Methods for total petroleum hydrocarbons

Table 7.3. EPA Test Methods for Total Petroleum Hydrocarbons...

Two methods, EPA SW-846 8015 and 8015A, were, in the past, often quoted as the source of gas chromatography-based methods for measurement of the total petroleum hydrocarbons in a sample. However, the original methods were developed for nonhalogenated volatile organic compounds and were designed to measure a short target list of chemical solvents rather than petroleum hydrocarbons. Thus, because there was no universal method for total petroleum hydrocarbons, there were many variations of these methods. Recently, an updated method... 

In contrast to traditional methods for total petroleum hydrocarbons that report a single concentration number for complex mixtures, the fractionation methods report separate concentrations for discrete aliphatic and aromatic fractions. The petroleum fraction methods available are GC-based and are thus sensitive to a broad range of hydrocarbons. Identification and quantification of aliphatic and aromatic fractions allows one to identify petroleum products and evaluate the extent of product weathering. These fraction data also can be used in risk assessment. 

An important feature of the analytical methods for total petroleum hydrocarbons is the use of an equivalent carbon number index (EC index, ECN index, or ECNl), which represents equivalent boiling points for hydrocarbons and is the physical characteristic that is the basis for separating petroleum (and other) components in chemical analysis. Petroleum fractions as discussed in this profile are defined by the ECN index. 

Because of the relative complexity of the analytical methods for total petroleum hydrocarbons, there is a need for devising methods for the determination of total petroleum hydrocarbons. But the major problem lies in the range of compounds covered by the term hydrocarbons. Again, the most notable variation is in the relative volatility and other properties of the hydrocarbons under investigation. Although instrumental detection methods are available (Sadler and Connell, 2003), another approach involves collection of the contaminated soil and sealing it in a container, where the soil gas can accumulate. This gas is then analyzed by one of several reliable instrumental procedures. 

The ASTM method for total petroleum hydrocarbons (ASTM book) is similar to the standard EPA method (EPA 418.1) and calls for extraction with Freon. The estimated variability of the test method is quesUonable and may leave room for serious errors in the calculation of total petroleum hydrocarbons. 

The character of fuel oil generally renders the usual test methods for total petroleum hydrocarbons (Chapters 7 and 8) ineffective since high proportions of the fuel oil (specifically, residual fuel oil) are insoluble in the usual solvents employed for the test. In particular, the asphaltene constituents are insoluble in hydrocarbon solvents and are only soluble in aromatic solvents and chlorinated hydrocarbons (chloroform, methylene dichloride, and the like). Residua and asphalt (Chapter 10) have high proportions of asphaltene constituents, which render any test for total petroleum hydrocarbons meaningless unless a suitable solvent is employed in the test method. 

Hawthorne ST, Miller DJ, Hegvik, KM. 1993. Field evaluation of the SFE-infrared method for total petroleum hydrocarbon (TPH) determinations. Journal of Chromatographic Science 31 26-30. 

Analysis for total petroleum hydrocarbons (EPA Method 418.1) provides a one-number value of the petroleum hydrocarbons in a given environmental medium. It does not, however, provide information on the composition (i.e., individual constituents) of the hydrocarbon mixture. The amount of hydrocarbon contaminants measured by this method depends on the ability of the solvent used to extract the hydrocarbon from the environmental media and the absorption of infrared light (infrared spectroscopy) by the hydrocarbons in the solvent extract. The method is not specific to hydrocarbons and does not always indicate petroleum contamination, since humic acid, a nonpetroleum material and a constituents of many soils, can be detected by this method. 

There are several reasons why the data for total petroleum hydrocarbons do not provide ideal information for investigating sites and establishing target cleanup criteria. For example, use of the term total petroleum hydrocarbons suggests that the analytical method measures the combined concentration of all petroleum-derived hydrocarbons, thereby giving an accurate indication of site contamination. But this is not always the case. Furthermore, target cleanup levels based on total petroleum hydrocarbons concentrations implicitly assume (1) that the data are an accurate measurement of petroleum-derived hydrocarbon concentration, and (2) the data also indicate the level of risk associated with the contamination. These assumptions are not correct due to many factors, including the nonspecificity of some of the methods used and the complex nature of petroleum hydrocarbons and their interaction with the environment over time. 

Furthermore, as a fuel evaporates or biodegrades, its pattern can change so radically that identification becomes difficult. Consequently, a gas chromatographic fingerprint is not a conclusive diagnostic tool. The methods used for total petroleum hydrocarbon analysis must stress calibration and quality control, whereas pattern recognition methods stress detail and comparability. 

Therefore, for infrared spectroscopic methods, the total petroleum hydrocarbons comprise any chemicals extracted by a solvent that are not removed by silica gel and can be detected by infrared spectroscopy at a specified wavelength. The primary advantage of the infrared-based methods is that they are simple and rapid. Detection limits (e.g., for EPA 418.1) are approximately 1 mg/L in water and 10 mg/kg in soil. However, the infrared method(s) often suffer from poor accuracy and precision, especially for heterogeneous soil samples. Also, the infrared methods give no information on the type of fuel present in the sample, and there is little, often no information about the presence or absence of toxic molecules, and no specific information about potential risk associated with the contamination. 

Immunoassay methods correlate total petroleum hydrocarbons with the response of antibodies to specific petroleum constituents. Many methods measure only aromatics that have an affinity for the antibody, benzene-toluene-ethylbenzene-xylene, and PAH analysis (EPA 4030, Petroleum Hydrocarbons by Immunoassay). 

The screening measures for total petroleum hydrocarbons screening are done differently in various states, regions, and individual laboratories. In fact, the data that are reported as total petroleum hydrocarbons are so variable that much caution should be exercised when attempting to compare or interpret the data. As analytical methods evolve in response to environmental needs, the definition of total petroleum hydrocarbons may become more closely related to reality rather than to the respective analytical method. 

Since the term total petroleum hydrocarbons (total petroleum hydrocarbons) includes any petroleum constituent that falls within the measurable amount of petroleum-based hydrocarbons in the enviromnent the information obtained for total petroleum hydrocarbons depends on the analytical method used. Therefore, the difficulty associated with measurement of the total petroleum hydrocarbons is that the scope of the methods varies greatly (Table 8.1). Some methods are nonspecific, whereas others provide results for hydrocarbons in a boiling-point range. Interpretation of analytical results requires an understanding of how the determination was made (Miller, 2000, and references cited therein Dean, 2003). 

Most of the analytical methods discussed here for total petroleum hydrocarbons have been developed within the framework of federal and state regulatory initiatives. The initial implementation of the Federal Water Pollution Control Act (FWPCA) focused on controlling conventional pollutants such as oil and... 

The conventional methods of analysis for total petroleum hydrocarbons (Chapter 7) have been used widely to investigate sites that may be contaminated with petroleum hydrocarbon products (see also EPA 418.1) for the determination of petroleum hydrocarbons. The important advantage of this method is that excellent sample reproducibility can be obtained, but the disadvantages are that... 

Methods are also available for determination of specific hydrocarbon compounds in biological samples. Some of these methods are shown in Table 3-5. Since these methods have not been demonstrated for total petroleum hydrocarbons, the analyst must verify that they are suitable prior to use. 

MICROWAVE-ASSISTED SOLVENT EXTRACTION AND A NEW METHOD FOR ISOLATION OF TOTAL PETROLEUM HYDROCARBONS (TPH) FROM PLANTS WITH COLUMN CHROMATOGRAPHY (SILICA GEL AND ALUMINA) AND DETERMINATION WITH SPECTROFLUOROPHOTOMETRY... 

There are many recommended sampling protocols (Table 6.2). The sampling methods used for petroleum hydrocarbons are generally thought of as methods for determination of the total petroleum hydrocarbons. In part due to the complexity of the components of the total petroleum hydrocarbons fractions, little is known about their potential for health or environmental impacts. As gross measures of petroleum contamination, the total petroleum hydrocarbons data simply show that petroleum hydrocarbons are present in the sampled media. Measured total petroleum hydrocarbons values suggest the relative potential for human exposure and therefore the relative potential for human health effects. 

Total petroleum hydrocarbon (TPH) (Chapter 4) analyses (Tables 7.1 and 7.2) are conducted to determine the total amount of hydrocarbon present in the environment. There are a wide variety of methods for measurement of the total petroleum hydrocarbon in a sample, but analytical inconsistencies must be recognized because of the definition of total petroleum hydrocarbons and the methods employed for analysis (Rhodes et al., 1994). Thus, in practice, the term total petroleum hydrocarbon is defined by the analytical method since different methods often give different results because they are designed to extract and measure slightly different subsets of petroleum hydrocarbons. 

Table 7.1. Summary of Analytical Methods for Determining the Total Petroleum Hydrocarbons in a Sample... 

The analysis for the total petroleum hydrocarbons (TPHs) in a sample as a means of evaluating petroleum-contaminated sites is also an analytical method in common use. The data are used to establish target cleanup levels for soil or water by regulatory agencies in the United States and in many other countries. 

The data obtained by the analysis have become key remediation criteria and it is essential that the environmental analyst (and others who may use the data) be knowledgeable about the various analytical methods. It is also important to know that minor method deviations may be found from region to region. For example, in terms of nomenclature, itself a complex and often ill-defined area of petroleum science (Chapter 1) (Speight, 1999), the analytical methods may refer to total petroleum hydrocarbons as mineral oil, hydrocarbon oil, extractable hydrocarbon, and oil and grease. 

Thus, as often occurs in petroleum science (Speight, 1999), the definition of total petroleum hydrocarbons depends on the analytical method used because the total petroleum hydrocarbons measurement is the total concentration of the hydrocarbons extracted and measured by a particular method. The same sample analyzed by different methods may produce different values. For this reason, it is important to know exactly how each determination is made since interpretation of the results depends on understanding the capabilities and limitations of the method. If used indiscriminately, measurement of the total petroleum hydrocarbons in a sample can be misleading, leading to an inaccurate assessment of risk. 

Some methods measure more compounds than other methods because they employ more rigorous extraction techniques or more efficient solvents for the extraction procedure(s). Other methods are subject to interferences from naturally occurring materials such as animal and vegetable oils, peat moss, or humic material, which may result in artificially high reported concentrations of the total petroleum hydrocarbons. Some methods use cleanup steps to minimize the effect of nonpetroleum hydrocarbons, with variable success. Ultimately, many of the methods are limited by the extraction efficiency and the detection limits of the instrumentation used for measurement. 

There are many analytical techniques available that measure total petroleum hydrocarbon concentrations in the environment, but no single method is satisfactory for measurement of the entire range of petroleum-derived hydrocarbons. In addition, and because the techniques vary in the manner in which hydrocarbons are extracted and detected, each method may be applicable to the measurement of different subsets of the petroleum-derived hydrocarbons present in a sample. The four most commonly used total petroleum hydrocarbon analytical methods include (1) gas chromatography (GC), (2) infrared spectrometry (IR), (3) gravimetric analysis, and (4) immunoassay (Table 7.1) (Miller, 2000, and references cited therein). 

Gas chromatographic methods are currently the preferred laboratory methods for measurement of total petroleum hydrocarbon measurement because they detect a broad range of hydrocarbons and provide both sensitivity and selectivity. In addition, identification and quantification of individual constituents of the total petroleum hydrocarbon mix is possible. 

Methods based on gravimetric analysis (Table 7.2) are also simple and rapid, but they suffer from the same limitations as those of infrared spectrometric methods (Table 7.2). Gravimetric-based methods may be useful for oily sludge and wastewaters, which will present analytical difficulties for other, more sensitive methods. Immunoassay methods for the measurement of total petroleum hydrocarbon are also popular for field testing because they offer a simple, quick technique for in situ quantification of the total petroleum hydrocarbons. 

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