Internal standards selection

The CE method was validated in terms of accuracy, precision, linearity, range, limit of detection, limit of quantitation, specificity, system suitability, and robustness. Improved reproducibility of the CZE method was obtained using area normalization to determine the purity and levels of potential impurities and degradation products of IB-367 drug substance. The internal standard compensated mainly for injection variability. Through the use of the internal standard, selected for its close mobility to IB-367, the method achieved reproducibility in relative migration time of 0.13% relative standard deviation (RSD), and relative peak area of 2.75% RSD. 

Internal standard. Selectivity, intrabatch precision and accuracy, recovery. 

With good samples, the transmission camera is capable of 0.3 % using an internal standard. Selected-area diffraction gives about 0.5 %, and reflection diffraction about 1 %. Electron diffraction patterns are sensitive to surface defects, particle size, sample position and orientation, and refraction effects, so that most materials are unsuited to precision measurements. Normally, electron diffraction is a poor second to X-ray diffraction as far as accuracy is concerned. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

The ISO 9000 series has been chosen because it is the most widely used Quality Management system and is a recognized international standard. The use of ISO 9000 in this book is not an endorsement of ISO 9000 over other systems. Rather, it is a practical decision based on the need to select one system to consistently illustrate the ideas contained in the book. 

In the internal standard method the intensity of the unknown line is measured relative to that of an internal standard line. The internal line may be a weak line of the main constituent. Alternatively, it may be a strong line of an element known not to be present in the sample and furnished by adding a fixed small amount of a compound of the element in question to the sample. The ratios of the intensities of these lines — the unknown line and the internal standard line — will be unaffected by the exposure and development conditions. This method will provide lines of suitable wavelength and intensity by variations of the added element and the amount added, due regard being paid to the relative volatility of the selected internal standard element. It is important to use as internal standard pairs only those lines of which the relative intensities are insensitive to variations in excitation conditions. The line selected as standard should have a wavelength close to that of the unknown and should, if possible, have roughly the same intensity. 

The plastics properties catalogue includes single-point data, multi-point data, processing data, product description texts and customer service information. You can select plastic products for your specific application by using the query options. The main feature of the CAMPUS philosophy is comparable data. The properties are based on the international standards ISO 10350 for Single-Point data and ISO 11403-1, -2 for Multi-Point data. CAMPUS is available in English, German, Spanish, French and Japanese. 

CAMPUS uses a uniform database structure and uniform interface for all participating suppliers, with frequent updates of the property data. It allows preselection or screening of materials, suitable for specific applications, from a worldwide range of commercial plastics, while continuously being developed further with respect to its properties base. CAMPUS is based on two international standards for comparable data, that use meaningful properties based on unambiguous selection of specimen types... 

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. 

It is possible to carry out a chromatographic separation, collect all, or selected, fractions and then, after removal of the majority of the volatile solvent, transfer the analyte to the mass spectrometer by using the conventional inlet (probe) for solid analytes. The direct coupling of the two techniques is advantageous in many respects, including the speed of analysis, the convenience, particularly for the analysis of multi-component mixtures, the reduced possibility of sample loss, the ability to carry out accurate quantitation using isotopically labelled internal standards, and the ability to carry out certain tasks, such as the evaluation of peak purity, which would not otherwise be possible. 

This selectivity allows the use of isotopically labelled analytes as internal standards and this, coupled with high sensitivity, allows very accurate and precise quantitative determinations to be carried out. 

How many ions should be monitored and how are appropriate ions for SIM selected There are a number of general guidelines and these should be applied equally to any internal standard(s) that may be employed, as described in the following ... 

Figure 5.66 Molecular structures of Idoxifene and its deutrated internal standard ds-Idoxifene. Reprinted from J. Chromatogr., B, 757, Comparison between liqnid chromatography-time-of-flightmass spectrometry and selected-reaction monitoring liqnid chromatography-mass spectrometry for qnantitative determination of Idoxifene in hnman plasma , Zhang, H. and Henion, I., 151-159, Copyright (2001), with permission from Elsevier Science.

Internal Standards. A compound selected as an internal standard ideally should behave in a manner identical to that of the analyte in all separation steps in the analytical process and should be measured by the same final determination method. Distillation from aqueous systems and solvent partition are the... 

It is clear that neither NMEA nor NDPA is appropriate for an internal standard in NDMA determination if criteria are interpreted strictly, but both compounds have been used for this purpose. Addition of a nitrosamine, not normally present in the sample, is helpful in detecting any gross errors in the procedure, but the addition should not be considered to be internal standardization. Utilization of NMEA or NDPA to indicate recovery of NDMA can lead to significant errors. In most reports of the application of these "internal standards", recovery of all nitrosamines was close to 100%. Under these conditions, any added compound would appear to be a good internal standard, but none is necessary. NDMA is a particularly difficult compound for use of internal standardization because of its anomalous distribution behavior. I mass j ectrometry is employed for quantitative determination, H- or N-labeled NDMA could be added as internal standard. Because the labeled material would coelute from GC columns with the unlabeled NDMA, this approach is unworkable when GC-TEA is employed or when high resolution MS selected ion monitoring is used with the equipment described above. 

Figure 5. Selected HPLC elution profile of products obtained after incubation of 0.25% polygalacturonate with PGII, upper trace, and PGII H223A, lower trace, respectively, demonstrating the effect of the mutation on catalysis. G1 to G3 indicate the peaks of the corresponding oligogalacturonates. IS indicates the internal standard, glucuronate. The vertical axis shows the pulsed amperometric detector response while the horizontal axis shows the retention time.

International standards for selected chemical, microbiology and other parameters. 

In both the GC/MS and the LC-FL analyses selected perdeuterated PAHs were added to the samples prior to extraction for use as internal standards for quantification. 

Alternatively, LC is used for the separation and quantification of PAHs using both UV and fluorescence detection. The analytes are identified based on their relative retention times and UV and/or fluorescence emission spectra. For UV detection an efficient cleanup is a prerequisite since this detection method is not very selective (almost universal for PAHs), and hence it also responds to many coeluting compounds. Due to the high specificity of fluorescence detection for most PAHs, this LC detection method is less susceptible to potential interferences. As in the case of GC the apphcation of internal standard(s) is mandatory since solvents have to be evaporated during the cleanup, which may result in partial losses of some of the more volatile analytes. 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

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