Methods of Soil Analysis – A summary of some of the many methods of soil analysis

Published: 10th Dec 2009

by Hazel Davidson

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Soils are an extremely complex matrix to analyse, particularly on contaminated sites – the actual soil matrix can vary from a sand (silica) to limestone (calcium carbonate) to clay (complexed minerals), or a mixture of many. In addition to this, the range of contaminants varies from fairly innocuous construction materials to toxic gasworks waste to highly toxic pharmaceutical waste/ mercury/explosives, etc.

Methods of chemical analysis need to cope with this very wide range of materials and compounds, which is difficult, and the introduction of MCERTS (the Environment Agency’s Monitoring Certification Scheme) has raised awareness of the issues associated with reliable testing over such diverse matrices.

Preparation of soil samples

This is an absolutely critical step, and tends to be overlooked in a review of methods, but if the preparation is not carefully done, then no amount of sophisticated instrumentation will improve the result.

Many tests cannot be performed on a dried and crushed sample, as some of the parameter of interest will be lost, so soils must be initially mixed (and tested) in a wet, as received state, and these include:

Volatiles, leachate testing, phenols, ammoniacal nitrogen, sulphides, cyanides, most organics, hexavalent chromium.

Homogenisation can be done by a classic cone and quartering technique, or using a jaw crusher to break up larger chunks, or kneading clay type samples. Fibrous material may need to be cut up or shredded. If a subsample can be dried, this is usually at a temperature of 35 – 40oC, or 105oC if a moisture content is required. Moisture contents will be needed for all testing performed on as received soils, as these must be adjusted back to a dry weight basis for reporting purposes. For contaminated soils, it is not regarded as advisable to remove anything from the sample, as potential hazardous material may coat the lumps (e.g. paint, electroplating fluids), or constitute the whole lump (e.g. tar), but if any component is removed, this must be noted on the final report to comply with MCERTS requirements.

Once a subsample of soil is weighed out, then for most methods, some form of liquid is added to the soil to extract the required parameter of interest. For example, anions and pH require a 2:1 water extraction, metals use an acidic digest, cyanides need an alkaline extraction, phenols use a methanol/water mix, and most organics need a solvent extraction. Samples must be shaken, refluxed, or digested for specific periods of time, then filtered or centrifuged, and the liquid extract then analysed by an instrument specific for the analyte in question.

“during the validation of a method, the robustness and applicability over a range of matrices must be determined, and this is usually done by using certified reference materials”

During the validation of a method, the robustness and applicability over a range of matrices must be determined, and this is usually done by using Certified Reference Materials (CRMs) or spike and recovery (spiking a range of soils with a known amount of a standard and then testing to see how much is recovered). The validation must cover the preparative stages of a method, as well as the instrumental analysis.

Analytical methods

Calibration

All analytical instruments require calibration, and this is usually performed by preparing a range of standards (five or six is common) at increasing concentrations, putting these through the instrument, which will then construct a calibration curve or graph. Samples, when run through the instrument, are read off this graph to give the concentration of the analyte within the sample. Alternatively, internal standards of a known concentration are added to the sample, and the concentration of compounds within the sample are calculated according to their response measured against that of the internal standard – this is more common in organic methods.

Methods fall into two main groups: spectroscopic methods for most inorganic analytes, and chromatographic methods for organic compounds.

Inorganic compounds

Anions

One of the most basic (and traditional) methods is colourimetric spectroscopy. This has been used by chemists for hundreds of years, although instrumentation is much more automated nowadays. This chemistry lends itself well to anions (compounds which are negatively charged and therefore attracted to an anode), and examples are:

Chloride, nitrate, nitrite, sulphate, phosphate, sulphide, ammoniacal nitrogen.

Most laboratories will use some form of automated spectrophotometer, such as a Kone or an Aqua. They are multi element analysers, so more than one compound can be determined in one run. All instruments will have an autosampler, and a system for sequentially adding the appropriate chemicals to develop the colour.

The final solution then passes into the spectrophotometer part of the system. The general rule (the Beer-Lambert Law) is that: ‘the absorbance of light at a given wavelength is directly proportional to the concentration of the absorbing species’, or in simpler words, the more intense the colour, the greater the concentration. These automated analysers are very efficient and can run up to two hundred samples per hour.

Metals

Metals comprise one of the most commonly requested groups of analytes in environmental analysis, and these are usually measured by an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP – OES).

The dried and crushed soil is weighed out (usually 5 g of sample), and then digested on a hot block with aqua regia (a 3:1 mix of concentrated hydrochloric and nitric acids) for up to two hours. The acid extract is filtered and then loaded into the autosampler rack on the ICP – OES, where it will be aspirated and pumped into the plasma (a very hot, ionised gas at 10,000oC). Here, energy causes electron excitation, and then as the sample passes out of the plasma, this energy will be emitted as the electrons return to their ground state. The energy emitted will be at specific wavelengths, according to the metals present in the sample, and this energy is detected by the spectrophotometer.

The beauty of the ICP is that it will measure several wavelengths at once, so over 20 metals can be analysed in about 4 minutes, ensuring this is a very efficient method. ICPMS can be used instead of ICPOES, but the samples require large dilutions due to the sensitivity of the instrument to the high acidic and dissolved solids content in the digests. An alternative method of metals analysis is atomic absorption, and there are several forms of this, but the downside is that only one element can be measured at once.

Sulphur compounds

There are several forms of sulphur containing compounds, and these have differing levels of importance in environmental analysis

• Total sulphur can be measured using an induction furnace, but as sulphur can exist in many forms, it will not give much information regarding risks on site

• Elemental (or free) sulphur is analysed using a solvent extraction, followed by HPLC (an organic method described later in this article). This is present on many ex gasworks sites and can cause a dermatitis like skin reaction

• Total sulphides can be analysed by acid digestion and ICP, but many of these are insoluble and relatively inert, so do not constitute an environmental hazard

• Acid Soluble Sulphide is considered more of an environmental risk, as if these compounds are present on a site with acid rain or groundwater, then hydrogen sulphide can be formed, which is highly toxic at low levels. These are analysed either by ion selective electrode, or acid digestion followed by colourimetric analysis of the impinger solution (which traps the evolved hydrogen sulphide)

• Acid Soluble Sulphate, sometimes referred to as total sulphate (which may not be strictly equivalent), is analysed using an acid digestion followed by ICP-OES. A value of 0.24% is considered a potential risk for concrete attack

• Water Soluble Sulphate, from a 2:1 water extract, is analysed either by ICP-OES or colourimetric spectrophotometry. This will determine if sulphate resisting cement is required by the construction company

Nitrogenous compounds

The interrelationship of nitrogen compounds can be confusing, and these are defined as follows:

• Nitrate and nitrite (total oxidised nitrogen) – these will be analysed on a 2:1 water extract of the soil by colourimetric spectroscopy

• Ammoniacal nitrogen – this will include both ammonia (NH3) and ammonium (NH4), and again is either a water extract, or distilled as exchangeable ammonia

• Kjeldahl nitrogen is a measure of the ammoniacal nitrogen and organic nitrogen, and is analysed by a distillation and titration method

• Total nitrogen involves a stronger acid digestion, and includes all of the above

“ICP is a very efficient method it can measure several wavelengths at once, so over 20 metals can be analysed in about 4 minutes”

Cyanide compounds

• Free cyanide represents simple cyanide salts, such as potassium cyanide. These are very water soluble and extremely toxic, and are measured by a water extraction, followed by a specific, automated distillation/colorimetric analysis such as a Skalar

• Easily liberated cyanide is still measuring free cyanide, but uses a more acidic digestion to cope with difficult matrices

• Total cyanide requires a much more aggressive digestion, but this is also performed on the Skalar system. This will include the complex ferri and ferro cyanides

• Thiocyanate is a separate colourimetric analysis, and is not included in the total cyanide, but again can be performed on the Skalar

Organic methods

These are commonly based on some form of chromatography, with the usual methods including:

• HPLC – High performance (or high pressure liquid chromatography)

• GCFID – Gas chromatography with flame ionisation detection

• GCMS – Gas chromatography with mass spectroscopy detection

“chromatography is the separation of a complex mixture by the use of partitioning between a stationary phase (the column) and the mobile phase (a liquid or gas)”

Chromatography is the separation of a complex mixture by the use of partitioning between a stationary phase (the column), and the mobile phase (a liquid or gas). The separation occurs because of different properties of the compounds – either their mass (so larger molecules take longer to pass through), or their polarity (the charge associated with a compound – the greater this is, the more reactive the compound will be and the more slowly it will move through the column). Water Analysis

High Performance Liquid Chromatography

Speciated phenols are analysed by HPLC, and will give results for phenol itself, xylenols, cresols, resorcinols and napthols. Depending on the site, clients need to request just phenol, total phenols, or speciated phenols.

“separation occurs because of different properties of the compounds – either their mass or their polarity”

Gas chromatography with FID

This method is mostly used for petroleum hydrocarbons, such as range organics (GRO) and extractable petroleum hydrocarbons (EPH). The pattern of the chromatogram enables the analyst to identify the type of product in the soil.

Gas chromatography with Mass Spectroscopy detection

This method is used for a very wide variety of compounds, some examples of which include:

Polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), and pesticides (chlorinated and phosphorylated, plus many others).

A mass spec can be run in two ways, either in full scan, where the instrument is set to look at every peak, or in selected ion monitoring (SIM), where it will only look for specific target compounds and ignore anything else which may be present. If a client is not sure which compounds to request, then it is better to ask for a VOC and/or SVOC full scan plus TICs. Each of these will provide a target list of 60 + compounds plus up to ten Tentatively Identified Compounds. If the likely compounds are known, then it is better to use the methods run in SIM, as these will provide lower detection limits.

Summary

This review gives only a summary of some of the many methods of soil analysis, but these cover the more widely used techniques. All methods must be fully validated and proven to work across a range of matrices, and are monitored on a batch by batch basis with the use of blanks and Analytical Quality Control (AQC) samples. In addition, laboratories accredited to ISO 17025 (and MCERTS) must participate in proficiency testing schemes, where blind samples are tested at regular intervals to ensure the laboratory methods are fit for purpose. Environmental analysis today is complex, time consuming, and is subject to rigorous quality procedures. Over 30% of samples analysed are QC or validation samples, and this represents a heavy cost to the laboratory, but data must be defensible. Laboratories constantly seek to improve their methods and processes, and further developments/ legislation will provide the impetus to continue with these improvements – this is an ever changing industry.

Author Hazel Davidson

Technical Sales Manager

ALcontrol Laboratories

www.alcontrol.com

ALcontrol aims to be the leading Environment and Food testing group in Europe. Through its network of laboratories it services customers in 11 European countries and provides support to customers globally.

ALcontrol tests soil, water, food, oil and air for contaminants or hazardous substances to allow our clients to ensure health and safety, comply with law or conserve the environment.

Most of our work is driven by legislation affecting our clients businesses. We hope our customers choose us because we are easy to deal with, provide fast accurate results giving our clients Confidence to Act on our results.

ALcontrol has over 2000 employees, 30 laboratories and service centres and over 100 logistics points for collection of samples in 11 countries.

Published: 10th Dec 2009 in AWE International

Author Details

Hazel Davidson

DETS is a private limited company, set up in 1999 and independently owned. Steady expansion, involving two moves to larger premises, has allowed the company to gain a larger market share, and to increase the scope of analyses offered in order to service a wide variety of industry requirements.

With over 30 years experience, and recently joining DETS from ALcontrol, Hazel is a well known chemist within the industry, and frequent speaker at conferences and seminars. Hazel is chair of the EIC Laboratories Working Group, and sits on several committees.

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