Environmental contamination by petroleum hydrocarbons such as crude oil, or diesel or motor oil, is currently measured using gas chromatography with flame ionization detection (GC-FID). This measurement is more commonly referred to as total petroleum hydrocarbon (TPH) determination and is one of the commonest and most important applications in environmental laboratories.
TPH analysis is usually carried out by the extraction of samples (water, soil, sediment) using a non-polar solvent (e.g. hexane) and the extract cleaned by passing over silica or florisil (to retain more polar solutes such as lipids). The extract is then concentrated by any of a variety of techniques (using a gentle stream of Nitrogen or using the Kuderna-Danish concentrator or simple evaporation at room temperature). The concentrated extract is then analysed using a GC-FID with a non-polar column (HP-1, HP-5, DB-1, DB-5 e.t.c) and the areas of the fraction eluting between the retention times of C10 and C40 are typically summed together and defined as total petroleum hydrocarbon.
To facilitate the elution of the higher hydrocarbons, C35-C40, and to ensure the separation of the extraction solvent from the first peak (C8 or C10), standard methods such as the ASTM, ISO and USEPA methods usually specify the use of a column with a thin film (high phase ratio) and GC oven initial temperatures that are as low as 35 °C to 40 °C. Typically, the analysis is performed using a 10 to 30 metre column using split/splitless, programmable temperature vaporizing or cool on-column injection and oven temperature programming from 35°C or 40 °C to above 300 °C. at typically 10 to 20 °C/min ramp rates. Considering oven cool-down time to the low initial temperature, typical total cycle time of the analysis is 30 minutes or longer. With sample loads sometimes up to 300 or more, TPH analysis can take several days or weeks to complete. Therefore, environmental laboratories are currently seeking ways to improve cycle time and lower cost per sample.
One of such ways of improving cycle time is by adding a Low Thermal Mass module (LTM) to an Agilent 7890A GC system equipped with a split/splitless inlet, fast automatic liquid sampler and FID. Using this technique, Agilent Technologies Inc. has demonstrated cycle times of 5 minutes, while still meeting all method requirements as specified by the ASTM, ISO or USEPA. The GC-FID conditions for this method are as follows:
The fast GC conditions are listed in Table 1.
Table 1. GC-FID Setpoints for Fast TPH Analysis Using a Low Thermal Mass Oven
Injection 1 μL, splitless
Inlet Temperature, 350 °C
SSl Inlet He pressure (constant flow mode]
LTM column module containing a 10 m × 0.32 mm, 0.1 μm DB-5HT column
LTM column program 40 °C (0.5 min), 200 °C/min to 240 °C, 100 °C/min to 340 °C (0.5 min), [total time = 3 min]
FID 340 °C, H2 = 40 mL/min, Air = 400 mL/min, Makeup = 20 mL/min
The chromatogram and a typical calibration curve obtained using this method are shown below:
Figure 1. n-Alkane test mixture run with fast oven program on LTM oven module.
Figure 2. Mineral oil calibration. Calibration from 40 – 1000 mg/L; Linearity: r² > 0.999;
Repeatability at 400 mg/L: RSD on peak area = 0.64%; LOD: < 25 mg/L
It is now possible to achieve TPH determination by GC-FID in less than 5 minutes! This has been demonstrated by Agilent Technologies and it only requires adding an LTM module to an Agilent 7890A GC. Analysis time for the separation of C10 to C40 alkanes is below 3 minutes while oven cool-down time to 40 °C was 2 minutes. This results in a total cycle time of 5 min. This increases sample throughput and saves a lot of costs.
If you are interested in this solution, kindly contact us at AAS Ltd.
David, F., and Klee, M.S. High Throughput Mineral Oil Analysis (Hydrocarbon Oil Index) by GC-FID
Using the Agilent Low Thermal Mass (LTM) System, Agilent Technologies Application Note
Written by Adebola Muyiwa
07084594001, [email protected]