Thiourea as Substrate for Gas Chromatography of Fluorocarbons SIR: We report here that thiourea, but not urea, is a good substrate in gas chromatography for the separation of the unsaturated compounds perfluoro2-methylpentene-2 and cis- and trans-perfluoro-2-met hylpentene-3 from perfluorohexanes and from one another. The use of urea and of thiourea was an attempt to makte use of the canal complexes that these chemicals form with numerous coimpounds ( 1 ) . If this phenomenon Rhould occur in a chromatographic column, a separation of isomers would be possible resulting in the elution of the most highlybranched structure first and the normal isomer last. With both of these substrates the saturated perfluorohexanes appear in an order which is the reverse of that found with n-hexadecane as substrate ( 2 ) . With the latter substrate the isomers appear strictly in order of increasing boiling point which is the order for increasing complexity of molecular chain branching in CsFla. On urea and on the n-hexadecane substrates the unsaturated molecules are not separated from the saturated ones.
EXPERIMENTAL
Sources for the perfluoroalkanes have been cited ( 2 ) . The perfluoroalkenes were prepared in this laboratory, purified by preparative-scale gas chromatography, and identified by nuclear magnetic resonance. The urea and thiourea packings were prepared from a water solution evaporated on a steam bath in contact with acid-washed Chromosorb-P. The packed 1/4-inch 0.d. columns were dried by helium flow at room temperature. Chromatograms were obtained in conventional apparatus, and the retention volumes were standardized for pressure corrections and void volumes. The carrier gas was helium. Hydrogen reacts with thiourea and destroys the separating ability of the column. RESULTS
Thiourea is a solid at temperatures below 180' C. This work was carried out at 25' C. and thus may be an example of gas-solid chromatography. Chromatogram peaks were symmetrical with no tailing for the sample sizes used, 1 to 2 ml. of liquid. The following standard retention volumes in cc.
of helium a t 1 atm. and 25" C. were obtained at 25' C. on 15.2 meters of 1/4-inch copper tube containing 31.2 grams of thiourea on 124.6 grams of Chromosorb-P, 35- to 80-mesh, with helium carrier at 34.7 p s i inlet and 16.8 p.s.i. outlet pressure flowing a t 29.9 cc. per minute when metered a t 1 atm. and 25' C . : perfluoro-2,3dimethylbutane, 344; perfluoro-2methylpentane, 368; perfluoro-3methylpentane, 385; perfluoro-nhexane, 410; trans - perfluoro - 2 methylpentene-3, 323; cis-perfluoro-2methylpentene-3, 377; perfluoro-2methylpentene-2, 451. LITERATURE CITED
(1) Brown, J. F., Jr., Sci. Am. 207, 82 (July, 1962). ( 2 ) Reed, T. M., 111, J. Chromatog. 9, 419 (1962).
JAMES C. MAILEN T. M. REEDI11 JOHN A. YOUNG Department of Chemical Engineering University of Florida Gainesville, Fla. WORKsu ported in part by the Atomic Energy &ommission and the National Science Foundation.
Infra red Mic r ocel I Richard M. Chrenko,, General Electric Research Laboratory, Schenectady, N. Y.
A
of fixed thickness microcell suitable for the measurement of infrared absorption spectra of liquids is described. . Its sandwich construction makes this microcell a scaled-down version of commercially available macrocells, except for several important features. I t has been used in the PerkinElmer 4-1 double-beam spectrophotometer with the Perkin-Elmer 4-1 beam condenser, and with the Beckman IR-7 double-beam spectrophotometer with the Beckman beam condenser. This microcell was designed to contain the liquids collected in (capillary tubes from gas chromatography columns. These liquids were volatile and reactive in air. Hence, the primary design requirements were based on the volumes that were obtained from the columns, the cell thickness needed to obtain meaningful NEW TYPE
spectra of neat samples, and the need to fill and seal the cell in a drybox. Typical samples were of the order of 5 pl. and, because of the volatility of the samples, the cell had to have approximately this total volume. This was necessary since the infrared beam illuminates and heats the central section of the cell, and if one did not have a completely filled cell, volatile samples would distill out of this section. However, the active volume of this central section of the actual cell was less than 0.2 pl. 4 constant and reproducible cell thickness of 1/2 mil was deemed appropriate for obtaining meaningful spectra. A number of secondary requirements also led to this design; namely, the cell had to be easy and quick to construct; easy to fill; capable of allowing an operator to observe the entire liquid volume during and after
filling; easy to seal; and easy to clean. A number of microcells are available commercially (Perkin Elmer Corp.; Beckman Instruments; Connecticut Instrument Corp.), or are described in the literature (1-S), that enable one to measure the infrared absorption spectra of small-volume liquid samples. However, each cell has a drawback that makes it unsuitable for the liquid samples from chromatography capillary tubes. These cells will not be described here. EXPERIMENTAL
Apparatus. T h e microcell can best be described by discussing first the liquid container and then the mechanism for sealing the filling holes. A blown-up view of the cell is shown in Figure l a . The dimensions given VOL. 36, NO. 9 , AUGUST 1964
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below are our compromise between small volume and ease of construction and use. The liquid container consists essentially of two pieces of NaC1 (other alkali halides can be used for extended range in the infrared) separated by a l/2-mil silver spacer. The bottom slab of NaCl is 1 cm. X 2 cm. X 2 mm. and is polished on both sides. The top slab is also 1 cm. X 2 cm. X 2 mm., is polished on both sides, and has two holes 1 cm. apart and 1 mm. in diameter. The Ag spacer is amalgamated and is 0 8 cm. X 1 8 cm. X ‘/2 mil with a slot 1 mm. X 1 cm. cut in the middle. Assembling. Place the amalgamated Ag sheet on the top slab of NaC1, being careful t h a t t h e Ag spacer doesn’t extend to the edges of the NaCl and t h a t the slot ends in the Ag spacer are aligned with the holes drilled in the NaC1. Next, place the bottom slab of NaCl on the Ag. Then clamp this sandwich firmly between two thick plexiglass plates (see Figure 16) in order to see that the contact between the amalgamated Ag and the NaCl slabs is good. Finally, spread Eastman 910 adhesive around the outside of the joint to bond the NaCl plates together. The 910 adhesive makes a permanent bond between the NaCl plates in a few minutes and the cell is ready to use. The sample, itself, is contained by the NaC1amalgamated -4g-NaC1 combination. The 910 adhesive is used only to hold the sandwich together, and the sample never touches the adhesive. The filling holes are sealed simply by pressing amalgamated P b balls 50 mils in diameter (Fisher Scientific Company; lead metal, No. 12 shot) onto the holes by means of phosphor bronze clips, which are normally swung to one side during the filling process. RESULTS
A microcell built to these specifications has met the original requirements. The calculated total volume of the cell is less than the 5-pl. specification. The slot in the Ag spacer contains approximately 0.13 pl. and the two filling holes contain 3.2 pl. total. However, excellent spectra of neat nonvolatile samples have been taken using only 0.4 pl. of sample. The ceIl is easy to construct, a complete cell taking approximately 1 hour to make. No particular problems have arisen in filling the cell from drawndown capillary tubes. The lack of any opaque materials on top allows one to observe the entire liquid path. The amalgamated P b balls make an excellent seal. Cleaning with solvents is easy and efficient because of the two filling holes and the straight-through flushing path. CONSTRUCTION HINTS
Several techniques have been used that make the construction of the cell easier and more efficient. The salt plates are usually cleaved to 1 cm. X 2 cm. and then ground and polished to 2 1884
e
ANALYTICAL CHEMISTRY
-
( a ) CELL
so DOUBLE STICKY TAPE METAL BASE PLATE
[ c ) POSITION
(b) CLAMP FOR CELL
OF SPACER
(SECTION THROUGH ) CENTER OF CELL
CORRECT
WRONG -
Figure 1 Exploded view of microcell plus holder Plexiglass clamp for assembling cell c. View showing filling hole and Ag spacer-positioned correctly and incorrectly a.
b.
mm. thickness. The grinding is done using No. 220 and No. 600 Rubwet paper, and polishing is done using 0.3micron powder (Macalaster Bicknell Company, Syracuse, N. Y. ; 0.3-micron Precisionite polishing powder) with methanol on a suede cloth material (Behr-Manning, Watervliet, N. Y. ; electrocoated Norzon fabric). The filling holes can be drilled with a machinist’s drill or dental bur, but some care must be taken since small cracks may appear a t the surfaces. This problem is remedied by making the top plate thicker than is necessary, drilling the holes, and then grinding the plate thinner until the cracks are gone. The slot in the Ag spacer should have no jagged edges that could trap part of a sample and, hence, possibly contaminate following samples. The Ag spacer itself, and the P b balls for sealing the filling holes, are amalgamated in a dish of H g covered with acetic acid, rinsed with water, and then blotted to remove any excess H g before they are used. During assembly of the cell, the slot ends in the Ag spacer should be carefully positioned with respect to the filling holes in the top slab so as to eliminate any dead-end traps that could not easily be reached by a cleaning solvent (See Figure IC.). The complete circumference of the slot should be a tight seal for the same reason, and also to prevent the cell from leaking. Eastman 910 adhesive is used to physically bond the sandwich, mainly because of its quick drying characteristics which enable the
cell to be used soon after assembling. Other adhesives should work equally well if immediate use of the cell is not required. The Ag spacer is made smaller than the NaCI plates so that the adhesive bonds KaC1 to NaCl. The 910 adhesive does not form a bond with amalgamated Ag. Filling the cell from a sealed capillary tube is accomplished by necking down the tube a t a point removed from the sample, breaking the tube a t the neck, and placing the broken-off necked-down portion in the filling holes. Judicious use of a soldering gun near the sealedoff end of the capillary will induce the liquid to enter the cell. The P b balls that seal the liquid are used only once because they deform slightly when used. The cell is cleaned by placing a piece of tubing attached to an aspirator over one hole, while squirting an appropriate solvent in the other hole. If the outer surface of the cell becomes dirty, it can be repolished without taking the cell apart. The volume of the cell might be reduced by either making the top slab thinner or making the filling holes with smaller diameter. Making the slab 1 mm. thick instead of 2 mm. reduces the total cell volume to under 2 pl.) but the top is appreciably more difficult to polish. Making the filling holes 0.5 mm. in diameter instead of 1 mm. reduces the total cell volume to 1 pl., but filling problems are greatly enhanced. The cell dimensions mentioned previously are considered a good com-
promise between small volume and ease of use and construction, resulting in a cell that is practical for measuring infrared absorption spect,ra of neat volatile liquids of a few microliters volume and of neat nonvolatile liquids of a few tenths of a microliter volume.
ACKNOWLEDGMENT
LITERATURE CITED
The author acknoffledges the technical advice of R. s. LfClhmld and the helpful comments from Dorothy McClung, who used the cell for infrared measurements.
E.R.1 Parrish, N.,Bird, G . R., Abbate, M.J., J . Opt. SOC.A m . 42, 966
( 1 ) Blout,
(1952). ( 2 ) Cole, A. R. H., Jones, R. N., Zbid., p. 348. (3) J ~ R. Norman, ~ ~ Nadeau, ~ , Armand, Spectrochim. Acta 12, 183 (1958).
Pneumatic Dewpoint Meter William S. Pappas, Technical Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Corp., Nuclear Division, Oak Ridge, Tenn. PNEUMATIC response due to Condensation of moisture a t a restriction in a flowing system has been used in a family of moisture analyzers: ( I ) a simple portable dewpoint meter, ( 2 ) a highly sensitive, accurate instrument, (3) a n automatic dewpoint alarm, and (4)a sensitive and accurate continuous moisture analyzer. ‘I’hese instruments are characterized by their sensitivity, low cost and maintenance, and relative independence of operator judgment. The analyzers provide pneumatic signals to be applied directly to control systems. The principles are generally applicable t’o determination of condensables, including corrosive compounds. These low cost analyzers were developed to meet a need for intermittent and continuous measurement of dewpoints, especially a t low moisture levels. Most methods ( I ) for moistme determination have limited sensitivity and accuracy a t low concentrations and some are dependent upon operator judgment. Subsequent to the completion of this work, Keidel introduced the Electrolytic Hggrometer ( 2 ) which i;s applicable to a wide variety of gas media; however, it is subject to several interferences (2). Also, trace contaminants such as methanol and basic gases are detrimental to the detector. While the Electrolytic Hygrometer is sensitive to moisture as low as 1 to 2 p.p.m., commercial instruments generally have limited application at lower levels. This report describes the development of a simple analyzer for the determination of condensables, with emphasis on its application to measuring trace moist>ure in air in the range of 0.02 to 10 p.p.m. THE
PRINCIPLE
Figure 1 represents a typical sensing system. For a gas passing at a controlled rate through a progressively cooled restriction, an abrupt increase in the differential pressure occurs when condensation causes plugging. The restriction temperature, when the pressure deflection occurs, corresponds closely to the dewpoint of the ?;as a t the system
Time lndicolor
COOLING CURVE
Figure 1.
Model I-manual
pressure. The measured dewpoint temperature can be corrected to standard (atmospheric) pressure conditions. For continuous measurement, the pressure signal may be used to operate the restriction heater to maintain a partial condensation plug. Under these conditions the temperature of the restriction follows the dewpoint temperature of the gas being tested. MANUAL DEWPOINT METER
Two portable analyzers, differing slightly from each other, were developed for intermittent dewpoint measurement.
Model I-Simple Portable Instrument. As shown in Figure 1, sample gas enters the analyzer at constant pressure P I and leaves a t constant pressure P 3 . Pressure P p , the variable pressure, is dependent principally upon the size of the “detector” restriction which is subject to rapid change when condensation occurs. T o make a dewpoint measurement, a cooling bath of liquid nitrogen is applied to the cold finger. With
dewpoint meter
sample gas flowing, the restriction is progressively cooled, resulting in a gradual decrease in pressure P , due to thermal effects on gas density and viscosity. An abrupt pressure change occurs a t the condensation point as shown in the cooling curve (Figure 1). The temperature a t the restriction when the pressure deflection occurs is taken as the dewpoint. DETECTING RESTRICTION.For dewpoint measurements above -90” F. a small capillary or porous plug restriction is satisfactory. More sensitive response is obtained with a slit-type restriction, prepared by four simple operations. I n the first, a 3/8- or inch copper tube is pinched with a standard pinch-off tool. In the second, the tube is sawed into two Iiieces, either one being useful as a restriction; in the third, the pinched end is smoothed with a file to seal the slit opening. In the final operation, the slit is opened by applying mechanical iiressure at the slit ends to allow about 15 cc. per minute flow a t 1 1i.s.i. pressure differential. For best sensitivity a slit of uniform rectangular cross section is desirable and is achieved with little VOL. 36, NO. 9, AUGUST 1964
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