Chromatography is a criminalist's dream tool. Developed just after the turn of the century by a Russian botanist though not much used until the 1930s, the device analyzes compounds such as foods, drugs, blood and trace elements and isolates the pure elements in them. There are a half-dozen variations on the process but the most common type used in forensic science is the gas chromatograph, which burns a sample of evidence. The resulting vapors are then separated to indicate the component substances that make up the sample. In a forensic science lab the chromatograph is us
ually connected to a mass spectrometer, which can identify many of the substances specifically.
The gas chromatograph will only work with materials that can be vaporized--burned--at relatively low temperatures. The limestone wouldn't ignite, of course. But Rhyme wasn't interested in the rock; he was interested in what trace materials had adhered to the dirt and gravel. This would narrow down more specifically the places Garrett had been.
"It'll take a little while," Rhyme said. "While we're waiting let's look at the dirt in the treads of Garrett's shoe. I tell you, Ben, I love treads. Shoes, and tires too. They're like sponges. Remember that."
"Yessir. I will, sir."
"Dig some out and let's see if it comes from someplace different from Blackwater Landing."
Ben scraped the dirt onto another subscription card, which he held in front of Rhyme, who examined it carefully. As a forensic scientist, he knew the importance of dirt. It sticks to clothes, it leaves trails like Hansel's and Gretel's bread crumbs to and from a perp's house and it links criminal and crime scene as if they were shackled together. There are approximately 1,100 different shades of soil and if a sample from a crime scene is the identical color to the dirt in the perp's backyard the odds are good that the perp was there. Similarity in the composition of the soils can bolster the connection too. Locard, the great French criminalist, developed a forensics principle named after him, which holds that in every crime there is always some transfer between the perpetrator and the victim or the crime scene. Rhyme had found that, second to blood in the case of an invasive homicide or assault, dirt is the substance most often transferred.
However, the problem with dirt as evidence is that it's too prevalent. In order for it to have any meaning forensically a bit of dirt whose source might be the criminal must be different from the dirt found naturally at the crime scene.
The first step in dirt analysis is to check known soil from the scene--an exemplar--against the sample the criminalist believes came from the perp.
Rhyme explained this to Ben and the big man picked up one bag of dirt, which Sachs had marked Exemplar soil--Blackwater Landing, along with the date and time of collection. There was also a notation in a hand that was not Sachs's. Collected by Deputy J. Corn. Rhyme pictured the young deputy eagerly scurrying off to do her bidding. Ben poured some of this dirt onto a third subscription card. He set it beside the dirt he'd dug out of Garrett's treads. "How do we compare them?" the young man asked, looking over the instruments.
"Your eyes."
"But--"
"Just look at them. See if the color of the unknown sample is different from the color of the known."
"How do I do that?"
Rhyme forced himself to answer calmly. "You just look at them."
Ben stared at one pile, then the other.
Back again. Once more.
And then once again.
Come on, come on ... it isn't that tricky. Rhyme struggled to be patient. One of the hardest things in the world for him.
"What do you see?" Rhyme asked. "Is the dirt from the two scenes different?"
"Well, I can't exactly tell, sir. I think one's lighter."
"'Scope them in the comparison."
Ben mounted the samples in a comparison microscope and looked through the eyepieces. "I'm not sure. Hard to say. I guess ... maybe there is some difference."
"Let me see."
Once again the massive muscles held the large microscope steady and Rhyme peered into the eyepieces. "Definitely different from the known," Rhyme said. "Lighter colored. And it has more crystal in it. More granite and clay and different types of vegetation. So it's not from Blackwater Landing. ... If we're lucky it came from his hidey-hole."
A faint smile crossed Ben's lips, the first Rhyme had seen.
"What?"
"Oh, well, that's what we call the cave a moray eel takes for his home ..." The young man's smile vanished as Rhyme's stare told him that this was not the time or place for anecdotes.
The criminalist said, "When you get the results of the limestone on the chromatograph run the dirt from the treads."
"Yessir."