Measurement-Procedure-Standardisations.md

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 | sulphur chemiluminescence - gas chromatography | Chemiluminescence is the emission of light (luminescence), as the result of a chemical reaction. Chemiluminescence occurs as a result of the reaction of SO and ozone (SO+O3 --> SO2*+O2). The return to a fundamental electronic state of the excited SO2* molecules is made by luminous radiation in a specific spectrum (SO2* --> SO2 + hv), which can be measured (by a photomultiplier tube). The concentration of sample total sulphur is directly proportional to the intensity of light emitted. This method can be mixed with gas chromatography (GC) to allow to determination of specific sulphur compounds (i.e. SO2). The gas sample is passed through a GC column before being ultimately measured by the photomultiplier detector. Gas chromatography (GC) is a method used for separating and analysing compounds that can be vaporized without decomposition. A sample solution is injected into a instrument, entering a gas stream which transports the sample (mobile phase) into a separation tube known as the "column". The mobile phase is a carrier gas, usually an inert gas such as helium or an unreactive gas such as nitrogen. Helium remains the most commonly used carrier gas in about 90% of instruments although hydrogen is preferred for improved separations. The column consists of a microscopic layer of liquid or polymer on an inert solid support a microscopic layer of liquid or polymer on an inert solid support (stationary phase), inside a piece of glass or metal tubing, placed inside a piece of glass or metal tubing. Once inside the column, the gaseous compounds being analysed interact with the walls of the column coated with a stationary phase. This causes each compound to elute at a different time, known as the retention time of the compound.  The comparison of retention times is what gives GC its analytical usefulness. If greater separation of compounds is required, multiple distinct columns can be used for this purpose. | | |
 | flame photometric detection (FPD) | Many elements give characteristic emission when burned in flame. Absorption of energy from the flame allows a ground state atom or molecule to reach an excited state. The atom/molecule may return to the ground state through emission of light (luminescence), which can be subsequently measured (by a photomultiplier tube). This process is a chemiluminescent process (where luminescence occurs as the result of a chemical reaction). The concentration of the sample gas is directly proportional to the intensity of light emitted. This method has been applied for the measurement of sulphur containing species (i.e. SO2). For specific measurement of solely SO2, the sample gas must be scrubbed of other sulphur species prior to measurement, and the photomultiplier detector measures emission centred near 394nm. | Other sulphur compounds | |
 | flame ionisation detection (FID) | This method is based on the principle of the generation of an electrical current that is proportional to the rate of ion formation, dependent on the concentrations of species in the sample gas. The method is typically the standard detection method for hydrocarbons, however the method is also sensitive to almost all compounds, mostly combustible ones. There are, however, a few compounds to which the method has very little, if any, sensitivity, including: O2, N2, SO2, NO, N2O, NO2, NH3, CO, CO2, and H2O. | Method is non-specific for different gases in the sample. | |
+| Conductimetry | Involves the absorption of a specific gas species in deionized water, to produce an acid, which can be detected by a conductivity cell. Through the use of the deionized water reagent, measurements are susceptible to interferences from CO2, salt aerosols, acid mists and basic gases.  Addition of hydrogen peroxide to the deionized water minimises interference (through reduced solubility of CO2 within the water). This method been used extensively to measure SO2. | Any gas that can form or remove ions (NO2, NH3, HCl, Cl2 are the worst known interferants). | |
+| second derivative spectroscopy | Derivative spectroscopy involves plotting the first, second or higher order derivatives of a spectrum with respect to wavelength. Usually this is obtained by a microprocessor connected in series with a spectrophotometer, which computes the derivative with respect to time as the spectrum is scanned at constant speed. The "true" wavelength derivative is linearly related to the time derivative recorded, the magnitude of which is directly affected by scanning speed and spectral band width. The derivative process provides two general advantages: first, an effective enhancement of resolution, which can be useful to separate two or more components with overlapping spectra; second, a discrimination in favour of the sharpest features of a spectrum, used to eliminate interferences by broad band constituents. However, this process results in a decrease in the signal to noise ratio. Both advantages and disadvantages increase with the derivative order. Generally, the second derivative is more useful than the first ones. | | |
+| coulometry | Measures the current necessary to maintain a low concentration of halide and halogen at equilibrium. Typical coulometric methods use solutions of potassium bromide (halide) and bromine (halogen) in dilute sulphuric acid. The concentration of bromine is measured as a voltage between an indicator electrode and a bromide reference electrode. When the measured species is present, the bromine concentration is decreased through reaction. A feedback system then compensates by generator bromine at a generator electrode. The current required to regenerate the bromine is directly proportional to the concentration of the measured species. The method is extremely sensitive to interference through reaction with other undesired species (typically NO, NO2, O3, sulphur species, Cl2). Other species are typically removed through pre-filters. This method been used to measure NO2 and SO2. | Reaction with other gases, typically: NO, NO2, O3, sulphur species, Cl2. | |
+| Polarography | An electrochemical technique where the cell current is measured as a function of time and as a function of the potential between the indicator and reference electrodes. The working electrode is a dropping mercury (or other liquid conductor) electrode and unstirred solutions are used. The potential is varied using pulses of linearly increasing amplitude (one pulse during each drop lifetime) and the current is sampled before and after each voltage pulse. | | |
+| ultraviolet fluorescence | Fluorescence is the emission of light (luminescence) by a substance that has absorbed light or other electromagnetic radiation (excitation). In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. Ultraviolet fluorescence specifically refers to the process of a species being excited by ultraviolet light (10nm to 400nm) (species + hv (UV) --> species*). The return to a fundamental electronic state of the excited species is made by luminous radiation on a longer wavelength spectrum (species* --> species + hv). SO2 is efficiently excited to an excited state (SO2*) by UV light between 190 nm-230 nm, subsequently fluorescing light at approximately 330nm (SO2* --> SO2 +hv(330nm)), which can be measured. | 3rd body quenching (NO, CO2, O2, H2O), light pollution, UV absorption by ozone, other species undergoing ultraviolet fluorescence (poly-nuclear aromatics, NO). | |
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