Herbicides volatilization measured by the relaxed eddy-accumulation technique using two trapping media

Abstract

Two relaxed eddy-accumulation (REA) systems were developed for measuring the flux of agrochemicals; one using polyurethane foam (PUF) plugs as a trapping medium with an air sampling flow rate of 60 1 min −1, and the other utilizing mini-tubes (MT) containing Tenax-TA resin and aspirated at 100–300 ml min −1. Analysis of agrochemicals trapped on PUF is time consuming in that Soxhlet extraction followed by extract concentration is required prior to quantification by gas chromatography (GC). In contrast, the MT technology provides enhanced sensitivity through direct thermal desorption of the entire trapped sample from the Tenax-TA resin onto a GC column. The objective of the study was to determine if the simpler-to-use MT technology was as effective as PUF, which has been traditionally used in the measurement of vapour losses of agrochemicals. The vapour fluxes of two soil-applied herbicides (170 mg active ingredient (a.i.) m −2 Z triallate and 115 mg a.i. m −2 trifluralin) were compared using PUF and Tenax-TA trapping media in the corresponding REA measuring systems over four days during autumn 1992. The aqueous emulsion of trifluralin and triallate was not soil incorporated as recommended, in order that a wider range of vapour fluxes could be studied. Vapour fluxes of both herbicides were highest on the day of application and were approximately 0.64 μg m −2 s −1 for trifluralin and 0.86 μg m −2 s −1 for triallate. On the following day, herbicide fluxes decreased by a factor of three. The temporal patterns of the fluxes obtained using the two trapping media were similar, but the magnitude of the flux maxima varied slightly. This was attributed to adsorption/desorption of the herbicides on the walls of the inlet tubing of the MT system, even though the inlet tubes were heated to minimize herbicide adsorption. The MT system was more sensitive to adsorption because the herbicide amounts trapped on the resin are much smaller. Cumulative fluxes from the two media agreed within 7% for triallate whereas a 22% higher flux was obtained with the MT system for trifluralin. Use of the REA system utilizing the MT technology with Tenax-TA resin is promising for measuring agrochemical fluxes. However, since the amounts of agrochemicals sampled are approximately 500 times less than those sampled using PUF, an improved system for controlling the temperature of the inlet tubing is required to minimize adsorption/desorption of the agrochemicals.