JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 19 Preconcentration and Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Metal Ions With On-line Chelating Ion Exchange Valerio Porta Corrado Sarzanini Ornella Abollino Edoardo Mentasti and Enzo Carlini Department of Analytical Chemistry University of Torino Via P. Giuria 5 10125 Turin Italy An on-line preconcentration method utilizing a microcolumn of XAD-2 resin functionalized with 1 -(2- thiazolylazo)-2-naphthol has been developed. Preconcentration factors of 1 25 were easily obtained for injection times of 5 min. The detection limit ranged between 2 ng I-l for Mn and 40 ng I-' for Ni. The resin has been used to preconcentrate Cd Cu Fe Mn Ni and Zn from river water and Antarctic sea-water (Ross Bay) prior to their determination by inductively coupled plasma atomic emission spectrometry.The precision of the technique is around 10% relative standard deviation at concentrations below the pg I-' level and 5% for higher concentrations. Keywords Sea- water; on-line enrichment; chelating resin; inductively coupled plasma atomic emission spectrometry; preconcen tra tion The determination of ultratrace levels of metal ions in freshwater samples usually requires a preconcentration step andor separation of the analyte from the matrix before the instrumental analysis. There are at present two principal methods of preconcentration i.e. off- and on-line proce- dures. With off-line preconcentration the enrichment manifold is completely separate from the measurement instrument and all chemical preconcentrations are con- ducted independently away from the spectrometer.In the second procedure the enrichment manifold is connected directly to the spectrometer and the preconcentration and measurement cannot be considered as two separate tech- niques as the specific instrumentation performs the chemi- cal pre-treatment of the sample and detection of the analyte. The separation of the metal ions from the liquid sample in both of these methods can be obtained in several ways however one of the most commonly used approaches consists of allowing the sample to flow through a column packed with an active material that is capable of retaining the analyte. The selective retention of metal ions can be obtained by two different procedures. (i) A ligand which can interact with the analytes is added to the sample; the resulting complex species are then retained by the station- ary phase of the column.(ii) The complexing ligand is immobilized on the stationary phase which then systemati- cally retains the metal ions as the sample flows through the column in an ion-exchange fashion. Both of these techniques1-14 have found widespread application in off- and on-line modes as they can provide the necessary detection power in particular for the case of sea-water analysis. Sturgeon and co-workers112 used 8- hydroxyquinoline or diethyldithiocarbamate and C1 silica in an off-line system for the determination of various metal ions in sea-water. RGiiEka and Arnda14 demonstrated the possibility of using the same ligands and a solid substrate in an on-line system with excellent results.On the other hand Sturgeon et a1.,8 Marshall and Mottolag and McLaren el al.,1° chose silica immobilized 8-hydroxyquinoline as an active solid substrate for off- and on-line systems. They showed that this solid is particularly useful for the precon- centration and matrix isolation of trace metals. In the present work a column of chelating resin has been used as part of an on-line manifold. As the active solid substrate 1 -(2-thiazolylazo)-2-naphthol (TAN) loadedI5 on XAD-2 a styrene-divinylbenzene copolymer was uti- lized. After uptake by the column the metal ions were then eluted directly into the nebulizer of the inductively coupled Fig. 1 Section of the enrichment column 1 PTFE tube ( 1.5 mm Ld.); 2 Tygon tube (1.3 mm id.2.0 mm 0.d.); 3 Tygon tube (2.1 mm Ld.); 4 PTFE net; and 5 resin plasma (ICP) atomic emission spectrometer by concen- trated acids. This arrangement allowed preconcentration and determination of Cdl* Cull Fell Mn" Nil1 and Znll in river water and sea-water with high precision and accuracy. Experimental Reagents and Apparatus High-purity water (HPW) was produced with Millipore Milli-Q equipment which was supplied with de-ionized water from a mixed-bed twin ion-exchange column. All the acids and ammonia solutions (E. Merck Darms- tadt Germany) were purified using sub-boiling distillation apparatus (IS. Kurner Rosenheim Germany). Concentrated metal standard solutions (Titrisol E. Merck) were diluted as desired for the standard additions and for method evaluations.Amberlite XAD-2 resin (Serva) 50- 100 pm was purified according to the following procedure portions of the resin were placed into poly(propy1ene) Bio-Rad Econocolumns and repeatedly washed with methanol 2.0 mol dm-3 HCl and 0.1 mol HN03 and HPW in this order. The microcolumn was prepared from a piece of Tygon tubing (2.1 mm i.d. 30 mm long). The resin was held in place by two balls of poly(tetrafluoroethy1ene) (PTFE) net (pore size 75 pm). The connection between the 1.5 mm i.d. PTFE tubing and the columns was achieved by using two other Tygon tubes (2.0 mm o.d. about 7 mm long). A section of the column is shown in Fig. 1. The resin bed was 25 mm long for a total volume of solid substrate of 0.1 ml. The TAN (Fluka Buchs Switzerland) was used as a sequestration agent.The XAD-2 resin was loaded in situ with TAN by flowing a 1.0 x rnol dm-3 solution of TAN (water + MeOH 50 + 50 v/v) through a micro- column packed with the required amount of purified20 JOURNAL OF ANALYrICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 n7 Waste To ICP plasma fl Eluent Bufferd- I 0.5 ml min-' Sample 10.0 mI min-' Table 1 Typical preconcentration sequence Sequence Time/s Flow ratelm1 min-' Start - - Conditioning 20 6.0 Switch valve 1 - - Sample injection 180 10.0 Switch valve 1 - - Washing 20 6.0 Switch valve 2 - - Elution 60 1 .o Switch valve 2 - - End - - Fig. 2 Schematic diagram of the preconcentration manifold resin.15 After loading the ligand the column was washed with water and subsequently with 2.0 mol dm-3 HCl in order to remove any metallic impurities.The present column design did not show on use any channelling problems which can arise due to the reduced swelling and shrinking of the fuctionalized adsorbent during uptake of the trace metals from aqueous samples. Water from the river Po was collected in the centre of the City of Turin and immediately filtered on a 0.45 pm cellulose membrane filter. It was then frozen (-20 "C) without the addition of any reagents. As part of the activities of the Italian 'Progetto Antar- tide-Impatto Ambientale' associated with an expedition during the Antarctic summer of 1987-1 988 this operation unit received sea-water samples which had been filtered though 0.45 pm membranes a few hours after collection and then kept frozen at -20 "C.Acidification of the samples before preconcentration was required. Hence 2.0 ml 1-1 of concentrated ultrapure nitric acid were added to both the river water and sea-water samples. An atomic emission ICP spectrometer (IL Plasma 300) was used. All the instrumental parameters were the same as reported in previous work.I6 Off-line background correction on one side of the emission line was utilized. As the instrumentation used did not have a program for flow injection the emission was registered by discrete sampling and the peak areas were computed with a Lotus 123 spreadsheet program. For some measurements electrothermal atomic absorp- tion spectrometry (ETAAS) with Zeeman-effect back- ground correction (Model Zeeman 5 100 Perkin-Elmer equipped with an HGA-600) was used.Perkin-Elmer pyrolytic graphite coated graphite tubes were normally employed. Preconce n t rat ion Procedure All sample manipulations and preparations were conducted under a laminar flow fume hood. A schematic diagram of the manifold is given in Fig. 2. Valve 1 facilitates the alternate flow of the sample and the washing solution whereas valve 2 enables the column to be encorporated as a loop. While the column is being loaded a flow of the eluent is maintained to the nebulizer. The sample and washing solutions were buffered on-line through connection at a T-junction of the sample washing and buffer solution lines. A cartridge packed with Chelex 100 was inserted in the buffer h e to remove all metallic impurities from the buffer solution which was the main source of contamination.The samples and washing solution were pumped by a peristaltic pump at flow rates of 10 and 6 ml min-l respectively. The buffer solution was pumped by another peristaltic pump at 0.5 ml min-l. The final pH of the samples and of the washing solution was 8.4k0.3. The metal ions retained on the column were eluted with 2.0 mol dm-3 HC1 + 0.1 mol dm-3 HN03. The column was first conditioned to a suitable pH value by flowing the washing solution through after which valve 1 was switched on and the sample injected. The column was then washed again in order to eliminate any sample remaining in the line and in the column. By switching on valve 2 the metal ions were eluted from the column. A typical preconcentration sequence is reported in Table 1.For the determination of the metal ion concentration measuring the area of the elution peak was preferred to peak height measurements. The peak area was correlated to the concentration both with the standard additions method ,and with calibration against a steady-state signal of a :standard solution prepared in 2.0 mol dm-3 HCl + 0.1 mol dm-3 HN03. The algorithm used for the calculation 'was where c,=concentration of the analyte (ng ml-I); A=peak area of analyte emission signal (counts min); T= reading nnterval (min); F=eluent flow rate (ml min-l); Cstd concen- 1:ration of the standard solution (ng m1-l); &d=emission of the standard (counts rnin); Sbkg= background emission (counts rnin); and V,=volume injected (ml). Results and Discussion Performance of Method A s previously reported,15 an efficient chelating ion-ex- change resin can be prepared with a commercial adsorbent resin XAD-2 or XAD-4 and a water insoluble complexing agent TAN or 1 -(2-pyridylazo)-2-naphthol (PAN).The resin can be functionalized in a short time < I h and maintains a high efficiency even after treatment with concentrated acids. The chelating resin was able to retain selectively many transition metal ions such as Cu" Ni" ;SnI1 Cd" Fe" CoI1 and other ions such as U022+ and AP while Call or Mg" are not retained. The batch capacity of the resin was about 0.1 mmol g-l of Cul* for particles with dimensions of between 20 and 50 mesh but in this work an adsorbent resin with smaller particles and conseqeuntly with a higher loading capacity was used.The pH was chosen (8.4 k 0.3) after considering that at lower values complexation and retention may be incom- plete and at higher values (especially at pH>9.2) the performance of the chelating resin decreases ra~id1y.l~ The low internal diameter of the column did not cause a.ny problems in terms of sample flow resistance. It wasJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 21 400 1500 A r I v) +- 200 P t PI tn .- 1000 5 8 v) C U > C 500 2 n 30 " 0 10 20 Time/s Fig. 3 Elution profile for A Mn and B Fe obtained from water from the river Po Table 2 Recovery of elements from HPW river water and sea- water Recovery yield (Yo) Element HPW Sea-water River water Cd 92 100 100 c u 103 98 105 Fe 106 92 96 Mn 90 93 93 Ni 97 99 99 Zn 101 105 100 *Standard deviation was always within 10%.found that the maximum flow rate was around 10 ml rnin-l and this could be maintained for a high number of preconcentration cycles. When the flow did start to decrease slowly this was probably because of a long-term tightening of the resin bed. The elution profiles for Fe and Mn are given in Fig. 3. It can be seen that Fe gave larger peaks compared with Mn. The peak width for Fe is just over 20 s which is equivalent to 0.4 ml; this means that with an injection volume of 50 ml a preconcentration factor of 125 could be obtained. In terms of concentration efficiency,ll defined as the product of the preconcentration factor and the sampling frequency (Le. the number of samples analysed per minute) a value of 18 was obtained. This result is much higher than the one reported by Beauchemin and Berman14 and is only slightly lower than those reported by Fang et al." obtained with a dual column system.The efficiency of the chelating resin was evaluated in an off-line mode.15 In previous work,15 recoveries from HPW with functionalized XAD-2 with a greater particle size proved to be incomplete. With the smaller XAD-2 resin this problem was not observed and the ions were totally recovered from HPW river water and sea-water (see Table 2). Analytical Blanks Absolute blank values were evaluated for 50 ml samples. For each metal no sample solution was pumped during the preconcentration sequence but all the other steps were the same. This procedure allowed the evaluation of the contri- bution of metal ions present in the reagents used.The results are reported in Table 3. All data refer to three times Table 3 Absolute blanks and detection limits for the proposed method; sample volume 50 ml Element BlanWng Detection limithg 1-I Cd 0.4 c u 0.6 Fe 0.6 Mn 0.1 Ni 2 Zn 0.6 8 12 12 2 40 12 Table 4 Analytical data for the adopted enrichment procedures for the analysis of a river water sample Concentration/pg l-l* Element ETAASt XAD-2 +TAN XAD-2 +OX Cd 0.064 0.03 k 0.01 0.033 f 0.004 cu 1.5 1.30 f 0.10 1.3 +_O. 1 Fe 5.4 5.9 k 0.7 5.6 t- 0.4 Mn 0.82 0.80 +_ 0.05 0.87 k 0.05 Ni 5.4 5.5 k0.3 5.8 f 0.3 Zn NDS 8 8.2 k 0.2 *Mean of at least five determinations. ?Standard deviations within 10%. SND not determined. the noise of the baseline but no background peaks were observed in this instance.For this reason the results are shown without standard deviations. The insertion of the Chelex column in the buffer solution line significantly reduces the amount of metal ions intro- duced with the buffer or avoids large blank values for certain metals due to accidental contamination of the buffer. This is particularly valid for Zn which usually gives high blank levels and is often subject to contamination from the laboratory environment. The detection limits reported were calculated from the blank value. However it must be pointed out that since no blank signal was observed an increase in the sample volume can further decrease the detection limit. Analytical Results Results for the analysis of river water samples are given in Table 4 as mean values of at least five determinations made on samples from different bottles.The river-water data can be compared with the results from direct analysis by ETAAS and with those obtained with a different on-line preconcentration method obtained using 8-hydroxyquinol- ine when the ligand is added directly to the sample.' Calibration was always obtained both with standard addi- tions and external calibration even if total recovery was assumed. This was effected in order to account for any unexpected degradation of the column which could have given the wrong results when using external calibration and to avoid a change in sensitivity during the long analysis time required for some elements (Cd) which would have affected the standard additions results.The accuracy of the values found is acceptable with the only exception being for Cd for which there is agreement between the precon- centration data but they differ from the ETAAS concentra- tions. No plausible explanation for this has been found. The results for the Antarctic sea-water analysis are reported in Table 5. The values for the concentrations of the metals studied are in the same range as others found with22 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 5 Analytical data for the analysis of two Antarctic sea-water samples (1987-1988 compaign). Sample A (SW 31) 75" 06' 24" south 165" 29' 06" east. Sample B (SW 46) 75" 29' 0 0 south 165" 3 1' 48" east Concentration/pg 1-' Element A B - * 0.020 2 0.0 10 Cd c u 0.30 f 0.02 0.25 2 0.04 Fe 0.29 f 0.03 0.34 2 0.02 Mn Ni 0.43 f 0.04 0.49 2 0.04 Zn 0.34 f 0.02 0.20 2 0.0 1 - * 0.01 3 2 0.005 *Not determined in this sample.different preconcentration methods.I7 Preconcentration from the same sample performed with the XAD-2-oxine method7 and the procedure reported here gave similar results. The precision of the analysis of sea-water (see Table 5) is fairly good but is not as good for elements present at very low concentrations (e.g. Cd) as for those for which the ratio of the concentration to the detection limit of the plasma is low (e.g. Ni). Conclusions The present work confirmed that on-line preconcentration is one of the best methods of sample pre-treatment in atomic spectrometry. The procedure developed requires very few sample manipulations and reagent additions and for this reason very low blank levels are obtained.More- over the concentration efficiency of the method which combines a preconcentration factor and analysis time was about 18 one of the highest reported in the literature. The functionalized XAD-2 can easily be substituted with a different active substrate such as silica immobilized 8- hydroxyquinoline or a more selective reagent. An adaptation of the present procedure for use with ETAAS and ICP mass spectrometry should be fairly easy especially for the determination of Cd" CuII Fell Mn" Nil1 and Zn" in sea-water where these elements are present at very low concentrations. The financial support from Minister0 dell'Universita e della Ricerca Scientifica e Tecnologica (MURST Rome) and from the Italian National Research Council (CNR Rome) is kindly acknowledged.1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 1 '7 References Sturgeon R. E. Berman S. 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Chem. 1989 13 463. Porta V. Sarzanini C. and Mentasti E. Mikrochim. Ada 1989 111 247. Mentasti E. Porta V. Abollino O. and Sarzanini C. Ann. Chim. (Rome) 1989 79 629. Paper I /O I5 6 7B Received April 3 1991 Accepted August 29 I991