Questions: Gas Chromatography: Quantitative Analysis and Calibration

Question 1

Two analysts measure the same analyte mixture using identical GC conditions. Analyst A uses external standard calibration; Analyst B uses the internal standard method. Their results diverge by 12%. What is the most likely cause?

• A. Analyst A's injections varied in volume across samples, introducing proportional error that the internal standard method corrects
• B. The internal standard co-eluted with the analyte in Analyst B's analysis, distorting peak areas
• C. Analyst A used peak area while Analyst B used peak height, which always diverge
• D. Different detector response factors applied to the two analysts' instruments

Answer: (A)

Injection volume variability is the primary limitation of external standard calibration. Even small differences in injected volume produce proportional differences in peak area, introducing scatter across samples and standards. The internal standard method corrects for this by adding a fixed amount of a non-analyte compound to every sample and standard — both the analyte and the IS experience the same injection volume variation, so their area ratio cancels the error. Options B and C describe real issues but wouldn't produce systematic 12% divergence between methods on the same samples. Option D would affect both analysts equally.

Question 2

An analyst injects equal masses of hexane and toluene into a GC-FID system and observes that hexane gives a larger peak area despite equal mass. What does this demonstrate?

• A. The column retained toluene longer, spreading its peak and reducing height but not area
• B. Hexane eluted as a sharper peak, so its height is larger even though area is the same
• C. FID response factors differ between compounds — equal masses produce different detector signals
• D. Toluene partially decomposed on the column, reducing the amount reaching the detector

Answer: (C)

FID response is roughly proportional to the number of carbon-hydrogen bonds involved in combustion, not simply mass. Hexane (fully aliphatic) has a higher FID response per unit mass than toluene (aromatic ring carbons respond less efficiently to FID). This is the relative response factor: the ratio of detector signal to analyte mass differs between compounds. Ignoring this and treating all peak areas as directly comparable masses would introduce systematic errors in multicomponent analysis. Options A and B confuse peak shape effects with area. Option D is a different phenomenon.

Question 3

Peak area is generally preferred over peak height for GC quantitation because it is more reproducible under most chromatographic conditions.

• T. True
• F. False

Answer: False

Peak area is generally preferred because it is proportional to the total mass of analyte reaching the detector regardless of peak shape. However, peak height can outperform area when peaks partially overlap: integrating the area of two merged peaks introduces larger errors than reading the heights of partially resolved maxima. The claim that area is *always* preferred is the misconception — the choice depends on peak resolution. When peaks are fully resolved, area is more robust; when peaks partially overlap, height measurement at the apex may be more accurate.

Question 4

In the internal standard method, the ratio of analyte peak area to internal standard peak area is used instead of raw analyte area because the ratio corrects for injection volume variation.

• T. True
• F. False

Answer: True

This is the core logic of the internal standard method. If injection volume varies by 5% between runs, both the analyte signal and the IS signal change by ~5% — so their ratio remains constant. The calibration curve plots this area ratio versus analyte concentration, meaning any injection volume variation affects numerator and denominator equally and cancels out. This is why the IS must be added at the same concentration to every sample and standard before injection — it must experience exactly the same injection variability as the analyte to function as a correction factor.

Question 5

Why must an internal standard be chemically similar to the analyte, and why must it be fully resolved chromatographically from the analyte?

Model answer: Chemical similarity is required so the IS behaves like the analyte during sample preparation and injection — if the IS has very different volatility or solubility, it may be lost or enriched at different rates than the analyte during sample handling, breaking the correction logic. The IS must also have a similar detector response factor so that its signal scales predictably with the analyte's. Full chromatographic resolution is required so that the two peaks can be integrated independently — if the IS co-elutes with the analyte, their peaks overlap and neither can be accurately measured.

The internal standard method's validity depends on the IS being a perfect surrogate for the analyte everywhere except the concentration axis. Chemical similarity ensures it behaves identically during injection; similar response factors ensure detector behavior is comparable; chromatographic resolution ensures the two peaks can be distinguished. Violating any of these requirements introduces systematic bias that defeats the purpose of using an IS in the first place.