Making it elemental

Advancements in spectroscopy have meant that precision and accuracy are the watchwords of modern metals analysis – here is our guide to the wide range of techniques available, and which is best for your lab

Tradition has it that modern chemistry arose out of alchemy, in which one of the alchemist's primary aims was to transform a base metal into a noble metal. However, that same tradition has less to say about how the product of the alchemist's experiment was tested, even if every chemist will agree that every experiment will have failed.

However, had the alchemist had access to modern analytical techniques, it's amusing to speculate whether a trace amount of gold or silver might have been found in the end product, albeit that it would also have been found in the starting materials too. For modern metals analysis has moved on a great deal since the wet chemistry, titration techniques of the early 1900s and beyond the atomic absorption spectroscopy of the 1950s such that sensitivity, reliability and precision are much improved. Today metals can be detected in all manner of matrices, down to parts per billion levels – but with a wide range of techniques available, it can be hard to choose the right instrument for the job.

Each of the elemental analysis techniques has its own pros and cons, and understanding the requirements for testing helps to make a clear decision on which technique is best for any given analysis.

Some techniques can offer ultra-low levels of detection at the expense of high initial purchase costs and high running costs. An alternative technique or instrument may have lower sensitivity, and hence lower purchase costs, but can be more labour intensive. There may also be health and safety concerns around use and storage of the gases needed to operate these instruments.

There are three main techniques used for elemental analysis, which are summarised below. Understanding these instruments at a basic level can help guide you towards choosing the most appropriate technique.

Atomic absorption spectroscopy (AAS)

This is one of the earliest examples of instrumental elemental analysis, developed in the 1950s and still routinely used to this day. A sample is aspirated into a flame where the atoms are excited. A beam of light from a lamp carrying the element of interest is passed through the flame, the excited atoms absorb some of the light from the lamp and the signal is measured which allows, through use of a calibrations solution, to evaluate the level of analyte in the samples.

Typically this technique offers detection limits in the parts per million (ppm) range, and as it is well established, and the chemical/physical properties of the elements are well understood and documented, the technique is considered robust and easy to use. The instruments themselves are also relatively cheap to buy.

The downside is that a source of light needs to be provided for each element, thereby creating an additional running cost, and typically limiting the analysis to one element at a time. In a commercial environment, this can have a big impact on time scales for analysis.

Another stumbling block for the introduction of AAS instruments into a workplace is the use of an open flame for analysis. The fuel for the flame is almost exclusively acetylene, which, as a flammable product can cause issues for certain sites, especially if these kinds of materials are difficult to accommodate.

Inductively coupled plasma optical emission spectroscopy (ICP-OES)

With ICP-OES, samples are typically aspirated into an argon plasma which has a temperature of 10,000oC. The elements of interest are excited and as they cool they emit light at specific wavelengths. These emissions are detected on a device similar to that found in a digital camera, and this is then used to calculate the concentration of element in a given sample. This technique offers limits of detection in approximately high parts per billion (ppb) to low ppm levels.

The key advantage of this technique is that is relatively quick in that it can analyse many elements simultaneously and offers good levels of detection. However, this comes at a price as the initial instrument costs are higher than that of the AAS. Also, since a supply of argon gas is required, the running costs are also to be considered.

 Inductively coupled plasma mass spectrometry (ICP-MS)

This instrument also uses an argon plasma, but instead of an optical detector, a mass detector is employed to detect the ions generated in the plasma. Mass detectors are very sensitive and as such it is easy to have detection limits in the sub ppb range, which is the key benefit of ICP-MS, along with the ability to analyse samples very quickly due to simultaneous detection. Of the three techniques this is easily the most expensive of the instruments to purchase, and also has the same problem as the ICP-OES in that it uses argon gas to run the plasma.

The three instrument types discussed above are by no means exhaustive, and even then there are many variants that allow specific analyses to take place, or remove the interferences that may give erroneous results.

If a need for elemental analysis has been identified, it is important to consider many factors before making the purchase of the instrument, such as initial outlay and subsequent running costs, the required detection limits and safety requirements. The expertise and skills required of the operating chemist should not be overlooked in this calculation. Most laboratory equipment is only 'easy to use' if you know what you are doing! The following case studies illustrate some different factors that might be relevant.

Food manufacturer

Consider a food manufacturer that requires analysis of its products as they come off the production line to ensure that each batch has been produced correctly and has compliance with the label claims stated on the packaging. In this case, sodium and calcium are the elements of interest that need to be measured.

As both elements are intended to be present at % levels, ICP-MS is far too sensitive a technique to use for this analysis. The high initial cost is also prohibitive, as is the fact that calcium is a tricky element to analyse by ICP-MS (its mass is too similar to the argon used in the incineration stage). Calcium and sodium are better suited to ICP-OES, but as they are present in relatively high levels, high dilutions may be required to make the technique worth using. Also the initial and running costs may be too high.

That leaves AAS as probably the best option in this case. Both elements are well suited to this technique, and as they are present in high levels then low detection limits aren't required. The initial outlay is smaller than other techniques and operation is relatively simple. Perhaps the only question for the manufacturer is whether they want to have flammable gases in use on site.

Pharmaceutical manufacturer

Many drugs are manufactured using catalysts such as palladium. Hence the final product needs to be checked for the absence of the catalyst. However, with Pharmacopoeial requirements due to change, the manufacturer might be keen to have one eye on future requirements, as well as what is currently permissible.

Since low detection limits (ppb) are required AAS is not a suitable technique for this analysis when run in a standard flame mode. Graphite furnace AAS (where a small amount of sample is rapidly heated in a small graphite furnace to generate a signal) can offer the required level of detection, but is very slow to run. It also has high running costs as the graphite furnace tubes are expensive and need to be frequently replaced.

ICP-OES can offer improved detection limits compared to that of the AAS, but may still not have the required sensitivity. If the cost of the catalyst is high, then it is important that the absence of the material in the final product is confirmed.

So the perfect instrument for this application is ICP-MS. The sub ppb detection limits will ensure that any catalyst present in the final product can be easily detected, also the multi-element capabilities with low detection limits means that the instrument can be used for any further elemental analysis that may be required in the future.

Supplement manufacturer

Imagine a company producing a multi-mineral supplement containing calcium, magnesium, iron and phosphorus, which will want to monitor the levels of the minerals in the final product to ensure that the label claims are being adhered to.

AAS might appear to be a good analysis instrument for calcium, magnesium and iron as the levels are chosen to be high and therefore in a suitable range for detection. However, as there are three elements, this could prove quite time consuming as each element will need to be analysed separately. Phosphorus by AAS is not possible so another method of analysis would need to be used.

ICP-MS could be used to analyse for all these elements, but as they will be present in high levels then a large dilution of the samples will need to be carried out. There is also the consideration of interferences as both calcium and iron are known to have significant interference when analysed by ICP-MS. As the instrument will not be required for low level analysis and the high initial cost, this is not the most ideal instrument.

In this case, ICP-OES would be the best choice. Simultaneous analysis means that all the required elements can be analysed quickly, the levels of detection are appropriate, the initial cost is less than that of ICP-MS but will still have similar running costs. The instrument can also be left to run unsupervised so efficient 24/7 use can be made.

Elemental analysis is a very broad field and this is only a very simplified look at the main options available. The market is constantly changing and new developments appear on a monthly basis. Each instrument manufacturer has its own refinements to the instruments mentioned above to improve sampling times or remove interference.

There are also a wide range of other instruments available that aren't discussed above, such as instruments specifically designed for the analysis of mercury or arsenic, x-ray microfluorescence (XRF), spark emission and laser ablation techniques, which allow the analysis of solid samples without the need to prepare the samples into an analysis solution. There have also been developments in the use of nitrogen gas plasmas that eliminate the need for argon and vastly reduce running costs.

Of course, the other alternative to buying an instrument is to contract out analysis to a specialist, accredited and experienced laboratory. In many cases this can also be an economic way of satisfying analysis needs without the high set-up and maintenance costs associated with the latest technology.

Author: Alan Cross, Metals Lab, RSSL enquiries@rssl.com