VPH Analysis by the New GC/MS Method
Alpha Analytical is now offering VPH Analysis by the new MassDEP procedure – “Method for the Determination of Volatile Petroleum Hydrocarbons (VPH) by Gas Chromatography / Mass Spectrometry; January, 2017”. This new GC/MS VPH method does not replace the original GC/PID-FID VPH method, Alpha continues to offer VPH analysis by both methodologies. The GC/MS VPH does represent another option for VPH analysis however, and as such there are some advantages to the new method, depending on your project application.
Given the greater degree of qualitative certainty provided by GC/MS over GC/PID-FID, the GC/MS VPH analysis can exclude contributions from non-petroleum hydrocarbons. Non-petroleum hydrocarbons can range from compounds of biogenic origin such as terpenes ; or alcohols and ketones, which may or may not have a natural origin; as well as chlorinated solvents which are present due to a release. The GC/MS VPH method may be particularly useful for site applications where comingled plumes of petroleum hydrocarbons and chlorinated solvents exist.
The presence of these non-petroleum compounds would not be included in the VPH aliphatic and aromatic hydrocarbon range totals if they can be identified as non-petroleum hydrocarbons and they are not co-eluting with another hydrocarbon chromatographic peak. If the VPH by GC/MS method is requested, Alpha will automatically exclude those non-petroleum compounds that meet this criteria. The report narrative will list the excluded compounds that were identified as non-petroleum hydrocarbons and state that they were not included in the associated VPH range total concentrations.
Similar to the original GC/PID-FID VPH method, the GC/MS VPH method also could be subject to potential positive biases. An evaluation of the MassDEP Round Robin Study that was conducted to validate the method showed that the GC/MS C9 – C12 aliphatic range could be biased high due to the difference in the mass spectrometer’s response between aliphatic and aromatic hydrocarbons. The following table summarizes the MassDEP’s observations of the Round Robin data:
|Target VPH Analytes||Equivalent||Equivalent|
|C5 – C8 Aliphatics||Equivalent||Equivalent|
|C9 – C12 Aliphatics||Potential high bias from MS response to aromatics in range|
|C9– C10 Aromatics||Potential high bias from PID response to aliphatics in range|