0.o.t
0.0.
Figure 3. Quantitative estimation of carvyl acetate spots using densitometer
could be minimized. However, only acetone gave improved results, the blue colors fading much more slowly in this solvent. Unfortunately, the yellow colors from anthranilate esters disappeared instantly with all solvents tested. All colors were totally insoluble in both benzene and carbon tetrachloride. ACKNOWLEDGMENT
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from the R/ value. I t was observed that acetates always had the lowest R, values and that these values slowly increased with increasing chain length of the acid moiety. However, formates, having R, values corresponding closely to the butyrates, were an exception to this rule (Figure 1). It was also observed that when the chain length of the parent acid was held constant, there was an increase in R j value with increasing chain length of the alcohol moiety (Figure 2). To minimize differences between individual chromatoplates, standard Rj values of the eaters were computed using citronellal as an internal standard. Table I shows the standard Ri values in benzene and TF->IC (6G40), respectively. Quantitative Analysis. Quantitative estimations were based on optical density measurement using the densitometer for spots of lowest concentration, 0.5 to 5 nl. per spot (nl. = nanoliter = one-billionth liter), and on qectrophotometric measurements using either the Beckman D t - or DK-2 for spots of higher concentration. Only those compounds reacting with vanillin/ HzS04could be studied acrurately with the densitometer as the spots formed with IiRInO4/H2SO4 could not be measured by the light transmission method. The spectrophotometric procedure was not only limited to materials
76
ANALYTICAL CHEMISTRY
I2
I3
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IS
Conc. n l f Spot
sensitive to vanillin, but could only be used satisfactorily with a portion of the vanillin-sensitive substances. Figure 3 shows a typical curve obtained by reading a set of varying concentration spots using the deniitonieter. The graph is seen to be linear to a concentration of about 5 nl./spot. Plots of data from spectrophotometric measurement give a straight line up to a relatively high concentration. However, as mentioned above, not all compounds could be measured by this method. The colorimetric procedure involved the careful removal of the spot from the chromatoplate 4 to 6 hours after spraying. The colored silica gel was placed in a 12-ml. centrifuge tube and 2 ml. of Skellysolve 13 was added with stirring to elute the color. The color was centrifuged to separate the silica gel and then decanted into a cell for the colorimetric reading taking care to avoid evaporation. S o difficulties were encountered with geranyl, neryl, linalyl, and other esters giving green spots. However, the blue color obtained from carvyl, terpinyl, and some other esters faded too rapidly in Skellysolve 13 solution for use in quantitative measurement. The yellow color from anthranilate esters dkappeared immediately on contact with this solvent. h number of solvents were investigated to determine if these undesirable effects
Appreciation is expressed to Fritzsche Brothers, Inc., New York, for supplying samples of many of the esters. LITERATURE CITED
(1) Attaway, J . A,, Wolford, R. W., Alberding, G. E., Edwards, G . J., ANAL.CHEY.34, 671 (1962). ( 2 ) Zbid., 35, 234 (1963). (3) Attaway, J. A , , Wolford, R. W., Edwards, G. J., J . i l g r . Food Chem. 10, 102 (1962). (4) Attaway, J. A,, Wolford, R. W.,
Alberding, G. E., Edwards, G . J., Ibid., 12, 118 (1964).
( 5 ) Nigam, I. C., Sahasrabitdhe, M., Levi, Leo, Can. J . (‘hen,. 41, 1535 (1963).
(6) Randerath, Kurt, “Thin Layer Chromatography,” p. 56, Academic Press, New York, 1963. (7) Shriner, R. L., Fuson, R. C., “Systematic Identification of Orranic Compounds,” 3rd ed., p. 171, OU’iley, New York, 1958. RECEIVEDfor review Jrily 2 , 1964. Accepted September 30, 1964.
Correction Piezoelectric Sorption Detector I n this article by 11‘. H . King, Jr.,
[ANALCHICM. 36, 1735 (1964)] on page 1739, column 1, reference (2) is incorrect. I t should read as follows: VanDyke, K.E.,I-.S. Patent 2,571,171 (July 4, 1945).
