Crude oils are some of the most complex mixtures routinely analyzed by gas chromatography (GC). They contain significant amounts
of organic compounds, ranging from light hydrocarbons to complex biomolecules derived from the remains of ancient marine organisms
and the bacteria that feed on them.
Figure 1
The compounds of most interest to the petroleum industry are relatively volatile (with boiling points generally below 400
°C) and nonpolar, leading to separations predominantly being performed by GC with a nonpolar column. Unsurprisingly, the resulting
chromatograms are complex and usually characterized by a matrix of unresolved material that appears as a significant background
"hump" beneath the partially resolved nonpolar compound peaks (Figure 1). A suitable background subtraction and library search
algorithm can identify the nonpolar compound peaks, but for those lost in the matrix, there is little hope for a positive
identification.
The inability of simple GC separations to cope with this level of complexity has led to a system of classification based upon
the presence of discrete biomarker compounds. However, such an incomplete characterization limits understanding of the oil
sample's geochemistry and provides no insight into issues that might arise during extraction, transport, or refining. This
problem is becoming more acute as the more easily extracted "light" crude oils become depleted and oil companies move to reservoirs
containing "heavier" crudes, bituminous shales, and tar sands, which contain higher proportions of involatile, polar compounds
that can complicate the analysis (1).
Two-Dimensional Gas Chromatography
Two-dimensional chromatography (GC×GC) is an important technique for the petrochemical industry due to its ability to separate
very complex mixtures (2,3). The usual configuration is a conventional nonpolar column connected to a short length of a polar
column with a GC×GC modulator in between. The modulator collects time slices of effluent from the first column (typically
5-s wide) and reinjects them onto the short polar column. The result is a separation that combines both polar and nonpolar
elements. In the case of crude oil analysis, GC×GC can start to tease apart the complexity present in the matrix and identify
the major components present there. With two levels of separation, GC×GC has the resolution required to isolate many of the
compounds of interest in complex samples.
Figure 2
Figure 2 shows a relief representation of a GC×GC separation of the same lubricating oil sample shown in Figure 1. The silhouette
outline on the top left is a representation of how the sample would look on a conventional, single-dimensional GC separation.
The colored portion in the figure represents the GC×GC separation with the nonpolar aspect being from bottom to top and the
polar aspect being from right to left. This separation allows us to visualize compound peaks invisible in a conventional GC
separation.
Mass Spectrometry Detectors for GC×GC
Figure 3
GC×GC peak widths in this example analysis are typically 50–200 ms wide, which presents a problem for traditional GC–mass
spectrometry (MS) systems. The maximum scan rate of a quadrupole mass detector will be in the order of 10,000 amu/s. A system
set to scan from 50 to 550 m/z would imply a maximum of 20 scans/s. When an interscan delay is taken into account, it is rarely possible to acquire more
than 10 scans/s, implying a very poor definition of even the 200-ms wide peaks. To monitor such narrow peaks effectively,
a much faster mass detector is required, and time-of-flight (TOF)-MS is currently the only viable technology.
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