Reference standard additions method

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

R. Sundberg, Interplay between chemistry and statistics, with special reference to calibration and the generalized standard addition method. Chemom. Intell. Lab. Syst., 4 (1988) 299-305. 

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]. 

In the context of the voltammetry of microparticles methodology, the H-point standard addition method has been adapted for determining organic dyes [241] as well as lead and tin in ceramics [242]. Let us consider a mixture of material containing unknown amounts of two electroactive compounds, A and B, and a reference compound, R. It is assumed that weighted amounts of both materials are accurately powdered and thoroughly mixed so that the mass ratio between the A,B-containing material and the reference compound, m/mR, is known. 

Table 41 Application of the H-point standard addition method to a mixture of a commercial madder pigment (Kremer, 0.03325 g) plus silica (0.25836 g) and moiin (0.02486 g), this last acting as an auxiliary reference compound... 

Table 4.2 Statistical parameters for H-point standard addition method apphed to commercial madder pigment using silica as a diluent and morin as a reference compound...

Data in Table 4.2 corresponds to the application of the H-point standard addition method to a mixture of a commercial madder pigment diluted with silica, using morin as a reference compound. Calculations were performed by taking m/niR = 10.246, using square-wave voltammetric currents measured for sample-modified PIGEs in contact with an acetate buffer of pH 4.90. Linear plots of ii/ip(R) (squares) and i2/ip(R) (solid squares) vs. mA/mp for additions of purpurin are shown in Fig. 4.17. 

As might be anticipated, the reduction in flame temperature has a deleterious effect upon the incidence and extent of matrix interferences when such boat techniques are used. As a consequence, precise matrix matching is necessary for accurate results, or the standard additions method may be employed.6 If the user is in any doubt as to whether matrix matching alone is sufficient, the adequacy of this approach may be confirmed by the analysis of certified reference materials and/or by applying the standard additions technique as well to a selection of samples to make sure that both techniques give the same results. For bismuth, cadmium, lead, silver, and thallium, detection limits by AAS are a few ng ml -1 or better.6 For arsenic, selenium, and tellurium they tend to lie in the range 10-30 ng ml-1, depending upon the source used. 

The direct determination of trace elements (Al, Ba, Cu, I, Mn, Mo, Pb, Rb, Se, Sr, and Zn) by ICP-MS in powdered milk was reported [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. Mass intensity losses were not observed. The quantitative ICP-MS procedure was applied the standard additions method with a Y internal reference. This direct technique is as fast as the slurry approach without particle size effects or sensitivity losses. 

Hence, the standard additions method is unique in that it actually employs the very material under analysis as a reference matrix material, thus providing for efficient elimination of very complex matrix effects even when the final material is the result of a multi-step preparative procedure and the composition of the matrix of the original material is completely unknown. These advantageous features of the standard additions technique have been discussed and verified in context with quantitative headspace gas analysis [68]. 

There is one more, very important and relatively simple method to use when an interference effect is difficult to explore but its occurrence is probable and poses a threat to the reliability of analytical results. This refers to the case when an analyzed series of samples have similar chemical composition (at least in terms of the composition of interferents) and the determined component is present in all samples in a similar quantity. In this situation, the standard addition method can be used for analysis of one selected sample and the constructed calibration graph employed for interpolative determination of analyte in the remaining samples. This combined procedure is depicted in Fig. 3.16. Thus obtained results are, as a mle, more accurate than those obtained after application of the set of standards method to all the samples. In addition, the analyses are conducted faster than when all the samples are analyzed using the standard addition method. 

The instrument is calibrated for a given element for each series of samples. Direct calibration requires detailed knowledge of the milieu to be analysed. Precise results depend on the composition of the calibration solutions being as close as possible to that of the solutions to be analysed. The standard addition method can be u.sed in cases where it is not possible to produce external reference solutions similar to the solutions to be analysed. This method should be used with considerable caution because it assumes that the absorption is due solely to the element under analysis and, in particular, that the non specific absorption is fully corrected for. If this is not the case, any interfering absorption leads to an overestimate of the values observed. On the other hand, this method does have the advantage of eliminating the matrix effect. 

Chemical interference is practically non existent as a result of the high temperature of the plasma. On the other hand, physical interference may be observed. This stems from variations in the sample atomisation speed which is usually due to changes in nebulisation efficiency caused by differences in the physical properties of the solutions. Such effects may be caused by differences in viscosity or vapour tension between the sample solutions and the standards due, for example, to differences in acidity or total salt content. The technique most commonly used to correct this physical interference is the use of internal standards. In this technique a reference element is added at an identical concentration level to all the solutions under analysis, standards, blank and samples. For each element, the ratio of simultaneous measurements of the lines of the element and the internal standard is then determined in order to compensate for any deviation in the response of the plasma. If the internal standard behaves in the same way as the element to be determined, this method can be used to improve the reliability of the result by a factor of 2 to 5. It can also, however, introduce significant errors because not all elements behave in the same way. It is thus necessary to take care when using it. Alternatives to the internal standard method include incorporating the matrix into the standards and the blank, sample dilution, and the standard addition method. 

The variation of sensitivity between different sensors was also checked. Calibration curves with five different sensors were performed. A Relative Standard Deviation of 13, 13 and 42% of calibration slopes (sensitivity) were obtained for Cu, Pb and Cd respectively. These variations should have limited consequence on bias and precision when the standard addition method is used. However, for Cd, variations in the limit of quantification between two electrodes could be expected. Finally, the accuracy of the method was evaluated by the measurement of a SWIFT reference material used during the 2nd SWIFT-WFD Proficiency Testing exercise (Table 4.2.2). The reference value was chosen as the consensus value of the selected data population obtained after excluding the outliers. The performances of the device were estimated according to the Z-score (Z) calculation. Based on this score, results obtained with the SPEs/PalmSens method were consistent with those obtained by all methods for Pb and Cu ( Z < 2) while the result was less satisfactory for Cd (2 < Z < 3). 

When standard reference materials and different analytical methods are not applicable, the standard-addition method may prove useful. Here, in addition to being... 

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 standard addition method is used taking into account the poor precision of the reference procedure (MBAS), and the concentration of the added LAS was included between 0 to 25 mg/L, and was chosen in agreement with the common values encountered in... 

The standard addition method can be efficiently implemented in a flow system with multi-site detection, as demonstrated by the amperometric determination of ascorbic acid in fruit juices [87], Only one sample aliquot was inserted, and the sample zone was monitored under two different conditions, before and after the standard addition by confluence. Measurements related to the first and second detection sites were normalised and the matrix effect was efficiently circumvented. About 55 samples were rim per hour, yielding precise results in agreement with a reference procedure. 

The standard-addition method has been applied to the determination of chloride and fluoride in samples of commercial phosphors. In this application, solid-state indicator electrtKies for chloride and fluoride were used in conjunction with a reference electrode the added standard contained known quantities of the two anions. I he relative standard deviation for the measurement of replicate standard samples was 0.7% for fluoride and 0.4% for chloride. In routine use with real samples, the standard-addition method yielded relative standard deviations of l.l k) for fluoride and 0.8% for chloride. When similar samples were ana-Ivred bv the usual electrode calibration methods, the... 

After the measurement stage has been completed the concentrations of determined elements are calculated using calibration curves, multielement standards, standard reference materials, standard addition methods and single comparator approaches. 

The standard-addition method is similar to the internal-standard method, except that the element being determined serves as the reference standard itself. Analysis is accomplished by measuring the intensity of the characteristic spectral line before and after the addition of a known amount of the element being determined. It is assumed that a linear relationship exists between line intensity and concentration (over a limited concentration range), and that the matrix is not significantly altered by the addition. In general, the addition method is limited to the determination of trace and minor elements. All of the comments regarding sample preparation for internal standards apply directly to the standard-addition approach. 

Ouinolin-8-ol immobilized on silica was used for on-line preconcentration purposes with an ICP-MS system [17]. Isotope dilution and standard addition methods were applied for the determination of trace metals in a sea water standard reference material. A similar ion-exchanger was used for on-line preconcentration of cobalt with catalytic sp>ectrophotometric determination [18]. 

ICP-MS is a powerful technique for multi-element trace determinations in water samples. However, its application to the analysis of saline waters is limited to those with dissolved solid contents below 0.2%. in order to avoid instrumental drifts caused by solid deposition on the orifice. The analysis of sea water therefore demands a separation of the salt matrix prior to determination by ICP-MS. Beauchmin and Berman [19] used an on-line column packed with silica immobilized quinolin-8-oI ion-exchanger to separate the matrix and determine Mn, Co, Ni. Cu. Pb and U in the standard reference open ocean water NASS-2 using an isotope dilution technique and a standard addition method. 

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. 

The aim of our projeet was to identify and determine the fatty acid content of these oils by means of C H NMR spectroscopy. The C H NMR spectra of locally produced marula, apricot kernel, avocado pear, grape seed, macadamia nut, and mango kernel oils are very similar to those of the well-studied olive oil, we attempted to assign their C H NMR spectra by the method used by Mannina et al. for olive oil (1). Mannina et al aehieved assignment of the C H spectra in part by addition of pure Standard triaeylglyeerols we shall refer to as the standard-addition method. For our locally produced oils we found however, that full assignment of the spectra was... 

The standard addition method (often referred to as spiking the sample) is commonly used to determine the concentration of analytes in a complex matrix, such as biological fluid, when the matrix contains the substance investigated or other component(s) that may interfere with the analyte signal, causing inaccuracy in the determined concentration. 

Perhaps the greatest problem in trace analysis is assurance of the accuracy of the results (i.e., the avoidance of systematic errors). Systematic sources of error are possible in every step of an analytical process. The most reliable method for detecting systematic errors is continuous and comprehensive quality assurance, particularly by occasional analysis of (certified) standard reference materials. Strictly speaking, an analytical method cannot be calibrated if suitable (i.e., representative) standard reference materials adequately representing the matrix of the expected test samples are not available. However, internal laboratoiy reference materials can then usually be prepared, whose matrix largely resembles the matrix of the test portions expected. If problems occur in the preparation of such reference samples, the standard addition method (SAM) can be applied, in which internal laboratory standards are added stepwise to the test sample (analyte and matrix)... 

The concentrations of the alkali metals Cs, Li, and Rb were detected by LEI in different types of reference rock samples. Rock samples were dissolved by standard methods and analyses performed without preconcentration, with pure aqueous standards for calibration. Concentrations were determined by both calibration curve and standard additions methods. The results obtained by the two techniques coincided within experimental errors. Concentrations varied in the range... 

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... 

It is crucial in quantitative GC to obtain a good separation of the components of interest. Although this is not critical when a mass spectrometer is used as the detector (because ions for identification can be mass selected), it is nevertheless good practice. If the GC effluent is split between the mass spectrometer and FID detector, either detector can be used for quantitation. Because the response for any individual compound will differ, it is necessary to obtain relative response factors for those compounds for which quantitation is needed. Care should be taken to prevent contamination of the sample with the reference standards. This is a major source of error in trace quantitative analysis. To prevent such contamination, a method blank should be run, following all steps in the method of preparation of a sample except the addition of the sample. To ensure that there is no contamination or carryover in the GC column or the ion source, the method blank should be run prior to each sample. 

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