Quadrupole mass analyzer

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/z). A quadrupole mass analyzer is essentially a mass 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/z 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.

Triple quadrupoles
A linear series of three quadrupoles can be used; known as a triple quadrupole mass spectrometer. The first (Q1) and third (Q3) quadrupoles act as mass filters, and the middle (q2) quadrupole is employed as a collision cell. This collision cell is an RF only quadrupole (non-mass filtering) using He or N gas (~10-3 Torr, ~30 eV) to induce collisional dissociation of selected parent ion(s) from Q1. Subsequent fragments are passed through to Q3 where they may be filtered or scanned fully.

This process allows for the study of fragments (daughter ions) which are crucial in structural elucidation. For example, the Q1 may be set to "filter" for a drug ion of a known mass, which is fragmented in q2. The third quadrupole (Q3) 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.

The arrangement of three quadrupoles was first developed by Jim Morrizon of LaTrobe University, Australia for the purpose of studying the photodissociation of gas-phase ions. Yet, the first triple-quadrupole for mass analysis was developed at Michigan State University by Dr. Christie Enke and graduate student Richard Yost in the late 1970's.