In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
W
A Small-Scale Low-Cost Gas Chromatograph Natas ˇa Gros* Faculty of Chemistry and Chemical Technology, University of Ljubljana, Asˇkercˇeva, 1000 Ljubljana, Slovenia; *
[email protected] Margareta Vrtac ˇnik Faculty of Natural Sciences and Engineering, University of Ljubljana, Vegova 4, 1000 Ljubljana, Slovenia
The hands-on approach in teaching and learning is believed to contribute to better understanding of the concepts taught in the classroom. However, this approach is difficult to introduce if costly instrumentation is required. The reasons why analytical chemistry is usually neglected at lower levels of chemical education are varied. Most commercial instruments are expensive and their operation and maintenance are demanding; therefore, they are not readily available at most high schools. Possibilities that enable some of these drawbacks to be overcome are simplification and miniaturization of the analytical instruments. The design and application of low-cost, simplified gas chromatographs are discussed in many articles, for example, Wollrab (1), Wollrab and Doyle (2), Bricker, Taylor, and Kolb (3), Thompson (4), Furton and Mantilla (5), Fox and Shaner (6), and Smith, Thorne, and Nadler (7). The detection of analytes, hydrocarbons or halogenated hydrocarbons, is enabled by the use of flammable carrier gas that burns at the top of the capillary tube that is at the end of the instrument.
For the instrumental detection of hydrocarbons an IR sensor is used, while the spectrometric detection of halogenated hydrocarbons is based on the Beilstein test in which blue– green light is emitted when substances containing chlorine come into contact with red-hot heated copper wire. In this article we propose the design and application of a small-scale portable gas chromatograph for hands-on learning of the basic concepts of chromatography. We optimized the operation of the instrument so that it offers a low-cost solution that does not require the use of a computer for recording data. The design actively engages students in the measuring process. Three experiments have been developed for the introduction of chromatographic parameters at the high school level. The experimental procedures are described in the student manual in the Supplemental Material.W However, more interested high school students or first-year college students could start a complete project in gas chromatography by building the apparatus and getting a wider overview of the chromatographic parameters (Table 1).
Table 1. Results of Chromatographic Separation and Basic Chromatographic Parameters Parameter
Chromatographic Peak CH2Cl2
Formula
tRi/s
---
hi/mV
---
33.3
6.5
Peak area/(arb unit)
---
253.9
109.6
Wi/s α/s
---
64
136
---
25
61
β/s
---
36
77
Rs
Rs =
(
2 t R2 − t R 1
)
309
1.8
W 2 + W1
N
N = 16
As
As =
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Chromatographic Peak CHCl3
•
t Ri Wi
β α
2
68
83
1.4
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Description of the Instrument
Figure 1. Small-scale low-cost gas chromatograph for hands-on learning of the basic concepts of chromatography.
RLPS
RS F
RT
CuW
OF
MS
RS
C
RT-I
RX Figure 2. Schematic representation of the chromatographic unit (C –column, CuW–copper wire, F–flame, MS–metallic stand, OF–optic fiber, RLPS–removable light-protection shield, RS–rubber stopper, RT–rubber tube, RT-I–rubber tube-injector, RX–photoresistor).
10 kΩ
10 kΩ
2.2 kΩ BC 548 B 4.7 kΩ
Only a brief description on assembling the instrument is given here, while hints and precautions regarding the construction of the apparatus and troubleshooting of the apparatus can be found in the teacher’s guide in the Supplemental Material.W The apparatus consists of two basic separable units: a chromatographic unit and an electronic unit (Figures 2 and 3). The portability of the system is ensured by the introduction of a 600-mL gas supply cylinder (propane 40%兾butane 60%). The gas supply is attached by a rubber tube (RT) to a short piece of a glass tube, which is further connected to the column (C) by a rubber tube (RT-I) that functions as an injection port. The glass column is packed with anti-limescale powder. A burner is made of a glass tube with the same diameter as the column and ends in a tip. A copper coil (CuW) is positioned over the tip. The transmission of the light from the Beilstein detector to a photoresistor (RX) is made possible by optic fibers. To prevent interference from the surrounding light, a metallic black, easily removable light-protection shield (RLPS) is placed over the optical setup. A photoresistor functions as a sensor. Measured values are directly read from the display of a low-cost digital voltmeter (measuring range 200 mV). A 9-V battery is required for the operation of the electronic setup. The electronic unit could be easily assembled in the teaching laboratory on a prototype board according to the scheme in Figure 3. More details are given in the teacher’s guide in the Supplemental Material.W Hazards The carrier gas is a mixture of 40% propane and 60% butane. The mixture is a highly flammable liquefied gas and is kept under pressure in a 600-mL gas-supply cylinder. Handling of the gas cylinder is described in the Supplemental Material.W Dichloromethane and trichloromethane irritate the eyes and skin, cause dizziness, nausea, and headaches; they are also both potential occupational carcinogens. Even in very small quantities, careful handling is required; a detailed description is given in the Supplemental Material.W
1 kΩ Rx
2
1 kΩ
ICLM 741
3
–+
4
6
voltmeter
7
1 kΩ
220 µF 220 Ω LED 3 mm ⫺
9V
⫹
Figure 3. Scheme of the electronic unit of the gas chromatograph.
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Hands-on Approach for Learning the Basics of Chromatography For the introduction of the basic chromatographic parameters by a hands-on approach, students use the liquid mixture of CHCl3 and CH2Cl2 in a volume ratio of 3:2. For the actual procedure see the student manual in the Supplemental Material.W Students inject the sample of the vapor phase (250 µL) into the chromatograph, follow the changes in the color of the flame and observe the separation of the two components of the mixture by noticing the appearance and disappearance of blue–green light in two different time intervals. In the next step the experiment is repeated, but this time the instrumental approach is introduced. The chromatogram
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tR1 30
120
25
100
Response / mV
Response / mV
35
20
h1
15
0.1h1
10
α β
5
W1
50 µL 100 µL 150 µL 200 µL 250 µL
80 60 40 20
0
0 0
50
100
150
200
250
300
350
400
450
0
25
50
75
Time / s
100
125
150
175
200
Time / s
Figure 4. Chromatogram obtained for 250 µL of a vapor phase of the liquid mixture of dichloromethane and trichloromethane (volume ratio 2:3) at room temperature.
Figure 5. Chromatograms for the vapor phase of dichloromethane at different injection volumes.
resulting from this repeated experiment is shown in Figure 4. From the chromatogram, at the basic level, students determine the retention time (tRi ), peak height (hi ), and calculate peak area. While at the higher level the peak width at the base (Wi ), the value of α and β at 10% of the peak height, and calculations of the resolution (Rs ), chromatographic efficiency (N ), and peak asymmetry (As ) can be introduced The results we obtained are summarized in Table 1. The quantitative aspect of chromatographic analysis is further explored by examining the chromatograms of five different volumes of dichloromethane vapor phase. The resulting chromatographic peaks and calibration function are presented in Figures 5 and 6. This experiment is discussed in detail in the student manual in the Supplemental Material.W Concluding Remarks A low-cost portable gas chromatograph is described and used for separation and determination of chlorinated hydrocarbons. Three high school laboratory experiments for handson introduction of the basic chromatographic parameters are described. The portability and miniaturization of the instrument were ensured by the application of a small propane兾butane cylinder as a supply for the carrier gas and by the use of optic fibers for light transmission to the electronic unit with the photoresistor as a sensor. Since the flame over the Beilstein burner is so small that no severe injuries can be expected, and the user is able to inspect the flame at all times during the operation of the chromatograph, hazards are minimized in handling the instrument. The instrument does not act as a black box since it enables the students an easy transition between the observations of the flame and measurements. In this way a better understanding of the chromatographic process can be achieved. The response of the electronic unit of the instrument is optimized in such a way that readings from the voltmeter cannot cause problems if two students are working together—one student reading the responses at specified time intervals and the other student writing them up.
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Peak Area (arb unit)
0.8
y = 0.0034x − 0.1348
0.6
R 2 = 0.9988 0.4
0.2
0.0 0
50
100
150
200
250
300
Volume / µL Figure 6. Calibration function for dichloromethane.
The instrument setup can be further upgraded by a computer for data acquisition and measurements. Since the students had previously experienced manual data reading, the role of the computer as a tool would be clearly understood. WSupplemental
Material
A detailed student manual and a teacher’s guide are available in this issue of JCE Online. Literature Cited 1. Wollrab, Adalbert. J. Chem. Educ. 1975, 52, 200–201. 2. Wollrab, Adalbert; Doyle, Richard R. J. Chem. Educ. 1982, 59, 1042–1043. 3. Bricker, Clark E.; Taylor, Max A.; Kolb, Kenneth E. J. Chem. Educ. 1981, 58, 41. 4. Thompson, S. CHEMTREK: Small Scale Experiments for General Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990; pp 421–441. 5. Furton, Kenneth G.; Mantilla, Adriana. J. Chem. Educ. 1991, 68, 74–77. 6. Fox, John N.; Shaner, Robert A. J. Chem. Educ. 1990, 67, 694–695. 7. Smith, Allan L.; Thorne, Edward J.; Nadler, Wolfgang. J. Chem. Educ. 1998, 75, 1129–1132.
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