Detection of agricultural chemical residues on plant leaf surfaces with secondary ion mass spectrometry

The high sensitivity of secondary ion mass spectrometry (SIMS) for materials residing on a sample surface makes it well suited for detecting the presence of chemicals sprayed onto surfaces. To investigate the possibility that agricultural chemicals could be detected by direct analysis of a plant leaf surface, two common agricultural chemicals, 2,4-D and malathion, were sprayed onto separate plants at label-recommended concentrations (6.7 p.p.t. and 5.0 p.p.t., respectively) and individual plant leaves were analyzed using SIMS over a period of up to 9 weeks after spraying. The plants were located in a greenhouse and were not exposed to rain (or overhead watering) during the experiment. Characteristic negative fragment ions of the chemicals were used as marker peaks; the molecular ions were not obtained. The 2,4-D was easily detectable during the first few days after spraying but only marginally detectable thereafter. The malathion was easily detected up to 9 weeks after spraying. The minimum detectable limit for malathion on this particular plant leaf type was determined to be ∼10 ng. The results indicate that SIMS may be a useful analysis technique when screening for residual chemicals on plant surfaces.