Originally published at http://www.yearofscience2009.org/ (November 2009) in a month long series of articles about chemistry during the 2009 Year of Science celebration. An original essay by Barbara Belmont. All rights reserved.
 Part Three: Adventures of a Chemical Detective
Before I exited the commercial laboratory business, which was investigative chemical analysis, many projects started out as big a mystery to me as they were to my client. Something - a process, a product, or an effect - isn't the way it's expected to be, and chemistry is involved in some way. My job is to find out the chemical cause of the problem. Over the years, I came to understand the chemistry of many different products, but reaching that understanding is never as easy as suggested on our favorite television detective shows. On television, the detectives and forensic chemists come to quick conclusions about the mysterious substance as the data pours out of the analysis machines, and rush to save the day. Consumers of commercial chemical consulting services have been mislead by that portrayal of scientific problem solving. While we may have our "Eureka" moments, solving the problem requires methodical and thorough investigation. To illustrate, I'm going to share an interesting story about an adhesive failure, and explain what I did to find out the cause of the problem.
The scenario is this: Some very large granite dimension stones had come loose from their concrete setting bed at a newly constructed plaza at the base of a Los Angeles high-rise building. An architectural consultant had recommended gluing the stones back to their setting bed by using epoxy adhesive. Despite pumping hundreds of gallons of epoxy beneath the stones, the stones were still loose, and some of the stones had a gooey substance oozing up between their junctions. On hot days, a very unpleasant odor permeated the air. My company was hired to identify the ooze, identify the source of the odor, and determine why the glue did not work.
My survey of the situation revealed that there was no history of odor in the area until after the epoxy system had been used, and that there was no history of the site being a previous oil drilling field or waste dump. So there was a pretty compelling correlation between the presence of the epoxy system and the odor. But epoxy systems usually don't smell after they've cured, so further investigation was necessary. (Epoxy is a two-part adhesive system. "Part A" is the liquid epoxy resin, and "Part B" is called the curing agent. When the two components are mixed together in the right proportions, they react to form a hard material. )
When the stones were pried up, I saw two conditions:
pools of hard translucent amber substance on top of concave setting bed, with no such substance on the stone
gooey amber substance stuck to the setting bed and the bottom of the stone
Without yet performing any chemical analysis, I recognized the physical traits of two kinds of adhesive failure. Condition one is called "adhesive" failure, where the adhesive doesn't properly stick to the surface. Condition two is called "cohesive" failure, where the adhesive doesn't have enough internal strength to stay in one piece. This site inspection gave a very compelling correlation between gross failure of the epoxy and the looseness of the stones on the setting bed. The epoxy installers blamed the epoxy, claiming it was defective. The contracting company blamed the building maintenance people, claiming that cleaning chemicals were either the cause of the epoxy failure, or that the gooey stuff was maintenance chemicals instead of epoxy. The property owner blamed them all.
I took samples back to the lab, and procured samples of the epoxy systems that had been used, as well as samples of all the maintenance products used by the maintenance people. After inventorying and inspecting everything, I prepared what are called "exemplar samples". This means I mixed the epoxy systems exactly the way they should be mixed, and let them cure according to the manufacturer's directions. Then I used my favorite analytical instrumental method, called infrared spectroscopy. In this technique, a beam of infrared light is passed through the sample. Certain classes of chemicals absorb infrared energy at specific wavelengths. A map the response of the sample over a range of wavelengths is called an infrared spectrum. While the technique can't positively identify a substance, it is quite useful to use like to classifying, characterizing, and comparing chemicals. I compared the site samples with my exemplar samples, trying to find any differences between them that would give me clues as to the cause of the problem.
The first thing I could tell was that the hard translucent amber substance (from Condition One) was chemically the same as the exemplar. This substance was in fact properly cured epoxy. It just hadn't stuck to the bottom of the stone. Close inspection of the stone samples revealed a layer of powdery material on the bottom - where the epoxy had never touched, or no powdery material at all - where the epoxy had once been, but did not stick. This physical condition told me that in some places, the epoxy never got to touch the bottom of the stone (where the powder was). And where the epoxy had touched the stone, it could not stick, because the powdery substance prevented adequate contact.
I scraped some of the powdery substance off of the stone, and applied some moist pH paper to it. The powder was very alkaline. When treated with dilute acid, the powder fizzed a little. This meant that carbonate was present, among other things. I analyzed the powder for its elements using a technique called Energy Dispersive X-Ray Spectroscopy. In this technique, a beam of electrons is used to cause elements to fluoresce x-rays. Each element has its own typical x-ray fluorescence spectrum. My analysis showed that the powder contained calcium, silicon, oxygen, and carbon. Further analysis by x-ray diffraction showed calcium silicate (cured cement), silica (sand), and calcium carbonate (hard water residue). In x-ray diffraction, the crystal structure of a chemical compound causes x-rays to bend at very specific angles. This "powder pattern", as it's called, is used to identify the chemical compound.
That part was relatively straightforward. One clue led to another, to the logical conclusion that the failure in this area was due to contaminated surface preventing good contact between the epoxy and stone. In addition, observations about the setting bed being concave demonstrated that the concrete had not been properly leveled before laying the stone.
But the gooey substance of Condition Two was another story. Its infrared spectrum was very similar to the epoxy exemplar, with some differences so subtle that I didn't notice them right away. What I did notice right away was that this gooey substance had a very strong odor, quite similar to the Part B curing agent used in the epoxy system, as well as similar to the odor in the air at the site. This odor led me to hypothesize that the gooey substance might represent a mis-proportioned mixture of Part A and Part B. Normally, the epoxy system was mixed two volumes of Part A to one volume of Part B. To test my hypothesis, I mixed Parts A and B in varying proportions, allowed them to cure, then observed their properties and measured their infrared spectra. Methodically testing each version of the mixture helped me see the nuanced differences in their spectra. It turned out that mixing one volume of Part A with two volumes of Part B formed a gooey smelly substance that had an infrared spectrum identical to that of my sample!
This was my "Eureka" moment, because I now had proof that someone had incorrectly mixed this product - at least for some of the operation. I advised my client to question the epoxy installers directly, rather than speak to their supervisors, to find out what might have happened on the job site. Guess what was found out! The epoxy installers used a special machine to meter, mix, and pump the components under the stone surface. One day, after several hours of work, they discovered that they had reversed the connections. They hadn't told anybody about the problem at the time, which is why their supervisors and the general contractor had sworn that the epoxy had been installed correctly.
Unfortunately, the only way to remedy the problem at the plaza was to demolish it and rebuild it. But thanks to my investigation, the causes of the problem were determined to be poor workmanship (the uneven setting bed), improper preparation of surface (the powder stuck to the bottom of the stone), and mis-proportioning of the epoxy system. A settlement in favor of my client was reached the morning the matter went to jury trial.
And I got to solve a puzzle, explain chemistry, and help people!