Samples, contamination cutting down

Speed. For applications where a great number of analyses must be performed on a routine basis, speed is perhaps the most important criterion for a system. Analytical procedures which often take more time than the actual analysis itself include such sample pretreatment procedures as dissolution and digestion. If these pretreatment steps could be kept to a minimum, analysis time would be cut down considerably. Also, mimimum pretreatment reduces the chances of sample contamination by the reagents used, or the loss of volatile elements during pretreatment. Thus the ideal system would require no sample pretreatment at all real systems should strive for minimum sample pretreatment. 

If highly resistant items (e.g. metal bars, rubbers or plastics) are to be included in the sample, they require preparation by a more appropriate method. Metals may be carefully cut down to <3 mm using a water-cooled saw or metal shear followed by cleaning to remove any contaminating swarf. Milling using... 

It is important to emphasize that whatever containers, vessels, beakers, volumetric ware, or equipment is used to prepare the sample for ICP-MS analysis, it is absolutely critical that when not in use, they be soaked and washed in a dilute acid (1-2% HNO3 is typical). In addition, if they are not being used for extended periods, they should be stored with dilute acid in them. Wherever possible, disposable equipment such as autosampler tubes and pipette tips should be used and then thrown away after use, to cut down on contamination. 

The expertise of the analyst who actually prepares the samples and carries out the analysis can be a major factor in getting the right result by ICP-MS. Even if aU precautions have been taken to cut down on contamination, if the analyst is not experienced in working with ICP-MS and does not nnderstand all the potential pit-falls, the analysis could be doomed. For example, analysts have to be aware of all the potential contaminants that are generated by their own bodies or the clothes or jewelry they are wearing. Table 15.3 shows some common trace elements found on the hnman body. It is by no means an exhaustive list, but at least it gives you an idea of the problem. 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

---