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.

Introduction

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.

Methodology

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

Conclusions

The prototype ambient ionisation source was used to consistently detect sub picogram levels of pesticides in methanol and tea.