The Empty Chair by Jeffery Deaver

this rule.
    He shook the shoe, peered into it. "Looks like there's gravel or something inside."
    "Hell, I didn't have Amelia ask for sterile examining boards." Rhyme looked around the room. "See that magazine there? People ?"
    Ben picked it up. Shook his head. "It's three weeks old."
    "I don't care how current the stories about Leonardo DiCaprio's love life are," Rhyme muttered. "Pull out the subscription inserts inside . . . Don't you hate those things? But they're good for us – they come off the printing press nice and sterile, so they make good mini-examining boards."
    Ben did as instructed and poured the dirt and stones onto the card.
    "Put a sample in the microscope and let me take a look at it." Rhyme wheeled close to the table but the ocular piece was a few inches too high for him. "Damn."
    Ben assessed the problem. "Maybe I could hold it for you to look in."
    Rhyme gave a faint laugh. "It weighs close to thirty pounds. No, we'll have to find a – "
    But the zoologist picked up the instrument and, with his massive arms, held the 'scope very steady. Rhyme couldn't, of course, turn the focusing knobs but he saw enough to give him an idea of what the evidence was. "Limestone chips and dust. Would that've come from Blackwater Landing?"
    "Uhm," Ben said slowly, "doubt it. Mostly just mud and stuff."
    "Run a sample of it through the chromatograph. I want to see what else is in there."
    Ben mounted the sample inside and pressed the test button.
    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 usually 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