GC Second Column Confirmation Fallacy

GC (EPA Method 8020) results, even with second column confirmation, is of little value for BTEX analysis of samples containing hydrocarbons. GCMS (EPA Method 8260) provides definitive qualitative and quantitative results.

EPA Methods 8010 and 8020, especially 8020, has been a concern of ours for many years because you cannot rely on identification based on GC retention times alone. This is especially true of 8020 in which a photoionization detector (PID) is used. The problem is that the PID is not selective enough; it responds to hydrocarbons of all types, not just aromatic hydrocarbons. We have known this for many years and have always recommended that our clients rely on GCMS (EPA Method 8260) rather than GC for confirmation. Finally we decided to provide you with real data in hopes that this will convince clients and regulators not to rely on EPA Method 8020 for BTEX results.

Second column confirmation of GC results is generally a good idea. One exception is for petroleum hydrocarbons using EPA Method 8020 (GC with PID). Petroleum products contain a large number of components, and capillary GC is capable of resolving over 100 components in many distillate fuel products. With a nonspecific detector such as a PID, even using a lower energy lamp (10.0 ev), there is a continuum of peaks which may lead to falsely identifying some peaks as an aromatic hydrocarbon.

In our example data presented on this page, three samples were analyzed by GC-PID (EPA 8020) on two different capillary columns and by GCMS (Table 1). Samples of soil contaminated with petroleum confirmed aromatic hydrocarbons on both GC columns because there were peaks within the retention time windows on both columns. In all but one case the GCMS results are either much less than the GC results or "Not Detected".

The GC and GCMS chromatograms are presented in Figure 1 and Figure 2. With GCMS, target analyte identity is based both on retention time and mass spectrum confirmation. Using quantitation ions which are specific to the target analyte provides much more definitive qualitative and quantitative results. For example the peak for ethylbenzene in the GCMS analysis of Sample #2 is shown in Figure 3. Mass 106 is the molecular ion for ethylbenzene expected to appear at 12:30 min. (Figure 4). The sample spectrum at this retention time clearly shows that mass 106, ethylbenzene, is a fraction of the component(s) eluting at that time (Figure 5). Of course the PID could not tell you this, only GCMS is capable of distinguishing between these hydrocarbons.

In conclusion, don’t trust EPA 8020 results when there are hydrocarbons present. They could be false positives. Rely on GCMS (EPA 8260) for correct BTEX data.

Table 1. Comparison of GC and GCMS for BTEX Analysis (Parts per billion, ppm, ug/kg)
Sample #1	GC#1	GC#2	GC#2
Benzene	ND<100	ND<100	ND<100
Toluene	ND<100	300	ND<100
Ethylbenzene	1900	460	ND<100
Xylenes	1100	2600	ND<100
Surrogate Recovery	75%	114%	99%
Sample #2	GC#1	GC#2	GC#3
Benzene	ND<100	100	ND<100
Toluene	ND<100	180	ND<100
Ethylbenzene	6200	1900	800
Xylenes	3400	9500	ND<100
Surrogate Recovery	82%	195%	104%
Sample #3	GC#1	GC#2	GC#3
Benzene	ND<1	ND<1	ND<1
Toluene	10	20	30
Ethylbenzene	130	20	4
Xylenes	83	100	30
Surrogate Recovery	85%	148%	97%
Soil samples 1 and 2 were extracted 10g to 10ml in methanol, and a portion of the extract subjected to purge trap analysis. All analyses of Samples 1 and 2 were performed on same extract. One gram of Sample 3 was purged directly for each analysis.
GC #1 Column-DB-624 0.53mm x 75m, 3um film. GC #2 Column-RTX-5 0.32mm x 50m, 3um film. GCMS Column-DB-624 0.53mm x 75m, 3um film.

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