Device for Isolation of Components Separated by Gas Chromatography Joseph M. Lesser, Barretf Division, Allied Chemical Corp., Philadelphia , Pa.
of gas chromatogT raphy is particularly applicable to HE TECHNIQUE
some analytical problems arising in the laboratory. While some of the separations achieved are remarkable, the location of the peak on the chromatogram does not gii e an unequivocal qualitative identification of the component in yucstion. It therefore became necessary to isolate the individual components and examine them by mass spectrometry. For gas chromatography, this laboratory uses the Burrell Kromo-Tog, n hich ha5 a conventional hot wire detector cell. The cell is so designed that the effluent gas vents directly from the cell bath into the atmosphere. I n cells of older design a piece of steel tubing n a s provided from the cell exit to the atmosphere. This tubing should be removed, because it creates a certain back pressure, which contributes to instrument instability. For collecting fractions, this tube must be removed or be heated as condensation Tvill occur in the cool section, Considerable equipment has been developed for collecting the individual components as separated by chromatography. Generally, the collection devices have been somewhat complex. A very simple technique has been developed in this laboratory for this purpose. The method is applicable to any gas chromatographic device where the effluent gas exit is close to the detector. It may also be applied where
EXIT FROM
-2% &3
DRY ICE HOLDER 70 MM. /
c~~~~~~
METAL CONNECTOR
/
Figure 1.
Collection device
gas exit lines are heated to preyent condensation. The following equipment is required.
Two one-hole silicone stoppers, Burre11 No. 261-9. Two-hole nietal connector, Burrell KO.340-140. Collector tube, borosilicate glass, i mm. in outside diameter, approximately 7 5 mm. long. Holder for d r y ice made from a 25-mm. borosilicate glass test tube cut down to 70 mm. length. The equipment, i5 assembled as indicated in Figure 1. The assembly as
first prepared has one hole of the metal connector in the silicone stopper, thus preyenting the condensation of atmospheric moisture. No moisture is condensed via the open end due t o the positive carbon dioxide pressure. T h e n the desired peak appears on the chromatogram, the assembly is attached via the metal connector to the effluent gas exit, care being taken that both holes are open. After the elution of this component, the collector is removed and sealed a t both ends with suitable plugs. Duplicate assemblies can be used to collect other componcnts from the same run.
A Molecular Distillation Unit for Organic Particulates in Air George R. Sanders and H.
L. Helwig, Division of
or short-path, distillais a successful method for separating, purifying, and handling complex mixtures and delicate or unstable organic compounds of relatively high molecular Tveight. Thus, it seems reasonable to apply it to the organic particulate portions of air-borne pollutants following solvent extraction from the air filter and gross chemical group separation. A short-path still has been designed for this specific purpose. The design is unique in that it includes several features which made its application to this problem feasible. The capacity is such that 1 mg. to 5 grams of material may be handled. The fraction collecting is simple and precise. The temperatures and pressures are measOLECULAR.
M tion
484
ANALYTICAL CHEMISTRY
Laboratories, California State Department of Public Health, Berkeley, Calif.
ured 111 ail absolute manner. The distillation chamber is exhausted from the bottom, thus preventing a n y of the distillate from getting entrained in the vacuum system. The chamber is easily dismantled and cleaned. The visibility is unobstructed. The vacuum system and chamber occupy a space 26 inches wide. The vacuum line is made of 35-mm. glass tubing and is articulated by two 50/30 spherical joints. A large right-angle stopcock with a 15-mm. stopper base serves as the main shutoff valre between the pumps and the system. The chamber itself, 14 inches high and 4 inches in diameter, comprises two parts. The top part, 6 inches long, contains the cold finger that is the condenser for the distillate. The bottom part, 8 inches long, contains a ther-
riionieter, a sensing element for the ionization gage, a steel grid for holding the sample dish, and a heating element. The two parts are connected by ground lips. The tip of the cold finger is 5 cm. above the dish. This distance may be varied by constructing condensers of longer or shorter lengths as demanded by the character and the type of molecules that are being distilled. I n all cases, however, this distance must he rvithin the mean free path of the distilling molecules. The heating element is constructed of four sheets of mica which are cut in a circle 3 inches in diameter and are bolted together. Small holes are drilled around the entire periphery and resistance ribbon is laced across it. It is suspended in the chamber by two stiff wires, vihich are in turn welded into the male portion of a 10/30 standard-taper joint. The female part
0-
3
-““-0330 HEAT
Figure 1. chamber
Molecular
uc
CLLII*T
distillation
of the joints is welded on opposite sides of the chamber. I n operation, the material in a solvent is transferred to the dish and the solvent is completely evaporated. The dish is placed on the steel grid in the chamber. The condenser is connected,
the trap and condenser are refrigerated with d r y ice and acetone, and the system is evacuated to a pressure 5 X 10-6 min. of mercury or lower. Initially the pressure measurements must be the same on both the McLeod and ionization gages. The ionization gage measures condensable as well as noncondensable gas pressures, while the NcLeod gage measures only the noncondensable gas pressures. This is important because the “base line,” or residual pressure, must be known so that the rise in pressure during distillation, as measured by the ionization gage, is the vapor pressure of the distilling substance at that temperature. The pressure measurement on the McLeod gage remains constant (base line) throughout the distillation. After the base line pressure has been attained (0.5-half to 1 hour), a definite amount of energy is supplied, as heat from the resistance heater. The energy is related to the voltage, which is controlled by B variable transformer. The temperature will, over a period of time, rise to a maximum. During this period the pressure \Til1 increase and then gradually drop back t o the base line pressure. The didillate collected during this cycle
is considered BS a fraction. The chamber is cooled with a blast of air, and the system is brought back to atmospheric prcssure. T h e condenser is removed, and the distillate is dissolved off the cold finger with a solvent, generally ether, and is ready for further examination by other means. The compound or compounds distilling for a given amount of energy are within limits of a definite molecular weight range. The next fraction will be a repeat of the cycle, but with a greater energy input. As the differences of the molecular weights in a mixture of compounds increase, the apparatus is able to perform sharper separation and greater purification of the compounds. The organic particulate matter froin air-borne pollutants n-ere distilled in the manner described above. The mixture was knon-n to contain aliphatic and polynuclear hydrocarbons. The distillates n ere characterized by ultraviolet spectra. From this mixture, distillates were obtained which, on the basis of ultraviolet absorption spectra, were benzpyrene and coronene in their pure states.
Sapphire-Windowed Spectrophotometer Absorption Cell for Use with Corrosive liquids Elliot Raisen,l Propellants Laboratory, Bell Aircraft Corp., Buffalo 5, N. Y. EAR-INFRARED
The assembled cell is shown in Figure 1. Construction details are given in Figure 2. T h e gaskets are placed between the windows and the spacer. The ground surfaces of the frames are placed against the n indow.
analysis for water in
N fuming nitric acid according t o the
method of White and Barrett [ K h i t e L., Jr., Barrett, Jv. s., A I ~ A LCHEhl. . 28, 1538 (1956)l has been used in the Bell Aircraft Propellants Laboratory for some time. Although not as accurate as titration, its relative speed and simplicity make it a very useful procedure. A change in analytical procedure was necessitated v h e n the practice of adding a corrosion inhibitor, hydrofluoric acid, was adopted. The hydrofluoric acid \\-as removed by reaction with soft glass beads, making it possible to continue to use a glass absorption cell. The procedure was impractical, because of the excessive amount of time required for the reaction with the glass beads, and the difficulty of obtaining a clear sample due to the presence of a precipitate of silicic acid. Therefore, a cell was designed which could be used in the presence of fuming nitric acid and hydrofluoric acid and n hich would not require any alteration of the Beckman DU spectrophotometer. The cell, howel-er, may also be used with other instruments that have similar cell carriers-e.g., Beckman DK, PerkinPresent address, Physical Chemistry Sertion, Armour Hece irch Foundation of Illinois Institute of Teciiriology, Chicago, Ill.
Figure 1.
Assembled cell
Elmer spectracords, and Cary spectrophotometers. The cell consists of a stopper, and sapphire windows. Although rather expensive, the sapphire FTindows are an excellent choice, because they are virtually impervious to attack by strong acids (including hydrofluoric), and strong bases, and have optical and physical properties which are slightly superior to those of quartz. If the cell is not subjected to strong alkali or hydrofluoric acid, quartz or glass windows can be used with a resultant saving in cost.
To become familiar n-ith the assembly procedure, and to avoid breakage of the expensive sapphire TT indows by excessive tightening of the screws, the cell should first be assembled using glass nindows. The glass n-indows can be readily cut from a microscope slide. Tighten the screws evenly to a finger-tight pressure. Then fill the cell with acetone to check for leakage. If the seal leaks, remove acetone, loosen the s c r e m slightly, dry with air or nitrogen, retighten the screws, and test again. Repeat this procedure until the tension of the screns is sufficient to form a leakproof seal. Because of the low surface tension and viscosity of acetone (and of fuming nitric acid) the screws must be fairly tight. With other solvents-e.g., n-ater, methanol, etc.-the cell can usually be made leakproof on the first attempt. Primarily because of minor nonuniformity of the sapphire, the cellr should be calibrated against a standard. The calibration should be checked after reassembling a cell. A medicine dropper with a long, thin tip is used to fill and empty the cell. Because nitric acid attacks rubber, and VOL. 31,
NO. 3, MARCH 1959
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