In the Laboratory
An Inexpensive Detector for Gas Chromatography Allan L. Smith and Edward J. Thorne Department of Chemistry, College of Arts & Sciences, Drexel University, Philadelphia, PA 19104 Wolfgang Nadler College of Arts & Sciences, Drexel University, Philadelphia, PA 19104
Gas chromatography (GC) is an extremely powerful tool for the separation of complex mixtures, and its usefulness in chemistry is widespread and ever-expanding. Since chemistry deals with the composition of matter, experimental methods such as GC that determine composition should be part of a laboratory program in general chemistry. The power and utility of GC as a separation tool are becoming so widespread that this process is discussed, albeit in a cursory qualitative fashion, in general chemistry textbooks (1–4). The details of GC are far beyond the scope of an introductory-level course, but some experimental exposure to GC at this early stage is beneficial because of its widespread application. In many instances, people not directly associated with technical fields of study are becoming exposed to this method with increasing regularity. Most urine tests to detect the presence of illegal narcotics are accomplished by GC, as are blood tests to determine alcohol levels of people suspected of being intoxicated; but the laboratory equipment required for such analyses often is not readily available in undergraduate laboratories, particularly at the introductory level. Since GC experiments in laboratory manuals and the literature typically require the use of a commercial instrument, large introductory lab courses do not usually include GC. We have modified an existing low-cost method of demonstrating GC techniques. These modifications enable collection and quantitative analysis of data taken by freshmen. The experimental apparatus described below allows us to run multiple gas chromatographs simultaneously in freshman lab sections. Previous Introductory Use of GC Wollrab (5) has described a simple gas chromatograph for separating mixtures of hydrocarbons using a 25–30-inch glass column (1/2 inch diameter) packed with NaCl coated with hexadecane. The carrier gas, hydrogen, is burned at the point of exit from the chromatographic column into a capillary tube. As the separated hydrocarbon vapors emerge from the column a visually observed change in the intensity of the flame is the basis of the qualitative detection. Wollrab and Doyle (6 ) modified this procedure to detect halogenated hydrocarbons using a piece of copper wire turned into a coil and inserted in the flame (a Beilstein detector). The burning halogenated hydrocarbon reacts with the copper wire, creating volatile copper halides, which have a characteristic green color. By monitoring the time necessary for the green color to appear, the retention time is determined. Fox and Shaner (7) described a simple computerized gas chromatograph using a 4-foot glass column packed with laundry detergent. Detection is made with an infrared detector sensing changes in the flame temperature. Liquid samples of pentane and hexane are injected
directly onto the column, but the detector response is not quantitative for the amount of liquid injected. Thompson (8) described a simple method of demonstrating the general process of GC using a “homemade chromatograph” constructed from a 12-inch length of glass tubing packed with laundry detergent. Self-sealing latex tubing is connected to both ends of the column. One end is connected to a supply of natural gas, which is used as the carrier gas; the other end is connected to a piece of bent glass tubing fitted with a Beilstein detector. The retention time is determined by visually noting the amount of time necessary for the green flame to appear. Each of the above methods is qualitative, or semiquantitative at best. We have developed and constructed a detector box that is specific for the green color produced from the Beilstein detector and is proportional to the halogenated organic analyte emerging from the column. We have interfaced the detector box to an Apple Macintosh IIci computer to construct the chromatograms and perform quantitative analysis on the data generated. Experimental Apparatus The chromatograph was constructed from a 12 × 1/4-in. o.d. packed glass tube. The inlet side was connected to the natural gas supply on the bench with latex tubing and the exit side was connected to the detector box. We found sporadic and irreproducible results using laundry detergent (7, 8) as the column packing, despite sieving. We then switched to a commercially available column packing,1 which gave much more reproducible results. The cost of the packing is about $4.00 per column and these columns can be used repeatedly. A detector box is constructed from an ammunition box fitted with a copper inlet tube that bends upward to serve as both a flame burner/nozzle and a support for the copper coil. When properly inserted, the copper coil will emit a blue flame when only the carrier gas (natural gas) is eluting from the column. A coil made from standard insulated copper wire can be used repeatedly for as many as 20 classes without replacement. The effluent from the column enters the detector box through the latex tubing connector, is burned in a flame, and passes over the copper coil. The light emitted is focused through the lens, passes through a green plastic filter that removes all colors but green, and then passes on to a CdS detector, which shows maximum sensitivity for green light. A schematic for the detector is shown in Figure 1. Power for the detector circuit is supplied by a 9-V dc adapter (Archer 273-1455c), although no exact supply voltage is required. Q1 provides an output voltage on terminal 1 and is precisely 1/2 of the voltage applied to terminals 2 and 3. Therefore terminal 1 can be considered a virtual ground for the detector amplifier
JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education
1129
In the Laboratory
Q2. The single adapter voltage splits into +/᎑ tracking supply voltages. The VT 50 was chosen for its large area, allowing the light to fall on the detector in a large defocused spot. This reduces the effects of flame flicker on the signal. A gelatintype filter with 60% transmission at 520 nm further enhances sensitivity by reducing stray light coming though the chimney. R1 and the CdS cell form a voltage divider. The resistance of the CdS cell varies from approximately 200 k Ω in total darkness to 20 k Ω with a bright flame. Hence the voltage at point A, with respect to ground, can vary approximately from +0.5 to +2.5 V, giving a maximum signal range of 2 V. With switch S2 closed, R2 and R4/R5 in the feedback of operational amplifier Q2 form an amplifier with a gain of ᎑0.83. With switch S2 in the open position the gain is ᎑5. It is possible to reduce the response time of the amplifier to approximately 1 s depending on the gain setting, response being faster at the lower gain setting. Pin 2 is the summing junction of the amplifier. Because R2 = R3, a voltage on potentiometer R6 equal and opposite to the voltage on point A allows the output voltage to be adjusted to zero for any flame intensity. The ammunition box used to build the detector box has a hinged lock that can be used to open the box to light the flame, insert the copper coil, etc. The top of the box is fitted with a chimney to allow for ventilation and the escape of column effluent combustion products. Once the detector box has been closed and locked, it may be difficult to know if the flame has been extinguished. The main problem is that if the flame is no longer lit but the carrier gas continues to flow the room may become filled with natural gas, which is an obvious safety hazard. As a safety feature, the detector box has been fitted with a sensor that will emit a high pitched sound if the flame has been extinguished once the cover is closed and locked. The alarm circuit also senses the voltage at Point A. Q3 is operating as a comparator. The voltage on pin 2 is set to be slightly more positive than point A when the flame is lit. When the flame is extinguished, the voltage at point A will rise and exceed the threshold set on pin 2. The output of Q3
will become positive and sound the alarm. The sensor is adjustable to take into account very small flames that can result if the line pressure of the natural gas is slight enough that the back pressure developed by the packed column restricts the gas flow to a relatively low value. On the front face of the detector box are plugs to connect the detector output to an appropriate voltage measuring device (Vout in Fig. 1), filter and gain switches, and zero offset. The final parts cost for the detector box having all of these features is about $70. Diagrams of both the inside and the outside face of the detector box are shown in Figure 2. Experimental Procedure Using this simple experimental setup, mixtures of halogenated hydrocarbons such as CH2Cl2, CHCl3, and C2HCl3 can be readily separated. These materials all have an appreciable vapor pressure at room temperature. Use of materials that are vapors at room temperature eliminates the necessity of having a heated injector port to vaporize the sample. Detection is made by the Beilstein detector and a detector box. As the halogenated hydrocarbon vapors emerge from the packed column and are burned in the flame, volatile copper halides are formed on the detector surface and the excited copper atoms generated emit a characteristic green light as they return to their ground state. It is this green light that we use as an indication of the arrival of the halogenated species from the column. Using a gas-tight plastic syringe, the sample vapor corresponding to one of the above materials is injected directly into the latex tubing immediately before the packed column and a stopwatch is started. The time to first appearance of the green light is recorded as the retention time. This can be repeated with each of the other materials, demonstrating the fact that each will have its own unique retention time. At this point, a mixture of the three can be run, demonstrating that such a mixture will give three different retention times, one for each component. The carrier gas flow rate can also be determined without the use of a flowmeter. The simplest procedure is to inject a considerable volume (about 3–4 mL) of air into the packed column. The presence
DETECTOR C4 0.22 µFS1
+4.5V
to ALARM
3
2
V IN9 VDC
F
1
TLE2 425 COM
C3 0.1 µF
2
VT 50
Chimney
a Cu Coil
6
Column Effluent
3
Lens
VOUT
4 R6 1KΩ
GAIN
C5 0.01 µF
7 Q2 OP 07
IN C1 Q1 10 µF
S2
R5 5 M Ω
A R2 1 M Ω
C2 0.1 µF
FILTER
R4 1 M Ω
R1 200 KΩ
Filter
CdS Detector
R3 1 MΩ
LATCH
b
ZERO
-4.5V ALARM
+4.5V
OFFSET
R8 10 KΩ R7 1 M to DETECTOR
Q3 TLC2201
A
6 + 8
2
HI LOW
PIEZO BUZZER
7
3
FILTER GAIN
OUTPUT
-
POWER
4
LEVEL R9 100 KΩ
Figure 1. Schematic of detector circuit.
1130
-4.5V
Figure 2. Diagram of detector box. (a) Inside view. (b) Outside face.
Journal of Chemical Education • Vol. 75 No. 9 September 1998 • JChemEd.chem.wisc.edu
In the Laboratory
Results In the freshman course at Drexel, we connect the output ports from the CdS detector to an Apple Macintosh IIci or SE30 computer connected to a Strawberry Tree Analog Connection Workbench2 terminal panel and associated software to acquire the data. The data, taken at 0.2-s intervals, are stored in a text file that is readily opened and analyzed by standard spreadsheet software. The GC apparatus described here provides freshman students an opportunity to use computerassisted methods of analysis for the interpretation of their laboratory data. Figure 3 shows the chromatogram collected for a 0.10-mL injection of CH2Cl2 vapor into the column. The data are evaluated quantitatively by using the statistical capabilities of Excel. Using the data before the onset of the peak, an average value for the baseline signal, , was calculated using Excel’s AVERAGE command. The standard deviation (σ) of the same data was obtained using STDEV. The onset time (t1), time of maximum signal ( tmax), and ending time (t 2) of the peak were identified and its area, A, was determined by summation of all data elements between t1 and t 2. With the unit of time in seconds and the detector response in volts, the units of area will be volt-seconds. The signal, S, is defined as S = A – (t 2 – t1) The noise associated with same signal is given as N = σ (t 2 – t1) The signal-to-noise ratio (S/N) is therefore
S = A – < B > t2 – t1 N σ t2 – t1 The detection limit (DL) is evaluated from both the mass of sample injected and the signal-to-noise ratio: mass DL = = m signal / noise S / N The chromatogram presented in Figure 3 gives the following experimental values for the parameters listed above: t1 = 18.4 s, t2 = 32.0 s, tmax = 20.6 s, tair = 5.6 s, A = 3.559 volt-seconds, = ᎑ 0.08549 V, and σ = 0.00517 V. The mass of sample injected, m, can be determined from the volume injected and the equilibrium vapor pressure of CH2Cl2 at room temperature (428 torr, or 0.563 atm). For 0.10 mL of vapor at 298 K, this gives a mass of 0.195 mg. The results are as follows: S = 4.721 volt-seconds N = 0.0703 volt-seconds S/N = 67.14
Depending on the stability of the flame and the halogen content of the analyte, detection limits of 0.5–5 µ g are routinely achieved for volatile halocarbons. Figure 4 shows the chromatogram resulting from a mixture of 0.60 mL each of the vapor phase of CH2Cl2, CHCl3, and C2HCl3 injected into the chromatograph. The vapor pressures at 25 °C are 428, 194, and 69 torr, respectively. The vapor phase of each component is sequentially admitted to a plastic gas-tight syringe. In Figure 4 we see that the area of each peak decreases as the vapor pressure (and hence, the mass) of the component decreases. Reducing the amount of the more volatile component(s) will reduce the size of the corresponding peak and make the less volatile components easier to detect. The detection system shows good linearity over a sixfold change in concentration. This can be determined by comparing Figures 3 and 4. The signal, S, for the CH 2Cl2 peak in Figure 4 (a 0.60-mL injection of CH2Cl2 vapor) is 28.19 volt-seconds. Assuming linearity, the “expected” signal can be determined from the Figure 3 signal (4.721 volt-seconds for a 0.10-mL vapor injection): Sexpected = (6.0)(4.721 volt-seconds) = 28.33 volt-seconds This expected value is within 0.5% of the observed value. It is also interesting to note the excellent reproducibility of the retention time in comparing Figures 3 and 4. The response factor and detector sensitivity are heavily dependent 0.10 mL vapor injected
1.2
Detector box response / V
tr = tx – tair
0.195 mg DL = m = = 0.0029 mg = 2.9 µg S/N 67.14
1.0 0.8 0.6 0.4 0.2 0.0 0
5
10
15
20
25
30
35
40
45
50
55
Time / s
Figure 3. Chromatogram of methylene chloride. 5.0
Detector box response / V
of air eluting from the column will cause a disturbance of the fuel/air ratio, which will be observed as a decrease in the size of the flame on the detector coil. The time necessary for the decrease in flame height and the distance between the point of injection and the detector coil can be used to calculate the carrier gas flow rate. Representing the time for the air peak to be observed as tair, the retention time, tr, for a peak noted at some finite time tx is actually
Methylene chloride 4.0 3.0
Chloroform
2.0 Trichloroethylene
1.0 0.0 0
10
20
30
40
50
60
70
80
90 100 110
Time / s
Figure 4. Chromatogram of mixture of methylene chloride, chloroform, and trichloroethylene, 0.60 mL each.
JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education
1131
In the Laboratory
upon the construction of the copper coil used to generate the green color. A tightly wound coil gives best results and a loose coil that has only a few turns of metal shows little response. After generating the chromatogram shown in Figure 4, we normally supply an unknown for the students to analyze. One interesting unknown that has given good results is a commercial brake cleaner, which is composed of CH 2Cl2 and C2HCl3. The students gain a better appreciation for the method and data interpretation when they can relate it to some common material that they may actually use. The students construct a chromatogram for each of the pure vapors, the mixture of the three, and the unknown. From this they are required to determine the composition of the unknown by comparing retention times. As time permits, chromatograms of a single pure vapor using different injection volumes are also constructed and area calculations as described above are assigned.
Literature Cited 1. Olmsted, J.; Williams, G. M. Chemistry: The Molecular Science; Mosby-Year Book: St. Louis, 1994; pp 478–479. 2. Masterton, W. L.; Hurley, C. N. Chemistry: Principles and Reactions, 2nd ed.; pp. 5-6, Saunders: Philadelphia, 1993; pp 5–6. 3. Ebbing, D. D. General Chemistry, 4th ed.; Houghton Mifflin: Boston, 1993; pp 16–18. 4. Gillespie, R. J.; Eaton, D. R.; Humphreys, D. A.; Robinson, E. A. Atoms, Molecules, and Reactions: An Introduction to Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1994; p 541. 5. Wollrab, A. J. Chem. Educ. 1975, 52, 200. 6. Wollrab, A.; Doyle, R. R. J. Chem. Educ. 1982, 59, 1042. 7. Fox, J. N.; Shaner, R. A. J. Chem. Educ. 1990, 67, 694. 8. Thompson, S. CHEMTREK: Small Scale Experiments for General Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990; pp 421–441.
Notes 1. 10% SP-1000 on 80/100 mesh Supelcoport (Cat. No. 1-1872 from Supelco, Inc., Bellefonte, PA 16823). 2. Strawberry Tree, Inc., 160 South Wolfe Road, Sunnyvale, CA 90486; 408/736-8800.
1132
Journal of Chemical Education • Vol. 75 No. 9 September 1998 • JChemEd.chem.wisc.edu
