He placed the soil in a centrifuge tube and added water.

'I use deionized water in a ratio of two to one.'

The tube went onto a vortex for one minute to mix the solution. Then he transferred it to a centrifuge and closed the lid.

'The temperature inside is held at five degrees. I'l centrifuge for forty minutes, then filter the sample to remove any remaining microorganisms. After that it's simple. I'l check the pH, acidify with a formic acid solution, and pop the thing into the gas chromatography.'

'How about a crash course.'

Laslo finished adjusting settings, then gestured to a desk and we both sat.

'O.K. As you know, I'm looking at the products of muscle and fat breakdown caled volatile fatty acids. Are you familiar with the four stages of decomposition?'

Anthropologists and death investigators think of corpses as being in one of four broad stages: fresh, bloated, decayed, or skeletal.

I nodded.

'There's little change in VFAs in a fresh corpse. In the second stage, a body bloats due to anaerobic fermentation, primarily in the gut. This causes skin breakage and the leakage of fermentation byproducts rich in butyric acids.'

'Butyric acids?'

'Volatile fatty acids include forty-one different organic compounds, of which butyric acid is one. Butyric, formic, acetic, propionic, valeric, caproic, and heptanoic are detectable in soil solution because they're soluble in water. Two of them, formic and acetic, are too abundant in nature to be of much use.'

'Formic is the one that causes pain from ant bites, right?'

'That's the one. Caproic and heptanoic are only found in significant amounts during the colder months. Propionic, butyric, and valeric are my boys. They're released from a decomposing corpse and deposited in soil solutions in specific ratios.'

I felt like I was back in Biochem 101.

'Since butyric and propionic acids are formed by anaerobic bacteria in the gut, the levels are high during the bloat stage.'

I nodded.

'Later, during decay, aerobic bacteria join the act.'

'So at stage three there's a surge in al VFA formation.'

'Yes. Then there's a rapid fal-off at the onset of stage four.'

'No flesh, no bacteria.'

'The soup kitchen closes.'

Behind us the centrifuge hummed softly.

'I've also found that al fatty acid values are highest just after maggot migration.'

'When larvae abandon the corpse to pupate.'

'Yes. Until that point the presence of the insects tends to restrict the flow of body fluids into the soil.'

'Doesn't pupation occur at approximately four hundred ADD?' ADD stands for 'accumulated degree days,' a figure calculated by summing average daily temperatures.

'With some variation. Which brings up a good point. VFA production is temperature dependent. That's why it can be used to determine time since death.'

'Because a corpse wil produce the same ratios of propionic, butyric, and valeric acids for any given accumulated degree days.'