Definition/Introduction
Xenon is an element with the symbol Xe and an atomic number 54 (group 18 of the periodic table). It is an inert mono-atomic gas first 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).[1] 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,” underlining its extreme rarity. Although there are naturally 9 isotopes, the most abundant is Xe 132.
Among its physical and chemical properties, Xenon has a boiling point of 166.6 K and a melting point of 161.7 K, a density of 5.851 g/dm2, and a blue-green color spectrum. Because its oil-gas partition coefficient is 1.9, it is among the noble gases and the most soluble gas in oil (lipids).
Even if Xenon does not react with any chemical element, it can always form particular compounds with water, hydroquinone, and phenol. This noble gas can be oxidized by extremely electronegative groups, forming salts. For instance, the compound called xenon hexafluoplatinate was first synthesized in 1962 by the chemist Neil Bartlett (1932-2008). 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).
The fractional distillation of liquefied air can manufacture Xenon. High costs limit the use of Xenon in the industry. Among these applications is the xenon lamp, a particular arc lamp that uses xenon gas to produce intense white light similar to sunlight. There are several xenon lamps, all consisting of a glass or quartz tube with 2 tungsten electrodes at the ends and filled with xenon gas after vacuuming. 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 in the food industry to kill microorganisms and in aerospace.
Xenon in Medicine
The element's main role concerns its use as a radioactive diagnostic agent in clinical imaging and an inhaled anesthetic in general anesthesia. Other applications concern organ protection, ophthalmology, and dermatology.
Clinical imaging
Xenon is indicated for cerebral flow assessment (xenon-enhanced computed tomography), pulmonary function evaluation, and lung imaging. It has also found use in nuclear medicine with computed tomography, single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI). In summary, the gas can be useful for measuring cerebral blood flow, whole-brain scans, and ventilation studies of the lungs through MRI (131 Xe), SPECT (133 Xe), and CT (129 Xe).
General anesthesia
The anesthetic proprieties of Xenon were discovered in 1939 and applied in mice in 1940 (JH Lawrence) and subsequently in 2 human volunteers (Cullen and Gross) in 1951.[2] 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.[3]
Organ protection
An important field of study concerns xenon-induced organ protection. For example, xenon use has been proposed for preventing ischemia/reperfusion damage after Stanford Type-A acute aortic dissection surgery.[4] 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[5][6]. The combination of Xenon with hypothermia is a fascinating hypothesis. Because Xenon seems to have neuroprotective properties already in sub-anesthetic concentrations, these effects can be achieved independently from the anesthetic effect.[7] Regardless of the precise mechanism, potential clinical applications of Xenon for organ protection could be manifold. For instance, Jia et al showed intermittent xenon exposure protects against gentamicin-induced nephrotoxicity.[8] This renal protection is of fundamental importance in kidney transplantation, preventing ischemia/reperfusion damages and delaying rejection and chronic nephropathy.[9] Xenon has had promising results with neurobehavioral dysfunction caused by brain insult [10], cardiac arrest-induced cerebral ischemia [11], and even neonatal hypoxia-ischemia.[12]
Other clinical uses
Apart from this research, there have been studies with the gas for pathologies, including dementia, epilepsy, Alzheimer disease, and obsessive-compulsive disorders.[13][14][15][16] Moreover, Xenon is used in ophthalmology for laser therapy and dermatology to remove skin lesions. Despite all these potential applications, the biggest limitation is the high cost of use. For example, the market price in anesthesia is approximately 6-12£ per liter.