Gas Phase Enrichment Of Ultratrace Arsenic By Dielectric Barrier Discharge And Its Application To Determination In Foodstuff

Arsenic?As?is widespread in nature,designated as Class IA carcinogen by the International Agency for Research on Cancer.Due to ultratrace arsenic at?g/kg levels in foods,it requires a high sensitivity for the analytical instrumental method.Atomic fluorescence spectrometry?AFS?is a good spectrometric instrument famous for Chinese intellectual property rights,with a good analytical sensitivity,anti-interference ability and easy operation.However,the absolute sensitivity of AFS is indeed lower than inductively coupled plasma mass spectrometry?ICP-MS?,sometimes hard to fulfill the analysis of ultratrace elements.Gas phase enrichment?GPE?,such as quartz tubes,graphite,and tungsten coil,is an available pre-concetration approach with proven efficiency,simplicity and low cost,could significantly enrich As by more than 10-fold.The necessity of heating for the methods mentioned above can consume large energy and goes against the miniaturization.Dielectric barrier discharge?DBD?is an alternative GPE means at ambient temperature.Here,DBD was firstly used for the As enrichment,and combined with HG and AFS to form off-situ HG-DBD-AFS systems.On the basis of DBD mechanism study,an in-situ HG-DBD-AFS system was established to further enhance the efficiency for As pre-concentration.Finally,the analytical methods of ultratrace arsenic in agri-food were established.The detailed contents are as follows:1.The off-situ HG-DBD-AFS system and the analytical methods for ultratrace As in food were established.The off-situ HG-DBD-AFS system comprised the coaxial DBD reactor,HG system and AFS,whose atmosphere condition,discharge voltage and sweeping time were optimized here.The optimized conditions were as follows:for trapping,40 mL/min O₂ and 9.2 kV trapping voltage;before releasing,sweeping for 180 s by Ar;for releasing,200 mL/min H₂ and 9.8 kV discharge voltage.Under the optimal conditions,the limit of detection?LOD?was 1.0 ng/L?sampling volume was 20mL?,the linear ranges was 0.1 to 5?g/L,R²>0.995,and the enrichment factor was 8.For the established method of As in food,the limit of quantification?LOQ?for water samples was 3.3ng/L,the spiked recoveries were 98%-103%,the RSD values of repeated measurements for vaious food samples were 1.4%-3.6%?n=7?,and the testing results were not significantly different?p>0.05?from that by ICP-MS method.2.The mechanism of DBD system were studied to improve the DBD reactor further.The As species and the spatial distribution of As during trapping/releasing by DBD were investigated by fiber optic spectrometry,X-ray Photoelectron Spectrometry and other methods.The preliminary mechanism results were as follows:the arsenic was trapped as oxide on the quartz surface;then arsenic atoms were released but soon changed to a stable form?may be atom cluster?for transport.The As concentrations on the discharge zone of quartz inner surface was the highest,which proved arsenic inclined to be trapped by stronger discharge.However,an amount of arsenic remained retained on the inner surface of the back of the quartz tube,which could caused the loss of As into AFS.Thus,an in-situ DBD system could further enhance the absolute sensitivity of off-situ HG-DBD-AFS.3.The DBD reactor was further reformed to assemble a novel in-situ HG-DBD-AFS system,and the analytical method of ultratrace As in food was established.Based on the DBD mechanism studies,the back section of quartz tube of the off-situ HG-DBD-AFS was cut off to to assemble a novel in-situ HG-DBD-AFS system,whose discharge gas and other conditions were optimized as follows:for trapping,50 mL/min air and 9.2 kV trapping voltage;before releasing,sweeping for 180s by carrier gas;for releasing,200 mL/min H₂ and 9.8 kV discharge voltage.Under the optimal condition,the LOD for water samples was 2.8pg?sampling volume was 2 mL?,the linear ranges was 0.1 to 8?g/L,R²>0.999and the sensitivity increasing factor was 7.For the established method of As in food,the LOQ was9.2pg,the spiked recoveries were 100%-106%,the RSD values of repeated measurements were 3.2%-6.1%?n=7?,and the testing results was not significantly different?p>0.05?from that by ICP-MS method.In a word,the DBD enrichment technique and the proposed HG-DBD-AFS method were suitable to ultratrace As determination in foods accurately and stably.