Absorption, interferences

Absorption interferences would arise from compounds other than leucovorin that convert to anhydroleucovorin at acidic pH. An example of such a compound is 10fH F, although it is not expected to contribute to absorption due to its instability. However, 10-formyl-7,8-dihydrofolate is a possible contaminant. Additional absorption could also result from unsaturated pteridines. 

According to Stoeppler et al. [15], severe errors up to a factor of two may result from ETA—AAS analysis of biological materials without some form of sample pretreatment. The approaches that will be discussed here are (a) the use of diluent solutions to minimise matrix and molecular absorption interferences (b) partial decomposition techniques in which metals are extracted from proteins with acids (c) dissolution of tissue samples without complete oxidation (d) complete oxidation procedures such as dry ashing, wet digestion at ambient and elevated pressures, and low temperature ashing with reactive gases at low pressures. 

A few simple chemical reagents may be added to biological samples in order to minimise molecular absorption interferences by providing a more complete oxidation within the atomisation cell, or to minimise the effect of the matrix upon analyte sensitivity either by modifying the volatility of the analyte, or the matrix, or both. 

Although it is possible to measure directly Mn in body fluids at normal concentrations of 18—180 nmol 1 I (1—lOpgl-1) it is difficult to achieve a precision of better than 0.10 RSD. The elimination of the considerable molecular absorption interferences requires strict control of ETA ashing temperatures and a good background correction system, and the variable condensed phase matrix interferences from the inorganic constituents necessitates the use of standard additions for calibration [64], Even this approach may not yield a viable method due to curvature of the calibration graph at very low absorbances, particularly with a diluted blood matrix [65],... 

Few simple direct procedures exist for the measurement of selenium in body tissues and fluids using ETA—AAS. Severe matrix and molecular absorption interferences usually require some form of separation procedure to be used. Saeed et al. [70] recently reported an excellent method for measuring Se in 20 pi volumes of serum. The samples were diluted with 20 pi volumes of 0.1% Ag or Ni nitrate solutions, which effected some degree of matrix modification prior to direct analysis by ETA—AAS. The Ag or Ni... 

The measurement of Bi in body fluids and tissues may be achieved using either ETA—AAS or hydride generation techniques. Rooney [101] compared these procedures and reported that the latter was the method of choice. The severe molecular absorption interferences at 213.2 nm necessitate some form of chemical pretreatment for ETA—AAS. Thomas et al. 

Molecular absorption interferences can be reduced considerably by ashing within the atomiser itself, although care must be taken to avoid preatomisation losses of lead which occur at 350—400°C with untreated blood samples. These losses may be avoided at higher temperatures by the addition of a solution of (NH4)2HP04 [111]. The net effect of the sample matrix on... 

Bismuth added to urine was recovered by Willis (W14) with solvent extraction and determined by atomic absorption spectroscopy. An absorption interference rarely encountered in atomic absorption spectroscopy was seen from the absorption of the 3068-A line of bismuth by the OH radical in the air-coal gas flame. 

Cholesteric reflectors for all wavelengths of the visible spectrum [26-29] will be used as reflectors and polarizers for back lights of LC displays to get a higher yield of the emitted light Classical dyes containing polarizers absorb 50% of the polarized light in contrast to the cholesteric systems based on a non-absorptive interference mechanism. 

Fat-soluble vitamins are absorbed by the intestinal tract using the same mechanism that absorbs fat. Conditions that interfere with fat absorption interfere with fat-soluble vitamin absorption. Fat-soluble vitamins are stored in the liver, fatty tissues, and muscle, and are excreted in urine. They remain in the body longer than water-soluble vitamins. 

If use of an alternate wavelength is not possible, the composition of the reference and sample solutions can be made equivalent by adding an appropriate amount of the placebo matrix (formulation mixture without the active ingredient) into the reference standard solution mix. The absorption interference from the excipient will be equal in both the sample and the standard solution and is thus eliminated. 

The CL spectrum can be compared to the absorption spectra of reactants, products, and suspected interfering species to determine if spectral absorption interferences or if self-absorption is significant. 

Bengtsson, P. E., Alden, M., Kroll, S., and Nilsson, D. "Vibrational CARS Thermometry in Sooty Flames Quantitative Evaluation of Cj Absorption Interference." Combustion and Flame 82 (1990) 199. 

Manning, D.C. (1970b) Compensation for broad-band absorption interference in the flameless atomic absorption determination of mercury. Atom. Abs. Newsl., 5,109-110. 

Windham, R.L. (1972) Simple device for compensation of broad-band absorption interference in flameless atomic absorption determination of mercury. Anal. Chem., 44, 1334-1336. 

There is a considerable literature on atomic absorption interferences, which fall generally into three types, and can be controlled to some extent by choosing the proper sampling system. The three forms of interference are classifiable as chemical, ionization, and Tbulk, or matrix. ... 

Molecular-absorption interference, although less common, does occur. For example, a CaOH absorption band occurs in the region of the barium 553.6-nm line. A 1% calcium solution has been reported to give an absorption equivalent to that expected from 75 ppm barium, well within the analytical measurement range for barium (see Table 10.4). High concentrations of alkali-metal and other salts can exhibit appreciable molecular absorption, particularly at wavelengths less than 300 nm. 

Atomic spectrometric methods are relative methods. The signals produced by the sample solutions are compared to the signals caused by the reference solutions. If the samples and references are behaving differently during the measurements, interferences will be seen. Interferences in the flame AA techniques may be divided into chemical, ionization, physical, and background absorption interferences. 

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