As regulations set lower limits for contamination of the air, the development of ion chromatography grows, as Rajmund Michalski explains.

Introduction

Air pollution is a severe local, national and global problem due to its adverse effects on human health, its strong ecological impact through wet and dry deposition, and its influence on global climate change.

Emissions of air pollutants connected to energy production, the large variety of industrial plant, traffic, agricultural activities and other sources are under strict legal control in many countries and as a result of this the quality of air has been substantially improved.

Atmospheric pollutants can occur in a variety of forms, such as dust, fumes, gases, vapours, mists and aerosols. Knowledge of the distribution of air pollutants between the gaseous and the particulate phases is a very important goal for the environmental analytical chemist.

Among the many analytical techniques available for air pollution control and atmospheric research, the role of ion chromatography is increasing. Ion chromatography is well established as a regulatory method for the analysis of anions and cations in environmental samples, as there are few alternative methods that can determine multiple ions in a single analysis. Ion chromatography offers an enormous range of possibilities for the selection of stationary and mobile phases and, in combination with different detection techniques, is usually able to solve even difficult separation problems.

Ion analysis of air samples can be divided into two branches. One is gas analysis after trapping of the gaseous analyte in a liquid or on a solid sorbent and dissociation/conversion of the gaseous compounds into ionic species. The other is the characterisation of the chemical composition of aerosols, i.e. analysis of particulate matter, hydrometeors and wet precipitation.

Airborne particulate matter is partially composed of water soluble salts and wet precipitates may contain a considerable concentration of different ionic species.

In general, sample preparation methods are based on converting a real, complex matrix into a sample in a format that is suitable for analysis by a specific analytical technique. They have a common aim:

• Removal of potential interferences from the sample, thus increasing the selectivity of the method • Increasing the concentration of the analyte(s) and, thus, the sensitivity of the determination • Converting the analyte into a more suitable form, if necessary • Providing a robust and reproducible method that is independent of variations in the sample matrix

In modern analytical chemistry and, particularly, in trace analysis, sample preparation is usually more important than the determination method itself for the accuracy and reproducibility of the results. The most important sample preparation methods for the determination of air pollution are absorption in liquids, adsorption on solid phases, membrane sampling, and chemical conversion.

Analysis of gaseous compounds

A substantial number of gas analyses by using ion chromatography are based on trapping the analyte of interest in a liquid solution or collection on a solid adsorber. Gases dissociating into ions when dissolved in aqueous solutions can be quantified by the determination of the respective ions formed. Ammonia can be converted to the ammonium cation, sulphur dioxide to more stable sulphate anion, and nitrogen dioxide to nitrite and nitrate ions. In addition, organic anions which include carboxylic acids and amines can be determined.

Suitable sampling conditions for ambient air, indoor air as well as flue gases and other emissions are described by organisations such as NIOSH, OSHA, ASTM, US EPA and ISO.

In the collection of trace levels of reactive gases like SO2, NH3, NOx, HCl, HNO2, HNO3, H2SO4 as well as short chain volatile organic acids and amines, gas filtration cannot be employed without the high risk of losses due to absorption or conversion of the gaseous species on the filter material itself, or the particulate matter accumulated on the filter with time.

Sulphur dioxide, which results primarily from the combustion of coal and petroleum, is one of the six major air pollutants. It is therefore necessary to have a reliable analytical method for its determination.

The US EPA has been evaluating the ion chromatography method for ambient SO2 monitoring. It was noted that this method has no temperature stability problems and eliminates the use of the toxic chemical, potassium tetrachloromercurate, used in the previously recommended methods. The hydrogen peroxide absorbing reagent is a very efficient collector of sulphur dioxide.

Gas sampling and analysis has been widely applied because of its instrumental simplicity and low cost and also, partially because of missing alternatives.

An alternative to gas collection by trapping in solution is to pass the sample gas through a filter or cartridge impregnated with a suitable reagent. Passive sampling of gaseous contaminants has a long tradition in occupational health studies and workplace monitoring, but has also gained considerable interest in ambient air monitoring.

Badge-type samplers are typically employed in personal sampling and rely on the controlled permeation of the gaseous compound of interest through a thin, semi-permeable membrane. After sampling the badges are dismounted and the collecting surface eluted by an appropriate solvent. These devices need to be calibrated by exposure to a known gas concentration because the respective permeation constants are generally unknown.

This method has several advantages. External energy is not required, the samplers are simple to construct and relatively cheap, can generally be reused, are small sized and can be easily mounted at almost any site. Also, it is possible to prepare the samplers in the laboratory, send them by mail to the measuring site for remote sampling, and after exposure send them back to the laboratory for analysis. Disadvantages are the relatively long exposure times required to obtain adequate detection limits for trace gases, and only moderate precision achievable due to the variability of the collection rate caused by atmospheric conditions.

Applications of passive sampling encompass the determination of a large variety of volatile organic compounds, as well as many inorganic gases. With respect to the application of ion chromatography as the finishing analytical method, examples often reported are the determinations of acidic gases (e.g. SO2, NOx, HCl) as well as ammonia and short-chain amines, which can be collected on suitable alkaline or acidic sorbents, respectively6. Passive sampling has also been introduced in Standard Methods of Analysis, as is the case, for example, for the determination of NH3 and NO2 in ambient air.

The simultaneous collection of gases and particulate matter and the quasi-simultaneous determination of the respective ionogenic constituents can be done by using denuders, wet scrubbers and membrane based devices.

In their simplest form denuders used for gas analysis are tubes, internally coated with a sorbent. The selection of the coating material of denuders depends on the intended application. For the determination of acidic gases and acid vapours, generally alkaline absorber surfaces (e.g. Na2CO3, KOH) are employed. After collection has been finished the analyte(s) together with the coating are washed off with water and the extract is analysed by using ion chromatography.

Examples in atmospheric research include the trace determination of SO2, nitric acid, nitrous acid, organic acids and ammonia among others. Recently, denuder sampling has been introduced in Official Standard Methods of Analysis, as is the case, for example, for acidic and basic gases in ambient air.

A significant number of different designs of wet-effluent denuders have been developed in the past. They include single tube wet-effluent denuders, rotating wet-annular denuders and parallel plate wet-effluent denuders, all with different geometrical dimensions and with different wettable surfaces, respectively.

Scientists from the Netherlands Energy Research Foundation developed a number of highly sophisticated and fully automated instruments based on this principle and in co-operation with a well known manufacturer have developed a gas-aerosol IC-system which has been made commercially available.

Interesting alternatives to wet effluent denuders are membrane-based gas sampling devices in which a gas sample and the liquid absorber solution flow along the two sides of a semi-permeable membrane, and the analyte gas is transferred across the membrane and trapped in the liquid phase.

The types of membranes used for gas sampling can be classified into microporous hydrophobic, non-porous hydrophilic and ion-exchange membranes. Most of the applications reported involve microporous membrane devices. Typical membrane materials are polypropylene (PP), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). They are available in different thicknesses, porosity and pore-size.

Analysis of wet atmospheric precipitation

Wet precipitates provide information about atmospheric reactions that occur at the gas-liquid interface. This holds even more for fog, mist, or clouds which are in ultimate contact with the gas phase, thus mirroring the dynamic gas-liquid equilibria.

The chemical characterisation of wet precipitations like rain, snow, hail and sleet provides invaluable data on wet-deposition of pollutants with possible impacts on water and soil as well as outside surfaces of buildings and works of art. The main analytes of interest in this respect are nitrogen and sulphur species, as well as metals and metalloids. Examples of application are the determination of common inorganic anions and ammonium as well as short-chain organic acids in various wet precipitates.

Analysis of atmospheric particulate matter

The main sources of atmospheric particles are suspension, condensation of hot vapours stemming from combustion processes and gas-to-particle conversions in the atmosphere. With respect to health impacts of particulate matter the size and the chemical composition are also of concern.

With decreasing particle diameter they can penetrate deeper into the aspiration tract and can be deposited in the lung. Ultrafine particles of <0.1 µm are even able to enter the blood circuit and can pass the blood-brain-barrier.

For the determination of the mass concentration and the chemical characterisation of particulate matter the amount collected on filters or impactor plates must be sufficiently high. An important aspect of particle sampling by filtration is the selection of the appropriate filter material which on the one hand must provide complete collection of all particles, and on the other hand fulfil requirements of easy handling, good weighing properties and, in case of chemical characterisation of the particulates, low blank values.

The filter material must be inert in order to prevent possible chemical conversion of the collected particulate matter. With regard to the chemical characterisation of filter collected particulates, two potential problems associated with filter collection need to be considered. One is the possible reaction of the particulate matter with gaseous compounds, while the second is that of their solid constituents with each other. Gas-to-particle conversion during filtration is a common problem when reactive gases such as ammonia, nitrogen dioxide, sulphur dioxide or acid vapours are present.

In such analysis a very useful instrument called a ‘Particle into Liquid Sampler’, or PILS, was initially developed by Weber et al8 at the Georgia Institute of Technology. It has now been re-engineered and made available through distributors worldwide.

Conclusions

Since its introduction in 1975, ion chromatography has been used in most areas of analytical chemistry and has become a versatile and powerful technique for the analysis of a vast number of ions present in the environment.

The most important aims in ion chromatography development are new stationary phases, miniaturised systems, enhanced peak capacity through the use of complex eluent profiles and associated computer tools for simulation and prediction of retention, and hyphenated IC systems.

The development of ion chromatography allows the determination of ionic contaminants not only in water, but also in air pollution control. Ion chromatography will continue to be developed as more and more ionic contaminants become regulated at lower and lower limits in the future.

Published: 27th Feb 2014 in AWE International