Tea protection racket: Protectants compensate for matrix effects in pesticides analysis of tea

Matrix effects on pesticides

There are many established methods for analysing pesticides by GC/MS but many of them require manipulation to compensate for interferences caused by other substances in complex matrices, like soil and plants. These interferents are co-extracted with the pesticides and injected onto the gas chromatography column where they become absorbed by the coating and block active sites for the target analytes. Their presence leads to more frequent maintenance of the column and imposes a greater burden on the analyst.

In order to counter the matrix effects, several different measures have been adopted. One of the most common is matrix-matched standardisation, in which an extract of the blank material that does not contain pesticides is used for calibration purposes. This technique is approved for use by the EU but is not permitted in food testing by the US EPA and FDA, partly because it can be difficult to find pure blank samples. Other countermeasures include the use of deuterated internal standards and the standard additions technique.

A further method gaining favour is the use of analyte protectants which interact strongly with the active sites on the column and reduce degradation and adsorption of the analytes. They are added to both the sample extract and the matrix-free calibration standards and have proved to be effective in minimising on-column losses and reducing peak tailing to improve the peak shapes. Their use is not prohibited by the US agencies.

In reported studies to date, the introduction of analyte protectants has been studied in the GC/MS analysis of pesticides in various foodstuffs such as honey, juice, vegetables and herbs. Now, scientists in China have applied the methodology to pesticides in tea, one of the countries principal crops.

Polyol protectants

Guofang Pang and colleagues from Yanshan University, Qinhuangdao, the Chinese Academy of Inspection and Quarantine, Beijing, and Beijing Technology and Business University, began by selecting 186 pesticides which might be used on tea plants. They included organo-phosphorus and organo-chlorine compounds, pyrethroids and carbamates. Three types of tea were analysed with green, oolong and black tea chosen to represent unfermented, partially fermented and fermented tea, respectively.

A solution containing all of the pesticides was used to spike the teas by soaking before they were extracted with acetonitrile and cleaned up by solid-phase extraction. Then the analyte protectants were added. The researchers selected a panel of 11 protectants for testing, based on previous reports. They all contained hydroxy groups and comprised sugars, alkanols, poly(ethylene glycol) and olive oil.

The final solutions were analysed by GC/MS using a (cyanopropyl-phenyl) methylpolysiloxane column for pesticide resolution and selected ion monitoring mode under electron ionisation. Quantitation was achieved by comparing the peak areas with those of heptachlor epoxide which was added as an internal standard.

Pesticide recoveries improved

The matrix effects were measured by comparing the signal responses of each single pesticide in a tea extract with that for a sample prepared in solvent alone, in the absence of tea. These experiments were conducted in the absence of the protectants. The responses were dependent on the type of tea. For green tea, 74% of the pesticides showed a strong signal enhancement in the tea extract, but this figure rose to 94% for the other types of tea. So, it is clear that the effects are widespread.

Then the individual protectants as well as various combinations were checked to see which ones gave the greatest protection against the matrix effects. It turned out that a mixture of triglycerol and D-ribonic acid gamma-lactone was the most powerful, generating recoveries of 70-120% for 96% of the pesticides.

The effects of the analyte protectants on the performance of the GC/MS system in the long term were estimated by running 95 consecutive samples, interspersing solvent standards between tea extracts and matrix-matched standards. The researchers found that the protectants slowed down the decrease in pesticide peak intensities over time, thereby extending column life before it needs to be cleaned.

The use of analyte protectants appears to be an effective way to counteract the matrix effects in tea which affect the GC/MS analysis of pesticides, although the team note that the method has yet to be validated. They also plan to carry out a comparative analysis of tea samples from different areas of China.