Two Detector Solution to Analyzing Sulfur

Dan Li, Katarina Oden, Chris English, and Jason Herrington

Sulfur compounds are reactive, corrosive to pipes, and destructive to catalysts in petroleum refineries. Sulfur emission are strictly regulated globally. When released into the atmosphere sulfur dioxide converts to sulfuric acid resulting in adverse effects on human health and the environment. Sulfur detection is found useful in many other industries; therefore, detection of sulfur compounds in matrices serves a vital role in many application areas.

Sulfur analysis is typically done by gas chromatography (GC). The frequently used sulfur detectors are sulfur chemiluminescence detector (SCD), flame photometric detector (FPD), and mass spectrometry detector (MSD). As sulfur selective detectors, SCD and PFPD have the advantages of measuring components of interest and providing an equimolar response for sulfurs. Compared to SCD, FPD is more robust, less expensive and less complicated for maintenance. One challenge of FPD or PFPD is the hydrocarbon interference, especially those from the chromatographic coelutions, which can cause quenching or signal suppression. In order to minimize the quenching effect, one can either use high split ratio to reduce the amount of hydrocarbon injected or resolve the sulfur species from the hydrocarbons chromatographically. In most cases, small injection volumes or high split ratios are unsuitable for trace-level detection. If cryogenic cooling cannot be used, it is difficult to avoid coelutions using a single detector.

Mass spectrometry is a universal detector widely used in many applications. It provides structural information of the analytes in full scan mode and enhanced selectivity / sensitivity in selected ion mode (SIM). For some volatile sulfur compounds, unique qualifier ions are not available in the presence of impurities; however, selected ion monitoring can reduce the coelution problems in many applications.

Using the MS in tandem with the FPD mitigates the disadvantages of both detectors. FPD provides accurate sulfur amount due to the equimolarity characteristics while MS gives a total profile to include matrix not seen by the FPD. This paper describes the design of a parallel GC FPD-MS and demonstrates its applications.

Analysis of sulfur samples was performed on a Shimadzu GC-MS QP 2010 Plus system equipped with an FPD. Sample introduction was done by manual injection through a split/splitless injector at 200 °C. The injection volume was 1 mL. The GC analytical column was a 15 m × 0.25 mm × 0.25 µm Rtx-1 (Cat # 10120). A three-port SilFlow device (SGE Analytical Science) was employed to split the flow at the end of analytical column to both MS and FPD. Two deactivated, uncoated fused-silica transfer lines (restrictors) were employed to couple the splitter device with MS and FPD, one with a dimension of 84 cm × 100 µm I.D (connected to MS) and the other 75 cm × 250 µm I.D (connected to FPD). Figure 1 shows a schematic drawing of the GC with parallel FPD and MS. Details of instrumentation are listed in Table 1. Both FPD and MS chromatograms for the gas samples were acquired simultaneously.

Figure 1. A schematic diagram of GC-MS-FPD setup.

Table 1. Instrument Conditions

Sulfur standards were purchased from DCG Partnership 1, LTD. (Pearland, TX). They were prepared in two blends due to the stability issue. The components and concentrations are listed in Table 2.

Table 2. Sulfur Standards

Peak shapes are greatly impacted by the inertness of the column. Rtx-1 column offers great inertness as well as sufficient resolution for heavy matrix sulfur analysis, as seen in Figures 2-4.

By using the dual detectors system, a sulfur chromatogram and a simultaneous hydrocarbon chromatogram can be generated from a single injection. For the following examples, the top chromatogram displays the FPD signals and the bottom window displays the corresponding MS profiles. The retention time from both chromatograms matched well. This hardware configuration can be applied to other sulfur application areas.

Figure 2 shows a set of FPD and MS Total Ion Chromatograms (TIC) of sulfurs and hydrocarbons. In this case, a series of hydrocarbons coeluted with sulfur components. Recognizing specific sulfur compounds using FPD makes it possible to accurately pinpoint the retention time in complex TIC. Quenching, which is caused by the coelution of hydrocarbons, is illustrated in Figure 2. Sulfur signals can be significantly suppressed by the matrices, leading to inaccurate quantification. With the assistance of cryogenic devices or longer columns, sulfurs with limited number of hydrocarbons may be resolved. It is impossible to avoid quenching or signal suppression in gasoline samples containing hundreds of different hydrocarbon compounds with a wide range of concentrations. The use of the MSD in SIM mode can reduce this problem in many cases, while operating in scan mode assists in initial method development, unknown matrices identification, and finding the retention time of interferences.

Figure 2. GC-FPD-MS (full scan mode) detection of sulfur standards (Blend 1) in hydrocarbon matrices.

Figure 3 is a display of sulfur analysis with FPD and MS under SIM conditions. The SIM ions are listed below the chromatograms. The ions are carefully chosen to avoid interferences from hydrocarbons; however, the confirmation of peaks 1, 3, and 4 may be questionable because the qualifier ions were also shared by matrice ions. When a unique ion is not available, different chromatographic column / conditions should be tried to resolve the analytes. Fortunately, we have the ability to use Restek’s free ProEZGC library tool which will allow us to model the elution times of both sulfurs and hydrocarbon interferences under a specific set of conditions, in this case, using the Rtx-1. Conditions can be optimized for specific conditions and allow the use of SIM and the specific retention time as a reliable means of compound identification.

Figure 3. GC-FPD-MS (SIM) detection of sulfur components (Blend 1) in hydrocarbon matrices.

In Figure 4, both FPD and extracted ion chromatograms were collected for all 20 sulfur compounds (Blends 1 and 2). The SIM ions used for each sulfur compound are listed. The relative abundances are different in FPD and SIM responses, which results from the different detection mechanisms. The SIM chromatogram showed improved resolution on hydrogen sulfide and carbonyl sulfide.

Figure 4. GC-FPD-MS (SIM) detection of sulfur components (Blends 1 and 2).

The GC-FPD-MS coupling allows positive identification of sulfurs in complex matrixes and eliminates the need for multiple injections using different columns and detectors.

Dimethyl polysiloxane stationary phases (Rtx-1) provide good retention and resolution for sulfurs. Historically, thick film columns are used since they provide excellent inertness and peak shapes since analytes spend less time in contact with the deactivated fused silica surface. Columns with thinner films demand excellent surface inertness, for example, a thin-film short column (15 m × 0.25 mm ×1µm) was employed, resulting in a 10-minute analysis time. Conditions were optimized using ProEZGC resolving 16 out of 20 compounds (Figure 4) to include low-molecular-weight volatile sulfurs.

The FPD-MS combination is a powerful tool for unknown compounds, especially in the presence of complex matrices. Using tandem FPD/MS detectors provides an additional measure of confirmation not available from using either detector alone.

This paper demonstrates the capabilities of this hardware configuration for sulfur analysis. Further optimizations can be done on different column dimensions, oven temperatures, flow rates, and other parameters by using EZGC programs.

Acknowledgements

The authors would like to thank Shimadzu Corporation for their consultation with the operation of the QP2010 Plus GC-MS instrument and the FPD. http://www.ssi.shimadzu.com/