Standard addition calibration

In order to validate sliding spark spectrometry results, plastic material was collected and the element concentration was determined via AAS after digestion. The samples were used as calibration standards. Additional standards were obtained by manufacturing known amounts of additives in the polymer matrix. Calibrations were made for Cd, Cr, Pb, Zn, Sb, Si and Ti in chlorine-free polymers Al, Ba, Ca, Cd, Pb, Sn, Ti, Zn in PVC chlorine (as PVC) and bromine in polyurethane (PUR). A calibration curve for Br as a flame retardant in PUR is shown in Figure 8.5. 

Quantitative analysis in ICP-MS is typically achieved by several univariate calibration strategies external calibration, standard addition calibration or internal standardisation. Nevertheless multivariate calibration has also been applied, as will be presented in Chapters 3 and 4. 

Finally, the combined influence of the LC conditions on the herbicide response, and the ion suppression by matrix effect into the API interface, make the quantification procedure quite tricky. The former needs much more theoretical and experimental data to be clearly understood, whereas the use of in-matrix calibrations (standard additions) often helps to take into account matrix effects. 

For precise and accurate quantification, it is essential to obtain a calibration curve to accurately define the relation between a known concentration of the analyte and the mass spectrometry signal. Calibration is performed with the external calibration, standard addition, or internal standard method. The last method is more accurate because an internal standard can account for deviation in the mass spectrometry response and the sample losses that might occur in various samplehandling and chromatographic steps. An internal standard is any compound that has chemical and physical properties similar to those of the analyte or homologous to the analyte or a stable isotope-labeled analog of the analyte. The last type of standard provides more accurate results because its chemical and physical properties are virtually identical to those of the analyte. 

Several solution-based calibration techniques (external calibration, standard addition and isotope dilution technique) developed in the author s laboratory represent new calibration strategies for the quantification of analytical data in LA-ICP-MS (1). During the laser ablation of a thin section of biological tissue, defined standard solutions with increasing concentration were nebulized, whereby the calibration of the analytical method was performed by a standard addition mode (21). 

Fig. B.9 Polagram of the zinc standard addition experiment originally from Ref. [15] where the figure caption reads Determination of Zinc by calibration (standard) addition, a unknown and supporting electrolyte, b same solution after calibration addition. Reproduced with permission from Ref. [12]. Copyright Springer Science and Business Media...

FIGURE 4 Graphical representation of external calibration, standard addition and internal standard strategies related to obtaining the quantitative information from the C matrix provided by multiset analysis. 

Medium Calibration (gradient calibration, standard addition, isotopic dilution)... 

Colorplate 2 shows an example of a set of standard additions and their corresponding standard additions calibration curve. 

Examples of calibration curves for the method of standard additions. In (a) the signal is plotted versus the volume of the added standard, and in (b) the signal is plotted versus the concentration of the added standard after dilution. 

A fifth spectrophotometric method for the quantitative determination of the concentration of Pb + in blood uses a multiple-point standard addition based on equation 5.6. The original blood sample has a volume of 1.00 mb, and the standard used for spiking the sample has a concentration of 1560 ppb Pb +. All samples were diluted to 5.00 mb before measuring the signal. A calibration curve of Sjpike versus Vj is described by... 

Figure 5.7(b) shows the relevant relationships when Sspike is plotted versus the concentrations of the spiked standards after dilution. Standard addition calibration curves based on equation 5.8 are also possible. 

Since a standard additions calibration curve is constructed in the sample, it cannot be extended to the analysis of another sample. Each sample, therefore, requires its own standard additions calibration curve. This is a serious drawback to the routine application of the method of standard additions, particularly in laboratories that must handle many samples or that require a quick turnaround time. For example, suppose you need to analyze ten samples using a three-point calibration curve. For a normal calibration curve using external standards, only 13 solutions need to be analyzed (3 standards and 10 samples). Using the method of standard additions, however, requires the analysis of 30 solutions, since each of the 10 samples must be analyzed three times (once before spiking and two times after adding successive spikes). 

The method of standard additions can be used to check the validity of an external standardization when matrix matching is not feasible. To do this, a normal calibration curve of Sjtand versus Cs is constructed, and the value of k is determined from its slope. A standard additions calibration curve is then constructed using equation 5.6, plotting the data as shown in Figure 5.7(b). The slope of this standard additions calibration curve gives an independent determination of k. If the two values of k are identical, then any difference between the sample s matrix and that of the external standards can be ignored. When the values of k are different, a proportional determinate error is introduced if the normal calibration curve is used. 

In a standard addition the analyte s concentration is determined by extrapolating the calibration curve to find the x-intercept. In this case the value of X is... 

An appropriate standard additions calibration curve based on equation 5.8 plots Sspi elVo + Vs) on they-axis and CsVs on the x-axis. Clearly explain why you cannot plot Sspike on the y-axis and Cs[ Vs/( Vo Vj)] on the x-axis. Derive equations for the slope and y-intercept, and explain how the amount of analyte in a sample can be determined from the calibration curve. 

Construct an appropriate standard additions calibration curve, and use a linear regression analysis to determine the concentration of analyte in the original sample and its 95% confidence interval. 

Quantitative Analysis for a Single Analyte The concentration of a single analyte is determined by measuring the absorbance of the sample and applying Beer s law (equation 10.5) using any of the standardization methods described in Chapter 5. The most common methods are the normal calibration curve and the method of standard additions. Single-point standardizations also can be used, provided that the validity of Beer s law has been demonstrated. 

A standard addition calibration curve of emission versus the concentration of added sodium gives, by linear regression, an equation of... 

Description of Method. The amount of chlorpromazine in a pharmaceutical formulation is determined voltammetrically at a graphite working electrode in a nonstirred solution. Calibration is achieved using the method of standard additions. 

Note. The calibration procedure is, however, of limited accuracy and a more accurate result may be obtained using the method of standard addition (Section 9.4)... 

As an alternative to plotting a calibration curve, the method of standard addition may be used. The appropriate ion-selective electrode is first set up, together with a suitable reference electrode in a known volume (Ft) of the test solution, and then the resultant e.m.f. ( t) is measured. Applying the usual Nernst equation, we can say... 

Before dealing with the experimental details of A AS or FES determinations it is necessary to consider the mode of treatment of the experimental data obtained. To convert the measured absorption values into the concentration of the substance being determined it is necessary either to make use of a calibration curve, or to carry out the standard addition procedure. 

Throughout this book the use of a number of standard analytical samples is recommended in order that practical experience may be gained on substances of known composition. In addition, standard reference materials of environmental samples for trace analysis are used for calibration standards, and pure organic compounds are employed as standard materials for elemental analysis. 

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