High mass resolution TOF-MS Instrument for process gas analysis at atmospheric pressures
The Kore high mass resolution time-of-flight mass analyser (TOF-MS) is designed for analysis of process gases for production and research applications. The 19 inch rack-mounted instrument has anti-vibration suspension for easy and safe transportation between different sites.
The TOF-MS's ability to quantitatively analyse inorganic gases as well as identify and analyse volatile organic compounds (VOCs) at pressures from millibars to10 bars makes the equipment well suited to process monitoring as well as catalysis studies and combustion or pyrolysis reaction research.
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.
Mass resolution is often expressed as the mass of the peak divided by its width, in this case at half height, hence the phrase 'Full Width Half Maximum', or FWHM. High resolution permits the separation of so-called isobaric species, i.e. species that have the same nominal mass. However, owing to the small differences in the exact masses of the constituent atoms, dissimilar molecules with the same nominal mass will have different exact masses. As the resolving power of the instrument increases, it becomes possible to separate two peaks with exact masses that are closer and closer.
For instance, C2H3O+ and C3H7+ both appear at mass 43, but C2H3O+ has an exact mass of 43.0184 whereas C3H7+ has an exact mass of 43.0546. If both peaks have equal intensity, then to resolve these peaks into two separate peaks down to half their height requires a resolving power of approximately 1,200. To resolve them down to the baseline requires much greater resolving power.
As a second example, H2S+ has the same nominal mass (34) as an isotope of oxygen, namely16O18O+. The exact masses are 33.9878 and 33.9940 respectively. Now the mass difference is smaller, and so the resolving power needs to be greater. 5,500 resolution is required to separate two peaks of equal intensity down to their half heights.
The TOF-MS system used is highly attractive because it offers real-time, high time resolution, high sensitivity detection of all gas species in parallel. All masses are collected, preserving the maximum amount of information.
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.
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.
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.
The Data System
Data is displayed on a PC, using the Windows® based GRAMS/AI™ software from Thermo Scientific Corporation. This industry standard software has been customised by Kore to make the collection of spectra a simple press of a screen button. The software allows the user to monitor trends in multiple gases over long periods of time.
|Mass Range||unlimited in principle|
|Mass resolution||≥ 6,000 (FWHM)|
|Sensitivity||Xenon in ambient air 90ppb in 1 minute|
|Mass Accuracy1||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||61x165x80cm (width, height, depth)|
|Data acquisition||Rack-mounted PC or laptop|
© Kore Technology Limited 2011