An improved solvent-extraction/infrared-analysis technique has been devised to replace an older technique for measuring very small concentrations of nonvolatile residues of industrial hydraulic fluids, oils, and greases on hardware that is required to be cleansed of such residues. The older technique involves solvent extraction of nonvolatile residues followed by gravimetric determination of the quantity of dissolved residues.

The older technique entails two major disadvantages: The first disadvantage is that the solvent is 1,1,2-trichloro-1,2,2-trifluoroethane (also known by the trade name "Freon 113"). This and other chlorofluorocarbons have been found to contribute to depletion of ozone in the upper atmosphere, and therefore the law requires that they be phased out of production and use. The second major disadvantage is that the gravimetric method is susceptible to large errors at the low concentrations of interest in the original application. In terms of areal mass density on the hardware, these concentrations are typically a few milligrams per square foot (1 mg/ft2 = 11 mg/m3); in terms of volume mass densities in solution, these concentrations are typically a few milligrams per liter.

The Infrared Absorbance Spectrum of a perchloroethylene solution can be analyzed to find a peak characteristic of C=O ester groups in small amounts of greasy and oily residues dissolved in the solution.

The improved solvent-extraction/infrared-analysis technique features (1) the use of a less-harmful solvent and of (2) Fourier-transform infrared (FT-IR) analysis of an infrared spectral peak specific to the dissolved residues that one seeks to detect. The solvent in this technique is perchloroethylene; in comparison with 1,1,2-trichloro-1,2,2-trifluoroethane, perchloroethylene is relatively environmentally benign and nontoxic. Perchloroethylene is also less volatile; it boils at a temperature of 121 °C, whereas 1,1,2-trichloro-1,2,2-trifluoroethane boils at 48 °C.

The spectral peak in question is one attributable to ester C=O groups conjugated with C=C groups or aromatic rings in organic molecules. This ester peak is suitable because even at relatively low spectral resolution, it stands out from other spectral peaks attributable to C-H bonds (see upper part of figure) and because the residues of interest contain such ester C=O groups. With higher spectral resolution, the ester peak of a typical residue of interest dissolved in perchloroethylene can be seen to be split into two peaks: one at wave numbers from ~1,753 to ~1,724 cm-1 and one at wave numbers from ~1,724 to ~1,708 cm-1 (see lower part of figure). The splitting has been conjectured to be caused by interactions between the residue and perchloroethylene molecules.

The technique has been tested in experiments on solutions of various industrial hydraulic fluids dissolved in perchloroethylene at known concentrations. The solutions were analyzed on an apparatus that comprised a standard high-intensity infrared source, a Fourier-transform infrared (FT-IR) spectrometer containing a Michelson interferometer, and an HgCdTe photodetector cooled by liquid nitrogen. The output of the spectrometer was digitized and processed by a spectral-analysis computer program. The results of the experiments were interpreted as signifying that the ester spectral peaks can indicate the presence of the residues of interest at the low concentrations of interest, and that at areal concentrations as low as ~1 to ~5 mg/ft2(~11 to ~54 mg/m2), the areas under the two ester spectral peaks are indicative of the concentrations within a factor of 2.

This work was done by Narinder K. Mehta of the University of Puerto Rico for Kennedy Space Center.