What is a quadrupole mass spectrometer?
A quadrupole mass spectrometer consists of four parallel rods, arranged similarly to the four poles on a compass. The radio frequency and direct current voltages across the north-south pair have one polarity, while the east-west pair has the opposite polarity. Hexapoles and octapoles are similar, but have six and eight rods, respectively. They are used as collision and/or storage cells. In tandem mass spectrometers (e.g., triple quadrupoles or Q-Tofs), quadrupoles can be used as a mass spectrometer or an ion transmission mode. To obtain an MS spectrum, the quadrupole operates in the so-called rf mode, where its function is to transmit ions from the ion source to the third quadrupole or TOF that records the spectrum. In the parent ion/product ion MS/MS mode, the quadrupole is used to select a specific ion for CAD (collision-activated dissociation) in a static mode. The resulting MS spectrum can be used to determine partial amino acid sequences of peptides. In precursor ion/neutral loss MS/MS and neutral loss MS/MS modes, the quadrupole scans in specific relation to the third quadrupole or TOF. Both types of scans can be used to detect phosphorylated peptides.
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