Quantification standard addition

In relation to quantification, standard addition methodology is usually used due to the presence of matrix effect [75]. For that, spiking samples with different amounts of standard compounds were prepared, in order to obtain a concentration range in agreement with the levels observed in assayed samples. Tentatively identified compounds (with no standard available) were quantified as equivalents of the most similar compound, taking into account their molecular weight [94]. In the other cases, triphenyl phosphate (TPP) was used as an internal standard for quantification purposes... 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

Quantification by standard addition using pentane internal standard... 

Headpace GC with electron-capture or flame-ionisation detection. Quantification by standard addition using 1-chloropropane internal standard Headspace GC of the polymer dissolved in N,N-dimethylacetamide with detection by electron-capture or flame-ionisation detection. Headpspace GC of the plastic if not soluble... 

Matrix effects were evaluated by Gago-Ferrero et al. [23]. Both signal suppression and signal enhancement were observed. The extent of these effects was found to be fairly dependent on the UV filter. Thus, quantification should be performed by standard addition or internal standard calibration. Since standard addition is a high time-consuming procedure, internal standard calibration with appropriate isotopi-cally labeled compounds is the best option. 

Because of this complexation capacity, any standard addition performed at high pH will not return 100% of the spike, so a true value for the copper concentration cannot be calculated. Therefore, after an initial measurement at high pH the sample was acidified to pH 1.0 with 0.5 ml acid and another trace obtained. This compared the amount of copper released at low pH with the labile fraction at high pH. Standard additions were performed on the sample at low pH so almost all of the spike was returned. This allowed an estimate to be made of the percentage of total copper that was labile at high pH, and the quantification of this fraction in pg/1. This is illustrated graphically in Fig. 5.9. 

Quantification is usually achieved by a standard addition method, use of labeled internal standards, and/or external calibration curves. In order to allow for matrix interferences the most reliable method for a correct quantitation of the analytes is the isotope dilution method, which takes into account intrinsic matrix responses, using a deuterated internal standard or carbon-13-labeled internal standard with the same chemistry as the pesticide being analyzed (i.e., d-5 atrazine for atrazine analysis). Quality analytical parameters are usually achieved by participation in interlaboratory exercises and/or the analysis of certified reference materials [21]. 

The standard addition procedure is another method for recognising and overcoming potential matrix effects in quantification. Both alternatives, FIA—MS or FIA—MS—MS, can be performed using this procedure. Despite the increased expenditure because of a multiplication in analyses, the FIA approach combined with standard addition remains the faster technique even with the application of specific analytical MS—MS techniques such as product-, parent- or neutral loss scans applying selected reaction monitoring (SRM). The greatest drawback of this technique is that the compounds to be quantified must... 

Obstacles of ionisation interferences in the quantitative determination of the N-containing surfactants from industrial blends dissolved together with AE compounds in methanol applying FIA-MS can be minimised or even eliminated if quantification was performed in the standard addition mode. So the standard deviations (SD) observed now reached a maximum of 7% for N-containing compounds whereas AE could be quantified with a SD of 4%. In parallel, the time investment for FIA-MS quantification in the standard addition mode, however, increased considerably and reached a factor of 3-4. 

The quantification of N-containing surfactants under MS-MS conditions but without standard addition resulted in a SD of 14%. In parallel, an increase in time by a factor of 1.5 was found compared with FIA-MS analysis without standard addition. The results obtained with MS-MS using product, parent or neutral loss scan are more... 

The alternative to FIA—MS performed in the standard addition mode is the quantification in FIA—MS—MS mode combined with a standard addition procedure. Here, the analytical specificity and the approach of a rapid analysis is combined, however, analysis time is expanded by a factor of 4.5 compared with FIA-MS. 

With this technique, the sample is injected hy applying a range of a few volts to the sample for a certain period. The ions that are ionized will migrate into the capillary as a function of their mobility, as will a small amount of sample mobilized by the EOF that is generated due to the voltage applied. Jackson and Haddad" was able to analyze low levels (ng/mL) of anions in samples of moderate ionic strength. For quantification, he recommended the use of one or two internal standards or a standard addition. 

The detection and quantification capabilities of analytical methods often are important if they are used at trace levels of analytes. The description of the standard addition method, a special calibration in the sample finatises the chapter. 

The advantage of IDMS in comparison to other quantification strategies (external calibration or standard addition) is that analyte recovery does not have to be quantitative, providing that isotopic equilibrium has been achieved between all of the analyte and the added spike material. High accuracy analysis by IDMS is now well established.53 In the following paragraphs selected examples of IDMS use will be discussed briefly. 

QUANTIFICATION. The addition of the internal standard (10 yg) to the urine before extraction is necessary for the quantification procedures. Known amounts of amphetamine and diphenylamine in the range 0-5 yg are chromatographed and calibration curves are plotted for peak area (y axis) versus concentration (x axis). 

Degassing allows the elimination of residual oxygen that may modify the nature of certain chemical compounds. A better amma recovery is obtained when the aqueous phase is saturated with salt, while an internal standard addition allows semi-quantification. 

While the exact recovery depends on the volatility of the flavor compounds, most compounds can be detected with this method when present at ppb (mg/liter) concentrations. Reproducibility (CV) is between 5% and 10%. To quantify the amount trapped, an internal standard curve can be made by adding the standards in solvent directly to the trap just before thermal desorption on the side of gas entry during thermal desorption. For liquid homogeneous samples, quantification of the amount in the matrix can be done by a standard addition methodology. 

Pesticide quantification by means of standard addition algorithm... 

In this method, portions (2 g) of the plant material were digested in 30 ml of digestion acid until fumes of nitric acid are produced then the solution is cooled, and diluted to 50 ml in 15% hydrochloric acid-10% sulfuric acid vlv. This solution was used for analysis. Recovery studies were made by spiking the digestion acid or sample just prior to the digestion procedure with varying amounts of standard solution. Quantification of the six elements was made from linear calibration curves verified by the method of standard additions. 

Where ETAAS is the technique used for quantification, the method of standard additions is strongly recommended for calibration. 

One of the important aspects of any analytical method is its calibration and, therefore, much effort has been put into SPME calibration. As it is not always practicable to employ traditional calibration methods (external standards, internal standards, and standard addition) owing to the sometimes significant matrix effects in complex samples, equilibrium calibration has been suggested as an alternative. In SPME, however, it would normally take rather a long time to achieve equilibrium calibration. If sensitivity were not a concern in an analysis, reduction of extraction time would be desirable, that is, the extraction could be stopped before equilibrium but this would thus demand a new approach to calibration. In this regard, as a way of circumventing matrix effects in environmental analysis, several diffusion-based calibration methods have been recently developed for quantification in SPME.30... 

P T. Headspace does not have problems with water, and memory effects are greatly reduced from those found with P T. Problems will be encountered if the sample contains matrix modifiers that change the analyte pKas. In this case, quantification should be based on standard additions methods. 

Whereas carbonyl chloride itself is very moisture sensitive and requires the corresponding precautions such as efficiently dried glassware and solvents, the Box derivative is very stable and can be analysed by high performance liquid chromatography (HPLC) with ultra violet (UV) detection at 270 nm. Quantification is achieved by the standard addition procedure spiking carbonyl chloride into the test polymer solution. However, since Box is a commercially available chemical, it is advisable to work also with Box standards, especially when the method is used for the first time and when problems are experienced in the HPLC determination or the derivation procedure. The standards of the carbonyl chloride derivative are particularly useful to establish the analytical system and to check linearity of detector response as well as for the recovery check. 

A microcystin can be identified according to its mass spectram, as long as a standard is available. In MS/MS, the fragmentation pattern can be used to greatly assist in determining the identities of unknown microcystins (Bateman 1995 Lawton 1995 Yuan 1998, 1999). However, the problem of the lack of standards is still present as with any other analytical procedure since standards are necessary for accurate quantification. In addition toxicity data are required in order that toxicity equivalents can be calculated. 

Quantification can take place by the normalized percent method or by standard addition using the integrated peak areas of the fluorosilanes, taking account of the substance-specific correction factors. Calibration and checks of linearity were carried out using mixtures of siloxanes of known purity, e.g., methyltris(trimethylsiloxy)silane (M3T) in octamethylcyclotetrasiloxane (D4). 

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