the research of the EMSG Back to the index page
 

link to the home page

link to the profile page

link to the research page

link to the people page

link to the collaborators page

link to the links page

link to the home page of the Department of Chemistry

link to the University of Edinburgh home page

  

Instrument Development

A Versatile Tandem Time-of-Flight Mass Spectrometer

This page is taken from a poster that I presented in August 1998 at the annual meeting of the Physical Chemistry Section that takes place at the university's outdoor centre near Loch Tay every year.

1. Introduction

The past decade has witnessed explosive growth in the use of Time-of-Flight Mass Spectrometry (TOF-MS) [1], particularly using Matrix-Assisted Laser Desorption / Ionisation (MALDI) for analysis of high molecular weight biomolecules and synthetic polymers.

An alternative technique developed for detection and fingerprinting of molecules directly from their host environment is L2MS. Here, desorption of intact molecules directly from the host matrix is achieved using a pulsed laser, followed by photoionisation using a second laser. L2MS enables the analysis of large involatile organic molecules in complex samples mixtures.

Such spectra are usually recorded under soft ionisation conditions and frequently display an intense peak due to the molecular ion with little fragmentation. Tandem L2MS would permit compound identification with a higher degree of confidence by using daughter ion spectra and also allow fundamental studies on the fragmentation dynamics of large molecules as well as other systems.

An existing TOF-MS instrument has therefore been converted to allow Tandem (TOF/TOF)-MS studies of large organic molecules as well as elemental clusters.

Photograph of the instrument used

2. Experimental

The tandem instrument consists of:

  1. Molecular beam chamber (MB) housing laser vaporisation (LVS) and pulsed arc cluster ion sources (PACIS) for neutral and charged cluster generation
  2. Laser desorption/laser ionisation chamber (L2). A pulsed CO2 laser (10.6 um) can be used to desorb intact neutral molecules, subsequently ionised by a UV laser (eg 266 nm harmonic of an Nd:YAG laser)
  3. Linear (TOF1) drift region, fitted with a micro channel plate (MCP) detector, for mass analysis of precursor ions
  4. Electronic mass gate situated in the linear TOF region, driven by a pulse generator
  5. Interaction chamber (IC), where the mass-selected precursor ion can be subjected to photo-induced dissociation, or collision-induced dissociation
  6. Reflectron (RE-TOF) mass spectrometer for mass analysis of the product ions
  7. CAMAC-based data acquisition and control system

3. Precursor Ion Selection (Ion Gating)

A device for mass selection of a target precursor ion has been designed. The operating principle is illustrated below. All ions are scattered as they pass through a series of wires, some at ground potential and some at a high positive voltage. This voltage is pulsed to ground for a short time to allow unhindered passage of the ion of interest through the gate.

Photograph showing the mass gate in the linear flight tube:

Photograph showing the mass gate in the linear flight tube

The program SIMION [2] has been used to simulate the trajectories of ions passing through the mass gate, as shown below:

Gate closed: ions are scattered

Diagram of ion paths for closed gate

Gate open: ions pass through

Diagram of ion paths for closed gate

4. Preliminary Results

Graph showing preliminary results

5. Future Experiments

Improved analytical capability

Unambiguous structural characterisation (selection of one target molecular ion, identified by its secondary fragmentation spectrum).

Photofragmentation dynamics of large organic molecules, e.g. porphyrins, PAHs

Photoionisation of certain molecules results in the production of fragments which cannot be explained by stepwise fragmentation of the molecular ion. Tandem TOF-MS will be used to map in detail the precursor ion - product ions relationships resulting from photo-induced dissociation, providing an understanding of the fragmentation mechanisms involved.

Photofragmentation studies of mass-selected clusters

Virtually all cluster sources produce a distribution of sizes, making it difficult to carry out experiments on a particular size of cluster. Tandem TOF-MS would enable selection of one cluster size, which can then be characterised using the second RE-TOF mass analyser.

6. Conclusion

Development of time-of-flight mass spectrometry into a tandem technique makes it a more powerful analytical tool. The novel tandem (TOF/TOF) MS instrument described here should permit a wide range of analytical and fundamental studies to be carried out.

7. References

  1. M Guilhaus, V Mlynski, D Selby, Rapid Comm. Mass Spectrom., 11, 951, 1997.
  2. D A Dahl, SIMION 3D Version 6.0 (Idaho National Engineering Laboratory, PO Box 1625, Idaho Falls, ID 83415, 1995).

8. Acknowledgements

  • Robert Maier and Paul Donaldson for technical assistance
  • EPSRC
  • British Mass Spectrometry Society for the award of a John Beynon Studentship

Last updated 14th November 1998