Mass Spectrometry Ion Sources: Selection and Engineering Considerations for VUV Photoionisation Tubes

In mass spectrometry imaging ion sources, there is virtually only one answer when it comes to selecting the core light source for vacuum ultraviolet (VUV) photoionisation: a krypton (Kr) discharge lamp. The rationale behind this choice warrants thorough explanation—as it determines the upper limit of post-photoionisation performance.

Why a Kr lamp, rather than an Ar or Xe lamp?

VUV photoionisation requires fulfilment of a simple condition: the photon energy (hν) must exceed the ionisation energy (IE) of the analyte. The IEs of common organic molecules fall within the range of 8–11 eV—lipids at approximately 8.5–10 eV, polycyclic aromatic hydrocarbons at approximately 7.5–8.5 eV, fatty acids at approximately 9.8–10.2 eV, and sterols at approximately 9.5–9.7 eV.

Photon energies of several rare-gas discharge lamps:

• Ar lamp: 11.6 eV (primary emission line) / 11.8 eV (secondary line)

• Kr lamp: 10.0 eV (secondary line) / 10.6 eV (primary emission line)

• Xe lamp: 8.4 eV (primary emission line) / 9.6 eV (secondary line)

The Ar lamp has the highest energy, but it exceeds the ionisation energy (IE) of most organic molecules by too much, making excessive fragmentation likely (photons above 10.6 eV may cause the molecular backbone to break). The Xe lamp has the lowest energy; many target molecules (fatty acids, sterols, with IE > 9.5 eV) simply do not reach the ionisation threshold.

The 10.6 eV from the Kr lamp represents a ‘sweet spot’—just above the IE of most organic molecules, yet below the bond-breaking energies of typical chemical bonds (approximately 3.6 eV for C–C bonds and 4.3 eV for C–H bonds; however, the dissociation threshold after intramolecular vibrational redistribution is typically higher than 11 eV). In other words, photons at 10.6 eV have sufficient energy to knock an electron out of a molecule, but are generally insufficient to break the molecular backbone. This makes Kr lamps the de facto standard for soft-ionisation VUV light sources.

Engineering Limits of Photon Flux

The photon output of a Kr lamp is determined by the discharge power, gas pressure and window material. Typical commercial Kr lamps have a VUV output of approximately 10¹⁵–10¹⁶ photons/s (over all solid angles) at a discharge power of approximately 10–30 W. After passing through an MgF₂ window (cutoff wavelength approximately 115 nm, with a transmittance of approximately 60–70 per cent for 10.6 eV/116.5 nm photons) and undergoing optical transmission, the effective photon flux reaching the ionisation region is approximately 10¹³–10¹⁴ photons/s/cm².

This constitutes the fundamental limit on post-photoionisation efficiency: if the photon flux is not sufficiently high, the probability of neutral molecules being ionised cannot be increased. Ways to increase the photon flux include increasing the discharge power (limited by lamp life; a typical Kr lamp has a lifespan of approximately 2,000–5,000 hours), optimising the optical geometry (to ensure the beam passes through the ionisation zone in a more concentrated manner), and replacing the lamp with a higher-power model—though the trade-off between increased power and the thermal effects on lamp life is an engineering consideration that must be weighed up.

Practical challenges of window contamination

As the VUV beam travels from the lamp to the ionisation region, it must pass through at least one optical window (typically MgF₂). In a mass spectrometry imaging environment, this window inevitably comes into contact with high-boiling-point solvent vapours and even minute droplets generated by DESI spraying. The deposition of organic compounds on the window surface leads to a gradual decline in VUV transmittance—a progressive yet irreversible process.

The MSI DPI and MSI LDPI (Neo Source INSTRUMENT) incorporate several design measures to mitigate window contamination: a ‘curtain gas’ is positioned in front of the window, using a stream of dry nitrogen to prevent solvent vapours from approaching the window surface; the VUV photocurrent is monitored regularly to determine whether window transmittance has fallen to a threshold requiring cleaning.

Selectivity of photoionisation: not all molecules are ‘visible’

Where does the ‘selectivity’ of VUV photoionisation come from? It is not a form of intelligent screening, but rather a simple matter of energy matching.

For any given analyte, the photoionisation cross-section (σ) is a function of photon energy. When hν is exactly equal to the ionisation energy (IE), σ begins to rise from zero; when hν exceeds the IE by approximately 1–3 eV, σ reaches its peak (typically a maximum of about 10–100 Mbarn, i.e. 10⁻¹⁷–10⁻¹⁶ cm²). For molecules with an IE in the range of 7.5–10 eV, the photoionisation cross-section for 10.6 eV photons is in the upper-middle range—which means that Kr lamps are effective for ‘most’ organic molecules.

However, some molecules fall through the net: if the IE of the analyte exceeds 10.6 eV (for example, some small molecules such as CO₂ have an IE of 13.8 eV, and H₂O has an IE of 12.6 eV), 10.6 eV photons are completely incapable of ionising them. This is not a ‘flaw’ in post-photoionisation, but rather a law of physics—however, it does mean that if a particular molecule cannot be detected in the interpretation of VUV photoionisation data, it may simply be because its IE exceeds the photon energy, rather than because the molecule does not exist.

As part of a mass spectrometry imaging source

The VUV lamp is merely one component of the imaging source’s optical system, but it is the key limiting factor for post-photoionisation performance. From a selection perspective, the Kr lamp represents the current balance between technology and cost; should VUV light sources with higher photon flux and longer lifespans emerge in the future (such as VUV lasers), the efficiency of post-photoionisation may well take a significant leap forward.

Related products: Neo Source INSTRUMENT’s MSI DPI electrospray/photochemical ionisation mass spectrometry imaging source and MSI LDPI laser/photochemical ionisation mass spectrometry imaging source both incorporate a Kr VUV lamp as the post-ionisation light source and are compatible with mass spectrometers from Agilent, AB SCIEX and Thermo.