Sulphur and Halide Determination by Combustion Ion Chromatography

C. Emmenegger, A. Wille and A. Steinbach, Metrohm International Headquarters, Herisau, Switzerland.

The fully automated combustion ion chromatography (CIC) system presented here combines a highly efficient combustion system with the separation power of ion chromatography (IC). CIC allows for the simultaneous, speciated trace analysis of halide (F, Cl, Br and I) and sulphur compounds (as sulphate) from sub-ppm to percent levels in any nonaqueous sample matrix.

Chloride and sulphur recoveries in the certified polymer standard ERM-EC681k and a S-benzyl thiouronium chloride sample were between 96...103% and 97...101%, respectively. While sulphur concentrations in the diesel fuel and the unleaded gasoline 95 sample were around 10 mg/kg, the biodiesel sample showed a lower sulphur concentration of 3.8 mg/kg. Chloride concentrations in all fuel samples ranged between 4 and 8 mg/kg. Because multiple burns can be collected in the same absorption solution — demonstrated by means of chloride and sulphate data (R² ≥ 0.998) — CIC achieves detection limits in the sub-ppm range.

Introduction

In response to the decreasing lifetime and the skyrocketing production of electronic and electrical equipment, the "Restriction of the use of certain hazardous substances in electrical and electronic equipment" (RoHS) limits the content of some hazardous species, such as polybrominated biphenyls, in these types of equipment. The electronics industry, driven by several international regulations, is striving to obtain halogen-free products. Because of upcoming international regulations, the rapid monitoring of partly corrosive and toxic organohalogen compounds in challenging matrices such as coal, crude oil, naphtha, liquefied petroleum gas and so forth, will gain importance.

The most prominent techniques for halogen determination in these materials include the determination of absorbable/total organic halogens (AOX/TOX), inductively coupled plasma mass spectrometry (ICP–MS), ion-selective electrodes (ISE) and ion chromatography (IC). However, either results are unspeciated sum parameters, or samples are difficult to bring into solution and thus require complex and time-consuming sample preparation procedures.

These drawbacks can be overcome by using combustion IC (CIC). This presentation highlights the applicability of automated CIC to the determination of trace halide and sulphur compounds in different sample matrices.

Experimental

Instrumentation

The fully automated combustion ion chromatography system consists of the following devices:

• 881 Compact IC pro – Anion – MCS

• Mitsubishi ABC-100

• Mitsubishi AQF-100

• Mitsubishi GA-100

While the ABC-100 transfers the samples into the furnace (AQF 100), the GA-100 absorbs the gaseous compounds produced and transfers the resulting solution to the ion chromatograph. The combustion unit is controlled by the system software of Mitsubishi's AQF-100. Operational parameters for the combustion and the subsequent chromatographic separation are indicated in Table 1.

Table 1: Operational parameters for the determination of chloride, bromide and sulphur by CIC.

Standard solution, gases and eluents

All reagents used in this work were of the highest purity grade (puriss. p.a.). Oxygen and argon gas were purchased from Pangas (Dagmersellen, Switzerland) and have a purity better than 99.999 and 99.996% (v/v), respectively. All solutions were prepared with deionized water with a specific resistance higher than 18 MΩ·cm. The polymer certified reference material ERM-EC681k was purchased from the Institute for European Reference Materials (ERM) in Geel, Belgium. S-benzyl thiouronium chloride stems from Merck Schuchardt OHG in Hohenbrunn, Germany. The fuel samples were purchased from several Swiss gas stations.

Principle

According to Figure 1, sample pyrolysis takes place in the combustion system. Combustible samples are oxidized by O₂ at temperatures above 900 °C. While sulphur-containing compounds are oxidized to sulphur dioxide, halogen-containing compounds are converted to hydrogen halides or gaseous halogens. The gaseous compounds formed are led into an oxidizing absorption solution, where sulphur dioxide is further oxidized to form sulphate and the halogens form halide ions. The absorption solution is then fed into the sample loop of the Metrohm 881 Compact IC pro. All kinds of combustible samples can be analysed.

Figure 1: Schematic illustration of the CIC system with sample introduction, combustion unit, gas absorption and the Metrohm IC system.

While one chromatogram is being recorded, pyrolysis of the next sample is already under way. The MagIC Net software reliably controls the analysis and automatically calculates the sulphur and mass fractions for each sample. Both liquid and solid samples can be analysed efficiently in a short time by using the sample changer.

Halide and sulphur determination

(a) in a certified polymer standard

The certified polymer reference material is a low-density polyethylene granulate spiked with known amounts of chloride, bromide and sulphur (Table 2). It is used to test the performance of the CIC unit.

Table 2: Certified and experimentally determined chloride, bromide and sulphur concentrations of the certified polymer standard ERM-EC681k.

The results for chloride, bromide and sulphur (Figure 2) are well within the certified concentrations of the ERM-EC681k standard. Recoveries between 96 and 103% demonstrate complete matrix destruction during combustion and the quantitative capture of gaseous chloride-, bromide- and sulphur-containing compounds by the absorption solution.

Figure 2: Ion chromatogram for the separation of analyte anions using conductivity detection after pyrolysis of the certified polymer standard ERM-EC681k.

(b) in S-benzyl thiouronium chloride

A known amount of S-benzyl thiouronium chloride was automatically combusted and analysed for its chloride and sulphur content (Figure 3). Recoveries between 97 and 101% demonstrate the applicability of the combustion procedure (Table 3).

Figure 3: Ion chromatogram using conductivity detection obtained after pyrolysis of S-benzyl thiouronium chloride.

(c) in different fuel samples

Sulphur in fossil fuels is present in both inorganic and organic form. While the first fraction mainly affects the motor performance in terms of precipitating salts, organosulphur compounds, when combusted, emit detrimental sulphur dioxide with severe adverse environmental impacts. In view of these drawbacks, the sulphur content, whether in the inorganic or in the chemically bound organic form, has to be controlled. Whereas the water-soluble inorganic ions can be selectively determined by IC coupled to upstream in-line dialysis or extraction, the total sulphur content can be determined by CIC.

Table 3: Theoretical and experimentally determined chloride and sulphur contents of S-benzyl thiouronium chloride.

Chloride concentration in all investigated fuels range between 4 and 8 mg/L (Figure 4, Table 4). While sulphur concentrations in the unleaded gasoline and the two diesel samples are approximately 10 mg/kg, the sulphur content of the biodiesel sample is significantly lower (3.8 mg/kg). When multiple combustions are collected in the same absorption solution, as displayed in Figure 5, there is a linear correlation between the concentration of the combustion products in the absorption solution and the number of performed combustion cycles. By collecting multiple 'burns' into a single absorption solution, combustion IC achieves detection limits in the sub-ppm range.

Figure 4: Ion chromatograms of the combustion products of (a) unleaded gasoline 95, (b) diesel, (c) MIGROL Diesel Greenlife Plus and (d) rapeseed biodiesel.

Conclusions

The automated CIC system allows straightforward in-line combustion and is designed to quantitatively trap the gaseous combustion products in an oxidizing absorption solution for subsequent ion chromatographic analysis. The method excels by the possibility to simultaneously determine sulphur and each individual halogen (fluorine, chlorine, bromine and iodine) with high precision and accuracy. Sample enrichment is achieved through the possibility to trap analytes from multiple combustion cycles in the same absorption solvent. CIC is capable to determine sulphur and total speciated halides down to 0.5 ppm.

Table 4: Chloride and sulphur contents in different fuel types determined by CIC.

Application fields include the petroleum, coal, plastic, semi-conductor and power generation industries, which are obliged to monitor banned toxic compounds as they are sources of corrosion or poison.

Figure 5: Chloride and sulphate concentrations of combusted gasoline 95 against the number of combustion cycles. Chlorine and sulphur species from multiple combustion cycles of unleaded gasoline were absorbed in a single absorption solution.

References

1. ASTM D 7359-08, Standard test method for total fluorine, chlorine and sulphur in aromatic hydrocarbons and their mixtures by oxidative pyrohydrolytic combustion followed by ion chromatography detection (combustion ion chromatography-CIC).

2. E.M.M. Flores et al., Analytical Chemistry, 80, 1865–1870 (2008).

3. G. Bogenschütz and T. Kolb, Ion chromatographic determinations of anions, cations and organic acids in biofuels, downloadable under http://products.metrohm.com (search for 8.000.6011EN).

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