Determination Of Nitrate In High Conductivity Water Samples By Photoreduction-Capillary Electrophoresis-Contactless Conductivity Method

Capillary electrophoresis coupled with contactless conductivity detection（CE-C⁴D）is a technique that can be used for on-site separation and simultaneous detection of various ions in water samples.However,it is not suitable for direct detection of NO₃^-in high conductivity water samples.Since Cl^-,SO₄^2-,and NO₃^-have similar mobilities in CE.The signal peak of Cl^-and SO₄^2-in high concentration overlaps with NO₃^-signal peak when detected by C⁴D,which seriously interferes with the determination of NO₃^-.In view of this,this thesis proposes to combine photochemical reactions with CE-C⁴D technology to convert NO₃^-into NO₂^-by photochemical reduction,and then indirectly quantify NO₃^-based on the content of NO₂^-.In acidic buffer solution,NO₂^-has a slower mobility than Cl^-and SO₄^2-,and can achieve baseline separation with Cl^-and SO₄^2-for detection using C⁴D.This allows for the detection of NO₃^-and total dissolved nitrogen（TDN）in high conductivity water samples.（1）An offline photochemical reduction-CE-C⁴D method was established and used for the detection of NO₃^-in high conductivity water samples.A circulating offline photochemical reduction system was constructed by winding a perfluoroalkoxy（PFA）tube around a 254 nm low-pressure mercury lamp.The sample was driven by a peristaltic pump and circulated through the PFA tubing,where NO₃^-was reduced to NO₂^-by photochemical reduction,and then separated and detected using CE-C⁴D in a buffer solution containing 20m M TRIS and 210 m M acetic acid.This CE-C⁴D method achieved baseline separation and detection of 200 m M Cl^-and trace amounts of NO₂^-,with good anti-interference ability.The various factors that affect the photochemical reduction efficiency of NO₃^-were optimized.The linear working range of the method was 0～800μM,the detection limit was 13μM,the RSD was 2.7%（n=11）,and the sample spiked recovery rate was between 93.7%and 116.7%.The nitrate-to-nitrite conversion efficiency was 72±2%.The method was applied to the determination of TDN in water samples.After the water samples were digested by potassium sulfate,nitrogen in various forms was converted into NO₃^-,and nearly 100 m M of SO₄^2-was produced by persulfate thermal decomposition.This method can be used to determine NO₃^-in high concentrations of SO₄^2-,and thus achieve the determination of TDN.（2）Based on the offline method,a capillary column photochemical reduction-CE-C⁴D method was developed to detect NO₃^-.Using the latest deep ultraviolet light LED as the light source,the sample was directly irradiated at the head of the capillary column to achieve photochemical reactions,and then detected by CE-C⁴D,realizing"photochemical reduction-separation-detection"in the same capillary.LEDs at 230,255,and 280 nm were used as the light sources for NO₃^-photochemical reduction,and the kinetics of the photochemical reduction reaction of NO₃^-at different wavelengths were investigated.When two 255 nm LEDs were used as the light source,the linear working range of the method was 0～300μM,RSD was 4.9%（n=11）,spike recovery rate was between 97%and 109%.The photochemical reduction efficiency of the method for NO₃^-was 66±3%.This method was successfully applied to the detection of TDN in water samples.（3）An automatic CE-C⁴D system was developed to realize automatic detection of NO₃^-.An automatic injection system of CE was developed using valves,pumps,etc.The capillary injection end was inserted into a PFA tube,which was placed outside with a low-pressure mercury lamp.After the sample reacted to the light at the capillary inlet,it was immediately pushed into the capillary and subjected to electrophoretic separation.The linear working range of this method was 0～900μM,RSD was 3.5%（n=10）,detection limit was 20μM,and the recovery rate was between 89～97%.The photo-reduction efficiency of NO₃^-was 41±1%.