Multiple standard addition method

Walters [24] examined the effect of chloride on the use of bromide and iodide solid state membrane electrodes, and he calculated selectivity constants. Multiple linear regression analysis was used to determine the concentrations of bromide, fluorine, and iodide in geothermal brines, and indicated high interferences at high salt concentrations. The standard curve method was preferred to the multiple standard addition method because of ... 

The multiple standard addition method is time-consuming although it usually yields superior results to those of the other methods outlined in Table 3.3. 

Due to the possible toxic effects of Hg, alternative electrodes have been sought, such as bismuth, which has recently been used by Kadara and Tothill [155,156] for the determination of Pb2+ and Cd2+ at an SPCE. In this investigation, stripping chronopotentiometric measurements were carried out by depositing a metallic film of bismuth in situ with the target metal ions (lead and cadmium). A deposition potential was applied to preconcentrate the analytes, after which, a constant current was applied to strip the preconcentrated analytes until a limit of —0.2 V. The concentrations for Pb2+ and Cd2+ in the wastewater samples and acetic acid extracts of soils were quantified by the use of a multiple standard addition method. 

Multiple standard addition method (three additions of 50ppb of Cd and Pb solution) is used to detect Cd and Pb in the sample. 

Multiple standard addition method (three additions of 50 ppb of Cd and Pb solution) was utilised to recover two concentration levels of 40 and 80 mg/L of spiked metals into the solution composed of lmL tap water and 24 mL electrolytic cell (0.1M acetate buffer, pH 4.5). 

Spreadsheet Summary In Chapter 12 ot Applications of Microsoft Excel in Analytical Chemistry, we investigate the multiple standard additions method for determining solution concentration. A least-squares analysis of the data leads to the determination of the concentration of the analyte as well as the uncertainty of the measured concentration. 

Generalised (multiple) Standard Addition Method Equation (2.20) can be applied to a data set obtained by spiking the unknown samples with known amounts of the components to be determined. In this case, the cs/ values will correspond to the added concentrations of each component in each standard. As above, the regressions performed for each measuring channel will provide the kj11 and zm values, which will represent estimates of the sensitivity of each component and the sample signal (in the absence of additive interferences), respectively. Finally, by using Eq. (2.20) with the zm values, one can calculate the concentration of each component in the unknown sample. 

Among the referred algorithms, the multilinear regression method has often been used because of its easy implementation, and good results have been obtained in most cases. The chemical interferents are the main limitation of such techniques, because prior knowledge of each substance that contributes to the overall signal is needed. In this case, multiplicative interferences can easily be addressed using the multiple standard addition method. The elimination of additive interferences has not been achieved. 

The multiple standard addition method, also known as the generalized standard addition method, involves scanning the unknown sample and then supplying it with known volumes of a mixture of standards in order to obtain a new scan without altering the experimental setup. 

The alcoholic content of the samples affects the potential of the ion-selective nitrate electrodes. Therefore, for analyzing the nitrate content of wine samples, the multiple standard addition method gives more accurate results than calibration curve-based potentiometric... 

In Section 8.2.8 we have discussed the standard addition method as a means to quantitate an analyte in the presence of unknown matrix effects cf. Section 13.9). While the matrix effect is corrected for, the presence of other emalytes may still interfere with the analysis. The method can be generalized, however, to the simultaneous analysis of p analytes. Multiple standard additions are applied in order to determine the analytes of interest using many q > p) analytical sensors. It... 

Probably the best way to overcome the problem of poor selectivity is via the multiple standard addition approach in which known amounts of analyte are added to the sample in increasingly high concentrations. Using this method, a... 

Add additional analyte to the solution to swamp the interferent. The ratio of analyte to interferent will therefore increase, thus improving the precision of the measurement. This method is termed multiple standard addition... 

Employing the method known as multiple standard addition helps when trying to discern the effects of the sample matrix in which an analyte is dissolved. In this technique, the emf is determined as a function of the amount of standard solution added to the sample. 

In addition techniques, the test substance concentration is determined from the difference in the ISE potentials obtained before and after a change in the sample solution concentration. The main advantage Ues in the fact that the whole measurement is carried out in the presence of the sample matrix, so that results with satisfactory accuracy and precision can be obtained even if a substantial portion of the test substance is complexed. Several addition techniques can be used, namely, single, double or multiple known addition methods, in which the sample concentration is increased by additions of a test substance standard solution single, double or multiple known subtraction methods, in which the sample concentration is decreased by additions of a standard solution of a substance that reacts stoichiometrically with the determinand and analyte addition and subtraction methods, in which the sample is added to a test substance solution or to a reagent solution. 

A comparison of three calibration methods based on the use of CRMs, generalized simplified standard additions and multiple standard additions revealed the last to be the most practical and reliable [72]. 

In the method of standard additions, a known amount of a standard solution of analyte is added to one portion of the sample. The responses before and after the addition are measured and used to obtain the analyte concentration. Alternatively, multiple additions are made to several portions of the sample. The standard additions method assumes a linear response. This should always be confirmed or the multiple additions method used to check linearity. 

We use the method of standard additions when it is difficult or impossible to duplicate the sample matrix. In general, the sample is spiked with a known amount or amounts of a standard solution of the analyte. In the single-point standard addition method, two portions of the sample are taken. One portion is measured as usual, but a known amount of standard analyte solution is added to the second portion. The responses for the two portions are then used to calculate the unknown concentration, assuming a linear relationship between response and analyte concentration (see Example 8-8). In the multiple additions method, additions of known amounts of standard analyte solution are made to several portions of the sample, and a multiple additions cahbration eurve is obtained. The multiple additions method gives some... 

The standard-addition method (see Section 8C-3) involves determining the potential of the electrode system before and after a measured volume of a standard has been added to a known volume of the analyte solution. Multiple additions can also be made. Often, an excess of an electrolyte is incorporated into the analyte solution at the outset to prevent any major shift in ionic strength that might accompany the addition of standard. It is also necessary to assume that the junction potential remains constant during the measurements. 

The standard addition method can take several forms as discussed in Section 8C-3 the single-point method was described in Example 8-8. The multiple additions method is often chosen for photometric or spectrophotometric analyses, and this method will be described here. This technique involves adding several increments of a standard solution to sample aliquots of the same size. Each solution is then diluted to a fixed volume before measuring its absorbance. When the amount of sample is limited, standard additions can be carried out by successive addition of increments of the standard to a single measured aliquot of the unknown. The measurements are made on the original solution and after each addition of standard analyte. This procedure is often more convenient for voltammetry. 

Single-point standard addition methods are inherently more risky than multiple-point methods. There is no check on linearity with single-point methods, and results depend strongly on the reliability of one measurement. 

The moderate dilution factors attainable with fully automated flow injection or sequential injection systems may impose severe difficulties for the online development of accurate partitioning studies in environmental solids of relative complexity. The contribution to the analytical results of multiplicative (nonspectral) matrix interferences has been solved elegantly by the use of in-line standard addition methods (Dong and Yan, 2005), although at the expense of a threefold decrease in sampling frequency. 

Several measurements can be performed on the same sample for e.g., wide-range spectrophotometry, simultaneous determinations, selectivity evaluation, accuracy assessment or compensation of matrix effects by the standard addition method. Each measurement normally relates to a different sample (or standard) volumetric fraction, hence a different degree of dilution. To this end, different approaches involving single or multiple sample (or standard) insertions and/or exploitation of concentration gradients have been used, as emphasised in Chapter 7. 

From sx the 95% confidence interval on the estimate may be determined by multiplication by the appropriate /-value (/o.o5",n-2)-Standard addition is used when there are potential interferents that would lead to a systematic error that is proportional to concentration. Calculation of the concentration by the standard addition method causes these errors in the measurements to cancel. It is also useful if the analyte cannot be extracted from its matrix, and there is not a matrix matched calibrant available. This may be the case in environmental analysis. Note, however, that standard addition does not compensate for a constant additive interferent. 

Calcium in beer can be determined by filtering the sample, adjusting the pH to 6, and employing a known addition of Ca. The copper-ion content of tap water can be determined with a solid-state crystalline copper electrode using a multiple standard-addition procedure [20]. Tap water is mixed 1 1 with a complexing antioxidant buffer (sodium acetate, acetic acid, sodium fluoride, and formaldehyde) to buffer the pH at 4.8, to complex the Cu uniformly with acetate, and to complex the Fe " interferant with fluoride. Copper in tap water can be determined down to about 9 ppm with a standard deviation of about + 8%. The recovery of Cu + added to natural waters, an indication of the accuracy of the method, averaged 103% for samples in the range of 3 to 60 ppm Cu. 

Detection of the molecules produced, consumed, and secreted by the cells described here is challenging for two main reasons. First, the cell is dynamic and constantly tries to maintain balance. As such, molecules concentrations or speciation are usually changing. Second, the matrix in which these measurements are typically performed is very complex. Thus, the technique of choice needs to have some built-in feature that enables the analyst to overcome the matrix. To date, a variety of measurements have been employed to learn more about the roles of the cells in the microcirculation. Specifically, fluorescence, chemiluminescence, and amperometry have all been used extensively. Not surprisingly, all three of these detection schemes are readily employed in capillary electrophoresis-based determinations. Therefore, many of the measurements employ technology from the CE field. However, due to the cell matrix complexity, techniques are required to overcome potential interfer-ents. Eor example, Kovarik et al. employed a Nafion coating over a micromolded ink electrode for selectivity in detecting dopamine in the presence of an anion interferent (ascorbate). Eor similar reasons, Ku" ° employed the classic method of multiple standard additions to quantitatively determine the amount of NO released from activated platelets in a flowing stream. 

Application of ion-selective electrodes to determine copper by standard addition method in nickel plating bath was suggested by Frant. Later Hulanicki et al." using a copper ion-selective electrode proposed a method based on multiple standard addition in presence of a copper complexing agent to prevent a harmful influence of chloride ions. In this work a similar method is used to determine copper also in zinc and cobalt baths. 

One of the weaknesses of HSGC is that quantitation is not as straightforward as in classical GC. Calibration of a HSGC system relies either on the availability of the matrix void of the analyte or on the composition of the matrix being known so that it can be simulated by individual ingredients. If neither option works, for instance if the matrix is not available and cannot be simulated, the so-called standard addition method has to be used. Even in situations where a standard addition method may lead to uncertainties, the multiple headspace extraction (MHE) technique allows a reliable quantification of trace constituents. 

Overspotting can be performed (also with the Linomat), in which more than one sample can be applied to a single initial zone position. These samples can include multiple standard reference compounds from different vials to prepare an in situ mixture, sample plus spiking solution, for validation of quantitative analysis by the standard addition method, or sample plus reagent, for in situ prechroma-tographic derivatization. 

Obtaining absolute quantity or concentration of an analyte typically requires constmction of a calibration plot based on analyses of standard samples with different concentrations. When the number of analyzed samples is small, or when the separation is not perfect, and it does not completely eliminate interference, it is advised to carry out quantification using multiple-standard addition. In the case of the standard addition method, the sample is analyzed first, followed by analyses of the same sample supplemented with the chemical standard at different concentrations. If the amounts of added standards are chosen correctly, one can estimate the amount of target analyte initially present in the sample. When carrying out quantification by multiple standard addition, one still has to make sure that the peaks of the supplemented samples are not overloaded. Verification of this point may require conducting additional preliminary tests. 

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