Development of analytical methods for elemental analysis

Transcription

Development of analytical methods
for elemental analysis of durum wheat
in support to food safety
M. Menelao, C. Zoani
ENEA - Italian National Agency for New Technologies, Energy and Sustainable Economic
Development; Technical Unit for Sustainable Development and Innovation of Agro-Industrial
System (UTAGRI);Casaccia Research Centre - Via Anguillarese, 301- 00123 ROMA (Italy)
Corresponding author: Marta Menelao – ENEA UTAGRI (host for degree thesis), C.R. Casaccia – Via
Anguillarese301, 00123 Roma – Tel. +39 06 3048 6202/3738, Fax: +39 06 3048 6258, martamenelao@
gmail.com
Riassunto
Scopo del presente lavoro è lo sviluppo di metodologie analitiche finalizzate alla valutazione del
grado di contaminazione da elementi tossici o potenzialmente tossici di materie prime e prodotti
alimentari, con particolare riguardo alla filiera cerealicola. A tal fine, dopo un esame approfondito
della disponibilità attuale di metodi e materiali di riferimento da impiegare per la caratterizzazione
dei cereali, sono state messe a punto metodologie analitiche basate su tecniche spettroscopiche e
spettrometriche finalizzate all’analisi elementare del grano duro. In particolare sono state definite
le condizioni per la preparazione ed il pretrattamento dei campioni, con specifico riferimento alle
procedure di omogeneizzazione, macinazione e dissoluzione. Sono state poi messe a punto le metodologie per l’analisi strumentale mediante Spettroscopia di Emissione Atomica a Plasma Accoppiato
Induttivamente (ICP-AES) e Spettrometria di Massa a Plasma Accoppiato Induttivamente (ICP-MS),
approfondendo anche gli aspetti legati al controllo delle contaminazioni e delle interferenze spettrali
e di matrice. Le metodologie messe a punto sono state infine applicate per la caratterizzazione di
alcuni campioni di grano duro provenienti da cinque diverse Regioni italiane raccolti nella stagione
2013-2014. Considerando i Limiti di Rivelabilità (DL) ottenuti ed i tenori massimi di elementi tossici
consentiti nel grano duro, le metodologie sviluppate sono risultate idonee ad individuare la contaminazione da elementi tossici o potenzialmente tossici nei cereali e nei prodotti a base di cereali e – in
generale – per gli studi sulla sicurezza alimentare.
Parole chiave: grano duro, analisi elementale, ICP-AES, ICP-MS
Abstract
The aim of this work was the development of analytical methods aimed at evaluating the degree of
contamination from toxic or potentially toxic elements of raw materials and food products, particularly referring to the cereal sector. With this purpose, after a thorough examination of the current
availability of reference materials and methods to be used for cereal characterization, analytical methodologies based on spectroscopic and spectrometric techniques for the elemental of durum wheat
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La Rivista di Scienza dell’Alimentazione, numero 1,
settembre-dicembre
2014, ANNO 43
analysis were developed. In particular, the conditions for sample preparation and pretreatment were
defined, with specific reference to the homogenization, grinding and dissolution procedures. Methodologies for instrumental analysis by Atomic Emission Spectroscopy Inductively Coupled Plasma
(ICP-AES) and Mass Spectrometry Inductively Coupled Plasma (ICP-MS) were then set up, deepening the aspects related to contamination control and spectral and matrix interferences. Finally these
methodologies were applied for characterizing some durum wheat samples arising from five different Italian Regions, cropped in the 2013-2014 agronomic season. Considering the obtained Detection
Limits (DLs) and the maximum permitted levels of toxic elements in durum wheat, the developed
methodologies resulted suitable for identifying cereal and cereal based product contamination from
toxic or potentially toxic elements and – in general – for food safety studies.
Keywords: durum wheat, elemental analysis, ICP-AES, ICP-MS
Introduction
Food safety represents a key issue, especially
because it is strictly associated to the concept of
risk for the consumer health. For this reason, and
to allow the free trade of goods, the European
Union adopted a policy of food protection aimed
at fostering consumer health and food safety,
guaranteeing the regular functioning of the internal market and harmonizing the safety requirements for food and feedstuff, while taking
into account the diversity and the internal differences in food product availability and the important role of traditional products (EU-DG AGRI,
2007). Starting from the “Green Paper on European Food Law” (1997) and the “White Paper on
Food Safety” (2000), on 2002 the EC Reg. n.178
– which introduced the fundamental principle of
an integrated supply chain approach – was implemented, while in 2006 the so-called “hygiene
package” definitively established the principles
of the EU law on hygiene and official controls of
foodstuffs and feedingstuffs intended for human
consumption: integrated controls over the entire
supply chain; interventions based on risk analysis; direct responsibility of the food business
operator for any produced, processed, imported,
marketed, labeled, advertised product; product
traceability along the entire supply chain; active role of consumers infood safety. To do so,
the EU improved the rules on food labelling so
that consumers receive essential, readable and
42
understandable information to make informed
purchase decisions. In addition, for public health
reasons, the new rules reinforce the protection
against allergens (EC Reg. 1169/2011).
Food contamination can be due to many different sources of chemical, physical and biological origin. As far as chemical contamination is
concerned, since the ‘70s WHO identified the
classes of chemicals as a priority and the diet
represent one of the main exposure pathways.In
this framework, toxic and potentially toxic elements are particularly relevant. Several studies
(Armar-Klemesu M, 2000; EU-DG SANCO 2007;
Jia L, 2010; Contam,2014)on the environmental
and/or technological contamination degree and
on the risk of translocation of these substances
along the food chain were conducted, leading to
the definition of intervention strategies aimed at
the reduction of human exposure through diet.
Toxic and potentially toxic elements are among
the more diffuse contaminants commonly present in soil, with important impacts on food safety and consumer health, also considering the
harmful effects that they could produce if accumulated in excess into the organism. They can
be present at different concentration levels in the
different environmental compartments (soils,
surface waters, groundwater, atmosphere) and
can have both natural (e.g. geological substrate,
volcanic emissions, organic substance decomposition, forest fires) and anthropogenic origin.
Development of analytical methods for elemental analysis of durum wheat in...........
They could be introduced in food products also
as residues from primary production or through
contamination from packaging, transformation
and conservation processes.
In order to detect the contamination sources
along the supply chain, prevent or reduce the
risk of foodstuff contamination and introduce
adequate control and alarm systems in productive systems, it is necessary to have sensitive and
accurate analytical methodologies able to identify and quantify chemical and biological contaminants of different origin. Specifically concerning
elemental analysis, atomic spectroscopy (AAS,
ICP-AES) and mass spectrometry (ICP-MS) techniques are of particular importance, because they
permit to define the composition of raw materials and products, determining – simultaneously
- both nutritive elements and toxic and potentially toxic elements (environmental contaminants;
contaminants arising from primary production,
transformation processes and storage).
Durum wheat (Triticumdurum Desf., Triticumturgidum L. subsp. Durum) is a tetraploid
wheat of the Poaceae family, widely cropped for
its transformation in flour. Durum wheat is the
exclusive raw material for the production of
pasta, a product of the Mediterranean diet appreciated all over the World; it represents a large
share of national exports and is one of the major symbols of the Made in Italy. Domestic wheat
production is about 4 million tons/year on average and covers a 55%÷70% market share of the
processing industry needs. In Mediterranean
Countries, durum wheat is the main raw material for bread and couscous preparation; in other Countries the same cereal is used to produce
a wide range of products and often is the main
food source [Flagella, 2006].
The EC Reg. 1881/2006 defines maximum levels for certain contaminants in foodstuffs, among
which Cd, Pb, Hg and Sn. In particular, a maximum level of 0.2 mg/kg wet weight is stated for
Pb, while for Cd it is 0.1 mg/kg wet weight in cereals Cereal grains excluding wheat and rice, 0.2
mg/kg wet weight in wheat grains, rice grains,
wheat bran and wheat germ for direct consumption, soy beans, 0.040 mg/kg wet weight in processed cereal-based foods (starting from January
1st, 2015); lastly, for tin the maximum allowable concentration is 50 mg/kg wet weight for
canned baby foods and processed cereal-based
foods for infants and young children, excluding
dried and powdered products.
In recent years several studies and researches
aimed at the chemical characterization of cereals
and derivatives have been conducted, mainly referring to the identification of contaminants arising from the primary production environment
and from transformation, conservation, transport
and consumption processes and to the development of traceability systems. Particular attention
has been paid also to the identification of markers
to be employed for the authentication and geographical and/or botanical origin demonstration
of raw materials and products. The most employed techniques for elemental analysis include:
spectroscopic techniques, such as ET-AAS and
ICP-AES (Bittencourt , 2014; Ficco, 2009); spectrometric techniques, such as ICP-MS and IRMS
(McBeath, 2013; Kelly, 2005);separative and ifenated techniques, such as HPLC-ICP-MS (D’Amato, 2011; Fang-Jie, 2010); and some methods for
different cereals and cereal products have been
developed: wheat (Podio, 2013), barley (Husted,
2004), rice (Wei, 2014), cereals in general (Quetel,
2011), cereal products (Beltrami, 2011).Reference
methods specific for cereal analysis are produced
by the ISO technical committees TC 34/SC 3 Fruit
and vegetable products, TC 34/SC 4 Cereals and pulses, TC 34/SC 10 Animal feeding stuffs, TC 34/SC
14 Fresh, dry and dried fruits and vegetables and the
International Association for Cereal Science and Technology (ICC). The regulatory references to be considered are: Reg. 152/2009/CE on methods of sampling and analysis for the official control of feed, Reg.
687/2008/CE on procedures for the taking-over
of cereals by intervention agencies or paying
agencies and methods of analysis for determining
the quality of cereals; D.M. 23/07/94 MIPAAF
(Italian Regulation) on official methods of analysis
43
of the cereals and derivatives. Reference methods
specific for toxic and potentially toxic element
analysis in cereals and cereals products have been
developed only by the ICC and concern mineralization of Hg [ICC, 1984], Cd and Pb [ICC, 1990].
Considering the current availability of Reference Materials (RMs) [Zappa G. et al., 2000;
Zoani C. 2014; COMAR dB), against an overall production of about 10400 RMs in total and
about 600 Matrix-RM specific for the agrofood
sector, cereals are the class of foodstuff mainly
represented in the production of RMs for food
and beverages. 163 Matrix-RMs of cereals are
currently available, produced by IRMM (EU 76 RMs), Romerlab-Biopure (Austria - 14 RMs),
IAEA (Austria - 1 RM), BAM (Germany - 3 RM),
WAU (Holland - 3 RMs), IHP (Holland- 1 RM),
LGC (UK - 2 RMs), INRA (France - 2 RMs), IMCT
(Poland - 1 RM), BelGIM (Byelorussian - 2 RM),
FGUP-UNIIM (Russian Federation - 1 RM),
NIST (USA - 12 RM), some Chinese producers
(NRC - 8 RMs, NACIS - 5 RMs, BMEMC - 1 RM,
COCRI - 1 RM, FDSI - 1 RM), Korea (KRISS - 3
RM) and Japanese (NMIJ - 4 RMs, NIES - 3 RM)s.
Largely represented matrices are: maize, wheat
and wheat flour, rice and rice flour. As concerns
certified parameters, they are mainly elements
– constituents or in trace - (145 RMs), followed
in number by mycotoxins (15 RMs) and nutritional properties (12 RMs). 125 RMs produced
by IRMM, certified for the percentage of genetically modified material are then available. Specifically concerning the determination of toxic or
potentially toxic elements in wheat, 9 RMs are
currently available (GBW 08503, NCS ZC73009
and GBW 10011 of wheat flour produced by COCRI – China; B214 and B227 of wheat (straw)
produced by WAU – the Netherland; SRM 1567a
and RM 8346 of wheat flour, RM 8418 of wheat
gluten produced by NIST – USA; ERM-BC382 of
wheat flour, produced by IRMM).
This work was aimed to develop analytical methodologies for evaluating the degree of contamination from toxic or potentially toxic elements of raw
materials and products, with particular reference
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2014, ANNO 43
to the cereal supply chain. Analytical methods
based on spectroscopic and spectrometric techniques for the elemental analysis of durum wheat
were set up. In particular, the operative conditions
for sample preparation and pretreatment, specifically concerning the procedures for homogenization, milling and dissolution, were established.
The methodologies for instrumental analysis by
ICP-AES and ICP-MS were then developed, also
deepening the aspects referred to the control of
contaminations and spectral and matrix interferences. These methodologies were then applied for
characterizing durum wheat samples arising from
five different Italian Regions.
Materials and methods
Specific tests finalized to the development and
set-up of sample pretreatment procedures and instrumental analysis by ICP-AES and ICP-MS were
performed, using a durum wheat test-sample employed also for conditioning all the equipment for
homogenization and grinding. Particular attention was paid to guarantee the homogeneity and
representativeness of sub-aliquots and to avoid
contaminations from contact materials, therefore
all the used vessels, glassware and accessories
were previously decontaminated with an automatic acid (HNO3conc.) reflux system (MilestoneTraceCLEAN®) and then rinsed with ultrapure
H2O (resistivity = 20 MΩ). All reagents were analytical grade or higher. The developed methodologies were then applied to durum wheat samples of different geographical origin. In particular,
5 samples cropped in the 2013-2014 season in 5
different Italian Regions were analysed (W1 –
Puglia, W2 – Abruzzo, W3 – Marche, W4 – Tuscany, W5 – Emilia Romagna). In order to evaluate
the accuracy of results, the Certified Reference
Material (CRM) NIST20 SRM 1567a - Wheat Flour
[NIST, 1988] was submitted to the same analytical
procedure of durum wheat samples.
20 NIST – National Institute of Standards (http://www.
nist.gov/srm/)
Sample pretreatment and dissolution
A sample pretreatment procedure aimed at obtaining durum wheat samples in powder form to
be submitted to the subsequent dissolution procedure was applied. In order to avoid contaminations, all the equipments were cleaned with
ultrapure water (resistivity = 20 MΩ) and then
conditioned with durum wheat, while for milling containers coated with agate and agate balls
were used. Each sample was weighted, homogenized and partitioned in 8 sub-aliquots of about
25 g by means of a Fritsch rotary cone sample divider. Then 1 of the 8 sub-aliquots representative
of each sample was milled, at first more coarsely
by means of a Fritsh vibratory micro mill Pulverisette 0 and then more finely by means of a Retsh
planetary ball mill. Then the dissolution procedure for the complete solubilization was set up
on the finely milled samples. It was performed
by means of an acid attack in a high pressure microwave system Milestone MLS 1200MEGA. The
procedure was optimized by testing different reagent mixtures in different operative conditions,
so to define the better operative parameters for
obtaining a complete dissolution of the samples.
In particular, aliquots of different weights (about
0.100 g, 0.200 g and 0.500 g accurately weighed)
were submitted to a microwave assisted dissolution with HNO3(69.9%V/V) alone ranged from
2 ml up to 6 ml, and with different mixtures of
HNO3 (69.9%V/V) and H2O2 (30%V/V) (HNO3
ranged from 2 ml up to 6 ml, while H2O2 ranged
from 1 ml up to 2 ml), in different conditions:
Wmax =600÷650 W, total time = 20÷25 min, free
pressure rice. The durum wheat samples and the
CRM were then submitted to dissolution by employing 4 ml HNO3 on 0.500 g samples and applying the following digestion cycle: 1 min at 250
W, 2 min at 0 W, 5 min at 250 W, 5 min at 400 W,
5 in at 650 W and 5 min vent (with free pressure
rice). The obtained solutions were then filled up
to the 25 ml final volume with ultrapure water
in glass volumetric flasks and submitted to ICPAES and ICP-MS analysis. All dissolutions were
performed at least in triplicate, while for the W1
sample and the CRM 7 replicates were made in
order to better assess the analytical procedure
reproducibility. Process blank solutions were
prepared and submitted to elemental analysis
under identical conditions.
Elemental analysis
Methodologies for elemental analysis by ICPAES e ICP-MS were set up. ICP-AES analysis
was performed by a Varian Vista MPX (Axial
configuration; simultaneous 1.12 Mpixel CCD
detector). First of all tests to optimize gas flows
basing on signal-to-background ratio (S/B) were
conducted. Then samples were analyzed with a
method allowing to perform the study of spectral interferences and the preliminary qualitative analysis (simultaneous detection of 72 elements). Signal stability tests were carried out,
making consecutive replicate measurements on
ultrapure water and replicate measurements
on multi-elemental standard solutions of the
elements to be quantified. Finally, the following elements were quantified: Al, As, B, Ba, Be,
Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Mo, Na, Ni,
P, Pb, S, Sb, Se, Si, Sn, Sr, V, Zn, Zr. Conditions
for quantitative analysis are reported in table1.
For calibration, multi-elemental solutions were
used. In order to study matrix interferences, calibration solutions were prepared both in H2O and
in HNO3 (69.9%V/V) at the same concentration
as that used for dissolution.
ICP-MS analysis was performed by a Bruker
Aurora M90 (90 degree ion mirror ion optics; Collision Reaction Interface). First of all the ICP-MS
was tuned with 5 μg/l Be, Mg, Co, In, Ba, Ce, Ti,
Pb and Th solution for sensitivity and resolution
optimization and mass calibration. The level of
oxide ions was checked by the CeO+/ Ce+ratio (<
2%), while double charged ions were monitored
by the signal 137Ba++/137Ba+ (< 3%).Signal stability tests were carried out, making consecutive
replicate measurements on a multi-elemental
standard solution of the elements to be quantified and consecutive replicate measurements on a
standard multi-elemental solution of Be, Mg, Co,
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In, Ce e Th (conc. = 10 μg/l), selected so to cover a
wide mass range (Be9, Mg25, Co59, In115, Ce140,
Th232).The following elements (selected considering the ones resulted < DL or near the DL in the
ICP-AES quantitative analysis) were quantified:
As, Cd, Co, Cr, Ni, Pb, Rb, Sb, Se, Sn, Sr, Ti, V e Zr.
In particular, two different methods were developed: the first one in Normal Sensitivity Mode for
Cd, Co, Cr, Ni, Pb, Rb, Sb, Sn, Sr, Ti, V and Zr determination (table 2), and the second one specific
for As and Se determination using the Collision
Reaction Interface for the reduction of interferences (table 3).The isotopes to be investigated were
selected basing on potential interferences and
relative abundances. In order to check/correct
the presence of interferences, for some elements
the methods were optimized by analyzing more
isotopes, even applying correction equations with
respect to isotopes of other elements potentially
interfering.
Results and discussion
Analytical methodologies for elemental analysis (constituents, trace and ultra-trace elements) of durum wheat by ICP-AES and ICPMS were set up.
As concerns ICP-AES analysis, nebulizer, auxiliary and plasma gas fluxes were optimized basing on the effects produced on the S/B ratio of
some elements selected as reference (Table 4). Table 5 shows a comparison between the Intensity
of Emission (Iem) signals obtained by analyzing
the multi-elemental standard solutions prepared
in ultrapure water and those containing HNO3 at
the same concentration used for sample dissolution, so to study matrix effects and evaluate the
effects produced on the signal by the HNO3 itself.
As can be observed, the HNO3 addition tends to
depress the signal intensity for all the elements
(on average between 10 and 30%), except for B for which there are no effects - and for Sn - for
which, on the contrary, a signal increase could
be highlighted. For calibration, standard multi-elemental solutions at a single concentration
level for each element were used; this level was
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established basing on a semi-quantitative analysis carried out by the same method used for the
qualitative analysis and the study of spectral interferences. For some elements, and in particular
for those elements present at an high concentration level or with concentration levels very different among the analyzed samples (Ca, Na, P, S),
the linearity was verified by analyzing standard
solutions at different concentration levels. In the
studied intervals, excellent levels of linear correlation were obtained, with the following coefficients: Ca – 5 ÷ 20 mg/l, R = 0.9996; Na – 5 ÷ 50
mg/l, R = 0.9999; P – 2 ÷ 50 mg/l, R = 0.9998; S = 5
÷ 20 mg/l, R = 0.9995. Elemental DLs in solution,
valuated as valuated as 3sb/Sb, for ICP-AES analysis were: Al – 0.10 mg/l; As – 0.10 mg/l; B – 0.03
mg/l; Ba – 0.001 mg/l; Be – 0.002 mg/l; Ca – 0.001
mg/l; Cd –0.003mg/l; Co – 0.005 mg/l; Cr – 0.003
mg/l; Cu – 0.004 mg/l; Fe – 0.005 mg/l; Li – 0.003
mg/l; Mg – 0.0005 mg/l; Mn – 0.0005 mg/l; Mo
– 0.008 mg/l; Na – 0.01 mg/l; Ni – 0.01 mg/l; P
– 0.05 mg/l; Pb – 0.03 mg/l; S – 0.5 mg/l; Sb –
0.05 mg/l; Se – 0.05 mg/l; Si – 0.05 mg/l; Sn – 0.1
mg/l; Sr – 0.0005 mg/l; V – 0.005 mg/l; Zn – 0.03
mg/l; Zr – 0.005 mg/l.
As concerns ICP-MS analysis, for all elements
external calibration was performed by employing 5 standard solutions and a blank solution;
working ranges were: 0.05 ÷ 5 μg/l for As, Cd,
Cr, Pb, Sb, Se, V; 0.1 ÷ 10 μg/l for Co, Ni, Sn,
Ti, Zr; 0.5 ÷ 50 μg/l for Sr e Rb. In these intervals, excellent levels of linear correlation were
obtained, with correlation coefficient R> 0.9999.
Elemental DLs in solution, valuated as3sb/Sb,for ICP-MS analysis were: As – 0.05 μg/l; Cd –
0.005 μg/l; Co – 0.05μg/l; Cr – 0.05 μg/l; Ni 0.02
μg/l; Pb – 0.04 μg/l; Rb – 0.03 μg/l; Sb – 0.02
μg/l; Se – 0.02 μg/l; Si – 0.01 mg/l; Sn – 0.05
μg/l; Sr – 0.01 μg/l; Ti – 0.05 μg/l; V – 0.02 μg/l;
Zr – 0.08 μg/l.
The used CRM permitted to evaluate the accuracy for analyses both by ICP-AES and ICP-MS in the
two different modes, considering that the certified
values are referred either to elements for which
quantification the ICP-AES technique was applied
(Ca, Mg, P, S, Al, Cu, Fe, Mn, Mo, Na, Zn), and to
Cd, Se, for which quantification the ICP-MS technique was applied. It should be highlighted that,
as reported in the CRM certificate[NIST, 1988], the
minimum sample sizes of 500 mg, equal to the employed analytical aliquot. For all elements, good
recoveries were obtained and measured values lie,
within 1s, in the intervals of the certified values
(except for Mg, where the certified value lies in the
interval of the certified within 2s).
Figure 1 shows the diagrams representing the
elemental profiles (mean value ± standard deviation for the quantified elements) of each durum
wheat sample. These diagrams were built on a
logarithmic scale so to allow the simultaneous
representation of all the elements, also at concentrations of different order of magnitude.The only
one element quantified with both by ICP-AES
and ICP-MS was Sr; the results obtained with
the two techniques were fully comparable for all
samples. In order to highlight the concentration
differences of the different elements among the
samples arising from the different geographical
areas, a histogram on a logarithmic scale showing the total concentrations of all the elements
for all the samples was realized. Figure 2 shows
this graph, together with a scheme summarizing samples in which each element is present at
higher concentrations (where any indication is
reported, no relevant differences are observed
among concentrations in the different samples).
With the aim to detect further correlations, a
Principal Component Analysis (PCA) was performed with the XLSTAT®2014software package
(Addinsoft) used as a Microsoft Excel plug-in(figure 3).In particular, at first a PCA on all the elements (fig. 3a) and them a PCA on a selection of
the elements (fig.3b) – the ones for which more
differences among the samples were highlighted:
Al, As, Ba, Cd, Co, Cr, Fe, Ni, P, Pb, Rb, Sb, Se , Si,
Sn, Ti, V and Zr – were performed. From a first
analysis, samples of the different areas show significant differences in trace elements concentration In fact it is possible to describe the 94% of the
total variability through theF1 and F2. The graphs
show that the major differences are between the
W2 and W5 samples, while for the W1 and W3
samples the used descriptors don’t allow to highlight the different origin. Further evaluations
based on multivariate analysis are underway, in
order to highlight other distinctness and study
the inter-elemental relations, to be put – in case –
in relation with the sources and the mechanisms
of element assimilation by plants.
In order to evaluate the suitability of the developed methodologies for studies on food safety
and therefore for identifying cereal contamination from toxic or potentially toxic elements, a
comparison between the maximum levels set
by the current regulation (EC Reg. 1881/2006)
and the DLs of the developed methodologies
was carried out. Where no limit values are established, in order to evaluate this suitability a
methodology for estimating the maximum levels starting from the WHO recommendations
- and in particular from the Tolerable Daily Intake (ADI) and/or the Tolerable Weekly Intake
(TWI) – was applied: first of all for the elements
regulated by the EU Reg. (Cd, Pb) the ration between the WHO values and the maximum permitted values was calculated; then this factor
was applied to the other elements. Considering
that two different factors were obtained for Cd
(0.035) and Pb (0.125), the limit values obtained
for cereals were expressed as interval: the lower extreme was established by applying the factor of Pb (0.125), and the higher extreme by the
factor of Cd (0.035). Comparing the limit values
with the ICP-AES and ICP-MS DLs, it could be
observed how the developed methodologies allow to detect contaminant concentrations also
well below the maximum permitted levels (table
6); therefore these methods are suitable for food
safety evaluation.
Conclusions
This study allowed to develop and set up analytical methodologies based on ICP-AES and
ICP-MS for elemental analysis of durum wheat
to be applied for food safety and qualification of
47
raw materials and products. The operative conditions for sample preparation (homogenization
and milling) were defined and the conditions
for dissolution were optimized, so to obtain the
sample complete mineralization with the minimum dilution – with advantages in term of sensitivity – and with the minimum use of acidic
reagents, reducing the blanks and the potential
interferences with advantages in terms of detection limits. They were developed:
• an ICP-AES method for the simultaneous
determination of Al, As, B, Ba, Be, Ca, Cd,
Co, Cr, Cu, Fe, Li, Mg, Mn, Mo, Na, Ni, P,
Pb, S, Sb, Se, Si, Sn, Sr, V, Zn and Zr
• an ICP-MS method for the determination
of Cd, Co, Cr, Ni, Pb, Rb, Sb, Sn, Sr, Ti, V
and Zr
• an ICP-MS method for the determination
of As and Se using the Collision Reaction
Interface for the correction of interferences
In all cases the signal stability and the DLswere evaluate and the study of matrix, spectral
(ICP-AES) and isobaric (ICP-MS) interferences
was carried out. For all elements excellent levels
of linear correlation in the studied concentration
ranges were obtained. The analyses on the CRM
NIST 1567a-Wheat Flour showed excellent recovery for all certified elements, including both
elements analyzed by ICP-AES and elements
analyzed by ICP-MS with two different methods.
The developed methodologies were then applied for the characterization of durum wheat
samples arising from five different Italian regions and the obtained results were submitted
to PCA analysis. First results of the PCA analysis
show significant differences in the concentration
of trace elements in the samples of the different
areas. This allows to assume that it is possible to
develop methods for sample recognition based
on elemental patterns.
Performed activities allowed – on the whole
- to make available analytical methods to be
used for characterizing wheat and assessing the
contamination all along the production chain.
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2014, ANNO 43
These methods could be directly applied to durum wheat and durum wheat flour characterization; in addition their applicability (direct or
with adaptation) to other cereals or intermediate
process or processed products along the cereal
supply chain could be evaluated. Considering
the obtained DLs, well below to the maximum
levels (set by the current regulation or calculated
starting from current regulation and TWI/ADI),
the developed methodologies resulted suitable
for identifying cereal and cereal based product
contamination from toxic or potentially toxic elements and – in general – for food safety studies.
Further developments will concern the evaluation of the possibility to directly apply the dissolution procedure on wheat samples as they are
in grain form (without milling) and to employ
analytical techniques permitting the direct analysis of solid samples (durum wheat grains) by
coupling ICP-AES and ICP-MS with Laser Ablation systems.
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settembre-dicembre
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Tables and figures
Table 1 – Operating parameters for ICP-AES quantitative analysis
50
Table 2 – Operative conditions for elemental analysis by ICP-MS
(Bruker Aurora M90) and analyzed isotopes [Normal Sensitivity Mode]
Table 3 – Operative conditions for elemental analysis by ICP-MS (Bruker Aurora M90)
and analyzed isotopes [Normal Sensitivity Mode - Collision Reaction Interface]
51
settembre-dicembre
2014, ANNO 43
Table 4 – Optimization of the gas flows for ICP-AES analysis according to the signal-to-background ratio
(S/B)(Varian Vista MPX; Power = 1.2 W, plasma gas flow = 15 lmin-1)
Table 5 – Study of the matrix effects on ICP-AES analysis - comparison betweensignals (Iem)
for standard solutions prepared in H2O and standard solutions containing HNO3
52
Table 6 – Comparison among instrumental DLs and maximum permitted concentrations of toxic
and potentially toxic elements
a) Wheat grains; b) cereals; c) cereal products for infants
*) Calculated with respect to TWI; **) calculated with respect to ADI
Figure 1 – Elemental profiles of durum wheat samples (mg/kgfresh weight)
53
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2014, ANNO 43
Figure 2 – Element distribution in durum wheat samples (mg/kgfresh weight)
54
Figure 3 – PCA analysis on durum wheat samples
3a) all the elements - 5 variables, 28 observations
3b) elements for which the major differences
between samples were highlighted – 5 variables, 18 observations
55

Development of analytical methods for elemental analysis

Transcription

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