Principle of Operation
The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically, and a radio frequency (RF) voltage is applied between one pair of rods and the other. A direct current voltage is then superimposed on the RF voltage. Ions travel down the quadrupole between the rods. Only ions of a certain mass-to-charge ratio m/z will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods. This permits selection of an ion with a particular m/z or allows the operator to scan for a range of m/z-values by continuously varying the applied voltage. Mathematically this can be modeled with the help of the Hill differential equation.
Ideally the rods are hyperbolic. Circular rods with a specific ratio of rod diameter-to-spacing provide an easier-to-manufacture adequate approximation to hyperbolas. Small variations in the ratio have large effects on resolution and peak shape. Different manufacturers choose slightly different ratios to fine-tune operating characteristics in context of anticipated application requirements. In recent decades some manufacturers have produced quadrupole mass spectrometers with true hyperbolic rods.
Read more about this topic: Quadrupole Mass Analyzer
Famous quotes containing the words principle of, principle and/or operation:
“The principle of the brotherhood of man is ... narcissistic ... for the grounds for that love have always been the assumption that we ought to realize that we are the same the whole world over.”
—Germaine Greer (b. 1939)
“All Protestantism, even the most cold and passive, is a sort of dissent. But the religion most prevalent in our northern colonies is a refinement on the principle of resistance; it is the dissidence of dissent, and the Protestantism of the Protestant religion.”
—Edmund Burke (1729–1797)
“Human knowledge and human power meet in one; for where the cause is not known the effect cannot be produced. Nature to be commanded must be obeyed; and that which in contemplation is as the cause is in operation as the rule.”
—Francis Bacon (1560–1626)