Oxygen saturation is a relative measure of the concentration of oxygen, dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media water; the standard unit of oxygen saturation is percent. Oxygen saturation can be measured noninvasively. Arterial oxygen saturation is measured using pulse oximetry. Tissue saturation at peripheral scale can be measured using NIRS; this technique can be applied on both brain. In medicine, oxygen saturation refers to oxygenation, or when oxygen molecules enter the tissues of the body. In this case blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygen saturation measure the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Fish, invertebrates and aerobic bacteria all require oxygen for respiration. In aquatic environments, oxygen saturation is a ratio of the concentration of dissolved oxygen, to the maximum amount of oxygen that will dissolve in that water body, at the temperature and pressure which constitute stable equilibrium conditions.
Well-aerated water without oxygen producers or consumers is 100% saturated. It is possible for stagnant water to become somewhat supersaturated with oxygen either because of the presence of photosynthetic aquatic oxygen producers or because of a slow equilibration after a change of atmospheric conditions. Stagnant water in the presence of decaying matter will have an oxygen concentration much less than 100%, due to anaerobic bacteria being much less efficient at breaking down organic material. Similar to water, oxygen concentration plays a key role in the break down of organic matter in soils. Higher levels of oxygen saturation allow for aerobic bacteria to persist, which break down decaying organic material in soils much more efficiently than anaerobic bacteria, thus soils with high oxygen saturation will have less organic matter per volume than those with low oxygen saturation. Environmental oxygenation can be important to the sustainability of a particular ecosystem; the US Environmental Protection Agency has published a table of maximum equilibrium dissolved oxygen concentration versus temperature at atmospheric pressure.
The optimal levels in an estuary for dissolved oxygen is higher than 6 ppm. Insufficient oxygen caused by the decomposition of organic matter and/or nutrient pollution, may occur in bodies of water such as ponds and rivers, tending to suppress the presence of aerobic organisms such as fish. Deoxygenation increases the relative population of anaerobic organisms such as plants and some bacteria, resulting in fish kills and other adverse events; the net effect is to alter the balance of nature by increasing the concentration of anaerobic over aerobic species. Oxygen deficiency
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure. Various units are used to express pressure; some of these derive from a unit of force divided by a unit of area. Pressure may be expressed in terms of standard atmospheric pressure. Manometric units such as the centimetre of water, millimetre of mercury, inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer. Pressure is the amount of force applied at right angles to the surface of an object per unit area; the symbol for it is p or P. The IUPAC recommendation for pressure is a lower-case p. However, upper-case P is used; the usage of P vs p depends upon the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, on writing style. Mathematically: p = F A, where: p is the pressure, F is the magnitude of the normal force, A is the area of the surface on contact.
Pressure is a scalar quantity. It relates the vector surface element with the normal force acting on it; the pressure is the scalar proportionality constant that relates the two normal vectors: d F n = − p d A = − p n d A. The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outward; the equation has meaning in that, for any surface S in contact with the fluid, the total force exerted by the fluid on that surface is the surface integral over S of the right-hand side of the above equation. It is incorrect to say "the pressure is directed in such or such direction"; the pressure, as a scalar, has no direction. The force given by the previous relationship to the quantity has a direction, but the pressure does not. If we change the orientation of the surface element, the direction of the normal force changes accordingly, but the pressure remains the same. Pressure is distributed to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point.
It is a fundamental parameter in thermodynamics, it is conjugate to volume. The SI unit for pressure is the pascal, equal to one newton per square metre; this name for the unit was added in 1971. Other units of pressure, such as pounds per square inch and bar, are in common use; the CGS unit of pressure is 0.1 Pa.. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre and the like without properly identifying the force units, but using the names kilogram, kilogram-force, or gram-force as units of force is expressly forbidden in SI. The technical atmosphere is 1 kgf/cm2. Since a system under pressure has the potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume, it is therefore related to energy density and may be expressed in units such as joules per cubic metre. Mathematically: p =; some meteorologists prefer the hectopascal for atmospheric air pressure, equivalent to the older unit millibar. Similar pressures are given in kilopascals in most other fields, where the hecto- prefix is used.
The inch of mercury is still used in the United States. Oceanographers measure underwater pressure in decibars because pressure in the ocean increases by one decibar per metre depth; the standard atmosphere is an established constant. It is equal to typical air pressure at Earth mean sea level and is defined as 101325 Pa; because pressure is measured by its ability to displace a column of liquid in a manometer, pressures are expressed as a depth of a particular fluid. The most common choices are water; the pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation p = ρgh, where g is the gravitational acceleration. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, therefore lower energy, than the absorbed radiation; the most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can be seen only when exposed to UV light. Fluorescent materials cease to glow nearly when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after. Fluorescence has many practical applications, including mineralogy, medicine, chemical sensors, fluorescent labelling, biological detectors, cosmic-ray detection, most fluorescent lamps. Fluorescence occurs in nature in some minerals and in various biological states in many branches of the animal kingdom.
An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as lignum nephriticum. It was derived from the wood of Pterocarpus indicus and Eysenhardtia polystachya; the chemical compound responsible for this fluorescence is matlaline, the oxidation product of one of the flavonoids found in this wood. In 1819, Edward D. Clarke and in 1822 René Just Haüy described fluorescence in fluorites, Sir David Brewster described the phenomenon for chlorophyll in 1833 and Sir John Herschel did the same for quinine in 1845. In his 1852 paper on the "Refrangibility" of light, George Gabriel Stokes described the ability of fluorspar and uranium glass to change invisible light beyond the violet end of the visible spectrum into blue light, he named this phenomenon fluorescence: "I am inclined to coin a word, call the appearance fluorescence, from fluor-spar, as the analogous term opalescence is derived from the name of a mineral." The name was derived from the mineral fluorite, some examples of which contain traces of divalent europium, which serves as the fluorescent activator to emit blue light.
In a key experiment he used a prism to isolate ultraviolet radiation from sunlight and observed blue light emitted by an ethanol solution of quinine exposed by it. Fluorescence occurs when an orbital electron of a molecule, atom, or nanostructure, relaxes to its ground state by emitting a photon from an excited singlet state: Excitation: S 0 + h ν e x → S 1 Fluorescence: S 1 → S 0 + h ν e m + h e a t Here h ν is a generic term for photon energy with h = Planck's constant and ν = frequency of light; the specific frequencies of exciting and emitted lights are depended on the particular system. S0 is called the ground state of the fluorophore, S1 is its first excited singlet state. A molecule in S1 can relax by various competing pathways, it can undergo non-radiative relaxation in which the excitation energy is dissipated as heat to the solvent. Excited organic molecules can relax via conversion to a triplet state, which may subsequently relax via phosphorescence, or by a secondary non-radiative relaxation step.
Relaxation from S1 can occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen is an efficient quencher of fluorescence just because of its unusual triplet ground state. In most cases, the emitted light has a longer wavelength, therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; the emitted radiation may be of the same wavelength as the absorbed radiation, termed "resonance fluorescence". Molecules that are excited through light absorption or via a different process can transfer energy to a second'sensitized' molecule, converted to its excited state and can fluoresce; the fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed. Φ = Number of photons emitted Number of photons absorbed The maximum possible fluorescence quantum yield is 1.0.
Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence is by the rate of excited state decay: Φ = k f ∑ i k i where k f is the rate constant of spontaneous emission of radiation and ∑ i k i is the sum of all rates of
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
Acrylamide is an organic compound with the chemical formula CH2=CHCNH2. It is a white odorless solid, soluble in water and several organic solvents, it is produced industrially as a precursor to polyacrylamides, which find many uses as water-soluble thickeners and flocculation agents. It is toxic, for that reason it is handled as an aqueous solution; the discovery that some cooked foods contain acrylamide had attracted significant attention to its possible biological effects. Acrylamide can be prepared by the hydrolysis of acrylonitrile; the reaction is catalyzed by sulfuric acid as well as various metal salts. It is catalyzed by the enzyme nitrile hydratase. US demand for acrylamide was 253,000,000 pounds as of 2007, increased from 245,000,000 pounds in 2006. Acrylamide arises in some cooked foods via a series of steps initiated by the condensation of the amino acid asparagine and glucose; this condensation, one of the Maillard reactions followed by dehydrogenation produces N--L-asparagine, which upon pyrolysis generates some acylamide.
The majority of acrylamide is used to manufacture various polymers polyacrylamide used as a thickening agent and in water treatment. Acrylamide is classified as an hazardous substance in the United States as defined in Section 302 of the U. S. Emergency Planning and Community Right-to-Know Act, is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities. Acrylamide is considered a potential occupational carcinogen by U. S. government agencies and classified as a Group 2A carcinogen by the IARC. The Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health have set dermal occupational exposure limits at 0.03 mg/m3 over an eight-hour workday. In animal models, exposure to acrylamide causes tumors in the adrenal glands, thyroid and testes. Acrylamide is absorbed by the skin and distributed throughout the organism. Acrylamide can be metabolically-activated by cytochrome P450 to a genotoxic metabolite, considered to be a critical mode of action to the carcinogenesis of acrylamide.
On the other hand and glycidamide can be detoxified via conjugation with glutathione to form acrylamide- and isomeric glycidamide-glutathione conjugates, subsequently metabolized to mercapturic acids and excreted in urine. Acrylamide has been found to have neurotoxic effects in humans who have been exposed. Animal studies show neurotoxic effects as well as mutations in sperm. Acrylamide is a skin irritant and may be a tumor initiator in the skin increasing risk for skin cancer. Symptoms of acrylamide exposure include dermatitis in the exposed area, peripheral neuropathy. Laboratory research has found that some phytochemicals may have the potential to be developed into drugs which could alleviate the toxicity of acrylamide. Acrylamide was discovered in foods in April 2002 by Eritrean scientist Eden Tareke in Sweden when she found the chemical in starchy foods, such as potato chips, French fries, bread, heated higher than 120 °C, it was not found in food, boiled or in foods that were not heated.
Acrylamide has been found in roasted barley tea, called mugicha in Japanese. The barley is roasted; the roasting process produced 200–600 micrograms/kg of acrylamide in mugicha. This is less than the >1000 micrograms/kg found in potato crisps and other fried whole potato snack foods cited in the same study and it is unclear how much of this is ingested after the drink is prepared. Rice cracker and sweet potato levels were lower than in potatoes. Potatoes cooked whole were found to have lower acrylamide levels than the others, suggesting a link between food preparation method and acrylamide levels. Acrylamide levels appear to rise. Although researchers are still unsure of the precise mechanisms by which acrylamide forms in foods, many believe it is a byproduct of the Maillard reaction. In fried or baked goods, acrylamide may be produced by the reaction between asparagine and reducing sugars or reactive carbonyls at temperatures above 120 °C. Studies have found acrylamide in black olives, dried prunes, dried pears and peanutsThe US FDA has analyzed a variety of U.
S. food products for levels of acrylamide since 2002. According to the EFSA, the main toxicity risks of acrylamide are "Neurotoxicity, adverse effects on male reproduction, developmental toxicity and carcinogenicity". However, according to their research, there is no concern on non-neoplastic effects. Furthermore, while the relation between consumption of acrylamide and cancer in rats and mice has been shown, it is still not clear whether acrylamide consumption has an effect on the risk of developing cancer in humans, existing epidemological studies in humans are limited and don't show any relation between acrylamide and cancer in humans. Food industry workers exposed to twice the average level of acrylamide do not exhibit higher cancer rates. Although acrylamide has known toxic effects on the nervous system and on fertility, a June 2002 report by the Food and Agriculture Organization of the United Nations and the World Health Organization attempting to establish basic toxicology conclud
A spectrum is a condition, not limited to a specific set of values but can vary, without steps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors in visible light after passing through a prism; as scientific understanding of light advanced, it came to apply to the entire electromagnetic spectrum. Spectrum has since been applied by analogy to topics outside optics. Thus, one might talk about the "spectrum of political opinion", or the "spectrum of activity" of a drug, or the "autism spectrum". In these uses, values within a spectrum may not be associated with quantifiable numbers or definitions; such uses imply a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion. Nonscientific uses of the term spectrum are sometimes misleading. For instance, a single left–right spectrum of political opinion does not capture the full range of people's political beliefs. Political scientists use a variety of biaxial and multiaxial systems to more characterize political opinion.
In most modern usages of spectrum there is a unifying theme between the extremes at either end. This was not always true in older usage. In Latin, spectrum means "image" or "apparition", including the meaning "spectre". Spectral evidence is testimony about what was done by spectres of persons not present physically, or hearsay evidence about what ghosts or apparitions of Satan said, it was used to convict a number of persons of witchcraft at Salem, Massachusetts in the late 17th century. The word "spectrum" was used to designate a ghostly optical afterimage by Goethe in his Theory of Colors and Schopenhauer in On Vision and Colors; the prefix "spectro-" is used to form words relating to spectra. For example, a spectrometer is a device used to record spectra and spectroscopy is the use of a spectrometer for chemical analysis. In the 17th century, the word spectrum was introduced into optics by Isaac Newton, referring to the range of colors observed when white light was dispersed through a prism.
Soon the term referred to a plot of light intensity or power as a function of frequency or wavelength known as a spectral density plot. The term spectrum was expanded to apply to other waves, such as sound waves that could be measured as a function of frequency, frequency spectrum and power spectrum of a signal; the term now applies to any signal that can be measured or decomposed along a continuous variable such as energy in electron spectroscopy or mass-to-charge ratio in mass spectrometry. Spectrum is used to refer to a graphical representation of the signal as a function of the dependent variable. Electromagnetic spectrum refers to the full range of all frequencies of electromagnetic radiation and to the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object. Devices used to measure an electromagnetic spectrum are called spectrometer; the visible spectrum is the part of the electromagnetic spectrum. The wavelength of visible light ranges from 390 to 700 nm.
The absorption spectrum of a chemical element or chemical compound is the spectrum of frequencies or wavelengths of incident radiation that are absorbed by the compound due to electron transitions from a lower to a higher energy state. The emission spectrum refers to the spectrum of radiation emitted by the compound due to electron transitions from a higher to a lower energy state. Light from many different sources contains various colors, each with its own brightness or intensity. A rainbow, or prism, sends these component colors in different directions, making them individually visible at different angles. A graph of the intensity plotted against the frequency is the frequency spectrum of the light; when all the visible frequencies are present the perceived color of the light is white, the spectrum is a flat line. Therefore, flat-line spectra in general are referred to as white, whether they represent light or another type of wave phenomenon. In radio and telecommunications, the frequency spectrum can be shared among many different broadcasters.
The radio spectrum is the part of the electromagnetic spectrum corresponding to frequencies lower below 300 GHz, which corresponds to wavelengths longer than about 1 mm. The microwave spectrum corresponds to frequencies between 300 MHz and 300 GHz and wavelengths between one meter and one millimeter; each broadcast radio and TV station transmits a wave on an assigned frequency range, called a channel. When many broadcasters are present, the radio spectrum consists of the sum of all the individual channels, each carrying separate information, spread across a wide frequency spectrum. Any particular radio receiver will detect a single function of amplitude vs. time. The radio uses a tuned circuit or tuner to select a single channel or frequency band and demodulate or decode the information from that broadcaster. If we made a graph of the strength of each channel vs. the frequency of the tuner, it would be the frequency spectrum of the antenna signal. In astronomical spectroscopy, the strength and position of absorption and emission lines, as well as the overall spectral energy distribution of the continuum, reveal many properties of astronomical objects.
Stellar classification is the categorisation of stars based on their characteristic electromagnetic spectra. The spectral flux density is used to represent the spectrum such as a star. In radiometry and colorimetry, the spectral power distribution of a light source is a measure o
Dexter electron transfer
Dexter electron transfer is a fluorescence quenching mechanism in which an excited electron is transferred from one molecule to a second molecule via a non radiative path. This process requires a wavefunction overlap between the donor and acceptor, which means it can only occur at short distances; the excited state may be exchanged in two separate charge exchange steps. This short range energy transfer process was first theoretically proposed by D. L. Dexter in 1953; the Dexter energy transfer rate, k E T, is indicated by the proportionality k E T ∝ J e x p where r is the separation of the donor from the acceptor, L is the sum of the Van der Waals radii of the donor and the acceptor, J is the spectral overlap integral defined by J = ∫ f D ϵ A λ 4 d λ Fluorescence Quenching Förster resonance energy transfer Surface energy transfer