Video transcript Have you ever watched a TV show where to catch the criminal they take a sample of the liquid found at the crime scene, run it through this big fancy-looking box, and find out that that liquid was actually some gasoline and are able to suddenly trace the criminal back to the nearest gas station? That fancy looking box you saw is probably something that they were trying to use for gas chromatography, but in real life, gas chromatography doesn't really work like that.
It's a slower process for separating out compounds that have different boiling points and a few other properties. But let's take a step back and figure out how does the gas chromatograph work. First, what you need to have is a place to inject your sample. Even though you'll be injecting it as a liquid, what happens is it gets to this box, and it gets vaporized into a gas.
When it's in a gas, let's say that this particular mixture was made up of two different kinds of gas. I'll show that as some green dots and some orange gas particles. You can't really see these though, because usually the amount you're injecting is so small, on the order of microliters, in fact. And in gas chromatography, we've talked about how the mobile phase is a gas, which means that you need to have an inert carrier gas to push these through.
And it's important that this is inert, because you don't want it to react with whatever it is that you're trying to separate. Once it's passed through that, it'll get heated up and then go through a long tube. In order to make it fit into the box, they usually just coil a long length of tube, and the longer the tube, the better separation you'll get. And once it's finished passing through the tube, there needs to be some kind of detector that picks up how many particles of the green compound were found versus how many particles of the orange compound reached it.
And they'll be reaching the detector at different rates, which I'll explain shortly. From there, the detector will be able to take these signals and display them in a way that you can analyze on your computer. Often what you'll get is something that looks like this. This is known as a chromatogram, which is just a way of saying, a graph for gas chromatography, and we'll also be explaining this later on.
So to recap, we injected our liquid sample, which was vaporized into gas, then it joined up with the stream of inert gas that was already flowing and was pushed onto the long column. But what's going on inside that coiled column? Let's take a closer look. Pretend that this is stretched out, just into a straight column that's horizontal. The resistance within the wire depends upon temperature, which is dependent upon the thermal conductivity of the gas.
TCDs usually employ two detectors, one of which is used as the reference for the carrier gas and the other which monitors the thermal conductivity of the carrier gas and sample mixture. Carrier gases such as helium and hydrogen has very high thermal conductivities so the addition of even a small amount of sample is readily detected. The advantages of TCDs are the ease and simplicity of use, the devices' broad application to inorganic and organic compounds, and the ability of the analyte to be collected after separation and detection.
The greatest drawback of the TCD is the low sensitivity of the instrument in relation to other detection methods, in addition to flow rate and concentration dependency. Figure 13 represents a standard chromatogram produced by a TCD detector. In a standard chromatogram regardless of the type detector, the x-axis is the time and the y-axis is the abundance or the absorbance. From these chromatograms, retention times and the peak heights are determined and used to further investigate the chemical properties or the abundance of the samples.
Electron-capture detectors ECD are highly selective detectors commonly used for detecting environmental samples as the device selectively detects organic compounds with moieties such as halogens, peroxides, quinones and nitro groups and gives little to no response for all other compounds.
Therefore, this method is best suited in applications where traces quantities of chemicals such as pesticides are to be detected and other chromatographic methods are unfeasible. The simplest form of ECD involves gaseous electrons from a radioactive? As the analyte leaves the GC column, it is passed over this? The electrons from the?
In the absence of organic compounds, a constant standing current is maintained between two electrodes. With the addition of organic compounds with electronegative functional groups, the current decreases significantly as the functional groups capture the electrons.
The advantages of ECDs are the high selectivity and sensitivity towards certain organic species with electronegative functional groups. However, the detector has a limited signal range and is potentially dangerous owing to its radioactivity. In addition, the signal-to-noise ratio is limited by radioactive decay and the presence of O2 within the detector. Atomic emission detectors AED , one of the newest addition to the gas chromatographer's arsenal, are element-selective detectors that utilize plasma, which is a partially ionized gas, to atomize all of the elements of a sample and excite their characteristic atomic emission spectra.
AED is an extremely powerful alternative that has a wider applicability due to its based on the detection of atomic emissions. MIP is the most commonly employed form and is used with a positionable diode array to simultaneously monitor the atomic emission spectra of several elements. The components of the Atomic emission detectors include 1 an interface for the incoming capillary GC column to induce plasma chamber,2 a microwave chamber, 3 a cooling system, 4 a diffration grating that associated optics, and 5 a position adjustable photodiode array interfaced to a computer.
Chemiluminescence spectroscopy CS is a process in which both qualitative and quantitative properties can be be determined using the optical emission from excited chemical species.
It is very similar to AES, but the difference is that it utilizes the light emitted from the energized molecules rather than just excited molecules. Moreover, chemiluminescence can occur in either the solution or gas phase whereas AES is designed for gaseous phases. The light source for chemiluminescence comes from the reactions of the chemicals such that it produces light energy as a product.
This light band is used instead of a separate source of light such as a light beam. Like other methods, CS also has its limitations and the major limitation to the detection limits of CS concerns with the use of a photomultiplier tube PMT. A PMT requires a dark current in it to detect the light emitted from the analyte. Another different kind of detector for GC is the photoionization detector which utilizes the properties of chemiluminescence spectroscopy.
Photoionization detector PID is a portable vapor and gas detector that has selective determination of aromatic hydrocarbons, organo-heteroatom, inorganice species and other organic compounds. PID comprise of an ultrviolet lamp to emit photons that are absorbed by the compounds in an ionization chamber exiting from a GC column.
Small fraction of the analyte molecules are actually ionized, nondestructive, allowing confirmation analytical results through other detectors. In addition, PIDs are available in portable hand-held models and in a number of lamp configurations. Results are almost immediate. PID is used commonly to detect VOCs in soil, sediment, air and water, which is often used to detect contaminants in ambient air and soil.
The disavantage of PID is unable to detect certain hydrocarbon that has low molecular weight, such as methane and ethane.
Gas chromatography is a physical separation method in where volatile mixtures are separated. It can be used in many different fields such as pharmaceuticals, cosmetics and even environmental toxins. Since the samples have to be volatile, human breathe, blood, saliva and other secretions containing large amounts of organic volatiles can be easily analyzed using GC. Knowing the amount of which compound is in a given sample gives a huge advantage in studying the effects of human health and of the environment as well.
Air samples can be analyzed using GC. Most of the time, air quality control units use GC coupled with FID in order to determine the components of a given air sample. Although other detectors are useful as well, FID is the most appropriate because of its sensitivity and resolution and also because it can detect very small molecules as well. This method be applied to many pharmaceutical applications such as identifying the amount of chemicals in drugs.
Moreover, cosmetic manufacturers also use this method to effectively measure how much of each chemical is used for their products. Some application, HETP concepts is used in industrial practice to convert number of theoretical plates to packing height.
Introduction In early s, Gas chromatography GC was discovered by Mikhail Semenovich Tsvett as a separation technique to separate compounds. Instrumentation Sample Injection A sample port is necessary for introducing the sample at the head of the column.
Figure 1: A cross-sectional view of a microflash vaporizer direct injector. Carrier Gas The carrier gas plays an important role, and varies in the GC used. Figure 3. Gas Recommendations for Packed Columns.
Column Oven The thermostatted oven serves to control the temperature of the column within a few tenths of a degree to conduct precise work. The effect of column temperature on the shape of the peaks.
Open Tubular Columns and Packed Columns Open tubular columns, which are also known as capillary columns, come in two basic forms. Figure 4. An inert carrier gas is also flowing through the chromatograph.
This gas shouldn't react with any components of the mixture. Common carrier gases include argon, helium, and sometimes hydrogen. The sample and carrier gas are heated and enter a long tube, which is typically coiled to keep the size of the chromatograph manageable.
The tube may be open called tubular or capillary or filled with a divided inert support material a packed column. The tube is long to allow for a better separation of components. At the end of the tube is the detector, which records the amount of sample hitting it.
In some cases, the sample may be recovered at the end of the column, too. The signals from the detector are used to produce a graph, the chromatogram, which shows the amount of sample reaching the detector on the y-axis and generally how quickly it reached the detector on the x-axis depending on what exactly the detector detects.
The chromatogram shows a series of peaks. The size of the peaks is directly proportional to the amount of each component, although it can't be used to quantify the number of molecules in a sample. Usually, the first peak is from the inert carrier gas and the next peak is the solvent used to make the sample. Subsequent peaks represent compounds in a mixture. In order to identify the peaks on a gas chromatogram, the graph needs to be compared to a chromatogram from a standard known mixture, to see where the peaks occur.
At this point, you may be wondering why the components of the mixture separate while they are pushed along the tube. The inside of the tube is coated with a thin layer of liquid the stationary phase. Gas or vapor in the interior of the tube the vapor phase moves along more quickly than molecules that interact with the liquid phase. Compounds that interact better with the gas phase tend to have lower boiling points are volatile and low molecular weights, while compounds that prefer the stationary phase tend to have higher boiling points or are heavier.
Other factors that affect the rate at which a compound progresses down the column called the elution time include polarity and the temperature of the column. Because temperature is so important, it is usually controlled within tenths of a degree and is selected based on the boiling point of the mixture.
Unlike other methods, the mobile phase in GC does not interact with chemicals and only serves to carry them. Because of this, the carrier gas must be inert. Some examples are helium, nitrogen, and argon. The type of detector on the GC usually determines which gas is used. It is not something that you would have to decide since a working GC should already have a gas tank connected to it. Like any other oven, a GC oven provides heat. But instead of baking goods, what this oven gives you is vaporized material right after injection.
In addition, it keeps the column heated so that you continue to have gaseous molecules traveling through. A temperature program can be set electronically to maintain a constant temperature or to gradually increase ramping.
The program that you select will depend on the nature of the sample. This is a device at the end of the column that senses each compound as it comes out. The data recorded by the detector is transmitted into a computer and produces a two-dimensional plot, called chromatogram. There are several types of detectors with varying detection methods and limits.
A particular powerful detector is the mass spectrometer MS. A GC chromatogram Figure 2 is a visual output of the data recorded by the detector. It is presented as a plot of detector response y -axis versus retention time x -axis. Figure 2.
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