Threshold Ionisation Mass Spectrometry (TIMS): A Complementary Technique for Gas Analysis
Threshold Ionisation Mass Spectrometry (TIMS) is an advanced analytical technique used with quadrupole mass spectrometers to improve the selectivity and quantification of gas mixtures. By controlling the electron energy used for ionisation, TIMS enables separation of species with overlapping mass-to-charge ratios that are difficult to resolve using conventional mass spectrometry.
This makes TIMS particularly valuable for applications such as trace gas analysis, plasma diagnostics, and isotopic measurements, where conventional mass-resolved techniques may struggle.
What is Threshold Ionisation Mass Spectrometry?
TIMS is based on the principle that each gas species has a characteristic ionisation threshold energy. By scanning electron energy rather than only mass-to-charge ratio, TIMS provides an additional dimension of selectivity.
Unlike conventional quadrupole mass spectrometry, which typically operates at a fixed electron energy (~70 eV), TIMS uses variable electron energy to distinguish between species with similar masses but different ionisation characteristics.
This enables more accurate identification and quantification of gas components.
Key Advantages of TIMS
Improved Selectivity
TIMS allows separation of species that overlap in conventional mass spectra by exploiting differences in ionisation energy.
Enhanced Quantification
By analysing ionisation curves, TIMS enables more accurate determination of relative gas concentrations.
Detection of Trace Species
TIMS is effective for detecting trace gases and low concentration species in complex mixtures.
Complementary to Conventional MS
TIMS can be used alongside standard mass spectrometry to provide additional analytical insight.
Applications of TIMS
Separation of Overlapping Species
TIMS can resolve species with similar mass peaks, such as nitrogen and carbon monoxide at m/z = 28, which are difficult to distinguish using conventional mass spectrometry.
Isotopic and Gas Mixture Analysis
The technique enables accurate measurement of isotopic ratios, such as helium/deuterium mixtures, even when mass differences are very small.
Trace Gas Detection in Air
As shown in the data on page 2, TIMS can detect trace organic vapours in air by operating below the ionisation threshold of background gases such as nitrogen.
Plasma Diagnostics
TIMS can be used to detect metastable species in plasma environments, providing insight into plasma processes and reaction kinetics. The results on pages 5–6 show detection of metastable helium species in RF plasma systems.
Gas Analysis at Elevated Pressure
TIMS can also be applied at higher pressures, enabling analysis of gas mixtures in process environments and plasma systems.
How TIMS Improves Conventional Mass Spectrometry
In conventional quadrupole mass spectrometry, species are identified based on mass-to-charge ratio alone. However, many gases share similar or identical mass peaks.
TIMS introduces electron energy as a second analytical variable, allowing:
separation of overlapping peaks
reduction of spectral interferences
improved identification of species
more accurate quantification
This makes TIMS a powerful complementary technique rather than a replacement for conventional mass spectrometry.
TIMS for Plasma and Gas Analysis Applications
TIMS is particularly valuable in applications where:
complex gas mixtures are present
metastable species need to be detected
high sensitivity and selectivity are required
conventional MS cannot resolve overlapping species
The technique is widely used in:
plasma processing and diagnostics
fusion research
environmental gas analysis
industrial process monitoring
Conclusion
Threshold Ionisation Mass Spectrometry provides significant advantages for analysing complex gas mixtures. By combining mass analysis with controlled ionisation energy, TIMS enables improved selectivity, enhanced quantification, and detection of species that are difficult to resolve using conventional techniques.
As a complementary method, TIMS expands the capabilities of quadrupole mass spectrometry and is particularly useful in advanced gas analysis and plasma applications.
Download the Full Paper
For full experimental details and results download the full paper:
Davies, S., Rees, J.A. and Seymour, D.L. (2014) Threshold ionisation mass spectrometry (TIMS); a complementary quantitative technique to conventional mass resolved mass spectrometry. Vacuum, 101, pp. 416–422.
