Mass spectrometer single-focusing

Figure Bl.7.17. (a) Schematic diagram of a single acceleration zone time-of-flight mass spectrometer, (b) Schematic diagram showing the time focusing of ions with different initial velocities (and hence initial kinetic energies) onto the detector by the use of a reflecting ion mirror, (c) Wiley-McLaren type two stage acceleration zone time-of-flight mass spectrometer.

In the modem scanning mass spectrometer, it is more convenient that ions arrive at a single point for monitoring (collection), so r (or r ) is kept constant. Therefore, B or V must be varied to bring all ions to the same focus viz., one of the relationships in Equation 24.5 must apply ... 

Quadmpole mass spectrometers (mass filters) allow ions at each m/z value to pass through the analyzer sequentially. For example, ions at m/z 100, 101, and 102 are allowed to pass one after the other through the quadmpole assembly so that first m/z 100 is transmitted, then m/z 101, then m/z 102, and so on. Therefore, the ion collector at the end of the quadmpole unit needs to cover only one point or focus in space and can be placed immediately behind the analyzer (Figure 30.1). A complete mass spectram is recorded over a period of time (temporally), which is set by the voltages on the quadmpole analyzer. In this mode of operation, the ions are said to be scanned sequentially. The resolution of m/z values is dependent solely on the analyzer and not on the detector. The single-point collector is discussed in detail in Chapter 28. 

Single-focusing mass spectrometer. An instrument in which ions with a given mass-to-charge ratio (m/z) are brought to a focus although the initial directions of the ions diverge. 

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. 

The mass spectrum of niclosamide was obtained utilizing an AEI single focusing mass spectrometer model MS 12 with VG micromass 2S8, using an ion source temperature of 100-300°C, an electron energy of 70 eV, and with a trap current of 100 pA. 

Hites, R.A. Biemann, K. A Computer-Compatible Digital Data Acquisition System for Fast-Scanning, Single-Focusing Mass Spectrometers. Anal. Chem. 1967, 39, 965-970. 

Fig. 1.7 Single stage pumping atmospheric pressure ionization interface with curtain gas. The size of the orifice is ca. 100 pm, qO acts as a focusing quadrupole and the nitrogen curtain gas prevents neutrai molecules being introduced into the mass spectrometer. T = Temperature of the cryoshells (in Keivin) p = pressure.

Double-focusing mass spectrometers with high mass resolution are rather bulky and expensive instruments so that the development of single magnetic sector field mass spectrometers was of significance in the following years. 

The ions formed are extracted by the extraction lens, EL, perpendicular to the electron beam. Due to the low energy spread of ions formed in an electron ionization source (< 1 eV) one single-focusing magnetic sector field mass spectrometer or quadrupole analyzer is sufficient for... 

An overview of commercial ICP mass spectrometers from different companies (quadrupole based ICP-MS with and without collision/reaction cell, double-focusing sector field instrumentation with single and multiple ion collectors, time-of-flight (ToF), ICP-ion trap-MS and non-commercial ICP-Fourier transform ion cyclotron resonance (FTICR) mass spectrometers is given in Figure 5.2. By using ion traps and FTICR mass spectrometers in ICP-MS isobaric interferences of atomic ions... 

Double-focusing Sector Field ICP Mass Spectrometers with Single Ion Collector (ICP-SFMS)... 

---