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Determination of trace metals in seawater by graphite furnace atomic absorption following on-line separation and preconcentration
Spectrochimica
Acta, Vol. 488, No. 1, pp. 91-98,
1993
0.584-8547/93 sfl.al + .oo Pergamon Press Ltd
Printed in Great Britain.
Determination of trace metals in seawater by graphite furnace atomic absorption following on-line separation and preconcentration LAERTE C. AZEREW,* RALPH E. SruRGEoNt and ADILSON J. CURTIUS$ Institute for Environmental
Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR9 (Received 14 July 1992; accepted 2.5 July 1992)
Abstract-A discontinuous microscale preconcentration system, based on the chelation of trace metals by a 20 ul column of silica-immobilized 8hydroxyquinoline, was interfaced with a graphite furnace atomic absorption spectrometer. Volume-based loading of 8%SOOO ul sample loops, followed by elution of the sequestered elements directly into the furnace in 48 ~1 of acid, provided quantitative recovery of Fe, Cd, Zn, Cu, Ni, Mn and Pb from open ocean seawater with a frequency of lo-20 per hour. Absolute detection limits were 0.3, 6.9, 4.2, 1.8, 10.2, 5.7 and 1.8 pg for Cd, Cu, Fe, Mn, Ni, Pb and Zn, respectively. The sensitivity of the graphite furnace technique is enhanced up to 2%fold compared to a standard 20 ul injection volume. Results are presented for the analysis of these elements in NRCC seawater Reference Materials NASS3 and CASS-2 using simple calibration curves for quantitation.
FLOW injection (FI) technology has significantly enhanced the performance of atomic spectrometry [l]. An injector, pump and flow manifold in a variety of configurations may be used to replace conventional sample introduction systems. In general, the requirements of matrix removal and analyte preconcentration are priorities which have often driven developments in this area. FI on-line column preconcentration approaches have recently been reviewed by FANG [2]. Compared to their off-line batch counterparts, these systems offer a number of significant advantages for ultratrace determinations: greater efficiency; lower consumption of sample and reagent; improved precision; reduced risk of contamination; and increased sampling frequency and throughput as well as ease of automation. The first three factors serve to enhance the detection power of the system. Despite the impressive detection power of the technique, graphite furnace atomic absorption spectrometry (GFAAS) often cannot be routinely used for the direct trace element analysis of many natural samples of environmental interest [3]. Concentration of the desired trace elements can extend the detection limits, remove interfering concomitants and improve the precision and accuracy of the analytical results. On-line column preconcentration manifolds for GFAAS must operate in a discrete fashion as sample presentation is inherently discontinuous. Due to the increased difficulties encountered as a result of this requirement, few papers describe the successful interfacing of on-line columns to the auto-sampling arm of the spectrometer. FANG et al. [4] and SPERLING et al. [5] demonstrated the advantages to be gained with this approach. These authors utilized solution chelation of the trace metals of interest with diethyldithiocarbamate followed by their sorption onto a minicolumn (15 ~1) packed with (&bonded silica reversed-phase sorbent. Although good results were obtained for the determination of Cd, Cu, Pb and Ni in saline water, the approach was less than ideal because all of the analyte placed on the column could not be quantitatively
* On leave from UFRRJ, Departamento de Quimica, Km 47 Antiga, Estr. Rio-SP., Rio de Janeiro, Brazil. t Author to whom correspondence should be addressed. $ Depto. de Quimica da PUCYRio, 22453 Rio de Janeiro-RI, Braxil. NRCC No. 34223. 01992 Crown Copyright. 91
92
L. C. AZEREDO et al.
eluted in a solvent volume (ethanol) accommodated by the furnace. This necessitated reproducible introduction of a volume slice of eluant into the furnace that contained about 50% of the adsorbed analyte. A similar approach was taken by PORTA et al. [6], although the complexing agent was slightly different (pyrrolidinedithiocarbamate) and the analytes were eluted with 80 ~1 acetonitrile. Unfortunately, a double elution step was required for good recovery of a number of elements. BEINROHR et al. [7] utilized a short column packed with 79 l.~lof Spheron oxine (&hydroxyquinoline immobilized onto a methacrylate gel) for the determination of Pb in NaCl by GFAAS. Difficulties were encountered in the elution step and processing times were comparatively slow (5 min for 100 ~1 samples). Although FANG and WELZ [8] also reported recovery problems when using a similar material (8-quinolinol immobilized on controlled pore glass-CPG/8-Q ion exchanger) for on-line preconcentration of several elements for flame AAS, immobilized chelates offer a number of significant advantages over solution phase chelants [9]: these materials are reusable; they can be easily precleaned; no excess chelating agent is released into the elution solvent; their chelation properties are very similar to their free molecule counterparts; and they have sufficient capacity that small volume columns can be made very efficient. We report here on the analytical performance of a mini-column of silica-immobilized 8-hydroxyquinoline for the preconcentration of Cd, Cu, Fe, Mn, Ni, Pb and Zn from seawater using a discontinuous on-line FI-GFAAS system.
EXPERIMENTAL Reagents and samples All reagents were purified prior to use. Deionized, distilled water (DDW) was produced with a commercial mixed-bed ion-exchange system (Barnstead Nanopure) fed with distilled water. Concentrated HNOs and HCl were prepared by sub-boiling distillation in a quartz still using reagent grade feedstocks. From these, an acid mixture consisting of 2 M HCl and 0.8 M HNOs was prepared for elution of trace metals from the column. A saturated ammonia solution (28% v/v) was obtained by isothermal distillation of reagent grade ammonia liquor with collection of the evolved NH3 in chilled DDW. A 1 M NH&l buffer was prepared, adjusted to pH 9 with excess NI&OH, and used for subsequent DDW wash pH adjustment (50 p.1 ml-‘). Silicaimmobilized 8-hydroxyquinoline (I-8HOQ) was synthesized in-house [9] from 37-75 pm particle size Porasil B (Waters Associates, Milford, MA, U.S.A.) silica and used for the sequestration of trace metals from the samples. Stock solutions of all metals of interest were prepared by dissolution of the high-purity metals or their salts (Spex Ind., Edison, NJ, U.S.A.). All of the above operations were conducted in a clean room having class 10 workstations. Reference materials from the National Research Council of Canada (NRCC), NASS-3 open ocean seawater and CASS-2 coastal seawater, were analysed and used for validation of accuracy. Samples were adjusted to pH 9 by addition of NI-&OH. The required amount was ascertained beforehand using a small test portion so that in this manner no pH electrode was ever in contact with the analytical sample. Znstrumentation All analyses were performed using a Perk&Elmer model 5000 spectrometer equipped with an HGAJOO furnace, Zeeman background correction and an AS-40 autosampling device. Pyrolytic graphite coated tubes (without platforms) were used exclusively. The FI manifold consisted of a single four-channel Minipuls 3 model M 312 peristaltic pump (Gilson, Villeurs le Bel, France) and three metal-free Rheodyne Type 50 rotary injection TeflonR valves (Rheodyne Inc., Cotati, CA, U.S.A.). Three 125 ml TeflonR separatory funnels served as sample and reagent reservoirs. With the exception of tygon pump tubes used for solution delivery (0.051 in i.d.), all other conduits and loops consisted of nominally 0.063 in o.d. x 0.040 in i.d. TeflonR tubing interconnected via flangeless TefzelR nuts and ferrules (Upchurch Scientific). Figure 1 schematically illustrates the experimental set-up. An injection loop was fitted to each of the three rotary valves: 300 ~1 volume for DDW wash; 48 ~1 for acid eluant; and 0.835-5.0 ml for sample. A guard column for reagent clean-up was placed in the DDW line (pump channel No. 1) which consisted of a 6 cm long x 0.20 cm i.d. 200 ~1 volume of I-8HOQ. The material was retained within the tube using a small wad of precleaned quartz wool
Fig. 1. Flow injection manifold and sequence of operations.
LOADING
t
GC: guard column; AC: analytical column.
t
WASTE
ELUTION
r
94
L. C. AZEREDO et al.
sandwiched between the resin bed and a 2 mm length of 0.20 cm o.d. x 0.1 cm i.d. TeflonR tube inserted at either end. The analytical column (20 ~1) was similarly constructed from a 2.55 cm resin bed of I-SHOQ placed within a 0.1 cm i.d. TeflonR tube and positioned within the system such that it could be conveniently fastened to the sampling arm of the AS-40, thereby forming the interface between the FI system and the furnace. Procedure The FI manifold
and the sequence of its operation are shown in Fig. 1. A complete cycle of preconcentration and elution required approximately 240 s with a sample loading time of 100 s (corresponding to 0.835 ml of sample). In the “stand by” position the wash, acid and sample loops are filled using arbitrary flow rates of 300, 48 and 500 ~1 mm-‘, respectively. Air is used as the moving fluid in this system and in this position air is passing through the analytical column at a flow rate of 1.2 ml min-‘. In the “loading” step, the sample valve is manually actuated to deliver the contents of the sample loop to the column using air as the carrier at a flow rate of 250-500 ~1 min-’ (adjusted by changing the pump speed). Following passage of the seawater, the sample valve is restored to its initial position and the wash valve activated to deliver 300 t~,l of pH 9 DDW to the analytical column at an air flow rate of 300 p.1 mm-i. During this step, excess alkali and alkaline earth elements are removed from the resin bed and passage of “excess” air leaves the column free of any interstitial water so as to eliminate any dilution of the minimum volume of acid used in the subsequent elution step. With the wash valve returned to its initial position so as to refill the loop, the acid loop is brought on-line and pumped through the column by an air flow rate of 130 ~1 min-‘. At this stage, the effluent from the column containing the trace metals of interest is directed into the graphite tube by manually turning the AS-40 sampling arm into the inject position and holding it there for the duration of the elution phase (22 s). The various air flow rates indicated above arise, not as a result of intentional changes to the pump speed (unless indicated) or compression of the tygon pump tubing by the rotors, but rather in response to the various back pressures and nominally different inner diameters of the loops used (i.e. 1.0 mm sample loop; 0.9 mm acid loop; 0.7 mm wash loop). Following elution, the furnace program is manually initiated and the absorbance transient recorded and quantitated by both peak height and area measurement using in-house software compatible with the output from the Model 5000. As all potential matrix interferents are removed during the preconcentration step, quantitation was performed using peak height measurements and calibration was achieved by manual injection of standards, prepared in a 48 u.1 volume of the elution acid, directly into the furnace (atomization programs used for the elements were not substantially different from those recommended by the manufacturer). The acid elution valve was then returned to its “fill loop” position and the “wash” valve reactivated so as to flush the column of any remaining acid and return it to an alkaline pH optimal for loading the next sample.
RESULTS AND DISCUSSION
I-8HOQ was selected as the analytical column material for this study because, in addition to our lengthy experience in working with it in the batch mode [9], we have also found it to perform well in an off-line FI mode using small sample volumes [lo]. Several desirable properties arise which make it attractive for FI work. In addition to the advantages alluded to earlier, which are inherent to the use of immobilized chelating agents, I-8HOQ exhibits good stability in acidic media (necessary for elution of the analytes), high exchange capacity (0.06 mmol Cu g-l), excellent rejection of alkali and alkaline earth elements (> 99.9999% Na, 99.998% Mg and 99.95% Ca removal), large stability constants for metal chelates and non-specificity for a wide range of trace metals. The high exchange capacity results in a total capacity of the analytical column equivalent to 20 bg Cu-more than adequate for the trace levels of heavy metals encountered in small volumes of natural waters. Moreover, the kinetics of ion exchange with chelating sites is rapid [9], permitting relatively large sample flow rates to be used without deterioration of performance. Although reliance on the use of sample injection loops limits sample volumes to discrete amounts and requires multiple individual loops, this option was determined to be preferable to time-based sampling because the reproducibility of the speed of the peristaltic pump from sample-to-sample and day-to-day was poor.
Seawater trace metal determination
by GFAAS
95
Table 1. Recovery of trace elements from DDW and NASS-3 Recovery* (%) Element Cd cu Fe Mn Ni Pb Zn
DDW 100 97 93 99 98 92 99
f 2 rt 2 + f f
7(6) B(6) 5(6) 3(6) 4(6) 12(6) 9(6)
NASS3 97 98 93 94 96 93 98
I ? _’ 2 f 2 f
7(5) 7(5) 2(S) ll(5) 3(4) 6(7) 6(5)
* Mean and standard deviation. Number in parentheses is number of replicate measurements.
Column performance Table 1 summarizes data characterizing the recovery of trace elements from samples of spiked NASS3 and DDW. Quantitative recovery is evident for all elements from both media, illustrating that the saline matrix does not influence performance. Additionally, quantitation was achieved using simple standards prepared in the elution acid and manually injected into the furnace using a micropipette. No interferences are present since the eluant is virtually matrix-free, thereby permitting peak-height evaluation of the signals generated by atomization from the furnace wall. Conditions for optimal multielement sequestration onto I-8HOQ occur at pH > 8. In this study, samples were adjusted to both pH 8 (NASS-3) and 9 (CASS-2) prior to analysis with no difference in performance. Sample pH > 8 is usually avoided when large volumes are processed off-line in the batch mode of operation due to hydrolysis of the silica substrate and increase in the iron blank [9]. This problem was not evident here, likely as a consequence of the small sample volumes used. Sample loading onto the column was unaffected by changes in flow rate between 250-500 t~1min-l. At 1000 ~1 min-i the recovery of Mn decreased to 75%. Manganese was selected as the target element for all studies relating to the sequestration of trace metals onto the column as it is the most weakly bound of the suite of elements of interest and therefore most susceptible to loss because of poor exchange kinetics [9]. Thus, although sample loading was limited to flow rates below 500 p,l mitt-’ in this study in order to quantitatively recover Mn, significantly higher flow rates could undoubtedly be used if Mn was not an element of concern. Small volume elution of the sequestered elements required an acid mixture somewhat more concentrated than that used for off-line batch preconcentration with this same material. Copper was selected as the target element for all studies relating to the elution of trace metals ofl the column as it is the most strongly bound of those studied [9]. Although all metals could be quantitatively recovered from the column in a 20 ~1 volume of elution acid, a minimum volume of 48 t~,lhad to be used because it was not possible to attach a sample loop onto the rotary valve which was smaller than this. The high capacity of this mini-column permitted large sample volumes to be preconcentrated without degradation in performance. Quantitative recovery of Mn was evident from seawater sample volumes up to 5000 ~1. This was the upper limit selected for evaluation because larger volumes required unrealistically long loading times (> 10 min) which detracted from the utility of the FI methodology. The low stability constants of alkali and alkaline earth quinoline complexes results in excellent rejection of these salts from the column and ease of operation with saline samples. Despite this, the column becomes loaded with Ca and Mg on sites not occupied by heavy metals. The majority of these can be easily washed from the resin using 300 ~1 of pH 9 DDW. Omission of this step results in the formation of significant non-specific background absorption in the furnace.
L. C.
96
AZEREDO et al.
Table 2. Absohrte procedural blank Element
Blank*(pg)
Cd CU Fe Mn Ni Pb Zn
0.4 12.1 23.3 0.7 14.1 3.7 4.6
+ 0.1 rt 2.3 f 1.4 +- 0.6 rt 3.4 I? 1.9 * 0.6
*Mean and standard deviation of three replicates.
The high stability of I-8HOQ resulted in a long lifetime for the column, permitting hundreds of “load” and “elute” cycles to be performed without loss of analytical performance. Figures of merit The absolute blank obtained for the analysis of seawater is summarized in Table 2. These values are orders of magnitude lower than those reported earlier using off-line batch techniques [ll] and lo-RIO-fold lower than those attained with an on-line FI system using solution phase chelation and sorption onto C&-reversed-phase silica [6]. This observation supports the conclusion that on-line FI systems are inherently cleaner than their off-line counterparts, as it is possible to process the sample and transfer it to the furnace with minimum exposure to the environment. Additionally, it is clear that immobilized ligands are also capable of providing lower system blanks as they are easily “self-cleaned” during each elution step. In all cases, the major source of the blank was determined to be the elution acid, despite its preparation from high purity concentrates. In several cases (Cd, Mn, Pb and Ni), the analytical blank is approaching the detection limit of GFAAS and is thus instrument noise limited (this is evidently the case for Mn). The guard or clean-up column placed on the buffered DDW line was effective in reducing the blank for Zn and Ni by 4-fold. Detection limits (LOD) are summarized in Table 3 and are defined as the absolute mass of element that gives a response equivalent to three times the standard deviation of the blank. Relative LODs were calculated assuming a 5 ml sample volume, which is the practical upper limit of the technique. Extrapolation of the relative LOD to other volumes can be confidently done as the blank is independent of the sample volume loaded and recovery is quantitative. These LODs are 6-2O-fold lower than
Table 3. Detection limit Element Cd cu Fe Mn Ni Pb Zn
Absolute*(pg)
Relativet(ng 1-l)
0.3 6.9 4.2 1.8 10.2 5.7 1.8
0.06 1.38 0.84 0.36 2.04 1.14 0.36
* Defined as the mass of analyte that gives a response equivalent to three times the standard deviation of the blank. t Based on a 5 ml sample volume.
Seawater trace metal determination by GFAAS
97
Table 4. Analytical results for NASS3 Concentration (pg 1-r) Element
a CU Fe Mn Mnt Ni Pb
Pbt Zn
This study*
0.031 0.108 0.349 0.024 0.0180 0.228 0.038 0.0409 0.173
f 0.003 (4) f 0.010 (10) + 0.009 (4) 2 0.097 (6) + 0.0904(6) + 0.020 (3) + 0.007 (4) rf:0.0006(4) +: 0.013 (6)
Certified value 0.029 0.109 0.327 0.022 0.022 0.257 0.039 0.039 0.178
2 + + f k + + * 2
0.004 0.011 0.022 0.007 0.007 0.027 0.006 0.006 0.025
* 835 p,f sample loop. t 5ooo ~1 sample loop. Values in parentheses are the numbers of replicate measurements.
those reported by SPERLINGet al. [5] and lo-lOO-fold lower than those reported by [6] who used on-line FI with solution phase chelation techniques. Due to the quantitative recovery of all trace elements of interest, an enrichment factor [2] (EF) of up to 125 can be realized with a 5 ml sample loading, thereby increasing the sensitivity of the graphite furnace technique by up to 250-fold compared with a standard 20 l.~linjection volume. With the present arrangement, sample throughput is approximately 12 per hour based on a 1 ml sample. This figure is degraded to 5 per hour with 5 ml sample volumes. The concentration efficiency [2] is thus 10 and 20 EF min-l, respectively, for 1 and 5 ml sample volumes. This figure of merit can only be improved by decreasing the sample processing time. PORTAet al.
Analytical results
The accuracy of the procedure was assessed by the analysis of several reference materials. Aqueous standard solutions (prepared in 48 ~1 volumes of elution acid) were used for calibration. These were not run through the FI manifold but injected directly into the furnace. Tables 4 and 5 summarize the results of replicate analyses. Excellent agreement with certified values is evident in all cases. Due to the extremely low levels of Pb and Mn in NAB-2, a 5 ml sample loop was used to improve precision of determination. The precision of determination of this element in CASS-2 could have been likewise improved had a 5 ml loop been used here as well. As a result of the relatively high concentration of Fe and Mn in CASS-2, a 200 ~1 sample loop was used Table 5. Analytical results for CASS-2 Concentration (jog 1-r) Element Cd cu Fet Mnt Ni Pb
This study* 0.017 0.664 1.15 2.09 0.303 0.018
” 0.002(S) zk 0.011(8) f 0.06 (6) + 0.10 (6) 2 0.011(9) + 0.004(5)
Certified value 0.019 0.675 1.20 1.99 0.298 0.019
+ + f * + f
0.004 0.039 0.12 0.15 0.036 0.006
* 835 ELIsample loop. t 209 pl sample loop. Values in parentheses are the number of replicate measurements.
98
L. C. AZERELW et al.
for these elements so as to avoid excessively large absorbances. In general, precision of replicate determination varies from 1510% (mean 6%) under conditions of optimum sample volume usage.
CONCLUSION The results presented here demonstrate the utility of use of immobilized reagents for on-line FI with GFAAS. The extremely low blanks make the technique particularly attractive for ultratrace determinations. Automation of the procedure, reduction of the acid elution loop volume to 20 l.~l, reduction of the wash volume to 100 ~1 (with increased buffer content) and the use of cleaner acid will serve to increase throughput and further decrease analytical blanks. If Mn is not an element of interest, the throughput could be considerably enhanced simply by increasing loading flow rates. Similarly, if calibration standards are processed on-line as well, compensating for any minor reductions in recovery, sample throughput could easily be improved more than S-fold [9] to 60 per hour (for a 1 ml sample). Acknow1edgemenbL.C.A. support while in Ottawa.
thanks the CNPq and the National Research Council of Canada for partial
REFERENCBS [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo]
J.F. Tyson, Spectrochim. Acta Rev. 14, 169 (1991). Z. Fang, Specfrochim. Acta Rev. 14, 235 (1991). R. E. Sturgeon, Spectrochim. Acta 44B, 1209 (1989). Z. Fang, M. Sperling and B. Welz, J. Anal. At. Spectrom. 5, 639 (1990). M. Sperling, X. Yin and B. Welz, J. Anal. At. Spectrom. 6, 295 (1991). V. Porta, 0. Abollino, E. Mentasti and C. Saaanini, J. Anal. At. Spectrom. 6, 119 (1991). E. Beinrohr, M. Cakrt, M. Rapta and P. Tarapci, Fresenius’ Z. Anal. Chem. 335, 1005 (1989). Z. Fang and B. Welz, J. Anal. At. Spectrom. 4, 543 (1989). R. E. Sturgeon, S. S. Berman, S. N. Willie and J. A. H. Desaulniers, Anal. Chem. 53, 2337 (1981). S. Nakashima, R. E. Sturgeon, S. N. Willie and S. S. Berman, Fresenius’ Z. Anal. Chem. 330, 592 (1988). [ll] S. N. Willie, R. E. Sturgeon and S. S. Berman, Anal. Chim. Acta 149, 59 (1983).
Acta, Vol. 488, No. 1, pp. 91-98,
1993
0.584-8547/93 sfl.al + .oo Pergamon Press Ltd
Printed in Great Britain.
Determination of trace metals in seawater by graphite furnace atomic absorption following on-line separation and preconcentration LAERTE C. AZEREW,* RALPH E. SruRGEoNt and ADILSON J. CURTIUS$ Institute for Environmental
Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KlA OR9 (Received 14 July 1992; accepted 2.5 July 1992)
Abstract-A discontinuous microscale preconcentration system, based on the chelation of trace metals by a 20 ul column of silica-immobilized 8hydroxyquinoline, was interfaced with a graphite furnace atomic absorption spectrometer. Volume-based loading of 8%SOOO ul sample loops, followed by elution of the sequestered elements directly into the furnace in 48 ~1 of acid, provided quantitative recovery of Fe, Cd, Zn, Cu, Ni, Mn and Pb from open ocean seawater with a frequency of lo-20 per hour. Absolute detection limits were 0.3, 6.9, 4.2, 1.8, 10.2, 5.7 and 1.8 pg for Cd, Cu, Fe, Mn, Ni, Pb and Zn, respectively. The sensitivity of the graphite furnace technique is enhanced up to 2%fold compared to a standard 20 ul injection volume. Results are presented for the analysis of these elements in NRCC seawater Reference Materials NASS3 and CASS-2 using simple calibration curves for quantitation.
FLOW injection (FI) technology has significantly enhanced the performance of atomic spectrometry [l]. An injector, pump and flow manifold in a variety of configurations may be used to replace conventional sample introduction systems. In general, the requirements of matrix removal and analyte preconcentration are priorities which have often driven developments in this area. FI on-line column preconcentration approaches have recently been reviewed by FANG [2]. Compared to their off-line batch counterparts, these systems offer a number of significant advantages for ultratrace determinations: greater efficiency; lower consumption of sample and reagent; improved precision; reduced risk of contamination; and increased sampling frequency and throughput as well as ease of automation. The first three factors serve to enhance the detection power of the system. Despite the impressive detection power of the technique, graphite furnace atomic absorption spectrometry (GFAAS) often cannot be routinely used for the direct trace element analysis of many natural samples of environmental interest [3]. Concentration of the desired trace elements can extend the detection limits, remove interfering concomitants and improve the precision and accuracy of the analytical results. On-line column preconcentration manifolds for GFAAS must operate in a discrete fashion as sample presentation is inherently discontinuous. Due to the increased difficulties encountered as a result of this requirement, few papers describe the successful interfacing of on-line columns to the auto-sampling arm of the spectrometer. FANG et al. [4] and SPERLING et al. [5] demonstrated the advantages to be gained with this approach. These authors utilized solution chelation of the trace metals of interest with diethyldithiocarbamate followed by their sorption onto a minicolumn (15 ~1) packed with (&bonded silica reversed-phase sorbent. Although good results were obtained for the determination of Cd, Cu, Pb and Ni in saline water, the approach was less than ideal because all of the analyte placed on the column could not be quantitatively
* On leave from UFRRJ, Departamento de Quimica, Km 47 Antiga, Estr. Rio-SP., Rio de Janeiro, Brazil. t Author to whom correspondence should be addressed. $ Depto. de Quimica da PUCYRio, 22453 Rio de Janeiro-RI, Braxil. NRCC No. 34223. 01992 Crown Copyright. 91
92
L. C. AZEREDO et al.
eluted in a solvent volume (ethanol) accommodated by the furnace. This necessitated reproducible introduction of a volume slice of eluant into the furnace that contained about 50% of the adsorbed analyte. A similar approach was taken by PORTA et al. [6], although the complexing agent was slightly different (pyrrolidinedithiocarbamate) and the analytes were eluted with 80 ~1 acetonitrile. Unfortunately, a double elution step was required for good recovery of a number of elements. BEINROHR et al. [7] utilized a short column packed with 79 l.~lof Spheron oxine (&hydroxyquinoline immobilized onto a methacrylate gel) for the determination of Pb in NaCl by GFAAS. Difficulties were encountered in the elution step and processing times were comparatively slow (5 min for 100 ~1 samples). Although FANG and WELZ [8] also reported recovery problems when using a similar material (8-quinolinol immobilized on controlled pore glass-CPG/8-Q ion exchanger) for on-line preconcentration of several elements for flame AAS, immobilized chelates offer a number of significant advantages over solution phase chelants [9]: these materials are reusable; they can be easily precleaned; no excess chelating agent is released into the elution solvent; their chelation properties are very similar to their free molecule counterparts; and they have sufficient capacity that small volume columns can be made very efficient. We report here on the analytical performance of a mini-column of silica-immobilized 8-hydroxyquinoline for the preconcentration of Cd, Cu, Fe, Mn, Ni, Pb and Zn from seawater using a discontinuous on-line FI-GFAAS system.
EXPERIMENTAL Reagents and samples All reagents were purified prior to use. Deionized, distilled water (DDW) was produced with a commercial mixed-bed ion-exchange system (Barnstead Nanopure) fed with distilled water. Concentrated HNOs and HCl were prepared by sub-boiling distillation in a quartz still using reagent grade feedstocks. From these, an acid mixture consisting of 2 M HCl and 0.8 M HNOs was prepared for elution of trace metals from the column. A saturated ammonia solution (28% v/v) was obtained by isothermal distillation of reagent grade ammonia liquor with collection of the evolved NH3 in chilled DDW. A 1 M NH&l buffer was prepared, adjusted to pH 9 with excess NI&OH, and used for subsequent DDW wash pH adjustment (50 p.1 ml-‘). Silicaimmobilized 8-hydroxyquinoline (I-8HOQ) was synthesized in-house [9] from 37-75 pm particle size Porasil B (Waters Associates, Milford, MA, U.S.A.) silica and used for the sequestration of trace metals from the samples. Stock solutions of all metals of interest were prepared by dissolution of the high-purity metals or their salts (Spex Ind., Edison, NJ, U.S.A.). All of the above operations were conducted in a clean room having class 10 workstations. Reference materials from the National Research Council of Canada (NRCC), NASS-3 open ocean seawater and CASS-2 coastal seawater, were analysed and used for validation of accuracy. Samples were adjusted to pH 9 by addition of NI-&OH. The required amount was ascertained beforehand using a small test portion so that in this manner no pH electrode was ever in contact with the analytical sample. Znstrumentation All analyses were performed using a Perk&Elmer model 5000 spectrometer equipped with an HGAJOO furnace, Zeeman background correction and an AS-40 autosampling device. Pyrolytic graphite coated tubes (without platforms) were used exclusively. The FI manifold consisted of a single four-channel Minipuls 3 model M 312 peristaltic pump (Gilson, Villeurs le Bel, France) and three metal-free Rheodyne Type 50 rotary injection TeflonR valves (Rheodyne Inc., Cotati, CA, U.S.A.). Three 125 ml TeflonR separatory funnels served as sample and reagent reservoirs. With the exception of tygon pump tubes used for solution delivery (0.051 in i.d.), all other conduits and loops consisted of nominally 0.063 in o.d. x 0.040 in i.d. TeflonR tubing interconnected via flangeless TefzelR nuts and ferrules (Upchurch Scientific). Figure 1 schematically illustrates the experimental set-up. An injection loop was fitted to each of the three rotary valves: 300 ~1 volume for DDW wash; 48 ~1 for acid eluant; and 0.835-5.0 ml for sample. A guard column for reagent clean-up was placed in the DDW line (pump channel No. 1) which consisted of a 6 cm long x 0.20 cm i.d. 200 ~1 volume of I-8HOQ. The material was retained within the tube using a small wad of precleaned quartz wool
Fig. 1. Flow injection manifold and sequence of operations.
LOADING
t
GC: guard column; AC: analytical column.
t
WASTE
ELUTION
r
94
L. C. AZEREDO et al.
sandwiched between the resin bed and a 2 mm length of 0.20 cm o.d. x 0.1 cm i.d. TeflonR tube inserted at either end. The analytical column (20 ~1) was similarly constructed from a 2.55 cm resin bed of I-SHOQ placed within a 0.1 cm i.d. TeflonR tube and positioned within the system such that it could be conveniently fastened to the sampling arm of the AS-40, thereby forming the interface between the FI system and the furnace. Procedure The FI manifold
and the sequence of its operation are shown in Fig. 1. A complete cycle of preconcentration and elution required approximately 240 s with a sample loading time of 100 s (corresponding to 0.835 ml of sample). In the “stand by” position the wash, acid and sample loops are filled using arbitrary flow rates of 300, 48 and 500 ~1 mm-‘, respectively. Air is used as the moving fluid in this system and in this position air is passing through the analytical column at a flow rate of 1.2 ml min-‘. In the “loading” step, the sample valve is manually actuated to deliver the contents of the sample loop to the column using air as the carrier at a flow rate of 250-500 ~1 min-’ (adjusted by changing the pump speed). Following passage of the seawater, the sample valve is restored to its initial position and the wash valve activated to deliver 300 t~,l of pH 9 DDW to the analytical column at an air flow rate of 300 p.1 mm-i. During this step, excess alkali and alkaline earth elements are removed from the resin bed and passage of “excess” air leaves the column free of any interstitial water so as to eliminate any dilution of the minimum volume of acid used in the subsequent elution step. With the wash valve returned to its initial position so as to refill the loop, the acid loop is brought on-line and pumped through the column by an air flow rate of 130 ~1 min-‘. At this stage, the effluent from the column containing the trace metals of interest is directed into the graphite tube by manually turning the AS-40 sampling arm into the inject position and holding it there for the duration of the elution phase (22 s). The various air flow rates indicated above arise, not as a result of intentional changes to the pump speed (unless indicated) or compression of the tygon pump tubing by the rotors, but rather in response to the various back pressures and nominally different inner diameters of the loops used (i.e. 1.0 mm sample loop; 0.9 mm acid loop; 0.7 mm wash loop). Following elution, the furnace program is manually initiated and the absorbance transient recorded and quantitated by both peak height and area measurement using in-house software compatible with the output from the Model 5000. As all potential matrix interferents are removed during the preconcentration step, quantitation was performed using peak height measurements and calibration was achieved by manual injection of standards, prepared in a 48 u.1 volume of the elution acid, directly into the furnace (atomization programs used for the elements were not substantially different from those recommended by the manufacturer). The acid elution valve was then returned to its “fill loop” position and the “wash” valve reactivated so as to flush the column of any remaining acid and return it to an alkaline pH optimal for loading the next sample.
RESULTS AND DISCUSSION
I-8HOQ was selected as the analytical column material for this study because, in addition to our lengthy experience in working with it in the batch mode [9], we have also found it to perform well in an off-line FI mode using small sample volumes [lo]. Several desirable properties arise which make it attractive for FI work. In addition to the advantages alluded to earlier, which are inherent to the use of immobilized chelating agents, I-8HOQ exhibits good stability in acidic media (necessary for elution of the analytes), high exchange capacity (0.06 mmol Cu g-l), excellent rejection of alkali and alkaline earth elements (> 99.9999% Na, 99.998% Mg and 99.95% Ca removal), large stability constants for metal chelates and non-specificity for a wide range of trace metals. The high exchange capacity results in a total capacity of the analytical column equivalent to 20 bg Cu-more than adequate for the trace levels of heavy metals encountered in small volumes of natural waters. Moreover, the kinetics of ion exchange with chelating sites is rapid [9], permitting relatively large sample flow rates to be used without deterioration of performance. Although reliance on the use of sample injection loops limits sample volumes to discrete amounts and requires multiple individual loops, this option was determined to be preferable to time-based sampling because the reproducibility of the speed of the peristaltic pump from sample-to-sample and day-to-day was poor.
Seawater trace metal determination
by GFAAS
95
Table 1. Recovery of trace elements from DDW and NASS-3 Recovery* (%) Element Cd cu Fe Mn Ni Pb Zn
DDW 100 97 93 99 98 92 99
f 2 rt 2 + f f
7(6) B(6) 5(6) 3(6) 4(6) 12(6) 9(6)
NASS3 97 98 93 94 96 93 98
I ? _’ 2 f 2 f
7(5) 7(5) 2(S) ll(5) 3(4) 6(7) 6(5)
* Mean and standard deviation. Number in parentheses is number of replicate measurements.
Column performance Table 1 summarizes data characterizing the recovery of trace elements from samples of spiked NASS3 and DDW. Quantitative recovery is evident for all elements from both media, illustrating that the saline matrix does not influence performance. Additionally, quantitation was achieved using simple standards prepared in the elution acid and manually injected into the furnace using a micropipette. No interferences are present since the eluant is virtually matrix-free, thereby permitting peak-height evaluation of the signals generated by atomization from the furnace wall. Conditions for optimal multielement sequestration onto I-8HOQ occur at pH > 8. In this study, samples were adjusted to both pH 8 (NASS-3) and 9 (CASS-2) prior to analysis with no difference in performance. Sample pH > 8 is usually avoided when large volumes are processed off-line in the batch mode of operation due to hydrolysis of the silica substrate and increase in the iron blank [9]. This problem was not evident here, likely as a consequence of the small sample volumes used. Sample loading onto the column was unaffected by changes in flow rate between 250-500 t~1min-l. At 1000 ~1 min-i the recovery of Mn decreased to 75%. Manganese was selected as the target element for all studies relating to the sequestration of trace metals onto the column as it is the most weakly bound of the suite of elements of interest and therefore most susceptible to loss because of poor exchange kinetics [9]. Thus, although sample loading was limited to flow rates below 500 p,l mitt-’ in this study in order to quantitatively recover Mn, significantly higher flow rates could undoubtedly be used if Mn was not an element of concern. Small volume elution of the sequestered elements required an acid mixture somewhat more concentrated than that used for off-line batch preconcentration with this same material. Copper was selected as the target element for all studies relating to the elution of trace metals ofl the column as it is the most strongly bound of those studied [9]. Although all metals could be quantitatively recovered from the column in a 20 ~1 volume of elution acid, a minimum volume of 48 t~,lhad to be used because it was not possible to attach a sample loop onto the rotary valve which was smaller than this. The high capacity of this mini-column permitted large sample volumes to be preconcentrated without degradation in performance. Quantitative recovery of Mn was evident from seawater sample volumes up to 5000 ~1. This was the upper limit selected for evaluation because larger volumes required unrealistically long loading times (> 10 min) which detracted from the utility of the FI methodology. The low stability constants of alkali and alkaline earth quinoline complexes results in excellent rejection of these salts from the column and ease of operation with saline samples. Despite this, the column becomes loaded with Ca and Mg on sites not occupied by heavy metals. The majority of these can be easily washed from the resin using 300 ~1 of pH 9 DDW. Omission of this step results in the formation of significant non-specific background absorption in the furnace.
L. C.
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AZEREDO et al.
Table 2. Absohrte procedural blank Element
Blank*(pg)
Cd CU Fe Mn Ni Pb Zn
0.4 12.1 23.3 0.7 14.1 3.7 4.6
+ 0.1 rt 2.3 f 1.4 +- 0.6 rt 3.4 I? 1.9 * 0.6
*Mean and standard deviation of three replicates.
The high stability of I-8HOQ resulted in a long lifetime for the column, permitting hundreds of “load” and “elute” cycles to be performed without loss of analytical performance. Figures of merit The absolute blank obtained for the analysis of seawater is summarized in Table 2. These values are orders of magnitude lower than those reported earlier using off-line batch techniques [ll] and lo-RIO-fold lower than those attained with an on-line FI system using solution phase chelation and sorption onto C&-reversed-phase silica [6]. This observation supports the conclusion that on-line FI systems are inherently cleaner than their off-line counterparts, as it is possible to process the sample and transfer it to the furnace with minimum exposure to the environment. Additionally, it is clear that immobilized ligands are also capable of providing lower system blanks as they are easily “self-cleaned” during each elution step. In all cases, the major source of the blank was determined to be the elution acid, despite its preparation from high purity concentrates. In several cases (Cd, Mn, Pb and Ni), the analytical blank is approaching the detection limit of GFAAS and is thus instrument noise limited (this is evidently the case for Mn). The guard or clean-up column placed on the buffered DDW line was effective in reducing the blank for Zn and Ni by 4-fold. Detection limits (LOD) are summarized in Table 3 and are defined as the absolute mass of element that gives a response equivalent to three times the standard deviation of the blank. Relative LODs were calculated assuming a 5 ml sample volume, which is the practical upper limit of the technique. Extrapolation of the relative LOD to other volumes can be confidently done as the blank is independent of the sample volume loaded and recovery is quantitative. These LODs are 6-2O-fold lower than
Table 3. Detection limit Element Cd cu Fe Mn Ni Pb Zn
Absolute*(pg)
Relativet(ng 1-l)
0.3 6.9 4.2 1.8 10.2 5.7 1.8
0.06 1.38 0.84 0.36 2.04 1.14 0.36
* Defined as the mass of analyte that gives a response equivalent to three times the standard deviation of the blank. t Based on a 5 ml sample volume.
Seawater trace metal determination by GFAAS
97
Table 4. Analytical results for NASS3 Concentration (pg 1-r) Element
a CU Fe Mn Mnt Ni Pb
Pbt Zn
This study*
0.031 0.108 0.349 0.024 0.0180 0.228 0.038 0.0409 0.173
f 0.003 (4) f 0.010 (10) + 0.009 (4) 2 0.097 (6) + 0.0904(6) + 0.020 (3) + 0.007 (4) rf:0.0006(4) +: 0.013 (6)
Certified value 0.029 0.109 0.327 0.022 0.022 0.257 0.039 0.039 0.178
2 + + f k + + * 2
0.004 0.011 0.022 0.007 0.007 0.027 0.006 0.006 0.025
* 835 p,f sample loop. t 5ooo ~1 sample loop. Values in parentheses are the numbers of replicate measurements.
those reported by SPERLINGet al. [5] and lo-lOO-fold lower than those reported by [6] who used on-line FI with solution phase chelation techniques. Due to the quantitative recovery of all trace elements of interest, an enrichment factor [2] (EF) of up to 125 can be realized with a 5 ml sample loading, thereby increasing the sensitivity of the graphite furnace technique by up to 250-fold compared with a standard 20 l.~linjection volume. With the present arrangement, sample throughput is approximately 12 per hour based on a 1 ml sample. This figure is degraded to 5 per hour with 5 ml sample volumes. The concentration efficiency [2] is thus 10 and 20 EF min-l, respectively, for 1 and 5 ml sample volumes. This figure of merit can only be improved by decreasing the sample processing time. PORTAet al.
Analytical results
The accuracy of the procedure was assessed by the analysis of several reference materials. Aqueous standard solutions (prepared in 48 ~1 volumes of elution acid) were used for calibration. These were not run through the FI manifold but injected directly into the furnace. Tables 4 and 5 summarize the results of replicate analyses. Excellent agreement with certified values is evident in all cases. Due to the extremely low levels of Pb and Mn in NAB-2, a 5 ml sample loop was used to improve precision of determination. The precision of determination of this element in CASS-2 could have been likewise improved had a 5 ml loop been used here as well. As a result of the relatively high concentration of Fe and Mn in CASS-2, a 200 ~1 sample loop was used Table 5. Analytical results for CASS-2 Concentration (jog 1-r) Element Cd cu Fet Mnt Ni Pb
This study* 0.017 0.664 1.15 2.09 0.303 0.018
” 0.002(S) zk 0.011(8) f 0.06 (6) + 0.10 (6) 2 0.011(9) + 0.004(5)
Certified value 0.019 0.675 1.20 1.99 0.298 0.019
+ + f * + f
0.004 0.039 0.12 0.15 0.036 0.006
* 835 ELIsample loop. t 209 pl sample loop. Values in parentheses are the number of replicate measurements.
98
L. C. AZERELW et al.
for these elements so as to avoid excessively large absorbances. In general, precision of replicate determination varies from 1510% (mean 6%) under conditions of optimum sample volume usage.
CONCLUSION The results presented here demonstrate the utility of use of immobilized reagents for on-line FI with GFAAS. The extremely low blanks make the technique particularly attractive for ultratrace determinations. Automation of the procedure, reduction of the acid elution loop volume to 20 l.~l, reduction of the wash volume to 100 ~1 (with increased buffer content) and the use of cleaner acid will serve to increase throughput and further decrease analytical blanks. If Mn is not an element of interest, the throughput could be considerably enhanced simply by increasing loading flow rates. Similarly, if calibration standards are processed on-line as well, compensating for any minor reductions in recovery, sample throughput could easily be improved more than S-fold [9] to 60 per hour (for a 1 ml sample). Acknow1edgemenbL.C.A. support while in Ottawa.
thanks the CNPq and the National Research Council of Canada for partial
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