areas immersed in the same solution of a redox couple and shorted together externally. Take, as boundary conditions of the Fick's law equations describing each, the common conditions of semiinfinite diffusion and equality of fluxes of the oxidizd and reduced forms. Restrict the ratios of the concentrations of the oxidized and reduced forms a t the electrodes to be equal to each other a t all times, thereby implicitly assuming the obedience of the reaction to the Nernst equation and the absence of ohmic potential drop through the solution between the electrodes. -4s a last boundary condition restrict the total current applied to these electrodes from some third counter electrode to be constant. Solution of this problem by Laplace transform methods shows that the ratio, R, of the currents a t the plane and sphere is given by
correspond to the sphere above; the exp ( D t / 4 r 2 )erfc ( 4 z / 2 r ) 2 - exp (Dt/4r2)erfc (4z/2~) slopes of the unevenness correspond to the plane; the valleys would correspond to diffusion out of rather than into a where D is the diffusion coefficient (assphere. The analogy is not a perfect sumed, for simplicity, to be the same one, because a t a rough electrode the diffor oxidized and reduced forms); r fusion gradients from the various points is the radius of the spherical electrode; merge to form a gradient which on a t is time; and erfc represents the error macro scale approaches that correspondfunction complement. ing to the gross geometry of the elecThe physical interpretation of the trode, when the diffusion layer thickresult is that at short times, when the ness becomes large compared to the amplitude of the surface roughness. diffusion layer thickness is Even when this occurs, inhomogeneities small compared with r, the current is equally divided between plane and in current density are present. at the electrode because of its roughness, the sphere (exp a2 erfc a approaches one as a approaches zero) ; but a t longer times current tending to favor the "high a progressively larger fraction (and in points." WILLIAMH. REINMUTH the limit all of it) passes through the spherical electrode (exp u2 erfc a apDepartment of Chemistry proaches infinity). Columbia University The analogy to a rough electrode is New York 27, N. Y. evident. The peaks of the unevenness
E =
(6)
Gas-Liquid Chromatography of Aroma of Vegetables and Fruit Direct lniection of Aqueous Vapors SIR: We wish to report the use of the dual-flame ionization detector of McWilliam and Dewar (3) for the GLC evaluation of aroma by direct injection of the vapor above hot aqueous vegetable and fruit products. Such a method could be easily applied to the analysis of aroma qualities in quality control. The organoleptic evaluation of aroma of vegetable and fruit products is difficult, particularly in regard to obtaining reliable quantitative measurements. Gas-liquid chromatography has introduced some possibility of quantitative and reproducible analyses. Since the thermal conductivity detectors are very insensitive as compared to the sense of smell and the volatile materials are present in most products in minute amounts, a concentration step requiring considerable time and a large sample is necessary with conventional gasliquid chromatography equipment. To examine the vapor above food samples, the GLC apparatus must meet three criteria: Very sensitive detectors must be used; these detectors must be insensitive to water; and the columns used should not be unduly disturbed b y the injection of the relatively large amounts of air and water taken with the sample. Three types of ionization detectors were tried in this work: the beta-ray argon triode of Lovelock (a), the single flame of Thompson (4), and the dual flame of McWilliam and Dewar ( 3 ) . It was found that the dual-flame detector was the most suitable for this type of analysis. The
argon detector is desensitized by oxygen and water, and the single-flame detector as employed by us was not as stable as the dual-flame detector a t the extreme sensitivities required. This is apparently due to the balancing out of environmental changes by the dualflame detector. Although it is desirable to use capillary columns with high separation efficiencies for such complex mixtures as food aromas, little success was found in applying these columns without some concentration step. The difficulty is in the very small volumetric flow rate in the capillary columns (0.01-inch i.d.) used, and the resulting undesirable time lag for the relatively large vapor sample to get on the columns. Separation efficiency was therefore sacrificed for large flow rate and large volume vapor injections of packed columns. Some applications of the method to differentiate samples of dehydrated potatoes, carrots, and pears are given. EXPERIMENTAL
Materials. Vegetable and fruit samples were obtained from current studies a t Western Regional Research Laboratory or from local retail stores. Apparatus. The GLC apparatus was constructed in this laboratory. The dual-flame ionization detector and electrometer were constructed according t o the design of McWilliam and Dewar (3). The simple electrom-
eter circuit used for the present work was built around that using M E 1400 tubes with 1010-ohm control grid resistors, Voltage to the recorder was taken from across a 1000-ohm resistor placed across the electrometer output. A minor difference in the present detector was the use of ceramic and Teflon insulation inside the detector cell (i.d. 85 mm.) instead of glass. The column was a &foot long, 0.25-inch o.d., 0.20inch i.d. stainless steel tube packed with 30% Apiezon M high vacuum grease on 60-80-mesh diatomaceous earth firebrick. This column was well aged for a t least a week a t 250" C. Column temperature was maintained within i ~ 0 . 5 "C. Nitrogen was used as the carrier gas; hydrogen was introduced a t the end of the column just before the detector. The nitrogen was partially saturated with water by passing it over distilled water before it entered the apparatus. The GLC conditions used for all results reported were: column tempera0.5" C., inlet pressure 7 ture 115" p.s.i. nitrogen, flow rate 30 cc. per minute, hydrogen flow rate 30 cc. per minute, air flow rate 600 cc. per minute. Method. The general method as to place a weighed amount of the sample in a measured volume of boiling distilled water in a 250-ml. Erlenmeyer flask, cover the flask with clean aluminum foil, swirl the flask a few times, and gently heat t o the boiling point. At this point 10 cc. of the vapor was removed with a clean syringe and injected directly into the GLC unit. Variations of the product weight and water volume were used depending upon the product.
*
VOL. 33, NO. 10, SEPTEMBER 1961
1439
Sample
Kater
Produrt Weight, G. Volume, M1 Potato granules 20.0 150 Diced carrots 50.0 75 Pears 60 0 125 RESULTS A N D DISCUSSION
POTATO GRANULES STORED ONE YEAR IN OXYGEN ATMOSPHERE
POTATO GRANULES STORED ONE
YEAR I N
AT
160 m 75'F.
NITROGEN ATMOSPHERE AT
-3o'F.
Figure 1. GLC curves of hot aqueous vapors above off-flavored and fresh dehydrated potato granules
FROZEN, DICED C A R R O T S , STORED TWO YEARS AT -1O'C.
F i i E S H , DICLS C A K r l C T S
GLC curves of hot aqueous vapors above off-flavored and fresh carrots
Figure 2.
7 \
'\ \ ---t]
j
4omr
ionr
0
COMICE
PEAR
n
h 1 4 0 mr
D'ANJOU
I,'
IOmr.
hmv
h---e
30
10 111"
PEAR
Figure 3. GLC curves of hot aqueous vapors above fresh Cornice and fresh D'Anjou pears
1440
ANALYTICAL CHEMISTRY
Apieaon M was chosen for the stationary liquid phase because of its low vapor pressure, thermal stability, and inertness to water vapor. The normal drawback of Apiezon M on firebrick of tailing with carbonyls and alcohols was considerably reduced by partially saturating the nitrogen carrier gas with water vapor (1). The polar stationary liquid phases such as diethylene glycol succinate polyester and Carbowax gave relatively high noise levels from "bleeding" when heated above 100' C. and, therefore, could not be used. Figures 1 to 3 show examples of analyses carried out with the described equipment and procedure. Figures 1 and 2 compare the effect of different storage conditions on dehydrated potato granules and on fresh and stored, diced carrots. Figure 3 shows the comparison of Cornice and D'Anjou pears. In the case of the potato granules, storage in oxygen was used to give an advanced degree of oxidative deterioration. Large changes were also found on storage in air for much shorter periods. The GLC conditions for the curves shown were chosen for a n intermediate range of compouiids. A lower column temperature would have permitted better resolution of the lower boiling compounds. A higher column temperature would have shortened the time needed for the determination of the higher boiling components. Programmed temperature runs would yield optimum resolution in a shorter time, but this technique presents base line shift problems a t the sensitivity necessary for this work with the present type of equipment. Instead of using the hot aqueous product, one can sample the vapor above the cold product. This, however, limits the chromatogram to the lower boiling components, usually to compounds smaller than n-octannl. No attempt was made to identify the individual peaks in this work. However, t o obtain some itlca of the range of compounds covered by this method, n-hexanal, n-octannl, and n-decanal were run through the GLC apparatus under the conditions stated and had the retention times of 10, 34, and 111 minutes, respectively. Complete identification would not seem necessary for the method to be uscful in quality control work. The chromatogram obtained could be considered a n "aroma profile." Relation to aroma qualities could be obtained by empirical means for
each product, according to the appearance or disappearance of certain peaks. Such a chromatogram is only a portion of the whole profile, however, and instrument resolution and sensitivity are still not comprehensive. The present method Seems to be largely limited to compounds boiling lower than n-decanal (b.p. 209' C.) for food aromas. Because of the low vapor pressure Of the higher compounds, a greater sensitivity iS needed for their detection.
ACKNOWLEDGMENT
LITERATURE CITED
The authors are indebted to representatives of the member firms of the Instant Potato Granule Manufacturers Association, and to J. Come and C. E. Hendel for helpful discussion and suggestions. RONG. BUTTERY
ROYTERANISHI Western.&gional Research Laboratory u. s. Department of Agriculture Albany 10, Calif.
(1)(1958). Knight, H.
% ANAL.
CHEW
301 2030
(2 Lovelock, J. E., Ibid., 33, 162 (1961). (31 McWilliam, I. G. !?ewar, R. A.
"Gas Chromatography, D. H. Desty,
ed', p' 142, .Butterworths~ 1958. (4) Thompson, A. E., J . Chromtog. 2 , 148 (1959).
MENTIONof specific products does not imply recommendationby the Department of Agriculture over others of a similar nature not mentioned.
Detection of Acids, Bases, and Salts at Micronormal Concentrations in Organic Solvents solid caustic soda) in the dark a t about 2.0 p N ) concentration, and stronger rmm temperature. However, if the acids even better. The sensitivity of the rhodamine reagent turns pink because reagent can be increased nearly 10 times of aging or exposure to light, it can be -i.e., near 0.2 p N by adding a f r a revived somewhat by a fresh extraction drops of absolute alcohol to the solid with a buffer of pH 10. A few other NaOH-dried reagent just before testing rhodamine and cationic dyes are also and detecting under anhydrous consuitable for this purpose under well ditions. Phenols, alcohols, etc., being defined conditions of extraction, but the common rhodamine dyes-e.g., rhodaweak acids, also resFond to the above mine I3, G, &.-cannot be used. test but a t a rather high concentration BASE DETECTION,ROSE BENGAL of about 1% and so can be distinguished REAGENT.Rose Bengal (1 mg.) i p easily from carboxylic compounds. dissolved in about 5 ml. of a buffer The strong acids-iiz., the mineriil (potassium acid phthalate or sodium acids, sulfonic acids, etc.-give the phosphate-citric acid) of pH 4 (or above test, but they can be distinlower) and is extracted with 10 ml. of guished from weak organic acids by the benzene. Though insoluble in benzene, fact that they producc strong fluoresthe dye dissolves practically completely to form a colorless solution under the cence down to almost the micronorabove conditions, thus giving a conccnmal level. The hj*drosyl group of tration of about 10 mg. of dye per 100 silanols responds as a stiong acid, in ml. of reagent. This benzene extract agreement with the fact that silicic acid is the required reagent which produces a is a strong acid; therefore, this method rose color with traces of bases. Unlike can be used for the detection of OH the rhodamine reagent preparation, groups in silicones. where the given conditions are quite Base Detection. The Rose Bengal critical, the proportion in this case reagent gives a rose coloration in the admits variation over a wide range. EXPERIMENTAL A few other halogenated phthaleins such presence of bases. Freshly prepared as eosin, etc., can also be used successdiethylamine solution in benzene of REAGENTS. ACID DETECTION, fully, but Rose Bengal (and also somew-hat less than micronormal conRHODAMINE REAGENT.Rhodamine 6 Erythrosin J, extracted a t about pH 3) centration (ca. 0.5 p N ) can be detected Gx (Color Index 45160), 3 mg., is is somewhat better. Test tubes coated easily by this reagent. The sensitivity, dissolved in about 2.5 ml. of a buffer with silicones are preferred for the (sodium phosphate-sodium hydroxhowever, is almost 100 times less for Preparation, preservation of, and experiide) of p H 10 or higher and is exweaker bases like pyridine, aniline, mentation with these reagents, as the tracted immediatrly with 100 ml. etc., and far less for electron donors slightest imperfection or impurity on the of benzene. The dye, although norlike nitriles, ketones, etc. The senglass surface tends to get colored upon mally insoluble in benzene, goes almost contact. sitivity of the reagent increases with wholly into the benzene layer to give a higher concentration of the dye. Since brownish-orange solution which is the RESULTS the Rose Bengal reagent is insensitive required reagent. The extraction Acid Detection. The rhodamine to weak organic acids in dilute solution, should be done in subdued light a t about reagent gives a pink color with all 25' C. and under oxygen-free conditions it also detects ampliolytes as bases. for best results; however, for fairly types of organic acids down to almost The eolor produced by w r y dilute solugood results these precautions are unmicronormal concentrations. I n the tions of bases is somewhat unstable, necessary. This reagent turns pink in micronormal range the color takes fading out considerably in the course the presence of minute amounts of acids some time for full development; of a few hours to days. and also by exposure to light or upon therefore i t is advisable to leave the Salt Detection. The rhodamine heating. A properly prepared reagent test solution plus reagent along with a reagent is also extremely scnsitive t o should give a distinct pink color immediblank in darkness for a few hours and benzene-soluble salts, producing a ately as cornparid against the blank compare the color of the two later. pink color like the acids. Quaternary with 0.0002% stearic acid in benzene. The above test easily detects stearic The reagent is unstable and has to be ammonium salts can be detected preserved over a buffer of pH 10 (or over acid down to near micronormal (ca. easily, and also inorganic salts suffi-
SIR: The need often arises to test for the presence of acidic or basic groups a t the micronormal level in waterinsoluble organic compounds, either as such or as impurities. Existing procedures (1) are mostly rather insensitive, cumbrous, or unreliable. The present method depends on the finding by the author ( 2 ) thnt some dyes dissolved in a suitable buffer under well defined conditions can be extracted with benzcnc, and this benzene extract changes color in the presence of a minute trace of acids, bases, or salts. This may happen even if the dye is not a conventional acid-base indicator in aqueous media, or if it is insoluble in benzene and similar neutral solvents. The method is briiig used routinely in this laboratory for the detection of the acid-base nature of benzene solutions of organic compounds including various types of dctergrnts and high polymers, and has yielded satisfactory results.
VOL 33, NO. 10, SEPTEMBER 1961
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