Phosmet: A challenge in canola
Phosmet is a phthalimide-derived, nonsystemic organophosphate insecticide. Its main application is for controlling coddling moth on apple trees, although it is also used on a variety of other fruit crops. The compound is recognized by many agencies in the U.S. and around the globe as having acute toxicity and human health implications.
Figure 4 - Phosmet: 20, 10, 4, and 2 ppb in a) barley matrix overlaid with matrix blank, b) barley matrix-matched Australian standards, c) wheat matrix overlaid with matrix blank (MRL, 50 ppb), and d) wheat matrix-matched Australian standards (MRL, 50 ppb).
In barley and wheat matrix, phosmet poses no problem, with little or no matrix interference even at a level of 2 ppb (Figure 4a and b) with a “textbook” standard curve (Figure 4c and d). The Australian MRL standard for phosmet is 50 ppb; therefore, carrying out this analysis in barley or wheat is not difficult.
Figure 5 - Phosmet: a) 80 ppb in canola matrix overlaid with matrix blank (no response), and b) 40 ppb in barley (red = no response) and 80 ppb in canola (green = response).
In contrast, analysis of phosmet in canola does pose a significant challenge. In the trace displayed (Figure 5a), canola demonstrates a massive matrix signal where phosmet should be at 17.489 min. The potential for substantial retention time shift caused by matrix was investigated together with analysis by CI with full scan, yet phosmet could not be detected. This problem is ongoing, and Figure 5b illustrates its extent, with the barley (40 ppb) and canola (80 ppb) traces overlaid (i.e., the canola signal is hidden because of the matrix interference). The obvious answer to this problem is to use LC-MS as an alternative analysis method, but it would be preferable to develop a reliable GC-MS methodology because that technique works very well with every other matrix examined in this study.
The impact of matrix-matched standards on limits of detection (LOD)
This study has demonstrated that the MRM transitions cannot be utilized in complex matrices without taking matrix interference into account. The authors have tested each matrix one by one and developed a database of reliable MRM transitions on routinely analyzed grain commodities. In some cases, alternative MRM transitions have been necessary to address the issue of false negatives and false positives. The alternative option of using chemical ionization has also been examined, and was found to be extremely useful in certain cases, with enormous potential for future ongoing method development.
The study has illustrated how the matrix can greatly affect GC-MS analysis of some pesticides and how awareness of these “limitations” can 1) help to establish achievable LOD for each compound and matrix, and 2) minimize false positives when using matrix-specific transitions.
- DEFRA Report. Development of Methods for the Multi-Residue Analysis of Pesticides in Animal Feeds. http://randd.defra.gov.uk/Document.aspx?Document=ps2543_9933_FRP.pdf
- Cervera, M.I.; Medina, C. et al. Multi-residue determination of 130 multiclass pesticides in fruits and vegetables by gas chromatography coupled to triple quadrupole tandem mass spectrometry. Anal. Bioanal. Chem. Aug 2010, 397(7), 2873–91. Epub 2010 Mar 17.
- Agency for Toxic Substances and Disease Registry. Public Health Statement for Malathion: http://www.atsdr.cdc.gov/phs/phs.asp?id=520&tid=92
Robert Trengove, Bruce Peebles, Katherine Rousetty, and Sze Bong are with the Separation Science & Metabolomics Laboratory, Murdoch University, Murdoch, South St., Western Australia, 6150, Australia; e-mail: R.Trengove@murdoch.edu.au. Felician Muntean, Steven Schachterle, Chris Kellog, and Patrick Jeanville are with Bruker Chemical & Applied Markets, Fremont, CA, U.S.A.; e-mail: Patrick.firstname.lastname@example.org.