition to other general requirements such as well-matched cells that are positioned accurately, the solution should be made homogeneous, and the instrumental stray radiation at the wavelength of interest should be extremely small. 

Stray radiation, commonly called stray light, is defined as radiation from the instillment that is outside the nominal wavelength band chosen for the determination. This stray radiation often is the result of scattering and reflection off the surfaces of gratings, lenses or mirrors, filters, and windows. When measurements are made in the presence of stray light, the observed absorbance is given by... 

Application of Eqs. 4.9 and 4.10 presupposes an ideal situation where the concentration and size distribution of the particles in the monitored suspension are not limiting factors, scattering is the only process attenuating the incident beam, the radiation is monochromatic, sample discontinuities are not present, chemical deviations do not manifest themselves, rate of turbidity formation is not critical, temperature is maintained, stray radiation is not detectable and the detector is ideal. These aspects are discussed in detail below. 

Analogously to spectrophotometry, stray radiation can alter the measured turbidance (Eq. 4.6), the limitation becoming more severe at lower transmitted power. The presence of suspended matter in the processed sample is inherent to turbidimetry and leads to an amplification of stray light due to scattering effects. This effect has not been systematically investigated in flow analysis. 

The scattered radiation, however, is often made up of wavelengths to which the instrument is highly sensitive. Thus, the effects of stray radiation can be greatly enhanced. Indeed, in some instances the output signal produced by the stray radiation may exceed that produced by the monochromator output beam. In such cases, the component of the measured transmittance due to the stray radiation may be as large as or exceed the true transmittance. 

In ICP-MS, the background intensities are mostly low and are mainly due to the dark current of the detector, and to signals produced by ions scattered inside the mass spectrometer as a result of collisions with residual gas species or reflection. In order to shield the direct UV radiation, a beam stop is supplied often in the mass spectrometer. The plasma contributes little to the background. in contrast to ICP-AES where the background continuum stems from interactions between electrons and ions in the plasma, from molecular bands, from the wings of broad matrix lines, and from stray radiation. 

Scattered Radiation, Scattered radiation, also called stray radiation, is radiation of the wrong frequency impinging on the detector. The... 

The effects of scattered radiation will show up as false intensity data, usually a decrease in the peak intensity, and the accuracy of the results will obviously suffer. As stated in Chapter 2 (Section 2.2B), the intensity of the scattered radiation can be measured and a correction factor applied to the data. If this is done, the data should be redetermined and the factor recalculated periodically. It is better to eliminate or reduce an error than to correct for it a short-wavelength filter placed in the sample beam will cut down on the stray radiation. 

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