the value drops below that critical value although a specific sampling method is described here other methods such as an optical probe inserted in the process or spectral acquisition at elevated temperatures could also be used with suitable calibration similarly other common quantitation techniques such as quadratic baseline calculations and peak area measurements could also be used a hydroisomerized fischer tropsch diesel fuel nominally boiling from 250 700 u00b0 f was spiked successively with 20 80 and 2000 ppm hexanoic acid the mid infrared spectra were measured using a 1 mm pathlength cell and 2 cm 1 spectral resolution a linear baseline correction drawn between 1755 cm 1 and 1685 cm 1 was used other cell pathlengths spectral resolutions and baseline corrections could be used the peak absorbance was taken to be the highest absorbance value in the 1711 1715 cm 1 range the absorbance value reported was determined by measuring the absorbance at the peak maximum relative to the baseline absorbance value at the frequency the linear correlation between hexanoic acid concentration and the absorbance at 1713 cm 1 is shown in fig2 the following example demonstrates that monitoring the ir absorbance at 1713 cm 1 is useful for predicting the corrosivity of that fuel fuel corrosivity was measured using the standard cu strip corrosion test astm d130 with the following modifications 1 the cu strip was weighed both before and after the experiment to detect any weight loss from the coupon due to corrosion 2 icp analysis was performed on the used fuel after the test to detect dissolved corroded cu in solution 3 the test was run at 100.