CPR to resuscitate polluted produce

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  • Published: Dec 13, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
thumbnail image: CPR to resuscitate polluted produce

Noxious nourishment

Chronic consumption of pesticides is detrimental to the wellbeing of both wildlife and humans. Spraying of toxins, pollutants and carcinogens onto our vegetative produce has allowed their passage to consumers, with consequences arising throughout the food chain. Direct effects on humans, aside from food contamination, include the pollution of drinking water through irrigation and rain-induced run off. Such problems caused by agrarian practices are now receiving attention, with those who disrespect the regulations instated by preventative programs finding themselves subjected to fines or other punishments.

To get the ball rolling, government bodies placed responsibility on scientific researchers as key players in the development of now established chromatographic techniques for pesticide analytics. The use of chromatography, alongside mass spectrometry, has become rudimentary for compound qualification and quantitation. Advances in post-processing procedures are, however, ongoing.

Track the trace

GC/TOFMS is a sensitive technique used to produce accurate, high-resolution data by the separation of volatile samples. Applying GC/TOFMS to pesticide analysis has now become routine.

Hindrances regarding noise, interference and lack of trace level-sensitivity are largely owing to the inherent complexity of matrices being observed. Researchers look to background subtraction algorithms as a method of eliminating these problems to keep false or implied signals off the field. The ‘ion trace detection’ algorithm and ‘unique ion filter’ are among other common processes implemented for more accurate results. Many of these, however, are incapable of accurate analysis of trace samples; signal amplification and complete elimination of interference reduce the sensitivity of such methods.

This season’s best

A group of Chinese chemists, captained by Xiu-Ping Chen, have won ‘Team of the Month’ for developing what they describe to be “a novel post-processing method for data from a high-resolution MS instrument, the Chromatographic Peak Reconstruction (CPR) method, to analyze trace ions in complex matrices.”

Their original technique, capable of pesticide quantification and qualification from recorded GC/TOFMS data, comprises a five-step process. The first involves extraction of chromatograms corresponding to single ions (mass window width ±10 mDa). These are then sorted by scan number to create a mass spectrometry EIC peak table for each target ion. The inversely proportional relationship between adjacent scanned m/z and MS peak intensity provides foundations for the third step: calculation of relative deviations between adjacent m/z values.

Next, Chen and his team estimate “threshold of deviation” values within specific regions of intensity. These define the confidence limits for each suspicious scan-point, allowing for “suspicious scan-point screening”. The points are then modified but, importantly, never removed entirely to avoid distorting the data. Origin 8 software is implemented in the final stage, to reconstruct an EIC peak for each target ion. The area of this peak allows for both qualitative and quantitative analysis of target analytes.

Auspicious apples

Application of Chen’s new technique to GC/TOFMS data from an apple sample spiked with 118 pesticides yielded positive results. At 5 ppb, CPR processing flagged up 107 of these analytes, within the stipulated limits. Although low ion signals and inadequate confidence values left 11 unconfirmed, the team still claim the algorithm offers “higher accuracy, resolution and detection sensitivity” than previous methods.

Of the 107 successfully identified pesticides, CPR was applied to 10 random samples at 5 ppb. The relative observed error values were less than ±1.55%, down from ±2.23% upon application of Gaussian function and 2.29% from raw data. The Chinese team’s method clearly has advantages, but can it stand up to high levels of scrutiny?

Setbacks in the technique have been identified. The chemists from Shanghai University stated that, “the reconstructed results of tolclofos-methyl, delta-BHC, quinalphos, and tebufenpyrad showed some negative aspects of the CPR algorithm since incorrect assignment may have been made.”

Limitations aside, Chen’s novel post-processing tool, CPR, could provide significant improvements in the detection of pesticides at trace level in complex matrices.

Related Links

Rapid Communications in Mass Spectrometry, 2016, 30, 2655-2663. Chen et al. Chromatographic peak reconstruction algorithm to improve qualitative and quantitative analysis of trace pesticide residues.

Pesticide regulations – Health and Safety Executive, UK

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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