Direct real time analysis of sub picogram levels of pesticides in drinks
Scott Campbell1, William Greenwood2, Claudio dos Santos2, Peter Luke3, David Douce4, David Megson2
1 SpectralWorks Limited, UK. 2Manchester Metropolitan University, UK. 3Mass Spec Analytical, UK. 4Waters Corporation, UK.
First presented at BMSS 2023 Manchester, UK. September 2023.
Pesticides are widely used in agriculture to control pests and improve food security and yields. However, they are also toxic to humans and so are regularly monitored in foods and beverages to protect consumers. Safe levels of pesticides in many products are not based on human health thresholds but on minimum reporting levels of analytical instruments at levels of approximately 1 ppb (1 pg ul-1 or 1 pg g-1). Food and beverage matrices can be challenging and so often require lengthy clean-up which leads to long processing times.
Aim: to investigate if a prototype ambient ionisation source is capable of detecting sub picogram levels of pesticides in drinks through analysis by direct injection in real time.
Analysis was performed on 7 triazine pesticides (ametryn, atrazine, prometon, prometryn, propazine, simazine and terbutryn). The standard was diluted in methanol to produce solutions at 500 pg uL-1, 50 pg uL-1, 5 pg uL-1 and 0.5 pg uL-1. Samples were injected onto a prototype ambient ionisation source developed by Mass Spec Analytical (MSA) (Figure 1). This was connected to a Waters triple quadrupole mass spectrometer.
Figure 1. Prototype ambient ionisation source developed by MSX
•A 100 pg uL-1 standard was injected into the instrument in full scan mode to confirm parent ion masses.
•The mass spectrometer was then operated in MRM mode to identify and select appropriate transitions.
•injection volume, source flow rate, source temperature, cone voltage all investigated to optimise detection limits, peak shape and reproducibility.
•All samples were injected five times to test reproducibility.
Results and Discussion
Sub-picogram detection limits recorded in solvent solutions
Concentrations off all 7 pesticides were consistently detected with a S:N>3 in all five injections at an on instrument mass of 0.5 pg (Figure 2).
Figure 2. Chromatogram of 5 injections of propazine in methanol
Sub-picogram detection limits recorded in Green tea matrix
Concentrations off all 7 pesticides were consistently detected with a S:N>3 in all five injections at an on instrument mass of 0.5 pg (Figure 3).
Figure 3. Chromatogram of 5 injections of propazine in green tea
Isomeric pesticides distinguished by unique MRM transitions
Prometryn and terbutryn are structural isomers of C10 H19 N5 S. These could be differentiated by two unique transitions highlighting the benefits of qqqMS (figure 4).
Prometryn: m/z 242 to m/z 158 and m/z 242 to m/z 200
Terbutryn: m/z 242 to m/z 71 and m/z 242 to m/z 186
Figure 4. Structures of prometryn and terbutryn resulting in unique fragments
The prototype ambient ionisation source was used to consistently detect sub picogram levels of pesticides in methanol and tea.