Last Month's Most Accessed Feature: The MIDAS touch for TLC: Novel ambient mass spec source

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  • Published: Apr 8, 2016
  • Categories: Base Peak
thumbnail image: Last Month's Most Accessed Feature: The MIDAS touch for TLC: Novel ambient mass spec source

Ambient ionisation source for surfaces

An ambient ionisation source based on desorption APCI has been developed for the analysis of compounds on surfaces, illustrated by the identification and measurement of different types of compounds separated on TLC plates.

A new ionisation source for ambient mass spectrometry has been developed by researchers at the University of Maryland who adapted an earlier source of theirs to make it capable of analysing planar surfaces that might also be irregular. In 2015, William LaCourse, Gregory Winter and Joshua Wilhide published details of their direct sample analysis source which operated under desorption atmospheric pressure chemical ionisation (DAPCI) conditions. A corona discharge produced charged water clusters that were directed towards a sample held on the plate.

This design has now been modified to narrow the nitrogen gas flow that sped through a heated inlet towards the corona pin where water molecules present in the naturally humid air were ionised into clusters. The overall effect was to produce the higher resolution that is required when surfaces such as TLC plates are analysed by tracking across the top. In addition, the source was mounted on a moveable sample stage so that it could be traversed across samples in the x and y directions.

The device was named molecular ionisation-desorption analysis source, generating the catchy acronym MIDAS. Its added precision and flexibility of movement were demonstrated with the analysis of several types of compound on TLC plates, showing that it is not necessary to visualise them for analysis.

Thin-layer chromatography is a common separation technique in many areas of work but some of the techniques used to detect the separated compounds on the plates, such as staining or charring, can be destructive. MIDAS produced no visible damage to the plates so that they can be removed for further examination or processing, which is an important advantage if the experiments are being conducted on the preparatory scale.

TLC plate analysis

In the first experiments, five amino acids were spotted onto different positions on a conventional TLC plate. They were ionised by tracking the source across the surface at a height of a few millimetres and a speed of 1.6 mm/s. The desorbed ions were drawn into a short capillary that was attached to the inlet of a time-of-flight mass spectrometer.

Under positive ionisation, arginine, leucine, lysine, methionine and proline were all detected, showing strong extracted ion chromatograms of the most intense peaks. At the relatively high concentrations employed, amino acid dimers were also readily detected. The positions of the amino acids spots on the plate were confirmed by ninhydrin staining.

In a second illustration, the components of an analgesic tablet were dissolved in methanol, separated on the plate and analysed by MIDAS. Caffeine, acetaminophen, acetylsalicylic acid and salicylamide were all detected, some in positive and negative modes and others in just one mode.

Acetylsalicylic acid and salicylamide co-migrated on the plate, ending up at the same position which would make visual detection difficult. However, they were easily distinguished from their mass spectra using the MIDAS system as acetylsalicylic acid migrated slightly slower and appeared at the back edge of the merged spot.

A spot of blue chemical marker was positioned at each end of the plate after the compounds had been separated. Although the team made no attempt to identify the compounds in the marker, their positions in the chromatogram allowed the retention factors of the analgesic components to be calculated.

Quantitative capability

The ability of the MIDAS system to allow quantitative studies was demonstrated by analysing separate spots containing 50-1000 ng caffeine. The peak areas in the extracted ion chromatograms were plotted, giving a linear calibration curve without the necessity for added standards of any type. The accuracy could be improved further using an automated TLC spotter that would give more precise sample positions.

The MIDAS examples all used TLC plates but the researchers pointed out that other flat samples could be used due to the interchangeable sample plates. So, the commonly used 96- and 384-well plates could be attached. Mass spectrometers with atmospheric pressure inlets from a range of manufacturers can also be used in conjunction with MIDAS due to its independent mode of operation.

One key disadvantage is the inability to analyse large biomolecules such as proteins and DNA, since APCI sources are heated and can denature or degrade the compounds. In these cases, electrospray ionisation is more suitable, as it also produces a range of multicharged ions for each analyte.

Nevertheless, the few examples provided by LaCourse and his colleagues showed that MIDAS deserves further examination with a wider range of analyte and surfaces. Its simple mode of operation in the open air could provide an easy and rapid way to analyse compounds on various types of surface.

Related Links

Journal of the American Society for Mass Spectrometry 2016, 27, 352-358: "Molecular ionization-desorption analysis source (MIDAS) for mass spectrometry: Thin-layer chromatography"

Journal of the American Society for Mass Spectrometry 2015, 26, 1502-1507: "Characterization of a direct sample analysis (DSA) ambient ionization source"

Article by Steve Down

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

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