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Measuring cobalt and chromium ions

Chromium and cobalt and other metals present in surgical implants are usually measured by inductively coupled plasma mass spectrometry (ICPMS) using either quadruple (QICPMS) or high resolution mass spectrometry (HR-ICPMS). Both are capable of accurate analysis, but only HR-ICPMS instruments will allow the measurement of some other metal ions such as titanium and nickel. Electro thermal atomisation atomic absorption spectrometry may also be used, but is less common now in the leading trace element analysis laboratories.

Blood samples for trace element analysis must be collected in trace element free tubes. Tubes are available with either EDTA anticoagulant for the analysis of whole blood samples or with no additive for the analysis of serum samples. There is a small difference in results obtained from whole blood and serum, but both can be used to assess release of laboratory without the need for separation of serum, a step which may allow potential for sample contamination. Some laboratories may advise against the use of stainless steel needles for sample collection, but the amount of contamination introduced via this route is usually low relative to the amount of chromium and cobalt released from high wear joints.

Synovial fluid samples should be collected into the same blood collection tubes or into sterile plastic ‘universal’ containers. Urine samples should be random collections voided directly into a plastic universal container, although in rare circumstances a timed 24-hour collection may be appropriate. In this case the laboratory should be contacted for advice before sample collection is commenced. In all circumstances glass and metal-containing containers must be avoided.

Trace element assays are available from the Supra-Regional Trace Element laboratories. In all cases the samples must be referred to the analytical laboratory via the local clinical biochemistry laboratory. Most laboratories will be unable to accept referrals from individual surgeons. All laboratories use QICPMS. HR-ICPMS is also available at London (Imperial College).

All laboratories participate in the national QC programme TEQAS, run from the School of Molecular and Biomedical Sciences, University of Surrey. This includes assessment of chromium and cobalt. Most laboratories will also participate in other international EQA schemes.

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Methods approved by groups such as the Association of
Official Analytical Chemists (AOAC) and the American Public Health Association (APHA).

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Inductively coupled plasma mass spectrometry (ICP-MS)

is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 1012 (part per trillion). It is based on coupling together an inductively coupled plasma as a method of producing ions (ionization) with a mass spectrometer as a method of separating and detecting the ions.

In trace elemental analysis, the method has advantages of high speed, precision and sensitivity compared to atomic absorption techniques. Analysis of lower concentrations at the same time is more prone to disruption by trace contaminants in labware and reagents used. Specific analytes suffer from interferences exclusive to ICP-MS technique. Verification of analysis results requires additional effort.

The variety of applications exceeds that of ICP-OES and includes isotopic speciation