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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
the V,, plot. This is significantly better than the *3% by volume for ASTM 1319 (FIA). Although slight aberrations in the base line occurred when the backflush valve was operated, no corrections were made for this deviation. T h e aromatic content of 25 recycled petroleum naphtha hydrocarbon fractions was determined by the gas chromatographic method. The results are given in Table I1 and compared to the aromatic content determined by the FIA method. The greatest difference between the two methods was no larger than 2% with the average difference being 1%. Only the samples from the Reedly recycling center gave consistently large differences from the FIA method. However, these samples contained percentages of aromatic hydrocarbons which are well below the maximum allowed by law. This gas chromatographic method could be utilized to determine the aromatic content of lower boiling petroleum naphtha fractions. Fractions having a boiling point similar to benzene would contain light molecular weight saturated hydr~carbons-c~and lower. Because benzene is eluted after n-CI5,the benzene content and the content of heavier molecular weight alkylbenzenes present in the light petroleum naphtha fraction could be determined by operating the
backflush valve after the elution of benzene. The nonaromatic hydrocarbons would be eluted first followed by benzene which in turn would be followed by the c8 and heavier alkylbenzenes. Such a procedure could be used to determine the aromatic content of gasoline. The gas chromatographic procedure described here offers a quick and reasonably accurate determination of C8 and heavier molecular weight alkylbenzenes contained in petroleum naphtha. Where the time required for the analysis of aromatic content must be kept to a minimum, the gas chromatographic method is far superior to the FIA method.
LITERATURE CITED (1) "Hydrocarbon Types in Liquid Petdeum Products by Fluorescent Indicator Absorption", ASTM Stand., D1319-77, Part 23, 693 (1978). (2) J. C. Sautoni, H.R. Garber, and 8. E. Davis, J . Chromatogr. Sci., 13, 367 (1975). (3) H. Boer and P. Van Arkel, Hydrocarbon Process., 51, 80 (1972). (4) C. L. Stuckey, J . Chromatogr. Sci., 7 , 177 (1969). (5) "Aromatics in Light Napthas and Aviation Gasolines by Gas Chromatography", ASTM Stand., D2267-66, Part 24, 251 (1978). (6) "Benzene and Toluene in Finished Motor and Aviation Gasolines by Gas Chromatography", ASTM Stand., D3606-77, Part 25,371 (1978).
RECEIVED for review April 23,1979. Accepted June 18,1979.
Calibration of Methanol and Ethylene Glycol Nuclear Magnetic Resonance Thermometers David S. Raiford, Cherie L. Fisk, and Edwin D. Becker' National Institutes of Health, Bethesda, Maryland 20205
In high resolution proton NMR the temperature of the sample is frequently determined by measurement of the chemical shifts of methanol at low temperature and ethylene glycol a t high temperature. As the temperature rises, the amount of hydrogen bonding diminishes, and the OH proton resonance moves upfield toward the CH3 or CH2 resonance. The relation between chemical shift and temperature has been reported in several calibration curves (1-4), the most extensive and reliable of which are generally accepted as those of Van Geet (3, 4 ) . Van Geet reported a quadratic equation describing the difference in temperature of a methanol sample as a function of the separation (in Hz) between the OH proton and the CH3 protons ( 3 ) ,and a linear relationship between temperature and separation of the OH protons and CH, protons of ethylene glycol ( 4 ) . Both of these studies were performed a t 60 MHz. With the advent of higher field spectrometers, it has been necessary to scale up the results, with possible attendant magnification of any errors. We report below the results of measurements a t 220 MHz, which are entirely compatible with Van Geet's and indicate that the errors in his measurements are probably even smaller than stated in his papers.
EXPERIMENTAL A Varian HR-220 spectrometer with pulse Fourier transform capability was used. A calibrated copper-constantan thermocouple was held in place in a spinning NMR tube at a uniform depth to eliminate temperature gradient effects. Methanol or glycol (reagent grade) was used in an open sample tube for thermocouple measurements. The thermocouple was secured by a Plexiglas adaptor which screwed on top of the probe. Into the adaptor a 1-mm open-ended capillary tube was inserted, through which the thermocouple, attached to a Doric Trendicator 400 digital display, extended into the solution. After careful equilibration at a specific temperature (1.5-2.5 h), the thermocouple was removed and a standard Varian 5-mm sealed temperature calibration sample was
Table I. Separation (in H z ) of CH,(CH,) and OH Proton Lines of Methanol and Glycol a t 220 MHz as a Function of Temperature ( K ) T(K)
238.8 257.9 267.2 278.4 292.2 293.0 308.8
methanol A u (Hz) 459.7 424.8 405.2 383.8, 383.5a 356.4 356.2 320.8
glycol
T(K)
303.4 314.4 327.6 339.9 352.8 372.4
Av
(Hz)
353.3, 353.2b 330.0 300.1, 300.0b 279.0 246.8 202.5
a An unsealed sample tube containing methanol, into which a capillary tube was subsequently inserted as during temperature calibration, was used for these two measurements to eliminate the possibility that the apparatus itself changed the sample temperature. Measurements on two Varian glycol samples.
introduced for the NMR measurements and allowed to equilibrate. Once equilibrated, the probe temperature was stable to zk0.14.2 K over a period of several hours in the temperature range below 320 K. Above that temperature, stability was good only to f1.0 K over several hours. The frequency differences between the hydroxyl resonance and either the CH3 or CH2 resonance were measured with a digital resolution of 0.18 Hz. All samples were spinning during measurement. Experimental data were fitted to either two- or three-parameter curves by a least-squares regression using the NIH DEC-10 MLAB program ( 5 ) .
RESULTS Table I gives the observed data. For methanol the least squares fit gives the equation
T ( K ) = 429.2
-
0.283 JAvl - 2.862 X
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
(Av)'
(1)
ANALYTICAL CHEMISTRY, VOL 51, NO 12, OCTOBER 1979
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330
our data are plotted along with Van Geet's equation. Our ethylene glycol data give a least-squares fit of
T(K) = 466.5 - 0.461 (hJ
TEllP I K
I
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(3)
which is in excellent agreement with Van Geet'c; equation ( 4 ) , scaled t o 220 MHz,
t
T(K) = 466.0- 0.462 I A v ~
(4)
: ' . . . T
2sc 230
Our new data thus demonstrate that Van Geet's 60-MHz calibration equations may be confidently scaled u p to 220 MHz, and the agreement suggests that scaling to even higher frequencies should not introduce appreciably larger errors.
300
350
400
450
500
LITERATURE CITED
SHIFT I N HZ 12201
Figure 1. Temperature dependence of the chemical shift separation of CH3 and OH proton lines of methanol at 220 MHz. The data points are taken from Table I; the line represents Van Geet's 60-MHz equation scaled to 220 MHz (Equation 2 in text)
where Av is in Hz a t 220 MHz. Van Geet's equation ( 3 ) ,scaled t o 220 MHz, is T(K) = 403.0 - 0.134 IAvl - 4.92 X (Av)'
(1) Variable Temperature Operation in HR-220 Spectrometer System, Technical Manual , Publication no 87 122-003, Section 2 4, Varian Associates, Palo Alto, Calif. (2) R . R. Shoup, quoted in R. R. Shoup, M. L. McNeel, and E. D. Becker, J . Phys. Chem., 76, 71 (1972). (3) A. L. Van Geet, Anal. Chem., 42, 679 (1970) (4) A. L. Van Geet, Anal. Chem., 40, 2227 (1968). (5) G. L. Knott, "MLAB: An On-Line Modeling Laboratory", 7th ed.,Juv 1977, Division of Computer Research and Technology, National Institutes of
Health, Bethesda, Md.
(2)
The apparent differences turn out to be insignificant over the temperature range 24C-310 K, as indicated in Figure 1 where
RECEIVED for review April 4, 1979. Accepted July 10, 1979.
Determination of Cobalt by Lophine Chemiluminescence Dean F. Marino, Fred Wolff, and J. D. Ingle, Jr." Department of Chemistry, Oregon State University, Corvallis, Oregon 9733 7
Interest has recently been generated in the applications of solution chemiluminescence (CL) t o trace metal analysis because of the simplicity of CL instrumentation and the low detection limits available for some metals via this approach ( 1 4 ) . Systems based on luminol and lucigenin appear to be the most popular (I+?), although other CL reagents such as gallic acid and pyrogallol have been utilized for trace metal analysis (9, 10). T h e CL of lophine (2,4,5-triphenylimidazole)has been known since 1877 (11)and the mechanism of the CL reaction of lophine and some of its derivatives have been studied (12-14). A preliminary study (15) indicated that lophine CL might be useful for trace metal determinations although Co(I1) was not identified as an activator. Our initial studies indicated that, under proper conditions, ultratrace Co(I1) concentrations enhanced the CL of lophine in basic H,Oz solutions. Thus reagent concentrations were optimized for a low Co(I1) detection limit and the interference from other species was investigated.
EXPERIMENTAL All measurements were obtained with a discrete sampling CL photometer system reported earlier (16) and with the modifications and approximate experimental conditions previously described (7, 10). Lophine (Aldrich) was used without further purification. Lophine solution preparation and storage proved to be somewhat critical. Best results were obtained by degassing reagent grade methanol via boiling followed by dissolution of the lophine in the hot methanol. Lophine solutions were refrigerated when not in use. All other solutions (e.g., HzOz,Co(II), and other metals) were prepared as previously described (7, IO). The general analysis procedure consisted of addition with Eppendorf pipets of the following quantities of the equilibrated solutions into the reaction cell: 1.0 mL sample or blank, 0.5 mL 0003-2700/79/0351-2051$01.00/0
lophine solution, and 0.5 mL H202solution. The contents of the reaction cell were allowed to mix for 10 s prior to injection of 0.5 mL of KOH solution with an automatic dispensing syringe to initiate the reaction. The CL analytical signal is taken as the difference in the CL peak height between a blank and analyte run. The cell was then evacuated and rinsed twice with 0.1 M "OB followed by two Millipore water rinses. This wash solution proved to be critical in elimination of memory effect!, and ensuring reproducibility. Typical peak shapes for blank and Co runs are shown in Figure 1. Interference studies were conducted as previously described to determine detection limits for all species and intaference levels for some species with respect to the determiniition of Co(I1). The detection limit (DL) is defined as the concentration of analyte solution yielding an analytical signal equal to twice the standard deviation of the reagent blank CL signal. For this work, the standard deviation of the blank signal was determined by the irreproducibility of the blank signal (about 5 1 0 % relative standard deviation (RSD)) and not by noise in the dark current or background CL signal. The interference level is defined as the concentration of the species which causes the mean 1 ppb Co(I1) CL signal in the presence of the species to differ from the mean Co(I1) CL signal in the absence of the species by two standard deviations in the 1 ppb Co(I1) signal.
RESULTS AND DISCUSSION Optimization Studies. Because lophine is sparingly soluble in water, a water miscible organic solverit must be used to make the lophine stock solution. The use of organic solvents in analytical CL measurements has not been explored to a great extent, even though there are potential advantages: (i) higher concentrations of the CL reagent in the reaction mixture may provide better detection limits, (ii) water insoluble reaction products may be kept in solution and increase 0 1979 American Chemical Society
