Ambient mass spectrometry source uses high air flow rates

The ion collection tube in the novel source was 50 cm long with an unusually large internal diameter of 3 mm. It led into a refluence tube which was attached to the mass spectrometer and was pumped using an aspirator as a supplementary pump to generate high air flow rates up to 25 L/min. The collection tube inlet was positioned near the sample to collect desorbed ions and the other end was held 5-10 mm from the mass spectrometer orifice.

The effect of the air flow rate was illustrated in initial experiments with the dye rhodamine 6G on a glass plate using desorption electrospray ionisation (DESI) and a time of flight mass spectrometer. The intensity of the characteristic ion at m/z 443 increased rapidly with rising air flow rate and was still rising at the maximum flow rate, changing 75-fold over the range.

The researchers attributed this vast improvement to higher collection efficiency due to better pumping combined with reduced levels of neutralisation of the charged droplets on the transport tube walls, even though the transport distance to the orifice was relatively long.

With the signal intensity of conventional ESI set to 100%, the relative intensity for remote ESI, in which the source was at the open end of the 50-cm transport tube, was just 7%. However that for AFA-ESI was 150%. So, the ion intensity losses in remote ESI can be countered with AFA-ESI.

Further studies on the influence of the static electric field applied between the transport tube and an ion funnel in its open end showed that air flow was the dominant effect.

In AFA-ESI experiments with the protein cytochrome c, ion intensities again increased with air flow rate and shifted to higher charge states, although they remained lower than those in conventional ESI. This shift was interpreted in terms of increased desolvation of the ions at higher flow rates, which was supported by other experiments.

In addition, the variation of water cluster intensities with air flow rate in AFA-APCI studies confirmed that clusters are promoted by higher flow rates, preventing fragmentation.

This combination of reduced fragmentation and improved ion capture and transport using the long transport tube enhanced the sensitivity of remote sampling ambient mass spectrometry, which the researchers illustrated with a series of analytes and sampling conditions.