approximately 75" C. and then add 10 nil. of the reagent to the sample solution. Stir well and allow to stand until the mixture is a t room temperature (approximately 30 minutes). Filter by suct,ion through a dried and weighed fritted glass filter. Rinse the beaker with the wash solution (957, alcohol saturated with the triple acetate) and wash t,he precipitate with the same solution. Wash the precipitate once with ether to remove the alcohol and 1)ull air through the filter for 1 minute to remove the ether. Dry the outside of the crucible and count the sodium activity of the precipitate by placing the crucible on the 3- X 3-inch K a I crystal. Reweigh the crucible to obtain the weight of precipitate. Calculate the radiochemical yield by dividing the activity of sodium in the precipitate by the total activity of the initial tracer. The total activity of the tracer is determined by evaporating on a small circle of filter paper an aliquot of the sodium tracer equivalent to that used in the analysis, placing the paper on the frit of a glass crucible, and counting under the same conditions used for the samples. The weight of the precipitate is divided by t'he radiochemical yield, and the corrected weight is multiplied by 0.01495 to produce the weight of sodium. RESULTS AND DISCUSSION
When the method described by Furman (2) was used to precipitate 1 mg. of sodium and NaZ4tracer a t room temperature (23" to 25" C.), only 77% recovery was found by gravimetric and radiometric methods after 0.5 hour of aging. The recovery increased to 85%, S9%, and 97% after 2.5, 5, and 16
A
8 $? 8 0 1
f
A
8
I
I 0
70
0 0
60 GRAVIMETRIC
25OC 0 , O 75OC A
RADIOMETRIC 25OC 0,m 75OC A
0.5
Figure 1 .
1.0 1.5 V O L U M E , ml
Effect of volume of sample
1 mg. of N a , 1 0-ml reagent, 2.5 hours aging
hours, respectively; however, 24 hours of aging was required to ensure quantitative precipitation. I n addition, the amount of sodium being precipitated was observed to affect the degree of recovery after specified aging timesLe., the percentages of 0.5, 1.0, and 2 mg. of sodium precipitated in 3 hours were 73, 87, and 91, respectively. A constant volume of sample, 1 ml., is specified in the standard method because of the possibility of loss of precipitate through partial solubility of sodium zinc uranyl acetate ( I ) . Although results obtained from gravimetric and radiometric measurements
agreed closely when a sample voluine of 1 ml. was used, the agreement was poor when the sample volume was 0.5 ml. or 2.0 ml. (Figure 1). These discrepancies indicated an additional source of error-coprecipitation of reagent with the triple acetate. Much better correlation between gravimetric and radiometric measurements, as well as greater recovery of sodium, can be achieved by adding the reagent a t an elevated temperature and allowing sodium zinc uranyl acetate to precipitate as the system cools to room temperature (Figure 1). As a result of t,hese experiments, we recommend the modified version of this standard method wherein radiotracers are included for yield correction, and the zinc uranyl acetate reagent is preheated to improve the purity of the precipitate and to increase the recovery of sodium. The time required per analysis is decreased significantly and the accuracy increased, thereby enhancing the usefulness of this method for routine analysis. LITERATURE CITED
(1) Barber, H. H., Kolthoff, J. iM., J . Am. Chem. SOC.50, 1625 (1928); 5 1 ,
3233 (1929). (2) Furman, N. H., "Standard Methods of Chemicals Analvsis." Vol. I. ww. 13-14, \'an Nostrand, New York,'19'69 W. J. Ross Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tenn. RESEARCX sponsored by the U. S. Atomic Energy Commission under contract with the Union Carhide Corporation.
Determination of Lanthanum in Actinide-Lanthanide Mixtures by Isotopic Dilution and Mass Spectrometry SIR: We have successfully applied isotopic dilution and mass spectrometry to the determination of lanthanum in highly radioactive solutions of americium, curium, and rare earths. Conventional gravimetric or titrimetric techniques either lacked specificity for lanthanum or involved chemical separations that would have required use of extensively shielded analytical facilities. In the pilot scale test of the CmZ4' separations process at the Savannah River Plant, lanthanum was added early in the process as a carrier to increase recovery of the trivalent actinides. The feed solution to the Tramex process ( 4 ) that separates the actinides from the rare earths therefore contained about 1 gram of curium and 30 g a m s of lanthanum per liter. T o evaluate the performance of the pilot
scale operation, it was necessary to determine curium-lanthanum weight ratios that ranged from 0.03 to 1000 in process samples. EXPERIMENTAL
A quantity of pure La203, 2.16% enriched in La1%, was obtained from Oak Ridge National Laboratory. A nitric acid solution of known lanthanum concentration was prepared from this material for use as the internal standard. Isotopic analyses of lanthanum samples before and after isotopic dilution were made with a single-stage surface einission mass spectrometer, Cugsolidated Electrodynamics Gorp.> Model 21-702, equipped with an electron multiplier and vibrating reed electrometer for the detection system. Magnetic scanning was used and a correction (5) for mass
discrimination of the electron multiplier was applied to the results. To avoid interference from I3a1Sn (6.31y0 thermal neutron fission yield from Pu239) (S), a single filament surface ionization source was used and the L a 0 + peaks at masses 154 and 155 were measured. With this type of source, lanthanum evaporated as L a o + , whereas barium did not evaporate as the oxide ( I ) . A small amount of borax added to the samples enhanced the formation of L a o + . To reduce radiation exposure, the Trames feed solution was diluted by a factor of lo3 with 1M HXO,. Equal aliquots of the dilution were pipetted into four glass vials, and a known quantity of the enriched lanthanum standard solution waf then added to each of two of these vials, Finally, equal aliquots of the lanthanum standard were pipetted into two additional vials. The solutions in each VOL. 37, NO. 1, JANUARY 1965.
169
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
