1, gravimetric method (can be used to measure total oil)
The gravimetric method for measuring petroleum contaminants is a method of weighing the measured components directly without the need for a standard oil sample. After extracting the petroleum contaminant from the sample to be tested with an extractant, the extractant is evaporated by evaporation or the like. The remaining components are then weighed to obtain the weight of petroleum contaminants in the sample.
The advantage of the gravimetric method is that it is not limited by the oil type, no standard sample is required, the equipment is simple, and the less toxic hexane or petroleum ether can be used as the extractant, which can reduce the "secondary pollution" caused to the environment to a large extent; The disadvantage is that by removing the extractant by evaporation, the organic matter having a boiling point lower or closer to the extracting agent is evaporated along with the solvent while evaporating the extracting agent, so that the measured value is lower than the true value. In addition, the weight method is low in automation, complicated in operation, and easy to generate large systematic errors during the measurement process. This method is only suitable for measuring oil samples of 10 mg/L or more.
2, turbidity method
The turbidity method for measuring petroleum contaminants is a method for detecting oil beads dispersed in a solvent based on the principle of light scattering.
When the sample to be tested is sufficiently oscillated or sonicated, the petroleum-like contaminants dispersed in the sample form small oil beads uniformly suspended in the sample solution. When the light beam passes through it, part of the transmission occurs, and a part of the scattering occurs, according to Rayleigh scattering. Under certain conditions, the transmitted light is proportional to the particle concentration, and the scattered light is inversely proportional to the particle concentration. Detecting the intensity of transmitted light and scattered light can quantitatively determine the content of petroleum pollutants in the sample. From the perspective of instrument structure, the turbidity method can be further divided into a transmission turbidity method and a scattering turbidity method. The projection turbidimetric method measures the relationship between transmitted light and turbidity. The instrument has a simple structure and easy design, but it is greatly affected by incident light and has low sensitivity. The scattering turbidimetric method measures the relationship between scattered light and turbidity, and has high sensitivity. However, due to the different particle size and nature, the scattered light may be heterogeneously scattered, that is, the intensity of the scattered light at different angles is different. Therefore, when measuring by the turbidity method, different angles will be measured at different angles. result.
The instrument for measuring oil using the turbidity method has the advantages of simple optical structure and easy miniaturization of the instrument, and the detection of suspended oil in the water body has outstanding advantages over other methods. The disadvantages are poor optical specificity, low sensitivity, and still problem with solution extraction.
3 , chromatography ( GC )
Gas chromatography for the measurement of petroleum contaminants is a method in which the sample to be tested is separated by a chromatographic column, and the different components are sequentially introduced into the detector.
The GC method has the advantages of high sensitivity and qualitative detection of oil contaminant components. However, due to the complex composition of petroleum, the selection of standard oil is very difficult when it is measured by chromatography, which is difficult to achieve petroleum pollution. Detection of the total amount of matter. In addition, the traditional GC structure is complex, and it is difficult to implement on-site and online analysis, which limits the application of the method to some extent.
4 , the resistance method
The electric resistance method for measuring petroleum contaminants is a method for quantitatively measuring the change in electrical resistance after the petroleum-based contaminants in the sample are accumulated in the lipophilic film. Specifically, a pair of electrodes are placed in the sample tank, and an oleophilic membrane is placed between the electrodes. When the sample to be tested flows through the oleophilic membrane, the oil accumulates on the membrane. At this time, between the two electrodes The resistance changes, causing a corresponding change in the current, and the petroleum-like pollutants in the sample to be tested can be quantitatively determined according to the magnitude of the current change.
The method has the advantages that the solvent extraction process is not needed, the “secondary pollution†to the environment is avoided, and online monitoring can be realized, but its disadvantage is that the sensitivity is not high enough, the electrode has the problem of being easily contaminated, and the lipophilic film is Regeneration is required after each measurement. So far, no commercial instrument using this method has appeared.
5, pyrolysis
Pyrolysis of petroleum pollutants is the use of thermal extraction to separate petroleum pollutants from the sample to be analyzed, and at the same time, the organic matter is decomposed at high temperatures, and then the flame ionization detector is used for petroleum pollution. Quantitative determination of matter, this method is suitable for the determination of petroleum pollutants in the soil. Since the petroleum pollutants in the soil are complex and the pyrolysis temperatures are different, this method can measure different components by means of program temperature control. The method has the advantages that solvent extraction is not required, and the disadvantage is that the operation is cumbersome and the instrument structure is complicated.
6, ultraviolet absorption spectrophotometry
Ultraviolet absorption spectrophotometry utilizes the organic compound of CC conjugated double bond in petroleum contaminants to have characteristic absorption in the ultraviolet region at 215~230nm ; and contains a simple, non-conjugated double bond with n- electron chromophore The organic compound of the group exhibits the characteristics of the low-intensity absorption band in the range of 250 to 300 nm . Therefore, in the ultraviolet absorption photometry, the scanning is generally performed in the range of 215 to 300 nm , and then the absorption peak having the largest absorption is selected. measuring. In order to avoid interference from other factors, the ultraviolet absorption method often uses a dual wavelength measurement method.
The advantage of the ultraviolet absorption spectrophotometry is that it is easy to operate and is suitable for the determination of oil-containing samples of 0.05~50mg/L . At the same time, the method can also use low-toxic petroleum ether as the extractant, which has small "secondary pollution" to the environment; The main ones are organic compounds having a conjugated double bond and a chromophore group having n electrons, and do not include saturated hydrocarbons. Therefore, the measurement results are not representative, and the standard oil of the method is difficult to obtain. Poor comparability.
7. Fluorescence spectrophotometry
Fluorescence spectrophotometric determination of petroleum-based contaminants is based on the fact that the benzene-based compounds have fluorescent properties, since the content of petroleum-based contaminants can be quantitatively determined based on the intensity of the emitted fluorescence.
The advantage of fluorometry is its high sensitivity, with a measurement range of 0.002~20mg/L . Because of the high sensitivity of the method, it is usually not necessary to extract the enrichment to detect the content of petroleum contaminants in the water. Therefore, it is easy to realize on-line detection. In addition, since the sample to be analyzed is perpendicular to the optical device and has no direct contact, There is also no cleaning problem of the optical device; it has the disadvantage that only the case of the benzene-based compound having the fluorescent property in the petroleum-based contaminant is examined, and the linear alkane having no fluorescent property therein cannot be determined. If the sample (water sample) is not directly measured, it usually responds very sensitively to petroleum-like pollutants dissolved in water, and has little response to the composition suspended in water, so the measurement result is often lower than the true value.
8, immunoassay
Immunoassay for petroleum-based contaminants is a method for the qualitative and quantitative analysis of petroleum compounds using antibodies as biochemical probes. Immunoassay has the advantages of high specificity, high sensitivity (low detection limit of 1~1000ng/mL ), convenient and fast, large analytical throughput, low detection cost, safe and reliable. This method is the standard method for TPH detection in American soil. .
The method generally does not require expensive instruments, and can simplify or even eliminate the cumbersome sample pretreatment process. The immunoassay method can be developed to widely use the sample kit for quantitative and rapid detection in the field, and the professional quality of the user is not high and easy. Popularization and promotion. Therefore, it is said that immunoassay technology is the most competitive and challenging detection and analysis technology in the 21st century.
9, the transition time method
The method is based on the specificity of the reflection of light by oil beads of different sizes and different compositions, thereby realizing quantitative analysis of petroleum pollutants.
The principle of the method is to irradiate the detected sample with a light beam rotating at a certain angular velocity, and the detector receives the reflected light signal of the oil bead, and the size of the oil bead particles can be calculated according to the emission time of the light beam and the acceptance time difference of the light signal and the rotational angular velocity of the light. From the peak shape of the signal, the distribution density of the oil beads in the sample, the shape of the oil beads and its optical properties can be calculated. The intensity of the optical signal obtained by the method is directly related to the size of the oil bead, and the influence of other factors such as the reflection index of the oil bead on the light, the viscosity of the oil bead and the thermal motion of the oil bead are not related, so the measurement interference is small. In addition, the method can directly detect the sample without performing extraction and the like, and avoids "secondary pollution" caused to the environment during the measurement process.
10. Photoacoustic spectroscopy
Photoacoustic spectroscopy for the detection of petroleum contaminants is the use of different organic materials with different photoacoustic spectroscopy to achieve quantitative detection of organic pollutants in environmental samples.
The photoacoustic signal of water is very small, and the photoacoustic signal of oil is very large. Therefore, when the photoacoustic spectroscopy measures the petroleum pollutants in the water, the matrix interference is very small, and at the same time, the photoacoustic response of the dissolved oil and the suspended oil is different. It can be used to distinguish them so as to measure the content of dissolved oil and suspended oil in the water body separately. In addition, the method can be directly detected without extracting the sample. The drawback of this method is that the sample has a certain pollution effect on the detector, and the accuracy of the measurement will be greatly affected.
11 , infrared absorption spectrophotometry
Infrared absorption spectrophotometry for the detection of petroleum contaminants is based on the shrinkage vibration of the CH bond of its components, which absorbs certain characteristic wavelengths of the infrared region, so it can be absorbed at the characteristic wavelength according to the infrared radiation passing through the sample. Corresponding petroleum pollutants are quantitatively determined. This method has been designated as the national standard for monitoring petroleum pollutants in water in China. Infrared absorption spectrophotometry includes non-dispersive infrared absorption spectrophotometry and infrared spectrophotometry.
( 1 ) Non-dispersive infrared absorption spectrophotometry
Since the petroleum contaminants linear and cyclic alkanes stretching vibration of CH bond is present in the absorption band around 2942cm -1, and thus the number of waves can be measured in a single set at the wave number of 2942cm -1 to about petroleum contaminants measured. This method is a standard method for the US EPA to measure petroleum pollutants in soil and water.
The advantage of non-dispersive infrared absorption spectrophotometry for measuring petroleum pollutants is that the instrument has a simple structure and good reproducibility. The disadvantage is that only the linear or naphthenes in petroleum pollutants are referenced, and the Benzene, which affects the representation of the data.
( 2 ) Infrared spectrophotometry
Since the main components in petroleum pollutants are composed of compounds containing -CH 2 - , CH 3 - , =CH-, etc., this method comprehensively investigates the stretching and vibration of hydrocarbon chains in petroleum pollutants, which can be accurate. The total content of petroleum pollutants is obtained; the disadvantage is that the method involves solvent extraction, and a small amount of volatile organic substances are inevitably lost during the extraction process.
12 , combined technology
The detection methods of petroleum pollutants in environmental samples have their own advantages and disadvantages. If they are combined to make their advantages complementary and overcome their shortcomings, the accuracy of detection will be greatly improved. The combined technology mainly includes the combination of GC and other detection technologies, and the combination of fluorescence and turbidity technology.
( 1 ) Combination of GC and other detection technologies
The combination of GC and other detection techniques usually uses a chromatographic column to separate the petroleum contaminants in the sample and then use different detection techniques such as MS , infrared, fluorescence, etc. to obtain more detailed and accurate contaminants. information.
( 2 ) Combination of fluorescence spectrometry and turbidimetry
Fluorescence spectrometry is one of the most sensitive methods for the detection of petroleum contaminants dissolved in water or solvents, and it is difficult to detect petroleum contaminants suspended in water or other solvents. . Although the turbidity method cannot detect oil dissolved in water, it has a good ability to detect petroleum pollutants suspended in water. If these two detection technologies are used together, the method will be improved for various water bodies. Or the accuracy of the detection of petroleum contaminants in solvents.
In summary, although there are many test methods for petroleum pollutants at present, the enzyme-linked immunoassay method is an excellent on-site emergency and semi-quantitative detection method because of its simple operation and low cost. In addition, instruments based on infrared absorption spectrometry (especially infrared spectrophotometric instruments) have a simple structure and high sensitivity. By changing the length of the absorption cell and changing the sampling amount, it can be suitable for the detection of samples in a large concentration range, and comprehensive detection. The components of petroleum pollutants, if added with appropriate accessories, can also measure the advantages of animal and vegetable oils and petroleum in oils. These instruments occupy a dominant position in China's petroleum pollution detection market.
Yingnuo instrument finishing (excerpted from "Research Methods and Testing Instruments for Petroleum Pollutants")
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