V O L U M E 27, N O . 6, J U N E 1 9 5 5 vious storage history of the propellant. Those unfamiliar with considerations of propellant stability should be cautioned that autoignition is a function of many factors, including composition, temperature, pressure, size, and others. Even a fresh, stable propellant may be ignited, indeed this is a requirement for its use. ACKNOWLEDGMENT
Unpublished data by H . 11. Spurlin and .4. G. Sandhoff of the Hercules Powder Co. lvere responsible for the interest of the senior author in constant-volume tests. The encouragement of F. C. Thames during the course of these investigations is much appreciated. 11. E. Baicar, C. I-.Jamen, and Celia J. Wright assisted in carrying out the tPst9. LITERATU-RE CITED
(1) Davis. T. L., "Chemistry of Powder and Explosives," pp. 30713, Wiley, Sew York, 1913.
961 Phillips, L., Nature, 160, 753 (1947). Robertson, R., and Napper, S. S., J . Chem. SOC.,91, 764 (1907). Schroeder, W-.A . , hlalmberg, E. W., Fong, L. L., Trueblood, K. X . , Landerl, J. D., and Hoerger, Earl, Ind. Eng. Chem., 41, 2818 (1949).
Schroeder, W.A., Wilson, 11.K., Green, C., Wilcox, P. E., Mills, R. S.,and Trueblood, K. K.,Ihid., 42, 539 (1950). Taliani, AI., Garz. chim.ital., 51(1), 184 (1921). U. S. Navy, Bureau of Ordnance, "Taliani Test for Determination of Stability of Solid Propellants," NavOrd OS 7904 (-4pril 5, 1951). Wiggam, D. R., and Goodyear, E. ED.,4, 73 (1932).
S.,IND.EXG.CHEM.,ANAL.
R E C E I V Efor D review November 2 , 1954. Accepted January 27, 1955. Published with perniission of the Bureau of Ordnance, Kavy Department. The opinions and conclusions are those of the authors. Presented before the Propellant Power Symposium, Ameriran Institute of Cheniical Engineers, Louisville, Ky., M a r r h 23, 1'355.
Fluorimetric Determinations of Aluminum and Gallium in Mixtures of Their Oxinates JUSTIN W. COLLAT'
and
L. B. ROGERS
Department o f Chemistry and Laboratory o f Nuclear Science, Massachusetts lnstitute o f Technology, Cambridge 39, Mass,
This study was designed to test the feasibilitj of determining tw-o substances in a mixture, when each has nearlj the same fluorescence spectrum, by taking advantage of the difference in their sensitivities to different wave lengths of exciting radiation. A Beclcnian DU spectrophotometer was modified to enable one to determine the fluorescence spectra of the individual components, while another was employed to provide monochromatic exciting radiation. Aluminum and gallium oxinates, which have nearl!, the same fluorescence spectra in chloroform, hate been analjzed with moderate success bj this technique. Determination of mixtures of substances having different fluorescence spectra can probablj be facilitated bj taking advantage of this additional variable.
A
Y.1LYTICAL use of fluorescence has been based almost entirely on the measurement of fluorescent light emitted from a sample under excitation by one or more mercury lines selected bv means of a glass filter. The 3650 A. emission line has been used most frequent11 because it can pass through g1a.F optics, whereas shorter wave lengths require fused silica or quartz. Furthermore, fluorescence hay heen limited to the estimation of a single constituent in a medium which does not contain any other interfering fluorescent compounds. Little effort has been directed tonard the determination of mixtures of fluorescent materials by taking advantage either of differences in their fluorescent spectra or in the differing intensities of fluorescence produced by different exciting wave lengths. The anal! tical possibilities of fluorescence spectra were realized by Huke, Heidel, and Fassel ( 4 ) , who modified a Beckman Model DE spectrophotometer to obtain fluorescence gpectra for the study of rare-earth solutions; by Peattie ( 6 ) , mho determined samarium and europium in ignited calcium sulfate; and by Aitken and Preedy ( I ) , who studied the fluorescence spectra cf estrone, estradiol l i p , and estriol compounds. The present study was designed to explore further the feasibility of fluorescence analysis for the determination of more than one 1 Present address, Department of Chemistry, Ohio State Unix eraity, Columbus 10, Ohio
compound in a mixture, using differences in the responses of the compounds t o different exciting wave lengths. The system chosen for study was a mixture of aluminum and gallium oxinates (salts of 8-quinolinol), both of which are easily extracted from water into chloroform, in which solvent they have essentially identical fluorescence spectra. The fact that one can determine these two compounds in this unfavorable case indicates t h a t t h e determination of two constituents which possess different fluorescence spectra should be even more readily attacked in this way. The determination of gallium by the fluorescence of its oxinate in chloroform and conditions for extracting the oxinate into chloroform have been described by Sandell ( 7 ) . The oxinate of aluminum has been applied most recently in fluorimetric analysis by Goon and coworkers ( 3 ) . Both oxinates emit a strong yellow-green fluorescence in chloroform solution \Then irradiated with the 3650-A. incrcnrj. line. EXPERIMENTAL DETAILS
Reagents and Solutions. +4Lciwivux STOCK SOLUTION. Aluminum chloride hexahydrate was used to prepare a stock solution of aluminum containing about 1.00 mg. of metal per ml. Solutions containing 100, 10, 1.0, 2.5, 0.28 y of aluminum per ml. were prepared as needed by dilution of the stock solution with 0.05M hydrochloric acid. GALLIUMSTOCKSOLUTIOS. A sample of gallium oxide which was shown spectroscopically to contain less than 100 p.p.m. of aluminum was used t o prepare a stock solution by dissolving it in hydrochloric acid. The final gallium concentration was 100 y per ml. ; the final hydrochloric acid concentration was O.05M. This solut'ion was diluted with 0.05Jf hydrochloric acid to prepare a solution with 10 y of gallium per ml. as required. OXINE SOLUTION.A 0.1% solution of oxine was prepared according to directions given by Sandell ( 7 ) . XMUOSI~M ACETATE. A 1M solution was added to the acidic solutions of gallium and aluminum to provide buffering action. Ten milligrams of quinine QUINISE SULFATESTANDARDS. sulfate U.S.P. was dissolved in 1 liter of 0.1M sulfuric acid to S. Pharmacopoeia prepare the stock solution described in the I?. (8). A portion of this was diluted with 0.1M sulfuric acid daily to obtain solutions containing 3.0 and 0.3 p.p.m. of quinine for comparison with the oxinate extracts. CHLOROFORM. Reagent grade chloroform was used for all extractions. METAL OXINATES. Solid aluminum and zinc oxinates were prepared by the method of Rolthoff and Sandell ( 5 ) . These were
ANALYTICAL CHEMISTRY
962 dissolved in chloroform and used t o obtain absorption and fluorescence spectra in the preliminary studies. Apparatus. A schematic drawing of the apparatus used to obtain fluorescence spectra is shown in Figure 1. Two Beckman Model DU spectrophotometers were used, one to isolate a monochromatic beam of light for excitation, the other t o determine the spectrum of the fluorescence. A Beckman Model 4300 photomultiplier attachment, equipped with an RCA 1P28 photomultiplier tube, was used instead of the blue-sensitive phototube usually supplied with the instrument. The phototube housing was placed in the position normally occupied by the source, as in the apparatus constructed by Huke and coworkers (4).The cell compartment was modified by placing a solid aluminum plate over the side usually fitted with the phototube compartment and by replacing the standard cover with one having a hole to permit entrance of a vertical beam of esciting radiation. Uncovered Beckman fused silica cells (1 cm.) were used as vessels to hold the samples and standards. Thus, in operation, light from the esciting source passed into the usual exit slit of the monochromator spectrophotometer, through the prism, and out through the usual entrance slit where it was reflected by the entrance mirror down into the sample. Fluorescent light from the sample traversed the analyzer spectrophotometer similarly and emerged a t the photomultiplier tube. The source of ultraviolet excitation radiation as a General Electric photochemical lamp, Type UA-2. This lamp, a mercury
,A’S 3u R CE 4 , ’
in hydrochloric acid, was placed in an extraction flask with 3.0 m!. of oxine solution and 10 ml. of ammonium acetate solution. The resulting solution was extracted with two 10-ml. portions of chloroform, and the combined extracts diluted to 50.0 ml. with chloroform. h 3.00-ml. aliquot of this solution was taken for meaurement in a l-cm. Beckman fused silica absorption cell. MEASUREMENT OF FLUORESCENCE. hbout an hour before measurements were to be made, the source, the analyzer spectrophotometer, and the photomultiplier power supply were turned on to ensure maximum stability. For routine measurements of fluorescence intensity, the wave-length dial of the analyzer spectrophotometer was set a t 520 mp, the nominal slit nidth a t 1.00 mm. (an effective band width of 23 mp), and the sensitivity knob a t the limit of counterclockmise rotation. The slit width of the monochromator spectrophotometer was set at 2.00 mm. for all measurements; this setting ‘IT as not critical, as any slit width wide enough to pass the mercury line used for escitation would suffice. Readings were taken in arbitrary units on the “per cent transmittance” scale of the spectrophotometer, with the monochromator spectrophotometer set a t 365 or 436 nip. Tl-hen samples were being excited by the 3650-A. mercury line, readings were also taken using the quinine standards. From these readings and the fact that quinine fluorescence was found to be linear Kith concentration in this range, a “quinine equivalent” of the oxinate fluorescence was calculated. This number, derived from simple interpolation, is the concentration of quinine which would give the same fluorescence as the oxinate solution. As quinine does not fluoresce under 4358-;1. radiation] the quinine readings obtained with 3650-A. excitation Fere also used t o calculate the quinine equivalent of fluorescence under 4358-A. excitation. Spectral distribution curves of the fluorescent light were taken in a manner similar t o that described above, except that no quinine standards were used and the slit width of the analyzer spectrophotometer was continually adjusted t o maintain an effective band width of 30 mp throughout the spectrum. RESULTS
ANALYZER
DU
Preliminary Studies. The apparatus was tested by using it to examine the fluorescence of an oxinate solution containing 0.1 p.p m. of aluminum. Excitation nave lengths of 2537, 3131, 3650, and 4358 A . nere tried, but it was found that the only excitation n ave lengths n h1c.h produced appreciable fluorescence were the 3650- and 4358-L4. lines. Absorption and fluorescence spectra of gallium and aluminum oxinates are shoIvn in Figures 2 and 3 . The fluorescence spectra are similar for both compounds, and they consist of a single broad band. S o attempt has bern made t o correct the fluorescence data for variatlons in the sensitivity of the photomultiplier tube a t diffelent wave lengths or for absorption of the fluorcqcent light as it traversed
”\
Figure 1. Beckman Rlodel DU spectrophotometers modified for obtaining fluorescence spectra
vapor type rated at 250 watts, has a quartz bulb and transmits wave Iengths greater than 2200 A. The source was placed 20 inches from the monochromator spectrophotometer and was focused on it by a quartz lens of 5-inch focal length. It was found convenient t o mount the source in a hood so that ozone formed in the air by the strong ultraviolet radiation could be continuously drawn off. Aluminum and gallium osinates were extracted from aqueous solutions into chloroform in 50-ml. glass-stoppered Erlenmeyer flasks to which side arms of capillary tubing 1 mm. in inside diameter had been attached in order to drain off the chloroform layer after an extraction. This apparatus eliminated any possibility of contaminating the sample with stopcock grease from an ordinary separatory funnel, although the experience of Goon and coworkers indicates that this precaution was not necessary. Procedures. EXTRACTION OF 0x1XATES. T h e conditions finally selected for extracting thegalliumand aluminum oxinates from aqueous solutions were different from those suggested by Sandell and by Goon and coworkers. Up to 10 ml. of the sample, 7%-hichwas 0.05M
A I ( C 9 H e O N ) 3 , 2 p p m A l below 3 2 0 m p , 2 Oppm AI above 330mp
12~
I
I
- 3.2 ppm 8-quinolinol .(exine)
___
OL I
Go I C 9 & ON), ,excess oxine present 2 ppm Go below 2 9 0 r n p I 8 ppm Go obove 2 9 0 rnp
\
240
I 300
\
\
\
I 350
I 400
WAVELENGTH ( m p )
Figure 2.
Abmrption spectra of metal oxinates in chloroform solution
4, 0
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5
963 tration ranges which donot fulfill the ideal conditions, one can empirically calibrate the method, so that, nonlinearity of fluorescence intensity or interaction effects are compensated for in the calibration. One way of doing this is sho-rn in Figure 6, which is a plot of fluorescence intensity expressed as quinine equivalents 2's. the concentrations of aluminum and gallium. With measurements of fluorescent intensity a t both wave lengths one can, 11)- interpolation, locate the only concentrations of aluminum and gallium which would give rise to the measured intensities. An example of the use of Figure G to ohtain the concentrations of aluminum and gallium from quinine equivalent data is given in the first line of Tahle I.
~
~
I ~
~
WAVELENGTH ( m p )
Fluorescence spectra of metal oxinates in chloroform solution
Figure 3. .____-
Table I.
Analyses of Synthetic Rlixtures of Aluminum and Galliunl Quinine Equivalents
Taken, P.P.11. Aluminum Gallium 0 03 1 70 0 03
0.30
0 10
1.10
0.17
0.30
0 17
1.70
f o r Excitation by-~ 43.58 A. 0.75 0.75 0.72 0.16 0.16 0.18 0.63 0.63 0.60 0.40 0.43 0.43 0.94 0 89
3650 A . 2.14 2.13 2.10 0.93 0.84 0.8fi
2.43 2.39 2.33 2.37 2.20 2.33 3.03 2.91
Found, P.P.11. Aluminum Gallium 0.044 ,58 0.043 1.58 0 044 1.49 0 041 0.20 0.030 0 23 0.023 0.30 0.17 0.64 0.17 0.66 0.13 0.94 0.19 0.18 0.17 0 18 0.18 0.12 0.20 1.24 0 19 1.30
Av. of Replicates L i t h .%I.. Deviations, P.P.AI. Aluminum Gallium rt o,ooo . j 5 rt o , o
~
0 . 0 3 1 rt 0 . 0 0 6 0 . 2 4 & 0 . 0 4
= 0 13
0.155 i 0.017
0.73
0.175 It 0 006
0 16 i 0 03
0.192
the sample cell. A change in exciting wave length changed the intensity of fluorescence, though it did not change the shapes of the spectra. As the maximum of the fluorescence occurred at about 520 mM for both compounds, this wave length was used for making quantitative mcasurement. Calibration Data. Figures 4 and 5 show fluorescence intensities plotted as functions of the aluminum and gallium concentrations. Each figure is made up of a famill- of curve8 showing, for different amounts of aluminum, the effect on the total fluorescence of various concentrations of gallium. The fluorescence of a mistare of the oxinates is smaller than the sum of the fluorescences for solutions of the individual components. Much of the decrease is undoubtedly due to greater absorption of the exciting radiation before reaching the portion of the solution sern b ~ the analyzer spectrophotometer. However, there remains the popsibility of still another source of decrease ryhich may be the result of interaction of the oxinates. Thus, under 4358 -4.excitation, it is impossible to distinguish hetween solutions having concentrations of aluminum from 0.15 to 0.20 p,p.m. Even a t a level of 0.10 p.p.m. of .aluminum the curves leave much t o be desired. This situation results in poorer precision and accurac>for the method JThen applied to mixtures containing high concentrations of both aluminum and gallium. For practical analytical purposes it is advantageous, but not necessary, to have the fluorescences additive and linear with concentration so that in a two-component system the concentrations of the components can he determined hp solution of simultaneous equations using data obtained with two different excitation wave lengths. I n the present case, rather than discard the concen-
=0
006
As the number of quinine equivalents arising from 4358 -1. excitation is 0.75 an that prodnced by 3650 ;i.escitation 2.14, the area of interest in Figure 6 bounded by the dotted lines lahlrd 0. and 0.8 and the solid likes labeled 2.0 and 2.2. TO estimate the concentrations correPponding t o the experimental value of 0.75, locate on a vertical (or horizontal] line a point corresponding to the fraction of the distance b e b e e n the calibration lines-Le. (0.80-0.75),'(0.80-0.G0). Constriict the approximate curve for 0.75 quinine equivalents by connecting severa1 of these points. Then construct a similar curve for the value 2.11. The coordinates of the intersection of the curves for 0.75 and 2.14 quinine equivalents give the concentrations of aluminum and gallium in the sample.
1 27 rt 0 . 0 3
Analyses. The precision of thc method was determined Xvith gallium and aluminum separately a t hoth high and low levels of concentrations. Reference to Table I1 shows that the precision of determination of the individual elements with the modified spectrophotometer is satisfactory but not, quite so good as that reported by other investigators using conventional fluorimeters. Table I shows the results obtained in the anaIyses of several synthetic mixtures of aluminum and gallium. As one might predict from the curves of Figures 4 and 5, the precision and accuracy of the method decline a t high aluminum and galliuni concentrations.
Table 11. Determinations of Aluminum and of Gallium Alone Taken,
P.P.R.I. .1I
0,020
AI
0.180
Ga
0.30
Ga
1.80
Found, P. P. RI. 0.026 0.023 0.021 0 019 0.17 0.16 0.16 0.17 0.15 0.33 0.33 0.35 0.35 0.30 1.6 1.7 1.8 2.1
Standard Deviation
A\.. rt, P.P.I\I. 0.022 =t0 . 0 0 3
0 . 1 0 rt 0 01
0.33 rtO.02
1.8
~~
= 0.2
ANALYTICAL CHEMISTRY
964
0
1.0 ppm GALLIUM
I
tometer. Beyond a depth of 2.0 cm. further addition of sample resulted in a decrease in fluorescence because some exciting radiation was absorbed by that portion of the sample above the field. The blank shows appreciable “fluorescence” for a sample about 1.7 em. deep. This light is probably not true fluorescence, but light which is refracted into the analyzer spectrophotometer by the meniscus of the sample. T o minimize the contribution of this factor in measurements on fluorescent samples, a volume of 3.00 ml. was selected and used for all measurements. This volume effectively filled the cell and presented the analyzer spectrophotometer iTith a uniform sample area. Because the vertical dimensions of absorption cells vary slightly, the same cells uere used for quinine standards and for the unknown solutions throughout the work. I t made no difference which windon- of a given cell was placed next t o the entrance window of the analyzer spectrophotometer. Effect of Amount of Oxine. Goon and coworkers ( 3 ) pointed out that the stahility of chloroform solutions of oxinates and reproducibility of fluorescence measurements are enhanced by the presence of excess oxine in the chloroform. I n the present work, a t least a fourfold ewes‘ of oxine was present. The low fluorescence obtained with appropriate blanks indicated that the excess oxine did not interfere by fluorescing. However, when the 2 ml. of 2% oxine recommended by Goon and coworkers was used ( S ) , the fluorescence under 3650 A. excitation of a solution containing 2.0 p.p.m. of gallium u a s 3 i % lower than when the usual procedure was employed, indicating interference by absorption of the exciting radiation. Because the precision did not
Figure 4. Calibration curves for gallium i n presence of varying amounts of aluminum using 3650 A. excitation radiation
-0
.05
.20
.IO .I5 ppm ALUMINUM
Figure 6. Calibration graph for mixtures of aluminum and gallium Solid lines are lines of identical fluorescenceintensity under 3650 A. excitation: dotted lines are those for 4358 A . excitation. I\-umbers beside lines are quinine equivalents
Figure 5, Calibration curves for gallium in presence of varying amounts of aluminum using 4358 A. excitation radiation Effect of Volume of Sample in Cell. T o obtain a linear relationship between fluorescence and concentration the depth of the sample should be adjusted so that the intensity of the exciting radiation, a t some fiducial point in the cell, would be standard for all measurements. As this type of compensation is impractical in routine work, it was decided to use a constant sample volume (and hence, constant length of light path). Figure 7 shows the dependence of the intensity of fluorescence on depth of sample. As the cell was filled t o a depth of 2.0 em. the fluorescence increased, as a result of a larger amount of the sample being within the field of view of the analyzer spectropho-
10
2.0 DEPTH OF SAMPLE ,cm.
30
Figure 7 . Variation with depth of sample of measured fluorescence intensity under 3650 A. excitation
V O L U M E 2 7 , N O . 6, J U N E 1 9 5 5
965
appear to be improved, the use of a large excess of oxine did not seem advisable in the procedure. DISCUSS103
Completeness of Extraction. The extractions in the present work were made from aqueous solutions of pH 5 . 7 . Experiments were conducted n-hich showed that the extraction of aluminum was as complete at this pH as in the pH range of 8.0 f 1.5 recommended hy Goon and coivorkers ( 3 ) . The estraction of gallium v a s more than 907, complete after a single extraction and was essentially 1007, complete after the second extraction, from a solution of pH ,5.i, while in Sandell’s original method ( 7 ) , where selectivity rather than completeness was desired, the extraction from :i solution nf pH 2.6 to 3.0 was 70 t o 80% romplete. Intensity of Mercury Lines. D a t a given by the General Electric Co. on the U-1-2 pliotocheniical lamp ( 8 ) show that the intenxitie3 of the variow lines of wave length less than 5000 A. dec r e a x in tlie following oi,tier: 3650, 3131, 2530, and 4358 A. Of these, tlie 3650- and 4358-A. lines were found to lie most useful in esciting fluorcrcence so that, beside3 intrinsic line intensity, alxorption and scattering it1 the optical system and alisorption characteristics of the fluorrrcent compounds are responsible for the efficary of the 4358-.1. line over the more intense C 3 6 - and 313 L-.l. lines. Although the readings for a given standard solution were essentially constant over a 3- t o 5-hour period, the readings often differed by as much as l5Y0 from day to day (see Table 111). For that reason, the use of‘ quinine was adopted as a secondary standard. The observed variations may have been due to variations iu voltage and temperature of the mercury lamp or t o changes in the operating characteristics of the photomultiplier. Zinc Oxinate. Solid zinc osinate was prepared and found to be highly fluorescent a.hen tlisolved in chloroform solution. Its fluorescence spectrum is almost identical with those of gallium arid aluminum. Though mistures of zinc and aluminum osinates were determined after dissolution in chloroform, more complete results on zinc are not included in the present study because its
osinatc was not extractable from aqueous solution into chloroform. This behavior apparently is due to water of hydration in the zinc oxinate which renders it insoluble in chloroform, even though the oven-dried product is soluble.
Table 111.
Day-to-Day Variations of ,ibsolute Values of Fluorescence Oxinate“ Quinine, P.P.lI. 0.3 3.0
Uar
r\bsolate
Quinine equir.
10.3 87.3 (i? .7 2.14 10.4 85.2 10.1 77.2 35 n 2 10 10.0 78 8 * Oxinate e x t r a c t s contained 1.7 p.p.m. of galliuni and 0.030 i1.p.m. of ali~~~iini~iir. 1
2
ACKNOWLEDGMENT
The authors are indebted t o the Xtomic Energ), Commission for partiul support of this study. LITERATURE CITED
(1) ditken, E. H., and Preedy, J. R. K., J . Endoc~irtol., 9, 251
(19531. ~I
( 2 ) General Electric Co., Lamp Dirkion, Application Eng. Dept.,
Bull. LS-103 (June 1953). ( 3 ) Goon. E., Petley, J. E., ;\IcA\lullen, JV. H.. and Wlberley, S.E., h K . i L . CHEhI., 25, 608 (1953). (4) Huke, F.B., Heidel, K.H., and Fassel, V. A , J . Opt. SOC.Amer., 43, 400 (1953); A N h L . CHEM., 26, 1134 (1954). (5) Kolthoff, I. AI., and Sandell, E. B., “Textbook of Quantitative Chemical Analysis,” 3rded., p. 321, hIacmillan, S e w York, 1953. ( 6 ) Peattie, C. G., Ph.D. thesis, Massachusetts Institute of Technology,
Cambridge, Alass., 1952.
(7) Sandell, E. B., d K . i L . CHEhf., 19, G3 (1947). (8) U. S. Pharmacopoeia, XIV Rev., p. 772. Alack Publishing Co.,
Easton, Pa., 1950.
R E C E I V Efor D reriew August 11, 1954. Accepted February 11, 1955, Presented before tile Diyision of Analytical Chemistry a t tlie lSGtli 1Ieeting of the . ~ I I E R I C . A S CHEIII(.ILS O C I E T Y , S e w I-ork, Sel,tember 19%.
Adsorption-Dialysis, an Extraction Technique Use in Recovery of Amino Acids IRVING R. HUNTER, DAVID F. HOUSTON, and ERNEST 6. KESTER W e s t e r n Utilization Research Branch,
U. 5. Department o f Agriculture, A l b a n y 70, Calif.
.Sn extraction procedure useful for recovering amino compounds from biological materials involves the combined use of adsorption and dialysis. Slurries of an adsorbent and the material are separated by a semipermeable membrane and agitated. Amino acids and other substances of low molecular weight diffuse out of the material and through the membrane. When a cation exchange resin is used as an adsorbent, compounds containing basic groups such as amino acids are selectively removed from solution and nonbasic components are rejected. The concentration of adsorbable material in the dialyzate above the resin is accordingly reduced, permitting additional amounts to diffuse through the membrane. The adsorbed substances are easily recovered from the resin by elution with ammonia. The method yields considerably larger amounts of amino nitrogen compounds in higher concentration than extraction with water or aqueous alcohol.
F
REE amino acids occurring in small amounts in biological
substances must be separated before they can be analyzed by physical or biochemical methods. Estrartions with water or 80% alcohol have the disadvantage of dissolving relatively large amounts of other soluble ingredients and requiring large volumes of estracting liquid arid tedious evaporations. The procedure described herein was devised to overcome these difficulties. It has been used in an investigation of parboiled rice and may be generally applicable to biological materials. Modifications might be upel‘ul for compounds other than amino acids. Briefly, the method comprises concurrent dialysis and adsorption. Dialysis permits entry of compounds of low molecular weight into a compartment containing a resin that adsorbs one or more classes of compounds and rejects others, depending on type. Dialysis is continued until substantially all the compounds of a specific group have migrated from the starting material to the adsorbent. Gilbert and Swallow ( 1 ) used a similar system for purifying solutions of proteins and enzymes but vere
