Reduction of interferences in graphite furnace atomic absorption spectrometry by multiple linear regression modelling

Spectrochimica Acta Part B 55 Ž2000. 1847᎐1860

Reduction of interferences in graphite furnace atomic absorption spectrometry by multiple linear regression modelling Marco GrottiU , Maria Luisa Abelmoschi, Francesco Soggia, Christian Tiberiade, Roberto Frache Dipartimento di Chimica e Chimica Industriale, Sezione di Chimica Analitica e Ambientale, Uni¨ ersita ` di Geno¨ a, Via Dodecaneso 31, 16146 Geno¨ a, Italy Received 16 November 1999; accepted 8 September 2000

Abstract The multivariate effects of Na, K, Mg and Ca as nitrates on the electrothermal atomisation of manganese, cadmium and iron were studied by multiple linear regression modelling. Since the models proved to efficiently predict the effects of the considered matrix elements in a wide range of concentrations, they were applied to correct the interferences occurring in the determination of trace elements in seawater after pre-concentration of the analytes. In order to obtain a statistically significant number of samples, a large volume of the certified seawater reference materials CASS-3 and NASS-3 was treated with Chelex-100 resin; then, the chelating resin was separated from the solution, divided into several sub-samples, each of them was eluted with nitric acid and analysed by electrothermal atomic absorption spectrometry Žfor trace element determinations. and inductively coupled plasma optical emission spectrometry Žfor matrix element determinations.. To minimise any other systematic error besides that due to matrix effects, accuracy of the pre-concentration step and contamination levels of the procedure were checked by inductively coupled plasma mass spectrometric measurements. Analytical results obtained by applying the multiple linear regression models were compared with those obtained with other calibration methods, such as external calibration using acid-based standards, external calibration using matrix-matched standards and the analyte addition technique. Empirical models proved to efficiently reduce interferences occurring in the analysis of real samples, allowing an improvement of accuracy better than for other calibration methods. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Multivariate analysis; Empirical modelling; Electrothermal atomisation atomic absorption spectrometry; Interferences; Calibration

U

Corresponding author. Fax: q39-10-353-6190. E-mail address: [email protected] ŽM. Grotti.. 0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 2 9 0 - 1

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M. Grotti et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1847᎐1860

1. Introduction

Table 2 Temperature programs for GTA 96 graphite tube atomiser

There is great interest in the determination of trace elements in seawater, including environmental quality evaluation, speciation studies and investigation on biogeochemical cycles of the elements. Because of the extremely low concentration of heavy metals in seawater, the most common analytical procedures include a pre-concentration step w1᎐21x, although methods for direct determination have also been proposed w22᎐28x. One of the widely employed chelating agents for trace metals pre-concentration is Chelex-100, the selectivity for heavy metals of which secures elimination of most of the marine salts, and good recovery. Nevertheless, elements eluted together with the analyte may interfere in the following determination by electrothermal atomic absorption spectrometry ŽETAAS., and a suitable method to overcome matrix effects must be applied. Empirical modelling of interferences in atomic spectrometry could present a simple and satisfactory method, both to obtain a multivariate evaluation of the complex interfering effects due to a composite matrix, and to provide a computational correction of matrix effects w29᎐33x. The aim of the present work was to study the applicability of an empirical modelling approach in order to reduce matrix effects in the ETAAS determination of trace elements in seawater after a pre-concentration step. The interferences of Na, K, Mg and Ca as nitrates on the electrothermal atomisation of manganese, cadmium and iron were modelled by using the multiple linear re-

Step

Temperature Ž⬚C.

Time Žs.

Ar flow rate Žl miny1 .

Read

1 2 3 4 5 6 7 8 9

50 95 120 TPyra TPyra TPyra TAtom b TAtom b TClean c

5 60 10 10 10 2 0 2 2

3.0 3.0 3.0 3.0 3.0 0 0 0 3.0

No No No No No No Yes Yes No

Table 1 Instrumental parameters for ETAAS measurements Parameter

Cd

Mn

Fe

Wavelength Žnm. Lamp current ŽmA. Slit width Žnm. Background correction Signal measurement Integration time Žs. Time constant Žs. Injection volume Ž␮l.

228.8 279.5 248.3 7 8 10 0.5 0.2 0.2 Zeeman effect Integrated absorbance 3.0 3.0 3.0 0.05 0.05 0.05 20 20 20

a Pyrolysis temperature: 250⬚C for Cd, 800⬚C for Mn and 800 ⬚C for Fe. b Atomisation temperature: 2000⬚C for Cd, 2400⬚C for Mn, and 2300 ⬚C for Fe. c Cleaning temperature, set 100⬚C higher than the corresponding atomisation temperature.

gression ŽMLR. method in conjunction with a suitable experimental design. Afterwards, the models were applied to correct the matrix effects occurring in the ETAAS determination. Analytical results obtained by applying the MLR models were compared with those obtained with other calibration techniques, such as external calibration using acid-based or matrixmatched standards and the analyte addition technique.

2. Experimental 2.1. Instrumentation ETAAS measurements were carried out using a Varian ŽSpringvale, Australia. SpectrAA 300 atomic absorption spectrometer, equipped with a GTA 96 graphite atomiser and a PSC 56 programmable sample changer. Pyrolytic graphitecoated tubes were used. Instrument parameters, reported in Table 1, were chosen to provide optimum analytical performance. The temperature programs, reported in Table 2, were optimised in order to get the highest integrated absorbance value and maximum pyrolysis temperature without loss of analyte.

M. Grotti et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1847᎐1860

Inductively coupled plasma optical emission spectrometric ŽICP-OES. analyses were performed using a Varian ŽSpringvale, Australia. Liberty 100 atomic emission spectrometer, while inductively coupled plasma mass spectrometric ŽICP-MS. measurements were carried out on a Perkin Elmer Sciex Elan 6000 ICP mass spectrometer. 2.2. Reagents and samples Standard solutions Ž1 and 10 mg mly1 . of Na, K, Mg and Ca nitrate ŽBDH Chemicals. were used as matrix stock solutions. Standard solutions Ž1 mg mly1 . of Mn, Cd and Fe nitrates ŽBDH Chemicals. were used as the analyte stock solutions. Synthetic samples were prepared daily by dilution with Milli-Q water ŽMillipore.. All solutions were in 0.1 mol ly1 HNO3 . Chelex-100 ŽNaq form., 50᎐100 mesh, was obtained from Bio-Rad Laboratories. Nitric acid and sodium carbonate were of suprapure grade quality ŽMerck.. The certified seawater reference materials CASS-3 and NASS-3 ŽNational Research Council, Canada. were used. 2.3. Analytical procedure Pre-concentration was effected by a batch equilibration procedure reported earlier w34x. Chelex100 Ž2.4 g., prepared in Hq form, was added to

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exactly 3200 ml of seawater, after attaining a pH of 6᎐7 by the addition of approximately 5.76 g of Na 2 CO 3 . The water and the exchanger were shaken for 24 h using a self-made rotating apparatus. The exchanger was separated by filtration through a 60᎐␮m nylon filter, washed with 500 ml of Milli-Q water and dried under a laminar flux for 3 days. The resin was divided into 16 sub-samples of the same weight Ž0.150 g., and each of them was eluted with 2 ml of 1 mol ly1 HNO3 left in contact for 1 h. Finally, the solutions were diluted with Milli-Q water in order to obtain an optimal concentration for the analysis by ETAAS and ICP OES. Concentrations of standards and the linear regression coefficients of ETAAS calibration curves are reported in Table 3. Data obtained when using matrix-matched standards were corrected for contamination of matrix-element standard solutions. 2.4. Multi¨ ariate experiments and data processing The multivariate study of matrix effects was performed according to a method previously reported w30,31x. The adopted experimental design is presented in Table 4. According to this, the experiments were performed on a spherical domain, at the vertices of the hypercube corresponding to a 2 5y 1 fractional design Žpoints 1᎐16., at the so-called ‘star points’ Žpoints 17᎐26. and at the centre of the domain Žpoints 27᎐32.. The replicates at the centre point give the estimate of

Table 3 Parameters of the calibration techniques Calibration technique

External calibration, using acid-based standards External calibration using matrix-matched standardsa Analyte addition technique a

Concentrations of standards Žng mly1 .

Linear regression coefficientsd

Fe

Fe

0; 8; 16 0; 8; 16 0; 8; 16

Mn

Cd b

0; 3; 6 0; 1; 2c 0; 3; 6b 0; 1; 2c 0; 3; 6b 0; 1; 2c

0; 1; 2

0.999

0; 1; 2

0.997

0; 1; 2

0.999

Matrix-matched standards were prepared according to matrix composition reported in Table 6. Data referred to CASS-3 analyses. c Data referred to NASS-3 analyses. d Mean value relative to 16 calibration curves. b

Mn

Cd b

1.000 1.000c 0.998b 0.986c 1.000b 0.991c

0.998 1.000 0.998

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Table 4 Experimental design matrix Run

ŽNa.

ŽK.

ŽMg.

ŽCa.

ŽAnalyte.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

y1 q1 y1 q1 y1 q1 y1 q1 y1 q1 y1 q1 y1 q1 y1 q1

y1 y1 q1 q1 y1 y1 q1 q1 y1 y1 q1 q1 y1 y1 q1 q1

y1 y1 y1 y1 q1 q1 q1 q1 y1 y1 y1 y1 q1 q1 q1 q1

y1 y1 y1 y1 y1 y1 y1 y1 q1 q1 q1 q1 q1 q1 q1 q1

q1 y1 y1 q1 y1 q1 q1 y1 y1 q1 q1 y1 q1 y1 y1 q1

17

y'5 q'5

0

0

0

0

0

0

0

0

18 19

0

y'5

0

0

0

20

0

q'5

0

0

0

21

0

0

y'5

0

0

22

0

0

q'5

0

0

23

0

0

0

y'5

24

0

0

0

q'5

25

0

0

0

0

y'5

26 27 28 29 30 31 32

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

q'5 0 0 0 0 0 0

0 0

the experimental variance, taking into account both the instrumental precision and the sample preparation procedure. By fixing the ranges of the variables and the scale Ža logarithmic scale for the interfering elements and a linear scale for the analytes ., the coded values were replaced by real values and the experimental plan was obtained ŽTable 5.. According to this, each solution was prepared and analysed by ETAAS, using a calibration curve based on standard solutions in 0.1 mol ly1 HNO3 . Data were processed by performing an MLR analysis, in which the added element concentrations were considered as independent variables

and the measured values as the dependent variable. The quality of the MLR analyses was tested by performing a ‘cross-validation’ procedure w35x, according to which, each experiment was removed from the training set and the model recalculated. Then, the predicted value of the missing experiment was computed by the new model and compared with the true one. This procedure was repeated for all the experiments and the explained variance ŽEV. was calculated: N

Ý Ž ˆyi y yi .

2

i

EV s 100 y

N

)100

N

Ý Ž yi y yi . i

Ž 14.

2

Ny1

where yi is the experimental value of experiment i, ˆ yi the predicted one and N the number of experiments. Multiple linear regression analyses and the other statistical calculations were performed using the PARVUS 1.2 package of programs w36x.

3. Results and discussion 3.1. Multi¨ ariate study of the matrix effects In every analytical determination in which an interference occurs, the measured concentration differs from the true value, depending on the concentration of interfering elements. This relationship can be efficiently modelled by the following equation: n

cf s b0 q

n

n

Ý bi c i q Ý bii c i2 q Ý is1

is1

bi j c i c j

Ž2.

i/j;i , js1

where c f is the found concentration, c i the true concentration of the i elements Žincluding the analyte and matrix elements. and bi the coefficients of the model. These coefficients can be easily deduced from a limited number of experimental data by per-

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Table 5 Experimental plan and analytical results of the interference multivariate study Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Matrix element concentrations Ž␮g mly1 .

Analyte concentrations Žng mly1 .

Na

Iron

4.1 122.5 4.1 122.5 4.1 122.5 4.1 122.5 4.1 122.5 4.1 122.5 4.1 122.5 4.1 122.5 0.5 1000 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 a

K

4.1 4.1 122.5 122.5 4.1 4.1 122.5 122.5 4.1 4.1 122.5 122.5 4.1 4.1 122.5 122.5 22.4 22.4 0.5 1000 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4

Mg

4.1 4.1 4.1 4.1 122.5 122.5 122.5 122.5 4.1 4.1 4.1 4.1 122.5 122.5 122.5 122.5 22.4 22.4 22.4 22.4 0.5 1000 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4

Ca

4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 122.5 122.5 122.5 122.5 122.5 122.5 122.5 122.5 22.4 22.4 22.4 22.4 22.4 22.4 0.5 1000 22.4 22.4 22.4 22.4 22.4 22.4 22.4 22.4

Manganese

Cadmium

Added

Found

Added

Found

Added

Found

14.5 5.5 5.5 14.5 5.5 14.5 14.5 5.5 5.5 14.5 14.5 5.5 14.5 5.5 5.5 14.5 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 0.0 20.0 10.0 10.0 10.0 10.0 10.0 10.0

14.2 6.0 5.7 15.3 7.7 17.7 17.4 7.6 6.0 15.3 15.0 6.5 16.0 7.8 6.8 16.2 11.0 11.1 11.7 11.9 10.4 12.1 11.7 11.9 0.8 21.2 11.1 10.9 11.2 11.3 11.1 11.3

10.0 4.0 4.0 10.0 4.0 10.0 10.0 4.0 4.0 10.0 10.0 4.0 10.0 4.0 4.0 10.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 0.3 13.7 7.0 7.0 7.0 7.0 7.0 7.0

9.1 4.5 4.3 10.8 5.0 12.5 12.1 5.8 4.1 10.5 10.0 5.1 9.5 5.1 4.8 10.8 7.6 8.7 7.5 9.4 7.7 6.6a 8.9 7.5 0.4 12.4 7.6 7.8 7.7 7.8 7.8 8.2

1.45 0.55 0.55 1.45 0.55 1.45 1.45 0.55 0.55 1.45 1.45 0.55 1.45 0.55 0.55 1.45 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.00 2.00 1.00 1.00 1.00 1.00 1.00 1.00

1.94a 0.66 0.64 1.77 0.74a 1.68 1.79 0.63 0.72 1.68 1.73 0.48 1.94 0.76 0.66 1.83 1.40 0.78 1.43 1.13 1.43 0.98 1.40 1.25 0.00 2.75 1.17 1.32 1.38 1.40 1.42 1.43

Measurement not included in the computation, corresponds to an outlier Ž P - 0.01..

forming an MLR analysis, according to the procedure extensively described in the experimental section. In the present study, we have investigated the multivariate effect of Na, K, Mg, and Ca, as nitrates, on the ETAAS determination of Fe, Mn and Cd, when the matrix elements are present simultaneously at concentration levels ranging from 0.5 to 1000 ␮g mly1 . The interfering elements were selected because they are the major

constituents in water samples after the pre-concentration step of the considered analytical procedure. The experimental plan and the relative analytical results Žaverage of four instrumental readings. are reported in Table 5. By comparing the values of the analyte concentrations found with the concentrations added, it is evident that systematic errors occur, the magnitude of which depends on matrix element concentrations. By processing data

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reported in Table 5, the following models were deduced Žonly terms having a probability level ) 99.9% are reported.: w Fex f s 1.1q 4.57 Ž Fe. q 0.66 Ž Mg.

Ž3.

w Mn x f s 8.10q 2.83 Ž Mn . y 0.34 Ž Mn . 2 q q0.33 Ž Na. q 0.30 Ž K . q q0.38 Ž Mg. y 0.28 Ž Ca . q y0.38 Ž Mg.Ž Ca .

Ž4.

w Cdx f s 1.304q 0.596 Ž Cd. y 0.148 Ž Na. q y0.092 Ž Mg. q 2

y0.048 Ž Na. q 0.140 Ž Na.Ž K . q Ž5. q0.116 Ž Na.Ž Ca . q y0.125 Ž Na.Ž Cd. q q0.122 Ž K .Ž Mg. q 0.158 Ž Mg.Ž Ca . q y0.122 Ž Mg.Ž Cd. where the symbols enclosed between round brackets indicate the theoretical Žadded. concentrations, expressed as coded values, and wFex f , wMnx f and wCdx f are the concentrations found for Fe, Mn and Cd, respectively, expressed in ng mly1 . The model relative to iron ŽEq. Ž3.. is very simple. The iron concentration found when using external calibration differs from the true Žadded. one because of an interference due to magnesium nitrate, which causes an overestimation of the analyte concentration. This analytical error increases linearly Žon a logarithmic scale. as magnesium concentration increases. The model of manganese ŽEq. Ž4.. shows that the analytical result is greatly affected by the presence of all the matrix elements, leading to an under- or over-estimation of the analyte concentration, depending on the amount of the interfering salts. In fact, the analytical signal is decreased by calcium, while it is increased by the other

three elements. The quadratic term including the analyte concentration indicates that a non-linear dependence between the absorbance signal and the analyte concentration exists in the presence of the matrix. Moreover, the interaction between magnesium and calcium reflects that the effect of the former depends on the concentration of the latter. The complexity of the observed interference effect could represent a theoretical limit to the application of the analyte addition technique, and of the matrix-matched calibration curve technique, both based on simple linear relationships. The model relative to cadmium ŽEq. Ž5.. includes several terms, showing how complex the relationship between the interference effect and matrix composition can be. The concentration of cadmium found when an external calibration is used is strongly affected by the presence of sodium and magnesium, their effect being dependent on cadmium concentration Žin fact, the interactions between Na or Mg and Cd are significant.. Moreover, magnesium shows a linear interfering effect, while the sodium effect is modelled by a quadratic term. Finally, further significant interactions between matrix elements were deduced and must be considered to obtain a complete quantification of interfering effects. Finally, in order to verify the validity of the models, the MLR coefficients Ž r 2 . and EV values Žsee Eq. Ž1.. were computed. It was found that the models in Eqs. Ž3. ᎐ Ž5. are characterised by r 2 values of 0.99, 0.98, 0.98 and EV values of 99, 97, and 96%, respectively. From these values, it was concluded that all the models are satisfactory, both to fit experimental data, and to predict the response inside the experimental domain. 3.2. Reduction of matrix effects by multiple linear regression modelling Since the empirical models in Eqs. Ž3. ᎐ Ž5. proved to be capable of predicting the interference effects of the considered matrix elements on the ETAAS determination of Fe, Mn and Cd, it may be expected that the models can be successfully used to correct for these matrix effects.

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Fig. 1. Analytical procedure.

In order to verify this assumption, determination of the considered trace elements in the certified seawater reference materials CASS-3 and NASS-3 was performed. The analytical procedure is described in the Section 2 and schematically shown in Fig. 1. The results are summarised in Table 6, in which mean values of the measured concentrations of matrix elements and analytes in the 16 sub-samples are reported, using an external calibration. A good repeatability of the analyt-

ical technique can be deduced from the values of the standard deviation. Although the pre-concentration procedure allowed the elimination of most of the marine salts, their removal was not complete, and interferences in the following ETAAS determinations occurred. In order to minimise any other systematic error beside that due to matrix effects, the accuracy of the pre-concentration step and contamination levels of the procedure were checked by ICP-MS and ETAAS

M. Grotti et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1847᎐1860

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Table 6 Mean values of the concentrations found in the 16 sub-samples a After pre-concentration

Dilution

CASS-3b

NASS-3c

CASS-3

NASS-3

CASS-3

In graphite furnace NASS-3

Fe Na Mg K Ca

130 " 8 369 " 28 1381 " 100 17.4" 1.8 1248 " 114

39 " 3 416 " 16 1615 " 60 14.0" 0.5 1232 " 42

1:10

1:4

12.9" 0.9 37 " 3 138 " 10 1.7" 0.2 125 " 11

9.7" 0.8 104 " 4 404 " 15 3.5" 0.1 308 " 11

Mn Na Mg K Ca

264 " 17 369 " 28 1381 " 100 17.4" 1.8 1248 " 114

2.3" 0.3 416 " 16 1615 " 60 14.0" 0.5 1232 " 42

1:50

3:8

5.3" 0.3 7.4" 0.6 28 " 2 0.35" 0.04 525 " 2

0.9" 0.1 156 " 6 606 " 23 5.3" 0.2 462 " 16

Cd Na Mg K Ca

4.5" 0.5 369 " 28 1381 " 100 17.4" 1.8 1248 " 114

3.6" 0.4 416 " 16 1615 " 60 14.0" 0.5 1232 " 42

3:10

3:8

1.3" 0.2 111 " 8 414 " 30 5.2" 0.5 374 " 34

1.3" 0.1 156 " 6 606 " 23 5.3" 0.2 462 " 16

Concentrations expressed in ␮g mly1 for matrix elements and ng mly1 for the analytes. Salinity 30.2‰. c Salinity 35.1‰. a

b

measurements. The results are reported in Tables 7 and 8. From these data, it was concluded that the sample preparation procedure was accurately carried out and that any difference between the concentration found and the certified value can be ascribed to matrix effects. Analytical results concerning each sub-sample are plotted in Figs. 2᎐7, together with the certified values and the 95% confidence intervals. Tolerance intervals, obtained by considering a mean confidence interval of experimental data equal to 10%, are also reported. Points outside the tolerance intervals indicate a result statistically different from the certified value, at a confidence level of 95%. For each sample, analytical data were obtained by applying four calibration methods: 1. external calibration, using acid-based standards; 2. external calibration and following correction by MLR models in Eqs. Ž3. ᎐ Ž5.; 3. external calibration using matrix-matched standards; and

4. analyte addition technique. It should be noted that, in the common procedure for heavy metals in seawater, only one sample for each water mass is usually picked up and treated with an aliquot of Chelex-100 resin, leading to a single pre-concentrate for each sample. Therefore, to discuss the results shown in Figs. 2᎐7, data relative to the 16 sub-samples should be considered as the results of 16 analyses rather than 16 replicates of a single one. Iron determination in the reference seawater CASS-3 is presented in Fig. 2. As it can be seen, Table 7 Contamination levels measured by ETAAS Analytes

Blank concentration a Žng mly1 .

Fe Mn Cd

0.073" 0.030 0.020" 0.004 0.001" 0.001 a

16..

The uncertainties represent 95% confidence limits Ž n s

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Table 8 Pre-concentration accuracy checked by ICP-MS measurements a Analytes

Fe Mn Cd a b

CASS-3

NASS-3

Found

Certified

Found

Certified

ᎏb 2.68" 0.13 0.038" 0.002

1.26" 0.17 2.51" 0.36 0.030" 0.005

ᎏb 0.025" 0.001 0.035" 0.001

0.327" 0.022 0.022" 0.007 0.029" 0.004

Concentrations expressed in ng mly1 ; the uncertainties represent 95% confidence limits Ž n s 16.. Date not available due to the well-known ArOq interference on Feq using a quadrupole ICP-MS.

Fig. 2. Fe determination in the certified reference seawater CASS-3 using: Ž䢇. external calibration, with acid-based standards; ŽB. external calibration and following correction by multiple linear regression models in Eqs. Ž3. ᎐ Ž5.; Ž ⌬ . external calibration using matrix-matched standards; and Ž=. analyte addition technique. The lines represent the 95% confidence interval of the certified value. Dotted lines represent the tolerance interval, considering a mean confidence interval of experimental data equal to10%.

Fig. 3. Fe determination in the certified reference seawater NASS-3. For symbols and further explanations see Fig. 2.

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Fig. 4. Mn determination in the certified reference seawater CASS-3. For symbols and further explanations see Fig. 2.

accurate data were obtained using both acid-based and matrix-matched standards for calibration. This result clearly shows that, in this case, interfering effects are low, as predicted by the model in Eq. Ž3., the application of which led to corrected values not significantly different from the original ones and inside the tolerance interval of the certified value. On the contrary, the iron concentration was over-estimated when applying the analyte addition technique. This result is not easy to justify. A possible explanation could be that the additions exceeded

the linear calibration range, regardless of the high correlation coefficients ŽTable 3.. Indeed, a reasonable risk of obtaining erroneous information with the correlation coefficient has been stressed w37᎐39x. Moreover, it should be considered that the analyte addition technique is an extrapolation method, and small variations in experimental data can drastically influence the calculation of the intercept. The iron determination in reference seawater NASS-3 is characterised by a lower analyte con-

Fig. 5. Mn determination in the certified reference seawater NASS-3. For symbols and further explanations see Fig. 2.

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Fig. 6. Cd determination in the certified reference seawater CASS-3. For symbols and further explanations see Fig. 2.

centration. Therefore, a lower dilution is required and a higher concentration of matrix elements in the graphite furnace obtained ŽTable 6.. Analytical data are shown in Fig. 3. As expected, matrix effects were greater than in the analysis of CASS3, and the use of an acid-based external calibration led to incorrect analytical data Žnine samples resulted outside the tolerance interval.. The situation is partially corrected by MLR modelling: for 10 samples the model in Eq. Ž3. was able to efficiently predict and correct the matrix effects, while over-correction occurred for the other six.

However, the MLR modelling resulted in the optimal calibration method, since less accurate data were obtained by calibrating with matrixmatched standards and the analyte addition technique. As found for iron determination in the same reference material, manganese determination was not significantly affected by the matrix, and accurate data were obtained by applying both acidbased and matrix-matched calibration ŽFig. 4.. This situation was accurately predicted by the model in Eq. Ž4. for the matrix composition

Fig. 7. Cd determination in the certified reference seawater NASS-3. For symbols and further explanations see Fig. 2.

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Table 9 Comparison of the different calibration methodsa Analyte

Fe Mn Cd

Sample

CASS-3 NASS-3 CASS-3 NASS-3 CASS-3 NASS-3

Analyte concentrations Žng mly1 .b

Certified

n

2.38" 0.18 0.590" 0.033 4.55" 0.34 0.015" 0.002 0.043" 0.002 0.038" 0.001

1.26" 0.17 0.327" 0.022 2.51" 0.36 0.022" 0.007 0.030" 0.005 0.029" 0.004

24 16 16 12 32 24

MM

AAT

␣ s 0.05

␣ s 0.01

1.282 7.124 1.379 0.995 3.231 1.810

8.624 12.973 8.124 2.159 3.364 3.413

2.025 2.042 2.042 2.056 2.015 2.027

2.713 2.750 2.750 2.779 2.691 2.718

EC

MC

MM

AAT

1.29" 0.04 0.387" 0.017 2.82 " 0.09 0.006" 0.002 0.047" 0.003 0.039" 0.002

1.09" 0.04 0.290" 0.017 2.70" 0.08 0.029" 0.001 0.037" 0.004 0.029" 0.004

1.40" 0.05 0.433" 0.019 2.77" 0.10 0.019" 0.002 0.042" 0.003 0.034" 0.002

EC

MC

0.283 4.225 1.619 4.901 4.565 3.653

1.543 2.623 1.023 2.275 1.878 0.077

t values

Fe Mn Cd

CASS-3 NASS-3 CASS-3 NASS-3 CASS-3 NASS-3

t-tab

a EC, external calibration, using acid-based standards; MC, external calibration and following correction by MLR models in Eqs. Ž3. ᎐ Ž5.; MM, external calibration using matrix-matched standards; and AAT, analyte addition technique. b The uncertainties represent 95% confidence limits Ž n s 16, except for Cd in NASS-3, where n s 15..

resulting from the pre-concentration step and reported in Table 6. Therefore, data obtained after MLR correction are not significantly different from the other ones. Again, application of the analyte addition technique led to an over-estimation of the analyte concentration. As expected from the data in Table 6, the determination of manganese in NASS-3 is complicated by its low concentration and the high amount of interfering salts. The resulting effect was a severe under-estimation of the analyte concentration, as shown in Fig. 5. This effect is well predicted by the model in Eq. Ž4., and a good correction was obtained. In addition, the use of matrix-matched standards and of the analyte addition technique caused a shift of data towards the certified value. Cadmium determination in the reference seawaters CASS-3 and NASS-3 is characterised by similar analyte concentration and matrix composition ŽTable 6.. In both cases, an over-estimation of cadmium was observed ŽFigs. 6 and 7., as predicted by the model in Eq. Ž5., the application

of which led to a significant improvement of trueness in most of the determinations. Finally, in order to better compare the different calibration techniques, the student’s t-test was performed ŽTable 9.. Remembering that a t value lower than the t tabulated means a measured concentration not statistically different from the certified one, at the corresponding confidence level, it can be observed that a lower t value indicates a better accordance between the found and certified values, i.e. a better trueness. By comparing t values obtained for the different calibration techniques, it can be seen that the empirical modelling approach was able to provide accurate data in all cases; moreover, it resulted in the optimal calibration method for Fe determination in NASS-3, Mn determination in NASS-3 and Cd determination in both seawater reference materials. Acknowledgements We are indebted to Prof Bernhard Welz for his

M. Grotti et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1847᎐1860

useful suggestions and discussions and to Dr Ornella Abollino for her collaboration in the ICP-OES measurements.

w12x

References w1x M. Nicolai, C. Rosin, N. Tousset, Y. Nicolai, Trace metals analysis in estuarine and seawater by ICP-MS using on line pre-concentration and matrix elimination with chelating resin, Talanta 50 Ž1999. 433᎐444. w2x J. Komarek, J. Holy, Determination of heavy metals by electrothermal atomic absorption spectrometry after electrodeposition on a graphite probe, Spectrochim. Acta Part B 54 Ž1999. 733᎐738. w3x Q.H. Jin, X.G. Su, F. Liang, H.Q. Zhang, Flow injection semi-online sorbent extraction pre-concentration for graphite furnace atomic absorption spectrometry, Microchem. J. 62 Ž1999. 316᎐327. w4x T.S. Cordero, E.I.V. Alonso, P.C. Rudner, A.G. de Torres, J.M.C. Pavon, Computer-assisted SIMPLEX optimisation of an on-line pre-concentration system for determination of nickel in sea-water by electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 14 Ž1999. 1033᎐1037. w5x M.L. Wells, K.W. Bruland, An improved method for rapid pre-concentration and determination of bioactive trace metals in seawater using solid phase extraction and high resolution inductively coupled plasma mass spectrometry, Mar. Chem. 63 Ž1998. 145᎐153. w6x A. Ali, X.F. Yin, H. Shen, Y.X. Ye, X. Gu, 1,10phenanthroline as a complexing agent for on-line sorbent extraction pre-concentration for flow injectionflame atomic absorption spectrometry, Anal. Chim. Acta 392 Ž1999. 283᎐289. w7x E. Hosten, B. Welz, Evaluation of an immobilised macrocyclic material for on-line column pre-concentration and separation of cadmium, copper and lead for electrothermal atomic absorption spectrometry, Anal. Chim. Acta 392 Ž1999. 55᎐65. w8x P. BermejoBarrera, M. FerronNovais, G. GonzalezCampos, A. BermejoBarrera, Tin determination in seawater by flow injection hydride generation atomic absorption spectroscopy, At. Spectrosc. 20 Ž1999. 120᎐125. w9x C.B. Ke, K.C. Lin, Laser-enhanced ionization detection of Pb in seawater by flow injection analysis with on-line pre-concentration and separation, Anal.Chem. 71 Ž1999. 1561᎐1567. w10x A.T. Rangel, R.C. de-Campos, I. Moreira, Comparative study of different procedures for the determination of total manganese in coastal waters by graphite furnace atomic absorption spectrometry, At. Spectrosc. 19 Ž1998. 133᎐136. w11x P. Su, S. Huang, Use of 4-Ž2-pyridylazo.resorcinol or 2-Ž2-pyridylazo.-5-dimethylaminophenol as chelating agent for determination of cadmium in seawater by

w13x

w14x

w15x

w16x

w17x

w18x

w19x

w20x

w21x

w22x

1859

atomic absorption spectrometry with on-line flow-injection sorbent extraction, Anal. Chim. Acta 376 Ž1998. 305᎐311. D. Colbert, K.S. Johnson, K.H. Coale, Determination of cadmium in seawater using automated on-line pre-concentration and direct injection graphite furnace atomic absorption spectrometry, Anal. Chim. Acta 377 Ž1998. 255᎐262. T.P. Rao, S. Kathikeyan, B. Vijayalekshmy, C.S.P. Iyer, Speciative determination of chromium ŽVI . and chromiumŽIII. using flow-injection on-line pre-concentration and flame atomic-absorption spectrometric detection, Anal. Chim. Acta 369 Ž1998. 69᎐77. K. Akatsuka, Y. Yoshida, N. Nobuyama, U. Hoshi, S. Nakamura, T. Kato, Preconcentration of trace cadmium from seawater using a dynamically coated column of quaternary ammonium salt on C-18-bonded silica gel and determination by graphite-furnace atomic absorption spectrometry, Anal. Sci. 14 Ž1998. 529᎐533. D. Pozebon, V.L. Dressler, A.J. Curtius, Determination of copper, cadmium, lead, bismuth and seleniumŽIV. in sea-water by electrothermal vaporization inductively coupled plasma mass spectrometry after on-line separation, J. Anal. At. Spectrom. 13 Ž1998. 363᎐369. Y.C. Sun, Y.L. Tu, J. Mierzwa, Chelating resin pre-concentration and HgŽII.-assisted elution followed by differential-pulse anodic stripping voltammetry for the determination of Cd, Eu, and Pb in seawater, Fresenius J. Anal. Chem. 360 Ž1998. 550᎐555. A.L.R. Sekaly, M.H. Back, C.L. Chakrabarti, D.C. Gregoire, J.Y. Lu, W.H. Schroeder, Measurements and analysis of dissociation rate constants of metal᎐fulvic acid complexes in aqueous solutions part I: simulation of decay curves obtained by inductively coupled plasmamass spectrometry to evaluate a method to measure the distribution of first-order rate constants, Spectrochim. Acta Part B 53 Ž1998. 837᎐846. C. DiazRomero, Comparison of APDCrfreon extraction and chelex-100 ion exchange chromatography for preconcentration of heavy metals from seawater, Int. J. Environ. Anal. Chem. 64 Ž1996. 163᎐170. M.T.S.D. Vasconcelos, M.A.O. Azenha, Additional evidence of the operational character of the lability of complexes of copperŽII. with simple organic ligands, Anal. Chim. Acta 333 Ž1996. 97᎐105. M.M. Silva, F.J. Krug, P.V. Oliveira, J.A. Nobrega, B.F. Reis, D.A.G. Penteado, Separation and pre-concentration by flow injection coupled to tungsten coil electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 51 Ž1996. 1925᎐1934. E. Rubi, M.S. Jiminez, F.B. deMirabo, R. Forteza, V. Cerda, Preconcentration and atomic absorption determination of iron by sequential injection analysis, Talanta 44 Ž1997. 553᎐562. H.W. Liu, S.J. Jiang, S.H. Liu, Determination of cadmium, mercury and lead in seawater by electrother-

1860

w23x

w24x

w25x

w26x

w27x

w28x

w29x

w30x

M. Grotti et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1847᎐1860 mal vaporization isotope dilution inductively coupled plasma mass spectrometry, Spectrochim. Acta Part B 54 Ž1999. 1367᎐1375. O. Acar, A.R Turker, Z. Kilic, Direct determination of bismuth, indium and lead in sea water by Zeeman ETAAS using molybdenum containing chemical modifiers, Talanta 49 Ž1999. 135᎐142. E. Rosland, W. Lund, Direct determination of trace metals in sea-water by inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 13 Ž1998. 1239᎐1244. P. BermejoBarrera, J. MoredaPineiro, A. MoredaPineiro, A. BermejoBarrera, Usefulness of the chemical modification and the multi-injection technique approaches in the electrothermal atomic absorption spectrometric determination of silver, arsenic, cadmium, chromium, mercury, nickel and lead in sea-water, J. Anal. At. Spectrom. 8 Ž1998. 777᎐786. P.G. Su, S.D. Huang, Direct and simultaneous determination of molybdenum and vanadium in sea-water using a multi-element electrothermal atomic absorption spectrometer, J. Anal. At. Spectrom. 13 Ž1998. 641᎐645. P.G. Su, S.D. Huang, Direct and simultaneous determination of copper and manganese in seawater with a multi-element graphite furnace atomic absorption spectrometer, Spectrochim. Acta Part B 53 Ž1998. 699᎐708. J.Y. Cabon, A. Le Bihan, The determination of chromium, copper and manganese in seawater with transversely heated graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 50 Ž1995. 1703᎐1716. M. Grotti, E. Magi, R. Leardi, Study of interferences in graphite furnace atomic absorption spectrometry by means of experimental design, Anal. Chim. Acta 327 Ž1996. 47᎐51. M. Grotti, R. Leardi, R. Frache, Empirical modelling of interferences in electrothermal atomisation atomic absorption spectrometry, Anal. Chim. Acta 376 Ž1998. 293᎐304.

w31x M. Grotti, R. Leardi, C. Gnecco, R. Frache, Determination of manganese by graphite furnace atomic absorption spectrometry: matrix effect control by multiple linear regression model, Spectrochim. Acta Part B 54 Ž1999. 845᎐851. w32x M. Grotti, C. Gnecco, F. Bonfiglioli, Multivariate quantification of spectroscopic interferences caused by sodium, calcium, chlorine and sulfur in inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 14 Ž1999. 1171᎐1175. w33x M. Grotti, R. Frache, Multivariate investigation of matrix effects in inductively coupled plasma atomic emission spectrometry using pneumatic or ultrasonic nebulization, J. Anal. At. Spectrom. 15 Ž1999. 89᎐95. w34x F. Baffi, A.M. Cardinale, R. Bruzzone, Preconcenrtation of chromium, copper and manganese from seawater on pre-treated solid materials for determination by atomic absorption spectrometry, Anal. Chim. Acta 270 Ž1992. 79. w35x M. Stone, Cross-validatory choice and assessment of statistical predictions, J. R. Stat. Soc. B 36 Ž1974. 111᎐147. w36x M. Forina, S. Lanteri, C. Armanino, R. Leardi, G. Drava, PARVUS 1.2 An Extendable Package of Programs for Data Explorative Analysis, Classification and Regression Analysis, Dipartimento di Chimica e Tecnologie Farmaceutiche e Alimentari, Universita ` di Genova, Genoa, Italy, 1995. w37x M.D. VanArendonk, R.K. Skogerboe, C.L. Grant, Correlation coefficients for evaluation of analytical calibration curves, Anal. Chem. 53 Ž1981. 2349᎐2350. w38x Analytical Methods Committee, Uses Žproper and improper. of correlation coefficients, Analyst 113 Ž1988. 1469᎐1471. w39x B. Welz, M. Sperling, Atomic Absorption Spectrometry, 3rd, Wiley-VCH, Weinheim, New York, 1999.