Ion Transport: Why ‘ionisation’ does not equate to ‘detectability’

Many people assume that once a molecule has been ionised, it will inevitably be detected by mass spectrometry. In fact, there is a crucial intermediate stage—ion transmission. This refers to the entire process of efficiently and with minimal loss transporting ions generated by the ion source to the mass analyser, involving a series of ion optical components and vacuum channels. From the moment they are generated until they are detected, ions undergo deflection, focusing, and differential pumping from ambient pressure or low vacuum to high vacuum; at each stage, ions may collide with walls or diverge, resulting in loss. The higher the transmission efficiency, the more ions reach the detector, leading to better sensitivity and a better signal-to-noise ratio; conversely, if a large number of ions are lost along the way, the ionisation process is effectively wasted.

Ion transport is particularly challenging for open-space imaging at atmospheric pressure. Analytes are ionised in an open space at atmospheric pressure but must be transported into a high-vacuum mass analyser; the pressure differential is extreme, and the path must be as short as possible to minimise ion loss. The design of this ‘atmospheric pressure to vacuum’ transport section often directly determines the practical sensitivity limit of an imaging source—no matter how effective the ionisation, without adequate transport, it is impossible to produce high-quality imaging.

Neo Source INSTRUMENT’s MSI DPI features a dedicated ‘high-efficiency ion transfer conduit’ installed downstream of the conventional DESI spray system, which efficiently directs ions—having been desorbed and photoionised—into the mass spectrometer. This is one of the key factors behind its signal improvement of 1–3 orders of magnitude compared to traditional DESI—the additional ions generated by supplementary ionisation can only be effectively converted into detectable signals through efficient transfer.

It is worth noting the engineering details of this conduit: the MSI DPI utilises a proprietary titanium alloy ion transfer tube, the front end of which has been polished to ensure it does not damage the sample surface upon contact, guaranteeing that tissue sections remain intact for H&E histopathological examination following imaging; simultaneously, the tube is removable, allowing for easy cleaning and maintenance, whilst balancing long-term stability with serviceability. For further information on the ‘ion lens + vertical ion introduction’ ion optical chain on the online mass spectrometry side, please refer to the dedicated article ‘Ion Lenses, Vertical Ion Introduction and Efficient Ion Transport’. Both the MSI DPI and MSI LDPI can be retrofitted to mass spectrometers from Agilent, AB SCIEX and Thermo.