the present time are high, and the sample maximum is maintained for only 5 or 10 minutes. The high blank readings are derived mainly from the fluorescent oxidation products of benzoin, m-hich is extremely sensitive to oxygen in the presence of sodium hydroxide. The hydroaylaniine reagent is used to reduce the blank reading, but higher concentrations cannot be used without srriously reducing the sample readings also. Typical development curves for a 10-pg. silicon standard and the blank are shown in Figure 1. On our apparatus a difference reading of 162 units is equivalent to 10 pg. of silicon. I n comparison a i t h the benzoin method of estimating boron ( 2 ) the silicon method is at present much less sensitive. A final test solution containing 1 pg. per ml. of silicon gives a fluorometer reading of 324 units, while a solution containing 1 pg. per ml. of boron would give a reading of 10,000 when operating at the same sensitivity. There is no indication of significant silicon contamination from the borosilicate glass used in the experiments. Some control tests have been made by preparing the blank solution in platinum and holding it there until it was almost time for the reading to be taken, then transferring rapidly to the fluorometer cell. No significant change in the blank reading was observed, confirming that the blank reading was derived from the oxidation products of benzoin, as suggested above. At the present time
the blank to be further reduced mithout seriously affecting the silicon fluorescence. I t should be possible eventually to make measurements below the 1pg. level, which would be valuable in the testing of high purity metals and certain compound semiconductors. We hope to report more complete methods for silicon in these materials a t a later date. ACKNOWLEDGMENT
Figure 1. Fluorescence development curve for sample containing 10 pg. of silicon
it is possible to make accurate measurements down to 2 pg.. and a linear relationship between silicon concentration and fluorescence in the 2- to 10-pg. range has been found. Deosygenation, by bubbling the solution with oxygen-free hydrogen or nitrogen instead of employing a reducing agent, does not appear to be effective in reducing the blank readings or in stabilizing the sample readings in this method. DISCUSSION
Using the fluorescent reaction described, accurate measurements of silicon down to 2 pg. have been made. The fluorescent reaction for silicon is more specific than the molybdenum blue reaction, which is also given by arsenic and phosphorus. Further work is in progress on reducing agents other than hydrosylamine, which may enable
This work is part of a program on the analysis of semiconducting materials sponsored by Standard Telecommunication Laboratories Ltd., Harlow, England, and we not only acknowledge their permission to publish this communication, but also the discussion with Henry Kolfson and E . H. Cornish of their staff for constructive suggestions. LITERATURE CITED
.(1) Cali, J. P., Xeiner, J. R., J . Electrochem. SOC.107,1015 (1960). ( 2 ) Elliott, G., Radley, J. A., Analyst
86,62 (1961). (3) Feigl, F., "Spot Tests, Vol. 1, Inorganic Applications," pp. 307-8, Elsevier, New York, 1954. (4) Sonnenschein, W., 2. -4nal. Chem. 168, 18 (1959). ( 5 ) Koods, J. T., Mellon, M. G., IND. E x . CHEM..-4h-a~. ED. 13, 760 (1941). RECEIVEDfor review June 21, 1961. Accepted August 3, 1961. GEORGE ELLIOTT J. A. RADLEY Radley Research Institute 220-222 Elgar Road Reading, Berkshire, England '
Acylated Cyclodextrins as Polar Stationary Phases for Gas-Liquid Chromatography SIR: The polyesters commonly used as polar stationary phases in gas-liquid chromatography of fatty esters have C : O ratios similar to those of carbohydrate esters. Some compounds of the latter type have been used as phases, but either their applicability was limited to temperatures brlow 180" C. (S, 6 ) or their performance was otherwise poor ( 4 ) . Tests reported here nere carried out v i t h 8-cyclodextrin ( O X ) acetate ( I ) icycloheptaamylose henricosaacetate, n1.w. 2018, m.p. 199-201°), the propionate (m.w. 2312, m.p. 169O), (1-cyclodestrin acetate ( 1 ) (cyclohcuaamylose octadecaacetate, m.w. 1730, m.p. 243-G0), and their mixtures. 1624
0
ANALYTICAL CHEMISTRY
These fully acylated carbohydrates are as efficient as the polyester phases for separation of fatty csters. Their heat stability is very satisfactory. PROCEDURE
Ten grams of 8-CDX acetate in 50 ml. of acetone was deposited on 40 grams of Chromosorb R, 30- to 60-mesh, and the mixture was dried in a rotating evaporator in high vacuum a t SO". A 10-foot aluminum column, '/l-inch o.d., was packed with 31 grams of the mixture and tempered a t 236" for 16 hours under the flow of 50 ml. of He per minute. A Beckman GC-2 instrument with a thermal conductivity detector was used with samples of 1 to
3 ul. to obtain chromatograms A and C in' Figure 1. The conditions were 236" with a flow of 57 ml. of He Der minute at a n inlet pressure of 47 p.&. and a temperature of 270" a t the inlet chamber. In esperiments B and D, butanediol succinate polyester, 20% on acid-washed Chromosorb R, 30- to 60-mesh (Wilkens Instrument & Research, Inc.), was used in a column of the same dimensions under conditions that were identical but for a n inlet pressure of 30 p.s.i. The same resolutions were obtained with p-CDX acetate a t 206O, but QC D X acetate having a higher melting point was not useful at either temperature. Lo\\-er melting mixtures of 0-
and 8-CDX acetate, with p-CDX propionate as such and in mixtures with P-CDX acetate, operated well a t 206’ and belon . Both polyesters and CDX esters have low C : O ratios, but differ in t h a t the latter lack end groups, have uniform molecular \\eight, and are asymmetric. The polyesters are more easily prepared, but the cyclodextrin esters are more reproducible. Cyclodextrins are cumbersome to prepare, but easily esterified, and the products discusscd here are e a 4 y crystallized. K h e n polyester phascs bleed it may be caused bl- chemical destruction and/ or randomization of the molecular w i g h t s of the polymer species without change of its chemical character. Both processcs give ribe to volatile materials, but only the f o r m u applies to C D X esters. The heat resistance of the phases nas frequently testcd. The data on the P-CDX acctntcl column are representative and may serve for comparison IIitii other phase materials. During the initial 16 hours of tempering a t 2 3 6 O , 6.3 iiig. of liquid material condensed in a receiving tube a t room tempcrature. K h e n collection was repeated after the column had been used for several days. 2.2 mg. condensed within 16 hours under identical conditions. Howel-er, this time a dry ice trap was connected in series with the condensing tube and an additional 25 mg. of acetic acid was found in i t Corresponding 1-aluw for the butanediol succinate poiyrstcr column n-cre higher. In accord n i t h this, it was possible to use a more sensitive hydrogrn flarnc. clr3tector I\ ith C D S cster
penoids (s),phenols (3), and methylated sugars ( 4 ) . On the other hand, it has been shown in connection with steroids, sapogenins, and alkaloids that almost any polar phase may serve for separations, provided its stability permits temperatures of 200’ or higher ( 2 ) . The equality of polyesters and C D X esters in regard t o separation of fatty esters is demonstrated here. I t appears likelj that derivatives of C D X can serw also for separation of the aforementioned types of compounds when stability of the polar phase a t high t e m p m ture is required. ACKNOWLEDGMENT
We are indebted to W. E. Link for the comparative experiments with the hydrogen flame detector. D
LITERATURE CITED
(1) French, D., Advances in Carbohydrate
MirJ’es
Figure 1 . Recordings of chromatograms obtained with 0-CDX acetate ( A and C) and butanediol-succinic acid polyester (6 and D) a t 236” Sequence in A and 6: methyl esters of Clo, Clz, to C ~ O acids; in C and D: methyl esters of myristic, palmitic, stearic, oleic, linoleic, and linolenic (with some isomeric) acids
phases a t attenuation 100 times higher than the other column m-ould permit. The literature describes the use of carbohydrate derivatives as stationary phsses for the chromatography of ter-
Chem. 12, 189 (1957). ( 2 ) Haahti, E. 0. A., Van den Heuvel. TV. J. A., Horning, E. C., J . Org. Cheni
26,626 (1961). (3) Janitk, J., Komers, R., “Proceeding? of 2nd SvmDosium on Gas Chromatography, Amstkrdam 1958,” p. 343, Butterworths, London, 1958. (4) Kircher, H. W., ANAL. CHEM. 32, 1103 (1960). ( 5 ) Smith, D. M., Bartlet, J. C., Levi, L., Ibid., 32, 568 (1960). DONALD 11.S A N D HERMAYTN SCHLESK The Hormel Institute University of hTinnesota Austin, Minn. WORKsupported by a research grant from the Sational Institutes of Health (USPHS H-5363) and by The Hormel Foundation.
Effect of Silica Flour in Dow Corning High Vacuum Grease on Gas Chromatographic Retention Volumes SIR: Dow Corning high vacuum grease has been used b y a number of norkers as a gas chromatographic partition agent. Cropper and Heywood have reported on a method for improving this material by precipitating it from ethyl acetate solution with methanol (3). This procedure was developed “in order t o remove the lower molecular weight silicones which otherwise tended to volatilize from the column and condense in the katharometer” ( 2 ) . It was apparently not noticed that this treatmcnt also served to partially remove an iriorganic “silica flour” (sic) filler 1% hich I S deliberately added to this Iroduct by its manufacturer ( 7 ; . The present work has shown that this filler is a fairly strong genrral adsorbent and should b t rompletely removed for nt-t results \I ith reactivr cuinlmwntt
capable of interaction with the filler. For example, it seems likely that some of the variable results and conditioning effects noted by Cason and Xiller were partially due to the presence of this filler ( I ) . While the obvious and best way to circumvent these problems is to use the silicone fluid (DC 200, 12,500 centistokes viscosity) without the added filler, in view of the popularity of this grease as a partition agent it is felt desirable to publish the information contained herein. EXPERIMENTAL
Separation of Filler. Ten grams of Don, Corning high vacuum grrase were diqsolved in about 200 mi. of boiling ethyl acetate T h e solution was filtered a i t h suction through IVhatmtri hu. 1 f l l t c I papei, anti the
precipitated filler was thoroughl.~ washed with several portions of fresh hot ethyl acetate. T h e d r y powder thus obtained was practically free of silicone grease and weighed 1.5 grams (15% of original weight of grease). Preparation of Bare G a s Chromatographic Columns. To compare the adsorption strength of silica floui with t h a t of a commonly used gas chromatographic substrate, two I/*meter gas chromatographic columns were prepared from standard ’/r-inch diameter copper tubing. One was packed only with Chromosorb I T (Johns-Manville 30- to GO-mesh) and the other with a mixture of the same' Chroniosorb JV plus 27y0 ( v . 1 ~ ~of’ )thc dry silica flour. In this way the p r w sure drop across the columns was abour the same and only about 1.0 gram o: silica flour was required. Both coiumrlb were conditioned at 150” C for w v w x VOL
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