The composition of petroleum is a complex mixture of hundreds of different hydrocarbon compounds. The resultant makeup of hydrocarbons released into the environment is variable, and dependent on the conditions to which it is subsequently exposed. The fractionation of the total extractable petroleum hydrocarbons (EPH) is necessary to determine the concentration of the aliphatic versus aromatic compounds. This application note describes the use of unbonded silica gel solid phase extraction (SPE) cartridges to fractionate the C9-C18 aliphatic hydrocarbons (n-nonane to n-octadecane), C19-C36 aliphatic hydrocarbons (n-nonadecane to hexatriacontane), and the C11-C22 aromatic hydrocarbons (naphthalene to benzo(ghi)perylene).
1. Cartridge Conditioning
a) Remove the caps from both ends of the silica gel SPE cartridges and attach the SPE cartridges to the stopcocks on a 16-position glass block manifold. Add 10 mL of n-hexane immediately into the cartridges to prevent silica gel from adsorbing the ambient moisture and allow to pass through via gravity.
b) Add 2 more aliquots of 10 mL hexane. Close the stopcock to stop the flow once the hexane level reaches the top frit. Do not allow the top frit or the silica gel sorbent to go dry.
2. Sample Loading
a) Add 1 mL EPH standard prepared in hexane or sample extract (exchanged into 1 mL hexane) to the SPE cartridges; 0.5 mL hexane rinse can be used to ensure quantitative transfer.
b) Let samples pass by gravity until the level reaches the top frit, then close the stopcock.
a) Insert the 12-position collection rack with 40-mL VOA glass vials into the manifold.
b) Elute the aliphatic fraction with 20* mL of n-hexane (2 ×10 mL) by gravity; collect and label the eluates as "aliphatic."
c) Remove the vials with aliphatic fractions from the collection rack, and insert new VOA vials to collect the aromatic fractions.
d) Elute the aromatic fraction with 20 mL of DCM (2 × 10 mL) by gravity; collect and label the eluates as "aromatic."
e) Concentrate the eluates to 1 mL or a higher volume if the desired detection limits can be achieved and analyze the two fractions separately by gas chromatography–mass spectrometry.
*The volume of n-hexane should be optimized so that only the aliphatic hydrocarbons are eluted without breakthrough of the aromatic hydrocarbons into the aliphatic fraction (the naphthalene and 2-methyl naphthalene breakthrough should be <5%). The optimum hexane volume may vary from lab to lab, depending on the moisture content in the lab environment.
Excellent analytical performance has been obtained using UCT's EPH silica gel SPE cartridges (heat treated, large particle size) for the fractionation of aliphatic and aromatic hydrocarbons. Recoveries were ranged from 90.7 to 94.6% for three EPH fractions. The naphthalene and 2-methyl naphthalene breakthrough was < 1%, also meeting the required <5% breakthrough of the aromatics into the aliphatic fraction.