Determination of residual carbon by inductively-coupled plasma optical emission spectrometry with axial and radial view configurations

Transcription

1 Analytica Chimica Acta 445 (2001) Determination of residual carbon by inductively-coupled plasma optical emission spectrometry with axial and radial view configurations Sandro T. Gouveia a, Fernando V. Silva b,c, Letícia M. Costa b, Ana Rita A. Nogueira c, Joaquim A. Nóbrega b, a Universidade Federal do Ceará, Fortaleza, CE, Brazil b Departamento de Química, Universidade Federal de São Carlos, P.O. Box 676, São Carlos, SP, Brazil c Embrapa Pecuária Sudeste, São Carlos, SP, Brazil Received 8 February 2001; received in revised form 3 July 2001; accepted 3 July 2001 Abstract In this work it was evaluated the performance of inductively-coupled plasma optical emission spectrometers (ICP-OESs) with axial and radial view configurations for residual carbon content (RCC) determination. The effects of carbon compound source (urea, l-cysteine, and glucose), sample medium, and internal standards on RCC determination were systematically evaluated. All measurements were carried out with two ICP spectrometers using the carbon atomic emission lines: and nm. The results obtained using axial and radial configurations showed that both the carbon source and the sample medium did not affect significantly the emission intensities. The sample medium only caused drastic influence when H 2 SO 4 was employed probably due to transport interference that can be corrected employing Y as internal standard. The sensitivity attained using axial view ICP-OES was 20-fold better than that reached using radial view ICP-OES based on the slopes of the analytical curves at the most sensitive wavelength ( nm). Using radial and axial ICP-OESs, high concentrations of Fe (>100 mg l 1 ) interfered at nm wavelength. An addition-recovery experiment was made by adding urea to an acid-digested sample and all recoveries were in the 100 ± 5% range for axial and radial measurements. At this wavelength, R.S.D. <2.0% (n = 10) and detection limits of 33 and 34 gml 1 C, were measured for ICP-OESs with radial and axial configurations, respectively. Biological samples were acid-digested using a closed-vessel microwave-assisted procedure and RCC was determined using both ICP-OES configurations Elsevier Science B.V. All rights reserved. Keywords: Residual carbon content; Microwave-assisted digestion; Inductively-coupled plasma optical emission spectrometry; Radial and axial configurations 1. Introduction Corresponding author. Fax: address: djan@zaz.com.br (J.A. Nóbrega). Frequently instrumental techniques require complete sample decomposition before measurements. Usually acid wet digestion is implemented combining oxidant agents and heating for destroying the /01/$ see front matter 2001 Elsevier ScienceB.V. All rights reserved. PII: S (01)

2 270 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) organic fraction of the sample [1]. However, due to the high stability of some organic compounds present in samples or formed during decomposition, incomplete oxidation is generally observed. Spectroanalytical techniques may not be critically affected by the residual carbon content (RCC) [2,3]. On the other hand, the application of electroanalytical techniques can be severely limited [4 6]. Therefore, RCC is an important parameter to be controlled depending on the instrumental technique used. Additionally, RCC measurement is an important parameter to evaluate the efficacy of sample decomposition procedures. Different approaches were proposed for RCC determination [6 8]. Elemental analysis or spectrophotometric titration of the organic matter with chromic acid can be used for this purpose [6]. In this latter procedure, controlled reaction conditions and experienced analysts are required to reach suitable accuracy. Inductively-coupled plasma optical emission spectrometry (ICP-OES) was also applied for RCC determination [7,8]. The RCC presents in natural waters was determined by Emteryd et al. [7] using a flow injection system coupled to an ICP-OES spectrometer [7]. The measurements were made at nm wavelength and solutions prepared from citric acid or potassium hydrogen phthalate were used for calibration. The obtained results were in agreement with those determined by elemental analysis. Krushevska et al. [8] also used the C emission line nm to perform RCC determination in milk sample digests. Recoveries of aliphatic and aromatic compounds were evaluated in different sample media. Recovery values around 100% were achieved using Sc as internal standard. The authors also mentioned memory effects in the spray-chamber when measuring aromatic compounds. Long washout times were required to reduce this effect. Considering the emerging of solid state detectors and changes of the optical system in ICP-OESs, the present work investigated the determination of RCC in acid-digested biological samples using ICP-OESs with axial and radial view configurations. The main experimental parameters and figures of merit were systematically evaluated and further correlated with the employed configuration. The axial and radial ICP-OESs developed procedures were compared to TOC analyzer in order to assess the accuracy of obtained results. The developed procedures were also applied to assess the digestion efficiency of microwave-assisted digestion procedures. 2. Experimental 2.1. Instrumentation Axial and radial view simultaneous ICP-OESs (Vista AX and RL, Varian, Mulgrave, Australia) equipped with CCD detectors were used in this study. The spectrometers provided wavelength coverage from 167 to 785 nm with the optical system purged with argon and the Echelle polychromator thermostated at 34 C. In the axial arrangement the cool plasma tail was removed from the optical path using an end-on gas to purge the plasma spectrometer interface. An argon snout purge system was employed in the radial configuration to produce an argon purged environment between the pre-optical system and the plasma in order to allow readings below 190 nm wavelength. The operational parameters established for RCC determination in each configuration are listed in Table 1. All measurements were carried out using liquid argon to decrease signal blank caused by plasma gas contamination [8]. The RCC was monitored at C I and nm wavelengths. The same instrumental conditions and nebulizer system (V-groove) were used in both ICP-OES spectrometers to facilitate the comparison of performance of axial and radial configurations. Table 1 Instrumental parameters for RCC determination using axial and radial ICP-OESs Instrumental parameter Axial and radial (kw) 1.0 Plasma gas flow (l min 1 ) 15.0 Auxiliary gas flow (l min 1 ) 1.5 Observation height a 9 Nebulizer gas flow (l min 1 ) 0.90 Spray chamber Sturman Masters Nebulizer V-groove Sample flow rate (ml min 1 ) 0.80 Analytical wavelengths (nm) C I C I Y II a Only for radial view configuration.

3 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) The acid digestions were performed in a microwaveoven (ETHOS 1600, Milestone, Sorisole, Italy) equipped with 10 perfluoralkoxy Teflon (PFA) closed vessels with calibrated resealing pressure relief mechanism (maximum operating pressure 110 atm). The vessels were put on a rotating turntable inside the microwave oven cavity. Before using, the PFA vessels were acid cleaned and rinsed with deionized water. The heating programs used for acid digestions are described further on. A sub-boiling apparatus (subpur, Milestone) was also used to distill the concentrated nitric acid. The total carbon determination used to evaluate the accuracy of the proposed procedure was carried out in a total carbon analyzer (TOC 5000 Shimadzu, Japan) Reagents and solutions All solutions were prepared using analytical grade reagents and Milli-Q distilled and deionized water (Millipore, Bedford, MA, USA). Sub-boiled distilled nitric acid and hydrogen peroxide (Mallinckrodt, Mexico) have also been used to perform the microwave-assisted digestions. Stock solutions containing 5.0% m/v C in aqueous medium were prepared from glucose (C 6 H 12 O 6, Merck, Germany), urea (CH 4 N 2 O, Reagen, Brazil) and l-cysteine (C 3 H 7 NSO 2, Sigma, USA). Test solutions containing 0.05 and 0.25% m/v C were prepared in HNO 3 (1.4 mol l 1 ), HNO 3 + H 2 O 2 (1.4 mol l % v/v) and H 2 SO 4 (1.8 mol l 1 ) media and used to evaluate the influence of carbon source and sample medium in C emission intensities. Evaluation of Fe interference was carried out using solutions containing 0.05% m/v C plus 10, 100 and 500 mg l 1 Fe, respectively. Carbon addition-recovery experiments were performed using 0.05% m/v C reference solutions and additions of 0.10% m/v C as urea, glucose, l-cysteine and citric acid. Addition of carbon to an acid-digested sample was also performed. For RCC determination the analytical curve used (0.05, 0.10 and 0.25% m/v C) was prepared in 1.4 mol l 1 HNO 3 using urea stock solution. Yttrium as internal standard was added to all reference solutions and samples in a final concentration of 1.0 mg l Samples Standard reference materials NIST-1577b Bovine liver, NIST-8435 Whole milk powder, NIST-1515 Apple leaves, and NIST-1570a Spinach leaves (National Institute of Standards and Technology, Gaithersburg, MD, USA) were digested and RCC was determined using TOC analyzer in order to check the accuracy of the proposed procedures. Lyophilized bovine liver and soybeans samples were microwave-assisted acid-digested using different heating programs. The RCC in all diluted digests was determined using ICP-OESs with axial and radial view configurations Sample preparation The microwave-assisted digestions were carried out using 250 mg of sample and an oxidant mixture containing 2 ml of HNO 3 plus 1 ml of H 2 O 2. According to the procedure recommended by Krushevska et al. the digests were transferred to 10 ml glass beakers and Table 2 Microwave-assisted digestion programs employed to decompose lyophilized bovine liver and soybeans samples Step Microwave digestion programs a a A sixth step consisting of 5 min of ventilation without any applied power was implemented in all tested programs.

4 272 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) evaporated gently at 120 C to remove the volatile carbon compounds [8]. After, the digests were quantitatively transferred to 10 ml volumetric flasks and the volume was made up with H 2 O. It was used the microwave digestion program 1 described in Table 2. The time and power parameters in the last step of the microwave heating program 1 were systematically modified to evaluate its effect on the digestion of lyophilized bovine liver and soybeans samples. All evaluated programs are presented in Table 2. The digestion efficiency was evaluated by determining the RCC in the digests Total carbon analyzer The accuracy was evaluated comparing the obtained results with those established using a total carbon analyzer (TOC). In the comparative method, the residual carbon presents in the sample digests was thermally converted into CO 2 and detected by an infrared sensor. The analytical curve was obtained using reference solutions containing 1.0, 5.0, 11 and 17 mg l 1 C prepared using potassium hydrogen phthalate (C 8 H 5 O 4 K, Nacalai Tesque, Japan) in aqueous medium. The sample digests were diluted according to the analytical curve concentration range. 3. Results and discussions 3.1. Carbon source and sample medium evaluation The influence of carbon source on C emission intensities was evaluated using reference solutions Table 4 Ratio of carbon emission intensities in axial and radial view configurations ICP-OESs λ (nm) Ratio (axial/radial) Glucose Urea l-cystein prepared from different organic compounds. Glucose, urea and l-cysteine were used to prepare reference solutions in the % m/v C concentration range. This concentration range showed a non-linear behavior in axial configuration at the nm wavelength. Probably, the elevated carbon concentration caused self-absorption effects at the most sensitive wavelength. Thus, measurements were repeated using solutions containing from 0.05 up to 0.25% m/v C. The parameters of the analytical curves obtained by ICP-OES axial and radial view configurations for each evaluated carbon source are shown in Table 3. The sensitivity of the measurements was not affected by carbon source in both studied configurations. The relative standard deviations of the slopes for curves obtained using glucose, urea and l-cysteine reference solutions were around 10 and 3% for axial and radial view configurations, respectively. We can conclude that any tested compound could be used for calibration owing to the low differences observed, however, it should be mentioned that aromatic compounds can generate memory effects and therefore aliphatic compounds are recommended for preparation of standard solutions [8]. All further measurements were carried out using urea. Table 4 shows that axial view improved sensitivities for all C sources. Table 3 Analytical curves parameters obtained for glucose, urea and l-cysteine reference solutions established by ICP-OESs with axial and radial view configurations Reference solution λ (nm) Slope Linear coefficient Linear correlation coefficient Axial Radial Axial Radial Axial Radial Glucose Urea l-cystein

5 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) The sample medium can influence the analytical signal either by physical effects, such as changes in nebulization efficiency, or by chemical processes, such as alteration of excitation mechanisms in the plasma. The carbon emission intensities were evaluated in different media (HNO 3, HNO 3 /H 2 O 2 and H 2 SO 4 ). All results were compared to those obtained using reference solutions prepared in aqueous medium. In axial and radial view configurations, deviations minor than 10% were observed when HNO 3 or HNO 3 /H 2 O 2 medium was used. Sulfuric acid medium caused a pronounced decrease in C emission intensities with both configurations. This could be related to the higher viscosity of this solution that affects the efficiency of sample transport to the plasma. This undesired effect of H 2 SO 4 can be corrected employing Y as internal standard. The analytical curve for RCC determination was prepared in 1.4 mol l 1 HNO 3 taking into account the final acid concentration of diluted digests Iron interference The measurements of carbon at nm emission line for RCC determination can be spectrally interfered by Fe II nm. This Fe ionic line is two-fold more intense than the C atomic line at this same wavelength. Based on this effect, the influence of Fe on carbon measurements at and nm wavelengths in both configurations was evaluated. For axial configuration, at nm wavelength, iron caused positive interferences (Table 5). At nm emission line, the signal variation was lower than 10%. The same behavior was observed for radial configuration, however, at observation height of 14 mm the Fe interference on Table 6 LOD and BEC for axial and radial view configurations λ (nm) Axial Radial BEC LOD BEC LOD nm wavelength was slightly more pronounced than that observed at 9 mm Limits of detection and memory effects The limits of detection (LOD) at and nm wavelengths were determined considering the background equivalent concentration (BEC) [9] and the results for both configurations are presented in Table 6. The background repeatability was similar in both plasma views, but the measurements at 247 nm in radial view configuration presented higher R.S.D. due to the low sensitivity at this wavelength. Despite the highest intensities obtained using axial configuration, the highest background equivalent concentration also increased and affected negatively the LOD. Therefore, the LODs were similar using axial and radial configurations. It should be pointed out that the detection limits with both arrangements could be slightly deteriorated by C contamination of the plasma gas despite of the use of liquid argon. The occurrence of memory effects was investigated by continuous monitoring of the emission signals of glucose, urea and l-cysteine solutions intercalated with blank solution aspiration. For all carbon sources the C emission intensities decreased quickly after Table 5 Effect of Fe on C recovery a Iron Recovery (%) Axial Radial nm nm nm nm 9mm a 17 mm a 9mm a 17 mm a a Observation heights.

6 274 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) Table 7 RCC determined using ICP-OESs with axial and radial view configurations and TOC analyzer in biological sample digests a Sample RCC (wt.%) Axial Radial TOC NIST-1577b bovine liver 9.82 ± ± ± 0.3 NIST-8435 whole milk powder 14.5 ± ± ± 1.0 NIST-1515 apple leaves 7.79 ± ± ± 0.12 NIST-1570a spinach leaves 8.42 ± ± ± 0.15 a Standard deviation based on sample in quadruplicate (n = 4). Table 8 Effect of microwave-assisted heating program on RCC in acid digests of lyophilized bovine liver and soybeans a Microwave program RCC (wt.%) Lyophilized bovine liver Soybean Axial Radial Axial Radial ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.84 a Standard deviation based on sample in quadruplicate measurements (n = 4). stopping their nebulization. Thus, the Sturman Masters chamber was effective for avoiding memory effects Carbon recovery Additions of 0.05% m/v C to reference solutions generated recovery values around 100 ± 5% for all tested carbon compounds in both ICP-OESs configurations. Similar results were obtained when C was added to a lyophilized bovine liver digest, indicating the absence of matrix effects Residual carbon content determination The RCCs for biological samples digested using the microwave digestion programs showed in Table 2 were determined by ICP-OESs with axial and radial view configurations and TOC analyzer. All measurements with ICP-OESs were carried-out at nm. Table 7 summarizes the obtained results. According to a paired t-test all results are in agreement at 95 or 99% confidence levels. The results obtained for lyophilized bovine liver and soybeans samples digested using different microwave heating programs are shown in Table 8. It can be seen that when nominal power was increased from 400 to 800 W in the last step (programs 2 and 3), the RCC decreased 45% in both samples. On the other hand, when the heating time was increased from 2.5 to 5 min (programs 1 and 4), the same pronounced effect was not observed. Increasing the heating time from 2.5 to 7.5 min (programs 4 and 5), the RCC decreased 38 and 26% for lyophilized bovine liver and soybeans samples, respectively. These results indicate a more pronounced effect of nominal power than heating time on the efficiency of decomposition. Lower RCCs were reached using simultaneously higher nominal power and longer heating time. 4. Conclusions The developed procedures were suitable for RCC determination in biological sample acid digests. When

7 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) compared to RCC determination using TOC analyzer, axial and radial ICP-OESs procedures reduced the analysis time and decreased sample manipulation. Additionally, the ICP-OES multi-elemental characteristics enable the simultaneous monitoring of other analytes. Both configurations evaluated presented results in agreement with those obtained using TOC analyzer. However, the determination carried out with axial view configuration presented higher sensitivity and similar deviations compared to the radial one. Considering the procedures tested to perform microwave-assisted acid digestions, it was observed a more pronounced effect of applied power on the efficiency of organic compounds decomposition. Acknowledgements The authors are grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo by the financial support (98/ ) and by the fellowship provided to F.V.S. (00/ ). A.R.A.N., J.A.N. and L.M.C. are grateful to CNPq by researchships and fellowship provided. S.T.G. is grateful to CAPES-PICDT by fellowship provided. References [1] H.M. Skip Kingston, S.J. Haswell (Eds.), Microwave- Enhanced Chemistry Fundamentals, Sample Preparation and Applications, American Chemical Society, Washington, DC, [2] G. Knapp, B. Maichin, At. Spectrosc. 19 (1998) 220. [3] P. Allain, L. Jaunault, Y. Mauras, J.M. Mermet, T. Delaporte, Anal. Chem. 63 (1991) [4] P. Mader, J. Száková, E. Curduvá, Talanta 43 (1996) 521. [5] S.B. Adeloju, Analyst 114 (1989) 455. [6] M. Würfles, E. Jackwerth, M. Stoeppler, Anal. Chim. Acta 226 (1989) 31. [7] O. Emteryd, B. Anderson, H. Wallmark, Microchem. J. 43 (1991) 87. [8] A. Krushevska, R.M. Barnes, C.J. Amarasiriwaradena, H. Foner, L. Martines, J. Anal. At. Spectrom. 7 (1992) 845. [9] V. Thomsen, G. Robert, K. Burgess, Spectroscopy 15 (2000) 33.