Analytical chemistry has an ever-increasing impact upon many diverse professions, not least those involved in site investigation. With drivers such as the Environment Agency and the new NHBC Guidelines on land quality, the need for such professions to procure analytical services is on the increase. Often, the uninitiated can find some of the jargon associated with environmental analysis a mystery.
While there are still some tests that use traditional 'wet chemistry' techniques with relatively unsophisticated equip- ment (eg the BS 1377 test for sulphate, often performed by geotechnical laboratories with otherwise limited chemical testing capabilities), most geochemical analytical techniques now rely on sophisticated instrumentation. Advantages include speed, selectivity and sensitivity, improved accuracy and precision and cost effectiveness.
However, highly computerised and automated instrumentation can have its drawbacks, not least of which is that it can engender a 'black box' mentality. So it is even more important to have well trained chemists to interpret the data, and to ensure that the customer or ultimate user of the data knows what the results mean.
Environmental analytical chemistry can be conveniently separated into organics and inorganics (although obviously there are areas where they overlap).
The ability of the carbon atom to join with four other atoms, including itself, has produced thousands upon thousands of organic compounds, limited only by the ingenuity of mankind and nature. As a result, analysis for organic compounds can take many routes.
We have fortunately progressed from the simple solvent (often toluene) extractable matter test for determining contamination by organic compounds, though it is still used as a screening technique. However, it is a crude and non-selective technique, simply weighing out the residue left after the solvent has been evaporated (and unfortunately any of the lighter organics that may have been in the sample). Note also that many of the organic compounds extracted, such as humic acids, are not contaminants or anthropogenic (manmade) in origin.
Take this screening technique a stage further, extract the organics and shine infra red light on the extract and the IR will be absorbed by the various bonds in the various organic molecules by varying amounts, dependent upon the frequency of the IR and the chemical environment the bond is in.
The bond commonly targeted for environmental analysis is the C-H bond. However, if toluene is chosen as the extracting solvent for subsequent IR analysis, the bonds in the toluene molecule absorb much of the IR, swamping the signal.
Fortunately, solvents without a C-H bond can be used, avoiding this effect. Unfortunately such solvents are CFCs (eg standard methods for analysis for mineral oil refer to Freon, which is in diminishing supply and hardly the reagent of choice in
the environmental protection business), so IR analysis for environmental pol- lutants may have had its day. IR is sometimes referred to as FTIR. The FT stands for fourier transform, mathematical wizardry built-in to many modern IR spectro- meters to maximise the information and minimise the noise.
Happily, there are alternatives. The work- horses of modern organics analysis involve chromatographic tech- niques. Chromatography involves separating a mixture into its components, by partition of the components between two phases, one of which is stationary. Perhaps the simplest used for environmental analysis is thin layer chromatography (TLC). All chromatographic techniques are based on similar principles, although different forms of instrumentation are used to obtain the information.
A powerful and versatile tool, gas chromatography (GC) involves the injection of the sample in the gaseous phase (either naturally gaseous at ambient temperature or volatilised by a heated injector) onto a column coated with a liquid stationary phase, chosen to suit the chemistries of the compounds of interest. A carrier gas is then swept through this column, and partition occurs between the two phases. Detection and quantification is achieved through detectors sited at the end of the column, also suited to the compounds of interest. These include a flame ionisation detector (FID), perhaps the most versatile detector there is, which is particularly suited to hydrocarbon analysis, for example from leaking underground storage tanks. Other GC Detectors include the electron capture detector (ECD), the flame photometric detector (FPD) and the thermal conductivity detector (TCD). Figure 1 shows a GC-FID trace for diesel contamination.
Such GC techniques are useful, but once the appropriate detector and column has been selected for the determinands of interest, identification solely depends on the retention time (the time taken for the compound to emerge after injection). Pattern recognition can yield valuable information and sample clean-up procedures before analysis can remove potential interferences and hence reduce the chance of false identification.
However, for the chemical cocktails often encountered on particularly polluted sites, an even more sophisticated technique is often necessary to avoid misdiagnosis (for example assigning a chromatographic peak to the presence of a compound when it is in fact due to another - compounds behaving in this way are said to 'co-elute'). This technique is gas chromatography-mass spectrometry (or GC-MS for short).
With GC-MS, peak identification is not only achieved through retention time, but also from each peak's mass spectrum. Put simply, each compound has its own mass spectrum, so once separated from the chemical crowd by chromatography, the compound's mass spectrum is then compared with a computer based library of mass spectra and a match obtained.
Another variation on the chromatography theme is high performance liquid chromatography, again quite versatile and with various column and detector options to suit the application, eg electrochemical detection for phenols analysis.
Another method is ion chromatography, which leads neatly into inorganics. This uses ion exchange columns to do the chromatography. A typical application is analysis for anions such as sulphate, nitrate, nitrite, fluoride and chloride.
But perhaps the most common use of environmental inorganic analysis is in the analysis of metals.
Atomic absorption and emission
When atomised, different metals will absorb light of different wavelengths, and the amount absorbed is dependent upon the concentration. This forms the basis of atomic absorption spectrometry (AAS). Usually the metals have to be brought into solution, which is fine for waters but the common approach for soils is acid digestion (usually aqua regia, a mixture of nitric and hydrochloric acids), which gives a worst case estimate of risk (fortunately acid rain is not that strong just yet).
Alternative extracting agents and leaching tests, such as the Environment Agency Protocol, are available for metals as for other species. AAS instruments can take a number of forms, flame AAS, graphite furnace AAS and hydride generation or cold vapour AAS, chosen to suit the metal of interest and/or sensitivity.
One disadvantage of AAS is that it can only analyse one metallic element at a time. With inductively coupled plasma atomic emission spectroscopy (ICP-AES), this is not the case, since the sample is atomised and the atoms elevated to an excited state in a very hot plasma (typically around 7500degreesK). Once in an excited state, the atoms then emit energy, again at a wavelength specific to the element. With diode arrays, emission measurements can then be made at many wavelengths for many elements simultaneously, saving sample, time and money.
Spectrometric techniques can involve the use of electromagnetic radiation and in the case of environmental inorganic analysis, this is usually ultraviolet (UV) or visible (Vis) light. This is used in conjunction with chemical reagents which react with the compound of interest to produce an absorbing species, often after some isolation technique such as distillation is used to remove interferences.
Spectrometry is often used to analyse a variety of inorganic species. The 'colouring up' process can be automated with fancy plumbing and coupled
with spectrometry forms the basis of modern flow injection analysers .
Paul Board is a chartered chemist and business development manager of Robertson Laboratories.