Internal standard
An internal standard is an aid in quantitative analyzes to detect sample losses during the analysis process or to determine the amount of a molecule in a sample by comparing the amount of the molecule with the known amount of an internal standard contained in the sample. In contrast to the external standard , the internal standard is in the sample to be measured and is therefore treated in the same way. However, it is only suitable for some procedures.
properties
The internal standard is usually added to a sample; the addition is sometimes colloquially referred to as spiking (from spike , tip, after the form of a measurement signal). The internal standard serves as a relative reference value , which should show the influence of the procedure on the result and allow an assessment of the quality of the procedure. For this purpose, substances that are as similar as possible to the analyte but do not appear in the analysis sample should be used. An internal standard is added to each sample (and the calibration solutions ) in a defined amount at the earliest possible stage of the analysis process.
After the analysis, the result of the internal standard is compared with the expected amount (determination of the recovery rate ). If the concentration of the internal standard has changed due to the procedure , it is assumed that the concentration of the analyte has changed in the same way. In the optimal case, systematic errors (e.g. losses due to sample preparation) and random errors (measurement errors due to fluctuations in the device parameters) can be corrected with the recovery rate of the internal standard.
The prerequisite for the use of internal standards is an analytical method with which the internal standard and the analytes can be determined simultaneously (e.g. chromatography , ICP mass spectrometry ). The choice of the internal standard must be very precisely tailored to the examined sample and the analytical method. The selection must ensure that each step of the process has the same or similar effects on the analyte and internal standard. This usually requires a certain amount of prior knowledge of the sample. In the optimal case, the amount of the internal standard is also similar to the amount of the analyte in the sample (same order of magnitude).
example
The lead content of a sediment sample is to be determined. For this purpose, a known amount of a metal can be added to the sediment sample immediately after sampling, which is similar to Pb, but does not occur in the sediment (e.g. rare metals such as Tl, Rh). Thereafter, the analytical procedure is carried out normally, e.g. B. dry the sediment, crush it, digest it with concentrated acid, smoke the acid, absorb the residue and dilute it in a defined manner, take an aliquot and determine the lead content and the recovery rate of the internal standard using ICP-MS . The lead content in the sample is corrected by recovering the internal standard.
Individual evidence
- ↑ Entry on internal standard . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.I03108 Version: 2.3.