cases, a reference cell could be adjusted to cornpensate an analytical peak until the recorder d r a w a straight line within O.5Y0 transmittance either side of a straight line. These limits are equivalent to a n absorbance of about 0.004. Thus, greatest accuracy was achieved when working n i t h absorption bands of high intensity. However, unless there n a s a t least about 10% trnnsniission in the two beams a t the absorption m a h i u m of a peak, there was insufficimt energy falling on the detertor to actuate the servomechanism.
ACKNOWLEDGMENT
The author acknonledges the suggestion of I. K. \Talker that a second variable-length cell be used in infrared differential spectrophotometry to tliniinate interfering absorption. LITERATURE CITED
(1) Bellamy, L. J., J . .-1ppl. (‘hem. 3 , 421 (1953). ( 2 ) Cleverlej; B., .-IYAL. CHEM.32, 128 (1960). (3) McI>onald, I. R. C., n’atson, C. C., Ibzd., 29, 339 (1957). (4) Manno, R . P., I’araslievopoulos, N.,
llatsuguma, H. J., .4ppl. Spectroscopy 13, 57 (1959). ( 5 ) Ponell, H., J . A p p l . C‘heni. 6, 488 (1956). ( 6 ) Rotinson, 11. Z., XXAL.CHEAT. 24, 610 (1952). ( 7 ) Tercnin, A . iY.j Yaroslnvskii, S . d c t a Physzcothint. U.R.S.S. 17, 240 ( l W 2 ). ( S j Kashburn, M-,H., SIahone); 11. J., ASAL.CHEL 30, 1 0 3 (1958). ( 9 ) ITillis, H. A , , lliller, R. G , J., Spectiochin[.d c t a . 14, 119 (lY59).
BARRY CLEVERLEY Dominion 1,ahoratorv Department of Scientific arid Industrial Research IVellington, Xew Zealand
N e w Fluorescent Reaction for Silicon S I R : Tlic estiniation of traces of silicon in crrtain high purity metals and semiconductors is important. Silicon may be determined in a number of materials by the t’echnique of neutron activation analysis and radiochemical separation ( 1 ) . However, difficulties may hc experienced in this method if the pile is not near a t hand, because of the short half life (2.63 hours) of the Si31 produced on irradiation. Furthermore, laboratories without “hot” facilities could not handle the analysis of many materials, in which high activities are induced on irradiaton. The standard colorimetric nirthod of estimating traces of silicon employs the molybdenum blue reaction, using sodium sulfite or benzidine as a reductant. Vsing the molybdenum blue reaction for the detection or estimation of silicon by test book methods does not always give adequate sensitivity. It is gencrally agreed that the lower limit of rc.liable estimation is about 1 p.1i.m. of Si in the final tcst solution ( 3 , 5 ) . Thus for an absorption cell of 20-ml. capacity, a minimum of 20 fig. of silicon must be available. I n spectrographic estimations, a concentration of the same order is required. Howevc>r, recent work by Sonnenschclin ( 4 ) has shown that it is possibk to determine about 0.005 pg. of Si per ml. of final solution by a molybdenum blue method, using chloroform est,raction. EXPERIMENTAL
I n these laboratories we have coiicentrated on the investigation of 110ssiblP fluorescent reactions, which hare giron highly srnsitive methods for boron, beryllium, magnesium, arid aluminum. =\n indicntion of a possible mcthod for silicon was obtained during work on the boron-benzoin fluorescence, reported cJlsewhere ( 2 , . During the estimation of boroii in silicon tetra-
chloride, a small quantity of mannitol was added to a formamide tcst solution, containing benzoin and sodium hydroxide. A strong green fluorescence was obtained, which was later shown to be independent of the boron concentration and proportional to the amount of sodium silicate present. The diffwence between the silicon and boron tests lies only in the addition of mannitol for the silicon reaction. Mannitol destroys the boron fluorescrnce with benzoin. and so no response is obtained froin any boron present in the silicon estimation. Sorbitol behaves similarly to mannitol, but the fluorescence output for a given concmtration of silicon is lower; glycerol produces no significant reaction. Keaker bases, e.g., isobutylamine or tetrabutylammonium hydroside are ineffective when substituted for sodium hydroxide. Apparatus. T h e fluorcscencr is rnmsured in a 20-1111. capacity glass cell, illuminated by a stabilized 125watt’ mercury lamp, using a Chance OX1 filter, t’o select the 366-nip radiation. The secondary filter, to exclude scattered ultraviolet radiation, consists of Chance OB2 and J\-ratten 2.U. filter plates. Output is measured with 931 -4. multiplier phototube feeding a sensitive galvanometer. Reagents. Laboratory reagent grade benzoin is recrystallized first from high purity benzene, then twice from high purity methanol, to remove fluorescent impurities. The formamide we used, supplied by the British Drug Houses Ltd., was virtually free from fluorescent impurities and required no further treatment. H o w v e r , other grades of formamide may be highly fluorescent. and the reagent should be examined before use. and if necessary redistilled under reduced pressurr. Silicate-free sodium hydroxide is prepared from a 15c7, solution of normal reagent quality sodium hydroxide. using
a polythenc divided cell and electrolyzing against an anion-permeable membrane, placing the solution in the cathode compartment. Platinum clertrodes are used. Alfter several hours rlectrolysis at, 2 to 3 amperes, a sodium hydroxide solution, virtually free from silicate, borate, and other anions is obtained. dilute sodium hydroside solution is used in the anode comlxirtnient. Analytical Procedure. T h e tentative mcthod for silicon determination is as follo~vs: T h e silicon is isolatecl as sodium silicate and t h e aqueous solution is evaporated g w t l y to dryness in platinum. T h e solid residue is dissolved in 1 nil. of water and 0.5 nil. of 15% sodium hydrosidc s Ilution (silicate frcle). Mannitol (0.03 gram) is dissolwd with stirring in t h c x solution, which is then rinsed into a 25ml. calibrated flask with about 15 ml. of formamide, followed by the addition of 0.5 nil. of saturated benzoin solution in methanol a,nd 0.5 mi. of a 27c aqueous solution of hydrosylaniine hydrochloride. The solution is finally made up to 25 ml. with formamide and thoroughly mixed. blank is preparcd by a similar procedure, using all thc reagents employed in the sample detvrmination. Interferences. Arsenio and phosphorus in milligram concentrations give only a faint fluorescence, and so produce no serious interference except a t very high concentrations A $
RESULTS
I t is necessary to plot the development of fluorescence for both sample and blank, conimenr minutes after mising. rescence is gcnerally obtained aftrr :i develolmient, time of about 60 niiniitr.: a t room tenlperature The fiuorescril:’t duc to silicate is thc tiifitrrenct~ in Iwah. height het\\ecn the, siimple and M s n h eurv(’s, 1tiis nicasurenirnt prww i;ris rrquirecc.
a5
the blank ieaciingh ai
V O l 33, NO 1 1 . OCTOBEk 1 Y b 1
1623
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-
