The EESI way: Ambient mass spectrometry for lead in aqueous samples

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  • Published: Aug 16, 2012
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: The EESI way: Ambient mass spectrometry for lead in aqueous samples

Testing for lead

A novel ambient mass spectrometry method for the measurement of lead in water is based on the formation of complexes with EDTA in intersecting sprays of sample and reagent.

The incidence of lead poisoning is falling around the world due to the introduction of lead-free food containers, petrol and paint but it remains at levels that cause concern. Older housing that contains lead water pipes, occupational exposure in places like smelters, and environmental exposure from contaminated land and water are just some of the remaining sources of lead.

The fall in reported cases in the western world has removed lead from the headlines and people tend to forget how nasty the chronic effects of lead poisoning can be. It attacks the nervous system, causes kidney damage and hypertension, and can reduce the IQ of children and cause disruptive behaviour.

Over the years, there have been many tests for lead in water, one of the principal routes of exposure, but a team of Chinese scientists has been drawn towards a new type of mass spectrometry and the advantages that it might give. Extractive electrospray ionisation (EESI) is one of the recently developed ambient mass spectrometry methods that operate in the open air in the absence of vacuum and it is especially useful for complex matrices.

During EESI, a spray of a liquid sample is crossed with a spray containing a reagent that brings about ionisation of the target compounds in the sample. The length of the sample spray from the spray outlet to the mass spectrometer inlet disperses the various components in the matrix, effectively diluting them so that they cause less interference. This factor, combined with the minimal sample preparation required, has led to the analysis of compounds in complex matrices like biological fluids, skin and living objects.

Now, Haiwei Gu and colleagues from the East China Institute of Technology, Nanchang, the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Beijing, the Beijing Entry-Exit Inspection and Quarantine Bureau, and the Administrative Centre for China's Agenda 21 (ACCA21), Beijing, have tested EESI for the measurement of lead in various types of aqueous samples including beverages and lake water.

A complex solution

The researchers took advantage of the fact that lead forms strong complexes with ethylenediaminetetraacetic acid (EDTA), a property which is used in the treatment of lead poisoning. In set-up experiments, a solution of lead acetate was sprayed from a capillary and intersected by a spray of EDTA solution. Both sprays were encompassed in a flow of nitrogen gas to aid droplet formation.

The tips of the capillaries were in the open air just 8 mm from the inlet of a mass spectrometer but that allowed sufficient time for the lead-EDTA complexes to form. They were drawn into the mass spectrometer for ionisation in negative-ion mode and the fragmentation patterns and tandem mass spectra confirmed that lead acetate was produced in the sprays.

For the purposes of quantitation, the team compared MS/MS and MS/MS/MS spectra of the complex. They selected the MS/MS/MS spectra of [EDTA-208Pb-3H]3 ions because they were less noisy than those of the MS/MS spectra, although they gave a slightly worse detection limit of 3 x 10–14 g/mL for lead in deionised water. The other analytical characteristics were all acceptable and a big bonus was that each analysis could be completed within two minutes.

Drinks testing

The EESI method was applied to a variety of waters like mineral water, tap water and lake water, as well as beverages such as beer, orange juice, brewed tea, soft drinks and energy drinks. No lead was found in any of the samples, so small amounts were added to each sample for analysis, to test the applicability of the method on different kinds of liquids.

The recoveries ranged from 91.5-129.0%, depending on the sample matrix, and the other method characteristics were sufficient to permit a semi-quantitative estimation of the levels in lead. All of the linear calibration ranges stretched over at least two orders of magnitude. However, the widest ranges were achieved for Woolong tea and Coca Cola, which are two of the more complex matrices, illustrating the high matrix tolerance of EESI.

The method compared favourably with a standard ICPMS method for lead speciation but has the advantages that it does not require sample preparation and it maintains structural information that allows for the lead species to be identified in the mass spectrometer.

The speed of the reaction between lead ions and EDTA and the fast analysis times suggest that EESI could be set up for high-throughput analysis, possibly in quality control environments. However, each type of sample should be examined individually for the presence of organic acids and other compounds that that would also react with lead ions and distort the results.

Related Links

Talanta 2012 (Article in Press): "Detection of trace levels of lead in aqueous liquids using extractive electrospray ionization tandem 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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