d r y 100- to 120-mesh Chromosorb was retained with Ebmm. wads of glass wool, cut flush with the ends of the column. I n the second case, finely perforated screens (125T perforated nickel, Pyramid Screcn Corp., 181 Harvard St., Brookline 49, Mass.) were installed as flow distributors in place of the glass wool plugs. The screens were secured 0.5 mm. in from the ends of the column, and the column space between screens was tightly packed with the same Chromosorb packing. PROCEDURE
Since thc greatest asymmetry and largest values of A had previously been found with air peaks, the present measurements were made with an air sample. The carrier gas was helium. Peak asymmetry was measured as follows: The tangents to the inflection points of the peak wcre extrapolated to their own intersection to define the peak center. The distances from peak center to each side at half height were measured. Peak asymmetry’ was expressed as the distance to the trailing side divided by the distance to the lcading side. A series of air peak measurements was made with each column. Peak asymmetry was measured in each c m , and plate height data were used to compute the value of the A tei m, using Equation 1 corrected for column pressure drop ( 4 ) . Data were not corrected for the
effects of the small volumes external t o the column. RESULTS
The value A term for the column using glass wool plugs waa 0.02 cm. T h a t for the column using nickel screens waa 0.004 cm. The standard error of estimate was 0.002 cm. in both cases. Peak asymmetry in the first case was 1.40 at an average gas velocity of 20 cm. per second. In the second case, asymmetry was within experimental (Asymmetry in error (1.02 - 1.04). the second case was 1.15 at a velocity of 3 cm. per second. The reason for the greater asymmetry at low velocity was not investigated, since such velocities are well below optimum for high-performance columns.) Thus it appears that a major source of the .4 term lies in peak asymmetry, which can result from mixing. It seems probable that this factor is a major one in the performsnce of large-bore prcparative columns, where the cross scction of the column ordinarily is much greater than that of the inlet piping. Thc mathematical treatment of plate height thpory assumes peaks having the shape of a gaussian distribution. The observed mixing effect produces a nongaussian peak, which is incompatible with the usual treatment of peak var-
iance. Consequrntly, i t appears desirable to correct for any observed peak asymmetry beforo using experimental data for theoretical analysis. The small residual A term of 0.004 cm. reported above may represent the error introduced by the increased peak asymmetry at low velocity. It might also represent the effect of the meuhanisms previously ascribed to the A term. Further study will be required t o establish ita nature. LITERATURE CITED
(1) Bohemen, J., prnell, J. H., “Garr
Chromatography, pp. 10, 11, D. H. Desty, Ed., Academic Press, New York,
1958. (2) Giddings, J. C., Nature 184, 357 ( 1959). (3) Knrasek, F. W., Ayers, B. O., Fifth National SvmDosium on Instrumental Methods of Ann1v.k Instrument Society of Americn, “Houston, Tex., May 18-20, 1959. (4) Kieselbach, R., ANAL. CHEM.33, 23 (19G1). ( 5 ) Klinkenberg, A., Sjenitaer, F., Giddings, J. C., A’alure 187 1023 (19GO).
(6) van Deemter, J. J., Zuiderweg, F. J., Klinkenbera. A.. Chem. Ena. Sci. 5, 271 (195G).-’
‘
RICHARD KIESELBACH Engineering Research Laboratory Engineering Department E. I. du Pont de Nemours & Co., Inc. Wilmington, Del. RECEIVEDfor review January 27, 1961. Accepted February 28, 1961.
Lead Determination Using an Anion Exchanger and Sodium Chloranilate SIR: This method was developed to eliminate the tedious preparation accompanying lead determination. It is essentially a general method, combining the techniques of three separate research teams: the method of Strel’nikova and Pavlova ( 4 ) which involves conversion of lead to an anionic complex with 1.ON hydrochloric acid, absorption on a strongly basic anion exchanger, and subsequent elution with distilled water; the Frost-Jones and Yardley (3) technique of precipitating lead with t h e sodium salt of 2,5-dichloro-3,6dihydrosy-p-benzoquinone; and, lastly, t h e Ferro and H a m (2) use of disodium EDTA t,o release the acid chlora~iilate ion by chelation of the metal cation. The authors are aware of the simplified extraction procedure of Diaper and Xuksis (1) and other methods which increase the specd of analysis of lead using dithizone. Inherent in all these a r e the requirements for scrupulously clean glassware, and deleading the
solvents prior to the test. Such precautions are not necessary with the method described below. EXPERIMENTAL
Reagents. Sodium chloranilate solution, 1% w./v.; disodium (ethylenedinitri1o)tetraacetate solution, 5% w./v.; strongly basic anion exchanger; lead standards, 50 t o 20 pg. of P b ; acetate buffer, p H 4.63; methyl Cellosolve. Procedure. Prepare exchange column by slurrying 30 grams of a strongly basic anion exchanger in a 1N solution of hydrochloric acid. Transfer slurry t o column and drain t h e excess liquid until i t is about 1 ml. above t h e anion exchanger. Convert t h e lead t o complex b y adding 1N H C l t o t h e lead solution. Pass this through t h e prepared exchanger at a rate of 1 drop per second. Wash with 40 ml. of 1N hydrochloric acid and discard eluate. T o release t h e lead complex, pass 140 ml. of distilled water through the column. Adjust t h e eluate t o a p H
of 4.0 with 500/, sodium hydroxide and make up to a volume of 150 ml. with distilled water. Pipet 10 ml. of the solution into a centrifuge tube. Add 1 ml. of buffer, 2 ml. of methyl Cellosolve, and shake well. Then add 1 ml. of a freshly prepared 1% aqueous sodium chloranilate solution. Shake by twirling and let stand for 30 minutes. Centrifuge for 30 minutes, using an apparatus which does not have a constant angle. Decant carefully and add 5 ml. of a 1 t o 1 isopropyl alcoholwater mixture. Stopper the centrifuge tube and shake, recentrifuge for 30 minutes, then decant again with care, blotting excess supernatant with filter paper. To t h e precipitate, add 1 ml. of buffer and 5 ml. of a 5% solution of disodium EDTA. Stopper, and shake t o chelate the lead comuletelv. Let stand for 10 minutes, t h e i read on spectrophotometer (Beckman D U or Fisher Electrophotometer), comparing against a reagent blank run at the same time. A standard curve may be prepared SO that the results may be interpreted VOL. 33, NO. 6, MAY 1961
807
directly. If tlw Fishcr model is used, thc 525B (525 nip) green filter is requirt- I. With tliv Ueckman DU, take readings a t 530 nip. DISCUSSION
Lincarity of from 50 to 250 pg. of lmd W ~ Robtriined. Using n constant angle centrifuge, consirlcr:hlc loss that amounted up to 7% was eupericnred in the recovery of the lend. 1Sliition frnm the anion exchanger p r o w d to be almost 100% as determined by washing the column with more distilled water, ronccntrating the t,luate, and tcsting with chromritc solution.
Our inability to determine less than 50 pg. by this method is due to the fact that the l e d chloranilate did not pre-
cipitate in the substrate used here. Solubility tests of lead chlornnilate in various solvents indicated that, by increasing the concentration of methyl or butyl Cello~olve, the range below 50 fig. can be analyzed in the same manner 8,s above. This, then, will give the clinical chemist, and most importantly, the toxicologist and industrial hygiene chemist, a lesfi tedious method than the dithizone technique. LITERATURE CITED
(1) Diaper, D. C. M., Kuksis, A., Can. J . Chenz. 35, 1278 (1957).
(2) Ferro, P. V., Hsiii, A. n., Teed. 1~1111. Regi8lry itied. Technoloqisla 27, 247 ( 1957). (3) FrosbJonea, It. E. U., Tardlcy, J . T., Analyst 77, 468 (1952). (4) Strel'nikova, N. P., l'avlnva, V. N., Zwotlskaya Lab.26,63 (1960).
EDWIN A. WYNNE RUMELL D. BURDICK LEONARD H. FINE^ Chemical Manufacturing DiviRioii Fisher Scientific Co. Fair Lawn, N. J.
RECEIVED for review Decrinbrr 27, 1980. Accepted February 28, 1961. 1. PreRent address, Seniironductor Di. vislon, General Electric Po., SyracuRe, N. Y.
Determination of Trace Quantities of Acetylene in Ethylene by Gas Chromatography SIR: .4 recently puhlishcd gas chromatogrciphic rncthod for the determination of traw quantities of acctylene in rthylrne rrqiiirrs ti preliminary conwntration s t t y to r('niovc niost of the (%hylm: prior to chromatographic analysis ( 2 ) . A method which prinitP tlircct itijoction of the sample woiild provido an ohvioiis saving of timr and labor. One requirement for direct antilysis i.; :i detoctor capable of adequate response to w r y low concentrations. .4 flame ionization detector merts this requircmrnt. 11 srcond obstacle to the direcat determination of tracrs of acetylene in et,liylrnc: is the tendency of the 1:~rpt: cthylcm peak to overlap and obscure the small acctylene peak. T o overromc this problem, a stationary phase having a high separation factor (ratio of rotcrition volumes measured from air pc:ik) for acetylene and rthylcne is rcquired. Kent (3) has recently reportcd a direct method for a similar prohlcm, the detcrmination of acctylene in ethane in which separation of the minor peak is achieved by an 80-foot dimcthglsulfolane column a t l o o c. l>uring rcxxnt work 011 the srparation of hydrocarbon gases the material hexametli~~lphos~~lioramide(HMPA) (Fishcr Scientific Co.) was found to hrive a st,p:iration factor of 40 for acctylene and ethylcne. This high separation factor suggrstcd its evaluation for thc direct dct,ermination of traces of acctylmc i n ethj.lene. Using a short column of this material and a scnsitive flamc. ionization detector (I), traccs of acctylene in ethylcne can be determined without preliminary ,sample treatment. 808
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Thc column was preparcd by packing a 5.5-foot length of '/,-inch 0.D. copper tubing with 20% HMPA on 60- to 80-mesh Chromosorb solid support. Using an electric vibrator, a packing density of 0.53 gram per cc. was ohtained. The column had an cfficicncy of 2000 theoretical d a t e s for acetylene at 25' C. and was usrd a t room temperature (27' f 1 . 5 O C.) without a bath or thrrmostating device. At this temperature, the background signal due to traces of HMPA strilq~edfrom
Figure 1.
the column was very high. In ordcr to eliminate this background signal n small trap immemcd in dry iceacetone bath was inserted bctwrcn the column and the detector. Gas .standards for instrunirlit calibration were prepared by scrirtl dilution in nitrogen-filled bulbs fitted nith rubber serum stoppen. Aliquots mcrc withdrawn and injected tlirorigh the stopprrs with a prerision syringe. The gnsc's were mixed by agitating \\ith g1:~ss beads placed in the bulbs. A rigorous cleanup procedure consistilig of repeated evac-uation and flushing nlth
Typical chromatogram of acetylene in ethylene
Operating Conditions Detector. Flame ionization cell gain = 10'" ohms ( I ) Recorder. 1 L N Speedomax Type-G, 0-1 0 mv., chart speed 48 in./hr. Nitrogen flow fate. 40 ml./min. @ 4 p.s.1.g. Column flow rob?.. Some as for nitrogen Hydrogen flow rote. 35 ml./min. @ 10 p.r.i.g. Air flow rate. 2 cu. ft./hour ( 5 6 literr/hour) @ 5 p.s.1.g.
