Mass Spectrometric Detector (MSD):
This image shows a side-view of the quadrupole-mass spectrometer contained in the detector module on the side of the GC in the main image. The column is connected to the module through the hole visible towards the front (left side of the image). The pumps necessary to maintain a vacuum within the MS are visible at the bottom of the unit, while the actual detector is located at the rear (right side of the image).
The effluent from the column is directed into an ionization region, where the mobile phase and analyte components are ionized either by high-energy electrons (electron ionization, EI) or a high-energy reagent gas (chemical ionization, CI). The ionized particles enter a mass filter of some kind (such as a quadrupole) which allows only ion fragments with a specific mass-to-charge ratio (m/z) through to the detector at a time.
A mass spectrum is obtained by measuring the total number of ions hitting the detector in a given time for each m/z value in turn over a specififed range. This range is often chosen so that no signal is obtained from the GC mobile phase or potential contaminants (trace atmospheric gases that leak into the detector from outside or from the ion source.)
Chromatograms may be plotted in terms of the total ion current (or total ion count) from all species detected in each scan. or in terms of the counts for certain specific m/z fragments (selected ion monitoring). The individual mass spectra at any point in time can also be viewed as an aid to identifying sample components eluting from the column.
Quadrupole MSD:
These images show the quadrupole mass filter contained in the detector module shown above. The lower section contains the power supplies and electronics. In the second image, the ions enter from the source on the right-hand side, pass through the quadrupole mass filter in the centre, and exit to the detector on the left.
A quadrupole works by applying a varying electric field between each pair of electrodes; this affects the trajectory of the ions passing through the quadrupole so that only selected m/z fragments pass through at any one time. The electrodes are the long metal rods seen in the centre of the second image. The copper bus wires that carry this signal to the electrodes can also be seen clearly in the image.