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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
Anal. Chem. 1981, 53, 1963-1965 TO 'VACUUM SYSTEM
c
Figure 1. Simplified sample tube cracker: (A) indentation to prevent plug from sliding up tubing; (13) Teflon disks, '/e in. thick with a 1-2 mm diameter hole through the middle; (C) disk cut from 200-mesh screen; (D) size 1817 groundl-glass ball joint; (E) size 18/9 groundglass socket joint; (F) position of scratch on sample tubing; (G) FETFE O-ring; (H)size 7 Ace-Thred connector; (I) nylon bushing from Ace-Thred connector; (J) sarrple ampule made from 6 mm 0.d. glass tubing. The ball and socket jolnt must be held together with a spring-tension clamp; the clamp is not shown.
flexible tubing and requires some machined parts. Although this method is effective, sometimes the tubing shatters if it is not positioned properly within the tube cracker. Also, the cost becomes a significant €actorif multiple tube crackers are needed. Consequently, another method has been developed that consistently results in clean breakage of the glass tubing and utilizes readily available, low-cost materials. The apparatus shown in Figure 1,for cracking 6-mm 0.d. tubing (J),utilizes a size 18/7 ground-glass ball joint (D) and an adaptor consisting of a size 18/9 ground-glass socket (E), attached to a size 7 Ace-thred connector (H) (Ace Glass Inc., Vineland, NJ). The two halves of the ball and socket joint, are greased with
1963
high-vacuum stopcock grease and must be held together with a spring-tension clamp (this is not shown in the figure). Thle tubing to be cracked is scratched about 3 cm from the end and then inserted through the O-ring (G)until the scratch is positioned at the base of the ball joint (F).The bushing (I) is tightened to seal the O-ring aroung the tubing and the system is evacuated. The tubing may then be cracked by slightly flexing the adapter; if the tubing is correctly positioned, little force is needed to crack it. If the tubing doer3 not break easily, it cain be repositioned without opening the system by sliding it up or down through the O-ring seal. Because the scratch can be seen through the glass adapter, correct positioning is easily achieved. A Teflon plug (B) with a small (1-2 mm diameter) center hole may be pressed into the connecting tube of the ball joinit to prevent the upper portion of the tubing from being blown up into the vacuum system when the tubing is cracked or when the bushing (I) is loosened to vent the system. In Figure 1, a small disk of 200-mesh screen (C) has been sandwiched between two center-drilled disks (B) that were cut from a l/s in. thick sheet of Teflon with a stopper-boring tool. The fine mesh screen prevents any small chips of glass from entering the vacuum system. Changing sample tubes is easiest if the bushing (I) is completely removed. The O-ring will come out with the lower portion of the sample tube and allow the upper portion of the sample tube to fall out. Although the upper ball joint may be a size 18/9, the smaller inside diameter of a size 18/7 ball joint results in a tighter fit around the 6-mm tubing and thus less flexure is required to crack the tubing. The use of excessive grease on the ground joints should be avoided to prevent grease from squeezing out at the base of the ball andl causing the upper portion of the cracked tubing to stick behind! when the lower portion is removed. Although there should be no problems with leakage with high-quality ball and socket, joints, if problems do develop, the joints can be reground withi a slurry of grinding compound. The use of O-ring type ball. joints is not recommended as they are more likely to leak when flexed during the fracture operation. This system has been used successfully for cracking both Pyrex and quartz tubing. The method is equally effective for 9-mm 0.d. tubing if a size 11Ace-Thred connector and a size 18/9 ball joint are used. However, the inside diameters of the ground joints vary from one manufacturer to another and, because of the closer tollerances encountered when using 9-mm tubing, joints should be chosen that easily accept the tubing that is to be cracked. LITERATURE CITED (1) DesMarais, D. J.; Hayes, J. M Anal. Chem. 1978, 48, 1651-1652.
RECEIVED for review April 27, 1981. Accepted July 6, 1981.
Micro Liquid Chrormatography/Mass Spectrometry Diaphragm Probe Interface Jack D. Henion" Dlagnostlc Laboratory, New York State College of Veterinary Medicine, Cornell University, Ithaca, New York 14853
Timothy Wachs Department of Chemlstty, Cornell University, Ithaca, New York 14853
Several groups have reported LC/MS results on a variety of compounds (1-4)in the recent past. The referenced work generally utilizes either the moving belt introduced by Scott (5) or the direct liquid introduction (DLI) LC/MS interface
first reported by McLafferty (6). These and other approaches have provided an increasingly viable means of accomplishing LC/MS, but routine sensitivity has not been comparable to that afforded by GC/MS. Many resesarchers involved with
0003-2700/81/0353-1963$01.25/00 1981 American Chemical Soclety
