How does photoionisation mass spectrometry imaging eliminate polarity bias?

To understand ‘eliminating polarity bias’, we must first examine its origins. Taking traditional DESI as an example, this technique relies on charged droplets to desorb sample molecules, and the ionisation efficiency is highly dependent on molecular polarity: highly polar components are easily charged and enter the gas phase, whilst weakly polar and non-polar components are difficult to ionise, leading to a ‘skew’ in the imaging spectrum— —this is polarity discrimination, which causes the signals from non-polar components to be systematically underestimated or even lost.

The breakthrough in photoionisation mass spectrometry imaging lies in ‘post-ionisation’. Neo Source INSTRUMENT’s MSI DPI, following DESI spraying, and MSI LDPI, following laser desorption, both incorporate a vacuum ultraviolet photochemical ionisation system. Regardless of their polarity, desorbed molecules are first released in a neutral state and are then secondarily ionised by vacuum ultraviolet photons; as the photon energy exceeds the ionisation energy of the vast majority of organic compounds, neutral molecules (including non-polar lipids, terpenes, flavonoids, etc.) can be efficiently ionised, thereby circumventing the polarity bias inherent in DESI, which relies solely on the charging of the liquid phase.

The results are quantifiable: compared with conventional DESI, the overall metabolite signal intensity from the two new sources developed by Ceyuan has increased by 1–3 orders of magnitude; the cholesterol signal in mouse brain has increased by approximately 205-fold, the theanine signal in tea leaves by 14.5-fold, and the caffeine signal by 61-fold; in both positive and negative ion modes, hundreds of new lipid, terpenoid, flavonoid, amino acid and glycoside species can be detected.

This also explains why the New Ceyuan approach complements MALDI: whilst MALDI can also detect certain non-polar macromolecules, it requires the application of a matrix and suffers from matrix peak interference in the low-mass region; photoionisation, on the other hand, is matrix-free, produces a clean low-mass region, and offers more natural coverage of non-polar small molecules. In the study of the vast number of non-polar components in metabolomics and lipidomics, this ‘non-selective’ characteristic directly translates into the detection of more metabolites and more reliable spatial distribution.

In a nutshell: traditional ionisation is ‘selective’, whilst photoionisation is ‘non-selective’; this is precisely the physical essence of non-polarity-discriminatory imaging.