The globalization of food markets and the relative ease with which food commodities are transported between countries, means that consumers are increasingly concerned about the origin of the foods they eat.
The food and drink industry is constantly striving to attain improved quality and security for its products. The demands of nutritional value for the consumer and the protection of food brands against counterfeiting and adulteration1 mean that the industry needs to employ state-of-the-art analytical techniques such as multi-element analysis by XRF.
In general, XRF is the only technique that can provide definitive data on the identity and the quantity of essential trace elements and micro-nutrients in food stuffs.2 This can work from a nutritional perspective to check that foods such as cereals, bread and vegetable products contain the stated quantities of iron, zinc, phosphorus and other trace elements. And also for quality assurance/safety, where the food materials may be checked for their point of origin by their geographic trace element profile and concentration, or more importantly checked for potentially poisonous materials such as lead, nickel or arsenic.
Heavy metals in food
Heavy metals can be extremely hazardous contaminants in the human food chain which are are non-biodegradable and have long biological half-lives, meaning they can accumulate in the body. According to the World Health Organization3,4 heavy metals must be strictly controlled in food sources in order to guarantee public health safety. Disproportionate concentrations of heavy metals in food is associated with the etiology of many diseases, specifically cardiovascular, renal, neural, and also bone disease.4
A key reason to monitor the levels of toxic elements in food comes from the increased contamination of the environment in which food is grown.5,6 These metals can reach the food chain through various biochemical processes and ultimately be consumed, biomagnified and threaten human health. Heavy metals are major contaminants of food and the problem is getting increasingly serious on a global scale5 because of poor regulation, contaminated land and adulterated fertilizer. Sea fishing is essential in some areas of the world to feed the population. However, industrial contamination of heavy metals such as mercury and nickel have meant that staple fish such as tuna and particularly shell fish need to examined using methods such as XRF to check for heavy metal contamination.6
Determining trace elements in food with XRF
Molybdenum is an essential trace element vital for both plants and animals in very small amounts. It is a component of several mammalian metalloenzymes including xanthine oxidase, aldehyde oxidase and sulfite oxidase. The relationship between molybdenum deficiency and dental caries, esophageal carcinoma in humans has been established, however there is also an expected relationship between a high intake of molybdenum and a high level of incidence of gout.3 Lead is also a major public health problem in many industrialized countries and is perhaps one of the most preventable environmental diseases.3 Its effects include anemia, neurological dysfunction, and renal impairment.
Trace elements, which can be essential or toxic, play a significant role in human health and diseases, and their main pathway to reach the human body is the food chain. To safeguard human health and to protect environment it is, for example, important to know the level of Mo and Pb in foods and their dietary intake. Hence analytical methods for rapid and routine determinations of Mo and Pb in a complex matrix such as food have been developed.
Determination of elements in a sample by XRF provides better sensitivity than inductively coupled plasma (ICP) methods and sample preparation is much easier as well. The detection limit of the XRF method is usually higher in solution than it is in solid homogeneous fine samples. Solid samples in a pellet form are the most suitable for XRF as thick targets are faster to prepare than thin targets, and have less risk of contamination and loss of elements to be analyzed. For thick solid biological targets Energy Dispersive X-ray Fluorescence (EDXRF) is one of the best methods for the determination of Mo and Pb in foods without prolonged pre- treatment procedures, which are usual in the analysis of biological samples. However, the XRF method needs to consider the quantitative evaluation of the matrix effects,6,7 although by using suitable reference materials this is not a problem.
Food forensics and the role of XRF
Food forensics is a growing and highly technical area of the food industry,16 which aims to determine the source of contamination in a food system.9 Examples of this include the identification of foreign materials or foreign chemicals, detection of unpredicted ingredient interactions, food quality issues, viscosity problems, and food safety issues.
Energy Dispersive Micro X-ray Fluorescence (μXRF) is growing in importance in this field. Similar to more traditional electron-based Energy Dispersive Spectroscopy it uses an EDS- based system for elemental analysis, but differs in three ways:10
Instead of the electron probe typically used for EDS, μXRF uses a focused X-ray probe to produce sample characteristic X-rays
There is less ‘background’ Bremsstrahlung radiation with μXRF because of the lack of atomic particle-based interaction associated with electron probes
Photon-based μXRF displays lower detection limits for heavier elements than electron-based, or SEM EDS
One of the basic advantages of μXRF over bulk XRF is its ability to analyze small particles or small features in conjunction with an SEM. μXRF is typically much better for the analysis of heavier elements. A good example of this is the analysis of foreign material on the surface of raw meat.10 Using cooperative techniques light microscopy revealed the foreign matter as mostly organic material, but each piece also had several small flecks of metal.
A backscatter electron SEM image was able to differentiate metal pieces from organics as bright against the darker organic material allowing alignment of the EDS system and the μXRF probe to the small flecks and to identify the metal present.10 XRF is an excellent method to determine the identity and quantity of heavy elements.
Using XRF to trace food fraud
The food industry has been rocked by a number of scandals in recent years. Some examples are the melamine in infant milk scandal in China and the horsemeat scandal in the UK. Although these last two examples rely mostly on the identification and quantification of organic materials there are also areas where XRF is of great use.
In the spice industry11 common spices such as sisal, pepper, ginger and cinnamon may be found with heavy metal contamination by lead, mercury, iron, copper and zinc. These materials may result from bioaccumulation by the spice plant during cultivation or could result from adulteration of the processed spice.11 In these cases XRF of samples can determine the presence of metal, its quantity and also provide a provenance for the geographical origin of the product.
Tea (Camellia sinensis) is a major agricultural product in Sri Lanka, which produces more than 300 million of kilograms per year, approximately 2% of national GDP. In a recent study12 trace metals and stable isotope ratios in tea samples originating from various regions in Sri Lanka were determined by using X-ray fluorescence analysis and isotope-ratio mass spectrometry to classifying the tea according to its geographical origin. Some 13 elements (Mg, P, S, Cl, K, Ca, Mn, Fe, Cu, Zn, Br, Rb and Sr) were measured in 58 tea samples originating from four production districts in Sri Lanka (Hantana, Thalawakelle, Passara and Ratnapura). The study showed the differentiation and classification of tea samples according to the four regions of origin is possible by analysis of the teas elemental content.
Nutritional analysis in the dairy industry
Milk analysis, dairy analysis, and milk powder testing for iron, phosphorus and calcium, sodium and potassium are some of the essential processes in the regulation of the dairy industry and the QA of infant formula. X-ray fluorescence analysis (XRF) of milk and dairy products is not yet widespread in dairy industry, although the method has a lot of potential, as dried samples may be analyzed directly without any chemical treatment and XRF equipment is easily available.13
XRF spectrometry is a comparative technique and it requires a set of calibration standards in order to perform its quantitative measurements and this can be a difficulty in the dairy industry because of the wide range of sample types. In a recent study13 the concentrations of minerals (Na, Mg, P, S, Cl, K, and Ca) and trace elements (Mn, Fe, Ni, Cu, Zn, Rb, Sr, and Br) in different types of milk, dairy products, and infant formulas were determined using wavelength dispersive X-ray fluorescence analysis (WDXRF).
The technique used freeze dried samples pressed as tablets of 4 g and calibrations were established using available plant and milk standard reference materials. The results demonstrated the suitability of WDXRF for the quantitative analysis of Na, Mg, P, S, Cl, K, Ca, Zn, Rb, Sr, and Br in all of the dried samples. Trace elements such as Mn, Fe, Ni, and Cu were at the limit of detection (LOD) in most samples. The main disadvantage of XRF was the high LODs for trace elements (Cd, Hg, Pb, and As).
Sample preparation for the XRF analysis of food
Food can be a difficult matrix to handle in analyses as there may be wet ingredients or specific areas to examine. In the majority of cases food can be freeze dried and pelleted, at a consistent size, using a Specac manual press, for small scale studies, or an automatic hydraulic press for large samples. Examples of these are the tea and milk studies12,13 mentioned previously.
It is also possible that food forensics requires samples to be analyzed in situ and in these cases a particle can be located in a food item using microscopy (e.g., metal fragments in cheese or sausage) and then analyzed by using a μXRF method. These examples show that by using handheld XRF, μXRF and laboratory based WD XRF and ED XRF the food industry has a powerful technique with a wide range of applications.