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The quadrupole mass analyzer is one type of mass analyzer used in mass spectrometry. In a quadrupole mass spectrometer the quadrupole mass analyzer is the component of the instrument responsible for filtering sample ions, based on their mass-to-charge ratio (m/q). A quadrupole mass analyzer is essentially a m/q filter that is capable of transmitting only the ion of choice. A mass spectrum is obtained by scanning through the mass range of interest over time.
How it works
The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically and a radio frequency voltage is applied between one pair of rods, and the other. A direct current voltage is then superimposed on the R.F. voltage. Ions travel down the quadrupole in between the rods. Only ions of a certain m/q will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods. This allows selection of a particular ion, or scanning by varying the voltages.
Applications
These types of mass spectrometers excel at applications where particular ions of interest are being studied because they can stay tuned on a single ion for extended periods of time. One place where this is useful is in liquid chromatography-mass spectrometry or gas chromatography-mass spectrometry where they serve as exceptionally high specificity detectors. Quadrupole instruments are often reasonably priced and make good multi-purpose instruments. This instrument consist of a ion gauge like front end, the analyzer, an electron multiplier, and electronics.
Triple quadrupoles
A series of three quadrupoles can be used, this is known as Triple Quadrupole mass spectrometry. The first and third quadrupoles are mass filters, and the middle one is a collision cell. This allows the study of fragments, useful in structural studies.
For example, the first quadrupole may be set to "filter" for a drug ion of a known mass, which is fragmented in the second quadrupole. The third quadrupole can then be set to scan the entire m/z range, giving information on the sizes of the fragments made. Thus, the structure of the original ion can be deduced.
See also: other mass analyzers
- Quadrupole ion trap A very closely related mass analyzer
- Time of flight
- magnetic/electric sectors
- Fourier transform ion cyclotron resonance
External links
- Quadrupole mass spectrometry Page from the Chemistry Hypermedia Project, summary of the technique and useful diagram
"Quadrupole mass analyzer"