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Noble gases have very low boiling and melting points, which makes them useful as cryogenicrefrigerants.[1] In particular, liquid helium, which boils at 4.2 K (−268.95 °C; −452.11 °F), is used for superconducting magnets, such as those needed in nuclear magnetic resonance imaging and nuclear magnetic resonance.[2] Liquid neon, although it does not reach temperatures as low as liquid helium, also finds use in cryogenics because it has over 40 times more refrigerating capacity than liquid helium and over three times more than liquid hydrogen.[3]
Helium is used as a component of breathing gases to replace nitrogen, due its low solubility in fluids, especially in lipids. Gases are absorbed by the blood and body tissues when under pressure like in scuba diving, which causes an anesthetic effect known as nitrogen narcosis.[4] Due to its reduced solubility, little helium is taken into cell membranes, and when helium is used to replace part of the breathing mixtures, such as in trimix or heliox, a decrease in the narcotic effect of the gas at depth is obtained.[5] Helium's reduced solubility offers further advantages for the condition known as decompression sickness, or the bends.[6][7] The reduced amount of dissolved gas in the body means that fewer gas bubbles form during the decrease in pressure of the ascent. Another noble gas, argon, is considered the best option for use as a drysuit inflation gas for scuba diving.[8] Helium is also used as filling gas in nuclear fuel rods for nuclear reactors.[9]
Noble gases are commonly used in lighting because of their lack of chemical reactivity. Argon, mixed with nitrogen, is used as a filler gas for incandescent light bulbs.[3] Krypton is used in high-performance light bulbs, which have higher color temperatures and greater efficiency, because it reduces the rate of evaporation of the filament more than argon; halogen lamps, in particular, use krypton mixed with small amounts of compounds of iodine or bromine.[3] The noble gases glow in distinctive colors when used inside gas-discharge lamps, such as "neon lights". These lights are called after neon but often contain other gases and phosphors, which add various hues to the orange-red color of neon. Xenon is commonly used in xenon arc lamps, which, due to their nearly continuous spectrum that resembles daylight, find application in film projectors and as automobile headlamps.[3]
The noble gases are used in excimer lasers, which are based on short-lived electronically excited molecules known as excimers. The excimers used for lasers may be noble gas dimers such as Ar2, Kr2 or Xe2, or more commonly, the noble gas is combined with a halogen in excimers such as ArF, KrF, XeF, or XeCl. These lasers produce ultraviolet light, which, due to its short wavelength (193 nm for ArF and 248 nm for KrF), allows for high-precision imaging. Excimer lasers have many industrial, medical, and scientific applications. They are used for microlithography and microfabrication, which are essential for integrated circuit manufacture, and for laser surgery, including laser angioplasty and eye surgery.[12]
Some noble gases have direct application in medicine. Helium is sometimes used to improve the ease of breathing of people with asthma.[3] Xenon is used as an anesthetic because of its high solubility in lipids, which makes it more potent than the usual nitrous oxide, and because it is readily eliminated from the body, resulting in faster recovery.[13] Xenon finds application in medical imaging of the lungs through hyperpolarized MRI.[14] Radon, which is highly radioactive and is only available in minute amounts, is used in radiotherapy.[6]
Noble gases, particularly xenon, are predominantly used in ion engines due to their inertness. Since ion engines are not driven by chemical reactions, chemically inert fuels are desired to prevent unwanted reaction between the fuel and anything else on the engine.
Oganesson is too unstable to work with and has no known application other than research.
In many applications, the noble gases are used to provide an inert atmosphere. Argon is used in the synthesis of air-sensitive compounds that are sensitive to nitrogen. Solid argon is also used for the study of very unstable compounds, such as reactive intermediates, by trapping them in an inert matrix at very low temperatures.[15] Helium is used as the carrier medium in gas chromatography, as a filler gas for thermometers, and in devices for measuring radiation, such as the Geiger counter and the bubble chamber.[16] Helium and argon are both commonly used to shield welding arcs and the surrounding base metal from the atmosphere during welding and cutting, as well as in other metallurgical processes and in the production of silicon for the semiconductor industry.[3]
Noble gases have a significant and broad use as geochemical tracing tool in earth science[17]. They provide important information on the small to large scale degassing history of the Earth and the resulting effect to the atmosphere composition. Due to their inert nature, noble gases retain their source signature as it interacts with other volatile reservoirs while migrating through the earth crust, deeper part of the earth and to the atmosphere[18] Owing to their low abundances in different geological settings, changes to their concentration can be used to resolve the processes that influenced their current signature[17][19]. To make their use more convenient, geochemist uses ratios to describe their quantities and relationship between different noble gases. Isotopic ratios takes account of multiple sources from various geochemical reservoir, the ratio is represented by a specific or a range value to reflect the influence from a particular source(s).
Helium has two abundant isotopes, helium-3 and helium-4. Helium-3 is primordial, which means it originated from accretional process during earth formation where it was trapped in the earth's core and mantle. Helium-4 is an alpha particle that originates from the decay of radionuclides (232Th, 235,238U), which are abundant in the earth's crust. Isotopic ratios of helium are represented by RA value, which signifies a relative measurement to air 3He/4He = 1.39*10-6[20]. Volatiles that originate from the earth's crust have a 0.02-0.05 RA, which indicate an enrichment of helium-4[21]. Volatiles that originate from deeper sources such as subcontinental lithospheric mantle (SCLM), have a 6.1± 0.9 RA[22]. Volatiles that get transported from upper mantle, represented by mid-oceanic ridge basalts (MORB) has a signature with 8 ± 1 RA and anything with deeper sources (i.e., mantle plume) have an > 8 RA[22][23] . Solar wind, which represent a primordial signature records a 3He/4He ratio of ~ 330 RA[24]. In comparison, plume sample from picrite rock in Iceland has been reported to have ~50 RA indicating less crustal contamination effect[25]. A RA in-between the reported values indicate mixing processes among the endmember sources (i.e., binary mixing), which indicate either an addition or loss of 3He or 4He from their respective sources.
Helium isotope measurements has been perfected over the years to unravel different geochemical processes, nevertheless, isotopes of other noble gases (neon, argon, krypton, and xenon), which can provide a different information that is not readily available from helium isotopes.
Neon has three main stable isotopes, each sourced from different geochemical reservoirs and processes[19][26]. 20Ne is mainly produced by cosmic nucleogenic reactions and trapped during atmosphere formation, which explain its high abundance in air compared to other geologic settings. 21Ne and 22Ne are produced in the earth's crust as a result of interactions between alpha and neutron particles with light elements; 18O, 19F and 24,25Mg[27]. The ratios between the neon isotopes represent geochemical endmembers that unravel the source and evolution pathway of the volatile system. For example, neon ratios (20Ne/22Ne and 21Ne/22Ne) are widely used to discern the heterogeneity in the Earth's mantle. Reported high 20Ne/22Ne in Iceland (12.88±0.12)[28] or south Atlantic (13.10± 0.5)[29] reflect a trapped nebular gas in the deep mantle[30] A lower 20Ne/22Ne value in MORB (12.5) reflect a different process other than mass-dependent fractionation that resulted to escape of 20Ne. Similarly, air 20Ne/22Ne = 9.8 reflect a comparative huge loss of 20Ne, possibly during mass-dependent escape of light isotopes accompanying massive hydrodynamic flow of H2 from Earth's system[20]. All of these has an implication on the thermal evolution and volatile budget of Earth's systems, complimenting the information provided by helium isotopes.
Argon has three stable isotopes: 36Ar, 38Ar and 40Ar. 36Ar and 38Ar are primordial, with their inventory on the earth's crust dependent on the equilibration of meteoric water with the crustal fluids[19]. This explains huge inventory of 36Ar in the atmosphere. Production of these two isotopes (36Ar and 38Ar) is negligible within the earth's crust, only limited concentrations of 38Ar can be produced by interaction between alpha particles from decay of 235,238U and 232Th and light elements (37Cl and 41K). While 36Ar is continuously being produced by Beta-decay of 36Cl[35][36]. 40Ar is a product of radiogenic decay of 40K. Different endmembers values for 40Ar/36Ar have been reported; Air = 295.5[37], MORB = 40,000[37], and crust = 3000[19].
Kypton has several isotopes, some of which are mainly primordial; 78, 80, 82Kr while other are produced by spontaneous fission of 244Pu and radiogenic decay of 238U; 83,84, 86Kr [38][19]. Together with argon and xenon, these are known as heavy noble gases, with key geochemical characteristic of their composition in mantle reservoirs resembling the modern atmosphere[39]. Unlike, light noble gases (helium and neon), krypton preserves the solar-like primordial signature. Krypton isotopes have been used to decipher the mechanism of volatiles delivery to earth's system, which had great implication to evolution of earth (nitrogen, oxygen, and oxygen) and emergence of life[40]. This is largely due to a clear distinction of krypton isotope signature from various sources such as chondritic material, solar wind and cometary[41][42].
Xenon has nine isotopes, most of which are produced by the radiogenic decay. Detailed information on Xenon isotope geochemistry can be found in the linked resource. Nevertheless, krypton and xenon noble gases requires pristine, robust geochemical sampling protocol due to ease of entrapment of atmospheric component rendering their use questionable[43]. Sophisticated Instrumentation are required to resolve mass peaks among vast isotopes with narrow mass difference during analysis, which makes their use limited.
Elemental ratios for noble gases can also reflect the different endmembers source and processes. They reflect different degree of mixing between endmembers or evolution of specific noble gases through time.
The 4He/20Ne ratios is a powerful tool used to correct for air helium contribution to the collected samples[51]. This becomes relevant due to challenge of isolating air contamination during sampling or from equilibrated air with the other geochemical systems via meteoric recharge. Below equation is used for correction[51]. 4He/40Ar values ratio is a function of (U + Th)/K, and therefore it reflects the level of fractionation during thermal release from their mineral sites, transport processes and the time the system remained closed for 4He, 40Ar, U, Th and K [19][52].
Various media can be sampled for noble gas measurements, including volcanic fluids, springs, geothermal wells, natural gases and trapped gas in rocks[53]. Volcanic fluids that are actively released from volcanic centers such as fumaroles or passively degassing along faults provide a window to sample deep sources signature. Generally, to collect a sample a funnel or a titanium tube connected to a Tygon tubing on other end is thrusted in the discharging source. The specific sampling protocol is given below for various sampling container.
A field setup for collecting gas sample intended for noble gas analysis. The sampling setup includes the inverted funnel on top of the hot spring with macro seep, two copper tubes connected with TygonⓇ tube.Copper tubes - These are standard copper tubes piece cut in ~10 cm3 with outer diameter of 3/8” (9.53mm). The sampling technique involves an inverted funnel that is placed on top of a volatile discharging vent/spring, which help to focus and concentrate the sample towards the copper tubes. The copper tube is connected to the funnel using a TygonⓇ tube, if duplicate sample is needed, a second piece of TygonⓇ which is connected to the second copper tubes. The other end of copper tube is connected to another TygonⓇ tube, which is submerged in a water container. This setup allows one-way positive inflow from the sample source and isolate air contamination. Because copper tubes are malleable they can be cold welded to seal both end of the tube or pinched off using refrigeration clamps after enough time has passed to allow flushing of the copper tube with pristine sample.
Sampling of noble gases using a Giggenbach bottle, a funnel is placed on top of the hot spring to focus the stream of sample towards the bottle via the Tygon tube. A geochemist is controlling the flow of the sample inlet using a Teflon valve. Note the condensation process inside the evacuated Giggenbach bottle.Giggenbach bottles - These are evacuated glass flask fitted with a Teflon stopcock, designed for sampling and storing gases. They can be filled with sodium hydroxide (NaOH) solution, which remove reactive gas species (SO2, CO2, H2S, HF, HCL), which could have unintended effect of corroding metal-container. Unlike copper tubes, which require no preparation prior to sampling, giggenbach bottles needs to pre-evacuated prior to sample collection[54]. The non-reactive gases such as noble gases are allowed accumulate in the headspace of the bottle. These bottles were first invented and distributed by a Werner F. Giggenbach, a German chemist[55]. Diffusion of light noble gases through glass has been thoroughly constrained to establish the time limit of giggenbach bottles effectiveness[56][57]. A decrease of RA value of up to 4% within a few months from sample collection has been reported, highlighting a need for rapid analysis turnaround[17].
Tedlar bags/ Multi-layered foil bags - For immediate analysis in the field, these ready to use cost-effective bags can be deployed and adapted to instrument for analysis.
Noble gases have a vast number of isotopes, with some having a subtle variation which have driven a need to develop high precision, sensitive and robust systems to discern between extremely low concentrations noble gases. Through the years, the main system to detect the noble gas isotopes ratios used a magnetic sector mass spectrometer, which operated by changing magnetic field strength that focuses the ion beam sequentially for different ion masses to a single collector[58][59]. The main challenge with this is the time consumption and "peak jumping mode" which result to low sensitivity in measurements.
Multiple-collector mass spectrometer allows different ion beams for different isotopes mass to be detected by positioning multiple detectors. This allows stable, flat-topped peaks and high sensitivity at a higher sample throughput[59]. A good example of such systems is the Quadrupole mass spectrometer (QMS).
Before analysis, preparation of the noble gases sample for analysis is necessary, involving extraction and purification stages. This is crucial due to low abundance of noble gases in most of geological materials [17]. Most noble gas laboratories have both system's protocols in place for extraction and purification processes. Extraction allows liberation of noble gases from their carrier (major phase; fluid or solid). Different extraction protocols exist depending on the analysis objective. For fluid samples, noble gases insolubility results to preferential expansion to a volume under static vacuum that has been created and maintained by pumps connected to the extraction line. For solid samples such as melt inclusions, noble gases can be extracted using specific solutions treatment, crushing or heating under vacuum systems[17][60].
Noble gases are rarely found in isolation in natural systems, and purification process is necessary to remove impurities and improve concentration per unit sample volume that gets analyzed. This process makes use of the inertness of noble gases by subjecting the sample volume to reactive metal surfaces "Getters" that break-down and adsorb reactive species. Prior to expansion of sample to the getters, the sample is first introduced to a pre heated Titanium -sponge furnace at 850 - 900o C, which break down species such as CH4, CO2, N2, H2, and O2[61][62]. Different getters exist in various lab's purification lines such as SAES GP-50 and SAES NP-10[62]. Using cryogenic traps, noble gases can be sequentially analyzed in the mass spectrometer detector without peak interference that can results from mass/charge similarity (40Ar++ with 20Ne or 44CO2++ and 22Ne)[61]. Stepwise raising of temperature is done to permit sequence analysis in the mass spectrometer.
Extraction and purification (clean up) mass spectrometer line.
Miniaturizing sophisticated lab-based noble gas mass spectrometer and maintaining operation and sensitivity performance out in the field is a challenging endeavor. Several research labs such as Stanford Research Systems have managed to invent gas analyzer that can be assembled with low-cost vacuum systems, and other component into a full-fledged field-mass spectrometer system. A modern example is a portable mass spectrometer (miniRuedi), capable of quantifying He, Ne, Ar, Kr, and other elements/compounds using the miniaturized quadrupole mass analyzer and still maintaining analytical uncertainty of 1-3%[63].
A typical quadrupole mass analyzer mode of function that is used in lab-based mass spectrometer and field-based mass spectrometer.
^Maiken, Eric (1 August 2004). "Why Argon?". Decompression. Retrieved 26 June 2008.
^Horhoianu, G.; Ionescu, D. V.; Olteanu, G. (1999). "Thermal behaviour of CANDU type fuel rods during steady state and transient operating conditions". Annals of Nuclear Energy. 26 (16): 1437–1445. doi:10.1016/S0306-4549(99)00022-5.
^"Disaster Ascribed to Gas by Experts". The New York Times. 7 May 1937. p. 1.
^Dunkin, I. R. (1980). "The matrix isolation technique and its application to organic chemistry". Chem. Soc. Rev. 9: 1–23. doi:10.1039/CS9800900001.
^Hwang, Shuen-Chen; Lein, Robert D.; Morgan, Daniel A. (2005). "Noble Gases". Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383. doi:10.1002/0471238961.0701190508230114.a01.
