Trace metal analysis by mass spectrometry

Trace metal summary for the general public (see the last paragraph).

For trace metals analysis, we employ Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to allow us to determine the precise concentrations of metals in biofluid or tissue samples.

Inorganic elements are extracted by digestion in high acid concentrations. Samples are diluted and vaporised in the inductively coupled plasma before measurement of their mass and abundance by mass spectrometry.   We employ a 25 element standard to generate calibration curves (R>0.99) for each analyte, and which is used for quality control throughout the run. Of these elements, up to 16 are biologically relevant and can be measured reliably (dependent on the matrix) in concentrations from 1 μmol/kg to 100 mmol/kg. These elements are Sodium, Magnesium, Potassium, Calcium, Chromium, Manganese, Iron, Cobalt, Nickel, Copper, Zinc, Arsenic, Selenium, Molybdenum, Cadmium and Lead, and include their various isotopes. We have validated our protocol by analysis of certified standard reference materials – our method resulted in values within 4% of the NIST SRM 1950 standard which is certified for seven elements.

Careful and exacting sample collection and preparation are critical for ICP-MS as everything is measured in the solution. For example, it is recommended that you don’t use ‘Head and Shoulders’ while working with samples for ICP-MS, especially if you are interested in selenium, as levels are exceedingly high! Where possible, trace metal-free certified materials should be used for sample collection and storage. Frequently we realise this is not practicable – in which case it is important to analyse the consumables that have been used to assess potential levels of contaminating trace metals. Below is a table where a range of standard consumables have been tested for trace metal levels. For tubes/vials/tips, ICP-MS digestion solution was added for ~30s, removed and then measured immediately. For gloves, a 1 cm piece from a fingertip was washed in the solution for 30s (so won’t reflect normal laboratory usage). Concentrations are μg/L.

Laboratory consumables 23Na [He] 63Cu [He] 66Zn [He]
1.5 ml microcentrifuge tube 48.4 0.2 17.8
2.0 ml microcentrifuge tube 54.0 0.0 38.9
15 ml centrifuge tube 39.3 2.1 11.0
50 ml centrifuge tube 6.6 -0.3 2.7
2 ml cryovial 59.0 2.5 9.0
Glass vial 1441.0 1.9 108.1
LC-MS grade water 5.1 -0.5 -1.2
Blue nitrile glove 2978.2 0.3 2598.0
Orange nitrile glove 221.8 0.3 48.1
Purple nitrile glove 217.3 1.6 761.2
Red neoprene nitrile glove 1540.3 0.6 6613.3
1000 μl blue pipette tip 36.9 2.2 12.2

 


In the illustrative example shown below, ten elements were compared between control and dried-defatted livers from ob/ob mice and non-obese controls. Copper showed the biggest difference with three other metals also showing significant differences (Na, Mn and Se) [1].

Copper concentrations in hepatic tissue dried to constant weight and defatted from ob/ob (n=10) and C57BL/6 control (n=6) mice.

[1] Church SJ, Begley P, et al. ‘Deficient copper concentrations in dried-defatted hepatic tissue from ob/ob mice: A potential model for study of defective copper regulation in metabolic liver disease.’ Biochemical and Biophysical Research Communications 2015 460:549-554 [http://www.ncbi.nlm.nih.gov/pubmed/?term= 25797622]

Instrument

Agilent 7700x Inductively Coupled Plasma-Mass Spectrometer

Data processing software

MassHunter for ICP-MS raw data processing (Agilent)