Quantitative external standards procedure

The labor requirements of GC methods are also being addressed. Traditionally, trace analyses by GC have been quantitated using an internal standard for calibration. These test methods require careful weighing procedures for each sample. Now, with the routine use of autosamplers to provide repeatable injections, an external standard procedure is preferred as a means of saving analyst time. Trace impurities by GC, ASTM Test Method D5917, was written as an equivalent to the internal standard GC method D2360. 

Quantitative analysis of AP/APEO by HPLC-FL can be performed with external standard solutions of mixtures of AP or APEO. Initially quantification of oligomeric mixtures was based on the elaborate procedure of normal-phase analysis with subsequent quantification of all oligomeric peaks [27]. Kiewiet et al. [28] have described the general principle of quantification of ethoxymers in reversed-phase LC with spectroscopic detection in detail using the example of derivatised alcohol ethoxylates. Based on this method the quantitative analysis of... 

Variation in sample volume is the factor that most affects the precision of quantitative measurements and the use of an injection valve may overcome this and permit the use of external standards. However, it is still often desirable to use an internal standardization procedure as this will reduce the effects of any variation in the detector responsiveness over a period of time. 

An external standard method is used when the standard is analyzed on a separate chromatogram from the sample. Quantitation is based on a comparison of the peak area/height (HPLC or GC) of the sample to that of the reference standard for the analyte of interest. The external standard method is more appropriate for samples with a single target analyte and narrow concentration range, where there is a simple sampling procedure, and for the analysis of hydrocarbon fractions. The calculation requires an accurate extract final volume and constant injection size. The peak area of an analyte is compared with that from a standard or standard curve and corrected for volume ... 

The most accurate quantitative results in GC and LC are probably obtained by the use of an internal standard. However, it can be difficult to employ, as the procedure depends upon finding an appropriate substance that will elute in a position on the chromatogram where it will not interfere or merge with any of the natural components of the mixture. Unfortunately, for a multi-component sample, this can become difficult to the extent of being virtually impossible, under which circumstances the external standard method must be used. Having identified an appropriate reference standard, the response factors for each component of interest in the mixture to be analyzed must be determined. It should be noted that this usually does not include all the components. In many instances only certain components of the mixture need to be assayed. A synthetic mixture is then made up containing known concentrations of each of the components of interest together with the standard. 

Prerequisites for the Calibration Types. It depends on the design of the analytical procedure as to which regression parameters are meaningful and which results are acceptable. In other words, the model to be used for quantitation must be justified. For a singlepoint calibration (external standardization), a linear function, zero intercept, and the homogeneity of variances are required. The prerequisites for a linear multiple-point calibration are a linear function and in case of an unweighted calibration also the homogeneity of variances. A non-linear calibration requires only a continuous function. With respect to the 100%... 

In general, quantitative analyses are carried out as usual, using internal or external standards and calibration curves. Detection limits can be achieved in the lower picogram range dependent on the selected derivatization procedure. 

This HPLC procedure utilizes direct injection of juice samples on two reverse phase C-18 columns connected in series. Tartaric, quinic, malic, citric and fumaric acids were quantitated by the external standard method. Table II summarizes the results for the pear juice samples. Ripening had little influence on the total titratable acidity, however, quantitative differences in the compositional profile were evident. Most noticeably, the level of quinic acid was higher in the ripened fruit. 

It is clear from both interlaboratory studies that differences in measurement data on SCCPs (and likely CPs of other chain length) can be notable. The choice of quantitative procedures used by laboratories and also choice of external standard employed (this was certainly the case in the Tomy et al. (1999) smdy [62]) can contribute to unreliable data. The recent commercial availability of impurity-fi ee synthetic C10-C13 solutions should eliminate the uncertainty in preparation of working standard solutions. However, other confounding variables like choice of extraction and clean-up procedures have not been quantitatively assessed and are likely to contribute to the uncertainty in CP data. 

External standard, quantitation by using external standardization. The analyte itself is used for quantitative calibration as a clean standard or added to a blank standard matrix. The signal height for a known concentration of the analyte is used for the calibration procedure. The calibration runs are carried out separately (externally) from the analysis of the sample. 

Quantification of the relative abundance of crystalline phases in a multiphase mixture is an everyday problem in a wide range of applications. Common examples are evaluation of the yield in inorganic synthesis and catalytic processes, characterization of raw mineral materials for industrial processes, quality check of fired ceramic products, and many more. While in most cases the required accuracy level of the analysis is a few percent at best, in particular cases such as in the quantification of phase contaminants in technologically important materials, or of hazardous and toxic phases in environmentally dispersed aerosols, the required level of accuracy must be substantially lower than 1 wt% relative abundance. Accuracy levels of 2-3 wt% are commonly reached if standard procedures of quantitative phase analysis by diffraction data are properly performed. Generally employed analytical methods include the internal or external standard method, the matrix flushing method, and the reference intensity ratio method. Very recently, the availability of analysis techniques of powder diffraction data based on full-profile (Rietveld method), originally developed... 

The isotope dilution analysis method provides a definitive technique that does not depend on the comparison to one or more external calibration standards for quantitation. The method is similar to the standard addition technique in that a known quantity of analyte material is added to the sample to be analyzed, thereby performing the quantitation in situ. This approach ehminates matrix interference problems. The method is also similar to the internal standardization procedure, with the exception that a form of the analyte itself serves as the internal standard. AU of these factors combine to provide analysis characteristics comparable to the most basic gravimetric methods. 

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