Method and apparatus for simultaneous online assay of nitrites and nitrates in water samples

Abstract

The invention relates to an apparatus for simultaneous online assay of nitrites and nitrates in water samples with improved sensitivity and accuracy, enhanced capacity of anti-interference against salinity, reduced assay costs, and simplified operation.

Claims

1. An apparatus for simultaneous online assay of nitrites and nitrates in water samples, comprising a low pressure pump (1), a 6-way injection valve (2), a low pressure anion separation column (3), a mixer (6), a reduction column (7), a reactor (8), a sample vessel (11), an eluent vessel (12), a color developer solution vessel (13), and a waste liquid vessel (14), wherein the sample vessel (11) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the eluent vessel (12) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the color developer solution vessel (13) is connected with an inlet of the mixer (6) via the low pressure pump (1), the other inlet of the mixer (6) is connected with a liquid outlet of the 6-way injection valve (2), the liquid inlet and the liquid outlet of the low pressure anion separation column (3) are connected with a liquid outlet and a liquid inlet of the 6-way injection valve (2), respectively, the liquid inlet and the liquid outlet of the reduction column (7) are connected with the outlet of the mixer (6) and the inlet of the reactor (8), respectively, and a liquid outlet of the 6-way injection valve (2) is connected with the waste liquid vessel (14), wherein all connections are effected by ducts, and the reduction column (7) comprises a column body, filter membranes (7-4) located at the liquid inlet and the liquid outlet of the column body, and column fillers (7-3) in the inner cavity of the column body, wherein the column fillers (7-3) are copperized cadmium particles, or a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin; wherein the column fillers (7-3) are copperized cadmium particles, in which the cadmium particles have a particle size of 0.5-1.0 mm and the thickness of the copper layer is 0.05-0.1 mm.

2. An apparatus for simultaneous online assay of nitrites and nitrates in water samples, comprising a low pressure pump (1), a 6-way injection valve (2), a low pressure anion separation column (3), a mixer (6), a reduction column (7), a reactor (8), a sample vessel (11), an eluent vessel (12), a color developer solution vessel (13), and a waste liquid vessel (14), wherein the sample vessel (11) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the eluent vessel (12) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the color developer solution vessel (13) is connected with an inlet of the mixer (6) via the low pressure pump (1), the other inlet of the mixer (6) is connected with a liquid outlet of the 6-way injection valve (2), the liquid inlet and the liquid outlet of the low pressure anion separation column (3) are connected with a liquid outlet and a liquid inlet of the 6-way injection valve (2), respectively, the liquid inlet and the liquid outlet of the reduction column (7) are connected with the outlet of the mixer (6) and the inlet of the reactor (8), respectively, and a liquid outlet of the 6-way injection valve (2) is connected with the waste liquid vessel (14), wherein all connections are effected by ducts, and the reduction column (7) comprises a column body, filter membranes (7-4) located at the liquid inlet and the liquid outlet of the column body, and column fillers (7-3) in the inner cavity of the column body, wherein the column fillers (7-3) are copperized cadmium particles, or a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin; wherein the column fillers (7-3) are a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin in a weight ratio of 1:1-1:2.

3. The apparatus according to claim 1, further comprising an optical flow cell (9) downstream of the reactor (8), an optical detector (5) attached to the optical flow cell, and a computer system (4) connected to the optical detector.

4. An apparatus for simultaneous online assay of nitrites and nitrates in water samples, comprising a low pressure pump (1), a 6-way injection valve (2), a low pressure anion separation column (3), a mixer (6), a reduction column (7), a reactor (8), a sample loop (10), a sample vessel (11), an eluent vessel (12), a color developer solution vessel (13), and a waste liquid vessel (14), wherein the sample vessel (11) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the eluent vessel (12) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the color developer solution vessel (13) is connected with an inlet of the mixer (6) via the low pressure pump (1), the liquid inlet and the liquid outlet of the sample loop (10) are connected with a liquid outlet and a liquid inlet of the 6-way injection valve (2), respectively, the liquid inlet of the low pressure anion separation column (3) is connected with a liquid outlet of the 6-way injection valve (2), the liquid inlet and the liquid outlet of the reduction column (7) are connected with the liquid outlet of the low pressure anion separation column (3) and the other inlet of the mixer (6), respectively, the outlet of the mixer (6) is connected with the inlet of the reactor (8), and a liquid outlet of the 6-way injection valve (2) is connected with the waste liquid vessel (14), wherein all connections are effected by ducts, and the reduction column (7) comprises a column body, filter membranes (7-4) located at the liquid inlet and the liquid outlet of the column body, and column fillers (7-3) in the inner cavity of the column body, wherein the column fillers (7-3) are copperized cadmium particles, or a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin; wherein the column fillers (7-3) are copperized cadmium particles, in which the cadmium particles have a particle size of 0.5-1.0 mm and the thickness of copper layer is 0.05-0.1 mm.

5. An apparatus for simultaneous online assay of nitrites and nitrates in water samples, comprising a low pressure pump (1) a 6-way injection valve (2), a low pressure anion separation column (3), a mixer (6), a reduction column (7), a reactor (8), a sample loop (10), a sample vessel (11), an eluent vessel (12), a color developer solution vessel (13), and a waste liquid vessel (14), wherein the sample vessel (11) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the eluent vessel (12) is connected with a liquid inlet of the 6-way injection valve (2) via the low pressure pump (1), the color developer solution vessel (13) is connected with an inlet of the mixer (6) via the low pressure pump (1), the liquid inlet and the liquid outlet of the sample loop (10) are connected with a liquid outlet and a liquid inlet of the 6-way injection valve (2), respectively, the liquid inlet of the low pressure anion separation column (3) is connected with a liquid outlet of the 6-way injection valve (2), the liquid inlet and the liquid outlet of the reduction column (7) are connected with the liquid outlet of the low pressure anion separation column (3) and the other inlet of the mixer (6), respectively, the outlet of the mixer (6) is connected with the inlet of the reactor (8), and a liquid outlet of the 6-way injection valve (2) is connected with the waste liquid vessel (14), wherein all connections are effected by ducts, and the reduction column (7) comprises a column body, filter membranes (7-4) located at the liquid inlet and the liquid outlet of the column body, and column fillers (7-3) in the inner cavity of the column body, wherein the column fillers (7-3) are copperized cadmium particles, or a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin; wherein the column fillers (7-3) are a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin in a weight ratio of 1:1-1:2.

6. The apparatus according to claim 4, further comprising an optical flow cell (9) downstream of the reactor (8), an optical detector (5) attached to the optical flow cell, and a computer system (4) connected to the optical detector.

7. The apparatus according to claim 2, further comprising an optical flow cell (9) downstream of the reactor (8), an optical detector (5) attached to the optical flow cell, and a computer system (4) connected to the optical detector.

8. The apparatus according to claim 5, further comprising an optical flow cell (9) downstream of the reactor (8), an optical detector (5) attached to the optical flow cell, and a computer system (4) connected to the optical detector.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a structural schematic illustration of the reduction column in the apparatus of the invention.

(2) FIG. 2 is a structural schematic illustration of the apparatus according to the first aspect of the invention.

(3) FIG. 3 is a schematic illustration of the flow path of the apparatus according to the first aspect of the invention in injection state when used in the method for simultaneous online assay of nitrites and nitrates in water samples.

(4) FIG. 4 is a schematic illustration of the flow path of the apparatus according to the first aspect of the invention in analytical state when used in the method for simultaneous online assay method of nitrites and nitrates in water samples.

(5) FIG. 5 is the spectrogram of accuracy of the standard sample obtained in Example 4.

(6) FIG. 6 is a structural schematic illustration of the apparatus according to the second aspect of the invention.

(7) FIG. 7 is a schematic illustration of the flow path of the apparatus according to the second aspect of the invention in injection state when used in the method for simultaneous online assay of nitrites and nitrates in water samples.

(8) FIG. 8 is a schematic illustration of the flow path of the apparatus according to the second aspect of the invention in analytical state when used in the method for simultaneous online assay of nitrites and nitrates in water samples.

(9) FIG. 9 is the spectrogram of accuracy of the standard sample obtained in Example 8.

(10) FIG. 10 is the working curve of nitrites obtained in Example 5.

(11) FIG. 11 is the working curve of nitrates obtained in Example 5.

(12) FIG. 12 is the working curve of nitrites obtained in Example 9.

(13) FIG. 13 is the working curve of nitrates obtained in Example 9.

(14) The symbols in the drawings are as follows: 1low pressure pump, 26-way injection valve, 3low pressure anion separation column, 4computer system, 5optical detector, 6mixer, 7reduction column, 7-1fixed hub, 7-2column tube, 7-3 column fillers, 7-4filter membrane, 7-5plug, 7-6duct, 8reactor, 9optical flow cell, 10sample loop, 11sample vessel, 12eluent vessel, 13color developer solution vessel, 14waste liquid vessel, 15testing components, S.sub.1test sample, S.sub.2standard sample, Eeluent, Rcolor developer solution, Wwaste liquid.

DETAILED DESCRIPTION OF THE INVENTION

(15) The invention will be further illustrated by way of examples below. These examples are only illustrations of the invention and do not limit it. The chemical reagents such as sodium nitrite, sodium nitrate, sulfonamide, hydrochloric acid, N-(1-naphthyl)ethylenediamine dihydrochloride, sodium chloride, ammonium chloride, are all analytically pure. The concentrations of nitrites and nitrates mentioned in the examples are all based on N.

Example 1

Preparation of Low Pressure Anion Exchange Resin

(16) (1) The raw materials, styrene and divinylbenzene, and the catalyst, benzoyl peroxide, were added in a reaction vessel in a weight ratio of 600:60:1, and a polymerization reaction was performed for 15 h at a temperature of 90 C. and under normal pressure, obtaining spherical styrene-divinylbenzene copolymer beads.

(17) (2) The copolymer beads from step (1) were added in a reaction vessel, and a 70 wt. % alcoholic solution of chloromethyl methyl ether was added in a ratio of weight of copolymer beads (kg):volume of alcoholic solution of chloromethyl methyl ether (L)=1:4 to react for 8 h at a temperature of 50 C. for chloromethylation. Then a 30 wt. % aqueous solution of trimethylamine was added in a ratio of weight of copolymer beads (kg):volume of aqueous solution of trimethylamine (L)=1:4 to react for 12 h at a temperature of 40 C. for amination, obtaining anion exchange resin beads with quaternary ammonium functional groups.

Example 2

Structure of Reduction Column and Preparation of Column Fillers

(18) The reduction column in this example has an inner diameter of 3 mm and a column length of 40 mm. The structure thereof is shown in FIG. 1. It mainly comprises a column body, filter membranes 7-4 located at the liquid inlet and the liquid outlet of the column body, and column fillers in the inner cavity of the column body. The column body comprises a column tube 7-2, plugs 7-5, fixed hubs 7-1, and ducts 7-6. The plugs 7-5 are located on the outside of the filter membranes 7-4, and have center holes for the insertion of the ducts 7-6. The fixed hubs 7-1 are located at the junctions of the plugs 7-5 and the column tube 7-2, for fixing the plugs 7-5 on the column tube 7-2. The filter membranes 7-4 are made of ninon, and are mounted between the column tube 7-2 and the plugs 7-5 at both ends. The column tube 7-2 is made of glass, and is filled with the column fillers 7-3. The column fillers 7-3 are copperized cadmium particles, in which the cadmium particles have a particle size of 0.5 mm and the thickness of copper layer is 0.05 mm. Both the fixing plugs 7-5 and the fixed hubs 7-1 are made of silicon rubber, and the ducts 7-6 are polytetrafluoroethylene ducts.

(19) The column fillers 7-3 were prepared as follows. Cadmium blocks were hammered to pieces having a thickness of 0.51.0 mm, which were then cut into particles having a particle size of 0.51.0 mm. The cadmium particles were filled into the column tube 7-2, and then flushed with a mixed aqueous solution of copper sulfate and EDTA, in which the concentration of copper sulfate was 6.64 g/L, and that of EDTA was 14.88 g/L, at a flow rate of 0.81.2 mL/min for 30 min to obtain the product.

Example 3

Apparatus (I) for Simultaneous Online Assay of Nitrites and Nitrates in Water Samples

(20) The structure of Apparatus (I) is shown in FIG. 2. It comprises a low pressure pump 1, a 6-way injection valve 2, a low pressure anion separation column 3, a mixer 6, a reduction column 7, a reactor 8, a sample vessel 11, an eluent vessel 12, a color developer solution vessel 13, and a waste liquid vessel 14. The reactor 8 is of a coil structure, and was made by winding a polytetrafluoroethylene tube having a length of 3.0 m and an inner diameter of 0.5 mm. The low pressure anion separation column 3 has an inner diameter of 5 mm and a column length of 40 mm, and the column fillers therein were prepared according to Example 1. The reduction column 7 is the one described in Example 2. The working pressure of the low pressure pump 1 is 2310.sup.5 Pa. The components of Apparatus (I) are assembled in the following way.

(21) The sample vessel 11 is connected with a liquid inlet of the 6-way injection valve 2 via the low pressure pump 1. The eluent vessel 12 is connected with a liquid inlet of the 6-way injection valve 2 via the low pressure pump 1. The color developer solution vessel 13 is connected with an inlet of the mixer 6 via the low pressure pump 1. The other inlet of the mixer 6 is connected with a liquid outlet of the 6-way injection valve 2. The liquid inlet and the liquid outlet of the low pressure anion separation column 3 are connected with a liquid outlet and a liquid inlet of the 6-way injection valve 2, respectively. The liquid inlet and the liquid outlet of the reduction column 7 are connected with the outlet of the mixer 6 and the inlet of the reactor 8, respectively. A liquid outlet of the 6-way injection valve 2 is connected with the waste liquid vessel 14. All connections are effected by ducts.

Example 4

Accuracy of Assay Using Apparatus (I)

(22) A standard sample was assayed using Apparatus (I) described in Example 3 to evaluate the accuracy of the assay. The steps were as follows:

(23) 1. A mixed solution, as standard sample S.sub.2, was prepared with deionized water, sodium nitrate, and sodium nitrite, having a nitrate concentration of 20 g/L and a nitrite concentration of 5 g/L.

(24) 2. Color developer solution R was prepared as follows. 2.50 g sulfonamide was added in a 1000 mL volumetric flask, and 600 ml deionized water was added. Then 100 mL 8 mol/L hydrochloric acid was added without waiting for complete dissolution of the sulfonamide. After shaking, 0.250 g N-(1-naphthyl)ethylenediamine dihydrochloride was added and dissolved, and then deionized water was added to the volume 1000 mL, obtaining the product.

(25) 3. A mixed aqueous solution having a sodium chloride concentration of 1.50 wt. % and an ammonium chloride concentration of 1.50 wt. % was prepared, as eluent E.

(26) 4. Assay was carried out and a spectrogram of the standard sample was mapped.

(27) The assay was carried out using Apparatus (I) described in Example 3 (further comprising testing components 15, comprising an optical flow cell 9, an optical detector 5 and a computer system 4), and the flow paths as shown in FIG. 3 and FIG. 4 were employed. In the low pressure pump 1, the flow rate in the pump line of standard sample S.sub.2 was 0.20.4 mL/min, the flow rate in the pump line of color developer solution R was 0.81.2 mL/mm, the flow rate in the pump line of eluent E was 0.81.2 mL/min, and the working pressure was 2310.sup.5 Pa. The standard sample S.sub.2, the eluent E and the color developer solution R were filled in the sample vessel 11, the eluent vessel 12, and the color developer solution vessel 13, respectively. The optical path of the optical flow cell 9 was 28 mm, and the detection wavelength of the optical detector 5 was 530 nm. The computer system 4 was a conventional computer installed with HW-2000 chromatography workstation (Shanghai Qianpu Software Company Ltd.).

(28) The assay included the following steps:

(29) (1) Baseline mapping. Apparatus (I) was set in injection state, and the flow path is shown in FIG. 3. The apparatus was turned on. Driven by the low pressure pump 1, eluent E entered the mixer 6 via the low pressure pump 1 and the 6-way injection valve 2, color developer solution R entered the mixer 6 via the low pressure pump 1, and they were mixed and formed a mixed solution in the mixer 6. The mixed solution entered the optical flow cell 9 via the reduction column 7 and the reactor 8, and the signal produced by the optical detector 5 was transferred to the computer system 4 for processing to obtain a baseline. Simultaneously, standard sample S.sub.2 entered the low pressure anion separation column 3 from the liquid inlet thereof via the low pressure pump 1 and the 6-way injection valve 2, and saturated the low pressure anion separation 3. Excess standard sample S.sub.2 was discharged into the waste liquid vessel 14 via a waste liquid outlet.

(30) (2) Mapping of the spectrogram of the standard sample. Apparatus (I) was switched to analytical state, and the flow path is shown in FIG. 4. Driven by the low pressure pump 1, color developer solution R entered the mixer 6 via the low pressure pump 1, and eluent E entered the low pressure anion separation column 3 from the liquid outlet thereof via the low pressure pump 1 and the 6-way injection valve 2, to backwash the nitrites and nitrates in the low pressure anion separation column 3. Driven by eluent E, the nitrites, which had low affinity to the column fillers, and the nitrates, which had high affinity to the column fillers, successively, flowed out of the low pressure anion separation column 3 and entered the mixer 6, where the nitrites were mixed with color developer solution R and then entered the reactor 8 via the reduction column 7 to accomplish a color development reaction forming a reaction solution, while the nitrates were mixed with color developer solution R and then entered the reduction column 7 to be reduced to nitrites, which, in the form of a mixed solution with color developer solution R, entered the reactor 8 to accomplish a color development reaction forming a reaction solution. The two reaction solutions entered, successively, the optical flow cell 9. The signal produced by the optical detector was transferred to the computer system 4 for processing to obtain a spectrogram of nitrites and nitrates in standard sample S.sub.2.

(31) The above steps (1) and (2) were repeated for 8 times, and a spectrogram as shown in FIG. 5 was obtained, wherein the relative standard deviation of the peak heights of nitrites and nitrates were 1.51% and 1.34%, respectively, demonstrating good accuracy of the assay of the invention.

Example 5

Assay of Water Samples Using Apparatus (I)

(32) Water samples (commercially available pure drinking water) were assayed using Apparatus (I) described in Example 3. A total of 3 samples (test sample 1#, test sample 2#, and test sample 3#) were assayed with the following steps:

(33) 1. Standard samples 1# to 5# were prepared with deionized water, sodium nitrate, and sodium nitrite, wherein standard sample 1# was deionized water; standard sample 2# was a mixed solution in which the concentrations of nitrites and nitrates were 0.1 g/l, and 1.0 g/L, respectively; standard sample 3# was a mixed solution in which the concentrations of nitrites and nitrates were 0.2 g/L and 2.0 g/L, respectively; standard sample 4# was a mixed solution in which the concentrations of nitrites and nitrates were 0.5 g/L and 5.0 g/L, respectively; and standard sample 5# was a mixed solution in which the concentrations of nitrites and nitrates were 2.0 g/L and 10.0 g/L, respectively.

(34) 2. Color developer solution R was prepared as follows. 2.00 g sulfonamide was added in a 1000 mL volumetric flask, and about 600 ml deionized water was added. Then, 100 mL 6 mol/L hydrochloric acid was added without waiting for complete dissolution of the sulfonamide. After shaking, 0.200 g N-(1-naphthyl)ethylenediamine dihydrochloride was added and dissolved, and then deionized water was added to the volume 1000 mL, obtaining the product.

(35) 3. A mixed aqueous solution having a sodium chloride concentration of 3.09 wt. % and an ammonium chloride concentration of 3.00 wt. % was prepared, as eluent E.

(36) 4. Assay was carried out and spectrograms of the standard samples were mapped.

(37) The assay was carried out according to step 4 in Example 4, with steps (1) and (2) being performed using test sample 1#, test sample 2#, test sample 3#, standard sample 1#, standard sample 2#, standard sample 3#, standard sample 4#, and standard sample 5#, respectively, instead of standard sample S.sub.2 in Example 4, obtaining a spectrogram of each of the test samples and the standard samples.

(38) 5. Working curves were plotted using the concentrations (g/L) of standard samples 1#-5# on the X-axis, and the peak heights (mV) in the spectrograms of standard samples 1#-5# on the Y-axis. The resulting working curves of nitrites and nitrates were shown in FIG. 10 and FIG. 11, respectively.

(39) 6. Regression equations were established. The regression equation obtained from the working curve of nitrites as shown in FIG. 10 was H=5.7919C+1.7336, wherein H represented peak height (mV) and C represented concentration (g/L) of nitrites in standard samples, and the correlation coefficient R was 0.999. The regression equation obtained from the working curve of nitrates as shown in FIG. 11 was H=2.2957C+0.1755, wherein H, C and R were as defined above for FIG. 10. The baseline noise in the assay was 0.32 mV, and it was calculated that the detection limits of nitrites and nitrates were 0.47 g/L and 0.34 g/L, respectively.

(40) 7. The concentrations of nitrites and nitrates in test samples 1#3# were calculated by substituting each of the peak heights of nitrites and nitrates in the spectrograms of test samples 1#3# into the regression equations obtained in step 6. The concentrations of nitrites and nitrates in test samples 1#3# were obtained, as shown in Table 1.

(41) TABLE-US-00001 TABLE 1 Concentration of Concentration of Recovery Sample nitrites (g/L) nitrates (g/L) rate (%) No. Tagged Measured Tagged Measured Nitrites Nitrates 1# 0 2.96 0 2.65 0.50 3.48 2.00 4.70 103.1 102.3 2.00 4.94 4.00 6.61 98.9 99.1 2# 0 0 0 1.92 0.10 0.10 1.00 2.93 102.4 100.6 0.20 0.21 2.00 3.86 104.1 96.9 3# 0 0 0 4.43 0.10 0.10 3.00 7.50 101.9 102.3 0.20 0.19 6.00 10.51 96.8 101.3

Example 6

Structure of Reduction Column and Preparation of Column Fillers

(42) The reduction column in this example is substantially the same as that in Example 2, except that the inner diameter thereof is 4 mm, the column length thereof is 50 mm, and the column fillers are a mixture of copperized cadmium powders and polystyrene-divinylbenzene resin.

(43) The column fillers were prepared as follows. 5 g cadmium powders having a particle size of 75150 m were added in a 100 mL beaker. 100 mL a mixed aqueous solution of copper sulfate and EDTA, in which the concentration of copper sulfate was 6.14 g/L and that of EDTA was 14.38 g/L, was then added slowly. The reaction was stirred for 10 min for copperization. After filtration, the resulting copperized cadmium powders were mixed with styrene-divinylbenzene resin having a particle size of 100200 m at a weight ratio of 1:1 to obtain the product.

Example 7

Apparatus (II) for Simultaneous Online Assay of Nitrites and Nitrates in Water Samples

(44) The structure of Apparatus (II) is shown in FIG. 6. It comprises a low pressure pump 1, a 6-way injection valve 2, a low pressure anion separation column 3, a mixer 6, a reduction column 7, a reactor 8, a sample loop 10, a sample vessel 11, an eluent vessel 12, a color developer solution vessel 13, and a waste liquid vessel 14. The reactor 8 and the low pressure anion separation column 3 are the same as those described in Example 3. The reduction column 7 is the one described in Example 6. Sample loop 10 is a polytetrafluoroethylene tube having a volume of 150 L. The components of Apparatus (II) are assembled in the following way.

(45) The sample vessel 11 is connected with a liquid inlet of the 6-way injection valve 2 via the low pressure pump 1. The eluant vessel 12 is connected with a liquid inlet of the 6-way injection valve 2 via the low pressure pump 1. The color developer solution vessel 13 is connected with an inlet of the mixer 6 via the low pressure pump 1. The liquid inlet and the liquid outlet of the sample loop 10 are connected with a liquid outlet and a liquid inlet of the 6-way injection valve 2, respectively. The liquid inlet of the low pressure anion separation column 3 is connected with a liquid outlet of the 6-way injection valve 2. The liquid inlet and the liquid outlet of the reduction column 7 are connected with the liquid outlet of the low pressure anion separation column 3 and the other inlet of the mixer 6, respectively. The outlet of the mixer 6 is connected with the inlet of the reactor 8. A liquid outlet of the 6-way injection valve 2 is connected with the waste liquid vessel 14. All connections are effected by ducts.

Example 8

Accuracy of Assay Using Apparatus (II)

(46) A standard sample was assayed using Apparatus (II) described in Example 7 to evaluate the accuracy of the assay. The steps were as follows:

(47) 1. A mixed solution, as standard sample S.sub.2, was prepared with deionized water, sodium nitrate, and sodium nitrite, having a nitrate concentration of 100 g/L and a nitrite concentration of 50 g/L.

(48) 2. Color developer solution R was prepared according to step 2 in Example 4.

(49) 3. A mixed aqueous solution having a sodium chloride concentration of 3.09 wt. % and an ammonium chloride concentration of 3.00 wt. % was prepared, as eluent E.

(50) 4. Assay was carried out and a spectrogram of the standard sample was mapped.

(51) The assay was carried out using Apparatus (II) described in Example 7 (further comprising testing components 15, comprising an optical flow cell 9, an optical detector 5 and a computer system 4), and the flow paths as shown in FIG. 7 and FIG. 8 were employed. In the low pressure pump 1, the flow rate in the pump line of standard sample S.sub.2 was 0.20.4 mL/min, the flow rate in the pump line of color developer solution R was 0.81.2 mL/min, the flow rate in the pump line of eluent E was 0.81.2 mL/min, and the working pressure was 2310.sup.5 Pa. The standard sample S.sub.2, the eluent E and the color developer solution R were filled in the sample vessel 11, the eluent vessel 12, and the color developer solution vessel 13, respectively. The optical path of the optical flow cell 9 was 28 mm, and the detection wavelength of the optical detector 5 was 530 nm. The computer system 4 was a conventional computer installed with HW-2000 chromatography workstation (Shanghai Qianpu Software Company Ltd.).

(52) The assay included the following steps:

(53) (1) Baseline mapping. Apparatus (II) was set in injection state, and the flow path is shown in FIG. 7. The apparatus was turned on. Driven by the low pressure pump 1, eluent E entered the mixer 6 via the low pressure pump 1, the 6-way injection valve 2, the low pressure anion separation column 3, and the reduction column 7, color developer solution ft entered the mixer 6 via the low pressure pump 1, and they were mixed and formed a mixed solution in the mixer 6. The mixed solution entered the optical flow cell 9 via the reactor 8, and the signal produced by the optical detector 5 was transferred to the computer system 4 for processing to obtain a baseline. Simultaneously, standard sample S.sub.2 entered the sample loop 10 via the low pressure pump 1 and the 6-way injection valve 2, and filled the sample loop 10. Excess standard sample S.sub.2 was discharged into the waste liquid vessel 14 via a waste liquid outlet.

(54) (2) Mapping of the spectrogram of the standard samples. Apparatus (II) was switched to analytical state, and the flow path is shown in FIG. 8. Driven by the low pressure pump 1, color developer solution R entered into the mixer 6 via the low pressure pump 1, and eluent E entered the sample loop 10 via the low pressure pump 1 and the 6-way injection valve 2 and brought the standard sample S.sub.2 in the sample loop 10 to the low pressure anion separation column 3 via the 6-way injection valve 2. Driven by eluent E, the nitrites, which had low affinity to the column fillers, and the nitrates, which had high affinity to the column fillers, successively, flowed out of the low pressure anion separation column 3 and entered the reduction column 7. The nitrites entered the mixer 6 via the reduction column 7 and were mixed with color developer solution R, and then entered the reactor 8, where a color development reaction occurred to form a reaction solution. The nitrates were reduced to nitrites in the reduction column 7, the resulting nitrites entered the mixer 6 and were mixed with color developer solution R, and then entered the reactor 8, where a color development reaction occurred to form a reaction solution. The two reaction solutions entered, successively, the optical flow cell 9. The signal produced by the optical detector 5 was transferred to the computer system 4 for processing to obtain a spectrogram of the nitrites and the nitrates.

(55) The above steps (1) and (2) were repeated for 8 times, and a spectrogram as shown in FIG. 9 was obtained, wherein the relative standard deviation of the peak heights of nitrites and nitrates were 1.17% and 1.77%, respectively, demonstrating good accuracy of the assay of the invention.

Example 9

Assay of Water Samples Using Apparatus (II)

(56) Water samples (seawater) were assayed using Apparatus (II) described in Example 7. A total of 2 samples (test sample 1# and test sample 2#) were assayed with the following steps:

(57) 1. Standard samples 1# to 5# were prepared with deionized water, sodium nitrate, and sodium nitrite, wherein standard sample 1# was an aqueous solution (3.09 wt. %) of sodium chloride; standard sample 2# was a mixed solution in which the concentrations of nitrites and nitrates were 5.0 g/L and 20.0 g/L, respectively; standard sample 3# was a mixed solution in which the concentrations of nitrites and nitrates were 10.0 g/L and 100.0 g/L, respectively; standard sample 4# was a mixed solution in which the concentrations of nitrites and nitrates were 30.0 g/L and 500.0 g/L, respectively; and standard sample 5# was a mixed solution in which the concentrations of nitrites and nitrates were 50.0 g/L and 1000.0 g/L, respectively.

(58) 2. Color developer solution R was prepared according to step 2 in Example 5.

(59) 3. A mixed aqueous solution having a sodium chloride concentration of 2.00 wt. % and an ammonium chloride concentration of 2.00 wt. % was prepared, as eluent E.

(60) 4. Assay was carried out and spectrograms of the standard samples were mapped.

(61) The assay was carried out according to step 4 in Example 8, with steps (1) and (2) being performed using test sample 1#, test sample 2#, standard sample 1#, standard sample 2#, standard sample 3#, standard sample 4#, and standard sample 5#, respectively, in stead of standard sample S.sub.2 in Example 8, obtaining a spectrogram of each of the test samples and the standard samples.

(62) 5. Working curves were plotted using the concentrations (g/L) of standard samples 1#-5# on the X-axis, and the peak heights (mV) in the spectrograms of standard samples 1#-5# on the Y-axis. The resulting working curves of nitrites and nitrates were shown in FIG. 12 and FIG. 13, respectively.

(63) 6. Regression equations were established. The regression equation obtained from the working curve of nitrites as shown in FIG. 12 was H=0.2093C+1.4138, wherein H represented peak height (mV) and C represented concentration (g/L) of nitrites in standard samples, and the correlation coefficient R was 0.999. The regression equation obtained from the working curve of nitrates as shown in FIG. 13 was H=0.1032C0.6307, wherein H, C and R were as defined above for FIG. 12.

(64) 7. The concentrations of nitrites and nitrates in test samples 1# and 2# were calculated by substituting each of the peak heights of nitrites and nitrates in the spectrograms of test samples 1# and 2# into the regression equations obtained in step 6. The concentrations of nitrites and nitrates in test samples 1# and 2# were obtained, as shown in Table 2.

(65) TABLE-US-00002 TABLE 2 Concentration of Concentration of Recovery Sample nitrites (g/L) nitrates (g/L) rate (%) No. Tagged Measured Tagged Measured Nitrites Nitrates 1# 0 21.4 0 81.7 15.0 36.7 50.0 133.0 101.9 102.7 20.0 41.5 100.0 182.5 100.3 100.8 2# 0 5.67 0 141.8 5.00 10.5 100.0 240.6 97.5 98.8 10.0 15.8 200.0 346.2 101.3 102.2