Quadrupole mass spectrometer

Rod system of a quadrupole mass spectrometer

A quadrupole mass spectrometer (QMS) is a mass spectrometer whose analyzer is an electric quadrupole .

The ions are first accelerated by a static electric field and then fly along the axis between four parallel rod electrodes, the intersections of which form a square with a plane perpendicular to the cylinder axis. In the alternating field between the quadrupole rods, an m / q selection takes place, so that only particles with a certain ratio of their mass m to charge q can pass through the field. The ions hit a detector with a measuring amplifier that measures the ion current and is converted into counting rates or partial pressure by the software of a connected computer .

The ionization units, quadrupoles and detectors are available in various versions for different applications. The quadrupole mass spectrometer - in principle a linear Paul trap - is available in expensive, high-resolution, but also in inexpensive versions (as a residual gas analyzer ) and is widely used in research and development .

Other types of mass spectrometers also use quadrupole filters, such as. B. ion traps , which are storage mass spectrometers . The term “quadrupole mass spectrometer” is usually only used technically for systems with four quadrupole rods.

Quadrupole mass spectrometer Arrangement of electrodes and example of a flight path

Mass selection in the quadrupole field

The opposing electrodes of the quadrupole are each at the same potential. Between adjacent electrodes is a voltage having a DC voltage - and a high-frequency AC moiety with the amplitude applied, d. H. . The path of the ions in the QMS is described by the Mathieu differential equation. From systematic investigations of these differential equations it is known that there are certain stable and unstable areas. The work line, d. That is, the straight line on which all observable masses lie is determined by the ratio . In order to achieve the best possible resolution (values from R = 1,000 to 4,000), the following must apply. The intersection of the working line and the stable area of Mathieu's differential equations is then very small. The value 0.1678 must not be exceeded under any circumstances, otherwise all ions are unstable, i.e. that is, they collide with one of the four rods during the run. ${\ displaystyle (U)}$ ${\ displaystyle HF}$ ${\ displaystyle V}$ ${\ displaystyle u (t) = U + V \ cos \ left (\ omega t \ right)}$ ${\ displaystyle {\ frac {U} {V}}}$ ${\ displaystyle {\ frac {U} {V}} \ approx 0 {,} 1678}$

By setting the frequency or the voltages , it is possible to determine which particles with which mass-to-charge ratio reach the detector via the central trajectory. The path of a particle with the correct ratio is sinusoidal with constant distances from the central path of the quadrupole. All other ions fly through the alternating field around this nominal orbit area in a sine cycle, but are increasingly accelerated further, so that at some point they shoot out to the side outside the quadrupole and leave the area of influence of the EM field . Only particles within the acceptance range thus reach the end of the mass spectrometer. ${\ displaystyle \ omega}$ ${\ displaystyle U, V}$ ${\ displaystyle {\ frac {m} {q}}}$ ${\ displaystyle {\ frac {m} {q}}}$

This mass selection in the quadrupole field is referred to as a “mass filter” because of its functionality, but actually the mass-to-charge ratio ( m / z ) of the ions is the decisive factor. ${\ displaystyle {\ frac {m} {q}}}$

Technical construction of a quadrupole filter

In practice, four precisely manufactured round rods made of stainless steel or molybdenum , which are stored in ceramic holders in order to minimize the influence of temperature changes on the geometry of the quadrupole filter, are usually used to build the quadrupole filter for quadrupole mass spectrometers . The bars are arranged at a distance r from the axis of symmetry. The opposing bars are at the same potential. As already stated, a high-frequency voltage with a superimposed DC voltage is applied between the rod pairs . The ratio is usually controlled in such a way that the quadrupole filter does not work with a constant resolving power R , but with a constant line width (typically ∆m = 0.7–1), the so-called “unit resolution”. With ∆m = 1, m / q = 200 results in a resolution R = 200, with m / q = 1000 R = 1000. For the detection of positive ions , as a rule, the ion source in which the ions are generated is located also has a positive potential compared to the quadrupole. The potential difference here is usually between 2 and 5 kV. ${\ displaystyle HF}$ ${\ displaystyle U}$ ${\ displaystyle {\ frac {U} {V}}}$

Individual manufacturers use hyperbolically shaped gold-coated quartz glass rods instead of four round rods made of stainless steel in order to get closer to the ideal shape of a quadrupole field , according to their own statements .

Examples of the use of quadrupole mass spectrometers

Residual gas analyzers

The simplest application of quadrupole mass spectrometers is residual gas analyzers. They are usually used as a measuring instrument for assessing the residual gas composition in vacuum systems and are flanged directly to this. In the high vacuum an is often Faraday detector sufficient for ultra-high vacuum are often secondary electron multiplier used as detectors.

Quadrupole mass spectrometer in GC-MS coupling (GC-QMS)

The most widespread GC-MS type in organic analysis is the so-called GC-Single Quadrupole Mass Spectrometer (GC-QMS). In general, with GC-MS the substance mixtures to be examined are vaporized in the injector of the gas chromatograph . After passing through the GC column , the substances separated by interaction with the stationary phase of the column are ionized in the subsequent ion source of the mass spectrometer. For ionization of the substances is usually the EI ( electron impact - electron impact ), but also the CI ( chemical ionization ) or other ionization techniques used - the procedures are in the article mass spectrometry further explained. A quadrupole filter with “unit resolution” is used as the mass analyzer in the QMS. Usually with upstream so-called “pre-rods”, which are intended to minimize the influence of contamination. A secondary electron multiplier with or without a conversion dynode is usually used as the detector .

The devices are operated either in scan mode over a defined m / q range (“mass range”) or in selected ion monitoring mode (SIM). Due to the physico-chemical properties of gas chromatography, only vaporizable substances with a correspondingly relatively low molecular mass (m approx. <1000 u ) can be examined.

The areas of application of GC-QMS are qualitative and quantitative analysis or trace analysis . GC single quadrupole mass spectrometers can usually detect amounts of substances below a picogram. They are considered robust and linear over a wide range.

Quadrupole of a triple quadrupole mass spectrometer

Triple quads

Triple quads are used for tandem mass spectrometry (MS-MS). Two quadrupole analyzers are coupled in series, separated by a collision cell or collision cell, which can be designed as a quadrupole or octapole. The collision-induced dissociation in the collision cell leads to additional information about the examined compounds (mass fragmentograms) or is used to increase the selectivity of the measurement (SRM, selected reaction monitoring). Since the HPLC-MS coupling in the ion source is predominantly ionized with few fragments, the devices are robust, have a wide linearity range and can analyze amounts of substances in the nano-mole range, triple quads are the most widely used today in HPLC-MS Mass spectrometer for quantitative analysis.

For certain applications such as B. Multi-methods in pesticide analysis are often used, but GC triple quads are also used.

Further coupling techniques with quadrupole filters

Other coupling techniques with quadrupole filters are also used, e.g. B. QTOF ( time-of-flight mass spectrometer with upstream quadrupole filter).

literature

Michael Oehme: Practical introduction to GC / MS analysis with quadrupoles. Basics and Applications. Hüthig Verlag, Heidelberg, 1996.
Stavros Kromidas (editor): The HPLC-MS book for users. Wiley-VCH, Weinheim, 2017.

Individual evidence

^ Raymond E. March, Richard J. Hughes: Quadrupole Storage Mass Spectrometry. Wiley-Interscience, New York, 1989.

↑ Gerald Teschl : Ordinary Differential Equations and Dynamical Systems (= Graduate Studies in Mathematics . Volume 140 ). American Mathematical Society, Providence 2012, ISBN 978-0-8218-8328-0 ( mat.univie.ac.at ).

↑ Agilent Tech .: The New Agilent 5975C Series GC / MSD , Brochure 7641EN, 2011

^ Pfeiffer Vacuum: Residual gas analysis

↑ H.-J. Hübschmann: GC / MS manual. VCH Weilheim, 1996

↑ cf. DIN EN 12393-1: 2014-01

[1] Raymond E. March, Richard J. Hughes: Quadrupole Storage Mass Spectrometry. Wiley-Interscience, New York, 1989.

[2] Gerald Teschl : Ordinary Differential Equations and Dynamical Systems (= Graduate Studies in Mathematics . Volume 140 ). American Mathematical Society, Providence 2012, ISBN 978-0-8218-8328-0 ( mat.univie.ac.at ).