Elsevier

Analytica Chimica Acta

Volume 442, Issue 2, 5 September 2001, Pages 191-200
Analytica Chimica Acta

The potential of double focusing-ICP-MS for studying elemental distribution patterns in whole milk, skimmed milk and milk whey of different milks

https://doi.org/10.1016/S0003-2670(01)01170-9Get rights and content

Abstract

The aim of this work was to investigate the potential of DF-ICP-MS for studying in a straightforward manner multielemental distribution patterns in human, cow and formula milks. Thus, the multielemental determination of essential and toxic elements, of primary importance, such as Na, Ca, Mg, Al, Cr, Mn, Fe, Ni, Cu, Zn, Se, Sr, Cd, Hg and Pb in whole milk, skimmed milk and milk whey by DF-ICP-MS is described. Skimmed milk and milk whey were obtained from whole milk by means of physical procedures (ultracentrifugation), without any chemical treatment. In order to destroy organic matrix, each sample was digested (before ICP-MS final analysis) by using a microwave assisted digestion method. A reference material, BCR natural skim milk powder (CRM 063) was analysed for testing the accuracy of the proposed methodology. After validation, the method has been applied to the multielemental analysis of different milk types and fractions. Results of total concentrations and distribution patterns of the above-mentioned elements in the different fractions (whole milk, skimmed milk and milk whey) of human milk, cow milk and formula milk, are presented.

Introduction

Milk is recognised as the most complete food in human diet, because it contains all macronutrients (such as proteins, lipids and carbohydrates) and all micronutrients (elements, vitamins and enzymes). This fact is particularly true in the case of the early childhood, because milk (human, cow or formula) is the only source of nutrients during the first months of a baby’s life.

What is more, infancy is characterised by a very high rate of synthesis of tissue cells, which leads to double the infant’s birth mass in a period of about 5 months. It is therefore essential that all macronutrients and micronutrients are contained in milk in adequate quantities in order to assure an appropriate development of infant’s functions, organs and systems. On the other hand, it is well accepted today that human milk provides such an adequate supply of all nutrients for infants. Therefore, human milk can be used as a reference for nutritional purposes in order to develop cow’s milk and soy based formulas which can replace or supplement human milk in infant’s nutrition.

In this sense, a lot of information has been accumulated concerning the composition of milk in terms of fat, protein and vitamins [1], [2]. However, less attention has been paid to the elemental composition of milk in spite of the great importance of essential elements for nutritional purposes [3], [4].

Essential elements (e.g. iron, copper and zinc) are involved in the form of metalloproteins and enzymes in many biochemical process which play essential roles in human’s life. These roles are more important in the case of infant’s growth and development because during the early infancy the human body shows special sensitivity to essential element deficiency and also to possible excess of these essential elements. Thus, the intake of essential elements from milk has to be adequate in order to assure human health. Moreover, milk can also be a source of exposure to toxic elements which could produce long-term health risks, especially for young children [5], [6]. All the above facts explain the present growing interest in the determination of trace elements in milk owing to the critical importance of milk in human’s diet [4], [6], [7], [8].

Nowadays, inductively coupled plasmas (ICP)-MS has become the standard analytical technique for rapid multielemental analysis of biological samples [9], [10], [11], offering multielemental capability, extreme sensitivity and selectivity. However, this technique suffers from spectral interferences which cannot be resolved by quadrupole ICP-MS instruments [12], [13], [14], [15]. In this sense, the use of a double focusing (DF)-ICP-MS allows to reduce the influence of spectral interferences by using measurements at medium resolution settings (e.g. R=3000) [15], [16], [17]. Furthermore, the DF-ICP-MS working at low resolution (e.g. R=300) provides better sensitivity, superior by one or two orders of magnitude to that of quadrupole ICP-MS [18], [19]. In fact, the use of DF-ICP-MS has grown exponentially in the last years for multielemental analysis of biological samples, including milk [7], [15], [16], [20].

On the other hand, it is accepted world-wide nowadays that the bioavailability of a given trace element depends on the chemical form in which this element occurs. Therefore, speciation studies, focused to obtain a more complete information about the form of trace elements in foods, are today mandatory also in the nutritional field [21], [22].

As a previous step to study the bioavailability of metals in milk samples, to know elemental distributions between the different fractions of milk (including whole milk, skimmed milk and milk whey [23], [24] seems essential as it gives a first idea of elemental speciation (fractionation) in milk. Thus, the aim of this work was to investigate the potential of DF-ICP-MS for studying in a fast and straightforward mode multielemental distribution patterns in the different fractions of human, cow and formula milks. Multielemental determinations in each fraction (whole milk, skimmed milk and milk whey) of Na, Ca, Mg, Al, Cr, Mn, Fe, Ni, Cu, Zn, Se, Sr, Cd, Hg and Pb have been carried out by DF-ICP-MS for this purpose.

Section snippets

Instrumentation

The ICP-MS instrument used was a double focusing inductively coupled plasma mass spectrometer (ELEMENT, Finnigan MAT, Bremen, Germany). This instrument can be operated at three resolution settings (mm=300, 3000, 7500). Optimum instrumental conditions used are shown in Table 1.

An ultracentrifuge, Mod. Jouan MR 22 I, was used in order to obtain skimmed milk and milk whey from whole milk samples. A closed microwave digestion unit (milestone MLS 1200) equipped with a rotor for six Teflon vessels

Mass selection

The development of a method for the rapid multielemental determination of Na, Ca, Mg, Al, Cr, Mn, Fe, Ni, Cu, Zn, Se, Sr, Cd, Hg and Pb by DF-ICP-MS demands a previous selection of isotope masses for each element. Ions of Ga, Y, Rh, In and Tl were selected as internal standards in order to correct for the matrix effects and instrumental drift. When the sought element had two or more isotopes, at least two isotopes per element were monitored. The selected isotopes were the most abundant ones of

Conclusions

The use of a DF-ICP-MS instrument, after microwave assisted digestion, has proved to be a very useful tool for the simultaneous multielemental analysis of major, minor and trace elements in milky samples. DF-ICP-MS instruments allow to overcome many polyatomic interferences (that could not be solved by using quadrupole ICP-MS) by measuring at medium resolution setting (mm=3000).

Skimmed milk and milk whey were obtained from whole milk samples by standard ultracentrifugation methodologies in

Acknowledgements

The authors are grateful to Fundación para la Investigación Cientı́fica Aplicada y la Tecnologı́a, Principado de Asturias (FICYT) for the Grant FC-98-BECA-012 to FARM. Provision of human milk samples from the Department of Neonatology of the Hospital Central de Asturias, is highly appreciated. Finally, the authors are also grateful to J.B. Cannata and his group in the Bone and Mineral Research Unit of the Hospital Central de Asturias for strategic help in milk fractionation.

References (35)

  • E.D Strange et al.

    J. Chromatogr.

    (1992)
  • P.J Mckinstry et al.

    Food Chem.

    (1999)
  • M Krachler et al.

    J. Trace Elements Med. Biol.

    (1999)
  • J.M Marchante-Gayón et al.

    Anal. Chim. Acta

    (1999)
  • A Sanz-Medel

    Spectrochim. Acta B

    (1998)
  • J Szpunar

    Trends Anal. Chem.

    (2000)
  • P Bermejo et al.

    Talanta

    (1997)
  • F Vanhaecke et al.

    Talanta

    (1992)
  • G Bellomonte et al.

    Ann. Ist. Super. Sanitá

    (1990)
  • E. Coni, A. Alimonti, A. Bocca, F. La Torre, D. Pizzuti, S. Caroli, in: S. Caroli (Ed.), Element Speciation in...
  • P Brätter et al.

    Analyst

    (1998)
  • A. Astrup Jensen, S.A. Slorach, Chemical Contaminants in Human Milk, CRC Press, Boca Raton, FL,...
  • A Orkarsson et al.

    Analyst

    (1995)
  • T Prohaska et al.

    J. Anal. At. Spectrom.

    (2000)
  • T.D.B Lyon et al.

    J. Anal. At. Spectrom.

    (1988)
  • C Sariego Muñiz et al.

    J. Anal. At. Spectrom.

    (1998)
  • H Vanhoe et al.

    J. Anal. At. Spectrom.

    (1994)
  • Cited by (0)

    View full text