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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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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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Flgure 2. Separation of 1:200 and 300:l NO,-:NO,PRP-1. See Figure 1 for column condltlons.
mixtures on
mL Flgure 1. Separation of a 1:l mixture of NOz- and NO3- on PRP-1. The mobile phase conditions are 1:3 CH3CHHz0, 1.00 X lo3 M TPeAF at a flow rate of 1 mL/mln using UV detection. A IO-yL aliquot containing 10 pg each of NO2- and NO3- is the sample.
eluting condition is minimized. This is illustrated in Figure 1 where about 10 ng of NOz- and NO3- as sodium salts are base line resolved in well-defined peaks in less than 15 min. Several mobile phase variables which can be altered will change selectively, resolution, and subsequently analysis time. These are the following. (1) A different R4N+salt can be used; increasing the alkyl chain length increases retention. Adjustment of the R4N+concentration also influences retention; in this case analyte retention passes through a maximum (5). (2) The CH3CN/H20ratio can be varied; retention decreases as CH3CN increases. (3) Ionic strength can be increased by the addition of NaF; this causes retention to decrease. (4) A stronger eluent anion (see eq 1) can be used to decrease analyte retention. These trends and their experimental verification are discussed elsewhere (5, 6). In Figure 1 10 pL of a sample that is 10 ppm in NaNOz and 10 ppm in NaN03 was injected. Peak areas clearly indicate that concentrations well below this level could be easily detected. However, since the quality of the water used contained NO, at about 170 ppb and NOz- at
