Various methods are used to determine the mass to charge ratio. The instrument available to this laboratory uses a quadrupole mass filter that consists of four electrically conducting rods in a high vacuum chamber. The ions travel at constant velocity along the direction of the four rods, the z-direction, but undergo complex oscillations in the x and y directions in response to direct current (dc) voltages and radio frequency (rf) voltages applied to the rods. A particular ion with m/z = M+ will pass through the mass filter without striking any of the rods at a unique combination of the two voltages. Ions passing through the mass filter at the correct dc and rf voltages strike the detector and a signal is recorded. The mass spectrum consisting of the molecular ion and its fragment ions is recorded by varying the dc and rf voltages needed to focus in succession ions of increasing mass to charge ratio.
Figure 7. Diagram of a quadrupole mass filter.
Another method of measuring the mass to charge ratio is with a variable magnetic field created with a magnetic sector. With this method the ions are accelerated with a specific acceleration potential into a curved tube of specific radius under high vacuum. The curved tube is located within the magnetic sector. An ion of specific mass to charge ratio will pass through the tube at a specific magnetic field and be detected at the end of the tube. If the magnetic field is incorrect, the ion will collide into the side of the tube and be lost. The mass to charge ratio for the ion is calculated from the magnetic field that allows the ion to pass completely through the tube. A nice animation of mass spectrometry with this method of separating ions is presented at the following web site created by Dr. Tom Chasteen, a former graduate student at the University of Colorado.
Next section: Fragmentation Mechanisms
Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.