MSI DPI: How electrospray desorption and photoionisation work together spatially

In mass spectrometry imaging, the two ionisation modes are used in tandem; it is not simply a matter of “A first, then B”. The spatial geometry of the ionisation sources directly determines whether these two modes work synergistically or interfere with one another.

Neo Source INSTRUMENT’s MSI DPI electrospray/photochemical ionisation mass spectrometry imaging source integrates both DESI (desorption electrospray ionisation) and PI (photoionisation) modes, which operate sequentially within the same ionisation chamber. The core engineering challenge here is not “whether there are two ionisation sources”, but rather “how they are positioned and how the plumes are transported between them”.

Plume dynamics of DESI spraying

Let us first consider the DESI side. The solvent spray (typically methanol/water, flow rate 2–5 μL/min) is atomised by high voltage to form charged droplets, which strike the sample surface at a velocity of approximately 100–200 m/s. The point of impact creates an elliptical sputtering zone of approximately 100–200 μm on the sample surface; the sputtered analytes are distributed across two regions:

• The core jet region (angle approximately 15–20°), containing larger droplets and ionised primary ions, travels at a relatively high velocity directly towards the mass spectrometer inlet;

• the diffusion plume region (at an angle of approximately 40–60°), which contains smaller droplets and a large number of neutral molecules; these move more slowly and disperse in a fan-shaped pattern.

In conventional DESI, only the core jet region contributes to the ion signal—the neutral molecules in the diffusion plume region are largely evacuated by the vacuum pump. The MSI DPI strategy is as follows: leave the core jet region undisturbed (DESI has already done its job), and instead position the VUV beam along the path of the diffusion plume region.

Position of the VUV beam

In MSI DPI, the direction of the VUV beam (10.6 eV, Kr discharge lamp) intersects the axis of the DESI spray at approximately a right angle. The beam passes through the middle section of the diffusion plume—by this point, the capillary core jet has already moved beyond the beam’s range, carrying away the vast majority of the ESI ions; whilst the neutral molecular cloud in the diffusion zone has just reached the photoionisation ‘window’.

The physical parameters of this intersection region are approximately as follows:

• Approximately 5–10 mm from the sample surface (depending on the DESI nozzle angle and the position of the mass spectrometer inlet);

• The concentration of neutral molecules in this region is approximately 10⁸–10¹⁰ molecules/cm³ (depending on the sample concentration);

• The time taken for molecules to traverse the photoionisation zone is approximately 10⁻⁵–10⁻⁴ seconds (depending on the gas flow velocity and beam width).

Under these parameters, the probability of photoionisation of neutral molecules is sufficiently high, whilst the probability of ESI ions being irradiated by VUV photons is sufficiently low—this forms the engineering basis for “mutual non-interference”.

The temporal sequence is not precise

It should be noted that the notion of “DESI ionisation first, PI ionisation second” is merely a conceptual simplification. In practice, the time windows of the two processes overlap—the DESI spray is continuous, as is the VUV beam, and the plume undergoes steady-state transport rather than batch processing.

However, spatial separation does indeed achieve the effect of temporal separation: molecules require a flight time of approximately 50–200 microseconds from being sputtered out to reaching the position of the VUV beam (depending on distance and gas flow velocity). During this time, the ESI ionisation process is essentially complete (the typical timescale for droplet evaporation, Coulombic fragmentation and ion release is on the order of microseconds). Therefore, even if the DESI spray never stops, the fate of the molecules in each ‘batch’ upon entering the ionisation chamber is indeed as follows: they first undergo ESI in the core jet region, and then traverse the PI region in a neutral state.

Why is the PI not used in the core region?

A common question: why isn’t the VUV beam directed directly at the sputtering point on the sample surface? Wouldn’t that mean all molecules—both neutral and ionised—would be ‘swept away’ by the VUV photons, resulting in a stronger signal?

In practice, directing VUV irradiation at the sputtering point on the sample leads to three problems:

• Formed ESI ions are fragmented by 10.6 eV photons, increasing fragmentation;

• VUV photons heat the sample surface, potentially causing thermal degradation;

• Solvent vapour near the sputtering point absorbs VUV photons, reducing the effective photon flux.

The MSI DPI’s choice to “intercept” neutral molecules in the diffusion plume region is an engineered compromise: sacrificing a small amount of absolute ionisation efficiency (as some neutral molecules have already been pumped out in the upstream section) in exchange for cleaner mass spectra and lower chemical background.

Practical operation of dual-mode switching

The DESI and PI modes of MSI DPI can be switched on and off independently. The common operating logic in practical use is as follows:

• Routine analysis: Enable DESI only to obtain high-contrast imaging of polar molecules;

• Comprehensive coverage: Activate dual mode to detect both polar and non-polar molecules simultaneously;

• Targeting neutral molecules: Compare the differences between DESI-only and DESI+PI images to identify molecules ‘illuminated by PI’—these are often targets overlooked by conventional imaging.

Related products: Neo Source INSTRUMENT MSI DPI electrospray/photochemical ionisation mass spectrometry imaging source, with a spatial resolution of 20–200 μm, compatible with Agilent, AB SCIEX and Thermo mass spectrometers.