Evaluation of inductively coupled plasma optical emission spectrometers with axially and radially viewed configurations

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1 Spectrochimic Act Prt B 57 (2002) Evlution of inductively coupled plsm opticl emission spectrometers with xilly nd rdilly viewed configurtions,b b Fernndo V. Silv, Lilin C. Trevizn, Cınti S. Silv, An Rit A. Nogueir, Joquim A. Nobreg, * Grupo de Anlise Instrumentl Aplicd, Deprtmento de Quımic, Universidde Federl de So Crlos, Cix Postl 676, So Crlos, SP, Brzil b Embrp Pecuri Sudeste, So Crlos, SP, Brzil Received 14 My 2002; ccepted 27 August 2002 Abstrct The nlyticl performnce of two inductively coupled plsm opticl emission spectrometers with xilly nd rdilly viewed configurtions, equipped with chrge coupled device solid-stte detectors ws evluted using Ar, B, Mg, nd Ni s test elements. Both instruments hve similr Echelle opticl rrngements nd rdio-frequency genertors, differing only in the viewing mode nd in the dimeter of the centrl injector tube, i.e. 2.3 for the xilly nd 1.4 mm for the rdilly viewed ICP. The figures of merit evluted were: wrm-up time, short- nd long-term stbilities, UV nd VIS spectrl resolution nd limit of detection (LOD) for Ni in 0.14 moll HNO3 nd 1000 mg l Cr medi, respectively. The influence of residul crbon content on bckground equivlent concentrtion (BEC) nd sensitivity ttined for Al, As, C, Cd, Co, Cr, Cu, Fe, Mg, Mn, Se nd Zn ws lso investigted in both viewing modes. The robustness, short- nd long-term stbilities, nd UV nd VIS spectrl resolutions were similr for both configurtions. For the rdilly viewed equipment, the wrm-up time ws t lest fctor of 2 shorter thn tht for the xilly viewed configurtion. On the other hnd, the sensitivity ttined for Ni with the xilly viewed rrngement ws pproximtely 20 times better thn tht with the rdilly viewed. In both viewing modes, nd for most of the evluted elements, the vlues of BEC nd LOD were similr for ll studied crbon concentrtions (from 10 to mg lc) when working with robust plsm conditions. The Mg IIyMg I rtio t n pplied power of 1.3 kw nd nebulizer gs flow-rte of 0.90 nd 0.70 l min for xilly nd rdilly viewed configurtions were 10.6 nd 13.7, respectively. Quntittive determintions were successfully performed using both systems Elsevier Science B.V. All rights reserved. Keywords: Figures of merit; Axilviewing; Rdilviewing; ICP OES This pper ws presented t the 7th Rio Symposium on Atomic Spectrometry, held in Florinopolis, Brzil, April 2002 nd is published in the Specil Issue of Spectrochimic Act Prt B, dedicted to tht conference. *Corresponding uthor. Tel.: q ; fx: q E-mil ddress: djn@terr.com.br (J.A. Nobreg) /02/$ - see front mtter 2002 Elsevier Science B.V. All rights reserved. PII: S Ž

2 1906 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Introduction Inductively coupled plsm opticl emission spectrometry (ICP OES) is well estblished method for multielementl nlysis. This method hd rebirth in the lst decde due to the development of equipments with xilly viewed configurtion nd solid-stte detectors tht improved flexibility nd sensitivity w1x. The incorportion of segmented chrged device or chrge coupled device (CCD) solid-stte detectors enbled to rech spectrl resolution, liner response rnge nd simultneous reding of wide rnge of electromgnetic spectrum, required by n ICP OES. The use of this kind of detector becme possible fter development of new mterils in the semi-conductors technology w2,3x. The ide of xilly viewed mesurements ws proposed in the 1970s w4x, however, in spite of n improvement in sensitivity, the system performnce ws degrded due to incresed interferences. After two decdes this proposl becomes vible due to the development of proper interfce systems bsed on sher gs or end-on gs w1x. However, common sense in this re still recommends ICP OES with xilly viewed configurtion for pplictions requiring better sensitivity, but without complex mtrix. For smples with complex mtrices, the best option ppers to be to use n ICP OES with rdilly viewed configurtion. This generlopinion needs more criticl evlution. Dubuisson et l. w5x showed tht neither the effect cused by esily ionizble elements, nor the gin in sensitivity ws so impressive in n ICP OES with xilly viewed configurtion. These uthors employed three ICP OES systems with different configurtions. Ivldi nd Tyson w6x presented similr conclusions nd pointed out tht n xilly viewed plsm might be successfully used when lower detection limits re desired, but reduced plsm robustness could be problem. In recent comprehensive review, Brenner nd Znder w1x pointed out tht the interfces used in ICP OES with xilly viewed configurtion should be criticlly compred to gin better understnding of the performnce of ech rrngement. Similrly, ICP OES with the sme opticlrrngement nd solid-stte detector, but with different torch positions should lso be compred to generte criticl dt bout the performnce of ech system. The work here described delt with this evlution using plsm dignostics ccording experiments proposed by Mermet w7x, Pousselet l. w8x, nd Mermet nd Poussel w9x. 2. Experimentl 2.1. Instrumenttion Two simultneous ICP OES instruments with xilly nd rdilly viewed configurtions Vist AX-CCD nd RL-CCD (Vrin, Mulgrve, Austrli) were used in this investigtion. Both systems re equipped with simultneous CCD solid-stte detector tht llows mesurements from 167 to 785 nm. The pre-opticlsystems re purged with rgon in order to enble redings below 180 nm. The polychromtor ws thermostted t 34 8C nd purged with rgon. An end-on gs interfce ws used in the xilly viewed configurtion to minimize interferences cused by self-bsorption nd recombintion, to provide wide liner dynmic rnge nd low bckground. In the rdilly viewed ICP OES, snout ws used to purge the environment round the plsm with rgon. The sme smple introduction system, composed by cyclonic spry chmber nd concentric nebulizer, ws employed in both systems in order to obtin comprble results. However, the injector tube dimeter of the torch used in ech equipment ws different; i.e. 2.3 mm in the xilly nd 1.4 mm in the rdilly viewed configurtion. The observtion height used in the rdilly viewed configurtion ws djusted in order to rech the highest signl-to-bckground rtio (SBR) for most elements. The observtion zone in the xilly viewed system ws utomticlly set by the progrm supplied with the Vist AX control softwre using the Mn II nm emission line. The detiled description of both spectrometers is presented in Tble 1 nd opertionl prmeters re listed in Tble 2. All emission signls were integrted using the utomted dpttive integrtion time tht is performed in VISTA equipments ccording to line intensities nd nlyte concentrtions. All mes-

3 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Tble 1 Chrcteristics of Vist AX nd RL ICP OES Chrcteristic Genertor frequency Opticl sytem Polycromtor Grting density groove Focllength Entrnce slit Smple introduction system Nebulizer Spry chmber Detector Torch injector tube dimeter Vist AX nd RL 40 MHz Echelle-grtingqCF2 cross dispersing prism 95 grooves mm 400 mm Heights0.029 mm nd widths0.051 mm Concentric Cyclonic Peltier cooled CCD pixels spred cross 70 non-liner rrys Wvelength rnges nm 2.3 (xilly viewed) nd 1.4 mm (rdilly viewed) urements were performed using two pixels for ech wvelength. Microwve-ssisted cid digestion of two certified reference mterils ws performed using cvity-type oven (ETHOS 1600, Milestone, Sorisole, Itly) equipped with ten closed perfluorlkoxy Teflon vessels with pressure relief mechnism. The power pplied ws controlled by monitoring the temperture in single vessel using fiber opticlsensor Solutions A multielement solution contining 5.0 mg l B nd Mg, nd 10 mg lni in 0.14 mo l HNO3 medium ws used to evlute wrm-up time, short nd long-term stbilities, robustness, UV nd VIS spectrl resolutions nd the limit of detection (LOD) for Ni II 231 nm ttined by ech configurtion. In order to check mtrix effects on sensitivity nd selectivity solution contining Tble 2 Opertionl prmeters used in xilly nd rdilly viewed ICP OES Opertionlprmeter RF power (kw) 1.2 Plsm gs-flow rte (lmin ) 15.0 Auxiliry gs-flow rte (lmin ) 1.5 Nebulizer gs-flow rte (lmin ) 0.8 Smple flow-rte (mlmin ) 0.8 Observtion height (mm) 13 Emission lines (nm) AlI Cr II Ar I Cu I Ar I Fe II As I Mg II B II Mg I B II Mn II C II Ni II Cd II Se I Co II Zn II Only for rdilly viewed configurtion.

4 1908 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Tble 3 Summry of dignostic procedure used for evlution of xilly- nd rdilly-viewed ICP OES w5 7x Figure of merit Prmeter UV spectrl resolution Profile of B II 230 nm line VIS spectrl resolution Profile of B II 455 nm line Robustness Mg II 280 ymg I 285 nm rtio Short-term stbility R.S.D. for Mg I 285 nm emission signl (ns15) Long-term stbility R.S.D. for Mg I 285 nm emission signl (ns8; ts2 h) Sensitivity LOD for Ni II 231 nm line Wrm-up time R.S.D. for Ar, B nd Mg emission signls 10 mg lni in 1000 mg lcr medium nd multielement solutions contining 20 mg l of Al, As, C, Cd, Co, Cr, Cu, Fe, Mg, Mn, Se nd Zn in 10, 100, 1000 nd mg lc medium, respectively, were employed. All solutions were prepred by suitble dilution of 1000 or 5000 mg l stock solutions of ech element (Spex Smple Preprtion, Metuchen, NJ) with Milli- Q distilled nd deionized wter (Millipore, Bedford, MA) nd sub-boiling distilled nitric cid (Milestone). Ure(CH4N2O, Regen, Brzil) ws used s crbon source for investigtion of residul crbon content effects on sensitivity. The use of this compound ws bsed on previously described study w10x. An nlyticl curve Blnk: 0.14 mol l of HNO 3; Reference Solution 1: 2.0 mg l of Cu, Fe, Mn nd Znq20 mg lmgq50 mg lc, nd Reference Solution 2: 4.0 mg l of Cu, Fe, Mn nd Znq40 mg lmgq100 mg lc ws used to ssess the precision nd ccurcy of mesurements performed in both xilly nd rdilly viewed configurtions Procedure Dignostic tests performed were bsed on experiments proposed by Mermet w7x, Pousselet l. w8x nd Mermet nd Poussel w9x nd they re summrized in Tble 3. The bckground equivlent concentrtion (BEC) nd LOD were clculted ccording to IUPAC w11x. Crs IrsyIblnk BECs, SBRs, SBR I blnk 3=BEC=R.S.D. LODs 100 where, C rs, concentrtion of multielementl refer ence solution (20 mg l ); SBR, signl-to-bck- ground rtio; Irs nd I blnk, emission intensities for multielementl reference nd blnk solutions, respectively, nd R.S.D., reltive stndrd devition for 10 mesurements of blnk solution Smple preprtion A mss of 250 mg of two certified reference mterils, NIST 1577b Bovine liver nd NIST 1515 Apple leves (NtionlInstitute of Stndrds nd Technology, Githersburg, MD), ws digested using 2 ml of concentrted sub-boiling distilled HNO3 plus 1 ml of H2O2 30% vyv. The heting progrm used is shown in Tble Results nd discussion All mesurements were crried out using instrumentlprmeters ccording to Tbles 1 nd 2. The performnce of xilly nd rdilly viewed ICP OES configurtions ws evluted using simple experiments proposed by Mermet nd Poussel w9x. The emission lines of Ar I nd Tble 4 Heting progrm used to perform closed-vessel microwvessisted cid digestions Step Time (min) Power (W) T (8C) A sixth step ws implemented to perform 5 min of ventiltion without heting.

5 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Fig. 1. Wrm-up time for xilly nd rdilly viewed ICP OES. () Axilly nd (b) rdilly viewed configurtions nm, B II nm nd Mg II nm were used for evlution of wrm-up time. The reltive intensity observed for both equipment used is shown in Fig. 1. In the ICP OES with xilly viewed configurtion, high vrition of emission signls ws observed during the initil5 min. Ar I emission intensities decresed while B II nd Mg II intensities incresed. This behvior ws not observed when using rdilly viewed ICP OES configurtion. In this cse, emission signls were prcticlly stble during the entire mesurement time (90 min). According to Fig. 1, wrm-up times for xilly nd rdilly viewed configurtions were 20

6 1910 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Tble 5 Emission intensities nd Mg IIyMg I rtios for xilly nd rdilly viewed configurtions Configurtion Mg I nm Mg II nm Mg IIyMg I Axilly viewed Rdilly viewed Multiplied by 1.8 ccording to Ref. w12x. nd 10 min, respectively. These times were estblished by considering emission intensity devitions less thn 5%. The short-term stbility ws determined by performing 15 consecutive mesurements of emission intensity t the Mg II nm line. For xilly nd rdilly viewed configurtions, R.S.D. were 0.70 nd 0.60%, respectively. When the emission signlws monitored during 2-h period (ns8, i.e. mesurements t time intervls of 15 min), R.S.D. observed for rdilly nd xilly viewed configurtions were 1.5 nd 1.4%, respectively. The Mg IIyMg I rtio is prcticlcriterion to evlute plsm robustness nd nlyticl performnce. The term robustness ws proposed to represent the efficiency of energy trnsfer nd response of the plsm to chnges in tomiztion nd excittion conditions nd chemiclcomposition of the spirted solution w7x. This experiment ws mde by pplying rdio-frequency power of 1.3 kw in both ICP, nd fterwrds by djusting the nebulizer gs flow-rte in ech one to rech the highest Mg IIyMg I rtio. This prmeter ws set t 0.90 nd 0.70 l min for xilly nd rdilly viewed ICP OES, respectively. The obtined rtios were multiplied by 1.8 to correct response intensities tking into ccount the use of n Echelle grting nd solid-stte detector. This vlue ws previously estblished by Dennud et l. w12x, lso working with Vist ICP; ccording to these uthors this vlue cn vry from 1.5 to 1.8. As expected, the results showed higher Mg IIyMg I rtio for the rdilly viewed configurtion (Tble 5). The elevted rtios obtined indicte tht both instruments were operted t robust conditions w1x. It should lso be mentioned tht robust conditions re effective to llevite mtrix effects but the sensitivity is slightly degrded w6x. Spectrl resolution ws similr in both Echelle polychromtors. Using B II nd B II nm emission lines, spectrl resolution vlues of 8 nd 30 pm were mesured, respectively. It should be mentioned tht the selected B lines re positioned t the center of the order in the CCD detector tht is chrcterized by better resolution. LOD in ech configurtion were evluted using Ni II nm line. The LODs were clculted bsed on BEC. The effect of mtrix on the LODs ws evluted determining this prmeter in 0.14 moll HNO3 nd 1000 mg l Cr medi. Results re presented in Tble 6. LOD were pproximtely fold better when the xilly viewed configurtion ws employed. These dt re in greement with Brenner nd Znder w1x tht the men detection limit improvement fctor for xilly viewed, compred to rdilly viewed configurtions, vried from 2 to 20, using conventionlnebuliztion w1x. The deteriortion of detection power ws slightly higher in the medium contining Cr, prticulrly Tble 6 LOD nd BEC for Ni II ( nm) in 0.14 mol l HNO3 nd 1000 mg l of Cr medi for xilly nd rdilly viewed configurtions Smple medium Axilly viewed Rdilly viewed BEC (mgl ) LOD (mgl ) BEC (mgl ) LOD (mgl ) 0.14 moll HNO mg lof Cr

7 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Tble 7 LOD nd BEC for Al, As, C, Cd, Co, Cr, Cu, Fe, Mg, Mn, Se nd Zn in solutions contining different crbon concentrtions in xilly viewed ICP OES Element Crbon concentrtion (mg l ) 0.14 moll HNO 3 BEC LOD BEC LOD BEC LOD BEC LOD BEC LOD AlI As I C II Cd II Co II Cr II Cu I Fe II Mg II Mn II Se I Zn II Vlues of BEC nd LOD re expressed in mgl. in the rdilly viewed configurtion. These results seem to indicte tht the end-on gs interfce used to eliminte the plsm til recombintion zone in the xilly viewed configurtion ws efficient. The performnce of the end-on gs interfce cn even proportionte lower mtrix interferences in xilly viewed compred to the rdilly viewed configurtion, s recently demonstrted for Dy, Eu nd Yb determintion in sheep feces w13x. Results obtined for the influence of the crbon content on the sensitivity of mesurements lso confirms this observtion (Tbles 7 nd 8). The xilly viewed configurtion did not present significnt vritions in BEC nd LOD vlues, Tble 8 LOD nd BEC for Al, As, C, Cd, Co, Cr, Cu, Fe, Mg, Mn, Se nd Zn in solutions contining different crbon concentrtions in rdilly viewed ICP OES Element Crbon concentrtion (mg l ) 0.14 moll HNO 3 BEC LOD BEC LOD BEC LOD BEC LOD BEC LOD AlI As I C II Cd II Co II Cr II Cu I Fe II Mg II Mn II Se I Zn II Vlues of BEC nd LOD re expressed in mgl.

8 1912 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) Tble 9 Results for C, Cu, Fe, Mg, Mn nd Zn determintion bsed on dry mtter, using xilly nd rdilly viewed ICP OES Smple C (% mm ) Cu (mg kg ) Fe (mg kg ) Mg (% mgkg ) Mn (mg kg ) Zn (mg kg ) NIST 1577b Bovine liver Axilly viewed 146 "12 142"1 164"4 546"5 7.20" "1 Rdilly viewed 107"5 148"1 156"4 523"4 9.14" "3 Certified vlue 116"4 160"8 184"15 601" " "16 NIST 1515 Apple leves Axilly viewed 1.32" " " " " "0.3 Rdilly viewed 1.46" " " " " "2.7 Certified vlue 1.526" "0.24 (80) b 0.271" "3 12.5"0.3 Concentrtion expressed in mg kg for C nd % m m for Mg. b Estimted vlue. when the crbon concentrtion of multielement solution vried from 10 to mg l. In generl, the devition of results ws less thn 15%. The suitble performnce presented by this configurtion cn be ssocited to the efficiency of the end-on gs interfce to eliminte the plsm til recombintion zone. The increse of crbon concentrtion could produce non-specific emission nd deteriorte the power of detection due to the increse of bckground rdition (BEC). It is expected tht this drwbck could be more pronounced for the xilly viewed configurtion. However, this undesirble effect ws not observed in the evluted crbon concentrtion rnge using the ICP OES with n end-on gs interfce. Similr behvior ws observed using rdilly viewed configurtion when operting the ICP t robust conditions. In this cse the presence of crbon in multielement solutions only ffected the BEC nd LOD for Se I nm, for which vrition pproximtely 30% ws observed over the crbon concentrtion rnge evluted. The mtrix effect cused by residulcrbon on ICP OES mesurements of Se ws previously described by Knpp et l. w14x, nd it cn lso cuse problems in inductively coupled plsm mss spectrometry w15,16x. Recovery experiments were performed using both instruments to ssess the ccurcy nd precision of mesurements. All results for C, Cu, Fe, Mg, Mn nd Zn were in greement with certified vlues t 95% confidence level (Tble 9). Other elements, such s Al, As, Cd, Co, Cr nd Se, were not determined becuse their concentrtion in digesttes were below the detection limit, even when using ICP OES with xilly viewed configurtion. 4. Conclusion Axilly nd rdilly viewed configurtions presented similr figures of merit. Despite differences in wrm-up time nd LODs, nlyticl performnce of both configurtions ws not significntly compromised. It cn be inferred tht quntittive determintions cn be crried out with suitble precision nd ccurcy using ny viewing configurtion. Tking into ccount the obtined results, the xilly viewed configurtion presented higher detection power thn tht chieved by the rdilly viewed configurtion, independent of the complexity of the smple medium. In spite of some prticulrities, such s the longer wrm-up time observed for the xilly-viewed configurtion, it could be supposed tht the efficiency of the endon gs interfce improved the performnce of this rrngement. It cn lso be supposed tht most pplictions could perfectly be performed using n ICP OES with this rrngement without ny serious degrdtion of nlyticl performnce, s previously suggested by Brenner nd Znder w1x. Acknowledgments The uthors re grtefulto Fundco de Ampro ` Pesquis do Estdo de So Pulo for finncil

9 F.V. Silv et l. / Spectrochimic Act Prt B 57 (2002) support provided (Process ). F.V. Silv, L.C. Trevizn, nd C.S. Silv re lso grteful to FAPESP nd CNPq, respectively, for fellowships provided. A.R.A. Nogueir nd J.A. Nobreg re thnkfulto CNPq for reserch scholrships provided. References w1x I.B. Brenner, A.T. Znder, Axilly nd rdilly viewed inductively coupled plsms criticl review, Spectrochim. Act Prt B 55 (2000) w2x T.W. Brnrd, M.I. Crockett, J.C. Ivldi, P.L. Lundberg, D.A. Ytes, P.A. Levine, D.J. Suer, Solid-stte detector for ICP-OES, Anl. Chem. 65 (1993) w3x S. de Goy, D. Lnziser, K. Lopez, J. Noonm, J. Rebello, Which CCD detector is right for you?, Spectroscopy 13 (1998) w4x M.H. Abdllh, R. Diemiszonek, J. Jrosz, J.M. Mermet, J. Robin, C. Trssy, Spectrometric study on rdiofrequency induced plsm. 1. Anlyticl performnce, Anl. Chim. Act 84 (1976) w5x C. Dubuisson, E. Poussel, J.M. Mermet, Comprison of xilly nd rdilly viewed inductively coupled plsm tomic emission spectrometry in terms of signl-tobckground rtio nd mtrix effects, J. Anl. At. Spectrom. 12 (1997) w6x J.C. Ivldi, J.F. Tyson, Performnce evlution of n xilly-viewed horizontl inductively coupled plsm for opticlemission spectrometry, Spectrochim. Act Prt B 50 (1995) w7x J.M. Mermet, Use of mgnesium s test element for inductively coupled plsm tomic emission spectrometry dignostics, Anl. Chim. Act 250 (1991) w8x E. Poussel, J.M. Mermet, O. Smuel, Simple experiments for the control, the evlution nd the dignosis of inductively coupled plsm sequentil systems, Spectrochim. Act Prt B 48 (1993) w9x J.M. Mermet, E. Poussel, ICP emission spectrometers: nlyticl figures of merit, Appl. Spectrosc. 49 (1995) w10x S.T. Gouvei, F.V. Silv, L.M. Cost, A.R.A. Nogueir, J.A. Nobreg, Determintion of residulcrbon by inductively-coupled plsm opticl emission spectrometry with xilnd rdilview configurtions, Anl. Chim. Act 445 (2001) w11x IUPAC, Anlyticl Chemistry Division, Commission on Spectrochemiclnd OpticlProcedures for Anlysis, Nomenclture, symbols, units nd their usge in spectrochemiclnlysis-ii. Dt interprettion, Spectrochim. Act Prt B 33 (1978) w12x J. Dennud, A. Howes, E. Poussel, J.M. Mermet, Study of ionic-to-tomic line intensity rtios for two xil viewing-bsed inductively coupled plsm tomic emission spectrometers, Spectrochim. Act Prt B 56 (2001) w13x E.E. Grci, A.R.A. Nogueir, J.A. Nobreg, Mtrix effects on the determintion of dysprosium, europium nd ytterbium used s nimlfeclmrkers by inductively coupled plsm opticl emission spectrometry with xilly- nd rdilly-viewed configurtions, J. Anl. At. Spectrom. 16 (2001) w14x G. Knpp, B. Michin, U. Bumgrtner, Interferences in ICP-OES by orgnic residue fter microwve-ssisted smple digestion, At. Spectroc. 19 (1998) w15x P. Allin, L. Junult, Y. Murs, J.M. Mermet, T. Delporte, Signl enhncement of elements due to presence of crbon-contining compounds in inductively coupled plsm mss spectrometry, Anl. Chem. 63 (1991) w16x E.H. Lrsen, S. Sturup, Crbon-enhnced inductively coupled plsm mss spectrometric detection of rsenic nd selenium nd its ppliction to rsenic specition, J. Anl. At. Spectrom. 9 (1994)