Chromotography is a technique used to separate out a mixture
of chemicals. There are two essential parts of chromatography. The first is a
mobile phase, the gas or liquid that moves over the surface of the
stationary phase. The stationary phase is the phase that stays in place during
the separation. An easy way to think of chromatography is as a race. All the
chemicals start out at the same position, however due to their different
"athletic ability" or ability to travel over the stationary solid,
they begin to spread out. As the liquid or gas moves over the solid phase, some
of the molecules get pulled into the stationary phase for a short amount of
time before being let back into the liquid or gas phase. Each of the different
chemicals in the mixture experience this interaction with the stationary phase
in different ways. When the analytes
pass through the column they are separated by two factors, volatility, the tendency of a compound to vaporize and
polarity, tendency of a molecule to be attracted or repelled by electrical charges. More volatile and less polar samples travel through the column quickly. The below video is an awesome animation of what happens in chromatography and shows you what I mean by "racing." Skip to 32 seconds in.
In gas chromatography, we start with a liquid sample that is
vaporized before it is transferred into the column. The column is a very thin
capillary tube anywhere from 15-60 meters long, that is coiled inside
the oven of the instrument. The oven is heated before the sample is injected
and is usually programmed to raise the temperature a certain amount every so
often. The reason for this is the less volatile chemicals need an extra push to
make it through the column in a reasonable amount of time.
As the compounds elute, they are quantified
by the detector. Retention time is the time it took for an analyte to elute
from the column. A chromatogram is the plot of the intensity and
the retention time. For a specific column and program, each analyte will have a
comparable retention time which can be used to identify the compounds in the
sample.
When paired with mass spectrometry, this can be an extremely
powerful tool in analytical chemistry. A mass spectrometer ionizes molecules, or
breaks them apart into smaller pieces. The ions, small charged pieces of molecules,
are then accelerated so they have the same kinetic energy and then deflected by
a magnetic field. The lighter a chunk of a molecule is, the more it is deflected.
These ions are then detected by the machine. Each organic molecule has a
different mass spectra due to different fragmentation, which allows the ability to
identify them in unknown samples.
When paired with gas chromatography, we can obtain mass spectras for each analyte that gets separated. It is then possible to definitively identify each compound in a sample. Although mass spectra seem like they would be tough to analyze, in analytical chemistry it is fairly simple. It only takes a fraction of a second to do a library search to match the spectra of the unknown with a known spectra. Then it's as simple as matching and confirming peaks.
GCMS spectra of Cannabinoids(Tetrahydrocannabinol, Cannabidiol, Cannabinol), the active ingredients in marijuana.
Above is a chromatogram and spectra I obtained of a cannabinoid sample using GCMS. There are three major peaks, being tetrahydrocannabinol, cannabidiol, and cannabinol, which as we can see separated successfully. The correlating mass spectra for the middle peak at 8.152 is below it. Looks crazy right? It really isn't so bad. Below are the sample mass spectra and the mass spectra from the library. All of the major peaks we expect to see for THC are there, so it is a positive match. See, not so bad.
Mass spectra of the middle peak (8.152) above the library search that matched the spectra. A match for delta-9-tetrahydrocannabinol or THC
Hopefully you're leaving with a better understanding of one way chemists identify chemicals and if I'm lucky, maybe you'll even agree with me on how awesome this technology is.
In chemistry, strides are always being made to make
technology better, faster, smaller, and stronger. Analytical chemistry can tell
us so much about the world we live in across a variety of different fields. Due to the heavy hollywoodization of the forensic science in television crime dramas, people have unrealistic expectations of the field.
This was deemed the "CSI effect," defined by criminologist Monica
Robbers as "the phenomenon in which jurors hold unrealistic expectations
of forensic evidence and investigation techniques...” The backlash of this
effect is huge in the justice system. On TV analytical results take minutes to
run, as a result the general population have unrealistic expectations. They
want lab results and an arrest to be made within days. In the real world, these
results may take weeks to acquire. The effects in the court room are
detrimental as well. A study in 2008 showed that about 60% of defense lawyers
and nearly 70% of judges believed that jurors had unrealistic expectations of
forensic science. People are wanting stronger forensic evidence to make a conviction.What may have been a
quick conviction in the old days is now much more likely to become an
acquittal. This created a need to improve technology in the forensic science field.
A recent
technological advancement may just live up to jurors expectations. Fingerprints have always been evidence of great interest for obvious reasons. With
the use of time-of-flight secondary ion imaging mass spectrometry (TOF-SIMS),
fingerprints can be age dated. This would help determine the relevance of a
fingerprint that was found at the crime scene. A development of this nature
would be a game changer in the court room. TOF-SIMS is used to analyze the
surface of solid materials. This method works by looking at the surface
diffusion of biomolecules in the fingerprint over time. Researchers looked at
fingerprints 1, 24, 48, 72, and 96 hours after they had been deposited onto a surface. In the early hours after the fingerprint
are deposited, the ridge patterns are extremely clear. At this time, molecules,
such as palmitic acid, are still along the edge of the ridges, the valleys are
unoccupied. After 24 hours the ridge patterns begin to lose their clarity as
the molecules diffuse into the valley regions. It is possible to estimate the
time it takes for these acids to reach position x. As a result, researchers were able
to look at a fingerprint sample and estimate how old it was.
Age dating of fingerprints could be a monumental advancement in forensic science. However, this new advancement has limits. Currently, the method of age dating of fingerprints is only
useful for prints that are less than 92 hours old, but their next goal in their
research is to expand this timeline to up to 240 hours.
