Determination of trace metals in spirits by total reflection X-ray fluorescence spectrometry

Accepted Manuscript Determination of trace metals in spirits by total reflection X-ray fluorescence spectrometry

G. Siviero, A. Cinosi, D. Monticelli, L. Seralessandri PII: DOI: Reference:

S0584-8547(17)30627-4 doi:10.1016/j.sab.2018.03.006 SAB 5394

To appear in:

Spectrochimica Acta Part B: Atomic Spectroscopy

Received date: Revised date: Accepted date:

20 December 2017 7 March 2018 12 March 2018

Please cite this article as: G. Siviero, A. Cinosi, D. Monticelli, L. Seralessandri , Determination of trace metals in spirits by total reflection X-ray fluorescence spectrometry. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sab(2017), doi:10.1016/j.sab.2018.03.006

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ACCEPTED MANUSCRIPT Determination of trace metals in spirits by Total Reflection X-Ray Fluorescence Spectrometry G. Sivieroa*, A. Cinosia, D. Monticellib, L. Seralessandria a

GNR s.r.l., via Torino 7, 28010 Agrate Conturbia (NO), Italy

b

Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, via Valleggio 11,

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22100 Como, Italy

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*Corresponding author: [email protected]

Abstract

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Eight spirituous samples were analyzed for trace metal content with Horizon Total Reflection XRay Fluorescence (TXRF) Spectrometer. The expected single metal amount is at the ng/g level in a mixed aqueous/organic matrix, thus requiring a sample preparation method capable of achieving

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suitable limits of detection. On-site enrichment and Atmospheric Pressure-Vapor Phase Decomposition allowed to detect Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Sr and Pb with detection limits

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ranging from 0.1 ng/g to 4.6 ng/g. These results highlight how the synergy between instrument and sample preparation strategy may foster the use of TXRF as a fast and reliable technique for the determination of trace elements in spirituous samples, either for quality control or risk assessment

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purposes.

1. Introduction

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Keywords: TXRF, spirits, metal traces, Vapor Phase Decomposition, on-site enrichment.

Determination of trace metals in spirits, which are characterized by a complex matrix, usually

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requires sample treatment and dedicated instrument calibration[1]. Ethanol is above 15% in volume, whereas the rest consists of water, higher alcohols, sugars, added flavorings and minor components below 1%[2]. Contaminants can enter the product through raw materials, as additional ingredients during the process or as impurities from the process equipment itself, including aging and storage[3]. Despite some of these metals are nutrients essential to humans, an excessive intake may cause adverse effects[4]. Moreover, others are classified as carcinogenic (Cr(VI), As, Cd) or possibly carcinogenic (Ni, Co, Pb)[5].

In spirits, the presence of trace metals and metalloids is poorly regulated:

according to current European Union (EU) legislation for contaminants in food[6], maximum limits are set for As, Sn, Cd, Hg, Pb for several categories, not including spirits. Moreover, European Council Directive 98/83, regarding the quality of water for human consumption[7], set additional 1

ACCEPTED MANUSCRIPT maximum levels for Cr, Mn, Fe, Ni, Cu, Se; however, applying these limits to spirits would be clearly incorrect, considering the difference in expected daily consumption. In some cases, either national legislation has set maximum levels, e.g. for Cu (1 mg/l) and Fe (8 mg/l)[8], or extrapolation from maximum limits in water has been proposed (EU AMPHORA) [9]: nevertheless, no globally recognized thresholds have been set. Additional concern is generated by unrecorded alcohol, which accounted for the 25% of the global consumption in 2012[10], being prone to increased contamination levels.

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Beyond consumer safety, trace metal concentration was investigated also as a possible fingerprint for counterfeit detection in a wealthy business such as Scotch Whisky[1][11] manufacturing.

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Analytical techniques of choice for trace metal investigations in spirits are Electrothermal and Flame Atomic Absorption Spectroscopy (ETAAS, FAAS), Inductively Coupled Plasma Atomic

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Emission Spectroscopy and Mass Spectrometry (ICP-AES, ICP-MS) and electrochemical methods: to the best of our knowledge, only a few publications involving TXRF spectrometry determined the metal content in distilled alcoholic beverages. Moreover, the investigated sample preparation

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required in some cases either incineration and nitric acid leaching[12] or Cold Plasma Ashing[11]. Application of TXRF to alcoholic beverages has been mainly focused on wines[13], which however

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differentiate from distilled products by lower ethanol content and higher amount of mineral elements. According to literature[14], concentration of K and Ca for wines ranges from 300 to 1500 mg/l and from 10 to 1000 mg/l, respectively, while those for spirits from 0.3 to 38 mg/l and from 0.4

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to 4 mg/l.

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Digestion procedures do not affect the cations content, thus their large amount is detrimental to the detection capabilities of metals of interest, because of the increase of both the residual mass on the reflector and the background signal [15]. On the other hand, spirits are suited to on-site enrichment coupled with Atmospheric Pressure-

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Vapor Phase Decomposition (AP-VPD), proposed previously in the petrochemical field by some of the authors: together they could be an effective treatment approach, combining improved limits of

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detection[16] and the minimization of contamination from reagents and vessel[17]. The aim of the present work is to show how the synergy between an efficient sample preparation and TXRF spectrometry can fit the purpose of determining some key trace metals and metalloids in spirits.

2. Materials and Methods

2.1 Ethanol blank spiking and spirits Before analyzing spirits, ethanol blank measurement (96 vol%, Carlo Erba Reagents) and recovery tests for the elements of interest were performed. The latter was carried out by spiking ethanol with an ICP multi-element standard solution IV Certipur®, comprising Ag, Al, B, Ba, Bi, Ca, Cd, Co, Cr, 2

ACCEPTED MANUSCRIPT Cu, Fe, Ga, In, K, Li, Mg, Mn, Na, Ni, Pb, Sr, Tl, Zn at 1000 mg/l in 6% nitric acid. Recovery for arsenic was performed employing an arsenic ICP standard solution Certipur® at 1000 mg/l in 3% nitric acid. According to previous studies regarding trace metals in distilled beverages[1][18], concentrations were set at 24 and 88 ng/g. Eight spirituous beverages were collected from different markets: two Chinese Bai jiu (1 and 2), two Italian Grappa (1 and 2), one homemade Albanian Raki, one Flavored Ukrainian Vodka and

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two Whiskies (American and Scotch). 2.2 Spectrometer

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A benchtop TXRF spectrometer (Horizon, G.N.R. Italy), equipped with a 600 W Molybdenum XRay tube was used to collect the data. The emission spectrum from the source was

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monochromatized to Mo-K (17.44 keV) by a multilayer before exciting the sample, whose fluorescence signal was then collected by a 25 mm2 Silicon Drift Detector. The automatic sample

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changer could host up to 12 samples. Counting time for each specimen was 600 s, after correction for detector dead time.

Analysis software featured automatic background and spectrum identification and fitting;

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quantification was achieved through the Internal Standard method and relative sensitivity curves. In contrast to traditional analytical techniques for trace metal identification, no matrix-matched

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calibration curves are required. Outlier detection was performed by using modified Z-score[19].

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2.3. Sample preparation

Sample preparation steps followed ISO/TS 18507[20]: the sample was spiked with gallium internal standard (Gallium ICP standard in HNO₃ 3%, 1000 mg/l Certipur®) in a centrifuge tube. After homogenization, 8 microliters were pipetted onto a siliconized quartz reflector (Serva Silicone

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solution) and dried at 80 °C on a hot plate. In order to reduce the Limit of Detection (LOD), on-site enrichment was performed[16] by repeating

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the deposition on the same reflector up to 10 times: this is a distinctive advantage, related to the deposited mass-dependence of the fluorescence signal in thin film approximation[21], which is one of the basic requirements for the TXRF analysis to be valid. In this approximation, according to Klockenkämper and von Bohlen[22], the net area (intensity) Nx of a characteristic peak belonging to an analyte spectrum is related to its concentration cx (mass fraction) through a sensitivity factor Sx given by equations 1a and 1b, where  is the sample density and t its thickness. Ex and G are factors related to fluorescence excitation efficiency and geometry, respectively. Thus, it is straightforward to see that intensity is linearly dependent on analyte mass per unit area, mx/A (1d). 𝑁𝑥 = 𝑆𝑥 𝑐𝑥

(1𝑎)

𝑆𝑥 = 𝐸𝑥 ∙ 𝐺 ∙ 𝜌 ∙ 𝑡

(1𝑏) 3

ACCEPTED MANUSCRIPT 𝑚𝑥 = 𝑐𝑥 𝑚

(1𝑐)

𝑁𝑥 = 𝐸𝑥 ∙ 𝐺 ∙ 𝜌 ∙ 𝑡 ∙ 𝑐𝑥 = 𝐸𝑥 ∙ 𝐺 ∙ 𝑚𝑥 ⁄𝐴

(1𝑑)

The limit of detection for the analyte x is calculated as three times the standard deviation of background signal area Nback below its fluorescence peak, according to equation (2)[23]. By using equation (1a) and (1b) in (2), the LOD can be expressed as a function of the mass per unit area m/A on the reflector, thus showing that on-site enrichment allows to improve the detection

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capabilities, as long as thin film approximation is valid[16].

(2)

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𝐿𝑂𝐷𝑥 = (3𝑐𝑥 √𝑁𝑏𝑎𝑐𝑘 )⁄(𝑁𝑥 ) = (3√𝑁𝑏𝑎𝑐𝑘 )⁄(𝐸𝑥 𝐺 ∙ 𝑚/𝐴)

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It is well known that, after alcohol evaporation, non-volatile organic matter constitutes a fraction of the residue left on the reflector. Taking into account the increased amount due to on-site

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enrichment, in some cases it was apparent, even by naked eye observation, that the deposited mass was far beyond the critical one for the TXRF technique to work, i.e. matrix effect could not be avoided[22].

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Accordingly, AP-VPD was performed directly on some of the samples deposited on the reflectors, by placing them at the bottom edges of a large beaker. A small beaker filled with a HNO3+H2O2 (3:1) mixture was placed at the center of the larger one, so that it was surrounded by the reflectors.

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A watch glass was placed as a lid on the large beaker before moving the assembly on a hot plate

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at 120° C[17]. In this way, a protected reaction environment was obtained: moreover, the nitric acid vapor treatment is almost contaminant free. Typical digestion time was between 20 and 60 minutes. The procedure can be alternated with on-

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site enrichment steps in order to get a more effective digestion. Each sample was prepared in triplicate, both for the treated and untreated specimens. In comparison with other digestion techniques used in analysis of spirits by TXRF, AP-VPD

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appears less time-consuming: Cold Plasma Ashing took 2 hours at 75W [11], while incineration 24 hours at 500 °C, followed by nitric acid leaching.

3. Results and discussion

3.1 Recovery test Recovery test results for the solutions described in section 2.1 are reported in Table 1 for the elements of interest: all are within the range 80%-110% set by Codex Alimentarius Commission Procedural Manual[24] for 100 ng/g concentration level. The best results are obtained for the 88

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ACCEPTED MANUSCRIPT ng/g one, while the 24 ng/g ones show a tendency of underestimating elemental concentrations, especially Mn, Co and Ni. As far as contamination is concerned, ethanol blank measurements showed levels of Fe, Cu, Zn below 4 ng/g. Corresponding LODs, calculated according to equation 2, were compared with those set by EU for drinking water[7], which can be considered as the lowest reference levels for beverages: the detection capabilities are compatible. As expected from theory[25], LOD improves with atomic number (Z) for the same fluorescence line series, K in this case. The value for lead

Ethanol

EU water 5 5 20

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0.58 0.37 0.32 0.27 0.22 0.22 0.19 0.09 0.14 0.27

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LOD (ng/g)

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Cr Mn Fe Co Ni Cu Zn As Sr Pb

Recovery Value (%) 88 24 ng/g ng/g 102 100 96 82 86 98 97 84 93 86 95 92 98 87 96 96 98 87 105 97

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breaks this trend, because the characteristic peak belongs to L lines series.

2 200 1 1

Table 1. Recovery test values and calculated LODs. Required values in drinking water according to EU

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legislation are reported for comparison[7].

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Given the chosen excitation energy (Mo K), cadmium and antimony were not considered, because their available fluorescence lines (L) have a low sensitivity and are strongly interfered by K and Ca K lines, usually expected in real spirits. As regards mercury, the described sample

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preparation may induce evaporation and loss of this element, due to the high vapor pressure of

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elemental mercury and some of its compounds[26].

3.2 Spirituous samples

AP-VPD treatment was necessary for Grappa 1 and 2, Vodka and the Whisky samples, according to the procedure described in section 2.3. The spectra after 10 on-site enrichment only (labelled “on-site”) and after additional VPD treatment (labelled “on-site + VPD”) for sample Grappa-1 are reported in Fig.1. A high background and a pronounced Compton scattering peak are visible for the ”on-site” sample, whereas

a strongly improved signal-to-noise ratio and the appearance of

additional fluorescence peaks were registered when the Vapor Phase Decomposition was applied. Limits of detection remarkably improve, e.g. for Cu LOD is reduced from 6 ng/g to 0.2 ng/g (see 5

ACCEPTED MANUSCRIPT Table 3 for a collection of calculated LODs). It is worth noting that, in addition to the fluorescence peaks characteristic of the elements of interest, i.e. those for which the recovery test was performed, the simultaneous multi-elemental detection capability of the technique provides information on additional analytes, among which K, Ca, Ba, V, Br. Although they may be of limited interest for risk assessment, they could be exploited for additional investigations such as

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authenticity studies.

Figure 1. Example of spirit spectrum before (red dash-dot) and after (black solid) AP-VPD treatment. The

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improvement in detection capabilities after treatment is apparent. Note that the y-axis is in log scale.

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Results for analyte concentrations are reported in Table 2, while LOD values in Table 3 (ranges refer to LOD calculated in different samples): it is apparent that there is a large variability in both concentrations and limits of detection, spanning one or two orders of magnitude. Remarkably, for each analyte the LOD range minimum value is similar to that of the recovery test samples, showing that the most of the organic matrix has been effectively removed. Incidentally, reported LOD values in wines are much higher, between 0.01 and 0.1 mg/l [15], for the reasons outlined in section 1. LOD median values for spirits are compared to those available from the literature: in particular European Food Safety Authority (EFSA) results, regarding the data collection call DATEX-20080002 [27] and -001222 [28] for Pb and As in foodstuffs, show that the median LOD values for alcoholic beverages are of the same order as our experimental ones. The point to note here is that 61% of that data was collected employing AAS or ICP-MS, which are the recommended 6

ACCEPTED MANUSCRIPT techniques for trace metal determination in foodstuff [29][30]. Bai-jiu samples, investigated by ETAAS after treatment with HNO3 and HClO4 [18], have detection limits one order of magnitude higher. As regards Mn, Fe, Cu and Zn, the detection limit ranges are similar or slightly better than those reported in the literature on Whisky after Cold Plasma Ashing[11] and Venezuelan Firewater with TXRF[8]. Similarly, Cunha e Silva et al.[12] reported LOD from 8 to 35 ng/g for these analytes in sugar-cane spirits after incineration.

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On the one hand it must be pointed out that this comparison is purely indicative, taking into account that the procedures for LOD estimation are different from one technique to the other, e.g.

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for ICP-MS, ICP-AES it is usually derived from standard solutions (even single element) irrespective of the real matrix. On the other hand, comparison among results from TXRF

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measurements in similar matrices may be sound, if the formula for calculation is reported, as usually done.

As far as concentrations are concerned, the average Relative Standard Deviation (RSD) for the

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triplicate measurements of analytes of each sample is below 20%, while its median value is below 11%. Taking into account the small concentration values, the dispersion of data is compatible with

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a minimal or null contamination effect due to the sample preparation procedure. Considering the criterion for Limit of Quantification (LOQ) set by Codex Alimentarius[24] and assuming that current concentration corresponds to the Maximum Level (ML) for the analyte, 81% times the corresponding LOD, thus showing a good

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of the measured values are above two

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compliance.

Having a look at the data, a few considerations can be performed: arsenic was detected in both Bai-jius, Grappa 1 and Vodka at levels compatible with previous findings[18], which seems not to be a concern. The likely source for Bai-jiu samples is the raw material, i.e. rice[28]. On the other

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hand, lead was present in considerable amount in Homemade Raki sample, five times larger than set limit for drinking water (10 ng/g)[7] but still below that extrapolated by AMPHORA project[9].

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Interestingly, the corresponding copper amount is about 9 g/g: the likely source of both metals is the still and the plumbing used for distillation. In fact, traditional distillation of pomace (marc) spirits, such as Raki and Grappa, is performed in a copper still, as traditional Scotch Whisky, according to Ibanez et al.[3]. The amount of lead could be a hint of the quality or age of the apparatus.

Cr Mn Fe Co

Bai jiu 1

Bai jiu 2

Grappa 1*

16.3±1.6 28.1±0.7 185.7±0.1 2.3±0.8

2.2±0.2 5.1±0.8 79.5±1.6 0.9±0.5

1.7±0.3 3.8±0.4 34.5±0.2 0.5±0.02

Grappa 2*

Hom. Raki

(ng/g) 2.1±0.4 2.1±0.8 6.7±0.1 1.8±0.5 22.7±14.2 16.4±2.2 1.2±0.3 <0.8

Vodka*

American Whisky*

Scotch Whisky*

<0.9 246.4±8.7 22.3±5.1 0.91±0.02

2.5±0.7 5.0±0.5 14.9±0.5 <0.8

8.4±0.7 43.2±0.3 69.3±4.7 1.31±0.01

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33.2±1.3 31.3±0.9 61.6±3.3 10.1±3.8 115.2±2.3 17.7±0.3

3.5±0.1 6.8±0.4 11.8±0.5 2.1±0.8 15.0±0.1 6.0±0.1

1.9±0.1 199.5±9.2 61.8±1.0 1.1±0.1 4.2±0.1 4.6±0.4

22.7±0.1 1234±73 258.8±19.5 <0.34 2.5±0.3 2.3±0.5

11.2±0.6 8977±703 64.4±7.5 <2.1 1.6±0.1 52.2±2.7

3.9±1.9 3.8±0.2 8.3±0.01 1.9±0.1 7.2±0.2 1.1±0.3

11.0±0.8 31.2±0.4 11.4±0.2 <2.7 55.1±1.5 1.1±0.06

7.14±0.04 152.9±0.2 11.0±4.2 <1.6 5.73±0.04 1.3±0.5

Table 2. Average results of the triplicate measurement for spirits in ng/g with corresponding standard deviation. “<” denotes value below detection limit, while “*“ treatment with AP-VPD.

Whisky ICP [11]

2 2

1 2

1 1 1 2

2 4

Venez. TXRF [8]

Bai jiu ETAAS [18]

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1.0 0.6 0.6 0.6 0.4 0.3 0.3 1.8 0.3 0.7

Whisky TXRF [11] (ng/g)

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0.3-2.5 0.2-1.4 0.2-1.2 0.2-1 0.2-0.8 0.2-0.8 0.2-0.7 0.2-4.6 0.1-0.6 0.3-1.8

EFSA median [27][28]

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Exp. median

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20 19

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Cr Mn Fe Co Ni Cu Zn As Sr Pb

Exp. range

210 10

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Table 3. Comparison of Limits of Detection for this work (Exp.) and literature. Details in text.

4. Conclusions

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It was shown that the Horizon TXRF spectrometer, coupled with AP-VPD sample treatment and on-site enrichment, is an effective means of determining metal traces in spirits, with multi-element

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detection capabilities and LOD as low as 0.1 ng/g. Knowledge of the raw materials and production processes made the formulation of hypotheses about their origin possible. On-site enrichment exploits the advantages of thin film approximation in TXRF analysis, i.e.

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inverse proportionality between limits of detection and deposited mass on reflector. AP-VPD sample treatment allows to perform the digestion process directly on the deposited

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specimen, with minimum risk of contamination from reagents, vessels and environment: with one reflector it is possible to measure the spectrum before and after treatment, thus giving the user the chance of removing organic matrix contribution only when really needed. In this way, volatile elements such as Cl and Br could be preserved in the sample. Moreover, the required experimental setup, consisting of a hot plate and affordable glassware and reagents available in the market, is simple and cheap, thus suitable for low-budget laboratories. Extension of the procedure to the quantification of elements of toxicological relevance like Cd and Sb requires a different X-Ray source, capable of exciting their K lines: this will be the object of scheduled investigation with the Horizon spectrometer. More generally, the sample preparation method used in this work is peculiar to TXRF technique: it has been successfully exploited for spirits and, previously, in the petrochemical field. It is thus 8

ACCEPTED MANUSCRIPT foreseen that this procedure could be extended to other application fields, featuring samples characterized by an organic matrix with low amount of residual trace metals. As a final remark, the growing concern regarding manifest or suspect toxic effects of some elements demands risk assessment studies, where collection of a lot of data is needed. The simultaneous multi-element detection capabilities of TXRF spectrometer, the independence from calibration curves and the internal standard quantification method coupled with low detection limits, granted by the sample preparation method, may constitute an ideal analytical solution aimed at fast

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and reliable data collection.

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ACCEPTED MANUSCRIPT

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights for manuscript “Determination of trace metals in spirits by Total Reflection X-Ray Fluorescence Spectrometry”.

Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Sr and Pb were detected in eight spirits by TXRF

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On-site enrichment and AP-VPD allowed to get detection limits from 0.1 to 4.6 ng/g

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TXRF efficiently combines easy detection and minimal sample preparation

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Application to quality control or risk assessment is envisaged.

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