An internal standard in analytical chemistry is a chemical added in constant amounts for a sample, a blank standard and a calibration in chemical analysis. This substance can then be used for calibration by plotting the ratio of the analyte signal to the internal standard signal as a function of standard concentration analytes. This is done to correct the loss of the analyte during sample preparation or sample inlet. Internal standards are very similar compounds, but not identical to chemical species of interest in the sample, because the effects of sample preparation must, relative to the number of each species, become the same for signals from internal standards. as for signal (s) of the species of interest in the ideal case. Adding the number of known analyte (s) is a different technique called standard addition, which is done to correct the effect of the matrix.
This ratio for the samples is then used to obtain the concentration of their analyte from the calibration curve. The internal standard used needs to provide a signal similar to the analytical signal in many ways but quite different so that the two signals can be easily distinguished by the instrument. For example, deuterised chlorobenzene (C 6 D 5 Cl) is an internal standard used in volatile analysis on GC-MS because it is similar to Chlorobenzene but does not occur. Of course. Norleucine is also a popular internal standard for amino acid analysis through GC-MS.
In NMR spectroscopy, e.g. from the 1 H, 13 C and 29 Si, the frequency depends on the magnetic field, which is not the same in all experiments. Therefore, the frequency is reported as a relative difference to tetramethylsilane (TMS) internal standard. This relative difference to the TMS is called a chemical shift , and is measured in parts per million.
In practice, the distinction between common solvent signals and TMS is known, and since modern instruments are able to detect small amounts of protonated solvents present in commercial deuterated solvents, no TMSs need to be added. By determining the key solvents to be used, modern spectrometers can correctly reference samples; in essence, the solvent itself serves as an internal standard.
In chromatography, an internal standard is used to determine the concentration of other analyte by calculating the response factor. The selected internal standard should be again similar to the analyte and have the same retention time and similar derivitisation. It should be stable and should not interfere with the sample component.
Video Internal standard
References
- Skoog, Douglas A. (1998). Principles of Instrumental Analysis : Introduction, pp.Ã, 18. Harcourt Brace. ISBN: 0-03-002078-6
- IUPAC, Chemical Terminology Summary , second edition. ("The Golden Book") (1997). Online correction version: Ã, (2006-) "internal standard".
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