Direct Analysis and Quantification of Metaldehyde in Water using Reactive Paper Spray Mass Spectrometry

Abstract

Metaldehyde is extensively used worldwide as a contact and systemic molluscicide for controlling slugs and snails in a wide range of agricultural and horticultural crops. Contamination of surface waters due to run-off, coupled with its moderate solubility in water, has led to increased concentration of the pesticide in the environment. In this study, for the first time, rapid analysis (<~1 minute) of metaldehyde residues in water is demonstrated using paper spray mass spectrometry (PS-MS). The observed precursor molecular ions of metaldehyde were confirmed from tandem mass spectrometry (MS/MS) experiments by studying the fragmentation patterns produced via collision-induced dissociation. The signal intensity ratios of the most abundant MS/MS transitions for metaldehyde (177 → 149 for protonated ion) and atrazine (221 → 179) were found to be linear in the range 0.01 to 5 ng/mL. Metaldehyde residues were detectable in environmental water samples at low concentration (LOD < 0.1 ng/mL using reactive PS-MS), with a relative standard deviation <10% and an R² value >0.99, without any pre-concentration/separation steps. This result is of particular importance for environmental monitoring and water quality analysis providing a potential means of rapid screening to ensure safe drinking water.

Figure 1. Schematic of the paper spray mass spectrometry experimental setup used for rapid detection of metaldehyde in water samples

Figure 2. Positive ion mode paper spray mass spectrum of metaldehyde recorded using a bench-top ion trap mass spectrometer.

5 μg of the analyte in 1 μL of deionized water was spotted onto filter paper and ionized in air by application of a positive electric potential (3.5 kV) using methanol as the paper spray solvent. (a) The sodiated molecular ion [M + Na]⁺ peak of metaldehyde (MW 176) in deionized water produced the dominant ion signal intensity (m/z 199), and (b) Sodiated molecular ion [M + Na]⁺ of deuterated metaldehyde-d₁₆ (MW 192) in deionized water produced the dominant ion peak (m/z 215). Inserts (i–ii) show the isotopic distribution of the metaldehyde and metadehyde-d₁₆ sodiated [M + Na]⁺ ion adducts at m/z 199 and 215 respectively. Note that in insert (ii) the relatively large signal intensity for m/z 214 is likely a consequence of D-H back-exchanges occurring in the ambient environment (and 99% isotopic enrichment). Inserts (iii–v) show the tandem MS CID data for the selected ions of metaldehyde and metadehyde-d₁₆.

Figure 3. Positive ion mode paper spray mass spectrum using a bench-top ion trap mass spectrometer with MeOH:(H₂O + 0.1% formic acid) (1:1, v/v) spray solvent application.

5 μg of the analyte in 1 μL of deionized water was spotted onto filter paper and ionized in air by application of a positive electric potential (3.5 kV); (a) metaldehyde and (b) paraldehyde. Tandem MS CID data for the m/z 177 and m/z 133 ions are shown in inserts (i) and (ii) respectively.

Figure 4. Positive ion mode paper spray mass spectra for rapid detection of metaldehyde in raw water samples (supplied by Northumbrian Water) whereby a volume of ~10 μL of the sample was deposited onto the paper substrate and ionized in the open environment by application of an electric potential of +3.5 kV.

Abberton Raw was analyzed according to (a) the ‘normal PS-MS’ method and (b) with reactive PS-MS. Similarly for Chigwell Raw, ‘normal PS-MS’ analysis is shown in (c) and reactive PS-MS in (d). Inserts (i) & (ii) are the MS/MS CID mass spectra for the protonated metaldehyde ion at m/z 177 from each water sample analyzed using the reactive methodology.

Figure 5. Proposed mechanism of acid catalyzed metaldehyde ring opening.

Figure 6. Illustrative diagram showing reactive and “normal” PS-MS analysis of metaldehyde generating different ion types.