Are a mass spectrometry imaging source and a mass spectrometry ion source the same thing?

A mass spectrometry imaging source and a mass spectrometry ion source are not the same concept. A mass spectrometry ion source is the component within a mass spectrometer that generates ions and converts sample molecules into gaseous ions; its focus is on ‘how to ionise’. A mass spectrometry imaging source builds upon the ion source by adding spatial localisation and point-by-point sampling capabilities, enabling the mass spectrometer to determine the spatial distribution of substances; its focus is on ‘where and in what order to ionise’. The two concepts are related in a way where one encompasses the other and is an extension of it.

By definition, a mass spectrometry ion source is an essential component of any mass spectrometry system, whose function is to convert neutral molecules into ions that can be detected by the mass analyser. Common ionisation methods include electrospray ionisation (ESI), matrix-assisted laser desorption/ionisation (MALDI), electron impact (EI), chemical ionisation (CI) and photoionisation (PI), amongst others. It merely addresses the step of ‘converting molecules into ions’ and does not carry any spatial information in itself.

The reason an imaging source is able to ‘produce images’ is that a spatial dimension has been superimposed upon the ion source. It enables the probe (spray, laser or ion beam) to scan the sample surface point by point, collecting a mass spectrum at each spatial position, and ultimately stitching together the coordinates corresponding to each spectrum to form a two-dimensional or three-dimensional molecular image. In other words, an imaging source = ionisation method + spatial scanning mechanism + synchronised control of point-by-point triggered acquisition. Without spatial scanning, the ion source alone cannot produce images.

Take Neo Source INSTRUMENT’s products as an example. The MSI DPI is an electrospray/photochemical ionisation mass spectrometry imaging source, which introduces photochemical ionisation (PI) based on DESI (dispersive electrospray ionisation), with a resolution of 20–200 μm; the MSI LDPI is a laser/photochemical ionisation mass spectrometry imaging source, utilising transmission laser desorption combined with photoionisation, with a resolution of 2–3 μm—a world-first innovation. Both integrate scanning and positioning capabilities directly into the ‘ion source’, thereby upgrading from a standard ion source to an ‘imaging source’.

Returning to the purchasing rationale: if an existing mass spectrometer is equipped only with a general-purpose ion source (such as ESI), it can only perform routine mass spectrometry analysis; to obtain spatial distribution data, it is necessary to replace or retrofit the instrument with an imaging ion source capable of scanning. New Source’s MSI DPI / MSI LDPI are designed to provide retrofit solutions for mass spectrometers from brands such as Agilent, AB SCIEX and Thermo. In other words, imaging ion sources are a specialised form of mass spectrometry ion source; not all ion sources are capable of imaging.

A common misconception is to equate ‘imaging ion sources’ with a specific ionisation technique (such as DESI sources); this actually confuses ‘ionisation method’ with ‘imaging capability’. The same ionisation principle can be used for both conventional and imaging sources; the difference lies in whether the engineering design incorporates controllable spatial scanning and point-by-point synchronisation.

Can a standard ESI source be used directly as an imaging source?
No. Conventional ESI sources lack the scanning mechanism required to move the spray point by point across the sample surface whilst simultaneously acquiring data; they are therefore unable to obtain spatial coordinates. Imaging requires the use of a DESI-type scanning electrospray imaging source, or the retrofitting of a scanning module to the existing system.

Can an imaging source only use one ionisation method?
Not necessarily. For example, the MSI DPI features both DESI electrospray and photochemical ionisation (PI) modes, allowing different ionisation methods to be switched between or combined on the same imaging hardware to accommodate molecules with different properties, such as polar and non-polar ones.

When purchasing, should one decide on the ion source or the imaging source first?
One should first clarify whether spatial distribution information is required: if only qualitative and quantitative analysis of components is needed, a conventional ion source will suffice; if imaging is to be performed, one must first determine which ionisation method is suitable for the sample, then select an imaging source with the corresponding scanning capabilities, and confirm its compatibility with the host model.