Chao Yan Zhou

Orthogonal array design for the optimization of closed-vessel microwave digestion parameters for the determination of trace metals in sediments

Abstract Orthogonal array design was applied to the optimization of the closed-vessel microwave digestion of sediment samples. Five variables related to the digestion efficiency were studied: the addition of HF in digestion acid, the ratio between HCl and HNO 3 , the maximum pressure setting, the output power and the digestion time. In the experimental assignment, the digestion time at two levels and other variables at four levels were tested using an OA 16 (4 4 × 2 3 ) matrix. The interactions between the parameters were neglected. From the results, the optimum conditions for the closed vessel microwave digestion for sediment samples were established.

Microwave digestion of biological samples with tetramethylammonium hydroxide and ethylenediaminetetraacetic acid for element determination

A microwave-digestion system with a closed PTFE vessel was used to improve the leaching of inorganic constituents from biological samples with tetramethylammonium hydroxide (TMAH) and ethylenediaminetetraacetic acid (EDTA). The effects of microwave parameter settings and the quantities of TMAH and EDTA used on leaching efficiency were evaluated. This new digestion method has been applied to the standard reference materials NIST SRM 1577 B Bovine Liver 1515 Apple Leaves and NIES CRM No. 1 Pepperbush, No. 3 Chorella, No. 6 Mussel and No. 7 Tea Leaves. The major and minor elements in the digests were analyzed by flame atomic absorption spectrometry or graphite furnace atomic absorption spectrometry. Good agreement of the analytical results with the certified values was obtained.

The behaviour of leached aluminium in tea infusions

Abstract The behavior of leached aluminium (Al) was studied using GF-AAS with different sample pretreatments and RP-HPLC with lumogallion as the precolumn reagent. The results showed that Al species in tea infusion could be categorized into three groups: large organic compounds, small stable organic compounds and free form Al. The compositions of Al species in a tea infusion would vary with the method of the tea production. For non-fermented teas such as Long-Jin and Moli-Hua, most of the leached Al in the first tea infusion was in large organic compounds and small stable organic compounds, and free form Al consisting of less than 10%. In the case of semi-fermented and fully fermented teas such as Shuihsien and Pu-Erh, the main compositions of Al species in the first infusion were free form Al and small stable organic compounds, while Al in large organic compounds made up about 14% and 4%, respectively. When 1-g tea leaves were infused with 100 ml boiling water, about 21% Al in the tea leaves was leached into the water in the first infusion and a total of about 27% of the Al in the tea leaves was leached into the water in four infusions.

High performance liquid chromatographic determination of aluminium in natural waters in the form of its lumogallion chelate.

A high performance liquid chromatographic method for the determination of ultra trace amount of aluminium in natural waters has been developed using lumogallion as a precolumn reagent for fluorimetric detection. The highly fluorescent Al-lumogallion chelate (lambda(ex) 500 nm, lambda(em) 574 nm) was separated on a LiChrosorb RP 18 column with an eluent containing 3:7 acetonitrile/0.02M potassium hydrogen phthalate buffer (pH 4.7) containing 10(-5)M lumogallion. The proposed system provides a simple, quick, selective and sensitive method for the determination of ultra-trace amount of aluminium in water samples. The detection limit defined as three times the standard deviation of the blank signal, was 0.05 mug/l. in water samples for 100 mul injection. The tolerance limits were 5 mg/l. for Fe(III) and F(-) and over 10 mg/l. for other foreign ions. The sensitivity of the method was independent of salinity. This method had been used for the direct determination of aluminium in both tap and coastal sea-waters without any preconcentration steps.

Microwave-assisted dilute acid extraction of trace metals from biological samples for atomic absorption spectrometric determination

A microwave extraction method with dilute nitric acid for biological samples was developed using a pressure feedback microwave extraction system. The concentration of nitric acid, pressure setting and extraction time were optimized by a mixed level orthogonal array design. The interactions among the selected parameters for the extraction were investigated. The effect of sample mass was also studied. Six replicate analyses of NIST SRM 1515 Apple Leaves were performed under the optimized extraction conditions (5 ml of 14% v/v nitric acid; pressure setting, 1104 kPa; extraction time, 30 min; sample mass, 0.3 g). Eight elements, viz., Ca, Cu, Fe, K, Mg, Mn, Ni and Zn, were determined by FAAS or ETAAS. Recoveries of 96–103% were achieved for all the elements. The RSDs of the test elements were less than 3.8%, except for Ni. The extraction method was also employed to extract four other biological SRMs and CRMs. With the exception of Ni, the recoveries of the eight test elements were 91–107% in all the reference materials.

Soil lead and other metal levels in industrial, residential and nature reserve areas in Singapore

The use of pressure feedback microwave digestion technique has permitted rapid and efficient digestion of soil and sediment samples. The evaluation of different acid mixtures to digest soil samples were studied by mixed-level orthogonal array design. The selected acid mixture of HCl-HNO3-HF was employed in the survey of Pb, Zn, Cu, Cr and Mn in soil samples. Surface soil samples were collected from industrial, residential and nature reserve areas in Singapore. The five metal concentrations were determined by flame atomic absorption spectrometry and graphite furnace atomic absorption spectrometry. The lead and other metal contents in NBS SRM 1645, NIES CRM No. 2 and NRCC BCSS-1 sediment standard references were determined concurrently with the survey samples. Five measured metal loading on the surface soils was in the order: industrial area > residential area > nature reserve area. The trace metal concentrations in surface soil from areas of heavy traffic are higher than those from residential areas. The main sources of trace metal pollution are vehicular exhaust and industrial activities.

Orthogonal array design (OAD) for the optimization of mercury extraction from soils by dilute acid with microwave heating

The use of dilute acid for mercury extraction from geological samples with microwave assisted heating was investigated. An orthogonal array design OA(16)(4(1)x2(12)) was applied to select the optimum conditions. The effects of the concentration of HNO(3), the additions of HCl and H(2)S0(4), the extraction time and their interactions were evaluated by the mixed-level orthogonal array design (OAD). The results indicated that the addition of small amount of HCl would improve significantly the extraction of mercury from soil matrices. From the study, it is seen that the mercury in the soil matrices could be extracted completely by 14% (v/v) HNO(3) with small quantity of HCl. The effects of the pressure within the digestion vessel and the sample mass were studied under the optimum conditions derived from the OAD. The extraction methods with optimum conditions were evaluated by determining mercury in NIST SRM 1645 River Sediment, NIES CRM No. 2 Pond Sediment and NRCC BCSS-1 Marine Sediment. Recoveries of 78-109% were achieved. Good mean recoveries of 95-98% were also obtained from real soil samples spiked with different species of mercury.

Evaluation of high-pressure microwave digestion methods for hydride generation atomic absorption spectrometric determination of total selenium and arsenic in sediment

Five closed-vessel microwave digestion methods were compared for the accurate determination of arsenic and selenium in NIST SRM 1645 River Sediment by flow-injection hydride-generation atomic absorption spectrometric methods. The digestion methods using five different acid mixtures (HNO3/ H2SO4, HNO3/HCl04, HNO3/HCl, HNO3/HCl/HF, HNO3/H2SO4/HClO4) were all found to be reliable for the determination of the analytes. Taking into consideration the safety and suitability for the analysis of other metals, the methods based on the use ofaqua regia are recommended for closed vessel microwave digestion with pressure control. Using the quick digestion program, the presence of up to 10% organic content in soil samples did not adversely affect the closed vessel digestion and did not cause the loss of volatile analytes. After digestion, opening the vessel under an inner pressure of below 345 kPa (50 psi) had no effect on the accuracy of the results. The recommended digestion methods (HNO3/HCl and HNO3/ HCl/HF) for the reliable determination of arsenic and selenium in different sediment samples were demonstrated. The calculated detection limits (3σb) were less than 0.030 μg/g and 0.033 μg/g for arsenic and selenium, respectively. All analytical results for arsenic and selenium in SRM 1645 River sediment, NRCC BCSS-1 Marine Sediment and NIES CRM Pond Sediment were within or near the certified and reported ranges, with the exception of selenium in NIES CRM No. 2 Pond Sediment.Five closed-vessel microwave digestion methods were compared for the accurate determination of arsenic and selenium in NIST SRM 1645 River Sediment by flow-injection hydride-generation atomic absorption spectrometric methods. The digestion methods using five different acid mixtures (HNO3/ H2SO4, HNO3/HCl04, HNO3/HCl, HNO3/HCl/HF, HNO3/H2SO4/HClO4) were all found to be reliable for the determination of the analytes. Taking into consideration the safety and suitability for the analysis of other metals, the methods based on the use ofaqua regia are recommended for closed vessel microwave digestion with pressure control. Using the quick digestion program, the presence of up to 10% organic content in soil samples did not adversely affect the closed vessel digestion and did not cause the loss of volatile analytes. After digestion, opening the vessel under an inner pressure of below 345 kPa (50 psi) had no effect on the accuracy of the results. The recommended digestion methods (HNO3/HCl and HNO3/ HCl/HF) for the reliable determination of arsenic and selenium in different sediment samples were demonstrated. The calculated detection limits (3σ b ) were less than 0.030 μg/g and 0.033 μg/g for arsenic and selenium, respectively. All analytical results for arsenic and selenium in SRM 1645 River sediment, NRCC BCSS-1 Marine Sediment and NIES CRM Pond Sediment were within or near the certified and reported ranges, with the exception of selenium in NIES CRM No. 2 Pond Sediment.

Urine levels of aluminum after drinking tea

A microwave-assisted acid digestion procedure coupled with a graphite furnace atomic absorption method has been applied in the determination of aluminum (Al) in urine to verify the correlation of free forms of Al in tea infusions and urinary excretion of Al. Significant urinary Al excretion has been found in 24-h urine of four volunteers after tea drinking. However, the difference in amount of Al excretion in urine between the consumption of Oolong (black tea) and Long-Jin (green tea), each of them with unique Al contents and species, was not significant. These findings indicated that the high levels of free Al species in tea infusions did not result in significant change in urinary excretion of the metal, possibly owing to the transformation by ligands present in food and the gastrointestinal tract (GIT). However, it could not be assumed that there was no big difference in absorption of the metal in the human body if fractions of consumed Al retained in the body or excreted by bile or feces were considered.