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GC/MS-TOF Description

The most commonly used mass spectrometers in gas chromatography are systems based on quadrupole analyzers. Other analyzers used in GC/MS are ion traps. Both analyzers used the same principle. Four poles are connected into the electrical circuit that generates radiofrequency field. Change of this electrical field enables to generate scanning in time. Fragments comming from ion source through ion optics enters the analyzer. The ions separation is accomplished by applying alternating RF frequency and DC voltage to diagonally opposite ends of the quadrupole, which in turn allows aspecific mass fragment to pass through the quadrupole filter.

Quadrupole analyzer

Picture 1: Scheme of quadrupole analyzer

Time of Flight Analyzer (TOF)

Time of Flight analyzer separates ions on different principle to quadropole system. TOF instruments utilize the times taken by ions to pass (fly) along an evacuated tube as a means of measuring m/z values and therefore of obtaining a mass spectrum.

  • All ions leaving “starting line” at the same time
  • The ions separation process is rather like a sprint race
  • The ions arrive at the detector in procession and strictly in the order of increasing m/z values
  • Ions of the smallest m/z values arrive first, followed successively by others of increasing m/z value

Tme of flight analyzer

Picture 2: Scheme of TOF analyzer

Advantages of TOF GC/MS systems
  • Flight times are extremely short for all of the ions
  • The acquisition (NO “SCANNING”) of the total mass spectrum from m/z 4 to 1,024 is instantaneous on a human time scale
  • Because GC/MS-TOF spectra are obtained in such a short time frame, in one second it is possible to accumulate several thousand mass spectra
  • GC/MS-TOF rapid accumulation of spectra leads to excellent reproducibility and better signal-to-noise characteristics, and makes full use of small quantities of sample

Acquisition performed by means of continuous spectra, providing increased selectivity of correct mass and no ion loss during the separation process in the flight tube. In principle, m/z range is unlimited. In one scan the whole mass range can be analyzed with incredibly high resolution and sensitivity. GC/MS-TOF provides constant ion abundance ratios across a chromatographic peak. In case of co-eluting compounds, overlapping peaks are detected and deconvolution is performed. Trace level detection is achieved. GC/MS TOF systems are ideal for Fast GC and GCxGC applications due to its fast acquisition rate.