Doing the groundwork

Soil testing is crucial, both to comply with environmental regulations and to diagnose ‘soil health’. Here we learn that the latest ICP-OES offers a reliable technique for the difficult task of detecting and quantifying potentially hazardous elements  

It is suggested that soils available for cultivation and farming are becoming increasingly contaminated with toxic heavy metals. Contributing factors may include increased use of pesticides, careless waste disposal and increased industrial activity, all of which can have a major impact on the surrounding land. Contaminants in soil can lead to damage and loss of function and may go on to cross-contaminate water sources with detrimental impacts on the food chain and drinking water.

The UK Environment agency published a new set of Soil Guideline Values (SGV) in 2009, detailing representative average levels of chemicals in soil, below which the likely long-term health risks will be minimal. SGV reports are available for several heavy metals and other inorganic compounds found in soil samples, including arsenic, mercury, selenium and cadmium¹. As a result of this and similar guidelines, there is a regulatory need to monitor levels of potentially harmful elements in soils. However, reliable measurement is not always straightforward.

Fundamentally, the make-up of the soil itself may present a challenge to good analysis. Soil can often be very coarse, even when suspended in liquid for testing, and producing a sample that is suitable for analysis can be complicated and time consuming. Aggressive sample preparation methods may themselves have an impact on the sample, affecting the viability of subsequent results. This means that, ideally, analytical instruments should be capable of processing samples without the need for complex preparation procedures.

Another complicating factor is that soil typically has a very complex sample matrix. Because of this, components other than the analyte of interest will have an influence on the way the analysis must be conducted. Matrices can vary considerably between individual soil samples and in some cases may be responsible for very high levels of interference when attempting to quantify distinct elements.

when analysing soils for the presence of toxic metals, the analytical technique or instrument used must be capable of operating over a wide range of concentrations, from the ppb range right up to high levels of contamination

Furthermore, the permitted levels of contaminants in soil are often dependant on the element in question and the use of the land. Therefore, when analysing soils for the presence of toxic metals, the analytical technique or instrument used must be capable of operating over a wide range of concentrations, from the ppb range right up to high levels of contamination.

One technique that is widely used in the elemental analysis of soils is inductively coupled plasma optical emission spectrometry (ICP-OES).  Having the ability to simultaneously measure approximately 70 elements in aqueous solution, it is a technique that delivers comprehensive analysis. It is challenging working with difficult soil samples, but the application of new ICP-OES technologies is supporting efficient and reliable analysis.

Many laboratories undertaking this kind of soil testing have to deal with large numbers of samples every day. High throughput requirements mean that running costs are also a consideration when selecting laboratory instrumentation for accurate soil testing.

In ICP-OES, the emission source is an Inductively Coupled Plasma, an electrically powered, high temperature discharge into which the sample is introduced as an aerosol. When a gas is heated to a certain point, the thermal energy becomes high enough to ionise the atoms (or molecules) to form what is known as plasma. The ions and free electrons in the plasma cause it to become electrically conducting, making it possible to magnetically couple radio frequency energy into the plasma and maintain it continuously. In the plasma the sample atoms are excited to emit element-specific radiation. The intensity of this radiation is proportional to the number of atoms present (i.e. the concentration), which is the basis of quantitative analysis by Optical Emission Spectrometry.

ICP-OES is recognised as a valuable method for the analysis of heavy metals. It is described in the standard procedures for soil testing from several regulatory agencies, such as BSI standard EN 13346:2000 and ISO standard ISO 11885:1996. With a capacity for multi-element determination, a broad measurement range and high sensitivity, ICP-OES has been widely adopted in soil analysis and it effectively meets the challenges outlined earlier.

Advances in today’s instruments continue to improve the speed and effectiveness of soil analysis.

Due to their solid nature and physical properties, soil samples can prove very difficult to prepare for elemental analysis. ICP-OES is rugged enough to handle particles with sizes up to 15µm in solution. This reduces the amount of sample preparation and dilution procedures. For particles greater than 15µm, an accurate ICP-OES analysis relies on solutions or suspensions with a defined particle size distribution. In these cases the samples are either diluted or digested. Remaining residues are typically filtered prior to the analysis to avoid possible clogging of the nebulisation system. With the ability to analyse various sizes of particles, ICP-OES provides a solution across a range of soil types and textures.

Once samples are prepared, the presence and levels of many elements must be measured. Such complex analysis can be performed economically using ICP-OES, which provides a complete scan of the sample and its components simultaneously, eliminating the need for multiple analysis runs or the use of separate instruments.

Achieving the required Instrument sensitivity and working with disruptive sample matrices are common issues during the analysis of complex soil samples, especially when working with minute amounts of heavy metals. The two viewing options offered by ICP-OES enable users to manage these challenges by observing the plasma in different ways. The first of these is radial or ‘side-on’ plasma viewing; the second is axial or ‘end-on’ viewing. Modern ICP-OES systems incorporate dual plasma observation technologies, permitting easy selection and switching between these two options without the need for two separate systems.

Radially viewed ICP-OES is the traditional operation mode for ICP-OES spectrometers. Here the plasma is in a vertical orientation, and the sample is observed from the side. Due to a small volume of the emission zone being observed, there is less opportunity for interference from the sample matrix.  However, reducing the viewed area also decreases the ability to detect elements present in very small quantities. This reduction of sensitivity can be especially important in the determination of heavy metals in soil.

In contrast, axially viewed ICP-OES rotates the plasma to a horizontal position, enabling the spectrometer to measure straight down the axis. The use of axially viewed ICP-OES provides a longer viewing path down the complete emission zone, allowing extension of the measurable concentration range to sub ppb levels for many elements. Detection limits are improved by a factor of 3 to 10. However, there is an increased risk of interference from sample matrices.

Twin interface instruments have the advantage of providing a radial and an axial plasma observation mode in one instrument, allowing users to choose the setting which will work best for their sample. In many cases this could be the choice of using both plasma observations on the same sample.

As well as improving analysis itself, laboratories are increasingly under pressure to drive down the costs of many procedures.  The latest ICP-OES systems, such as SPECTROBLUE TI, feature robust free running 27MHz generators that are capable of handling high plasma loads with high stability, and are reliable with exceptional uptime. In laboratories processing large numbers of samples, with limited time for maintenance and repair, these new instruments can create significant financial savings.

Moving forward, it is safe to assume that ICP-OES instruments will continue to develop with the incorporation of a growing range of innovative technologies, tailored towards making multi-element analysis even more efficient and effective.

In the area of soil analysis, particularly in the case of toxic heavy metals, this provides the opportunity to detect levels of toxic heavy metals that may be of concern at an earlier stage. Correct precautions to maintain soil health can then be taken in a timely manner.

References

1) The Environment Agency, 2009. Soil Guideline Values, http://www.environment-agency.gov.uk/research/planning/64015.aspx 

Author Steve Allott is Manager at SPECTRO UK and Ireland                                     

Contact  www.spectro.com