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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Mass Analyzer

The positive molecular ions and neutral and ionized fragments are drawn out of the source and into the mass analyzer (quadrupole) by a series of voltages applied to electrostatic lenses known as the repeller, the ion focus, and the entrance lens. The quadrupole is a mass filter constructed of four metallic bars of opposing polarity. A radio frequency (rf) current is applied to two poles opposite each other and direct current (dc) current is applied to the other two opposing poles. A voltage differential between the rf and dc currents send the masses of choice on an unimpeded trajectory within the boundaries of the quadrupoles. Masses that were not selected by the operator either collide with the poles or spin off between the poles where they are evacuated from the instrument under high vacuum.
After exiting the quadrupole, a voltage pushes the ion masses into a structure that resembles a horn, called the electron multiplier. A photomultiplier tube can also be used as the multiplier. For each ion that hits the emissive surface of the electron multiplier, two electrons are emitted. These electrons ricochet off the surface and emit two more electrons each. This continues down through the multiplier so that by the time the electrons reach the signal recorder, the signal has been multiplied several million times.
The operator views the data collected by the GC/MS in the form of a chromatogram and mass spectrum. The chromatogram is a graphical representation of the signal produced by each ionized analyte. Time is displayed on the x axis and signal intensity on the y axis. The area within each peak can be measured and used to calculate the analyte’s concentration. The retention time is used in the identification of the analyte. The mass spectrum is a graphical representation of the mass to charge ratio (m/z) on the x axis and abundance on the y axis. Identification based on mass spectra is very specific.