Paper power

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  • Published: Mar 1, 2010
  • Author: Steve Down
  • Channels: Base Peak
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Since its conception in 2004, ambient ionisation mass spectrometry has come on in leaps and bounds. The original duo of techniques, desorption electrospray ionisation (DESI) and direct analysis in real time (DART), have both been commercialised and a succession of ambient ionisation methods have been developed over the last few years.

Extractive electrospray ionisation (EESI), easy ambient sonic spray ionisation (EASI), desorption APCI (DAPCI), atmospheric pressure solids analysis probe (ASAP), jet desorption ionisation (JeDI) and low-temperature plasma (LTP) are just some of the variations on ambient ionisation. It has been estimated that there are now more than 20 such methods for analysing unextracted samples in their native state.

Early in 2010, paper spray ionisation was announced as a new form of ambient ionisation by the Graham Cooks lab at Purdue University in the US, from which DESI originated. In this case, the sample is loaded onto a small triangular piece of paper, with the apex facing the mass spectrometer. The paper is subsequently wetted before a high voltage is applied to generate ions for analysis. Unlike other forms of ambient ionisation, no sheath gas or sheath liquid is required, making it one of the more simple variations.

Although the ionisation and spray mechanisms have not been studied extensively, it is believed that the high electric potential between the paper and the mass spectrometer inlet induces charges that accumulate at the paper apex adjacent to the inlet. Coulombic forces then produce charged droplets which desolvate to form dry ions, as in electrospray ionisation.

The use of paper in analytical chemistry is not unusual and paper spray ionisation can take advantage of some of its applications. For instance, the original report from the Cooks lab analysed drugs in dried blood spots on paper, as well as chemicals in situ after separation by paper chromatography.

Now the team has carried out an extensive examination of paper spray ionisation to study the range of compounds that can be detected, the opportunities for sampling using a paper medium, and the potential for analysis at the point of sampling.

The work was carried out by Jiangjiang Liu and Jin-Ming Lin from Tsinghua University, Beijing, and Graham Cooks, He Wang, Nicholas Manicke and Zheng Ouyang from Purdue University, using a linear ion trap mass spectrometer for detection. In initial tests, aqueous methanol solutions of heroin were added to chromatography paper and the voltage applied. At 1 ng/mL, mass spectra were recorded and the identity of heroin was confirmed by tandem mass spectrometry.

The spray duration increased slightly with increasing analyte concentrations and larger sample volumes. However, the most effective way to extend spray time was to cover the paper with PTFE film, presumably by preventing solvent losses by evaporation.

Different types of paper produced spectra with different chemical backgrounds, due to the various additives in the paper, such as the plasticiser dibutyl phthalate. Glass fibre paper gave far poorer spectra than filter paper and chromatography paper. The total intensity was two orders of magnitude below any of the other papers, so low, in fact, that the cocaine peak molecular ion could not be identified. Chromatography paper gave the best performance and was used for most of the subsequent experiments.

The position of the paper source was not critical and exact alignment was not necessary to produce good signals. The paper tip could be positioned anywhere within a 5 mm x 10 mm area in front of the mass spectrometer inlet.

The researchers ran spectra from a broad range of compound classes, including amino acids, peptides, proteins, pesticides, drugs and fatty acids. Good spectra were obtained in both positive and negative modes, with judicious adjustment of the solvents. Most analytes gave strong signals at 1 µg/mL, but this concentration could be reduced by 1-2 orders of magnitude for tandem mass spectrometry.

In "real" applications, dried blood spots and dried urine spots were analysed after wetting with aqueous methanol. In both cases, drugs were detected at ng/mL levels. Blood spots are a standard way of storing neonatal samples and paper spray ionisation presents a rapid way for subsequent analysis, without sample preparation or operator intervention during analysis. The paper triangle simply has to be placed in front of the mass spectrometer, wetted, and the voltage applied.

Sample collection and analysis can be neatly linked when objects are wiped with the paper substrate. The researchers used pre-wetted paper triangles to wipe a desktop that had been contaminated with heroin and also the peel of a lemon purchased at a local store. As little as 100 pg heroin was detected and the peel revealed the presence of a commercial pesticide.

The ease of sampling and subsequent analysis opens up the opportunity for real-time on-site applications. Paper wipes are used at security checkpoints. If the wipes are triangular, they could be analysed in situ on a miniature mass spectrometer to search for traces of drugs and explosives. With the correct mass spectrometer set up as a "black box", results would be obtained within seconds and suspects restrained. In the same way, the set up could be used in the clinical world for point-of-care diagnosis.

The low cost of paper raises the possibility of producing cheap, disposable cartridges for use in paper spray ionisation and the technique has enormous potential in a range of lab and real-world applications.

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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