The most common type of mass spectrometer used in environmental GC/MS field analysis is the quadrupole mass analyzer. Quadrupole mass spectrometers are usually less expensive and more rugged than their magnetic sector counterparts. They are generally more compact and commonly found in commercial bench top spectrometers. They also offer the advantage of low scan times, on the order of less than 100 milliseconds. This is useful for real-time scanning of chromatographic peaks.
The heart of a quadrupole mass analyzer is four parallel cylindrical rods that serve as electrodes. Opposite rods are connected electrically with one pair being attached to the positive side of a variable direct current (dc) source and the other pair to the negative terminal. Variable radio-frequency alternating current (ac) potentials, 180 degrees out of phase, are applied to each pair of rods. Ions are accelerated into the space between the rods by a potential of 5 to 10 volts. The ac and dc voltages on the rods are increased simultaneously while maintaining a ratio constant. All the ions except those with a certain mass-to-charge ratio strike the rods and converted to neutral molecules. Only ions with a limited range of mass values reach the transducer. Quadrupole instruments typically resolve ions that differ by one mass unit.