4.21  Chemical speciation

4.21.1  Introduction

The inorganic fraction of the aerosol mass consists mainly of sulphate, ammonium, and nitrate containing particles. Other ions contribute to a less extent to the inorganic fraction. It is recommended to measure all soluble base cations, sodium, potassium, calcium, and magnesium, as well as chloride. Participants applying ion chromatography for analysis of the EMEP filterpack aerosol filter for sulphate, nitrate and ammonium (see chapter 3.2) should be able to obtain the base cations and chloride concentrations on a daily basis.

The recommended measurement program also includes EC/OC measurements. The carbon fraction typically constitutes ~ 30 per cent of the mass and contains elemental carbon (EC) and a huge number of different organic carbon (OC) compounds. It is recommended to determine the amounts of EC and OC in at least one sample every week from each station in the start phase. A (partial) speciation of the organic fraction is also of great interest, but this will mainly be a research activity applying advanced laboratory equipment and rather complicated chemical procedures and not applicable on all EMEP sites

4.21.2  Extraction

For extraction of water-soluble constituents from the PM10 filters, it is recommended to use a punch to remove an accurately defined part of the exposed area of the sample filter. The diameter should be chosen to allow for a similar sub-sample to be taken for the determination of elemental and organic carbon (see below). A suitable size would be a circular punch, of diameter 3-5 cm. The extraction volume should be at least 10 ml. The filters are put in tubes and deionised water is added, the tubes should be kept in ultrasonic bath for at least 30 minutes to obtain a complete extraction. When quartz or other fibre filters are used, be careful to avoid breaking up the filter by unnecessary stirring, because loose fibres in the solution do not go well with the ion chromatograph! Filtration of the extract may be necessary.

If heavy metals are to be determined by e.g. ICP-MS it is necessary to use acidic extraction agent, see chapter 3.11.5.

4.21.3  Determination of the inorganic components

Chemical speciation should primarily include determination of sulphate, nitrate, ammonium and other water-soluble ions in filter samples. The latter will include sea-salt, which contributes significantly to the PM10 in coastal regions in Western Europe. The concentrations of aerosol sulphate, nitrate and ammonium ions are usually determined in connection with the normal EMEP measurement programme, and the determination of these water-soluble ions by ion chromatography of filter extracts can be used to give data also for sodium, potassium, magnesium, calcium and chloride, particularly if the recommendations for sampling given in the chapter 4.1-4.6 are followed (see also Třrseth et al., 1999).

4.21.4  Determination of heavy metals

Heavy metals do not usually give a significant contribution to the particulate mass, and it is not important to measure these components just to obtain a mass closure. But aerosols play an important role as a carrier of heavy metals. Sampling methods of heavy metals in particles are found in chapter 3.11 and the analytical methods are found in 4.17. In addition, heavy metals may be determined using PIXE and INAA described under mineral dust, 4.21.7.

4.21.5  EC/OC determination

The quantification of elementary carbon and organic compounds (EC/OC) in aerosol particles is of considerable interest. The ratio between EC and OC is often used as a valuable tool for the elucidation of the origin of the air masses investigated. Elementary carbon is present in the form of chain aggregates of small soot globules, and is responsible for the light absorption of the material collected on filters. Unfortunately this light absorption depends on the size distribution of the soot particles and on the association of the soot particles with other substances in the aerosol particles and on sample filters. Optical methods to determine EC are therefore only semi-quantitative, and calibration factors may vary from one situation to another, see e.g. (Liousse and Jennings, 1993).

The recommended method to determine elementary carbon is therefore by successive volatilization and oxidation of the sample, and to determine the evolved CO2, either directly or after conversion to CH4 by a flame ionization detector (FID). This procedure also gives the total carbon content, and a quantification of the amount of organic materials through the organic carbon content of the aerosol particles. The method is not free of artefacts, particularly the charring or incomplete removal of organic compounds may lead to the overestimation of EC. To compensate for this, optical detection of a darkening of the filter during the last stage of the OC volatilization is recommended (Chow et al.  1993; Huntzicker et al.  1982). This method is now part of the USEPA programme, and the equipment described by Birch and Cary (Birch and Cary, 1996) is commercially available (Sunset Laboratory Inc., USA). CCC has opportunity to analyse samples collected on quartz fibre filters and the equipment is also available in other laboratories in Europe. A factor of 1.4 is tentatively recommended to convert the measured OC content to total organic particulate mass.

Chemical analyses for further speciation of the organic component in aerosol particle samples are much more demanding, although some advances have been made in determining the water-soluble organic aerosol mass, and specific fractions of this mass. Quantification of selected chemical compounds by gas chromatography and other methods is also possible, but the number of individual compounds is very large, and chemical analyses should therefore be directed to determination of ”signature” compounds, which are indicative of certain groups or specific emission sources (e.g. wood combustion).

4.21.6  Chemical characterization of the OC fraction

High performance liquid chromatography (HPLC) combined with mass spectrometry (MS) have reached a state where identification of unknown compounds has become possible at quantities about 1 ng. From the accurate mass determined, the elemental composition of an unknown can be calculated. Combination of retention time data obtained during a HPLC run, the corresponding UV spectra, and the isotope pattern in the mass spectrum, makes possible calculations of possible elemental compositions. Commercially available chemistry databases should allow the identification of unknown compounds present in the aerosol samples. Based on the solubility, particulate matter can be defined as water soluble organic carbon fraction (WSOC) and water insoluble organic carbon fraction (WINSOC). According to Zappoli and co-workers (Zappoli et al.  1999), the WINSOC fraction can be further separated into solvent extractable polar organic compounds (SEPOC); solvent extractable non-polar organic compounds (SENOC) and non extractable organic compounds (NEC).

Derivatisation with alkylchloroformates is helpful for trace analytical purposes. Alkylchloroformates have unique properties which allows to derivatise nearly all polar compound classes simultaneously; phenols, organic acids, hydroxylated acids, amines etc. Derivatisation with alkylchloroformates will be used in order to obtain a complementary sample preparation to the approaches below.

For polar organic compounds, the following sample preparation and analytical methods can be used:

In order to distinguish between the water-soluble OC and water insoluble OC compounds the following techniques can be used:

  1. A gentle filter washing with water giving a water-soluble fraction and water insoluble fraction.
  2. The water insoluble fraction remaining on the filter is dried and divided into two parts. One part is analysed for cellulose. The other part is extracted with sodium hydroxide (NaOH) in two increasing concentration steps in order to obtain the "humic acids" and the "humin" fractions according to the procedure in Havers et al. (Havers et al.  1998). The quantification of the humic acids and humic fractions will be performed via micro-combustion analysis. Combustion method is described in Puxbaum et al.(Puxbaum et al.  2000).
  3. The water-soluble fraction will be separated into three fractions by two-step solid phase extraction: weakly polar compounds (fatty acids, fatty aldehydes, fatty alcohols, esters), strong polar compounds (dicarboxylic acids and other multifunctional compounds), and macromolecular water soluble compounds ("fulvic acids"). Weakly and strong polar compounds can be determined as described by Limbeck and Puxbaum (Limbeck and Puxbaum, 1999). The macromolecular fraction is determined by the micro-combustion technique as mentioned above.

4.21.7  Analysis of mineral dust

Mineral dust may often contribute significantly to the particle mass and to generate a total mass closure it is important to determine this fraction. Monitoring of air pollution has mainly been focused on anthropogenic sources, mineral dust has therefore traditionally not been measured because of its more natural origin, even though atmospheric dust can be an indirect result of land use and human activities. Mineral dust is a mirror of the earth crust and consists mainly of silicates and oxides of silicon, aluminium and iron. The relative importance of mineral dust in particulate matter depends on location, season and particle size, it is mainly concentrated in the coarse fraction. There can be large local variations depending on the source, e.g. Sahara dust can give a large contribution of the PM10 concentration in southern Europe.

Mineral dust has in general low solubility and is therefore difficult to analyze using instruments like ICP-MS, ICP-AES, AAS etc. To dissolve e.g. silicon it is usually necessary to use strong solvents as hydrofluoric acid. This solvent is however not very practical for most instruments and needs special precautions. As a consequence XRF, INAA and PIXE are the most commonly used techniques for analyzing mineral dust, Table 4.21.1. These techniques have a major advantage that the sample can be analyzed directly from the filter avoiding uncertainties of whether everything is dissolved when using solvent techniques. An additional advantage using PIXE, INAA or XRF is the multielement analysis thereby the possibility to get information of the concentrations of heavy metals in particulate matter as well.

Table 4.21.1: Analytical methods used for analyzing mineral dust.

Analytical method

Disadvantages

Advantages

Proton induced X-ray emission (PIXE)

Demanding

Sensitive, multielement analysis

Neutron activation analysis (INAA)

Demanding, silicon cannot be analysed, time consuming

Sensitive, multielement analysis

X-ray fluorescence (XRF)

High detection limit for silicon, absorption

Multielement analysis

X-ray diffraction (XRD)

Insensitive

Composition of species

Microscopy

Difficult to quantify the species

Characterization of particles

Proton induced X-ray emission (PIXE) and neutron activation analysis (INAA) are excellent instruments for dust analysis, but they are demanding methods needing a proton and neutron accelerator respectively, and for most laboratories X-ray-fluorescence (XRF) is easier accessible, XRF is however less sensitive. The theory behind the techniques are found in numerous textbooks (see e.g. a review by Török et al, 1996) and will not be described here, neither will a detailed analytical description, since it is dependent on the instrument, and the user manual from the manufacture should be used to set up a standard operational procedure.

One major problem analysing the content of mineral dust is the low sensitivity for silicon in most analytical techniques. PIXE is the only method that has proven to be suitable for this element. However, it is possible to analyse e.g. aluminium or iron to estimate the amount of crustal mass in the sample using the known composition of the earth’s crust (Mason, 1966); although one should bare in mind that the chemical composition of the mineral dust is not necessarily consistent because of influence from sources where some of the crustal elements are enriched (Rahn, 1976 and 1999).

X-ray fluorescence (XRF) analysis can be used for all elements passed the first row in the periodic table and the detection limit is dependent on element ranging from 20 to 200 ng/cm2 for 44 of 49 elements (Willeke and Baron, 1993). Two different types of instrumentation can be used, wavelength dispersive (WD-XRF) and energy dispersive (ED-XRF). ED-XRF provides simultaneous determination of multiple elements, whereas WD-XRF usually determines one element at a time. The latter technique has somewhat lower detection limits for elements with low atomic number and it has better spectral resolution compared with ED-XRF where interference and line overlap may be a problem (Claes et al, 1998). Absorption of primary and emitted X-rays can be a problem, but if the deposition is thin, X-ray is not absorbed in the matrix and conversion into concentrations is simplified considerably. The filters may also absorb X-rays; membrane filters where the aerosols are collected on the surface are much better compared to filters where the aerosol is collected in the material. Filters of low mass are also preferable to minimize the background scattering. Teflon membrane filters are frequently used. Glass fibre filter should not be used due to higher absorption and since the content of silicon then can’t be determined. Nucleopore filters may also be used (Willeke and Baron, 1993; Claes et al., 1998). The particles should preferably be quite small and the deposition should be homogenous. This is even more critical for the PIXE technique where only a very small part of the filter is analyzed. PIXE differ from XRF in excitation source for X-ray fluorescence, using high-energy protons. Nucleopore filters should be used because fluoride in Teflon filter causes problems for PIXE analysis. This technique is described in more detail by e.g. Maenhaut (1987). INAA is similar to PIXE regarding limits of detection and it is also suitable to determine a large number of elements in the samples (Willeke and Baron, 1993). The advantage of INAA is the almost absence of matrix effects, self-absorption and interferences. It can be used to analyze thick and inhomogeneous samples; the disadvantage is of course the need for a nuclear reactor and special expertise.

X-ray diffraction (XRD) can sometimes be used depending on the concentration level. The great advantage of XRD is that it gives the composition of the minerals, which is not possible with the above mentioned element analysis. The most important use of XRD has been for silica (Lodge, 1989). The problem with this technique is the low sensitivity and for background sites the concentration levels are usually too low. To improve the low peak intensity/background ratio the dust can be deposited on Ag-filters or silicon (5 1 0) plates (Queralt et al., 2001).

Microscopy can also be a powerful tool to identify the different minerals in particles; though, in practice not easy to use for quantification.

4.21.8  References

Birch, M.E. and Cary, R.A. (1996) Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust. Aerosol Sci. Technol., 25, 221-241.

CEN (1998) Air quality. Determination of the PM10 fraction of suspended particulate matter. Reference method and field test procedure to demonstrate reference equivalence of measurement methods. Brussels (EN 12341).

Chow, J.C., Watson, J.G., Pritchett, L.C., Pierson, W.R., Frazier, C.A. and Urcell, R.G. (1993) The DRI thermal/optical reflectance carbon analysis system: description, evaluation and application in U.S. air quality studies. Atmos. Environ., 27A, 1185-1201.

Claes, M., Gysels, K., van Grieken, R. and Harrison, R.M. (1998) Inorganic composition of atmospheric aerosols. In: Atmospheric particles. Ed. by R.M. Harrison and R. van Grieken. Chichester, Wiley. p. 95-146.

Havers, N., Burba, P., Lambert, J. and Klockow, D. (1998) Spectroscopic characterisation of humic-like substances in airborne particulate matter. J. Atmos. Chem., 29, 45-54.

Huntzicker, J.J., Johnson, R. Lo., Shah, J.J. and Cary, R.A. (1982) Analysis of organic and elemental carbon in ambient aerosol by a thermal-optical method. In: Particulate carbon. Atmospheric life cycle. Ed. by G.T. Wolff and R.L. Klimisch. New York, Plenum. p. 79-88.

Limbeck, A. and Puxbaum, H. (1999) Organic Acids in continental background aerosols. Atmos. Environ, 33, 1847-1852.

Liousse, C. and Jennings, S.G. (1993) Optical and thermal measurements of black carbon aerosol in different environments. Atmos. Environ., 27A, 1203-1211.

Lodge Jr., J.P. (1989) Methods of air sampling and analysis. 3rd ed. Chelsea, Mi: Lewis.

Maenhaut, W. (1987) Particle induced X-ray emission spectrometry: An accurate technique in the analysis of biological, environmental and geological samples.  Anal. Chem. Acta, 195, 125-140.

Mason, B. (1966) Principles of geochemistry. 3rd ed. New York, Wiley.

Puxbaum, H., Rendl, J., Allabashi, R., Otter, L. and Scholes, M.C. (2000) Mass balance of atmospheric aerosol in a South-African subtropical savanna (Nylsvley, May 1997). J. Geophys. Res., 105, 20697-20706.

Queralt, I., Sanfeliu, T., Gomez, E., Alvarez, C. (2001) X-ray diffraction analysis of atmospheric dust using low-background support. J. Aerosol Sci., 32, 453-459.

Rahn, K.A. (1976) Silicon and aluminium in atmospheric aerosols: Crust-air fractionation?  Atmos. Environ., 10, 597-601.

Rahn, K.A. (1999) A graphical technique for determining major components in a mixed aerosol I. Descriptive aspects? Atmos. Environ., 10, 597-601

Třrseth, K., Hanssen, J.E. and Semb, A. (1999) Temporal and spatial variations of airborne Mg, Cl, Na, Ca and K in rural areas of Norway. Sci. Total Environ., 234, 1-3, 75-85.

Török, S.B., Lábár, J., Imjuk, J. and van Grieken, R.E. (1996) X-ray spectrometry. Analytical Chem., 68, 467R-485R.

Willeke, K. and Baron, P.A. (1993) Aerosol measurement. Principles, techniques and applications. New York, Van Nostrand Reinhold.

Zappoli, S., Andracchio, A., Fuzzi, S., Facchini, M.C., Gelecser, A., Kiss, G., Krivacsy, Z., Molnar, A., Meszaros, E., Hansson, H.-C., Rosman, K. and Zebühr, Y. (1999) Inorganic, organic and macromolecular components of fine aerosol in different areas of Europe in relation to their water solubility. Atmos. Environ., 33, 273-2743.


Last revision: May 2002