In this edition of the blog, we will continue to examine various methods of sample preparation.
Samples containing a high percentage of organic matter (biologicals and foodstuffs, for example) can be prepared by a procedure known as "dry ashing" With this technique, an amount of sample is heated in a crucible over a flame or in a muffle furnace. The use of a muffle furnace is preferred as it permits greater control of the temperature. The sample residue is then dissolved, and the analysis is performed.
The strengths of this method are similar to those of Wet Digestion:
- Dry ashing usually requires relatively simple apparatus - a source of heat and crucibles. For some samples, more sophisticated equipment may be required, however.
- Only a minimum amount of working time is required making this method suitable for large numbers of samples. The ashing can procedure with little or no attention.
- Since additional reagents are not always added, preparation blanks are generally very good. In some instances, the addition of "ashing aids" is required to prevent loss of analyte or speed up the ashing time. Blank values can then become a concern.
- Large amounts of sample can be prepared. (Some procedures can call for as much as 0.5 - 1.0 kg) This is particularly helpful when working at the trace level.
Analysts using the dry ashing technique must contend with a number of fairly severe disadvantages. It may be the simplest method, but it is also the most unreliable.
- The potential for losing volatile species is quite real and thus limits the maximum temperature at which the ashing can occur. The limited upper temperature may result in the incomplete ashing of some sample types. Generally, ashing temperatures are usually around 550 0 The list of elements that can be lost at temperature of about 600 oC is fairly lengthy.
- Reaction with the crucible material can occur depending upon the sample matrix.
- The ashing process may take a long time. Efforts to speed up the ashing may introduce contaminants, through ashing aids, or expense and complexity through addition of flowing gas streams.
Many samples are extremely resistant to attack by acids. These include geological, metallurgical, glass and ceramic. These types of samples are put into solution by a technique known as "fusion". This consists of mixing the sample with a "fluxing agent" The mixture is heated to a temperature that is above the melting point of the fluxing material. Once the sample has been completely dissolved, the melt is cooled and then is dissolved in an acid solution. For the fusion method to be used, the sample must contain chemically bound oxygen, e.g., oxides, carbonates and silicates. Depending on the type of sample to be fused, various fluxing agents are available.
The points in favor of utilizing fusions:
- Samples can be fused with very simple equipment.
- Can prepare samples that could not easily be done by any of the above methods.
- Fusions can take from several minutes to several hours.
Fusions are generally used only when absolutely necessary. Though it is a powerful technique, it possesses some serious drawbacks.
- The high temperatures necessary to reach the melting points of the various fluxes used in fusions may result in the loss of volatile components.
- It is not always obvious when the fusion is complete. The fusion should be as short as possible to minimize the attack on the crucible and to avoid decomposing the flux material.
- Large quantities of flux material are required for fusion, generally 10 times the sample weight. Contamination from the flux material is a potential problem.
- The crucible in which the fusion is performed is inevitably attacked, resulting in contamination of the sample.
- Because of the amount of flux required, the resulting solution will have a high level of dissolved solids, which can present problems for analysis by ICP.
- Manual preparation of large numbers of samples can be extremely slow. Automated systems can substantially increase sample preparation throughput.
In this technique, samples in a closed vessel are subjected to a microwave field. This field heats the sample in a much more uniform manner than a hot plate, for example, which heats by time consuming conduction. Consequently, in the microwave, the sample reaches the boiling point much more rapidly. The closed vessel allows higher temperatures and pressures to be attained. These combine to allow most samples to be digested in considerably less time than by the traditional methods.
The application of microwave technology to sample preparation for atomic spectroscopy represents a significant advance over the traditional methods. Microwave technology provides a number of advantages:
- Sample preparation times can be reduced by up to 90%
- The closed vessels reduce the loss of volatile species and provide better recoveries. The closed vessels also reduce the chance for sample contamination.
- Microwaves permit precise and accurate control of pressure and temperature which result in reproducible digestion conditions and better long term precisions.
- Microwave digestion requires less acid than traditional techniques resulting in superior blanks. Digested sample will also have a consistent acid matrix.
- Samples can be digested without constant supervision.
Microwave digestion is a useful and popular method to prepare samples. Like all the other method discussed, however, it too has its drawbacks:
- Closed vessel systems are limited to about 1 gram of sample, which may pose a problem for trace level analysis. Open vessel systems can handle larger sample sizes (~10 grams), but these systems generally cannot prepare any many samples simultaneously as the closed vessel systems.
- Methods to digest new and unknown samples may by time consuming to develop.
- Some sample types still resist acid dissolution even with the microwave.
- Assembly and cleaning of vessels can be time consuming.
- Microwave digestion equipment represents a significant investment.
The modern analytical laboratory and chemist have a number of sample preparation options at their disposal. Some of these techniques are as old as chemistry itself and others are quite new. What experience shows us is that none of the techniques are effective on every sample type. To get our samples into solution, we may need to use a variety of techniques or even those techniques in combination.
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