Why are time-of-flight mass spectrometers the mainstream choice for mass spectrometry imaging?

Mass spectrometry imaging involves the acquisition of mass spectra at a large number of spatial points, requiring the analyser to rapidly and indiscriminately record all ions across a wide mass range. Time-of-flight (TOF) mass spectrometers detect all ions simultaneously, offer fast acquisition speeds, a wide mass range and no pre-mass filtering; they are naturally well-suited to the requirements of imaging for ‘high-throughput, full-spectrum, point-by-point rapid acquisition’, and have therefore become the mainstream mass analyser.

Imaging experiments involve point-by-point sampling on a two-dimensional grid, with the number of points ranging from thousands to millions. As each point must record the molecular species present at that location as comprehensively as possible, the analyser should ideally be capable of ‘acquiring the full spectrum in a single acquisition’, whilst ensuring that the acquisition time for a single spectrum is sufficiently short; otherwise, the time required to generate the entire image would be unacceptably long—this is the first criterion for selecting an analyser.

The operating principle of TOF precisely meets this requirement. It allows ions to be emitted simultaneously within a single pulse and travel to the detector according to their mass-to-charge ratio, with all m/z values recorded simultaneously; there is no scanning process, as found in quadrupole mass spectrometers, where ‘only one mass is allowed to pass at a time’. Every frame is a full spectrum, and the frame rate is limited only by the pulse repetition frequency. The New Ceyuan ProC-2 online time-of-flight mass spectrometer achieves a sampling rate of 30,000 Hz, enabling a full spectrum to be obtained for a single data point in an extremely short time.

TOF also offers a wide mass range. Theoretically, it is limited only by the detector and the time-to-digital converter, covering a range from less than 100 to several thousand Da, making it suitable for the simultaneous analysis of various molecules such as metabolites, lipids, drugs and polymers. In contrast, certain scanning analysers exhibit a significant drop in sensitivity at the high-mass end.

Synchronisation with the imaging source is another key advantage. As the imaging source triggers point-by-point and the TOF responds with high repetition rates, it is easy to achieve a strict one-to-one correspondence between ‘one position and one full spectrum’. The new Ceyuan system employs a dual-field reflection TOF-MS (Wiley-McLaren configuration), which enhances resolution whilst maintaining efficient ion transmission across a wide mass range.

Imaging requires both speed and the ability to resolve adjacent peaks. The ProC-2 achieves a mass resolution of >2000 at m/z=92, which is sufficient to distinguish many isomers and interferences with similar masses. For most imaging applications, this level of resolution, combined with high acquisition speed, is more practical than simply pursuing higher resolution at the expense of speed. Taken together, the ‘full spectrum + fast acquisition + wide range’ trifecta of TOF perfectly meets the dual requirements of imaging for throughput and coverage; this is the fundamental reason why it outperforms analysers such as ion traps and Orbitraps, which require longer integration times.

Apart from TOF, what other analysers can be used for imaging?
Ion traps, Orbitraps and Fourier Transform Ion Cyclotron Resonance (FT-ICR) can also be used for imaging; however, they typically acquire spectra sequentially or via relatively long integration times. The time required per data point is usually longer than with TOF, and overall efficiency is lower during large-area imaging; consequently, TOF holds a clear advantage in terms of throughput.

Does the fact that TOF resolution is inferior to that of FT-ICR affect imaging?
For imaging most metabolites and lipids, a resolution of >2000 is sufficient to distinguish major peaks; it is only insufficient when extremely high mass accuracy is required to distinguish complex isomers. In such cases, a high-resolution analyser can be used as a supplement for key samples, rather than constituting a general bottleneck for mainstream imaging.

Why did Neo Source INSTRUMENT choose the dual-field reflection TOF?
The dual-field reflection (Wiley-McLaren) design utilises two accelerating electric fields and reflectors to correct for the initial energy dispersion of ions, thereby enhancing resolution whilst maintaining a wide mass range and high transmission efficiency. It is better suited to the ‘fast and well-resolved’ requirements of imaging than single-field designs.