The mirror is then positioned to use the hydrogen lamp and the absorbance measured again at the same wavelength. The difference between the two ahsorbances is the amount contributed by the test element. Table I gives the results from the determination of ainc in solutions of redistilled sulfuric acid, and Table 11, those from cadmium in sodium salts. These combinations were chosen because of particularly severe background interferences. The results clearly demonstrate that it is possible to correct for the absorption by matrix salts if both a cathode lamp and a continuous light source are used, and that absorption of the continuous radiation by the test element will need to he considered only in eases requiring the highest accuracy.
The corrections will be valid only if the background absorption at the precise wavelength of the elemental line is the same as the average absorption in the wavelength interval passed by the monochromator. Fine structure in the background (perhaps unresolved) could cause errors. In determining magnesium in the presence of large amounts of sodium, for example, the ground state sodium line a t 2852.8 A. would ahsorb part of the continuous radiation passed by the monochromator set for the Mg 2852.1-A. line and cause the correction to he too large. Corrections made in this manner are for absorption by matrix salts and not for any effect the salts might have on the slope of the working curve of the test element. However, the present method permits the use of the standard
additions method to correct for such effects. Standard additions cannot be used in the presence of an unknown amount of background absorption. LITERATURE CITED
(1) Fuwa, K., Vdlee, B. L., ANAL.CHEM. 35, Y42 (1963). (2) Koirtvohann. S. R.. Feldman. C..
copy,” ‘Vol. 3, J. g.. Forre&, ed., Plenum Press, New York, 1964. (3) Werner, G. K., Smith, D. D., Dvenshine, S. J., Rudolph, 0. B., iMcN~.lly, J. R., J. O p t . Soe. Am. 45, 203 (19.55).
S. R. KOIRTYOHANN E. E. PIcuEn
University of Missouri Department of Agricultural Chemistry Columbia, Mo.
Rapid, Quantitative Determination of Tertiary Amines in Long Chain Amine Oxides by Thin Layer Chromatography SIR: In the manufacture of long chain tertiary amine oxides, the main irnpurity is the starting amine. A number of procedures ( 1 , 8 , 7 ) have already been developed to determine the amount of unreacted starting material. However, a simple process control of this product was required and even though most methods are acceptable, they either lose precision in the lower concentration ranges or are too time consuming. In recent years, thin layer chromatography (TLC) has been making rapid strides toward being a precise analytical tool (3, 4, 9) besides being a most valuable qualitative analytical aid. Information can he obtained from standard TLC plates, usually within 30 to 45 minutes. However, by adopting the quantitative method of Purdy and Truter (8)to smaller plates prepared on 1.5- x 3-inch glass microscope slides, the time necessary t o chromatograph a sample can be reduced to a few minutes and an accurate quantitative analysis can he made. Quantitation is based on a linear relationship existing between the square root of the area of a comvonent after chromatographing and the iogarithm of the weight of samc,le applied to the plate. This relationship is valid for specific weight ranges of different chromatographed components. In the case of long chain tertiary amines, we found this range to be approlximately 200 pg.
The ultraviolet source used is a hand mineral lamp. A IO-pl. syringe i s used for spotting the plates and a 250-ml. indicator spray bottle for spraying the plates after development. Reagents. Silica Gel G was obtained from Research Specialities, Richmond, Calif. Chloroform, methanol, and isopropanol are C.P. grade, while ammonium hydroxide and 95% ethanol are reagent grade. The indicator, 2‘7‘dichlorofluorescein, wae obtained from Eastman Organic Chemicals. All water is deionized before using. Distilled, dimethyl “coco” amine was used in the preparation of standard solutions. Gas and thin layer chro-
EXPEIRIMENTAL
Figure 1. Separation of tertiary amine from amine oxide
Apparatus. G lass microscope slides (1.5 x 3 inches) :are used in preparing the thin layer plates. Eight-ounce glass jars are use d as chromatographing tanks.
component “sored ,OI“.”t
front I, *e
tSrtl0.y
amine
Component which moved only D short distonce from origin i s the amine oxide
matography and wet analysis showed the material to be 100% tertiary amine. Developing Solvent. The developing solvent system is one similar to that recommended by Mangold (6). It is prepared in the followin8 manner: SO’% chloroform equilibrated for 1 hour with 20% concentrated ammonium hydroxide, The organic layer is separated and measured and taking its volume as 97’%, 3% methanol is added. Standard Curve. The microscope slides are coated with Silica Gel G adsorbent by dipping them once into a slurry of 50 grams of Silica Gel in 150 ml. of chloroform (6). They are then air-dried for 5 minutes and any remaining adsorbent on the reverse side of each plate is removed with a tissue. Six different concentrations of the tertiary amine are prepared in 10-ml. volumetric flasks using isopropanol as a solvent. These amine solutions represent the following percentages of unreacted tertiary amine in a 5-gram sample of tertiary amine oxide diluted in a similar fashion: 0.28%, 0.73Y0, l.39%, 2.13%, 4.32%, and 8.28%. Five-microliter sliquots from these standard solutions are transferred to the microscope slide plates u$ing a microsyringe. All plates are spotted 1 cm. from the bottom of the plate. A few milliliters of the developing solvent are placed in the S-ounce glars jars that serve as chromatographing tanks and the plates are inserted into the tanks. The solvent front is allowed to travel to within 1 ern. of the top of the plate before the d a t e is removed. The time required f i r the development of earh chromatogram ranges from 3 to 5 minutes. All standards are run m quadruplirate. Upon removal from the tank, the plates are sprayed with a 0.057, soloVOL. 37, NO. 4, APRIL 1965
* 603
RESULTS A N D DISCUSSION
Table I. Results of Potentiometric Determinations and TLC Determinations for Tertiary Amine in Tertiary Amine Oxide Samples
Potentiometric 0 557,
0 70% 0 62yc 1 03yc
TLC 0 54% 0 77% 0 69% 1 00%
tion of dichlorofluorescein in methanol and placed under the ultraviolet lamp. An outline of the tertiary amine spot is made with a pencil and an area of the spot is obtained by measuring and multiplying the height by width. The average area of the spots representing the various concentrations are taken and plotted, using the appropriate relationship. Sample Procedure. A 5-gram sample of the tertiary amine oxide is weighed into a 10-ml. volumetric flask and made up to volume with isopropanol. The sample is thoroughly mixed and a 5+1. aliquot of the solution is spotted onto a small T L C plate. The same procedure that is used to prepare the standard curve is then followed. After the chromatogram is developed, the plate is sprayed with the indicator. The area of the tertiary amine spot is determined and the weight of unreacted tertiary amine is obtained from the standard curve.
Figure 1 shows the separation obtained between the tertiary amine and the tertiary amine oxide. The standard curve obtained is shown in Figure 2. tb Four different dimethyl “coco” amine oxide samples were analyzed for unreacted starting tertiary amine. The anresults obtained were compared to re- $ . sults obtained using a potentiometric determination ( 7 ) , and compared quite -0I ” favorably (Table I). When used in a process control, a 1.0quantitative determination by thin layer chromatography has several advantages over other methods. Since it does not necessitate elaborate equip0.2ment, its cost is nominal. It is possible onJ o t v ’ 8.0 ” 10.0 ‘ “ to obtain very good precision and not E sacrifice time. An experienced operFigure 2. Standard curve ator can perform 25 to 30 determinations in an hour. Also, a small corner of a work bench is sufficient to handle all (6) LIangold, H. K., Kanimereck, R., Ibid., 39, 202 (1962). necessary equipment. (7) LIetcalfe, L. D., AKAL. CHEM. 34, 1849 (1962). (8) Purdy, S. J., Truter, E. Y.,Analyst LITERATURE CITED 87, 802 (1962). (1) Feigl, F., Amoral, J. R., Mikrochim (9) T’annier, S . H., Stanley, W. L., J . A d a . 1958 337--41. Assoc. Ofic. ilgr. Chemzsts 41, 432 (2) Glynn, E., Analyst 72, 248 (1947). (1958). (3) Hefendehl, F. W., Planta Medica JOHN R PELKA 8 , 65 (1960). LINCOLN I). METCALFE (4) Kirchner, J. G., hliller, J. M., Rice, Armour Industrial Chemical Co. R. G., J . Agr. Food Chem. 2, 1031 (1954). 8401 W. 47th St. (5) Mangold, H. K., J . Am. Oil Chemists2 McCook, Ill. 60529 SOC.38, 708 (1961). I
E
Determination of Van Slyke Factors by Gas Chromatography SIR: Van Slyke factors (VSF) have been determined successfully for many years ( I , 3, 6-8). However, a solution of more than one gas requires the use of gas-free reagents to induce suitable reactions and thus to eliminate the vapor pressure of one or more of the gas components. Even for a solution of one gas, partial pressure must be determined in the presence of water where small pressure changes are difficult to measure; also, by separate experiment, the amount of gas that becomes redissolved during a pressure measurement must be determined and an appropriate correction must be applied to the data. Ramsey (6) applied gas chromatography to the analysis of gas-containing solutions by connecting a gas chromatograph (GC) to a Van Slyke extraction apparatus. The net response of two estractions was taken to represent the release of all dissolved gas. This innovation results in greater sensitivity and allows the experimenter to analyze for a particular gas in a gas mixture. Gordon and Adams ( 2 ) of Argonne S a tional Laboratory have used this equipment for gas analysis where they multiply the GC peak area of the first extrac604
0
ANALYTICAL CHEMISTRY
tion by the appropriate VSF, as determined in the conventional manner (1, ?, 6-8), to calculate the response of all the gas in the sample. This communication extends previous work by relating the two GC peak areas obtained by two successive extractions from a sample in such a way as to enable calculation of Van Slyke factors, Henry’s constant, and Bunsen’s solubility coefficient. If one successively extracts a sample and analyzes the gases on a GC until all the gas is removed from the liquid sample, the sum of the peak areas for any gaseous components can be represented as m
A =
Ea< i=l
where ai is the area related to the ith extraction of the gaseous component in question. With Henry’s law and the assumption of ideal gas-solution behavior it can be shown that a1
az
=
=
(A
Ak
(2)
- ai)k
(3)
where k is the fraction of gas molecules removed from the liquid phase into the gas phase during the extraction process; this ratio is related to the VSF applicable to the experimental conditions employed. Equations 2 and 3 can be solved simultaneously for k and A; this solution yields k
A
-
a2)/a1
(4)
= al”(a1
- az)
(5)
=
(a1
EXPERIMENTAL
A Perkin-Elmer vapor fractometer, Model l54-DG, was connected to a Thomas Van Slyke apparatus by a four-way valve arrangement (Figure 1). All parts of the apparatus were either glass or copper. For COz analysis, a 0.25-inch diameter, 2-meter, silica gel (50-mesh) column was operated a t 110’ C. Helium was used as carrier gas a t an input pressure of 7.0 pounds per sq. inch to obtain an exit flow rate of 50 cubic em. per minute. The thermal conductivity detector was operated a t 8.0 volts and the output signal was fed into a 5-mv. recorder with a chart speed of 4 inches per minute. For oxygen analysis a 0.25-inch Apparatus.
