• Introduction
• Common Questions and Answers
• Dr. Ding Yung Liu

The inductively coupled plasma mass spectrometer (ICP-MS) was purchased as a result of a Joint Research Equipment Initiative award from the Medical Research Council in 2000 (MRC grant number G0000618; ID 53061). The principal applicant was Dr Jonathan Powell from the Department of Nutrition, School of Health and Life Sciences. Dr Powell, and co-applicants from the iron metabolism IRG and from GKT, co-ordinated their grant application through the MS Steering Committee. The instrument is housed in a customised 'clean air' laboratory within the MS Facility.

ICP-MS spectrometry is a rapid, sensitive way of measuring the elemental concentrations of solutions. More than 75 elements can be determined, most of them at detection limits less than 1 part per billion.

The first step in the procedure is conversion of the molecules in the sample to individual atoms and ions using a high temperature radio frequency induced argon plasma. The sample is introduced into the plasma as a solution. Sample is pumped using a peristaltic pump to a nebulizer, where it is converted to a fine spray and mixed with argon in a spray chamber. The purpose of the spray chamber is to make sure that only droplets in a narrow size range make it through into the plasma. Most of the sample drains away from the chamber, the rest is carried into the plasma and instantly excited by the high temperatures (5000-10,000 K). Atoms become ionised with 99% efficiency (arsenic and selenium are a couple of exceptions, ionising only at 52% and 33%). Either ICP-optical emission spectrometry (ICP-OES) or ICP mass spectrometry (ICP-MS) can be used to analyse samples. ICP-OES utilises UV and visible spectrometry to image the plasma at the exact wavelength of ionic excitation of the element of interest. This is a well-established technique. ICP-MS is a somewhat newer tool in the biological sciences and is described in detail below.

Singly charged positive ions predominate - an important state of affairs as the mass spectrometer can only measure mass to charge ratio - a doubly charged cation has the same mass charge ratio as a singly charged cation weighing half as much, so the two are indistinguishable. After the plasma the ions enter an ion transport system. This is designed to remove the argon and focus the ion beam. A nickel/platinum cone with a very fine orifice in the centre lets a sample of the ion beam pass through. The ions are now in a low-pressure region where they are accelerated beyond the speed of sound (the zone of silence) before passing through another, similar cone into a vacuum. Photons and anything non-ionic within the beam are excluded here, and the ions are focused into the mass spectrometer.

The Perkin Elmer Elan mass spectrometer has a quadruple mass filter: altering a combination of RF and DC voltages in four rods, positioned around the ion beam, alters the combined electric field, allowing only ions of a certain mass charge ratio to pass directly through for a given RF/DC ratio. Ions with different m/z ratios will follow unstable trajectories. The instrument can switch from one voltage to the next in microseconds so analysis times are often very short (a few milliseconds).

Finally the ions reach the detector. In its most sensitive mode (pulse counting) the detector measures individual ions ('counts'). If the number of ions is too high for the pulse counting (above 2000 ppb, depending on the element) the detector automatically switches to analogue mode, dampening the sensitivity by a factor of 10,000 to protect the detector. Even so, the detector has a finite life (1027 counts).

Some instruments, like Elan DRC, have a 'reaction cell' between the ion transport system and the mass spectrometer. A low molecular weight gas and another quadrupole are inside the cell. Interfering molecular ions (for example ArN+ which interferes with 54Fe) react with the gas molecules to produce ions of non-interfering m/z. These non-interfering ions are then expelled by the pre-quadrupole, working in tandem with the main quadrupole.

ICP-MS is also suited to isotope work. For example the uptake of 58Fe into blood has been measured using this technique. In clinical studies this is especially valuable as it allows the analysis of stable (non-radioactive) isotopes.

Common Questions and Answers

• What can ICP spectrometry do?
• Can all elements be analysed?
• Should I use ICP-OES or ICP-MS?
• What sample types are suitable?
• What standards and blanks do I need?
• What is the precision and accuracy?
• Can isotopes be measured?
• I thought ICP-MS suffered too many interferences for most biological work?
• Can chemical speciation data be obtained?
• How many samples can be analysed per day?
• Can any technician/student use the instrument?

What can ICP spectrometry do?
The concentration of an element in solution can be measured. Detection limits are sub-parts per billion and diluted samples (e.g. plasma or whole blood), acid digested samples (e.g. tissues) or whole fluids (e.g. tissue culture medium) can be used.

Can all elements be analysed?
No, but at least 75 can and almost all are the biologically important ones. Notable exceptions are hydrogen, carbon, nitrogen and oxygen but other techniques are suitable for these. Halides are also difficult.

Should I use ICP-OES or ICP-MS?
The Mass Spectrometry Centre will decide which is the most appropriate technique for the samples and elements of choice. As a very approximate guide, ICP-OES is better for the lower atomic weight elements and has limits of detection of a part per billion, while ICP-MS is better for the higher atomic weight elements and has limits of detection of 10 parts per trillion.

What sample types are suitable?
Any sample can be analysed providing it can be brought into solution by acid/peroxide digestion, for example. Liquids can be analysed directly, sometimes with prior dilution. Advice on this can be gained from the Mass Spectrometry Centre.

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.

What is the precision and accuracy?
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 %.

Can isotopes be measured?
Yes. Isotope and isotope ratio analysis is common by ICP-MS. However, the laboratory is not set up to handle radio-isotopes so only stable isotopes should be investigated by this technique.

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 above) means that interference-free ICP-MS can now be developed for most sample types and elements.

Can chemical speciation data be obtained?
No. The technique measures total element, so any separation of the sample for speciation will need to be carried out prior to analysis (e.g. HPLC separation, ultrafiltration etc).

How many samples can be analysed per day?
First a method has to be developed suitable for your element(s) and sample type(s) and this can take up to a day or occasionally longer. Thereafter, a conservative estimate of 80 samples/day can be analysed.

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.

Dr. Ding Yung Lui

Dr. Ding Yung Lui is a Senior Research fellow in the Pharmacy department, who organises the management of the ICP Suite. For access to this instrument, please contact Dr. Yung Ding Liu 020 7848 4844.

Document compiled by Dr Jonathan Powell, Dept of Nutrition and Mr Mark Sykes (Analytical Chemist) - January 2002