Background absorption

The analysis of clinical samples is often complicated by the complexity of the sample matrix, which may contribute a significant background absorption at the desired wavelength. The determination of serum barbiturates provides one example of how this problem is overcome. The barbiturates are extracted from a sample of serum with CHCI3, and extracted from the CHCI3 into 0.45 M NaOH (pH 13). The absorbance of the aqueous extract is measured at 260 nm and includes contributions from the barbiturates as well as other components extracted from the serum sample. The pH of the sample is then lowered to approximately 10 by adding NH4CI, and the absorbance remeasured. Since the barbiturates do not absorb at this pH, the absorbance at pH 10 is used to correct the absorbance at pH 13 thus... 

Background correction is used to compensate for background absorption and scattering due to interferents in the sample. Such interferences are most severe for analytes, such as Zn, that absorb at wavelengths of less than 300 nm. 

When using the DIAL method to measure the concentration of a molecule with discrete absorption the wavelengths of the two laser beams, on and off a narrow absorption line, must be very similar (less than 1 nm separation) so that the background absorption and backscatter is the same for both. 

The deuterium arc continuum travels the same double-beam path as does the light from the resonance source (see Fig. 21.9). The background absorption affects both the sample and reference beams and so when the ratio of the intensities of the two beams is taken, the background effects are eliminated. 

Ideally, the background absorption should be measured as near as possible to that of the analyte line. This approach has been achieved in the subsequent methods described below. 

Here, B is the background absorption in the absence of chemical relaxation, ca the angular frequency of the inflection point of the absorption ca = 2nf (rad s ) and X the relaxation time (s). The excess absorption due to chemical relaxation A is obtained from the low-frequency absorption where ca x and is given by ... 

The low-frequency band in the spectrum of sample Af was superposed on a rather intense high background absorption band its exact shape could not be determined. The conclusion that these bands are due to the presence of CO at the surface follows from their close similarity to those found upon adsorption of CO on the same samples. Hence, dissociative chemisorption of C02 does indeed take place. However, the CO bands observed upon dissociative adsorption of C02 are much weaker than those produced by CO directly adsorbed as such. This becomes evident if we compare them with the nitrogen absorption bands in Figs. 8 and 9 where... 

Since in many applications minor absorption changes have to be detected against strong, interfering background absorptions of the matrix, advanced chemometric data treatment, involving techniques such as wavelet analysis, principle component analysis (PCA), partial least square (PLS) methods and artificial neural networks (ANN), is a prerequisite. 

The diffuse reflectance spectrum of CsReFg was studied between 10 and 40 kK. by Brown et al. (32), but no clearly defined peaks could be recognised above a high background absorption. Since Laporte-allowed charge-transfer bands usually yield well marked intense absorptions it is reasonable therefore to conclude that these are absent below 40 kK. (See also Section 5). 

The molybdenum concentration in the reference sample is rather high and a direct determination using the cookbook conditions [710] is very straightforward. There is no difference between peak area and peak height evaluation. In spite of 1800 °C for thermal pretreatment, a small background absorption signal is present. 

Devoto 115)has described an indirect procedure for the determination of 0.1 ppm arsenic in urine. The arsenomolybdic acid complex is formed and extracted from 1 ml of urine at pH 2 into 10 ml of cyclohexanone. The molybdenum in the complex is then measured. Before extracting the arsenic, phosphate in the urine is separated by extracting the phosphomolybdic acid complex at pH 1 into isobutyl acetate. The direct determination of arsenic in biological material and blood and urine is best done using a nitrous oxide-acetylene flame 116>. The background absorption by this flame is low at 1937 A, and interferences are minimized due to the high temperature of the flame. 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

Define background absorption and background correction. How does the use of a continuum fight source help with background correction ... 

Mobile phase should not show strong background absorption at the selected wavelength. 

Noise consists mainly of two elements instrumental noise, which includes the lamp and its age, and noise due to the high background absorptivity. As this is an important parameter, Yeung has proposed the term the dynamic reserve (D ) for this ... 

Optical properties of the material are less critical for microchips hyphenated with MS than for devices with on-chip optical detection where low background absorption or fluorescence is mandatory. Thus, completely opaque polymers like glassy carbon or polyimide " can be used as microfabrication substrates. Furthermore, polymer microchips are of great interest because their potentially low manufacturing costs may allow them to be disposable. Methods used for the fabrication of plastic chips include laser ablation and molding methods. 

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