In gas chromatographic analysis of oils, detection can be achieved using either Flame Ionization Detection (FID) or Mass Spectrometry (MS). These detection techniques are fundamentally distinct, rendering direct result comparisons impractical. When referencing established standards, it becomes crucial to employ analysis outcomes generated through the same method as those standards. Analyses of precisely defined compounds in water or soil should yield essentially equivalent results, whether performed through GC-FID or GC-MS.
However, in the analysis of compound groups, such as oils, there is no absolute truth in concentration due to the varying ways in which oil content can be defined.
In GC-FID analysis, oil content is related to the signal of a reference oil. The reference oil corresponds to a blend of diesel oil and a heavier oil. Conversely, in GC-MS analysis, molecules are fragmented within the detector by bombarding them with electrons. In aliphatic molecules, a fragment with a mass of 57 is formed. This fragment is used for the quantification of aliphatics. All molecules possessing an aliphatic group substantial enough to generate a mass 57 fragment will be reported as aliphatics.
In all analyses, a standard solution with known concentrations of the desired compounds is initially analyzed. For aliphatic analysis, this solution contains alternating straight-chain hydrocarbons (C8, C10, C12, etc.). The detector’s sensitivity to these hydrocarbons varies slightly based on factors like the compounds’ boiling points. When quantifying different aliphatic fractions, the hydrocarbons in the standard that most closely resemble those in the sample are used.
The proportion of fragment 57 can significantly vary depending on the type of oil. Fresh, unweathered oils often contain a high proportion of straight-chain hydrocarbons, leading to a relatively high proportion of fragment 57 in the detector. Bacterial degradation of oil primarily targets straight-chain hydrocarbons, resulting in a relatively small proportion of fragment 57. Aged contaminants may therefore exhibit low aliphatic content. Oils with a different composition than the common ones (e.g., cutting oils) may also yield very low results in GC-MS analysis if fragment 57 is not produced. It is common for GC-FID analysis to yield concentrations several times higher than those from GC-MS. Truly comparable values are only obtained from oils composed exclusively of straight-chain hydrocarbons.
Particularly in the analysis of compound groups (e.g., oils), it is crucial not to directly compare results obtained using different analytical techniques. When benchmarking against reference values, one should utilize the analytical technique employed in establishing those standards. Reference values for contaminated soil are determined using GC-MS. UV spectrometric as well as mass spectrometric analysis also enables the identification of unknown compounds in a sample through spectrum libraries, even without access to standard substances. This is utilized in screening methods.
The Case for Labio GC-UV as the Gold Standard in Oil Analysis
By measuring the absorption of specific wavelengths of UV light by target compounds, GC-UV offers unparalleled specificity and sensitivity, making it a robust choice for the characterization of various organic compounds within complex matrices like oils. The specificity of GC-UV lies in its ability to target compounds with unique UV absorption properties. This inherent selectivity allows for the precise identification and quantification of individual compounds, setting the stage for accurate and unambiguous results. In contrast to GC-FID and GC-MS, GC-UV is particularly well-suited for the analysis of specific functional groups and compounds that exhibit characteristic UV absorption patterns.
By taking the average of absorbance at certain wavelength intervals a chromatogram is obtained. If this interval is broad it represents a non specific detection and if it is narrow it can represent a specific detection for a certain group of compounds. This is similar to the total ion current (TIC) and the single ion monitoring (SIM) in GC-MS or the Gram Smidt (non specific chromatogram) and certain wavelength ranges, characteristic for functional groups in GC-FTIR.
