Anal. Chem. 1982, 54, 2599-2801
ically, they were greater than 20 nm width at half-maximum, when determined by measuring responses to equal masses injected at different slelected excitation wavelengths for a particular emission filter. Optimum excitation wavelengths are shown in Table I. Sensitivities also depended upon the location of the fiber optics bundle tip within the detector cavity. Insertion of the tip 1mm into the cavity (Figure 1)yielded the greatest sensitivity for all compouinds investigated. We found no unexpected interfering eluates. For example, 10-pL injections of nonaromatic technical grade solvents (hexane, cyclohexane, ]pentane, petroleum ether, methanol, ethanol, 2-propanol, 1-propanol, chloroform, carbon tetrachloride, methylene chloride, acetonitrile, ethyl acetate, and acetone) showed no interference. However, both benzene and biphenyl showed very low fluorescence, a t retention times preceding all the PNAs. The FPD modificatiion for GPF described here has sufficient sensitivity and LOD for use in environmental and toxicologic determinations of PNAs. The high selectivity of the detector, thus, may allolw analysts to avoid extensive sample
2599
cleanup procedures presently required (IO) for reliable quantitations of nanogram amounts of PNAs in complex sample matrices. LITERATURE C I T E D (1) Froehllch, P.; Wehry, E. L. “Modern Fluorescence Spectroscopy”; Wehry, E. L., Ed.; Plenum Press: New York, 1981; p 35. (2) Bowman, M. C.; Beroza, M. Anal. Chem. 1971, 40, 535. (3) Burchfield, H. P.; Wheeler, R. J.; Bernes, J. B. Anal. Chem. 1971, 43,
1976. (4) Freed, D. J.; Faulkner, L. R. Anal. Chem. 1972, 44, 1194. (5) Robinson, J. W.; Goodbread, J. P. Anal. Chlm. Acfa 1973, 66, 239. (6) Burchfleld, H. P.; Green, E. E.; Wheeler, R. J.; Bllledeau, S. M. J. Chromatogr. 1974, 9$, 697. (7) Muiik, J.; Cooke, M.; Guyer, M. F.; Semeniuk, G. M.; Sawlcki, E. Anal. Lett. 1975, 8, 511. (8) Cooney, R. P.; Winefordner, J. D. Anal. Chem. 1977, 49, 1057. (9) Cooney, R. P.; Vc-Dihn, T.; Winefordner, J. D. Anal. Chim. Acfa 1977, 89, 9. (10) MacLeod, W. D.; Prohaska, P. G.; Gennero, D. D.; Brown, D. W. Anal. Chem. W82, 54, 386.
RECEIVED for review July 6, 1982. Accepted September 3, 1982. This work was funded in part by the National Institutes of Health under Grant No. RR 07079.
Determination of Alkyl Nitrate Additives in Dlesel Fuel by Liquid Chromatography with Infrared Spectrametric Detection J. F. Schabron” and M. P. Fuller Phillips Petroleum Company, Bartlesvilie, Oklahoma 74004
The production of diesel fuel is expected to rise sharply to meet the demand created by an ever-increasing number of diesel powered vehicles. As this occurs, the utilization of chemical cetane improver additives should increase to maintain high quality fuel levels and thus avoid difficulties in cold starting and other performance problems associated with low cetane numbers. Amyl nitrate commonly has been used as a cetane improver additive in the past. Presently, hexyl nitrate and octyl nitrate are being utilized as well. The method currently used to determine amyl nitrate in diesel fuels is a colorimetric method prescribed by ASTM (I). The procedure involves hydrolysis of the alkyl nitrate in acid media followed by nitration of m-xylend by the nitric acid formed. Following furtlher sample treatment, including distillation and extraction steps, the nitrated xylenol is determined spectrophotometrically. By application of density corrections, the method can be used to determine volume percent of hexyl nitrate and octyl nitrate (1). High-performance liquid chromatography (HPLC) using a variable wavelength infrared (IR) detector provides very selective monitoring of a particular functional group. This selectivity permits the analysis for components that would be unresolved from the background signal using an ultraviolet or refractive index detector. Applications of variable wavelength HPLC/IR systems have included the separation of triglycerides (2), hydrocarbon group types (3),solvent refined coal liquids (4),and corn oil (5). In the area of polymer analysis, work with styrene-acrylonitrile copolymers (6), styrene-tert-butyl methacrylate block polymers (7), poly(ethylene oxide) (8),and polyethylene (9) has been reported. In the present work, a direct injection HPLC method was developed for the determination of amyl nitrate, hexyl nitrate, and octyl nitrate cetane improver additives in diesel fuel. The method described repiresents a unique application of HPLC/IR methodology. 0003-2700/82/0354-2599$0 1.25/0
EXPERIMENTAL SECTION Instrumentation. The liquid chromatograph used in this study consisted of a Model M-6000A pump and Wisp 710B autosampler, from Waters Associates, Milford, MA. Elution was monitored with a Model 785 variable wavelength detector from Micromeritics Instrument Corp., Norcross, GA, and Miran-1A infrared analyzer with a 9-pL, 3-mm pathlength sodium chloride cell from Foxboro Analytical, Norwalk, CT. A variable input dual channel strip chart recorder set at 0.25 in./min completed the system. The columns used were obtained from Waters Associates. These were 3.9 mm i.d. x 30 cm p-Porasil packed with 10-pm porous silica. Reagents. Cyclohexane was Phillips spectrogradefrom Phillips Chemicals, Borger, TX. Carbon tetrachloride was ACS grade from Fisher Scientific. Other solvents used in the preliminary work were from commercially available sources. All mobile phase solvents were filtered through Millipore type F-H 0.5-pm filters prior to use. Amyl nitrate was from Pfaltz and Bauer, Stamford, CT. Hexyl nitrate and octyl nitrate were from Ethyl Corp., Baton Rouge, LA. D-2 reference fuel was from Phillips Chemicals, Borger, TX. All additives were used without further purification. Procedure. Standard stock solutions of about 10 mg/mL for each additive were made up in cyclohexane. Solutions of 5, 3, 2, and 1mg/mL for the standards were obtained by appropriate dilution of aliquots of stock solutions with mobile phase. Samples were prepared by pipetting 2.0-mL portions of each sample into a 4-mL autosampler vial, along with 2.0 mL of mobile phase. An experiment showed that these volumes were additive. For both standards and samples, the injection volume was 50 pL. The run time was 17 min for standards and 25 min for samples. The autosampler was set to withdraw solutions at a slower than normal rate (system message 76-01). This was required with the mobile phase used for providing repeatable and accurate injections. The IR detector wavelength was set at 6.15 wm with an absorbance of 1.0, slit at 2 mm, and meter response at 1. The HPLC 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table I. Recoveries of Alkvl Nitrates from D-2Reference Fuel
nitrate 0.2 REFERENCE FUEL
nominal vol %
amyl
[-
hexyl
1 0.1 0.05 1 0.1
0.05 OCtYl
1 0.1
0.05 a
amt added,
amt found,
274 27.4 13.7 314 31.4 15.17 28 2 28.2 14.1
%
recovered
Ilg
Ilg
258 24.0 12.5 292 31.5 15.0 312 52.5
94 88 94 93 100 96 111 186
a
Not detected.
Table 11. Results of Analysis of Refinery D-2Samples 0 - 2 PLUS AMYLNITRATE 0
4
E 12 16 20 VOLUME FROM INJECTION.mL
nitrate
24
amyl
Flgure 1. Chromatograms showing IR and UV detector response for diesel fuel with and wlthout alkyl nitrate additives at levels of about 1 vol %. Chromatographic conditions are given in text.
system consisted of three p-Porasil columns in series with a cyc1ohexane:carbon tetrachloride (1:l) mobile phase at 1mL/min. The system pressure was about 3200 psi. Duplicate injections of standard and sample solutions were made. Peak areas were obtained by multiplying peak heights measured to the nearest 0.5 mm, by peak widths at half the peak height measured to the nearest 0.1 mm using a peak magnifier. Standard calibration curves were prepared by plotting peak area vs. micrograms injected. With the calibration curve, a value for microgram of additive per microliter of sample was obtained. Volume percent of additives was calculated using appropriate density values in grams per milliliter (1.00 for amyl nitrate, 0.970 for hexyl nitrate, and 0.964for octyl nitrate). For precision studies, three refinery D-2 diesel fuel samples containing a nominal 0.3 vol % of amyl nitrate, hexyl nitrate, and octyl nitrate were analyzed six times.
RESULTS AND DISCUSSION Preliminary Studies. The separation of amyl nitrate, hexyl nitrate, and octyl nitrate was studied with normal-phase HPLC systems. T o overcome the problem of UV absorbing components from the diesel fuel coeluting with the alkyl nitrates, we tried IR detection. The optimum IR wavelength window chosen for monitoring nitrates in methylene chloride was 6.15 fim, At about this wavelength, the intense asymmetrical -NOz stretch absorption occurs. The best normalphase system was p-Porasil with a cyc1ohexane:carbon tetrachloride (1:l) mobile phase. T o maximize the number of theoretical plates, we used three p-Porasil columns in series for sample analyses. Typical separations of alkyl nitrates and diesel fuel are illustrated in Figure 1. Diesel fuel samples had to be diluted 1:l with mobile phase to prevent the appearance of distorted peaks due to viscosity differences between the mobile phase and sample solutions. Accuracy and Precision. The method was carried out for D-2 reference fuel spiked individually with three different levels of each additive. The results are summarized in Table I. Recoveries were good except for low levels of octyl nitrate, where the presence of a peak due to diesel fuel makes small octyl nitrate peaks difficult to measure (Figure 1). Because of the presence of this IR absorbing material in diesel fuel, low values for octyl nitrate should be reported as estimated values unless a blank diesel fuel corresponding to the sample without additive is available for injection. In another experiment, three levels for each of the three additives were determined from a refinery D-2 diesel fuel
nominal vol % 0.3
0.3 0.15
0.30 0.16 0.08 0.33 0.16 0.06 0.31 0.14
0.05
a
0.15
hexyl octyl
a
determined vol %
0.05 0.3 0.15 0.05
Not detected.
Table 111. Precision Results for Six Replicate Analyses of Refinery D-2Samples with Alkyl Nitrates nitrate amyl hexyl octyl
x , vol%
0.34 ?; 0.01 (99%) 0.34 ?; 0.01 (99%) 0.31 * 0.01 (99%)
std dev 0.0084 0.0089 0.0075
sample to which the alkyl nitrates were added. The results are listed in Table 11. These results show good overall accuracy for the method. Precision results are listed in Table 111. These data indicate good precision for the method. Limits of Detection. Limits of detection for the method were estimated for peaks with a S I N ratio of 2. For amyl nitrate and hexyl nitrate, this was about 12 gg injected or 0.05 vol % in diesel fuel. For octyl nitrate there is some interference due to the absorbance of diesel fuel components. The results in Table I indicate that octyl nitrate probably cannot be measured accurately at levels below 0.1 vol % in diesel fuel. Calibration. The calibration curves for the alkyl nitrates were not linear. This probably results from working in a wavelength region where mobile phase solvent transmittance is only about 10%. Calibration should be checked each time a set of samples is run. Detector response may vary slightly over a period of days. If the appropriate level of alkyl nitrate is known, calibration need be performed with only two standards, a t levels below and above the expected value. Other Considerations. Since alkyl nitrate additive formulations used as cetane improvers can be mixtures of isomers, it would be best, if possible, to use a standard from the same source of additive as was used in preparing a particular lot of fuel. Mixtures of all three additives can be determined with this method. Since amyl nitrate and hexyl nitrate coelute and have similar response characteristics (Figure l),these two can be calculated as amyl plus hexyl nitrate. Octyl nitrate can be determined separately. The results obtained from a refinery
Anal. Chem. 1982, 54, 2601-2603
D-2 fuel with 0.1 vol %) each additive were 0.25 vol % amyl plus hexyl nitrate and 0.14 vol % octyl nitrate. The method described in this report represents a timesaving alternative to the wet chemical method for alkyl nitrates in diesel fuels.
(3) Baker, D. R. Hewlett Packard Application Note AN 232-9; HewlettPackard Co.: Palo Alto, CA, 1978. (4) Brown, R. S.; Hausler, D. W.; Taylor, L. T. Anal. Chem. 1980, 52, 1511. (5) Parris, N. A. J. Chromatogr. 1978, 149, 615. (6) Bartlck, E. G. J. Chromatogr. Scl. 1979, 17, 336. (7) Dawklns, J. V.; Hemmlng, M. J. Appl. Polym. Scl. 1975, 19, 3107. (8) Terry, S. L.; Rodrlguez, F. J. Polym. Sci., Part C 1968, 21, 191. (9) Ross, J. H.; Casto, M. E. J . Polym. Sci., Part C 1968, 21, 143.
LITEXATURE CITED (1) "Annual Book of ASTM Standards"; American Society for Testing and Materials: PhlladelPhia, PA, 1980; ASTM D 1839-809 Pari 24, PP 101-104. (2) Parris, N. A. J. Chron;tsrtogr. Sci. 1979, 17, 541.
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R~~~~~~~for review june 16, 182. ~
~September ~ 17, ~
1982.
Determination of Nitrite and Nitrate in Water and Food Samples by Ion Interaction Chromatography Zlad Iskandarani antd Donald J. PleWrzyk* Department of Chemistry, The University of lowa, Iowa City, Iowa 52242
Nitrite and nitrate are found in natural occurring salts and are part of many natural processes including the nitrogen cycle. Hence, their natural levels are affected by processes involved in this cycle. Since man has found many applications for these salts, particularly NO3- salts, these often have a greater influence on N02--N03- levels than do the natural occurring processes. Thus, N02--N03- analyses not only are industrially important but also are of concern to the general public because of excessive exposure and intake. This latter concern is mainly the result of the wide use of NO3- and NO, in agriculture and food industry, respectively. Of particular concern is their excessive introduction into water and food and their relationship, particularly for NO2-, to nitrosoamine production. The major methods dleveloped for NO2- and NO3- analyses have involved colorimetiric procedures. For NO3- the methods are based on either nitration of a phenol derivative, a nitrate oxidation of a suitable organic molecule, or reduction of NOY to NOz- which is used to convert sulfanilic acid to a diazonium salt followed by coupling to an aromatic amine. This latter procedure is also the one used for NO,. Recently, several other techniques, including fluorimetry, polarography, ionselective electrodes, chemiluminescence, and ion chromatography, have been shown to be useful for N02--N03- determinations. These, as well as agency apptoved methods, have been reviewed elsewheire (1-4). This report focuses on a liquid chromatographic (LC) method for the separation and subsequent determination of N02--N03- mixtures. A nonpolar poly(styrene-divinylbenzene) (PSDVB) copolymeric adsorbent, PRP-1, is used as the stationary phase and the separation ia affected by careful control of the equilibria that occur between the analyte anion and PRP-1 in the presence of a tetraalkylammonium (R4Nf) cation as a counterion, a coanion, and a mixed solvent. the two major equilibria influencing the analyte retention are (1) retention of the R4N+salt on the PRP-1 surface as a double layer where the R g + occupies the primary layer and a coanion the secondary layer and (2) an anion selectivity between the analyte anion and coanioms that comprise the secondary layer ( 5 6 ) . The anion elution orders are similar to those observed in ion chromatography where a strongly basic anion exchanger is used as the separator column (7). Since the procedure described here does not employ an ion exchanger stationary phase, this type of chromatography has been called ion interaction chromatography and can be used to separate organic (5) and inorganic anions (6). 0003-2700/82/0354-260 1$01.25/0
EXPERIMENTAL SECTION Reagents. Tetrapentylammonium bromide (TPeABr) was obtained from Pfaltz and Bauer and Eastman Kodak. TPeABr was converted into other anionic forms by anion exchange ( 5 ) . Analytes and other inorganic salts used, when possible, were analytical reagent grade sodium salts. LC quality CH&N was obtained from MCB. All water used was treated by taking distilled water and passing it through a mixed bed ion exchanger, an activated charcoal column, and 2 pm stainless steel filters. By use of a large sample aliquot and the LC procedure described here the residual NO3- and NO2- in this treated water was estimated to be about 170 ppb and I- > NO3- > Br- > NO2- > C1- > F- > OH- (1)
The selectivities are so favorable that separation of mixtures of closely related anions, such as NO3-, Br-, NO2-, and C1- (6), is readily achieved. If the sample contains only NO2- and NO,, eluting power can be increased to improve analysis time. Furthermore, by use of a UV detector at the appropriate wavelength potential interference of Br- and Cl- at the stronger 0 1982 American Chemlcal Soclety
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