The evaluation of the suitability of different alternative ...

FRIN-06034; No of Pages 9 Food Research International xxx (2015) xxx–xxx

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The evaluation of the suitability of different alternative sample preparation procedures prior to the multi-elemental analysis of brews of ground roasted and instant coffees by FAAS and ICP OES Anna Szymczycha-Madeja ⁎, Pawel Pohl, Maja Welna, Ewelina Stelmach, Dominika Jedryczko Wroclaw University of Technology, Wybrzeze Stanislawa Wyspianskiego 27, 50370, Wroclaw, Poland

a r t i c l e

i n f o

Article history: Received 9 July 2015 Received in revised form 21 September 2015 Accepted 23 September 2015 Available online xxxx Keywords: Brews Ground coffee Instant coffee Alternative sample preparation Leachability Major Minor and trace elements

a b s t r a c t A simple, non-digestion treatment of brews of ground roasted and instant coffees prior to their analysis on the content of Ca, Fe, K, Mg and Na (with flame atomic absorption spectrometry) and Al, Ba, Cd, Co, Cr, Mn, Ni, Pb and Sr (with inductively coupled plasma optical emission spectrometry) was proposed. Three different sample preparation procedures of brews, including acidification with HNO3 and aqua regia (both at 0.25, 0.50 and 1.0 mol L−1) and direct analysis of untreated beverages, were compared. Under selected conditions, i.e., using acidification of brews with HNO3 to 0.25 mol L−1, precision (as relative standard deviation) from 0.2 to 8.7% and detection limits between 0.036 (Sr) and 36 (Fe) ng mL−1 were achieved. Accuracy of the chosen procedure was verified by comparing obtained results with those assessed using open-vessel wet digestion. Eighteen different coffees marketed in Poland were analyzed with the use of the proposed procedure. Concentrations of selected metals (Al, Ca, Co, K, Mg, Mn, Ni) were effectively used to classify brews of analyzed coffees by principal component analysis, factor analysis and hierarchic cluster analysis. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Elemental analysis of coffee brews, although indifferently reported in the literature, is used to judge the nutritional value of consumed coffees and evaluate the degree of coverage of DRIs for dietary and physiologically important elements through the customary drinking of coffee brews (Pohl, Stelmach, & Szymczycha-Madeja, 2014a, 2014b; Oliveira et al., 2012; Ashu & Chandravanshi, 2011; Grembecka, Malinowska, & Szefer, 2007; Santos, Lauria, & Porto da Silveira, 2004). Flame atomic absorption spectrometry (FAAS) (Ozdestan, 2014; Pohl et al., 2014a; 2014b; Stelmach, Pohl, & Szymczycha-Madeja, 2013; Oliveira et al., 2012; Ashu & Chandravanshi, 2011; Grembecka et al., 2007), inductively coupled plasma optical emission spectrometry (ICP OES) (Welna, Szymczycha-Madeja, & Zyrnicki, 2013; Frankova, Drabek, Havlik, Szkova, & Vanek, 2009; Fernandes et al., 2005; Santos et al., 2004) or inductively coupled plasma mass spectrometry (ICP MS) (Nedzarek et al., 2013; Santos et al., 2004) are commonly used to measure concentrations of different elements, including macro-, micro-, and trace elements, in brews of ground roasted and instant coffees. Unfortunately, sample treatment for elemental analysis of this beverage by FAAS, ICP OES and ICP MS is typically laborious and requires ⁎ Corresponding author. E-mail address: [email protected] (A. Szymczycha-Madeja).

wet or dry ashings. Accordingly, for wet digestion, infusions of ground roasted coffees are decomposed in mixtures of concentrated HNO3 and HClO4 solutions (Nedzarek et al., 2013; Ashu & Chandravanshi, 2011) or concentrated HNO3 and 30% H2O2 solutions (Welna et al., 2013). Resulting digests are re-constituted in water. For dry ashing, infusions of ground roasted coffees are evaporated to dryness, then resulting residues are ashed and finally remaining ashes are digested in concentrated solutions of HCl (Ozdestan, 2014; Grembecka et al., 2007) or HNO3 (Santos et al., 2004). In both cases, time required for preparing respective sample solutions takes from 4 (Ashu & Chandravanshi, 2011) to even 96 h (Santos et al., 2004). In reference to abovementioned examples, it is evident that advances in simpler and faster alternative sample preparation procedures of coffee brews are important and noteworthy for future research. This is particularly valid concerning high consumption of the coffee brew in the world and a need for multi-elemental analyses of a large number of samples in case of controlling its quality and safety or for bromatological and mineral characteristics. Validated analytical methods for multi-elemental analysis of coffee brews with simplified sample pre-treatments would conveniently increase the sample throughput, decrease costs of such analyses and improve the quality of results because of a lower risk of contamination and losses of analytes. Simplified sample preparation procedures of ground roasted and instant coffees have been barely reported so far. Accordingly, Ca, Cu, Fe, K,

http://dx.doi.org/10.1016/j.foodres.2015.09.031 0963-9969/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: Szymczycha-Madeja, A., et al., The evaluation of the suitability of different alternative sample preparation procedures prior to the multi-elemental analysis of ..., Food Research International (2015), http://dx.doi.org/10.1016/j.foodres.2015.09.031

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A. Szymczycha-Madeja et al. / Food Research International xxx (2015) xxx–xxx

Mg, Mn, Na, Pb, S, Se, Si, Sn, Sr and Zn were determined by ICP OES in supernatants of extracts of ground and instant coffees after their acidification with HNO3 to a concentration of 0.014 mol L− 1, however, no validation data were provided (Fernandes et al., 2005). Similarly, for measurements of Al by ICP OES in untreated infusions of ground coffees (Frankova et al., 2009), no validation parameters were shown. In case of FAAS, alternative methods of elemental analysis have recently been developed in our group (Pohl et al., 2014a; 2014b; Stelmach et al., 2013). Brews of ground roasted and instant coffees were acidified with HNO3 to higher concentrations, i.e., 1.0 or 1.4 mol L−1, and introduced to an air-C2H2 flame. Oliveira et al. (2012) have also analyzed brews of instant coffees acidified only with HCl to a concentration of 1.0% (v/v). All mentioned methods were validated, however, they were applied to a limited number of elements, i.e., Ca, Cu, Fe, K, Mg, Mn, Na and Zn. For this reason, the main objective of this work was to compare three different simple and fast sample treatments of brews of ground roasted and instant coffees before their analysis on the content of Ca, Fe, K, Mg and Na (using FAAS) and Al, Ba, Cd, Co, Cr, Mn, Ni, Pb, Sr and Zn (using ICP OES). Procedures used included acidification of brews with aqua regia or HNO3 (at 0.25, 0.50 and 1.0 mol L−1 in both cases) and a direct measurement of untreated beverages. The suitability of compared procedures and accuracy of obtained results were evaluated by comparing them with concentrations of elements measured after preparation of samples by open-vessel wet digestion (reference procedure). Additionally, precision of results was evaluated and detection limits (DLs) of metals for proposed sample preparation procedures were assessed. For the procedure providing results consistent with those achieved using the reference procedure, the recovery study was carried out. The selected procedure was applied for multi-elemental analysis of different ground roasted (GCs) and instant (ICs) coffees available in the Polish market. 2. Experimental 2.1. Instrumentation A Bodenseewerk Perkin-Elmer GmbH single-beam spectrophotometer (model 1100B), equipped with a deuterium lamp for background correction, a Littrow mount, 267-mm optical length monochromator with a 1800 lines/mm grating, and a photomultiplier, was used to measure concentrations of Ca, Fe, K, Mg and Na (major and minor metals) by FAAS with a fuel lean air-C2H2 flame. The flame was sustained in a 10-cm, Ti, single-slot burner head mounted on an inert, plastic coated burner-mixing chamber ended with a nebulizer holder and a drainage assemblage. A self-aspirating steel nebulizer and a flow spoiler were used to introduce solutions to the burner-mixing chamber by pneumatic nebulization (PN). Concentrations of major and minor metals were measured using atomic absorption (Ca, Fe, Mg) and atomic emission (K, Na) modes of the instrument. Operation settings recommended by the producer of the apparatus were applied, i.e., analytical lines: 422.7 (Ca), 248.3 (Fe), 766.5 (K), 285.2 (Mg) and 589.0 nm (Na); spectral band-passes: 0.7 (Ca, K, Mg and Na) and 0.2 nm (Fe); flow rates of gases: 8.0 (air) and 2.4 L min−1 (fuel), the burner height: 6.0 mm, lamp currents: 12 (K, Na), 15 (Ca, Mg) and 30 mA (Fe). Averaged readings of background corrected absorbance (3 replicates), taken within 3.0 s in a time-average integration mode, were used for calibration. An Agilent bench-top optical emission spectrometer of an axially viewed Ar-ICP, model 720, was used to determine concentrations of trace metals, i.e., Al, Ba, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sr and Zn. The instrument was equipped with a 5-channel peristaltic pump, a highresolution echelle-type polychromator and a VistaChip II CCD detector. A standard, one-piece, low-flow, extended quartz torch with an injector tube (2.4 mm ID) was used to sustain the plasma. The torch was combined with a single-pass glass cyclonic spray chamber. A hightech engineering polymer (PFA and PEEK) OneNeb concentric nebulizer was mounted into the spray chamber and applied to introduce solutions

by PN. Operating instrument settings recommended by the manufacturer of the spectrometer for solutions containing high levels of dissolvedsolids were used, i.e., the RF power: 1.2 kW, gas flow rates: 15.0 (plasma), 1.5 (auxiliary) and 0.75 L min−1 (nebulizer), the sample flow rate: 0.75 ml min−1, stabilization and sample uptake delays: 15 and 30 s, rinse and replicate times: 10 and 1 s (3 replicates). Analytical lines selected for measurements were: Al I 396.2 nm, Ba II 455.4 nm, Cd II 214.4 nm, Co II 238.9 nm, Cr II 267.7 nm, Cu I 324.8 nm, Mn II 257.6 nm, Ni II 231.6 nm, Pb II 220.4 nm, Sr II 407.8 nm and Zn I 213.8 nm. A fitted background mode with 7 points per a line profile was applied for background correction. Background corrected intensities of analytical lines were used for calibration. A Fritsch (Germany) planetary micromill Pulverisette 7 premium with agate grinding bowl (20 mL) and agate balls (10 mm OD) was used to powder samples of GCs and ICs. A MPW-350 centrifuge (MPW Medical Instruments, Poland) was used to separate any particles from brews of ICs. An Anton Paar Multiwave Pro microwave sample preparation system with a 8NXF100 rotor and eight PTFE-TFM liners (100 mL) with lip-type seals, ceramic pressure jackets and protective casings were used to digest solid samples of coffees in a mixture of concentrated HNO3 and 30% H2O2. The device was equipped with a temperature probe and a remote IR control. 2.2. Reagents and samples EMSURE® ACS grade reagents, i.e., concentrated HNO3 (65%), HCl (36%) and H2O2 (30%) solutions, were purchased from Merck Millipore (Germany). Aqua regia was freshly prepared by mixing concentrated HCl and HNO3 solutions at volumetric ratio 3:1. De-ionized water from an EASYpure™ water purification system (Barnstead Corp., USA) was used throughout. A Merck Certipur® multi-elemental stock (1000 μg mL−1) ICP standard solution IV was used for preparing simple and matrix-matching standard solutions for calibration of FAAS and ICP OES instruments. Eight regular commercial samples of ground roasted coffees (GCs) and their eight instant equivalents (ICs), sold under the same name, were purchased in a local market, i.e., GC1 and IC1 (Jacobs), GC2 and IC2 (Tchibo), GC3 and IC3 (Maxwell House), GC4 and IC4 (Woseba), GC5 and IC5 (Prima), GC6 and IC6 (Pedros), GC7 and IC7 (Carte Noire), and GC8 and IC8 (Davidoff). Additionally, two instant coffees with admixtures of ground coffees were selected, i.e., IC9 (Nescafe) and IC10 (Jacobs). Before microwave-assisted wet digestion of coffees, samples were milled using a rotational speed of 650 for 10 (GCs) and 5 (ICs) min. The particle size of these samples was b250 (GCs) and b50 (ICs) μm. 2.3. Coffee brew preparation The mud coffee brewing method was used for preparing brews of coffees. Temperature of water was used according to recommendations given by producers/suppliers of GCs and ICs. In case of GCs, samples as received (6.0 g) were placed in 400-mL glass beakers, poured with 200 mL of boiling (100 °C) de-ionized water and left under the cover for 10 min to brew (at room temperature, 22 ± 1 °C). Resulting infusions were separated from settled grounds by filtering them through 390 grade quantitative filterpapers (Munktell & Filtrak, Germany). In case of ICs, samples as received (6.0 g) were placed in 400-mL glass beakers, poured with 250 mL of hot water (90–95 °C) and dissolved by mixing with a glass rod. Portions of resulting brews were placed into 30-mL PP centrifuge tubes (Equimed, Poland) and centrifuged for 10 min at 12,000 rpm (MPW 352, Poland). Supernatants were separated using PE syringes (Equimed, Poland). Collected filtrates (GCs) and supernatants (ICs) were suitably treated and analyzed by FAAS and ICP OES.



2.4. Sample treatment and analysis Four different procedures were used to prepare coffee brews prior to their elemental analysis. All experiments were carried out using brews of two selected coffees, i.e., GC2 and IC2. Open-vessel wet digestion procedure (P1, reference procedure): portions of coffee brews (20.0 g) were placed in 200-mL glass beakers and poured with 15 mL of concentrated HNO3. Beakers were covered with watch glasses and heated on a hot plate to make sample solutions gently boil and reflux for 3 h and reduce their volumes to ~2 mL. After cooling, 3.0 mL of 30% H2O2 was added and beakers were continuously heated until clear solutions were obtained and their volumes were reduced almost to dryness. Resulting aliquots were quantitatively transferred to 30-mL PP screw-capped containers (Equimed, Poland) and diluted with water to 25.0 g. Direct analysis (P2): prepared brews (10.0-g portions) were directly analyzed without any treatment. Acidification with HNO3 (P3): 10.0-g portions of coffee brews were placed into 10-mL PP tubes and acidified by adding appropriate amounts of concentrated HNO3. Final concentrations of HNO3 in coffee brews were 0.25, 0.50 and 1.0 mol L−1. Acidification with aqua regia (P4): 10.0-g portions of coffee brews were placed into 10-mL PP tubes and acidified by adding appropriate amounts of aqua regia solutions. Final concentrations in coffee brews were 0.25, 0.50 and 1.0 mol L− 1.To establish total concentrations of studied metals in solid coffee samples, closed-vessel microwaveassisted wet digestion was used. Milled samples of GCs and ICs (0.5 g) were weighted into liners and poured with 6.0 mL of concentrated HNO3 and 1.0 mL of 30% H2O2 solutions. Vessels were tightly closed,

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jacked and inserted into the rotor. An eight-step digestion program was used: 1 (250 W, 5 min, ramp), 2 (250 W, 10 min, hold), 3 (400 W, 5 min, ramp), 4 (400 W, 10 min, hold), 5 (400 W, 10 min, hold), 6 (650 W, 5 min, ramp), 7 (650 W, 10 min, hold) and 8 (ventilation, 30 min). After cooling and opening vessels, sample digests were transferred to 30-mL PP screw-capped containers and diluted with water to 25.0 g. All sample solutions were prepared and analyzed in triplicate (n = 3). For a given sample preparation procedure for coffee samples and brews, respective blanks were prepared and considered in final results. In case of the determination of Fe and Na by FAAS and Al, Ba, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sr and Zn by ICP OES, undiluted sample solutions were measured versus matrix-matching standard solutions. In case of the determination of Ca, K and Mg by FAAS, sample solutions were appropriately diluted, i.e., from 5 to 200 times, and analyzed against simple standard solutions. 3. Results and discussion 3.1. Comparison of different treatments of coffee brews The suitability of alternative sample preparation procedures (P2, P3, P4) of coffee brews was assessed by comparing results, i.e., mean concentrations and standard deviations (SDs, n = 3), achieved with these procedures to those obtained using wet digestion (P1), treated as the reference procedure (Szymczycha-Madeja & Welna, 2013; Szymczycha-Madeja, Welna, & Pohl, 2013). The significance of differences between SDs of concentrations of studied elements, indicating differences in precision of results, was tested using the F-test

Table 1 Calculated values of F- (Fcalculated) and t-tests (∣tcalculated ∣) for the comparison of standard deviations of means and mean concentrations of metals determined in brews of ground (GC2) and instant (IC2) coffees by FAAS (Ca, Fe, K, Mg, Na) and ICP OES (Al, Ba, Co, Cr, Cu, Mn, Ni, Sr, Zn) using wet digestion in concentrated HNO3 with 30% H2O2 solutions (P1), no treatment (P2) and acidification to 0.25, 0.50 and 1.0 mol L−1 HNO3 (P3) or aqua regia (P4). Significant differences are underlined. Metal

|tcalculated |

Fcalculated P2

P3 0.25

P4 0.50

P2

1.0

0.25

0.50

1.0

P3

P4

0.25

0.50

1.0

0.25

0.50

1.0

Brews of GC2 Al 2.88 Ba 1.64 Ca 1.00 Co 4.00 Cr 12.76 Cu 2.52 Fe 1.36 K 3.16 Mg 1.00 Mn 16.00 Na 7.37 Ni 1.23 Sr 16.00 Zn 2.25

1.70 6.25 1.31 1.70 1.09 3.24 1.41 2.34 29.86 4.00 25.91 1.56 5.76 1.00

1.96 1.44 4.46 6.25 1.08 1.51 1.80 5.77 1.36 2.25 35.68 4.00 2.51 4.00

2.58 3.24 3.58 – 9.77 2.54 13.06 2.02 4.42 16.00 5.92 11.11 36.00 3.36

1.48 4.00 3.64 6.25 3.70 19.43 19.67 0.75 34.35 16.00 2.25 1.00 5.76 1.00

1.84 1.44 2.06 2.45 1.93 3.24 0.52 1.41 1.37 16.00 33.43 1.69 1.44 1.36

1.75 2.56 1.80 1.14 1.93 7.31 1.60 4.16 3.63 4.00 12.93 2.78 1.00 3.36

2.665 2.185 0.175 8.366 9.007 10.803 2.067 6.510 5.307 0.420 0.855 5.665 2.941 4.564

0.025 0.322 2.461 0.046 0.250 0.168 1.764 0.306 3.614a 0.775 0.082a 0.271 0.266 0.204

0.629 1.109 1.994 10.399 2.833 4.575 1.541 1.574 2.657 2.162 0.089a 2.479 1.773 1.549

4.820 3.196 4.847 – 6.269 6.174 1.149 1.144 5.202 3.361 0.797 0.830 1.860a 2.626

1.960 2.634 0.286 9.435 9.712 0.081a 0.723a 1.140 0.587a 2.941 0.089 1.960 4.930 5.307

1.596 3.992 1.671 2.299 2.811 17.328 1.538 0.922 1.022 0.840 0.081a 0.739 2.218 1.879

7.652 4.039 5.527 4.699 0.787 2.982 3.778 0.790 2.633 0.775 0.848 20.942 2.143 1.659

Brews of IC2 Al 4.00 Ba 12.25 Ca 2.51 Co 10.24 Cr 1.19 Cu 5.06 Fe 1.15 K 1.34 Mg 4.00 Mn 1.00 Na 5.29 Ni 676.00 Sr 1.00 Zn 9.68

4.00 5.44 3.51 2.25 1.99 1.56 1.56 46.23 5.60 1.00 25.14 4.00 1.00 1.62

4.00 12.25 1.29 12.25 1.19 14.06 3.81 26.74 2.21 1.00 4.68 1089.00 1.00 1.36

4.00 5.44 1.27 6.25 2.56 64.00 3.09 44.83 2.40 9.00 5.40 36.00 1.00 12.25

4.00 10.80 2.61 4.00 64.00 – 12.68 2.82 2.09 5.44 0.98 441.00 16.00 2.71

1.44 13.80 1.73 11.90 9.00 – 1.11 2.86 1.48 18.78 1.49 144.00 49.00 1.16

11.11 8.16 2.57 2.10 1.19 52.56 1.27 16.76 1.20 11.11 3.39 441.00 28.09 4.64

5.422 10.468 16.032 5.786 8.140 4.385 0.253 1.609 0.904 1.225 1.727 2.500a 13.472 3.357

0.000 0.910 0.017 0.480 0.294 1.082 0.970 0.445a 0.564 1.225 0.025a 2.324 1.225 0.146

1.549 1.190 0.395 2.617 2.075 1.450 1.512 0.483a 0.140 0.816 1.725 3.427a 18.371 0.611

6.197 7.733 1.142 6.111 0.428 4.385a 0.183 0.576a 4.341 7.668 0.267 3.022a 26.944 2.379

10.070 3.098 0.245 1.162 4.093a – 0.154 1.557 0.125 4.094 0.119 1.682a 11.342 4.124

24.949 4.632 0.849 7.185 0.411 – 0.829 3.181 0.016 4.544 1.059 3.523a 7.000a 2.856

65.365 8.419 0.913 30.189 0.851 1.401a 3.221 9.527 1.215 8.129 0.083 11.368a 11.511a 5.330

The critical value of the F-test (Fcritical): 19.00 (p = 0.05). The critical value of the t-test (tcritical): 2.776 (p = 0.05). a The C-test was used with the critical value (Ccritical): 4.303 (p = 0.05).


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(Konieczka & Namiesnik, 2009) with the critical value of this test (Fcritical) equal to 19.0 (p = 0.05). Accuracy of results obtained for compared procedures was tested by comparing mean concentrations of studied elements. When SDs of results obtained for compared alternative procedures (P2, P3, P4) and the reference procedure (P1) did not statistically differed (Fcritical N Fcalculated), the t-test was used (Konieczka & Namiesnik, 2009) with its critical value (tcritical) of 2.776 (p = 0.05). In case when SDs achieved for compared procedures differed in a significant manner (Fcritical b Fcalculated), the Cochran-Cox C-test was used (Konieczka & Namiesnik, 2009) with the critical value (Ccritical) of 4.303 (p = 0.05). 3.1.1. Precision As can be seen from Table 1, values of Fcalculated were in majority of cases lower than Fcritical. This indicated that differences between SDs of results and their precision obtained using alternative procedures P2, P3, P4 did not differ in a statistically significant manner from those obtained using the procedure P1. Few exceptions were found for brews of GC2 treated with procedures P3 and P4, i.e., Mg (acidification to 0.25 mol L− 1 HNO3 and aqua regia), Na (acidification to 0.25 and 0.50 mol L−1 HNO3, and 0.50 mol L−1 aqua regia), Sr (acidification to 1.0 mol L− 1 HNO3) or Cu and Fe (acidification to 0.25 mol L− 1 aqua regia). For brews of IC2, the number of exceptions was higher, likely due to a more complex matrix as compared to GC2, and found for: Cr (acidification to 0.25 mol L− 1 aqua regia), Cu (acidification to 1.0 mol L− 1 HNO3 and aqua regia), K (acidification to 0.25, 0.50 and 1.0 mol L−1 HNO3), Na (acidification to 0.25 mol L−1 HNO3). Almost all compared sample preparation procedures, i.e., no treatment, acidification to 0.50 and 1.0 mol L−1 HNO3, and 0.25, 0.50 and 1.0 mol L−1 aqua regia, failed regarding SDs obtained for Ni.

In reference to relative standard deviations (RSDs), precision of results obtained for reference (P1) and alternative procedures (P2, P3, P4) is given in Table 2. For the procedure P1, RSDs of mean concentrations determined for both coffee brews were within 0.2–9.2% while overall precision for all metals was 3.8%. For other procedures, RSDs and overall precisions were respectively as follows: 0.3–14% and 4.0% (P2), 0.2–8.7% and 3.1% (P3, 0.25 mol L−1 HNO3), 0.3–12% and 4.1% (P3, 0.50 mol L− 1 HNO3), 0.2–16% and 4.6% (P3, 1.0 mol L− 1 HNO3), 0.3–23% and 5.7% (P4, 0.25 mol L− 1 aqua regia), 0.5–8.4% and 4.6% (P4, 0.50 mol L−1 aqua regia), 0.4–13% and 5.6% (P4, 1.0 mol L−1 aqua regia). Basing on these results, it seems that overall precision of results achieved using the procedure P3 with the lowest concentration of HNO3 in brews, i.e., 0.25 mol L−1, is the highest. Generally, precision decreases with an increase in the concentration of HNO3 in brews. For aqua regia, regardless of its concentration in analyzed brews, overall precision is lower than this obtained using wet digestion.

3.1.2. Accuracy Mean concentrations of studied metals determined in brews of GC2 and IC2 by FAAS (Ca, Fe, K, Mg, Na) and ICP OES (Al, Ba, Co, Cr, Cu, Mn, Ni, Sr, Zn) using different sample treatments are given in Table 2. Table 1 presents also modules of calculated values of the t-test (| tcalculated |) applied to test the significance of differences between mean concentrations of metals determined in differently prepared brews of both coffees and for pairs of results with no significant differences in SDs of means. Otherwise, the C-test was used; calculated values of this test (Ccalculated) are given in Table 1 as well. As reference values, mean concentrations acquired using wet digestion (P1) were taken.

Table 2 Concentrations of metals determined by FAAS (Ca, Fe, K, Mg, Na) and ICP OES (Al, Ba, Co, Cr, Cu, Mn, Ni, Sr, Zn) in brews of ground (GC2) and instant (IC2) coffees prepared before analysis by wet digestion in HNO3 with 30% H2O2 solutions (P1), no treatment (P2), acidification to 0.25, 0.50 and 1.0 mol L−1 HNO3 (P3) or aqua regia (P4). Metal

Concentration, in μg mL−1 P1

P2

P3

P4

0.25

0.50

1.0

0.25

0.50

1.0

Brews of GC2 Al/103 Ba/103 Ca Co/103 Cr/103 Cu/103 Fe/103 K Mg Mn/103 Na/103 Ni/103 Sr/103 Zn/103

9.95 (5.6) 17.0 (2.9) 30.9 (2.2) 5.11 (5.9) 2.71 (9.2) 5.21 (5.2) 138 (5.0) 500 (1.8) 40.6 (1.6) 218 (1.8) 835 (4.5) 24.3 (4.1) 57.5 (2.1) 21.1 (2.8)

8.95 (3.7) 17.8 (2.2) 31.0 (2.2) 3.49 (4.3) 1.36 (5.1) 7.20 (2.4) 127 (4.7) 431 (3.7) 43.2 (1.4) 219 (0.46) 855 (1.6) 28.7 (3.1) 59.6 (0.50) 19.2 (2.1)

9.94 (4.3) 17.1 (1.2) 32.4 (2.4) 5.10 (4.5) 2.76 (8.7) 5.18 (2.9) 129 (4.5) 498 (1.2) 42.2 (0.27) 216 (0.93) 884 (0.84) 24.1 (3.3) 57.3 (0.87) 21.2 (2.8)

10.2 (3.9) 17.5 (3.4) 31.8 (1.0) 3.17 (3.8) 2.12 (12) 4.29 (5.1) 130 (4.0) 509 (0.75) 42.1 (1.8) 209 (2.9) 888 (0.71) 22.7 (2.2) 55.2 (3.4) 21.7 (1.4)

12.9 (7.0) 18.9 (4.8) 35.0 (3.7) bDLa 1.76 (4.5) 7.02 (6.1) 155 (16) 511 (2.6) 42.7 (0.70) 210 (0.48) 854 (1.8) 24.8 (1.2) 55.9 (0.36) 23.0 (4.8)

9.13 (5.0) 18.7 (5.3) 30.8 (1.2) 6.87 (1.7) 1.13 (12) 5.14 (23) 134 (1.2) 491 (2.2) 40.9 (0.26) 211 (0.47) 837 (3.0) 22.7 (4.4) 53.8 (0.93) 18.5 (3.2)

9.08 (8.4) 18.8 (3.2) 30.1 (1.6) 5.85 (8.0) 2.21 (8.1) 2.12 (7.1) 127 (7.5) 494 (1.6) 41.2 (1.8) 216 (0.46) 883 (0.74) 23.6 (5.5) 55.5 (1.8) 22.1 (3.2)

5.85 (13) 19.2 (4.2) 28.2 (1.8) 3.92 (8.2) 2.85 (6.3) 6.55 (11) 162 (5.4) 495 (0.91) 41.7 (0.79) 216 (0.93) 854 (1.2) 10.2 (5.9) 55.4 (2.2) 22.3 (4.9)

Brews of IC2 Al/103 Ba/103 Ca Co/103 Cr/103 Cu/103 Fe/103 K Mg Mn Na Ni/103 Sr/103 Zn/103

116 (0.86) 59.0 (1.2) 63.7 (3.0) 11.2 (1.8) 3.12 (7.7) 2.50 (1.6) 827 (3.7) 643 (2.9) 69.9 (0.89) 322 (0.93) 67.7 (3.5) 41.9 (0.24) 141 (0.71) 89.1 (3.1)

123 (1.6) 63.4 (0.32) 42.9 (2.8) 8.96 (7.1) 1.59 (14) 2.99 (3.0) 833 (3.4) 616 (3.6) 69.2 (1.7) 325 (0.92) 65.2 (1.5) 46.5 (5.6) 152 (0.66) 83.4 (1.1)

116 (1.7) 59.4 (0.50) 63.7 (5.6) 11.1 (2.7) 3.07 (5.5) 2.54 (2.0) 849 (2.9) 658 (0.21) 70.4 (2.1) 325 (0.92) 66.6 (0.70) 41.6 (0.48) 142 (0.70) 89.4 (2.5)

114 (1.8) 58.5 (0.34) 64.4 (3.4) 10.1 (6.9) 3.51 (6.3) 2.37 (6.3) 857 (1.8) 671 (0.27) 69.9 (0.60) 324 (0.93) 65.2 (1.7) 49.9 (6.6) 126 (0.79) 90.4 (2.6)

108 (1.8) 55.6 (0.54) 61.8 (3.5) 9.30 (5.4) 3.19 (4.7) 3.50 (9.1) 823 (2.1) 704 (0.20) 72.8 (1.3) 308 (0.32) 68.1 (1.5) 43.2 (1.4) 119 (0.84) 85.1 (0.94)

103 (1.9) 54.7 (4.2) 64.2 (4.8) 10.9 (3.7) 2.42 (1.2) bDLa 830 (1.0) 656 (1.7) 70.0 (1.3) 304 (2.3) 68.0 (3.5) 44.4 (4.7) 114 (3.51) 81.3 (2.1)

93.5 (1.3) 51.8 (5.0) 62.5 (2.3) 8.22 (8.4) 3.06 (2.6) bDLa 807 (3.6) 670 (1.6) 69.9 (1.1) 287 (4.5) 65.5 (4.4) 44.9 (2.7) 106 (6.6) 82.8 (3.1)

76.6 (0.39) 48.7 (4.1) 61.8 (5.0) 5.06 (5.7) 2.96 (7.4) 2.79 (10) 0.913 (3.8) 700 (0.65) 70.5 (0.96) 273 (3.7) 67.9 (1.9) 25.0 (8.4) 97.1 (5.4) 79.6 (1.6)

Means (n = 3) with relative standard deviations (RSDs) in brackets. a Below the detection limit (DL).



5

Table 3 Detection limits of metals determined for FAAS (Ca, Fe, K, Mg, Na) and ICP OES (Al, Ba, Co, Cr, Cu, Mn, Ni, Sr, Zn) combined with different sample preparation procedures, i.e., wet digestion in HNO3 with 30% H2O2 solutions (P1), no treatment (P2), acidification to 0.25, 0.50 and 1.0 mol L−1 HNO3 (P3) or aqua regia (P4). Detection limit, ng mL−1

Metal

P1

P2

P3

P4 −1

Al Ba Ca Co Cr Cu Fe K Mg Mn Na Ni Sr Zn

2.1 0.47 42 1.4 0.76 0.73 65 8.2 7.3 0.40 6.3 5.4 0.27 1.9

2.5 0.091 20 1.4 0.79 0.64 36 4.2 3.5 0.16 3.3 2.2 0.039 0.80

−1

−1

0.25 mol L

0.50 mol L

1.0 mol L

0.25 mol L−1

0.50 mol L−1

1.0 mol L−1

1.6 0.085 22 0.90 0.38 0.42 36 4.4 3.3 0.15 3.5 2.0 0.036 0.46

1.9 0.057 27 0.88 0.57 0.65 40 5.2 3.4 0.16 3.7 2.4 0.066 0.87

2.2 0.14 30 2.5 0.45 0.96 43 5.0 3.9 0.13 4.3 2.0 0.061 1.4

2.6 0.086 21 1.6 0.32 0.98 32 4.1 3.8 0.36 4.0 3.6 0.060 0.87

2.6 0.11 26 1.6 0.60 0.52 39 4.1 3.5 0.16 4.2 3.4 0.023 0.54

1.5 0.12 33 1.1 0.69 0.76 45 4.7 4.6 0.19 5.0 2.3 0.078 2.4

results; statistically significant differences between mean concentrations for 3 (0.50 mol L− 1) to 7 (1.0 mol L−1) out of 14 metals were established. Similar observations were noted for mean concentrations of metals determined in brews of IC2. Accordingly, only the procedure P3, in which brews of IC2 were acidified with HNO3 to 0.25 mol L−1, gave mean concentrations of all studied metals that were statistically indifferent from those obtained with wet digestion (P1). Acidification of brews to 0.50 mol L−1 HNO3 also produced dependable results for all

According to both tests it appears that only results achieved using the procedure P3 with acidification of brews with HNO3 to a concentration 0.25 mol L−1 are consistent with those obtained with the procedure P1 for both types of coffee. For brews of GC2, the use of a higher concentration of HNO3 (0.50 mol L−1) was valid for all metals except for Co, Cr and Cu. Acidification with HNO3 to 1.0 mol L−1 results in lower accuracy for a greater number of metals, i.e., Al, Ba, Ca, Co, Cr, Cu, Mg and Mn. Acidification of brews of GC2 with aqua regia neither provides satisfying

Table 4 Concentrations (in μg mL−1) of metals in brews of ground (GCs) and instant (ICs) coffees determined by FAAS (Ca, Fe, K, Mg, Na) and ICP OES (Al, Ba, Co, Cr, Cu, Mn, Ni, Sr, Zn) using acidification of brews with HNO3 to 0.25 mol L−1. Concentration, μg mL−1

Metal

Brews of GCs

Al/103 Ba/103 Ca Co/103 Cr/103 Cu/103 Fe/103 K Mg Mn/103 Na/103 Ni/103 Sr/103 Zn/103

GC1

GC2

GC3

GC4

GC5

GC6

GC7

GC8

Meana (CV, %)

28.7 (4.2) 35.5 (1.1) 23.7 (1.1) b0.9b b0.4b 12.3 (0.81) 78.5 (3.0) 473 (0.68) 44.0 (1.2) 433 (0.46) 824 (1.5) b2.0b 107 (0.94) 52.2 (3.2)

9.94 (4.3) 17.1 (1.2) 32.4 (2.4) 5.10 (4.5) 2.76 (8.7) 5.18 (2.9) 129 (4.5) 498 (1.2) 42.2 (0.27) 216 (0.93) 884 (0.84) 24.1 (3.3) 57.3 (0.87) 21.2 (2.8)

12.5 (1.6) 19.0 (1.0) 18.5 (0.62) 6.70 (0.15) b0.4b 8.37 (3.3) 88.1 (5.3) 518 (0.25) 38.2 (1.4) 228 (0.44) 764 (0.87) 34.2 (3.5) 66.5 (0.60) 29.3 (2.7)

9.12 (0.55) 42.3 (1.4) 19.2 (0.28) b0.9b b0.4b 11.7 (3.4) 84.7 (3.5) 479 (0.52) 40.3 (1.2) 358 (1.4) 1510 (0.94) b2.0b 100 (1.0) 41.4 (1.4)

12.1 (0.83) 20.8 (0.96) 17.6 (0.74) b0.9b b0.4b 14.2 (3.5) 76.1 (1.6) 502 (1.1) 36.3 (1.8) 150 (0.67) 225 (0.64) b2.0b 77.9 (4.4) 29.0 (3.8)

21.2 (2.8) 16.2 (2.5) 14.6 (0.85) 4.03 (3.7) b0.4b 9.93 (1.0) 91.2 (2.2) 493 (0.20) 34.2 (0.60) 181 (1.1) 680 (0.69) 30.0 (0.33) 57.1 (0.88) 23.9 (4.2)

6.90 (0.87) 29.1 (1.0) 19.7 (1.4) b0.9b b0.4b 4.75 (5.7) 74.4 (3.2) 498 (1.0) 43.3 (1.4) 365 (1.1) 355 (1.4) b2.0b 63.1 (0.79) 29.1 (4.8)

7.84 (3.8) 62.7 (1.1) 21.4 (1.2) b0.9b b0.4b 3.52 (2.0) 74.4 (2.1) 463 (0.29) 43.4 (0.87) 419 (0.96) 310 (1.9) b2.0b 129 (0.78) 40.5 (1.5)

13.5 (55.9) 30.3 (53.1) 20.9 (25.7) 3.85 (35.8) 1.48 (34.8) 8.74 (45.1) 87.1 (20.8) 491 (3.6) 40.2 (9.1) 294 (38.2) 694 (59.8) 15.2 (79.3) 82.2 (32.5) 33.3 (31.3)

Brews of ICs

3

Al/10 Ba/103 Ca Co/103 Cr/103 Cu/103 Fe/103 K Mg Mn/103 Na Ni/103 Sr/103 Zn/103

IC1

IC2

IC3

IC4

IC5

IC6

IC7

IC8

IC9

IC10

Meana (CV, %)

35.6 (2.2) 62.4 (0.80) 65.8 (1.1) 7.55 (2.2) 1.78 (5.6) 3.22 (1.2) 372 (2.5) 700 (1.3) 74.2 (1.5) 515 (0.39) 24.9 (0.70) b2.0b 166 (0.60) 57.5 (1.7)

116 (1.7) 59.4 (0.50) 63.7 (5.6) 11.1 (2.7) 3.07 (5.5) 2.54 (2.0) 849 (2.9) 658 (0.21) 70.4 (2.1) 325 (0.92) 66.6 (0.70) 41.6 (0.48) 142 (0.70) 89.4 (2.5)

76.3 (2.0) 73.2 (1.2) 59.7 (0.91) 22.9 (2.6) 3.37 (3.6) 4.22 (2.8) 734 (1.2) 605 (0.95) 65.3 (1.2) 535 (1.3) 43.5 (0.23) 70.6 (2.8) 157 (1.3) 57.7 (1.6)

35.3 (2.8) 213 (0.94) 80.0 (1.3) b0.9b 2.75 (2.5) 4.86 (1.2) 668 (2.4) 727 (0.73) 83.9 (1.6) 907 (0.55) 9.52 (0.29) b2.0b 328 (0.61) 107 (0.94)

53.3 (0.75) 33.0 (3.3) 53.0 (1.0) 12.6 (4.0) 2.90 (5.5) 8.13 (2.7) 337 (4.4) 748 (0.61) 76.2 (1.2) 294 (0.34) 12.2 (0.50) 44.4 (1.1) 146 (0.68) 49.6 (2.6)

208 (0.48) 83.6 (1.7) 50.3 (1.2) 23.7 (3.8) 3.42 (3.2) 4.48 (4.2) 1333 (1.4) 712 (1.0) 74.7 (0.51) 350 (0.57) 9.77 (0.62) 80.8 (0.62) 182 (1.6) 46.0 (1.7)

21.3 (4.2) 82.9 (0.84) 64.7 (1.2) b0.9b 2.10 (1.9) 0.895 (4.0) 310 (1.7) 629 (1.0) 75.8 (1.1) 950 (0.84) 41.3 (1.6) b2.0b 118 (1.7) 61.0 (0.82)

41.3 (4.4) 187 (1.6) 71.4 (0.46) b0.9b 2.41 (1.7) 1.47 (4.1) 488 (3.0) 750 (1.4) 71.4 (1.4) 797 (0.75) 44.2 (1.3) b2.0b 293 (1.4) 96.6 (0.83)

43.6 (2.1) 45.7 (1.3) 60.0 (2.0) 18.5 (1.6) 2.62 (5.0) 7.10 (3.8) 366 (2.4) 640 (1.8) 60.0 (1.2) 366 (0.82) 0.501 (2.1) 36.9 (2.2) 140 (1.4) 42.5 (1.2)

21.0 (4.3) 74.5 (1.1) 63.9 (0.57) 5.58 (2.5) 2.56 (1.6) 13.6 (0.74) 268 (2.2) 636 (1.0) 63.9 (3.5) 611 (0.82) 35.7 (1.5) b2.0b 141 (0.71) 59.5 (2.7)

65.2 (88.5) 91.5 (65.2) 63.3 (13.5) 11.1 (73.3) 2.70 (19.4) 5.05 (74.5) 573 (58.1) 681 (7.8) 71.6 (9.8) 565 (43.4) 28.8 (72.3) 30.8 (92.7) 181 (39.0) 67.5 (35.3)

Mean values (n = 3) with relative standard deviations (RSDs) in brackets. a Mean values for all coffee samples with coefficients of variance (CVs) in brackets. b Below the detection limit (DL).


6


metals, except Sr. In case of aqua regia (P4), none of tested concentrations of this reagent was found suitable for further research because results for 5 (0.25 mol L−1) up to 9 (1.0 mol L−1) out of 14 metals significantly differed from those assessed for the procedure P1. Finally, analysis of untreated brews (P2) of both types of coffee (CG2 and IC2) was either inadequate. Mean concentrations of 8 (in case of GC2) and 9 (in case of IC2) out of 14 metals mismatched with those obtained using the procedure P1. Since acidification of brews with HNO3 to 0.25 mol L− 1 was established to serve the best for the analytical purpose of this work, the recovery test for brews of GC2 and IC2 was carried out only for this procedure. Brews were spiked with three different concentrations of metals, i.e., 0.10, 0.20 and 0.50 μg mL− 1, acidified with HNO3 to 0.25 mol L−1 and measured by ICP OES on the content of Al, Ba, Co, Cr, Cu, Mn, Ni, Sr and Zn. For FAAS, recognized less susceptible to interferences (Welz & Sperling, 1999), the recovery test was made only for Fe. Recoveries of added metals were assessed by analyzing unfortified and fortified brews. Values obtained for GC2 (ranges for all concentrations of added metals) were as follows: 92.1–94.1% (Al), 98.6–102% (Ba), 98.1–101% (Co), 98.4–102% (Cr), 98.0–99.7% (Cu), 99.3–103% (Fe), 100–101% (Mn), 102–104% (Ni), 98.5–99.8% (Sr) and 99.1–99.8% (Zn). Correspondingly quantitative recoveries were determined for brews of IC2, i.e., 95.5–99.6% (Al), 98.4–99.1% (Ba), 97.9–102% (Co), 98.6–102% (Cr), 99.0–104% (Cu), 97.9–103% (Fe), 98.7–99.9% (Mn), 99.2–103% (Ni), 98.5–103% (Sr) and 99.5–101% (Zn). All these data indicate that acidification of coffee brews with HNO3 to 0.25 mol L−1 provide dependable results of their multi-element analysis by FAAS and ICP OES.

trend was previously described by Grembecka et al. (2007), who studied both types of coffees. The only exception was Cu; a mean concentration of this metal was by 1.6-fold higher in GCs than its level in ICs. Concentrations of K, Mg and Ca were the highest in brews of both types of coffee, similarly as previously reported by Oliveira et al. (2012); Ashu and Chandravanshi (2011); Grembecka et al. (2007); Fernandes et al. (2005) and Suseela, Bhalke, Vinod Kumar, Tripathi, and Sastry (2001). These three metals were also characterized by the lowest variation of results within the group for both coffees. Also Na was found to be in a relatively high amount in brews of ICs. Sodium and Mn in GCs brews and Fe, Mn and Sr in ICs were present in moderate concentrations. Finally, trace metals present (in order of a downward concentration) in brews of both types of coffee were Fe, Sr, Zn, Ba, Al, Ni, Cu, Co and Cr (GCs) and Ba, Zn, Al, Ni, Co, Cu, Cr (ICs). For this group of metals, values of CVs were the highest and changed from 53% (Ba) to 124% (Cr) in case of GCs and from 58% (Fe) to 108% (Ni) in case of ICs. Regarding literature available for brews of GCs (Nedzarek et al., 2013; Stelmach et al., 2013; Ashu & Chandravanshi, 2011) and ICs (Pohl et al., 2014a; Oliveira et al., 2012; Suseela et al., 2001), outcomes achieved in the present work correspond well with concentration ranges reported by cited authors.

3.1.3. Detection limits Using FAAS and ICP OES along with different sample preparation procedures, DLs of metals were determined according to the 3 × SDblank criterion, where SDblank is SD of an analyte signal measured in a blank adequate for a given procedure. As can be seen from Table 3, DLs obtained using the procedure P3, where brews of coffees were acidified with HNO3 to 0.25 mol L−1, are in overwhelming cases the lowest among others. Accordingly, DLs of metals for FAAS and ICP OES assessed using this procedure were within 0.04–0.90 ng mL−1 (Ba, Co, Cr, Cu, Mn, Sr, Zn) and 1.6–4.4 ng mL−1 (Al, K, Mg, Na, Ni). The highest values of DLs were found for Ca (22 ng mL−1) and Fe (36 ng mL−1). Quoted DLs were lower by about 2–3- (Ca, Co, Cr, Cu, Fe, K, Mg, Na), 3–4- (Mn, Ni) and 4–8-fold (Ba, Sr, Zn) as compared to those obtained using the procedure P1. 3.2. Analytical application Considering important validation parameters, i.e., precision and accuracy of results, and DLs for studied metals assessed with FAAS and ICP OES combined with compared sample preparation procedures, acidification of brews with HNO3 (P3) to 0.25 mol L−1 was the procedure of choice for reliable multi-element analysis of brews of GCs and ICs. This procedure was simpler and faster than commonly applied wet (Nedzarek et al., 2013; Ashu & Chandravanshi, 2011; Welna et al., 2013) and dry (Ozdestan, 2014; Grembecka et al., 2007; Santos et al., 2004) ashings. Allowing to analyze a greater number of brews in a shorter time with significantly lower amounts of reagents, it is an alternative, green analytical sample preparation procedure that could be considered particularly important for multi-element screening and quality-control monitoring studies of coffee as served (Bendicho, Lavilla, Pena, & Costas, 2011). Results of analysis of brews of popular in the Polish market GCs and ICs, obtained using developed methods, are given in Table 4. Additionally, averages within the group along with coefficients of variance (CVs) are given. As can be seen, mean concentrations of all metals determined in ICs are higher by 1.4–2.0- (K, Mg, Mn, Zn), 2.3–4.0- (Ba, Ca, Cr, Ni, Sr), 4.3–6.5- (Al, Co, Fe) and 35-fold (Na) than those found in GCs. A similar

Fig. 1. Scatter plots for first two component loadings in PCA (a) and factors in FA (b). Mean concentrations of Al, Ca, Co, K, Mg, Mn and Ni determined in brews of GCs and ICs were used as variables in PCA. Mean concentrations of all studied metals determined in brews of GCs and ICs were used as variables in FA.



7

were also located in the latter group of brews. To reduce the number of variables and detect a structure in data, factor analysis (FA) was made. A classical type of factoring was selected while the data matrices were concentrations of all 14 studied metals. As in case of PCA, missing data were replaced with respective DLs. Factors were extracting from the data correlation matrix using the Kaiser criterion, i.e., dropping components with eigenvalues under 1.0. Two factors were extracted, i.e., F1 and F2, with eigenvalues of 7.68 and 3.14, respectively. They accounted for 91.6% of variability of original data (65.0% for F1 and 26.6% for F2). Initial factor loadings were rotated using a varimax criterion. Considering factor loadings obtained after the rotation (values are given in brackets), it seemed that concentrations of Mg (0.862), Mn (0.859), Sr (0.889), Zn (0.928), Ca (0.897) and Zn (0.780) were represented by the first factor (F1). The second factor (F2) strongly affected concentrations of Al (0.842), Ni (0.946), Co (0.936) and Fe (0.787). A scatter plot of both factors (F1 versus F2) is given in Fig. 1b. It can be seen that brews of GCs are well differentiated from brews of ICs on the basis of multielement analysis and the information about concentrations of all metals determined. As in case of PCA, brews of IC9 and IC10 were located in the group of ICs, likely because of a similarity in components used and the chemical composition. Finally, mean concentrations of Al, Ca, Co, K, Mg, Mn and Ni were also used for visualizing analyzed coffees by agglomerative hierarchic cluster analysis (HCA). The Ward linkage method, decreasing variance for clusters, with the Euclidean distance for measuring similarity among samples were used. As can be seen from Fig. 2, the resulting dendogram is consistent with results of PCA. Two main well-separated clusters corresponding to brews of GCs and ICs, respectively, were attained. As in case of PCA and FA, the first cluster representing brews of GCs was more homogenous (lower within-class variability) than the second one for ICs.

Fig. 2. A dendogram resulted from HCA for analyzed brews of GCs and ICs. Mean concentrations of Al, Ca, Co, K, Mg, Mn and Ni were used as variables.

3.3. Chemometric data evaluation To demonstrate feasibility of fast and simple sample preparation procedure and multi-element analysis described, principal component analysis (PCA) was used to categorize brews of studied coffees (Fig. 1a) A correlation matrix of concentrations of all metals (variables) was applied, missing data (metals not detected) were replaced with respective DLs (Paz-Rodriguez, Dominguez-Gonzalez, Aboal-Somoza, & Bermejo-Barrera, 2015). No rotation was applied to PCs. It was established that two first component loadings explained 56.3 (PC1) and 23.5% (PC2) of the whole data variance. Respective eigenvalues were 7.81 (PC1) and 3.27 (PC2). In the next trial, only variables with the highest contributions to PC1 and PC2 were used, i.e., Ca (11.1%), K (10.4%) and Mg (11.4%) for PC1 and Al (13.5%), Co (21.5%), Mn (12.2%) and Ni (24.9%) for PC2. Using these variables, both PCs were found to explain 90.1% of variance, i.e., 56.4% (PC1) and 33.8% (PC2). Respective eigenvalues were 3.94 (PC1) and 2.36 (PC2). As can be seen Fig. 1a with a scatter plot for PC1 and PC2, well-defined separation of brews of GCs and ICs is achieved. Brews of GCs were located within a narrow range of negative PC1 scores (− 2.05 to − 1.58), indicating a high similarity of examined samples. Brews of ICs were located in a much broaden range of positive PC1 scores, i.e., from 0.50 to 3.52. The range of PC2 scores for ICs was expanded as well (−2.77 to 3.02), indicating a high variability of these products. Interestingly, brews of IC9 and IC10, being instant coffees with admixtures of ground coffees,

3.4. Leachability Finally, percentages of metals leached into brews were assessed using results achieved after microwave-assisted wet digestion of solid coffee samples and results for brews described above. Considering masses of GCs and ICs taken to prepare respective brews and their final volumes, leaching efficiencies (in %) for studied metals were calculated (see Table 5). Box-and-whisker plots with results of FAAS and ICP OES analysis of samples of coffees (means, medians, minimal and maximal values and 1st and 3rd quartiles, in μg g−1) are given in Fig. 3. Mentioned mean concentrations for all analyzed solid coffees with CVs are given in Table 5.

Table 5 Mean concentrations (in μg g−1) of studied metals in GCs and ICs in addition to mean leachabilities (in %) of these metals into respective brews.

Al. Ba Ca Co Cr Cu Fe K Mg Mn Na Ni Sr Zn a b c d

Concentration, μg g−1

Leaching efficiency, %

Meana (CV, %)

Meana (CV, %)

Range

GCs

ICs

GCs

ICs

GCs

ICs

14.6 (73.0) 3.26 (55.8) 1.24 × 103 (13.8) 0.364 (117) 0.365 (25.6) 12.8 (14.5) 37.6 (28.8) 1.97 × 104 (8.2) 1.87 × 103 (7.4) 23.3 (27.9) 58.9 (54.9) 0.815 (135) 5.39 (29.5) 4.93 (8.9)

3.45 (86.4) 4.20 (65.1) 3.36 × 103 (10.3) 0.636 (87.4) 0.467 (20.5) 0.722 (107) 31.2 (42.4) 3.09 × 104 (12.1) 3.18 × 103 (7.8) 24.3 (42.9) 1.25 × 103 (70.9) 1.21 (107) 8.15 (35.0) 2.90 (32.7)

4.5 (78.6) 31.8 (19.1) 55.8 (17.2) 21.0 (34.8) 26.9d 2.2 (37.9) 8.1 (26.3) 83.3 (9.0) 72.2 (11.5) 41.0 (15.2) 40.9 (30.5) 67.0 (45.0) 50.8 (14.9) 22.3 (26.3)

79.7 (18.3) 90.9 (3.8) 77.2 (7.4) 73.1 (15.6) 24.9 (29.2) 68.5 (38.4) 70.1 (20.9) 92.8 (13.5) 92.7 (10.1) 97.0 (2.7) 95.8 (2.3) 98.7 (0.8) 91.6 (5.2) 95.5 (3.2)

1.8–12.7 24.3–42.1 39.2–68.5 15.4–29.3 26.9d 1.0–3.1 5.1–11.5 76.0–99.4 59.3–86.3 31.5–50.5 25.6–61.4 26.8–99.1 39.1–65.9 14.8–34.3

48.6–93.5 85.8–98.1 70.4–86.4 45.3–96.3 16.9–38.0 5.28–93.9 47.7–95.0 70.5–99.8 78.1–99.9 93.9–99.7 91.8–98.9 87.5–99.4 84.2–98.4 91.1–99.8

GCsb

ICsc

3–64 ~12 1.9–70.9 5.1–67.0 15.9–99.0 1.2–11.9 0.8–24.9 57.1–95.7 6.4–79.9 8.8–56.2 5.8–99.6 18.4–83.7 – 0.6–90.5

– – 86.5–103 81.4–107 72.7–108 73.7–110 80.1–96.1 90.1–109 88.5–100 88.6–119 90.3–103 50.9–106 – 69.4–106

Mean values for all coffee samples with coefficients of variance (CV) in brackets. Data taken from Pohl et al. (2014b), Welna et al. (2013), Stelmach et al. (2013); Ashu and Chandravanshi (2011); Frankova et al. (2009) and Grembecka et al. (2007). Data taken from Grembecka et al. (2007). In 7 out of 8 samples of GCs, Cr was not detected.


8


Fig. 3. Box-and-whisker plots of results of analysis of solid samples of GCs and ICs.

It was established that, except for Cr, leachabilities of metals into brews of ICs were high and changed from 68 to 80% (Al, Ca, Co, Cu, Fe) and 91–99% (Ba, K, Mg, Mn, Na, Ni, Sr and Zn). These results were coincident with outcomes of Grembecka et al. (2007), who, as the only ones, reported so far such data for ICs. Leachabilities of metals into brews of different coffees reported in literature varies a lot, likely as a consequence of different brewing procedures. Nevertheless, results achieved for GCs in the present study using the mug coffee preparation were well-matched to those obtained by other authors that also used a similar coffee preparation procedure (Pohl et al., 2014b; Stelmach et al., 2013; Welna et al., 2013).

4. Conclusions The non-digestion method for multi-element analysis by FAAS (Ca, Fe, K, Mg, Na) and ICP OES (Al, Ba, Cd, Co, Cr, Mn, Ni, Pb and Sr) in brews of ground roasted and instant coffees was proposed and its suitability was evaluated. The procedure based only on acidification of brews with HNO3 to a concentration of 0.25 mol L−1 produced precise and accurate results along with sensitivity and DLs adequate for determining macro-, micro- and trace elements. The acidification of brews to a higher concentration of HNO3 (0.50, 1.0 mol L−1), and the use of aqua regia for this purpose or no treatment (and direct analysis of brews) were useless. The application of the proposed procedure was faster, simpler, cheaper and safer in comparison to a typical sample treatment of brews based on wet digestion or dry ashing, where longer times and aggressive concentrated reagents were required. Thus, it could be an interesting alternative procedure applicable for routine multi-element of coffee brews in terms of the monitoring of their quality and safety, the evaluation of the mineral characteristics and finally, the classification due to type and/or geographical origin.

Acknowledgments This work was funded by The National Science Centre (2013/09/B/ NZ9/00122) (decision no. 2013/09/B/NZ9/00122).

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