Changes

==Limitations due to internal standards==

==Limitations due to internal standards==

An internal standard is commonly coinjected with the sample to aid in quantification during gas chromatography. If this internal standard is not clearly labeled on the chromatogram, it may be confused with an indigenous compound and erroneously used in correlation.

An internal standard is commonly coinjected with the sample to aid in quantification during gas chromatography. If this internal standard is not clearly labeled on the chromatogram, it may be confused with an indigenous compound and erroneously used in correlation.

===Examples of problems with internal standards===

===Examples of problems with internal standards===

[[file:oiloil-and-oilsource-rock-correlations_fig8-17.png|thumb|{{figure number|8-17}}See text for explanation.]]

[[file:oiloil-and-oilsource-rock-correlations_fig8-18.png|thumb|{{figure number|14}}. Copyright: Murray et al. (1993); courtesy Australian Geological Survey Organization.]]

The figure below shows a gas chromatogram of the saturated hydrocarbons from the Lufa seep oil, another seep oil from Papua New Guinea. Because this work was carried out by a different laboratory than the analyses in the previous figure, the internal standard used here is different. This standard elutes just before ''n''-C₂₂. The pristane–''n''-C₁₇ ratio suggests that this oil is different from those in Figure 8-17, a fact confirmed by other geochemical evidence.

[[:file:oiloil-and-oilsource-rock-correlations_fig8-17.png|Figure 13]] shows gas chromatograms of the saturated hydrocarbons from two seep oils from Papua New Guinea. Both samples contain an internal standard that elutes just before ''n''-C₂₁. Because the left oil is biodegraded, the detector sensitivity was set very high to record the trace amounts of remaining ''n''-alkanes. Consequently, the internal standard peak is very tall. In contrast, the right oil (undegraded) shows a much smaller internal standard peak because the detector sensitivity was lower.

[[file:oiloil-and-oilsource-rock-correlations_fig8-18.png|thumb|{{figure number|8-18}}. Copyright: Murray et al. (1993); courtesy Australian Geological Survey Organization.]]

[[:file:oiloil-and-oilsource-rock-correlations_fig8-18.png|figure 14]] shows a gas chromatogram of the saturated hydrocarbons from the Lufa seep oil, another seep oil from Papua New Guinea. Because this work was carried out by a different laboratory than the analyses in the previous figure, the internal standard used here is different. This standard elutes just before ''n''-C₂₂. The pristane–''n''-C₁₇ ratio suggests that this oil is different from those in [[:file:oiloil-and-oilsource-rock-correlations_fig8-17.png|Figure 13]], a fact confirmed by other geochemical evidence.

==Limitations due to contaminated samples==

==Limitations due to contaminated samples==