Study On Determination Of Cadmium And Mercury In Agri-foods By Solid Sampling Electrothermal Vaporizationspectrometry Using Atomic Traps

At present, agricultural environment in China has been heavily polluted by heavy metals, which leaded to heavy metal contamination of agri-products, especially for cadmium(Cd) and mercury(Hg) in grain, vegetables, tea, animal organs, fishes, shellfish etc. The heavy metal contamination in agri-foods caused not only potential risks for consumers’ health, but also adverse effects on governments’ food safety management, agricultural production, agri-products export, social harmonization and so on. Presently, the spectrometric methods using nebulization sampling system were always employed to measure Cd and Hg in agri-foods for food safety monitoring. But, the methods mentioned above were difficult to be minimized and simplified, as well consumed more times and labors, because of sample digestion and pretreatments. So the spectrometric instruments with nebulization sampling system were not suitable to on-field analysis for heavy metals, and they could not support fast methods for agri-foods monitoring during field output and market entrance.In order to determine Cd and Hg quickly and simply, atomic traps, on-line ashing and porous carbon electrothermal vaporization(ETV) techniques were employed to design and assemble solid sampling(SS) devices. Then the SS devices were coupled with atomic fluorescence spectrometry(AFS) and inductively coupled plasma mass spectrometry(ICP-MS), furthermore the direct SS methods for Cd and Hg determination in agri-foods were established. The established methods without digestion treatments and chemical reagents were simple, sensitive and stable, and the analytical time including sample pretreatments was within 10 min. Therefore, the established methods were able to provide available supports for quickly determining Cd and Hg in agri-foods. The main contents and results of this paper were as followed:1. The SS-AFS instrument was assembled cooperatively with a tungsten coil trap(TCT), on-line ashing and porous carbon ETV. The instrument was mainly composed of sampling unit, ashing unit, ETV, tungsten coil trap, elastic sealing unit, gas channel, electric circuit and detector(AFS). In this experiment, instrumental conditions were optimized, and the mechanism of Cd trapping and releasing of TCT was also studied. The linear ranges of the SS-AFS instrument were 0.3 pg- 800 ng(at the lowest sensitivity of AFS), the regression coefficient(R2) exceeded 0.997, the mixture of Ar and H2 with 10 % H2(v:v) was chosen as carrier gas, the limit of detection(LOD) was 0.3 pg, and the values of relative standard deviation(RSD) were less than 5 %. Besides, it was proved that vaporized, trapped and released Cd speciation by ETV and TCT were almost atoms.2. Using the assembled SS-AFS instrument, the fast analytical methods of Cd in grain, spinach, tea, pork liver and shellfish samples were established. In this experiment, the equations of relative homogeneity factor(HE) and Pauwels were firstly employed to evaluate the homogeneity and minimum sampling mass(M) for fresh spinach and pork liver samples. The method limit of quantification(LOQ) was 1.6 pg; the spiked recoveries were 84 %- 113 %; the RSD values of repeated measurements were less than 10 %(powdered samples) and 15 %(fresh samples), respectively; the measured values by the established SS-AFS method were not significantly different(p > 0.05) from that by microwave digestion GFAAS and ICP-MS methods or certified reference materials(CRMs). Besides, the M values of various agri-food samples were studied, and the results were showed as followed: the homogeneity of grain samples sieved by 60 mesh and powdered samples of spinach, tea, pork liver and shellfish sieved by 80 mesh were well(HE < 10), and their minimum sample mass values were from 1 to 5 mg; the homogeneity of fresh samples was poor(HE > 10), and the minimum sample mass values of frozenly grinded spinach, shellfish and pork liver samples by liquid nitrogen were from 20 to 25 mg. So, the minimum sample mass mentioned above all satisfied the largest capacity of a sampling boat.3. The on-line ashing device, porous carbon ETV and TCT techniques were integrated into ICP-MS. On the basis of SS device for AFS, some crucial problems such as the connecting interface, gas line design and gas composition were solved in this experiment. For the SS-ICP-MS instrument, the mixture of Ar and H2 with 4 % H2(v:v) was chosen as carrier gas and auxiliary gas(double gas lines design); besides, the instrumental linear ranges were 0.1 pg- 800 ng, R2 values exceeded 0.996, LOD was 0.1 pg, and the RSD values were less than 5 %.4. Using the assembled SS-ICP-MS instrument, the fast analytical methods of Cd in grain, spinach, pork liver samples were established. The method LOQ was 0.5 pg; the spiked recoveries were 88%- 112%; the RSD values of repeated measurements were less than 10 %(powdered samples) and 15 %(fresh samples), respectively; the measurement results by the established method were not significantly different(p > 0.05) from that by microwave digestion ICP-MS methods or CRMs. It was proved that our SS device using on-line ashing, porous carbon ETV and TCT techniques together was also suitable for fast Cd analysis by ICP-MS.5. The instrument of SS-AFS with gold and tungsten traps was assembled cooperatively. On the basis of vaporization temperature differences between Hg and Cd elements, a gold coil and tungsten coil were employed to trap Hg and Cd, respectively. The instrument was mainly composed of sampling unit, ashing unit, ETV and TCT for Cd, catalyzer unit, Hg trapping unit, gas channel and detector(AFS). Hg vaporization and sample ashing could be completed in air ambient by the ashing unit, and Hg was trapped at room temperature in air ambient and released at 600 ℃ in the mixture of Ar and H2 with 10 % H2(v:v)。6. Using the assembled SS-AFS instrument with a gold and tungsten trap, the fast analytical methods of Hg and Cd in grain, spinach, tea and animal product samples were established. The method LOQs were 4 pg(Hg) and 1.6 pg(Cd); the spiked recoveries for Hg were 95 %- 105 %; the RSD values of repeated measurements were less than 10 %(powdered samples) and 15 %(fresh samples), respectively; the measurement results by the established SS-AFS method for Hg and Cd were not significantly different(p > 0.05) from that by the microwave digestion ICP-MS method or CRMs. It was proved that the established methods could meet the demands for fast determination of ultratrace Hg and Cd in agri-foods simultaneously.