- Email: [email protected]
Direct analysis of silica by means of solid sampling graphite furnace atomic absorption spectrometry
Spectrochimica Acta Part B 71–72 (2012) 24–30
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Direct analysis of silica by means of solid sampling graphite furnace atomic absorption spectrometry M. Resano a,⁎, E. Mozas b, C. Crespo b, J. Pérez c, E. García-Ruiz a, M.A. Belarra a a b c
Department of Analytical Chemistry, University of Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain Technological Institute of Aragon, Laboratory of Mechanics and New Materials, María de Luna 8, E-50018 Zaragoza, Spain Grupo IQE.Polígono Industrial Malpica, Calle “D”, no 97, E-50057 Zaragoza, Spain
a r t i c l e
i n f o
Available online 19 March 2012 Keywords: Solid sampling Graphite furnace atomic absorption spectrometry Hazardous elements Analysis of silica
a b s t r a c t This paper reports on the use of solid sampling-graphite furnace atomic absorption spectrometry for the direct analysis of synthetic amorphous silica. In particular, determination of hazardous elements such As, Cd, Cr, Cu, Pb and Sb is investigated, as required by regulations of the food industry. The conclusion of the work is that, after proper optimization of the working conditions, paying particular attention to the atomization temperature and the use of proper modifiers (graphite powder, HNO3 or Pd), it is possible to develop suitable procedures that rely on the use of aqueous standard solutions to construct the calibration curves for all the elements investigated. The proposed method shows important benefits for the cost-effective analysis of such difficult samples in routine labs, permitting fast screening of those elements that are very rarely present in this type of sample, but also accurate quantification of those often found, while offering low limits of detection (always below 0.1 mg g− 1) that comply well with legal requirements, and precision levels that are fit for the purpose (approx. 6–9% R.S.D.). © 2012 Elsevier B.V. All rights reserved.
1. Introduction Synthetic amorphous silica (SAS) is a high purity, crystalline-free, synthetic silicon dioxide, which may be produced as precipitated, silica gel, fumed or pyrogenic silica [1]. Even though the manufacturing process may differ to some extent, all types of SAS are similar in their purity, physico-chemical properties and toxicological requirements [2]. Therefore, they are considered as the same substance. SAS is a nanostructured material widely used in many industrial applications, mainly as reinforcement and thickening agent. Commercial SAS grades are part of industrial formulations such as elastomers (tires, shoe soles and technical pieces), resins, inks and paints. SAS grades are also incorporated in consumer products such as cosmetics, medical drugs or toothpastes. Furthermore, SAS is used in the food industry as anticaking, stabilizing, thickener and gelling agent, and has also been approved as a food and feed additive (EU Directive 95/2/EC, amendment number E551). As a result of this intensive use, the global demand for this product was forecasted to exceed 1.5 million metric tons in 2011. In particular, the use of SAS in food products makes it necessary to control the level of potentially toxic elements. Nowadays, following EU Directive 2008/84/EC, SAS products comply with legal standards only when As, Pb and Hg contents are below 3, 5 and 1 μg g − 1,
⁎ Corresponding author. Fax: + 34 976761292. E-mail address: [email protected] (M. Resano). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.03.005
respectively, while EU Directive 2002/32/EC introduced a limit of 2 μg g − 1 for Cd. The traditional methods for the analysis of metal impurities in SAS substances can be found either in the silica monographs of the Food Chemical Codex [3] or in the JECFA (Joint Expert Committee on Food Additives) monograph for synthetic amorphous silica [4]. All these procedures for trace metal determination begin with the dissolution of the sample, which for this type of sample requires the use of hydrofluoric acid. Later, metals in solution are determined by suitable atomic spectrometric techniques, such as atomic absorption spectrometry (AAS) or inductively coupled plasma (ICP)-based techniques. The development of analytical methods that permit the determination of the metals of interest directly in SAS samples could be an appealing alternative to control these materials. In this way, it would be possible to: (i) improve the sensitivity (by avoiding the dilution that accompanies sample digestion); (ii) considerably increase the sample throughput; (iii) minimize the risk of contamination/losses; and (iv) implement much “greener” (environmentally-friendly) analytical approaches [5–7]. There are different analytical techniques that can be used for that purpose, but many of them require some sort of sample pretreatment for this type of materials (e.g., for X-ray fluorescence is typical to carry out a borate fusion [8–10]), and rely on the availability of suitable solid standards for calibration [11]. In contrast, graphite furnace AAS (GFAAS) shows interesting advantages, such as potential for direct analysis, an excellent sensitivity and selectivity and, also very important in this context, the possibility to use aqueous standard solutions
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
for calibration [12–15]. To the best of the authors' knowledge there is no previous report in the use of this technique for SAS analysis, but previous works focused in the analysis of soils and other geological samples [16–23] have reported satisfactory results, at least for some elements. In addition, a work describing the use of slurry sampling for analysis of high-purity quartz also reported suitable accuracy and low limits of detection (LOD) for a variety of analytes [24]. It is the purpose of this work to evaluate the use of solid sampling (SS)-GFAAS for the direct analysis of SAS samples in the context of a cooperation with a manufacturing company that required control of a number of elements, namely As, Cd, Cr, Cu, Pb and Sb. The final goal is to develop straightforward procedures relying on the use of aqueous standards for calibration.
2. Experimental 2.1. Reagents Standard solutions containing appropriate amounts of As, Cd, Cr, Cu, Pb, Pd and Sb were daily prepared from commercially available (Merck, Darmstadt, Germany) 1 g L − 1 single-element standards by dilution with 0.14 mol L − 1 HNO3. Purified water was obtained from a milli-Q system (Millipore, Billerica, USA). 65% HNO3 and 37% HCl were purchased from Carlo Erba (Sabadell, Spain) and 48% HF from Scharlau (Barcelona, Spain).
25
2.4. Procedure for SS-GFAAS Sample analysis required no prior treatment other that checking the humidity level. Samples were kept at 105 °C during 24 hours prior to analysis. The moisture content was always at the 3–4% level. The solid sampling device used allows automatic weighing and transport of the samples into the furnace. The empty platform was first transported to the microbalance using a pair of tweezers. After taring, an appropriate amount of graphite powder (approx. 1.5 mg, except for Cd determination, for which graphite addition was not required) was deposited onto the platform. Afterwards, the sample was also deposited onto the platform and weighed. Typical values for the sample masses used are shown in Table 1. Finally, the chemical modifier (Pd or HNO3) was added when needed. The platform was then transferred to the graphite furnace and subjected to the temperature program. All these operations were fully controlled from a keyboard, except for the deposition of the sample and modifiers onto the platform, which were carried out manually. The operating conditions used are summarized in Table 1. The calibration was carried out using 10 μL of aqueous solutions of the appropriate concentration, added with a micropipette onto the sampling platform. For every determination, five solid samples were analyzed and the median of the five results was taken as a representative value [28]. However, when it was clear that the sample contained no measurable amount of analyte, only three replicates were carried out. Integrated absorbance (Aint) was selected as the measurement mode in all circumstances. 2.5. Dissolution and analysis of the samples for comparison purposes
2.2. Samples The samples analyzed were obtained from Industrias Químicas del Ebro (IQE, Zaragoza, Spain). SAS samples were available as fine powders, with a mean particle size of 250 μm. The certified reference material (CRM) NCS DC73304 Rock-Constituents (China National Center for Iron and Steel, Beijing, China) was used for validation purposes, since the chemical composition of this material is similar to those of the samples. This material shows a SiO2 content of 90.36 ± 0.15%, other major constituents being Al2O3 (3.52 ± 0.09%) and Fe2O3 (3.22 ± 0.07%). Its particle size is lower than 74 μm.
For evaluating the results obtained, one SAS sample was dissolved and the solutions obtained were subsequently analyzed by means of ICP OES or HR-CS GFAAS. The process of sample dissolution consisted of: a) weighing of approximately 500 mg; b) digestion with 1 mL of 37% HCl, 1 mL of 65% HNO3 and 4 mL of 48% HF for 40 min in the microwave oven; c) cooling; and d) dilution to 50 mL. Approximately 3 hours were required for the entire process. 3. Results and discussion 3.1. Method development
2.3. Instrumentation An AAS ZEEnit 600 atomic absorption spectrometer from Analytik Jena (Jena, Germany) was used throughout the work. This instrument is equipped with a transversely heated graphite tube atomizer, an automatic solid sampling accessory (SSA 61), including a Sartorius (Göttingen, Germany) M2P microbalance with a precision of 0.001 mg, and a Zeeman-effect background correction system [14]. Pyrolytic graphite coated graphite tubes (with dosing hole) and solid sampling platforms were used for this instrument. A Horiba JobinYvon (Kyoto, Japan) ACTIVA™ ICP optical emission spectrometer (OES) was used for the solution ICP OES measurements. It is equipped with a Czerny-Turner optical system and a low-noise high-speed CCD detector, which covers the spectral range of 165– 800 nm. A HF-resistant torch (made of alumina), nebulizer, spray chamber (made of polytetrafluoroethylene) and injector (made of polyether ether ketone) were used to avoid the deleterious effect of such acid. For Pb determination in digested silica samples, a highresolution continuum source atomic absorption spectrometer (HRCS AAS), ContrAA 700, commercially available from Analytik Jena AG (Jena, Germany) was used. More details on this instrument can be found elsewhere [25–27]. A microwave oven Ethos One from Milestone Inc. (Shelton, USA) was used for sample digestion. The different subsamples were weighed in a Sartorius (model R-200 D) microbalance with a precision of 0.001 mg.
In order to develop and validate the solid sampling methodology investigated in this work, a CRM was selected. Since no certified silica material is available for this purpose, a rock material that contains approx. 90% of SiO2 was considered as the most similar of the commercially available materials and was used instead. 3.1.1. Sensitivity adjustment Since, for As, Cd and Sb the goal was to develop a method capable of carrying out determinations below the μg g − 1 level, maximum sensitivity conditions were used for those analytes. That translates into using the most sensitive resonance lines and stop flow conditions during atomization. Sensitivity can then be adjusted to some extent by selecting the appropriate sample mass, even though using very high amounts (over 1 mg) is not recommended, as it typically results in higher background signals and faster platform deterioration. For Cu, Cr and Pb, higher analyte levels are expected in both the reference material and the samples. Thus, for Cu and Pb slightly less sensitive lines (approx. a factor of two) than the main lines were selected (283.3 nm instead of 217.0 nm for Pb and 327.4 nm instead of 324.7 nm for Cu). For Cr, owing to the typical high sensitivity of the technique for this element, a four times less sensitive line (427.5 nm instead of 357.9 nm) and medium flow (1 L min− 1) conditions during atomization were chosen. It should be mentioned that Cr is a priori the most difficult analyte of the list to be determined in a refractory matrix, owing its low volatility [29], and the maintenance of Ar during atomization was
26
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
Table 1 Optimum conditions used during SS-GFAAS analysis of all the materials. Elements
Wavelength / nm Background Correction Pyrolysis Temperature / °C Ramp / °C s− 1 Hold / s Gas flow Atomization Temperature / °C Ramp / °C s− 1 Hold / s Gas flow Modifier Platform protection Chemical interaction Sample mass / mg NCS DC73304 SAS 1
Cu
Cr
Cd
As
Pb
Sb
327.4 Zeeman
427.5 Zeeman
228.8 Zeeman
197.2 Zeeman
283.3 Zeeman
217.6 Zeeman
1000 200 30 Max
1000 300 20 Max
600 100 20 Max
800 100 20 Max
700 300 30 Max
1500 500 10 Max
2500 2000 6 Stop Graphite powder1
2500 1200 6 1 L min− 1 Graphite powder1 15 μL 65% HNO3
2200 2000 4 Stop None
2400 2000 4 Stop Graphite powder1 5 μg Pd
2400 2000 3 Stop Graphite powder1
2500 2000 2.5 Stop Graphite powder1 5 μg Pd
≈ 0.1–0.2 0.5–1
≈ 0.2–0.5 0.5–1
≈ 0.2–0.5 ≈1
≈ 0.1–0.3 ≈1
≈ 0.2–0.5 0.5–1
≈ 0.4–0.7 ≈1
Approximately 1.5 mg.
preferred because it has been reported for similar matrices that it can help in reducing particle condensation in the tube cooler ends, thus minimizing the corresponding light scattering issues [17,30]. 3.1.2. Atomization of the different elements from the solid CRM Optimal atomization conditions were investigated taking into account the following aspects: i) considering the low volatility of the main matrix component (SiO2), it does not seem feasible to achieve a significant degree of matrix removal during the pyrolysis step; thus, the pyrolysis step is not considered as very critical for this sample [13]; ii) background is typically lower for higher wavelengths, because light scattering is less of an issue and fewer diatomic species absorb, and thus fewer problems in this regard are expected for Cu and Cr monitoring; for the rest of the analytes, the higher the atomization temperature, the higher the probability of matrix co-vaporization (as SiO), thus optimization of the atomization temperature is important to minimize problems in this aspect; iii) however, when analytes are embedded in this type of sample they are often retained to some extent, making it necessary to use higher atomization temperatures compared with the work with solutions. In other words, the matrix itself may act as a modifier, making redundant the use of chemical modifiers, at least for some elements. Since the goal of the work is to develop procedures as simple as possible, chemical modifiers will only be used when they are considered indispensible; iv) very often for this kind of sample the atomization process for the solid sample and for an aqueous standard solution is not exactly the same, which results in differences in the signal profiles. However, the use of integrated peak areas may still make it feasible to use aqueous standards for calibration [31–33]. Cu and Pb atomization from the CRM resulted to be relatively simple. Pyrolysis temperature shows little effect on the final signal. It is not necessary to use any chemical modifier to stabilize the analytes since the use of pyrolysis temperatures sufficiently low to prevent any losses is possible. For the atomization temperature, on the other hand, it is necessary to use 300 °C more than typically recommended for solutions, as expected. However, under the conditions shown in Table 1, the signals for solid samples and aqueous standard solutions are practically identical in the case of Cu (see Fig. 1a), and also quite similar (a bit more broadened for the solid) in the case of Pb (see Fig. 1b). In both cases, the areas of the solid samples and aqueous solutions were comparable. Pb signal for the CRM exhibited a higher background signal, but it was clearly retarded with respect to the analyte signal. The integration time was restricted to 3 s for Pb, to minimize the potentially deleterious influence of this background absorbance.
Cd shows a similar behavior to that of Pb, but the stabilizing effect of the matrix can be clearly observed for this analyte. This is an analyte for which using atomization temperatures below 2000 °C is the rule, even for direct solid sampling analysis [21,34–38]. However, for the CRM investigated in this work it is necessary to use an atomization temperature of at least 2200 °C to obtain a well-defined signal profile. Still this signal is delayed (0.5 s) and broadened compared with that of an aqueous standard solution (see Fig. 1c), but peak areas are analogous. No chemical modifier was needed for this analyte. Arsenic atomization can also be carried out, in principle, directly without the concourse of any chemical modifier. Very similar signal shapes with comparable peak areas were obtained for the solid samples and the aqueous standard solutions (with the solid sample signal appearing a bit before that of the aqueous standard, see Fig. 1d). The background signal behaves in a similar way as described for Pb, even though it does not increase significantly up to 4 s. However, under those working conditions, and despite the excellent signal definition achieved, low precision values for this element were found, and a sensitivity drift for consecutive atomizations of solid samples was also detected. The use of Pd as chemical modifier was observed to alleviate these effects. Identical atomization conditions could be used when adding Pd, and the signal profiles obtained were very similar to those shown in Fig. 1d, but delayed 0.2 s, which clearly indicates analyte interaction with the modifier. Very similar considerations can be made for Sb. For this element, best repeatability was attained when using Pd as chemical modifier. The background signal increases significantly with time (see Fig. 1e), as a consequence of the high atomization temperature needed for proper atomization (2500 °C) and the short wavelength, so the integration time was restricted to 2.5 s. Under these conditions, use of aqueous standards for calibration also appears to be feasible, as the areas obtained for solid samples and standard solutions are identical. Finally, Cr determination proved to be the most challenging one. Even though, when measuring aqueous standards or even digested samples, it is feasible to obtain well-defined single-peak signals for atomization temperatures of 2400–2500 °C, very low, poorly defined and unreproducible signals were obtained for direct analysis of the solid sample. Optimization of the atomization temperature did not result in any significant improvement, even when the highest atomization temperature available was used (2700 °C). This fact is consistent with a previous work in which carrying out a micro-fusion directly in the graphite furnace was necessary to properly atomize Cr from a
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
A
B
C
D
E
F
27
Fig. 1. Comparison of the signal obtained for the atomization of the analytes from CRM NCS DC73304 Rock-Constituents and from aqueous standard solutions when monitoring: A) Cu; B) Pb; C) Cd; D) As; E) Sb; F) Cr. BG stands for background.
sewage sludge with a high level of silicates [29]. Less problems were encountered, however, in other work [18]. In this particular case, the addition of 15 μL of 65% HNO3 as chemical modifier was found sufficient to help in releasing Cr from the matrix. This signal is clearly
delayed (approx. 1.5 s) in comparison with that of an aqueous standard solution (see Fig. 1f), and shows considerably more background, but it is well defined, reproducible and shows an area comparable to that of an aqueous standard solution.
28
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
Table 2 Results obtained using SS-GFAAS analysis of the rock CRM (90% SiO2) NCS DC73304, following the procedure described in Section 2.4. (n = 8–10). Analyte
As Cd Cu Cr Pb Sb
Reference
SS-GFAAS results
Certified value / μg g− 1
Result ± 95% C.I. / μg g− 1
RSD / %
LOD / ng g− 1
9.1 ± 1.2 0.060 ± 0.016 19 ± 2 20 ± 3 7.6 ± 0.8 0.60 ± 0.11
8.8 ± 0.4 0.059 ± 0.003 18.6 ± 1.3 21.2 ± 1.1 7.1 ± 0.4 0.57 ± 0.03
6.6 7.8 8.6 6.5 6.5 8.7
41 2 26 70 18 43
mass used (see Fig. 1c). This aspect further proves the potential of SSGFAAS for ultratrace analysis. It can also be mentioned that even lower limits could be obtained for Cu and Pb, and particularly for Cr, using more sensitive lines [29]. This is, however, not necessary in the context of the current application.
3.3. Analysis of SAS samples
3.1.3. Platform protection During the optimizations carried out before, it was observed that the lifetime of the platforms was very short for those elements for which an atomization temperature of 2400 °C or more was needed (all of them but Cd). After monitoring 20–30 consecutive solid samples, signals of platform deteriorations were evident, eventually leading to the development of an orifice, likely as a consequence of the reaction between SiO2 and C (reduction at high temperature), in a similar way as described for direct analysis of alumina [39]. The sample mass also plays a role, as a larger mass further accelerates the process. Thus, in order to improve the lifetime of this important part, addition of graphite powder is greatly recommended. This addition does not significantly alter the shape and the magnitude of the signals discussed before, but it permits using the same platform for at least 50–60 replicates for sample masses up to 1 mg. 3.2. Analysis of the CRM. Performance of the method Analysis of the rock CRM NCS DC73304 was carried out using solid sampling- GFAAS and the conditions shown in Table 1. The procedure is described in Section 2.4. In all cases, the calibration curve was constructed simply using aqueous standard solution. Five replicate measurements were made for each determination, and the median of them was taken as representative value instead of the mean, in order to minimize the possible influence of outliers [28]. The results of eight to ten determinations carried out on three different days are summarized in Table 2, and compared with the reference values. As can be seen, there is good agreement for all the elements in terms of accuracy and no significant differences could be established, which confirms that aqueous standard solutions can be successfully used for calibration. The precision obtained by means of SS-GFAAS (between 6% and 9% RSD) can also be regarded as satisfactory for the purpose intended. The limits of detection, are also fit-for-purpose, as contents below 0.1 μg g− 1 already comply with legal requirements in all cases. The detection level achieved for Cd is extremely low owing to the excellent sensitivity of the technique for this element. It can be stressed that the results obtained for this element are very comparable in terms of accuracy and precision to those obtained for the other analytes, despite the fact that the sample contains only 15–30 pg Cd with the sample
After optimization and validation with the CRM, the methodology developed was used for analysis of real SAS samples during the last 2 years, a period in which more than 100 samples from the company IQE (that produces SAS) were analyzed. During that period, the method provided a robust and consistent performance. Results for a representative sample are shown in Table 3. As can be seen, the first noticeable aspect is that As, Sb and Cd were not found in the sample at detectable levels. This was also the case for all the other samples investigated. This aspect illustrates a situation often encountered in real life analysis, where it is necessary to check that a sample does not contain detectable levels of some hazardous elements that are not expected to be present, according to the manufacturer's experience. The use of SS-GFAAS for this case is particularly advantageous, since in 10 min per analyte (if no signal was obtained only 3 and not 5 replicates per sample were carried out) the sample can be screened, avoiding the time and effort required for its dissolution. Moreover, the SS-GFAAS approach guaranties sufficiently low detection limits, such that the sample can be cleared for its use, whether it is going to be destined to the food industry as additive (with the corresponding astringent requirements) or will be used in a different context (see introduction for examples). It should be mentioned that in some cases also Hg was monitored in the samples, but the procedure used for Hg was already described in a previous work [40], so that this element is not discussed in the current paper. Even though some elements were not present in any of the samples, a recovery test was carried out to further validate the methodology proposed. For this purpose, the sample (Ebrosil 001) was spiked with known amounts of As, Cd and Sb to obtain approx. 1 μg g− 1 of As and Sb, and 0.1 μg g− 1 of Cd. The recovery test proved satisfactory, as shown in Table 3. The other three elements could be determined in all the samples. The signal profiles obtained for these elements are shown in Fig. 2. They are very similar to those obtained for the CRM, proving that the former could be considered as a good choice for validation. In all the cases, very well-defined single-peak signals were attained. As stated before, the values obtained for Ebrosil 001 and shown in Table 3 can be considered as representative in terms of precision and analyte level. The precision is also similar to that observed for the CRM (6–9% RSD) and quite normal for the SS-GFAAS technique [13]. This is typically regarded as a weak characteristic of SS-GFAAS, owing to the low sample masses that are analyzed. It has to be stressed, however, that when determinations at the 1 μg g − 1 level or below for a complex samples are aimed, it is also not easy to improve this aspect when
Table 3 Results obtained using SS-GFAAS analysis of the of a silica sample (Ebrosil 001), following the procedure described in Section 2.4 (n = 5). Reference values were obtained as described in Section 2.4 (n = 4). Digestion-ICP OES results Analyte
Result ± 95% C.I. / μg g− 1
As Cd Cu Cr Pb Sb
b0.5 b0.02 1.39 ± 0.14 1.34 ± 0.21 1.04 ± 0.141 b0.4
1
SS-GFAAS results RSD / %
10.3 9.5 8.5
LOD / μg g− 1
Result ± 95% C.I. / μg g− 1
0.5 0.02 0.2 0.2 0.4 0.4
b 0.04 b 0.002 1.30 ± 0.13 1.28 ± 0.10 1.17 ± 0.11 b 0.04
SS-GFAAS recovery assay RSD / %
8.3 6.5 7.8
Owing to the high LOD obtained with ICP OES for this element, these measurements were carried out with HR-CS GFAAS.
LOD / μg g− 1
Analyte added / pg
Analyte found / pg
0.04 0.002 0.03 0.07 0.02 0.04
1000 100
943 ± 87 102 ± 9
1000
993 ± 75
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
A
B
29
Table 4 Results for the direct determination of Pb in sand samples used as precursor of SAS by means of SS-GFAAS analysis, following the procedure described in Section 2.4 (n = 5). Reference values were obtained as described in Section 2.4 (n = 4), with HR-CS GFAAS. Sample
Reference value / μg g− 1
SS-GFAAS Result ± 95% C.I. / μg g− 1
Sand 001 Sand 002 Sand 003
2.79 ± 0.25 1.33 ± 0.13 0.94 ± 0.12
2.90 ± 0.27 1.38 ± 0.06 0.85 ± 0.10
Of these elements, perhaps the greatest concern in terms of toxicity and regulations is Pb. Moreover, this element could not be properly determined by means of digestion and ICP OES analysis at the level found in most of the samples (limit of quantification 1.3 μg g− 1). That is the reason why the reference value shown in Table 3 was obtained with HR-CS GFAAS (to further assess the absence of interferences) after sample digestion. These two aspects, very fast screening of As, Cd and Sb, and direct determination of Pb at the μg g − 1 level, would be the main reasons to opt for the SS-GFAAS approach, which, of course, also shows some disadvantages, such as lack of multi-element capability. Finally, the origin of the Pb found in the samples was suspected to be the sand used as precursor for producing the SAS samples. Thus, direct determination of Pb in sand could be of interest in this context. It can be stressed that the same method developed before could be applied for the analysis of sand samples, as demonstrated in Table 4. Even more, the typical Pb signal profiles obtained for sand were remarkably similar to those obtained for SAS. 4. Conclusions
C
This work explores the possibilities of SS-GFAAS for direct analysis of silica samples. This method combines the inherent advantages of most solid sampling methods (good sensitivity, speed, no need to use hazardous reagents) with the possibility to obtain accurate results by using aqueous standards for calibration, and precision values that are fit-forpurpose (below 10% RSD). In this way, it is possible to carry out the analysis of these samples in a cost-effective way, avoiding laborious digestion procedures that require the use of hazardous reagents (e.g., HF), and offering detection limits that are sufficiently low to meet legal requirements, thus complying well with the requisites of industrial laboratories. Acknowledgements This work has been funded by the Spanish Ministry of Science and Innovation (Project CTQ2009-08606) and the Aragon Government (Project PM024/20006 for multidisciplinary research and Departamento de Ciencia, Tecnología y Universidad del Gobierno de Aragón y Fondo Social Europeo). References
Fig. 2. Comparison of the signal obtained for the atomization of the analytes from a SAS sample (Ebrosil 0001) and from aqueous standard solutions when monitoring: A) Cu; B) Pb; C) Cr. BG stands for background.
dissolution of the sample is carried out, as demonstrated by the values shown in Table 3. The analyte levels varied for different samples in the range 0.7 to 1.2 μg g− 1 for Cu, 1.0 to 1.3 μg g− 1 for Cr and 1.1 to 2.2 μg g− 1 for Pb.
[1] R.K. Iler, The Chemistry of silica: solubility, polymerization, colloids and surface properties and biochemistry of silica, Wiley-Interscience, New-York, 1979. [2] Synthetic Amorphous Silica (CAS Nº 7631-86-9), Joint Assessment of Commodity Chemicals Report Nº 51, European Centre for Ecotoxicology and Toxicology of Chemicals, Brussels, 2006, pp. 1–237. [3] Food Chemicals Codex, 7th ed. United Book Press Inc, Baltimore, 2010. [4] Joint FAO/WHO Expert Committee on Food Additives, Combined Compendium of Food Additive Specifications, Volume 4 Analytical Methods, Test Procedures and Laboratory Solutions Used by and Referenced in the Food Additive Specifications, Food and Agriculture Organization (FAO) of the United Nations, Rome, 2006. [5] M. Resano, E. García-Ruiz, M.A. Belarra, F. Vanhaecke, K.S. McIntosh, Solid sampling in the determination of precious metals at ultratrace levels, Trends Anal. Chem. 26 (2007) 385–395. [6] M. de la Guardia, Green analytical chemistry, Trends Anal. Chem. 29 (2010) 577.
30
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
[7] S.L.C. Ferreira, M. Miró, E.G.P. da Silva, G.D. Matos, P.S. dos Reis, G.C. Brandao, W.N.L. dos Santos, A.T. Duarte, M.G.R. Vale, R.G.O. Araujo, Slurry Sampling—An Analytical Strategy for the Determination of Metals and Metalloids by Spectroanalytical Techniques, Appl. Spectrosc. Rev. 45 (2010) 44–62. [8] M.H. Ramsey, P.J. Potts, P.C. Webb, P. Watkins, J.S. Watson, B.J. Coles, An objective assessment of analytical method precision: comparison of ICP-AES and XRF for the analysis of silicate rocks, Chem. Geol. 124 (1995) 1–19. [9] J. Enzweiler, P.C. Webb, Determination of trace elements in silicate rocks by X-ray fluorescence spectrometry on 1:5 glass discs: comparison of accuracy and precision with pressed powder pellet analysis, Chem. Geol. 130 (1996) 195–202. [10] R. Falcone, S. Hreglich, M. Vallotto, M. Verità, X-ray fluorescence analysis of raw materials for the glass and ceramic industries, Glass Technol.: Eur. J. Glass Sci. Technol. Part A 43 (2002) 39–48. [11] M. Pouzar, T. Kratochvíl, L. Čapek, L. Smoláková, T. Černohorský, A. Krejčová, L. Hromádko, Quantitative LIBS analysis of vanadium in samples of hexagonal mesoporous silica catalysts, Talanta 83 (2011) 1659–1664. [12] B. Welz, M.G.R. Vale, D.L.G. Borges, U. Heitmann, Progress in direct solid sampling analysis using line source and high-resolution continuum source electrothermal atomic absorption spectrometry, Anal. Bioanal. Chem. 389 (2007) 2085–2095. [13] M.A. Belarra, M. Resano, F. Vanhaecke, L. Moens, Direct solid sampling with electrothermal atomization/vaporization. What for and how? Trends Anal. Chem. 21 (2002) 828–839. [14] M.G.R. Vale, N. Oleszczuk, W.N.L. dos Santos, Current status of direct solid sampling for electrothermal atomic absorption spectrometry—A critical review of the development between 1995 and 2005, Appl. Spectrosc. Rev. 41 (2006) 377–400. [15] M. Resano, F. Vanhaecke, M.T.C. de Loos-Vollebregt, Electrothermal vaporization for sample introduction in atomic absorption, atomic emission and plasma mass spectrometry—a critical review with focus on solid sampling and slurry analysis, J. Anal. At. Spectrom. 23 (2008) 1450–1475. [16] M. Resano, M. Aramendía, E. Garcia-Ruiz, C. Crespo, M.A. Belarra, Solid samplinggraphite furnace atomic absorption spectrometry for the direct determination of silver at trace and ultratrace levels, Anal. Chim. Acta 571 (2006) 142–149. [17] M.A. Belarra, C. Crespo, M.P. Martínez-Garbayo, M. Resano, Direct determination of cobalt and zinc in samples of different volatility by means of solid sampling – graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 58 (2003) 1847–1858. [18] P. Török, M. Žemberyová, A study of the direct determination of Cd, Cr, Cu, Pb and Zn in certified reference materials of soils by solid sampling electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 66 (2011) 93–97. [19] P. Török, M. Žemberyová, Utilization of W/Mg(NO3)2 modifiers for the direct determination of As and Sb in soils, sewage sludge and sediments by solid sampling electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 65 (2010) 291–296. [20] I.M. Dittert, D.L.G. Borges, B. Welz, A.J. Curtius, H. Becker-Ross, Determination of silver in geological samples using high-resolution continuum source electrothermal atomic absorption spectrometry and direct solid sampling, Microchim. Acta 167 (2009) 1–6. [21] L.F. Rodrigues, J.C.P. de Mattos, V.L. Dressler, D. Pozebon, E.M.M. Flores, Determination of cadmium, copper and lead in alumina based catalysts by direct solid sampling graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 62 (2007) 933–938. [22] R.C. Bolzan, L.F. Rodrigues, J.C.P. de Mattos, V.L. Dressler, E.M.M. Flores, Chromium determination in pharmaceutical grade barium sulfate by solid sampling electrothermal atomic absorption spectrometry with Zeeman-effect background correction, Talanta 74 (2007) 119–124. [23] M.G.R. Vale, M.M. Silva, B. Welz, R. Nowka, Control of spectral and non-spectral interferences in the determination of thallium in river and marine sediments using solid sampling electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 17 (2002) 38–45.
[24] S. Hauptkorn, V. Krivan, Solution and slurry sampling electrothermal atomic absorption spectrometry for the analysis of high purity quartz, Spectrochim. Acta Part B 51 (1996) 1197–1210. [25] M. Resano, E. García-Ruiz, High-resolution continuum source graphite furnace atomic absorption spectrometry: Is it as good as it sounds? A critical review, Anal. Bioanal. Chem. 399 (2011) 323–330. [26] B. Welz, S. Morés, E. Carasek, M.G.R. Vale, M. Okruss, H. Becker-Ross, HighResolution Continuum Source Atomic and Molecular Absorption Spectrometry A Review, Appl. Spectrosc. Rev. 45 (2010) 327–354. [27] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution Continuum Source AAS-The Better Way to Do Atomic Absorption Spectrometry, Wiley-VCH, Weinheim, 2005. [28] M.A. Belarra, M. Resano, J.R. Castillo, Theoretical evaluation of solid samplingelectrothermal atomic absorption spectrometry for screening purposes, J. Anal. At. Spectrom. 14 (1999) 547–552. [29] M. Resano, E. Garcia-Ruiz, F. Vanhaecke, C. Crespo, M.A. Belarra, Evaluation of solid sampling-electrothermal vaporization-inductively coupled plasma mass spectrometry and solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of Cr in various materials using solution- based calibration approaches, J. Anal. At. Spectrom. 19 (2004) 958–965. [30] M.A. Belarra, C. Crespo, M. Resano, J.R. Castillo, Direct determination of copper and lead in sewage sludge by solid sampling-graphite furnace atomic absorption spectrometry-study of the interference reduction in the gaseous phase working in non-stop flow conditions, Spectrochim. Acta Part B 55 (2000) 865–874. [31] M. Resano, J. Briceño, M. Aramendía, M.A. Belarra, Solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of boron in plant tissues, Anal. Chim. Acta 582 (2007) 214–222. [32] M. Resano, M. Aramendía, E. Garcia-Ruiz, M.A. Belarra, Solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of Au in samples of various natures, J. Anal. At. Spectrom. 20 (2005) 479–481. [33] M. Resano, E. Garcia-Ruiz, C. Crespo, F. Vanhaecke, M.A. Belarra, Solid samplinggraphite furnace atomic absorption spectrometry for palladium determination at trace and ultratrace levels, J. Anal. At. Spectrom. 18 (2003) 1477–1484. [34] F. Vignola, D.L.G. Borges, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of Cd and Fe in sewage sludge by high-resolution continuum source electrothermal atomic absorption spectrometry with slurry sampling, Microchem. J. 95 (2010) 333–336. [35] A. Detcheva, K.H. Grobecker, Determination of trace elements in aquatic plants by means of solid sampling Zeeman atomic absorption spectrometry (SS-ZAAS), Environ. Chem. Lett. 6 (2008) 183–187. [36] M.G.R. Vale, M.M. Silva, B. Welz, É.C. Lima, Determination of cadmium, copper and lead in mineral coal using solid sampling graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 56 (2001) 1859–1873. [37] M.A. Belarra, M. Resano, S. Rodríguez, J. Urchaga, J.R. Castillo, The use of chemical modifiers in the determination of cadmium in sewage sludge and tin in PVC by solid sampling-graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 54 (1999) 787–795. [38] I. López-García, P. Viñas, J. Arroyo-Cortéz, M. Hernández-Córdoba, Rapid determination of lead and cadmium in sewage sludge samples using electrothermal atomic absorption spectrometry with slurry sample introduction, Fresenius J. Anal. Chem. 367 (2000) 727–732. [39] M. Lucic, V. Krivan, Solid sampling electrothermal atomic absorption spectrometry for analysis of aluminium oxide powders, J. Anal. At. Spectrom. 13 (1998) 1133–1139. [40] M. Resano, E. García-Ruiz, M. Aramendía, M.A. Belarra, Solid sampling-graphite furnace atomic absorption spectrometry for Hg monitoring in soils. Performance as a quantitative and as a screening method, J. Anal. At. Spectrom. 20 (2005) 1374–1390.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Direct analysis of silica by means of solid sampling graphite furnace atomic absorption spectrometry M. Resano a,⁎, E. Mozas b, C. Crespo b, J. Pérez c, E. García-Ruiz a, M.A. Belarra a a b c
Department of Analytical Chemistry, University of Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain Technological Institute of Aragon, Laboratory of Mechanics and New Materials, María de Luna 8, E-50018 Zaragoza, Spain Grupo IQE.Polígono Industrial Malpica, Calle “D”, no 97, E-50057 Zaragoza, Spain
a r t i c l e
i n f o
Available online 19 March 2012 Keywords: Solid sampling Graphite furnace atomic absorption spectrometry Hazardous elements Analysis of silica
a b s t r a c t This paper reports on the use of solid sampling-graphite furnace atomic absorption spectrometry for the direct analysis of synthetic amorphous silica. In particular, determination of hazardous elements such As, Cd, Cr, Cu, Pb and Sb is investigated, as required by regulations of the food industry. The conclusion of the work is that, after proper optimization of the working conditions, paying particular attention to the atomization temperature and the use of proper modifiers (graphite powder, HNO3 or Pd), it is possible to develop suitable procedures that rely on the use of aqueous standard solutions to construct the calibration curves for all the elements investigated. The proposed method shows important benefits for the cost-effective analysis of such difficult samples in routine labs, permitting fast screening of those elements that are very rarely present in this type of sample, but also accurate quantification of those often found, while offering low limits of detection (always below 0.1 mg g− 1) that comply well with legal requirements, and precision levels that are fit for the purpose (approx. 6–9% R.S.D.). © 2012 Elsevier B.V. All rights reserved.
1. Introduction Synthetic amorphous silica (SAS) is a high purity, crystalline-free, synthetic silicon dioxide, which may be produced as precipitated, silica gel, fumed or pyrogenic silica [1]. Even though the manufacturing process may differ to some extent, all types of SAS are similar in their purity, physico-chemical properties and toxicological requirements [2]. Therefore, they are considered as the same substance. SAS is a nanostructured material widely used in many industrial applications, mainly as reinforcement and thickening agent. Commercial SAS grades are part of industrial formulations such as elastomers (tires, shoe soles and technical pieces), resins, inks and paints. SAS grades are also incorporated in consumer products such as cosmetics, medical drugs or toothpastes. Furthermore, SAS is used in the food industry as anticaking, stabilizing, thickener and gelling agent, and has also been approved as a food and feed additive (EU Directive 95/2/EC, amendment number E551). As a result of this intensive use, the global demand for this product was forecasted to exceed 1.5 million metric tons in 2011. In particular, the use of SAS in food products makes it necessary to control the level of potentially toxic elements. Nowadays, following EU Directive 2008/84/EC, SAS products comply with legal standards only when As, Pb and Hg contents are below 3, 5 and 1 μg g − 1,
⁎ Corresponding author. Fax: + 34 976761292. E-mail address: [email protected] (M. Resano). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.03.005
respectively, while EU Directive 2002/32/EC introduced a limit of 2 μg g − 1 for Cd. The traditional methods for the analysis of metal impurities in SAS substances can be found either in the silica monographs of the Food Chemical Codex [3] or in the JECFA (Joint Expert Committee on Food Additives) monograph for synthetic amorphous silica [4]. All these procedures for trace metal determination begin with the dissolution of the sample, which for this type of sample requires the use of hydrofluoric acid. Later, metals in solution are determined by suitable atomic spectrometric techniques, such as atomic absorption spectrometry (AAS) or inductively coupled plasma (ICP)-based techniques. The development of analytical methods that permit the determination of the metals of interest directly in SAS samples could be an appealing alternative to control these materials. In this way, it would be possible to: (i) improve the sensitivity (by avoiding the dilution that accompanies sample digestion); (ii) considerably increase the sample throughput; (iii) minimize the risk of contamination/losses; and (iv) implement much “greener” (environmentally-friendly) analytical approaches [5–7]. There are different analytical techniques that can be used for that purpose, but many of them require some sort of sample pretreatment for this type of materials (e.g., for X-ray fluorescence is typical to carry out a borate fusion [8–10]), and rely on the availability of suitable solid standards for calibration [11]. In contrast, graphite furnace AAS (GFAAS) shows interesting advantages, such as potential for direct analysis, an excellent sensitivity and selectivity and, also very important in this context, the possibility to use aqueous standard solutions
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
for calibration [12–15]. To the best of the authors' knowledge there is no previous report in the use of this technique for SAS analysis, but previous works focused in the analysis of soils and other geological samples [16–23] have reported satisfactory results, at least for some elements. In addition, a work describing the use of slurry sampling for analysis of high-purity quartz also reported suitable accuracy and low limits of detection (LOD) for a variety of analytes [24]. It is the purpose of this work to evaluate the use of solid sampling (SS)-GFAAS for the direct analysis of SAS samples in the context of a cooperation with a manufacturing company that required control of a number of elements, namely As, Cd, Cr, Cu, Pb and Sb. The final goal is to develop straightforward procedures relying on the use of aqueous standards for calibration.
2. Experimental 2.1. Reagents Standard solutions containing appropriate amounts of As, Cd, Cr, Cu, Pb, Pd and Sb were daily prepared from commercially available (Merck, Darmstadt, Germany) 1 g L − 1 single-element standards by dilution with 0.14 mol L − 1 HNO3. Purified water was obtained from a milli-Q system (Millipore, Billerica, USA). 65% HNO3 and 37% HCl were purchased from Carlo Erba (Sabadell, Spain) and 48% HF from Scharlau (Barcelona, Spain).
25
2.4. Procedure for SS-GFAAS Sample analysis required no prior treatment other that checking the humidity level. Samples were kept at 105 °C during 24 hours prior to analysis. The moisture content was always at the 3–4% level. The solid sampling device used allows automatic weighing and transport of the samples into the furnace. The empty platform was first transported to the microbalance using a pair of tweezers. After taring, an appropriate amount of graphite powder (approx. 1.5 mg, except for Cd determination, for which graphite addition was not required) was deposited onto the platform. Afterwards, the sample was also deposited onto the platform and weighed. Typical values for the sample masses used are shown in Table 1. Finally, the chemical modifier (Pd or HNO3) was added when needed. The platform was then transferred to the graphite furnace and subjected to the temperature program. All these operations were fully controlled from a keyboard, except for the deposition of the sample and modifiers onto the platform, which were carried out manually. The operating conditions used are summarized in Table 1. The calibration was carried out using 10 μL of aqueous solutions of the appropriate concentration, added with a micropipette onto the sampling platform. For every determination, five solid samples were analyzed and the median of the five results was taken as a representative value [28]. However, when it was clear that the sample contained no measurable amount of analyte, only three replicates were carried out. Integrated absorbance (Aint) was selected as the measurement mode in all circumstances. 2.5. Dissolution and analysis of the samples for comparison purposes
2.2. Samples The samples analyzed were obtained from Industrias Químicas del Ebro (IQE, Zaragoza, Spain). SAS samples were available as fine powders, with a mean particle size of 250 μm. The certified reference material (CRM) NCS DC73304 Rock-Constituents (China National Center for Iron and Steel, Beijing, China) was used for validation purposes, since the chemical composition of this material is similar to those of the samples. This material shows a SiO2 content of 90.36 ± 0.15%, other major constituents being Al2O3 (3.52 ± 0.09%) and Fe2O3 (3.22 ± 0.07%). Its particle size is lower than 74 μm.
For evaluating the results obtained, one SAS sample was dissolved and the solutions obtained were subsequently analyzed by means of ICP OES or HR-CS GFAAS. The process of sample dissolution consisted of: a) weighing of approximately 500 mg; b) digestion with 1 mL of 37% HCl, 1 mL of 65% HNO3 and 4 mL of 48% HF for 40 min in the microwave oven; c) cooling; and d) dilution to 50 mL. Approximately 3 hours were required for the entire process. 3. Results and discussion 3.1. Method development
2.3. Instrumentation An AAS ZEEnit 600 atomic absorption spectrometer from Analytik Jena (Jena, Germany) was used throughout the work. This instrument is equipped with a transversely heated graphite tube atomizer, an automatic solid sampling accessory (SSA 61), including a Sartorius (Göttingen, Germany) M2P microbalance with a precision of 0.001 mg, and a Zeeman-effect background correction system [14]. Pyrolytic graphite coated graphite tubes (with dosing hole) and solid sampling platforms were used for this instrument. A Horiba JobinYvon (Kyoto, Japan) ACTIVA™ ICP optical emission spectrometer (OES) was used for the solution ICP OES measurements. It is equipped with a Czerny-Turner optical system and a low-noise high-speed CCD detector, which covers the spectral range of 165– 800 nm. A HF-resistant torch (made of alumina), nebulizer, spray chamber (made of polytetrafluoroethylene) and injector (made of polyether ether ketone) were used to avoid the deleterious effect of such acid. For Pb determination in digested silica samples, a highresolution continuum source atomic absorption spectrometer (HRCS AAS), ContrAA 700, commercially available from Analytik Jena AG (Jena, Germany) was used. More details on this instrument can be found elsewhere [25–27]. A microwave oven Ethos One from Milestone Inc. (Shelton, USA) was used for sample digestion. The different subsamples were weighed in a Sartorius (model R-200 D) microbalance with a precision of 0.001 mg.
In order to develop and validate the solid sampling methodology investigated in this work, a CRM was selected. Since no certified silica material is available for this purpose, a rock material that contains approx. 90% of SiO2 was considered as the most similar of the commercially available materials and was used instead. 3.1.1. Sensitivity adjustment Since, for As, Cd and Sb the goal was to develop a method capable of carrying out determinations below the μg g − 1 level, maximum sensitivity conditions were used for those analytes. That translates into using the most sensitive resonance lines and stop flow conditions during atomization. Sensitivity can then be adjusted to some extent by selecting the appropriate sample mass, even though using very high amounts (over 1 mg) is not recommended, as it typically results in higher background signals and faster platform deterioration. For Cu, Cr and Pb, higher analyte levels are expected in both the reference material and the samples. Thus, for Cu and Pb slightly less sensitive lines (approx. a factor of two) than the main lines were selected (283.3 nm instead of 217.0 nm for Pb and 327.4 nm instead of 324.7 nm for Cu). For Cr, owing to the typical high sensitivity of the technique for this element, a four times less sensitive line (427.5 nm instead of 357.9 nm) and medium flow (1 L min− 1) conditions during atomization were chosen. It should be mentioned that Cr is a priori the most difficult analyte of the list to be determined in a refractory matrix, owing its low volatility [29], and the maintenance of Ar during atomization was
26
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
Table 1 Optimum conditions used during SS-GFAAS analysis of all the materials. Elements
Wavelength / nm Background Correction Pyrolysis Temperature / °C Ramp / °C s− 1 Hold / s Gas flow Atomization Temperature / °C Ramp / °C s− 1 Hold / s Gas flow Modifier Platform protection Chemical interaction Sample mass / mg NCS DC73304 SAS 1
Cu
Cr
Cd
As
Pb
Sb
327.4 Zeeman
427.5 Zeeman
228.8 Zeeman
197.2 Zeeman
283.3 Zeeman
217.6 Zeeman
1000 200 30 Max
1000 300 20 Max
600 100 20 Max
800 100 20 Max
700 300 30 Max
1500 500 10 Max
2500 2000 6 Stop Graphite powder1
2500 1200 6 1 L min− 1 Graphite powder1 15 μL 65% HNO3
2200 2000 4 Stop None
2400 2000 4 Stop Graphite powder1 5 μg Pd
2400 2000 3 Stop Graphite powder1
2500 2000 2.5 Stop Graphite powder1 5 μg Pd
≈ 0.1–0.2 0.5–1
≈ 0.2–0.5 0.5–1
≈ 0.2–0.5 ≈1
≈ 0.1–0.3 ≈1
≈ 0.2–0.5 0.5–1
≈ 0.4–0.7 ≈1
Approximately 1.5 mg.
preferred because it has been reported for similar matrices that it can help in reducing particle condensation in the tube cooler ends, thus minimizing the corresponding light scattering issues [17,30]. 3.1.2. Atomization of the different elements from the solid CRM Optimal atomization conditions were investigated taking into account the following aspects: i) considering the low volatility of the main matrix component (SiO2), it does not seem feasible to achieve a significant degree of matrix removal during the pyrolysis step; thus, the pyrolysis step is not considered as very critical for this sample [13]; ii) background is typically lower for higher wavelengths, because light scattering is less of an issue and fewer diatomic species absorb, and thus fewer problems in this regard are expected for Cu and Cr monitoring; for the rest of the analytes, the higher the atomization temperature, the higher the probability of matrix co-vaporization (as SiO), thus optimization of the atomization temperature is important to minimize problems in this aspect; iii) however, when analytes are embedded in this type of sample they are often retained to some extent, making it necessary to use higher atomization temperatures compared with the work with solutions. In other words, the matrix itself may act as a modifier, making redundant the use of chemical modifiers, at least for some elements. Since the goal of the work is to develop procedures as simple as possible, chemical modifiers will only be used when they are considered indispensible; iv) very often for this kind of sample the atomization process for the solid sample and for an aqueous standard solution is not exactly the same, which results in differences in the signal profiles. However, the use of integrated peak areas may still make it feasible to use aqueous standards for calibration [31–33]. Cu and Pb atomization from the CRM resulted to be relatively simple. Pyrolysis temperature shows little effect on the final signal. It is not necessary to use any chemical modifier to stabilize the analytes since the use of pyrolysis temperatures sufficiently low to prevent any losses is possible. For the atomization temperature, on the other hand, it is necessary to use 300 °C more than typically recommended for solutions, as expected. However, under the conditions shown in Table 1, the signals for solid samples and aqueous standard solutions are practically identical in the case of Cu (see Fig. 1a), and also quite similar (a bit more broadened for the solid) in the case of Pb (see Fig. 1b). In both cases, the areas of the solid samples and aqueous solutions were comparable. Pb signal for the CRM exhibited a higher background signal, but it was clearly retarded with respect to the analyte signal. The integration time was restricted to 3 s for Pb, to minimize the potentially deleterious influence of this background absorbance.
Cd shows a similar behavior to that of Pb, but the stabilizing effect of the matrix can be clearly observed for this analyte. This is an analyte for which using atomization temperatures below 2000 °C is the rule, even for direct solid sampling analysis [21,34–38]. However, for the CRM investigated in this work it is necessary to use an atomization temperature of at least 2200 °C to obtain a well-defined signal profile. Still this signal is delayed (0.5 s) and broadened compared with that of an aqueous standard solution (see Fig. 1c), but peak areas are analogous. No chemical modifier was needed for this analyte. Arsenic atomization can also be carried out, in principle, directly without the concourse of any chemical modifier. Very similar signal shapes with comparable peak areas were obtained for the solid samples and the aqueous standard solutions (with the solid sample signal appearing a bit before that of the aqueous standard, see Fig. 1d). The background signal behaves in a similar way as described for Pb, even though it does not increase significantly up to 4 s. However, under those working conditions, and despite the excellent signal definition achieved, low precision values for this element were found, and a sensitivity drift for consecutive atomizations of solid samples was also detected. The use of Pd as chemical modifier was observed to alleviate these effects. Identical atomization conditions could be used when adding Pd, and the signal profiles obtained were very similar to those shown in Fig. 1d, but delayed 0.2 s, which clearly indicates analyte interaction with the modifier. Very similar considerations can be made for Sb. For this element, best repeatability was attained when using Pd as chemical modifier. The background signal increases significantly with time (see Fig. 1e), as a consequence of the high atomization temperature needed for proper atomization (2500 °C) and the short wavelength, so the integration time was restricted to 2.5 s. Under these conditions, use of aqueous standards for calibration also appears to be feasible, as the areas obtained for solid samples and standard solutions are identical. Finally, Cr determination proved to be the most challenging one. Even though, when measuring aqueous standards or even digested samples, it is feasible to obtain well-defined single-peak signals for atomization temperatures of 2400–2500 °C, very low, poorly defined and unreproducible signals were obtained for direct analysis of the solid sample. Optimization of the atomization temperature did not result in any significant improvement, even when the highest atomization temperature available was used (2700 °C). This fact is consistent with a previous work in which carrying out a micro-fusion directly in the graphite furnace was necessary to properly atomize Cr from a
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
A
B
C
D
E
F
27
Fig. 1. Comparison of the signal obtained for the atomization of the analytes from CRM NCS DC73304 Rock-Constituents and from aqueous standard solutions when monitoring: A) Cu; B) Pb; C) Cd; D) As; E) Sb; F) Cr. BG stands for background.
sewage sludge with a high level of silicates [29]. Less problems were encountered, however, in other work [18]. In this particular case, the addition of 15 μL of 65% HNO3 as chemical modifier was found sufficient to help in releasing Cr from the matrix. This signal is clearly
delayed (approx. 1.5 s) in comparison with that of an aqueous standard solution (see Fig. 1f), and shows considerably more background, but it is well defined, reproducible and shows an area comparable to that of an aqueous standard solution.
28
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
Table 2 Results obtained using SS-GFAAS analysis of the rock CRM (90% SiO2) NCS DC73304, following the procedure described in Section 2.4. (n = 8–10). Analyte
As Cd Cu Cr Pb Sb
Reference
SS-GFAAS results
Certified value / μg g− 1
Result ± 95% C.I. / μg g− 1
RSD / %
LOD / ng g− 1
9.1 ± 1.2 0.060 ± 0.016 19 ± 2 20 ± 3 7.6 ± 0.8 0.60 ± 0.11
8.8 ± 0.4 0.059 ± 0.003 18.6 ± 1.3 21.2 ± 1.1 7.1 ± 0.4 0.57 ± 0.03
6.6 7.8 8.6 6.5 6.5 8.7
41 2 26 70 18 43
mass used (see Fig. 1c). This aspect further proves the potential of SSGFAAS for ultratrace analysis. It can also be mentioned that even lower limits could be obtained for Cu and Pb, and particularly for Cr, using more sensitive lines [29]. This is, however, not necessary in the context of the current application.
3.3. Analysis of SAS samples
3.1.3. Platform protection During the optimizations carried out before, it was observed that the lifetime of the platforms was very short for those elements for which an atomization temperature of 2400 °C or more was needed (all of them but Cd). After monitoring 20–30 consecutive solid samples, signals of platform deteriorations were evident, eventually leading to the development of an orifice, likely as a consequence of the reaction between SiO2 and C (reduction at high temperature), in a similar way as described for direct analysis of alumina [39]. The sample mass also plays a role, as a larger mass further accelerates the process. Thus, in order to improve the lifetime of this important part, addition of graphite powder is greatly recommended. This addition does not significantly alter the shape and the magnitude of the signals discussed before, but it permits using the same platform for at least 50–60 replicates for sample masses up to 1 mg. 3.2. Analysis of the CRM. Performance of the method Analysis of the rock CRM NCS DC73304 was carried out using solid sampling- GFAAS and the conditions shown in Table 1. The procedure is described in Section 2.4. In all cases, the calibration curve was constructed simply using aqueous standard solution. Five replicate measurements were made for each determination, and the median of them was taken as representative value instead of the mean, in order to minimize the possible influence of outliers [28]. The results of eight to ten determinations carried out on three different days are summarized in Table 2, and compared with the reference values. As can be seen, there is good agreement for all the elements in terms of accuracy and no significant differences could be established, which confirms that aqueous standard solutions can be successfully used for calibration. The precision obtained by means of SS-GFAAS (between 6% and 9% RSD) can also be regarded as satisfactory for the purpose intended. The limits of detection, are also fit-for-purpose, as contents below 0.1 μg g− 1 already comply with legal requirements in all cases. The detection level achieved for Cd is extremely low owing to the excellent sensitivity of the technique for this element. It can be stressed that the results obtained for this element are very comparable in terms of accuracy and precision to those obtained for the other analytes, despite the fact that the sample contains only 15–30 pg Cd with the sample
After optimization and validation with the CRM, the methodology developed was used for analysis of real SAS samples during the last 2 years, a period in which more than 100 samples from the company IQE (that produces SAS) were analyzed. During that period, the method provided a robust and consistent performance. Results for a representative sample are shown in Table 3. As can be seen, the first noticeable aspect is that As, Sb and Cd were not found in the sample at detectable levels. This was also the case for all the other samples investigated. This aspect illustrates a situation often encountered in real life analysis, where it is necessary to check that a sample does not contain detectable levels of some hazardous elements that are not expected to be present, according to the manufacturer's experience. The use of SS-GFAAS for this case is particularly advantageous, since in 10 min per analyte (if no signal was obtained only 3 and not 5 replicates per sample were carried out) the sample can be screened, avoiding the time and effort required for its dissolution. Moreover, the SS-GFAAS approach guaranties sufficiently low detection limits, such that the sample can be cleared for its use, whether it is going to be destined to the food industry as additive (with the corresponding astringent requirements) or will be used in a different context (see introduction for examples). It should be mentioned that in some cases also Hg was monitored in the samples, but the procedure used for Hg was already described in a previous work [40], so that this element is not discussed in the current paper. Even though some elements were not present in any of the samples, a recovery test was carried out to further validate the methodology proposed. For this purpose, the sample (Ebrosil 001) was spiked with known amounts of As, Cd and Sb to obtain approx. 1 μg g− 1 of As and Sb, and 0.1 μg g− 1 of Cd. The recovery test proved satisfactory, as shown in Table 3. The other three elements could be determined in all the samples. The signal profiles obtained for these elements are shown in Fig. 2. They are very similar to those obtained for the CRM, proving that the former could be considered as a good choice for validation. In all the cases, very well-defined single-peak signals were attained. As stated before, the values obtained for Ebrosil 001 and shown in Table 3 can be considered as representative in terms of precision and analyte level. The precision is also similar to that observed for the CRM (6–9% RSD) and quite normal for the SS-GFAAS technique [13]. This is typically regarded as a weak characteristic of SS-GFAAS, owing to the low sample masses that are analyzed. It has to be stressed, however, that when determinations at the 1 μg g − 1 level or below for a complex samples are aimed, it is also not easy to improve this aspect when
Table 3 Results obtained using SS-GFAAS analysis of the of a silica sample (Ebrosil 001), following the procedure described in Section 2.4 (n = 5). Reference values were obtained as described in Section 2.4 (n = 4). Digestion-ICP OES results Analyte
Result ± 95% C.I. / μg g− 1
As Cd Cu Cr Pb Sb
b0.5 b0.02 1.39 ± 0.14 1.34 ± 0.21 1.04 ± 0.141 b0.4
1
SS-GFAAS results RSD / %
10.3 9.5 8.5
LOD / μg g− 1
Result ± 95% C.I. / μg g− 1
0.5 0.02 0.2 0.2 0.4 0.4
b 0.04 b 0.002 1.30 ± 0.13 1.28 ± 0.10 1.17 ± 0.11 b 0.04
SS-GFAAS recovery assay RSD / %
8.3 6.5 7.8
Owing to the high LOD obtained with ICP OES for this element, these measurements were carried out with HR-CS GFAAS.
LOD / μg g− 1
Analyte added / pg
Analyte found / pg
0.04 0.002 0.03 0.07 0.02 0.04
1000 100
943 ± 87 102 ± 9
1000
993 ± 75
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
A
B
29
Table 4 Results for the direct determination of Pb in sand samples used as precursor of SAS by means of SS-GFAAS analysis, following the procedure described in Section 2.4 (n = 5). Reference values were obtained as described in Section 2.4 (n = 4), with HR-CS GFAAS. Sample
Reference value / μg g− 1
SS-GFAAS Result ± 95% C.I. / μg g− 1
Sand 001 Sand 002 Sand 003
2.79 ± 0.25 1.33 ± 0.13 0.94 ± 0.12
2.90 ± 0.27 1.38 ± 0.06 0.85 ± 0.10
Of these elements, perhaps the greatest concern in terms of toxicity and regulations is Pb. Moreover, this element could not be properly determined by means of digestion and ICP OES analysis at the level found in most of the samples (limit of quantification 1.3 μg g− 1). That is the reason why the reference value shown in Table 3 was obtained with HR-CS GFAAS (to further assess the absence of interferences) after sample digestion. These two aspects, very fast screening of As, Cd and Sb, and direct determination of Pb at the μg g − 1 level, would be the main reasons to opt for the SS-GFAAS approach, which, of course, also shows some disadvantages, such as lack of multi-element capability. Finally, the origin of the Pb found in the samples was suspected to be the sand used as precursor for producing the SAS samples. Thus, direct determination of Pb in sand could be of interest in this context. It can be stressed that the same method developed before could be applied for the analysis of sand samples, as demonstrated in Table 4. Even more, the typical Pb signal profiles obtained for sand were remarkably similar to those obtained for SAS. 4. Conclusions
C
This work explores the possibilities of SS-GFAAS for direct analysis of silica samples. This method combines the inherent advantages of most solid sampling methods (good sensitivity, speed, no need to use hazardous reagents) with the possibility to obtain accurate results by using aqueous standards for calibration, and precision values that are fit-forpurpose (below 10% RSD). In this way, it is possible to carry out the analysis of these samples in a cost-effective way, avoiding laborious digestion procedures that require the use of hazardous reagents (e.g., HF), and offering detection limits that are sufficiently low to meet legal requirements, thus complying well with the requisites of industrial laboratories. Acknowledgements This work has been funded by the Spanish Ministry of Science and Innovation (Project CTQ2009-08606) and the Aragon Government (Project PM024/20006 for multidisciplinary research and Departamento de Ciencia, Tecnología y Universidad del Gobierno de Aragón y Fondo Social Europeo). References
Fig. 2. Comparison of the signal obtained for the atomization of the analytes from a SAS sample (Ebrosil 0001) and from aqueous standard solutions when monitoring: A) Cu; B) Pb; C) Cr. BG stands for background.
dissolution of the sample is carried out, as demonstrated by the values shown in Table 3. The analyte levels varied for different samples in the range 0.7 to 1.2 μg g− 1 for Cu, 1.0 to 1.3 μg g− 1 for Cr and 1.1 to 2.2 μg g− 1 for Pb.
[1] R.K. Iler, The Chemistry of silica: solubility, polymerization, colloids and surface properties and biochemistry of silica, Wiley-Interscience, New-York, 1979. [2] Synthetic Amorphous Silica (CAS Nº 7631-86-9), Joint Assessment of Commodity Chemicals Report Nº 51, European Centre for Ecotoxicology and Toxicology of Chemicals, Brussels, 2006, pp. 1–237. [3] Food Chemicals Codex, 7th ed. United Book Press Inc, Baltimore, 2010. [4] Joint FAO/WHO Expert Committee on Food Additives, Combined Compendium of Food Additive Specifications, Volume 4 Analytical Methods, Test Procedures and Laboratory Solutions Used by and Referenced in the Food Additive Specifications, Food and Agriculture Organization (FAO) of the United Nations, Rome, 2006. [5] M. Resano, E. García-Ruiz, M.A. Belarra, F. Vanhaecke, K.S. McIntosh, Solid sampling in the determination of precious metals at ultratrace levels, Trends Anal. Chem. 26 (2007) 385–395. [6] M. de la Guardia, Green analytical chemistry, Trends Anal. Chem. 29 (2010) 577.
30
M. Resano et al. / Spectrochimica Acta Part B 71–72 (2012) 24–30
[7] S.L.C. Ferreira, M. Miró, E.G.P. da Silva, G.D. Matos, P.S. dos Reis, G.C. Brandao, W.N.L. dos Santos, A.T. Duarte, M.G.R. Vale, R.G.O. Araujo, Slurry Sampling—An Analytical Strategy for the Determination of Metals and Metalloids by Spectroanalytical Techniques, Appl. Spectrosc. Rev. 45 (2010) 44–62. [8] M.H. Ramsey, P.J. Potts, P.C. Webb, P. Watkins, J.S. Watson, B.J. Coles, An objective assessment of analytical method precision: comparison of ICP-AES and XRF for the analysis of silicate rocks, Chem. Geol. 124 (1995) 1–19. [9] J. Enzweiler, P.C. Webb, Determination of trace elements in silicate rocks by X-ray fluorescence spectrometry on 1:5 glass discs: comparison of accuracy and precision with pressed powder pellet analysis, Chem. Geol. 130 (1996) 195–202. [10] R. Falcone, S. Hreglich, M. Vallotto, M. Verità, X-ray fluorescence analysis of raw materials for the glass and ceramic industries, Glass Technol.: Eur. J. Glass Sci. Technol. Part A 43 (2002) 39–48. [11] M. Pouzar, T. Kratochvíl, L. Čapek, L. Smoláková, T. Černohorský, A. Krejčová, L. Hromádko, Quantitative LIBS analysis of vanadium in samples of hexagonal mesoporous silica catalysts, Talanta 83 (2011) 1659–1664. [12] B. Welz, M.G.R. Vale, D.L.G. Borges, U. Heitmann, Progress in direct solid sampling analysis using line source and high-resolution continuum source electrothermal atomic absorption spectrometry, Anal. Bioanal. Chem. 389 (2007) 2085–2095. [13] M.A. Belarra, M. Resano, F. Vanhaecke, L. Moens, Direct solid sampling with electrothermal atomization/vaporization. What for and how? Trends Anal. Chem. 21 (2002) 828–839. [14] M.G.R. Vale, N. Oleszczuk, W.N.L. dos Santos, Current status of direct solid sampling for electrothermal atomic absorption spectrometry—A critical review of the development between 1995 and 2005, Appl. Spectrosc. Rev. 41 (2006) 377–400. [15] M. Resano, F. Vanhaecke, M.T.C. de Loos-Vollebregt, Electrothermal vaporization for sample introduction in atomic absorption, atomic emission and plasma mass spectrometry—a critical review with focus on solid sampling and slurry analysis, J. Anal. At. Spectrom. 23 (2008) 1450–1475. [16] M. Resano, M. Aramendía, E. Garcia-Ruiz, C. Crespo, M.A. Belarra, Solid samplinggraphite furnace atomic absorption spectrometry for the direct determination of silver at trace and ultratrace levels, Anal. Chim. Acta 571 (2006) 142–149. [17] M.A. Belarra, C. Crespo, M.P. Martínez-Garbayo, M. Resano, Direct determination of cobalt and zinc in samples of different volatility by means of solid sampling – graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 58 (2003) 1847–1858. [18] P. Török, M. Žemberyová, A study of the direct determination of Cd, Cr, Cu, Pb and Zn in certified reference materials of soils by solid sampling electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 66 (2011) 93–97. [19] P. Török, M. Žemberyová, Utilization of W/Mg(NO3)2 modifiers for the direct determination of As and Sb in soils, sewage sludge and sediments by solid sampling electrothermal atomic absorption spectrometry, Spectrochim. Acta Part B 65 (2010) 291–296. [20] I.M. Dittert, D.L.G. Borges, B. Welz, A.J. Curtius, H. Becker-Ross, Determination of silver in geological samples using high-resolution continuum source electrothermal atomic absorption spectrometry and direct solid sampling, Microchim. Acta 167 (2009) 1–6. [21] L.F. Rodrigues, J.C.P. de Mattos, V.L. Dressler, D. Pozebon, E.M.M. Flores, Determination of cadmium, copper and lead in alumina based catalysts by direct solid sampling graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 62 (2007) 933–938. [22] R.C. Bolzan, L.F. Rodrigues, J.C.P. de Mattos, V.L. Dressler, E.M.M. Flores, Chromium determination in pharmaceutical grade barium sulfate by solid sampling electrothermal atomic absorption spectrometry with Zeeman-effect background correction, Talanta 74 (2007) 119–124. [23] M.G.R. Vale, M.M. Silva, B. Welz, R. Nowka, Control of spectral and non-spectral interferences in the determination of thallium in river and marine sediments using solid sampling electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 17 (2002) 38–45.
[24] S. Hauptkorn, V. Krivan, Solution and slurry sampling electrothermal atomic absorption spectrometry for the analysis of high purity quartz, Spectrochim. Acta Part B 51 (1996) 1197–1210. [25] M. Resano, E. García-Ruiz, High-resolution continuum source graphite furnace atomic absorption spectrometry: Is it as good as it sounds? A critical review, Anal. Bioanal. Chem. 399 (2011) 323–330. [26] B. Welz, S. Morés, E. Carasek, M.G.R. Vale, M. Okruss, H. Becker-Ross, HighResolution Continuum Source Atomic and Molecular Absorption Spectrometry A Review, Appl. Spectrosc. Rev. 45 (2010) 327–354. [27] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-Resolution Continuum Source AAS-The Better Way to Do Atomic Absorption Spectrometry, Wiley-VCH, Weinheim, 2005. [28] M.A. Belarra, M. Resano, J.R. Castillo, Theoretical evaluation of solid samplingelectrothermal atomic absorption spectrometry for screening purposes, J. Anal. At. Spectrom. 14 (1999) 547–552. [29] M. Resano, E. Garcia-Ruiz, F. Vanhaecke, C. Crespo, M.A. Belarra, Evaluation of solid sampling-electrothermal vaporization-inductively coupled plasma mass spectrometry and solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of Cr in various materials using solution- based calibration approaches, J. Anal. At. Spectrom. 19 (2004) 958–965. [30] M.A. Belarra, C. Crespo, M. Resano, J.R. Castillo, Direct determination of copper and lead in sewage sludge by solid sampling-graphite furnace atomic absorption spectrometry-study of the interference reduction in the gaseous phase working in non-stop flow conditions, Spectrochim. Acta Part B 55 (2000) 865–874. [31] M. Resano, J. Briceño, M. Aramendía, M.A. Belarra, Solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of boron in plant tissues, Anal. Chim. Acta 582 (2007) 214–222. [32] M. Resano, M. Aramendía, E. Garcia-Ruiz, M.A. Belarra, Solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of Au in samples of various natures, J. Anal. At. Spectrom. 20 (2005) 479–481. [33] M. Resano, E. Garcia-Ruiz, C. Crespo, F. Vanhaecke, M.A. Belarra, Solid samplinggraphite furnace atomic absorption spectrometry for palladium determination at trace and ultratrace levels, J. Anal. At. Spectrom. 18 (2003) 1477–1484. [34] F. Vignola, D.L.G. Borges, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of Cd and Fe in sewage sludge by high-resolution continuum source electrothermal atomic absorption spectrometry with slurry sampling, Microchem. J. 95 (2010) 333–336. [35] A. Detcheva, K.H. Grobecker, Determination of trace elements in aquatic plants by means of solid sampling Zeeman atomic absorption spectrometry (SS-ZAAS), Environ. Chem. Lett. 6 (2008) 183–187. [36] M.G.R. Vale, M.M. Silva, B. Welz, É.C. Lima, Determination of cadmium, copper and lead in mineral coal using solid sampling graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 56 (2001) 1859–1873. [37] M.A. Belarra, M. Resano, S. Rodríguez, J. Urchaga, J.R. Castillo, The use of chemical modifiers in the determination of cadmium in sewage sludge and tin in PVC by solid sampling-graphite furnace atomic absorption spectrometry, Spectrochim. Acta Part B 54 (1999) 787–795. [38] I. López-García, P. Viñas, J. Arroyo-Cortéz, M. Hernández-Córdoba, Rapid determination of lead and cadmium in sewage sludge samples using electrothermal atomic absorption spectrometry with slurry sample introduction, Fresenius J. Anal. Chem. 367 (2000) 727–732. [39] M. Lucic, V. Krivan, Solid sampling electrothermal atomic absorption spectrometry for analysis of aluminium oxide powders, J. Anal. At. Spectrom. 13 (1998) 1133–1139. [40] M. Resano, E. García-Ruiz, M. Aramendía, M.A. Belarra, Solid sampling-graphite furnace atomic absorption spectrometry for Hg monitoring in soils. Performance as a quantitative and as a screening method, J. Anal. At. Spectrom. 20 (2005) 1374–1390.