ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Several other examples can be added involving soluble oxidants which are reduced to an insoluble oxidation form which becomes attached to the HMDE and yields a film current in a certain potential range. The HMDE was kept in solutions of 0.1 M potassium perchlorate and chromic acid of concentrations between M for 1 or 5 min and 2 X a t -0.6 V. Upon cathodic scanning, large film currents with peak potentials between -1.78 and -1.86 V were observed. These currents were proportional to concentration in the range between and M chromic acid. A solution 2 x M in potassium chromate and 0.1 M in perchlorate was kept for 1min a t potentials between 0 and -0.8 V; upon cathodic scanning they yielded the same film current with peak potential a t -1.86 V, as was observed in the above solutions of chromic acid. The HMDE was placed for 1 to 5 min a t 0 V in solutions 0.1 M in potassium perchlorate and to 2 x M in permanganate. Large film currents with peak
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potentials varying between -1.75 and -1.9 V were observed. From the observations presented in this paper, it would appear that the HMDE can provide interesting information in studies of complex redox reactions in which an insoluble reducible or oxidizable product is formed which yields a film on the surface of the hanging but not of the dropping mercury electrode.
I. M. Kolthoff* Sorin Kihara Department of Chemistry University of Minnesota Minneapolis, Minnesota 55455 RECEIVED for review October 24, 1977. Accepted March 3, 1978. This work was supported by Public Health Service Grant CA16466-03 from the National Cancer Institute.
Comments on Unrecognized Systematic Errors in Quantitative Analysis in Gas Chromatography Sir: A recent paper ( I ) in this journal attempts to show the dangers of using the “internal standard” method for quantitative gas chromatography unless volumes of solution injected and component concentrations are the same as those used for calibration. The authors correctly state that the method of internal standards assumes that the area ratio of analyte/internal standard ( R A ) is independent of the volume injected. They have presented an abundance of data to show that a change in the volume injected or a change in both analyte and internal standard concentrations (at a constant weight ratio), produces a dramatic change in the observed R A . Such unrecognized systematic errors would indeed lead to very poor quantitative gas chromatography. We have conducted several experiments designed to test the validity of these authors’ conclusions as related to hydrocarbons (Figure 9) ( I ) . I t is our belief that the reported large changes in R A could easily be demonstrated by relatively few tests. A Hewlett-Packard 5711 A (FID) was equipped with a 6 f t X l/s-in. column packed with 10% OV-101 on 100/120 mesh Chromosorb W AW-DMCS and operated a t 130 “C. Injector and detector temperatures were 250 OC. Gas flows were: H2, 30 cm3/min; N2, 25 cm3/min; and air, 240 cm3/min. Area measurements were made with a Perkin-Elmer Model 1 computing integrator. The electrometer was kept on the 100 range for all tests. A solution containing 2.16 x g/cm3 n-nonane and 3.41 x g/cm3 n-decane in chloroform was used to determine the effect of the volume injected upon the C9/Cloarea ratio ( R A ) . All injections were made with a Hamilton 701 N syringe. Data below show no significant variation in R A as a function of a tenfold variation in injection volume. Injection volume, p L R A found 0.5 1.o
2.0 3.0
5.0
0.642 0.636 0.639 0.643 0.642 Mean R A = 0.640 s = 0.003 0003-2700/78/0350-1005$01 .OO/O
For the following tests, solutions were prepared by diluting n-C9 and n-Clo stock solutions (10 g hydrocarbon/100 mL CHC13)to the concentrations shown below. Injection volume was 1.0 pL in all cases. (C, = G o ) Concentrations, gIcm3 4 x 10-3 8x 2 x 10-2 4 x 10-2
R A found 1.003 0.997
1.015 1.013
Mean RA = 1.01
s = 0.008
These results show that a tenfold variation in concentration had no significant effect on R A , Sir: We We would like to make a few additional comments in response to Shatkay’s reply to our previous correspondence. In his letter to us, he replied that with the all-hydrocarbon system the “volume effect” should be so negligible as to easily escape detection. However, in his paper ( I ) he stated that while the phenomenon illustrated by Figure 9 (Le., the dependence of R A on volume injected) “is much less noticeable than for the pair trimethyl phosphate-undecane (cf., Figure 2), it is still quite significant.” The data we presented in our first correspondence were directed at the above conclusion. We did not find a significant variation in R A as a result of changing the injection volume. LITERATURE C I T E D (1) A. Shatkay and S. Flavian. Anal. Chem, 49, 2222 (1977).
G . C. Royston Research Department Copolymer Rubber and Chemical Corporation Baton Rouge, Louisiana 70821 RECEIVED for review January 16, 1978. Accepted March 10, 1978. 0 1978 American Chemical Society
