The ChromaBLOGraphy series continues for the use of internal standards with medical cannabis potency testing by GC-FID (I’ve listed the first two parts in the series immediately below as web-links). This third part demonstrates the positive impact an internal standard can have on quantitative accuracy.
The chromatograms below show a calibration standard at 50 ng/µL cannabinoids and internal standards (Phencyclidine, or PCP, was used for calibration and quantification) analyzed during initial calibration on a Friday and then again on Monday after sitting on the autosampler all weekend. Now first, I would never leave calibration standards of any type out all weekend and plan to use them again, except in this case the evaporative loss of solvent and resulting concentration of analytes serve to illustrate one of the main benefits of an internal standard: taking care of a proportional loss or gain in peak areas so that quantitative accuracy is not compromised. A peak area gain or loss could arise from injecting too much, injecting too little, or evaporative loss of solvent over the course of analyzing a large sample queue.
As you can see in the orange chromatogram, especially since I marked PCP and delta-9-THC with green lines, the peak heights (and thus areas) are higher across the board for this standard, which is the same one analyzed previously. However, when the internal standard technique is used for quantification, the analyzed values are very close to the expected values. If internal standard (ISTD) quantification is not used, in favor of external standard (ESTD) quantification, the values are not accurate.
The above situation could be considered an extreme case, so I put together a table that compares ISTD and ESTD values for check standards (those standards analyzed during/after a sample queue to make sure the calibration is holding) and standards analyzed after sitting on the autosampler for the weekend. In almost all cases the ISTD quantification method produces a better value for the standards. Hopefully this motivates you to consider using internal standards in your medical cannabis analyses.