a) In capillary GC-MS, the components are eluting from the chromatographic column comparatively fast. Specify the typical half width of such a peak in seconds.
Elution of a single component takes 0.5-2 s depending on chromatographic conditions.
b) Now, you have to find suitable settings for the scan speed of your mass spectrometer. Suggest a reasonable value.
One should obtain at least three scans over a chromatographic peak, thus scan cycle times in the order of 0.3-1 s are normally employed in GC-MS.
c) Assume your instrument is scanning from low to high m/z. How will your spectra change as elution proceeds? Use the spectrum of benzoquinone below to illustrate the effect.
Scans on the side of rising concentration (onset of elution) exhibit too high intensities in the higher mass range. Scans on top of the chromatographic peak show almost normal relative intensities and scans when the compound fades out exhibit falling intensity values towards higher mass.
d) What can you do to avoid distorted spectra?
Averaging or accumulation of spectra over the whole chromatographic peak compensates for the distortion of relative intensities due to rapid changes. Background subtraction avoids wrong intensity values resulting from isobaric interferences with column bleed.