Selecting an Atmospheric Pressure Mass Spectrometry Imaging Source: A Comparison of the Technical Advantages of DESI and Secondary Photoionisation Imaging Sources

The value of ambient mass spectrometry imaging (Ambient MSI) lies in the fact that samples can be imaged directly under atmospheric conditions, eliminating the need for extensive pre-treatment such as vacuum sampling and matrix spraying. However, amongst ambient-pressure sources, there are significant differences between DESI (desorption electrospray ionisation) and imaging sources incorporating secondary photoionisation (post-photoionisation) in terms of the types of molecules they are suitable for and their sensitivity. Clarifying the principles and limitations of these two technical approaches before selecting a system can save a great deal of time and effort.

Strengths and weaknesses of conventional DESI

DESI utilises charged aerosolised droplets to impact the sample surface, dissolving and carrying away molecules for mass spectrometric analysis. Its greatest advantages are that it operates at atmospheric pressure, is intuitive to use, is well-suited to polar metabolites and small molecules, and requires virtually no sample preparation. However, DESI’s ionisation relies fundamentally on charge transfer via electrospray, resulting in relatively low ionisation efficiency for weakly polar and non-polar species (such as certain lipids, sterols and terpenoids), which leads to weak or even absent signals for these molecules in the imaging maps.

How does secondary photoionisation (post-ionisation) address this shortcoming?

Secondary photoionisation, also known as post-ionisation, involves using photons to ionise molecules that have already been desorbed into the gas phase once more, after DESI desorption but before the ions enter the mass analyser. This step does not alter the desorption process; it merely enhances ionisation efficiency and coverage. By separating the processes of ‘desorption’ and ‘ionisation’, the detection capability for non-polar and weakly polar species is significantly improved.

How to use the DESI/PI dual-mode on the DPI

The New Ceyuan MSI DPI employs a DESI/PI dual-mode: within a single imaging source, it retains the ambient-pressure desorption capability of DESI whilst incorporating a photochemical ionisation (PI) channel. Users can switch between or combine the two modes on the same instrument depending on the sample type. DESI is sufficient for polar metabolites, whilst the PI channel can be activated for non-polar species such as lipids, thereby covering a broader range of molecular types.

The benefits of the dual-mode approach are quantifiable. In validated samples, post-photodissociation increased the ionic signal of caffeic acid by approximately 61-fold (approximately 60-fold in magnitude), theanine by approximately 14.5-fold, and mouse brain cholesterol by approximately 205-fold. These figures demonstrate the actual increase in sensitivity achieved through ‘dual-mode coverage’, rather than exaggerated claims about any single technology.

Three factors to consider when selecting a system

Firstly, consider the polarity distribution of the samples. If the research subject consists primarily of polar metabolites, traditional DESI is sufficient; if it involves a large number of non-polar species such as lipids and steroids, a source with post-laser photoionisation is more suitable. Secondly, consider the resolution requirements. DPI offers spatial resolution of 20–200 μm, making it suitable for imaging surface distributions in tissues, plants and food; for finer gradients, consider the 2–3 μm solution provided by MSI LDPI. Thirdly, consider pre-treatment tolerance. Both types of sources are matrix-free, but if completely zero pre-treatment is desired, dual-mode sources offer greater flexibility in terms of coverage.

Matrix-free operation is a shared fundamental advantage

Whether using DESI or post-photoionisation approaches, ambient-pressure imaging sources generally eliminate the need for matrix spraying. This not only reduces the suppression of signals at the low-mass end caused by matrix background but also avoids spatial artefacts resulting from matrix non-uniformity, making it more suitable for both qualitative and quantitative analysis. When selecting a system, treat ‘matrix-free’ as a fundamental criterion, then differentiate based on ionisation coverage and resolution; this will lead to a clearer assessment.

Summary

DESI and secondary photoionisation (PI) imaging sources are not substitutes for one another, but rather complementary technologies with overlapping coverage. The former is well-established and stable for polar species, whilst the latter utilises secondary photoionisation to bring non-polar species within the scope of ambient pressure imaging. The new Ceyuan MSI DPI’s DESI/PI dual-mode system integrates both approaches into a single instrument, allowing users to switch flexibly between them depending on the sample.