Xenon was discovered in England by the Scottish chemist
William Ramsay and English chemist
Morris Travers on July 12, 1898,[28] shortly after their discovery of the elements
krypton and
neon. They found xenon in the residue left over from evaporating components of
liquid air.[29][30] Ramsay suggested the name xenon for this gas from the
Greek word ξένον xénon, neuter singular form of ξένος xénos, meaning 'foreign(er)', 'strange(r)', or 'guest'.[31][32] In 1902, Ramsay estimated the proportion of xenon in the Earth's atmosphere to be one part in 20 million.[33]
During the 1930s, American engineer
Harold Edgerton began exploring
strobe light technology for
high speed photography. This led him to the invention of the xenon
flash lamp in which light is generated by passing brief electric current through a tube filled with xenon gas. In 1934, Edgerton was able to generate flashes as brief as one
microsecond with this method.[19][34][35]
In 1939, American physician
Albert R. Behnke Jr. began exploring the causes of "drunkenness" in deep-sea divers. He tested the effects of varying the breathing mixtures on his subjects, and discovered that this caused the divers to perceive a change in depth. From his results, he deduced that xenon gas could serve as an
anesthetic. Although Russian toxicologist
Nikolay V. Lazarev apparently studied xenon anesthesia in 1941, the first published report confirming xenon anesthesia was in 1946 by American medical researcher John H. Lawrence, who experimented on mice. Xenon was first used as a surgical anesthetic in 1951 by American anesthesiologist Stuart C. Cullen, who successfully used it with two patients.[36]
Xenon and the other noble gases were for a long time considered to be completely chemically inert and not able to form
compounds. However, while teaching at the
University of British Columbia,
Neil Bartlett discovered that the gas
platinum hexafluoride (PtF6) was a powerful
oxidizing agent that could oxidize oxygen gas (O2) to form
dioxygenyl hexafluoroplatinate (O+ 2[PtF 6− ).[37] Since O2 (1165 kJ/mol) and xenon (1170 kJ/mol) have almost the same first
ionization potential, Bartlett realized that platinum hexafluoride might also be able to oxidize xenon. On March 23, 1962, he mixed the two gases and produced the first known compound of a noble gas,
xenon hexafluoroplatinate.[38][18]
Bartlett thought its composition to be Xe+[PtF6−, but later work revealed that it was probably a mixture of various xenon-containing salts.[39][40][41] Since then, many other xenon compounds have been discovered,[42] in addition to some compounds of the noble gases
argon,
krypton, and
radon, including
argon fluorohydride (HArF),[43]krypton difluoride (KrF2),[44][45] and
radon fluoride.[46] By 1971, more than 80 xenon compounds were known.[47][48]
In November 1989,
IBM scientists demonstrated a technology capable of manipulating individual
atoms. The program, called
IBM in atoms, used a
scanning tunneling microscope to arrange 35 individual xenon atoms on a substrate of chilled crystal of
nickel to spell out the three letter company initialism. It was the first time atoms had been precisely positioned on a flat surface.[49]
Characteristics
Xenon has
atomic number 54; that is, its nucleus contains 54
protons. At
standard temperature and pressure, pure xenon gas has a density of 5.894 kg/m3, about 4.5 times the density of the Earth's atmosphere at sea level, 1.217 kg/m3.[50] As a liquid, xenon has a density of up to 3.100 g/mL, with the density maximum occurring at the triple point.[51] Liquid xenon has a high polarizability due to its large atomic volume, and thus is an excellent solvent. It can dissolve hydrocarbons, biological molecules, and even water.[52] Under the same conditions, the density of solid xenon, 3.640 g/cm3, is greater than the average density of
granite, 2.75 g/cm3.[51] Under
gigapascals of
pressure, xenon forms a metallic phase.[53]
Solid xenon changes from
face-centered cubic (fcc) to
hexagonal close packed (hcp) crystal phase under pressure and begins to turn metallic at about 140 GPa, with no noticeable volume change in the hcp phase. It is completely metallic at 155 GPa. When metallized, xenon appears sky blue because it absorbs red light and transmits other visible frequencies. Such behavior is unusual for a metal and is explained by the relatively small width of the electron bands in that state.[54][55]
Liquid or solid xenon
nanoparticles can be formed at room temperature by implanting Xe+ ions into a solid matrix. Many solids have lattice constants smaller than solid Xe. This results in compression of the implanted Xe to pressures that may be sufficient for its liquefaction or solidification.[56]
Xenon is a member of the zero-
valence elements that are called
noble or
inert gases. It is inert to most common chemical reactions (such as combustion, for example) because the outer
valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.[57]
Xenon is a
trace gas in
Earth's atmosphere, occurring at a volume fraction of 87±1 nL/L (
parts per billion), or approximately 1 part per 11.5 million.[60] It is also found as a component of gases emitted from some
mineral springs. Given a total mass of the atmosphere of 5.15×1018 kilograms (1.135×1019 lb), the atmosphere contains on the order of 2.03 gigatonnes (2.00×109 long tons; 2.24×109 short tons) of xenon in total when taking the average molar mass of the atmosphere as 28.96 g/mol which is equivalent to some 394 mass ppb.
Commercial
Xenon is obtained commercially as a by-product of the
separation of air into
oxygen and
nitrogen.[61] After this separation, generally performed by
fractional distillation in a double-column plant, the
liquid oxygen produced will contain small quantities of
krypton and xenon. By additional fractional distillation, the liquid oxygen may be enriched to contain 0.1–0.2% of a krypton/xenon mixture, which is extracted either by
adsorption onto
silica gel or by distillation. Finally, the krypton/xenon mixture may be separated into krypton and xenon by further distillation.[62][63]
Worldwide production of xenon in 1998 was estimated at 5,000–7,000 cubic metres (180,000–250,000 cu ft).[64] At a density of 5.894 grams per litre (0.0002129 lb/cu in) this is equivalent to roughly 30 to 40 tonnes (30 to 39 long tons; 33 to 44 short tons). Because of its scarcity, xenon is much more expensive than the lighter noble gases—approximate prices for the purchase of small quantities in Europe in 1999 were 10
€/L (=~€1.7/g) for xenon, 1 €/L (=~€0.27/g) for krypton, and 0.20 €/L (=~€0.22/g) for neon,[64] while the much more plentiful argon, which makes up over 1% by volume of earth's atmosphere, costs less than a cent per liter.
Solar System
Within the Solar System, the
nucleon fraction of xenon is 1.56×10−8, for an
abundance of approximately one part in 630 thousand of the total mass.[65] Xenon is relatively rare in the
Sun's atmosphere, on
Earth, and in
asteroids and
comets. The abundance of xenon in the atmosphere of planet
Jupiter is unusually high, about 2.6 times that of the Sun.[66][a] This abundance remains unexplained, but may have been caused by an early and rapid buildup of
planetesimals—small, subplanetary bodies—before the heating of the
presolar disk;[67] otherwise, xenon would not have been trapped in the planetesimal ices. The problem of the low terrestrial xenon may be explained by
covalent bonding of xenon to oxygen within
quartz, reducing the
outgassing of xenon into the atmosphere.[68]
Xenon-135 is a notable
neutron poison with a high
fission product yield. As it is relatively short lived, it decays at the same rate it is produced during steady operation of a nuclear reactor. However, if power is reduced or the reactor is
scramed, less xenon is destroyed than is produced from the beta decay of its
parent nuclides. This phenomenon called
xenon poisoning can cause significant problems in restarting a reactor after a scram or increasing power after it had been reduced and it was one of several contributing factors in the
Chernobyl nuclear accident.[73][74]
Stable or extremely long lived isotopes of xenon are also produced in appreciable quantities in nuclear fission. Xenon-136 is produced when xenon-135 undergoes
neutron capture before it can decay. The ratio of xenon-136 to xenon-135 (or its decay products) can give hints as to the power history of a given reactor and the absence of xenon-136 is a "fingerprint" for nuclear explosions, as xenon-135 is not produced directly but as a product of successive beta decays and thus it cannot absorb any neutrons in a nuclear explosion which occurs in fractions of a second.[75]
The stable isotope xenon-132 has a fission product yield of over 4% in the
thermal neutron fission of 235 U which means that stable or nearly stable xenon isotopes have a higher mass fraction in
spent nuclear fuel (which is about 3% fission products) than it does in air. However, there is as of 2022 no commercial effort to extract xenon from spent fuel during
nuclear reprocessing.[76][77]
Naturally occurring xenon is composed of seven
stableisotopes: 126Xe, 128–132Xe, and 134Xe. The isotopes 126Xe and 134Xe are predicted by theory to undergo
double beta decay, but this has never been observed so they are considered stable.[78] In addition, more than 40 unstable isotopes have been studied. The longest-lived of these isotopes are the
primordial124Xe, which undergoes
double electron capture with a half-life of 1.8×1022 yr,[79] and 136Xe, which undergoes double beta decay with a half-life of 2.11 × 1021 yr.[80]129Xe is produced by
beta decay of 129I, which has a
half-life of 16 million years. 131mXe, 133Xe, 133mXe, and 135Xe are some of the
fission products of 235U and 239Pu,[72] and are used to detect and monitor nuclear explosions.
Nuclear spin
Nuclei of two of the stable
isotopes of xenon, 129Xe and 131Xe (both stable isotopes with odd mass numbers), have non-zero intrinsic
angular momenta (
nuclear spins, suitable for
nuclear magnetic resonance). The nuclear spins can be aligned beyond ordinary polarization levels by means of circularly polarized light and
rubidium vapor.[81] The resulting
spin polarization of xenon
nuclei can surpass 50% of its maximum possible value, greatly exceeding the thermal equilibrium value dictated by
paramagnetic statistics (typically 0.001% of the maximum value at
room temperature, even in the strongest
magnets). Such non-equilibrium alignment of spins is a temporary condition, and is called hyperpolarization. The process of hyperpolarizing the xenon is called optical pumping (although the process is different from
pumping a laser).[82]
Because a 129Xe nucleus has a
spin of 1/2, and therefore a zero
electricquadrupole moment, the 129Xe nucleus does not experience any quadrupolar interactions during collisions with other atoms, and the hyperpolarization persists for long periods even after the engendering light and vapor have been removed. Spin polarization of 129Xe can persist from several
seconds for xenon atoms dissolved in
blood[83] to several hours in the
gas phase[84] and several days in deeply frozen solid xenon.[85] In contrast,
131Xe has a nuclear spin value of 3⁄2 and a nonzero
quadrupole moment, and has t1 relaxation times in the
millisecond and
second ranges.[86]
From fission
Some radioactive isotopes of xenon (for example, 133Xe and 135Xe) are produced by
neutron irradiation of fissionable material within
nuclear reactors.[16]135Xe is of considerable significance in the operation of
nuclear fission reactors. 135Xe has a huge
cross section for
thermal neutrons, 2.6×106barns,[27] and operates as a
neutron absorber or "
poison" that can slow or stop the chain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by the American
Manhattan Project for
plutonium production. However, the designers had made provisions in the design to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of
nuclear fuel).[87]135Xe reactor poisoning was a major factor in the
Chernobyl disaster.[88] A shutdown or decrease of power of a reactor can result in buildup of 135Xe, with reactor operation going into a condition known as the
iodine pit. Under adverse conditions, relatively high concentrations of radioactive xenon isotopes may emanate from cracked
fuel rods,[89] or fissioning of uranium in
cooling water.[90]
Isotope ratios of xenon produced in
natural nuclear fission reactors at
Oklo in Gabon reveal the reactor properties during chain reaction that took place about 2 billion years ago.[91]
Because the half-life of 129I is comparatively short on a cosmological time scale (16 million years), this demonstrated that only a short time had passed between the supernova and the time the meteorites had solidified and trapped the 129I. These two events (supernova and solidification of gas cloud) were inferred to have happened during the early history of the
Solar System, because the 129I isotope was likely generated shortly before the Solar System was formed, seeding the solar gas cloud with isotopes from a second source. This supernova source may also have caused collapse of the solar gas cloud.[92][93]
In a similar way, xenon isotopic ratios such as 129Xe/130Xe and 136Xe/130Xe are a powerful tool for understanding planetary differentiation and early outgassing.[26] For example, the
atmosphere of Mars shows a xenon abundance similar to that of Earth (0.08 parts per million[94]) but Mars shows a greater abundance of 129Xe than the Earth or the Sun. Since this isotope is generated by radioactive decay, the result may indicate that Mars lost most of its primordial atmosphere, possibly within the first 100 million years after the planet was formed.[95][96] In another example, excess 129Xe found in
carbon dioxide well gases from
New Mexico is believed to be from the decay of
mantle-derived gases from soon after Earth's formation.[72][97]
After Neil Bartlett's discovery in 1962 that xenon can form chemical compounds, a large number of xenon compounds have been discovered and described. Almost all known xenon compounds contain the
electronegative atoms fluorine or oxygen. The chemistry of xenon in each oxidation state is analogous to that of the neighboring element
iodine in the immediately lower oxidation state.[98]
Halides
Three
fluorides are known:
XeF 2,
XeF 4, and
XeF 6. XeF is theorized to be unstable.[99] These are the starting points for the synthesis of almost all xenon compounds.
The solid, crystalline difluoride XeF 2 is formed when a mixture of
fluorine and xenon gases is exposed to ultraviolet light.[100] The ultraviolet component of ordinary daylight is sufficient.[101] Long-term heating of XeF 2 at high temperatures under an NiF 2 catalyst yields XeF 6.[102] Pyrolysis of XeF 6 in the presence of
NaF yields high-purity XeF 4.[103]
The xenon fluorides behave as both fluoride acceptors and fluoride donors, forming salts that contain such cations as XeF+ and Xe 2F+ 3, and anions such as XeF− 5, XeF− 7, and XeF2− 8. The green, paramagnetic Xe+ 2 is formed by the reduction of XeF 2 by xenon gas.[98]
XeF 2 also forms
coordination complexes with transition metal ions. More than 30 such complexes have been synthesized and characterized.[102]
Whereas the xenon fluorides are well characterized, the other halides are not.
Xenon dichloride, formed by the high-frequency irradiation of a mixture of xenon, fluorine, and
silicon or
carbon tetrachloride,[104] is reported to be an endothermic, colorless, crystalline compound that decomposes into the elements at 80 °C. However, XeCl 2 may be merely a
van der Waals molecule of weakly bound Xe atoms and Cl 2 molecules and not a real compound.[105] Theoretical calculations indicate that the linear molecule XeCl 2 is less stable than the van der Waals complex.[106]Xenon tetrachloride and
xenon dibromide are more unstable that they cannot be synthesized by chemical reactions. They were created by
radioactive decay of 129 ICl− 4 and 129 IBr− 2, respectively.[107][108]
Oxides and oxohalides
Three oxides of xenon are known:
xenon trioxide (XeO 3) and
xenon tetroxide (XeO 4), both of which are dangerously explosive and powerful oxidizing agents, and
xenon dioxide (XeO2), which was reported in 2011 with a
coordination number of four.[109] XeO2 forms when xenon tetrafluoride is poured over ice. Its crystal structure may allow it to replace silicon in silicate minerals.[110] The XeOO+ cation has been identified by
infrared spectroscopy in solid
argon.[111]
Xenon does not react with oxygen directly; the trioxide is formed by the hydrolysis of XeF 6:[112]
XeF 6 + 3 H 2O → XeO 3 + 6 HF
XeO 3 is weakly acidic, dissolving in alkali to form unstable xenate salts containing the HXeO− 4 anion. These unstable salts easily
disproportionate into xenon gas and
perxenate salts, containing the XeO4− 6 anion.[113]
Barium perxenate, when treated with concentrated
sulfuric acid, yields gaseous xenon tetroxide:[104]
Ba 2XeO 6 + 2 H 2SO 4 → 2 BaSO 4 + 2 H 2O + XeO 4
To prevent decomposition, the xenon tetroxide thus formed is quickly cooled into a pale-yellow solid. It explodes above −35.9 °C into xenon and oxygen gas, but is otherwise stable.
A number of xenon oxyfluorides are known, including XeOF 2,
XeOF 4, XeO 2F 2, and XeO 3F 2. XeOF 2 is formed by reacting
OF 2 with xenon gas at low temperatures. It may also be obtained by partial hydrolysis of XeF 4. It disproportionates at −20 °C into XeF 2 and XeO 2F 2.[114]XeOF 4 is formed by the partial hydrolysis of XeF 6...[115]
XeF 6 + H 2O → XeOF 4 + 2 HF
...or the reaction of XeF 6 with sodium perxenate, Na 4XeO 6. The latter reaction also produces a small amount of XeO 3F 2.
XeO 2F 2 is also formed by partial hydrolysis of XeF 6.[116]
XeF 6 + 2 H 2O → XeO 2F 2 + 4 HF
XeOF 4 reacts with
CsF to form the XeOF− 5 anion,[114][117] while XeOF3 reacts with the alkali metal fluorides
KF,
RbF and CsF to form the XeOF− 4 anion.[118]
Other compounds
Xenon can be directly bonded to a less electronegative element than fluorine or oxygen, particularly
carbon.[119] Electron-withdrawing groups, such as groups with fluorine substitution, are necessary to stabilize these compounds.[113] Numerous such compounds have been characterized, including:[114][120]
C 6F 5–Xe+ –N≡C–CH 3, where C6F5 is the pentafluorophenyl group.
[C 6F 5 2Xe
C 6F 5–Xe–C≡N
C 6F 5–Xe–F
C 6F 5–Xe–Cl
C 2F 5–C≡C–Xe+
[CH 3 3C–C≡C–Xe+
C 6F 5–XeF+ 2
(C 6F 5Xe) 2Cl+
Other compounds containing xenon bonded to a less electronegative element include F–Xe–N(SO 2F) 2 and F–Xe–BF 2. The latter is synthesized from
dioxygenyl tetrafluoroborate, O 2BF 4, at −100 °C.[114][121]
An unusual ion containing xenon is the
tetraxenonogold(II) cation, AuXe2+ 4, which contains Xe–Au bonds.[122] This ion occurs in the compound AuXe 4(Sb 2F 11) 2, and is remarkable in having direct chemical bonds between two notoriously unreactive atoms, xenon and
gold, with xenon acting as a transition metal ligand. A similar mercury complex (HgXe)(Sb3F17) (formulated as [HgXe2+][Sb2F11–][SbF6–]) is also known.[123]
The compound Xe 2Sb 2F 11 contains a Xe–Xe bond, the longest element-element bond known (308.71 pm = 3.0871
Å).[124]
In 1995, M. Räsänen and co-workers, scientists at the
University of Helsinki in
Finland, announced the preparation of xenon dihydride (HXeH), and later xenon hydride-hydroxide (HXeOH), hydroxenoacetylene (HXeCCH), and other Xe-containing molecules.[125] In 2008, Khriachtchev et al. reported the preparation of HXeOXeH by the
photolysis of water within a
cryogenic xenon matrix.[126]Deuterated molecules, HXeOD and DXeOH, have also been produced.[127]
In addition to compounds where xenon forms a
chemical bond, xenon can form
clathrates—substances where xenon atoms or pairs are trapped by the
crystalline lattice of another compound. One example is
xenon hydrate (Xe·5+3⁄4H2O), where xenon atoms occupy vacancies in a lattice of water molecules.[128] This clathrate has a melting point of 24 °C.[129] The
deuterated version of this hydrate has also been produced.[130] Another example is xenon
hydride (Xe(H2)8), in which xenon pairs (
dimers) are trapped inside
solid hydrogen.[131] Such
clathrate hydrates can occur naturally under conditions of high pressure, such as in
Lake Vostok underneath the
Antarctic ice sheet.[132] Clathrate formation can be used to fractionally distill xenon, argon and krypton.[133]
Xenon can also form
endohedral fullerene compounds, where a xenon atom is trapped inside a
fullerene molecule. The xenon atom trapped in the fullerene can be observed by 129Xe
nuclear magnetic resonance (NMR) spectroscopy. Through the sensitive
chemical shift of the xenon atom to its environment, chemical reactions on the fullerene molecule can be analyzed. These observations are not without caveat, however, because the xenon atom has an electronic influence on the reactivity of the fullerene.[134]
When xenon atoms are in the
ground energy state, they repel each other and will not form a bond. When xenon atoms becomes energized, however, they can form an
excimer (excited dimer) until the electrons return to the
ground state. This entity is formed because the xenon atom tends to complete the outermost
electronic shell by adding an electron from a neighboring xenon atom. The typical lifetime of a xenon excimer is 1–5 nanoseconds, and the decay releases
photons with
wavelengths of about 150 and 173
nm.[135][136] Xenon can also form excimers with other elements, such as the
halogensbromine,
chlorine, and
fluorine.[137]
Applications
Although xenon is rare and relatively expensive to extract from the
Earth's atmosphere, it has a number of applications.
The individual cells in a
plasma display contain a mixture of xenon and neon ionized with
electrodes. The interaction of this plasma with the electrodes generates ultraviolet
photons, which then excite the
phosphor coating on the front of the display.[141][142]
Xenon is used as a "starter gas" in
high pressure sodium lamps. It has the lowest
thermal conductivity and lowest
ionization potential of all the non-radioactive noble gases. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the
breakdown voltage of the gas to be relatively low in the cold state, which allows the lamp to be more easily started.[143]
Lasers
In 1962, a group of researchers at
Bell Laboratories discovered laser action in xenon,[144] and later found that the laser gain was improved by adding
helium to the lasing medium.[145][146] The first
excimer laser used a xenon
dimer (Xe2) energized by a beam of electrons to produce
stimulated emission at an
ultraviolet wavelength of 176
nm.[22]
Xenon chloride and xenon fluoride have also been used in excimer (or, more accurately, exciplex) lasers.[147]
Xenon has been used as a
general anesthetic, but it is more expensive than conventional anesthetics.[148]
Xenon interacts with many different receptors and ion channels, and like many theoretically multi-modal inhalation anesthetics, these interactions are likely complementary. Xenon is a high-affinity glycine-site
NMDA receptor antagonist.[149] However, xenon is different from certain other NMDA receptor antagonists in that it is not
neurotoxic and it inhibits the neurotoxicity of
ketamine and
nitrous oxide (N2O), while actually producing
neuroprotective effects.[150][151] Unlike ketamine and nitrous oxide, xenon does not stimulate a dopamine efflux in the
nucleus accumbens.[152]
Like nitrous oxide and
cyclopropane, xenon activates the two-pore domain potassium channel
TREK-1. A related channel
TASK-3 also implicated in the actions of inhalation anesthetics is insensitive to xenon.[153] Xenon inhibits nicotinic acetylcholine
α4β2 receptors which contribute to spinally mediated analgesia.[154][155] Xenon is an effective inhibitor of
plasma membrane Ca2+ ATPase. Xenon inhibits Ca2+ ATPase by binding to a hydrophobic pore within the enzyme and preventing the enzyme from assuming active conformations.[156]
Xenon is a competitive inhibitor of the
serotonin5-HT3 receptor. While neither anesthetic nor antinociceptive, this reduces anesthesia-emergent nausea and vomiting.[157]
Xenon has a
minimum alveolar concentration (MAC) of 72% at age 40, making it 44% more potent than N2O as an anesthetic.[158] Thus, it can be used with oxygen in concentrations that have a lower risk of
hypoxia. Unlike nitrous oxide, xenon is not a
greenhouse gas and is viewed as
environmentally friendly.[159] Though recycled in modern systems, xenon vented to the atmosphere is only returning to its original source, without environmental impact.
Neuroprotectant
Xenon induces robust
cardioprotection and
neuroprotection through a variety of mechanisms. Through its influence on Ca2+, K+,
KATP\HIF, and NMDA antagonism, xenon is neuroprotective when administered before, during and after
ischemic insults.[160][161] Xenon is a high affinity antagonist at the NMDA receptor glycine site.[149] Xenon is cardioprotective in ischemia-reperfusion conditions by inducing
pharmacologic non-ischemic preconditioning. Xenon is cardioprotective by activating PKC-epsilon and downstream p38-MAPK.[162] Xenon mimics neuronal ischemic preconditioning by activating ATP sensitive potassium channels.[163] Xenon allosterically reduces ATP mediated channel activation inhibition independently of the sulfonylurea receptor1 subunit, increasing KATP open-channel time and frequency.[164]
Sports doping
Inhaling a xenon/oxygen mixture activates production of the
transcription factorHIF-1-alpha, which may lead to increased production of
erythropoietin. The latter hormone is known to increase
red blood cell production and athletic performance. Reportedly, doping with xenon inhalation has been used in Russia since 2004 and perhaps earlier.[165] On August 31, 2014, the
World Anti Doping Agency (WADA) added xenon (and
argon) to the list of prohibited substances and methods, although no reliable doping tests for these gases have yet been developed.[166] In addition, effects of xenon on erythropoietin production in humans have not been demonstrated, so far.[167]
Xenon, particularly hyperpolarized 129Xe, is a useful contrast agent for
magnetic resonance imaging (MRI). In the gas phase, it can image cavities in a porous sample, alveoli in lungs, or the flow of gases within the lungs.[171][172] Because xenon is
soluble both in water and in hydrophobic solvents, it can image various soft living tissues.[173][174][175]
Xenon-129 is currently being used as a visualization agent in MRI scans. When a patient inhales hyperpolarized xenon-129 ventilation and gas exchange in the lungs can be imaged and quantified. Unlike xenon-133, xenon-129 is non-ionizing and is safe to be inhaled with no adverse effects.[176]
Because of the xenon atom's large, flexible outer electron shell, the
NMR spectrum changes in response to surrounding conditions and can be used to monitor the surrounding chemical circumstances. For instance, xenon dissolved in water, xenon dissolved in hydrophobic solvent, and xenon associated with certain proteins can be distinguished by NMR.[178][179]
Hyperpolarized xenon can be used by
surface chemists. Normally, it is difficult to characterize surfaces with NMR because signals from a surface are overwhelmed by signals from the atomic nuclei in the bulk of the sample, which are much more numerous than surface nuclei. However, nuclear spins on solid surfaces can be selectively polarized by
transferring spin polarization to them from hyperpolarized xenon gas. This makes the surface signals strong enough to measure and distinguish from bulk signals.[180][181]
Liquid xenon is used in
calorimeters[185] to measure
gamma rays, and as a detector of hypothetical
weakly interacting massive particles, or WIMPs. When a WIMP collides with a xenon nucleus, theory predicts it will impart enough energy to cause ionization and
scintillation. Liquid xenon is useful for these experiments because its density makes dark matter interaction more likely and it permits a quiet detector through self-shielding.
Xenon gas can be safely kept in normal sealed glass or metal containers at
standard temperature and pressure. However, it readily dissolves in most plastics and rubber, and will gradually escape from a container sealed with such materials.[193] Xenon is non-
toxic, although it does dissolve in blood and belongs to a select group of substances that penetrate the
blood–brain barrier, causing mild to full surgical
anesthesia when inhaled in high concentrations with oxygen.[194]
The
speed of sound in xenon gas (169 m/s) is less than that in air[195] because the average velocity of the heavy xenon atoms is less than that of nitrogen and oxygen molecules in air. Hence, xenon vibrates more slowly in the
vocal cords when exhaled and produces lowered voice tones (low-frequency-enhanced sounds, but the
fundamental frequency or
pitch does not change), an effect opposite to the high-toned voice produced in
helium. Specifically, when the
vocal tract is filled with xenon gas, its natural resonant frequency becomes lower than when it is filled with air. Thus, the low frequencies of the sound wave produced by the same direct vibration of the
vocal cords would be enhanced, resulting in a change of the
timbre of the sound amplified by the vocal tract. Like helium, xenon does not satisfy the body's need for oxygen, and it is both a simple
asphyxiant and an anesthetic more powerful than nitrous oxide; consequently, and because xenon is expensive, many universities have prohibited the voice stunt as a general chemistry demonstration.[citation needed] The gas
sulfur hexafluoride is similar to xenon in molecular weight (146 versus 131), less expensive, and though an asphyxiant, not toxic or anesthetic; it is often substituted in these demonstrations.[196]
Dense gases such as xenon and sulfur hexafluoride can be breathed safely when mixed with at least 20% oxygen. Xenon at 80% concentration along with 20% oxygen rapidly produces the unconsciousness of general anesthesia. Breathing mixes gases of different densities very effectively and rapidly so that heavier gases are purged along with the oxygen, and do not accumulate at the bottom of the lungs.[197] There is, however, a danger associated with any heavy gas in large quantities: it may sit invisibly in a container, and a person who enters an area filled with an odorless, colorless gas may be asphyxiated without warning. Xenon is rarely used in large enough quantities for this to be a concern, though the potential for danger exists any time a tank or container of xenon is kept in an unventilated space.[198]
Water-soluble xenon compounds such as
monosodium xenate are moderately toxic, but have a very short half-life of the body –
intravenously injected xenate is reduced to elemental xenon in about a minute.[194]
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