A
B
Figure 1. (a) Sketch of Microslide cross-section indicating internal dimensions; (b) Cross-section sketch of “sandwich” arrangement of Microslides
The signal to noise ratio was 2.5 f 0.2 times as good for the Microslide in the Teflon holder as for the melting point capillary in the quartz tube. This ratio is based on measurements on each of the three peaks in the spectrum for eight capillary tubes and eight Microslides. Average peak heights for the sample in the capillaries were 60 f 2,60 f 2,50 f 2, and in the Microslides were 154 f 11, 152 f 11, and 144 f 11 (in arbitrary units). The major uncertainty appeared to be a systematic bias introduced by differences in the two Teflon sample holders used. Thus it appears that even though the procedure is proposed as a “quick and dirty” survey technique, it will easily provide results quantitatively useful to better than 10%. For comparison, the same sample solution in the Varian flat cell yielded peak heights of 159,154, and 145. Thus the Microslide gives essentially the same results as the expensive flat cell. The Microslides are easier to use than either capillary tubes or the flat cell. The Microslides can be filled by dipping the end in the sample solution; they fill by capillary action.
A rather striking result was obtained with the three Microslide “sandwich” assembly. With the Microslides parallel to the nodal plane of zero rf electric field, the spectrometer could not be tuned up. However, when the Microslides were rotated by 90’ so that they were perpendicular to the nodal plane of the rf electric field, the spectrometer tuned quite easily and yielded a signal 3.8 f 0.4 times as good as a capillary and 1.5 times as good as a single 0.4-mm Microslide or the Varian flat cell in the usual (parallel to node) orientation. This experiment was stimulated by Hyde’s similar finding with specially machined Rexolite sample cells in a TMllo cavity ( 2 ) . Hyde obtained a 4.9 improvement relative to a 1.1-mm capillary, using larger cells. The TElozcavity has only a 11-mm opening so the larger flat cell for which Hyde obtained a 6.25 improvement relative to a capillary cannot be used (2). Thus for the cavity we used, the Microslides in the perpendicular orientation give the best improvement relative to the capillary tube and the flat cell that has been observed to date. The Microslide exhibits the signals expected for glass-a strong resonance at -1600 G and a weak broad resonance near 3400 G-but these do not obtrude significantly for most spectra. The peak near 3400 G increased above noise level when the power was increased to 20 mW and the modulation amplitude was increased to 2 G. These are much higher settings than would be used with organic radicals such as the nitroxyl radical used in this study. LITERATURE CITED (1) R. S. Aiger, “Electron Paramagnetic Resonance: Techniques and Applications”, Interscience, New York, 1968, p 504. (2) J. S. Hyde, Rev. Sci. Instrum., 43, 629 (1972). (3) G. Brown, J. fhys. E, Sci. Instrum., 7, 635 (1974). (4) C. Klopfenstein, P. Jost, and 0. H. Griffith, Comput. Chem. Biochem. Res., 1, 175 (1972).
RECEIVED for review February 28, 1977. Accepted April 8, 1977. Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, the Research Corporation, and the National Institutes of Health (GM 21156) for partial support of this research.
Advances in Assembling Permeation Tubes Andrew E. O’Keeffe Environmental Sciences Research Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 277 1 1
Since publication of our initial article (I) on permeation tubes, these simple devices have attained wide acceptance as accurate primary standards for the calibration of air pollutant measurement methods and instruments. Concurrently, certain improvements have been made in the method of assembling permeation tubes, resulting in a simpler procedure and a more reliable product, Several of these are described below: 1. Tube Seals. In place of the steel ball originally used, we now recommend FEP Teflon rod (or other polymer rod to match the tubing used) slightly larger than the lumen of the tubing used. Figure 1is a dimensional sketch of a typical tube in a size having broad application. 2. Filling. The valving technique originally described for steel ball seals and later ( 2 , 3 )for rod-shaped seals is difficult to apply in the latter case. An alternate technique that is both simple and virtually free of difficulty due to spillage consists of the following steps (also shown in Figure 1): (a). Assemble 1278
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8 , JULY 1977
tube with a rod seal a t each end. (b). Place a 1-cm collar of gum rubber tubing (3-mm wall) around the rod at one end, its center aligned with the inboard end of rod. (c). Place a worm-gear tubing clamp around above assembly, tightening moderately. It is important that this clamp engage some portion of the length of the rod seal. (d). Fill through a hypodermic needle [No. 27, 0.018-in. (0.046-cm) diameter] inserted just inboard of rod. (e). Turn off gas supply and withdraw needle; rubber collar forms a temporary seal. (f). Push rod inward 1cm to final position (Figure 2) (g). Remove hose clamp and rubber collar. This will be easier if, prior to assembly, the collar is slit from end to end along a line approximately opposite the point a t which needle will be inserted. 3. Reinforcing Ferrules. It is recommended that the seals of permeation tubes made as described above be reinforced with Type 304 stainless steel ferrules ( 2 , 3 )as otherwise some
HYPODERMIC NEFDLE
RUBBER TUBING OlZ5mIJOrnrnlWALL
C R l M P l u l i FERRULE
Oin IO 1 6 m m i WALL
FEPROD mmi 014
*BEFORE CRIMPING W O R M GEAR HOSE CLAMP
Figure
1.
Recommended seal and filling technique RUBBER TUBING
CRIMPING FERRULE
(1125 in!] OmmIWALL m(6 5mml I D
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/ 0260
FEPTUBING 0 010 8" (0 16 nml WALL
/ O i 8 1 5 m 14 15 m
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.SECOND QLUGWLABEL, FEPROD 0.201n 1508mml DIA
'BEFORE CRIMPING
Flgure 2. Final sealing and labeling ,A-
0 30 in 17 5 2 mmi D I A .
0.301n I1 6 2 mm) D l A
4. Liquid vs. Gas Filling? We usually prefer to carry out the above-described filling procedure by distillation. Supply cylinder is held upright, gas distils through the hypodermic needle into the permeation tube. A few degrees' warming of the supply and/or cooling of the tube will initiate distillation. Either prior evacuation of the tube or repetitive (partial) fiiing and venting will overcome the tendency to form an air lock. Some polymer-forming gases ( 4 5 )e.g., 1,3-butadiene,vinyl halides, tetrafluoroethylene, are supplied with polymerization inhibitor(s) added. Liquid filling-i.e., filling with liquid withdrawn from an inverted supply cylinder-is recommended for such gases, as the finished permeation tube will then contain sufficient inhibitor to minimize any decline of permeation rate attributable to decreasing monomer partial pressure. 5. Labeling. Each tube should carry its own unique identification. A bit of pressure-sensitive adhesive paper affixed to a Teflon rod and inserted in the space vacated in the final adjustment of the sealing rod (paragraph 2 (fl above) affords an effective, visible, and protected means of accomplishing this end.
ACKNOWLEDGMENT Thanks are due Lester L. Spiller for obtaining the fabrication of the crimping tool described herein. LITERATURE CITED (1) A. E. O'Keeffe and G. C . Ortman, Anal. Chem.,38, 760 (1966); 41, 1598 (1969). (2) F. P. Scaringelli, A. E. OKeeffe, E. Rosenberg, and J. P. Bell, Anal. Chem., 42, 871 (1970). (3) B. E. Sakzmn, W. R. Burg, and G. Ramaswamy, Environ. Sci. Techno/., 5 , 1121 (1971). (4) W . A. McClenny, B. E. Martin, R. E. Baumgardner, R. K. Stevens, and A. E. O'Keeffe, Environ. Sci. Techno/.,10, 810 (1976). (5) W. R. Burg, S. R. Burch, J. E. Cuddeback, and B. E. Saltzman, Environ. Sci. Techno/.,10, 1233 (1976).
W E L D A U X I L I A R Y CRIMPING JAWS I N PLACE
Figure 3. Crimping tool
RECEIVED for review February 18,1977. Accepted March 25, 1977. Teflon and Vise-Grip are trademarks of E. I. du Pont
materials (notably NOz) tend to creep through the seals, in effect disturbing the normally steady permeation process. Figure 3 illustrates a simple modification of a Vise-Grip wrench that is very effective in applying such reinforcements.
de Nemours and Company, Inc., and Petersen Manufacturing Company, Inc., respectively. Mention of commerical products does not imply endorsement by the Environmental Protection Agency.
Extension of the Low Temperature Range of a Microprocessor-Controlled Gas Chromatograph Willfrled Dulson Institut fur Wasser, Boden-und Luehygiene, des Bundesgesundheitsamtes, 7 Berlin 33, Postfach, Federal Republic of Germany
The use of the thermogradient tube has established itself in modern environmental analysis as a satisfactory sampling procedure prior to the determination of volatile pollutants in air and water samples. According to this procedure, the air to be sampled is drawn through a length of intensely cooled metal tubing filled with adsorbent material (1). The pollutants present in the air remain trapped on the surface of the cold adsorbents and are steadily concentrated (2). When sampling is complete, the cold tube is sealed and may be conveniently transported to the laboratory. In order to separate and determine the adsorbed contaminants, they must first be transferred to the gas chro-
matograph. This is accomplished by opening the recooled sample tube and connecting to a suitably modified GC injector block. The tube is subsequently warmed by means of a hot-air blower or inductive heating, and the desorbed contaminants are carried in a gas stream onto the separating column. At this stage, the oven temperature must be sufficiently low to ensure that the sample condenses in a narrow band at the head of the column. On completion of the transfer process, the oven temperature is boosted up by a suitable program so that the column rapidly attains its working temperature range. The further analysis now proceeds as though the sample had been injected in the normal manner. ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
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