Gas Chromatography Questions and Answers

Answer: A

Reporting each peak as a percentage of the total area is correct only if the detector responds equally per unit mass to everything present, which is rarely exactly true. Injection volume cancels, identification is not required, and linearity is a separate assumption. A method that needs no standards at all is attractive until its hidden assumption is examined, and response factors are what correct it. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Because detectors respond unequally, each compound's area must be scaled by a factor measured against a reference before the areas can be compared. Area to height, transfer efficiency and signal decay are other quantities. Determining response factors is what turns an area normalisation from an approximation into a quantitative result, and it is the step most often omitted when a composition is quoted from a bare chromatogram. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

Dividing analyte by standard:

\[ \frac{4500}{3000} = 1.5 \]

giving 1.5, which is compared against a calibration of the same ratio to obtain the concentration. Inverting gives 0.67, and the sum and difference are not used. That the ratio rather than either area is carried forward is exactly what makes the injection volume irrelevant.

Answer: D

Software applies a consistent baseline and a consistent rule for splitting overlapping peaks, which two analysts measuring by hand would not. Retention times are readable either way, identification needs a standard or a spectrum, and symmetry helps but is not required manually. Removing operator judgement from a routine step is what makes results comparable between people and between days.

Answer: A

On a sloping or noisy baseline the software's chosen start and end points can differ substantially from what an experienced eye would pick, and the area changes with them. Retention times are robust, identification is a separate step, and linearity is an assumption of the calibration rather than the integration. That an automated step still needs reviewing, particularly where the trace is poor, is the practical caution to carry. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

The first two are correct and the third hides a substantial assumption: area normalisation is valid only if every component responds equally per unit mass, which detectors rarely do. Response factors correct it. A method requiring no standards is attractive precisely because its assumption is invisible, which is why it is worth stating explicitly.

Answer: C

The first three are correct and the fourth is not, so three match. Selectivity is the ratio of two capacity factors, a retention property, and has nothing to do with peak heights, which reflect amount and shape. Each quantity is read from a different feature of the same trace, and knowing which feature carries which information is most of reading a chromatogram properly.

Answer: D

This is the capacity factor, often written as k, and it measures how many times longer a compound stays than the carrier does. A response factor corrects detector sensitivity, a separation factor is another name for selectivity, and a symmetry factor describes peak shape. Four factors describing four quite different aspects of one chromatogram, and using the right name keeps a method description unambiguous. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Answer: A

Detector response time broadens peaks in practice but is not part of the equation, which describes processes inside the column alone. The other three are its three terms. Distinguishing extra-column broadening - from the injector, the detector and the connecting tubing - from the column's own contribution is what tells you whether a better column would actually help.

Answer: B

Equilibrating a sample in a sealed vial and injecting only the vapour above it delivers the volatile compounds while leaving the involatile matrix behind entirely. Residues, dissolved gases and early fractions are other things. Sampling only what is volatile is both a separation and a clean-up in one step, which is why blood alcohol and residual solvent determinations are done this way.