Standard addition procedure

The result may be checked by adding four successive portions (2mL) of standard solution C to the test solution of which the e.m.f. has already been determined, and measuring the e.m.f. after each addition the calculation for this standard addition procedure is as described above. 

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. 

Salgado Ordonez et al. [28] used di-2-pyridylketone 2-furoyl-hydrazone as a reagent for the fluorometric determination of down to 0.2 pg aluminium in seawater. A buffer solution at pH 6.3, and 1 ml of the reagent solution were added to the samples containing between 0.25 to 2.50 pg aluminium. Fluorescence was measured at 465 nm, and the aluminium in the sample determined either from a calibration graph prepared under the same conditions or a standard addition procedure. Aluminium could be determined in the 10-100 pg/1 range. The method was satisfactorily applied to spiked and natural seawater samples. 

Carr [562] has studied the effects of salinity on the determination of strontium in seawater by atomic absorption spectrometry using an air-acetylene flame. Using solutions containing 7.5 mg/1 strontium and between 5 and 14% sodium chloride, he demonstrated a decrease in absorption with increasing sodium chloride concentration. To overcome this effect a standard additions procedure is recommended. 

This analytical method, based on TXRF, enables a large number of trace elements to be determined simultaneously. The range is suitable for different areas of the sea. The motivation to use TXRF resulted mainly from the characteristic features of the method its high detection power, its universal calibration curve, which eliminates the need for matrix-dependent standard samples or standard-addition procedures, the simple preparation of the sample films, and of course the possibility of multielement determination. 

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... 

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. 

D. (a) The higher result in experiment 2 compared with experiment I is probably the effect of diluting interfering species, so they do not interfere as much in experiment 2 as in experiment 1. Dilution lowers the concentration of species that might react with Li or make smoke that scatters light. In experiment 3, interference is present to the same extent as in experiment 2, but the standard addition procedure corrects for the interference. The whole... 

When the total isolated trans fat levels are relatively low (below 10%), a potentially significant interference may be found in products containing conjugated unsaturation, e.g., 1 % in ruminant fat or 5% in tung oil (Firestone and Sheppard, 1992). This is due to the fact that conjugated trans,trans (near 990 cm ) and/or cis/trans (near 990 and 950 cm"1) double bonds exhibit absorption bands that are sufficiently close to 966 cm"1. This interference can be eliminated by applying a standard addition procedure (Mossoba et al., 2001 a). This procedure also applies to interferences from the food matrix. 

Chemical interferences, which can be highly dependent on matrix type and the specific analyte element, are characterized by molecular compound formation, ionization effects, and solute vaporization effects. If such effects are observed, they can be minimized by careful selection of operating conditions, by buffering the sample, by matrix matching, and by standard addition procedures. 

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. 

For illustration purposes, in the following the determination of carbonyl chloride in test sample is described in detail. From the measurements of the calibration samples prepared according to the standard addition procedure a calibration graph is obtained as depicted in Fig. 10-9. 

Figure 10-9 Calibration graph obtained from the standard addition procedure.

To date, slurry nebulization has not found widespread or routine application in environmental analysis by flame spectrometry. In the author s experience the time saving is small or even non-existent, because the use of standard additions procedures is time consuming, and samples often have to be processed one at a time when slurry atomization is to be used. 

The ET-AAS technique is especially prone to nonspectral interferences, that is, effects on the formation of atoms. Such interferences cannot be eliminated by BC. Some of these effects can, however, be corrected by the use of the standard addition procedure. To investigate whether standard addition is necessary, the... 

Aluminium 309.3 nitrous oxide/ acetylene Add 2000Mg ml-1 potassium to sample solutions to suppress ionisation. Use a standard additions procedure to avoid interference from the zinc matrix... 

By means of this CD-enhanced fluorescence method, the authors could determine 11-MeBPHT in spiked human urine samples [23]. The standard addition procedure, based on the addition of a constant volume of the spiked urine sample to solutions of increasing methyl-BPHT concentration, was applied using spectrofluorimetric measurements. The linear calibration curves and standard addition linear plots were parallel, which suggested the absence of matrix interferences in the urine samples and permitted the validation of the method. Good linearity and precision were also obtained for the standard addition plots, with correlation coefficients close to unity. Satisfactory recovery percentage values ranging from 97 to 108% were found in the urine samples for the determination of 11-MeBPHT concentrations at low xg/mL levels. 

Spreadsheet Summary In Chapter 4 of Applications of Microsoft Excel in Analytical Chemistiy, a multiple standard additions procedure is illustrated. The determination of strontium in sea water with inductively coupled plasma atomic emission spectrometry is used as an example. The worksheet is prepared, and the standard additions plot is made. The unknown Sr concentration and its standard deviation are obtained. 

DIN 32633 Chemical analysis -Methods of standard addition - Procedure evaluation, Beuth, Berlin, 1998. 

UV). Consequently the reliability of the signal relies on the prior chromatographic separation. Matrix matching of the calibration solution or standard addition procedure can help to overcome matrix influences. An example of the too often neglected matrix effect is given in Table 2.1. These results were obtained in an interlaboratory certification study of trace elements in soils. It demonstrates the importance of matrix matching for the calibration [25]. Except for Co and Cu, for which another source of error remained, the matrix matching of the calibrant solutions allowed the laboratory to come closer to the certified values. 

The validation of the extraction step is difficult and often unsatisfactory. For solids it is nearly impossible to prove that all the compounds have been extracted. Two approaches are usually applied. One is based on successive extractions with fresh solvents and determination of the residual traces of PCBs in the successive extracts. When no PCBs can be extracted any more one may assume (but not guarantee) that the extraction is complete. The second approach consists in a standard addition procedure with increasing spikes [29]. A more detailed discussion on this issue is given in Chapter 5.1.4. After extraction the analyst verifies that he recovers all added compounds. This method is only valid if the operator allows the spike to be in contact for a certain time with the material — at least overnight. By repeating the spiking at each level of enrichment the... 

Matrix effects were suspected to cause systematic errors in the determination of Mo and the use of standard addition procedures for calibration was strongly recommended. 

The verification of extraction recoveries was also discussed. Most of the laboratories performed standard addition procedures and hence did not need to correct for recovery. 

In the case of e.g. voltammetric techniques, the high content of organic matter could lead to erroneous (too high) results even in case standard addition procedures are applied. 

Widner et al. (1998) used ICP-OES to determine silicon in spot urine samples at the three most sensitive emission vavelengths for silicon that is, 212.412, 251.611 and 288.158 nm. The standard-addition procedure vas applied to meet the requirements of matrix compensation in a frequently changing environment. Addition of EDTA to the urine sample vas recommended to chelate cations, preferentially Ca ", in order to prevent precipitation of poorly soluble salts. 

Identification problems arise when the identity of a species is determined solely by retention times in a low-resolving separation system. More-dimensional analysis with different and independent methods is again the best solution. Capillary electrophoresis offers a fast and useful tool in this field (Michalke 1996), but specific problems (e.g., migration time shifts) must be taken into consideration. Standard addition procedures are therefore required. Precautions are also necessary concerning species altera-... 

Figure 8.1 shows how these data were used to calculate the nickel content of the synthetic unknown by the standard addition procedure. Although these data gave adequate results we found that the experiment was difficult to repeat since the background absorbance has a tendency to increase with the amount of nickel added. 

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