LABORATORY ANALYSIS METHODS

Transcription

1 LABORATORY ANALYSIS METHODS INFRARED (IR) ANALYSIS ON RESIDUE USING AN INFRACAL INSTRUMENT The technique involves an extraction of the acidified sample with a solvent, which does not have to be C-H free as required for the conventional IR method. The extract is then deposited on a sapphire window or a cubic Zirconia horizontal attenuated total reflection (HATR-T) sample stage. After the evaporation of the solvent, IR analysis is carried out on the residues at the wavelength equivalent to 2930 cm -1. Solvents that may be used in the method could include Hexane, Vertrel MCA. The sapphire window can be used on all types of IR instruments on the market; it is simple, cost effective and quick to run; it does not require IR transparent solvents. Loss of light components; detection limit at low end is around 5 parts per million (ppm). INFRARED (IR) USING AN ALTERNATIVE SOLVENT TO FREON Instead of using Freon, two other solvents, Perklone and S-316 could be used. There are no changes to the analysis procedure, i.e. solvent extraction followed by direct IR quantification of the hydrocarbons. In ASTM D IR Method, S-316 has been officially approved as the solvent replacing Freon Retains the current methodology; no need to purchase new instrument; simple, cost effective and results reproducible; quantification of aromatic and aliphatic components can be achieved with proper IR instrument. Uses halogenated solvents SUPERCRITICAL FLUID EXTRACTION AND INFRARED (SFE-IR) This Oil-in-Water analysis method has been developed based on using supercritical CO 2 fluid extraction and Infrared quantification. The method is essentially the same as the old IR method, but instead of using Freon or Perklone to extract the oil from water samples, it uses liquid carbon dioxide. Details of the technique and recent study results can be found in the paper entitled "Development by an automated method for the determining oil in water by direct aqueous supercritical fluid extraction coupled on-line with infrared spectroscopy" by Brian Minty, Edward Ramsey and Ieuan Davies.

2 Environmentally clean; simple operation; same IR quantification and therefore easy legislation continuation; suitable to analyse both volatile and non-volatile hydrocarbons; tolerant to dirty samples; minimum amount of sample handling; comparative analysis time to the traditional IR analysis. Relatively new method; involves high pressure liquid CO 2 handling; the unit is comparatively bulky; suitability for analysing offshore produced water oil content is still to be proven. GRAVIMETRIC The Gravimetric method also requires solvent extraction as the first stage of sample preparation. The extract is then evaporated at certain temperature over a steam or water bath. After the evaporation of the solvent, the residue oil is weighed. Standard methods include EPA and EPA Solvents may use hexane, or mixture of methyltertiarybutylether (MTBE) and hexane. Robust and reproducible. Loss of volatile components Not suitable for routine monitoring. ULTRAVIOLET (UV) ABSORPTION Aromatic components absorb strongly in the UV region of the magnetic spectrum ( nm). UV absorbance can then be used to determine the aromatic content of water samples. Provided that the ratio of aromatic to the total hydrocarbon remains constant, the total hydrocarbon content can then be calculated. There are two ways of using UV absorption for Oil-in-Water analysis; i.e. preextraction and direction analysis. Pre-extraction Method In the pre-extraction method, solvent extraction with the water sample is carried out. UV absorbance analysis is then conducted on the extract. Using a suitable calibrant, UV absorbing aromatic hydrocarbon content are detected and measured. The solvent used for extraction must be sufficiently transparent in the UV spectral region. Hexane, iso-octane, trichloroethane may be used. Direct Analysis Method

3 The direct analysis method involves mixing the sample with a non UV absorbing surfactant or a co-solvent to solubilize the oil. UV absorbance analysis is then directly carried out on the solution. Very sensitive; simple and cost effective to run; the direct analysis method does not have to use a hazardous solvent. It is an indirect method; the ratio of aromatic to the total hydrocarbons must remain relatively constant; the presence of Fe 3+ in the water sample may interfere with the direct analysis method; turbidity could also interfere with the direct method; ULTRAVIOLET (UV) FLUORESCENCE When aromatic components are irradiated with UV, they emit light at a longer wavelength. The phenomenon is called florescence. By measuring the fluorescence intensity, one can determine the concentration of aromatic components. Therefore, provided that the ratio of aromatic components to the total hydrocarbon stays constant, the total amount of hydrocarbon can be calculated. Like the UV absorbance methods, there are two ways of using UV fluorescence for Oil-in-Water measurement, i.e. pre-extraction and direction. Sample treatment and procedures are very similar to the UV absorbance based methods. UV Fluorescence methods have got similar advantages and disadvantages to the UV absorbance methods. However direct UV fluorescence method is less sensitive to the presence of Fe 3+ in the water sample than UV absorbance. 1. Colorimetry This method generally involves extraction of the acidified sample with a solvent (e.g. trichloroethane) that is sufficiently transparent in the visible region of the magnetic spectrum. Oil in the extract absorbs visible light to allow its detection and measurement using a suitable calibration. Simple, fast and cost effective to run. It requires a solvent, It does not quantify the hydrocarbons directly. 2. Gas Chromatography and Flame Ionisation Detection (GC-FID) Gas Chromatography (GC) is a quantitative technique based on separation of the various hydrocarbon compounds into their compounds using gas chromatography. Detection of these compounds of interest can then be measured by mass spectroscopy (MS) or flame ionization detection (FID). It involves extraction of acidified water sample with hexane or petroleum ether, removal of the polar

4 substances by clean-up using florisil. The extract is then analysed using GC-FID or GC-MS. An international standard based on this GC- FID method has been developed. The standard measures hydrocarbons in the C 10 to C 40 range. According to the specification, concentration range from 0.1 to 100 mg/l can be measured. Both aliphatic and aromatic hydrocarbons can be directly quantified; with the GC-MS method, it is possible to separate and analyse a large number of different components simultaneously, which include BETX and PAHs. It requires a solvent. It is expensive and time consuming and operation requires experience. It is not suitable for routine analysis. SOLID PHASE EXTRACTION (SPE) Solid phase extraction (SPE) is performed with commercially available disks. A certain volume of acidified sample is filtered through a SPE disk impregnated with a dedicated non-polar stationary phase. The non-polar compounds in the water are absorbed onto the stationary phase of the disk, while the more polar compounds pass through. A solvent (e.g. isooctane) is then used to remove the non-hydrocarbon compounds from the disk. Analysis of the aliquot is then carried out using a non-polar capillary column coupled with a mass spectrometer detector. Measurement of the total peak area (using specific ions of mineral oil) between n-nonane and n-tetracontane delivering the concentration on mineral oil in water which is called hydrocarbon oil index. Quantification is based on a calibration curve using a standard mixture consisting of two different types of mineral oil and a standard compound introduced after extraction to ensure reproducibility of the mass spectrometer response. It does not require halogenated solvent; it quantifies the hydrocarbons directly; it is accurate and able to identify a large number of different components simultaneously, which include the BTEX and PAHs. it is expensive and takes long time to do; it requires an experienced operator; it is not suitable for routine monitoring purposes. FIBRE OPTIC CHEMICAL SENSOR (FOCS) The FOCS technique takes advantage of the interaction between the light travelling through a fibre (with a proprietary coating) and a water solution containing petroleum hydrocarbons. The coating is sensitive to the types and as well as the concentration of hydrocarbons in the water sample. It can respond reversibly to the increasing or decreasing levels of petroleum hydrocarbons. When FOCS probe is used, the presence of hydrocarbons in the water will absorb to the surface of the coating, which results in change of its refractive index. The resultant change of the refractive index will

5 then change the amount of transmitted light. Therefore by measuring and calibrating such transmitted light using a range of samples containing known concentration of hydrocarbons; it is possible to determine the concentration of hydrocarbons in real samples. No solvent is required. Easy to use and calibrate. An indirect method; it is sensitive to the types of hydrocarbons, it is also an equilibrium based technique and therefore accurate measurement will depend on mass transfer of hydrocarbon in the water to the coating.