Accurate Determination of the Chemical Composition of Nickel-Titanium Binary Alloys by NIST High Performance Inductively Coupled Plasma – Optical Emission Spectroscopy Method

Tight control of the atomic ratio in Ni-Ti binary alloys, also known as Nitinol, where the two elements are present in roughly equal atomic ratios, is essential for their quality control in medical devices. For NiTi alloys with ≥ 55.0wt% Ni, 1% deviation in absolute nickel and/or titanium content could lead to a change in the transformation temperature of ~100 °C. Thus, the elemental composition of Nitinol has to be determined exceptionally accurately. This is not a trivial task by common analytical techniques, such as XRF, EDX, flame atomic absorption or classical ICP-OES, which are known to carry analytical errors above the needed ≤1% level.

In this study we are demonstrating that with the NIST high performance ICP-OES method the composition of Nitinol can be determined more precisely and accurately than the above analytical techniques. First, the main sources of measurement uncertainty were examined and the method parameters were tightened accordingly. The following factors were visited: reference standards of Ni and Ti, sampling amounts and drifting in quantification, which is based on signal ratio of sample to matrix-matching standard. With optimization of these measures, we could achieve extended measurement uncertainty of 0.2 - 0.3% for analyzing Nitinol samples.