CHM Gas Chromatography: Our Instrument Charles Taylor (r10)

Transcription

1 1/8 Introduction You should already be familiar with the basic concepts of gas chromatography. The purpose of this note pack is to familiarize you with the specific instrument that we will use in our labs. Even though most gas chromatographs work the same way, each instrument manufacturer makes their instruments slightly differently - controls may be in different places, the instrument may/may not be computerized, etc. Diagram vs. the real instrument In a previous note pack, we discussed a diagram of a simple gas chromatograph. For reference, here is the diagram: Illustration 1 - Diagram of a simple gas chromatograph. For comparison, here is a picture of our lab instrument.

2 2/8 Illustration 2 - Outside view of the Perkin-Elmer instrument So where are these parts? The instrument is large, with a control panel on top and a large door on the front. What we're primarily seeing here is the outside of the oven. If you open the oven door, you will be able to see the column, where separation takes place.

3 3/8 Illustration 3 - Inside the oven If you look carefully, you will notice that there are actually two columns inside the oven. The one in front (small brown tube) is a capillary column, and the one in back (thick silver tube) is a packed column. What's the difference? Packed columns - packed with solid support holding the stationary phase - can separate relatively large volumes of sample (because they're wide and have more room for sample to flow). Capillary column are much longer and thinner than packed columns, and cannot separate large samples. However, the quality of separation in a capillary column is typically better than in a packed column (because the sample is in contact with the stationary phase longer). The instrument is designed to hold two columns so two different kinds of experiments can be done on the instrument without having to get out the wrench and remove/replace the column. The back of the oven contains a fan. The fan allows the instrument to either heat or cool rapidly. Maintaining temperature at known values is important in gas chromatography, as the retention time of a component of a sample mixture depends on temperature! Where does the sample go in? On top of the instrument is the injection port. Since our instrument has two columns, it has two injection ports.

4 4/8 Illustration 4 - Injection ports Each sample injection port contains its own heater. Usually, the injection port is kept at a high temperature so that the sample vaporizes immediately - before making it to the column. Samples usually are provided as liquids, and these liquids are literally injected into the injection port with a syringe. These syringes can reproducibly inject very small samples into the instrument. Typical sample sizes for a packed column like the one installed in our instrument range from approximately 1 L to2 L (one microliter is 10-6 L).

5 5/8 Illustration 5 - Syringe So where's the detector? Our instrument actually has two of them inside the metal box on top of the instrument. In the picture, you can see the detectors. Illustration 6 - Detectors The shiny metal box at the top of the picture is the thermal conductivity detector, which we discussed in a previous note pack. The small "nozzle" at the bottom of the instrument is another type of detector called a flame ionization detector. In our experiments, we will use the thermal conductivity detector, so we don't need to worry about the other one.

6 6/8 Setting up the instrument Recall that several factors affect how a separation proceeds, and that we are able to control them To run a successful gas chromatography experiment, we need to make sure we set up the temperature and the carrier gas flow rate. Setting the temperature is easy - it's done via the digital control panel on top of the instrument. Illustration 7 - Instrument control panel How about the carrier gas flow rate? Unfortunately for us, that's not quite as simple. The carrier gas flow rate is adjustable via some knobs on top of the instrument. Illustration 8 - Gas flow adjustment knobs

7 7/8 The instrument isn't able to read the flow rate on its own, so we have to assist it using a device called a bubble flowmeter. The bubble flowmeter is a simple device that allows us to measure the flow rate of a gas by the time it takes a gas bubble to travel a certain distance. How does the bubble flowmeter work? You hook the bubble flowmeter up to the gas stream you want to analyze, squeeze the bulb, and time the (flat) bubble formed as it moves from the bottom line to the top line. Gas flow rates in gas chromatography are usually measured in terms of ml/min (milliliters per minute). To find gas flow rate using the flowmeter pictured on the left, time a bubble as is goes from the lower dotted line to the upper one. The volume of the space between the lines is 10 ml, so divide 10 ml by the number of minutes it took for the bubble to travel between the two dotted lines. Bubble flowmeters typically have different regions for measuring different flow rates. Often, you will see flowmeters with 1 ml, 10 ml, and 100 ml regions for measuring both high and low flow rates. Still other bubble flowmeters are marked in regular intervals like a graduated cylinder. Illustration 9 - A simple bubble flowmeter We will use the bubble flowmeter when setting up our instrument to verify the flow rate of our carrier gas, and use the flow rate adjustment controls to adjust the flow rate if necessary.

8 8/8 Running an experiment To run an experiment on our instrument, you inject a sample into the injection port using your syringe. At the same time, you press the "RUN" key on the control panel. Our computerized instrument then takes care of the rest, recording the data and then printing a graph (chromatogram). The retention times for each peak are labeled, and the area of each peak is calculated. Summary This note pack gives you a brief overview of our gas chromatograph and its features. You should be familiar now with the parts of our instrument and where they are located. You should also have an idea of how to use the instrument, though you will be given more detailed instruction when you actually go to use the instrument.