Stray-light measurement

Table 2.3 Stray light measurements for various instruments measured at 370 nm...

PERFORMANCE ATTRIBUTES Table 10.5. Stray Light Measurement... 

Droplet size distributions of aerosols can be determined by stray-light measure-... 

Aerosol droplet size di.stributions can be determined by stray-light measurements [87], [88]. Data obtained from sampling with cascade impac-tors may suffer from evaporation of the droplets on their way through the device, especially as sampling may require the application of underpressure [89]. Data for different nebulizers are available [90]. Particle size distributions depend on the nebulizer and its working conditions, on the liquid nebulized, and on the nebulization chamber. These relations have been studied in depth for organic solutions [91], [92], for which, at low gas flow, pneumatic nebulizers often... 

Stray light measurements are made using a sharp cutoff filter. Examples of these filter materials include saturated solutions of such compounds as potassium ferromanganate or lithium carbonate. Other solutions exhibiting abrupt cutoff wavelengths include KBr, KCl, Nal, NaN03 solutions, and acetone. Refer to ASTM E 169-87, Practice for General Techniques of Ultraviolet-Visible Quantitative Analysis. ... 

Figure 2. Optical schematic of the chamber for grazing angle measurements. Heaters and one thermocouple are located directly behind the sample (A). Gas inlet and outlet are near the IR transmitting windows (B). The mirrors (C) can be rotated and tilted to maximize signal and eliminate stray light. The entire cell (D) can be translated to change the angle of incidence.

Only one example will be given. It concerns a specific form of stray light that is observed when the entrance face of the CCD is protected by a transparent window. This situation arises mostly for cooled CCDs. A small fraction of the measured light is reflected off the surface of the CCD chip, hits the window, and bounces back toward another pixel of the CCD chip (Fig. 5). So the reading on the first pixel is lowered and that on the second is increased, the overall effect distorting the tme distribution in intensity. 

To take account of stray light in spectrophotometric measurements, the true absorbance values should be calculated using the formula ... 

Imperfections in monochromators result in the presence of a small proportion of unwanted wavelengths in the incident radiation. Such stray light results in a deviation from a Beer-Lambert relationship (Figure 2.14) and the effect is that absorbance measurements are lower than they should be. 

It is possible to assess the proportion of stray light by measuring the amount of radiation transmitted by samples that are optically opaque at the wavelength to be assessed but that transmit radiation of other wavelengths. The instrument is set to zero and 100% transmittance in the normal way and the opaque substance introduced into the sample compartment. The amount of light transmitted by the sample, measured in percentage transmittance, is... 

Figure 2.14 Effect of stray light. In the absence of stray light samples of increasing concentration result in a linear plot against the measured absorbance value. Increasing proportions of stray light result in deviations from this linear relationship above the absorbance values indicated (---).

The effect of multiple-order diffraction, if not corrected, is to introdnce a stray light signal at the detector, which badly affects the measnred linearity of the required absorbance signal. In the example above, if the required measurement is at 2000nm, and only 0.1% of stray light at lOOOnm reaches the detector, then appreciable absorbance nonlinearity is seen at absorbances above 2.0 AU. 

Practical Considerations. Typical absorption assay methods utilize ultraviolet (UV) or visible (vis) wavelengths. With most spectrophotometers, the measured absorbance should be less than 1.2 to obtain a strictly linear relationship (/.c., to obey the Beer-Lambert Law). Nonlinear A versus c plots can result from micelle formation, sample turbidity, the presence of stray light (see below), bubble formation, stacking of aromatic chromophores, and even the presence of fine cotton strands from tissue used to clean the faces of cuvettes. One is well advised to confirm the linearity of absorbance with respect to product (or substrate) concentration under the exact assay conditions to be employed in... 

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