Analyzers, mass time-of-flight

H. Wollnik, Time-of-flight mass analyzers, Mass Spectrom. Rev., 12 (1993) 89-114. 

The proportionality of time-of-flight to the square root of m/z causes At for a given Am/z to decrease with increasing m/z under otherwise the same conditions At per 1 u is calculated as 114 ns at m/z 20, 36 ns at m/z 200, and just 11 ns at m/z 2000. Therefore, the realization of a time-of-flight mass analyzer depends on the ability to measure short time intervals with sufficient accuracy. [32-34] At this point it becomes clear that the performance of the early TOE analyzers - among other reasons - suffered from the too slow electronics of their time. It took until the mid-1990s to overcome this barrier. 

Various mass spectrometer configurations have been used for the detection of explosives, such as ion traps, quadrupoles and time-of flight mass analyzers and combinations as MS/MS systems. The ionization method is usually APCI with corona discharge [24, 25]. An example is given in Figure 20, which shows the schematic diagram of an explosive mass spectrometer detector [25]. It is based on an ion trap mass analyzer, an APCI source with corona discharge and a counter-flow introduction (CFI) system. The direction of the sample gas flow introduced into the ion source is opposite to that of the ion flow produced by the ion source. 

Papers on Matrix-Assisted Laser Desorption Ionization Mass Spectrometry with Time of Flight Mass Analyzer Applied to Flavonoid Analysis... 

Time of flight ion probes (TOF SIMS) have unique capabilities not found in other mass spectrometers. A pulsed ion beam, typically either cesium or gallium, ejects atoms and molecules from the sample. Ionized species are accelerated down the flight tube and the arrival time in the detector is recorded, giving the mass of the species (see discussion of time-of-flight mass analyzers above). TOF SIMS instruments used in cosmochemistry have spatial resolutions of <1 pm. They are used to determine elemental abundances in IDPs and Stardust samples. The spatial distribution of elements within a small sample can also be determined. TOF SIMS instruments can obtain good data with very little consumption of sample. 

Magnetic sector field (B) Combination of magnetic (B) and electric sector fields (E) Quadrupole mass analyzer (Q) Time-of-flight mass analyzer (ToF)... 

Figure 3.10 Diagram of a linear time-of-flight mass analyzer separation of ions is shown with three different masses (heavy ions - Of i°ns of medium mass and light ions - ). The flight time of heavy ions is longer than that of light ions due to their lower kinetic energy.

Figure 3.11 Schematic of a time-of-flight mass analyzer with reflectron proposed by Mamyrin,11 Ions of higher kinetic energy penetrate more deeply into the reflecting field and are delayed in comparison to ions of lower kinetic energy. The delay compensates for differences in transit time of the field-free regions. Ions produced e.g., by focused laser or primary ion beam.

In general, all mass spectrometers share at least three distinct structures the source, the analyzer, and the detector. Differences in these three structures identify the multitude of MS systems. The source is perhaps the most crucial element of the mass spectrometer. Therefore, the selection of the source primarily differentiates the various MS systems. Although specific analyzers or detectors may be preferable for a particular MS application, mass spectrometrists will often refer to different systems solely by the source. Only occasionally, when a time-of-flight mass analyzer is used, for example, analysts refer to the method by the type of the mass analyzer. 

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

Finally, one concept that must be included in assessing quantitation by HRMS is the effective scan rate of the system. Quadrupole and time of flight mass analyzer are capable of rapid scan rates for SRM-type quantitation, with individual dwell times (quad) or scans (TOF) at 10-50 milliseconds possible. This permits acquisition of numerous data points across a chromatographic peak, which is critical for accurate and precise quantitation. Mass resolution is unaffected by changes in dwell time/scan... 

Fig. 8.12. Schematic illustration of a time-of-flight mass analyzer with an orthogonal electrospray ion source.

Figure 1 General diagram of a time-of-flight mass analyzer. R, repeller electrode G0, entry grid Gp acceleration grid, E, and E2, electric fields s0, distance to the midpoint of the first acceleration field sa and shaded circles, ion starting position closer to G0 sb and closed circles, ion starting position more distant from G() D, total drift length.

Boesl U, Bassmann C, Kaesmeier R (2001). Time of flight mass analyzer for anion mass spectrometry and anion photoelectron spectroscopy. Int J Mass Spect 206 231—244. 

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