Absolute Quantitation, Standard Addition Method

The main drawback for determining the absolute composition of solid samples using the voltammetry of microparticles approach is that there is no possibility of controlling the exact amount of sample deposited on the electrode surface. To solve 

Let us consider a solid sample containing an unknown mass concentration, /, of an electroactive species, A. Assuming, as before, that under the selected electrochemical conditions (technique, parameters, electrolyte) the voltammetric peak current (or peak area), 1a, is proportional to the amount of A transferred to the electrode, one can write 

The deposited sample also contains a mass mr of a R-containing compound (of molecular mass Mr), in which a mass fraction Jr of R exists. Then, the R-centered voltammetric signal should verify 

Samples should be prepared by weighting the appropriate amounts of the original sample, the A-containing standard and the R-containing auxiliary compound. Conducting the experiment so that a constant and known m/triR ratio exists for different samples containing known ma/mr ratios, one can write 

Application of the standard addition method for the determination of the content of zirconium in ceramics is illustrated in Fig. 4.13. Here, Zr02 was used as 

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In order to obtain quantitative results by HS-GC, the system must be calibrated. Absolute quantitation is not possible. Quantification can be done by the conventional external calibration method with liquids containing the analytes concerned in known concentrations or by means of standard addition. Pausch et al. [958] have developed an internal standard method for solid headspace analysis of residuals in polymers in order to overcome the limitations of external standardisation cfr. Chp. 4.2.2 of ref. [213a]). Use of an internal standard works quite well, as shown in case of the determination of residual hydrocarbon solvent in poly(acrylic acid) using the solid HS-GC-FID approach [959]. In the comparison made by Lattimer et al. [959] the concentrations determined by solid HS-GC exceeded those from either solution GC or extraction UV methods. Solid HS-GC-FID allows sub-ppm detection. For quantitative analysis, both in equilibrium and non-equilibrium conditions, cfr. ref. [960]. Multiple headspace extraction (MHE) has the advantage that by extracting the whole amount of the analyte, any effect of the sample matrix is eliminated the technique is normally used only for method development and validation. 

Calibration of the instrument is the first essential step of a quantitative procedure. Like any other analytical instrument, the response of a mass spectrometer is not absolute and might deviate with time. In addition, the sample matrix has a variable influence on the mass spectrometry response. Calibration involves determination of the correlation between a known concentration of the analyte and the resulting mass spectrometry signal. In ideal situations, the sample and the analyte standards are both analyzed under identical experimental conditions. Depending on the levels of accuracy and precision that are required, the calibration might be performed by one of the methods described below. 

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