Question

Gas chromotography (GC) typically uses the port in the split mode in which about 10% of the sample reaches the detector. Why is the use of the internal standard method particularly important with GC?

Answer 1

IS is important in improving the accuracy of result when GC is performed for unknown sample, comparing with ES (external standard) IS is best in accuracy predeiction.

Internal Standard (IS)

For IS calibration, an IS is chosen. For example, 900 µL of a 4-ng/mL solution of reference standard plus 100 µL of the 100-ng/mL IS solution would yield 1 mL of a solution with a ratio of 0.36 C _A /C _IS , which is equal to ([4 ng/mL × 900 µL]/1000 µL)/([100 ng/mL × 100 µL]/1000 µL). When this solution is injected, it will give a chromatogram with two peaks, one each for the reference standard and IS. The area ratio for these two peaks is paired with the concentration ratio to construct the calibration plot, where the arrows highlight the 0.36 C _A /C _IS concentration ratio and its corresponding 0.36 R _A /R _IS response ratio (I've assumed the analyte and internal standard have the same response characteristics). Now, if the same sample preparation procedure is used to add IS to unknown samples, the R _A /R _IS ratio can be used to determine the concentration ratio (arrows for 0.8 R _A/R _IS leading to 0.8 C _A /C _IS in Figure 1). Because the concentration of IS (C _IS) added to the sample is known, the concentration of analyte in the sample (C _A ) can be calculated.