A comparative study of acid-extractable and total digestion methods ...

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C. Yafa, J.G. Farmer / Analytica Chimica Acta 557 (2006) 296–303 297Table 1Operating conditions for the Thermo Jarrell Ash IRIS ICP-OES instrumentPower (kW) 1.40Plasma gas flow (L min −1 ) 15.0Auxiliary gas flow (L min −1 ) 0.75Nebuliser gas flow (L min −1 ) 0.75Pump speed (rpm) 15Sample delay (s) 40Rinse time between each sample (s) 40Replicates 3Replicate time (s) 30Stabilisation time (s) 10results from different studies arise from the nature of the samplepreparation methods adopted.Different research groups have used a variety of sample digestionschemes, which provide acid-extractable or total elementalconcentrations, without standardisation of methods. In general,the acid-extractable concentrations of elements in environmentalmatrices are conventionally defined by procedures involvingextraction with aqua regia or various concentrations of nitricacid (HNO 3 ) and hydrochloric acid (HCl) in combination oralone [20]. Procedures for determination of the total concentrationsof elements frequently employ mixtures of mineral acidswith hydrofluoric acid (HF) or tetrafluoroboric acid (HBF 4 ) fordirect wet ashing or dissolution after dry ashing [21–25]. Bothacid-extractable and total approaches have a number of versions,differing not only in the reagents used, but also in the sequenceand form of their use and the process parameters, as well as inquantitative parameters, such as the size of sample, the concentrationand amounts of reagents and the final volume of solution.To some extent, this is illustrated by the range of preparationand analytical procedures used by different laboratories in therecent development and certification of an ombrotrophic peatbog (low ash) reference material (NIMT/UOE/FM/001) for thedetermination of elemental concentrations, in which our laboratoryplayed a central coordinating role (cf. Table 1 in Ref.[26]).Notwithstanding the development of this reference material,the lack of common procedures for sample preparation hindersnot only the quality assurance of the generated analytical data butalso the interpretation and comparison of peat core metal profilesfrom different laboratories in the international community. Forthese reasons and at the same time as the reference material wasbeing developed, we undertook a comparative study of digestionmethods for preparation of peat material for analysis.Our aims were: (i) to develop/optimise methods for determinationof acid-extractable and total concentrations of inorganicelements using a previously distributed, but ultimately, noncertified,Canadian peat (OGS 1878 P-6) reference material [27]and to apply these in (ii) the development of the new certifiedreference material for peat (NIMT/UOE/FM/001) [26] and (iii)the determination and comparison of concentration profiles forvarious elements (some ‘lithogenic’, some ‘pollutant’) in a peatcore from Flanders Moss, Scotland. As a starting point, we consideredfour different published preparation procedures based onHNO 3 (USEPA 3051) [28], HNO 3 /HCl (USEPA 3051a) [29],aqua regia HNO 3 /HCl (ISO 11466) [30] and HNO 3 /HF (USEPA3052) [31].2. Experimental2.1. MaterialsIn the developmental stage of the work, a Canadian peat material,OGS 1878 P-6, was used. This material, a Carex (sedge)fen peat (∼20% ash content), was produced by the OntarioGeological Survey in 1982. More recently, as yet uncompletedattempts have been made to develop this as a peat referencematerial for quality control use by laboratories in the internationalpeat bog community. Provisional certified values arisingfrom an inter-laboratory comparison exercise were presented atan international conference in 2000 [27].2.2. Digestion methods2.2.1. HNO 3 (adapted USEPA 3051)Conventional HNO 3 digestion is commonly employed toachieve acid-extractable elemental concentrations in geologicalsamples. In this study, a modified version of the USEPAmethod 3051 protocol [28] was employed for the digestionof peat material, using microwave-assisted HNO 3 digestion.The modification, designed to enable some final analyses withICP-MS, where appropriate, entails ashing samples in a mufflefurnace to remove organic material before digestion. The overallmethod consisted of a representative sample of up to 0.25 g(the initial weight) being ashed at 450 ◦ C for 4 h prior to digestionin 10 mL concentrated HNO 3 (Aristar, Merck, Poole, UK)in Teflon microwave digestion vessels using microwave heatingwith a suitable laboratory microwave system, e.g. MARS 5(CEM, Buckingham, UK). The programme used had the followingfeatures: maximum power 1200 W, 100%, ramp 30 min, hold20 min, 150 PSI, 205 ◦ C. Upon cooling, the solution was filteredthrough Whatman No. 542 filter paper to remove any remainingsolid material. The solution was evaporated to approximately1 mL on a hotplate and then made up to 25 mL with 2% (v/v)Aristar HNO 3 prior to analysis by ICP-OES.2.2.2. HNO 3 /HCl (adapted USEPA 3051a)A modified version of the USEPA method 3051a protocol forHNO 3 /HCl digestion [29] was used in this study. As with theconventional HNO 3 digestion (Section 2.2.1), a representativesample of 0.25 g was ashed at 450 ◦ C for 4 h prior to digestionin 9 mL concentrated HNO 3 (Aristar) and 3 mL concentratedHCl (Aristar) in the microwave digestion system. Thereafter,the procedure was the same as in Section 2.2.1.2.2.3. Aqua regia (HNO 3 /HCl) (ISO 11466)In the absence of a microwave digestion system, the standardaqua regia extraction procedure according to ISO Standard11466 is commonly used to achieve acid-extractable elementalconcentrations [30]. A representative sample of 1.5 g peatmaterial was accurately weighed out into a 100 mL conical
298 C. Yafa, J.G. Farmer / Analytica Chimica Acta 557 (2006) 296–303flask. A small amount of water (2–3 mL) was added to generatea slurry and then 4.5 mL concentrated HNO 3 (Aristar)Table 2was added. After intensive foaming, 11.5 mL concentrated HCl Element(Aristar) was added in several portions. The mixture was manuallyshaken. The flask was covered with a watch glass and leftfor 16 h (overnight). A reflux condenser (a Dimroth type) ofabout 40 cm length was mounted on the top of the flask and thesolution gently boiled under reflux for 2 h. After the sampleshad cooled down, the condenser was rinsed with 10 mL of 2%(v/v) Aristar HNO 3 and the rinses were collected in the roundbottomedflask. The condenser was removed and the samplefiltered through Whatman No. 542 filter papers to remove anyundigested solid material. The filtrate was collected in a 50 mLvolumetric flask. The filter and the residue were rinsed threetimes with 5 mL of 2% (v/v) Aristar HNO 3 and the solution wasthen made up to 50 mL with 2% (v/v) Aristar HNO 3 prior tofinal analysis by ICP-OES.2.2.4. HNO 3 /HF (adapted USEPA 3052)A modified version of the microwave-assisted USEPAmethod 3052 protocol [31] was employed to achieve total dissolutionof peat material. As before (Sections 2.2.1 and 2.2.2), arepresentative sample of 0.25 g was ashed at 450 ◦ C for 4 h priorto digestion in 9 mL concentrated HNO 3 (Aristar) and 0.5 mLconcentrated HF (Aristar, Merck, Poole, UK) in the microwavedigestion system. Based on the USEPA method 3052 protocol,the amount of HF required is less than 1 mL when the samplecontains less than 20% silicate. So, the amount of HF used inthis procedure was 0.5 mL (for approximately 0.25 g sample)to ensure that all aluminosilicates present were completely dissolved.Thereafter, with the exception that a Teflon beaker wasused in the evaporation stage, the procedure was the same as inSections 2.2.1 and 2.2.2.2.3. Analysis2.3.1. Reagents and standardsHigh purity 18.2 M cm water (Milli-Q Element system,Millipore, Watford, UK) was used throughout. Acids (HNO 3 ,HCl, HF) employed were all of Aristar quality. All volumetricware and digestion vessels were acid washed and rinsed withMilli-Q water.Standards for ICP-OES calibration were prepared from1000 mg L −1 stock solutions (Fisher Scientific UK, Loughborough,UK).2.3.2. ICP-OESThe determination of elemental concentrations in the digestedCanadian peat OGS 1878 P-6 samples was carried out by ICP-OES using a Thermo Jarrell Ash IRIS instrument (ThermoElemental, Cambridge, UK) fitted with a cross-flow nebuliserand a cyclonic spray chamber. Table 1 summarises the instrumentaloperating conditions and Table 2 lists the selected analyticalemission lines and estimated solution detection limits. Forconstruction of the emission intensity/concentration calibrationcurves, concentrations of 1.00, 10.0 and 100 mg L −1 of all measuredelements in standard solutions prepared in 2% (v/v) HNO 3Wavelengths and detection limits for elemental analysis by ICP-OESWavelength(nm)Detection limit a(gL −1 )Al 308.2 30 3Co 228.6 5 0.5Cr 283.5 5 0.5Cu 224.7 4 0.4Fe 238.2 4 0.4Mn 257.6 1 0.1P 214.9 51 5Pb 216.9 28 3S 182.0 50 5Ti 338.3 5 0.5V 311.0 5 0.5Zn 213.8 2 0.2Method detectionlimit (mg kg −1 )a The solution detection limit was taken to be three times the standard deviationof the mean of 10 analyses of the sample blank.were used. Reagent and sample processing blanks were includedin each batch of analyses for quality control purposes and 2%(v/v) HNO 3 was used as the rinse solution between samples.2.4. Application to other samplesUltimately, after consideration of results (Section 3.1) fromthe comparative study of the four digestion procedures, theadapted USEPA method 3051 (HNO 3 ) and the adapted USEPAmethod 3052 (HNO 3 /HF) were selected for application to severaldifferent materials: (i) coal NBS SRM 1635, for which certifiedvalues of Al and Ti were available; (ii) NIMT/UOE/FM/001,the new ombrotrophic peat bog (low ash) reference materialdeveloped in our laboratory [26]; (iii) individual 1 cm sections ofa 43 cm core, FM99M, collected on 8th July 1999 by the monolithcoring technique from the ombrotrophic peat bog, FlandersMoss, in central Scotland [32].3. Results and discussion3.1. Canadian peat OGS 1878 P-6The concentrations of eight elements (Al, Co, Cr, Cu, Fe,Mn, Pb and Zn) determined in aliquots (n =7−9) of the Canadianpeat OGS 1878 P-6 reference material prepared accordingto the four digestion procedures are summarised and comparedwith ‘certified’ values in Table 3. Analysis was not carried out forP, S, Ti and V. The acid-extractable procedures (i.e. HNO 3 andthe two HNO 3 /HCl methods) generally yielded similar values tothose of the total digestion method (HNO 3 /HF) for all of the elementsdetermined except Al. The total Al concentration obtainedby the adapted USEPA method 3052 (HNO 3 /HF) was clearlyhigher than the acid-extractable concentrations, reflecting therequirement for HF to completely dissolve the aluminosilicatespresent in this comparatively high-ash peat material. Ourtotal Al value of 6000 ± 350 mg kg −1 is intermediate betweenthe 4500 ± 300 mg kg −1 (ICP-OES) and 7500 ± 100 mg kg −1(INAA) values recently reported by Sapkota et al. [25]. Theagreement for the other elements probably arises from the
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