Quadrupole mass analyzer

A quadrupole mass spectrometer (QMS) is a mass spectrometer, the analyzer is an electric quadrupole. The ions are first accelerated by a static electric field, and then to fly along the axis between four parallel- bar electrodes, which form points of intersection with a plane perpendicular to the cylinder axis a square. In the field between the quadrupole rods an m / q triage takes place, can go through the field so that only particles with a specific ratio of its mass m to the charge q. The ions strike a detector with amplifier, which measures the ion current and the software of the PC connected to the counting rate and the partial pressure is converted.

The Ionisationseinheiten, quadrupoles and detectors are available for different applications in different variations. The quadrupole mass spectrometer - an application of the Paul trap - is to get in expensive, high-resolution, but also in low-cost variants ( as a residual gas analyzer ) and is widely used in research and development.

Mass triage in the quadrupole field

The opposing electrodes of the quadrupole are located at the same potential. Between adjacent electrodes, a voltage of a DC and a high-frequency AC component is applied, that is. The path of the ions in the QMS is described by the Mathieu equation. For systematic studies of these differential equations is known that there are certain stable and unstable areas. The load line, that is, the straight line, lying on the all observable mass is determined by the ratio. To the best possible resolution (R = in practice from 1000 to 4000 ) to reach, must apply. The intersection of load line and stable region of the Mathieu differential equations is then very small. The value of 0.1678 may however be exceeded under any circumstances, otherwise all ions are unstable, ie they collide during the run with one of the four bars.

About setting the frequency or voltages allows to program with which mass-to- charge ratio which particles reach the detector via the central trajectory. The path of a particle to correct m / q ratio is sinusoidal with constant intervals to the central web of the quadrupole. Even all other ions fly in the sinus cycle by the alternating field to this set path area, but always far out accelerated so that they will eventually side shoot out outside of the quadrupole and leave the sphere of influence of the EM field.

Since the ionization occurs by electron impact, occur in almost all atoms / molecules instead of a peak at the corresponding mass of the atom / molecule several peaks in the mass spectrum. This has several reasons:

• The molecules can in the collision with the electrons into different components break ( fragmentation). Thus, for example, retrieves ethanol ( C2H5OH ) ( mass 46.07 g / mol) a weak peak at 46 Th ( C2H5OH ) produced a stronger peak at 45 Th ( a hydrogen atom missing) and the strongest peak (ca. 10 - times higher than the other peaks ) at 31 Th ( CH 2 OH ). Other peaks are, inter alia, 17 Th ( OH).
• A recombination or formation of new molecules is very rare. It can not be excluded because of the high accuracy of the quadrupole mass spectrometer that corresponding count rates can occur.
• The fragments of the molecules or atoms to be ionized twice or even three times. Since the quadrupole mass spectrometer - like other mass spectrometer - inherently measures only the m / q ratio, there are by definition more peaks, eg, argon at 40 Th and 20 Th
• At high concentrations of inert gases in a sample of the so-called clusters are shown, for example, argon 40 has a peak at 80AMU.