Informing Spectroscopists for Over 40 Years

Determination of inorganic elements in foods

Kenneth Neubauer, PhD

PerkinElmer, Inc., 710 Bridgeport Avenue, Shelton, CT 06484, USA.

The determination of inorganic elements in food substances is critical for assessing nutritional composition and identifying food contamination sources. The measurement of inorganic elements is challenging because there is a wide variety of edible substances. As a result, highly diverse matrices must be analysed accurately. The different matrices can lead to numerous interferences on inorganic elements of interest and the interferences can vary with matrix composition.

A key consideration in food analysis is the complex nature of the matrix, both in composition and form. Food substances can be composed of a wide variety of components including fats, proteins, sugars etc. Additionally, most foods are in solid form. The combination of complex matrices and solid samples means that most food substances have to be digested to break down the matrices and convert the samples to liquids, which are more easily analysed by inorganic techniques than solids.

Other complexities involve the inorganic elements of interest, which can be divided into two classes: nutritional and toxic. It is important to accurately determine the levels of both sets of elements to assess both the nutritional and harmful impacts of food substances. Nutritional elements (such as Mg, P and Fe) are present at high levels (mg kg–1), while toxic elements (such as Pb, Hg and Cd) should only be present at trace levels (ng kg–1 or µg kg–1).

Nutritional elements are best determined by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) because of the technique’s ability to handle difficult matrices and measure high concentrations accurately. However, ICP-OES is not useful for toxic element determination because it does not have the sensitivity necessary to measure low levels. Instead, toxic elements are usually analysed by Inductively Coupled-Mass Spectrometry (ICP-MS) because of its ability to effectively measure low-levels of inorganic elements. The complex nature of foods, however, can lead to numerous interferences on the trace elements. Two separate analyses are typically needed to determine both nutritional and toxic elements in foods; the results for toxic elements may require greater flexibility in measuring detection limits.

These challenges can be overcome using cell-based ICP-MS, which has the ability to measure both high and low levels while removing the effects of interferences. Cell-based ICP-MS systems work by introducing a gas into a cell located in the path of the ion beam emitted from the ICP. Interferences and analytes interact differently with the gas, resulting in reduction of the interfering species. As a result, low-levels of analytes can be detected and measured.
Cell-based ICP-MS also facilitates the analysis of elements present at high concentrations. A limitation of conventional ICP-MS is its ability to measure very high concentrations due to detector saturation. However, cell-based ICP-MS allows sensitivity to be detuned for selected elements during an analysis, thus allowing both high and low levels to be determined in the same sample.

An example of cell-based ICP-MS addressing these issues is the analysis of milk powder. Because of the complex nature of this matrix, the milk powder cannot simply be dissolved and analysed. Instead, the powder must be digested and diluted prior to analysis. The results of a typical analysis appear in Table 1, which shows data for both toxic and nutritional elements. Nutritional elements were measured by selectively attenuating their signals (without affecting the low-level analytes), while several of the toxic elements were determined in cell-mode, as indicated in the table. As a result, all elements were measured in a single analysis.

Table 1. Analysis of milk powder (units in mg kg–1).
Analyte m/z Concentration (mg kg–1) Target (mg kg–1)
Na 23 390 399–461
Mg 25 767 759–877
Al 27 0.49 0.46–1.56
P 31 143 160–182
K 39 525 491–341
Ca 42 405 382–458
Cr 52 0.063 0.110
Fe 54 15.2 13.0–16.6
Mn 55 2.22 2.26–2.84
Cu 63 3.57 2.07–2.84
Zn 66 19.0 16.4–20.8
AsO 91 0.003 0.002
Se 78 0.035 0.037–0.057
Mo 98 0.22 0.213–0.291
Cd 114 0.009 0.007–0.017
Sn 120 0.078 0.010–0.100
Hg 202 0.003 0.002–0.004
Pb 208 0.14 0.106–0.174

Bold: toxic elements; Bold italic: analysed in cell mode; Normal: nutritional elements

Cell-based ICP-MS is currently being used in a number of labs to analyse food substances for both nutritional and toxic elements. As the technique matures, its use in the food analysis industry will grow to meet increasing food safety regulations and requirements.