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:
“On principle I dislike an oath which requires a man to swear he has not done wrong. It rejects the Christian principle of forgiveness on terms of repentance. I think it is enough if the man does no wrong hereafter.”
—Abraham Lincoln (1809–1865)
“Life is a game in which the rules are constantly changing; nothing spoils a game more than those who take it seriously. Adultery? Phooey! You should never subjugate yourself to another nor seek the subjugation of someone else to yourself. If you follow that Crispian principle you will be able to say “Phooey,” too, instead of reaching for your gun when you fancy yourself betrayed.”
—Quentin Crisp (b. 1908)
“You may read any quantity of books, and you may almost as ignorant as you were at starting, if you don’t have, at the back of your minds, the change for words in definite images which can only be acquired through the operation of your observing faculties on the phenomena of nature.”
—Thomas Henry Huxley (1825–95)