1. How do you prepare and store samples to be sent for analysis by ICP?
You should consult the literature in your field of work for specific sample preservation and storage conditions. With that, the following rules usually apply. Soil samples are air-dried and ground to <2 mm. Composts and sewage sludges are similarly air-dried and ground to minimal size. Plant material is dried at 65-70°C and ground to fine powder. Water samples are stored at +4°C and should usually be acidified by nitric acid to pH<2 as soon as possible. Biological samples are to be kept at -20° to -80°C.
When you sample and store liquids or solids for ICP analysis please avoid metal containers (including aluminum foil!), in order to prevent sample contamination. Also note that liquid samples kept in glass vessels can become contaminated by B, by Si, and at low pH by other elements also. Try to use plastic containers only.
2. How much sample is needed for sample- preparation and analysis by ICP ?
About five ml of the tested solution is consumed by the ICP in a typical measurement. Water samples and dilute aqueous solutions are usually analyzed as received, i.e. without further dilutions. With that, we ask for 10-25 ml volumes of such samples to facilitate repetitions and dilutions.
Practically all other liquid or solid samples have to be digested in preparation for analysis. A typical protocol involves digestion of the sample in strong mineral acids using a microwave assisted system. The following table lists the minimal amount required for different sample types. The final volume of the resultant solution is typically made up with water to 25-50 ml.
| Sample Type | Minimal Amount |
|---|---|
| Water and clear dilute aqueous solutions. | 10-25 ml |
| Aqueous solutions with high salt or organic matter content. | 5-10 ml |
| Metals and alloys | 0.1-1 g |
| Oils and non-aqueous liquids. | 20g. |
| Plant material, Composts, Sludges | 500 mg - dry weight |
| Biological tissue, blood, feces. | 1g. - fresh weight |
3. Which sample types are difficult to prepare for analysis by ICP?
Samples have to be completely mineralized and solubilized to be amenable for analysis by ICP. Mixtures of nitric acid, hydrochloric acid and hydrogen peroxide are usually used successfully for digestion of most samples, but some matrices are hard to digest.
Milk and milk-based drinks which contain casein require a special Kjeldahl-type digestion with H2SO4. Hydrofluoric acid (HF) can be used to dissolve tenacious siliceous matrices, but we don't use it in our system as even traces of HF can damage the quartz spray chamber and torch. HF might also contaminate the sample with elements such as B, K, and Si which leach out from the glassware. HF may be neutralized by the addition of CaCO3.
Mineral and vegetable oils are also hard to digest. We usually dilute such samples with kerosene and analyze them against organo-metallic standards in kerosene, without further sample preparation. Many organic solvents present another problem being incompatible with the strong mineral acids used for digestion. In some cases the organic phase can be is evaporated and the residue dissolved in acid.
Please note that the special methods mentioned above usually involve increased matrix effects and background levels for many of the tested elements.
4. How do you submit samples for analysis by ICP?
Please download the Sample Submission Form as a pdf file or a Microsoft-Word file and fill it out. Consult the instructions page as necessary. Bring the samples and form to our offices. For additional information call Vasiliy at 054-88-20558, or 08-94-89975.