Detection Target
Determination of Total Recoverable Petroleum Hydrocarbons in Solid Waste
Overview
This solution conforms to the EPA Method 8440 Total Recoverable Petroleum Hydrocarbons by Infrared Spectrophotometry. This standard is used for the measurement of total recoverable petroleum hydrocarbons (TRPHs) extracted with supercritical carbon dioxide from sediment, soil and sludge samples. It is not applicable to the measurement of gasoline and other volatile petroleum fractions, because of evaporative losses.
Soil samples are extracted with supercritical carbon dioxide. Interferences are removed with silica gel, either by shaking the extract with loose silica gel, or by passing it through a silica gel solid-phase extraction cartridge. After infrared (IR) analysis of the extract, TRPHs are quantitated by direct comparison with standards.
Scope of Application
This method is applicable to the determination of 10 N-methylcarbamates, including Aldicarb (Temik), Aldicarb Sulfone, Carbaryl (Sevin), Carbofuran (Furadan), Dioxacarb, 3-Hydroxycarbofuran, Methiocarb (Mesurol), Methomyl (Lannate), Promecarb, and Propoxur (Baygon), in soil, water, and waste matrices using infrared spectroscopy.
This method is also suitable for the determination of total recoverable petroleum hydrocarbons (TRPHS) in solid waste extracts obtained by supercritical fluid chromatography. However, it is not applicable for the analysis of gasoline or other volatile components.
The method can detect extractable compounds at concentrations of 10 mg/L. When extracting a 3 g sample (assuming a 100% extraction efficiency), this corresponds to a detection limit of 10 mg/kg in soil.
Principle
The sample is extracted using Supercritical Fluid Extraction (SFE), and interfering substances are removed using bulk silica gel or silica gel solid-phase extraction (SPE) cartridges. The sample is analyzed by infrared spectroscopy (IR) by comparison with standard samples.
Reagents and Materials
Tetrachloroethylene, spectroscopic grade.
Reference oil mixture materials, spectroscopic grade.
1) n-Hexadecane
2) Isooctane
3) Chlorobenzene
Silica gel
1) Silica gel SPE cartridges (40 μm particle size, 60 Å pores), 0.5 g.
2) Silica gel, 60-200 mesh (deactivated with 112% water).
Calibration mixtures
1) Reference oil: Add 15.0 ml n-hexane, 15.0 ml isooctane, and 10.0 ml chlorobenzene into a 50 ml glass-stoppered bottle. Tighten the stopper to prevent volatilization losses. Store at 4°C.
2) Stock standard solution: Transfer 0.5 ml of the reference oil into a pre-weighed 100 ml volumetric flask and immediately seal. Weigh and dilute to volume with Tetrachloroethylene.
3) Working standard solution: Transfer an appropriate volume of the stock standard solution into a 100 ml volumetric flask based on cuvette size. Dilute to volume with Tetrachloroethylene. Calculate the concentration based on the stock standard solution.
Silica gel cleanup calibration
1) Prepare stock solutions of corn oil and mineral oil by adding 1 ml (0.5-1 g) of each into a pre-weighed 100 ml volumetric flask. Weigh to the nearest milligram. Dilute to the mark with Tetrachloroethylene, mix well, and dissolve until all contents are completely solubilized.
2) Prepare dilutions to the desired target concentrations as needed
3) Transfer 2 ml (or an appropriate volume) of the diluted corn oil/mineral oil sample into a sample vial. Add 0.3 g of bulk silica gel and shake the mixture for 5 minutes, or pass it through a 0.5 g silica gel SPE cartridge. If using an SPE cartridge, pre-condition it with 5 ml Tetrachloroethylene. Elute with Tetrachloroethylene and collect 3 ml of eluate. If using bulk silica gel, filter the extract through clean glass wool (using a disposable glass pipette).
4) Transfer the eluate or extract into a clean IR cuvette. Determine the elution fractions containing hydrocarbons (2800-3000 cm-1) without corn oil interference (1600-1800 cm-1, ester band). If the IR scan indicates that the silica gel's adsorption capacity is either too strong or insufficient (i.e., corn oil co-elutes with target hydrocarbons), select a new batch of silica gel or SPE cartridge.
Instruments and Accessories
HKL-8440 FTIR Spectrometer for total recoverable petroleum hydrocarbons, scanning or fixed-wavelength type, capable of scanning near 950 cm-1.
Cuvettes, 10 mm, 50 mm, and 100 mm path lengths, made of sodium chloride or IR-grade glass.
Magnetic stirrer, equipped with PTFE-coated stirring bars.
Test Procedure
1. Prepare the sample using liquid-liquid extraction (LLE) or normal-phase solid-phase extraction (SPE).
2. Add 0.3 g of bulk silica gel to the extract and shake the mixture for 5 minutes. Alternatively, pass the extract through a 0.5 g silica gel SPE cartridge (pre-conditioned with 5 ml Tetrachloroethylene). If bulk silica gel is used, filter the extract through clean glass wool (using a disposable glass pipette).
3. After silica gel cleanup, transfer the solution into an IR cuvette and measure its absorbance. If the absorbance exceeds the linear range of the IR spectrophotometer, dilute the sample appropriately and reanalyze. The adsorption efficiency of the silica gel can also be evaluated by repeating the cleanup and analysis process.
4. Select a working standard solution of appropriate concentration and choose a suitable cuvette size based on the following guidelines:
Path Length (mm) | Concentration Range (μg/ml, extract) | Volume (ml) |
10 | 5–500 | 3 |
50 | 1–100 | 15 |
100 | 0.5–50 | 30 |
5. Calibrate the instrument using a series of working standard solutions and the appropriate cuvette. Measure the absorbance of each solution directly at the maximum wavenumber (~2950 cm-1) and construct a calibration curve of petroleum hydrocarbon concentration versus absorbance.
Calculation of Results
The concentration of TRPHs in the sample is calculated using the following formula: ω (TRPHs)= (R×D×V)/W
Where:
R = TRPHs concentration (mg/ml), obtained from the calibration curve
V = Volume of extract (ml)
D = Dilution factor of the extract (if applicable)
W = Weight of the solid sample (kg)