Step 4: Mass analysis
The last step of GC/MS is mass analysis. The ions created at the ion source are electrically drawn from the source and into the mass analyzer (a quadrupole). The quadrupole is a mass filter constructed of four metallic bars of opposing polarity. A radio frequency current (rf) is applied to two poles opposite each other, and a direct current (dc) is applied to the other two opposing poles. A voltage differential between the rf and dc currents sends the masses of choice on an unimpeded trajectory within the boundaries of the quadrupoles. Think of the quadrupole as an adjustable mass filter. It can be tuned to allow only molecules of a certain charge and mass to go through, and in doing so, it will enable other ions with other masses to fly off into the vacuum waste, and not be detected. The ions, which were specifically selected for a given charge and mass, are then detected by a photomultiplier tube (or some other) detector.
The operator can then view 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 of each peak is proportional to the amount of analyte present. And, using known standards, can be used to calculate the analyte’s actual concentration. The retention time is used to identify 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 precise.
