I thought ICP-MS suffered too many interferences for most biological work?
Historically this was true, restricting ICP-MS usage to heavy metals (Cd, Pb etc) in biological samples, for which interferences are negligible, or very high resolution (and expensive) MS was required. However, the advent of the direct reaction cell (DRC; see below) means that interference-free ICP-MS can now be developed for most sample types and elements.
What elements may be analysed?
All of the naturally occurring elements, except for hydrogen, carbon, nitrogen, oxygen, fluorine, chlorine, helium, neon & argon may be analysed. In general, the most intense isotope will be monitored.
What is the system’s sensitivity?
This depends on the element being considered, the isotope monitored and the matrix in which it is presented. Perkin Elmer quotes detection limits as below:
B, Si, P, Br, I 1ppb
K, Ca, Se 100ppt
Other metals 10ppt or less
Typically precision and accuracy are within a few % although this figure increases as you approach the limits of detection for an element (see above). For isotope ratio analysis (see below), this figure falls to nearer 0.1 %.
How many samples can be run in a day?
This depends primarily on the number of elements to be measured, the techniques used and to a lesser extent on their concentration and the nature of the matrix. A typical analysis for 10 elements would take about 6 minutes per sample. The autosampler has room for 149 samples and the instrument can, if needed, be run overnight. There will also be a number of calibrant standards appropriate to your experiment to be run.
What standards and blanks do I need?
Blanks are very important. Ultrapure water should be treated in EXACTLY the same way as the sample and then analysed as an 'analytical blank'. Reagents used for digestion, dilution etc should also be checked for contamination. You may provide your own standards, in which case sample-based (i.e. spiked) standards are best. However, standards will also be prepared by the Centre. You may wish to provide a sample-like certified standard or reference material to be analysed along side your samples
How should the sample be prepared?
Organic solvents should be avoided as they damage the pump tubing. The most usual sample preparation involves an acid digestion, normally with nitric acid and subsequent dilution with water. It is essential that all containers are scrupulously clean and it is good practice to avoid glass vessels where possible. 18.2 M& water should always be used and other reagents should be of an ultra pure grade (e.g. Fluka ‘Trace Select). The sample is normally supplied to the MSF in a 15ml screw top test tube, preferably made of uncoloured plastic (e.g. Elkay 2086-500).
How much sample will I need?
The flow into the instrument is normally 0.4 ml/min, although this can be raised to 1 ml/min (max) if needed. At 0.4ml/min, the typical analysis above would consume 2.4 ml.
Are there any ‘problem’ elements?
Isobaric interferences are dealt with by the instrument’s software, which also warns of potential polyatomic interferences. Sometimes changing the isotope monitored may help (e.g. one normally monitors 44
Ca despite its low abundance, as 40
Ca is obscured by the massive 40
Ar signal from the plasma). Those elements which are found in normal laboratory background (e.g. sodium, potassium, silicon & aluminium) are very difficult to quantitate, especially at low levels. The instrument incorporates a ‘Dynamic Reaction Cell’ which may sometimes be used to separate elements from artefacts (e.g. 56
Fe from 40
) by reaction with ammonia.
Can I measure radioactive species?
Isotope and isotope ratio analysis is common by ICP/MS. but, the MSF is not equipped to handle radioisotopes.
Is HPLC/ICP/MS possible?
Yes, there is a HPLC system associated with the ICP instrument. Solvents should be aqueous; buffers are acceptable, but should be chosen so as not to interfere with the elements of interest and the maximum flow rate is 1ml/min. It is advisable to contact the MSF at an early stage in your method development to ensure compatibility with the instrument. Both samples and standards should be in normal 2 ml vials with septum caps.
Can any technician/student use the instrument?
Generally not. These are expensive and delicate instruments. However, where an individual will have a heavy analytical load and may wish to learn the technique as part of their training, then this can be negotiated with the Centre. However, it will not significantly alter the costs.