Wastewater Standards and Extraction Chemistry in Validation of Microwave-Assisted EPA Method 3015A

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1 Environ. Sci. Technol. 1999, 33, Wstewter Stndrds nd Extrction Chemistry in Vlidtion of Microwve-Assisted EPA Method 3015A DIRK D. LINK, PETER J. WALTER, AND H. M. KINGSTON* Deprtment of Chemistry nd Biochemistry, 308 Mellon Hll, Duquesne University, Pittsurgh, Pennsylvni The difficulties ssocited with the control nd trnsfer of environmentl lech methods re discussed. Optimized EPA Method 3015A, microwve-ssisted lech of wstewter nd drinking wter mtrices nd queous extrcts, is evluted. The option to dd HCl in ddition to HNO 3 provides etter complextion nd recovery of certin metls tht re regulted y the Resource Conservtion nd Recovery Act (RCRA) thn the originl HNO 3 -only Method Also discussed is the preprtion nd pproprite use of simulted wstewter stndrds. Stndrd reference mterils for wstewter mtrix re unville, nd this novel pproch provides NIST-trceility of results for the first time on this mtrix type. Lech concentrtions from these simulted stndrds were determined using oth the 5 ml HNO 3 nd the 4 ml HNO 3 nd1ml HCl lech options of new Method 3015A. Vlidtion of the new mixed-cid option of Method 3015A hs een provided y evluting its performnce on the 23 elements for which originl Method 3015 ws vlidted. In ddition, vlidtion is provided for oron, mercury, nd strontium, elements tht were not vlidted in originl Method Method 3015A hs een developed into method cple of evluting 26 elements in single, efficient, 20-min procedure. Introduction Lech methods pose unique quntifiction prolems when compring nlyte concentrtions. In contrst to totl decomposition methods, which give solute quntities of nlyte levels, lech methods provide only reltive concentrtions sed on the procedure used nd conditions chieved. Elementl lech methods re cid extrctions which re ment to provide mesure of nlyte soluiliztion under worst-cse scenrio environmentl conditions. Lech results re highly dependent on cid chemistry nd extrction conditions. Becuse no solute nswer is provided y lech methods, the precision of the results is used s n expression of the ccurcy. The dependence of lechle nlyte concentrtions on extrction conditions (1) mkes the reproducile control nd trnsfer of lech methods much more difficult thn for totl decomposition methods. Microwve digestion with temperture feedck control hs demonstrted more precise temperture control thn is possile on hot plte nd is more pproprite for * Corresponding uthor phone: (412) ; fx: (412) ; e-mil: kingston@duq.edu. TABLE 1. Comprison of EPA SW-846 Lech Methods for Aqueous Smples mtrices regents EPA Method 3015 queous smples nd extrcts 45 ml queous smple 5mLofHNO 3 EPA Method 3015A queous smples nd extrcts drinking wter 45 ml queous smple 5mLofHNO 3 or 4mLofHNO 3 + 1mLofHCl het to 170 ( 5 C heting profile het to 160 C in 10 min in 10 min slow rise to 170 C hold 170 ( 5 C in next 10 min for 10 min nlytes 23 elements 26 elements (not vlidted for (B, Hg, Sr dded) B, Hg, Sr) reproducile control nd trnsfer of environmentl lech methods. The Environmentl Protection Agency s SW-846 mnul of methods includes one microwve-ssisted cid lech method for the preprtion of queous smples nd extrcts for susequent elementl nlysis. This method, EPA Method 3015, is closed vessel microwve lech vlid for the nlysis of 23 elements. The dvntges of using closed vessels nd microwve heting hve een discussed (2, 3). The dded dvntge of reproducile temperture control, typiclly (2.5 C, mkes microwve heting the method of choice for optimum control nd trnsfer of stndrd methods mong nlysts, lortories, nd even foreign countries, compred to the wide vriility nd sujectivity ssocited with hotplte methods. Method 3015 hs een promulgted smple preprtion method for severl yers (4, 5). Originlly, this method ws limited y the EPA to the exclusive use of HNO 3. A HNO 3-HCl mixed-cid lech ws voided to reduce the potentil for interferences from HCl nd its resulting species on some nlyticl detection methods such s grphite furnce tomic sorption spectroscopy (GFAAS) nd inductively coupled plsm mss spectrometry (ICP-MS). However, y restricting the regents used in lech methods to HNO 3, limits re plced on the chemistry of the method (2). It hs een shown tht some nlytes re unstle either in the oxidizing environment of n HNO 3-only lech (6-11) or in environments contining trce mounts of chlorides (12). Solution stilities nd recoveries of these nlytes cn e incresed y rising the concentrtion of chlorides in the lech solution. To enhnce recoveries of certin nlytes through complextion y HCl, the new Method 3015A, which provides the option to perform either HNO 3-only or mixedcid HNO 3-HCl lech, ws proposed for the leching of queous smples nd extrcts. Tle 1 compres the protocols of originl Method 3015 nd the recently updted Method 3015A. This work descries the importnce of reproducile control nd trnsfer of lech methods nd evlutes the performnce of oth the HNO 3-HCl mixed-cid option nd the HNO 3-only option of the new EPA Method 3015A. One gol ws to provide vlidtion of the new method using severl stndrds consisting of vrious mtrix types. Concurrent with this vlidtion is discussion of the preprtion nd use of simulted wstewter stndrds consisting of stndrd reference mterils (SRMs) mixed with 18 MΩ cm wter. The sence of suitle wstewter stndrd /es990092w CCC: $ Americn Chemicl Society VOL. 33, NO. 14, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY Pulished on We 06/09/1999

2 reference mteril for these vlidtion studies prompted the development nd evlution of these simulted SRMwstewter stndrds. A second gol ws to provide method vlidtion for the elements oron, mercury, nd strontium, three nlytes which were never vlidted for the originl Method This study shows tht the mixed-cid option provides n pproprite chemistry for the recovery of 26 elements using single, 20-min microwve lech. Experimentl Section Microwve leches were performed using the MLS Meg with the Microwve Digestion Rotor MDR-300/10 from Milestone USA (Monroe, CT) nd the MDS-2100 with HDV (hevy duty vessels) from CEM Corp. (Mtthews, NC). Concentrted cids (ACS regent grde) were purchsed (Fisher Scientific, Pittsurgh, PA) nd were purified y suoiling distilltion using DuoPUR qurtz still (Milestone USA, Monroe, CT) prior to use. The SRMs used in this study, SRM 2704 (Bufflo River Sediment), SRM 4355 (Peruvin Soil), SRM 1084 (Wer-Metls in Luricting Oil), nd SRM 1634c (Trce Metls in Fuel Oil), were purchsed from the Ntionl Institute of Stndrds nd Technology (NIST) in Githersurg, MD. These SRMs were lso used in the study which vlidted EPA Method 3051A (2, 13) nd the round-roin study vlidting EPA Method 3051 ginst Method 3050B (14, 15). Therefore, n undnce of dt exists for comprison nd vlidtion of updted Method 3015A. To minimize the potentil for cross-contmintion, ll microwve vessels nd lwre were clened using either hot or room-temperture cid clening procedures etween uses. All procedures prior to nd following microwve leching were performed in clss 100 clen lortory (3). Prior to vlidtion of Method 3015A, n pproprite stndrd mteril hd to e developed. At the time this method ws under development, no SRMs representtive of wstewter mtrix were ville from NIST. Typiclly, wstewter mtrix is composed of n queous portion long with some percentge of n ssocited solid portion, which is usully suspended mtter nd some dissolved frction. This composition increses the complexity ssocited with quntifying lech results, which re ment to show the quntities of nlyte tht my ecome ville from the entire mtrix under the worst-cse scenrio conditions simulted y concentrted nitric (nd hydrochloric) cids. However, when the solid component of the wstewter is soil, sediment, or similr mtrix tht is susceptile to environmentl regents, elements my ecome ville from oth the solid portion nd the queous portion. Therefore, some wy of quntifying the lech recoveries from ech component of the wstewter ginst certified stndrd ws needed. For this reson, simulted wstewter stndrds were creted using solid SRMs nd ultrpure wter. These simulted stndrds were prepred y mixing n pproprite mount (typiclly round 400 mg) of the solid SRM with 45 ml of 18 MΩ cm wter in ech microwve vessel. Ech stndrd ws prepred individully rther thn prepring lrge tch of ulk wstewter stndrd nd using liquots of tht smple. Susmpling from slurry my e pproprite in the preprtion of new stndrd ut not in the use of these lredy-certified stndrds ecuse of smple heterogeneity issues. Susmpling would led to possile ltertion of the size nd content frctions of the certified mteril nd led to questionle trnsfer of the SRM. The degree of heterogeneity of solid SRMs is wellestlished nd is reported on the certificte of nlysis. By using mss greter thn the minimum certified homogeneous mount of SRM (pproximtely 250 mg) in ech vessel, representtive smple tht conforms to the certificte of nlysis ws produced in ech cse. Therefore, the trceility of the totl concentrtion in the smple is mintined. When FIGURE 1. Typicl temperture nd pressure profile for the leching of simulted wstewter smple ( 400 mg of SRM 2704 in 45 ml of 18 MΩ cm H 2O) using oth digest options of EPA Method 3015A (13). ttempting to susmple from lrger quntity of wstewter (or similr slurry-type of mtrix), it would e inherently more difficult to gurntee tht ech liquot contined enough prticles to ensure tht heterogeneity prolems re minimized. Minimizing heterogeneity prolems is especilly importnt when ttempting to sttisticlly vlidte the new method. Wstewters of three mtrix types were creted: sediment-wstewter (using SRM 2704), soil-wstewter (SRM 4355), nd n oil-wstewter (SRM 1084). These smples were leched using oth regent options of Method 3015A, ccording to method protocol (Tle 1). After pproprite filtrtion nd smple dilution, elementl nlysis ws performed using either flme tomic sorption spectroscopy (FAAS) (AAS 1100, Perkin-Elmer) or ICP-MS (PlsmQud II, VG Elementl). In ddition, mercury nlysis ws performed using the dedicted Hg nlyzer DMA-80 (Milestone USA, Monroe, CT). These Hg determintions followed procedures documented in drft Method 7473 in Updte IVA of EPA s SW-846 mnul of methods (4, 16, 17). Results nd Discussion Generl Sfety. The typicl temperture nd pressure profiles for oth the HNO 3-only nd the HNO 3-HCl leches of the SRM 2704-wstewter smple re shown in Figure 1. For comprison s ske, the profile for the heting of SRM 2704 in 18 MΩ cm H 2O is lso shown. Addition of cids cuses the pressure to increse y pproximtely 8 tm owing to the production of severl rective, gseous species upon heting. These species include chlorine gs (Cl 2) nd nitrosyl chloride (NOCl), which further decomposes into the gseous products Cl 2 nd NO (2, 3, 18, 19). It should e noted tht these conditions remin similr regrdless of the mtrix type used to crete the wstewter (i.e. soil-type wstewter or oiltype wstewter) nd regrdless of the cid rtio used in the lech. Also, the pressures ttined inside the vessel re highly dependent on the mteril nd thickness of the vessel nd csing components. Vessels from different mnufcturers nd different designs hve different het loss chrcteristics nd resultnt pressures t given temperture, s discussed elsewhere (2, 3). From sfety stndpoint, the pressures reched during the wstewter lech require vessels hving pressure cpilities of t lest 30 tm (435 psi). Prior vessel specifiction in Method 3015 ws 10 tm (110 psi). Another sfety precution specified in new Method 3015A is tht queous smples cnnot contin more thn 1% suspended solids or oxidizle mteril. This precution ws dded to prevent nlysts from mistkenly ttempting to lech 45 ml of highly rective, orgnic mtrix (for exmple, dding concentrted HNO 3 nd HCl to 45 ml of light liquid ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 14, 1999

3 oil or lcohol smple nd heting). Rections of this type re dngerous ecuse of the extreme mount of pressure tht would e produced in very short time. The shock wves resulting from this rection could potentilly led to ctstrophic vessel filure or n explosion. Sfety issues regrding microwve heting hve een recently compiled nd reviewed nd re ddressed in detil elsewhere (20). According to method specifictions, the mximum mount of orgnic solids present should e no more thn 0.45 g. Also, should vigorous rection occur upon ddition of the cids, the nlyst is instructed to tret the smple s potentil sfety hzrd nd to use nother method of smple preprtion such s EPA Methods 3051A or Vessel Mterils. In ddition to vessel pressure cpilities, the type of equipment specified in the method hs lso een updted. This is to llow the use of vessels constructed from dvnced mterils. The originl Method 3015 specified tht the digestion vessels e mde of fluorocron PFA or TFM mteril (4). Updted Method 3015A specifies only tht vessels e composed of microwve trnsprent nd regent resistnt mterils nd gives PFA, TFM, nd qurtz s exmples of vessel liner mterils (4). This permits the method to e virtully independent of microwve equipment nd mnufcturer, provided tht the equipment meets the minimum sfety nd performnce requirements of the method. Heting Profile. As shown in Tle 1, the temperture profile specified in Method 3015A hs lso een chnged. The profile previously specified in Method 3015 ws to het the smples to 160 ( 4 C in 10 min nd then to llow the smples to slowly rise to C during the next 10 min. The new specifiction in Method 3015A is to het the smples to 170 ( 5 C in 10 min nd to hold the smples t 170 ( 5 C for the remining 10 min. Not only does this new profile ccount for different mnufcturers methods of temperture control ut lso it llows more precise reproduction of the lech heting profile. Some mnufcturers tret the temperture specifiction s n upper limit. Microwve power is turned off once the controlling temperture hs een reched nd turned on fter the temperture hs dropped severl degrees elow the control temperture. Other mnufcturers tret the controlling temperture s men vlue, turning the power on nd off to mintin the temperture in certin rnge out the men controlling temperture. The new specified temperture of 170 ( 5 C llows etter complince y microwve mnufcturers. In nother chnge, the slow rise heting step in originl Method 3015 ws removed. Under the former conditions, it ws up to the nlyst to decide wht ws considered to e slow rise in temperture from 160 C up to 170 C, dding unwnted sujectivity to the method. With updted Method 3015A, reching nd holding the sme temperture llows more precise control of heting. This is importnt ecuse control of temperture hs een determined to e one of the most importnt prmeters in otining precise lech results (3). And since precision is used s ccurcy when compring lech results, this precise temperture control ecomes even more significnt. Development of Stndrds. As this ws the first time tht simulted wstewters were creted nd used s stndrds, the vlidity of this pproch ws evluted. Tle 2 shows the results using the mixed-cid option of queous microwve lech Method 3015A, compred to mixed-regent lech Methods 3051A nd 3050B for smples of SRM Methods 3051A nd 3050B re promulgted lech methods for solid mterils with n undnce of dt which cn e used to compre nd vlidte the procedure for creting simulted stndrds. Despite the presence of 45 ml of wter in Method 3015A, ll three lech methods consistently extrct the sme mount of metls from the mtrix t the 95% confidence TABLE 2. Comprison of Lechle Anlyte Concentrtions from SRM 4355 (Peruvin Soil) nd SRM 4355-Wstewter Using EPA Lech Methods 3015A, 3051A, nd 3050B (2, 13-15) element 3015A Lech 3051A Lech c 3050B Lech d Co 10.4 ( ( ( 2.8 Cd 1.28 ( ( ( 0.08 Cr 17.7 ( ( ( 0.9 P 136 ( ( ( 5 Ni 10.8 ( ( ( 1.45 Method 3015A smple ws 400 mg of SRM 4355 mixed with 45 ml of 18 MΩ cm H 2O; Methods 3051A nd 3050B smples were 500 mg of SRM Results reported in men µg/g nlyte (95% confidence intervl. c Results from Method 3051A vlidtion study (13). d Results from Method 3051 vs Method 3050B vlidtion study (14, 15). TABLE 3. Comprison of Lechle Anlyte Concentrtions from SRM 2704 (Bufflo River Sediment)-Wstewter Using Both Digest Options of EPA Method 3015A (13) element 5:0 Lech 4:1 Lech totl Ag 0.31 ( ( 0.09 <4 Co 12.0 ( ( ( 0.6 Cu 92.7 ( ( ( 5.0 Ni 39.6 ( ( ( 3.0 P 155 ( ( ( 17 V 6.18 ( ( ( 4 Zn 418 ( ( ( 12 Results reported in men µg/g nlyte (95% confidence intervl. Totl certified nlyte concentrtion tken from NIST SRM Certificte of Anlysis. intervl. These results demonstrted for the first time tht the pproch of creting simulted wstewters contining some mtrix-specific solid component ws pproprite. This pproch llows nlysts to crete simulted wstewter stndrds using solid reference mterils tht closely mtch the solid component of the smple wstewter mtrix. This pproch lso provides NIST-trcele lech results for stndrd of mtrix type which previously did not exist. For exmple, reference stndrd for wstewter consisting of suspended oil prticles could e simulted y mixing comprle mount of n oil SRM with ultrpure wter. The sme could e done with soils, sediments, nd other specific solids. Most environmentl SRMs contin frctions of cronceous mteril, chlorides, etc., portion of which my trnsfer to the queous portion when mixed with wter. Hving species present in oth the solid nd queous portions is typicl of wht my e expected with n ctul wstewter smple, thus providing stndrd tht is truly representtive of this smple type. Lech Chemistries. As mentioned, certin nlytes re not stle either in n oxidizing environment (S) or in environments contining trce quntities of chlorides (Ag). The chemistry of HNO 3-only lech is deficient when it comes to this type of nlyte. However, y including the option to dd HCl in the lech method, etter recoveries re chieved for those nlytes which re complexed y Cl - nd therefore more stle in solution. Specific discussion of the chemistry of these elements s it pertins to lech recoveries, including pertinent chemicl rections, nd overll improvement of the chemistries of lech methods hs een descried in detil elsewhere (2). Lech Recoveries. Lech recoveries using oth regent options of Method 3015A for the sediment-wstewter mtrix re shown in Tle 3, while recoveries for the soil-wstewter re shown in Tle 4. For oth of these similr mtrix types, the results demonstrte tht recoveries of some prolem VOL. 33, NO. 14, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

4 TABLE 4. Comprison of Lechle Anlyte Concentrtions from SRM 4355 (Peruvin Soil)-Wstewter Using Both Digest Options of EPA Method 3015A (13) element 5:0 Lech 4:1 Lech totl Ag 1.31 ( ( 0.11 (1.9) Cd 1.30 ( ( 0.33 (1.5) Co 10.5 ( ( ( 0.76 Cu 70.0 ( ( ( 4.7 Mo 0.99 ( ( 0.11 (1.7) Ni 12.2 ( ( 1.9 (13) S 3.7 ( ( ( 2.2 Results reported in men µg/g nlyte (95% confidence intervl. Totl certified nlyte concentrtion tken from NIST SRM Certificte of Anlysis. Vlues in prentheses indicte reference concentrtions. TABLE 5. Comprison of Lechle Anlyte Concentrtions from SRM 1084A (Wer-Metls in Luricting Oil)-Wstewter Using Both Digest Options of EPA Method 3015A (13) element 5:0 Lech 4:1 Lech totl Al 102 ( ( 7.0 (104) Cr 100 ( ( ( 0.8 Cu 93.6 ( ( ( 1.9 Mg 87.3 ( ( ( 1.7 Mo 83.8 ( ( ( 1.4 Ni 92.9 ( ( ( 1.6 P 88.0 ( ( ( 1.3 Results reported in men µg/g nlyte (95% confidence intervl. Totl certified nlyte concentrtion tken from NIST SRM Certificte of Anlysis. Vlues in prentheses indicte reference concentrtions. nlytes (e.g. Ag, S, V) hve incresed with the mixed-cid lech, while recoveries of other nlytes (e.g. Co, Ni, P) hve een preserved (i.e. losses hve not occurred). Lech recoveries for the oil-wstewter mtrix re shown in Tle 5. Recoveries using either lech option demonstrte sttisticl equivlence t the 95% confidence intervl, signifying tht the mixed-cid lech does not introduce negtive is in recoveries. Expnded Anlyte Vlidtion. Originlly, EPA Method 3015 ws not vlidted for the preprtion nd susequent evlution of oron, mercury, nd strontium. This mens tht some other smple preprtion method in ddition to Method 3015 would hve to e used for nlysis of these three elements. At one time, it ws cceptle to require mny different test methods to e used to evlute single wste mteril. Tody, the preferred strtegy is the development of more roust procedures cple of evluting ll the inorgnic elements of interest. Tle 6 demonstrtes the lech recoveries of these three nlytes using oth regent options for the sediment-wstewter nd the soil-wstewter. The recoveries using Method 3015A re compred to those using promulgted EPA Method 3051A to lech the solid sediment or soil smple. Becuse Method 3051A is vlidted for B, Hg, nd Sr, it ws used to vlidte the wstewter results. The tle shows tht recoveries for ll three nlytes re sttisticlly equivlent t the 95% confidence intervl. Similr recoveries re provided y oth the HNO 3-only nd the mixed-cid wstewter lech, demonstrting tht either regent comintion is pproprite for the nlysis of these three nlytes. Thus, the list of vlidted nlytes for wstewter preprtion hs een updted nd now includes 26 elements. Method Control nd Trnsfer. Becuse of the specil conditions under which lech results re compred, the reproduciility of the rection conditions is criticl. Also, it is the prctice of some testing ls nd gencies in foreign TABLE 6. Performnce nd Vlidtion of Method 3015A versus EPA Method 3051A for the Recovery of Previously Nonvlidted RCRA-Regulted Elements (13, 16) element 5:0 Lech 4:1 Lech EPA 3051A SRM 2704-Wstewter B 23.8 ( ( ( 4.4 Hg 1.49 ( ( ( 0.07 Sr 41.9 ( ( ( 4.5 SRM 4355-Wstewter B 32.9 ( ( ( 5.6 Hg 0.82 ( ( ( 0.15 Sr 136 ( ( ( 19.1 Results reported in men µg/g nlyte (95% confidence intervl. EPA Method 3051A leches were performed using the 9 ml of HNO 3 to 3 ml of HCl cid rtio option on 0.35 g smples of solid SRM 2704 nd SRM countries to dopt EPA methods s their stndrd methods. Thus, the development of reproducile stndrd methods hs oth ntionl nd worldwide importnce. This requires stndrd methods which provide reproducile control of extrction conditions nd ccurte trnsfer. Use of microwve energy nd temperture feedck control exceeds other methods in terms of the reproduciility of conditions nd ese of method trnsfer from one user to nother. Temperture, the prmeter which controls rection rtes, cn e mintined more ccurtely, resulting in more precise results. Summry The usefulness nd pplicility of creting specific types of simulted wstewter stndrds y mixing solid SRMs nd 18 MΩ cm wter hs een demonstrted. This technique estlishes NIST-trcele stndrds for mtrix which previously ws unville. This pproch my lso prove to e useful with reference mterils from other interntionl stndrds orgniztions, thus permitting wstewter methods to provide interntionlly referencele results. Also shown is the effectiveness of dding HCl in Method 3015A to enhnce the recoveries of specific nlytes for the leching of queous smples (wstewters, drinking wters, nd queous extrcts). Lech dt for three simulted wstewter mtrices using the mixed-cid lech Method 3015A shows tht this regent comintion provides enhncement in recoveries for prolem nlytes while preserving the recoveries of other nlytes. Thus, the mixed-cid option llows the nlyst to improve the chemistry of the wstewter lech method for recovery of certin nlytes, when necessry. In ddition, B, Hg, nd Sr hve een vlidted nd dded to the nlyte list for Method 3015A. This improves the efficiency of the method nd permits the smple preprtion for nlysis of 26 elements with single, reproducile, 20-min microwvessisted lech procedure. Literture Cited (1) Florin, D.; Brnes, R. M.; Knpp, G. Fresenius J. Anl. Chem. 1998, 362, (2) Link, D. D.; Wlter, P. J.; Kingston, H. M. Environ. Sci. Technol. 1998, 32, (3) Kingston, H. M.; Wlter, P. J.; Chlk, S. J.; Lorentzen, E. M.; Link, D. D. Environmentl Microwve Smple Preprtion: Fundmentls, Methods, nd Applictions. In Microwve Enhnced Chemistry: Fundmentls, Smple Preprtion, nd Applictions; Kingston, H. M., Hswell, S., Eds.; Wshington, DC, 1997; Chpter 3. (4) U.S. E.P.A. Test Methods for Evluting Solid Wste, Physicl/ Chemicl Methods; SW-846, 3rd ed.; U.S. Government Printing Office: Wshington, DC, (5) U.S. E.P.A. Fed. Regist. 1995, 60, (6) Hewitt, A. D.; Reynolds, C. M. Atom. Spectrosc. 1990, 11, ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 14, 1999

5 (7) Hewitt, A. D.; Crgin, J. H. Environ. Sci. Technol. 1991, 25, (8) Hewitt, A. D.; Crgin, J. H. Environ. Sci. Technol. 1992, 26, (9) Bjo, S.; Suter, U. Anl. Chem. 1982, 54, (10) Berry, S.; Brett, M. E. Inorg. Chim. Act 1984, 83, (11) Gllorini, M.; Greenerg, R. R.; Gillis, T. E. Anl. Chem. 1978, 50, (12) Bock, J. S. A Hndook of Decomposition Methods in Anlyticl Chemistry, 1st ed.; John Wiley nd Sons: New York, (13) Link, D. D.; Kingston, H. M.; Wlter, P. J. Presented t the Pittsurgh Conference, Atlnt, GA, 1997; Pper (14) Binstock, D. A.; Yeger, W. M.; Grohse, P. M.; Gskill, A. Vlidtion of Method for Determining Elements in Solid Wste y Microwve Digestion; 321U ; Reserch Tringle Institute: (15) Binstock, D. A.; Grohse, P. M.; Gskill, A.; Sellers, C.; Kingston, H. M.; Jssie, L. B. J. Assoc. Off. Anl. Chem. 1991, 74, (16) Boyln, H. M.; Wlter, P. J.; Kingston, H. M. Presented t the Pittsurgh Conference, New Orlens, LA, 1998; Pper 656. (17) U.S. E.P.A. Fed. Regist. 1998, 63, (18) Kingston, H. M.; Wlter, P. J. Spectroscopy 1992, 7, (19) Cotton, F. A.; Wilkinson, G. Advnced Inorgnic Chemistry, 5th ed.; John Wiley & Sons: New York, (20) Kingston, H. M.; Wlter, P. J.; Engelhrt, W. G.; Prsons, P. J. Lortory Microwve Sfety. In Microwve-Enhnced Chemistry: Fundmentls, Smple Preprtion, nd Applictions; Kingston, H. M., Hswell, S. J., Eds.; Americn Chemicl Society: Wshington, DC, 1997; Chpter 16. Received for review Jnury 27, Revised mnuscript received April 26, Accepted My 5, ES990092W VOL. 33, NO. 14, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY