The Economist - USA (2020-02-01)

The EconomistFebruary 1st 2020 Science & technology 69

1

S

ince thefirst use of fingerprints to

identify and convict a murderer, in 1892,

detectives the world over have come to rely

heavily on print evidence to build their

cases against suspects. One limit on the

value of fingerprints, though, is that it is

hard to work out how old they are. This is a

particular problem when a crime is com-

mitted somewhere that a suspect frequent-

ly and legitimately visits, such as a place of

work. In this case exactly when a print was

made might be crucial to establishing guilt

or innocence. But it is information that sci-

ence has, until now, been unable to provide

with any accuracy.

As they write in Analytical Chemistry,

Paige Hinners and Young Jin Lee of Iowa

State University believe they can remedy

this state of affairs. They knew from work

conducted by other laboratories that the

triglyceride oils contained in fingerprints

change by oxidation over the course of

time. That provides an obvious way to date

prints. The problem is that the techniques

which have been applied to analyse these

oils are able to distinguish age only

crudely. In practice, they can determine

whether or not a print is over a week old,

but nothing else.

Dr Hinners and Dr Lee wondered if

higher precision could be obtained by

thinking a bit more about oxidation. Oxy-

gen molecules in the air come in two vari-

eties. Most have a pair of atoms but some,

known as ozone, have three. Though far

rarer than diatomic oxygen, ozone is more

reactive and also reacts in ways different

from those of its two-atomed cousin. The

two researchers therefore decided to focus

their attentions on ozonolysis, as triatomic

oxidation is known.

Triglycerides, as their name suggests,

are three-tailed molecules. Each tail is a

chain of carbon atoms, with hydrogen at-

oms bonded to the carbons. The chains are

held together by bonds between the carbon

atoms. These are of two varieties, known as

single and double bonds. Single bonds are,

in chemistry-speak, saturated, and double

bonds unsaturated. By extension, mole-

cules with one or more double bonds in

them are also referred to as unsaturated,

while those with only single bonds are

called saturated.

Unsaturated bonds are more reactive,

and it is here that ozonolysis does its work.

Ozone breaks up triglycerides at their dou-

ble bonds, with one or more of the ozone’s

oxygen atoms becoming attached to the

carbon chain, to create new chemical spe-

cies. In principle, this should result in a

gradual loss of unsaturated triglycerides

and a concomitant rise in the reaction pro-

ducts of ozonolysis.

And that, in practice, is what Dr Hinners

and Dr Lee found. They asked three volun-

teers to leave their prints on a number of

glass slides. They then tested some of those

slides immediately, using a sensitive ana-

lytical technique called matrix-assisted la-

ser desorption/ionisation (maldi) to pro-

duce a spectrum of the prints’ chemical

contents. The other slides they left exposed

to the air for between one and seven days,

testing them at regular intervals.

As they suspected, as time progressed

the ratio of saturated to unsaturated tri-

glycerides in a sample rose, and so did the

quantities of two characteristic products of

ozonolysis—aldehydes and Criegee ions.

On the basis of what is, admittedly, a small

sample, Dr Hinners and Dr Lee therefore

think that maldianalysis of fingerprint

residues should prove accurate enough to

date to within 24 hours when a fingerprint

under a week old was made—and thus

whether it is associated with a crime tem-

porally, as well as spatially. 7

Fingerprints can now be dated to

within a day of when they were made

Forensic science

Whendunnit?

W

hen scanning for emissions from a

mud volcano in western Turkmenis-

tan in January 2019, a satellite called Claire

came across a large plume of methane

drifting across the landscape. The dis-

charge appeared to originate from a gas

pipeline at the nearby Korpezhe oil and

gasfield. Two more large plumes were also

spotted in the area, including one from a

compressor station. The company operat-

ing the satellite, ghgSat, based in Montre-

al, passed details via diplomats to officials

in Turkmenistan, and after a few months

the leaks stopped. This largely unknown

incident illustrates two things: that satel-

lites can play an important role in spotting

leaks of greenhouse gases and, rather wor-

ryingly, that the extent of such leaks is of-

ten greatly underestimated.

The reason for concern is that although

methane, the main constituent of natural

gas, does not linger in the atmosphere for

anywhere near as long as carbon dioxide

does, it is a far more potent heat-trapping

agent. About a quarter of man-made global

warming is thought to be caused by meth-

ane. And between a fifth and a third of the

methane involved is contributed by the oil

and gas industry.

The data from Clairesuggested the leak

in Turkmenistan had been a big one. To es-

tablish just how big, Daniel Jacob of Har-

vard University and his colleagues studied

the images obtained by this satellite along

with observations made of the area at the

time by the Tropospheric Monitoring In-

strument (tropomi), which is carried by an

atmospheric research satellite operated by

the European Space Agency. The results,

published in Geophysical Research Lettersin

November 2019, concluded that between

February 2018 and January 2019 the three

Methane is an important cause of global warming. Tracking its sources is crucial

Climate change

The methane hunters