2622
Anal. Chem. 1982, 5 4 , 2622-2623
The temperature distribution inside the quartz tube can be determined neither by the phototransistor method nor by a low mass thermocouple. Instead a film of Thermochrom crayon paste is internally applied into the tube surface. Such crayons (Faber Castell) indicate the temperature of hot objects by color changes. The reliability of these pastes was checked by applying them on a thermocouple, which was inserted in an oven with precise temperature control. The color changes appeared at the expected temperature settings f a few degrees. The pyrolysis temperature setting was gradually increased from 500 to 800 O C and the color inspected visually every 5 "C. The relation between the maximum temperature on the inner tube surface and the temperature setting is obtained by linear regression analysis on the data: Ti(OC)= -83.4 + 1.007T"r ("C) ' (r . = 0.9999). The axial temr>eratureDrofiles are shown in Figure 6. Until now the Thermochrom paste method is the most straightforward one available to estimate in situ the tem-
perature of the inside wall of a heated quartz tube used in the Pyroprobe system.
LITERATURE CITED (1) Walker, J. Q. Chromatographla 1872, 5 , 547-552. (2) Levy, R. L. Chromatogr. Rev. 1966, 8 , 48-89. (3) Walker, J. Q.;Wolf, C. J. J . Chromatogr. Scl. 1970, 8 , 513-518. (4) Coupe, B.; Jones, E. R.; Stockwell, B. Chromatographla 1973, 6 , 483-488. (5) Crighton, J. S. 3rd Conference on Analytlcal Pyrolysis, Amsterdam, Sept 1976. (6) Levy, R. L.; Fanter, D. L.; Wolf, C. J. Anal. Chem. 1972, 4 4 , 38-42. (7) Wolf, C. J.; Levy, R. L.; Fanter, D. L. J. Fire Flammability 1974, 5 , 76-84. (8) Tyden-Ericsson, J. Chromatographla 1973, 6, 353-358. Schmld, J. P.; Slmon, W. Anal. Chlm. Acta 1977, 89, 1-8. (9) (10) Wells, G.; Voorhees, K. J.; Futrell, J. H. Anal. Chem. 1980, 52, 1782-7784.
RECEIVED for review December 9,1980. Accepted September 3, 1982. The authors thank the "Prime Minister's Services of Science Policy Programming" for financial support.
Automated Variable Temperature Liquid Nitrogen Cold Trap C. A. M. Brennlnkmeller Isotope Physlcs Laboratoty, Unlversitv of Groningen, Westersingel 34, 97 18 CM Groningen, The Netherlands
Cold traps are frequently applied for the separation of condensable components from mixtures of gases and vapors. Liquid nitrogen and dry ice are commonly used for obtaining low temperatures. If liquid nitrogen or dry ice does not provide the required temperature, either can be used to prepare low temperature baths by freezing components having a suitable melting point (I). The main disadvantages of the low temperature media mentioned above, except for liquid nitrogen, are that they cannot be used in automated systems without considerable technical provisions and that the choice of temperatures is limited. Liquid nitrogen is an exception because it can be readily pumped to a cold trap (2) and any temperature can be obtained by combining it with a heat source ( 3 , 4 ) . Other options for reaching low temperatures are heat pumps based on thermoelectricity (Peltier effect), on evaporation (e.g., freon refrigerators), or on adiabatic expansion (Joule-Thompson effect). However, Peltier heat pumps exhibit impracticably low cooling capacities at temperatures below roughly -100 O C . Joule-Thompson effect refrigerators and evaporation refrigerators become increasingly expensive and bulky when temperatures approaching that of liquid nitrogen are required. In this report, we present a convenient, automated, temperature adjustable cold trap based on the use of liquid nitrogen for cooling and ohmic resistance for heating. During the operation of the low-temperature device, liquid nitrogen is forced upward from a large reservoir to the base of the trap by means of a two-phase lift pump. The gas phase for the pumping action is provided by evaporating some of the liquid nitrogen by means of electric heating. A diagram for a four-loop, glass coil cold trap is shown in Figure 1. The glass coil is situated in a copper sleeve of rectangular cross section, ending in a copper tray. Two copper-constantan thermocouplejunctions, one for monitoring and the other one for controlling the tempertature, are inserted into the copper sleeve as well. Thermal contact between the copper sleeve, the glass coil, and the thermocouple junctions is provided by copper powder. The liquid nitrogen is present in the lower compartment of the Dewar vessel, which is separated from the upper part containing the copper sleeve by a glass disk. 0003-2700/82/0354-2622$0 1.25/0
The liquid level is observed by means of a polystyrene foam float. The temperature of the copper sleeve is controlled by a commercial proportional-differential temperature controller, the sensing circuit of which is connected to one of the thermocouples. The desired upper temperature of the trap corresponds to the temperature selected on the controller. In order to reach the upper temperature, 60 W of heating power are generated in a coaxial-typeheating coil, which is soldered to the lower end of the copper sleeve. As the selected temperature is approached, the temperature controller starts heating intermittently until the desired temperature is reached and maintained to within f l "C. At the moment that the low-temperature mode is selected, three changes in the electric circuit are effected. Independently from the temperature controller, 6 W of electrical power are provided to the heating wire of the copper sleeve. Without this heating, the trap assembly would tend to cool down close to the temperature of liquid nitrogen. Secondly, a compensation voltage is connected in series with the thermocouple. This compensation voltage, which is adjustable from 0 to 10 mV, determines the temperature span of the trap assembly. In addition, the heater of the liquid nitrogen pump is connected via the relays of the controller to a 2-W power supply. Nitrogen gas bubbles, which are generated by the heater in the lower end of the rise-tube, lift liquid nitrogen into the copper tray. The excess of liquid nitrogen spills over the edge of the tray and returns to the reservoir. When the desired temperature is approached, the controller starts interrupting the power to the heater of the pump. The selection of a proportional bandwidth of 4% and a time constant of 10 s ensures a fast attainment of the set temperature without serious overshoot. The performance of the cold trap is illustrated in Figure 2, which shows four cooling-heating cycles. Curve 1 is the time-temperature relationship for an upper and lower temperature of 50 and -20 "C, respectively. A t t = 0, the cooling is started, and the cooling rate is about 1 "C s-l. However, it takes about 3 min to attain the desired temperature because an overshoot of 5 "C occurs. The overshoot becomes negligible at temperatures below -50 "C. The lower temperature settings 0 I982 American Chemical Soclety
Anal. Chem. 1982, 5 4 , 2623-2625
2623
GLASS TRAP POLYSTYRENE FOAW COVER
COPPER SLEEVE
COPPER TRAY THERMOCOUPLE iUNCTION COAXIAL HEATING WIRE I
COPPER FOWDEF
0
:
2
3
4
5
6
7
0
1
2
PYREX DISC
PYREX LIFT TUBE
LIQUID NITSOGEN
HEATER
Flgure 1. Cross section of the cold trap assembly. The dimensions of the copper sleeve and tray are 6 X 55 X 75 and 4 X 22 X 55 mm, respectively. The weight ad the copper sleeve, tray, and powder is 180 g. The inside diameter of the lift tube Is 4 mm. The heater consists of a wire-wound resistor. A Pyrex filling tube is not shown.
can be kept constant within f 2 "C. The upper temperature can be maintained for any length of time. The maximum duration of the low temperature depends on the rate at which the liquid nitrogen is consumed, unless the nitrogen is replenished. A temperature of -196 "C can be maintained for 1h. The heating rate of the trap assembly is 0.7 "C s-l, and no overshoot occurs. It should be noted that despite the good thermal conductivity of the copper sleeve, thermal gradients due to the fast heating and cooling occur. The steady-state situation is typically reached within 1min, exhibiting temperature differences of about 2 "C inside the copper sleeve. Curves 2 and 3 show the temperature vs. time for cooling down from 100 "C. At -80 "C in curve 2, a minor overshoot happens. The differenclg in the cooling rate of curves 2 and 3 illustrates the largest difference observed during many runs. In the four cases shown, the initial liquid nitrogen level was 0.5 cm below the bottom of the glass disk. Variations in this level of 0.5 cm have a negligible effect on the cooling rate. Care
3
4
5
l
l
6
7
tlme i m l r )
I
Flgure 2. The trap temperature during the cooling and heating initiated at t = 0 .
should be taken to remove water, which accumulates gradually, from the Dewar. A convenient method for accomplishing this task is to leave the trap at a high temperature setting overnight. Curve 4 shows the trap temperature during the cooling to liquid nitrogen temperature, which requires the maximum quantity of nitrogen, resulting in a 1.5-cm drop of the nitrogen level in the reservoir. The extreme cooling rate observed below -100 "C is due to the disappearance of the Leidenfrost effect, which at higher temperatures limits the heat transfer between the liquid nitrogen and the copper tray. A cold trap as depicted in Figure 1 has been operating satisfactorily in our laboratory for 3 years. Its application, combined with automatic high vacuum glass taps (5),has enabled the automation of a glass system that produces hydrogen from organic compounds for the determination of the D/H ratio of environmental samples. In this particular application, the lower temperature setting is -70 "C, which ensures complete trapping of the water of combustion and minimizes the collection of contaminating vapors.
LITERATURE CITED "Handbook of Chemistry and Physics", 53rd ed.; Chemical Rubber Co.: Cleveland, OH, 1972; p D177. Chopra, D.; Babb, H. Rev. Sci. Instrum. 1975, 46, 1126-1127. , Des Marais, D. J. Anal. Chem. 1978, 50, 1405-1406. (4) Stump, R. K.; Frazer, J. W. Lawrence Radiation Laboratory Report, UCRL-50318; University of California: Livermore, CA, 1967. (5) Brenninkmeijer, C. A. M. I n t . J. Appl. Radiat. Isot. 1981, 32, 679-680.
14 .
RECEIVEDfor review June 7 , 1982. Accepted September 7, 1982.
Comparison of Concentration Techniques for 2,3,7,8-Tet rachlorodibenzo-p -dioxin Patrick O'Keefe,* Cam1 Meyer, and Kathleen Dlllo~ Center for Laboratories and Research, New York State Department of Health, Albany, New York 12201
Pesticides and related compounds are generally present in dilute solution (>IO-mL samples) after cleanup from environmental samples. However, the final steps for quantitative determination require extract volumes ranging from 50 pL for high-performance liquid chromatography (HPLC) down to 1 to 3 pL for capillary gas chromatography (GC). Concentration of extracts to these small volumes may result in
loss of sample components through adsorption, decomposition, and evaporation (1-3). Efficient concentration is particularly important in the case of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), since there is a need to determine this very toxic compound at part-pertrillion concentrationsin environmental samples. Evaporation to dryness with an inert gas stream appears to be an accepted
0003-2700/82/0354-2623$01.25/00 1982 American Chemical Society
