Improvement Of Liquid Cathode Glow Discharge Device And Analysis And Detection Of Metal Elements

With the development of society and the continuous expansion of industry and manufacturing,people’s living standards have improved significantly,and according pollution problems have become increasingly prominent.However,heavy metal pollution has always been an eternal topic of concern for researchers in the fields of environment,food,soil,and water quality.A large amount of heavy metals are discharged into soil and rivers,which seriously endangering the ecological environment,diet safety and daily life.Therefore,it is necessary to construct a fast,efficient and portable metal element detection technology.At present,several conventional analytical techniques,i.e.,flame atomic absorption spectrometry（FAAS）,atomic fluorescence spectrometry（AFS）,inductively coupled plasma atomic emission spectrometry/mass spectrometry（ICP-AES/MS）,have been applied for the determination of metal elements in different matrices with high sensitivity and accuracy.However,these methods are bulky and costly,requiring high power input,high temperatures,and high vacuum,as well as inert/special gases,and thus they are typically employed in the laboratory and not suitable for real-time field analysis.In recent years,an electrolyte cathode atmospheric glow discharge（ELCAD）as one of the promising microplasma excitation source has attracted much attention because of its desirable advantages over conventional plasma sources,such as more compact and portable instrument,lower power consumption,no inert gas requirement and operation in atmospheric pressure.This paper mainly uses the improved LCGD-AES to detect metal ions in dairy products,zinc concentrate and water samples.The effects of solution flow rate,discharge voltage,supporting electrolyte,solution pH and chemical modifier on emission intensities were investigated in detail.The main research contents are as follows:（1）The emission mechanism of ELCAD and the research progress on the improvement of ELCAD devices at home and abroad are introduced,and the research status of this device in element detection is summarized.（2）A sample introduction of pump and valve system for LCGD was developed according to some previous reports for the determination of metal elements,in which buffer bottle was used to eliminate the pulsation of peristaltic pump and increase the stability of discharge,and six-port valve injection system was employed to ensure the continuity of discharge and improve the precision and speed of analysis.（3）The improved LCGD was employed to determine the metal elements in dairy products.The electron temperature（T_e）and electron density（N_e）of LCGD at different discharge voltages were calculated.Under the optimal experimental conditions,the standard curve method was used to determine the Ca,K,Na,Mg,Zn,Fe and Cu content in dairy products.The result is close to the value measured by ICP-AES,and the LOD is in the range of 0.003-0.255 mg L^-1.A standard substance（GBW（E）080397）was used to verify the analytical performance of the test results.It was found that the accuracy of the LCGD system measurement was high with an error of 0.82%.The measured results by LCGD-AES are in agreement with the reference value as well as verification values measured by ICP-AES.In addition,the recoveries of metal elements in dairy products are in the range of 90.0-115.0%,indicating that the measurement results by modified LCGD-AES are accurate and reliable.（4）The improved LCGD was used to determine the Cd in zinc concentrate.The study found that the presence of inorganic ions did not significantly interfere with the detection of Cd.Containing 0.15%cetyltrimethylammonium bromide（CTAB）and 18%formic acid could increase the net signal strength of Cd by 2.4 and 2.7 fold,respectively,and the sensitivity of Cd is improved by 2.6 and 4.5 fold.The LOD with18%formic acid for Cd was reduced to 0.0049 mg L^-1.The measured results by modified LCGD-AES are in accordance with the reference value as well as verification values measured by ICP-AES.In addition,the recoveries are in the range of96.5–105.4%.（5）The improved LCGD was employed for the determination of mercury in water samples.The effects of chemical modifiers on emission intensities of Hg were investigated.The results indicated that the addition of 4%methanol and 0.15%hexadecyltrimethylammonium bromide（CTAB）can enhance the net intensity of Hg about 15.5-fold and 7.7-fold,and the sensitivity of Hg about 15.2-fold and 5.6-fold,respectively.Adding 4%methanol markedly eliminates the interferences from the lower concentration Cl^-and higher concentration I^-and Fe³⁺.The limit of detection（LOD）is reduced from 0.35 mg L^-1 for no chemical modifier to 0.03 mg L^-1 for 4%methanol and 0.05 mg L^-1 for 0.15%CTAB.The relative standard deviation（RSD）for Hg with the addition of 4%methanol,0.15%CTAB and no chemical modifier is 2.38%,1.17%and 3.00%,respectively.Water samples containing high mercury(10-20 mg L^-1)and low mercury(0.2-5 mg L^-1)can be determined by modified LCGD-AES with no chemical modifier and 4%methanol,respectively.Adding 4%methanol significantly reduces the matrix effects from real water samples.The measurement results of spiked samples using modified LCGD-AES are in agreement with the spiked value.Moreover,the recoveries of Hg are ranged from 95.7%to 114.1%,suggesting that the measurement results of Hg by modified LCGD-AES are reliable and accurate.