is believed t,liat the polonium tracer extracted is in the quadrivalent oxidation state. Diisobubyl carbinol gives essentially the same results as diisopropyl carhinol and is readily available conimercially (Union Carbide Chemicals Co.). In a t'ypical experiment, a target solution containing 0.5X nitric acid, 0.311 aluminum nitrate, 0.03111 bismuth nitrate. 0.03N copper nitrate, and 5 X I O 3 alpha counts prr minute per ml. of poloniuni-XS \vas anal\-zed by thr, pro-
cedure given above. The recovery of the polonium-208 was essentially quantitative (>98%). The presence of the bismuth, aluminum, or copper ions did not interfere, either by extracting appreciably to cause self-absorption losses of the alpha particles, or by inhibiting the extraction of the polonium tracer. A slight amount of copper extracted but did not interfere. If desired, it could be removed readily from the organic phase b y washing for 3 minutcs n-ith a oiie-
half volume portion of 6JI hydrochloric acid solution. LITERATURE CITED
(1) Halperin, J., Miller, D. R., Oak Ridge Kational Laboratory Declassified Rept., CN-1639 (June 13, 1944).
FLETCHER 1,. XfOORE Oak Ridge National Lsboratory Oak Ridge, Tenn. RECEIVEDfor review April 13, 1960 rlccepted May 2 3 , 1SGO.
Preparation of Gas Chromatographic Packings by Frontal Analysis SIR: Gas chromatographic packings are generally prepared by simply slurrying the solid substrate with a calculat'ed weight of partition liquid in excess solvent. followed by evaporat'ion of the solvent with occasional stirring. This technique, while generally satisfactory, may lead to difficulties due t o nonuniform distribution of the liquid partition phase. This danger is especially pronounced when working with mixed partition liquids or when the partition phase has only limited solubility in t h r particular solvent being used. 'Yo avoid tliese difficulties i t has been advaiitagrous to use the chromatographic principle of "frontal analysis" ( 2 ) for preparing gas chroniat'ographic packings. In this technique t'he requirrd n-eiglit of solid substrate is pacakrd dry into a conventional glass chromatographic tube and a solution of tlie partition liquid in some volatile solvent is p e r d a t e d slowly through it. If tlie partition liquid is selectively adsorbed by the solid substrate, t,he first solvrnt cuts t,o emerge from the bottom of the coluniii are loiver in concentration of partition agent' than the feed solutioii. Eventually, however, a steady state is established betmeen tlie flowing solution and the solid subst'rate, so that tlic rfflucint from the bottom of the colunin ip of the same coiicent,ration as th(3 rolunin feed. Because tlie usual ga5 (,lironintographic substrates are very n-eak adsorbents, the required stcmly-statc conditions are generally r e a r l i d very quickly. Actual tests with a v:iric+y of partition liquids, solvents, and c,onventional gas rhroniatographic sulwtratcs. h a r e failed so far to show aiiy significniit chromatographic fractic~nation of the partitioning liquids. Hon-cwr. because. of t'his ever-present possilility, n-e reconinieiid the passage of at lrnst tn--o volumes of solution [ c p n t ' i t y rcqnirrd to wct the packing rompletc~ly( I ) ] donn the packcd column. IYhcw tlicrc is reason to expect adsorption of tlic partition liquid-e.g.q coat-
ing of active adsorbents such as silicic acid or carbon-the effluent should be analyzed by appropriate means t o be sure that the desired steady-state conditions have been achieved. At this point, the partitioning agent bhould be uniformly distributed over the available surfaces of the substrate. The last of the feed solution is then drained from the column, the solid packing extruded, and the residual solvent removed by evaporation. This final evaporation is generally carried out in a circulating air oven but, in special cases, could be carried out in a vacuum oven or in a heated tube under a flow of inert gas. The equilibrium concentration of partition agent on the solid substrate can best be obtained by reweighing the packing after solvent evaporation. Because, however, there is generally no appreciable adsorption of the partition agents owing to the inert nature of the conventional gas chromatographic supports, the approximate quantity of partition agent can be calculated from a knowledge of the feed composition and the effective free volume of the packed column. I n a typical experiment, 74 ml. (2 volumes) of a 3.69% TI( Jv.) solution of high vacuum silicone greaqe in ethyl acetate was passed d o n n a column containing 17.0 gram. of Celite. At the end of this time. the colunin n as n-et with. and theoretically in equilibrium u-ith, 37.0 ml. of the silicone grease solution. The packing was eytruded and spread out in a glass evaporating dish, and the residual solvent evaporated in a circulating air oven a t 60' C. overnight. The dried packing lveight was 18.35 grams, n-hich checked very closely n ith t h r calculated weight of 17.0 37 X 0.0369 = 18.36 grams. SThile we have not yet shom-n coiielusively that the above frontal analysis packing preparation procedure is actually superior to conventional niethods, there are several important practical and theoretical advantages. Chief among these are :
+
Frontal analysis packing preparation requires less operator time than direct addition packing, because allowing the column to fill and drain automatically eliminates the necessity for an operator to stir the conventional packing slurries during the initial solvent evaporation stages. Uniform packings are guaranteed by the frontal analysis technique, whereas conventional procedures may lead to uneven partition agent distribution. For example, where the partition agent is not extremely soluble in the solvent used, the partition agent may precipitate during evaporation, resulting in localiz~dareas of high concentration. Several partition agents may he applied simultaneously without dangei of uneven distribution (if adsorption effects are not encountered, as is generally the case). This may also be done h y conventional methods, if a good mutual solvent can be found. In many cases, however, considerable searching for such a solvent may be required. There is very little exposure of the packing to the air via the frontal analysis procedure, and th(. whole operation can be easily carried out in an inert atmosphere if desired. The prolonged heating required to evaporate large excesses of solvent is also avoided. The frontal analysis packing procedures described above have been in use in our laboratories for over a yew. The theoretical and practical advantages may \\-arrant its further investigation by other workers in the field of gas chroniatography. LITERATURE CITED
( 1 ) Cassidy, H.
vi,
"Adsorption and Chromatography, pp. 229-30, Interscience, New York, 1957. (2) Ibtd., pp. 258-9. The Chemstrand Corp. EDGAR I). SMITH I)ecatur, .41a. RECEIVEDfor review February 3, 1960. accepted April 25, 1960. Contribution Yo. 65 from the Research Center, Chemstrand Corp. VOL. 32, NO. 8, JULY 1960
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