Sniffing Out Trace Amounts of Sulfur

With its sickly yellow color and rotten-egg odor, the ancients called it brimstone. In modern times, we call it sulfur (or sulphur) and use it to make many valuable products, such as matches, gunpowder, fungicides, and even medicine. Pure sulfur is actually tasteless and odorless, but sulfur combines with nearly all elements and some of these compounds are extremely reactive.

Two of the most common sulfur compounds are hydrogen sulfide (H₂S), a poisonous gas with the odor of rotten eggs, and sulfur dioxide (SO₂) another poisonous gas that is formed when fuels containing even small amounts of sulfur are burned. Excessive SO₂ emissions result in poor air quality and associated adverse health effects, as well as acid rain, which corrodes buildings and damages lakes and forests.

Because sulfur occurs naturally in all petroleum products in varying degrees, countries around the world regulate the amount of sulfur in fuels and other petrochemical products. Although many of these regulations have existed for years, they continue to tighten as the environmental effects remain under constant scrutiny. As a result, hydrocarbon processors need increasingly sensitive and selective trace-sulfur measurements.

Careful monitoring is also necessary to detect and remove sulfur that occurs in feedstocks such as ethylene and propylene. These base chemicals are used to form polyethylene and polypropylene, which in turn are used to create a wide variety of products such as polyester, antifreeze, solvents, and synthetic rubber. Even trace amounts of sulfur compounds can deactivate polymerization catalysts, slow reaction rates and negatively affect the color and odor of the final products.

The need for low-level sulfur measurement exists in virtually all parts of the chemical industry. Examples include monitoring of odorants in natural gas, analysis of beverage-grade CO₂, and detection of contaminants in the feedstocks to fuel cells (an emerging source of alternative energy).

The challenges of trace-level detection

Even with gas chromatography, testing for small amounts of volatile sulfur compounds can be a difficult process. Samples are injected into a gas chromatograph (GC), where they separate into their components according to the length of time necessary for each compound to pass through (elute from) the column. When the gas chromatograph is used with a non-selective detector, it can be extremely difficult to distinguish between two or more sulfur-bearing or hydrocarbon compounds that have almost identical peak delay times. Another problem is the tendency for the sulfur compound to co-elute with the matrix or one of its components. To make it easier to characterize and quantify sulfur compounds, even at very low concentrations, the gas chromatograph must be used with a detector that exhibits an enhanced response to sulfur and sulfur compounds -- a sulfur-selective detector.

Some selective solutions

Agilent's flame photometric detector (FPD) is a widely used sulfur-selective detector. It is low cost, easy to use, and works well for many applications. The FPD identifies specific sulfur compounds by optically filtering and measuring a specific wavelength of the light emitted from sulfur compounds when the sample is burned in a hydrogen-rich flame. For sulfur levels that range from a few parts per million (ppm) to 50 parts per billion (ppb) range, the FPD is usually a good choice, although the results will be obscured if co-elution occurs.

Although the mass selective detector (MSD) is often overlooked as a tool for trace analysis of sulfur compounds, it is a sensitive and selective detector that can overcome the co-elution problem for many applications. This is particularly true when the MSD is operated with selective ion monitoring (SIM -- a method in which the intensities of specific ions are recorded rather than the entire mass spectrum). Another benefit is the structural information provided by the MSD. Sensitivities to less than 10 ppb can be achieved.

The benefits of a dynamic blending system

Calibration or blending of the gaseous analytes to be used in gas chromatographic systems is another challenge scientists face when they test for trace amounts of sulfur, and can be especially difficult when reactive compounds are involved. Permeation tubes solve some of these problems, but special hardware such as a gas standards generator is usually required for multi-component calibrations. Cylinder-based gas standards provide custom calibration mixes; however for many gases they can be difficult to use accurately below a few ppm. Agilent has developed a point-of-use/time-of-use dynamic blending system for its 6890N GC that allows easy preparation of gaseous samples in the low ppb range in numerous matrices using a single certified calibration cylinder. This system allows real-time, multi-point calibration and method development. Using a point-of-use system also allows reactive and cylinder-incompatible gases to be combined immediately prior to analysis. Another advantage is the ability to automate calibration with the ChemStation software.

For more information

Agilent's Chemical Analysis Group provides solutions for identifying, quantifying and analyzing the chemical properties of thousands of substances. For more information about how to set up the Agilent 6890N/5973N system for optimum sensitivity and selectivity of sulfur compounds, please view or download the application note, "Use of GC/MSD for Determination of Volatile Sulfur: Application in Natural Gas Fuel Cell Systems and Other Gaseous Streams." More detailed information about dynamic blending is available in the application note "Automated Dynamic Blending System for the Agilent 6890 Gas Chromatograph: Low Level Sulfur Detection." For general information, please return to the main page of the Chemical/Life Sciences section of our Web site.