An Introduction to Gas Chromatography
The increased popularity of forensic science has exposed the public to tools of analysis previously unknown outside of the scientific community. A wonderful example is gas chromatography (GC). GC is a technique which has been in use since the 1950’s, and is extremely useful when analyzing volatile compounds. (Volatile means that the substance can be easily converted to a gas.) It is employed in a number of industries, including (but certainly not limited to) the following few examples. The forensics world uses GC in the analysis of explosives. Environmental agencies use GC to quantify pollution levels in our air, water and soil. The modern fragrance industry relies heavily on GC and, to a lesser extent, so does much of the food and beverage industry.
What exactly is gas chromatography though? The first half of the name is easy. It is a technique that deals with gas. Chromatography is a fancy word that put simply means separation. In fact, most people get a chance to experience a form of chromatography at a young age in school. Think back to an experiment with a strip of paper with a dot or line of marker/pen ink near the bottom. The paper is suspended over an alcohol solution, with just the bottom submerged. Over time, the liquid climbs the paper, separating the ink into separate bands of color. That technique is paper chromatography, and while the apparatus is different, the concept is the same. In GC, a mixture of gasses is separated into separate segments, ideally each consisting of a single gas which can then be identified.
Separating gasses might sound like a daunting task. After all, gasses mix easily, you usually can’t even see them, and they don’t stay put. Surprisingly, GC turns out to be one of the simplest techniques out there. For the most part, a gas chromatograph (the name of the instrument) is an automated machine that handles samples with ease. The only real tasks that the analyst is responsible for is sample preparation and interpretation of the data. Let’s take a look at the steps that happen in between.
Whatever the sample may be – a vial of air, a volatile liquid, or a glob of hazardous sludge that gives off fumes – a small sample is required. The GC (the name refers to the instrument as well as to the technique) takes a small amount of sample by syringe and injects it into a heated chamber that ensures the sample is all in the gas state and at a known starting temperature. A small amount of that sample is passed on to the column. (The amount will vary depending on the concentration of the sample, but in general, a very small amount is required.)
The aforementioned column is where all the separation work takes place. The “column” is usually a thin glass tube, with a coating outside to prevent it from breaking. While the setup can vary, a typical column is several meters long, coiled and hung in a temperature controlled oven. The column has a very small diameter, may have a coating inside, and may be hollow or packed with small particles. A constant stream of an inert carrier gas (commonly helium) flows through the column, and carries the sample through.
A few factors play a part in separating the gasses. Boiling temperature and size are usually the key variables. More volatile (lower boiling point) gasses tend to move more rapidly through the column, as do smaller gasses. (Size is especially important in a packed column, which is filled with small particles that impede flow.) The oven temperature can be held constant or varied (a process called ramping) to enhance the separation based on volatility. Chemical interactions can also play a part in how quickly a certain gas passes through the column. For example, if some of the gasses have polar groups (like alcohols and amines) they will be attracted to a polar coating on the walls of the column. This slows their passage through the column, separating them from non-polar gasses that flow through unrestricted. Ideally, by the time the gasses reach the far end of the column, they have been separated into individual bands. (Sometimes a bit of optimization of factors like temperature, carrier gas flow rate, and column type are needed to make this happen.)
As the separated gasses pass from the column, they enter a detector. There are a number of different detector types (see below), but they share a common purpose. They measure the amount of sample passing through them at a given time. This measurement is usually displayed as a graph of signal strength (proportional to the amount of gas detected) versus time. Looking at such a chromatogram, you can see a flat baseline (where nothing is detected) interrupted by bell-shaped peaks that indicate when a particular gas “eluted” (came out). That time is known as the “retention time” (RT), and is highly useful in identifying what chemical a particular peak belongs to. On a given GC system, RT is constant for each gas, so, for instance, acetone will always show up at exactly the same time, perhaps 4.3 minutes, while vanillin (vanilla) might take 7.1 minutes. Libraries of common compounds (and their retention times) make it easy to identify known samples. For an unknown gas, retention time can give some hints about the properties of that gas, but further testing is necessary to identify it. For this reason, GC is sometimes coupled with another technique called mass spectrometry (MS) which provides a more precise chemical identification.
(A Few Detector Types)
As I mentioned, detectors come in many flavors. Aside from MS, which is the most expensive option out there, here are a few of the most common:
FID – Flame Ionization Detector
A hydrogen flame burns sample gasses as they exit the column, providing a detectable signal. While widely used, this detector is limited only to combustible gasses. (Water vapor gets through undetected.)
TCD – Thermal Conductivity Detector
A thermocouple measures how well the emerging gas stream conducts heat. Any gas with a different thermal conductivity from the carrier gas will show a response.
ECD – Electrical Conductivity Detector
Similar to the TCD, this detector measures how well the gas stream conducts an electrical current.
RI – Refractive Index
RI measures how much the gas stream refracts (bends) a beam of light. If a gas has a different RI than the carrier gas, it is detected.
To analyze the peaks in the chromatogram, the size of the peak is compared against a standard – another sample with known concentrations of each gas of interest. Computers can do this automatically, so aside from actually preparing the standard, there is little for the analyst to do other than to make sure the computer did it right.
As you might imagine then, GC is a pretty quick test to wrap up. Since the samples are minute and gaseous, there is minimal cleaning. Most samples are finished in 5 to 30 minutes, and computer processing goes quickly. Such ease and speed helps to maintain the popularity of GC, its remarkable utility notwithstanding. On a small side note, despite how quick and painless GC really is, it is not instantaneous. When you find yourself watching various crime scene dramas on television and the analyst rattles off results moments after injecting the sample, that’s just Hollywood, not science. Fifteen minutes later though – that’s doable.