ASTM E334-01(R2021) pdf free download

ASTM E334-01(R2021) pdf free download.Standard Practice for General Techniques of Infrared Microanalysis
4. Contamination
4.1 Although the presence of contaminants is a general problem in any type of analysis, contamination can be particu- larly severe in micro work. For example, minor impurities in a solvent can become major components of a residue remaining after solvent evaporation. Materials extracted from thin-layer chromatographic materials, from the paper used in paper chromatography, and from solid adsorbents in general, may include particular contaminants of concern. It should also be noted that the gas-chromatographic stationary phase may lead to significant contamination. Consideration of these and other sources of contamination must always enter interpretation of results in microanalysis. Erroneous results can be minimized by the use of pure reagents, extreme care in sample handling, and the frequent use of“blanks” in the course ofseparation and subsequent recording of spectra.
5. General Microspectroscopic Techniques
5.1 Spectroscopic techniques used for the examination of microsamples are usually adaptations of comparable macro techniques, and many have been described in the literature (1, 2). 3 5.2 In computerized dispersive spectrometers or Fourier transform-infrared (FT-IR) instruments, computer routines for multiple scanning, signal averaging, absorbance subtraction, and scale expansion can be used very effectively to enhance the observed signal-to-noise ratio of weak bands and increase sensitivity (3, 4). Absorbance subtraction is also commonly used to eliminate interfering bands from the sample matrix and thus lower the limits of detection (see Practices E168). 5.3 Use of Masking Apertures—The aperture of sample holders used for microspectroscopic study (without the use of an infrared microscope) are usually significantly smaller than the beam at the sample position of the instrument. As a consequence of these small apertures, steps need to be taken to ensure that the best quality spectra be obtained, and the techniques used will depend on the type of spectrometer being used. In general, the use of a beam condensing accessory will greatly improve the results obtained (see 5.4). 5.3.1 When a double-beam dispersive spectrometer that is not equipped for control by minicomputer is used, the refer- ence beam should be masked to a corresponding aperture. This can be accomplished by using an opaque sheet of stiff material punched with an appropriate opening, with reference screens, or with commercially available optical attenuators. Attenuation of the reference beam affects instrument performance, and appropriate adjustment ofthe instrument settings (that is, wider slits or higher gain) is necessary to produce reliable spectra at the lower energy levels. Enhancement of sensitivity can be attained by the ordinate scale expansion feature available on most spectrometers. 5.3.2 When using a single-beam spectrometer, the instru- ment background spectrum should be recorded through an aperture in the sample position that has dimensions no larger than those of the sample. Where appropriate, this can be done by using the empty sample holder itself.5.3.3 On some FT-IR spectrometers, insertion ofan aperture at the sample position will slightly change the observed frequency positions of bands, as a result of modification of the optical path. Hence, sample and reference aperture must be carefully aligned at the same position, particularly if computer differencing is to be done. 5.3.4 Some FT-IR spectrometers (especially those equipped with cooled mercury cadmium telluride (MCT) detectors) are so sensitive that under normal operating conditions (that is, when examining macro samples or recording the reference single beam spectrum) the energy throughput of the instrument needs to be restricted in order to avoid detector nonlinearity (5). This is typically done by insertion of an aperture or wire screen into the path of the beam. However, when the same instrument is employed to examine microsamples using a sample holder, which is in itself an aperture, this throughput restriction should be removed. 5.3.5 When using an infrared microscope, it is normal to record the reference spectrum through the same aperture as is used for a particular sample. To accomplish this, it is most convenient to use visual observation to select the aperture size required to mask the sample area of interest. The single-beam spectrum of this sample area is recorded, and the reference single-beam background spectrum is then recorded afterwards. The transmittance (or absorbance) spectrum of the sample is obtained by using the instrument software to calculate the ratio of the two single-beam spectra.