Determination of Arsenic in Palm Kernel Expeller using Microwave Digestion and Graphite Furnace Atomic Absorption Spectrometry Method Abdul Niefaizal Abdul Hammid, Ainie Kuntom, RazaIi Ismail, and Norazilah Pardi 1Malaysian Palm Oil Board, Persiaran Institusi, Bandar Baru Bangi, 43000,Kajang Selangor, Malaysia. niefaizal@.myAbstrak – A study on the method to determine arsenic in palm kernel expeller wascarried out. Microwave digestion technique is widely applied in the analyticalchemistry field. In comparison to conventional sample digestion method, themicrowave technique is simple, reduced contamination, usage of safe reagent andmatrix completely digested. A graphite furnace atomic absorption spectrometrymethod was used for the total determination of arsenic in palm kernel expeller.Arsenic was extracted from palm kernel expeller in a closed vessel digestion systemwith nitric acid and hydrogen peroxide. The results showed that the optimal ashingand atomizing temperatures were 800°C and 2400°C respectively. The limit ofdetection was estimated to be 0.001 ppm. The mean recoveries of arsenic forrepeatability and reproducibility for 1, 2 and 4 ppm were in the range between 79 –90%. Ten samples of palm kernel expeller from mills were analyzed to contain 0.18to 3.05 ppm of arsenic. Therefore, is proposed that this method be used to detectarsenic in palm kernel expellerKey Words –Microwave digestion, graphite furnace atomic absorptionspectrometry, palm kernel expeller, arsenic, mills.1 IntroductionArsenic has been considered as an essential trace element for normal growth and development of animals (Lasky et al, 2004). However, arsenic is more often regarded as a hazardous element rather than as essential element widely encountered in the environment and organisms (Cullen and Reimer, 1989). Arsenic is extensively distributed in the environment because of its presence naturally as well as from industrial production. Natural arsenic concentration in plants hardly exceeds 1 mg/kg (Porter and Peterson, 1975). This level increases to several folds when plants are coerced to grow in arsenic treated soils, but arsenic is primarily retained in the roots. Studies showed that vegetable grown in arsenic-spiked soils exhibited 7.1 and 5.0 mg/kg in the roots and shoots respectively Jones and Hatch, 1945). In tomato and bean plants, arsenic is primarily concentrated in the roots, and a small quantity is translocated to the pods (Cobb et al., 2000). A similar pattern is observed in Tamarik (Tamarix parviflora) and Eucalyptus (Eucalyptus camaldulensis) where the roots accumulate more arsenic compared to the shoots (Tossell et al., 2000).In Malaysia the level of arsenic in oil palm is not well established. Contamination of arsenic may come from the use of herbicides such a monosodium methyl arsenate (MSMA), disodium methyl arsenate (DSMA) and cacodylic acid (dimethylarsenic acid) in oil palm plantations. However these compounds are not in used anymore. Numerous methods are available for extracting arsenic from various matrices and analyzing total arsenic (Hudson-Edwards et al., 2004). The most common641methods for extracting total arsenic from soils and sediments involved wet ashing of sample using one or a combination of acids such as sulphuric acid, nitric acid, hydrochloric acid, boric acid, hydrogen fluoride and hydrogen peroxide. Ashing digestion can be carried out using hotplate or microwave oven (Mucci et al., 2003).During the past few decades, microwave digestion method has become widely used since they are more reproducible, more accurate and less time consuming than conventional digestion on hot plates in open crucibles and lost of analyte is minimum (Kingston and Jassie, 1988). Arsenic can be determined using the following methods: colorimetry, hydride generation system in combination with atomic absorption spectrometer (Slemer et al., 1976) and atomic fluorescence spectrometry (Chen et al., 2001). Graphite furnace atomic absorption spectrophotometer (GFAAS) is another analytical instrument used for trace element analysis. It has been widely used to determine lead in food (Chen et al., 1999), biological samples (Dabeka and McKenzie, 1992) and environmental samples (Cabrera et al., 1994).The main objective of this study was to test the effectiveness of microwave system for the digestion of palm kernel expeller and subsequently analyzing using graphite furnace atomic absorption spectrophotometer.2 Materials and Methods2.1 ReagentsAll reagents were of analytical grade unless otherwise stated. Double-distilled water (Milli Q Millipore 18.2 mΩ-cm resistivity) was used for dilution. Nitric acid (65% w/v) and hydrogen peroxide (30% w/v) were of suprapure quality (E.Merck, Darmstadt). All the plastic and glassware were cleaned by soaking in diluted nitric acid-distilled water (1+9) and were rinsed with distilled water prior to use.2.2 Arsenic Standard SolutionA standard stock solution of arsenic (1000 ppm) was purchased from BDH Laboratory Supplies.2.3 Working Standard SolutionWorking standard solution of 100 ppm and 1 ppm were prepared by diluting the standard solution with appropriate volumes of Milli Q water. Working standard for 100 ppb was prepared by adding 1 mL of1 ppm and2 mL of nitric acid.2.4 SampleAbout 0.5 g palm kernel expeller was spiked with 500 μL, 1 mL and 2 mL of 1 ppm working standard solution to produce 1 ppm, 2 ppm and 4 ppm respectively.2.5 Preparation of Standard CurveA working standard solution of 25 ppb, 50 ppb, 75 ppb and 100 ppb were prepared by diluting the 1 ppm working standard solution with appropriate volumes of Milli Q water.642 Insan Akademika Publications2.6 Analytical ProcedureMicrowave digestion procedure was applied for palm kernel expeller sample. About 0.5 g of palm kernel expeller was weighed. Then 6 mL of nitric acid (65% w/v) and 2 mL hydrogen peroxide (30% w/v) suprapure quality were added. A blank digest was carried out in the same way. The digestion conditions for the microwave system were shown in Table 1. After treatment, the contents were cooled down, then the resultant residue was dissolved in 25 mL Milli Q water for arsenic determination by graphite furnace atomic absorption spectrophotometer.2.7 ApparatusZeeman graphite furnace atomic absorption spectrophotometer AAnalyst 600 with standard transverse heated graphite atomizer (THGA) B3000641 and arsenic electrode less discharge lamp (As-EDL) were made by Perkin Elmer (Germany). Argon was used as the pure/inert gas. The instrument operating parameters are summarized in Table 2 and Table 3. For sample digestion, a Milestone Ethos MOD with Terminal 1024 closed vessel microwave digestion system with pressure and temperature controller was used.2.8 Analysis of Arsenic in Commercial Palm Kernel ExpellerTen different palm kernel expeller samples were collected from 10 different mills in Malaysia. The samples were preserved in covered polyethylene bags, tagged properly and kept in room temperature until analysis.2.9 Statistic AnalysisThe data obtained from the analysis were calculated using computer programme Microsoft Excel for Windows.Table 1: Microwave oven heating program for the decomposition of palm kernel expellerStep Time (Minute) Temperature 1 (˚C) Temperature 2 (˚C) Microwave Power(Watt)1 15 min 500 180 1002 15 min 500 180 1003 15 min 1000 200 1204 10 min 1000 200 1205 20 min 0 50 30 643644Insan Akademika PublicationsTable 2: Instrument parameters for the determination of arsenic in palm kernel expeller using graphitefurnace atomic absorption spectrophotometerParameterSettingWavelength (nm) 193.7Slit width (nm)0.7Signal measurementPeak areaLampElectrode less dischargeLamp current (mA)380Purge gas Argon Sample volume 20 μL Modifier volume5 μLTable 3: Temperature program for the determination of arsenic in palm kernel expeller by graphitefurnace atomic absorption spectrophotometerTemperature (˚C)Ramp TimeHold TimeInterna (ml/ l Flow Gas Type min)Read120 1 30 12 5 Argon800 10 30 25 0 Argon2400 0 5 5 0 Argon +25002325 0Argon* Temperature injects: 70 ˚C .3 Results and Discussion3.1Matrix ModificationMatrix modification is an essential step in the determination of easily volatile elements by graphite furnace atomic absorption spectrophotometer. The most common matrix modifiers used in arsenic determinations are palladium, palladium-magnesium nitrate and nickel nitrate.Palladium-magnesium nitrate was selected as a modifier since memory effects were observed whenthe nickel modifier was used (Bozsai et al., 1990). An interelement compound was formed between arsenic and palladium, which has a higher heat of vaporization than pure arsenic. Therefore, a higher ashing temperature can be used and the effects of interference are diminished. Magnesium nitrate behaves as an ashing aid during the thermal pretreatment step in graphite furnace determinations. Spectral interferences caused by aluminum and phosphate are possible at the primary resonance line (193.7 nm) of arsenic. It should be possible to almost eliminate these interferences with the Zeeman background correction technique (Bettinelli et al., 1989; Welz et al., 1988; Riley, 1982).Characteristic mass for arsenic is quite high, and therefore a rather large absolute mass of arsenic should be injected into a graphite tube in order to obtain a reasonable sensitivity. This means that a645P e a k A r e ahigher sample volume should be used or that a preconcentration step is necessary. However, with a larger sample volume, a longer sample drying phase is needed, and there is also a maximum sample volume that can be dispensed onto a platform.Therefore, a sample volume of 20 mL was selected for routine use. With very low arsenic concentrations, two or more sample dispensing-drying steps can be used in order to increase the absolute amount of arsenic in the graphite furnace. One should remember that this will also increase the amount of matrix introduced into the furnace. About 5 mL of modifier solution was injected onto a sample. Larger volumes of modifier had produced wider and flatter absorption signals, therefore 5 mL of modifier was determined to be the optimal volume in arsenic determinations.3.2Method PerformanceEvaluation of quality parameters such as the linearity, recovery percentage (repeatability, and reproducibility) and limits of detection are essential to assess the method performance (Zanella et al., 2000). Calibration curve was obtained by analyzing three times each, four different solutions of known concentrations of analyte included between 25 and 100 ppb. The curve equation y = bx + m calculated with linear regression method to determine samples concentration was utilized.The calibration curve data for standard arsenic is shown in Table 4 . The equation of the curve and the R2 value (0.999) shows the good linearity of the analytical method and the method is feasible to be used. Values of coefficients of variation are less than 5% for all concentration (25 ppb, 50 ppb, 75 ppb, and 100 ppb) and to be considered acceptable.Table 4: Parameters values obtained from the calibration curveConcentration of ArsenicStandard (ppb)Average Value(ppb)Mean of AreasStandard Deviation Coefficient of Variation (%)2526.14 0.0412 0.0003 0.715051.25 0.0807 0.0020 2.447573.96 0.1165 0.0036 3.0910099.830.15730.00120.74y = 0.0016x R² = 0.9994Concentration (ppb)International Journal of Basic and Applied Science V ol. 01, No. 03, Jan 2013, pp. 641-649 Millersmtih, et. al.Fig.1: Calibration curve for the determination of arsenic by means of thegraphite furnace atomic absorption spectrophotometer techniqueThe limit of quantification (LOQ) was stated as a concentration below which the method could not operate with an acceptable level of precision and trueness. The limit of detection (LOD) was the lowest concentration of arsenic in palm kernel expeller samples that was detectable but not necessarily quantified, distinguished from zero (signal/noise >3). These limits were established based on the mean recovery and relative standard deviation results obtained for the replicates of spiked samples. Limit of detection was found to be 0.001 ppm, limit of quantification was 0.006 ppm.Recovery test for repeatability and reproducibility were performed by spiking several concentrationsof arsenic standard to palm kernel expeller, which was then analyzed using the established method. Recoveries for repeatability from palm kernel expeller at 1, 2, and 4 ppm were found to be 80.40 ±4.55%, 90.59 ±4.74%, and 85.15 ±5.06%, respectively . Recoveries for reproducibility from palm kernel expeller at 1, 2, and 4 ppm were found to be 79.20 ± 3.18%, 89.10 ± 3.11%, and 80.35 ±6.25%, respectively .All recoveries were greater than 80% with coefficient of variation less than 10% and to be considered acceptable. The results obtained show that the established method is capable of yielding a satisfactory recovery.4 ConclusionThe microwave digestion method studied is suitable for the decomposition of palm kernel expeller, since good recoveries for arsenic were obtained. Digesting palm kernel expeller with microwave digester has the advantages of time-saving and more complete digestion. In addition, the graphite furnace atomic absorption spectrometry has high sensitivity and it only takes few minutes for the time taken to complete an analysis of an individual sample. 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