Standard addition technique

Situation Suppose a (monovalent) ionic species is to be measured in an aqueous matrix containing modifiers direct calibration with pure solutions of the ion (say, as its chloride salt) are viewed with suspicion because modifier/ion complexation and modifier/electrode interactions are a definite possibility. The analyst therefore opts for a standard addition technique using an ion-selective electrode. He intends to run a simulation to get a feeling for the numbers and interactions to expect. The following assumptions are made ... 

Figure 4.24. Estimated concentration of ion using the standard addition technique with an ion-selective electrode. The simulated signal traces are for DVM resolutions of 1, 0.1, 0.01, resp. 0.001 mV (left to right). For each resolution the added volume V2 is varied from 2.4 to 2.55 ml in increments of V2 = 10 /rl. The ordinate marks indicate the 95-105% SLs. The expanded traces for 0.1. .. 0.001 mV resolution are also given. The simulation was run for five different values of 0 = 300 + RND [mV]. The vertical drops (e.g., A B) occur at...

Fuller, C. W. "A Simple Standards Additions Technique Using the Model 306 Atomic Absorption Spectrophotometer". 

Suitable conditions for the quantitative polarographic determination of technetium as pertechnetate are given by Miller et al. who propose a 0.1 M KCl solution of pH 10 or a phosphate buffer solution of pH 7. Since in pH 7 buffer the current is directly proportional to the concentration of technetium over the range of 0.1 to 1.1 ppm, this medium has been used for the determination of low concentrations of technetium in solutions of fission products by the standard addition technique. The half-wave potential of the used wave is —0.68 V vs. SCE. The reaction appears to be irreversible (Fig. 13). It has been found that neither rhenium, ruthenium nor other fission products interfere. However, tetraphenyl-arsonium chloride is reduced at a more positive potential than is pertechnetate therefore, (QH5) AsCl, if present, must be separated. 

The accuracy of the ICE method was assessed by using the standard addition technique. Four mine samples containing measured concentrations of formic acid of about 0.8 to 1.1 yg/mL were spiked with a known volume for formate standard solution sufficient to double the sample concentration. The observed concentration indicated agreementof better than 12% with an average agreement within 5% (Table VI). 

Another calibration technique - standard addition - minimizes matrix effects because analytes with well defined increasing concentrations are added to a set of sample solutions to be analyzed. The measured calibration curve in the standard addition mode plots the measured ion intensities of analytes versus the concentration added to the sample solution. The concentration of analytes in the undoped sample is then determined by extrapolation of the calibration curve with the x-axis. Matrix matching is subsequently performed and the matrix effects (signal depression or interference problems) are considered. An example of the standard addition technique is described in Section 6.2.6 using solution based calibration in LA-ICP-MS. 

Three different calibration strategies for solution based calibration in LA-ICP-MS have been developed in our laboratory. These are similar to the solution calibration in solution analysis by ICP-MS external calibration if a high purity matrix-matched blank target is available,29,71 the standard addition technique (e.g., for high purity platinum)76 or the isotope dilution technique.43... 

Standard Addition Technique With Solution Based Calibration in LA-ICP-MS... 

Physical interferences are generally considered to be effects associated with such properties as change in viscosity, and surface tension can cause significant inaccuracies, especially in samples that may contain high dissolved solids, or acid concentrations, or both. If these types of interferences are operative, they must be reduced by dilution of the sample or utilization of standard addition techniques, or both. 

In principle, methods should be validated for each type of matrix and for the extraction agent applied. Matrix effects may affect strongly the calibration (e.g. loss of signal, interferences). Standard addition techniques are, therefore, the only way to control the validity of the detection, but only if the addition is performed with the proper identical form of the compound to be determined. As stressed above, it is of primary importance to evaluate this linear range before starting the analysis so that the spiked levels remain in the linear range of the detector response (Quevauviller et al., 1996a). 

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. 

Both the solution used to spike the filters and the mixed metal calibration standard were prepared from deliquescent Ni(. 6LL0 and had to be standardized (standard addition technique) to establish concentration. 2 ... 

With the various alternatives of the standard additions technique it is possible to carry out calculations with both peak areas and peak heights. 

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]. 

In practice, the value of k is never obtained as such, because the meter is adjusted so that the standard reads the correct value for its pX, the scale being Nernstian. As k contains in addition to the reference electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is always at least 0.001/(0.059// ) or 0.02 if n = I, i.e. a relative precision of about 2% at best. For the most precise work a standard addition technique (p. 32) and close temperature control are desirable. All measurements should be made at constant ionic strength because of its effect on activities. Likewise,... 

Many analyses depend on the comparison of a sample with a standard whose composition is as much like that of the sample as possible. Such standards are used in the evaluation of new methods, techniques, and procedures. A new procedure for nickel in an ore, for example, cannot be evaluated by a comparison of results from the sample with those from pure nickel the sample probably contains interferences, whereas the standard does not. The evaluation should be made with a standard containing the same kind and amount of impurities as the sample or through the use of the standard addition technique. A crucial problem in the preparation of standard samples is recognition of the significance of matrix effects. These effects result from often subtle influences of the chemical and physical composition of the sample on the magnitude of a measurement for the component to be assayed. Ideal standards eliminate some determinate errors in analyses by resembling as closely as possible the chemical and physical composition of the sample. This matrix problem is met in a serious form in spectroscopy. 

Good recoveries (82-98%) were obtained from spiked artificial seawaters, even at the 0.05-p.p.b. level, but for natural seawater samples the recoveries were lower (74-85%). Higher recoveries (100-102%) are obtained when the standard addition technique is used. 

The data and arguments presented indicate that the presence of sea salts alters the atomic absorption signal of these four elements from their response in de-ionized water-acid standards in a consistent manner for each particular element and sea salt concentration. This observation can be used to develop a modified standard addition technique. If a series of curves can be prepared that contain the range of metal and sea salt concentration expected in the samples, correction factors between actual and observed concentrations based on pure water-acid standards can be determined. This modified standard addition technique is illustrated for two of the elements discussed previously. For lead the actual concentration is plotted vs. the calculated concentration for a sea salt range of 1.0-5.0 parts per thousand sodium in Figure 4. This plot was prepared from solutions of known concentrations in a sea water medium. For a sample of unknown lead concentration, within the specified range. 

Typical solid samples representing a partition system include polymers heated above their glass transition point. These samples are amenable to application of the usual HS modes (e.g. the standard-addition technique). The standard can be added to the gas phase in the closed vial by injection through the septum, equilibrium being established by diffusion from both sides. If the regression analysis reveals the absence of linearity, then either some instrumental parameter (e.g. the sample temperature or equilibration time) will have to be adjusted or the system is not a partition but rather an adsorption system. 

Direct analysis or spiking with stock solutions to apply standard addition technique... 

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