UNDERSTANDING THE PETROLEUM ANALYTICAL PROCESS

FROM SAMPLE COLLECTION TO MEASUREMENT

This volume focuses on three types of petroleum analytical methods:
• methods that measure a TPH concentration;
• methods that measure a petroleum group type concentration; and
• methods that measure individual petroleum constituent concentrations.
These three types of methods measure different petroleum hydrocarbons that
might be present in petroleum-contaminated environmental media. TPH methods
generate a single number that represents the combined concentration of all petroleum
hydrocarbons in a sample, which are measurable by the particular method
(See discussion in Section 3 regarding limitations of TPH data). Petroleum group
type methods separate and quantify different categories of hydrocarbons (e.g., saturates,
aromatics, and polars/resins). The results of petroleum group type analyses
can be useful for product identification because different products (e.g., gasoline,
fuel oil no.2, and jet fuel) can have characteristic levels of various petroleum
groups (see Appendix II for a detailed characterization of petroleum products).
Individual constituent methods quantify concentrations of specific compounds
that might be present in petroleum-contaminated samples, such as benzene, ethylbenzene,
toluene, and xylenes (BTEX), and polycyclic aromatic hydrocarbons
(PAHs). Concentration data for individual petroleum constituents can be used to
evaluate human health risk, provided the necessary toxicity data are available.
Although these three method types measure different petroleum hydrocarbon
categories, there are several basic steps that are common to the analytical processes
for all methods, no matter the method type or the environmental matrix. This
section will focus on these basic steps. Sections 5, 6 and 7 review analytical methods
that can provide the three different types of petroleum concentration data.
Most of the common analytical steps are related to the separation of analytes of
interest from a sample matrix prior to their measurement. In general, these steps are:
• Collection and preservation - requirements specific to environmental matrix
and analytes of interest
• Extraction - separates the analytes of interest from the sample matrix
• Concentration - enhances the ability to detect analytes of interest
• Cleanup - may be necessary to remove interfering compounds
• Measurement - quantifies the analytes.
Each step affects the final result, and a basic understanding of the steps is vital
to data interpretation.

COLLECTION AND PRESERVATION OF ENVIRONMENTAL SAMPLES
The ability to collect and preserve a sample that is representative of the site is a
critically important step. Obtaining representative environmental samples is
always a challenge due to the heterogeneity of different sample matrices.
Additional difficulties are encountered with petroleum hydrocarbons due to the
wide range in volatility, solubility, biodegradation, and adsorption potential of
individual constituents.
Most site investigations for assessment of petroleum hydrocarbon contamination
in the environment are regulated by the states. However, sample collection and
preservation recommendations follow U.S. EPA guidelines. A summary of the most
commonly used guidelines is included in Table 1. It should be noted that there
might be additional requirements in any given state. Before a sample is collected,
the particular state requirements must be investigated. Because of holding time
considerations, the laboratory must be selected and notified prior to the collection
of the samples.

SAMPLE EXTRACTION
For most analyses, it is necessary to separate the analytes of interest from the matrix
(i.e. soil, sediment, and water). Extraction of analytes can be performed using one
or more of the following methods:
• Extracting the analytes into a solvent
• Heating the sample (used in the analyses of volatile compounds)
• Purging the sample with an inert gas (used in the analyses of volatile compounds).
There are a variety of common sample extraction techniques. See Table 2.
Soxhlet, sonication, supercritical fluid, subcritical or accelerated solvent, and
purge and trap extraction have been promulgated by the U.S. EPA as soil extraction
methods. Headspace is recommended as a screening method. Shaking/vortexing
is presently not approved by EPA, but is quite adequate for the extraction of
petroleum hydrocarbons in most environmental samples.
For these extraction methods, the ability to extract petroleum hydrocarbons
from soil and water samples depends on the solvent and the sample matrix.
Surrogates (compounds of known identity and quantity) are frequently added to
monitor extraction efficiency. Environmental laboratories also generally perform
matrix spikes (addition of target analytes) to determine if analytes are retained by
the soil or water matrix.
Solvents have different extraction efficiencies. Extracting the same sample in the
same manner by two different solvents may result in different concentrations. The
choice of solvents is determined by many factors such as cost, spectral qualities,
method regulations, extraction efficiency, toxicity, and availability. Methylene chloride
has been the solvent of choice for many semivolatile analyses due to its high
extraction efficiency, low cost, and specification by many state regulatory methods.
Chlorofluorocarbon solvents such as trichlorotrifluoroethane (Freon 113) have
been used in the past for oil and grease analyses because of their spectral qualities
(they do not absorb in the 2930 cm-1 infrared measurement wavelength) and low human toxicity. The EPA is phasing out use of chlorofluorocarbons, however, due
to their detrimental effects on stratospheric ozone. Tetrachloroethene and carbon
tetrachloride are possible replacements. Methanol is the most common solvent
used to preserve and extract volatiles such as BTEX in soils. Figure 2 illustrates the
typical solvents used for different analyses.
Water Samples
Water extraction methods in common use include the following:
For Volatiles:
• Purge and trap
• Headspace
For Semivolatiles:
• Separatory funnel extraction
• Continuous liquid-liquid extraction
• Solid phase extraction.
Volatile compounds (gasoline, solvents) in water are generally separated from
their matrix by purging with an inert gas and trapping the compounds on a
sorbent (EPA 5030, purge and trap analysis). The sorbent is later heated to
release the volatile compounds, and a carrier gas sweeps the compounds into a
gas chromatograph.
Headspace analysis is recommended as a screening method by EPA (Methods
3810, and 5021), although it performs well in particular situations, especially field
analysis. In this method, the water sample is placed in a closed vessel with a headspace
and heated to drive volatiles into the gas phase. Addition of salts or acids may
enhance this process. In headspace analysis, instrument contamination is minimized
because only volatile compounds are introduced into the instrument.
Samples containing heavy oils and high analyte concentrations can severely contaminate
purge and trap instrumentation.
The most commonly used water extraction method for semivolatiles is EPA
Method 3510, separatory funnel extraction. The sample is poured into a funnelshaped
piece of glassware, solvent is added, and the mixture is shaken vigorously.
After layer separation, the extract (i.e., the solvent layer) is removed, filtered, dried
with a desiccant, and concentrated. Multiple extractions on the same sample may
increase overall recovery.
Another commonly used water extraction method for semivolatiles is EPA 3520,
continuous liquid-liquid extraction. Rather than shaking solvent with the water
sample, the solvent is continuously heated, nebulized (broken into small droplets),
and sprayed on top of the water. Liquid-liquid extraction is excellent for samples
containing emulsion-forming solids, but it is more time-consuming than separatory
funnel extractions.
Solid phase extraction (SPE, EPA Method 3535) also can be used for extraction
and concentration of semivolatile material. The technique involves passing the
water sample through a cartridge or disk containing an adsorbent such as silica or alumina. The adsorbent is often coated with compounds that impart selectivity for
particular products or analytes such as PAHs. After extraction, the analytes are separated
from the solid phase by elution with a small amount of organic solvent. A
variant of SPE involves dipping a sorbent-coated fiber into the water (solid phase
micro-extraction, or SPME). Adsorbed analytes are thermally desorbed directly
into a heated chromatographic injection port. SPE uses much less solvent and
glassware than separatory funnel and liquid-liquid extraction.


source:
Analysis of Petroleum Hydrocarbons in Environmental Media (American Petroleum Institute,Association for the Environmental Health of Soils,Association of American Railroads,
British Petroleum,Chevron Research and Technology Company,Exxon Biomedical Sciences, Inc.,Retec, Inc., Shell Development Company, United States Air Force, Air Force Research Laboratory, University of Massachusetts)