Rapid Screening Method for the Total Petroleum Hydrocarbons in Water Samples by Solid-Phase Microextraction and GC-MS

Determination of total petroleum hydrocarbons (TPH) in water is an important tool for monitoring of contamination due to oil spills or leaking storage tanks. In this study, a screening method for the quantification of total petroleum hydrocarbons in water based on solid phase microextraction (SPME) in combination with gas chromatography – mass spectrometry (GC-MS) is presented. Extraction of hydrocarbons from water samples were conducted by SPME fiber coating placed into the headspace above water. Petroleum hydrocarbons were desorbed from the fiber coating in the injection port of gas chromatograph. The effect of the following parameters affecting the distribution of the analytes between three phases on the response of TPH were studied: SPME fiber coating type and dimensions, extraction temperature, extraction time and pH. The optimized method uses 100 μm polydimethylsiloxane fiber coating, extraction time 600 s, extraction temperature 80°C, without addition of salt at basic pH. The developed method was successfully applied for detection of total petroleum hydrocarbons in water taken from Koschagyl oil fields and Koschagyl village.


Introduction
As contaminants, petroleum and petroleum products pose a threat to the environment and its inhabitants. Covering vast areas of water surface with a thin film, petroleum is harmful to many living organisms and adversely affects all parts of the biological chain. Not only petroleum films on the water surface of the seas and oceans can disrupt the exchanges of energy, heat, moisture and gases between the atmosphere and water body; affect the physical and chemical processes, but petroleum settled on the bottom for a long time also can hurt all living things: the accumulation of petroleum can appear in the food chain of the simplest and higher animals. Aquatic toxicity is manifested at the concentration of more than 1 mg/L. Sources of TPH in the environment are diverse: oil spills at oil production, transportation and storage of fuel, oil pipelines and oil storage breakthrough; process disturbances and lack of wastewater treatment refineries, refueling engines, as well as emissions into the air of unburned fuel components from the internal combustion engine [1]. In surface water and groundwater, most petroleum products are the result of the transportation of oil from wastewater enterprises for domestic and municipal water [2]. Over time, the concentration of petroleum in the water under the influence of evaporation of the more volatile components, dissolution, oxidation, biodegradation, and emulsification is reduced. Oxidized residues of petroleum settle to the bottom of reservoirs [1].
Determination of petroleum products in the environment is quite complex task since the latter do not represent one particular substance, but the complex mixture of many different compounds, besides not having a constant composition. For analysis of TPH in water samples, methods based on different physical properties of petroleum products are used: gravimetric, IR spectrophotometric, fluorescence and chromatographic. Sample preparation is the most time-consuming and "dirty" stage of the analysis, which includes an extraction by organic solvent, cleanup and concentration.
Solid-phase microextraction has appeared recently, but immediately became one of the most popular "state-of-the-art" sampling/sample preparation methods due to its simplicity, speed and efficient extraction of toxic substances from various matrices. It became standard technique for many analytical tasks. SPME is based on extraction of analytes by a thin polymeric film coated to a thin quartz rod (fiber). SPME has been applied to the analysis of different petroleum-originated water pollutants including petroleum fuel contamination [3], polycyclic-aromatic and aliphatic hydrocarbons [4,5], bioavailable petroleum hydrocarbons [6] and gasoline range hydrocarbons [7].
The goal of this study was to develop a method of rapid screening of total petroleum hydrocarbons in water samples using GC-MS in combination with SPME.

Reagents and materials
Four SPME fibers, such as 100 μm polydimethylsiloxane (PDMS), 65 μm polydimethylsiloxane/divinylbenzen (PDMS/DVB), 85 μm carboxen/polydimethylsiloxane (CAR/PDMS), 7 μm PDMS were purchased from Supelco (Bellefonte, PA, USA). All fibers were conditioned in the hot injector port of the gas chromatograph according to recommendations of the manufacturer. Twenty mL vials with magnetic caps and PTFE/silicone septa (CTC Analytics AG) were used for extraction of TPH from water. Prior analyses, vials and septa were conditioned in a desiccator at a temperature of 150°C for 2 hrs. Chemically pure isopropanol and sodium chloride were obtained from Laborpharma LLC (Almaty, Kazakhstan). Helium (99.995 % purity) was obtained from Tehgaz (Orenburg, Russia). Crude oil was taken from Koschagyl oil field located in Atyrau oblast of Kazakhstan.

Parameters of SPME
The volume of water sample was 2 mL. SPME was done at a periodic shaking. The optimal SPME parameters for extracting petroleum hydrocarbons from water (fiber coating, extraction temperature and time, salt additive and pH) were determined experimentally.
Study of the effect of fiber coating was conducted under the following SPME parameters: extraction time 180 s, extraction temperature 80°C.
Study of the effect of extraction temperature was conducted under the following SPME parameters: fiber coating 100 µm PDMS, extraction time 60 s.

Parameters of GC-MS
Analyses by GC-MS were carried out using 6890/5973N (Agilent, USA) system equipped with a Combi-PAL autosampler (CTC Analytics AG, Switzerland). The autosampler was equipped with a 32-position 10/20 mL tray, 10/20 mL agitator, SPME fiber holder and a conditioning station. The GC was equipped with split/splitless inlet working in splitless mode to a 30 m x 0.250 mm DB-1MS column with a 0.25 µm film thickness (Agilent, USA). Helium was used as carrier with a constant flow rate of 1 mL/min. GC oven temperature program: 40°C (held for 10 min), heating to 240°C at 20°C/min (held for 20 min), the temperature of the MS interface was kept at 240°C. MS detection was conducted in the mode of scanning ions in the range of m/z 34-600.

Preparation of solutions of petroleum hydrocarbons in water
All the experiments were conducted on model water samples contaminated with crude oil taken from the Koschagyl oil field.
Distilled water was used for preparation of experimental samples. Distilled water was always checked for the presence of hydrocarbons by SPME-GC-MS before experiments, its pH was measured to be 6.0.
To prepare a standard solution of TPH (C = 10 mg/L), 6 µL of crude oil (ρ = 0,878 g/mL) was added to 500 mL of water. For better dissolution of crude oil, water was preheated close to its boiling point.

Results and Discussion
Selection of the optimal extraction coating SPME coating significantly affects the selectivity of SPME for organic compounds from showed that ature in the the response s, which is r pressure of s phase. In ams obtained res (Fig. 3

Application of the developed method
To verify the developed method, it was applied for the analysis of water samples taken from Koschagyl oil fields and Koschagyl village.
In Koschagyl oil fields, contamination is represented by a pure crude oil located inside old natural lakes and oil storage pits. In fall and spring, rain and ice water covers surface of oil contamination (Fig. 7) followed by a partial dissolution of hydrocarbons causing health risks for neighboring population and local animals represented by several small species and camels.
Analysis of water taken from Koschagyl oil fields using SPME-GC-MS showed the presence of TPH in all the studied samples. Quantitative analyses performed by standard method [8] showed TPH concentrations of up to 500 mg/L. Analysis of utility water taken from Koschagyl village showed presence of TPH while standard GC and FTIR based methods did not provide sufficient sensitivity for detection of such levels of TPH.

Conclusions
Thus, in the present work, a new optimized method for screening of total petroleum hydrocarbons in samples of water was developed using gas chromatography-mass spectrometry in combination with solid phase microextraction. The method sensitivity was optimized for water samples contaminated by crude oil from Koschagyl oil fields, the following optimum parameters were established: fiber coating 100 µm polydimethylsiloxane, extraction temperature 80°C, extraction time 600 s, no addition of salt, basic pH. The developed method was successfully applied for detection of TPH in water taken from Koschagyl oil fields and Koschagyl village.
The goal of the next stage will be development of SPME-GC-MS based method for quantitative determination of TPH in water samples.