Single Spike Method

The spike in most cases is single spike, but it can be double spike. Single spike method provides the information of the concentration of the sample, whereas the double-spike method can yield the concentration of the sample and the mass fractionation factor that can be used to calculate the true isotopic ratios in the sample. Take Pb isotope as an example. Natural Pb samples have 1.4% ° Pb, 24.4% ° Pb, 22.1% ° Pb and 52.4% ° Pb. A single spike is made by artificially concentrating one of the minor Pb isotopes, for example Pb. A double spike is made by artificially concentrating two minor isotopes. For example, we can concentrate Pb and b isotopes to make ° Pb- Pb double spike or concentrate Pb and Pb to make Pb- ° Pb double spike. 

Single spiking method is essentially a problem of two-component mixing. The ratio of the abundances of two isotopes A and B of an element in the mixture is (Faure, 1986)... 

This neat condition provides a good guidance for the single spike method. 

Based on the study results, decide how frequently and how rigorously the bias needs to be checked during routine use of the method (e.g. every batch using a single spike) and document it in the method as part of quality control. 

A single spike is enriched in one minor isotope whereas a double spike is enriched in two minor isotopes (Fig. 11.3). Double spike method needs two runs. The first run is to measure the un-spiked sample and the second run is to measure the spiked mixture. Double spike method not only provides the concentrations of the sample but also corrects the mass fractionation in mass spectrometers (and thus giving the true isotopic ratios). 

The use of several QA/QC methods is described in this article, including control charts for monitoring the concentration of solutions of thiosulfate that have been prepared and stored with and without proper preservation the use of method blanks and standard samples to determine the presence of determinate error and to establish single-operator characteristics and the use of spiked samples and recoveries to identify the presence of determinate errors associated with collecting and analyzing samples. 

Cross-Validation. Cross-validation is a comparison of validation parameters when two or more bioanalytical methods are used to generate data within the same study or across different studies. An example of cross-validation would be a situation where an original validated bioanalytical method serves as the reference and the revised bioanalytical method is the comparator. The comparisons should be done both ways. When sample analyses within a single study are conducted at more than one site or more than one laboratory, cross-validation with spiked matrix standards and subject samples should be conducted at each site or laboratory... 

The results of these interlaboratory studies are reported in USEPA Method Validation Studies 14 through 24 (14). The data were reduced to four statistical relationships related to the overall study 1, multilaboratory mean recovery for each sample 2, accuracy expressed as relative error or bias 3, multilaboratory standard deviation of the spike recovery for each sample and 4, multilaboratory relative standard deviation. In addition, single-analyst standard deviation and relative standard deviation were calculated. 

Cahill et al. [241] have developed a simple and sensitive analytical procedure for determining the concentration of trifluoroacetic acid in plant, soil, and water samples. The analysis involves extraction of trifluoroacetic acid by sulfuric acid and methanol followed by derivatisation to the methyl ester of trifluoroacetic acid. This is accomplished within a single vial without complex extraction procedures. The highly volatile methyl ester is then analysed using headspace gas chromatography. The spike recovery trials from all media ranged from a low of 86.7% to a high of 121.4%. The relative standard deviations were typically below 10%. The minimum detectable limit for the method was 34 ng/g for dry plant material, 0.20 ng/g for soil and 6.5 ng/1 for water. 

If the above approach creates practical problems, an alternative is to perform single analyses on a minimum of five test portions of the study sample. The standard deviation of replicate analysis results is an indicator of sample homogeneity and method precision. The disadvantage of this approach is that it does not provide a simultaneous measure of the analytical variance under the homogeneity test conditions. Analytical variance must be estimated from historical data (e.g. method validation) or spiked recoveries run with the homogeneity test samples. 

Isotope dilution mass spectrometry is a powerful method for determining the quantity of an element or an associated compound in a sample. It requires that a spike of the same element but with an isotopic composition different from that of the sample be introduced to it in a controlled manner. The mass spectrum of the mixture of spike and sample is then used to determine the concentration of the target element in the original sample. In most cases, a single ratio is used, one that incorporates the major isotope in the spike and the major isotope in the sample these must be different isotopes. The difference in the value of this ratio in the sample and in the mixture of sample and spike is proportional to the amount of the target element in the sample. 

For the purposes of this method, a water sample is defined as a single phase system that is primarily clear water but may contain very small amounts of floating, suspended and settled particulate matter. Multiple phases should not be present (see Section 8.4). Approximately 1 L of the water sample is spiked with the internal standard solution and filtered to separate the aqueous and particulate fractions. The filtered aqueous fraction is extracted with methylene chloride using a separatory funnel or continuous liquid-liquid extractor. The particulate fraction is extracted with toluene in a SDS extractor. The extracts of the two fractions are then combined for cleanup. 

Another standardization technique which is widely used in AAS is standard addition. It is especially useful for samples where the matrix is diflBcult to reproduce, such as samples decomposed by fusion. In this method, a known quantity of the element to be determined is added to a portion of the sample, preferably at the start of the decomposition procedure. This known amount then serves as the standard when the absorbances of the two solutions are compared. Because of the rather prevalent curvature of absorbance vs. concentration plots, it is usually necessary to run three standards as well as the unspiked sample. Thus, even if one could use a single multiple element spike as the solution for standard addition, this would quadruple the work load for sample decomposition and analysis. If such a solution were added after decomposition, it would require splitting the quantitatively diluted solution, spiking, and rediluting, and would quadruple the work load for analysts. 

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