Air flow-assisted ionisation: Novel technique facilitates remote sampling

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  • Published: Apr 15, 2011
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Air flow-assisted ionisation: Novel technique facilitates remote sampling

Improvements in ambient mass spectrometry

Since the advent of ambient mass spectrometry, in which ions are produced in the open air and withdrawn into the mass spectrometer for analysis, a number of ionisation modes have been developed. While they appear to have been successful on the whole, accomplishing the principal objective of external ionisation, they tend to suffer from some important limitations.

First of all, the early ambient ionisation techniques suffer from poor collection efficiency, so that the vast proportion of ionised species did not enter the mass spectrometer. This has the secondary effect of limiting sampling to short distances between the sample and orifice.

There have been some recent developments which have improved the charged particle capture efficiency, including the use of extra pumps, slow pumps, and enclosed sources. These, in turn, extended the sampling distance.

A novel approach has now been adopted by a team of scientists in China who studied the effect of air flow on ion transportation efficiency, leading to the development of the air flow-assisted ionisation (AFAI) source.

Zeper Abliz from the Key Laboratory of Bioactive Substances and Resource Utilization of Chinese Herbal Medicine and Peking Union Medical College, Beijing Xiaohao Wang from Tsinghua University, Beijing, reported their findings in Rapid Communications in Mass Spectrometry.

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.

Air flow-assisted ionisation applications

Many small molecules which are ionised by conventional APCI and ESI were readily analysed by AFA-APCI and AFA-DESI mass spectrometry, including drugs of abuse, pharmaceuticals, explosives, proteins and volatiles such as hydrocarbons, alcohols and chloroform.

The analysis of large objects was illustrated by the detection of heroin on the surface of a suitcase, the protruding transport tube reaching the case which was positioned under the ESI sprayer.

The system can also be used close to people without any danger of electric shock. A volunteer carefully opened a sample of injectable butyl phthalide before placing the fingers close to the transport tube. Clear signals of the phthalide were detected by AFA-APCI MS.

The final example illustrated analysis in real time, using four closed bottles of acetonitrile, methanol, acetone and acetic acid. The collection tube was placed close to the cap of each bottle in turn, for ten seconds at a time, repeated over five minutes. The extracted ion chromatograms in the AFA-APCI MS spectra repeatedly displayed peaks for each compound over the time frame, monitoring vapour leakage from the bottles.

One operational positive noted by Abliz and Wang was the relative ease of operation compared with other ambient ionisations sources due to the wide collection tube diameter and fast flow rates which eliminated any dependence on the positions and angles of the tube and the sample surface.

The good performance achieved by AFAI with the 50-cm collection tube suggests that longer tubes might also be practical but further tests will need to be carried out for confirmation.

Meanwhile, the new ambient ionisation technique should be particularly useful for the surface analysis of large objects and remote monitoring in real time, as a result of the combined effects of the high air flow rate and the long collection tube.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Ambient mass spectrometry has been boosted by the development of an air flow-assisted ionisation source which enhanced the capture and transport of charged species and provided sensitive remote sampling in real time

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