What is the difference between a mass spectrometer and a mass spectrometry imager?

A mass spectrometer produces a spectrum of the sample’s overall composition, answering the questions ‘What is it?’ and ‘How much is there?’; a mass spectrometry imager builds upon the mass spectrometer by adding spatial scanning and point-by-point sampling, producing a map of the spatial distribution of each component, answering the questions ‘Where is it?’ and ‘How is it distributed?’. The difference between the two lies in whether they possess spatial resolution capability, rather than in the principle of mass analysis itself.

What is a mass spectrometer?

This typically refers to a system comprising a sample introduction system, an ion source, a mass analyser and a detector, which produces a single mass spectrum or a series of spectra from a homogeneous sample for qualitative and quantitative analysis, as well as structural identification. It does not take into account positional differences within the sample, treating the sample as a ‘whole’.

Additional components of an imaging mass spectrometer

Imager = Mass spectrometer + spatial scanning mechanism + point-by-point synchronous acquisition + image processing. It lays out a grid across the sample surface, acquires spectra point by point whilst recording coordinates; the imaging software then maps the intensity of each m/z channel into a colour distribution map, thereby transforming ‘composition’ into a ‘map’.

Differences in hardware and data

The core of an analyser lies in the ion source and the detector; an imager additionally requires a precision translation stage or probe scanner, coordinate-synchronised triggering, and software to stitch vast numbers of point spectra into an image. New Source’s MSI DPI (20–200 μm) and MSI LDPI (2–3 μm) are imaging add-on modules that attach to host instruments from Agilent, AB SCIEX, Thermo and others. At the data level, analysers produce several spectral lines, whilst imaging mass spectrometers produce tens of thousands of coordinate-tagged spectral lines. This results in a steep increase in data volume, making them more reliant on fast, full-spectrum analysers such as TOF to manage timing.

How to Choose

If you are only performing compositional analysis, choose an analyser (or an online mass spectrometer); however, if you are investigating the penetration of drugs into tissues, the accumulation of contaminants on material surfaces, or the distribution of metabolites within organs, an imager is essential. The underlying principles of mass analysis for both can be identical (e.g. both using TOF); the only difference lies in the presence or absence of a ‘spatial dimension’.

Can an imaging system still be used as a standard analyser?

Yes. Imaging sources can usually also operate in single-point mode; disabling the scan function reduces it to a conventional ionisation source. Consequently, a single imaging system often serves both imaging and routine analysis purposes, eliminating the need for a separate analyser.

Can data from an analyser be converted into an image?

No. Spectra without spatial coordinates cannot be used to reconstruct a distribution; only point-by-point data that has been recorded with coordinates from the outset can be used for imaging. This is determined at the hardware level; post-processing software cannot generate positional information out of thin air.

Is there a significant difference in the level of expertise required for the two?

Imaging spectrometers involve more steps in sample preparation, scan parameters and image reconstruction; the operational and data interpretation requirements are higher than for conventional analysers, and additional knowledge of spatial calibration and image post-processing is required.