Quadrupole mass analyzer

The quadrupole mass analyzer (QMS) is one type of mass analyzer used in mass spectrometry. It is also known as a transmission quadrupole mass spectrometer, quadrupole mass filter, or quadrupole mass spectrometer. As the name implies, it consists of four cylindrical rods, set parallel to each other. In a quadrupole mass spectrometer the quadrupole is the component of the instrument responsible for filtering sample ions, based on their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.

The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically, and a radio frequency (RF) voltage with a DC offset voltage is applied between one pair of rods and the other. Ions travel down the quadrupole between the rods. Only ions of a certain mass-to-charge ratio 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 Mathieu differential equation.

Ideally, the rods are hyperbolic. Cylindrical 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.

A linear series of three quadrupoles is 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 Ar, He, or N₂ gas (~10⁻³ Torr, ~30 eV) for collision induced dissociation of selected parent ion(s) from Q₁. Subsequent fragments are passed through to Q₃ where they may be filtered or fully scanned.

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