Radiofrequency mass spectrometer

The ability to selectively excite a particular ion (or group of ions) by irradiating the cell with the appropriate radiofrequencies provides a level of flexibility unparalleled in any other mass spectrometer. The amplitude and duration of the applied RF pulse determine the ultimate radius of the ion trajectories. Thus, by simply turning on the appropriate radiofrequency, ions of a single m/z may be ejected from the cyclotron. In this way, a gas-phase separation of analyte from matrix is achieved. At a fixed radius of the ion trajectories the signal is proportional to the number of orbiting ions. Quantitation therefore requires precise RF control. 

Figure 5.29 Instrumental setup of a radiofrequency (rf) glow discharge mass spectrometer by combining an ion source developed in house with a commercial double-focusing sector field mass spectrometer with reverse Nier-Johnson geometry (Element, Thermo Fisher Scientific, Bremen, Germany), (f S. Becker et al., IntJ. Mass Spectrom., 164, 81 (1997). Produced by permission of Elsevier.)...

A. C. Muniz, J. Pisonero, L. Lobo, C. Gonzalez, N. Bordel, R. Pereiro, A. Tempez, P. Chapon, N. Tuccitto, A. Licciardello and A. Sanz-Medel, Pulsed radiofrequency glow discharge time of flight mass spectrometer for the direct analysis of bulk and thin coated glasses, J. Anal. At. Spectrom., 23, 2008, 1239-1246. 

This experiment presents the measurement of uranium with an inductively coupled plasma mass spectrometer (ICP-MS). In this system, a nebulizer converts the aqueous sample to an aerosol carried with argon gas. A torch heats the aerosol to vaporize and atomize the contents in quartz tubes. The atoms are ionized with an efficiency of about 95% by an RF (radiofrequency) coil. The plasma expands at a differentially-pumped air-vacuum interface into a vacuum chamber. The positive ions are focused and injected into the MS while the rest of the gas is removed by the pump. The ions are then accelerated, collected, and measured as a function of their mass. Losses at various stages, notably the vacuum interface, result in a detection efficiency of about 0.1 %, which is still sufficient to provide great sensitivity. The amounts of uranium isotopes in the sample are determined by comparisons to standards. Because different laboratories have different instruments, the instructor will provide instrument operating instmctions. Do not use the instrument until the instructor has checked the instrument and approved its use. 

In a guided ion beam experiment, reactant ions are created in the source region, mass selected by a mass spectrometer (a magnetic sector in our instruments), decelerated to a desired kinetic energy, and injected into a radiofrequency (rf) octopole ion beam guide [10,11]. This device comprises eight rods cylindrically surrounding the ion beam path. Opposite phases of an rf potential... 

Cable P. R. and Marcus R. K. (1994) Effects of target gas in collision-induced dissociation using a double quadrupole mass spectrometer and radiofrequency glow discharge, J Am Soc Mass Spectrom 5 845-851. 

The triple quadrupole (QqQ) mass spectrometer is comprised of two linear mass hlters, Q1 and Q3, and a radiofrequency (RE) only quadrupole, q2, as shown in Eigure 4.5(a). The QqQ analyzer is capable of MS and MS-MS scans and is commonly coupled to an ESI source for proteomic analysis. Each of the three quadrupoles consists of four linear rods that have a high voltage RE potential (I to 3 MHz) and a DC potential applied to them. A triple quadrupole instrument performs three types of MS-MS scan modes precursor, product, and neutral loss. The RF/DC... 

The rapid LC methods employed for array analysis produce typical peak widths at half-height of less than 2 s. It was immediately apparent that the scan rates achievable on a radiofrequency (RF) quadrupole mass spectrometer would not be sufficient to generate high-quality mass spectra that could be used to confirm the presence of a desired chemical entity. Alternatively, commercial orthogonal time-of-flight (TOF) mass spectrometers can acquire full scan profile data at rates approaching 10 Hz so that rapid switching between up to four streams becomes feasible. Therefore this approach was followed in our development of the novel interface based on the Micromass LCT instrument. 

Inductively coupled plasma-mass spectrometry is a very rapid technique for the determination of long-lived radionuclides. This technique is based on the ionization of elements in the plasma source. Typically, radiofrequency and argon are used to reach plasma excitation temperatures ranging from 4900 to 7000 K [18,19]. The ions produced are introduced through an interface into a vacuum chamber and are analyzed by a quadru-pole mass spectrometer. Other attempts are being made to use faster mass-spectrometer detectors, such as time-of-flight mass spectrometers, but methods are still not available. 

Originally the tool of physicists and physical chemists, now with improved electronics the quadrupole mass spectrometer has become an essential instrument for biological and biomedical research. Originally described as a mass filter, it operates by using a combination of a quadrupole static electric field and a radiofrequency field which combine to focus an ion beam on a collector. 

Figure 1 A schematic illustration of a quadrupole mass spectrometer. Ions that are to be filtered to identify the presence of a particular mass are injected in the direction of the axis of the instrument. If the combination of radiofrequency and direct voltages applied to the rods is correctly chosen, only ions of one particular mass to charge ratio (m/z) will be successfully transmitted to the detector.

Javahery, G. Thomson, B. A Segmented radiofrequency-only quadrupole collision cell for measurements of ion coUision cross section on a triple quadrupole mass spectrometer. J. Am. Soc. Mass Spectrom. 1997, 8, 697-702. 

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