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Imagine you have a sample extract with an unknown concentration of target analyte in a matrix you know has been difficult to extract from (leading to signal suppression) or which might conversely, have artificially enhanced the target response.
You don't have the time for a full method validation, so what do you do?
You use the method of Standard Additions in order to eliminate the matrix effect. A series of experiments will be carried out where equal volumes of the initial sample extract are taken, after which all but one are separately 'spiked' with known and different amounts of the target analyte, all are then diluted to the same volume and analysed in a sequence. The instrument signals are then determined for all these solutions and the results plotted.
Multiple calibration points are used and on construction of the calibration curve the absolute value of the point of intercept on the x-axis indicates the concentration of analyte originally in the sample.
Multiple calibration points in a curve constructed of concentration added vs. response of analyte. A trend line can be added and the point of intercept on the x-axis indicates the concentration of analyte originally in sample: here it is -0.75 which indicates an original concentration of 0.75 concentration units.
Standard Additions has positives and negatives:
It gives very accurate quantitation and takes into account any effects of sample matrix in analysis which could skew results but it must have enough sample to analyse at multiple levels (need at least 2 points), the sample must be homogenous and a new calibration curve must be constructed for each sample to get accurate results.