Micromass GCT TOF Mass Spectrometer

Available from KRSS a used fully refurbished Micromass/Waters GCT Time of Flight (TOF) mass spectrometer with installation and 6 months warranty included (Agilent 6890 GC available at additional cost).

The Micromass GCT is a benchtop, compact, fully integrated, high performance orthogonal acceleration time-of-flight (TOF) mass spectrometer designed for GC-MS and probe MS applications that offers exact mass measurement capabilities. The GCT’s high resolution and mass accuracy allow for the determination of elemental composition and increased specificity for the identification of unknowns. The high full scan sensitivity further extends it capability to profiling of complex chromatograms. High versatility is supplied in the form of various ionization and inlet options. These are GC, EI and CI. 

  • Overview
  • Ionisation Techniques
  • Inlets
  • Ion Optics
  • Find Out More

Overview

The system consists of a source and analyser housing separated by a pneumatic isolation valve and a differential pumping aperture. The source housing is fitted with a 250 L/sec high compression turbomolecular pump. The analyser housing is fitted with a 70 L/sec high compression turbomolecular pump. A single RV3 rotary pump backs both turbomolecular pumps.

Sample introduction on the GCT instrument occurs using a probe or GC interface. The TOF analyzer enables high resolution (5000-7000) mass measurements with an accuracy of ~5 ppm. EI/CI is most suitable for low molecular weight, low polarity compounds (m/z range ~1500). The probe may be heated to 650°C to vapourize low volatility compounds (although evaporation may be in competition with decomposition).

GCT includes Electron Impact (EI), Chemical Ionisation (CI) and Field Ionisation (FI) source options. The ion source is at ground potential to allow simple direct coupling to the GC inlet, thereby minimising the possibility of cold spots. A source probe allows easy exchange of the EI and CI removable inner volumes, or of the FI emitter.

Ions are accelerated from the grounded ion source to 40eV before being accelerated orthogonally into the time of flight (TOF) mass analyser. The TOF analyser has a two stage orthogonal acceleration region, followed by a single stage reflectron, giving an effective path length of 1.2 meter. Ions are detected using a dual microchannel plate assembly capable of detecting positive or negative ions. Ion arrival times are recorded using a time to digital converter (TDC) with a sampling rate of 1 or 3.6GHz. GCT TM produces high quality, full mass spectra with elevated resolution (~ 7000 FWHM). This elevated resolution reduces the likelihood of mass interferences. Furthermore the precise linear relationship between ion arrival time and the square root of its mass allows good mass measurement accuracy with only a single internal reference mass. The precision of mass measurement can provide elemental composition of unknowns and confirm identification of eluting compounds. The full mass spectral sensitivity of the GCT is comparable to that of a quadrupole mass spectrometer, operating in single ion recording mode and monitoring 10 - 20 masses.

In comparison to a quadrupole instrument when used to record full mass spectra, the GCT can be 10 - 100 times more sensitive, depending on the mass range acquired.

Ionisation Techniques

Electron Impact and Chemical Ionisation
The ion source consists of two assemblies. An inner, easily removable volume which comprises all the normally cleanable or replaceable parts, such as the filament, trap and repeller for an EI/CI source. The outer source comprises the source heater, thermocouple, focusing optics and other generally non-replaceable items, and is located on the source housing lid. Heaters in the outer source raise the source temperature to ensure sample vaporisation.

Electron Impact (EI)
Electron impact is the classical ionisation technique in which gas phase sample molecules are ionised in collisions with high energy electrons.

Chemical Ionisation (CI)
When the source is operated in the chemical ionisation mode, a reagent gas is admitted into the ion source at a relatively high pressure. The gas molecules are ionised by the electron beam. Sample ions are generated in reactions with these gas ions. CI is a ’softer’ ionisation technique than EI, producing less sample fragmentation and generally a stronger molecular ion.

 

Inlets

GC Interface
The GC interface provides a heated transfer line between the GC and the ion source. This ensures even heating in this region, so that the sample does not condense before it reaches the ion source. The interface is designed to be easily removable to allow simple and rapid conversion to solids probe operation.

The GC interface is capable of being heated to a temperature of 350°C. A spring loaded tip allows the interface to be in contact with the outer source block for CI operation, while allowing thermal expansion of the inner re-entrant tube to be accommodated.

Direct Insertion Probe
An optional direct insertion probe is available for the introduction of involatile materials. The probe lock is fitted in place of the GC interface. The probe has a maximum operating temperature of 650°C and is fully controlled from the MassLynx software.

DCI Probe
An optional Direct Chemical Ionisation (DCI) Probe is also available, which is introduced by means of the probe lock. The DCI probe current is controlled from MassLynx from 0 - 1.5A and is operated in CI mode. It is thus a fairly ’soft’ technique

The Heated Septum Interface
The septum interface is designed for the introduction of volatile reference materials for calibration and mass measurement. The interface consists of a heated 100ml chamber, 75µm I.D., fused silica capillary leak and a heated stainless steel transfer line. Reference material may be introduced via syringe through a septum into the chamber. A manual valve allows the chamber to be pumped to adjust the amount of reference material entering the source region. The interface is mounted on the source housing lid.

Ion Optics

The principal components of the ion optical system are shown here in schematic form. Ions generated in the ion source are accelerated and focused into the pusher region of the orthogonal TOF via a transfer lens.

A sudden voltage pulse is then applied to the pushout electrode, ejecting a section of the beam orthogonally. The ion packet then passes through a two stage acceleration region and enters the time of flight drift region. The reflectron reflects ions back to the dual microchannel plate detector. Ion arrivals are recorded using a time to digital converter (TDC).

As ions travel from the pusher to the detector they are separated in mass according to their flight times, with ions of the highest mass to charge ratio (_) arriving later in the spectrum.

The pusher may be operated at repetition frequencies of up to 30kHz, resulting in afull spectrum being recorded every 33 microseconds. Each spectrum is summed in the PC memory until the completed, histogrammed spectrum is transferred to the host PC. For an acquisition rate of 1 spectrum/second, each spectrum viewed on the host PC will be the result of summing up to 30,000 individual spectra recorded at the detector. Unlike scanning instruments, the TOF performs parallel detection of all masses within the spectrum at very high sensitivity and acquisition rates. This characteristic is of particular advantage when the instrument is coupled to fast chromatography, since each spectrum is representative of the sample composition at that point in time, irrespective of how rapidly the sample composition is changing.

Demo

 
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