Investigations of the effects of surface roughness on the utility of grazing-angle Fourier transform infrared reflection absorption spectroscopy (IRRAS) as a method for quantifying trace contamination of metal surfaces have been extended to acetaminophen, a model active pharmaceutical agent, on 316 stainless steel. The effects are more complicated than for the surfactant sodium dodecyl sulfate (SDS) on stainless steel; they include a strong surface-finish dependence of sensitivity and nonlinear behavior at surface loadings above ∼1–2 μg cm<sup>−2</sup>. Using data from samples in the loading range 0–0.5 μg cm<sup>−2</sup>, unbiased partial least squared calibrations can be readily achieved for individual surface finishes with detection limits of L<sub>D</sub> &ap; 0.15 &mu;g cm<sup>−2</sup>. However, as found for SDS on stainless steel, models built using data from samples of mixed surface roughness are more problematic.

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