Quantitative standard-additions procedure

A disadvantage of the split injection technique is the uncontrollable discrimination with regard to the sample composition. This applies particularly to samples with a wide boiling point range. Quantitation using external standardisation is particularly badly affected by this. Because of the deviation of the effective split ratio from that set up, this value should not be used in the calculation. Quantitation with an internal standard or alternatively the standard addition procedure should be used. 

Select one of the quantitation procedures we have discussed (response factor method, internal standard method, or standard addition method) and describe ... 

In the recent method of Zienmiak, et al3 3, a liquid chranatographic procedure has been developed which separates and permits quantitative analysis of cimetidine (I), cimetidine sulfoxide (II), its hydroxymethyl (111) and guanol urea derivatives (IV). Metiamide, SK F 92058, is used as the internal standard. Their procedure involves the precipitation of protein with acetonitrile, addition of anhydrous K2HPOit, extraction of the separated aqueous phase with methylene chloride and KH2POi, to saturate and salt out the solution. The methylene chloride is evaporated to dryness, the sample is reconstituted with mobile phase (CHaCbhCE OIbl OtNihOH,... 

The most extensive effort involves quality assurance. CDC provides common quality control (QC) materials and all members use standardized QC procedures. In addition, CDC regularly surveys the CRMLN laboratories to ensure they meet the required criteria. Initially, surveys were conducted monthly however, as the laboratories gained experience and performance improved, the survey schedule was changed to bimonthly. These surveys use CDC frozen serum secondary reference materials that have been prepared using NCCLS C37-A protocol [28], TC surveys include three levels analyzed in duplicate in two runs. HDLC surveys include four levels analyzed in duplicate in four runs. As an additional check on the quantitative step of the HDLC method, a low-total cholesterol material with cholesterol concentration <100 mg/dl is added to the TC survey. LDLC surveys include four levels analyzed in quadruplicate in four runs. 

Matisova and co-workers11 have suggested that the need for a reproducible sample volume can be eliminated by combining the standard addition method with an in situ internal standard method. In the quantitative analysis of hydrocarbons in petroleum, they chose ethyl benzene as the standard for addition, but they used an unknown neighboring peak as an internal standard to which they referenced their data. This procedure eliminated the dependency on sample size and provided better quantitation than the area normalization method they were using. 

The direct determination of trace elements (Al, Ba, Cu, I, Mn, Mo, Pb, Rb, Se, Sr, and Zn) by ICP-MS in powdered milk was reported [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. Mass intensity losses were not observed. The quantitative ICP-MS procedure was applied the standard additions method with a Y internal reference. This direct technique is as fast as the slurry approach without particle size effects or sensitivity losses. 

Hence, the standard additions method is unique in that it actually employs the very material under analysis as a reference matrix material, thus providing for efficient elimination of very complex matrix effects even when the final material is the result of a multi-step preparative procedure and the composition of the matrix of the original material is completely unknown. These advantageous features of the standard additions technique have been discussed and verified in context with quantitative headspace gas analysis [68]. 

Another standardization technique which is widely used in AAS is standard addition. It is especially useful for samples where the matrix is diflBcult to reproduce, such as samples decomposed by fusion. In this method, a known quantity of the element to be determined is added to a portion of the sample, preferably at the start of the decomposition procedure. This known amount then serves as the standard when the absorbances of the two solutions are compared. Because of the rather prevalent curvature of absorbance vs. concentration plots, it is usually necessary to run three standards as well as the unspiked sample. Thus, even if one could use a single multiple element spike as the solution for standard addition, this would quadruple the work load for sample decomposition and analysis. If such a solution were added after decomposition, it would require splitting the quantitatively diluted solution, spiking, and rediluting, and would quadruple the work load for analysts. 

The existence of reference materials and appropriate calibration procedures are two essential issues to be considered in quantifying the components of a sample. Quantitation has been substantially improved by the commercial availability of an increasing number of certified solid reference materials, especially for low concentration levels. Recently [42], NIST archival leaf standards were used as matrix-matched standards for reliable quantitative elemental analysis of Spanish moss samples. LA-ICP-MS was used with mixing standards in order to produce at least three data points for each calibration curve the results thus obtained were compared with those provided by microwave digestion ICP-MS/AES. Standard addition was also examined and found to be an effective method in the absence of matrix-matched standards. 

Py-GC/MS can be applied for both qualitative and quantitative purposes. One typical use of quantitative analysis using pyrolysis is the determination of the amount of a specific polymer in a given complex matrix, such as a composite material, inorganic matrix, etc. Since solubilization is frequently a very difficult task for these materials, pyrolysis can provide quantitative information based on the level of the polymer marker generated by the thermal decomposition. Calibration is typically necessary in these situations, and similarly to other analytical procedures this can be achieved using a standard addition type procedure (see e.g. [17]) or a calibration with known amounts of polymer in a similar or identical matrix. Another case where the quantitation can be necessary is the determination of the amount of a comonomer in a copolymer sample. Successful quantitation by Py-GC/MS is reported in literature for various copolymers [25-39], etc. 

For an exact characterization of the investigated systems, besides the isotope ratio measurement, one has to carry out the quantitative analysis of polyethers, and of the corresponding metal complexes as well. The analysis method which is most commonly described in the literature for polyethers is NMR spectrometry However, this spectroscopic method is not very precise for a quantitative determination. Therefore, other types of analysis have been developed, e.g. a gravimetric procedure and a titrimetric method using the standard addition technique with a potentiometric indication... 

Analytes may be quantitated using an internal standard or by the method of standard addition. An internal standard is a structural analog (e.g. deuterium-labeled compound), a known amount of which is added to specimens prior to any preparatory steps such as extraction or hydrolysis. Ideally, the internal standard should be as chemically similar to the analyte of interest as possible, so that throughout the analytical procedure (e.g. extraction, chromatographic separation and ionization processes) and under the specified conditions (e.g. derivatization), its recovery will be similar to the analyte s. The concentration of analyte can then be determined from the ratio of the analyte response to the internal standard response. 

Recommended Method. In the recommended method a 100-g sample is decomposed with concentrated sulfuric acid, ashed at 500°C, dissolved in dilute hydrochloric acid, and the concentration of nickel or vanadium measured by HVAA using the method of standard additions. The results of applying this procedure to several distillate fuels are shown in Table 14.IV. Each material was spiked to contain different levels of nickel and vanadium, which were added as the sulfonates. Essentially quantitative recoveries were obtained for both nickel and vanadium from all materials. The standard deviation for nickel was 6 over the range 50 to 100 ng Ni/g, and for vanadium 8 ng V/g over the range 30-100 ng V/g. The few tests at the higher level indicated a comparable precision. 

Quality control procedures should include blanks to indicate contamination. Replicate sample analysis indicates random contamination and precision. Accuracy can be assessed by quantitative recovery of added Al, including use of CRMs that are similar in nature to the samples, utilization of more than one procedure such as standard additions and comparison to aqueous standard curve, comparison to another analytical technique and/or participation in inter-laboratory comparison programs. 

Six general procedures are available to reduce or correct for the matrix dependence comparison standards, internal standards, standard addition, dilution, thin films, and scatter correction. Each of these is discussed below. One essential requirement for both samples and standards in all methods (other than those using thin films) is homogeneity on a micrometer scale because of the low penetration of incident x-ray photons. Other practical factors to be considered for quantitative results include surface preparation and particle-size distribution. 

Like all classical quantitative analysis methods, NMR spectroscopy needs calibration, calibration standards and a validation procedure. The standard techniques are used for calibration external calibration, the standard addition method and the internal standard method. A fourth is a special NMR calibration method, the tube-in-tube technique. A small glass tube (capillary) containing a defined amount of standard is put into the normal, larger NMR tube filled with the sample for analysis. In most cases, there are slight differences in the chemical shift of corresponding signals of the same molecule in the inner... 

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