Developments and strategies in the spectrochemical elemental analysis of fruit juices

Trends in Analytical Chemistry 55 (2014) 68–80

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Trends in Analytical Chemistry journal homepage: www.elsevier.com/locate/trac

Review

Developments and strategies in the spectrochemical elemental analysis of fruit juices Anna Szymczycha-Madeja, Maja Welna, Dominika Jedryczko, Pawel Pohl ⇑ Analytical Chemistry Division, Faculty of Chemistry, Wroclaw University of Technology, Wy Stanislawa Wyspianskiego 27, Wroclaw 50-370, Poland

a r t i c l e

i n f o

Article history: Available online 11 January 2014 Keywords: Atomic spectrometry Calibration Elemental analysis Fruit juice Mass spectrometry Quality assurance Sample preparation Sample treatment Spectrochemistry Trace element

a b s t r a c t Information on the concentration of major, minor and trace elements in fruit juices is very important because the popularity of these beverages and the rate of their consumption have rapidly increased in the past 20 years. For the overwhelming majority of cases, the elemental analysis of fruit juices is carried out using spectrochemical analytical methods, which normally require samples of fruit juices to be prepared by decomposing their organic matrix and releasing elements in a form suitable for measurement. This review covers different aspects of the elemental analysis of fruit juices and the societal implications related to the presence of various elements in these beverages. We review in detail sample-preparation procedures executed before the elemental analysis together with calibration strategies used, and quality assurance and quality control of results. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1.

2. 3.

4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Fruit juices as valuable sources of different elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Sources of elements in fruit juices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumental techniques for the elemental analysis of fruit juices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samples and their preparation prior to elemental analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Direct analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Dry ashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Wet digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Speciation analysis of elements in fruit juices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality assurance and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction A diet rich in fruit juices is recognized to be a good source of a wide range of physiologically and nutritionally important compounds, including carbohydrates, proteins, vitamins, carotenoids, pectins, flavonoids, glucarates, coumarins, monterpenes, limonids, triterpenes, phenolic acids and macroelements [1–10]. Due to the ⇑ Corresponding author. Tel.: +48 71 320 3445; Fax: +48 71 320 2494. E-mail address: [email protected] (P. Pohl). 0165-9936/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.trac.2013.12.005

68 69 69 72 74 74 75 75 77 78 78 79 79

presence of all these functional and bioactive phytochemicals, fruit juices are very popular beverages that offer great taste, flavor and color, while their intake provides many beneficial health effects, primarily antioxidative properties and other bioactivities [1–3,5,7,9,11–15]. Fruit juices are widely consumed all over the world by different age groups and their intake has rapidly increased in the past two decades because they are acknowledged to reduce the risks of many chronic and degenerative diseases [10,15,16]. In tropical and Mediterranean countries, they are the most widely consumed beverages, so they naturally help to

A. Szymczycha-Madeja et al. / Trends in Analytical Chemistry 55 (2014) 68–80

maintain and reinforce the health of the inhabitants of these regions [8,15,17].

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physicochemical species that are available for the human body [16,33]. 1.2. Sources of elements in fruit juices

1.1. Fruit juices as valuable sources of different elements As can be seen from Tables 1 and 2, which give the concentration ranges of different elements in commercially-available and freshly-prepared fruit juices, the contribution of these beverages to the total human diet cannot be neglected. Containing large amounts of essential and physiologically-important elements, fruit juices often prove to be rich sources of nutrients and valuable dietary supplements in cases of deficiencies of macroelements (e.g., Ca, K, and Mg) and other minor and trace elements (e.g., Cr, Fe, Mg, Mn, Mo and P) [3,4,8,10,12,13,16,18,19]. For this reason, it is not surprising that they are a subject of great concern and interest for many researchers, food analysts and nutritionists, who want to determine their nutritional quality and safety [15]. Considering total concentrations of elements, certain studies related to the elemental analysis of fruit juices reveal that the daily consumption of fruit juices may markedly cover recommended daily allowances (RDAs) for some nutritionally-important elements required by infants, children and adults {i.e. 0.8–19% (Al), 0.6–32% (Ca), 9.2–87% (Cr), 1.5–35% (Cu), 0.4–26% (Fe), 0.3–55% (K), 1.6–22% (Mg), 0.4–23% (Mn), 0.4–12% (Mo), 0.1–14% (Na) and 0.5–21% (Zn) [13,16,18,20–22]. In extreme cases, drinking fruit juices may even contribute to an unnecessary increase in the intake of Na [5,12] that is detrimental to health. The degree of coverage of RDA values strongly depends on the variety of juice and its provenience (i.e. the type of fruits from which it is produced and the region of the production). The impact of both factors is so strong that, in some cases, the consumption of certain fruit juices can only represent <0.01–0.7% of recommended or permissible levels of some elements {e.g., Al, Co, Cr, Cu, Fe, Mn, Ni, Zn and Se [7,8,13,18,20,23,24]}, and such beverages may not contribute at all to the dietary intake of the elements mentioned. Although concentrations of some minor and trace elements (e.g., Al, As, Cd, Co, Cu, Fe, Mn, Mo, Ni, Pb and Zn) in fruit juices are low, these elements can impose toxic effects when the share of the relevant beverages in the total diet is high [4,9,12,15,19,21,25–27]. When the content of Cd, Pb, As, Al, Co, Cr Cu, Hg and Ni in consumed fruit juices is low or not detectable at all, so conforming to national health and sanitary standards or food-policy regulations, it proves the high food safety and quality of these beverages [1,2,17,19,25,26,28–30]. Otherwise, when analyzed samples contain these elements above admissible levels stipulated in national and/or international safety standards, the contribution of certain fruit juices to the total dietary intake of As, Cd, Pb and other trace elements dangerously increases and becomes highly undesirable for consumers [12,21,26,31,32]. In this situation, it is understandable that regular examination, verification and tests of fruit juices with respect to the concentration of minor and trace elements are mandatory. The issue of the potability and the safety of fruit juices is especially crucial because the quality of harvested fruits is subject to seasonal variations provoked by climatic changes [4,12,22,26,29]. In view of this, it appears that the elemental analysis of fruit juices is significant for the beverage industry because it gives information about total concentrations of many elements and their average dietary intakes related to the consumption of these beverages [16]. However, fruit juices contain natural endogenous bioligands that can differentially bind elements and change their availability for the human body. Hence, the elemental analysis and the resultant information on total concentrations of elements can sometimes be less important than knowledge of ultimate

The variability of concentrations of elements in fruit juices is great [5,12,16,20,21,23,26,34–37]. Even within juices made of the same kind of fruit, the variation of concentrations of elements is high [8,13,38]. Usually, commercial fruit juices contain much higher quantities of elements than freshly prepared juices of the same fruit [39]. Several factors may contribute to observed variations in the levels of elements, including the availability of elements for uptake by plants (strictly related to the characteristics of soils, their mineral composition and the soil pH), agricultural practices and procedures applied during the growth of fruit plants (i.e. application of fertilizers and water irrigation, and climatic conditions), and, finally, plant variation in addition to the type and the maturity of fruits at harvest [4,5,8,13,16,17,20,22,34,35,37–43]. The effect of the soil mineralization can be of primary importance for macroelements (i.e. K, Mg and S) and some minor and trace elements (i.e. B, Fe, Mn and Zn) [43]. The soil type is recognized to be responsible for most regional differences observed in the content of elements present in fruit juices [22,40]. It is because specific groups and types of soil and rootstock can possess elevated concentrations of Ca, B, K, Na and Rb, and this is reflected in higher levels of these elements in fruit juices coming from these regions [40]. The proximity of rivers and any increase in their salinity can affect the concentration of Na in juices [40]. Due to similar properties of some elements and their natural co-occurrence or common exposure sources, several linear relationships between concentrations of these elements in different juices are found, including a very high positive correlation for Cr–Fe and high positive correlations for Al–Cr, Al–Pb, Ca–Cd, Ca–Mg, Ca–Sr, Cd–Ni, Cd– Sr, Cr–Mn, Cu–Fe, Fe–Mn and Mn–Zn [37]. Agricultural practices and techniques applied during the growth of plants and fruits are also important. Organic farming, which excludes the use of synthetic chemicals, such as fertilizers or pesticides in order to keep or to improve the fertility of soils, is recognized to have a significant effect on subsequent concentrations of selected macroelements (i.e. Ca, K and Mg) and microelements (Cu, Fe, Mn and Zn) in fruit juices [9,15,38]. In this case, the content of elements in juices made from organic fruits is higher [15,38]. The use of agrochemical products, e.g., insecticides or fungicides, employed during the growth of fruit plants, is responsible for diminishing the quality and the safety of these products because of an increased risk of the exposure of humans to heavy metals (e.g., Cd and Pb) [1,4,17,19,25]. Fruits themselves, their storage, handling and processing conditions and practices, especially the extraction procedure used to retrieve juices, are important post-harvest factors that affect the chemical composition of juices [1,4,5,20,23,31,34,39,42,44,45]. During the manufacture of juices, tap water [40,44] (increased concentrations of Al, Ca, Cu, Fe, Mg, Na, Si, Sr), peels [40] (increased concentrations of B, Cu, Fe, Mn, Na, Ni, Si, Sr), seeds [45] (increased concentration of Ca, Co, Fe, Mg, Mn and Zn), technological processes [13,23] (increased concentrations of Cr, K and Mn), packing [21,23,31] (increased concentrations of As and Al) and, in case of cans over glass bottles and carton packages, additives, pipes and containers used for processing and storage [44] can be sources of contamination. However, the production process and its steps may also lead to some loss of different elements [13] {i.e. Ca (54–55% compared to the concentration in fruits), Cr (53–77%), Cu (50%), Fe (45%), Mg (28%) and Mo (64–75%), due to the methods used for separating juices from pulps. Because the elemental profile of fruit juices varies according to differences in the chemical composition of fruits grown and

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Table 1 Concentrations (in lg mL

1

) of different elements in apple, apricot, aronia, blackcurrent, cherry, grape, grapefruit, lemon and lime, mandarin and mango juices

Apple

Aronia

3.4–347 [11,18,20,33,34,37,44,48,59]

26–76 [4,20]

Cd

ND-0.060 [1,11,26,29,37,44,48]

0.006–0.043 [1]

Co

ND-0.030 [7,11,26,29,44,48]

Cr

ND-0.14 [7,11,20,23,26,37,48]

0.006–0.009 [4] 0.003–0.005 [4] ND [20]

Cu

0.07–0.27 [4,20] 0.66–3.9 [4,20]

0.11–1.83 [1]

Fe

ND-1.8 [1,7,11,16,18,20,26,29,37,44,48,52,59] ND-1.8 [7,11,16,18,20,34,37,44,48]

Ga In K

ND [11] ND [11] 98–8088 [11,18,20,34,48,59]

La Li Mg

ND-0.04 [11] 12–349 [11,20,33,34,37,48,59]

Mn

ND-1.1 [7,11,18,20,29,37,44,48]

Mo Na Ni P

0.006 [44] 0.5–324 [11,18,20,34,44,59] ND-0.10 [7,11,26,29,37,44,48] 46–81 [11,37]

0.12–0.31 [4,20] 0.01–0.02 [4] 4.1–9.8 [20] 0.07–0.15 [4] 54–93 [4]

Pb

ND-0.67 [1,11,20,29,37,44,48]

ND [20]

Rb S Sb Sc Se Sn Sr

ND-1.1 [11,44]

Blackcurrent

Cherry

ND-0.07 [11,48] ND-0.83 [11,21,37,48,54] ND-0.014 [11,26,31]

Grape ND-0.48 [17,35,37]

Grapefruit

Lemon and lime

Mandarin

0.17 [37]

Mango ND-0.25 [17]

ND-0.01 [25,46] 1.4–3.0 [4]

1.3–2.0 [4]

3.2–3.6 [43] ND-0.049 [35,37]

0.64–0.69 [55] 0.093 [37]

19–323 [4,18,20] 0.006–0.008 [4]

19–680 [37,43,44]

75–163 [37,55] 0.005–0.041 [1,37]

0.003–0.007 [4]

ND-0.0003 [17,29,35,37,44] ND-0.004 [29,35,44]

ND [20]

ND-0.080 [17,29,35,37]

0.012 [37]

0.02–0.26 [4,18,20] 0.48–3.6 [4,18,20]

ND-1.7 [8,17,29,35,37,44] 0.14–9.5 [17,35,37,43,44]

0.11–1.56 [1,37,55] 0.56–2.6 [37,55]

ND-0.13 [11,37,48] ND-0.04 [11] ND [11]

0.03–0.10 [46] 336 [33] 0.010–0.049 [1]

0.24–1.84 [1]

475–1448 [4,20]

40–99 [9,15,38]

36–181 [30,44]

ND [29]

ND-0.0003 [17,44]

0.0001–0.002 [29,46] ND-0.050 [25,29,46] ND-0.11 [25,29,52]

ND-0.008 [7,44]

0.50–1.7 [25,46]

358–1639 [4,18,20]

ND-1.2 [7,17,30,44] 0.17–0.35 [9,15,38] 0.42–0.86 [9,15,38]

0.03–0.46 [7,16,17]

1197–3325 [9,15,38]

49–570 [30]

0.10–2.0 [7,16,17,30,44]

0.0003–0.001 [46] 19–48 [4,20]

90 [33]

0.02–0.43 [1]

0.06–0.26 [1]

26–56 [4,20]

49–114 [37,43]

96–137 [37,55]

0.16–0.53 [4,18,20] 0.01–0.02 [4] 13–324 [18,20] 0.03–0.08 [4] 61–80 [4]

0.08–1.2 [17,29,35,37,43,44] ND-0.007 [17,44] 29–165 [44] ND-0.20 [29,35,37,44] 136 [37]

0.22–0.32 [37,55]

ND [20]

ND-0.11 [17,29,35,37,44]

0.12 [37] 152–159 [37,55] 0.05–0.24 [1,37]

0.14–1.8 [44] 63–80 [43]

109–166 [9,15,38] 0.12–0.37 [9,15,38]

0.23 [29]

5.3–15.4 [46] ND-0.02 [25,29]

2.0–6.5 [9,38]

0.020–0.19 [7,17,44] ND-0.015 [17,44] 81–300 [30,44] 0.006–0.016 [7,44]

ND-0.57 [25,29]

ND-0.0003 [17,44]

0.5–2.9 [46]

0.19 [44] a

0.004–0.029 [46] 0.033–0.109a [46] ND-0.24 [11,51] 1.9 [29] ND-0.65 [11,37,44,48] ND [11] ND-0005 [26] ND-0.040 [11,44] ND-2.4 [1,7,11,16,18,20,26,29,37,44,48,52,59]

In ng mL

1

; ND, Not detected.

ND-1.8 [29,35] 0.090–2.0 [37,44]

0.18–0.74 [4,20]

0.31–1.96 [1]

0.36–3.17 [1]

0.10–0.45 [4,18,20]

0.004–0.033 [44] 0.071–5.2 [8,17,29,35,37,43,44]

34 [55] 0.53–1.7 [37,55]

1.8 [29]

0.26–3.39 [1,37,55]

0.10–0.93 [25,29,46,52]

0.19 [44]

0.27–0.48 [9,15,38]

0.004 [44] 0.05–0.70 [7,16,17,30,44]

A. Szymczycha-Madeja et al. / Trends in Analytical Chemistry 55 (2014) 68–80

Tl U V Zn a

Apricot

Ag Al As B Ba Be Bi Br Ca

Table 2 Concentrations (in lg mL

1

) of different elements in muskmelon, orange, passion fruit, pineapple, peach, pear, plum, pomegranate and strawberry juices

Muskmelon 5.9 [61]

44 [61]

Orange ND [11] 0.0004–3.1 [11,21,35,37,40,54,60,61] ND [11,25] ND-3.3 [4,11,40,55] ND-0.24 [11,35,37,40] ND-0.003 [11] ND [11] 13–711 [4,11,14,20,37,40,44,47,55,61] ND-0.040 [1,4,11,29,35,37,44]

Co Cr Cu

0.2 [61]

Fe

1.7 [61]

ND-0.020 [4,7,11,29,35,40,44] ND-0.018 [7,11,20,23,25,29,35,37] ND-1.8 [1,4,7,11,14,16,20,25,29,35,37,40,42,44,47,52,55,61] 0.008–16 [4,7,14,16,20,25,35,37,40,42,44,47,55,61]

Ga In K

1930 [61]

ND [11] ND [11] 291–2566 [4,11,14,40,47,61]

Li Mg Mn

33 [61] 0.2 [61]

ND-0.015 [11,40] 18–208 [4,11,14,20,37,40,47,55,61] ND-0.56 [4,7,11,14,20,29,35,37,40,44,47,55,61]

Mo Na Ni

239 [61] 0.1 [61]

ND-0.02 [4,11,40,44] ND-124 [11,14,20,40,44,47,61] ND-0.10 [4,7,11,25,29,35,37,40,44,61]

P Pb

140 [61]

Rb Se Si Sn

0.9 [61]

Sr Ti Tl V Zn

1.9 [61] 1.3 [61]

2.1 [61]

Zr

0.1 [61]

31 [61]

29–235 [4,11,37,40,47,55,61] ND-0.25 [1,11,20,25,29,35,37,42,44] ND-4.1 [11,40,44,61] ND-0.37 [11,51] ND-23 [11,40,61] ND-0.59 [29,35] 0.030–2.0 [11,37,40,44,55,61] ND-0.50 [11,40,61] ND [11] ND-0.004 [11,40,44] ND-4.7 [1,4,7,11,14,16,20,25,29,35,37,40,42,44,47,55,61] 0.1 [61]

Passion fruit

Pineapple

ND-0.37 [17,35]

ND-4.2 [21,35,37,54,61]

ND [35]

0.04–0.11 [35,37]

20–697 [44] ND-0.0005 [17,35,44] 0.001–0.28 [35,44] ND [17,35] ND-0.10 [17,35,44] 0.03–2.8 [17,35,44]

9.6–140 [37,41,44,61] ND-0.012 [29,35,37,44]

Peach ND-0.75 [17,21]

Pear

Plum

Pomegranate

Strawberry

0.39 [37]

1.3 [37]

ND [35]

0.16 [37]

0.22 [37]

ND [35]

22–80 [4,20,44] ND-0.010 [4,17,44]

15–124 [18,37,44] ND-0.0004 [37,44]

74–96 [18,37] 0.023 [37]

ND-0.004 [29,35,44] ND-0.017 [23,29,35,37] ND-0.21 [16,29,35,37,41,44,61] 0.13–9.1 [16,35,37,41,44,61]

ND-0.010 [4,44] ND-0.006 [17,20,23] 0.02–6.4 [4,17,20,42,44,52] 0.10–3.7 [4,17,20,37,42,44]

ND-0.002 [44] 0.011 [37] 0.058–0.71 [18,37,44] 0.06–9.7 [18,37,44]

0.024 [37] 0.16–0.51 [18,37] 0.16–1.5 [18,37]

40–702 [41,61]

588–1478 [4,20]

1230 [18]

1719 [18]

708–1735 [5]

7.0–75 [37,41,61] 0.06–23 [29,35,37,39,41,44,61] 0.049 [44] 1.5–75 [41,44,61] 0.044–0.29 [29,35,37,44,61] 71–198 [37,61] ND-0.24 [35,37,44]

30–54 [4,20] 0.010–0.35 [4,17,20,44]

51 [37] 0.46–0.83 [18,37]

35–97 [5] 0.31–1.68 [5]

194 [18] 0.11 [37]

51–209 [5] 0.02–0.06 [5]

129 [37] 0.79 [37]

38–104 [5] Up to 0.02 [5]

1.0–114 [4,44]

ND [35]

0.45–1.1 [44,61] 0.014–0.015 [51] 33 [61] ND-0.45 [29,35]

47 [37] 0.12–0.53 [18,37,44] 0.013–0.079 [44] 69–126 [18,44] 0.015–0.063 [37,44] 116 [37] 0.0007–0.19 [37,44] 0.20–0.27 [44]

0.088–0.62 [37,44,61] 0.30 [61]

0.19 [44]

0.24–0.80 [37,44]

0.75 [37]

0.14–2.1 [44]

0.005 [44] 0.04–5.7 [16,29,35,37,41,44,61] 0.2 [61]

0.002 [44] 0.070–1.1 [4,17,20,42,44,52]

0.004–0.005 [44] 0.29–5.2 [18,37,44]

0.43–0.47 [18,37]

1.9–3.2 [4]

0.02–0.29 [17,35,44] ND-0.017 [17,44] 8.4–400 [44] 0.012–0.10 [35,44]

ND-0.002 [17,35,44] 0.25–0.73 [44]

0.0007–0.006 [44] 0.046–0.42 [17,35,44]

ND-0.020 [4,17,44] 9.4–68 [20,44] 0.021–0.17 [4,44]

ND-0.27 [17,20,42,44]

60–172 [5] ND [5]

0.03–0.21 [5] 0.99–2.7 [5]

ND [35] ND [35] ND [35] ND [35] 0.19 [35]

0.04 [35]

0.08 [35]

ND [35]

0.58–0.84 [5]

ND [35]

Up to 0.24 [5]

ND [35]

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Ag Al As B Ba Be Bi Ca Cd

ND, Not detected.

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collected in different areas of production, information on total concentrations of elements can be used for classification and discrimination of their samples [26,46,47]. The chemometric evaluation of the data using principal-component analysis (PCA) [40,44,46], linear discriminant analysis (LDA) [46] and hierarchical cluster analysis [44] are helpful to differentiate juices based on their geographical or botanical origins. This strategy is also useful to control the truth-in-labeling of products or the origin of the production zone and to identify any potential adulteration of the beverages analyzed (e.g., resulting from substitution of juices with extracts or juices from other fruits, or addition of imported products in place of original substrates) [26,40,46,47]. In such cases, the elemental profile of adulterated juices is evidently skewed compared with unadulterated juices. Elements that are recognized as having the highest discriminating role are B, Ba, Br, Gd, La, Mn, Rb as well as As, Ca, Co, Fe, Na, Ni, Mo, Sr and Zn [40,44,46]. B, Ba, Ca, Fe, Na, Rb, Sr and Zn are recognized to be the best indicators of geographical origin because they make the strongest contribution to the variation of results and the highest power in grouping of unknown objects [40,44,46]. Certain elements, such as Co, Ni, Mn and Mo, are reported to be useful for grouping juices by brand, possibly due to a characteristic elemental composition of certain fruits and its dependence on the elemental soil composition in the region where these fruits are cultivated [44]. For all these reasons stated above, it appears that the elemental analysis of fruit juices can answer three important questions with reference to the contamination with toxic elements:  the nutritional importance;  the authenticity of these products in the market; and,  their wholesomeness and safety [5,11,14,47,48]. Indeed, the accurate and the precise elemental analysis of fruit juices is necessary for assessing the nutritional composition of these beverages, identifying the contamination source and the level of potentially toxic elements [49]. Hence, dependable and reliable determinations of various elements, including nutrients and trace elements, in these products are often used for monitoring their quality, authenticity and provenance (regions or countries of production) [11,40,46]. In provenance, this is achievable because there is strict correlation between concentrations of selected elements and the region of growing or production [4,34,40]. Since the analysis of fruit juices has gained considerable attention and importance with respect to their quality control and safety, our objective is to survey different aspects related to the presence of elements in these beverages and their analysis by atomic and mass spectrometry. We review papers devoted to this subject and reported in the past 20 years. 2. Instrumental techniques for the elemental analysis of fruit juices Spectrochemical techniques predominate in the elemental analysis of fruit juices. However, the matrix of these samples is complex and challenging due to the presence of high amounts of soluble solid substances, including sugars, proteins, amylums and different additives [44]. Fruit juices also contain large amounts of inorganic compounds (i.e. salts of Ca, K, Na and Mg, chlorides, phosphates and sulfates) [6]. Due to high viscosity and high content of dissolved solids, the direct analysis of fruit juices commonly gives many difficulties in sample introduction and non-spectral (matrix effects mainly) and spectral interferences in measurements by flame atomic absorption spectrometry (FAAS) [1], graphite furnace atomic absorption spectrometry (GF-AAS) [6,50], inductively-coupled plasma optical emission spectrometry

(ICP-OES) [35,40] or inductively-coupled plasma mass spectrometry (ICP-MS) [44]. As a remedy, samples of fruit juices have to be mineralized before the analysis to destroy the organic matter or diluted to decrease the content of concomitant substances [35,44]. Matrix effects, particularly in GF-AAS, may be overcome by using proper matrix modifiers [6,42,51]. However, digestion procedures, although very effective in removing the organic matrix of samples, commonly require use of large amounts of reagents that can be an additional source of contamination of the samples being analyzed [44]. We therefore also report attempts to analyze fruit juices by different spectrochemical techniques {i.e. FAAS [12,20,29,33,34,52], GF-AAS [51,53,54], hydride-generation atomic fluorescence spectrometry (HG-AFS) [31], ICP-OES [5,35,43,55] or ICP-MS [36,44,45]}, but without any special treatment of samples other than their dry or wet ashing. FAAS is usually used for the determination of major and minor elements of fruit juices (e.g., Ca, Cu, Fe, K, Mg, Mn, Na and Zn) [1,3,9,10,12–16,18–20,29,30,32–34,38,39,41,42,56]. However, FAAS can also be applied to the determination of trace elements (e.g., As, Al, Cd, Co, Cr, Hg, Li, Ni, Pb and Sn) [12,19,20,29,30,32,56–58] (see Table 3). In general, the use of different extraction methods to pre-concentrate elements prior to measurements by FAAS is quite limited. In the quantification of traces of Al and Pb, the enrichment of these elements by batch solid-phase extraction (SPE) can be made by complexing Al(III) and Pb(II) ions with cupferron and the retention of complexes formed on activated carbon, followed by their elution [58]. Liquid-liquid extraction of complexes of Cd and Pb formed with pyrrolidinodithiocarbamate ammonium (APDC) in sample solutions in the organic phase of methyl-iso-buthylketon (MIBK) can also be used to pre-concentrate traces of these elements prior to subsequent FAAS measurements [1]. An interesting FAAS-based technique, reported for the elemental analysis of fruit juices, is thermospray flame-furnace (TS-FF)AAS. TS-FF consists of a Ni tube, into which a sample solution is nebulized using a ceramic capillary, and a standard burner head of an FAAS instrument, onto which the tube is placed [52]. Compared to standard FAAS instruments, the TS-FF introduces a complete sample to the atomizer and provides a much longer residence time of the sample in the flame. As a result, the sensitivity of FAAS measurements increases by an order of magnitude. GF-AAS is typically used for determining low concentrations of elements (e.g., Al, As, Co, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Zn, Se and Sn) [2,6–8,13,17,18,21–24,27,42,50,51,53,54] (see Table 3). Measurements commonly apply to only one or two elements; three or four elements are determined infrequently. For multi-element analyses of fruit juices or determinations requiring high sensitivity (e.g., samples contaminated with traces of Al, As, Ba, Cu, Cr, Li, Ni, Sr), ICP-OES [4,5,11,22,25,35,37,40,43,47,48,55,59] and ICP-MS [13,26,36,40,44,45,49,60] are used (see Table 3). Both techniques are particularly helpful for measurements of trace and ultra-trace concentrations of elements, where high detection power is required. Axially-viewed plasma instruments are especially suitable for such analysis; they provide limits of detection (LODs) of elements comparable to those reported for GF-AAS but much-improved sensitivity and higher sample throughput [11]. Mercury can be determined using cold vapor-generation atomic absorption spectrometry (CV-AAS) [22,28], while, for As, HG-AFS [31] or HGAAS [22] can be applied. Occasionally, other spectrochemical techniques are used for the elemental analysis of fruit juices {i.e. X-ray fluorescence spectrometry (XRFS) [61]}, when organic soluble solid substances of samples are easily carbonized to form solid residues, and subsequently pelleted before the analysis. Instrumental neutron-activation analysis (INAA) is an example of a non-spectrochemical technique that

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A. Szymczycha-Madeja et al. / Trends in Analytical Chemistry 55 (2014) 68–80 Table 3 Elements determined in fruit juices by different instrumental techniques XRFS Ag As Al B Ba Be Bi Br Ca Cd Co Cr Cs Cu Fe Ga In Hf Hg K La Li Mg Mn Mo Na Ni P Pb Pd Rb S Sb Sc Se Si Sn Sr Ta Te Ti Tl U W V Y Zn Zr a b c d e

INAA [46]

[61]

ICP-OES

ICP-MS

[11,22,48] [11,25] [11,22,35,37,40,48,61] [2,4,22,33,40,43,55] [11,22,35,37,48] [11,22] [11]

[49] [13,26,45,49] [45,49,60] [13,49] [40,45,49] [49]

[4,5,11,22,37,40,43,47,48,55,59,61] [4,5,11,35,37,48] [4,11,35,48] [11,25,35,37,48]

[44,45,49] [26,36,44,49] [26,36,40,44,45,49] [13,26,36,45,49] [49,36] [13,26,36,40,44,45,49] [44,45,49]

FAAS

GF-AAS

[32,57] [12], [58]c

[2] [17,21,24,53,54]

[3,9,10,13–15,18,20,30,32–34,38,41,58,63] [1]a, [19,29,32] [2,19,29] [19,20,29,30,32,57]

[2,17,22,27] [7,22] [7,17,22,23]

[1,3,9,10,14–16,19,20,29,32,38,41], [52]b, [63] [3,9,10,12,14–16,19,20,30,32,34,38,41,56,58,63]

[2,7,8,17,18,22] [7,13,17,18,22]

[46] [61] [46] [46] [61] [61]

[46]

[61]

[4,5,11,25,35,37,47,48,55,59,61] [2,4,5,11,25,35,37,40,43,47,48,55,61] [11] [11]

[4,5,11,22,40,43,47,48,59,61] [46]

[61] [61] [61] [61] [61]

[61]

[46]

[46] [46] [46]

[11,22] [4,5,11,22,37,43,47,48,55,59,61] [4,5,11,35,37,43,47,48,55,61] [4,59] [5,11,22,40,47,48,61] [4,5,11,25,35,37,48,61] [4,5,11,22,37,40,43,47,55,59,61] [5,11,25,35,37,48] [11,61] [43] [22] [11] [40,61] [5,35,55] [11,22,37,40,48,55,61]

[61] [61]

[61]

[40,61] [11,22]

[11] [61] [61]

[46]

[2,4,5,11,25,35,37,43,47,48,55,59,61] [61]

[49] [49] [49] [45] [36]d, [49]d [40,45] [45] [13,36,40,44,45,49] [13,40,44,49] [44,45] [13,26,36,40,44,45,49] [26,36,44,49] [49] [36,40,44,49]

[2,32] [2,3,9,10,12–15,18,20,30,32,34,38,41,63] [12] [2,3,9,10,12–15,20,32–34,38,41,58,63] [2,3,9,10,12,14,15,20,29,32,38,39,41,56] [2,3,9,10,13,14,18,20,30,32,34,38,41,63] [19,29,32,56] [63] [1]a, [12,19,20,29,32], [58]c

[49] [49] [13,49]

[7,17,18,22] [17,22] [7,22] [2,17,22,27]

[6,22,50,51]

[36,40,49] [44,45,49] [49] [49] [49] [36,49] [26], [36]e, [49]e [49] [36,40,44,49] [36] [13,26,36,40,44,45,49] [49]

[29]

[22]

[1,3,9,10,12,14–16,19,20,29,30,32,38,41,52,58,63]

[8,17]

After pre-concentration by liquid–liquid extraction. A thermospray flame furnace was used. After pre-concentration by solid-phase extraction. Other lanthanides (Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). Other actinides (Th).

is successfully used for the multi-elemental analysis of fruit juices after their earlier carbonization and dry ashing [46]. Depending on the sample-preparation procedure executed and the detection technique used, different calibration strategies are applied to quantify concentrations of elements. In FAAS, GF-AAS, ICP-OES and ICP-MS, simple water standard solutions and the method of calibration curves are used, especially for previously digested samples [1,3,6,8,9,11,13–16,19,21,23–26,30–32,37– 44,47,49,53–58,60]. This kind of calibration is also used in direct analysis [5,20,31,33–36,42,45,50,52–54]. In direct analysis, the method of standard additions can also be applied to compensate for interferences originating from the presence of sugars and other matrix constituents in undigested samples [29]. The method of standard additions is also used for GF-AAS measurements [24,27,54] or to compare results with those achieved by the standard calibration method and to detect possible matrix effects

[6,8,21,23,24,31]. Matrix-matched solutions, in reference to the appropriate content of fructose, glucose and sucrose, were used in the analysis of fruit juices where samples were only diluted [48]. Internal standards, added to sample and standard solutions, are mostly used in measurements carried out by plasma-based instruments to compensate for possible variations in their performance {i.e. Lu [40] in ICP-OES, and In [40,60] or Rh [44,45,49] in ICP-MS}. This type of calibration can also be used in GF-AAS measurements (i.e. the addition of As to compensate for problems related to matrix constituents in the analysis of undigested sample solutions of fruit juices) [50]. In determinations of K and Na by FAAS, an ionization buffer (a 0.2% solution of Cs) is used [3]. In Ca measurements by FAAS, a releasing agent (a 0.1% solution of La) is applied [3]. A hotter N2O-acetylene flame is sometimes preferred for the determination of Ca [20]. Matrix modifiers (salts or oxides) are commonly used to

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overcome matrix effects (i.e. incomplete atomization and losses during the volatilization) in measurements made by GF-AAS. Accordingly, the following modifiers are used:  a 1% HNO3 solution for the determination of Al;  a mixture of HPtCl4 and Ni(NO3)2 for the determination of Se [6];  Pd(NO3)2 for the determination of Se [51];  a mixture of Pd(NO3)2 and Mg(NO3)2 for the determination of Pb [27];  a mixture of Ni(NO3)2, IrO2 dissolved in HNO3 and NH4NO3 for the determination of Cu and Pb [42];  (NH4)2H2PO4 for the determination of Cd [27]; and,  Mg(NO3)2 for the determination of Cu and Zn [8], Cr [23] or Al [21,54]. In addition, permanent chemical modifiers are used {i.e. Mo (as (NH4)2MoO4) [8,21,23], W-Rh (as Na4WO4 and (NH4)3RhCl6) [50] or W with a co-injection of Pd (as Pd(NO3)2) [50]}. These modifiers are used to improve the lifetime of GF atomizers and to avoid the formation of refractory carbides. In XRFS measurements, solid standards (pellets) are prepared using simple reagents {i.e. oxides of Al, Cu, Fe, Mg, Mn, Ni, Si, Ti, Zn and Zr, sulfate of Na, chlorides of K and Rb, carbonates of Ca and Sr and phosphates (P)} while adding a carbon powder as a filler [61]. The correction of matrix effects in this case is not required since, after the evaporation of samples to the dryness and their later carbonization, the content of the matrix in all samples is almost the same [61]. The instrument drift is often corrected using an internal standard (e.g., a Compton scattering peak or the analysis of a check sample) [61]. 3. Samples and their preparation prior to elemental analysis The following natural (freshly extracted) and commercially available juices, dispatched in tetrapacks, glass or plastic bottles and cans, are analyzed for their content of major, minor and trace elements: apple [1,7,11,12,16,18,20,21,23,26,29,31–34,36,37,44, 48,51,52,54,56,58,59], apricot [4,20,24], aronia [1], banana [32], berry [12], black current [1,3,33,56,57], black raspberry [3], cashew [52], cherry [4,18,20], grape [8,17,29,35,37,43,44,50], grapefruit

[1,37,55], guava [17,19,32,44,57], lemon and lime [25,29,46,52], mandarin [9,15,38], mango [7,12,16,17,30,32,44,50,56,57], morello cherry [58], muskmelon [61], orange [1,4,7,10–12,14,16,19–23,25, 29,32,35,37,40,42,44,47,49,51,52,54–58,60,61], passion fruit [17, 35,44,62], peach [4,17,20,21,23,24,42,44,52,58], pear [12,18,24,37, 44], pineapple [16,19,21,23,29,35,37,39,41,44,51,54,56,57,61,62], plum [18,32,37], pomegranate [2,5,32,63], raspberry [3], red currant [3], sea buckthorn [13], strawberry [12,35] and other exotic fruits [6,12,42,51,53,54,56,63,64]. Also, peel extracts and juice concentrates can be analyzed [2,40]. For freshly-prepared juices, fruits are primarily harvested, peels removed and the juice extracted [2,3,13–15,38,39,43,46,48,64]. After pressing, juices are centrifuged or filtered to remove any particles [3,14,25]. Before elemental analysis, juices or extracts are kept in sealed bottles and frozen at 14°C [46], 18°C [14] and 20°C [3,40,43], or cooled in refrigerators (0–4°C) [2,9,15,25,31,37,38,48], but rarely stored at ambient temperature [5,10]. For conservation, a 0.1% solution of sodium benzoate is added to samples [2]. 3.1. Direct analysis Since fruit juices are heterogeneous solutions with high concentrations of organic matter and dissolved total solids, possible matrix effects can affect their direct spectrometric measurement (without any destructive pretreatment) or make the introduction of undigested samples unfeasible due to blockages of nebulizer capillaries [47,48]. Nevertheless, other sample-preparation procedures that do not require the sample digestion are used and their benefits are not to be underestimated. Usually, different operating parameters of instruments used for the analysis are optimized to find compromise conditions (i.e. flow rates of gases, sample uptake rates, pyrolysis and atomization temperatures, dilution factors, types and amounts of buffers or modifiers) for all elements measured {e.g., ICP-OES [35,48], ICP-MS [44] or GF-AAS [50,51,54]}. To remove any particles and solid deposits that could disturb spectrochemical measurements, samples of juices are initially centrifuged [29,43,55], filtered [20,33,45] or centrifuged and filtered through 0.45-lm filters [39]. Although such a sample treatment is used in the determination of total concentrations of elements [20,29,43,52], it can also help, along with the digestion of the

Fig. 1. The fractionation analysis of elements in fruit juices based on the centrifugation and the filtration of samples (Adapted from [39], case of Mn in pineapple juices).

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whole juices and resulting filtrates, in retrieving information about the content of elements in particulate (pulp), dissolved and unbound fractions [39] (see the fractionation scheme in Fig. 1). In a similar way, supernatants resulting from the centrifugation of fruit juices can be acidified with aqua regia and diluted with water [55] or only acidified with a 1% HNO3 solution [24] and analyzed for the concentration of elements in the soluble fraction. Undiluted samples of clear juices or their filtrates and supernatants are rarely directly aspirated into the sample-introduction systems of atomic spectrometers [20,29,43,52,64]. When this happens, it relates to the determination of concentrations of minor and trace elements (e.g., Cu, Fe, Mn, or Zn) [20]. Otherwise, samples are diluted with water (in the determination of Ca, Cu, K, Mg, Na, Zn and many other elements), usually at ratios of 1:10 to 1:5 [20,36,52], and introduced to the flame or the plasma. Commonly, samples of juices are acidified in order to release elements bound by different organic matrix components. In this case, the sample-preparation strategy can be as follows. Juices are first diluted 5 [12] or 10 [34] times with water and acidified later with HCl [12] or HNO3 [34] to a final concentration of about 1 M. They can already be diluted 2 times using 0.2% or 2% HNO3 solutions and then centrifuged to remove any particles [5,35,37]. The dilution of samples, by 2 [50] or 20 [45,48] times, can be done with 0.9% [45], 1% [50] or 2% [48] HNO3 solutions. The dilution factor is usually a compromise between the required sensitivity and the maximum allowed organic matter dispensed in a sample aliquot [50]. Another means of preparing fruit juices is acidification with HNO3 to a concentration of 5% [33] or with HCl to a concentration of 2 M [31] and then appropriate dilutions {i.e. 10–25 [31] or 100– 200 [33] times}. Considering the possibility of complexation of elements by different natural bioligands of fruit juices, procedures in which samples are acidified seem to be the most reasonable, since this treatment facilitates the release of bound elements. All simplified sample-preparation procedures described above use limited amounts of reagents and do not require high-temperature treatments, so they help to avoid the risk of the sample contamination and losses of analytes, and they save considerable analysis time. An interesting alterative sample-preparation procedure was reported by Jalbani et al., who proposed ultrasound-assisted extraction of fruit juices [53]. In the procedure described, samples were treated with a mixture of concentrated regents (HNO3, H2SO4 and 30% H2O2) and subjected to sonication at 80°C for 20 min. However, after sonication, supernatant liquids were evaporated on a hot plate and made up to the volume with a 2 M HNO3 solution. 3.2. Dry ashing Sample preparation is a critical step in the elemental analysis of fruit juices, since it determines the time of the analysis and the occurrence of different systematic errors [48]. The evaporation of samples of juices in atmospheric air, usually followed by the dry ashing of resulting remnants, is not so common because it is long and cumbersome. Certainly, when applied, it pre-concentrates measured elements in analyzed juices [1,3,19,46,61], but the risk of losing trace elements due to their volatilization is quite high with this procedure. Dry ashing is used as the sample-preparation technique before determinations of elements in fruit juices by FAAS [1,19], GF-AAS [24,51,54], or HG-AFS [31]. Accordingly, samples of fruit juices are initially dried at 105°C in quartz and platinum crucibles or evaporating dishes and the residues are ashed in muffle furnaces at 400–450°C or even 600°C [54]. Ashing aids {i.e. solid MgO and Mg(NO3)2 or a solution of HNO3 can be added to sample aliquots to facilitate their incineration [31]}. Ashes resulting are dissolved in a 1 M HNO3 solution [1,19,24] or a 6.0 M HCl solution [31],

75

and then diluted to the required volumes or already diluted using a 0.1% HNO3 solution [51,54]. Drying portions of juices at 70–105°C can also be an initial sample treatment before wet digestion. In this case, the solid residues resulting are mineralized later using other protocols [3,4]. In XRFS analysis, large portions of fruit juices (150 mL) are slowly evaporated in special porcelain evaporating dishes on a hot plate at 150–200°C. The dry residues resulting are carbonized later at 300°C in a muffle furnace [61]. Carbonized samples are finally ground and pelleted with cellulose [61]. Such a procedure is also required for elemental analysis of fruit juices by INAA [46]. In this case, portions of juices are placed in porcelain crucibles and heated at 120–550°C first using a stove and then a furnace [46]. The ashes of samples resulting are put into polyethylene (PE) boxes for further analysis. 3.3. Wet digestion In most elemental analysis of fruit juices by spectrometric methods, the samples are wet digested in open-vessel or closedvessel systems. This procedure results in the destruction of the organic matrix and the release of elements into solutions in the form of simple ions. The main aim is to eliminate possible non-spectral interferences related to the presence of concomitant matrix compounds. Such treatment is preferred over dry ashing, but, in closed-vessel microwave-assisted systems, smaller amounts of samples are commonly handled at once, and some trace elements may not be detected at sufficiently low levels. Conventional wet digestion in open-vessel systems, facilitated using hot plates, digestion blocks, heating furnaces or oil and water baths, is quite often used for the decomposition of samples of fruit juices. Usually, relatively high amounts of juices (5–250 g or mL) are decomposed in a concentrated HNO3 solution [7,9,15,17,21,23,27,30,38,51,56,57], a 1% HCl solution [10], a mixture of concentrated HNO3 and HClO4 solutions [14,32], concentrated HNO3 and 30% H2O2 solutions [16,37], concentrated HNO3 and 30% H2O2 solutions with the addition of a concentrated HClO4 solution [58], aqua regia [6], aqua regia with the addition of a 30% H2O2 solution [25], a mixture of concentrated H2SO4 and HNO3 solutions [22,39] or a mixture of concentrated HNO3 and H2SO4 solutions and a 30% H2O2 solution [53]. Catalysts of digestion reactions are also added into reaction mixtures {e.g., V2O5 [7,8,21,23]}. Ratios of the mass or the volume of samples of juices to the volume of digestion reagents (in g/mL or mL/mL) are quite differentiated {i.e. 5:1 [7,8,21,23], 4:1 [32], 3.3:1 [16], 3:1 [9,15,38], 2:1 [30], 1.7:1 [14], 1.5:1 [17,27], 1:2 [22], 1:1 [37], 1:1.6 [25], 1:2 [6], 1:2.4 [51], 1:3 [56,57], 1:6 [58] or even 1:10 [10,53]}. Heating samples with digestion reagents is rather long. The digests resulting are diluted with water and ratios of the mass or the volume of the sample taken for digestion to the final solution volume are commonly 0.3–1.0 {i.e. 1:4.2 [17], 1:3.3 [27], 1:2.5 [6,14,30], 1:2 [7,8,21–23,51], 1:1.7 [9,15,38] or 1:1 [16,25,37,58]}. Higher dilution factors of original samples of fruit juices are also reported {e.g., 1:50 [10,53] or 1:10 [56,57]}, but this is rather disadvantageous due to the higher risk of not determining some trace elements. Closed-vessel microwave-assisted wet digestion is very often applied as a fast method to prepare sample solutions of fruit juices. Samples are exposed to the microwave irradiation in closed, pressurized vessels, so the thermal decomposition of the organic matter and the solubility of elements in solutions are very effective, short and safe. The main advantage of this type of the mineralization is also the relatively small consumption of chemical reagents and the absence of risk of the loss of elements [16]. The decomposition of samples (1–25 g or mL) [2,11,13,18,21,26,28,40,42,44,49,55,60] or dry residues of samples

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[3,4] by microwave-assisted digestion is usually realized in conventional microwave ovens using concentrated HNO3 solution [2,18,26,28,40,48,60], concentrated HNO3 solution with an added catalyst (V2O5) [21], concentrated HNO3 solution with the admixture of a 30% H2O2 solution [3,4,13,16,42,44,47,49,59], aqua regia [55] or a mixture of concentrated HNO3 and H2SO4 solutions followed by the addition of a 30% H2O2 solution [11]. Initial solubilization overnight with concentrated HNO3 is practiced rarely [28]. Instead, samples can initially be heated with an oxidizing reagent on a hot plate [42]. Ratios of the mass or the volume of samples of fruit juices to the volume of reagents used for their digestion (g/ mL or mL/mL) take different values {i.e. 5:1 [21], 2.5:1 [55], 1.9:1 [40], 1.7:1 [48], 1.1:1 [44], 1:1 [26,60], 1:1.5 [11,28,49], 1:2 [42,47], 1:5 [13,16] or 1:8 [2,59]}. Digests resulting are diluted

with water and final dilutions of original samples (in g or mL of a juice sample per mL of a sample solution) are also differentiated {i.e. 1:2 [21], 1:2.7 [40], 1:3 [44], 1:4 [48], 1:5 [42,47], 1:10 [11,60], 1:12.5 [28,49], or even 1:25 [13,26] or 1:50 [16,53,59]}. Minor and trace elements can be determined directly in the resulting solutions [2,40]. For macroelements (e.g., K), sample solutions are also diluted {e.g., 10 times [40]}. Fili et al. [47] reported use of a microwave-focused decomposition system with a reaction coil and delivery tubes for the on-line digestion of small portions of juices (0.5 mL in 1 mL of an 80% HNO3 solution) and the dilution of the digests resulting. In this system, elements were extracted by microwave-assisted pretreatment, which shortened the total time for digestion.

Fig. 2. The fractionation analysis of elements in fruit juices based on the centrifugation and the ultrafiltration of samples (adapted from [22], case of Ag, Al, B, Ba, Be, Ca, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Sb, Se, Sr, Tl and Zn in blood-orange juices).

Fig. 3. The chemical fractionation of elements in fruit juices based on the solid-phase extraction of samples (adapted from [33], case of Ca and Mg in apple and blackcurrant juices).

A. Szymczycha-Madeja et al. / Trends in Analytical Chemistry 55 (2014) 68–80

77

Fig. 4. The determination of the bioavailable fraction of elements in fruit juices based on the in-vitro digestion of samples along with their dialysis (on the basis of [62], case of Fe in citrus fruit juices).

4. Speciation analysis of elements in fruit juices Knowledge about the total concentrations of different elements is certainly valuable in terms of the wholesomeness and the acceptability of fruit juices, but it does not provide any appreciation of the functions of existing species of elements [33]. With respect to the bioavailability and the need for different species of elements, evaluation of the nutritional value of fruit juices made following the determination of total concentrations of elements is therefore rather incomplete [33]. Information about the speciation and the availability of elements in fruit juices is certainly important. Some carcinogenic or genotoxic effects may be mediated by interactions of juice components with transition metals and the presence of particular species [65]. Unfortunately, the overwhelming majority of works devoted to the elemental analysis of fruit juices in the past 20 years does not pay too much attention to this problem. Some authors point out that only carboxylic acids (i.e. citric, tartaric and malic acids) can form complexes with elements, making their availability in fruit juices different from the availability that can be anticipated from total concentrations [21,25]. In a few other works, measurements of available (dissolved) and bound (particulate) fractions of elements are operationally defined and based on initial filtration of samples or their centrifugation [22,39,51,54,55] (see fractionation schemes in Figs. 1 and 2). Accordingly, the analysis of filtered juices, without carrying out their destructive digestion, may provide information on the fraction of elements that is loosely bound. For Mn, this fraction may contribute to 40–100% of its total content

[39]; for Al, it can be about 70–80% [54]; and, for, Se 60–90% [51]. On the basis of measurements of elements in supernatants resulting from the centrifugation of juices, it is suggested that the availability of elements in citrus juices is impaired, probably due to the presence of their complexes with pectins that have a high capacity for binding them [22,55]. The contribution of such complexes of Cu and Zn can reach up to 35–50% of their total concentrations, and that corresponds to a 50–65% share of the free fraction of elements [55]. The share of the pectin complexes of other elements is much lower. Accordingly, Ca, Fe, Mg, Mn and Sr are bound by pectins only 0–20% with respect to the total concentrations of these elements [55]. Hence, the content of this group of carbohydrates has a negligible effect on the nutritional properties of macroelements contained in citrus juices. However, according to Cautela et al. [22], the presence of the free (unbound) fraction of elements can also be lower {i.e. 5% (Fe), 50% (Cu), and 40% (Mn, Zn)}. For non-citrus juices, results obtained for the operational chemical fractionation, based on the application of strong ion exchangers, indicate (see fractionation scheme in Fig. 3) that essential elements (e.g., Ca and Mg) are also primarily present in the form of cationic species [33]. The cationic fraction mentioned contributes 90–100% (Ca) and 96–100% (Mg) to total concentrations of these elements, which can mainly be present as simple ions of Ca(II) and Mg(II) in addition to their stable complexes with citric, malic, oxalic and tartaric acids [33]. Other works confirm that the share of the bioavailable fraction of some elements in fruit juices can be quite small. In these studies

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[22,62], in-vitro bioavailability assays are applied in conditions simulating gastro-intestinal digestion (see Fig. 4) and make a reasonable estimation of the availability of some elements (i.e. B, Cu, Fe, Mn, and Zn). Differences between these two works [22,62] relate to the manner of isolating potentially bioavailable species of elements {i.e. a dialysis bag [62] or a 3 kDa cut-off membrane filter [22]}. In both cases, the role of separation units is to imitate the penetration of the intestinal mucus layer by relatively small species of elements and their passive absorption. The results of these experiments are astonishing. They indicate that there is a great difference between the intake and the uptake of elements from fruit juices. It appears that, in conditions of gastro-intestinal digestion, the bioavailability of Cu, Fe and Zn is very small {i.e. 0– 30% (Fe) and 1–2% (Cu, Zn)}, so the contribution of these elements to RDAs supplied by the consumption of fruit juices is embarrassingly low. However, contributions of the bioavailable fractions of B and Mn are relatively high {i.e. 75–90% [22]}. 5. Quality assurance and control As part of the quality assurance and control of the elemental analysis of fruit juices, samples are carefully handled to avoid their contamination and losses of analytes. As a rule, solutions of reagent (procedural) blanks are prepared along with sample solutions and analyzed to correct instrument readings and final results. Developed and/or applied procedures and methods of the analysis are usually characterized by some selected validation parameters. They mostly include LODs and limits of quantification (LOQs) [6,8,11,16,21–23,27,28,31,35,37,40,42,44,47–54,57,58,60,61], precision [1,6–9,15,16,19,21–25,27,28,30–33,35,37,38,40–42,44– 54,58,60,61] and accuracy [1,6–9,11,15,16,19,21– 24,27,28,30,31,33,35,37,38,40,42,44–54,58,60,61]. The precision (or the reproducibility) of results is usually assessed by undertaking replicate measurements of the same standard or sample solutions. When assessed relative standard deviations of analytical results are below 15%, it is accepted that the precision of the method is satisfactory [49]. The accuracy of results is typically evaluated in two different ways. The most reliable approach to assess the accuracy of results requires the analysis of certified reference materials (CRMs) that match analyzed samples in reference to concentrations of matrix components and analytes [60]. Unfortunately, there is no commercially available CRM for fruit juices. Different water and agricultural CRMs are therefore commonly applied for this aim (i.e. digested at the same conditions as juice samples and analyzed). These CRMs are:  bush, branches and leaves GBW07603 from the Institute of Geophysical and Geochemical Exploration (IGGE) of China [9,15,38];  hay powder V-10 from the International Atomic Energy Agency (IAEA, Austria) [46];  lyophilized vegetable from the National Institute of Hygiene (NIH, Poland) [1];  rye grass BCR 281 from the Institute for Reference Materials and Measurements (IRMM) [28];  beech leaves BCR 100 from IRMM [24];  olive leaves BCR 62 from IRMM [24];  spruce needles BCR 101 from IRMM [24];  lucerne P-Alfaalfa 12-2-03 from PB-ANAL (Slovakia) [49];  apple leaves SRM 1515 from the Nation Institute of Standards and Technology (NIST) [28,42];  tomato leaves SRM 1573a from NIST [42];  citrus leaves SRM 1572 from the National Bureau of Standards (NBS) [8,21,23];

 wheat flour GBW08503 from the National Research Center for Certified Reference Materials (NRCCRM) of China [49];  wheat bread flour P-WBF 12-2-04 from PB-ANAL (Slovakia) [49];  wheat flour from the Central Laboratory of the Agricultural Research Center (ARC/CL) of Finland [27];  potato powder from ARC/CL [27];  rice flour SRM 1568 from NIST [28];  whole milk powder SRM 8435 from NIST [49,50,52];  non-fat milk powder SRM 1549 from NIST [28,50];  infant formula SRM 1846 from NIST [52];  bovine liver SRM 1577b from NIST [45];  bovine liver BCR 185R from IRMM [49];  bovine liver SMU 12-2-01 from PB-ANAL (Slovakia) [49];  bovine muscle BCR 184 from IRMM [49];  spiked skim milk powder BCR 150 from IRMM [49];  water SRM 1643e from NIST [45,48];  water SRM 1643d from NIST [50];  natural water SRM 1640 [48,50]; and,  river water SLRS-3 from the National Research Council of Canada (NRCC) [21,23]. When a good agreement between certified values and measured data is obtained (i.e. recoveries reflecting systematic errors are within the range ±1–15%), it is presumed that the concentrations of elements determined, especially at low levels, are accurate and the sample-preparation procedures and analytical methods used for the analysis are reliable [28,46,49]. Quite commonly, samples of juices are also analyzed by reference methods and results are compared {i.e. ICP-OES with XRFS [61]}. Two different procedures, one as reference and one tested, can also be used to prepare sample solutions by the same detection method, i.e.:  closed-vessel microwave-assisted digestion with conventional open-vessel digestion [16,21];  microwave-assisted extraction in an on-line focused microwave-assisted oven with closed-vessel microwave-assisted digestion in a conventional system [47];  ultrasound-assisted extraction with conventional wet digestion [53]; and,  dilution with a HNO3 solution with conventional open-vessel wet digestion [37,51] or dry ashing [54]. Normal procedure to confirm the accuracy of results is also to use spiked samples and blanks with elements at different concentrations. Spiked sample solutions are subjected to the digestion procedure or appropriately prepared prior to direct analysis and measured to evaluate recoveries of elements added [6– 9,11,15,16,19,21–25,30,31,33,35,37,38,40,42,44,45,47,49–54,60]. Acceptable recoveries that confirm the dependability of results are within the range 90–110%, but values within the range 85–117% also confirm good accuracy [44]. Such a strategy also enables monitoring any losses of elements during the digestion procedure or verifying the existence of matrix effects [11,35]. Finally, measured values for some major elements (K, Ca, Mg and Na) can be compared to values stated by producers on packages [20].

6. Future outlook The determination of total concentrations of elements in fruit juices by spectrometric methods usually requires the destruction of their samples in order to simplify the matrix and to release elements in forms suitable for measurement. This destruction is commonly realized by conventional dry-ashing and wet-ashing

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procedures in open-vessel systems. Unfortunately, such treatment of fruit juices prior to their elemental analysis increases the risk of losses of some of elements in conditions of high-temperature digestions and contamination of samples by the laboratory environment, reagents and vessels used. The sample-preparation procedures mentioned are also time and labor intensive. A more advantageous strategy is closed-vessel microwave digestion, which certainly takes less time and requires smaller quantities of reagents. Although masses of samples are smaller than in other methods, microwave-assisted digestion in closed pressurized vessels is much faster, less prone to contamination and loss of elements, and more reproducible. Interest in analytical methodology, based on the FAAS, GF-AAS, ICP-OES and ICP-MS detection, which avoids complete digestion of fruit juices using laborious, tedious procedures has increased considerably. We expect that this trend will be maintained in the future because simplified sample-preparation procedures, with appropriate dilutions and/or acidifications of samples, are highly desirable and advantageous from the viewpoint of routine analyses carried out to confirm the quality and the safety of fruit juices. This methodology can significantly reduce costs and errors of the elemental analysis and improve the precision and the accuracy of results. However, this approach will also require a suitable selection of measurement methods and operating conditions. We expect that the role of GF-AAS in this type of the analysis with a minimal sample preparation will increase. This method particularly allows for considerable dilutions of samples and the determination of various elements in fruit juices without any protracted sample treatment. Problems related to volatilization of elements can easily be solved by selection of appropriate matrix modifiers or application of internal standards. Also, a new generation of instruments with a continuous light source and an optical system based on an Echelle monochromator and a linear CCD array for spectrum acquisition, so-called high-resolution continuoussource (HR-CS)-GF-AAS, can improve the capability and the performance of this technique (i.e. simultaneous measurement of several elements, and lower LODs). Finally, we can hope to see an increase in the contribution of works, in which, apart from total concentrations of various elements, speciation and/or fractionation of these elements in fruit juices would be determined. It appears that these works would have increased value because they would enable certain functionalities and nutritional properties to be assigned to different species of elements. In this regard, we expect that ‘‘workhorses’’ (e.g., highperformance liquid chromatography (HPLC), with size exclusion and/or reversed-phase-separation mechanisms) would be combined with ICP-OES or ICP-MS detectors and applied for the speciation studies of elements. However, it would also be informative to have approaches to the fractionation analysis of elements by operationally-defined protocols, in which chromatographic and nonchromatographic methods of the separation would be used to differentiate species of elements according to their physical and chemical properties. Acknowledgments The work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry Wroclaw University of Technology. References [1] Z. Krejpcio, S. Sionkowski, J. Bartela, Safety of fresh fruits and juices available on the Polish market as determined by heavy metal residues, Polish J. Environ. Stud. 14 (2005) 877–881. [2] H. Hulya, Orak, Evaluation of antioxidant activity, colour and some nutritional characteristics of pomegranate (Punica granatum L.) juice and its sour

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