Analytical procedures

A common belief is that comparable analytical results are achieved by using the same standard method in all laboratories. Experience shows that this is not the case and the same method does not guarantee similar results. Many things affect to the analytical results, for example the purity

of reagents, apparatus type and procedure used in the laboratory, validation of the method in the laboratory and the professional skills of the laboratory personnel.

The European (CEN) and international (ISO) standardization organizations provide new

standards annually but they are not mandatory to take in to use by laboratories. It is not essential to use standardized methods, but it is essential to be sure that the method used is properly validated and its precision and accuracy are determined. Without a proper evaluation of the laboratory processes and methods used the laboratory cannot provide reliable services. With new methods it is often possible to analyse lower concentrations, and more accurate. This allows interpreting threshold values more strictly.

The development of the methods and equipments is fast, the sensitivities of methods gets higher and the LOQs become lower. The availability of methods developed for different substances is getting better all the time. The standards for the analytical method usually provide suggestive LOQs. In practice every laboratory will have to specify their own LOQs and they may be higher or lower than the suggested ones by the standard. This is caused by the quality system of

laboratories, the equipments used and the professional skills of the personnel in the laboratory.

9.4.1 Inorganic compounds Anions and ammonium

The chemical analysis of inorganic ions and ammonium has long traditions and the methods are quite stable. All methods have a European and international standards.

Chloride can be analysed by volumetric titration with a standardized titrant. The endpoint of titration can be identified by the colour resulting from reaction with an indicator, by change in pH-value or the change of electron potential.

The commonly used method is ion chromatography. It is a technique based on the liquid chromatography in which different ions go through an ion exchange column and the conductivity will be measured with detectors.

Potentiometric method measures the voltage of ions in the sample. Measurements are made with ion selective electrode and comparison electrode, which has constant potential. The

concentration of sample ions affects to the potential of ion selective electrode and it will be measured. With this method is possible to measure ions chloride, nitrate and sulphate.

The flow injection analysis (FIA) or the continuous flow analysis (CFA) methods are based on the separation of the ions, which are then detected with spectrometric detection.

Table 9.1 The existing methods for ions.

Metals

Metals are mostly analysed with the atomic absorption spectrometry (AAS) or inductively coupled plasma with either emission spectrometry (ES) or mass spectrometry (MS). The European and international standards are available for all metals.

The atomic absorption spectrometry is based on the fact that atoms of a known element, in gaseous state, absorb light in a wavelength that is characteristic to this element. The absorbance of light is comparable to the amount of this element.

In the flame atomic absorption spectrometry (FAAS), a sample is aspirated to a flame chamber and it is atomised. Light is passed through atomised vapour that is specific to the analysed element. Atoms of that element absorb part of the radiation. By the amount of absorption, the concentration of the element is measured. Each element needs its own light source and this method is not suitable for analysis of many elements simultaneously.

The graphite furnace atomic absorption spectrometry (EAAS) is based on the same principle than FAAS. The difference of the equipments is the heat source, which is a graphite furnace.

This method provides higher sensitivities and also lower limits of detection.

The atomic absorption spectrometry with cold vapour method is commonly used to analyse mercury. The mercury ions are reduced to free mercury and it is collected to a cold trap. From this trap, heating vaporizes mercury and vapour is lead with an inert gas to AAS-equipment. A new method with atomic fluorescence spectroscopy is also used and standards are almost completed. The method is more effective analysing low concentrations of mercury than the EAAS-method.

The inductively coupled plasma/emission spectrometry (ICP/ES) is a method in which the sample is carried as an aerosol with argon gas to plasma and it is heated. The high temperature of plasma causes the atoms to be exited. When the atoms are returning to a ground state, the excited atoms and ions produce an emission spectrum, which is specific to each element. A monochromator is used to separate specific wavelengths of the elements and a detector measures the intensity of the radiation of each wavelength. This method gives possibility to analyse several metals together.

The inductively coupled plasma/mass spectrometry (ICP/MS) is based on the same principle as ICP/ES. Elements are atomised and excited equally and then passed to a mass spectrometer.

Inside the mass spectrometer, the sample goes through electrical and magnetic fields. Finally, the sample goes to detector, which gives mass spectra of the different ions and the height of spectral peaks provides the amount of each ion in the sample.

Table 9.2 The existing analytical methods for trace elements.

Compound FAAS

9.4.2 Volatile organic compounds and pesticides

The volatile organic compounds and pesticides are most often analysed by chromatographic methods. Chromatography is a separation method in which gaseous or liquid flow of sample (mobile phase) goes through a column, either packed or coated with stationary phase. In the column, compounds of the sample are separated by the mobile phase based on the difference in interaction between compounds and the stationary phase. A suitable detector determines the compounds that elute from the end of the column.

The liquid chromatography (LC) is an analytical method using a liquid mobile phase and a packed column filled with a solid stationary phase. This is a widely used method for non-volatile compounds. High performance liquid chromatography (HPLC) uses high pressure to force the mobile phase pass through the column. A variety of detectors can be used with this technique, including ultraviolet (UV), fluorescence (FD), mass selective (MSD). Also a mass spectrometer (MS) can be used as the detector.

The gas chromatography (GC) permits the identification and the quantification of very small amounts of organic compounds. In GC, gas is used as mobile phase and the stationary phase is a liquid that is coated on the walls of a capillary column. Also thin packed columns are possible to use and then a liquid is coated on an inert granular solid.

Table 9.3 Some existing methods for organic compounds.

Compound

Brominated diphenylethers X X

C10-13-chloroalkanes Reliable methods do not exist yet

1,2-dichloroethane X X X

Hexachlorocyclohexane X X X

Naphtalene X X X X

Nonylphenols X X

Octylphenols X X

Pentachlorobenzene X X X

Pentachlorophenol X X X

Polyaromatic hydrocarbons X X X

Tetrachloroethylene X X X X X

Tributyltin compounds X X

Trichlorobenzene X X X

Trichloroethylene X X X X X

Trichloromethane X X X

The pre-treatment of samples is possible to perform in several ways, which depends on compounds to be analysed. With highly volatile compounds, either headspace injection or purge-and-trap method can be used. The headspace sample is in the vessel and it is heated so that the sample is vaporized. The sample of the gaseous phase is then injected into the column with carrier gas, which may be helium, argon, hydrogen or nitrogen. In purge-and-trap method the sample is bubbled with purge gas at ambient temperature. The vapour is collected to a cool trap. The trap is heated and sample is moved with the same gas to the column.

A sample can also be extracted straight to a suitable solvent. Depending on compounds examined, the solvent can be purified with different extraction methods. If compounds have high boiling point, they can be derivatized to more volatile form and then extracted by a solvent.

When the sample is injected into a column, the organic compounds are vaporized and they move with the carrier gas through the column at different rates. The moving rates depend on the differences in partition coefficients between the mobile and stationary phases. The gas through the column is passed to a suitable detector. There are several detectors that can be used

depending on the substances, which are to be analysed. For example, flame ionization (FID), electron capture (ECD) and mass selective (MSD) are frequently used. Also, GC can be used with MS as the detector.

In thin layer chromatography (TLC) a very thin plate of glass, metal or plastic is coated with a stationary phase, which is usually silica or alumina. A small amount of the sample to be analysed is spotted near the bottom of this plate. Then the TLC plate is located in a pool of solvent in a developing chamber so that only the bottom of the plate is in the liquid. This liquid (mobile phase) slowly rises up to the TLC plate by capillary action. The components will differ in the solubility and in the strength of their adsorption to the adsorbent and the rate in which the components will rise upward on the plate. Automated multiple development (AMD) technique, which is an instrumental technique for separation of compounds, can be used with TLC.

Table 9.4 Some existing methods for pesticides.

Compound