The case of the constipated system
As the director of an analytical services laboratory, my job is to solve customer problems. Usually our own products are involved i n some way, but occasionally a customer will ask us to lend expertise when they have a problem not related to our fluids.
One morning, I received a call from the senior engineer at a company that produces power steering units. They had recently tested a number of prototype units, and after the test was completed, some brown grease was found inside several of them. Since the power steering fluid was produced by a major oil company, a sample of the grease had been sent to them for analysis. Their lab identified it as hydrogenated castor oil, and since their power steering fluid didn’t contain this compound, they stated that their oil was not the source of the residue.
Our now thoroughly confused customer was left with the job of finding out what had caused the grease formation — so I agreed to help him try to solve the problem. When samples of the grease and the power steering fluid arrived in my lab, we scanned them with our Fourier Transform Infrared Spectroscopy (FTIR). The infrared spectrum of the power steering fluid indicated that it was a mixture of mineral oil and an ester (possibly vegetable oil). The spectrum of the grease showed that it contained a lot of ester. In fact, the spectrum did indeed resemble a spectrum of hydrogenated castor oil, but with some important differences, so further analysis was warranted.
We used column chromatography to separate the grease mixture into its component parts, and then each individual fraction was analyzed. Energy Dispersive X-Ray fluorescence spectroscopy was also used to find out what elements were present, and surprisingly, a large amount of chlorine was detected. After reviewing all the data, we finally concluded that the grease was a mixture of a high molecular weight chlorinated ester, a polyester-polystyrene copolymer, and some mineral oil.
We now knew the chemical composition of the grease, but still didn’t know where it had originated. I contacted our customer again and discussed possible sources of chlorine. After some investigation, he found out that the hoses on the power steering units were made of chlorinated polymer. It looked like the grease had formed when the hose material had begun to break down, probably because the power steering fluid had overheated during the test. He was happy with the information, and changes were made in their testing program to prevent it from happening again.
The question remained as to why the grease had been incorrectly identified as hydrogenated castor oil by the power steering fluid company’s laboratory. FTIR is a powerful tool for identifying an infinite number of possible chemical compounds. Every chemical has a distinctive spectrum, but when mixed with other chemicals, the resulting spectrum shows characteristics of each one.
Modern instruments come with databases of spectra, and the operator can easily run a computer search and get matches of similar spectra when dealing with an unknown. However, the database is simply a tool to be used by an experienced chemist to speed up the identification process. The computer will always provide a “hit” list, whether or not the actual compound is really in the database or not, and it takes years to become proficient in interpreting infrared spectra, and knowing when not to rely on the computer.
In this case, it looked like an inexperienced chemist had fallen into the trap of matching the grease spectrum with the first hit in the list of matches from the computer database instead of doing the complete analysis that was needed. A little knowledge can be a dangerous thing.
Robert Johnston is group leader, Analytical Services for Houghton International, Valley Forge, Pa. Contact Johnston at (610) 666-4114 or via email at [email protected].