In this work, they used a cross-flow nebulizer combined with a double-path spray chamber, and a cone spray associated with a cyclonic spray chamber. The vinegar samples were directly aspirated into the plasma after simple dilution with water. Even though the ionization energy for Sc 6. Some authors use physicochemical properties as a criterion for choosing an IS species Vanhaecke et al. According to a study by Finley-Jones et al. However, as frequently is the case in IS, there were several exceptions to this general rule. Salazar et al. Barros et al. Another strategy for selecting an IS species is to adopt a major constituent of either the sample or the plasma.
Scheffler et al. Unfortunately, this strategy is not applicable to correct for fluctuations due to aerosol generation and transport, as it only relates to effects on the Ar plasma. On the other hand, selecting an IS species which is a major constituent of the sample has been fairly successful in LIBS determinations. Lucena et al. In another work, Na was used as IS for Ca determination in biochar-based fertilizer for minimizing differences between the sample and the calibration standards, which were built using eucalyptus biochar as a matrix match Morais et al. In this same context, Fe has been used as IS to determine several elements in complex-matrix samples such as soil Kwak et al.
In all cases, IS led to improvement in the analytical calibration curve linearity. According to the authors, Ca concentrations were not homogeneous throughout the samples due to the different crystalline phases, which compromised the efficiency of the IS method.
ICP-MS: A versatile detection system for trace element and speciation analysis
Even though IS has been successfully used to correct for matrix effects in several applications involving different analytes and instrumental methods, the mechanisms by which it works must be better understood, especially considering the diverging reports in the literature on the criteria to select an effective IS species.
In this context, strategies such as similarities in physicochemical properties between analyte and IS, multivariate analysis, use of major constituents, and the use of validation results have been the main criteria for IS selection. Although effective in many applications, a better rationalization for IS selection and application would be highly useful in routine analyses. According to Kelly et al. However, it was used in XRF and atomic spectrometry only 20 years later.
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In SA, the calibration solutions are prepared by adding increasing concentrations of the analyte to the sample, and the sample solution itself is used as solvent to minimize matrix effects. The analyte concentration added to the sample solutions or the volume of the standard solution added to the sample is plotted as independent variable on the x -axis, with instrument response as the dependent variable on the y -axis. Similar to EC and IS, the calibration function is estimated by least squares fitting.
Figure 1 shows an example of a SA plot using five calibration points: no analyte added x 0 , and four analyte additions with increasing concentrations x 1 - x 4. It can also be determined by the interpolation method, which consists of interpolating C sam as twice the signal generated by the sample Andrade et al. A minor difficulty associated with the application of the SA method is the determination of the error in the measurement.
Figure 1. SA calibration plot using four addition points x 1 - x 4. Extrapolation is represented by a broken line, and x 0 is the sample with no standard addition. Several papers have applied the extrapolation strategy for SA, but according to Andrade et al. Additional details on the mathematical description and theory of the SA method can be found in the literature in works by Kolthoff and Lingane , Bader , Currie , Bergfeld et al.
There are several strategies to perform SA. In voltammetric analysis, frequently only two calibration points are used, with solution 1 containing the sample with no standard added, and solution 2 consisting of the sample with standard addition Kolthoff and Lingane, ; Kelly et al. In these cases, a mathematical equation, rather than graphical extrapolation, has been used to determine the analyte concentration in the sample. In spectrometric analysis, typically five or six calibration points and graphical extrapolation have been used, since the s, for calculating the analyte concentration in the sample Kelly et al.
Bader described a systematic approach to SA including five different strategies to perform this calibration method when analyte concentration and instrument response present a linear relationship. As discussed by the author, a different mathematical equation is used to calculate the unknown analyte concentration in the sample for each of the five proposed strategies.
Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
The mathematical equations use values for the intercept, slope, the concentration of the standard solution, and the volumes of both standard solution and sample. Kelly et al. Masses rather than volumes were used in this case, and according to the authors, such approach makes the SA method suitable not only for liquid solutions, but also for solid samples. When several solutions containing the same amount of sample and different amounts of standard solution are prepared separately, SA is known as conventional standard additions calibration C-SAC.
The first of these solutions contains only the sample, while increasing concentrations of the analyte are added the other solutions Figure 1. In sequential standard additions calibration S-SAC , different amounts of standard are added to a single flask containing the sample. In S-SAC, the original sample in a container is analyzed and the instrument response is recorded Brown et al. Then, a certain amount of standard solution is added to the same container and the analytical signal is recorded once again.
The procedure is repeated, with additional volumes of the standard solution sequentially added to the same container and the analytical signals recorded in each case. The SA calibration plot is then built with analyte concentrations or standard volumes added to the sample x -axis vs. The constant final volume strategy is only applicable to gas samples, but the extrapolation method used with it is the same as the one used with variable-final-volume S-SAC.
Brown , , Brown and Gillam a , b and Brown et al.
The authors have shown that S-SAC can replace C-SAC if an adequate quantity of standard solution is added to the sample, and if the appropriated equation which was proposed by the authors is used to determine the analyte concentration in the sample. However, the application of S-SAC may be limited to non-destructive instrumental methods, such as voltammetry and UV-visible spectrophotometry. So far, there is no report in the literature of the use of S-SAC in spectrometric instrumental methods.
However, it may be easily applied if the volume of sample consumed during the analysis is small, and the volume of standard solution added in each step is negligible in comparison with the total volume of the sample solution. For example, In this case, because the volumes of standard solution added and solution mixture consumed during the analysis have no significant effect on the final solution volume, S-SAC can be successfully applied.
It is important to note, though, that S-SAC is an irreversible method, i. Other approaches to SA include, for example, the bracket standard additions method described by Abbyad et al. It consists of recording the analytical signal before and after adding the standard solution to the sample, and using the mean of the two measurements in specific equations to compensate for instrument drift.
The authors also evaluated different levels of standard addition and showed that similar precisions are obtained when the analyte concentration in the standard solution is the same as, or 7- to fold higher than that of the sample. In another work, Boto and Murphy proposed modifications to the SA equation to overcome issues associated with its application to samples containing very high concentrations of analyte. Their method was used in XRF and may be applied to other instrumental analytical techniques.
Before the revision of Bader's equations by Kelly et al. In addition to its compatibility to solid samples as discussed earlier, the authors pointed out that the gravimetric approach may also improve accuracy when compared to the volumetric approach. Internal standardization was combined with the gravimetric approach to overcome the inconvenience associated with sample solution dilution when the standard is added.
The IS species and the analyte standard were gravimetrically added to each flask containing the sample usually a certified reference material , with concentrations varying from none to a high addition level. According to the authors, the main advantage of this approach is that quantitative adjustment of volume is not necessary, as the IS can correct for dilution effects.
Furthermore, if samples are homogenous and have identical matrix, it is not necessary to build a calibration curve for each sample, and the analyte concentration can be determined from the slope of a curve built with the certified reference material. Zhu and Chiba proposed combining the SA gravimetric approach with internal standardization using a single standard addition point for elemental analysis by ICP-MS. The analyte concentration in the sample is determined by using the appropriate equation and considering the signal and mass of analyte and IS in both the standard solutions and the sample.
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A different mathematical equation is proposed if the mass of the IS species is not known. As reported by the authors, any element can be used as IS.
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