In many chemical analyses, specifically where Gas Chromatography (GC) is utilised for chemical separation, your data can be represented and interpreted in the form of a chromatogram. The visual pattern of chromatograms can often be utilised to support the identification of contaminant sources and is particularly useful with petroleum hydrocarbons.
In some cases, results for a particular sample that was analysed for Petroleum Hydrocarbons F2-F4, will include a comment indicating that the "sample did not reach baseline" or on some occasions, " sample did not reach baseline, however did not indicate significant contamination after C50."
This is to indicate, that there may be additional hydrocarbon based substances or compounds in your sample matrix beyond the target range.
Let's look at a simplified example of F2-F4 petroleum hydrocarbons analysed via GC/FID. Figure 1 is what a very typical and chromatogram of F2-F4 analysis may look like for the carbon ranges of C10-C50.
In Figure 1, the response curve of target analytes, returns to the X-axis at or before Carbon-50.
Figure 2 represents a sample that is considered as not reaching the baseline. The chromatogram curve doesn't intercept the X-axis, thus indicating there may be heavier hydrocarbons present in the sample, such as: wax like substances, heavy oils, greases etc.
An F4G gravimetric procedure allows us to measure the amount of additional heavy hydrocarbons. This method consists of solvent extraction, followed by gravimetric (weight based) methods. Running an F4G will not allow us to identify the compounds present in the sample.
More information on F4G can be found in the follow article: What is F4G and when is it required?