Create a Gas Chromatograph

Introduction

Assemble an inexpensive working gas chromatograph in less than one hour! Teaching the principles of this workhorse instrument for chemical analysis has never been faster or easier.

Concepts

• Gas chromatography
• Retention time
• Detectors

Background

Chromatography refers to a broad range of techniques that are widely used in modern science labs for analyzing mixtures and separating the components in a mixture. Gas chromatography is the standard method used for separating mixtures of volitile compounds that are easily vaporized.

There are five basic parts to every gas chromatograph:

• Carrier gas supply and flow control
• Injection port
• Separation column
• Detector
• Recorder

The basic operation of a gas chromatograph is relatively simple and straightforward (see Figure 1). An inert carrier gas (1)—usually helium, argon, or nitrogen—is allowed to flow through the separation column and the detector. The mixture of compounds to be separated is introduced into the carrier gas stream at the injection port (2). The role of the injection port is the first important step in a successful separation. If the sample to be analyzed is a liquid, the injection port must be heated so that the components will quickly and completely vaporize before entering the separation column. The size of the sample is very important. If the sample is very large, it will overwhelm the column and separation will be poor. If the sample is too small, some minor components of the mixture may go undetected. The mixture of gases from the volatilized sample then enters the separation column (3), along with the carrier gas stream. The components of the mixture are separated inside the column based on polarity. The separation column is packed with a finely divided support material such as alumina or silica gel. The support is coated with an organic substance designed to separate the molecules in the mixture (see Figure 2). The coating may be a polar substance such as carbowax, a straight-chain polyethylene glycol, or it may be a nonpolar substance such as silicon oil (see Figure 3). Separation of the mixture occurs because, as the gaseous components pass over the surface of the coated particles, some components will be strongly adsorbed onto the coating, while others will not be adsorbed. Depending on the carrier gas flow, the temperature of the separation column, the concentration of the coating, and many other variables, the sample component will move at different rates between the stationary phase and the mobile phase. If a polar coating is used, the polar components will spend more time adsorbed on the coating (likes dissolve likes), while the nonpolar components will spend little, if any, time adsorbed on the coating. As a result, the components will separate into “bands” that flow through the column at different rates. In this scenario, the nonpolar components will pass through the column faster, followed by the polar components. The separation of a polar component A and a nonpolar component B is illustrated in Figure 4. Once separated, the components of the mixture flow into the detector (4). The recorder (5) produces a graph based on the detector responses, showing the time that each component exits the column. The area under the peak increases as the amount of the component increases. The retention time (the time it takes the component to reach the detector) and the area under the recorded peak are used to identify the compound and determine how much of it was present in the mixture. Detectors are designed to measure differences in the physical or chemical properties of the components and the carrier gas. Examples of suitable detector properties include thermal conductivity and ionization in a hydrogen flame.

In this demonstration, a gas chromatograph will be constructed to separate and detect the volatile halogenated hydrocarbons methylene chloride, CH₂Cl₂, and chloroform, CHCl₃.

A schematic of the gas chromatograph (GC) design is shown in Figure 5. The separation or chromatographic column consists of a glass tube filled with a packing material that will separate chlorinated hydrocarbons. Natural gas (methane) is the carrier gas. The normal laboratory gas jet has a useful line pressure and serves as a fine-tuning regulator for the carrier gas flow. The detector is called a Beilstein detector. The detector is a copper coil that is placed in a small flame generated at the column exit by burning natural gas. In commercial GC instruments the injection port has a little oven that quickly converts any liquid sample (often dissolved in a volatile solvent) into vapor. In this GC the samples will be introduced directly in the vapor phase, so that heating will not be necessary. The injection port is made from ordinary Bunsen burner tubing (latex tubing). The latex is a self-sealing material that will take repeated injections without leaking.

The Beilstein detector is a sensitive GC detector that emits a green-blue light when a halogenated hydrocarbon is burned in the flame. The detector is made by coiling copper wire into a small cylindrical shape and inserting this copper coil into the burning natural gas flame. When a halogenated hydrocarbon exits the column and enters the detector, the halogen atoms are converted to ions by the flame. The ions then react with the copper wire to produce copper halides, which are generated in an “excited” state. When these excited state copper halides “relax” or return to their ground state, they release energy in the form of greenish blue light. The green–blue light flame color serves as a positive test for the presence of a halogen-containing molecule.

Materials

Safety Precautions

Chloroform is a possible carcinogen, toxic and narcotic by inhalation; ingestion may be fatal. Methylene chloride is slightly toxic and can be narcotic at high concentrations; a possible carcinogen. Use only in an operating fume hood. The packing material is a slight respiratory irritant. It will stain both skin and clothing. Avoid contact of all chemicals with eyes and skin. Use care when using the syringe to avoid punctures. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Prelab Preparation

Part 1. Assembling the Glass Column

1. Cut a 7-cm length of glass tubing from the 12-inch glass tubing. Caution: The ends of the tubing may be sharp. Fire polishing is recommended. Place a small plug of polyester fiber inside one end of the 7-cm glass tubing. Note: Do not wash the 7-cm piece of glass tubing.
2. Insert a small cork into the same end of the tube that has the polyester fiber plug.
3. Using scissors, cut a 1" piece of latex tubing and attach one end to the funnel stem and one to the uncorked end of the 7-cm glass tubing (see Figure 6).
   {13952_Preparation_Figure_6}
4. Hold the funnel/glass tubing assembly vertically. Scoop up some of the packing material with the spatula and place it in the funnel. Deliver the packing material at an even rate—tap the tubing gently as it is filled. Note: If the funnel gets blocked, invert it over the packing material bottle, tap and start over. Do not attempt to force it through the funnel stem—it will become tightly packed in the stem and clog (see Figure 7).
   {13952_Preparation_Figure_7}
5. Keep adding packing material and tapping until the tube is completely filled.
6. Keep the tube vertical and gently bounce the tube on the table (at the cork end). The packing will settle a little. Add more packing until it is about 0.5 cm from the end.
7. Remove the funnel and latex tubing from the column. Place a plug of polyester fiber into the open end of the tubing to keep the packing material firmly in the tube (see Figure 8). Remove the cork.
   {13952_Preparation_Figure_8}

Part 2. Constructing the GC Detector

1. Obtain a glass Pasteur pipet.
2. Place the part where the pipet narrows down in a burner flame (or even a match flame). Rotate the pipet until the tip slowly starts to bend (see Figure 9).
   {13952_Preparation_Figure_9}
3. Stop rotating and let the tip bend, under gravity, until it forms a right angle. Place the pipet on the table to cool (see Figure 10).
   {13952_Preparation_Figure_10}
4. Holding the pipet firmly on the table, use a file to scratch and cut off the thin end so a tip 2–3 cm long is left (see Figure 11).
   {13952_Preparation_Figure_11}
5. Use the file to scratch and cut off the larger diameter end about 2–3 cm from the bend. Save the cutoff part. Fire polish the sharp, large diameter end. The detector burner is now assembled (see Figure 12).
   {13952_Preparation_Figure_12}
6. After the cut-off pipet has cooled, hold the straight, cut-off part and tightly wind the copper wire coil 10 times around the pipet. As the wire is wound, keep your thumb tightly on the end and also put tension on the wire as it is wound around the pipet (see Figure 13).
   {13952_Preparation_Figure_13}
7. Cut off the excess copper wire with wire cutters, leaving a tail of copper wire about 3 cm long (see Figure 14).
   {13952_Preparation_Figure_14}
8. Slip the wire coil off the glass tube and bend the tail so that it will be positioned down the axis of the coil (see Figure 15).
   {13952_Preparation_Figure_15}
9. Cut the tail off about 1-cm from the coil. Now slide the tail into the narrow part of the glass burner (see Figure 15).The burner and detector have been constructed. The gas chromatograph can now be assembled.

Part 3. Assembling the Gas Chromatograph

1. Using scissors, cut a 1-inch piece of the latex tubing.
2. Attach one end of the long length of latex tubing, saved from Part 1, onto the natural gas tap and gently push the other onto the glass column. Work carefully to avoid pulling the polyester fiber plug out.
3. Gently attach the 1" length of latex tubing onto the other end of the column. Attach the burner to the 1" piece of latex tubing. Carefully rotate the 1" piece of latex tubing, if necessary, so that the column will lie naturally on the table with the burner vertical. Clip the two clothespins or clamp holders onto the column to act as stabilizers (see Figure 16).
   {13952_Preparation_Figure_16}

Demonstration

1. (Optional) Make copies of the demonstration worksheet and graph paper masters. Pass out a worksheet and graph paper to each student before beginning the demonstration.

Procedure

Part 1. Calibration

1. Open the laboratory gas jet to allow natural gas to flow through the column and detector. After 5–10 seconds, light the gas where it exits the copper wire detector to produce a small flame.
2.  Adjust the gas flow rate as needed so that the height of the flame extents about one-half inch above the copper wire coil. The flame should create a smooth dome atop the coil, not an erratic one (see Figure 17).
   {13952_Preparation_Figure_17}
3. Let the flame burn for 60 seconds, then use a pair of tweezers to carefully remove the coil from the flame. Set the coil down on a ceramic pad to cool.
4. Light the burner if the flame has gone out, but do not change the gas flow rate. Note: The carrier gas flow rate must be constant because the rate at which components exit the column will depend on it. The faster the carrier gas is flowing, the faster the sample components will move through the column.
5. Use the 1.0-mL graduated syringe to obtain a 0.5-mL sample of air.
6. Place the syringe needle on the latex tubing near the beginning of the separation column. Push the needle into the middle of the tubing, then quickly inject the air sample and remove the needle from the tubing. Start timing.
7. Observe the flame—when the air that was injected onto the column reaches the detector, it will cause the flame to suddenly and briefly “dip” or flicker. Measure and record the time it takes for air to travel through the column. Have students record the retention time for air in Part 1 of the data table.
8. Repeat this step two more times to confirm data.

Part 2. Measuring Retention Times

1. Using tweezers, place the wire coil back into the opening of the pipet tip.
2. If the flame has gone out, relight the detector. Adjust the position of the coil to reestablish the correct flame condition (see Figure 17 in Part 1). Note: Do not readjust the gas flow.
3. Obtain the bottles of methylene chloride, CH₂Cl₂, and chloroform, CHCl₃. Place the bottles in an operating fume hood.
4. Obtain a 1-mL syringe. Press the syringe plunger to the zero mark.
5. Open the bottle of methylene chloride and insert the needle into the vapor space above the liquid. Do NOT allow the needle to touch the liquid!
6. Quickly pull the plunger to the 0.3-mL mark, then push the plunger back to the zero mark. Do this several times to remove all the air from the syringe.
7. Pull the plunger to the 0.2-mL mark to obtain a sample of pure methylene chloride vapor. Recap the bottle.
8. Take the syringe to the GC and place the syringe needle on the latex tubing near the beginning of the glass column. Push the needle into the middle of the tubing, then quickly inject the methylene chloride sample. Remove the syringe from the tubing and immediately start the timer.
9. Observe the flame. Have the students measure and record in the Part 2 Data Table:
   1. The time at which the greenish-blue flame color first appears
   2. The time at which the flame color reaches its maximum intensity
   3. The time at which the green-blue flame color disappears.
10. Have the students estimate, on a relative scale of 1–10, the color intensity of the maximum green-blue flame for each sample and record this number in Part 2 of the data table.
11. Repeat steps 3–10 two more times.
12. Using a new syringe, repeat steps 4–10 for chloroform, CHCl₃. Note: Always use separate syringes to inject the methylene chloride and chloroform.
13. (Optional) If time permits, repeat steps 4–10 with a mixture of methylene chloride and chloroform.
14. Have students perform the calculations outlined on the demonstration worksheet and discuss the results.

Student Worksheet PDF

13952_Student1.pdf

13952_Teacher1.pdf

Teacher Tips

• Make sure the column is not tightly packed. If the column is packed too tightly, there will be no carrier gas flow, and if the column is packed too loosely, channeling will occur, giving rise to poor or no separations. The longer the column, the longer the retention times will be for each compound, and the more the bands will spread. A column length of 7–10 cm is reasonable for the gas pressures encountered in most school labs. In this GC, the packed column will be used at ambient temperatures. The halocarbons used in this demonstration have a relatively high vapor pressure at room temperature, and elevated column temperatures are not necessary. Only halocarbon vapor samples will be separated by this column!
• Once conditioned, the detector is quite fragile. Be careful when handling the detector.
• The retention time for air may be used to calculate the linear gas velocity of the carrier gas. Gas flow rate is calculated by measuring the length of the column in centimeters and dividing this value by the retention time of air, in seconds.
• Use the first trial with the methylene chloride sample to make detailed observations of the process and subsequent trials to measure times and to estimate the maximum flame intensity. The elapsed time from injection to the maximum green-blue flame color is the retention time tR for the halocarbon. Measuring the time to the first appearance of the green-blue flame color will allow the students to calculate the bandwidth of the halocarbon peak.
• Other halogenated hydrocarbons, such as ethylene dichloride, C₂H₄C_l2, and trichloroethylene, C₂HC_l3, may also be used to demonstrate the separation of components in a mixture. Use more vapor for some of the less volatile halocarbons.

Sample Data

Part 1. Air Sample

Retention Time ___1___ sec
Retention Time ___1___ sec
Retention Time ___0.9___ sec
Average tR (air) ___1___ sec

Part 2. Halogenated Hydrocarbons

Answers to Questions

1. For each halocarbon tested, calculate the average values for the following data: the time the green-blue flame color first appears, the time the maximum color intensity is observed, the time the blue-green flame color disappears, the bandwidth and the maximum color intensity.

   The time green-blue color change first appears

   CH₂Cl₂ (18 + 18 + 18) sec / 3 = 18 sec
   CHCl₃ (91 + 89) sec / 2 = 90 sec

   The time the maximum green-blue flame color occurs

   CH₂Cl₂ (23 + 22 + 23) sec / 3 = 23 sec
   CHCl₃ (102 + 102) sec / 2 = 102 sec

   The time the green-blue flame color disappears

   CH₂Cl₂ (27 + 28 + 27) sec / 3 = 27 sec
   CHCl₃ (118 + 116) sec / 2 = 117 sec

   The bandwidth

   CH₂Cl₂ (27 – 18) sec = 9 sec
   CHCl₃ (117 – 90) sec = 27 sec

   The color intensity

   CH₂Cl₂ (10 + 10 + 10) / 3 = 10
   CHCl₃ (4 + 4) / 2 = 4

2. Reproduce Figure 18 on graph paper for each hydrocarbon sample used in this demonstration. Place time, in seconds on the x-axis and relative green-blue flame color intensity on the y-axis.
   {13952_Answers_Figure_18}
3. (Optional) Reproduce Figure 19 on graph paper for any mixed hydrocarbon sample used in this demonstration. Place time, in seconds, on the x-axis and relative green-blue flame color on the y-axis.
   {13952_Answers_Figure_19}