do we need to inject the ISTD every time while we have to analyze our analyte of unknown concn?
Hi, internal standard is must to calculate concentration of the unknown sample. Please note that you must 'add' an internal standard everytime into the sample which is to be analysed. Since, the peak area always varies with the amount of sample injected, finding out the response factor (area of IS/amount IS) solves this problem when you inject different volume each time. Please see attached document, how to calculate the concentration of unknown sample using internal standard method. Please note that you must also run standard compounds, including internal standard to find out conc. of unknown in the sample.
Internal standard is used to see the comparison of the instrument responses from the target compounds in the sample to the responses of reference standards added to the sample or sample extract before injection.
Yes, you should add the Internal Standard every time in order to keep out deviations caused by the stability of the equipment and errors in the sample handling
GC-FID is generally used for both quality and quantity analysis of unknown sample. Although major part of this analysis depends upon the GC column and method you choose for analysis. My suggestion would run one GC-MS analysis for your unknown sample there will get to identify most of your components in the component mixture. Now to quantify you can get the internal standards for the identified components and run it in GC-FID. Once you have both the spectra of your internal standard and sample you can easily analyze and quantify by comparing the area under the peak.
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