Using Large Volume Injections to Increase Sensitivity for Pesticide Detection in Water Samples

This application brief demonstrate the increase in sensitivity afforded by performing a larger volume injection using Waters Pesticide Screening Application Solution with UNIFI.

Benefits

Using a 100-μL injection volume of water matrices, the sub-ppb range LODs and LOQs are achievable for spiked pesticides.

Ever-decreasing detection limits due to advances in mass spectrometry have led to numerous studies on trace-level contaminants in the environment and food. Typically, tandem quadrupole MS is preferred for subppb level contaminant level analysis, but uses a targeted approach. Using Waters Xevo G2-S QTof, screening for both known and unknown compounds in the ppb range is easily achievable. However, by increasing the volume of injected sample, significant increases in sensitivity can be attained. When combined with the excellent mass accuracy (<3 ppm) of the Xevo G2-S QTof, the ability to perform screening experiments at such low levels is of great utility to many environmental analyses, where unknown or large numbers of contaminants make relying on MS/MS alone not ideal.

A 100-μL extension loop was installed on to the ACQUITY UPLC I-Class System to allow the increased sampling volume. Two different water samples, one bottled drinking water and local tap water, were used for this study.A calibration series, composed of 7 points ranging from 10 to 1000 ppt, was prepared by addition of the Waters Pesticide Screening Mix (PSM). This certified standard mix contains 20 pesticides of varying polarities and chemical classes. Tap water samples were filtered through a 0.45-μm PTFE filter prior to injection. The bottled drinking water was injected with no cleanup, due to its previously observed cleanliness and lack of matrix effect on the pesticides studied here. In a previous experiment not displayed here, the PTFE filter was found to be an appropriate filter for these compounds, based on results that assessed retention on the filter by comparing the pre- and post-filter spike response of the pesticides in matrix. Limits of detection (LODs) and limits of quantification (LOQs) were calculated for all 20 pesticides, Figure 1, and were based on peak to peak signal-to-noise (S/N) ratios of 3 and 10, respectively. As shown in Figure 1, sensitivity of the 20 pesticides was better in bottled water than tap water, most likely because of the increased complexity of the tap water matrix. Figure 2 shows the S/N calculation in UNIFI for the pesticide chlortoluron. Linearity was also investigated for those compounds which had at least 5 points of the calibration series over the LOQ. These results are displayed in Table 1.

The use of Waters Pesticide Screening Application Solution with UNIFI with larger injection volumes has been shown to be effective in the analysis of trace contaminants in water samples.