Micro magic: Microextraction and microfluidic techniques combined for urinary drug screening

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  • Published: Mar 26, 2012
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Micro magic: Microextraction and microfluidic techniques combined for urinary drug screening

Miniaturisation doubling up

Two miniaturised techniques have been combined for the extraction of drugs from urine and their transfer to an electrospray mass spectrometer, providing rapid screening with the potential for automation.

Two miniaturisation techniques have been combined for the extraction of drugs from urine and their transfer to an electrospray mass spectrometer, providing rapid screening with the potential for automation. Neither technique is novel, but their combination is, and the proof-of-principle study reported by Scandinavian researchers has illustrated the potential for large-scale drug screening applications.

The first technique is microextraction by packed sorbent (MEPS) also referred to as microextraction in a packed syringe, that was introduced in 2004. MEPS is effectively solid-phase extraction in a syringe and has the principle advantage that the volume of solvent used to elute the compounds from the adsorbent in the syringe is suitable for direct injection to a gas or liquid chromatograph.

Each packed syringe can also be used about 100 times, with suitable rinsing between samples, and produces similar results to those from SPE. MEPS has been applied successfully for the removal of many different classes of drugs from urine, blood and plasma.

The second miniaturisation technique is lidless micropillar electrospray ionisation (µPESI), which was first reported in 2007 by scientists from the University of Helsinki. It consists of an open microchip with an in-plane channel and tip. The samples are driven by capillary force, so that no pumps or sources are required. It is easily coupled to a mass spectrometer for electrospray ionisation.

The µPESI chip has since been developed into a multiple chip in which 60 individual chips are set in a circular system that is fitted to a rotating table for successive injection into the mass spectrometer. The design gives easier sample handling, reduces cross-contamination between samples, and accelerates the positioning of each chip from several minutes to a few seconds.

Now two of the Helsinki team, Raimo Ketola and Teemu Nissila, have teamed up with Katrine Nielsen and Frants Lauritsen from the University of Copenhagen to combine MEPS and µPESI for the screening of drugs in human urine and their results have been reported in Rapid Communications in Mass Spectrometry. The performance of the coordinated system was optimised using a trio of aromatic drugs: verapamil, propranolol and metoprolol.

Drug screening accelerated

When urine samples were injected into the µPESI system following centrifugation and filtering, but without MEPS, the channels in the chip became clogged by proteins and salts, confirming that extraction is required. No clogging problems were observed after MEPS.

A total of five MEPS adsorbents were tested. Syringes packed with C2, C8 and C18 performed better than pure silicate and a C8-strong cation exchange composite. In no cases did the mass spectrometric signal intensity increase after extraction, as was observed with a standard solution of the drugs in alkaline methanol. However, the signals for the protonated molecules remained sufficiently strong for easy detection.

This suppression effect was attributed to those same proteins and salts, which are thought to interfere with the sorbent-analyte interactions in the syringe. One way to improve the performance would be to dilute the urine samples, but that option was not pursued in this study.

The best MEPS eluent from five tested proved to be an alkaline solution of 96% methanol containing ammonium hydroxide. This agreed with published studies on the recovery of basic drugs from plasma samples by MEPS.

The eluates from MEPS were analysed by injection into one of the 60 individual µPESI chips forming the circular array, It was positioned 5 mm from the orifice of the mass spectrometer which was operated in positive ion mode. Clean unused µPESI tips were used for each sample and the spectra showed clear peaks for each compound.

Use of single µPESI tips for multiple samples had generated significant carryover from one sample to the next, but this was eliminated with the multichip system. There were some reproducibility problems with the mass spectrometric signals varying between sprayer tips. However, the main concern was to detect the drugs for screening purposes, rather than measure their concentrations, and this was always possible at levels of 1 µM in water or urine.

Although it is still a laborious task to load each sample onto one of the chips, the analysis time is reasonably low at 5 minutes per sample. However, with automation of the extraction and injection processes, which was deemed to be possible, the analysis time would reduce to about 1 minute and the sensitivity and reproducibility would improve. This would provide a rapid screening procedure for drugs in urine which would be suitable for clinical and forensic settings.

Related Links

Rapid Communications in Mass Spectrometry 2012, 26, 297-303: "Rapid screening of drug compounds in urine using a combination of microextraction by packed sorbent and rotating micropillar array electrospray ionization 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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