ICP-MS

The ICP-MS of the GI department is a model Xseries I from Thermo Fisher Scientific.

ICP-MS (inductively coupled plasma-mass-spectrometry) is a technique to determine low-concentrations (range: ppb = parts per billion = µg/l) and ultra-low-concentrations of elements (range: ptt = parts per trillion = ng/l). Atomic elements are lead through a plasma source where they become ionized. Then, these ions are sorted on account of their mass. The advantages of the ICP-MS technique above AAS (Atomic Absorption Spectroscopy) or ICP-OES (inductively coupled plasma optical emission spectrometry) are:

• Extremely low detection limits
• A large linear range
• Possibilities to detect isotope composition of elements

The ICP-MS technique has a multi-element character and a high sample throughput, like ICP-OES, but it allows one to perform more sensitive measurements. Disadvantages and weaknesses of the ICP-MS detection are the occurrence of spectral and non-spectral interferences and the high costs.

Principle

ICP part

Like for the ICP-OES, the sample solution is introduced into the device by means of a peristaltic pump. There it becomes nebulized in a spray chamber. The resulting aerosol is injected into an argon-plasma that has a temperature of 6000-8000 K. Inside the plasma torch, solution is removed from the sample and also atomization and ionization occur. Only a small amount part of the ions produced in the plasma further penetrate to the mass-spectrometer part.

Mass-spectrometer part
This part consists of:

• An interface (in particular a “sampler cone” and a skimmer cone), in which a small amount of the free ions generated by the plasma are transmitted. During this process the ions migrate from an environment with extremely high temperature and atmospheric pressure to a compartment at room temperature an high vacuum (< 0,001 Pa)
• Electrostatic lenses that focus (positive) ions onto the entry to the true mass-spectrometer
• The true mass-spectrometer in the GI device has a quadrupole, composed of 4 metal rods which separate the ions on account of their mass by a kind of resonance principle
• An electro-multiplier (a specific type of detector) containing active surfaces, which enhances the signal from one colliding ion so that a measurable pulse is generated
• Electronics that counts and sorts the pulses and relates them to the corresponding mass. This selection can be accomplished in milliseconds, so that a complete spectrum can be acquired within one second.

Sample preparation / how to deliver samples

• Solid material should be preferably destructed with nitric acid only (keep the concentration HNO₃ below 10%, ideally around 1%), or if necessary with HNO₃/H₂O_2. This step can be performed as a closed destruction in a microwave-oven.
• Samples should be acidified (preferably with 1-5 % HNO₃ in order to keep metals in solution)
• Deliver samples in 12 – 15 ml tubes.
• The content of salts in samples should be kept below 0.2% (=2 g/l)
• HCl can cause precipitates and the sulfur in sulfuric acid interferes in the analysis.
• The torch present at the GI is not resistant to HF.
• Samples should not contain any organic solvents.
• Indicate as far a possible which approximate concentration of the elements to analyse may be expected in the samples
• The ICP-MS can be employed to measure isotope ratios of enriched experiments, but the device is not appropriate to analyze the differences in natural abundance of isotopes