Study On Highly Sensitive Detection Of Metal Elements By Liquid Cathode Glow Discharge

As a new metal element detection method,electrolyte cathode glow discharge（ELCAD）technique has been widely used in metallurgy,food safety,environmental monitoring and other fields.Compared with traditional detection methods,such as atomic absorption spectroscopy（AAS）,inductively coupled plasma atomicemission spectroscopy（ICP-AES）,inductively coupled plasma mass spectrometry（ICP-MS）and flame atomic absorption spectrometry（FAAS）,ELCAD has some advantages,for example,simple operation,low cost,low power consumption,no need for flammable and special gases,and no need for vacuum environment.However,the initial ELCAD technique also has the disadvantages of high detection limit,unstable discharge and large sample consumption.Therefore,many researchers have modified the device to improve its performance,such as,solution cathode glow discharge（SCGD）,liquid sampling-atmospheric pressure glow discharge（LS-APGD）,direct current atmospheric pressure glow discharge（DC-APGD）,alternating current electrolyte atmospheric liquid discharge（AC-EALD）,liquid film dielectric barrier discharge（LF-DBD）,liquid electrode dielectric barrier discharge（LE-DBD）,and so on.At the same time,with the advent of miniaturized fiber optic spectrometers,ELCAD has become possible to develop small and portable analytical instrument.Based on the initial ELCAD principle,with improving the excitation source and the injection system,we have established a new spectral analysis method for the determination of metal element content,i.e.,liquid cathode glow discharge-atomic emission spectroscopy（LCGD-AES）,and then employed for the determination of metal elements in some samples.The research contents are as follows:（1）The content of K,Ca,Na and Mg in the salt ore samples was determined by liquid cathode glow discharge（LCGD）.The optimized analytical conditions of LCGD-AES are pH=1 HNO₃ as electrolyte,650 V discharge voltage and 3 mL min^–1solution flow rate.The limit of determinations（LODs）of K,Ca,Na and Mg are 0.390,0.054,0.048 and 0.032 mg L^–1,respectively.Measurement results of the LCGD-AES are in good agreement with the comparison value obtained by inductively coupled plasma（ICP）and ion chromatography（IC）.All results suggested that the developed portable analytical instrument can be used for on-site and real-time monitoring of metal elements in field with further improvement.（2）Liquid cathode glow discharge（LCGD）technique has been developed for the detection of Pb,Zn and Cu in digested soil samples.The matrix interferences of cations and anions have been investigated.High concentrations of Pb and Cu may interfere with each other,but there is no mutual interference between Pb and Zn.Zn did not affect the determination of Cu,but high concentrations of Cu could affect the determination of Zn.The presence of Fe could affect the signal intensity of Pb,Zn and Cu,while Cd only affected the Pb.The high concentrations of Ni could enhance the emission intensity of Pb and Zn.When SO₄^2-was added,the emission intensity of Pb,Zn and Cu significantly decreased,and Cl^-could strengthen the emission intensity of Cu.But the interference of SO₄^2-and Cl^-could be eliminated by adding the formic acid.Under the optimized analytical conditions of LCGD-AES,such as pH=1 HNO₃as electrolyte,3 mL min^–1 solution flow rate and 8%（V/V）formic acid as the organic additives,the calibration curves have good linearity in the range of 1³0 mg L^-1.The relative standard deviations（RSDs）are 7.8%for Pb,5.4%for Zn and 7.5%for Cu.The limits of determinations（LODs）of Pb,Zn and Cu are 4.97,2.51 and 1.50μg L^–1,respectively.The results obtained by LCGE-AES are generally consistent with ICP and certified reference material（GBW（E）080399）.By calculating the relative error（RE）,it is found that the RE of the remaining results is less than 15%.All results suggested that determinations of Pb,Zn and Cu in soil samples using LCGD-AES are reliable and accurate.（3）As a fast,efficient,low power,on-site and real-time metal element analysis method,LCGD-AES has been used for the determination of metal content in solution sample.To compare the accuracy of different analytical methods,in this study,we chose different quantitative analysis methods in LCGD-AES for determination.We used standard curve method and standard addition method to determine the content of Pb,Cu and Cd.Comparing the results,we found that using the standard addition method is more accurate than the other one.The relative error（RE）obtained by the standard addition method is 1%,4%and 3.8%for Pb,Cu and Cd,respectively,which are better than calculated by the standard curve method（5.3%,13.8%and 6.8%for Pb,Cu and Cd,respectively）.The recoveries are 98.76%¹04.62%for calibration curve method and 100.06%¹02.98%for standard addition method.It is suggested that standard curve method can effectively improve the accuracy of the measurement results.