DETERMINATION OF TRACE LEVEL IMPURITIES IN HIGH PURITY METALS USING MICROWAVE DIGESTION FOLLOWED BY SEQUENTIAL ICP-OES WITH AXIAL VIEWING

Introduction:

The determination of trace impurities in high purity metals is essential in order to control quality and to improve manufacturing technology. Trace impurities can have major effects on the properties of the finished products, and in many cases it is desirable to minimize, or at least to be able to control, the levels of certain trace impurities.

A variety of methods have been used for the analysis of high-purity metals. Flame atomic absorption spectrometry (FAAS) has been used for the analysis of Ultra pure metals but the applicability of this technique is often limited by inadequate detection limits. Further, AAS cannot be used for determination of certain nonmetals, like sulfur; because the most sensitive lines of such elements often lie in the vacuum ultraviolet region. Graphite furnace AAS (GFAAS) can be used for measurement of lower levels of trace elements, but suffers the additional limitation, like certain highly refractory elements like B and W are difficult or impossible to measure, because of chemical reactions with graphite. Another approach to improving the detection limit is hydride generation prior to introduction to the AA or AE spectrometer. Non-spectral interferences by transition and noble metals on the generation of hydrides are, however, serious, and require pretreatment of the sample to remove the Cu matrix, if hydride generation is to be used.

In this study, a sequential Inductively Coupled Plasma Optical Emission spectrometer (ICP-OES) have been used for determination of ultra trace level impurities in such highly pure metal like Aluminium, Zinc, Lead, Tin, Copper and Nickel. The plasma can be viewed either side-on (radially) or end-on (axially) in this equipment. The technique is usually more sensitive than flame AAS when axially viewed plasma is applied on the ultrapure metals. The detection limits can be improved still further with axial viewing of the plasma. The improvement factor depends on the element and wavelength, but the average improvement factor with axial viewing is 2–3 times, when the same sample introduction system is used. In each case, the sample preparation was carried out in a Microwave Digester which provided an efficient and clean sample preparation for multi-element analytical techniques in ICP-OES. This method is the most versatile and has been well proven. It allows variations in reagents and methodology, making it ideal for a variety of matrices and elements. The other advantages of Microwave digestion is its good reproducibility in digestion, improved QA/QC process, less dependency on operator skill factor and reduced sample preparation time.  

Experimental:

Sample Preparation

Figure 1 shows the appearance of the metal samples for which the trace impurities were determined. The foil samples were directly digested in appropriate acids after cleaning them in an ultrasonic bath using isopropyl alcohol followed by drying whereas the bar samples were drilled to extract foils out of them. The digestion of the metal samples (from their chip form) have been carried out in the microwave digester (Make: Anton Paar, Mode: Multiwave 3000) using the typical inorganic acids such as Hydrochloric, Nitric and Hydrofluoric where ever applicable. The pressure used during the digestion process is as high as 25bar and the power is kept typically at 100 Watt. Total time spent for such individual sample digestion is around 40 minutes. Figure 1 shows the digestion profile of these samples in Microwave Digester. Very clear solutions have been obtained from such digestion process without any sedimentation at the bottom. Samples were prepared from the certified reference materials BCS 194e (Zn), BCS 210e (Pb), BCS RM 192h (Sn), Merck GR Aluminium Foil (1.01057.0250), Sigma Aldrich Cu Foil (266744), and Sigma Aldrich Nickel Foil (357953). Distilled deionized (18 Mohm) water was used for all dilutions.

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Instrumental:

To ensure the best resolution available, an ICP-OES (Make: Perkin Elmer, Optima 7000 DV) was used for this study. Calibration was carried out by matrix matching the standards to the samples and was dissolved in similar acid medium by the same procedure as used for the samples. In all the cases, the correlation co-efficient values have been found to be greater that 0.99- revealing a true linear curve for all the parameters. The characteristics of the Instrument and of the operating conditions are given in Table 1 below:

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Results & Discussions:

A comparison of measured and certified values of the trace elements measured in each high-purity metal is given in Table 2-9. As can be seen, the agreement is quite satisfactory for all elements. In most of the cases, the standard deviation is quite low and the % recovery is within 90-115.

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However, in order to judge the actual level of competency in this field, the laboratory participated in a Round Robin Program co-ordinate by a reputed International Agency for a High purity Lead Samples. The results are found to be extremely close to the other participating Labs for all the parameters in terms of accuracy and repeatability. Table 8 shows the comparison of result for this Lead Samples among MSK and other recognized laboratories in this field.

Table 8: Performance of MSK lab in the International Round Robin Program for a High Purity Lead Sample

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Conclusion:

The analysis of pure metals for eight trace impurities has been carried out with a sequential ICP emission spectrometer and axial viewing of the plasma. A method based on matrix matching of standards was validated by comparison with results of the analysis of certified reference high-purity materials followed by participation in an International Round Robin Test.

The work has been jointly carried out by Dr. Saswati Ghosh. MSK Central Laboratory, Kolkata
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