Putting pen to paper: Nib-assisted paper spray mass spectrometry for drug screening

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  • Published: Oct 5, 2012
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Putting pen to paper: Nib-assisted paper spray mass spectrometry for drug screening

Enhancing paper spray times

A suite of amphetamines has been detected by paper spray mass spectrometry using a novel nib device as a sprayer that allows continuous elution for several minutes and provides high sensitivity.

The nib device next to the mass spectrometer inlet
Image: courtesy Cheng-Huang Lin

Paper spray mass spectrometry is one of the ambient mass spectrometry techniques that are designed to produce ions in the open air so that samples can be analysed directly with the least amount of sample preparation, perhaps even none. It was introduced in 2010 from the Cooks lab at Purdue University and incorporates a small paper triangle on which sample is loaded. Ionisation occurs when the paper is subsequently wetted and a high voltage is applied and the ions are drawn into the mass spectrometer for analysis.

Unlike other forms of ambient ionisation, no sheath liquid or sheath gas is needed to assist ionisation or ion desolvation. Subsequent studies in the Cooks lab have shown that two ionisation mechanisms are in play. At low solvent flow rates and relatively higher currents of about 1 µA, corona discharges are in operation but in solvent-rich systems, multiple Taylor cone jets are created to produce droplets of a range of sizes.

It is easy to identify compounds using paper spray, but less easy to quantify them because the solvent can evaporate from the paper, terminating the electrospray process. In order to extend the spray time, the Cooks group found that one of the most effective ways was to cover the paper with PTFE film, which prevents solvent evaporation. Now, a group of Chinese researchers has come up with an alternative way, which was reported in the Journal of Separation Science.

Nib makes a point

Cheng-Huang Lin, Hsun Lee, Chih-Sheng Jhang and Ju-Tsung Liu from the National Taiwan Normal University, Taipei, literally brought pen to paper by introducing a nib to the paper spray. In the first report of its use, the nib was fashioned from brass in the shape of a fountain pen nib and had a connection to a capillary at the opposite end to the nib point.

A triangular piece of paper measuring 5 mm in length and 3 mm across the base was wetted with drops of the sample solution and held on the nib with a copper clip. A methanolic solution was run through the capillary to soak into the paper and a high voltage of 3 kV was applied to the nib. As a result, ionisation occurred from the paper tip, as illustrated with a solution of p-chloroamphetamine.

The sharpness of the paper tip had a strong influence on the ionisation efficiency, which maximised when the angle of the point was 15°. This is a strong indication that the corona discharge ionisation mechanism is in play, said Lin.  

As long as the methanol solution continued to flow onto the paper, the ion intensity remained relatively steady. In the first 5 minutes, the amphetamine compound was continuously emitted from the tip at about 1 pg/s and the ion current was still at 20% of its initial level after a total of 10 minutes.

This stability led to the formation of high quality mass spectra and a detection limit of 0.1 µg/mL for p-chloroamphetamine. For the purposes of quantitation, linear calibration curves were obtained at 0.1-25 µg/mL.

Drug screening in saliva

The nib-assisted paper spray method was tested on a mixture of amphetamine-type designer drugs donated by the local Forensic Science Centre attached to the Military Police. The detection limits of the o-, m- and p-chloro- and fluoro- isomers of amphetamine were in the range 0.10-0.13 µg/mL which were better than those that the researchers obtained using MALDI mass spectrometry (7-8 µg/mL) and electrospray-assisted LDI mass spectrometry (4 µg/mL).

The method could also be suitable for the analysis of drugs in human saliva. When p-chloroamphetamine was added to the saliva of a volunteer, it was measured with a detection limit of 0.5 µg/mL, even though unknown matrix effects were observed.

Lin told spectroscopyNOW.com that the method is simple to operate in practice and concluded that “nib-assisted paper spray mass spectrometry is, under most circumstances, the most favourable ‘drug screening’ method for use under ambient conditions,” due to its high sensitivity, relatively long ion signal time and speed.

Since the submission of this paper, a second application of the technique has been published in Electrophoresis, in which both paper spray mass spectrometry and capillary electrophoresis-electrospray mass spectrometry are applied to the analysis of amphetamines in saliva. Analysis can be switched rapidly from one technique to the other simply by switching the syringe injector to another capillary and both analyses are carried out on the same mass spectrometer.

Related Links

Journal of Separation Science 2012 (Article in Press): "Rapid screening and determination of designer drugs in saliva by a nib-assisted paper spray-mass spectrometry and separation technique"

Electrophoresis 2012, 33 (Article in Press): "Rapid screening and determination of 4-chloroamphetamine in saliva by paper spray-mass spectrometry and capillary electrophoresis-mass spectrometry"

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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