Question

In choosing a compound to serve as IS(Internal Standard) for a GC analysis, what are the desirable properties to look for?

Answer 1

An internal standard is a substance which is blank solution used to callibrate the GC instrument.

1. An internal standard is added to the sample before analysis. This is composed of one or more compounds with sufficient similarity to the target analytes, so that they behave similarly, but must surely not appear in the samples.

2. Internal standard compounds must also be readily separated from any of the compounds in the sample. If an internal standard is added before such steps as concentration, extraction, or dilution, it is then not as necessary to make accurate volume measurements, and the injection volume is also not as critical.

3. It is important to keep the concentrations of the internal standard and the target compounds in a similar range, to keep linearity problems from arising. The area of the internal standard (A_is) is used to normalize the areas of all other sample peaks, thus eliminating the effect of differences in injection volumes or dilutions.

4. The internal standard must have the same chemical-physical characteristics of the compounds that need to be analyzed and is irrespective of the matrix(coloum).

There are two ways to proceed in this case:

1) Using only one internal standard. In this case, you should prepare a standard solution with all compounds found in your sample more internal standard (same amount used in real sample) and perform analysis by GC / MS in the same experimental conditions with which you have analyzed the actual sample. .

2) Use of more internal standards (one for each class of compounds that are in the real sample). Make an analysis by GC / MS to identify the compounds present in your actual sample. Subsequently add the real sample known amounts of internal standards that you have chosen and perform the analysis by GC / MS.