1222
Anal. Chem. 1980, 52, 1222-1224
Determination of Nitrite at Parts-per-Billion Levels by Derivatization and Electron Capture Gas Chromatography Koichi Funazo, Minoru Tanaka, and Toshiyuki Shono' Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-kami, Suita, Osaka 565,Japan
NRrite is first converted Into p-bromochlorobenzeneby reaction wlth p-bromoanillne and copper(11) chloride in a hydrochloric acid medium. The resulting p-bromochlorobenzene is extracted into toluene and determined by gas chromatography wlth electron capture detection. The detection limit for nitrlte with this method is 0.01 ppm, which is as low as that of the wldely used colorimetrlc method for the determination of nitrite. This method Is not affected by such inorganlc ions as CI-, Br-, F-, NO3-, H2PO4-, S042-, HCO,-, K+, Na', Mg2+, and NH,', which commonly coexlst with nitrite In environmental and biological samples. A varlety of samples containing nitrite (i.e., river water, human saliva, and foods) were analyzed by both the gas chromatographic and the colorimetric methods. Good agreement was found between the methods.
Recently, a great deal of interest has been generated concerning t h e toxicity of nitrites which are frequently used as food additives, etc. T h e toxicity of nitrites is due primarily to their oxidation of hemoglobin to form methemoglobin which is unable to transport oxygen (1,2). Secondarily, their toxicity relates to the possibility of reaction with amines and amides to form carcinogenic N-nitroso compounds, under conditions such as those in the human stomach (3,4). The determination of nitrite in a variety of matrices is, therefore, of vital importance. Numerous analytical methods have been developed for the determination of nitrite. T h e most widely used and applied methods are spectrophotometric ones. Some of these methods are based on t h e diazotization of sulfanilamide by nitrite and on subsequent coupling with an agent such as N-(1-naphthy1)ethylenediamineto produce a highly colored azo compound (5-10); others are based on other color reactions (11-13). Recently, the determination of nitrite by enthalpimetry (14),polarography (15), or ion chromatography (16-18) has also been reported. A gas chromatographic method for the determination of nitrite and nitrate has been reported by several groups (19-23). This method involved the reaction of benzene (or a benzene derivative) with nitrate in the presence of a n acid catalyst. The resulting nitrobenzene (or its derivative) was analyzed quantitatively by electron capture gas chromatography. Because this method is ineffective for nitrite itself, a n oxidizing agent which converts nitrite into nitrate is required t o determine nitrite. I n previous communications (24, 25), we reported a gas chromatographic procedure for the analysis of nitric oxide and nitrite. In that procedure, nitric oxide or nitrite was converted into aryl bromide according to the reaction published by Brackman et al. (26) or a similar reaction, and the resulting aryl bromide was determined by electron capture gas chromatography. The conversion reaction of nitrite was made to proceed in a neutral medium. In the present work, nitrite was converted into aryl chloride by using copper(I1) chloride in a hydrochloric acid (HC1) medium in order to avoid the interference of chloride and to raise the conversion yield. The reactions seem to be as follows:
NOz- + Br-C6H4-NH2
+
-
2H+ Br-C6H,-N+=N
Br-C6H,-N'=N
+ 2 H 2 0 (1)
cuc12
Br-c6H4-c1
(2)
Furthermore, the method has been adapted so t h a t the analysis of nitrite at sub-ppm levels can be performed with small amounts of sample. T h e sample volume needed for analysis is only 1 mL. Nitrite in river water, saliva, and foods was determined and the results obtained were compared with those of the colorimetric method which has been widely used. EXPERIMENTAL Apparatus. A Shimadzu GC-4BM gas chromatograph equipped with a 63Ni electron capture detector was used. A stainless steel column (2 m X 3 mm i.d.) was packed with 10% OV-17 on 60-80 mesh Shimalite W. The nitrogen flow-rate was 50 mL/min. The detector, injection port, and column temperatures were maintained at 250, 250, and 150 "C, respectively. The peak areas were measured by a digital integrator (Shimadzu Chromatopac-E1 A). Reagents. All reagents were of analytical-reagent grade and were used without further purification unless otherwise stated. Toluene and deionized water were distilled before use, for the analysis. Sodium nitrite was dried in an oven at 110 OC for 1 h before it was accurately weighed. Procedure. p-Bromoaniline solution, 1.0 mL of 1.0 X M, M p-dichlorodissolved in 1.0 N HCl and 1.0 mL of 2.0 X benzene (internal standard) solution in toluene were added to a 1.0-mL aliquot of aqueous sample in a reaction vessel (ca. 10 mL) with a glass stopper. After a few minutes, 1.0 mL of 2.0 X lo-' M CuC12aqueous solution was introduced into the vessel. Then the vessel was shaken at room temperature for 2 h with an electric shaker. A t the end of the reaction period, the toluene layer was separated from the aqueous layer and a 1.0-pL aliquot of the former was injected into the gas chromatograph. Resulting pbromochlorobenzene was determined by an internal standard method as usual. Samples. River Water. The sample was filtered to remove turbidity and was diluted with distilled water to a suitable nitrite concentration (
