of the six vials were evaporated to dryness, redissolved in 261 nitric acid, and the percentages Of La'38 and La'39 were determined. One micrograln Of lanthanum was sufficient for the mass spectrometric analysis. RESULTS
Results were calculated with an equation similar to the one given by Inghram ( 2 ) . A relative standard (n = 7, was deviation of *l.g% obtained for the analysis of a process
sample in which the concent,ration of lanthanum was 27.1 grams per liter as determined by total rare earth analvsis. Lanthanum constituted 99 to "IO0 weight per cent of the rare earths in this sample. The proposed method gave 28.5 grams of lanthanum per liter.
(3) Katcoff, S., A'ucleonics 18, 203 (1960). (4) Leuze, R. E., Baybarz, R. D., Weaver, B., Sucl. Sci. Eng. 17, 252 11963). (5) PIa&, w., waleher, w., R ~ sei, ~ , Instr. 22, 1028 (1951). WORK developed during the course of work under contract AT (07-2)-1 with the s. Atomic Energy Commission.
u.
Savannah River Plant
LITERATURE CITED
( 1 ) H-8, 1). C., Inghram, M. G., PhYs. Rev. 74, 1724 (1948). (2) Inghram, M. G., Ann. Rev. IVucl. Sci. 4, 82 (1954).
W. B. HESS
H. P. HOLCOMB
Savannah River Laboratgry E. I. du Pont de Nemours & Co. Aiken, S. C.
Trapping of Gas Chromatographic Effluents Using Carbon Dioxide as a Carrier Gas Irwin Hornstein and Patrick Crowe, Market Quality Research Division, Agricultural Research Service, United States Department
of Agriculture, Beltsville, Md. HE COLLECTION of
n
fractions obtained
T by gas chromatcgraphy for the further characterization of eluted compounds is usually required when separating an unknown mixture. Conventional cold traps are inefficient for trapping high-boiling solutes which tend to form aerosols that are swept through the trap. A nuinber of gas liquid chromatographic traps of varying complexity and success, designed t,o prevent the formation of aerosols, have been described n the literature. Such devices may depend on the maintenance of a temperature differential between a heated inner wall and the cooled outer wall (1, 3 ) ; or on the use of a cold trap and an elect'rostatic precipitat,or (4). A procedure for using argon as a carrier gas and condensing argon along with the sample in a trap surrounded by liquid nitrogen has also been described (8). Hcwever, t,he boiling point of argon, which is close t'o that of nitrogen (-186" us. - 196" C.), makes trapping of argon difficult,. In addition, the rapid boiling off of the argon when the trap is removed from the liquid nitrogen bath may result in the loss of trapped solute. A total trapping system is described here in which carbon dioxide, whose vapor pressure is less than 0.0001 micron of H g a t - 196" C., is used as the carrier gas in ccnjunction with a flame ionization detector. Although the carrier gas commonly used with a flame ionization detector is nitrogen, carbon dioxide ran be used equally well because this detector is also insensitive to COa. The use of this detector results in the total destruction of the elut.ed compounds. I t is t,herefore essential that' the gas stream be split between t'he column exit and the detector so that' 170
ANALYTICAL CHEMISTRY
.
PJmmiti
Figure 1 .
Trapping system
only a minor part of the stream goes to the flame and the rest to the collection device. Splitting arrangement's will vary for different instruments. In some instances, effluent splitters are available with the instrument purchased; in others, as in our own case, a relatively simple splitting system can be built. In this laboratory we have been using an Aerograph 1520. This gas chromatograph is a dual-column, dual-flame instrument that is also provided wit,h a thermal ccnductivity detector. The instrument. design is such that fractions may be trapped from the out'let of the thermal conductivity cell. We have constructed a splitter that feeds approximately 80% of the effluent from the column through the thermal conductivity cell for collection, and 20y0 through the flame for detection. The choice of a 4 : l ratio is arbitrary; the sensitivity of the flame det'ector permits splitting ratios considerably higher than 4 : 1 to be used if so desired. The trapping system is shown in Figure 1. The line to and from t.he thermal conductivity cell leading to the cold trap is heated in the detector oven
to a temperature a t least 20" C. above the maximum column temperature. The exit line from the thermal conductivity cell ends in a 1/2-inch Swagelok fitting closed with a silicone septum similar to that used in the injection port. A hole in the septum accommodates a short piece of 3-mm. 0.d. borosilicate glass tubing that leads to the collection trap by a one-hole silicone rubber stopper. A 2-cc. hypodermic syringe filled with anhydrous magnesium perchlorate and ending in a Yo. 19 needle is inserted through the stopper to serve both as a drying tube and a vent. The vent in turn is connected by an 18- x 150-mm. test tube and a two-hcle stopper to a soap bubble flowmeter. The test tube acts as a trap to keep the soap solution in the flowmeter from backing into the collection trap. A small Dewar flask, approximately 5.0-em. i.d. and 10.0 cm. in height, filled with liquid nitrogen is placed on a lab jack below the trap. When a sample is to be collected, the Dewar flask is raised. If the entire trapping tube were plunged into liquid nitrogen, the sudden pressure drop caused by the condensation of the effluent gas stream would suck air into the trap through the venting system. The Dewar flask is therefore raised slowly. The preferred condition for trapping is the equilibrium state wherein all the effluent is condensed and no air is pulled into the trap. The soap bubble flowmeter serves to adjust the system to this condition. As soon as the liquid nitrogen level rises above the bottom of the trap, the total effluent, solute plus carrier gas, starts condensing. At the same instant a soap film in the flowmeter immediately starts to descend, an indication that some air iq
being pulled into the trap. Equilibrium is very quickly attained and the soap film is then kept stationary in the flowmeter by slowly raising the Dewar flask. Some initial disturbance is observed in the base line when sample collection is begun, but with experience in using the technique, base line fluctuation is kept to a minimum. When sample collection is completed the entire trap is pulled from the chromatograph and the drying tube removed by pulling out the needle. If desired, another trap can be installed in a matter of seconds. Centrifugation speeds up the rate of C 0 2 sublimation. The trap plus inlet tube are immediately centrifuged a t approximately 1000 r.p.m. The use of a table model centrifuge is entirely satisfactory. Normally, all of the carrier gas is volatilized within 5 to 10 minutes, depending on the volume of gas collected. When a relatively volatile solute such as acetone has been trapped, care must be taken to stop centrifugation while the collection trap is still cold. The use of a refrigerated centrifuge, i f available, is helpful when volatile solutes are collected. The walls of the restricted section of the trap are not pulled too thin to avoid breakage during centrifugation. Xo particular precautions are needed to avoid loss of the trapped component. The trap contains solid carbon dioxide which sublimes a t so low a temperature that there is little possibility of losing the isolated compound by volatilization. In addition, because the carrier gas goes directly from solid to gas, entrainment of the trapped component by the liquid boiling-off is avoided. The efficiency of the trapping system approaches 100% for all of the exit gas that bypasses the flame because this entire gas
stream is trapped. Losses that may be incurred must take place during the removal of the carrier gas from the trapped solute or in subsequent transfer operations. The trap illustrated is useful, for example, in helping to obtain infrared spectra of trapped material. After carbon dioxide removal, the wall of the trap is rinsed with 2 pl. of carbon tetrachloride and the tube again centrifuged. The solution collects in the lower part of the trap. This section is broken off; the contents are transferred with a microsyringe to a microcell and a solution spectrum is obtained. With some high-boiling materials, this total trapping technique tends to cause the solute to condense in the 3-mm. tube immediately on emerging from the hot zone. It may, therefore, be necessary to wash the inlet tube with a solvent, preferably carbon tetrachloride, if a solution infrared spectrum is to be obtained. Three 5-p1. washes with a 1-minute centrifugation between washes is enough to transfer all material into the restricted portion of the trap. Once the latter is broken off, the excess solvent can be removed by gentle heating under a lamp. There is no danger of volatilization of the isolated compound because this removal of excess solvent is only needed when high-boiling components are trapped. This technique was evaluated for a 6-foot X '/*-inch 0.d. stainless steel column packed with 20y0 DEGS on acid washed 30- to 60-mesh Chromosorb W. Total trapping was achieved with flow rates ranging from 10 ml. to 100 ml. of carrier gas per minute, a t column temperatures varying from ambient to 200" C. and with samples of 5 pg. to 1 mg. Ten minute samples could easily be
collected even a t the highest flow rates. Recoveries of methyl oleate were determined at 185" C. and a t a carbon dioxide flow rate of 25 ml. per minute. Five microliters of a solution of 1% methyl oleate in methylene chloride were injected and the area was determined for the methyl oleate peak with total effluent passing through the detector. The area of this peak was assigned a standard value of 1. A similar 5-pl. sample was injected using the effluent splitter. The gas stream bypassing the detector was trapped during the identical time interval that the methyl oleate peak seen by the flame was being recorded. Carbon dioxide was removed and the inlet tube washed with solvent as described. I n this instance the restricted portion of the trap was not broken off; instead the solvent was evaporated, 20 pl. of methylene chloride were added to the restricted portion of the tube, and a 10-p1. aliquot (I/'* the trapped sample) was rechromatographed with he splitter closed. Twice the peak area of this methyl oleate peak was added to the area of the peak recorded while the sample was trapped. These combined areas were equal to 1.02 times the area of the similar 5-111. standard, giving a lO2y0 recovery. Apparent recoveries by this procedure in general range from 90 to 105%. LITERATURE CITED
(1) Stevens, R. K., Mold, J. D., J . Chromatog. 10, 398 (1963). (2) Swaboda, P. A. T., .Vature 199, 31
(1963).
(3) Teranishi, R., Corse, J. W., Day, J. C., Jennings, W. G., J . Chromatog. 9, 244 (1962). (4) Thompson, A. E., Ibid., 6, 454 (1961).
Isotope Dilution Analyses by Spark Source Mass Spectrography Fredric D. Leipziger, Sperry Rand Research Center, Sudbury, Mass.
commercial availability of source mass spectrographs has made possible mass analyses of elements which cannot be handled by electron impact or thermal techniques. In turn, this should permit stable isotope dilution analyses of elements which previously could not be handled by existing mass spectrometers. A comprehensive paper by Webster ( 2 ) surveys recent isotopic dilution analyses, and also notes that only monoisotopic elements are not amenable to this technique. The accuracy and sensitivity of the method make it useful in many fields wherever an accurate determination of a specific element is essential. ECEKT
R spark
01776
The capability of the spark source mass spectrograph for ionization of all elements leads to fulfillment of Webster's statement concerning isotope dilution analyses being possible for all polyisotopic elements or elements with long-lived radioisotopes. T o illustrate this, we have chosen to analyze several materials for trace amounts of copper and antimony by stable isotope dilution. Spark source instruments use phc tographic detectors which are a definite hindrance to rapid and accurate isotope ratio measurements. However, by a careful plate calibration and because of the ease with which the isotope ratio may be adjusted close to unity-where
precision of the measurement is bestwe have been able to obviate many of these difficulties. EXPERIMENTAL
Preparation of Spikes. T h e Cu65 and SblZ3spikes were obtained from Oak Ridge National Laboratory. They were dissolved in the appropriate acid and their concentration was determined by isotope dilution with pure copper and antimony solutions. Concentrations by this procedure agreed with the spike weights to within 1%. This method is described by Webster ( 2 ) . Plate Calibration. The method of Mattauch and Ewald (1) was used for calibration of the Ilford QrI plates, which are the detectors in the MetroVOL. 37, NO. 1, JANUARY 1965
* 171
