Sector field mass spectrometer

Sector field mass spectrometers are a type of mass spectrometry device . They are usually built in the form of high-resolution (double focusing) sector field mass spectrometers in which a magnetic field and an electric field are arranged sequentially.

Dempster's magnetic sector field analyzer

history

In 1918 Arthur Jeffrey Dempster designed and built the first modern mass spectrometer with a magnetic sector field analyzer, which was 100 times more accurate than all previous developments, and laid the foundation for the design of today's mass spectrometers. In 1934 Josef Mattauch and Richard Herzog described a double-focusing mass spectrometer (Mattauch-Herzog geometry, built by Mattauch and Herzog in 1936), which Mattauch used for the most precise atomic mass determinations of the time. Corresponding mass spectrometers were also built in the United States. Since the 1950s, more mass spectrometer types such as were flight mass spectrometer developed that made the sector field mass spectrometer competition. In the 1970s, however, sector field mass spectrometers coupled with gas chromatography were built whose detection sensitivity was up to 2 orders of magnitude higher than that of other types of devices. B. could be used excellently in the field of dioxin analysis. After environmentally relevant incidents such as the Sevesoung accident , a scientific focus was here and a corresponding number of devices were built.

Schematic drawing of a sector field mass spectrometer

Basic principle

A mass spectrometer (MS) consists of an ion source, an analyzer and a detector. The ion source of the sector field mass spectrometer must generate a sharply focused ion beam because, unlike the quadrupole mass spectrometer , the analyzer does not act as a mass filter. The requirements for the focusing unit are correspondingly demanding.

The analyzer in a sector field mass spectrometer works on the principle of deflecting accelerated ions in a magnetic field and / or an electric field. The radius of the circular paths that they traverse in the fields depends on the energy (in the electric field) and the momentum (in the magnetic field) of the ions. Knowing the charge, the energy and the momentum, the mass can then be determined. In the high-resolution sector field mass spectrometer, the ion beam is focussed twice by means of a magnetic field and an electric field, which are arranged one after the other. The reverse order is also possible.

When leaving the ion source , the kinetic energy of the ions is equal to the acceleration energy of the field in the ion source. The ion beam next enters the magnetic field, which is applied perpendicular to the direction of movement of the ions. The ions are deflected onto a circular path by the Lorentz force . The deflection radius depends on the mass / charge ratio of the ion and can be calculated by equating Lorentz force and centrifugal force:

${\displaystyle r={\frac {mv}{zeB}},}$

in which

${\displaystyle r}$ = Deflection radius

${\displaystyle m}$ = Mass of the ion

${\displaystyle v}$ = Speed ​​of the ion

${\displaystyle e}$ = Elementary charge

${\displaystyle B}$ = Magnetic flux density

${\displaystyle z}$ = Number of charges

The equation shows that the deflection radius depends on the ratio . The magnetic field not only causes the ions to be separated according to their ratio, but also to focus on the direction. This is helpful because, even in a sharply focused ion beam, not all ions have exactly the same direction of movement. By circulating through the magnetic field, the ions are united with like in one point. ${\displaystyle r}$ ${\displaystyle m/z}$ ${\displaystyle m/z}$ ${\displaystyle m/z}$

To record a mass spectrum, either the orbit radius can be measured or, if the detector is in a fixed position, the magnetic flux density or the acceleration voltage can be varied. The former is only rarely used, since then either a detector would have to be designed to be spatially displaceable or several detectors would have to be used permanently, which would be too expensive for analysis, especially in organic chemistry. In practice, this only occurs in the area of ​​isotope mass spectrometry. The variation of the voltage is technically easier and cheaper to implement than the variation of the magnetic flux density, but it provides poorer reproducibility, because repercussions on the ion source and thus on the ionization probability occur. The resolving power of a magnetic field is limited by Maxwell's velocity distribution of the ions. The ion beam is therefore allowed to pass through an additional field, the direction of which is both perpendicular to the magnetic field and perpendicular to the ion beam (see Wien filter ). A speed focusing takes place because the ions are separated according to their kinetic energy and independent of their mass. Only ions that meet the condition ${\displaystyle r}$ ${\displaystyle B}$ ${\displaystyle U}$

${\displaystyle v={\frac {E}{B}}}$

meet the filter can pass. The following applies now:

${\displaystyle r={\frac {mv^{2}}{zeE}},}$

${\displaystyle r}$ = Deflection radius

${\displaystyle m}$ = Mass of the ion

${\displaystyle v}$ = Speed ​​of the ion

${\displaystyle e}$ = Elementary charge

${\displaystyle E}$ = Electric field strength of the speed filter

${\displaystyle B}$ = Magnetic flux density (both in the velocity filter and in the mass analyzer)

${\displaystyle z}$ = Number of charges

Such double focusing sector field mass spectrometers achieve resolutions of up to 100,000 and were the mass spectrometers with the greatest resolution before the development of the FT-ICR mass spectrometer. In the area of ​​coupling with gas chromatography, one usually works with resolutions of 4,000 to 10,000. ${\displaystyle R}$ ${\displaystyle R}$

Device geometry

There are some common device geometries named after their respective developers.

Bainbridge-Jordan geometry

Geometry with a 127.30 ° electric sector field followed by a 60 ° magnetic field. ${\displaystyle \left({\frac {\pi }{\sqrt {2}}}\right)}$

Mattauch-Herzog geometry

Geometry with a 31.82 ° ( radians) electric sector field followed by a drift area and a 90 ° magnetic field in the opposite direction. ${\displaystyle \pi /4{\sqrt {2}}}$

Nier-Johnson geometry

Geometry with a 90 ° electric sector field followed by a long drift area and a 90 ° magnetic field in the same direction.

More geometries

Other known geometries are the Hinterberger-König geometry , the Takeshita geometry and the Matsuda geometry .

Areas of application

Double-focusing devices have long been the most important instrument in mass spectrometry. However, sector field devices are quite large, expensive and complex to operate and are therefore only rarely purchased today. In the area of ​​coupling with GC z. They are still used today, for example in dioxin analysis. In current devices, injecting only 20 femtograms of TCDD onto the column gives a signal-to-noise ratio of approximately 200: 1. They are also used in isotope mass spectrometry.

literature

• Jürgen H. Gross: Mass Spectrometry - A Textbook . Springer Verlag, Berlin / Heidelberg 2013, ISBN 978-3-8274-2980-3 .
• Herbert Budzikiewicz, Mathias Schäfer: Mass Spectrometry - An Introduction . Wiley-VCH, Weinheim 2005, ISBN 3-527-30822-9 .
• Hans-Joachim Hübschmann: Handbook of GC / MS, Fundamentals and Applications . 3. Edition. Wiley-VCH Verlagsgesellschaft, Weinheim 2015, ISBN 978-3-527-33474-2 , urn : nbn: de: 101: 1-20150601963 .

Individual evidence

1. ^ AJ Dempster: A New Method Of Positive Ray Analysis . In: Phys. Rev. . 11, 1918, p. 316.
2. ^ Josef Mattauch, Richard Herzog: About a new mass spectrograph . In: Journal of Physics . 89, 1934, pp. 786-795.
3. ↑ Stephen H. Safe: Dioxin– 1980– A Beginning . ( Memento of February 3, 2016 in the Internet Archive ) (PDF; 343 kB)
4. ↑ Alfred Klemm: On the theory of the double-focusing mass spectrograph for all masses . In: Journal for Nature Research . 1, 1946, pp. 137-41. bibcode : 1946ZNatA ... 1..137K .
5. ↑ J. De Laeter, MD Short: Alfred Nier and the sector field mass spectrometer . In: Journal of Mass Spectrometry . 41, No. 7, 2006, pp. 847-854. doi : 10.1002 / jms.1057 .
6. ^ Nier-Johnson geometry . In: IUPAC Compendium of Chemical Terminology . doi: 10.1351 / goldbook.N04141
7. ↑ patent US4553029 . ‌
8. ↑ DFS ™ high resolution GC-MS