Last Month's Most Accessed Feature: Paper spray ionisation: Zero volt version
- Published: Nov 9, 2015
- Categories: Base Peak
Paper spray ionisation improved
The paper slip held in place in front of the mass spectrometer inlet.
One of the recent variants of ambient mass spectrometry, in which ionisation takes place in the open air, is paper spray ionisation. As its name suggests, a piece of paper is used as the sample support. It is wetted with a suitable solvent to extract the target compounds which are driven to the tip of the paper when a high voltage, typically 3-4 kV, is applied to the paper. Here, the compounds are subjected to electrospray ionisation and drawn into the mass spectrometer.
The technique has proved to be versatile and has been illustrated for a variety of compounds in solid and liquid samples such as drugs, metabolites, alkaloids, food dyes and protein complexes. Now, two Japanese scientists have looked to simplify paper spray ionisation by removing the need for the high voltage. Akira Motoyama and Keishi Kihara from Shiseido Co. Ltd., Yokohama, achieved this by combining paper spray ionisation with the principles of solvent-assisted inlet ionisation (SAII).
In SAII, the samples are introduced to the mass spectrometer as a solution via a capillary that is inserted into the mass spectrometer inlet where ionisation occurs. The key advantages are that no extra voltage or nebulising gases are required but the technique is limited to solutions. The new combo, named zero-volt paper spray ionisation, delivers the best of both techniques.
Filter paper substrate
A rectangular piece of filter paper (10 x 2.5 mm) was used as the substrate compared with the triangle used in conventional paper spray. Liquid samples were dispensed onto the slip and analysed immediately, or allowed to dry to simulate solids. In the latter case, a few drops of an appropriate extraction solvent were added for analysis. The paper was held horizontally close to the orifice of the inlet capillary and it is this proximity that induced vacuum aspiration of the compounds from the paper followed by SAII.
The process worked well with filter paper, possibly due to its fibrous nature which regulates release of the droplets into the mass spectrometer. This was in contrast to a stainless steel spatula which could not give reproducible spectra as the signal intensities varied widely. Short videos of the system in operation are available in the supplementary information alongside the article in Rapid Communications in Mass Spectrometry.
The orientation of the rectangular filter paper was also important. In paper spray, the point of the triangular paper pointed towards the mass spectrometer but, in the zero-volt technique, the short edge of the rectangle was placed perpendicular to the orifice. This allowed a small reservoir of liquid to accumulate along the edge for controlled release. These are all small points but they ensured better performance.
The researchers conjectured that droplet release was affected by a number of factors including the solvent viscosity, surface tension, the surface area of the solvent along the edge and the aerodynamic force at the inlet capillary. Once the operating characteristics of the Orbitrap mass spectrometer were optimised, zero-volt paper spray ionisation was illustrated on a number of solid and liquid samples.
Direct analysis of peptides and lipids
Angiotensin II and insulin solutions were dried on the filter paper and analysed using acidified aqueous acetonitrile as the extraction solvent. Multiply charged ions of each compound were detected but the detection limits were not as low as those achieved with conventional SAII. The worsening performance was tentatively attributed to poor extraction recovery from the paper and reduced ion generation in the inlet due to the introduction of a relatively large volume of solvent over a shorter time.
However, Motoyama and Kihara were at pains to point out that this should not detract from the major advantage of zero-volt paper spray, which is the fact that runs are completed within a few seconds once the sample has been loaded and dried, and there is no need for high voltage, simplifying the process and making it safer.
A cosmetic lotion was also analysed as an example of a semi-solid. The oil-in-water emulsion revealed a series of polyols like glycerol, dipropylene glycol, polyethylene glycol and polymeric surfactant. They appeared as pairs of adducts with sodium and potassium ions which probably derived from their hydroxides which would be present in the emulsion as neutralisers.
Finally, an imprint of human skin was taken by pressing filter paper to a human cheek to extract the lipids. Analysis in positive- and negative-ion mode gave spectra of skin lipids such as mono-, di- and triglycerol, free fatty acids, ceramides and phospholipids.
The wide variety of compounds tested combined with the ease, flexibility and speed of operation make it an attractive option for the direct analysis of samples. Although the examples were used simply to identify compounds, the method could be extended to quantitative studies by the addition of an internal standard.
Rapid Communications in Mass Spectrometry 2015, 29, 1905-1916: "Zero volt paper spray ionization mass spectrometry for direct analysis of samples on filter paper substrate"
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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