Detecting Methanol in Light Crude Oil with GC-VUV
By: James Diekmann
I don’t know about you, but I DON’T like the idea of directly injecting crude oil into my gas chromatograph for determination of volatile compounds like methanol. No thank you. Instead, I’d rather use a less abrasive injection technique – sampling the headspace. For this blog post, I’m interested in detecting methanol in crude oil – a compound that is highly volatile and partitions easily into the headspace from crude oil.
For those of you wondering, methanol is used in crude oil to prevent gas hydrates forming in pipelines. Unfortunately, residual methanol in crude oil can lead to costly problems in refinery operations by negatively impacting water treatment facilities.
Traditional methods like ASTM D7059 involve direct injection of crude oil into a gas chromatograph, requiring backflush capability to properly analyze methanol with a flame ionization detector. To avoid a direct injection of crude oil, static headspace – gas chromatography – vacuum ultraviolet spectroscopy (SH-GC-VUV) can be used as an alternative method. This technique simplifies the GC configuration and saves the GC inlet and column from crude oil contamination, while also providing spectral identification and quantification of methanol.
Figure 1 shows an example chromatogram of methanol in an ASTM light crude oil, using SH-GC-VUV. The methanol peak is baseline resolved, meaning it is well separated from water and other compounds. The inset plot shows that methanol can be detected down to 5 ppm, which is comparable to existing methods. SH-GC-VUV calibration curves for methanol in crude oil from 5 to 1,000 ppm showed good linearity (Figure 2).
To emphasize the importance of VUV absorbance spectra, Figure 3 shows examples of methanol, 2-propanol, and 1-propanol. Even though they are all classified as alcohols, each VUV spectrum is unique, even for the two isomers.
In my opinion, if you want to avoid gunking up your GC, headspace GC-VUV is a viable alternative for analyzing volatiles in crude oil!