Determination of Halogens and Sulphur in Complex Matrices

This article presents a method that combines combustion digestion and ion chromatography into a single analysis (combustion ion chromatography [CIC]) making it possible to detect halogens and sulphur in complex matrices. The method is suitable for use in a wide range of application areas.

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There are a number of different methods available to determine fluorine, chlorine, bromine, iodine, and sulphur in flammable substances. The majority of these are based on independent analysis steps, beginning with sample digestion using high-temperature combustion followed by halogen and sulphur analysis. As an alternative, combustion digestion coupled with ion chromatography (CIC) combines sample digestion and analysis in one step. CIC can be performed on a wide range of sample types regardless of whether solid, liquid, or gas. The only condition is that the sample has to be flammable. In addition to the halogen and sulphur analysis processes discussed in this article, which are used in the plastics, power generation, petroleum, and fuel industries, CIC is also suitable for samples from the pharmaceutical, environmental, and food sector. It is for these advantages that many international standards refer to CIC when determining the presence of halogens and sulphur in complex matrices (Table 1).

Table 1: A selection of international standards that recommend combustion ion chromatography as a means of determining the halogen and sulphur content in flammable samples.

CIC Method Principles

Combustion: Combustion takes place in the presence of steam, which ensures that the gaseous halogen and sulphur compounds (HX, X₂, SO_x) are quantitatively collected by the absorption solution. Adding hydrogen peroxide to the absorption solution (for example, 90 mg/L) ensures that all sulphur compounds are present in the form of sulphate, which is necessary for ion chromatographic detection. However, when hydrogen peroxide is added as an oxidizing agent it can result in various interference peaks that overlay the fluoride peak. With in-line matrix elimination, these interferences disappear and the fluoride peak can be evaluated again (Figure 1).

Figure 1: Effect of 90 mg/L hydrogen peroxide in the absorption solution. Red: once with in-line matrix elimination; Black: once without.

An optical sensor in the pyrolysis oven indicates the progression of the combustion and regulates the feed of the sample boat. This ensures complete combustion by similarly reducing the sample's oven dwell time.

Instrumentation: The combustion unit is from Analytica Jena, while the liquid handling and IC are from Metrohm. The software used was MagIC Net from Metrohm.

Application Areas

The application areas described in this article illustrate the precision and accuracy of CIC. The samples analyzed were plastic and fuel samples as well as latex and vinyl gloves.

Figure 2: Determination of halogens and sulphur in polymer standard ERM–EC680k. Column: Metrosep A Supp 5 - 150/4.0, eluent: 3.2 mmol/L sodium carbonate, 1 mmol/L sodium hydrogen carbonate, 0.7 mL/min, column temperature: 30 °C.

Plastic Samples: The combustion of organic materials containing halogen and sulphur produces toxic gases that are harmful to humans and materials. To guarantee that plastics are halogen- and sulphur-free, a quick and reliable analysis is required. A certified polymer standard, ERM–EC680k (Institute for Reference Materials and Measurements, Geel, Belgium), is used to check the precision and accuracy of CIC. It is a low-density polyethylene granulate containing known quantities of chlorine, bromine, and sulphur. Figure 2 illustrates the analysis of halogen and sulphur content of the reference standard by CIC. The recovery rates, ranging between 99% and 102.4% (Table 2), demonstrate that the plastic has quantitatively combusted and that the combustion gases have been completely collected by the absorption solution.

Table 2: Certified chlorine, bromine, and sulphur content of the polymer standard ERM–EC680k determined using combustion ion chromatography.

Liquid Fuels: Burning fuels that contain sulphur produces sulphur dioxide that is harmful to the environment. In addition, high sulphur content deactivates catalysts in combustion engines and affects both the storage stability and ignition behaviour of fuels. The halogen content also plays an important role, as chloride in particular accelerates corrosion in the refinery process and in engines.

Figure 3: Determination of sulphur and halogen content in gasoline. Column: Metrosep A Supp 5 - 150/4.0, eluent: 3.2 mmol/L sodium carbonate, 1 mmol/L sodium hydrogen carbonate, 0.7 mL/min, column temperature: 30 °C.

The quality requirements for gasoline are defined in DIN EN 228 and DIN 51626-1. The sulphur content must not exceed a limit value of 10 mg/kg. Figure 3 shows that in the gasoline sample under investigation, the sulphur content was just below this limit value at 9.88 mg/kg.

Figure 4: Determination of halogen and sulphur in a coal sample. Column: Metrosep A Supp 5 - 150/4.0, eluent: 3.2 mmol/L sodium carbonate, 1 mmol/L sodium hydrogen carbonate, 0.7 mL/min, column temperature: 30 °C.

Solid Fuels: It is also possible to analyze halogens and sulphur in solid fuels such as wood pellets and brown or black coal (Figure 4). In the case of the analyzed coal sample, the results demonstrated similar precision to those for the liquid fuel. Recovery rates in the coal sample were 96.8% for sulphur and 103.4% for chlorine. Representative and, therefore, homogeneous samples are a prerequisite for reproducible results. In the case of solid samples, however, these are something of an exception, which is why any irregularities must be taken into consideration when selecting samples.

Figure 5: Determination of chloride and sulphate in latex gloves. Column: Metrosep A Supp 5 - 150/4.0, eluent: 3.2 mmol/L sodium carbonate, 1 mmol/L sodium hydrogen carbonate, 0.7 mL/min, column temperature: 30 °C.

Gloves in Clean Room Environments: Gloves are used in clean room environments to hold back the ionic contamination found in perspiration from hands. Only materials with low amounts of halogen and sulphur may be used in the water-steam cycle of power plants and the primary circuit of pressurized water reactors, so that no corrosive halogenides or sulphates can get into these systems. Halogen and sulphur content is therefore a key consideration when selecting appropriate materials for use in clean room environments. In this case, vinyl gloves are the better choice because of their significantly lower halogen and sulphur content (Figure 5 and Table 5).

Table 3: Determination of sulphur content in gasoline.

Summary

The CIC technique presented here provides a fully automated method of determining individual halogens and sulphur in a multitude of organic matrices — regardless of whether they are in liquid, solid, or gaseous form. Following combustion digestion of the samples, the combustion products containing halogens and sulphur are collected in an oxidizing absorption solution and determined as halogenides and sulphate during the subsequent ion chromatography process.

Table 4: Determination of chlorine and sulphur in solid fuels.

By measuring the light intensity, a sensor tracks the progress of the combustion digestion and ensures that the samples combust fully. Complex samples no longer require method development because the optimum parameters for combustion digestion are selected according to the type and quantity of the sample.

Table 5: Chlorine and sulphur content in latex and vinyl gloves.

References

1. A. Wagner, B. Raue, H.-J. Brauch, E. Worch, and T. Lange, J. Chromat. A 1295, 82–89 (2013).

2. M. Laeubli, C. Emmenegger, and B. Bogenschütz, Ion chromatographic determination of halogens and sulphur in solids using combustion as inline sample preparation, 7th Conference on Ion Analysis, Berlin, Germany, 2013 (downloadable from metrohm.com/com/Applications/, search for 8.000.6091).

Daniel Hemmler is an MSc student at Aalen University, Germany. His major fields of study are analytical and bioanalytical chemistry. Within his studies, he did an internship in the R&D Department of Ion Chromatography at Metrohm International Headquarters, based in Herisau, Switzerland.

Sophia Kaufmann is an R&D assistant in the Analytical Instrumentation Department of Analytik Jena AG in Langewiesen, Germany. She studied environmental chemistry at the Friedrich-Schiller-University in Jena, Germany, where she also obtained her diploma. Before joining Analytik Jena AG in 2010, she worked as scientific assistant at Saarland University, Germany, where she earned her Ph.D. in analytical chemistry.

Christian Emmenegger is a product manager ion chromatography in the Competence Center Ion Chromatography at Metrohm International Headquarters. After his diploma in chemistry, he obtained his Ph.D. in analytical environmental chemistry at the Swiss Federal Institute of Technology in Zürich, Switzerland.

Dirk Schmitz is a project manager in the Ion Chromatography Development of Metrohm International Headquarters. Dirk studied chemical engineering at the Fachhochschule Aachen in Germany.

Alfred Steinbach is a scientific technical writer and editor in the Marketing Department at the Metrohm International Headquarters. He obtained his MSc in nuclear chemistry from the University of Cologne, Germany, and his Ph.D. in environmental and analytical biogeochemistry from the University of Hamburg, Germany.

E-mail: ast@metrohm.com

Website: www.metrohm.com

This article is from The Column. The full issue can be found here:http://images2.advanstar.com/PixelMags/lctc/digitaledition/July07-2014-uk.html#2