The pyrethroid class of pesticides is widely used around the world for the control of insects in many areas, including forestry, animal care, agriculture, public health care and households. The EPA estimates that they are the active ingredients in more than 3500 registered products and their use is increasing as the more acutely toxic organophosphate-based pesticides are slowly being replaced.
That is not to say that pyrethroids are non-toxic. Their insecticidal effects are a result of altering the nerve function of the pests, which causes paralysis and leads to eventual death. In humans, the compounds act in a similar manner, affecting nerve and brain function to cause symptoms such as headaches, dizziness, nausea, convulsions and loss of consciousness. The effects can last several days.
In practice, pyrethroids are designed to degrade under ultraviolet light within a few days of application, so residues on plant surfaces or soil will not persist for very long. This was seen as an improvement over the related pyrethrins, from which they are derived. Pyrethrins are natural compounds originally extracted from the chrysanthemum but they are unstable under sunlight and quickly become inactive.
In soil, pyrethroids are retained by sorption and poor solubility in water and it is important to be able to estimate residual levels from an agricultural and environmental point of view. For samples containing trace amounts of pesticides, the obvious problem is extraction and preconcentration of the target compounds so that they can be detected during analysis. In general, this time-consuming step is carried out in several steps but Slovakian scientists have developed a single-step procedure that combines extraction and preconcentration and links it online with HPLC detection.
Milan Hutta, Maria Chalanyova and Martin Pagac from Comenius University, Bratislava, described their new procedure in the Journal of Separation Science for the recovery and analysis of three pyrethroids - kadethrin, cypermethrin and permethrin. Solid-liquid extraction, SPE and RP-HPLC were combined in series and the method was optimised using spiked soil samples.
In the first stage, an adsorbent was placed at the bottom of a short glass column and up to 200 mg of contaminated soil was loaded on top. The adsorbent had a dual role, acting as a sieve to retain the fine soil particles and an adsorbent to trap the pesticides. Silica was the preferred adsorbent of three tested because it prevented interfering coelutions and provided the best recoveries. In a flow-through extraction procedure, the best solvent was found to be a mixture of methanol and aqueous citric acid.
The liquid flow was directed towards an SPE column containing a phenyl-based adsorbent which trapped the three pyrethroids while the liquid flow was directed to waste. They were back-flushed with aqueous methanol to the HPLC system operating in large-volume injection mode, designed for low levels of analytes. Following separation on a C18 column, the compounds were monitored by UV detection at 270 nm for kadethrin and 235 nm for the others.
This approach led to recoveries of 81-84%, 56-59% and 58-63% for kadethrin, cypermethrin and permethrin, respectively. The corresponding detection limits achieved by large-volume injection of 1 mL solution were 4.5, 3.7 and 3.6 ng/mL and for dry soil they were 27, 32 and 29 ng/g, respectively.
The researchers described this procedure as "a solid sampling HPLC technique" which has rarely been implemented in the reported literature. They intend to refine it for the analysis of other types of compounds in soil, sediments and plants and declared that it will allow the analysis of soil samples that were collected after the suppression of a mosquito invasion in Bratislava in 2009.
The method will be particularly useful for the analysis of small samples such as sections of a soil column or the interface between soil and the roots of a plant. It could also be of assistance in environmental forensics, to measure small concentrations of pollutants across a contaminated site.
Related links:
* Journal of Separation Science 2010, 33, 134-142: "On-line flow-through extraction-preconcentration-large volume injection-RP LC for trace determination of pyrethroids in Slovak soil"
No comments:
Post a Comment