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139-452: GC–MS has been regarded as a " gold standard " for forensic substance identification because it is used to perform a 100% specific test, which positively identifies the presence of a particular substance. A nonspecific test merely indicates that any of several in a category of substances is present. Although a nonspecific test could statistically suggest the identity of the substance, this could lead to false positive identification. However,

278-514: A dinitrogen complex to be discovered was [Ru(NH 3 ) 5 (N 2 )] (see figure at right), and soon many other such complexes were discovered. These complexes , in which a nitrogen molecule donates at least one lone pair of electrons to a central metal cation, illustrate how N 2 might bind to the metal(s) in nitrogenase and the catalyst for the Haber process : these processes involving dinitrogen activation are vitally important in biology and in

417-406: A sensitivity of 100% concerning the presence of the disease (it identifies all individuals with a well-defined disease process; it does not have any false-negative results) and a specificity of 100% (it does not falsely identify someone with a condition that does not have the condition; it does not have any false-positive results). In practice, there are no true gold standard tests. Sometimes

556-473: A bridging ligand, donating all three electron pairs from the triple bond ( μ 3 -N 2 ). A few complexes feature multiple N 2 ligands and some feature N 2 bonded in multiple ways. Since N 2 is isoelectronic with carbon monoxide (CO) and acetylene (C 2 H 2 ), the bonding in dinitrogen complexes is closely allied to that in carbonyl compounds, although N 2 is a weaker σ -donor and π -acceptor than CO. Theoretical studies show that σ donation

695-414: A certain compound. This is a fast and efficient analysis, especially if the analyst has previous information about a sample or is only looking for a few specific substances. When the amount of information collected about the ions in a given gas chromatographic peak decreases, the sensitivity of the analysis increases. So, SIM analysis allows for a smaller quantity of a compound to be detected and measured, but

834-403: A characteristic retention time in a GC–MS analysis, it typically increases certainty that the analyte of interest is in the sample. For the analysis of volatile compounds, a purge and trap (P&T) concentrator system may be used to introduce samples. The target analytes are extracted by mixing the sample with water and purge with inert gas (e.g. Nitrogen gas ) into an airtight chamber, this

973-467: A chemical formula containing the various elements which are believed to be in the compound. The isotope pattern in the spectrum, which is unique for elements that have many natural isotopes, can also be used to identify the various elements present. Once a chemical formula has been matched to the spectrum, the molecular structure and bonding can be identified, and must be consistent with the characteristics recorded by GC–MS. Typically, this identification

1112-414: A definition. When referring to this meaning, gold standard tests are normally not performed at all. This is because the gold standard test may be difficult to perform or may be impossible to perform on a living person (i.e. the test is performed as part of an autopsy or may take too long for the results of the test to be available to be clinically useful). Other times, the "gold standard" does not refer to

1251-426: A dilute gas it is less dangerous and is thus used industrially to bleach and sterilise flour. Nitrogen tribromide (NBr 3 ), first prepared in 1975, is a deep red, temperature-sensitive, volatile solid that is explosive even at −100 °C. Nitrogen triiodide (NI 3 ) is still more unstable and was only prepared in 1990. Its adduct with ammonia, which was known earlier, is very shock-sensitive: it can be set off by

1390-653: A gas chromatograph and quadrupole mass spectrometer that can be used in tandem as a GC–MS. The material in the comet 67P/Churyumov–Gerasimenko was analysed by the Rosetta mission with a chiral GC–MS in 2014. Dozens of congenital metabolic diseases also known as inborn errors of metabolism (IEM) are now detectable by newborn screening tests, especially the testing using gas chromatography–mass spectrometry. GC–MS can determine compounds in urine even in minor concentration. These compounds are normally not present but appear in individuals suffering with metabolic disorders. This

1529-498: A liquid, it is a very good solvent with a high heat of vaporisation (enabling it to be used in vacuum flasks), that also has a low viscosity and electrical conductivity and high dielectric constant , and is less dense than water. However, the hydrogen bonding in NH 3 is weaker than that in H 2 O due to the lower electronegativity of nitrogen compared to oxygen and the presence of only one lone pair in NH 3 rather than two in H 2 O. It

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1668-483: A long time, sources of nitrogen compounds were limited. Natural sources originated either from biology or deposits of nitrates produced by atmospheric reactions. Nitrogen fixation by industrial processes like the Frank–Caro process (1895–1899) and Haber–Bosch process (1908–1913) eased this shortage of nitrogen compounds, to the extent that half of global food production now relies on synthetic nitrogen fertilisers. At

1807-607: A part of all US airports . These systems run on a host of technologies, many of them based on GC–MS. There are only three manufacturers certified by the FAA to provide these systems, one of which is Thermo Detection (formerly Thermedics), which produces the EGIS , a GC–MS-based line of explosives detectors. The other two manufacturers are Barringer Technologies, now owned by Smith's Detection Systems, and Ion Track Instruments, part of General Electric Infrastructure Security Systems. As part of

1946-494: A preference for forming multiple bonds, typically with carbon, oxygen, or other nitrogen atoms, through p π –p π interactions. Thus, for example, nitrogen occurs as diatomic molecules and therefore has very much lower melting (−210 °C) and boiling points (−196 °C) than the rest of its group, as the N 2 molecules are only held together by weak van der Waals interactions and there are very few electrons available to create significant instantaneous dipoles. This

2085-591: A problem which is only exacerbated by its low gyromagnetic ratio , (only 10.14% that of H). As a result, the signal-to-noise ratio for H is about 300 times as much as that for N at the same magnetic field strength. This may be somewhat alleviated by isotopic enrichment of N by chemical exchange or fractional distillation. N-enriched compounds have the advantage that under standard conditions, they do not undergo chemical exchange of their nitrogen atoms with atmospheric nitrogen, unlike compounds with labelled hydrogen , carbon, and oxygen isotopes that must be kept away from

2224-438: A prominent product ion which can be the product ion for the chosen precursor ion. The pair is called a "transition" and forms the basis for SRM. SRM is highly specific and virtually eliminates matrix background. After the molecules travel the length of the column, pass through the transfer line and enter into the mass spectrometer they are ionized by various methods with typically only one method being used at any given time. Once

2363-470: A promising ceramic if not for the difficulty of working with and sintering it. In particular, the group 13 nitrides, most of which are promising semiconductors , are isoelectronic with graphite, diamond, and silicon carbide and have similar structures: their bonding changes from covalent to partially ionic to metallic as the group is descended. In particular, since the B–N unit is isoelectronic to C–C, and carbon

2502-436: A second phase of mass fragmentation is added, for example using a second quadrupole in a quadrupole instrument, it is called tandem MS (MS/MS). MS/MS can sometimes be used to quantitate low levels of target compounds in the presence of a high sample matrix background. The first quadrupole (Q1) is connected with a collision cell (Q2) and another quadrupole (Q3). Both quadrupoles can be used in scanning or static mode, depending on

2641-463: A significant dynamic surface coverage on Pluto and outer moons of the Solar System such as Triton . Even at the low temperatures of solid nitrogen it is fairly volatile and can sublime to form an atmosphere, or condense back into nitrogen frost. It is very weak and flows in the form of glaciers, and on Triton geysers of nitrogen gas come from the polar ice cap region. The first example of

2780-412: A test becomes popular and is declared to be the gold standard without adequate consideration of alternatives or despite weaknesses. As new diagnostic methods become available, the "gold standard" test may change over time. For instance, for the diagnosis of aortic dissection , the gold standard test used to be the aortogram , which had a sensitivity as low as 83% and a specificity as low as 87%. Since

2919-606: A very high energy density, that could be used as powerful propellants or explosives. Under extremely high pressures (1.1 million  atm ) and high temperatures (2000 K), as produced in a diamond anvil cell , nitrogen polymerises into the single-bonded cubic gauche crystal structure. This structure is similar to that of diamond , and both have extremely strong covalent bonds , resulting in its nickname "nitrogen diamond". At atmospheric pressure , molecular nitrogen condenses ( liquefies ) at 77  K (−195.79 ° C ) and freezes at 63 K (−210.01 °C) into

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3058-488: A wide range of mass fragments. Full scan is useful in determining unknown compounds in a sample. It provides more information than SIM when it comes to confirming or resolving compounds in a sample. During instrument method development it may be common to first analyze test solutions in full scan mode to determine the retention time and the mass fragment fingerprint before moving to a SIM instrument method. In selective ion monitoring (SIM) certain ion fragments are entered into

3197-465: Is oxatetrazole (N 4 O), an aromatic ring. Nitrous oxide (N 2 O), better known as laughing gas, is made by thermal decomposition of molten ammonium nitrate at 250 °C. This is a redox reaction and thus nitric oxide and nitrogen are also produced as byproducts. It is mostly used as a propellant and aerating agent for sprayed canned whipped cream , and was formerly commonly used as an anaesthetic. Despite appearances, it cannot be considered to be

3336-439: Is ONF 3 , which has aroused interest due to the short N–O distance implying partial double bonding and the highly polar and long N–F bond. Tetrafluorohydrazine, unlike hydrazine itself, can dissociate at room temperature and above to give the radical NF 2 •. Fluorine azide (FN 3 ) is very explosive and thermally unstable. Dinitrogen difluoride (N 2 F 2 ) exists as thermally interconvertible cis and trans isomers, and

3475-419: Is a fuming, colourless liquid that smells similar to ammonia. Its physical properties are very similar to those of water (melting point 2.0 °C, boiling point 113.5 °C, density 1.00 g/cm ). Despite it being an endothermic compound, it is kinetically stable. It burns quickly and completely in air very exothermically to give nitrogen and water vapour. It is a very useful and versatile reducing agent and

3614-491: Is a more important factor allowing the formation of the M–N bond than π back-donation, which mostly only weakens the N–N bond, and end-on ( η ) donation is more readily accomplished than side-on ( η ) donation. Today, dinitrogen complexes are known for almost all the transition metals , accounting for several hundred compounds. They are normally prepared by three methods: Occasionally

3753-452: Is a quadrupole or the ion trap itself in an ion trap MS) where they are bombarded with free electrons emitted from a filament, not unlike the filament one would find in a standard light bulb. The electrons bombard the molecules, causing the molecule to fragment in a characteristic and reproducible way. This "hard ionization" technique results in the creation of more fragments of low mass-to-charge ratio (m/z) and few, if any, molecules approaching

3892-467: Is a weak base in aqueous solution ( p K b 4.74); its conjugate acid is ammonium , NH 4 . It can also act as an extremely weak acid, losing a proton to produce the amide anion, NH 2 . It thus undergoes self-dissociation, similar to water, to produce ammonium and amide. Ammonia burns in air or oxygen, though not readily, to produce nitrogen gas; it burns in fluorine with a greenish-yellow flame to give nitrogen trifluoride . Reactions with

4031-401: Is a weak diprotic acid with the structure HON=NOH (p K a1 6.9, p K a2 11.6). Acidic solutions are quite stable but above pH 4 base-catalysed decomposition occurs via [HONNO] to nitrous oxide and the hydroxide anion. Hyponitrites (involving the N 2 O 2 anion) are stable to reducing agents and more commonly act as reducing agents themselves. They are an intermediate step in

4170-399: Is a weaker base than ammonia. It is also commonly used as a rocket fuel. Hydrazine is generally made by reaction of ammonia with alkaline sodium hypochlorite in the presence of gelatin or glue: (The attacks by hydroxide and ammonia may be reversed, thus passing through the intermediate NHCl instead.) The reason for adding gelatin is that it removes metal ions such as Cu that catalyses

4309-556: Is also commonly used in forensic toxicology to find drugs and/or poisons in biological specimens of suspects, victims, or the deceased. In drug screening, GC–MS methods frequently utilize liquid-liquid extraction as a part of sample preparation, in which target compounds are extracted from blood plasma. GC–MS is the main tool used in sports anti-doping laboratories to test athletes' urine samples for prohibited performance-enhancing drugs, for example anabolic steroids . A post–September 11 development, explosive detection systems have become

Gas chromatography–mass spectrometry - Misplaced Pages Continue

4448-612: Is also evidence for the asymmetric red dimer O=N–O=N when nitric oxide is condensed with polar molecules. It reacts with oxygen to give brown nitrogen dioxide and with halogens to give nitrosyl halides. It also reacts with transition metal compounds to give nitrosyl complexes, most of which are deeply coloured. Blue dinitrogen trioxide (N 2 O 3 ) is only available as a solid because it rapidly dissociates above its melting point to give nitric oxide, nitrogen dioxide (NO 2 ), and dinitrogen tetroxide (N 2 O 4 ). The latter two compounds are somewhat difficult to study individually because of

4587-662: Is also used to detect and measure contaminants from spoilage or adulteration which may be harmful and which is often controlled by governmental agencies, for example pesticides . Several GC–MS systems have left earth. Two were brought to Mars by the Viking program . Venera 11 and 12 and Pioneer Venus analysed the atmosphere of Venus with GC–MS. The Huygens probe of the Cassini–Huygens mission landed one GC–MS on Saturn 's largest moon, Titan . The MSL Curiosity rover's Sample analysis at Mars (SAM) instrument contains both

4726-412: Is also utilized in the identification of continuous phase component in a smart material, magnetorheological (MR) fluid . Foods and beverages contain numerous aromatic compounds , some naturally present in the raw materials and some forming during processing. GC–MS is extensively used for the analysis of these compounds which include esters , fatty acids , alcohols , aldehydes , terpenes etc. It

4865-424: Is assumed to be the ground truth. Some authors use the term "golden standard". Claassen argues this usage is incorrect, as "golden standard" implies a level of perfection that is unattainable in medical science. Nitrogen gas Nitrogen is a chemical element ; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table , often called

5004-420: Is capable of performing both functions either individually or concomitantly, depending on the setup of the particular instrument. The primary goal of instrument analysis is to quantify an amount of substance. This is done by comparing the relative concentrations among the atomic masses in the generated spectrum. Two kinds of analysis are possible, comparative and original. Comparative analysis essentially compares

5143-415: Is done automatically by programs which come with the instrument, given a list of the elements which could be present in the sample. A "full spectrum" analysis considers all the "peaks" within a spectrum. Conversely, selective ion monitoring (SIM) only monitors selected ions associated with a specific substance. This is done on the assumption that at a given retention time, a set of ions is characteristic of

5282-400: Is especially important when a perfect test is available only by autopsy. A test has to meet some interobserver agreement, to avoid some bias induced by the study itself. Calibration errors can lead to misdiagnosis . Sometimes "gold standard test" refers to the best-performing test available. In these cases, there is no other criterion against which it can be compared and it is equivalent to

5421-404: Is essentially intermediate in size between boron and nitrogen, much of organic chemistry finds an echo in boron–nitrogen chemistry, such as in borazine ("inorganic benzene "). Nevertheless, the analogy is not exact due to the ease of nucleophilic attack at boron due to its deficiency in electrons, which is not possible in a wholly carbon-containing ring. The largest category of nitrides are

5560-464: Is increasingly becoming a common way to diagnose IEM for earlier diagnosis and institution of treatment eventually leading to a better outcome. It is now possible to test a newborn for over 100 genetic metabolic disorders by a urine test at birth based on GC–MS. In combination with isotopic labeling of metabolic compounds, the GC–MS is used for determining metabolic activity . Most applications are based on

5699-550: Is increasingly used for detection of illegal narcotics, and may eventually supplant drug-sniffing dogs. A simple and selective GC–MS method for detecting marijuana usage was recently developed by the Robert Koch Institute in Germany. It involves identifying an acid metabolite of tetrahydrocannabinol (THC), the active ingredient in marijuana, in urine samples by employing derivatization in the sample preparation. GC–MS

Gas chromatography–mass spectrometry - Misplaced Pages Continue

5838-414: Is known as purging or sparging . The volatile compounds move into the headspace above the water and are drawn along a pressure gradient (caused by the introduction of the purge gas) out of the chamber. The volatile compounds are drawn along a heated line onto a 'trap'. The trap is a column of adsorbent material at ambient temperature that holds the compounds by returning them to the liquid phase. The trap

5977-515: Is known. Industrially, ammonia (NH 3 ) is the most important compound of nitrogen and is prepared in larger amounts than any other compound because it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilisers. It is a colourless alkaline gas with a characteristic pungent smell. The presence of hydrogen bonding has very significant effects on ammonia, conferring on it its high melting (−78 °C) and boiling (−33 °C) points. As

6116-434: Is mildly toxic in concentrations above 100 mg/kg, but small amounts are often used to cure meat and as a preservative to avoid bacterial spoilage. It is also used to synthesise hydroxylamine and to diazotise primary aromatic amines as follows: Nitrite is also a common ligand that can coordinate in five ways. The most common are nitro (bonded from the nitrogen) and nitrito (bonded from an oxygen). Nitro-nitrito isomerism

6255-629: Is mostly unreactive at room temperature, but it will nevertheless react with lithium metal and some transition metal complexes. This is due to its bonding, which is unique among the diatomic elements at standard conditions in that it has an N≡N triple bond . Triple bonds have short bond lengths (in this case, 109.76 pm) and high dissociation energies (in this case, 945.41 kJ/mol), and are thus very strong, explaining dinitrogen's low level of chemical reactivity. Other nitrogen oligomers and polymers may be possible. If they could be synthesised, they may have potential applications as materials with

6394-489: Is much more common, making up 99.634% of natural nitrogen, and the second (which is slightly heavier) makes up the remaining 0.366%. This leads to an atomic weight of around 14.007 u. Both of these stable isotopes are produced in the CNO cycle in stars , but N is more common as its proton capture is the rate-limiting step. N is one of the five stable odd–odd nuclides (a nuclide having an odd number of protons and neutrons);

6533-682: Is not possible for its vertical neighbours; thus, the nitrogen oxides , nitrites , nitrates , nitro- , nitroso -, azo -, and diazo -compounds, azides , cyanates , thiocyanates , and imino -derivatives find no echo with phosphorus, arsenic, antimony, or bismuth. By the same token, however, the complexity of the phosphorus oxoacids finds no echo with nitrogen. Setting aside their differences, nitrogen and phosphorus form an extensive series of compounds with one another; these have chain, ring, and cage structures. Table of thermal and physical properties of nitrogen (N 2 ) at atmospheric pressure: Nitrogen has two stable isotopes : N and N. The first

6672-415: Is not possible to make an accurate identification of a particular molecule by gas chromatography or mass spectrometry alone. The mass spectrometry process normally requires a very pure sample while gas chromatography using a traditional detector (e.g. Flame ionization detector ) cannot differentiate between multiple molecules that happen to take the same amount of time to travel through the column ( i.e. have

6811-660: Is of interest for the preparation of explosives. It is a deliquescent , colourless crystalline solid that is sensitive to light. In the solid state it is ionic with structure [NO 2 ] [NO 3 ] ; as a gas and in solution it is molecular O 2 N–O–NO 2 . Hydration to nitric acid comes readily, as does analogous reaction with hydrogen peroxide giving peroxonitric acid (HOONO 2 ). It is a violent oxidising agent. Gaseous dinitrogen pentoxide decomposes as follows: Many nitrogen oxoacids are known, though most of them are unstable as pure compounds and are known only as aqueous solutions or as salts. Hyponitrous acid (H 2 N 2 O 2 )

6950-521: Is prepared by passing an electric discharge through nitrogen gas at 0.1–2 mmHg, which produces atomic nitrogen along with a peach-yellow emission that fades slowly as an afterglow for several minutes even after the discharge terminates. Given the great reactivity of atomic nitrogen, elemental nitrogen usually occurs as molecular N 2 , dinitrogen. This molecule is a colourless, odourless, and tasteless diamagnetic gas at standard conditions: it melts at −210 °C and boils at −196 °C. Dinitrogen

7089-443: Is produced from O (in water) via an (n,p) reaction , in which the O atom captures a neutron and expels a proton. It has a short half-life of about 7.1 s, but its decay back to O produces high-energy gamma radiation (5 to 7 MeV). Because of this, access to the primary coolant piping in a pressurised water reactor must be restricted during reactor power operation. It is a sensitive and immediate indicator of leaks from

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7228-491: Is relatively rare in the solid parts of the Earth. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772 and independently by Carl Wilhelm Scheele and Henry Cavendish at about the same time. The name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790 when it was found that nitrogen was present in nitric acid and nitrates . Antoine Lavoisier suggested instead

7367-474: Is significant. It is a weak acid with p K a 3.35 at 18 °C. They may be titrimetrically analysed by their oxidation to nitrate by permanganate . They are readily reduced to nitrous oxide and nitric oxide by sulfur dioxide , to hyponitrous acid with tin (II), and to ammonia with hydrogen sulfide . Salts of hydrazinium N 2 H 5 react with nitrous acid to produce azides which further react to give nitrous oxide and nitrogen. Sodium nitrite

7506-482: Is similar to that in nitrogen, but one extra electron is added to a π * antibonding orbital and thus the bond order has been reduced to approximately 2.5; hence dimerisation to O=N–N=O is unfavourable except below the boiling point (where the cis isomer is more stable) because it does not actually increase the total bond order and because the unpaired electron is delocalised across the NO molecule, granting it stability. There

7645-404: Is smaller than those of boron (84 pm) and carbon (76 pm), while it is larger than those of oxygen (66 pm) and fluorine (57 pm). The nitride anion, N , is much larger at 146 pm, similar to that of the oxide (O : 140 pm) and fluoride (F : 133 pm) anions. The first three ionisation energies of nitrogen are 1.402, 2.856, and 4.577 MJ·mol , and the sum of

7784-424: Is the ion trap mass spectrometer. Additionally one may find a magnetic sector mass spectrometer, however these particular instruments are expensive and bulky and not typically found in high-throughput service laboratories. Other detectors may be encountered such as time of flight (TOF), tandem quadrupoles (MS-MS) (see below), or in the case of an ion trap MS where n indicates the number mass spectrometry stages. When

7923-523: Is the most important nitrogen radioisotope, being relatively long-lived enough to use in positron emission tomography (PET), although its half-life is still short and thus it must be produced at the venue of the PET, for example in a cyclotron via proton bombardment of O producing N and an alpha particle . The radioisotope N is the dominant radionuclide in the coolant of pressurised water reactors or boiling water reactors during normal operation. It

8062-400: Is the simplest stable molecule with an odd number of electrons. In mammals, including humans, it is an important cellular signalling molecule involved in many physiological and pathological processes. It is formed by catalytic oxidation of ammonia. It is a colourless paramagnetic gas that, being thermodynamically unstable, decomposes to nitrogen and oxygen gas at 1100–1200 °C. Its bonding

8201-476: Is the strongest π donor known among ligands (the second-strongest is O ). Nitrido complexes are generally made by the thermal decomposition of azides or by deprotonating ammonia, and they usually involve a terminal {≡N} group. The linear azide anion ( N 3 ), being isoelectronic with nitrous oxide , carbon dioxide , and cyanate , forms many coordination complexes. Further catenation is rare, although N 4 (isoelectronic with carbonate and nitrate )

8340-469: Is the test against which new tests are compared to gauge their validity, and it is used to evaluate the efficacy of treatments. The meaning of "gold standard" may differ between practical medicine and the statistical ideal. With some medical conditions, only an autopsy can guarantee diagnostic certainty. In these cases, the gold standard test is the best test that keeps the patient alive, and even gold standard tests can require follow-up to confirm or refute

8479-399: Is then heated and the sample compounds are introduced to the GC–MS column via a volatiles interface, which is a split inlet system. P&T GC–MS is particularly suited to volatile organic compounds (VOCs) and BTEX compounds (aromatic compounds associated with petroleum). A faster alternative is the "purge-closed loop" system. In this system the inert gas is bubbled through the water until

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8618-640: Is thermodynamically stable, and most readily produced by the electrolysis of molten ammonium fluoride dissolved in anhydrous hydrogen fluoride . Like carbon tetrafluoride , it is not at all reactive and is stable in water or dilute aqueous acids or alkalis. Only when heated does it act as a fluorinating agent, and it reacts with copper , arsenic, antimony, and bismuth on contact at high temperatures to give tetrafluorohydrazine (N 2 F 4 ). The cations NF 4 and N 2 F 3 are also known (the latter from reacting tetrafluorohydrazine with strong fluoride-acceptors such as arsenic pentafluoride ), as

8757-512: The Greek word άζωτικός (azotikos), "no life", due to it being asphyxiant . In an atmosphere of pure nitrogen, animals died and flames were extinguished. Though Lavoisier's name was not accepted in English since it was pointed out that all gases but oxygen are either asphyxiant or outright toxic, it is used in many languages (French, Italian, Portuguese, Polish, Russian, Albanian, Turkish, etc.;

8896-435: The anhydride of hyponitrous acid (H 2 N 2 O 2 ) because that acid is not produced by the dissolution of nitrous oxide in water. It is rather unreactive (not reacting with the halogens, the alkali metals, or ozone at room temperature, although reactivity increases upon heating) and has the unsymmetrical structure N–N–O (N≡N O ↔ N=N =O): above 600 °C it dissociates by breaking the weaker N–O bond. Nitric oxide (NO)

9035-575: The nucleic acids ( DNA and RNA ) and in the energy transfer molecule adenosine triphosphate . The human body contains about 3% nitrogen by mass, the fourth most abundant element in the body after oxygen, carbon, and hydrogen. The nitrogen cycle describes the movement of the element from the air, into the biosphere and organic compounds, then back into the atmosphere. Nitrogen is a constituent of every major pharmacological drug class, including antibiotics . Many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules : for example,

9174-552: The pnictogens . It is a common element in the universe , estimated at seventh in total abundance in the Milky Way and the Solar System . At standard temperature and pressure , two atoms of the element bond to form N 2 , a colourless and odourless diatomic gas . N 2 forms about 78% of Earth's atmosphere , making it the most abundant chemical species in air. Because of the volatility of nitrogen compounds, nitrogen

9313-566: The 2s and 2p orbitals, three of which (the p-electrons) are unpaired. It has one of the highest electronegativities among the elements (3.04 on the Pauling scale), exceeded only by chlorine (3.16), oxygen (3.44), and fluorine (3.98). (The light noble gases , helium , neon , and argon , would presumably also be more electronegative, and in fact are on the Allen scale.) Following periodic trends, its single-bond covalent radius of 71 pm

9452-633: The French nitrogène , coined in 1790 by French chemist Jean-Antoine Chaptal (1756–1832), from the French nitre ( potassium nitrate , also called saltpetre ) and the French suffix -gène , "producing", from the Greek -γενής (-genes, "begotten"). Chaptal's meaning was that nitrogen is the essential part of nitric acid , which in turn was produced from nitre . In earlier times, nitre had been confused with Egyptian "natron" ( sodium carbonate ) – called νίτρον (nitron) in Greek ;– which, despite

9591-535: The German Stickstoff similarly refers to the same characteristic, viz. ersticken "to choke or suffocate") and still remains in English in the common names of many nitrogen compounds, such as hydrazine and compounds of the azide ion. Finally, it led to the name " pnictogens " for the group headed by nitrogen, from the Greek πνίγειν "to choke". The English word nitrogen (1794) entered the language from

9730-744: The Middle Ages. Alchemists knew nitric acid as aqua fortis (strong water), as well as other nitrogen compounds such as ammonium salts and nitrate salts. The mixture of nitric and hydrochloric acids was known as aqua regia (royal water), celebrated for its ability to dissolve gold , the king of metals. The discovery of nitrogen is attributed to the Scottish physician Daniel Rutherford in 1772, who called it noxious air . Though he did not recognise it as an entirely different chemical substance, he clearly distinguished it from Joseph Black's "fixed air" , or carbon dioxide. The fact that there

9869-860: The N anion, although charge separation is not actually complete even for these highly electropositive elements. However, the alkali metal azides NaN 3 and KN 3 , featuring the linear N 3 anion, are well-known, as are Sr(N 3 ) 2 and Ba(N 3 ) 2 . Azides of the B-subgroup metals (those in groups 11 through 16 ) are much less ionic, have more complicated structures, and detonate readily when shocked. Many covalent binary nitrides are known. Examples include cyanogen ((CN) 2 ), triphosphorus pentanitride (P 3 N 5 ), disulfur dinitride (S 2 N 2 ), and tetrasulfur tetranitride (S 4 N 4 ). The essentially covalent silicon nitride (Si 3 N 4 ) and germanium nitride (Ge 3 N 4 ) are also known: silicon nitride, in particular, would make

10008-527: The National Institute of Standards (NIST-USA). Spectral library searches employ matching algorithms such as Probability Based Matching and dot-product matching that are used with methods of analysis written by many method standardization agencies. Sources of libraries include NIST, Wiley, the AAFS, and instrument manufacturers. The "hard ionization" process of electron ionization can be softened by

10147-450: The N≡N bond may be formed directly within a metal complex, for example by directly reacting coordinated ammonia (NH 3 ) with nitrous acid (HNO 2 ), but this is not generally applicable. Most dinitrogen complexes have colours within the range white-yellow-orange-red-brown; a few exceptions are known, such as the blue [{Ti( η -C 5 H 5 ) 2 } 2 -(N 2 )]. Nitrogen bonds to almost all

10286-418: The ability to form coordination complexes by donating its lone pairs of electrons. There are some parallels between the chemistry of ammonia NH 3 and water H 2 O. For example, the capacity of both compounds to be protonated to give NH 4 and H 3 O or deprotonated to give NH 2 and OH , with all of these able to be isolated in solid compounds. Nitrogen shares with both its horizontal neighbours

10425-430: The advancements of magnetic resonance imaging , the magnetic resonance angiogram (MRA) has become the new gold standard test for aortic dissection, with a sensitivity of 95% and a specificity of 92%. Before the widespread acceptance of any new test, the former test retains its status as the "gold standard". Because tests can be incorrect (yielding a false-negative or a false-positive ), results should be interpreted in

10564-417: The atmosphere. The N: N ratio is commonly used in stable isotope analysis in the fields of geochemistry , hydrology , paleoclimatology and paleoceanography , where it is called δ N . Of the thirteen other isotopes produced synthetically, ranging from N to N, N has a half-life of ten minutes and the remaining isotopes have half-lives less than eight seconds. Given the half-life difference, N

10703-403: The best-performing test available, but the best available under reasonable conditions. For example, in this sense, an MRI is the gold standard for brain tumor diagnosis, though it is not as good as a biopsy. In this case, the sensitivity and specificity of the gold standard are not 100% and it is said to be an "imperfect gold standard" or "alloyed gold standard". The term ground truth refers to

10842-435: The beta hexagonal close-packed crystal allotropic form. Below 35.4 K (−237.6 °C) nitrogen assumes the cubic crystal allotropic form (called the alpha phase). Liquid nitrogen , a colourless fluid resembling water in appearance, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808 g/mL), is a common cryogen . Solid nitrogen has many crystalline modifications. It forms

10981-493: The concentrations of organic compounds in the vapor phase are at equilibrium with concentrations in the aqueous phase. The gas phase is then analysed directly. The most common type of mass spectrometer (MS) associated with a gas chromatograph (GC) is the quadrupole mass spectrometer, sometimes referred to by the Hewlett-Packard (now Agilent ) trade name "Mass Selective Detector" (MSD). Another relatively common detector

11120-416: The conjugate acid of the azide anion, and is similarly analogous to the hydrohalic acids . All four simple nitrogen trihalides are known. A few mixed halides and hydrohalides are known, but are mostly unstable; examples include NClF 2 , NCl 2 F, NBrF 2 , NF 2 H, NFH 2 , NCl 2 H , and NClH 2 . Nitrogen trifluoride (NF 3 , first prepared in 1928) is a colourless and odourless gas that

11259-400: The context of the history, physical findings, and other test results of the individual being tested. It is within this context that the sensitivity and specificity of the "gold standard" test is determined. When the gold standard is not a perfect one, its sensitivity and specificity must be calibrated against more accurate tests or against the definition of the condition. This calibration

11398-555: The continuity of bonding types instead of the discrete and separate types that it implies. They are normally prepared by directly reacting a metal with nitrogen or ammonia (sometimes after heating), or by thermal decomposition of metal amides: Many variants on these processes are possible. The most ionic of these nitrides are those of the alkali metals and alkaline earth metals , Li 3 N (Na, K, Rb, and Cs do not form stable nitrides for steric reasons) and M 3 N 2 (M = Be, Mg, Ca, Sr, Ba). These can formally be thought of as salts of

11537-470: The cooling of the molecules before their ionization, resulting in mass spectra that are richer in information. In this method named cold electron ionization (cold-EI) the molecules exit the GC column, mixed with added helium make up gas and expand into vacuum through a specially designed supersonic nozzle, forming a supersonic molecular beam (SMB). Collisions with the make up gas at the expanding supersonic jet reduce

11676-427: The degree of certainty about the identity of that compound is reduced. When collecting data in the full scan mode, a target range of mass fragments is determined and put into the instrument's method. An example of a typical broad range of mass fragments to monitor would be m/z 50 to m/z 400. The determination of what range to use is largely dictated by what one anticipates being in the sample while being cognizant of

11815-417: The destruction of hydrazine by reaction with monochloramine (NH 2 Cl) to produce ammonium chloride and nitrogen. Hydrogen azide (HN 3 ) was first produced in 1890 by the oxidation of aqueous hydrazine by nitrous acid. It is very explosive and even dilute solutions can be dangerous. It has a disagreeable and irritating smell and is a potentially lethal (but not cumulative) poison. It may be considered

11954-400: The diagnosis. The term 'gold standard' in its current sense in medical research was coined by Rudd in 1979, in reference to the monetary gold standard. "Gold standard" can refer to popular clinical endpoints by which scientific evidence is evaluated. For example, in resuscitation research, the "gold standard" test of a medication or procedure is whether or not it leads to an increase in

12093-555: The direction of Robert E. Finnigan . By 1966 Finnigan and collaborator Mike Uthe's EAI division had sold over 500 quadrupole residual gas-analyzer instruments. In 1967, Finnigan left EAI to form the Finnigan Instrument Corporation along with Roger Sant, T. Z. Chou, Michael Story, Lloyd Friedman, and William Fies. In early 1968, they delivered the first prototype quadrupole GC/MS instruments to Stanford and Purdue University. When Finnigan Instrument Corporation

12232-413: The electrons and analyte and cause a 'soft' ionization of the molecule of interest. A softer ionization fragments the molecule to a lower degree than the hard ionization of EI. One of the main benefits of using chemical ionization is that a mass fragment closely corresponding to the molecular weight of the analyte of interest is produced. In positive chemical ionization (PCI) the reagent gas interacts with

12371-734: The elements in the periodic table except the first two noble gases , helium and neon , and some of the very short-lived elements after bismuth , creating an immense variety of binary compounds with varying properties and applications. Many binary compounds are known: with the exception of the nitrogen hydrides, oxides, and fluorides, these are typically called nitrides . Many stoichiometric phases are usually present for most elements (e.g. MnN, Mn 6 N 5 , Mn 3 N 2 , Mn 2 N, Mn 4 N, and Mn x N for 9.2 < x < 25.3). They may be classified as "salt-like" (mostly ionic), covalent, "diamond-like", and metallic (or interstitial ), although this classification has limitations generally stemming from

12510-467: The equilibrium between them, although sometimes dinitrogen tetroxide can react by heterolytic fission to nitrosonium and nitrate in a medium with high dielectric constant. Nitrogen dioxide is an acrid, corrosive brown gas. Both compounds may be easily prepared by decomposing a dry metal nitrate. Both react with water to form nitric acid . Dinitrogen tetroxide is very useful for the preparation of anhydrous metal nitrates and nitrato complexes, and it became

12649-421: The first gases to be identified: N 2 O ( nitrous oxide ), NO ( nitric oxide ), N 2 O 3 ( dinitrogen trioxide ), NO 2 ( nitrogen dioxide ), N 2 O 4 ( dinitrogen tetroxide ), N 2 O 5 ( dinitrogen pentoxide ), N 4 O ( nitrosylazide ), and N(NO 2 ) 3 ( trinitramide ). All are thermally unstable towards decomposition to their elements. One other possible oxide that has not yet been synthesised

12788-496: The fourth and fifth is 16.920 MJ·mol . Due to these very high figures, nitrogen has no simple cationic chemistry. The lack of radial nodes in the 2p subshell is directly responsible for many of the anomalous properties of the first row of the p-block , especially in nitrogen, oxygen, and fluorine. The 2p subshell is very small and has a very similar radius to the 2s shell, facilitating orbital hybridisation . It also results in very large electrostatic forces of attraction between

12927-406: The given spectrum to a spectrum library to see if its characteristics are present for some sample in the library. This is best performed by a computer because there are a myriad of visual distortions that can take place due to variations in scale. Computers can also simultaneously correlate more data (such as the retention times identified by GC), to more accurately relate certain data. Deep learning

13066-437: The head of group 15 in the periodic table, its chemistry shows huge differences from that of its heavier congeners phosphorus , arsenic , antimony , and bismuth . Nitrogen may be usefully compared to its horizontal neighbours' carbon and oxygen as well as its vertical neighbours in the pnictogen column, phosphorus, arsenic, antimony, and bismuth. Although each period 2 element from lithium to oxygen shows some similarities to

13205-561: The high temperatures (300°C) used in the GC–MS injection port (and oven) can result in thermal degradation of injected molecules, thus resulting in the measurement of degradation products instead of the actual molecule(s) of interest. The first on-line coupling of gas chromatography to a mass spectrometer was reported in the late 1950s. An interest in coupling the methods had been suggested as early as December 1954, but conventional recording techniques had too poor temporal resolution. Fortunately, time-of-flight mass spectrometry developed around

13344-418: The identification probabilities of both known and unknown compounds, amplify isomer mass spectral effects and enable the use of isotope abundance analysis for the elucidation of elemental formulas. In chemical ionization (CI) a reagent gas, typically methane or ammonia is introduced into the mass spectrometer. Depending on the technique (positive CI or negative CI) chosen, this reagent gas will interact with

13483-431: The instrument method and only those mass fragments are detected by the mass spectrometer. The advantages of SIM are that the detection limit is lower since the instrument is only looking at a small number of fragments (e.g. three fragments) during each scan. More scans can take place each second. Since only a few mass fragments of interest are being monitored, matrix interferences are typically lower. To additionally confirm

13622-419: The internal vibrational (and rotational) energy of the analyte molecules, hence reducing the degree of fragmentation caused by the electrons during the ionization process. Cold-EI mass spectra are characterized by an abundant molecular ion while the usual fragmentation pattern is retained, thus making cold-EI mass spectra compatible with library search identification techniques. The enhanced molecular ions increase

13761-505: The interstitial nitrides of formulae MN, M 2 N, and M 4 N (although variable composition is perfectly possible), where the small nitrogen atoms are positioned in the gaps in a metallic cubic or hexagonal close-packed lattice. They are opaque, very hard, and chemically inert, melting only at very high temperatures (generally over 2500 °C). They have a metallic lustre and conduct electricity as do metals. They hydrolyse only very slowly to give ammonia or nitrogen. The nitride anion (N )

13900-478: The likelihood of a potentially positive result, it is relatively important to be sure that the ion ratios of the various mass fragments are comparable to a known reference standard. GC–MS is becoming the tool of choice for tracking organic pollutants in the environment. The cost of GC–MS equipment has decreased significantly, and the reliability has increased at the same time, which has contributed to its increased adoption in environmental studies . GC–MS can analyze

14039-484: The molecular mass unit. Hard ionization is considered by mass spectrometrists as the employ of molecular electron bombardment, whereas "soft ionization" is charge by molecular collision with an introduced gas. The molecular fragmentation pattern is dependent upon the electron energy applied to the system, typically 70 eV (electronvolts). The use of 70 eV facilitates comparison of generated spectra with library spectra using manufacturer-supplied software or software developed by

14178-553: The name azote , from the Ancient Greek : ἀζωτικός "no life", as it is an asphyxiant gas ; this name is used in a number of languages, and appears in the English names of some nitrogen compounds such as hydrazine , azides and azo compounds . Elemental nitrogen is usually produced from air by pressure swing adsorption technology. About 2/3 of commercially produced elemental nitrogen is used as an inert (oxygen-free) gas for commercial uses such as food packaging, and much of

14317-525: The name, contained no nitrate. The earliest military, industrial, and agricultural applications of nitrogen compounds used saltpetre ( sodium nitrate or potassium nitrate), most notably in gunpowder , and later as fertiliser . In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", a monatomic allotrope of nitrogen. The "whirling cloud of brilliant yellow light" produced by his apparatus reacted with mercury to produce explosive mercury nitride . For

14456-574: The nitryl halides (XNO 2 ). The first is very reactive gases that can be made by directly halogenating nitrous oxide. Nitrosyl fluoride (NOF) is colourless and a vigorous fluorinating agent. Nitrosyl chloride (NOCl) behaves in much the same way and has often been used as an ionising solvent. Nitrosyl bromide (NOBr) is red. The reactions of the nitryl halides are mostly similar: nitryl fluoride (FNO 2 ) and nitryl chloride (ClNO 2 ) are likewise reactive gases and vigorous halogenating agents. Nitrogen forms nine molecular oxides, some of which were

14595-436: The nucleus and the valence electrons in the 2s and 2p shells, resulting in very high electronegativities. Hypervalency is almost unknown in the 2p elements for the same reason, because the high electronegativity makes it difficult for a small nitrogen atom to be a central atom in an electron-rich three-center four-electron bond since it would tend to attract the electrons strongly to itself. Thus, despite nitrogen's position at

14734-537: The number of neurologically intact survivors that walk out of the hospital. Other types of medical research might regard a significant decrease in 30-day mortality as the gold standard. The AMA Style Guide has preferred the phrase criterion standard instead of "gold standard." Other journals have also issued mandates in their instructions for contributors. For instance, the Archives of Biological Medicine and Rehabilitation specifies this usage. In practice, however,

14873-410: The organic nitrates nitroglycerin and nitroprusside control blood pressure by metabolising into nitric oxide . Many notable nitrogen-containing drugs, such as the natural caffeine and morphine or the synthetic amphetamines , act on receptors of animal neurotransmitters . Nitrogen compounds have a very long history, ammonium chloride having been known to Herodotus . They were well-known by

15012-412: The other four are H , Li, B, and Ta. The relative abundance of N and N is practically constant in the atmosphere but can vary elsewhere, due to natural isotopic fractionation from biological redox reactions and the evaporation of natural ammonia or nitric acid . Biologically mediated reactions (e.g., assimilation , nitrification , and denitrification ) strongly control nitrogen dynamics in

15151-421: The other nonmetals are very complex and tend to lead to a mixture of products. Ammonia reacts on heating with metals to give nitrides. Many other binary nitrogen hydrides are known, but the most important are hydrazine (N 2 H 4 ) and hydrogen azide (HN 3 ). Although it is not a nitrogen hydride, hydroxylamine (NH 2 OH) is similar in properties and structure to ammonia and hydrazine as well. Hydrazine

15290-478: The oxidation of ammonia to nitrite, which occurs in the nitrogen cycle . Hyponitrite can act as a bridging or chelating bidentate ligand. Nitrous acid (HNO 2 ) is not known as a pure compound, but is a common component in gaseous equilibria and is an important aqueous reagent: its aqueous solutions may be made from acidifying cool aqueous nitrite ( NO 2 , bent) solutions, although already at room temperature disproportionation to nitrate and nitric oxide

15429-403: The particles from a human body in order to help link a criminal to a crime . The analysis of fire debris using GC–MS is well established, and there is even an established American Society for Testing and Materials (ASTM) standard for fire debris analysis. GCMS/MS is especially useful here as samples often contain very complex matrices, and results used in court need to be highly accurate. GC–MS

15568-704: The period 3 element in the next group (from magnesium to chlorine; these are known as diagonal relationships ), their degree drops off abruptly past the boron–silicon pair. The similarities of nitrogen to sulfur are mostly limited to sulfur nitride ring compounds when both elements are the only ones present. Nitrogen does not share the proclivity of carbon for catenation . Like carbon, nitrogen tends to form ionic or metallic compounds with metals. Nitrogen forms an extensive series of nitrides with carbon, including those with chain-, graphitic- , and fullerenic -like structures. It resembles oxygen with its high electronegativity and concomitant capability for hydrogen bonding and

15707-402: The phase properties (e.g. 5% phenyl polysiloxane). The difference in the chemical properties between different molecules in a mixture and their relative affinity for the stationary phase of the column will promote separation of the molecules as the sample travels the length of the column. The molecules are retained by the column and then elute (come off) from the column at different times (called

15846-610: The post-September 11 drive towards increased capability in homeland security and public health preparedness, traditional GC–MS units with transmission quadrupole mass spectrometers, as well as those with cylindrical ion trap (CIT-MS) and toroidal ion trap (T-ITMS) mass spectrometers have been modified for field portability and near real-time detection of chemical warfare agents (CWA) such as sarin, soman, and VX. These complex and large GC–MS systems have been modified and configured with resistively heated low thermal mass (LTM) gas chromatographs that reduce analysis time to less than ten percent of

15985-583: The primary coolant system to the secondary steam cycle and is the primary means of detection for such leaks. Atomic nitrogen, also known as active nitrogen, is highly reactive, being a triradical with three unpaired electrons. Free nitrogen atoms easily react with most elements to form nitrides, and even when two free nitrogen atoms collide to produce an excited N 2 molecule, they may release so much energy on collision with even such stable molecules as carbon dioxide and water to cause homolytic fission into radicals such as CO and O or OH and H. Atomic nitrogen

16124-522: The production of fertilisers. Dinitrogen is able to coordinate to metals in five different ways. The more well-characterised ways are the end-on M←N≡N ( η ) and M←N≡N→M ( μ , bis- η ), in which the lone pairs on the nitrogen atoms are donated to the metal cation. The less well-characterised ways involve dinitrogen donating electron pairs from the triple bond, either as a bridging ligand to two metal cations ( μ , bis- η ) or to just one ( η ). The fifth and unique method involves triple-coordination as

16263-508: The rest is used as liquid nitrogen in cryogenic applications. Many industrially important compounds, such as ammonia , nitric acid, organic nitrates ( propellants and explosives ), and cyanides , contain nitrogen. The extremely strong triple bond in elemental nitrogen (N≡N), the second strongest bond in any diatomic molecule after carbon monoxide (CO), dominates nitrogen chemistry. This causes difficulty for both organisms and industry in converting N 2 into useful compounds , but at

16402-432: The retention time), and this allows the mass spectrometer downstream to capture, ionize, accelerate, deflect, and detect the ionized molecules separately. The mass spectrometer does this by breaking each molecule into ionized fragments and detecting these fragments using their mass-to-charge ratio. These two components, used together, allow a much finer degree of substance identification than either unit used separately. It

16541-462: The same retention time), which results in two or more molecules that co-elute. Sometimes two different molecules can also have a similar pattern of ionized fragments in a mass spectrometer (mass spectrum). Combining the two processes reduces the possibility of error, as it is extremely unlikely that two different molecules will behave in the same way in both a gas chromatograph and a mass spectrometer. Therefore, when an identifying mass spectrum appears at

16680-456: The same time allowed to measure spectra thousands times a second. The development of affordable and miniaturized computers has helped in the simplification of the use of this instrument, as well as allowed great improvements in the amount of time it takes to analyze a sample. In 1964, Electronic Associates, Inc. (EAI) , a leading U.S. supplier of analog computers, began development of a computer controlled quadrupole mass spectrometer under

16819-599: The same time it means that burning, exploding, or decomposing nitrogen compounds to form nitrogen gas releases large amounts of often useful energy. Synthetically produced ammonia and nitrates are key industrial fertilisers , and fertiliser nitrates are key pollutants in the eutrophication of water systems. Apart from its use in fertilisers and energy stores, nitrogen is a constituent of organic compounds as diverse as aramids used in high-strength fabric and cyanoacrylate used in superglue . Nitrogen occurs in all organisms, primarily in amino acids (and thus proteins ), in

16958-583: The same time, use of the Ostwald process (1902) to produce nitrates from industrial nitrogen fixation allowed the large-scale industrial production of nitrates as feedstock in the manufacture of explosives in the World Wars of the 20th century. A nitrogen atom has seven electrons. In the ground state, they are arranged in the electron configuration 1s 2s 2p x 2p y 2p z . It, therefore, has five valence electrons in

17097-450: The sample is fragmented it will then be detected, usually by an electron multiplier , which essentially turns the ionized mass fragment into an electrical signal that is then detected. The ionization technique chosen is independent of using full scan or SIM. By far the most common and perhaps standard form of ionization is electron ionization (EI). The molecules enter into the MS (the source

17236-421: The smaller mass detectors. Depending on the system, the analytes can be introduced via liquid injection, desorbed from sorbent tubes through a thermal desorption process, or with solid-phase micro extraction (SPME). GC–MS is used for the analysis of unknown organic compound mixtures. One critical use of this technology is the use of GC–MS to determine the composition of bio-oils processed from raw biomass. GC–MS

17375-451: The soil. These reactions typically result in N enrichment of the substrate and depletion of the product . The heavy isotope N was first discovered by S. M. Naudé in 1929, and soon after heavy isotopes of the neighbouring elements oxygen and carbon were discovered. It presents one of the lowest thermal neutron capture cross-sections of all isotopes. It is frequently used in nuclear magnetic resonance (NMR) spectroscopy to determine

17514-401: The solvent and other possible interferences. A MS should not be set to look for mass fragments too low or else one may detect air (found as m/z 28 due to nitrogen), carbon dioxide ( m/z 44) or other possible interference. Additionally if one is to use a large scan range then sensitivity of the instrument is decreased due to performing fewer scans per second since each scan will have to detect

17653-520: The storable oxidiser of choice for many rockets in both the United States and USSR by the late 1950s. This is because it is a hypergolic propellant in combination with a hydrazine -based rocket fuel and can be easily stored since it is liquid at room temperature. The thermally unstable and very reactive dinitrogen pentoxide (N 2 O 5 ) is the anhydride of nitric acid , and can be made from it by dehydration with phosphorus pentoxide . It

17792-489: The structures of nitrogen-containing molecules, due to its fractional nuclear spin of one-half, which offers advantages for NMR such as narrower line width. N, though also theoretically usable, has an integer nuclear spin of one and thus has a quadrupole moment that leads to wider and less useful spectra. N NMR nevertheless has complications not encountered in the more common H and C NMR spectroscopy. The low natural abundance of N (0.36%) significantly reduces sensitivity,

17931-439: The target molecule, most often with a proton exchange. This produces the species in relatively high amounts. In negative chemical ionization (NCI) the reagent gas decreases the impact of the free electrons on the target analyte. This decreased energy typically leaves the fragment in great supply. A mass spectrometer is typically utilized in one of two ways: full scan or selective ion monitoring (SIM). The typical GC–MS instrument

18070-494: The time required in traditional laboratory systems. Additionally, the systems are smaller, and more mobile, including units that are mounted in mobile analytical laboratories (MAL), such as those used by the United States Marine Corps Chemical and Biological Incident Response Force MAL and other similar laboratories, and systems that are hand-carried by two-person teams or individuals, much ado to

18209-420: The touch of a feather, shifting air currents, or even alpha particles . For this reason, small amounts of nitrogen triiodide are sometimes synthesised as a demonstration to high school chemistry students or as an act of "chemical magic". Chlorine azide (ClN 3 ) and bromine azide (BrN 3 ) are extremely sensitive and explosive. Two series of nitrogen oxohalides are known: the nitrosyl halides (XNO) and

18348-498: The type of MS/MS analysis being performed. Types of analysis include product ion scan, precursor ion scan, selected reaction monitoring (SRM) (sometimes referred to as multiple reaction monitoring (MRM)) and neutral loss scan. For example: When Q1 is in static mode (looking at one mass only as in SIM), and Q3 is in scanning mode, one obtains a so-called product ion spectrum (also called "daughter spectrum"). From this spectrum, one can select

18487-400: The underlying absolute state of information; the gold standard strives to represent the ground truth as closely as possible. While the gold standard is the best effort to obtain the truth, ground truth is typically collected by direct observations. In machine learning and information retrieval , "ground truth" is the preferred term even when classifications may be imperfect; the gold standard

18626-419: The uptake of this term by authors, as well as enforcement by editorial staff, is notably poor, at least for AMA journals. When the criterion is a whole clinical testing procedure it is usually referred to as clinical case definition . Differing case definitions can produce wildly different results when used as the basis for evalulating a given diagnostic method. A hypothetical ideal "gold standard" test has

18765-447: The use of C as the labeling and the measurement of C-C ratios with an isotope ratio mass spectrometer (IRMS); an MS with a detector designed to measure a few select ions and return values as ratios. Gold standard (test) In medicine and medical statistics , the gold standard , criterion standard , or reference standard is the diagnostic test or benchmark that is the best available under reasonable conditions. It

18904-412: Was a component of air that does not support combustion was clear to Rutherford, although he was not aware that it was an element. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele , Henry Cavendish , and Joseph Priestley , who referred to it as burnt air or phlogisticated air . French chemist Antoine Lavoisier referred to nitrogen gas as " mephitic air " or azote , from

19043-445: Was acquired by Thermo Instrument Systems (later Thermo Fisher Scientific ) in 1990, it was considered "the world's leading manufacturer of mass spectrometers". The GC–MS is composed of two major building blocks: the gas chromatograph and the mass spectrometer . The gas chromatograph utilizes a capillary column whose properties regarding molecule separation depend on the column's dimensions (length, diameter, film thickness) as well as

19182-442: Was first found as a product of the thermal decomposition of FN 3 . Nitrogen trichloride (NCl 3 ) is a dense, volatile, and explosive liquid whose physical properties are similar to those of carbon tetrachloride , although one difference is that NCl 3 is easily hydrolysed by water while CCl 4 is not. It was first synthesised in 1811 by Pierre Louis Dulong , who lost three fingers and an eye to its explosive tendencies. As

19321-455: Was shown to lead to promising results in the identification of VOCs from raw GC–MS data. Another method of analysis measures the peaks in relation to one another. In this method, the tallest peak is assigned 100% of the value, and the other peaks being assigned proportionate values. All values above 3% are assigned. The total mass of the unknown compound is normally indicated by the parent peak. The value of this parent peak can be used to fit with

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