Spectroscopy is a classical analytical technique that makes use of the differing spectral properties of elements or compounds, thus allowing individual parameters to be identified and quantified, using an instrument known as a spectrometer.
It is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data are often represented by an emission spectrum, a plot of the response of interest as a function of wavelength or frequency.
Spectroscopy refers to a plethora of different techniques that employ radiation in order to obtain data on the structure and properties of matter, which is used for solving a wide variety of analytical problems. The term is derived from the Latin word “spectron” which means spirit or ghost, and a Greek word “skopein” which means looking onto the world.
The types of spectroscopy discussed in this article are atomic emission or absorption (generally used for metals), and colourimetric spectroscopy, which can be used for a wide range of (generally) inorganic compounds and elements. Mass spectroscopy can also be used when linked to atomic emissions.
Both AA and ICP spectrometers require the sample to be in a liquid form before introduction into the instrument. This is accomplished by digestion in concentrated acids, with aqua regia (HCl/HNO3) being the most common, although other acids can be used. It is important to note that this method of digestion may not result in a measure of total metals, as not all components may be soluble in the acid of choice, particularly those bound as silicates. If this is considered necessary (sometimes recommended for sediments), then it would be necessary to use hydrofluoric acid, which will digest the silicates. However, aqua regia is recognised as an industry standard and most commonly used, with the results sometimes referred to as ‘aqua regia soluble’.
Atomic absorption spectroscopy (AAS)
There are differing types of AA spectrometers, including:
- Flame atomic absorption (FAAS) – the most common technique for a wide range of metals
- Graphite furnace atomic absorption (GFAAS) – used for ultra trace levels
- Cold vapour atomic absorption (CV-AAS) or atomic fluorescence (CV-AFS) – used for the determination of mercury
- Hydride generation atomic absorption (HG-AAS) – used for trace levels of hydride forming elements As, Bi, Se, Sn, and Te
A description of FAAS will provide an understanding of the technique, as the other versions are refinements aimed at lowering detection limits or improving the sensitivity in specific target elements.
The main components of the system are as follows:
- A cathode lamp manufactured to emit radiation (light energy) at a wavelength specific to the element of interest
- A gas produced flame, often air/ acetylene, giving temperatures of 2,000 – 3,000°C
- A sample introduction system, to inject a nebulised spray of liquid into the flame
- A monchromator which detects the light energy after it passes through the flame, at the specific wavelength
- A detector to measure this light energy accurately
- A data handling system to calculate the concentration of the element of interest
The theory underpinning this method is that the element of interest aspirated into the flame will absorb energy from the lamp source at the element specific wavelength, and that the degree of absorption is directly proportional to the concentration of the absorbing element (Beer Lambert law).
The rate limiting factor with AAS is that with most lamps, only one element can be analysed at a time (although multi element lamps are available, they still analyse sequentially) – it is therefore very useful if only one metal is of interest, for example, lead, but if a multiple metal screen is required, then ICP, which allows up to 30 or more metals to be analysed in one run, would be the method of choice.
Inductively coupled plasma emission spectroscopy (ICP)
Atomic emission spectrometry (ICP-AES), sometimes termed OES – optical emission spectrometry, is a process whereby the light energy emitted by atoms or ions is measured and used to calculate the concentration of the element of interest. The emission occurs when thermal or electrical energy is available to excite a free atom or ion to an unstable energy state – this is what happens in the plasma – and the light energy is then released when the atom or ion returns to its stable ground state as it passes through the plasma.
The plasma is generated by a radio frequency field and a highly ionised gas, usually argon, and the temperature in the plasma is in the region of 10,000°C – much hotter than the flame in FAAS. This temperature allows complete atomisation of elements, thus minimising chemical interference effects. In addition, no primary light source is required, and up to 30 elements can commonly be determined in one run of about three to four minutes per sample.
The mechanics of the instrument are similar to FAAS, including a sample introduction system, a nebuliser/spray chamber, a monochromator or polychromator, a detector (often a photomultiplier tube), and axial or radial viewing systems, plus sophisticated data handling.
ICP-AES is accepted as the best overall multi-element atomic spectroscopy technique for soil metal analysis, with rapid sample throughput and a very wide analytical range of metals. A further benefit over AAS is the much greater range of linearity, as levels of metals vary considerably, and it is important that analysts do not have to repeatedly dilute samples to ensure they are within the calibration and linear range of the instrument.
ICP with mass spectrometry detection (ICP-MS)
This was introduced as an improvement to the method by combining an ICP as above, but linking it to a quadrupole mass spectrometer, by means of a sophisticated interface. The MS replaces the monochromator, and rather than separating light energy according to its wavelength, the MS separates the ions from the ICP according to their mass-to-charge ratio, and directs them to a detector that quantifies the number of ions present. An ICP-MS combines the multi-element capabilities and broad linear working range with exceptional detection limits.
However, it is more limited in the type of sample it can handle – there are constraints on the level of dissolved solids and acid concentration, so samples require diluting much more than with ICP-AES.
Even with this consideration, the detection limits achievable with ICPMS are still usually better than ICP-AES.
Speciation of metals
The methods above will provide a result of all forms of the metal found in the acid extract from the soil, but there are some instances when it is advisable to know the form of the metal. For example, chromium can exist in different valency states, commonly hexavalent (CrVI) and trivalent (CRIII). The hexavalent form is carcinogenic and far more toxic to humans, and is therefore of greater concern if present in contaminated soils. The usual method to differentiate these is to use ICP-AES for a ‘total’ chromium, and then a colourimetric method (see following section) for the hexavalent chromium, with the trivalent being determined by difference, if required.
The other metal commonly requiring speciation is mercury, and there are Environment Agency soil guideline values (SGVs) available for elemental, inorganic, and organic mercury (methyl mercury), which again, have differing levels of toxicity. This speciation is accomplished by using either cold vapour AAS or AFS (atomic fluorescence spectroscopy).
Speciated mercury analysis is performed on the wet, as received soils, so no losses are likely due to the preparative procedures.
- Inorganic mercury (II) and methyl mercury
Extraction of samples follows the USEPA Method 3200 guidelines for “Mercury species fractionation and quantification by microwave assisted extraction”.
“speciated mercury analysis is performed on the wet, as received soils, so no losses are likely due to the preparative procedures”
Samples are homogenised and taken through a two-step microwave extraction procedure to take both the extractable and non-extractable mercury (semi and non-mobile) compounds into solution. The use of microwave extraction in sealed vessels prevents the loss of any of the more volatile mercury components. Then the extracted species are separated by HPLC, oxidised to break down the organic complexes, followed by treatment with a reducing agent and analysis by atomic fluorescence spectroscopy. All stages are performed on a continuous ‘on-line’ setup directly linked to an atomic fluorescence detector. Quantification is performed by comparison to a specifically generated calibration curve.
- Elemental mercury
Samples are again tested on an as-received base. Samples are purged with argon, the volatilised elemental mercury is collected on a silica-gold vapour trap, and the collected elemental mercury analysed by atomic fluorescence spectroscopy. Quantification is performed by comparison to a specifically generated calibration curve.
Other metals sometimes requiring speciation are tin, selenium and arsenic.
Strictly speaking, colourimetric analysis is the measurement of the concentration of coloured solutions using a simple photometer to compare a sample against known coloured standards. However, by using a spectrometer (measuring specific wavelengths), this can be extended to include a wider range of spectra which would not appear ‘coloured’ to the human eye (e.g. UV). Often, the determinand of interest may not demonstrate a colour itself, but it can be reacted with a chemical to produce a coloured version of the compound.
In a basic spectrometer system, there is a light source, an absorption cell, and a detector. The specifically selected wavelength of light passes through the cell and is absorbed by the determinand of interest, and using the Beer-Lambert law again, the concentration of the determinand is directly proportional to the degree of absorption, taking into account the pathlength.
As with the metals analysis, it is necessary to extract the determinand of interest from the soil, as only liquids can be analysed by these spectrometers. The extractant is usually aqueous based, and can be simply water, pH adjusted water (either acid or alkaline), or a specific reagent such as EDTA (ethylene diamine tetra acetic acid). Following this extractant step, the samples are filtered and are then ready to be loaded onto an autosampler.
Examples of determinands
This technique is very versatile and can be applied to a wide range of determinands including chloride, nitrate, nitrite, ammonium, phosphate, sulphate, sulphide, hexavalent chromium, cyanide, and thiocyanate. Some examples of the chemistries are listed below.
Chloride – reaction of the water soluble extract – thiocyanate ion is liberated from mercuric thiocyanate by the formation of soluble mercuric chloride. In the presence of ferric ion, free thiocyanate ion forms highly coloured ferric thiocyanate, of which the intensity is proportional to the chloride concentrations.
“Laboratories would struggle without spectroscopy techniques, and they can easily be considered as the foundation of modern analytical chemistry”
Sulphate can be a water soluble, or acid soluble extraction from the soil, depending on client requirements – barium sulphate is formed by the reaction of the sulphate with barium chloride at a low pH. At high pH excess barium reacts with methylthymol blue to produce a blue chelate. The uncomplexed methylthymol blue is grey. The amount of grey uncomplexed methylthymol blue indicates the concentration of sulphate.
Total cyanide (includes complex cyanides such as ferri and ferro cyanides) – strongly acidic extraction, then distillation, followed by reaction with 2-aminobenzoic acid, or with formaldehyde and 3-methyl- 2-benzithiazolinone (particularly good for low level cyanide). For easily liberatable cyanide, the initial extraction is performed with a weak acid solution.
Ammoniacal nitrogen (ammonium) has an intense blue-green complex, related to indophenol blue, and is formed by the reaction of ammonia with hypochlorite and sodium salicylate, in the presence of sodium nitroprusside acting as a catalyst. The complex is measured spectrophotometrically at a wavelength of 655nm and is related to the ammonia concentration by means of a calibration curve. Sodium citrate is added to overcome interfering ions.
Multi analyte systems are now common in most environmental laboratories, either continuous flow or segmented flow, and a typical example is the Skalar San ++. This type of instrument allows samples to be loaded onto an autosampler, and then the various reagents for multiple analytes are added in sequence into the flow, so the instrument can perform, for example, chloride, sulphate and phosphate in one run. If only one determinand is required, this type of system is very fast, and can analyse up to 200 samples for chloride in an hour.
Spectroscopy therefore includes an extremely versatile, reliable series of methods for quantitative measurement of a wide range of determinands. It is quick, sensitive and does not always need complex or expensive equipment. However, all methods must be validated and proven to work across a range of matrices and concentrations, and to be able to cope with any possible interferences. The usual rules of good laboratory practice and adherence to quality systems will always apply. There is a vast array of literature on different methods, and published standard methods in common use. Laboratories would struggle without these techniques, and they can easily be considered as the foundation of modern analytical chemistry.