IH Quiz #6

Photoionization Detector (PID)

- A photoionization detector or PID is a type of gas detector. - Typical photoionization detectors measure volatile organic compounds and other gases in concentrations from sub parts per billion to 10 000 parts per million (ppm). The photoionization detector is an efficient and inexpensive detector for many gas and vapor analytes. A PID may produce instantaneous readings and operate continuously. These hand-held, battery-operated detectors are widely used in military, industrial, and confined working facilities for safety. - PIDs are used as monitoring solutions for: Lower explosive limit measurements Ammonia detection Hazardous materials handling Arson investigation Industrial hygiene and safety Indoor air quality Environmental contamination and remediation Cleanroom facility maintenance - In a photoionization detector high-energy photons, typically in the ultraviolet (UV) range, break molecules into positively charged ions. As compounds elute from the GC's column they are bombarded by high-energy photons and are ionized when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.

Combustible Gas Indicator (CGI) aka explosimeter

- An explosimeter is a device which is used to measure the amount of combustible gases present in a sample. When a percentage of the lower explosive limit (LEL) of an atmosphere is exceeded, an alarm signal on the instrument is activated. "Explosimeter" is a registered trademark of MSA. - The device, also called a combustible gas detector, operates on the principle of resistance proportional to heat—a wire is heated, and a sample of the gas is introduced to the hot wire. Combustible gases burn in the presence of the hot wire, thus increasing the resistance and disturbing a Wheatstone bridge, which gives the reading. - A flashback arrestor is installed in the device to avoid the explosimeter igniting the sample external to the device. Note, that the detection readings of an explosimeter are only accurate if the gas being sampled has the same characteristics and response as the calibration gas. Most explosimeters are calibrated to methane or hydrogen.

Infred Detector (IR)

- An infrared detector is a detector that reacts to infrared (IR) radiation. The two main types of detectors are thermal and photonic (photodetectors). - The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena. Bolometers and micro-bolometers are based on changes in resistance. Thermocouples and thermopiles use the thermoelectric effect. Golay cells follow thermal expansion. In IR spectrometers the pyroelectric detectors are the most widespread. - The response time and sensitivity of photonic detectors can be much higher, but usually these have to be cooled to cut thermal noise. The materials in these are semiconductors with narrow band gaps. Incident IR photons can cause electronic excitations. In photoconductive detectors, the resistivity of the detector element is monitored. Photovoltaic detectors contain a p-n junction on which photoelectric current appears upon illumination. Bolometer- An instrument that measures radiant energy by correlating the radiation-induced change in electrical resistance of a blackened metal foil with the amount of radiation absorbed. Thermocouple- a thermoelectric device used to measure temperatures accurately, especially one consisting of two dissimilar metals joined so that a potential difference generated between the points of contact is a measure of the temperature difference between the points. Thermopile- A device consisting of a number of thermocouples connected in series or parallel, used for measuring temperature or generating current. Thermal Noise- unwanted currents or voltages in an electronic component resulting from the agitation of electrons by heat.

Colormetric Tubes

- Detector tubes, or colorimetric indicator tubes, are the most widely used direct-reading devices due to ease of use, minimum training requirements, fast on-site results, and wide range of chemical sensitivities. - Hermetically sealed glass tube containing inert solid/granular materials impregnated with reagent(s) that change color based on chemical reaction(s). Filter and/or pre-layer to adsorb interferences. - Length of resulting color change or the intensity of the color change is compared with a reference to obtain the airborne concentration. Three methods of use: 1. calibration scaled marked on tube; 2. separate conversion chart;, and 3. separate comparison tube. - Break ends of tube and place in bellows/piston, or bulb-type pump which are specially designed by each manufacturer; therefore, interchanging equipment between manufacturer results in significant measurement errors. - Perform pump stroke to draw air through tube at a flow rate and volume determined by the manufacturer. A specified number of strokes are used for a given chemical and detection range. Total pumps stroke time can range from several seconds to several minutes. - Tube selection depends on the chemical(s) to be monitored and the concentration range. Most tubes react with more than one chemical that are structurally similar. Interferences are documented by manufacturers and should be understood. - Variety of tubes - different ranges; qualitative indicator tubes (not used regarding concentrations); presence/absence - poly tubes. Help to choose a more accurate method. Grab samples; variable; source monitoring, not compliance. - Sensitive to temperature, humidity, pressure, light, time, and presence of interferences. Reagents are chemically reactive and can degrade over time to heat/UV; limited shelf life. Recommended use in range of 0 to 40 degrees C. Sampling under different conditions [20 to 25 degrees C; 760 mm Hg; 50%RH]. OR corrections or conversions. Interferences - positive or negative. - Some tubes are designed to perform integrated sampling over long monitoring periods of up to 8 hours and use low-flow pumps. Lower limits of detection over longer sampling times. Length of stain is usually calibrated in microliters. Measurement can be converted to a TWA concentration. Diffusion tube results divided by exposure time. Temp/pressure corrections. Cross-sensitivities. Long-term tubes as screening device. Accuracy varies +/- 25 to 35%. Leak checks; volume/flow rate measurements.

Electro-chemistry

- Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor (a metal or a semiconductor) and an ionic conductor (the electrolyte), and which involve electron transfer between the electrode and the electrolyte or species in solution. - If a chemical reaction is driven by an external applied voltage, as in electrolysis, or if a voltage is created by a chemical reaction as in a battery, it is an electrochemical reaction. In contrast, chemical reactions where electrons are transferred between molecules are called oxidation/reduction (redox) reactions. In general, electrochemistry deals with situations where oxidation and reduction reactions are separated in space or time, connected by an external electric circuit to understand each process.

Flame Atomic Absorption (Flame AA)

- Flame atomic absorption is a very common technique for detecting metals and metalloid's s in environmental samples. It is very reliable and simple to use. The technique is based on the fact that ground state metals absorb light at specific wavelengths. Metal ions in a solution are converted to atomic state by means of a flame. Light of the appropriate wavelength is supplied and the amount of light absorbed can be measured against a standard curve. Nebulize- to reduce to fine spray; atomize.

Mass Spectrophotometer (GC- MS)

- Gas chromatography-mass spectrometry (GC-MS) is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, explosives investigation, and identification of unknown samples. GC-MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. GC-MS has been widely heralded as a "gold standard" for forensic substance identification because it is used to perform a specific test. A specific test positively identifies the actual presence of a particular substance in a given sample. A non-specific test merely indicates that a substance falls into a category of substances. Although a non-specific test could statistically suggest the identity of the substance, this could lead to false positive identification. Disadvantages: Limited to laboratory settings

Iductivity Coupled Plasma (ICP)

- Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry which is capable of detecting metals and several non-metals at concentrations as low as one part in 1012 (part per trillion). This is achieved by ionizing the sample with inductively coupled plasma and then using a mass spectrometer to separate and quantify those ions. - Compared to atomic absorption techniques, ICP-MS has greater speed, precision, and sensitivity. However, analysis by ICP-MS is also more susceptible to trace contaminants from glassware and reagents. In addition, the presence of some ions can interfere with the detection of other ions. - The variety of applications exceeds that of ICP-OES and includes isotopic speciation. Due to possible applications in nuclear technologies, ICP-MS hardware is a subject for special exporting regulations.

Thermal Conductivity Detector (TCD) aka Hotwire

- The TCD compares the thermal conductivities of two gas flows—pure carrier gas (also called the reference gas) and carrier gas plus sample components (also called column effluent). - This detector contains a filament that is heated electrically so that it is hotter than the detector body. The filament temperature is kept constant while alternate streams of reference gas and column effluent pass over it. When sample is added, the power required to keep the filament temperature constant changes. The two gas streams are switched over the filament five times per second and the power differences are measured and recorded. - When helium (or hydrogen) is used as carrier gas, the sample causes the thermal conductivity to fall. If nitrogen is used, the thermal conductivity usually goes up because most things are more conductive than nitrogen. Because the TCD does not destroy the sample during the detection process, this detector can be hooked up in series to a flame ionization detector or other detector.

Flame Ionization Detector (FID)

- Uses a hydrogen flame to produce ions. More difficult to operate than PIDs. Less sensitive to effects of humidity. Respond to greater number of organic chemicals (C-C or C-H bonds). Unit is linear over a greater range. Ionize materials with IP of 15.4 eV or less. Vapor sensitivity dependent on energy required to break chemical bonds. Response depends on particular chemical and functional groups affect sensitivity. Detector response is proportional to number of molecules; non-linear relationship. - FID ISSUES Insensitivity to ambient gases makes FID extremely useful in the analysis of atmospheric samples. Measurements are relative to calibrant gas, methane. FID response does not represent the concentrations of specific organic compounds, but rather an estimate of the total concentration of volatile organic compounds. One point calibration curve with methane is usually sufficient because instruments are linear up to 10,000 ppm. Zero in field by background reading obtained without flame being lit High purity hydrogen flame. Higher background reading than PID, since unit responds to more contaminants. Inlet particulate filters; GC-mode option.

Chromatography

- a technique used to separate the components of a chemical mixture by moving the mixture along a stationary material, such as gelatin. Different components of the mixture are caught by the material at different rates and form isolated bands that can then be analyzed.

High Pressure Liquid Chromotography (HPLC)

- can be described as a mass transfer process involving adsorption. - is a chromatographic technique used to separate a mixture of compounds in analytical chemistry and biochemistry with the purpose of identifying, quantifying or purifying the individual components of the mixture. HPLC is considered an instrumental technique of analytical chemistry (as opposed to a gravitimetric technique). HPLC has many uses including medical (e.g. detecting vitamin D levels in blood serum), legal (e.g. detecting performance enhancement drugs in urine), research (e.g. separating the components of a complex biological sample, or of similar synthetic chemicals from each other), and manufacturing (e.g. during the production process of pharmaceutical and biological products). - HPLC relies on pumps to pass a pressurized liquid and a sample mixture through a column filled with a sorbent, leading to the separation of the sample components. The active component of the column, the sorbent, is typically a granular material made of solid particles (e.g. silica, polymers, etc.), 2-50 micrometers in size. The components of the sample mixture are separated from each other due to their different degrees of interaction with the sorbent particles. The pressurized liquid is typically a mixture of solvents (e.g. water, acetonitrile and/or methanol) and is referred to as "mobile phase". Its composition and temperature plays a major role in the separation process by influencing the interactions taking place between sample components and sorbent. These interactions are physical in nature, such as hydrophobic (dispersive), dipole-dipole and ionic, most often a combination thereof.

Electron Capture Detector (ECD)

- is a device for detecting atoms and molecules in a gas through the attachment of electrons via electron capture ionization - used in gas chromatography to detect trace amounts of chemical compounds in a sample. - The electron capture detector is used for detecting electron-absorbing components (high electronegativity) such as halogenal compounds in the output stream of a gas chromatograph. The ECD uses a radioactive beta particle (electron) emitter in conjunction with a so-called makeup gas flowing through the detector chamber. The electron emitter typically consists of a metal foil holding 10 millicuries (370 MBq) of the radionuclide 63Ni. Usually, nitrogen is used as makeup gas, because it exhibits a low excitation energy, so it is easy to remove an electron from a nitrogen molecule. The electrons emitted from the electron emitter collide with the molecules of the makeup gas, resulting in many more free electrons. The electrons are accelerated towards a positively charged anode, generating a current. There is therefore always a background signal present in the chromatogram. As the sample is carried into the detector by the carrier gas, electron absorbing analyte molecules capture electrons and thereby reduce the current between the collector anode and a cathode. The analyte concentration is thus proportional to the degree of electron capture. ECD detectors are particularly sensitive to halogens, organometallic compounds, nitriles, or nitro-compounds. - Depending on the analyte, an ECD can be 10-1000 times more sensitive than a flame ionization detector (FID), and one million times more sensitive than a thermal conductivity detector (TCD). An ECD has a limited dynamic range and finds its greatest application in analysis of halogenated compounds.[5] The detection limit for electron capture detectors is 5 femtograms per second (fg/s), and the detector commonly exhibits a 10,000-fold linear range. This made it possible to detect halogenated compounds such as pesticides and CFCs, even at levels of only one part per trillion (ppt), thus revolutionizing our understanding of the atmosphere and pollutants. Anode- the electrode or terminal by which current enters an electrolytic cell, voltaic cell, battery, etc. Cathode- the electrode or terminal by which current leaves an electrolytic cell, voltaic cell, battery, etc.

Pneumatics

- the study of the mechanical properties of air and other gases.

Gas Chromatograph (GC)

A gas chromatograph is a chemical analysis instrument for separating chemicals in a complex sample. A gas chromatograph uses a flow-through narrow tube known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas, mobile phase) at different rates depending on their various chemical and physical properties and their interaction with a specific column filling, called the stationary phase. As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, and the temperature. - Detectors vary in sensitivity, selectively, and linearity. Choice depends on the chemicals investigated, the presence of other contaminants, and required sensitivity. Peaks of separated components; concentration determined by area under peaks; compare with calibration. Field operation of GC requires calibration with the chemical of interest under the same conditions as the chemical to be measured in field. Limitation is requirement of high degree of skill. Not unique retention times. QA/QC - repeatability and reproducibility. D- the technique of separating and analysing the components of a mixture of liquids or gases by selective adsorption in, for example, a column of powder ( column chromatography ) or on a strip of paper ( paper chromatography ) - can be described as a mass transfer process Includes the following methods: FID, PID, ECD, Mass Spectrometry (MS)