Mass spectrometers magnetic deflection

There are two important categories of magnetic-deflection mass spectrometers low (unit) resolution and high resolution. Low-resolution instruments can be defined arbitrarily as the instruments that separate unit masses up to m/z 2000[R = 2000/(2000 — 1999) = 2000]. A high-resolution instrument with R = 20,000 can distinguish between C16H2602 and C15H24N02 ... 

The three-compartment source was attached to the analyzer tube of a 6-inch radius 60° sector magnetic deflection mass spectrometer. Differential pumping was used between the source and analyzer regions. The ion detector was a 14-stage electron multiplier coupled to both a vibrat-ing-reed electrometer and a pulse counter (38). The electrometer was connected to a strip-chart recorder and the counter to a printer. This arrangement allowed any range of e/m to be scanned or a given peak to be monitored. 

In contrast to other studies, oxidation carried out in this department on a Pt/7-Al203 catalyst has not uncovered any oscillatory behaviour in the temperature range of 100-185 °C. Addition of a hydrocarbon like but-l-ene, but-2-ene, or propene induces sinusoidal or relaxation type oscillations at temperatures above 150 °C. The experimental set-up used consists of a continuous recycle reactor system. The catalyst is packed in the cylindrical tubes. The gas flow rates are precisely measured with a bubble flow-meter. The reactor outlet is connected to a magnetic deflection mass spectrometer. An electronic peak select unit allows up to four mass numbers to be continuously monitored. The output data are connected to a PDP 11/45 computer for automatic and fast data logging. The data thus stored in the computer can be analysed later. The line diagram of the experimental set up is given in Figure 1. 

In a magnetic deflection mass spectrometer, ions leave the ion source and are accelerated to a high velocity. The ions subsequently pass through a magnetic sector in which the magnetic field is applied in a direction perpendicular to the direction of ion motion. By applying an acceleration perpendicular to the direction of motion of an object, the velocity of the object remains constant, but the object travels in a... 

Inlet and outlet analyses were obtained by an on-line magnetic-deflection mass spectrometer equipped with a fast response continuous inlet system. An electronic peak select unit allowed up to four mass numbers to be continuously monitored. 

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

The mean free path is the average distance a molecule travels before colliding with another molecule. The mean free path, X, is given by X = kT/ jr2 itP). where k is Boltzmann s constant, Tis the temperature (K), P is the pressure (Pa), and cr is the collision cross section. For a molecule with a diameter d, the collision cross section is ltd2. The collision cross section is the area swept out by the molecule within which it will strike any other molecule it encounters. The magnetic sector mass spectrometer is maintained at a pressure of 10-5 Pa so that ions do not collide with land deflect) each other as they travel through the mass analyzer. What is the mean free path of a molecule with a diameter of 1 nm at 300 K in the mass analyzer ... 

Magnetic deflection mass analyzer Double focusing sector spectrometer Quadrapole mass analyzer Triple quadrapole analyzer... 

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

Mattauch-Herzog geometry. An arrangement for a double-focusing mass spectrometer in which a deflection of n/A radians in a radial electrostatic field is followed by a magnetic deflection of nil radians. 

Experimental. The mass spectra in Figures 1-8 are positive-ion spectra produced by electron impact and were obtained from a single-focusing, magnetic deflection Atlas CH4 Mass Spectrometer. The ionizing potential was 70 e.v. and the ionizing current 18/a a. An enamel reservoir heated to 120°C. was used from which the sample was leaked into the ion source. 

Historical That positive rays could be deflected in electric and magnetic fields was shown as early as 1898 by Wien, but it was not until 1912 that what was to become the forerunner of the modem mass spectrometers was built by JJ. Thompson, who became known as the father of mass spectrometry. The existence of two isotopes of neon (m/e 20 and 22) was demonstrated by Thompson with this instrument. The discovery of stable isotopes of elements has been generally considered the... 

The apparatus used, as described previously (7), has modifications as described by Maier (16). A description also appears in this volume (15). A mass spectrometer of special design with 2-inch round poles, using 90° deflection with 1-inch magnetic radius, provides momentum-selected beams of low energy ions. These ions proceed at full energy... 

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