Inductively Coupled Plasma Mass Spectrometry (ICP‑MS)

Overview

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is used in sample analysis to determine which elements are present and their concentrations.

ICP-MS is applied in a variety of industries such as:

  • Environmental
  • Pharmaceutical
  • Geochemical
  • Forensics

These may be for ensuring guidelines are adhered to in terms of purity of final products, sufficient treatment of waste product, or for exploration of biological matrices for example.

The greatest trait of the instrument is the degree of sensitivity it can offer, with the ability to measure some elements at part-per-billion (ppb) level. This measurement is defined as being one part in one billion units, in relative terms that is equivalent to 1mL of water in a 1000L swimming pool or 1 second in 31.7 years.

The instrument used for this analysis subjects the sample to a multi-stage process, the sample requires: introduction to the system, ionisation, ion-separation and detection.

The plasma's function is to ionise the sample and the mass spectrometer manipulates and quantifies the ions generated.

Plasma Ionisation

The method of sample introduction depends upon the state of the sample. ICP-MS is used for composition determination of solid and liquid samples, however only a minute amount of sample is sufficient for analysis. Prior to ionisation, solid samples are introduced using laser ablation; the focusing of a laser beam on the sample surface to generate fine particles for analysis whereas liquid samples undergo a nebulisation process.

The ionisation of an atom results in a positively charged ion (cation), this occurs when sufficient energy (ionisation energy) is applied to remove the valence (outer) electron from its orbital as shown in the following image.

An ionisation diagram, showing electron ejection on energy introduction.

The plasma is generated by applying an oscillating radiofrequency to a constant flow of argon gas, the argon atoms experience a molecular friction as a large quantity of energy results in the ionisation of the argon. Atoms that are then subjected to this high energy plasma are readily ionised in typical circumstances. This ionisation enables the manipulation of the charged particles for differentiation and selective quantification stages.

Analysis Stages

The block-diagram shows an overview of the complete analysis process, including the interface and mass spectrometer sections not covered previously.

A block diagram of the ICP-MS instrument.

The interface in this instrument is composed of a "sampler cone" and "skimmer cone", these act as a selective siphon, focusing the cations that are generated at the plasma stage to the mass spectrometer. The interface enables the transition from a high temperature, atmospheric pressure (plasma/ionisation stage) to a lower temperature, high vacuum environment (mass spectrometer).

The mass spectrometer is used to determine both the identity and subsequent concentration of ions by stages of differentiation followed by quantification.

The separation of ions is typically by use of a "quadrupole", an array of 4 electromagnetic rods that can deflect the ions in a curve towards the detector by varying the strength of the electric field. The separation is based on the mass:charge ratio of the ions, the degree of deflection varies between ions by both their mass (lighter being deflected more readily) and their charge (greater ionised resulting in greater deflection). In this method the mixed stream of ions can be separated into known ions by degree of deflection and then quantified by the amount of ions in the now filtered stream.

The quantification stage occurs at the detector. As an ion impacts the detector plate an electron is transferred from the metal to the ion, this results in a transfer of electrons from a connected wire to the detector plate. The flow of electrons through the wire is measured as an electric current, with a greater current generated when more electrons flow to the metal plate (a greater number of ions impacting the detector plate). By use of calibration standards of known concentrations it can be determined what amount of ions impacting the plate (measured by current) corresponds to the concentration of ions in the original sample.

Maintenance

The ICP-MS instrument requires significant maintenance in the form of upkeep and replacement of components such as the plasma torch, interface cones and moving parts required for the uptake of samples. These components are specialist items that are sold at prices reflective of such.

A constant flow of argon gas and a vacuum are required for instrument operation. The operator must be strategic in order to maximise value from the instrument, argon gas is relatively expensive and electricity is required to keep the pump, instrument and controlling computer running. It is important to consider the value of the information obtained each time the instrument is ran to ensure the instrument is being used optimally.

For optimal performance, analytical-grade hydrochloric and nitric acids are required as the carrier solutions. Standard purity acids are not suitable due to the sensitivity of the instrument, with any contaminants capable of affecting the sample results. Analytical-grade reagents are expensive but a necessary requirement for operation, their use should be factored into the cost of running the instrument.

If the instrument is not functioning properly and repair is outside of the skillset of the standard operator then an instrument engineer may be required, this is also true in the case of periodic servicing to ensure correct function.

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