Analytical Instrumentation - Terminology and Basics

Absolute Pressure – A quantity of pressure measured with respect to total vacuum. Equal to the sum of a pressure gauge reading and atmospheric pressure (14.69 psia or 1.01 bar at sea level).  

Absorption – The "soaking up" of gas, liquids or dissolved substances into a solid material.  

Acid – A chemical compound that dissociates in aqueous solution to form hydrogen ions; a proton donor that reacts with a base to form a salt.  

Adsorption – The condensation of gas, liquids or dissolved substances on the surface of solids. 

Aerobic Gas Mixture – Gas mixture containing oxygen; used for incubation of microorganisms that require oxygen for life.  

Air – The mixture of gases that surrounds the earth. The composition of air is 78.08% nitrogen, 20.95% oxygen, 0.03% carbon dioxide, and 0.93% argon. Standard air has a density of 0.075 lb/ft 3 (1.2 kg/m 3 ) measured dry at 70°F (21°C) and 760 mm Hg pressure, M.W. 28.3. 

Anaerobic Gas Mixture – A gas mixture containing no oxygen, used for incubation of microorganisms that do not require free oxygen for life.  

Anhydrous – A descriptive term meaning without water.  

Balance Gas – A gas used to "top off" a gas mixture after individual component gases at specified concentrations are added.  

Base – A chemical compound that can react with an acid to form a salt.  

Boiling Point – The temperature at which the vapor pressure of a liquid equals the atmospheric pressure (usually given at 760 mm Hg). 

Calibration Gas – A gas of accurately known concentration that is used as a comparative standard in analytical instrumentation.  

Carrier Gas – Pure gases or gas mixtures used to move a sample to be analyzed through the gas analysis system at an even rate and provide a zero reference (baseline) when a sample is not being detected.  

CGA – Short for Compressed Gas Association, an association that recommends cylinder valve outlet connections for specific gas services based on safety considerations.  

Compressed Gas – Any material or mixture having in the container either an absolute pressure exceeding 40 psia (3 bar) at 70°F (21°C) or an absolute pressure exceeding 104 psia (7 bar) at 130°F (54°C).  

Compressed Gas in Solution – A non liquefied compressed gas that is dissolved in a solvent. 

 Corrosive – Gases that corrode material or tissue with which they come in contact, or do so in the presence of water, are classified as corrosive. It is essential that equipment used for handling corrosive gases be constructed of proper materials. Proper protective clothing and equipment must be used to minimize exposure to corrosive materials.  

Critical Pressure – The pressure required to liquefy a gas at the critical temperature.  Critical 

Temperature – The temperature above which a gas cannot be liquefied by pressure alone.

Desorption – The leaching out of gas, liquids or dissolved substances from solid material.  

DIN – Deutsche Norm. A standard from the Deutsches Institut fur Normung. DIN 477 recommends cylinder valve outlet connections for specific gas services based upon safety considerations.

Explosion Proof – An enclosure for an electrical apparatus so designed that an explosion of flammable gas or vapor inside the enclosure will not ignite flammable gas or vapor outside. 

FID – Flame ionization detector.  

FID Fuels – Hydrogen-nitrogen and hydrogen-helium mixtures burned as a source of heat and power in FIDs.  

Flammable – A substance that will ignite easily and burn rapidly in the presence of an oxidizer. 

Flashback – The phenomenon characterized by vapor ignition and flame travel back to the vapor source.  

GC – Gas chromatograph.  

Impurity – Amount of foreign material or contaminants found in a pure material.
 
Inert – Gases that do not react with other materials under normal temperature and pressure. 

Liquefied Compressed Gas – A gas that is partially liquid at its charging pressure and a temperature of 70°F (21°C). 

Mole – Mass numerically equal to its molecular weight. A gram mole is the weight in grams equal to the molecular weight.  

Molecular Weight – The sum of the atomic weights of all the constituent atoms in a molecule. 

NEN – Nederlandse Norm. A standard from the Dutch Normalisation Institute. NEN 3260 recommends cylinder valve outlet connections for specific gas services based upon safety considerations

Nonliquefied Compressed Gas – A "nonliquefied compressed gas" is a gas, other than gas in solu-tion, that under the charged pressure is entirely gaseous at a temperature of 70°F (21°C).  

NTP – Normal temperature and pressure, 20°C and 760 torr.  

Off-gassing – The removal of gas, liquids or dissolved substances from the surface of solids.  

Oxidant – A gas that does not burn but will support combustion.  

Partial Pressure – In any gas mixture, the total pressure is equal to the sum of the pressures (partial) that each gas would exert were it alone in the volume occupied by the mixture.  

PPM - Parts per Million

PPB - Parts per Billion

PPT - Parts per trillion

Pyrophoric - Material that spontaneously ignite on contact with air at normal conditions.

Specific Gravity – The ratio of the weight of any volume to the weight of an equal volume of another substance taken as a standard. For solids or liquids, the standard is usually water and for gases, the standard is air.  

Specific Volume – The volume of a unit weight of a substance at a given temperature.  

Specific Heat – The amount of heat required to raise the unit weight of a substance one degree of temperature at constant pressure.  

STP – Standard temperature and pressure, 0°C and 760 torr.  

Sublimation – The process of passing from a solid state directly to a gaseous state.  

TLV – Threshold Limit Value, the time-weighted average concentration of an airborne substance that represents the condition under which it is believed nearly all workers may be exposed in a normal eight-hour day, five-day work week without suffering adverse effect. 

Toxic Gas – Gases that may chemically produce injurious or lethal effects to humans.  

Vapor Pressure – The pressure exerted when a solid or a liquid is in equilibrium with its own vapor at a particular temperature.

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