Determination of Sulphur-based Odourants in Commercially Available Natural Gas with Flow Modulated Comprehensive Two-Dimensional Gas Chromatography

Jan 01, 2014

Alkyl mercaptans, alkyl sulphides, and cyclic sulphides are added in various blends to commercial natural gas as a safety precaution to identify its presence by smell. Although several different analytical methods exist to measure these compounds, few are all-encompassing techniques that can measure all the sulphur compounds simultaneously at appropriate levels. Pulsed-flow-modulated comprehensive two-dimensional gas chromatography with flame ionization detection was used to separate and identify all common sulphur compounds found in natural gas. The column set used in this method consisted of a polydimethylsiloxane-like stationary-phase selectivity in the first dimension with a low-bleed wax stationary phase in the second dimension. This set was optimized to separate key sulphur compounds found in natural gas from this hydrocarbon matrix. Detection limits of this method were determined to be 0.1 ppm (v/v), and a linear calibration range spanning from 0.1 ppm to 30 ppm (v/v) was used to perform quantitative analysis on a residential natural gas supply. Reproducibility of peak volume of seven successive injections on these test analytes averaged 3.7%.

Natural gas is highly flammable, colourless, and odourless; therefore malodorous compounds are added as a method to identify the presence of this gas. Most commonly, these compounds consist of a blend of alkyl mercaptans, alkyl sulphides, or cyclic sulphides (1,2). These structures are prominent because of their desirable physical and chemical characteristics. For example, the blends need to be odorous, volatile, flammable liquids that have a low odour threshold and will not oxidize metal pipelines (1,2). The most common sulphur compounds used as odourants are tert-butyl mercaptan, tetrahydrothiophene, methylethyl sulphide, dimethyl sulphide, isopropyl mercaptan, n-propyl mercaptan, and sec-butyl mercaptan. Two or more of these compounds are added to natural gas before it is delivered to the consumer (1,2), and depending on environmental or economical factors, they will be blended in various combinations and percentages.

The ability to monitor these odourant blends in all their variant forms is beneficial to natural gas providers to certify their presence at detectable levels by smell and ensure consumer safety. North American regulations require an odourant to be detectable by smell when natural gas concentrations reach one-fifth of the lower explosive limit, which represents a level of about 1.25% in air (Federal Regulation, 49 CFR, 192.625). Most sulphur odourants have an odour threshold of approximately 1 ppb (3), and therefore are minimally required to be at concentrations of 100 ppb in natural gas. Generally, commercially available residential natural gas contains sulphur odourants with 0.5–10 ppm levels. At these levels, natural gas can be readily detected by smell at concentrations well below the lower explosive limit. It is also necessary to ensure that these odourants are not overdosed, which can lead to further expense to the natural gas supplier. Excessive additions in natural gas can cost companies millions of dollars in unnecessary chemical cost as well as investigations because of false positive alarms reported by consumers. Higher levels of sulphur species within the distribution pipelines may also corrode these pipes and create potentially hazardous leaks.

Currently, odourants in natural gas are monitored by various sensors such as olfactory detection (ASTM D6273-08) and lead acetate strip tests (4–6). Gas chromatography (GC) has also been demonstrated to detect these compounds, which usually involves a selective detection method such as flame photometric detection (FPD) (7–10), sulphur chemiluminescence detection (SCD) (11–13), mass spectrometry (MS) (14), or ion mobility spectrometry (15). The use of FPD requires the sulphur compounds of interest to be well separated from the hydrocarbons in the matrix to reduce the effects of quenching (16–18), which involves multiple switching valves. SCD affords great selectivity of sulphur response over carbon with low detection limits, but has a higher cost of ownership. Furthermore, this necessitates equipment specifically dedicated to sulphur detection. MS in selected ion monitoring mode can also be used; however, if there is insufficient separation between the analyte of interest and the interference there is a possibility of a false positive. For example, in selected ion monitoring mode an abundance of ions can cause peak broadening because of electrostatic repulsions within a cloud of similarly charged ions (19). Ultimately, this could offset the centroid of the sulphur species of interest or these coeluted hydrocarbons may overlap and create a false positive sulphur response (19).

Comprehensive two-dimensional gas chromatography (GC×GC) is an emerging technique that uses the separation power of two columns with different selectivity (20–22). All analytes in this method undergo separation by both columns, and the process is facilitated by a rapid re-injection from a modulator. Flow modulation was first described by Seeley (23) and Amirav (24). Basic operation of modulation includes effluent from the first column filling a collection channel within the plate, and then that effluent is rapidly injected into the second column by switching flow through the channel creating a pulsed flow. Not only does the separation power increase in GC×GC, but signals are enhanced as modulation creates sharp narrow peaks that elute off the second column.

GC×GC has been coupled to selective detection systems such as FPD and SCD to analyze sulphur species in complex matrices (25–27). Here, GC×GC is used to measure prominent sulphur odourants found in natural gas in one method. Utilization of this technique can be beneficial to separate sulphur compounds of interest from the hydrocarbon matrix as well as to identify the species added to a natural gas sample. Furthermore, signal enhancement from modulation allows detection at low levels meaning that these sulphur species can be monitored with flame ionization detection (FID) over an appropriate range.