Gas Inlet System
The instrument is fitted with a dual inlet system comprising a heated capillary inlet for the inorganic gases and a permeable membrane concentrator inlet assembly for the VOCs. The inlet system and capillary is heated to prevent moisture and other low temperature condensates separating from the gas phase before reaching the ion source.
Membrane assemblies provide long-life, low maintenance, rugged concentrating sample inlets that allows lower detection limits to be achieved for many VOC compounds This inlet system concentrates VOCs but not gases such as N2, Ar, CO, CO2 and H2O. We have used these membrane concentrators in our portable mass spectrometer ( MS-200 ) for many years and typically a single membrane will achieve a x100 concentration for a VOC such as benzene.
The Mass Spectrometer
At the heart of the instrument is a new time-of-flight mass spectrometer capable of delivering over 6,000 mass resolution.
In the instrument a separately pumped Electron Impact ( EI ) ion source feeds the TOF-MS via transfer optics in an orthogonal geometry. Electron impact ( EI ) ionisation is the best-known method for producing ions and thus gives data, which can be interpreted with well-established techniques and compared against extensive libraries using, automated methods. For example, the NIST EI mass spectral database contains almost 200,000 common compounds with many facilities for search and library organisation.
Unlike a quadrupole mass spectrometer, a TOF-MS offers the possibility of mass accuracy and mass resolution. Mass accuracy ( the position of the peak centroid ) is a matter of instrument stability. Interpolated mass accuracy ( 3 point calibration ) is 1mamu, extrapolated mass accuracy ( low mass, three point calibration ) can be as little as 2mamu.
1. Mass Spectra
The fundamental measurement 'unit' of the instrument is a mass spectrum. The instrument will generate a single mass spectrum from less than 0.5 seconds to many minutes' duration if required. Rapid spectral acquisition is key for verifying the correct operation of the instrument, and for deciding how to proceed further with the experiment of interest.
2. High Time Resolution Mode
This is a 'chromatogram' mode in which data is typically acquired over periods of up to a several minutes. The data is 'streamed to disk' for this period, and the data can be inspected retrospectively, looking at time intervals of as little as 100ms in the chromatogram direction.
3. Process Monitor Mode
The process monitor mode is more suited to long-term monitoring over hours or days. For the live display, the user views an updating plot of up to 16 species that have been selected beforehand. Spectra for each measurement are saved to disk, and once again the user can go back to the data to plot any species retrospectively.
Ion Source: Electron Impact at 70eV
Mass Range: unlimited in principle
Mass resolution: ≥ 6,000 ( FWHM )
Sensitivity:Xenon in ambient air 90ppb in 1 minute
Mass Accuracy:1mamu ( interpolated ), 2mamu ( extrapolated )
Inlet system: Dual capillary and membrane to 10 bar pressure gas inlet
Dynamic range:Better than 6 decades
Linearity:Better than 1%
Reproducibility:Better than 0.02%
Inlet Heater:up to 150°C
Power Supply:220-240 volts, 1kW
Dimensions:61 x 165 x 80 cm ( width, height, depth )
Data acquisition:Rack-mounted PC or laptop with NIST Library