Internal standard absorption correction

International or in-house standards in combination with fundamental parameters software, lead to the same accuracy as conventional analysis using regression analysis of standards. Provided that accurate standards are available, the main factors that determine the accuracy of XRF are the matrix absorption correction and (in the case of EDXRF) the spectrum evaluation programme, i.e. correction for spectral overlap and background. 

NMR characterization of the tautomers has shown that the pure keto isomer just above its melting point gives a single peak. This, of course, is required if structure assignment is correct. Absorption is at —2.67 ppm with reference to tetramethylsilane as an internal standard. This tautomer is extremely reactive and polymerized to an appreciable extent even at - 78° C in a few hours or in a few seconds at room temperature. 

Table I lists several XRD analytical methods recently developed in the NIOSH laboratories. For each analyte, the analytical range, detection limit and analytical precision are listed. The method numbers refer to the NIOSH Manual of Analytical Methods (2.). As indicated in the table, there are several NIOSH methods available for free silica analysis. Method No, P CAM 109 incorporates the internal standard approach as developed by Bumsted (3.), The other two methods S-315 and P CAM 259 are based on the substrate standard method. The major difference between the two is the direct sampling on silver membrane filters (S-315). This paper will address the various methods of quantitation, sample collection and procedures for matrix absorption corrections that have been used in this laboratory for the analysis of crystalline particulate contaminants in the workplace.

Proponents of the internal standard procedure have questioned the validity of the substrate standard method to adequately correct for matrix absorption, Leroux and coworkers JLA) have presented data which support the method in addition, several measurements were performed in this laboratory to verify the validity of the method ( ), Mixtures of chrysotile in talc (1-7%) were prepared and various quantities "spiked" on silver filters. Table III illustrates the results obtained after correcting for matrix absorption as compared with the uncorrected data. Overall, there is excellent agreement between the corrected weight and the "spiked" weight. 

Using the Compton line of the tube as an internal standard reference sometimes enables a thorough correction of the matrix effects. This is illustrated by the analysis of trace elements Ni and V in petroleum products where the sulphur content is variable. Whilst, in the absence of sulphur, the matrix is extremely light and thus not particularly absorbent, an increase in sulphur content leads to a noticeable absorption of the Ni and V signals. The first curve in Figure 4.11 is plotted from net intensity levels measured on vanadium, whereas, in the lower curve the net V K. intensity levels have been replaced by the intensity ratios ... 

These systems are very versatile. Absorption and emission measurements can be run by both channels. A dual-channel system permits the simultaneous determination of two elements. The sample amount and time needed are then only half of that required by a conventional AA system. This is of great use in analyses of small samples. The background correction can be performed by the other channel. Then the difference of the absorbance readings of the two channels are measured. It is also possible to obtain the absorbance ratio of the two channels, which can be used in working with the internal standards. 

The widely used clinical flame photometers have been described in the section above on flame spectrometers and filter photometers. Analyte concentrations and their inherent line intensities are such that samples may be diluted greatly and various difficulties avoided. The burners are small and round so that self-absorption is slight and calibration curves nearly linear. Use of lithium added at a high concentration as the internal standard helps to correct for some of the uncontrolled variables. 

See also Atomic Absorption Spectrometry Principles and Instrumentation Interferences and Background Correction Flame. Atomic Emission Spectrometry Inductively Coupled Plasma. Quality Assurance Internal Standards. 

XRD analysis provides a means by which different crystalline phases are characterized and identified. Samples are normally prepared as finely ground material and then presented to the X-ray beam in such a way that individual crystallites are randomly orientated. Comparison of diffraction traces with standard reference profiles enables the identification of phases to be made. Reference patterns are published by the International Centre for Diffraction Data. In some cases with a careful use of reference materials, internal standards and mass absorption corrections, some quantitative results are obtainable using a variety of mathematical formulas on the peak heights or areas. 

The first use of an internal standard technique was reported in 1936 by Clark and Reynolds. Studies in the 1930s and the 1940s by Brentano, Shaffer, Taylor, and Brindley provided the basis for the appropriate corrections in the experimental intensities, particularly for absorption, and the microabsorption phenomena. (Grain-related effect occurs in polyphase samples. It occurs when large crystals preferentially interact with the beam, causing an appreciable reduction in the ratio of the diffracted intensities.)... 

Highlights The determination of the uranium content and isotopic composition in surface water samples is normally easier than in ocean water due to the generally lower salt content in the former. Matrix effects could cause a bias in the nranium content measurements that can be corrected with an internal standard or separation of uranium and care must be taken to avoid precipitation or absorption of uraninm in the sample container. The 234u/23su ratio may yield interesting geological information. 

In addition, flux hardening (preferential absorption of low-energy neutrons) and seif-moderation (unmoderated neutrons are further moderated inside the sample) may occur. Since the correction is difficult to apply for samples with mixed composition and irregular shape, the effect is often avoided by using as small a sample and a. standard as possible or by diluting the sample with a material having a low absorption cross section (graphite. cellulose). An internal standard can also be applied. 

MnO, P2O5 were also measured for some samples. The measured X-ray intensities were corrected for matrix effects, absorption, and secondary fluorescence by the Bence-Albee correction program. Results were internally calibrated against international standards, and a laboratory reference material (hornblende) was repeatedly measured to insure consistency between analytical sessions. The results for each point analysis were normalized to total 99.00% (allowing 1 % for water and trace elements) and then the average calculated for each sample (Table 2). 

The absorption effect has already been referred to in Section II. Clearly, in a multiphase mixture, different phases will absorb the diffracted photons by different amounts. As an example, the mass absorption coefficient for CuKa, radiation is 308 cm/g for iron, but only 61 cm/g for silicon. Thus iron atoms are five times more efficient than silicon atoms in absorbing CuKo, photons. There is a variety of standard procedures for correcting for the absorption problem, of which by far the most common is the use of the internal standard. In this method a standard phase is chosen that has about the same mass absorption coefficient as the analyte phase, and a weighed amount of this material is added to the unknown sample. The relative intensities of lines from the analyte phase and the internal standard phase are then used to estimate the relative concentrations of internal standard and analyte phases. The relative sensitivity of the diffractometer for these two phases is determined by a separate experiment. Other procedures are available for the analysis of complex mixtures, but these are beyond the scope of this particular work. For further information the reader is referred to specific fexts dealing with the X-ray powder method. 

X-ray diffraction is adaptable to quantitative applications because the intensities of the diffraction peaks of a given compound in a mixture are proportional to the fraction of the material in the mixture. However, directly comparing the intensity of a diffraction peak in the pattern obtained from a mixrnre is difficult. Corrections are frequently necessary for differences in absorption coefficients between the compound being determined and the matrix. Preferred orientations must be avoided. Internal standards help, but they do not overcome the difficulties entirely. 

Infrared spectra were obtained with a Perkin-Elmer 1800 and a Nicolet Magna-IR 750 FTIR spectrophotometer, and the absorption frequencies are reported in wave numbers (cm ). NMR spectra were obtained with a BZH-300 and a CA-F-300 Bruker FT NMR 300MHz spectrometer, and a Varian EM390 90-MHz spectrometer. All chemical shifts are reported in parts per million downfield (positive) of the standard. H-NMR and C-NMR chemical shifts are reported relative to internal tetramethylsilane, while T-NMR chemical shifts are reported relative to internal fluorotrichloromethane. Unless specified, Rf values were obtained from silica gel thin-layer chromatography developed with a mixture of 1.5 ml of methylene chloride and three drops of acetone. Melting points were determined with an Electrothermal melting-point apparatus without correction. Elemental analyses were performed by Galbraith Laboratories, Inc., Knoxville, Tern. 

Finally, although temperature had a large effect on both the position (wavelength) and the intensity of the water absorption bands in the emulsion NIR spectra, careful experimentation demonstrated that the 1618 nm vinyl C—H band used in the calibration model did not shift in either position or intensity with temperature, in the temperature range used in these studies (25-75° C). Therefore, it was not necessary to correct the calibration model for temperature effects, either by the use of internal or external standards, or by including temperature variations in the calibration set. 

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