Determination of heavy metals by inductively coupled plasma mass spectrometry after on-line separation and preconcentration

Spectrochimica Acta Part B (1998) 1527–1539

Determination of heavy metals by inductively coupled plasma mass spectrometry after on-line separation and preconcentration Valderi L. Dressler, Dirce Pozebon, Adilson J. Curtius* Departamento de Quı´mica da Universidade Federal de Santa Catarina, 88040-900 Floriano´polis, S.C., Brazil Received 6 January 1998; accepted 1 June 1998

Abstract A method for the determination of Cu, As, Se, Cd, In, Hg, Tl, Pb and Bi in waters and in biological materials by inductively coupled plasma mass spectrometry, after an on-line separation, is described. The matrix separation and analyte preconcentration is accomplished by retention of the analytes complexed with the ammonium salt of O,O-diethyl dithiophosphoric acid in a HNO 3 solution on C 18 immobilized on silica in a minicolumn. Methanol, as eluent, is introduced in the conventional pneumatic nebulizer of the instrument. In order to use the best compromise conditions, concerning the ligand and acid concentrations, the analytes were determined in two separate groups. The enrichment factors were in the range from 5 to 61, depending on the analyte. The limits of detection varied from 0.43 ng L −1 for Bi to 33 ng L −1 for Cu. The sample consumption is only 2.3 mL for each group and the sampling frequency is 21 h −1. The accuracy was tested by analysing five certified reference materials: water, riverine water, urine, bovine muscle and bovine liver. The agreement between obtained and certified concentrations was very good, except for As. The relatively small volume of methanol, used as eluent, minimizes the problems produced by the introduction of organic solvent into the plasma. q 1998 Elsevier Science B.V. All rights reserved Keywords: inductively coupled plasma mass spectrometry; flow injection analysis; O,O-diethyl dithiophosphoric acid; preconcentration; certified reference material

1. Introduction The main interferences found in inductively coupled plasma mass spectrometry (ICP-MS) are signal supression by elements of low ionization potential, such as sodium and potassium, polyatomic ion formation, space charge effect, obstruction of the sampling system by samples of highly dissolved solids contents, and fluctuation and physical changes in the plasma due to organic solvents [1]. Several of these problems may be minimized by matrix * Corresponding author. Fax: +55-48-3319711, e-mail: curtius@ cfm.ufsc.br

separation [2–14], use of high resolution spectrometers [15–17] or by alternate sample introduction systems, such as electrothermal vaporization [18–25], laser ablation [26,27], hydride generation [28–31] and others [32,33]. Most of the works on matrix separation and analyte preconcentration used complexing agents containing imidoacetate or oxine functional groups immobilized on several supports, such as polystyrene, divinylbenzene, glass of controlled porosity, carboxy methylcellulose, etc. [2–14]. In most of these separation methods, the analytes retained in the column are eluted with strong acid mixtures, normally a mixture of HCl and HNO 3. However, depending on the case,

0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 18 0 -3

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the eluent itself may produce polyatomic ions that may interfere with some analytes. The process of preconcentration and separation by sorption of the complexes and subsequent elution with an organic solvent may bring difficulties in ICP-MS, since this eluent cannot be directly introduced into the plasma when the pneumatic nebulization is used, unless non-conventional means are adopted. Probably, owing to this problem, analytical methods by ICP-MS were not found in the literature dealing with separation and preconcentration using an organic solvent as eluent. Carbon deposits on the interface, ionic lenses and torch, fluctuation of the plasma due to the high energy necessary for the dissociation of organic molecules and interferences by polyatomic ion formation, are some of the inconveniences resulting from the direct introduction of organic solvents into the plasma [34]. However, organic compounds or solvents increase the signal intensities for some elements [35–37] and also influence the formation equilibrium of some polyatomic species [34,38,39]. The increase in the radiofrequency power of the plasma, the addition of oxygen to the nebulizer gas flow, the use of miniaturized or refrigerated nebulization chambers and alternate sample introduction systems are some of the expedients used to eliminate or to reduce the effects caused by organic compounds in analysis by ICP-MS [40–43]. The salts of dithiophosphoric acid O,O-diethyl ester (DDTP) form complexes with a great number of elements which, after sorption on some sorbents of low polarity, are quantitatively eluted with methanol or ethanol. The DDTP was studied in the 1960s by Bode and Arnswald [44,45], who demonstrated that this ligand complexes several transition metals and semi-metals in an acid medium, depending on the nature and concentration of the acid, but does not react with alkaline, alkaline earth elements and others, such as Mn, V, Ti, Co, Cr, Zn, etc. This agent was used for separation and preconcentration of Pb, Cu, Cd, Mo and Ag in a variety of samples for posterior determinations of the analytes by flame [46–48] and electrothermal [49–52] atomic absorption spectrometry. In these works, it was demonstrated that DDTP is properly stable in acid media and that their complexes adsorbed on C 18 or activated carbon are eluted with methanol, ethanol or treated with nitric acid. This ligand is also selective in relation to the

transition metals, which is an advantage when a minicolumn filled with a sorbent is used for sorption, since saturation of the column is less possible. The formation of polyatomic ions of P and S, mainly with oxygen [53,54], and the problems already discussed resulting from the introduction of organic solvent into the plasma, make it difficult to use DDTP with conventional nebulization in ICP-MS. In this work, a flow injection system (FI) coupled to the ICP-MS was developed in order to use DDTP as a complexing agent for the separation and preconcentration of several elements in a minicolumn filled with C 18, and elution with methanol. The small amount of eluent delivered to the plasma avoids most of the inconveniences of introducing an organic compound into the plasma, through conventional nebulization.

2. Experimental 2.1. Instrumentation An inductively coupled plasma mass spectrometer from Perkin Elmer–Sciex ELAN 6000 was used. For the pneumatic nebulization a cross-flow nebulizer, a Scott-type nebulization chamber and a Gilson peristaltic pump were used. An injector alumina tube of 1.3 mm i.d. was used inside the torch. The optimization of the ICP-MS parameters was done by adjusting the nebulizer gas flow and the alignment of the mass spectrometer in relation to the torch (x–y adjustment) in order to obtain the maximum production of ions M + and minimum signals for M ++, MO + and background at m/z 220. The operational conditions for the ELAN 6000 are given in Table 1. Argon with a purity of 99.996% (White Martins, Brazil) was used. The FI system, shown in Fig. 1, contains six threeway PTFE solenoid valves (Cole Parmer, catalogue No. 01367-72) and two peristaltic pumps (P2 is from Ismatec and P1 is from Gilson) with Tygon tubes, except the tube for the propulsion of methanol which was a PVC tube. All connections in the FI system were made with PTFE tubes of 0.8 mm i.d. The column used for the preconcentration/separation (PC) was from Perkin Elmer (Part No. B0504047), which contained about 30 mg of C 18 immobilized on silica, with particle size between 40 and 63 mm. Another 15-mm-long purification column (CC),

V.L. Dressler et al./Spectrochimica Acta Part B (1998) 1527–1539 Table 1 Instrumental conditions and data parameters RF power Gas flow rate: principal intermediate nebulizer Sampler and skimmer Signal measurement Resolution m/z measurement/cycle Readings/replicate Dwell time Auto lens Measurement mode

1150 W 15 L min −1 1.2 L min −1 0.84 L min −1 Pt peak area 0.7 amu (at 10% peak height) 4–5 100 100 ms on peak hopping

similar to the preconcentration/separation column, but with an internal diameter of 5 mm and also filled with the same sorbent, was used to purify the DDTP solution. This column was necessary to lower the blank signals, specially for some elements, like As. The retained free DDTP in this purification column does not prejudice further complex sorption. This last column was always cleaned with methanol after 4 h of use to remove the retained complexes and free DDTP. All the separation and preconcentration steps were online. The FI system was managed by a separate personal minicomputer 486 DX with a program writen in Visual Basic. The electronic circuit for the operation of the solenoid valves, containing the temporizer ULN-2004-A from SGS-Thomson-Microeletronics, was home built.

Fig. 1. FIA manifold. P1 and P2: peristaltic pumps; PC: preconcentration/ separation column; CC: cleaning column; V1,...V6: solenoid valves; R: solution recycling; W: waste; x, 50 cm; y, 3 cm; z, 25 cm; numbers in parenthesis: solution flow rates, in mL min −1.

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In Table 2, the steps of the operational sequency for the FI system are summarized. The following steps are performed during a preconcentration/separation cycle. 1. Preconditioning of the system: The valve V 1 is operated to transport the DDTP solution to the preconcentration/separation column PC. In the beginning of each cycle, a small volume of this solution washes the line of the sample solution, including the line x and the column PC. The time of this step must be sufficient to have the DDTP solution after the meeting point with the sample. 2. Preconcentration/separation: While valve V 1 is still actuated, valve V 2 is also operated, mixing the sample with the purified DDTP solution. The complexes are retained in the column PC and the effluent is discharged after valve V 6 (W). At the end of this step valve V 2 is turned off, while valve V 1 is still operating. 3. Cleaning the separation system: To remove the sample matrix from the lines x and y and from the interstitial volume of the column, the DDTP solution flows through valve V 1. 4. Removing the DDTP solution from lines x and z: By operating valves V 5 and V 4 simultaneously, the complexing agent solution is removed from line x, with a flow of methanol, while line z is cleaned with a flow of water. At the end of this step, valves V 3 and V 4 are turned off, while valve V 5 is kept on. 5. Elution: Valve V 5 stays on after the previous step until the elution of the complexes retained on column PC is completed. In a pre-selected time, valve V 6 is actuated, ensuring that only the methanolic fraction containing the analytes reaches the nebulizer of the spectrometer. The methanolic fraction is carried out to the nebulizer by the water from the stream through valve V 3. This is done, turning off valve V 5 and actuating valve V 3. Valves V 3 and V 6 remain on until the end of the measurement in the spectrometer. 6. Cleaning the system: Column PC and lines x and y are washed with methanol by turning on valve V 5. The effluent is discharged through valve V 6 (W). 2.2. Materials and solutions Methanol (Carlo Erba), nitric acid (Carlo Erba) and hydrochloric acid (Merck), all of analytical grade,

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Table 2 The FIA system conditions Step

Time (s)

Valves on

Solution

Flow rate (mL min −1)

Function

1 2

0–5 5–65 65–85 85–107

5

107–114 111–145 115–145 146–165

DDTP DDTP Sample DDTP Water Methanol Methanol

1.0 1.0 2.3 1.0 0.7 1.4 1.4

Preconditioning Preconcentration

3 4

V1 V1 V2 V1 V3 V4, V5 V5 V6 V3 V5

Water Methanol

0.7 1.4

6

were further purified by sub-boiling distillation in a quartz still before their use. The hydrogen peroxide was Suprapur from Merck. The 1% w/v stock solution of the ammmonium salt of O,O-diethyl dithiophosphoric acid (Aldrich No. 17779-2, 95% purity) was prepared daily and purified on-line by passing through a column filled with C 18 immobilized on silica after a convenient dilution and mixing with a 4% v/v nitric acid solution. Stock solutions containing 10 mg mL −1 of the analytes in 1% v/v nitric acid were prepared from Spex products of high purity, except the As(III) and Se(IV) stock solutions. These were prepared from arsenic oxide, As 2O 3 and sodium selenite, Na 2SeO 3.5H 2O (both from Merck, analytical grade), respectively, in 1% v/v hydrochloric acid. From these solutions, an intermediate multielemental stock solution containing 200 ng mL −1 of As, Se, Pb, Cd, Bi, Cu and Tl in 1% v/v hydrochloric acid was prepared weekly. The analytical solutions, containing the analytes in the range 0.05–0.5 ng mL −1, were prepared just before their use, by diluting the intermediate solution with nitric acid solutions of different concentrations. External calibration was used for all analytes. The used water was distilled and deionized using the Milli-Q system of Millipore (resistivity of 18 MQ cm). The glassware was left in concentrated nitric acid (Merck, Suprapur or sub-boiling distilled), and then exhaustively rinsed with deionized water.

Column washing DDTP removing

Elution Column washing

used: Riverine Water SLRS-3 from the National Research Council Canada (NRCC), Water 1643d, Urine 2670 (normal level, reconstituted according to the instructions given in the certificate), Bovine Muscle 8414 and Bovine Liver 1577a, all from the National Institute of Standards and Technology (NIST). The urine sample was diluted 1 + 9 with a nitric acid solution using the same amount of acid that was used to obtain the analytical curves, the spike being added to the sample already diluted. The sample NIST 1643d was diluted 1 + 49 and 1 + 99 and the sample SLRS-3 was diluted 1 + 9. The dilutions of the water samples were always made in the same conditions used to obtain the analytical curves. The Bovine Muscle (NIST 8414) and Bovine Liver (NIST 1577a) were digested in a Milestone MLS 1200 MEGA microwave system. About 250 mg of the dried solid samples were weighed in quartz tubes and 1 mL of concentrated nitric acid and 1 mL of hydrogen peroxide were added. The tubes were placed in the microwave oven, which was subjected to the following power program: 2 min at 250 W, 2 min at 0 W, 6 min at 250 W, 5 min at 500 W and 5 min at 600 W. The program was run three times. After digestion, the solution was diluted to 25 mL with nitric acid. The spike for In and Tl was added before the digestion of the samples. 2.4. Optimization of the method

2.3. Certified reference materials and sample preparation The following standard reference materials were

The operational conditions of the FIA system were optimized, starting with a nitric acid and DDTP concentration of 3.0% v/v and 0.25% m/v, respectively.

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Fig. 2. Elution of the analytes as a function of the time (for 0.25 ng mL −1 of the analytes in 2.0% v/v HNO 3 and 0.3% w/v DDTP).

The operational conditions (nebulizer gas flow rate, radiofrequency power and sample flow rate in the nebulizer) of the ICP-MS instrument were reoptimized for the FI system.

3. Results and discussion 3.1. Parameters of the FI system and complexation Initially, a study of the FI system and optimization of the program to activate the valves were performed using an analytical solution containing 0.5 ng mL −1 of the analytes in 3% v/v nitric acid. The washing of the residual sample solutions inside the tubing and column after the preconcentration step (Table 2, step 3), volume of eluent (Table 2, step 5) and final washing of the system with methanol (Table 2, step 6) are of great importance. As also found by other authors [46,47], it is necessary to have DDTP in the cleaning solution used in the washing of the preconcentration column (Table 2, step 5), to avoid elution of the retained complexes. It was found that the complexes, especially those of Cu, As, Cd and Tl, are partially eluted with either water or 3% v/v nitric acid solution. The DDTP solution used in the washing of the column cannot be diluted too much, otherwise the complexes are partially eluted and lost. Using the same DDTP concentration as for the complexation, it is possible to remove the residual matrix from the column without

eluting the analytes. On the other side, the large amount of DDTP that stays in the system after the cleaning of the column, may be washed with the eluent and introduced in an ICP-MS instrument, increasing the polyatomic ions formation of S and P [53,54], which may interfere with some analytes, besides affecting the peak shape of the transient signals. So the DDTP solution retained in the system must be removed as completely as possible, before the elution step. To achieve this condition, valves V 3 and V 4 were placed in the system, making possible the removal of the DDTP solution retained in lines x and z, before and after the PC column, respectively. In this way, line x is washed with methanol through valve V 4, while line z is washed with water through valve V 3. Only the PC column and the line between the confluence points of the valves V 3 and V 4 remain loaded with DDTP solution. The length of tube y must be as short as possible, so that less DDTP solution stays in the system before elution. In this way, the dispersion of the methanol is also reduced and so is the elution, avoiding the loading of the plasma with organic solvent. Besides, the adequate selection of the time to operate valve V 6, allows that practically only the volume of the eluent containing the analytes and a minimum amount of DDTP, is introduced into the plasma. The time to actuate valve V 5 was chosen, so that the volume of methanol is just enough to elute most of the analytes. The selected time was 7 s, but since it depends on the tube conditions, that is, if it is

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Fig. 3. Influence of the DDTP concentration on the preconcentration of the analytes (for 0.5 ng mL −1 of the analytes in 3.0% v/v HNO 3).

new or old, it has to be reoptimized after about every 20 h of use, because the tube becomes less flexible and the eluent flow rate diminishes with the time of use. However, it was verified that a longer elution time, 8–9 s, does not significantly increase the signal intensity, as shown in Fig. 2. With a time of 7 s, more than 90% of the retained analytes are eluted. With the FIA system proposed in this work, resulting in a small volume of methanol used for elution and its on-line dilution (1 + 1), there is no need to use oxygen mixed with the nebulizer gas. A higher on-line dilution of the methanol, which could reduce the effects of the organic solvent, was not done, because the concentrations of some analytes in the samples were quite low. However, even if carbon deposits were not visible on the parts of the instrument, a gradual change in the ionic lens potential was verified after about 2 weeks of intense use, needing recalibration. Anyway, for better detection limits and analytical practice of the analysis, the smallest possible volume of methanol should be introduced into the plasma, avoiding plasma fluctuation and peak shape distortion of the transient signals. A small memory effect was verified after each cycle if the PC column was not washed after the elution. In this way, at the end of each cycle, the system is washed with methanol, being the washing efficiency dependend on the analyte and on its concentration. The effect of the DDTP and acid concentrations were studied, keeping the flow rate at 1.5 mL min −1 for the DDTP plus 4% v/v nitric acid and at 2.3 mL min −1 for the sample solution. As can be

seen in Fig. 3, very small DDTP concentrations are able to complex most of the analytes. For As, In and Tl, however, the signal intensities increase significantly as a function of the DDTP concentration. However, to minimize the amount of DDTP introduced into the plasma, concentrations of 0.2% and 0.3% w/v were chosen. As for the nitric acid, also very small concentrations are sufficient, as shown in Fig. 4. As a matter of fact, the acid added to the DDTP solution in order to clean it, is already enough for the complexation of all the studied analytes, except As and Tl. While for As the signal increases in function of the acid concentration, for Tl the signal decreases. Due to these findings, the analytes were divided in two groups. For Cu, As, Se, Hg and Bi, the used concentrations were 0.2% w/v DDTP and 3% v/v nitric acid, while for Cd, In, Tl and Pb, the chosen concentrations were 0.3% w/v DDTP and 1% v/v nitric acid. However, for the certified solid samples, that is Bovine Muscle and Bovine Liver, a higher nitric acid concentration, 4% v/v was used, because of their acid digestion. The most abundant isotopes of Cd and In could not be monitored, because it was verified that contamination of Sn, which is also complexed, interfered with the two isotopes [53]. Even considering that the natural abundance of 114 Sn and 115Sn are low (0.34 and 0.65%, respectively) in relation to the abundances of the Cd and In isotopes at m/z 114 and 115, respectively, they could not be measured, because the Cd and In concentrations are quite low in the samples and the Sn contamination

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Fig. 4. Influence of the HNO 3 concentration on the preconcentration of the analytes (for 0.5 ng mL −1 of the analytes in 0.4% w/v DDTP).

was relatively high. The source of the Sn contamination could not be found. The possible sources are the acid, the methanol or the water. The study was repeated using hydrochloric acid instead of nitric acid. The signal intensities for most of the analytes are around 20% lower when hydrochloric acid is used in the preconcentration. However, for Tl, the signal intensity increases about 50% and for In, the signal is close to the blank, showing that it is not complexed in a hydrochloric acid medium. Because of these findings and also because of the used acid in the digestions of the solid samples, nitric acid was chosen. The fact that DDTP complexes the analytes in a nitric acid medium is a great advantage, since this is the acid that produces less interferences in ICP-MS [53]. Besides, it is not necessary to add a buffer to the solutions, which very often is a source of contamination and needs to be on-line purified. In this way, the FI system is much simpler. It was found that the DDTP in a nitric acid solution degrades slowly, as can be sensed by the smell of sulfydric acid (H 2S). To guarantee its stability, and also to avoid contamination, the acidification and purification was done on-line. To be efficiently purified, specially in relation to Cu, As and Pb, the DDTP needs to be previously acidified. Acid was added to the sample solution, just before being submitted to the preconcentration and separation cycle, to prevent the oxidation of As, since the DDTP only complexes As(III) [52]. The effect of the sample flow rate on the signal intensities of the analyte can be observed on Fig. 5.

The signals of the analytes increase pronouncedly as a function of the sample flow rate up to around 4 mL min −1, except for Tl and Cu. For Cu, the signal intensities remain almost constant as a function of the sample flow rate. Copper and Tl are hard acids, so their complexation with DDTP is less favorable. Besides, these analytes are reduced by the ligand [45] before their complexation and the nitric acid medium does not help their reduction. Probably owing to these facts, the kinetics of the complexation is slow and increasing the sample flow rate, the Tl signal intensities decrease. 3.2. Analytical results For the analysis of the certified reference materials, external calibration with analytical solutions in the range from 0.05 to 0.4 ng mL −1 was used. The analytical solutions were submitted to the same preconcentration procedure in the same FI system as the sample solutions. The limit of detection, LD, was defined as the ratio between three times the standard deviation of 10 consecutive measurements of the blank and the slope of the analytical curve of the analyte. The enrichment factor (EF) was obtained by comparing the signals for 80 mL of aqueous analytical solution containing 7.0 mg L −1 of the analytes, introduced directly in the nebulizer of the instrument and of an analytical solution containing 0.2 mg mL −1 of the analytes, introduced in the FI system coupled to the nebulizer. The comparison is not fair, because the signal in

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Fig. 5. Influence of the sample flow rate on the preconcentration of the analytes (for 0.25 ng mL −1 of the analytes in 3.0% v/v HNO 3 and 0.4% w/v DDTP).

the aquoeus medium is compared with the signal in the methanolic medium, which also contains unknown amount of DDTP. Assuming 100% analyte retention, the EF would be 23. Higher values were obtained, for As and Se, as shown in Table 3, owing to the organic enhacement effects. The low enrichment factor for Tl and In are due to the partial elution of the analyte during the washing of the PC column and to the Sn contamination, respectively. The formation of the polyatomic ion 31P16O 2 [54] and the partial elution of the complex during the washing of the column are responsible for the relatively high LD of 63 Cu. All figures of merit of the proposed method are shown on Tables 3 and 4. The precision is reasonable, Table 3 Figures of merit of the proposed method Isotope

Parameters LD (ng L −1)

63

Cu As(III) 77 Se(IV) 111 Cd 205 Tl 113 In 202 Hg 208 Pb 209 Bi 75

a

33 5.1 6.6 6.7 0.79 14.6 4.8 4.5 0.43

n = 6 (0.2 ng mL −1).

Enrichment r factor

RSD a

with a relative standard deviation (RSD) below 6% for six consecutive readings of the analytical solution containing 0.2 ng mL −1 of the analytes, as shown on Table 3. For the samples, the standard deviations are shown on Table 5. They were calculated using six sample replicates, the value of each replicate being the average of five consecutive measurements. More than 500 determinations can be performed with the same PC column, without significant change in the signal intensities. The matrix separation efficiency can be checked by the transient signals shown on Fig. 6. As can be seen, the signals of the analytical and sample solution are quite similar, even for the urine sample, which is very complex. It can also be observed that As does not complex quantitatively in the samples, probably because is partially present as As(V). Similar transient signals, not shown in Fig. 6, were also obtained for the other analysed certified materials (Water NIST 1643d, Riverine Water NRCC SLRS-3, Bovine Muscle NIST

(%) 22 27 61 19 5 9 16 22 20

0.9980 0.9990 0.9993 0.9990 0.9989 0.9999 0.9986 0.9998 0.9997

4.6 3.6 4.2 4.4 3.5 4.6 2.8 4.3 5.5

Table 4 The FI-ICP-MS parameters Sample loading rate, mL min −1 Preconcentration time, s Reagent consumption, mL: DDTP 0.6% (w/v) methanol HNO 3 4% (v/v) Sampling frequency, h −1

2.3 60 1.65 0.1 0.75 21

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Table 5 The analysis of the certified reference materials (mean of 6 replicates) a Analyte

Cu Cd Se As

Tl Pb Bi

Hg Analyte

Cu Cd Se As Hg Tl Pb Bi

In

a

Sample

Certified Measured Certified Measured Certified Measured Certified Measured Spike Recovered Certified Measured Certified Measured Certified Measured Spike Recovered Certified Measured

SLRS − 3 (mg L −1)

2670 (mg L −1)

20.5 6 3.8 18.4 (0.96) 6.47 6 0.37 6.93 (0.16) 11.43 6 0.17 12.66 (0.69) 56.02 6 0.73 26.64 (0.85) – – 7.28 6 0.25 7.02 (0.17) 18.15 6 0.64 18.41 (0.37) 13 b 13.22 (1.47) – – – –

1.35 6 0.07 1.47 (0.04) 0.013 6 0.002 0.010 (0.002) – – 0.72 6 0.05 0.15 (0.02) – – – – 0.068 6 0.007 0.046 (0.005) – – – – – –

0.13 6 0.02 0.11 (0.01) 0.00040 b 0.00060 (0.00006) 0.030 6 0.008 0.028 (0.001) 0.06 b 0.023 (0.004) 0.100 0.82 (0.002) – 0.0001 (0.00004) 0.01 b 0.0066 (0.0003) – , LD 0.100 0.101 (0.008) 0.002 b 0.0021 (0.0003)

1577a (mg g −1)

8414 (mg g −1)

158 6 7 – 0.44 6 0.06 0.41 (0.03) 0.71 6 0.07 0.71 (0.005) 0.047 6 0.006 , LD 0.004 6 0.002 0.005 (0.001) 0.003 b 0.003 (0.0004) 0.135 6 0.015 0.115 (0.001) – , LD 0.100 0.096 (0.002) – , LD 0.100 0.11 (0.03)

2.84 6 0.45 2.19 (0.20) 0.013 6 0.011 0.012 (0.001) 0.076 6 0.010 0.073 (0.014) 0.009 6 0.003 , LD 0.005 6 0.003 0.004 (0.001) – – 0.38 6 0.24 0.35 (0.03) – –

Sample

Certified Measured Certified Measured Certified Measured Certified Measured Certified Measured Certified Measured Certified Measured Certified Measured Spike Recovered Certified Measured Spike Recovered

Values in parenthesis are the standard deviations. Not certified.

b

1643d (mg L −1)

– – – –

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Fig. 6. Transient signals obtained with the FIA system. (a) and (a9) are for 0.3 ng mL −1 of analytical solution; (b) and (b9) are for the urine sample 10-fold diluted; (c) and (c9) are for the bovine liver spiked with 0.1 ng mL −1 of In and Bi.

V.L. Dressler et al./Spectrochimica Acta Part B (1998) 1527–1539

8414). Due to the high concentration of Cu in the samples, its signals are not shown in Fig. 6. Except for As, for the reason already presented, the concentrations obtained agree very well with the certified or informed concentration values, as shown in Table 5. Copper was not determined in the Bovine Liver sample, because of its high concentration.

4. Conclusions The results obtained demonstrate that the coupling FI-ICP-MS, using on-line complexation with DDTP and sorption of the complexes in a minicolumn filled with C 18 silica gel and elution with methanol is adequate for the separation and preconcentration of Cu, As, Se, Cd, In, Hg, Tl, Pb, Bi and probably of other elements, not investigated in this work, in a variety of samples. The proposed system, besides providing an increase in the sensitivity, allows the separation of the matrix, so that external calibration can be used. Also, low sample and chemicals consumption, high sampling frequency and relative standard deviation below 6% are achieved with the system. The complexation with DDTP with the studied elements occurs even at high acid concentrations and does not require the use of a buffer solution as is the case for most of the other complexing agents, immobilized or not. This is especially advantageous for samples digested with acids. Besides, complexing agents such as 8-hydroxyquinoline and Chelex-100, do not complex As, Se, In and Tl. The relatively small volume of methanol, used as eluent, and its introduction into the nebulizer of the instrument by a FI system, minimize the formation of carbon deposits on the interface and on other parts of the spectrometer, so that there is no need to use oxygen mixed with the nebulizer gas. In this way, changes were not found in the response of the instrument during several working days. However, the potential of the ionic lens should be monitored daily and eventually reoptimized after 500–600 cycles. Also, after a long period of use, even if a carbon deposit is not seen, torch, cones and lens should be cleaned.

Acknowledgements We thank Financiadora de Estudos e Projetos

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(FINEP) for financial support. D. Pozebon and V. L. Dressler have doctorate scholarships from Conselho Nacional de Pesquisas e Desenvolvimento Tecnolo´gico (CNPq)

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