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Unusual calibration curves observed for iron using high-resolution continuum source graphite furnace atomic absorption spectrometry
Spectrochimica Acta Part B 65 (2010) 258–262
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s a b
Analytical note
Unusual calibration curves observed for iron using high-resolution continuum source graphite furnace atomic absorption spectrometry Bernhard Welz a,b,⁎, Lísia M.G. dos Santos a,c, Rennan G.O. Araujo a,d, Silvana do C. Jacob c, Maria Goreti R. Vale b,e, Michael Okruss f, Helmut Becker-Ross f a
Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil Instituto Nacional de Ciência e Tecnologia do CMPq-INCT de Energia e Ambiente, Universidade Federal da Bahia, 40170-115 Salvador-BA, Brazil Instituto Nacional de Controle de Qualidade em Saúde-INCQS-Fiocruz, 21040-900 Rio de Janeiro-RJ, Brazil d Departamento de Química, Universidade Federal de Sergipe, 49100-000 São Cristóvão-SE, Brazil e Instituto de Química, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre-RS, Brazil f Leibniz-Institut für Analytische Wissenschaften-ISAS-Department Berlin, 12489 Berlin, Germany b c
a r t i c l e
i n f o
Article history: Received 14 July 2009 Accepted 27 January 2010 Available online 13 February 2010 Keywords: High-resolution continuum source AAS Electrothermal atomization Iron determination Calibration graphs Line wings
a b s t r a c t The simultaneous determination of cadmium and iron in plant and soil samples has been investigated using high-resolution continuum source graphite furnace atomic absorption spectrometry. The primary cadmium resonance line at 228.802 nm and an adjacent secondary iron line at 228.726 nm, which is within the spectral interval covered by the charge-coupled device (CCD) array detector, have been used for the investigations. Due to the very high iron content in most of the soil samples the possibility has been investigated to reduce the sensitivity and extend the working range by using side pixels for measurement at the line wings instead of the line core. It has been found that the calibration curves measured at all the analytically useful pixels of this line consisted of two linear parts with distinctly different slopes. This effect has been independent of the positioning of the wavelength, i.e., if the Cd line or the Fe line was in the center of the CCD array. The most likely explanation for this unusual behavior is a significant difference between the instrument width ΔλInstr and the absorption line width ΔλAbs, which is quite pronounced in the case of Fe. Using both parts of the calibration curves and simultaneous measurement at the line center and at the wings made it possible to extend the working range for the iron determination to more than three orders of magnitude. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Atomic absorption spectrometry (AAS) is known to have a relatively limited linear dynamic working range that typically does not exceed two orders of magnitude. Although other factors might contribute to the non-linearity of calibration curves in AAS [1], the most common reason is stray light or non-absorbable radiation emitted by the radiation source [2,3]. The calibration curves then consist of a linear part for low absorbance or integrated absorbance values, followed by a non-linear part that often can be described by a quadratic function, up to the “limiting absorbance”, where the absorbance does no longer increase with increasing analyte concentration or mass. The latter one is directly related with the amount of stray light and/or non-absorbable radiation. This situation does not change significantly when high-resolution continuum source AAS (HR-CS AAS) is used for measurement, ⁎ Corresponding author. Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil. Fax: +55 48 3721 6850. E-mail address: [email protected] (B. Welz). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.01.007
although it is often possible to extend the linear working range to lower absorbance values due to the significantly improved signal-tonoise (S/N) ratio obtained with this technique [4]. On the other hand, HR-CS AAS is offering possibilities to extend the linear working range, the most attractive one being measurement of the absorbance at the wings of the analytical line [5]; this option, however, has only been used in a few cases [6,7] and has not yet been explored systematically for the analysis of real samples. Another possibility to cope with high analyte concentrations is the measurement at secondary, less sensitive analytical lines. In line-source AAS this option is often limited, as the emission intensity of these lines from conventional hollow cathode lamps might be relatively low, resulting in an inferior S/N ratio. In HR-CS AAS, however, due to the use of a high-intensity continuum radiation source [4], all secondary lines have essentially the same intensity and may be used without compromises. Currently available instruments for HR-CS AAS are not designed for the simultaneous determination of more than one element or of the same analyte at more than one analytical line. This means that an additional measurement is necessary at an alternate line when analyte concentrations are very much different in the samples to be
B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262
investigated, which is not very attractive, at least not in graphite furnace AAS (GF AAS). Nevertheless, the simultaneous determination of more than one analyte or the same analyte at different analytical lines becomes feasible if more than one absorption line falls within the spectral band pass covered by the linear array detector. This way it has for example been possible to extend the working range for the determination of nickel in crude oil to more than three orders of magnitude by simultaneous determination at the primary resonance line at 232.003 nm and a secondary line at 232.138 nm [8]. The simultaneous determination of more than one analyte has been demonstrated for the determination of Cd and Fe in grain products [9] and in bean and soil samples [7], for Co and V [6] and Cr and Fe in crude oil [10]. Another condition that has to be fulfilled in order to make possible a simultaneous determination of more than one element is that the sensitivity ratio between the two analytical lines is within the same order of magnitude as the concentration ratio of the analytes in the samples to be investigated. This condition has been met reasonably well in the above-cited applications; however, when we tried to expand the simultaneous determination of Cd and Fe in grain products to soil samples, the low cadmium and high iron content of the samples exceeded the sensitivity ratio by more than two orders of magnitude. The goal of the present investigation has been to investigate the possibility to measure the absorbance at the wings of the secondary iron line at 228.726 nm in order to further decrease the sensitivity for the measurement of higher iron concentrations. The idea behind this investigation has been to use direct solid sample analysis in order to avoid tedious digestions; however, this approach practically excludes the possibility of dilution in the case of excessively high analyte concentrations. Part of the investigation has been to explore to which extent polynomial functions could be used for evaluation in order to increase the working range to the desired analyte concentrations. 2. Experimental 2.1. Instrumentation A prototype high-resolution continuum source atomic absorption spectrometer, built at ISAS, Berlin, has been used for all measurements in this work. The equipment is based on a Model AAS 6 Vario (Analytik Jena AG, Jena, Germany) from which all optical components, including the detector and control, have been removed and replaced by a HR-CS spectrometer similar to that described by Becker-Ross et al. [11]. This spectrometer consists of a high-intensity xenon short-arc lamp operating in a hot-spot mode, a high-resolution double monochromator and a CCD array detector. The double monochromator consists of a pre-dispersing prism monochromator and a high-resolution echelle grating monochromator, both in Littrow mounting, resulting in a resolution of λ/Δλ ≈ 140,000, corresponding to a resolution per pixel of ∼ 1.6 pm at the cadmium line at 228.803 nm. The system is controlled by a Pentium III personal computer (100 MHz), running a data acquisition program developed at ISAS Berlin. Details of this equipment have been described in previous publications of our group [4,8,12]. The secondary iron line at 228.726 nm has been used for all measurements unless otherwise stated. The integrated absorbance has been measured at all pixels that exhibited detectable absorbance over the profile of the iron line. The transversely heated graphite tube atomizer system supplied by Analytik Jena together with the Model AAS 6 Vario has been used throughout. All experiments were carried out using pyrolytically coated solid sampling (SS) graphite tubes without dosing hole (Analytik Jena Part No. 407-A81.303) and SS platforms (Analytik Jena Part No. 407152.023). Solid samples were weighed directly onto the SS platforms using an M2P microbalance (Sartorius, Göttingen, Germany, accuracy 0.001 mg) and inserted into the graphite tube using a pre-adjusted pair
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of tweezers, which is part of the SSA 5 manual solid sampling accessory (Analytik Jena). Aqueous standards were injected manually onto the SS platform using micropipettes with disposable tips. Argon (99.996%, White Martins, São Paulo, Brazil) was used as purge and protective gas throughout. The graphite furnace temperature program that has been optimized previously for the simultaneous determination of Cd and Fe in grain products [9] has been used throughout and is shown in Table 1. A two-step atomization has been used because of the significantly different volatilities of the two analytes, even when only Fe has been determined. 2.2. Reagents and standards The reagents were of analytical grade. The water with a resistivity of 18 MΩ cm was purified in a Milli-Q system (Millipore, Bedford, USA). The nitric acid (Merck, Darmstadt, Germany) used for preparation of the calibration solutions was further purified by subboiling distillation in a quartz still (Kürner Analysentechnik, Rosenheim, Germany). Calibration solutions were prepared in deionized water with 0.5% v/v HNO3 by serial dilution of the stock solutions with 1000 mg L− 1 Fe (as FeCl3) (Merck, Darmstadt, Germany). The following atomic absorption standard solutions were used for the permanent modifiers: 1000 mg L− 1 Ir and 1000 mg L− 1 W (both from Fluka, Buchs, Switzerland). In order to coat the platform with the W–Ir mixed permanent modifier, five aliquots of 40 µL of the W modifier solution were applied first, each one followed by the temperature program shown in Ref. [7,9], then five aliquots of 40 µL of the Ir modifier solution were applied, also followed by the same temperature program, resulting in a coating with 200 µg each of W and Ir. 3. Results and discussion The original idea of this work has been to transfer the procedure that has been developed for the simultaneous determination of Cd and Fe in grain products using direct solid sampling [9] to the analysis of bean and soil samples from different regions of Brazil [7]. While the procedure worked well for the bean samples, the soil samples turned out to be a problem due to their extremely high iron content. We therefore have been looking for a solution of the problem, and the obvious first approach has been to measure the absorbance at the line wings in order to reduce sensitivity and increase the working range, as has been proposed previously [5]. As for this application the center of the Cd line at 228.803 nm is focused on the center pixel (No. 250), the iron line is shifted and not exactly focused onto a specific pixel. The distribution of the iron line at 228.726 over the eight pixels that have a detectable contribution to the integrated absorbance is shown in Fig. 1. In an initial attempt calibration curves have been measured at all of these pixels individually using aqueous standards, and the corresponding linear regression equations together with the calibration range and the coefficient of correlation are summarized in Table 2. It is worth mentioning that all these data on the different pixels have been obtained from the same set of measurements for each iron standard,
Table 1 Temperature program for the simultaneous determination of cadmium and iron in food and soil samples by SS HR-CS AAS using W–Ir as the permanent modifier. Stage
Temperature/°C Ramp/°C s− 1 Hold time/s Ar flow rate/L min− 1
Drying 90 Drying 130 Pyrolysis 700 700 Auto zeroa a Atomization 1700 Atomizationa 2600 Cleaning 2600 a
10 5 50 100 2000 3000 1000
Signal registration in these stages.
10 5 15 1 3 12 3
2.0 2.0 2.0 0 0 0 2.0
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B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262 Table 3 Linear regression equations and correlation coefficient for calibration curves in the high analyte mass range obtained at various pixels in the vicinity of the 228.726 nm iron line; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C.
Fig. 1. Integrated absorbance measured at the various pixels in the vicinity of the iron absorption line at 228.726 nm for 100 ng Fe; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C.
and only the evaluation per pixel has been sequential. The results show that by choosing the appropriate pixel for evaluation, the sensitivity and also the working range can be increased or decreased by more than one order of magnitude. It is also worth mentioning that all of the calibration functions, except for that at pixel 207, do have correlation coefficients between 0.997 and 0.999, which means that the relations are linear. In an attempt to further extend the working range, the analyte mass has been increased up to 2000 ng Fe, corresponding to a concentration of 100 mg L− 1 Fe in 20 μL of solution, expecting that the calibration curve would become non-linear. To our surprise we discovered that all calibration curves showed kind of an inflection point, after which the curves continued to be linear, although with a distinctly lower slope. This behavior is shown in Table 3 for pixels 208 to 213, and the correlation coefficients between 0.9875 and 0.9994 confirm the linear correlation between integrated absorbance and analyte mass. Fig. 2 might serve as an example; it shows the summated integrated absorbance measured at pixels 209 + 213, i.e., the peak volume selected absorbance (PVSA) [5], and demonstrates that the linear range actually extends beyond the calibration range chosen in Table 3 at least up to 4000 ng Fe. The limit of quantification, determined as 10 times the standard deviation of a blank, divided by the slope of the calibration curve, in PVSA for pixels 210, 211 and 212 was found to be around 4 ng; this means that a concentration range of at least three orders of magnitude can be covered by selecting the proper pixel or combination of pixels for evaluation. In order to test the applicability of this calibration approach for the analysis of real samples, the certified reference material BCR 142 (light sandy soil) with an iron content, expressed as Fe2O3, of 2.80%
Pixel number
Linear regression equation
R
Calibration range/ng
208 209 210 211 212 213
Aint = 0.00016 mFe + 0.0842 Aint = 0.00058 mFe + 0.4196 Aint = 0.0011 mFe + 0.9111 Aint = 0.0013 mFe + 0.9945 Aint = 0.00094 mFe + 0.7026 Aint = 0.00036 mFe + 0.2078
0.9994 0.9984 0.9947 0.9953 0.9875 0.9902
250–2000 250–2000 250–2000 250–2000 250–2000 250–2000
has been analyzed using direct solid sampling. The result of 2.79 ± 0.59% (n = 10) is in excellent agreement with the certified value; the high standard deviation is due to the natural inhomogeneity of this kind of samples that becomes significant in this case, as the sample mass introduced into the furnace has been only between about 0.04 and 0.09 mg. In order to investigate the potential reasons for the unusual calibration curves, first of all the atomization signals obtained at the various pixels for a high iron mass have been consulted. As shown in Fig. 3, the signals do not return to the baseline within the atomization time of 12 s, i.e., part of the absorbance signal is lost under these conditions. However, increasing the atomization time until the signals returned to the baseline did not have any significant effect on the overall pattern; the slope of the high-mass calibration graphs increased slightly at all pixels and the intercept with the Y-axis decreased accordingly, but the calibration graphs remained linear with R values greater than 0.99. Another investigation that has been carried out was if the W–Ir permanent modifier, which had to be added to stabilize Cd, was causing kind of an “over-stabilization” of iron. However, determinations that have been carried out without this modifier under otherwise identical conditions did not show any significantly different results. Another investigation has been carried out to see if the fact that the measurements had been made at the edge of the detector and the line was not well centered on a pixel (refer to Fig. 1) had an influence on the results. Hence, the iron line at 228.726 has been focused on the center pixel 250 and the measurements have been repeated. Table 4 shows the regression equations using PVSA of pixels 248 + 252, which corresponds to the measurements shown in Fig. 2 at pixels 209 + 213;
Table 2 Linear regression equations and correlation coefficient for calibration curves in the low analyte mass range obtained at various pixels in the vicinity of the 228.726 nm iron line; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C. Pixel number
Linear regression equation
R
Calibration range/ng
207 208 209 210 211 212 213 214
Aint = 0.0001 mFe + 0.00052 Aint = 0.00037 mFe + 0.0239 Aint = 0.00179 mFe + 0.0218 Aint = 0.00418 mFe + 0.0712 Aint = 0.00439 mFe + 0.0507 Aint = 0.00267 mFe + 0.0202 Aint = 0.00048 mFe + 0.0309 Aint = 0.0001 mFe + 0.0131
0.9847 0.9977 0.9990 0.9989 0.9988 0.9960 0.9986 0.9974
75–800 50–600 50–350 10–100 10–100 10–100 40–400 50–800
Fig. 2. Calibration curves for iron in the vicinity of the 228.726 nm analytical line using PVSA at pixels 209 + 213 for evaluation; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C; linear regression equation for low mass range: A∑ ± 2,int = 0.0023 mFe + 0.0443; R = 0.9995; linear regression equation for high mass range: A∑ ± 2,int = 0.0007 mFe + 0.8478; R = 0.9972.
B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262
Fig. 3. Transient signals measured at pixels 208–213 in the vicinity of the Fe line at 228.725 nm; 1000 ng Fe from an aqueous standard solution; W–Ir as permanent modifier; pyrolysis temperature: 700 °C, atomization temperature: 2600 °C.
the similarity of the regression equations is striking, indicating that the off-center and asymmetric measurement had no influence on the result. In addition other secondary iron lines have been investigated in order to find out if they exhibit a similar behavior as the line at 228.726 nm; the corresponding regression equations are also shown in Table 4. The line at 360.880 nm is about a factor of 8 less sensitive than the line at 228.726 nm; for this reason the measurement has been made in the line core at pixel 250 in order to get comparable absorbance signals. There has been some non-linearity in the low absorbance range, which is not shown here, but the response has been linear for analyte masses between 250 and 4000 ng Fe, and the similarity of the regression equation with that measured at 228.726 nm at pixels 248 + 252 is again striking. Finally, another secondary iron line at 237.362 nm has been investigated as well, using the same pixels 248 + 252 at the wings as for the 228.726-nm line; the results are also shown in Table 4. It has again been possible to identify two linear parts, where the inclination of the first one has been identical to that found for the 228.726-nm line, whereas the second one, for analyte masses between 100 and 1000 ng had a lower inclination compared to the latter line. A distinct difference has been found for analyte masses higher than 1000 ng, or above a PVSA of 1 s, where the calibration curve started to exhibit pronounced curvature, i.e., kind of a “normal” behavior. It would actually have been possible to describe the entire calibration range from 20 to 4000 ng Fe (PVSA from 0.0797 to 2.429 s) using a quadratic equation, although with some inconsistencies in the curve. Harnly and colleagues published a series of papers about the shape of extended calibration curves for CS AAS, e.g. [13,14]. Without any exception their curves consist of two linear branches with significantly different inclination; however, all their curves are in a double logarithmic scale, which is quite different from the shape found in this work, so that their explanation cannot be applied for the present
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results. For further clarification of the unusual progression of the calibration curves, theoretical considerations reflecting the mode of measurement have been made. Initially, a “true” line profile has been generated by folding a Gauss-type with a Lorentz-type profile, both having the same width and area. The resulting Voigt-type profile was transformed to a transmittance spectrum by introducing an absorption length. Then, two absorbance values were calculated. The first one corresponds to the maximum absorption in the center of the profile and is assumed to be proportional to the analyte mass. The second value should simulate the measured integrated absorbance. It was gained by calculating the ratio between two integrals taken within the transmittance spectrum with and without absorption respectively. The integration limits were set symmetrically to the center profile and represent the instrument width. By variation of the absorption length over several orders of magnitude, a large set of related pairs of absorbance values was gathered. This procedure was repeated for different instrument widths and resulted in a set of theoretical calibration curves. Each curve was normalized to its maximum value in order to emphasize its inherent progression. Fig. 4 provides the results for three different instrument widths. The curves show that for growing instrument width the initially straight curve is increasingly bended, generating two distinguishable parts, which can be approximated by straight lines. This corresponds to the shape of the measured calibration curve in Fig. 2 and might be the explanation for it. The hypothesis is further supported by the fact that iron belongs to the elements with the narrowest absorption line widths. 4. Conclusion There are some similarities in the shape of the calibration curves of secondary, significantly less sensitive absorption lines of iron when high analyte concentrations are determined; however, there are also distinct differences for different lines using the same analytical conditions. The explanation for this unusual shape of the calibration curves might be in the difference between the instrument width ΔλInstr and the absorption line width ΔλAbs, which is quite pronounced in the case of Fe; however, other factors, such as stray light might also contribute to the phenomenon. It is obvious that more studies have to be carried out in order to investigate the observed phenomena, which also have to be extended to other elements. It appears to be worthwhile to carry out such investigations, as with the increasing use of HR-CS AAS equipment, particularly for GF AAS, there
Table 4 Linear regression equations and correlation coefficient for calibration curves obtained at various secondary analytical lines for iron; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C. Line/nm
Pixel
Linear regression equation
R
Calibration range/ng
228.726 228.726 360.880 237.362 237.362
248 + 252 248 + 252 250 248 + 252 248 + 252
A∑ ± 2, int = 0.0021 mFe + 0.0098 A∑ ± 2, int = 0.0007 mFe + 0.5629 A int = 0.0007 mFe + 0.8374 A∑ ± 2, int = 0.0021 mFe + 0.0004 A∑ ± 2, int = 0.0004 mFe + 0.1062
0.9982 0.9988 0.9995 0.9993 0.9971
20–200 400–4000 250–4000 20–100 100–1000
Fig. 4. Theoretical and normalized calibration curves for three different ratios between the instrument width ΔλInstr. and the absorption line width ΔλAbs..
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B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262
will be an increasing demand for using side pixels for measurement at the line wings in order to increase the dynamic range of this technique.
[7]
Acknowledgement [8]
The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support. B. W. and M.G.R.V. have research scholarships from CNPq and L.M.G.S. has a scholarship from CAPES. The authors are also grateful to Analytik Jena AG for the donation of the HR-CS AAS prototype.
[9]
[10]
References [1] A.Kh. Gilmutdinov, T.M. Abdullina, S.F. Gorbachev, V.L. Makarov, Concentration curves in atomic absorption spectrometry, Spectrochim. Acta Part B 47 (1992) 1075–1095. [2] Z.V. Gelder, Calculation of non-linearities of the calibration curves in atomic absorption flame spectroscopy, Spectrochim. Acta Part B 25 (1970) 669–681. [3] H.C. Wagenaar, L. de Galan, The influence of line profiles upon analytical curves for copper and silver in atomic absorption spectrometry, Spectrochim. Acta Part B 30 (1975) 361–381. [4] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution Continuum Source AAS, Wiley-VCH, Weinheim, 2005. [5] U. Heitmann, B. Welz, D.L.G. Borges, F.G. Lepri, Feasibility of peak volume, side pixel and multiple peak registration in high-resolution continuum source atomic absorption spectrometry, Spectrochim. Acta Part B 62 (2007) 1222–1230. [6] I.M. Dittert, J.S.A. Silva, R.G.O. Araujo, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of cobalt and vanadium in undiluted crude oil using
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high-resolution continuum-source graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. (2010), doi:10.1039/b915194j. L.M.G. dos Santos, B. Welz, R.G.O. Araujo, S.do.C. Jacob, M.G.R. Vale, A. Martens, I.B. G. Martens, H. Becker-Ross, Simultaneous determination of Cd and Fe in beans and soil of different regions of Brazil using high-resolution continuum-source graphite furnace atomic absorption spectrometry and direct solid sampling, J. Agric. Food Chem. 57 (2009) 10089–10094. M.G.R. Vale, I.C.F. Damin, A. Klassen, M.M. Silva, B. Welz, A.F. Silva, F.G. Lepri, D.L.G. Borges, U. Heitmann, Method development for the determination of nickel in petroleum using line-source and high-resolution continuum-source graphite furnace atomic absorption spectrometry, Microchem. J. 77 (2004) 131–140. L.M.G. dos Santos, R.G.O. Araujo, B. Welz, S.do.C. Jacob, M.G.R. Vale, H. Becker-Ross, Simultaneous determination of Cd and Fe in grain products using direct solid sampling and high-resolution continuum-source electrothermal atomic absorption spectrometry, Talanta 78 (2009) 577–583. I.M. Dittert, J.S.A. Silva, R.G.O. Araujo, A.J. Curtius, B. Welz, H. Becker-Ross, Direct and simultaneous determination of Cr and Fe in crude oil using high-resolution continuum-source graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 64 (2009). H. Becker-Ross, S. Florek, U. Heitmann, R. Weisse, Influence of the spectral bandwidth of the spectrometer on the sensitivity using continuum source AAS, Fresenius J. Anal. Chem. 355 (1996) 300–303. A.F. Silva, D.L.G. Borges, B. Welz, M.G.R. Vale, M.M. Silva, A. Klassen, U. Heitmann, Method development for the determination of thallium in coal using solid sampling graphite furnace atomic absorption spectrometry with continuum source, high-resolution monochromator and CCD array detector, Spectrochim. Acta Part B 59 (2004) 841–850. J.M. Harnly, T.C. O'Haver, Extension of analytical calibration curves in atomic absorption spectrometry, Anal. Chem. 53 (1981) 1291–1298. D.N. Wichems, R.E. Fields, J.M. Harnly, Characterization of hyperbolic calibration curves for continuum source atomic absorption spectrometry with array detection, J. Anal. At. Spectrom. 13 (1998) 1277–1284.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s a b
Analytical note
Unusual calibration curves observed for iron using high-resolution continuum source graphite furnace atomic absorption spectrometry Bernhard Welz a,b,⁎, Lísia M.G. dos Santos a,c, Rennan G.O. Araujo a,d, Silvana do C. Jacob c, Maria Goreti R. Vale b,e, Michael Okruss f, Helmut Becker-Ross f a
Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil Instituto Nacional de Ciência e Tecnologia do CMPq-INCT de Energia e Ambiente, Universidade Federal da Bahia, 40170-115 Salvador-BA, Brazil Instituto Nacional de Controle de Qualidade em Saúde-INCQS-Fiocruz, 21040-900 Rio de Janeiro-RJ, Brazil d Departamento de Química, Universidade Federal de Sergipe, 49100-000 São Cristóvão-SE, Brazil e Instituto de Química, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre-RS, Brazil f Leibniz-Institut für Analytische Wissenschaften-ISAS-Department Berlin, 12489 Berlin, Germany b c
a r t i c l e
i n f o
Article history: Received 14 July 2009 Accepted 27 January 2010 Available online 13 February 2010 Keywords: High-resolution continuum source AAS Electrothermal atomization Iron determination Calibration graphs Line wings
a b s t r a c t The simultaneous determination of cadmium and iron in plant and soil samples has been investigated using high-resolution continuum source graphite furnace atomic absorption spectrometry. The primary cadmium resonance line at 228.802 nm and an adjacent secondary iron line at 228.726 nm, which is within the spectral interval covered by the charge-coupled device (CCD) array detector, have been used for the investigations. Due to the very high iron content in most of the soil samples the possibility has been investigated to reduce the sensitivity and extend the working range by using side pixels for measurement at the line wings instead of the line core. It has been found that the calibration curves measured at all the analytically useful pixels of this line consisted of two linear parts with distinctly different slopes. This effect has been independent of the positioning of the wavelength, i.e., if the Cd line or the Fe line was in the center of the CCD array. The most likely explanation for this unusual behavior is a significant difference between the instrument width ΔλInstr and the absorption line width ΔλAbs, which is quite pronounced in the case of Fe. Using both parts of the calibration curves and simultaneous measurement at the line center and at the wings made it possible to extend the working range for the iron determination to more than three orders of magnitude. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Atomic absorption spectrometry (AAS) is known to have a relatively limited linear dynamic working range that typically does not exceed two orders of magnitude. Although other factors might contribute to the non-linearity of calibration curves in AAS [1], the most common reason is stray light or non-absorbable radiation emitted by the radiation source [2,3]. The calibration curves then consist of a linear part for low absorbance or integrated absorbance values, followed by a non-linear part that often can be described by a quadratic function, up to the “limiting absorbance”, where the absorbance does no longer increase with increasing analyte concentration or mass. The latter one is directly related with the amount of stray light and/or non-absorbable radiation. This situation does not change significantly when high-resolution continuum source AAS (HR-CS AAS) is used for measurement, ⁎ Corresponding author. Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-SC, Brazil. Fax: +55 48 3721 6850. E-mail address: [email protected] (B. Welz). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.01.007
although it is often possible to extend the linear working range to lower absorbance values due to the significantly improved signal-tonoise (S/N) ratio obtained with this technique [4]. On the other hand, HR-CS AAS is offering possibilities to extend the linear working range, the most attractive one being measurement of the absorbance at the wings of the analytical line [5]; this option, however, has only been used in a few cases [6,7] and has not yet been explored systematically for the analysis of real samples. Another possibility to cope with high analyte concentrations is the measurement at secondary, less sensitive analytical lines. In line-source AAS this option is often limited, as the emission intensity of these lines from conventional hollow cathode lamps might be relatively low, resulting in an inferior S/N ratio. In HR-CS AAS, however, due to the use of a high-intensity continuum radiation source [4], all secondary lines have essentially the same intensity and may be used without compromises. Currently available instruments for HR-CS AAS are not designed for the simultaneous determination of more than one element or of the same analyte at more than one analytical line. This means that an additional measurement is necessary at an alternate line when analyte concentrations are very much different in the samples to be
B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262
investigated, which is not very attractive, at least not in graphite furnace AAS (GF AAS). Nevertheless, the simultaneous determination of more than one analyte or the same analyte at different analytical lines becomes feasible if more than one absorption line falls within the spectral band pass covered by the linear array detector. This way it has for example been possible to extend the working range for the determination of nickel in crude oil to more than three orders of magnitude by simultaneous determination at the primary resonance line at 232.003 nm and a secondary line at 232.138 nm [8]. The simultaneous determination of more than one analyte has been demonstrated for the determination of Cd and Fe in grain products [9] and in bean and soil samples [7], for Co and V [6] and Cr and Fe in crude oil [10]. Another condition that has to be fulfilled in order to make possible a simultaneous determination of more than one element is that the sensitivity ratio between the two analytical lines is within the same order of magnitude as the concentration ratio of the analytes in the samples to be investigated. This condition has been met reasonably well in the above-cited applications; however, when we tried to expand the simultaneous determination of Cd and Fe in grain products to soil samples, the low cadmium and high iron content of the samples exceeded the sensitivity ratio by more than two orders of magnitude. The goal of the present investigation has been to investigate the possibility to measure the absorbance at the wings of the secondary iron line at 228.726 nm in order to further decrease the sensitivity for the measurement of higher iron concentrations. The idea behind this investigation has been to use direct solid sample analysis in order to avoid tedious digestions; however, this approach practically excludes the possibility of dilution in the case of excessively high analyte concentrations. Part of the investigation has been to explore to which extent polynomial functions could be used for evaluation in order to increase the working range to the desired analyte concentrations. 2. Experimental 2.1. Instrumentation A prototype high-resolution continuum source atomic absorption spectrometer, built at ISAS, Berlin, has been used for all measurements in this work. The equipment is based on a Model AAS 6 Vario (Analytik Jena AG, Jena, Germany) from which all optical components, including the detector and control, have been removed and replaced by a HR-CS spectrometer similar to that described by Becker-Ross et al. [11]. This spectrometer consists of a high-intensity xenon short-arc lamp operating in a hot-spot mode, a high-resolution double monochromator and a CCD array detector. The double monochromator consists of a pre-dispersing prism monochromator and a high-resolution echelle grating monochromator, both in Littrow mounting, resulting in a resolution of λ/Δλ ≈ 140,000, corresponding to a resolution per pixel of ∼ 1.6 pm at the cadmium line at 228.803 nm. The system is controlled by a Pentium III personal computer (100 MHz), running a data acquisition program developed at ISAS Berlin. Details of this equipment have been described in previous publications of our group [4,8,12]. The secondary iron line at 228.726 nm has been used for all measurements unless otherwise stated. The integrated absorbance has been measured at all pixels that exhibited detectable absorbance over the profile of the iron line. The transversely heated graphite tube atomizer system supplied by Analytik Jena together with the Model AAS 6 Vario has been used throughout. All experiments were carried out using pyrolytically coated solid sampling (SS) graphite tubes without dosing hole (Analytik Jena Part No. 407-A81.303) and SS platforms (Analytik Jena Part No. 407152.023). Solid samples were weighed directly onto the SS platforms using an M2P microbalance (Sartorius, Göttingen, Germany, accuracy 0.001 mg) and inserted into the graphite tube using a pre-adjusted pair
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of tweezers, which is part of the SSA 5 manual solid sampling accessory (Analytik Jena). Aqueous standards were injected manually onto the SS platform using micropipettes with disposable tips. Argon (99.996%, White Martins, São Paulo, Brazil) was used as purge and protective gas throughout. The graphite furnace temperature program that has been optimized previously for the simultaneous determination of Cd and Fe in grain products [9] has been used throughout and is shown in Table 1. A two-step atomization has been used because of the significantly different volatilities of the two analytes, even when only Fe has been determined. 2.2. Reagents and standards The reagents were of analytical grade. The water with a resistivity of 18 MΩ cm was purified in a Milli-Q system (Millipore, Bedford, USA). The nitric acid (Merck, Darmstadt, Germany) used for preparation of the calibration solutions was further purified by subboiling distillation in a quartz still (Kürner Analysentechnik, Rosenheim, Germany). Calibration solutions were prepared in deionized water with 0.5% v/v HNO3 by serial dilution of the stock solutions with 1000 mg L− 1 Fe (as FeCl3) (Merck, Darmstadt, Germany). The following atomic absorption standard solutions were used for the permanent modifiers: 1000 mg L− 1 Ir and 1000 mg L− 1 W (both from Fluka, Buchs, Switzerland). In order to coat the platform with the W–Ir mixed permanent modifier, five aliquots of 40 µL of the W modifier solution were applied first, each one followed by the temperature program shown in Ref. [7,9], then five aliquots of 40 µL of the Ir modifier solution were applied, also followed by the same temperature program, resulting in a coating with 200 µg each of W and Ir. 3. Results and discussion The original idea of this work has been to transfer the procedure that has been developed for the simultaneous determination of Cd and Fe in grain products using direct solid sampling [9] to the analysis of bean and soil samples from different regions of Brazil [7]. While the procedure worked well for the bean samples, the soil samples turned out to be a problem due to their extremely high iron content. We therefore have been looking for a solution of the problem, and the obvious first approach has been to measure the absorbance at the line wings in order to reduce sensitivity and increase the working range, as has been proposed previously [5]. As for this application the center of the Cd line at 228.803 nm is focused on the center pixel (No. 250), the iron line is shifted and not exactly focused onto a specific pixel. The distribution of the iron line at 228.726 over the eight pixels that have a detectable contribution to the integrated absorbance is shown in Fig. 1. In an initial attempt calibration curves have been measured at all of these pixels individually using aqueous standards, and the corresponding linear regression equations together with the calibration range and the coefficient of correlation are summarized in Table 2. It is worth mentioning that all these data on the different pixels have been obtained from the same set of measurements for each iron standard,
Table 1 Temperature program for the simultaneous determination of cadmium and iron in food and soil samples by SS HR-CS AAS using W–Ir as the permanent modifier. Stage
Temperature/°C Ramp/°C s− 1 Hold time/s Ar flow rate/L min− 1
Drying 90 Drying 130 Pyrolysis 700 700 Auto zeroa a Atomization 1700 Atomizationa 2600 Cleaning 2600 a
10 5 50 100 2000 3000 1000
Signal registration in these stages.
10 5 15 1 3 12 3
2.0 2.0 2.0 0 0 0 2.0
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B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262 Table 3 Linear regression equations and correlation coefficient for calibration curves in the high analyte mass range obtained at various pixels in the vicinity of the 228.726 nm iron line; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C.
Fig. 1. Integrated absorbance measured at the various pixels in the vicinity of the iron absorption line at 228.726 nm for 100 ng Fe; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C.
and only the evaluation per pixel has been sequential. The results show that by choosing the appropriate pixel for evaluation, the sensitivity and also the working range can be increased or decreased by more than one order of magnitude. It is also worth mentioning that all of the calibration functions, except for that at pixel 207, do have correlation coefficients between 0.997 and 0.999, which means that the relations are linear. In an attempt to further extend the working range, the analyte mass has been increased up to 2000 ng Fe, corresponding to a concentration of 100 mg L− 1 Fe in 20 μL of solution, expecting that the calibration curve would become non-linear. To our surprise we discovered that all calibration curves showed kind of an inflection point, after which the curves continued to be linear, although with a distinctly lower slope. This behavior is shown in Table 3 for pixels 208 to 213, and the correlation coefficients between 0.9875 and 0.9994 confirm the linear correlation between integrated absorbance and analyte mass. Fig. 2 might serve as an example; it shows the summated integrated absorbance measured at pixels 209 + 213, i.e., the peak volume selected absorbance (PVSA) [5], and demonstrates that the linear range actually extends beyond the calibration range chosen in Table 3 at least up to 4000 ng Fe. The limit of quantification, determined as 10 times the standard deviation of a blank, divided by the slope of the calibration curve, in PVSA for pixels 210, 211 and 212 was found to be around 4 ng; this means that a concentration range of at least three orders of magnitude can be covered by selecting the proper pixel or combination of pixels for evaluation. In order to test the applicability of this calibration approach for the analysis of real samples, the certified reference material BCR 142 (light sandy soil) with an iron content, expressed as Fe2O3, of 2.80%
Pixel number
Linear regression equation
R
Calibration range/ng
208 209 210 211 212 213
Aint = 0.00016 mFe + 0.0842 Aint = 0.00058 mFe + 0.4196 Aint = 0.0011 mFe + 0.9111 Aint = 0.0013 mFe + 0.9945 Aint = 0.00094 mFe + 0.7026 Aint = 0.00036 mFe + 0.2078
0.9994 0.9984 0.9947 0.9953 0.9875 0.9902
250–2000 250–2000 250–2000 250–2000 250–2000 250–2000
has been analyzed using direct solid sampling. The result of 2.79 ± 0.59% (n = 10) is in excellent agreement with the certified value; the high standard deviation is due to the natural inhomogeneity of this kind of samples that becomes significant in this case, as the sample mass introduced into the furnace has been only between about 0.04 and 0.09 mg. In order to investigate the potential reasons for the unusual calibration curves, first of all the atomization signals obtained at the various pixels for a high iron mass have been consulted. As shown in Fig. 3, the signals do not return to the baseline within the atomization time of 12 s, i.e., part of the absorbance signal is lost under these conditions. However, increasing the atomization time until the signals returned to the baseline did not have any significant effect on the overall pattern; the slope of the high-mass calibration graphs increased slightly at all pixels and the intercept with the Y-axis decreased accordingly, but the calibration graphs remained linear with R values greater than 0.99. Another investigation that has been carried out was if the W–Ir permanent modifier, which had to be added to stabilize Cd, was causing kind of an “over-stabilization” of iron. However, determinations that have been carried out without this modifier under otherwise identical conditions did not show any significantly different results. Another investigation has been carried out to see if the fact that the measurements had been made at the edge of the detector and the line was not well centered on a pixel (refer to Fig. 1) had an influence on the results. Hence, the iron line at 228.726 has been focused on the center pixel 250 and the measurements have been repeated. Table 4 shows the regression equations using PVSA of pixels 248 + 252, which corresponds to the measurements shown in Fig. 2 at pixels 209 + 213;
Table 2 Linear regression equations and correlation coefficient for calibration curves in the low analyte mass range obtained at various pixels in the vicinity of the 228.726 nm iron line; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C. Pixel number
Linear regression equation
R
Calibration range/ng
207 208 209 210 211 212 213 214
Aint = 0.0001 mFe + 0.00052 Aint = 0.00037 mFe + 0.0239 Aint = 0.00179 mFe + 0.0218 Aint = 0.00418 mFe + 0.0712 Aint = 0.00439 mFe + 0.0507 Aint = 0.00267 mFe + 0.0202 Aint = 0.00048 mFe + 0.0309 Aint = 0.0001 mFe + 0.0131
0.9847 0.9977 0.9990 0.9989 0.9988 0.9960 0.9986 0.9974
75–800 50–600 50–350 10–100 10–100 10–100 40–400 50–800
Fig. 2. Calibration curves for iron in the vicinity of the 228.726 nm analytical line using PVSA at pixels 209 + 213 for evaluation; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C; linear regression equation for low mass range: A∑ ± 2,int = 0.0023 mFe + 0.0443; R = 0.9995; linear regression equation for high mass range: A∑ ± 2,int = 0.0007 mFe + 0.8478; R = 0.9972.
B. Welz et al. / Spectrochimica Acta Part B 65 (2010) 258–262
Fig. 3. Transient signals measured at pixels 208–213 in the vicinity of the Fe line at 228.725 nm; 1000 ng Fe from an aqueous standard solution; W–Ir as permanent modifier; pyrolysis temperature: 700 °C, atomization temperature: 2600 °C.
the similarity of the regression equations is striking, indicating that the off-center and asymmetric measurement had no influence on the result. In addition other secondary iron lines have been investigated in order to find out if they exhibit a similar behavior as the line at 228.726 nm; the corresponding regression equations are also shown in Table 4. The line at 360.880 nm is about a factor of 8 less sensitive than the line at 228.726 nm; for this reason the measurement has been made in the line core at pixel 250 in order to get comparable absorbance signals. There has been some non-linearity in the low absorbance range, which is not shown here, but the response has been linear for analyte masses between 250 and 4000 ng Fe, and the similarity of the regression equation with that measured at 228.726 nm at pixels 248 + 252 is again striking. Finally, another secondary iron line at 237.362 nm has been investigated as well, using the same pixels 248 + 252 at the wings as for the 228.726-nm line; the results are also shown in Table 4. It has again been possible to identify two linear parts, where the inclination of the first one has been identical to that found for the 228.726-nm line, whereas the second one, for analyte masses between 100 and 1000 ng had a lower inclination compared to the latter line. A distinct difference has been found for analyte masses higher than 1000 ng, or above a PVSA of 1 s, where the calibration curve started to exhibit pronounced curvature, i.e., kind of a “normal” behavior. It would actually have been possible to describe the entire calibration range from 20 to 4000 ng Fe (PVSA from 0.0797 to 2.429 s) using a quadratic equation, although with some inconsistencies in the curve. Harnly and colleagues published a series of papers about the shape of extended calibration curves for CS AAS, e.g. [13,14]. Without any exception their curves consist of two linear branches with significantly different inclination; however, all their curves are in a double logarithmic scale, which is quite different from the shape found in this work, so that their explanation cannot be applied for the present
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results. For further clarification of the unusual progression of the calibration curves, theoretical considerations reflecting the mode of measurement have been made. Initially, a “true” line profile has been generated by folding a Gauss-type with a Lorentz-type profile, both having the same width and area. The resulting Voigt-type profile was transformed to a transmittance spectrum by introducing an absorption length. Then, two absorbance values were calculated. The first one corresponds to the maximum absorption in the center of the profile and is assumed to be proportional to the analyte mass. The second value should simulate the measured integrated absorbance. It was gained by calculating the ratio between two integrals taken within the transmittance spectrum with and without absorption respectively. The integration limits were set symmetrically to the center profile and represent the instrument width. By variation of the absorption length over several orders of magnitude, a large set of related pairs of absorbance values was gathered. This procedure was repeated for different instrument widths and resulted in a set of theoretical calibration curves. Each curve was normalized to its maximum value in order to emphasize its inherent progression. Fig. 4 provides the results for three different instrument widths. The curves show that for growing instrument width the initially straight curve is increasingly bended, generating two distinguishable parts, which can be approximated by straight lines. This corresponds to the shape of the measured calibration curve in Fig. 2 and might be the explanation for it. The hypothesis is further supported by the fact that iron belongs to the elements with the narrowest absorption line widths. 4. Conclusion There are some similarities in the shape of the calibration curves of secondary, significantly less sensitive absorption lines of iron when high analyte concentrations are determined; however, there are also distinct differences for different lines using the same analytical conditions. The explanation for this unusual shape of the calibration curves might be in the difference between the instrument width ΔλInstr and the absorption line width ΔλAbs, which is quite pronounced in the case of Fe; however, other factors, such as stray light might also contribute to the phenomenon. It is obvious that more studies have to be carried out in order to investigate the observed phenomena, which also have to be extended to other elements. It appears to be worthwhile to carry out such investigations, as with the increasing use of HR-CS AAS equipment, particularly for GF AAS, there
Table 4 Linear regression equations and correlation coefficient for calibration curves obtained at various secondary analytical lines for iron; W–Ir permanent modifier: pyrolysis temperature 700 °C; atomization temperature 2600 °C. Line/nm
Pixel
Linear regression equation
R
Calibration range/ng
228.726 228.726 360.880 237.362 237.362
248 + 252 248 + 252 250 248 + 252 248 + 252
A∑ ± 2, int = 0.0021 mFe + 0.0098 A∑ ± 2, int = 0.0007 mFe + 0.5629 A int = 0.0007 mFe + 0.8374 A∑ ± 2, int = 0.0021 mFe + 0.0004 A∑ ± 2, int = 0.0004 mFe + 0.1062
0.9982 0.9988 0.9995 0.9993 0.9971
20–200 400–4000 250–4000 20–100 100–1000
Fig. 4. Theoretical and normalized calibration curves for three different ratios between the instrument width ΔλInstr. and the absorption line width ΔλAbs..
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will be an increasing demand for using side pixels for measurement at the line wings in order to increase the dynamic range of this technique.
[7]
Acknowledgement [8]
The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support. B. W. and M.G.R.V. have research scholarships from CNPq and L.M.G.S. has a scholarship from CAPES. The authors are also grateful to Analytik Jena AG for the donation of the HR-CS AAS prototype.
[9]
[10]
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