following equation: lozt 10098
-
-E
96
-
Y
97-
3
90-
2
5 "
d
RON 0
p2 =
MON
94-
88-
$x 0684
-
80
I
I
82
84
I I I I I 86 88 90 92 94 OCTANE NUMBERS (ENGINE TESTS)
I
I
1
96
98
100
L IO:
Figure 2. Calculated RON and MON values vs. values obtained by engine test
obtained under more severe test engine operating conditions than those used for the RON determination (7). Quality of Fit. There are several measures of the quality of the fit that exist between the variables used in a regression analysis. One measure is the difference between the calculated RON and MON values and the input RON and MON values. A plot of the calculated RON and MON values vs. those obtained by engine test is shown in Figure 2. A standard deviation of f1.l octane number is obtained for both. The standard deviations encountered in engine testing' are approximately f0.25 and f0.45 octane number for RON and MON, respectively ( I , 7). Therefore, the fitted data are expected to show a t least this much scatter, and will show additional scatter caused by the combined uncertainties in the determined values of the four input variables for each sample. Intercorrelations between the input variables also may be contributing to the relatively high standard deviation of hl.1; however, for many purposes, this uncertainty is acceptable. The index of determination ( p 2 ) is another indicator of the quality of the fit that exists with the variables selected for analysis. The program MREGl calculates p2 from the
SSR:SST
(6)
where SSR is the sum of the squares from the regression line, and SST is the sum of the squares from the average value of the dependent variable (RON and MON). The values of p2 can range from 0 to 1. The closer p2 approaches 1.0, the better the fit. The values of p2 obtained in this analysis were 0.87 for RON and 0.90 for MON, indicating a satisfactory correlation with the variables selected. Precautions. 1) Equations 4 and 5 apply only to typical gasolines, which contain components providing a full boiling range. For example, calculations on 100% isooctane (RON = 100 and MON = 100) would give an erroneous RON of about 125 and a MON of about 118. 2) Some intercorrelation exists among the input variables, which are not entirely orthogonal (independent) as required for linear regression analysis to be strictly applicable. This problem is minimized to an extent by the relatively limited range covered by the dependent variable (RON values are from approximately 91 to 103) so that linear regression analysis still adequately predicts the octane numbers. Difficulties may be encountered in attempting to utilize this type of analysis for hydrocarbon mixtures that cover a wider range of octane values, for example, from 50 to 100 RON.
ACKNOWLEDGMENT We thank B. E. Nagel of the Laboratories for his helpful suggestions during the course of this work, and others who determined sulfur and lead in the gasoline samples. LITERATURE CITED (1) "ASTM Manual for Rating Motor, Diesel, and Aviation Fuels", ASTM Test Method D2699-70, American Society for Testing and Materials, Phiiadelphia, Pa., 1971. (2) "Reference Data for Hydrocarbons and Petrosulfur Compounds", Bulletin No. 521, Phillips Petroleum Company, Special Products Division, Bartiesville, Okla., 1962, pp 25-26. (3) P. C. Anderson, J. M. Sharkey, and R. P. Walsh, J. hsf. Pet., 58, 86
(1972). (4) M. E. Myers, Jr., J. Stollstelmer, and A. M. Wims. Anal. Chem., 47, 2010 (1975). (5) "1974 Annual Book of ASTM Standards-Part 23", Test Method D1319-70, American Society for Testing and Materials, Philadelphia, Pa., 1974. (6) Honeywell Series 400 Time-sharing Applications Library Operating Instructions, Document No. CPB-14876, Honeywell information System, Phoenix, Ariz.
(7) "ASTM Manual for Rating Motor, Diesel, and Aviation Fuels", ASTM Test Method D2700-70. American Society for Testing and Materials, Philadelphia, Pa., 1971.
RECEIVEDfor review May 5 , 1975. Accepted July 31, 1975.
Liquid Chromatography Detection by Current Semiintegral Electroanalysis in Flowing Streams G. H. Brilmyer,' Steven C. Lamey, and J. T. Maioy2 Department of Chemistry, West Virginia University, Morgantown, W. Va. 26506
Studies have shown that current semiintegral electroanalysis may offer advantages over other electrochemical techniques for applications such as kinetic studies ( I ) and .~ Present address, D e p a r t m e n t of Chemistry, U n i v e r s i t y of Texas a t Austin, Austin, Texas 78712. A u t h o r t o whom correspondence should be addressed.
*
2304
rapid quantitative analysis (2). It has been demonstrated that the semiintegrated current signal is proportional to the concentration of electroactive species uresent and is independent of the input signal applied to the electrochemical cell ( 3 ) .This signal also reaches a constant value within one second in the potential step experiment if the tech-
ANALYTICAL CHEMISTRY, VOL. 47. NO. 13, NOVEMBER 1975
50.0 21.0 17.2
14.2 12.5
Flgure 1. Flow cell
nique of current clipping is used (2). The method is relatively simple to use if the analog circuitry previously described (2, 4 ) is employed to obtain the current semiintegral. These characteristics suggest that current semiintegral electroanalysis may be used in the analysis of flowing streams where a rapid quantitative measurement of electroactive species is often desired. A flowing stream of particular interest is the eluant from a liquid chromatograph. Herein are presented the results of experiments to determine the variation of the current semiintegral obtained in the electrolysis of a substance carried in a flowing stream. An actual liquid chromatographic separation using an analog current semiintegrator with current clipper as the detector is reported. Experiments involving both a potential step and a linear sweep were conducted. The potential sweep method is useful because it not only provides a signal proportional to the concentration, but also provides a means of compound identification through the variation of the half-wave potentials of the components separated. Other similar studies have been reported (5, 6) utilizing chronocoulometric methods for detection. While chronocoulometry is a somewhat better choice than current semiintegration for quantitative measurements, it does not offer the qualitative advantages of the latter.
EXPERIMENTAL The electrochemical flow cell in Figure 1 was constructed and used in all studies. It consisted of a hanging mercury drop electrode (Princeton Applied Research Model 9323) inserted through a silver-silver chloride reference electrode; a silver wire downstream was used as the counter electrode. The electrodes were placed in a Y-shaped piece of glass tubing. The flowing solution passed up through the bottom of the “Y” and out the arm containing the counter electrode. A flow system was designed to pump solution through the cell a t various rates. The rate of solution flow (from a storage flask) was controlled by nitrogen gas pressure. The solution then flowed through a tube with a side-arm covered by a septum through which an air bubble could be injected into the stream. The solution containing the air bubble then passed through a calibrated tube and the time of transit for the bubble through the tube was measured by means of a stop watch. From this time, the flow rate was calculated. The solution then entered the cell. The electroactive material used in the flow studies was a 5mM solution of p-nitrobenzoic acid (Fisher Chemical Co.) in pH 6 phosphate buffer/DMF mixture (3 parts buffer to 1 part DMF by volume). All experiments utilized the Princeton Applied Research Model 170 electrochemistry system. Potential step and linear sweep input signals from 0.00 to -1.50 V vs. Ag/AgCl were applied. The chromatographic separation was carried out on a column packed with Sephadex G-10 (Pharmacia) dextran gel. The separation employed 6mM solutions of o-nitrobenzoic acid (Eastman Organic Chemicals), rn-nitrobenzoic acid (Fisher), and p-nitroben-
2
TIME (set) Flgure 2. Results of flow studies Relative current semiintegral responses from a pnitrobenzoic acid solution. Potential step from 0.00 to -1.15 V vs. Ag/AgCI. Flow rates (in mllmin) indicated by numbers adjacent to each line
zoic acid (Fisher) in pH 6 phosphate buffer. To three parts of this buffer solution was added one part N,N-dimethylformamide (DMF). The buffer-DMF mixture was also used as the eluant. The Sephadex packing was added to water and allowed to stand for one-half hour until the swelling process was complete and the slurry was then packed into the column. The column was attached to the flow cell with a short piece of tubing.
RESULTS AND DISCUSSION A series of potential step experiments was run with the p-nitrobenzoic acid solution flowing a t different rates through the cell. The relative current semiintegral responses are shown in Figure 2. The constant current semiintegral is changed very little by flow rates up to 5 ml/min in the duration of the electrolysis (1-5 sec), but the current semiintegral is no longer constant a t rates above 5 ml/min in this system. Flow rates up to 50 ml/min were examined. The type of current semiintegral response obtained appears to be determined by the mode of mass transport to the electrode surface. The constant current semiintegral can only be obtained when diffusion is the primary transport mode, while the sloping response occurs when convection becomes the dominant mode. Thus, for this particular flow cell geometry, diffusion appears to be the primary mode of mass transport during the first five seconds of electrolysis at flow rates up to 5 ml/min. In an attempt to eliminate the nonconstant current semiintegral behavior observed a t high flow rates, a solenoid valve was installed in the flow system after the calibrated tube. A switch was used to simultaneously stop the flow and turn on the current semiintegrating circuit. In this configuration, the flow was stopped instantly as the current semiintegral was recorded and the flow started again. The entire cycle took only 3 to 5 seconds. This permitted the use of flow rates up to 30 ml/min with no deviation in the constant current semiintegral. Since the current semiintegral cannot be used for continuous monitoring, this stopflow technique allows a wider range of flow rates to be sampled with minimum error. The effect of the stoppage on the chromatographic process would be minimal during a typical separation, for the flow would be stopped for only about 5% of the elution time. The system chosen for separation by liquid chromatogra-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2305
I
F
-0.90
POT
j'"'
-.995
t
'i
-I .a 0
VOLTS
Semiintegrated current traces with linear sweep input during the separation of o and pnitrobenzoic acid Flgure 3.
Potential sweep from 0.00 to -1.15 V vs. AgIAgCI. (a) current semiintegral from ortho isomer, ( b )current semiintegral from para isomer
phy has been previously reported by Brook (7) who separated the ortho and para isomers of nitrobenzoic acid using the column described above and a dropping mercury electrode (DME) detector. Attempts to separate the meta isomer were not reported. In the present experiments, linear sweep voltammetry from 0.00 to -1.50 V vs. Ag/AgCl was used rather than a potential step in an attempt to distinguish the isomers by their half-wave potentials. Linear sweep voltammetry of each compound in quiescent solution showed a sufficient difference in the peak potentials to make this method feasible. The first separation attempted was that of the ortho and para isomers. The column was operated at a flow rate of 2 ml/min. At this rate, it was unnecessary to use the stop-flow technique because it was possible to obtain a constant current semiintegral without it. Figure 3 illustrates the linear sweep current semiintegrals obtained for the ortho and para isomers a t the time at which their maximum response occurred. The isomers can be identified by their half-wave potentials and their concentrations calculated from the height of the signal. Figure 4 shows the results of this two-component separation. The lower portion of the figure is the chromatogram obtained by sampling the eluant every milliliter with a linear potential sweep. The resulting current semiintegral response is plotted as a function of volume of eluant. The chromatogram shows that detection is possible if a good separation is achieved. The upper portion of the figure shows the half-wave potential of the current semiintegral response as a function of volume of eluant. The upper and lower portions of Figure 4 show the expected correlation; the half-wave potential is constant while one species is being detected and then changes gradually to the half-wave potential of the other. The upper plot allows one to ascertain the species being detected without a prior standard chromatogram being run. Of course, this detection could have been achieved at a DME; however, conventional polarography could not have resulted in the compound characterization by half-wave potential in the chromatographic time frame. One might consider using modified forms of polarography but other problems would be encountered with these methods. For instance, the use of a rapidly dropping mercury electrode would add an additional mode of mass transport because of the vibration necessary to knock the drop, Rapid linear potential scans (-1 sec) on the drops would be feasible, but in this case, one loses the ability to compensate for any nonfaradaic currents by the current clipping technique ( 2 ) . The separation of the ortho, meta, and para isomers was also attempted. It proved possible to separate the ortho
2306
VOLUME Figure 4.
(ml)
Separation of o and pnitrobenzoic acid by liquid chroma-
tography Linear potential sweep of 100 mV/sec applied from 0.00 to -1.15 V vs. Ag/ ASCI. (Upper) Half wave response of current semiintegral as a function of eluant volume. (Lower) Chromatogram drawn point-by-point from maximum current semiintegral response with eluant volume. (a) o-nitrobenzoic acid, (b) pnitrobenzoic acid
isomer from the other two, but the para and meta isomers were inseparable. The half-wave potential measured during the attempted separation did not correspond to any of the isomers. This behavior may be due to mixed intermolecular hydrogen bonding between the meta and para isomers but, at this point, the cause of the behavior is open to conjecture. These studies do indicate that the technique may be useful for this type of application. The most apparent features are: (1) A constant signal independent of flow rate and proportional to concentration is obtained at flow rates less than 5 ml/min; and (2) Some means of component identification is available in the absence of a standard chromatogram if the linear sweep mode is used. These characteristics suggest that this method of detection may prove to be useful in chromatographic separations achieved using electrochemically compatible compounds and solvent systems.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
R. J. Lawson and J. T. Maloy, Anal. Chem., 48, 599 (1974). S.C. Lamey, R. D. Grypa, and J. T. Maloy, Anal. Chem.. 47, 610 (1975). K. B. Oldham. Anal. Chem., 44, 196 (1972). K. B. Oldham, Anal. Chem., 45, 39 (1973). D. C. Johnson and J. Larochelle, Talanta, 20, 959 (1973). Y. Takata and G. Muto. a/. Chem., 45, 1864 (1973). A. J. Brook, Cbem. lnd. (London), 1434 (1968).
RECEIVEDfor review May 19, 1975. Accepted August 4, 1975. This research was supported in part by the donors of the Petroleum Research Fund administered by the American Chemical Society. This paper was presented at the 169th National Meeting of the American Chemical Society in April 1975, during the First National Student Affiliate Research Symposium, Paper No. 25.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
