On 26 April 1986, a nuclear power plant in the Soviet Union republic of Ukraine (Chernobyl) exploded, resulting in a nuclear disaster that affected humans and both plant‐ and animal‐derived foods. The radiation released from the nuclear reactor had a relatively short half‐life but included rapidly ingested types of radionuclides such as iodine‐131. Others included radionuclide isotopes with longer half‐lives such cesium‐134 and cesium‐137 (source: http://www.livescience.com/39961‐chernobyl.html#sthash.NPIQczPe.dpuf; accessed 8 June 2016). According to Nesterenko et al. (2009), levels of I‐131, Cs‐134/137, Sr‐90, and other radionuclides in food products such as milk, dairy products, vegetables, grains, meat and fish were found to be significantly elevated. The detectable measurements of radionuclides were being recorded in food products from many European countries and these were the radionuclide isotopes that could be traced to the Chernobyl disaster, even after several years had passed (Nesterenko et al., 2009). Another nuclear disaster took place in Japan that was caused by the Great East Japan Earthquake that took place on 11 March 2011, which damaged nuclear reactors leading to leaks of radionuclides such as Co‐58, I‐131, I‐132, Cs‐134, Cs‐136 and Cs‐137 into the environment.
In separate incidents, the suspected politically motivated death of individuals targeted by authorities for having different views or ideology using the polonium‐210 radionuclide (a radionuclide with a short half‐life, 138.38 days) has been reported to poison the food to be ingested by the target person (http://nuclearweaponarchive.org/News/PoloniumPoison.html; accessed 7 June 2016). This implies that for polonium‐210 to be used in criminal or terrorist acts, it must be prepared shortly before the intended poisoning and in most cases it cannot be sourced from old discontinued radioisotope sources. However, other radionuclide isotopes with longer half‐lives, such as cesium‐137 and cobalt‐60, may present substances of major concern if they fall into the hands of criminals. This shows that nuclear power plants and radionuclide isotopes, if accessed by criminals, can potentially lead to a serious disaster in food products.
There are only a few radionuclide isotopes that may contribute significantly to human exposure through the consumption of contaminated foods. Mainly fission and activation processes are considered as the likely types of radionuclides that are capable of significantly contributing to human exposure in the event of food contamination, either directly or through nuclear accidents that may contaminate crops and water.
Food poisoning due to radioactive iodine (I‐131) is of high concern due of its rapid migration into milk from cows that graze on contaminated grass or feed. This radionuclide has a tendency to accumulate in the thyroid gland and thus elevates the probability of the risk of thyroid cancer. Another radionuclide isotope of concern in food poisoning includes radioactive cesium (Cs‐134 and Cs‐137), which unlike radioactive iodine, has a long half‐life. However, as with all radionuclide isotopes, exposure or consumption of food or water contaminated with cesium‐137 has the potential to elevate the risk of cancer.
There are other radionuclide isotopes that may be of great concern, if they are used to poison foods, due to their long half‐life. These include radionuclide strontium and radionuclide plutonium and unlike radionuclide iodine‐131 or radionuclide casium, both radionuclide isotopes of strontium and plutonium are relatively immobile in the environment and are therefore of concern more locally where the contamination or poisoning first took place.
Vegetable and plant crops which are grown outdoors may be highly susceptible to contamination by the atmospheric release of radionuclides, from sabotaged nuclear reactors or other sources of radionuclides causing massive radioactive contamination and even over a wide area covering more than one country. Another food product that is of concern is milk in which there is rapid transfer of radioactive iodine and radioactive cesium from contaminated grazing grass/pasture or feed into milk. Radionuclides may be transferred through soil into crops or animals, or into rivers, lakes and the sea where fish and other seafood consumed by humans can adsorb the radionuclides. In many instances, it may be plausible to relate the measurements of radionuclides (e.g. radionuclide iodine‐131) in pasture grass to that in milk, as the levels of pasture grass may provide a useful prediction of the type and concentration to be expected in milk and/or meat. Another group of foods of concern include those obtained from the wilder environment, which may include mushrooms, berries and game meat. Grain and rice may also be subjected to radionuclide contamination.
Radiation due to radionuclide particles may be classified into two main groups, such that the source of radiation may either be ionizing or non‐ionizing. The radionuclide types which are ionizing are further subdivided as described below.
Alpha particles are essentially helium nuclei consisting of two protons and two neutrons (Leikin et al., 2003a,b, 2007; McFee and Leikin, 2005). The alpha particles possess a significant mass and kinetic energy, enough to cause ionization of other atoms or molecules; however, due to the fact that they are massive and possess a +2 charge, they have relatively little penetrating power (compared to gamma rays or X‐rays), such that they cannot penetrate the epidermis of the skin. For this reason, exposure or poisoning due to alpha particles or alpha radiation may hardly imply an internal contamination and internal irradiation hazard, except in cases that involve alpha particle inhalation, ingestion or injection by penetrating wounds where they can thus damage tissues (Christensen and Sugarman, 2007; Leikin, 2005). Of the alpha emitters of concern, polonium, americium and plutonium (radionuclide produced from uranium (239U, 238U)) may attract considerable attention as far as food forensics is concerned (Dyer, 2007). There are more than 20 isotopes of Po with 210Po being the most stable form and unlike other radioactive elements, 210Po has been reported to be relatively safe to transport, but it is very toxic and if its toxicity is compared to hydrogen cyanide for example, then 210Po is several orders of magnitude more toxic than hydrogen cyanide. In terms of the radiotoxicity trend, 210Po > 228Ra > 210Pb > 226Ra > 234U > 238U > 224Ra > 235U.
Contamination due to alpha particles/alpha radiation can be detected using a number of devices including Geiger‐Muller detectors, of urine and blood samples from the victims.
Examples of beta particles include cesium‐137 (137Cs) (McFee and Leikin, 2005). Beta particles, unlike the alpha particles, potentially do pose a significant hazard to internal organs and tissues and they can cause severe burns to the skin and eyes on poisoning or contact occasions.
Gamma particles or gamma rays are high‐energy emitting uncharged particles, which consist of electromagnetic radiation (Leikin et al., 2007).
X‐rays are like gamma particles, in the sense that they are also capable of emitting high‐energy radiation.
Just as with other analytical procedures, the analysis of radionuclides in food and biological matrices (e.g. blood or urine) involves steps such as sampling, sample preparation and detection measurements.
Sampling for radionuclide analysis necessitates obtaining representative samples and the sampling regime is highly dependent on the purpose of investigation. For example, the analysts may be interested in establishing the source and type of poisoning, etc. Therefore samples can be collected from a localized area and/or from the single individual who may happen to be the victim or from a population if the act of terrorism has involved communities. Sampling may also involve collection from suspected routes where criminals may have traversed to the place where the radionuclide might have originated and the type of food that was contaminated.
Other precautions during sampling, such as the possibility for decomposition processes to occur and thus alter the evidence, are unlikely as the decomposition of food or biological matrices does not change the radioactivity. In order to maintain the integrity of samples during transport, freezing as well as the addition of formaldehyde and other additives are normally considered adequate as long as they are checked thoroughly that they do not contain significant amounts of the radionuclide being analyzed/suspected. After sampling, samples need to be subjected to treatment processes aimed at removing interfering and other unwanted molecules that may be present in the sample matrix and which may interfere with the signal of the analyte of interest during measurements. This step is known as the sample preparation step.
Sample preparation in the process of radionuclide analysis is intended to simplify the matrix by reducing the bulkiness of the sample analyte material and also to improve the efficiency of the measurement, lower the detection and also improve accuracy and precision of the measurements. For example, foods and biological specimens such as urine, contain high water content, which is undesirable and thus must be removed before the sample is introduced to the analytical system for measurement. During sample preparation procedures, water can be removed by either drying at room temperature, elevated temperatures, or even by freeze drying. It should be noted that most of the radionuclides of interest in food forensic analysis are not volatile under drying temperature conditions, with the exception of radionuclides such as tritium and iodine. Moreover, for most radionuclides (except tritium and iodine), the water content can be further reduced by ashing, a process which is normally carried out at elevated temperatures, by employing cold ashing using activated oxygen, or by wet ashing using oxidizing acids. The limitation of dry ashing is that it may result in losses, especially if volatile radionuclides such as carbon‐14, tritium, iodine, cesium, polonium and lead are the target analytes of interest and therefore extra care has to be taken to ensure that their integrity is retained.
The bulkiness of the sample can also be simplified by employing acids during the extraction procedures. The limitation of using acids in extraction is that they do not have the selectivity towards radionuclides and therefore all radionuclides (target and non target) will be extracted.
Another sample preparation procedure involves dissolution of the dried/ashed specimen in strong acids, especially if the specimen is to be subjected to chemical analysis. After sample preparation, the separation of the extracted radionuclides can begin.
The separation step plays a crucial role in converting the analytes of interest to a form suitable and compatible for detection and identification by the detecting counter device. The separation of radionuclides may be achieved through a number of processes such as elect redeposition and precipitation, to mention a few.
It should be noted that the real content of the radionuclide being analysed is normally decreasing and this may imply that chemical reactions that normally take place in other general chemistry setups such as precipitation do not have a chance under these circumstances. It is therefore plausible to incorporate small amounts of what is known as inert carrier material of the same element or incorporate a similar element to play a role as carrier (e.g. radium can be used in the place of barium), so that the inert form of the element will display the trend of the behavior of the element as it will proceed in the separation process, while carrying the radionuclide along with it. Another advantage of using the inert carrier is that it is vital in preventing undesirable co‐precipitation or absorption of the analytes to the glassware.
Biological specimens from victims may be urine or blood sample specimens. The selection of analytical measurement strategies and measurement protocols/methods depends entirely on the type of radiation/radionuclide particles, the form of the sample and/or matrix and the extent/quantity of radioactivity. The strategy for the complete analytical procedures and experimental design has to ensure that the sample is prepared to suit and meet the requirements for the intended analytical equipment to be used under its optimal conditions. For food forensic requirements, the measurements related to the total gamma, total beta, or even the total alpha activity on a sample of food are not desirable as far as radionuclide poisoning to humans is concerned. Normally, a proper and acceptable strategy and methods for radionuclide measurements and identification (e.g. for alpha and beta emitters) involve a radiochemical separation of the species (alpha and beta radionuclides) before the identification and it has to include (where necessary) the measurement of the magnitude of energy or half‐life of the separated material for accurate and acceptable radionuclide identification.
In order to generate reliable measurements, alpha emitters are normally prepared as thin sample specimens to minimize the possibility of self‐absorption phenomena, which results in spectra signals with poor resolution. There are a number of techniques for the measurements of alpha radiation emitters (measurement of alpha activity) and they include:
The techniques used for the measurement and counting of beta particles in foods or biological specimens include:
Liquid scintillators are attractive because they offer high efficiency, even for radionuclides with low energy emitting characteristics such as carbon‐14 and tritium.
Another technique that may be considered is the use of the beta spectrometer; however, this approach may not be ideal because each specific beta emitter species has a characteristic range of energies from a minimum of zero to a specific maximum that is applicable that particular beta emitting species.
Gamma radiation is highly energetic and very penetrating and for that reason the detection system to measure gamma/X‐ray emitting species must be built to handle a considerable mass capable of absorbing enough rays/radiation to generate a corresponding signal response. For this reason, solid detectors are the ones mostly used in gamma/X‐ray spectrometric measurements, as they are capable of complete absorption of gamma/X‐ray radiations. Sodium iodide is an example of such detectors; however, despite all the advantages which include high efficiency, sodium iodide detectors are characterized by poor energy resolution and this is their main shortcoming. To address the limitations of sodium iodide detectors, germanium diode detectors are normally employed to provide better resolution where the signal involves highly complex spectra.
Nearly all cases that have involved the use of radionuclides as food poisoning agents have been fatal resulting in terrible loss of life. It is thus expedient that efforts are made to come up with concrete ways of reversing the fatal effects of radionuclides. Moreover, due to their decaying nature, there is a need to have more reliable analytical methods to uncover the evidence precisely, even after the total decay of the radionuclide involved in the food poisoning.