## 1 Introduction
Co exposure is increasing as cobalt demand rises worldwide due to its use in enhancing rechargeable battery efficiency, super-alloys, and magnetic products. Co is considered to be a possible human carcinogen, with the lung being a primary target. However, few studies have considered cobalt-induced toxicity in human lung cells [23]. Although Co has a biologically necessary role as a metal constituent of vitamin B12, excessive exposure has been shown to induce various adverse health effects. The systemic health effects are characterized by a complex clinical syndrome, including mainly neurological (e.g., hearing and visual impairment), cardiovascular, and endocrine deficits [24]. Two of the most important trace elements playing a major role in the development of oxidative stress are Se and Cu. These elements are necessary for numerous oxidase and superoxide dismutase (SOD) reactions in the cytosol. Cu, in particular, is a critical functional component of several enzymes related to oxidative stress, e.g., SOD, cytochrome C oxidase (CCO) in the mitochondria, and tyrosinase [25; 26]. An excess amount of Mn, Cu, or iron is toxic for the human body. Unregulated copper homeostasis may lead to diseases such as [PERSON] disease--characterized by a deficiency of Cu, or to [PERSON]'s disease--characterized by an excessive amount of Cu [26]. Zn--which is also found in enzymes--catalyzes several synthetic biological reactions [25]. A Zn deficiency is characterized by acral dermatitis, alopecia, and diarrhea [27]. As, Cd and Pb are all highly toxic to humans in their ionic forms. Table 1 lists element concentration ranges that may be encountered in urine. The primary targets for the above-mentioned toxic elements (As, Cd and Pb) are not well understood, as they can damage different cellular structures and a variety of tissues and organs [33; 34]. However, one major cause of toxicity may be the strong interaction with sulfohydryl groups, and their interference with the homeostasis of essential elements [35]. Cd, for example, is known to interfere with the calcium (Ca) metabolism in humans [36]. Furthermore, Cd and Pb can replace Zn in proteins, adding another possible mechanism of toxicity. Indeed, Cd and inorganic As are both also classified as carcinogens, rendering their evaluation in urine highly relevant. In the past, atomic absorption spectrometry (AAS) was used for biomonitoring the levels of trace elements in various laboratories. Nowadays, many laboratories have moved towards methods based on inductively coupled plasma coupled with mass spectrometry (ICP-MS). ICP-MS has become the method of choice for analyzing trace elements in urine and blood due to its many advantages, including sequential multielement measurements, and lower cost- and time-consuming analysis [8; 9]. The typical amount of sample for trace element analysis in urine has varied amongst authors and number of trace elements. Currently, the sample volume of urine reported for the sequential analysis of trace elements is usually between 200 uL [37] and 2000 uL [38]. However, the amount of urine is often a limiting factor in clinical testing laboratories, especially when diagnosing newborns or infants. Therefore, well validated methods using a micro volume of sample with minimal hands-on time are crucial for such applications. In this study, we develop and validate a simple dilute and shoot method for the sequential determination of 16 trace elements--namely, Al; V; Cr; Mn; Fe; Co; Cu; Zn; As; Se; Mo; Cd; Sn; Ba; TI; and Pb--that are relevant in the monitoring of chelation therapy using a micro volume of human urine. We performed our experiments on an ICP-MS 7900 (Agilent Technologies, Tokyo, Japan) equipped with an octupole-based collision cell. As a further sign of quality, it is necessary to validate a method and participate in an external quality assessment in order to increase credibility with health agencies. Reducing the turnaround time and sample volume in routine analysis is also one of the most challenging parts in method development.