19’81
Anal. Chem. 1981, 53, 1961-1962
identical injector/detector/splitter/proportionalcounter combination was used to produce both traces; only the make-up gas flows were changed to ensure that the same total gas flow entered the splitter in both cases. Clearly capillary-RGC with a commercial gas flow proportional counter is feasible-in F‘igure 2.4 (that produced from the capillary system) well-defined peaks of mass and radioactivity are evident. Even more importantly, the increased resolution of which the capillary column is theoretically capable, is delivered to the RGC experiment. This is seen most palpably for the three sets of doublets (peaks 5/6,7/8,9/10). On the packed columns, base line resolution does not occur for any doublet set. With the capillary column, base line resolution is observed for all. If the concept of separation number [(retention time difference between two adjacent peaks/sum of half-bandwidths of these two peaks) - 11 can be applied legitimately to temperature programmed gas chromatography, the separation number corresponding to peaks 9/10 on the ]packed column would be zero, wlhile for the capillary column it would be 3. Figure 3 highlights some performance characteristics of capillary-RGC. Panel A shows the response corresponding to a sample of approximately 200 dpm (4ng, taking account of splitter) of n-[14C]dodecanol (peak 4 of the ‘he-ol” standard, see Figure 2). The radioactivity peak is 3.8 times the peak-to-peak background signal. We predict from this datum that our capillary-RGC unit could detect 1010 dpm comfortably. Two operational advantages of capillary-RGC over packed-column RGC are shown in the remainder of Figure 3. Panels B and C show, respectively, the corresponding section of RGC traces produced by packed and capillary columns from a radiolableled plant extract. Accurate attribution of radioactivity to mass in the area marked “a”, is not possible in the trace produced by the packed column (panel B); it is possible in that, produced by the capillary column (panel C). Panels D and E provide even more cogent proof
of the value of capillary RGC. Commercial samples of desmosterol, used by us in studies of cholesterol biosynthesis, consistently gave broad peaks on packed GC columns. When spiked with [14C]desmosterol,the radioactivity peak was not commensurately broad (panel D). The capillary RGC run of the spiked sample (panel E) shows that “commercial desmosterol” is in fad a mixture of two materials. The second substance is the true desmosterol (checked by mass spectrometry).
DISCUSSION Our results establish (a) that capillary-RGC with cornmercial gas flow proportional counters is feasible and (b) that the increased resolution that it brings to the RGC experiments can be valuable operationally. Indeed the commerciallybased unit is 4-fold more sensitive than the custom-designed system of Gross et al. in detecting 14C(100 vs. 400 dpm). One reason for this improved performance may be that the unit designed herein permits independent optimization of gas flow through GC column and proportional counter. LITERATURE CITED Kokes, R. I.; Tobln, M.; Emmett, P. H. J . Am. Chem. SOC.1855, i.7, 5860-5862. Campbell, I. M. Angal.Chem. 1978, 51, 1012A-1021A. Doerfler, D. L.; Rosenblum, E. R.; Malloy, J. M.; Naworal, J. D.; Mcklanus, I. R.; Campbell, I. M. Biomed. Mass Spectrom. 1980, 7 , 259-264. Hamnett, A. F.; Pratt, E. G. J . Chromatogr. 1878, 158, 387-399. Gross, D.; Gutekunst, H.; Blaser, A,; Hambock, H. J . Chromatogr. 1980, 198, 389-396. Campbell, I. M.; Doerfler, D. L.; Donahey, S. A.; Kadlec, R.; McGandy, E. L.; Naworal, J. D.; Nulton, C. P.; Venza-Raczka, M.; Wlmberly, F. Anal. Chem. 1877, 49, 1726-1734.
RECEIVED for review May 20, 1981. Accepted July 13, 1981. The financial support of the National Institutes of Health (Grant No. GM 25692), the National Science Foundation (Grant No. PCM 77-03966), and the Muscular Dystrophy Association of America is gratefully acknowledged.
Automatic Liquid Nitrogen Controller for the Thermal Energy Analyzer Cold Bath James L. Owens” and Oswald E. Kinast Monsanto Company, 800 M t f h Lindbergh Boulevard, St. Louis, Missouri 63 166
Since the introduction of the thermal energy analyzer (TEA) (I), the determination of N-nitrosamines has been greatly facilitated. This instrurnent is a selective and sensitive detector and can be interfaced with either a gas or liquid chromatograph. This fealture allows for the determination of both volatile and nonvolatile N-nitrosamines in many kinds of matrices. A cold trap and bath are essential elements of the TEA’S selectivity and sensitivity. After the catalytic disruption of the N-NO bond in the TEA pyrolyzer furnace, the fragments are directed into the cold trap which is maintained at -160 OC by using an isopentane/liquid nitrogen slush. This temperature is cold enough to freeze out most organic solvents and molecular fragments but allows the NO radical to pass into the reaction chamber. The preparation of the isopentane/liquid nitrogen cold bath slush is very tedious and time-consuming. Periodic addition of liquid nitrogen is requhed to maintain the temperature cold enough for proper operation. The storage and handling of isopentane are also safety hazards. The liquid boils at approximately 28 “C and is extremely flammable.
A desirable feature for analyzing large numbers of samplw for N-nitrosamines is automatic operation, i.e., autoinjection, especially for GC-TECA operation. Because of the problenis associated with handling isopentane, autooperation is not possible. In order to circumvent these problems, we investigated other alternatives. Liquid nitrogen alone proved to be too cold for our work, decreasing sensitivity by about 40%. In addition, liquid nitrogen boils off too rapidly. We tried without success to find other solvents which would produce the desired temperature, solve the handling problems, and be amenable for autoogeration. Another alternative was the use of cascade coolirig devices such as the RdC-4-130 Multi-Cool system (FTS Systems, Inc., Stone Ridge, NY). However, these systems are bulky, noisy, and exlpensive. Thermo Electron Corp. (Waltham, MA) has introduced a disposable cartridge trap called the CTR Gas Stream Filter. These filters replace the cold trap altogether and are comvenient to use. Depending upon sample type and load, they are good for 1-2 days. For certain analytical requirements they are as good as the cold trap and slush arrangement.
0003-2700/81/0353-196’I$O1.25/00 1981 American Chemical Society
1962
Anal. Chem. 1981, 53, 1962-1963
F i h Guide
Figure 1. EssenUal components for automatic liquid nitrogen controller.
The gas-liquid separator, wood dowel, float, and insulated stainless steel tubing are all contained within a 3.8-L Dewar flask.
However, we have found the cold trap more effective for producing a base line noise level low enough for the required sensitivity, precision, and accuracy in our analyses of meat samples for N-nitrosamines. Moreover, a CTR costs $10-12 each. The device described in this report uses 3-4 L of liquid nitrogen per day at a present cost of $0.26 per liter. We report here a simple, inexpensive device which allows automatic GC-TEA (and LC-TEA) operation without the need for hazardous solvents, periodic checking/adjustment of temperature, or a large investment of money for commercial devices. It eliminates most of the cold bath preparation time and operates with minimum attention. It is also economical in the consumption of liquid nitrogen, using only about 0.25 that required for the liquid nitrogen/isopentane cold bath slush. The controller consists of three parts (see Figure 1): trap insulation, float-valve,and gas-liquid separator. Since liquid nitrogen temperature is too cold for normal GC-TEA operation when the stainless steel trap is immersed directly into it, a suitable insulating material of appropriate wall thickness is required to maintain the trap temperature at a desired value (our device is designed for a trap temperature of -160 "C to -150 "C). The insulation is required to balance the heat input of the gas flow from the TEA vs. the cooling effect of the liquid nitrogen. The trap for GC mode of operation is made from a 56 cm length of standard 0.25 in. 0.d. stainless steel tubing. The tubing is bent at mid-point with a 3 cm radius. The insulating material is a 50 cm length of 0.25 in. bore, 0.25 in. wall seamless foam rubber pipe insulation (Rubatex, Rubatex Corp., Bedford, VA, or Armaflex, Armstrong Cork Co., Lancaster, PA) which is slipped onto the stainless steel tubing. The trap is connected to the TEA via Swagelok connectors and Vespel ferrules. The trap is then immersed about 6 cm into a 10 cm depth of liquid nitrogen in a 3.8-L Dewar flask (e.g., Pope Scientific Inc., Menomonee Falls, WI). The leads of a thermocouple were inserted inside the inlet of the stainless steel trap (exit from TEA). The temperature was monitored as a
function of the approximate distance the leads were from the bottom of the U-curve of the trap. At a height of about 10 cm above the U (5-6 cm of the trap is below the liquid nitrogen level) the temperature was -100 "C. The temperature dropped rapidly to -185 "C at the U. Between about 7.5-8 cm above the U on either side of the trap the temperature ranged from -130 to -185 "C. This temperature profile was quite sufficient to provide excellent TEA operation. The temperature profiie inside the stainless steel trap can be maintained at various temperatures by changing the wall thickness of the insulation and/or adjustment of the immersion depth in the liquid nitrogen. The float-valveassembly controls the flow of liquid nitrogen and maintains it at the desired level in the Dewar flask. The float is constructed from Styrofoam with a thickness of 2 cm and diameter of 10 cm and is weighted enough to activate a microswitch. The total weight including the float, shaft, and weight is 42 g. The float is suspended from the microswitch activating arm. To suppress laboratory electronic noise, a 0.1 MFD 600 V dc capacitor in series with a 0.5 W 100 Q resistor is connected around the switch. The microswitch and float guide are mounted on a 0.5 in. wood dowel. Wood was chosen to avoid frosting problems since it is partly submerged in the liquid nitrogen. The microswitch operates a standard cryogenic solenoid valve (e.g., Valcor 94C19C6, Valcor Engineering Corp., Springfield, NJ, or ASCO valve 8262D22, Automatic Switch Co., Florham Park, NJ). The valve controls the liquid nitrogen flow via a 0.0625 in. port from a large liquid nitrogen reservoir which is at a nominal pressure of 20 psi. The solenoid valve is well insulated to minimize heat transfer. The allowable liquid nitrogen level fluctuation is f 2 cm. The actual change with our device is only a few millimeters. The liquid nitrogen flows through an insulated 0.25 in. copper tube to a fabricated gas-liquid separator from which the liquid nitrogen flows by gravity to below the surface of the existing liquid nitrogen pool. The separator is constructed from Styrofoam with an inside chamber 2 X 2.5 X 20 cm and wall thickness of ca. 1cm. It is suspended in the Dewar flask on the 0.25 in. copper inlet tube which enters the separator at the top and terminates at mid-chamber. Gas vents through a 1 cm. i.d. Teflon tube at the top of the separator. It is important that the cold gas is vented out and away from the Dewar flask so as to not disturb the atmosphere in the flask around the stainless steel trap. A sudden flow of liquid nitrogen usually results in an initial blast of gas caused by the cooling effect of liquid nitrogen on the tubing, solenoid valve, and other hardware. This sudden, cold blast of gas can cool the temperature of the upper regions of the trap enough to cause a temporary decrease in sensitivity of the TEA. The liquid drains through two Teflon tubes (7 mm o.d., 3 mm i.d., 10 cm long) submerged in the pool of liquid nitrogen.
LITERATURE CITED (1) Fine, D. H.; Rufeh, F.; Lieb, D.; Rounbehler, D. P. Anal. Chern. 1975,
47, i i a a .
RECEIVED for review March 30,1981. Accepted July 16,1981.
Tube Cracker for Opening Samples Sealed in Glass Tubing Dennis D. Coleman Illinois State Geoiogical Survey, 615 East Peabody Drive, Champaign, Illinois 61820
The method developed by DesMarais and Hayes (I) for cracking ampules made of standard glass tubing in a closed system has greatly facilitated the storage and handling of small 0003-2700/8 1/0353-1962$01.25/0
gas samples and has in many cases eliminated the need for breakseals, which are more costly and troublesome. This method utilizes Cajon Ultra-Torr fittings and stainless steel 0 1981 American Chemical Society
