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HDXRF vs ICP for Plated Metals

Is your sample plated?

ABSTRACT

When measuring plated metal samples it is not uncommon for results from inductively coupled plasma (ICP) analysis to be different than results from HDXRF analysis. This application note addresses why the results may differ between ICP and HDXRF and how to identify whether a sample is plated. It also provides a method for minimizing the variation in results for plated metal samples.

WHY ICP AND HDXRF RESULTS MAY DIFFER

Figure 1 is a representation of a plated metal cut to expose the bulk alloy. Plated metal samples have at least two components: the bulk metal alloy and the plating material. There may be more than two components if the bulk alloy has multiple platings. Plating material is usually very thin when compared with bulk material.

Regardless of whether the sample is plated or homogenous, the ICP process for measuring the sample remains the same. A portion of the sample is cut and then ground to a powder as depicted in Figure 2. The powder is then digested in an acid solution and tested with an ICP analyzer. Due to the high mass ratio of bulk to plated material, the ICP result is heavily biased by the elemental composition of the bulk material.

HDXRF, on the other hand, is a surface-biased analysis technique as the tip of the HDXRF analyzer comes in direct contact with the surface of the sample, as shown in Figure 3. As a result, when measuring homogenous metal samples, analysis results for ICP and HDXRF will be very similar. However, when analyzing plated metal samples the HDXRF result will be more representative of the plating material, and the ICP result will be more representative of the bulk material.

Figure 1
Figure 1. A plated metal sample cut to expose the bulk alloy represented in blue. The plated alloy is represented in orange.

Figure 2
Figure 2. ICP analysis preparation requires a representative piece of the entire sample to be ground to a fine powder, causing the bulk material to predominate the elemental composition.

Figure 3
Figure 3. Analysis by HDXRF is surfacebiased and will yield an elemental concentration biased by the plating composition.
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HOW TO IDENTIFY A PLATED SAMPLE

Determining whether a sample is plated can be done quickly and easily with HDXRF. First, use the HDXRF analyzer to take a quick measurement of the surface of the metal sample. Next, sand, scrape, or cut the sample to expose the bulk material and then take a measurement of the bulk material. If the sample is homogenous, the results of the elemental composition analysis for both measurements will be the same. If the sample is plated, the results for the two measurements will be different. 
Figure 5

MINIMIZING THE VARIATION IN RESULTS FOR PLATED METAL SAMPLES

When comparing HDXRF and ICP results, it is important to measure the same individual sample with both techniques. This is because differences in elemental composition may exist among samples that look identical. Using one sample will eliminate this potential variation. Additionally, if the sample is plated and must be ground to a powder for ICP analysis, the metal powder can also be measured using HDXRF to provide a result similar to ICP.

GET THE LEAD OUT!

Methods currently specified for quantifying lead in plated metals advise grinding the sample as a whole and using the resulting concentration as the certifiable result. However, there are situations where the plating material may have a high lead concentration (over 100ppm lead) yet the substrate may contain minimal or no lead. If the plating is very thin, ICP may report lead as Not Detected, or ND, due to its bulk-material bias. This presents a major pitfall for those attempting to produce lead-free products. HDXRF can be a valuable tool in addressing this concern by qualitatively determining where areas of lead concentration exist in plated metal samples.

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