What are the uses of the default solution

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Calibration with standard solutions with internal standard

If losses of sample components or other systematic errors during sample preparation or analysis are to be expected, the use of an internal standard (= internal standard) is necessary.

The substance that is used as an internal standard is a non-sample component that is mostly chemically similar to the analyte, but not identical to it. It is added to each sample and standard in a known concentration. So it serves as a relative reference value. If the internal standard has changed its concentration, it is assumed that the concentration of the analyte has changed in the same way. Therefore, the choice of substance for the internal standard is of vital importance.

The mixtures that are measured as calibration standards therefore consist of the components that are to be determined (analytes) and the internal standard component.

The prerequisite for the use of internal standards is the use of an analytical method with which the internal standard and the analytes can be determined simultaneously. Chromatography is ideal for this technique. Individual component methods such as chloride-sensitive potentiometry or AAS cannot usually be calibrated using the internal standard method.

The calibration function

The calibration function is obtained from the signals of the measurements of one or more standard solutions (consisting of internal standard and analyte) of known concentration. For this purpose, a series of standard solutions (dilution series) of the analyte is prepared to which a known amount of internal standard is always added. The signals from both components are used to calculate the calibration function.

Calibration function of standard solutions (external standards)
The calibration function is expressed by the straight line equation:
Legend
-Absolute term of the calibration function (offset)
-Slope (sensitivity)
-signal
-concentration
-Analyte
-internal standard

The ratio of the relative intensity of an analyte signal to the signal of the internal standard serves as an analytical parameter. The signal ratio analyte / internal standard is thus used for calibration.

The analysis function

Analysis function from standard solutions with an internal standard
If the calibration function is changed over to the desired concentration, the result is called an analysis function.

This function is used to calculate the concentration of the sample. The application of the equation assumes a linear response behavior of the analytical method towards the analyte and also towards the standard.

The internal standard

The chemical and physical properties of the internal standard should be as similar as possible to those of the analyte so that both substances behave in the same way during the analysis. The internal standard should, if possible, show the following properties:

  • similar distribution ratios of atoms / molecules
  • similar response behavior in the analytical method
  • not an original part of the sample
  • same stability in calibration standards and samples
  • simultaneously determinable with the same method as the analyte

Under these conditions, fluctuations in sample preparation (e.g. volatilization of the solvent, adsorption on matrix components, incomplete reaction during derivatisation) and measurement (e.g. change in temperature in the measuring device, errors in the amount of sample injected) can have the same effect on the analyte and internal standard and can be adequately compensated for . Homologous "relatives" of the analyte are often used as internal standards, but also substituted or deuterated analyte molecules.

Summary

Tab. 1
Calibration with standard solutions with internal standard
requirementsadvantagesdisadvantage
▪ Suitable compound that meets the requirements of an internal standard ▪ Use of an analytical method for multi-component determination (e.g. chromatography) ▪ Possibility of even distribution of the internal standard in the sample - high reproducibility of adding the internal standard ▪ many systematic errors such as losses, accumulations, weighing and volume errors can be compensated; many samples can be measured without additional effort if the calibration is stable over a longer period; ▪ only a few special matrix effects can be corrected ▪ suitable compound with chemically and physically similar behavior is not readily available ▪ errors that only occur with the internal standard falsify the analytical result unnoticed ▪ the internal standard can only be incorporated into the sample to a limited extent (e.g. with a fixed matrix)

Further applications of the internal standard

Checking the constancy of an analyzer

In some cases it makes sense to add an internal standard to the sample solutions or the standard solutions in order to notice the changes in the response of the device. Then the internal standard is not used for quantification, but its signal value is written into a control card, for example, and its change is observed. If there is a significant change in this signal value, the system must be recalibrated. In this way of working, the internal standard serves as an indicator for a change in the conditions.

Retention time standard

An internal standard can also be useful to set a fixed reference point on the time axis of a chromatogram. This is necessary if the injection is not carried out in a defined manner by an autosampler, but is triggered manually. This means that the chromatogram does not have a start time. You can then get around by adding a foreign substance by choosing not the absolute retention time to identify the peaks, but the relative retention time based on the retention time of the internal standard.

Two internal standards can also be useful in chromatography to define a range between the two, where all peaks are integrated and used as a sum to determine the content. This technique is common for confusing, incompletely resolved chromatograms if the substances in the sample only belong to one substance class. An example of this is DIN for the determination of hydrocarbon mixtures such as diesel in the environment using gas chromatography.