Study Of Microwave Digestion Method On The Application Of Sanitary Analysis

In the field of sanitary analysis, sample digestion is a prerequisite for many analytical works, and it always has a decisive influence to the whole analytical work. Traditional digestion methods are time-consuming, and are always contacting with the outer environments, thereby increased the possibility of samples' pollution, and this kind of pollution is often fatal for the microanalysis or trace analysis. Microwave digestion technology is a new method for sample digestion developed in recent years. Even though, in the process of application, defects still exist. For example, the excessive usage amount of nitric acid, the nitrogen oxides in digestion fluid, and samples containing silicate couldn't be digested completely by nitric acid and hydrogen peroxide, resulting in not clarified digestion fluid. All of these are reasons that affect the results of the analysis. In this study, a series of microwave digestion programe with the advantage of rapidness, convenience and accuracy was researched. The results showed that these methods were applicable to the trace elements analysis of all type of samples in sanitary analytical works.The sample digestion programe of nitric acid, nitric acid-hydrogen peroxide system and nitric acid-hydrofluoric acid system was optimized with the use of MARS5 microwave digestion device of CEM Company of the United States, and the samples digested were accurately analyzed by flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry and atomic fluorescence spectrometry. These programes are accurate for both qualitative and quantitative analysis, and are reliable for trace analysis. The procedure was: 1. Selectting four standards: pork powder GBW08552, tea leaves GBW08513, human hair GBW09101 and powdery cosmetics GBW09301 for the optimization experiment of microwave digestion condition. 2. Preparation of sample solution: taking 0.5g (pork powder, tea leaves, human hair and powder) accurately into polytetrafiuoroethylene (PTFE) digestion tank. Samples were digested by different digestion fluid and procedures in accordance with the type of samples, and the digested fluid was metered and preserved for analysis. 3. Instruments: AA-6800 atomic absorption spectrophotometer (Shimadzu Corporation) with graphite furnace atomizer, S4 atomic absorption spectrophotometer (Thermo Electron Corporation) with flame atomizer and AF-610A atomic fluorescence spectrometer (The Second Optical Instrument Factory, Beijing). 4. Quality control: The national standard materials (the national research center for CRM'S) were used for precision and accuracy experiments to calculate the recovery rate and precision.Results: 1. Pork powder, tea leaves and human hair were digested by nitric acid or nitric acid - hydrogen peroxide system. The digestion effects of seven kinds of nitric acid dosage from 1.0 ml to 7.0 ml, and the two hydrogen peroxide dosage of 0.5 ml and 1.0 ml were compared. Samples could be digested completely and got clarified digestion fluid using nitric acid and hydrogen peroxide system. The satisfactory results could be obtained with 1.0 ml nitric acid and 0.5 ml hydrogen peroxide as digestion fluid. Nitrate - hydrofluoric acid digestion system were used for the digestion of powdery cosmetics containing silicate which could not be digested completely by nitric acid and hydrogen peroxide. The digestion effects of seven kinds of nitric acid dosage from 1.0 ml to 7.0 ml, and the two hydrofluoric acid dosage of 0.6 ml and 1.0 ml, were compared. The best programe of microwave digestion is to use 1.0 ml nitric acid and 0.6 ml hydrofluoric acid as digestion fluid. 2. The eight kinds of trace element in pork powder, tea leaves, human hair and powdery cosmetics were analyzed with flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry and atomic fluorescence spectrometry. The results of samples digested by the best microwave digestion programes were all in the marked range of standard substances. 3. Precision and recovery experiments. The 6 time continuous analysis averages of eight kinds of trace element (Fe, Mn, Cu, Zn, Pb, Cd, As and Hg) in five kinds of standard substance (GBW08552 pork powder, GBW 09101 human hair, GBW08513 tea leaves, GBW08508 rice flour and GBW09301 powdery cosmetics) were in the marked range, while the precision RSD was 0.63% -4.3% and the recovery rate was 92.10% - 108.70%。4. To compare the results of the methods above and the GB method。Fe and Mn were analyzed with the methods of GB12396-1990, Pb was analyzed using the HNO₃-HCIO₄ method in GB/T5009.12-1996 and As was digested by the wet method of GB/T5009.11-1996. The amount of Fe, Pb, As and Mn were determined in 20 samples and no significant difference was found among them analyzed by matching t-test.Conclusion: 1. It is capable to digest a variety of samples (excluding silicate) completely using 1ml nitric acid and 0.5ml hydrogen peroxide. Less usage of reagents and low blank value avoid the interference followed the large amount usage of acid. Samples could be determined after digestion without removing acid, which avoid the loss and pollution of determined elements. At the same time, the steps of operation were simplified and improved the efficiency. The digestion fluid was clarified and the results of analysis were satisfactory. 2. To digest samples containing silicate, the nitric acid - hydrofluoric acid digestion system was used, and with the using of lml nitric acid and 0.6ml hydrofluoric acid the satisfactory results could be obtained. 3. In this study, the required amount of nitric acid was less than lml, therefore, nitrogen oxide produced by digestion was lessen, reducing the pollution to environment and the hazard to health of experimenters. Significant economic benefit and ecological environmental protection benefit will be made.