Xenon is an element with the symbol Xe and an atomic number of 54 (group 18 of the periodic table). It is an inert mono-atomic gas firstly identified in 1898 by the British chemists William Ramsay (1852-1916) and Morris Travers (1872-1961) in the residue obtained by partial evaporation of liquid air (krypton impurity). The chemical element Xe is a colorless, odorless, non-pungent, nontoxic, nonexplosive, environmentally friendly, noble gas. It is found in the earth’s atmosphere in trace amounts as the concentration in the atmosphere is only 0.086 ppm. Yet, it is also found in the gases emitted by some mineral springs. Interestingly, the gas derives its name from the Greek word for “stranger” for underlining its extreme rarity. Although there are naturally 9 isotopes, the most abundant is Xe 132.
Among the physical and chemical properties of xenon, the boiling point is 166.6 K, the melting point 161.7 K, density is 5.851 g/dm2 whereas the colour spectrum is blue-green. Because the oil-gas partition coefficient of xenon is 1.9, it is among the noble gases the most soluble gas in oil (lipids).
Even if xenon does not react with any chemical element, it is always able to form quite particular compounds with water, hydroquinone, and phenol. This noble gas can be oxidized by groups that are extremely electronegative, forming salts. For instance, the compound, called xenon hexafluoplatinate, was first synthesized in 1962 by the chemist Neil Bartlett (1932-2008) and it was the first example of a noble gas chemical compound reported in the chemical literature. Other Xenon fluorides are the Xenon difluoride (XeF2), Xenon tetrafluoride (XeF4), and Xenon hexafluoride (XeF6).
Xenon can be manufactured by the fractional distillation of liquefied air. The use of xenon in the industry is limited by high costs. Among these applications, there is the xenon lamp, a particular arc lamp that uses xenon gas to produce very intense and white light similar to sunlight. There are several types of xenon lamps, all consisting of a glass or quartz tube with two tungsten electrodes at the ends and filled with xenon gas after vacuuming them. These lamps are used for street lighting, photo flashes and projectors, car headlights, and marine lighting. It is also used for lasers, and x-ray tubes as well as in the food industry for killing microorganism, and in aerospace.
Xenon in Medicine
The main role of the element concerns its use as a radioactive diagnostic agent in clinical imaging, and inhaled anesthetic in general anesthesia. Other applications regard the organ protection, ophthalmology, and in dermatology.
Xenon is indicated for cerebral flow assessment (xenon-enhanced computed tomography), pulmonary function evaluation, and lung imaging. Xenon has also found use in nuclear medicine with computed tomography as well as single-photon emission computed tomography (SPECT), and in magnetic resonance imaging (MRI). In summary, the gas can be useful for the measurement of cerebral blood flow, whole-brain scans, and ventilation studies of the lungs through MRI (131 Xe), SPECT (133 Xe), and CT (129 Xe).
The anesthetic proprieties of xenon were discovered in 1939 and applied in mice in 1940 (JH Lawrence) and subsequently in two humans volunteers (Cullen and Gross), in 1951. It is indicated in selected patients due to its cardiovascular stability, cerebral protection, and favorable pharmacokinetics including low solubility, and lack of metabolism. Moreover, its use is not associated with environmental impact.
An important field of study concerns the xenon-induced organ protection. For example, xenon use has been proposed for preventing ischemia/reperfusion damage after Stanford Type-A acute aortic dissection surgery. Again, several preclinical investigations conducted on different models subjected to preconditioning, real-time conditioning, and postconditioning have demonstrated that this gas may present important neuroprotective (in a dose-dependent manner) and cardioprotective effects by interfering with the glutamatergic transmission (glutamate receptors are implicated in both anesthesia and acute neurological injury through the apoptotic process) and by inhibiting the inflammatory cascade. The combination of xenon with hypothermia is a fascinating hypothesis. Of note, because xenon seems to have neuroprotective properties already in sub-anaesthetic concentrations, these effects can be achieved independently from the anaesthetic effect. Regardless the precise mechanism, potential clinical applications of xenon for organ protection could be manifold. For instance, Jia et al. showed that intermittent exposure to xenon is protective against gentamicin-induced nephrotoxicity. This renal protection is of fundamental importance in kidney transplantation, preventing ischemia/reperfusion damages, and delaying rejection and chronic nephropathy. Xenon has had promising results with neurobehavioral dysfunction caused by brain insult , cardiac arrest induced cerebral ischemia , and even against neonatal hypoxia-ischemia.
Other Clinical Uses
Apart from this research, there have been studies with the gas for pathologies including dementia, epilepsy, Alzheimer's disease, and obsessive-compulsive disorders. Moreover, xenon is used in ophthalmology for laser therapy, and in dermatology for removal of skin lesion. Despite all these potential applications, the biggest limitation is the high cost of use. For example, the market price in anesthesia is of approximately 6-12£ per liter.