Internal standard

An internal standard is a tool for quantitative analysis for the detection of sample loss during the analysis process or the quantification of a molecule in a sample by comparing the amount of the molecule to the known amount of internal standard present in the sample. In contrast to an external standard, the internal standard is in the sample to be measured, and is therefore treated the same, 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 (of English. Spike, tip ', after the shape of a measured signal). The internal standard is used as a relative reference, intended to reflect the impact of the process on the result and allow an assessment of the quality of the procedure. For this purpose, substances which are similar to the analyte as possible, in the analysis sample are not present should be used. An internal standard, each sample ( and the calibration solution ) was added at an early stage of the analytical process in a defined amount.

After the analysis is the result of the internal standard with the expected amount compared ( determination of the recovery rate). If the internal standard by the method of changing its concentration, it is believed that the concentration of the analyte is modified in the same manner. In the optimal case, systematic errors can be corrected with the recovery of the internal standard (eg losses due to sample preparation ) and random errors ( due to fluctuations in device parameters measurement error ).

Requirement for the use of the internal standard is an analysis method with which simultaneously the internal standard and the analyte may be determined ( e.g., chromatography, ICP mass spectrometry). The choice of the internal standard must be matched exactly to the sample under investigation and the analytical method. The selection must ensure that each step of the process on analyte and internal standard affects the same or similar. These usually some prior knowledge of the sample is necessary. In the optimum case, the amount of the internal standard similar to the amount of analyte in the sample (the same magnitude).

Example

The lead content of a sediment sample to be determined. This immediately after sampling to sediment sample, a known amount of a metal can be added, similar to the Pb sediment but not to occur ( for example, rare metals such as Tl, Rh ). Then the analytical procedure is normally performed, eg dry sediment, crushing, catch up with concentrated acid, the abrauchen acid, take up the residue and dilute defined, remove an aliquot and determine the lead content and the recovery of the internal standards by ICP- MS. About the recovery of the internal standard, the lead content in the sample is corrected.