Internal standard absorption

Description of Method. The water-soluble vitamins Bi (thiamine hydrochloride), B2 (riboflavin), B3 (niacinamide), and Be (pyridoxine hydrochloride) may be determined by CZE using a pH 9 sodium tetraborate/sodlum dIhydrogen phosphate buffer or by MEKC using the same buffer with the addition of sodium dodecyl-sulfate. Detection Is by UV absorption at 200 nm. An Internal standard of o-ethoxybenzamide Is used to standardize the method. 

To use KBr discs for quantitative measurements it is best to employ an internal standard procedure in which a substance possessing a prominent isolated infrared absorption band is mixed with the potassium bromide. The substance most commonly used is potassium thiocyanate, KSCN, which is intimately mixed and ground to give a uniform concentration, usually 0.1-0.2 per cent, in the potassium bromide. A KBr/KSCN disc will give a characteristic absorption band at 2125 cm 1. Before quantitative measurements can be carried out it is necessary to prepare a calibration curve from a series of standards made using different amounts of the pure organic compound with the KBr/KSCN. A practical application of this is given in Section 19.9. 

The deviations of Class I, here called absorption and enhancement effects, are known in the literature also as matrix effects, as self-absorption, and as interelement effects. The authors consider the most important objection to each of the last three names to be as. follows. To matrix effect the element sought (ncrt included in the matrix) contributes to the absorption effect for the sample in the same kind of way as any element (free or combined) in the matrix. To self-absorption the name makes no provision for enhancement effects. To interelement effects it fails to recognize that an absorption effect occurs even when only one element is present (Equation 7-4). The term matrix is useful but requires precise definition. What is the matrix when an internal standard is added, or when a powdered sample is dissolved ... 

The most interesting and important use of internal standards is to compensate absorption and enhancement effects the Class I deviations of Section 7.8. A discussion of this function of an internal standard can be profitably begun by considering two statements (1) An internal standard is fully satisfactory when the intensity ratio obeys the relation... 

To discuss the effectiveness of internal standards, it is helpful to recall the absorption and enhancement effects the standards are intended to compensate. In the present discussion, we shall ignore the incident beam even though this simplification is not always justified. We have indicated above that an internal standard will hot be completely effective if a change in composition influences the intensity ratio (Equation 7-12). Such influences could arise from differences in the extent to which the two analytical lines are absorbed or from differences in the extent to which the two lines are excited. Let us examine the way in... 

Because the two analytical lines differ in wavelength, an internal standard can never compensate absorption and enhancement effects completely. If Cases II and IV %re avoided in selecting ah internal standard, the use of such a standard will usually prove satisfactory. Special cases may require special calibration curves run with the disturbing elements present. 

It is sometimes convenient to use a properly chosen scattered line in the background for comparison to avoid having to add an internal standard to the sample.37 The experience of the Applied Research Laboratories shows that the effects of variations or fluctuations in the equipment and of particle size in powdered samples can be eliminated satisfactorily in this way. In some cases, absorption and enhancement effects are also adequately compensated. We have already mentioned (7.8) that scattered reference line and the analytical line will be subject to considerably different absorption effects if the two lines differ appreciably in wavelength. Everything depends upon the selection of a satisfactory scattered reference line, and this is done empirically. 

Recent results from the authors laboratory69 on the x-ray emission spectrography of tungsten or molybdenum in solution illustrate some of the points made in Section 7.13. The also show the usefulness of internal standards (7.12). Finally, the work on tungsten is closely related to the experiments on the absorption effect in sodium tungstate solutions, the results of which are summarized in Table 7-2. 

To test the effectiveness of the internal standard in compensating absorption effects attributable to added sodium hydroxide, the experiments of Table 7-8 were carried out. The internal standard is clearly effective Rt shows Ho trend attributable to an absorption effect. 

Broadening, of monochromatic beams by crystals, 115, 116, 118 of pulse-size range, 47 Bromine, as internal standard in tungsten analysis, 194-196 determination, by absorption-edge method, 138-142... 

As a result of different chemical surroundings the resonance frequency absorption will show definite peaks registered by Fourier transform technique. Compared to an internal standard the intensity of the signals is used for quantitative determination of different phosphorus-containing compounds in a given sample. 

Dowty, M. E. Dietsch, C. R., Improved prediction of in vivo peroral absorption from in vitro intestinal permeability using an internal standard to control for intra- inter-rat variability, Pharm. Res. 14, 1792-1797 (1997). 

From the uv absorption spectra, a suitable wavelength is found for the simultaneous detection of aspirin, phenacetin and caffeine. Using phenacetin as internal standard, response factors are calculated for aspirin and caffeine and the results are used for the quantitative determination of aspirin and caffeine in an analgesic tablet. 

Hasegawa et al. [76] measured miconazole serum concentration by a high performance liquid chromatographic method. The authors assessed whether the internal standard method produced an intra-assay error and found that the method gave more precise and more reproducible results compared to the absorption calibration curve method. With 0.5 pg/mL of miconazole, the coefficient of variation produced by that method was 3.41%, whereas that of the absorption calibration curve method was 5.20%. The concentration of absorptions calibration curve method showed higher values than the internal standard method. This indicated that the internal standard method was far more precise in measuring the miconazole serum concentrations than the absorption calibration curve method. 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Because most common solvents, including water, contain protons, and most NMR analyses involve the measurement of protons, a solvent without protons is generally used in NMR spectroscopy. Commonly, solvents in which the hydrogen atoms are replaced with deuterium (i.e., solvents that have been deuterated) are used, the most common being deuterochloroform. In addition, an internal standard, most commonly tetramethylsilane (TMS), is added to the sample in the NMR sample tube (see Figure 14.3, D) and all absorption features are recorded relative to the absorption due to TMS. 

Increasing standard amounts of analyte are added to the sample and the resulting peak areas, which should show an increase with concentration added, are measured. This method is not as useful in GC as it would be in atomic absorption (see Chapter 9), since the sample matrix is not an issue in GC as it is in atomic absorption, due to the fact that matrix components become separated. However, standard additions may be useful for convenience s sake, particularly when the sample to be analyzed already contains a component capable of serving as an internal standard. Thus, standard additions could be used in conjunction with the internal standard method (see Experiment 45), and the internal standard would not have to be independently added to the sample and to the series of standards — it is already present, a convenient circumstance. Area ratio would then be plotted vs. concentration added and the unknown concentration determined by extrapolation to zero area ratio. Please refer to Chapter 9 for other details of the method of standard additions. 

A graph is plotted between the concentration values and the ratios obtained from the physical value (i.e., peak area of absorption) of the internal standard and the series of known concentrations, thereby producing a straight line. Any unknown concentration may be determined effectively by adding the same amount of internal standard and locating exactly where the ratio obtained falls on the concentration scale. 

This particular method is often employed in a situation where the absorption bands of the analyte are found to be very close to those of the main constituent or the internal standard. 

Internal Standard for KBr-Disc Technique In quantitative analysis it is essential to examine absolutely uniform discs of identical weights. To achieve this, known weights of both KBr and analyte are required in the preparation of the KBr-disc and finally from the absorption data a calibration-curve may be obtained. In this process, it is a must to weigh the discs and also to measure their thickness at different points... 

General Procedure for the Hydroformylation/Electrophilic Substitution. Synthesis of 5,6-dihydroindolizines. A solution of 1-allylpyrroles (leq) and Rh4(CO)i2 (lmol%) in toluene was introduced by suction into an evacuated stainless-steel reaction vessel. CO (60 bar) was introduced, the autoclave was then rocked, heated to the desired temperature and H2 (60 bar) was introduced rapidly. When the gas absorption reached the value corresponding to the fixed conversion, the reaction mixture was siphoned out. The degree of conversion and the product distributions were determined by GC and GC-MS, by using acetophenone as an internal standard. 

Direct determination of portal blood flow rate is difficult and would generally require placement of an electronic flow probe in each animal. However the technique proposed by Hoffman et al. utilised tritiated water as an absorption probe (i.e. internal standard) [89], By dosing and sampling drug/ absorption probe concurrently, factors such as variable portal blood flow rate are normalised between experiments. 

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