Metal alloys are commonly analyzed using solid sampling techniques such as Arc/spark optical emission spectroscopy (A/S OES), X-ray Fluorescence Spectroscopy (both Energy Dispersive (ED-XRF) and Wavelength Dispersive (WD-XRF)), and X-ray Diffraction (XRD). These techniques provide fast analysis with little or no sample preparation required, and can be provided as portable units, and in the case of XRF, hand-held units, for maximum flexibility. These techniques also are considered non-destructive in that the metal alloy is left largely unchanged after analysis, and is available for further testing or archiving.
However, there can be limitations to these techniques, both in the terms of the total number of elements analyzed and in the sensitivity of the technique. The number of elements that are analyzed can be dictated by the spectrometer layout or by the standards available for calibration. Sensitivity in these techniques is commonly in the low ppm range.
ICP-OES or ICP-MS can provide expanded element coverage and sensitivity ranges several orders of magnitude lower than XRF or A/S OES. Synthetic standards can be easily prepared using aqueous multi-element mixes and allow the analysis of 70+ elements. However, both techniques require that the sample be presented as an aqueous solution. For metal alloys, this means that the sample must be digested completely prior to analysis. The following examples provide an overview of useful approaches for total dissolution.
The acid digestion of metal alloy samples can be performed using conventional hot-plate techniques or commercially available microwave digestion apparatus (NOTE: Use only microwave units specifically designed for laboratory use; kitchen microwave units are not suitable or safe for laboratory use and should never be utilized.) Always follow the specific instructions provided by the microwave digestion manufacturer. A third option is a new ColdBlock technology that uses shortwave IR lamps. You can read more about this in one of our other blog posts , The Evaluation of a New Sample Preparation Technique for Metal Alloys. Many manufacturers also provide application notes for various sample types, and may include metal alloys. For that reason, this discussion will focus primarily on conventional hot-plate digestion approaches, but these can also be applicable for microwave digestions.
Optimum sample sizes are typically 0.5-1.0 grams, and the sample should represent a homogeneous sub-sample of the metal alloy being sampled. Samples larger than 1g are more difficult to digest and require more time. Samples less than 0.5g may not be a representative sample of the alloy, particularly if there is any element segregation possible in the alloy. Digestions should be performed in acid-resistant labware, such as Teflon beakers. The use of glass is not recommended, as elements such as B or Si can leach during the digestion procedure and cause contamination of the sample.
Hot Block Digestion
In all digestions, it is very important to incorporate an appropriate QC sample, using the exact same sample digestion technique to prepare it as is used for the unknown samples. The QC sample should be a well characterized or certified reference material of the same type and general composition as the unknown samples, with as many of the analyte elements certified as possible. This will help to verify the suitability of the chosen digestion procedure for each analyte. Poor recoveries on the QC analytes can provide information as to possible incomplete dissolution or volatilization of those elements in the sample alloy. Refinements to the acid mix, heating, and pressure protocol can then be made to prevent such losses. In some circumstances, it may be necessary to prepare two different acid digestions of the sample to provide complete recovery of all analytes. Since each alloy type can have significant variations, these general approaches are guidelines – our laboratory chemists have many years of experience and can provide further customized solutions for successful digestion procedures for your specific alloy type.
In summary, ICP-OES and ICP-MS can provide significant advantages to conventional solid-sampling techniques. Although the sample preparation step is more time-consuming and requires the use of concentrated acids, the analysis can provide much improved sensitivity and a wider range of analytes. Samples characterized in this fashion can then be used as appropriate calibration or QC samples in a solid sampling based analysis procedure.