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The Hewlett-Packard 5890 gas chromatograph equipped with 5971 mass selective detector and Teckmar 3000 purge-and-trap unit. |
There are hundreds of potentially toxic organic compounds in soils, waters, and waste products, which makes organic compound analysis is much more challenging than for the limited number of inorganic (elemental) analytes. Organics are often regulated by EPA at ppt (parts per trillion) levels, and often occur in complex mixtures in contaminated samples that further complicate identification and quantification. Classes of organic compounds of environmental interest include aliphatic and aromatic petroleum hydrocarbons, industrial solvents (largely volatile compounds), pesticides, and high-molecular weight compounds (largely non-volatiles, such as PCB's, dioxins, etc.)
Sample preparation for organic analysis varies with the sample type and compound(s) of interest. For many compounds, samples are typically prepared by extracting the soil, sediment, or water sample with an organic solvent such as acetone or hexane; this solvent phase is then "cleaned up" by extraction into another organic solvent toremove interfering compounds. Alternatively, the Tekmar purge-and-trap unit available at the Lab can directly handle solid and liquid samples, applying a heating program to volatilize compounds directly into the analytical gas chromatograph (GC) unit.
The following is a partial list of compounds that may be routinely determined in water, feed, soil, or plant tissue analysis:
Pesticides: aldrin, chlordane, DDD, DDE, DDT, dieldrin, endrin, ethion, heptachlor, lindane, malathion, methoxychlor, methyl parathion, mirex, parathion, rabon, toxaphen.
Volatile Organics: vinly chloride, 1,1-dichlorethylene, methylene chloride, 1,1-dichloroethane, chloroform, 1,1,1-trichloroethylene, carbon tetrachloride, 1,1,2,2-tetrachloroethane, 1,2-dichloropropane, 1,1,2,2-tetrachloroethane, dibromochloromethane, chlorobenzene, bromoform, 1,2-dichlorobenzene, benzene, ethylbenzene.
Petroleum Products: n-decane, n-unedecane, n-dodecane, n-tridecane, n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, n-nonadecane, n-eicosane, n-henicosane, n-docasane, n-triosane, n-tetracosane, n-pentacosane, n-hexacosane, n-heptacosane, n-octacosane, n-nonacosane, n-tricontane, n-hentricontane, n-dotriacontane, n-tritriacontane, n-tetratricontane, n-penatriacontane, naphthalene, flourene, dibenzothiophene, phenanthrene, fluoranthene, pyrene, chrysene, benzo(b)fluoranthene, benzo(b)pyrene, perylene, indeno(1,2,3 cd)pyrene, dibenzo(a,h)anthracene, pristane, phytane, anthracene.
BTEX Compounds: benzene, toluene, ethylbenzene, o,m,p-xylenes.
For a given sample, a general scan of compounds present can be performed using
the purge-and-trap system or a simple solvent extraction followed by GC-MS
detection; this approach is semi-quantitative but allows a wide range of compounds
to be identified in a given sample. Analysis by convention GC with other detector
systems allows specific compounds to be quantified more accurately, and is
often performed following a GC-MS initial compound screening.
The primary unit used for screening and identification of organic compounds is a Hewlett-Packard (HP) 5890 series GC equipped with a 5971 mass selective detector (MSD) and an electron capture detector (ECD). A computer-based library of mass spectral information is available to identify peaks, and quantification is available via standard compound detector responses for a number of compounds. A Teckmar 3000 purge-and-trap unit is attached to this GC for automated sample volatilization and injection using solid or aqueous samples, eliminating the need for pre-extraction of samples.
| A range of column types is available for specific classes of compounds. Also in the lab is a newer HP 6890 GC (shown at right), which currently uses both flame ionization and thermal conductivity detectors; this instrument is used for quantitative analysis of samples with known types contaminant compounds. |  |
QA/QC is much more difficult with organic analysis compared to AA or ICP analysis of inorganics, due to the complexity and time required for GC runs, coupled with greater potential for error in extraction and quantification steps. Internal standards (spikes of similar non-analyte compounds) are routinely used to check efficiency of extraction and instrument performance in GC analysis; recoveries of such spikes should be >80% for high confidence in the resulting analysis. Blanks and replicates (1 in 10 samples) are also used to assess contamination and precision of analysis. In quantitative work, multipoint standard calibrations are used to assess the linear range of detector response, and to accurately determine the detection limit of the method.