X-Ray Fluorescence in Biological Sciences. Группа авторов

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6 Use of Energy Dispersive X‐Ray Fluorescence for Clinical Diagnosis

       Yeasmin Nahar Jolly

       Atmospheric and Environmental Chemistry Laboratory, Chemistry Division, Atomic Energy Centre, Dhaka, Bangladesh Atomic Energy Commission

6.1 Introduction

      X‐ray fluorescence (XRF) analysis is a method of qualitative and quantitative multi‐elemental detection which is non‐destructive. It is a nuclear analytical instrumental technique designed based on the determination of characteristic fluorescent radiation of a particular element. A suitable point source is applied to de‐excite the inner shell vacancies of sample elements by means of radiation. Of the numerous variants of XRF analysis, two major approaches are wavelength dispersive‐XRF (WDXRF) and energy dispersive‐XRF (EDXRF). They are distinguished from each other by the type of detector used to detect emission spectra, which is particular for each element. The characteristic wavelength of emitted X‐rays from elements obtained by the use of a diffracting crystal is the main focus of WDXRF whereas EDXRF relies on direct measurement of the energy of the X‐rays by collecting ionization produced in a suitable detecting medium. After the emergence of silicon drift detectors (SDD), EDXRF became the most widely used of the two. As compared with EDXRF, WDXRF is quite expensive and has few applications for testing materials in the steel or ceramics industries. In recent years, EDXRF has led over WDXRF and is a powerful tool for elemental analysis to determine major, minor, and trace elements in biological samples. The sample preparation for the EDXRF technique is much easier and therefore less prone to contamination. Generally, no chemicals are used, so system loss is relatively very small. A special feature of EDXRF is that only a very small amount of sample is required, which is in the range mg‐μg of a material. Hence it is suitable for clinical measurement of toxic elements in human body tissue, as the collection of human tissue in large volume/quantity is a big problem. Arsenicosis and autism spectrum disorder (ASD) are the conditions which are associated with the accumulation of toxic elements arsenic (As) and lead (Pb) in the human body respectively. Determining their concentrations in human tissues can give a clear picture of their presence (qualitatively and quantitatively) and evaluate their association with those diseases. In this chapter, EDXRF techniques for the determination of As concentration in human scalp hair for the diagnosis of arsenicosis, and determination of Pb concentration in whole blood samples for evaluating the relationship between lead exposure and blood lead concentration as a function of ASD have been discussed.

6.2 Determination of Arsenic Concentration in Human Scalp Hair for the Diagnosis of Arsenicosis Disease

      6.2.1 Background

      Drinking arsenic (As) contaminated water is the main cause of arsenicosis disease, that ultimately leads to a very painful death for the patient. As poisoning or arsenicosis is a medical condition that occurs due to elevated levels of As in the body. According to WHO the safe limit for As in drinking water is 10 μg/l and according to the government of Bangladesh’s standard it is 50 μg/l [1, 2]. Different studies showed that if a person drinks water with an As concentration more than 150 μg/l, the poisoning can enhance mortality in patients dealing with chronic diseases like cancer, heart disease, and diabetes by around 64%. Nevertheless, it can also be dangerous for patients suffering from normal, non‐chronic diseases.

      The occurrence of arsenic disease (arsenicosis) depends on the ingestion of As compounds and their excretion from the body. Generally As enters the human body in three different ways: (i) inhalation‐by air, (ii) ingestion by water and food, (iii) absorption through skin. It has been reported that 40–60% of As can be retained by the human body after entering into the body[3]. There are many reasons which cumulatively are responsible for the development of the symptoms of arsenicosis disease, viz.; exposure time, the body’s ability to defend against disease, food habits, concentration of As uptake by individuals, duration etc. and generally it is assumed that 2–20 years time is required to develop the symptoms of the disease [4].

      6.2.2 Role of EDXRF

      Arsenate and arsenite are the two arsenic compounds which are carcinogenic in nature and tend to deposit on human hair, nail, bone, skin, etc. As such, the toxic effects of As are usually observed in those parts of the body.

      Analyzing those organs can easily quantify the level of As exposure in that person. But collection of bone, skin, and nail samples from the human body is quite complicated and sometimes becomes impossible compared to scalp hair. More over it was observed that As has a strong tendency to deposit in human scalp hair, as it is composed of keratin, thus hair tissue contains the highest amount of Arsenic compared to other tissues of a suspected patient. At the time of exposure, As combines with hair keratin and becomes trapped in the hair, making it the best sample option for analysis. Nonetheless, hair tissue is a good biological indicator for chronic As exposure as the As excretes slowly through the hair. Therefore, by measuring the concentration of As in scalp hair, it is possible to have an accurate indication of As exposure levels in a body. There are many analytical methods suggested for determining As in scalp hair, but among them EDXRF is the safest and most simple technique, which gives more accurate results as no digestion or chemical treatment is required for sample preparation.

      6.2.3 Collection and Preparation of Hair Sample

      Two to four