Introduction. Experimental. A. N. Anthemidis* and V. G. Pliatsika

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1 ARTICLE On-line slurry formation and nebulization for inductively coupled plasma atomic emission spectrometry. Multi-element analysis of cocoa and coffee powder samples A. N. Anthemidis* and V. G. Pliatsika Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University, Thessaloniki, Greece. Fax: þ Received 10th May 2005, Accepted 12th August 2005 First published as an Advance Article on the web 1st September 2005 A simple on-line slurry formation and direct nebulization system for inductively coupled plasma atomic emission spectrometric multi-element analysis was developed. A laboratory made micro-chamber with a magnetic-stirrer was used for on-line stable slurry formation in a dispersant solution of 0.5% v/v Triton X-100 and 1% v/v HNO 3. A Babington-type nebulizer combined with cyclonic-type spray chamber was adopted for on-line slurry nebulization and atomization. All critical parameters were studied and optimized. The performance of the system was demonstrated for Al, Ca, Co, Cr, Cu, Fe, Mg, Mn, Ni, and Zn determination in cocoa and coffee powder samples. The recommended particle size was o70 mm and the slurry concentration was 0.6% m/v, while the working slurry concentration could be ranged from 0.3 to 3.3% m/v with proportional sensitivity. Excellent agreement was found between the standard addition calibration procedure and the calibration curves using simple aqueous standard solutions for almost all of the investigated elements. The reliability of the proposed method was confirmed by comparing it with FAAS and ETAAS wet digestion methods for the analysis of cocoa and coffee samples. No significant differences were observed between the two methods. DOI: /b506626c Introduction It is well established that the most conventional technique of introducing samples into plasmas is the nebulization of dissolved samples. 1 However, decomposition procedures are often time-consuming, requiring large volumes of aggressive and expensive reagents, and are usually prone to systematic errors arising from incomplete dissolution, contamination, adsorption and evaporative losses. Direct solid sample introduction (DSSI) into plasmas via laser ablation 2 and arc/spark discharges, 3 or slurry nebulization would overcome the above problems by combining matrix destruction and analyte emission in a single procedure. The major disadvantage of the DSSI technique is the nonhomogeneous discharge conditions and the lack of suitable calibration standards, while slurry nebulization has advantages because it involves minimum modification to the existing instrumentation and is potentially capable of calibration with aqueous standards. 4 6 Slurry nebulization into inductively coupled plasma atomic emission spectrometry (ICP-AES) using simple aqueous standard solutions for calibration requires that both the analyte transport efficiency of the slurry particle through the sample introduction system, and the atomization efficiency of that particle, must be identical with that of a simple aqueous solution. 7 The use of homogeneous stable slurries with proper dispersant is very important for accurate measurements. The most popular technique for slurry preparation is ultrasonic vibration, which is sometimes followed by magnetic stirring 8,9 or vortex mixing. 1 The above procedures increase the time of analysis, thus decreasing the sampling frequency. Coffee and cocoa are examples of products that are frequently consumed by large numbers of people all over the world. Heavy metal composition of foods is of interest because of their essential (Fe, Zn, Cu, Cr, Co, Mn) for nutritional purposes or toxic (Pb, Cd, Ni) nature. Thus, it is important to establish rapid and accurate analytical methods in order to ensure the quality of the final product. Usually, conventional digestion methods, which include low-temperature ashing or wet-acid digestion procedures followed by flame or electrothermal atomic absorption spectrometry (FAAS or ETAAS), 10,11 inductively coupled plasma atomic emission spectrometry (ICP-AES), 12,13 or mass spectrometry (ICP- MS), 14 and X-ray fluorescence spectroscopy (XRFS) 15 have been employed for the determination of mineral nutrients and toxic elements in coffee and cocoa samples. The estimated levels of the main constituents like Ca and Mg are in the range 1 6 mg g 1, while minor elements such as Fe, Zn, etc., range between mg g 1. In the present work a micro-chamber with a magnetic stirrer was developed and elaborated for on-line slurry formation and direct insertion into an ICP nebulizer for analyzing solid powder samples. To the best of our knowledge a stirring micro-chamber for on-line formation of slurries has not been reported in flow injection (FI) or continuous flow (CF) systems. The proposed manifold was optimized for multi-element determination of Ag, Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, Mg, Mn, Ni, Pb, Tl and Zn in coffee and cocoa powders using Triton X-100 in HNO 3 as a dispersant solution. The use of simple aqueous standard solutions for the calibration and quantification of cocoa and coffee samples has been tested and it was proved that they can be successfully applied. The proposed method was evaluated by comparing it with FAAS and ETAAS wet digestion methods for the analysis of cocoa and coffee samples. Experimental Instrumentation All experiments were carried out using a PerkinElmer Optima 3100 XL axial viewing inductively coupled plasma atomic 1280 J. Anal. At. Spectrom., 2005, 20, This journal is & The Royal Society of Chemistry 2005

2 Table 1 Operating conditions and description of the ICP-AES instrument RF generator RF incident power a Argon flow rates Nebulizer gas flow rate a Spray chamber Nebulizer type Nebulizer uptake flow rate a Detector 40 MHz, free-running 1500 W Auxiliary 0.5 l min 1 ; plasma 15 l min l min 1 Cyclonic Babington 1.0 ml min 1 Segmented-array charge-coupled (SCD) 235 sub-arrays Studied elements and their spectral lines: Ag / nm Fe / nm Al / nm Ga nm B / nm In / nm Ba nm Mg / nm Bi / nm Mn / nm Ca / nm Ni / nm Cd / nm Pb / nm Co / nm Tl / nm Cr / nm Zn / nm Cu / nm a Optimised value. emission spectrometer (ICP-AES), according to the operating conditions given in Table 1. The emission intensity signal was measured in peak area mode. The investigated elements and their spectral lines, which have been studied, are also presented in Table 1. For comparative studies a PerkinElmer Model 5100 PC atomic absorption spectrometer equipped with Zeemaneffect background correction and a transversely heated graphite tube atomizer (THGA) was used for Al, Co, Cr, and Ni determination, using the instrument conditions given in Table 2, while a PerkinElmer 1100B flame atomic absorption spectrometer was employed for the determination of Ca, Cu, Fe, Mg, Mn and Zn at 422.7, 324.8, 248.3, and nm resonance lines, respectively. The monochromator spectral bandpass was set at 0.7 nm for Ca, Cu, Mg and Zn and 0.2 nm for Fe and Mn. The operating conditions were chosen according to guidelines of the manufacture. The air flow rate was set at 8.0 l min 1 and the acetylene flow rate at 2.5 l min 1. The manifold for on-line slurry formation and for the subsequent insertion in the ICP- AES is presented in Fig. 1. It consists of two peristaltic pumps (P1 and P2, Gilson Minipuls 3), a six-port two-position injection valve (IV, Labpro, Reodyne, USA) and a laboratory made micro-chamber (MC) with a magnetic-stirrer, which was employed for on-line slurry formation. The developed micro-chamber (MC) is characterized by simple construction, small dead volume (1500 ml) and facilitates the successive formation of stable slurries in a sort time (90 s). It is composed of two polyethylene cylindrical parts, which are connected together by push-fit connection (Fig. 1). The lower part has a horizontal inlet (0.5 mm id) for the entry of the dispersant solution, while the upper part has a small conical head and a vertical outlet (0.7 mm id) at the top of the cone for the outlet of the formed slurry. An accurate weighted amount of powder sample is placed in the MC and the analysis cycle is started. A spinbar s magnetic stirring bar, 6 mm in length, was placed in the micro-chamber and was rotated by a magnetic stirrer at 1000 rpm. Reagents and samples All chemicals were of analytical reagent grade and were provided by Merck (Darmstadt, Germany). Ultra-pure quality water was used throughout and was provided by a Milli-Q system (Millipore, Bedford, MA, USA). Multi-element working standard solutions were prepared by appropriate dilution of the stock ICP multi-element standard IV (Merck) 1000 mg l 1 of Ag, Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, Mg, Mn, Ni, Pb, Tl and Zn in 1 mol l 1 HNO 3. Dilute solutions were prepared from Triton X % aqueous solution (Fluka). Cocoa and coffee powder samples were from different commercial brands and purchased from local markets. They were sieved before analysis, through 70 mm and 20 mm sieves, in order to collect three different types of particle fractions: o20 mm, mm and o70 mm. The cocoa powder samples are homogeneous with respect to trace element distributions and contain several organic materials, including fat, that are resistant to wet oxidation and that also readily forms slurries in water-based fluids. 5 In order to evaluate the proposed ICP-AES method, a wetacid digestion of the whole sample of cocoa and coffee was employed followed by ETAAS or FAAS determination. An accurate weighed (0.3 g) amount of cocoa or coffee sample was acid digested in the presence of 3 ml of HNO 3, 65% m/m, using a Teflon beaker in a pressurized bomb ( C, 2 h). After cooling, the mixture was diluted to 25 ml and the absorbance measured by ETAAS or FAAS. Procedure The operation sequences for slurry formation and ICP-AES multi-element determination in the measurement mode run through five steps that are summarized in Table 3. In step 1 (sample loading), a weighted amount of sample is placed into Table 2 Operation conditions of ETAAS instrument Al Co Cr Ni Wavelength/nm Pyrolysis temp./1c (ramp time; holding time/s) 1200 (10; 30) 1100 (15; 30) 1300 (10; 50) 1100 (5; 20) Atomization temp./1c (ramp time; holding time/s) 2300 (0; 6) 2400 (0; 8) 2500 (0; 8) 2300 (0; 8) Reading time/s J. Anal. At. Spectrom., 2005, 20,

3 Fig. 1 Schematic diagram of the manifold for on-line slurry formation and analysis by ICP-AES: P1, P2, peristaltic pumps; IV1, IV2, injection valves in A position; MC, micro-chamber with magnetic stirrer; DS, dispersant solution 0.5% v/v Triton X-100 in 1.0% v/v HNO 3 ; W, waste; WS, washing solution H 2 O; CA, compressed air. (a) Step 1, slurry formation/measurement. (b) Step 3, evacuation of MC, IV1 and IV2 are in B and A position, respectively. Table 3 Operation modes of on-line slurry formation system for ICP-AES a Step P1 P2 IV1 IV2 Delivered medium Flow rate/ml min 1 Time/s Operation 1 ON OFF B B Sample loading 2 ON OFF A A DS a Slurry formation/measurement 3 ON ON B A Air Evacuation of the MC 4 ON ON B B H 2 O Washing of the MC 5 ON ON B A Air Evacuation of the MC a Dispersant solution. MC. The concentration of the resulted slurry can be calculated from the sample amount and the capacity of the MC (1.5 ml). During the second step the dispersant solution (DS) is filling the MC, meanwhile the magnetic stirrer is rotated. When the MC is overfilled, the formatted slurry is pumped towards the ICP nebulizer for atomization and measuring. In the evacuation step, IV1 and IV2 are actuated in the B and A positions, respectively, and the peristaltic pumps P1, P2 are on. The MC is evacuated by an air flow supplied from an air compressor (CA). The cycling time is 200 s, the time for filling the MC and slurry formation is 130 s and the sampling frequency is 18 h 1. In the slurry calibration procedure (standard addition) a series of aqueous standard solutions of the studied elements in 0.5% v/v Triton X-100 and 1.0% v/v HNO 3 were used as dispersant solution in the same manifold (Fig. 1), with a fixed amount of sample in MC. In the aqueous calibration mode, a series of aqueous standard solutions of the studied elements in 0.5% v/v Triton X-100 and 1.0% v/v HNO 3 were also used as the dispersant solution. In this case, step 1 was eliminated and the procedure is running through four steps. Results and discussion Slurry formation The main contribution of this study was to develop a flow injection system for on-line slurry formation and subsequent multi-element analysis of solid samples by ICP-AES. In batch mode, slurries are prepared using ultrasonic vibration for 5 15 min 6 and subsequently magnetically stirred for another 10 min, 9,16 thus resulting in significant time and sample consumption. These problems were overcome using our laboratory made micro-chamber (MC) device with a magnetic-stirrer. A univariate optimization method was carried out for two types of powder sample: cocoa and coffee. Because of the adequate content of some elements in the investigated samples the optimization was carried out without spiked slurries. It is well known that the particle size distribution of a slurry is a limiting factor, controlling analyte transport efficiency through the introduction system and atomization efficiency of the particle and, finally, analytical recovery. 1 The effect of particle size on analytical performance was tested for three different type of particle fractions: o20 mm, mm and o70 mm, using a fixed 0.01 g of sample for each measurement. The recorded signals were higher for the fraction o20 mm and lower for mm. The recorded emission of the fraction o70 mm was only 80 95% of the fraction o20 mm signal. Thus, the fraction o70 mm was selected throughout, as a compromise between the sample preparation time and sensitivity. The dispersant solution plays a very important role concerning the stability and homogeneity of the slurry, as it helps to avoid the flocculation effect, which results in rapid sedimentation of the powder sample. For lyophilic surfaces, one of the most appropriate dispersant solutions is Triton X-100 in various concentrations. On the other hand, it has been proved that a considerable fraction of the analyte is extracted into the liquid phase of the suspending medium. 9,17 Thus, the Triton X-100 concentration was investigated in the range % v/ v, in the presence of various HNO 3 concentrations from 0.0 to 10% v/v. As is shown, the sensitivity was increased by increasing the Triton X-100 concentration up to 0.5% v/v, while for higher concentrations the emission remained almost constant. In addition, when HNO 3 was used the sensitivity was increased up to 1.0% v/v HNO 3, and after that the signals slightly decreased. Bearing the above observations in mind an aqueous Fig. 2 Effect of slurry concentration on the emission intensity of cocoa sample at: Fe ( nm) -&-, Cu ( nm) -J- and Al ( nm) -n J. Anal. At. Spectrom., 2005, 20,

4 Fig. 3 Effect of nebulizer gas flow rate on the emission intensity of Mn ( nm) -J-, Fe ( nm) -n- and Cu ( nm) -&- ina cocoa sample. Fig. 4 Effect of nebulizer uptake flow rate on the emission intensity of Cr ( nm) -n-, Al ( nm) -J- and Cu ( nm) -&-in a coffee sample. solution of Triton X % v/v in HNO 3 1.0% v/v was adopted as the dispersant solution throughout the experiments. The concentration of slurry is also an important parameter. Very diluted slurries may cause degradation of the precision, while at high concentration of slurry, the plasma stability and the atomization efficiency of sample may be reduced significantly. The effect of slurry concentration was studied in the range % m/v either for cocoa or for coffee samples. For this purpose the sample amounts which were loaded in the MC were varied from to 0.05 g, respectively (Fig. 2). The intensity was increased practically linearly by increasing the slurry concentration as is demonstrated in Fig. 2 for Fe Table 4 Regression results from the comparison of slurry calibration procedures versus aqueous one (see text for details) Cocoa Coffee Element/nm r Slope CI a Intercept CI Result b r Slope CI Intercept CI Result b Ag Ag Al Al B B þ þ Ba þ þ Bi þ þ Bi Ca Ca Cd Cd Co Co Cr Cr Cu Cu þ þ Fe Fe Ga In þ þ In Mg Mg Mn Mn Ni Ni Pb Pb Tl Tl þ þ Zn Zn þ þ a Confidence interval (for confidence level 95 %). b Statistically non-significant differences are noted with and significant differences are noted with þ. J. Anal. At. Spectrom., 2005, 20,

5 Table 5 Calibration data of each element determination, using aqueous, cocoa and coffee slurry calibration procedure Aqueous calibration Slurry cocoa calibration Slurry coffee calibration Element Line/nm r Slope CI a Intercept CI r Slope CI a Intercept CI r Slope CI Intercept CI Ag Ag Al Al B Bi Ca Ca Cd Cd Co Co Cr Cr Cu Fe Fe Ga In Mg Mg Mn Mn Ni Ni Pb Pb Tl Zn a 95% confidence interval. ( nm), Cu ( nm) and Al ( nm) in cocoa samples. The same phenomenon was observed for all analytes in cocoa and coffee samples. This fact shows that higher sensitivity can be obtained using a high slurry concentration (3.3% m/v). To compromise the sample consumption and the sensitivity of the method, the slurry concentration was fixed at 0.6% m/v (sample amount in MC, 0.01 g) for the rest of the study. Study of ICP parameters Radiofrequency (RF) incident power, nebulizer gas flow rate and nebulizer uptake flow rate seriously affect the transportation efficiency of slurry into the ICP and the atomization and excitation performance. RF power affects seriously the plasma temperature. The more RF power the hotter the plasma gets. Thus, the RF incident power was studied for 1300, 1400 and 1500 W. For lower values, 1100 and 1200 W, the plasma was being extinguished. The results indicate that the sensitivity for all elements is higher at 1500 W RF incident power, which also produces a more stable plasma. Consequently, 1500 W was used throughout. The effect of nebulizer gas flow rate was studied from 0.6 to 1.0 l min 1, a maximum appearing at 0.7 l min 1, as is presented for cocoa samples at the spectral lines Mn ( nm), Fe( nm) and Cu( nm) in Fig. 3. Thus, a 0.7 l min 1 nebulizer gas flow rate was adopted throughout. The effect of the nebulizer uptake flow rate was investigated in the range ml min 1. As is demonstrated in Fig. 4, for Cr ( nm), Al ( nm) and Cu ( nm) in a coffee sample, the intensity was increased by increasing the flow rate up to 1 ml min 1. For higher flow rates the intensity was slightly decreasing, probably due to the introduction of a higher organic load into the plasma. The nebulizer uptake flow rate 1 ml min 1 was adopted as optimal. This value is in the recommended range of the instrument for aqueous samples, which confirms the aqueous behavior of the slurry into ICP. Table 6 Limit of detection (c L ) and recovery (mean standard deviation, n ¼ 5) of each element at the most sensitive spectral lines in cocoa and coffee matrix Recovery (%) sd Element/nm c L /mg g 1 Cocoa Coffee Ag ( ) Al ( ) B ( ) Bi ( ) Ca ( ) Cd ( ) Co ( ) Cr ( ) Cu ( ) Fe ( ) Ga ( ) In ( ) Mg ( ) Mn ( ) Ni ( ) Pb ( ) Tl ( ) Zn ( ) J. Anal. At. Spectrom., 2005, 20,

6 Table 7 Analytical results (in mg kg 1 ) of main elements in cocoa and coffee by the proposed method and FAAS or ETAAS wet digestion method Cocoa Coffee Element Proposed method a (o70 fraction) AAS wet digestion (whole sample) Proposed method a (o70 fraction) AAS wet digestion (whole sample) Al 55 8 b Ca c Co b Cr b Cu 52 6 c Fe c Mg c Mn 42 7 c Ni b Zn c a Mean value of 3 sub-samples standard deviation; b By ETAAS; c By FAAS. b c b b c c 79 8 c c b c Calibration procedure The introduction of solid samples into a plasma in the form of slurry permits rapid analyses by leaving out the tedious digestion procedures, minimizing the time and the cost of analysis. The slurry technique can be easily employed with ICP-AES using aqueous standards for calibration. 1,6,18 Thus, the possibility of using aqueous standard solutions for calibration in order to quantify the powdered cocoa and coffee samples was investigated using the proposed manifold under the optimum conditions. The calibration curves, which have been obtained using cocoa and coffee slurry (standard addition calibration), were compared with the aqueous solutions calibration curves. The statistical test 19 in which the regression lines are used for comparing analytical methods was applied in order to estimate possible systematic differences between the two calibration procedures. According to this approach, the responses obtained from a series of aqueous standards are plotted against the responses obtained from an identical series of slurry standards. If the calculated slope and intercept from the regression line do not differ significantly from the ideal values of 1 and 0, respectively, then there is no evidence for systematic differences between the two calibration procedures. The calculated values of slope and intercept with their confidence interval (CI) at thee 95% confidence level which were obtained from all matrices (cocoa and coffee), at each spectral line of the studied elements, are presented in Table 4. The values are not rounded for statistical reasons. As is shown the slurry calibration curves did not differ significantly from aqueous ones in most of the investigated spectral lines. For the spectral lines B ( nm), Ba ( nm), Bi ( nm), Cu ( nm), In ( nm), Tl ( nm) and Zn ( nm), there are significant differences and the relevant correlation coefficients were not satisfactory, possibly due to matrix interference. The slope and intercept of the regression equations, Intensity ¼ slope [M] þ intercept ([M] in mg l 1 except Ca and Mg in mg l 1 ), at the optimum spectral lines using aqueous standard solutions and cocoa and coffee slurries, are presented in Table 5. The linearity of the proposed method was investigated for each element up to 1000 mg l 1 and up to 50 and 10 mg l 1 for Ca and Mg, respectively. The calibration curves were linear at least up to the maximum studied concentrations. The correlation coefficients were satisfactory and ranged between The values of detection limit (3s criterion) for each element at the optimum spectral lines are presented in Table 6. Recovery studies Recovery studies were carried out using spiked powdered cocoa and coffee samples at the most sensitive spectral lines, as no certified reference material was available. A dispersant solution (0.5% v/v Triton X-100 in 1.0% v/v HNO 3 ) containing 200 mg l 1 of each element and 4 mg l 1 of Ca and Mg was used for recovery estimation under the proposed procedure under optimum conditions. The concentrations 200 mg l 1 and 4mgl 1 correspond to 30 mg g 1 and 600 mg g 1, respectively, when an amount 0.01 g of sample is used. The results are presented in Table 6. The recovery varied in the range %. Low recovery values are attributed probably to a slight decrease in the atomization efficiency, due to the presence of the high organic load. The accuracy of the proposed ICP-AES method using o70 particle fraction was evaluated by comparing the results from the analysis of commercially available samples of cocoa and coffee with those obtained by use of flame atomic absorption spectrometry (FAAS) or electrothermal atomic absorption spectrometry (ETAAS) after wet digestion of the whole sample. The analytical results for Al, Ca, Co, Cr, Cu, Fe, Mg, Mn, Ni and Zn determination in cocoa and coffee samples are presented in Table 7. Some elements are not included, because their concentration was lower than the detection limit of the proposed method. Similar results were obtained from the proposed method and AAS methods. Conclusions A simple on-line slurry formation and direct nebulization system for ICP-AES multi-element analysis of powdered cocoa and coffee samples was developed. On-line slurry preparation is very effective, considerably less time consuming and less labor intensive than other methods. The proposed method facilitates the introduction of various slurry concentrations into an ICP-AES, thus affecting the sensitivity of the method. Multi-element analysis of powdered cocoa and coffee samples using aqueous standard solutions for calibration proved to be a promising approach. Comparison of the proposed method with the traditional atomic absorption wet digestion method showed that the two are significantly similar. Additional work is in progress on testing the micro-chamber for on-line slurry formation of other types of solid samples and analysis by ICP-AES. References 1 L. Ebdon, M. Foulkes and K. Sutton, J. Anal. At. Spectrom., 1997, 12, S. A. Baker, M. J. Dellavecchia, B. W. Smith and J. 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