The evaluation of a radio gas‐chromatographic system for the detection of trace amounts of labelled insecticides

Abstract

A radio gas‐chromatographic system consisting of a gas Chromatograph, combustion furnace, proportional counter and guard scintillation counter has been studied for the analysis of trace amounts of ¹⁴C‐labelled organophosphorus insecticides. The proportional detector is a concentration sensitive detector requiring strict flow control. Sensitivity depends on the concentration of the quenching gas used. Using ¹⁴C‐ethyl parathion and ¹⁴C‐metamidophos as model compounds a linear range of 3500–5000 was measured. A sensitivity of 0.98 cpm/pci and a detection limit of 24pCi were obtained which allows the detection of 0.33 ng if the specific activity of the insecticide is 21.5 mCi/mMol. The system must be decontaminated in order to achieve optimum detection levels by passage of oxygen through the combustion tube and by periodically injecting the corresponding non‐labelled compound into the column. A solvent effect was found to interfere in the analysis due to the sudden production and expansion of CO₂. This effect was eliminated by choosing column conditions to prevent elution near the solvent. The system was tested by analyzing a mixture of 37 ppb and 23 ppb of ethyl parathion and its oxygen analog in orange juice.

Key words:

• Analytical instrumentation
• analysis
• ¹⁴C‐ethyl parathion
• ¹⁴C‐metamidophos