Inductively coupled plasma (ICP) instruments rely on a argon plasma (a high-temperature ionized gas) to ionize samples. Beyond that, ICPs designs differ significantly. Major differences between ICP-OES, ICP-Q-MS, and ICP-SF-MS are described below.
iCap 7000 optical design; source.
In ICP-OES, the wavelength of light emitted by ions (and atoms) that have been excited by the plasma forms the basis of quantification. The emitted photons have discrete wavelengths that are characteristic of the element. The light is separated in two dimensions by an echelle grating and a prism and is then detected by a solid state device. Here's an illustration of the various emission lines on the periodic table of elements. Elements have multiple emission lines at different wavelengths with differing relative strengths. By careful selection of an element's wavelength(s) used for detection, analyses can be designed to minimize interference from light at nearby wavelengths from other elements.
Recent advances in optical design and detector technologies have lead to increases in sample throughout, accuracy, and precision.
In ICP-MS, ions created in the plasma are accelerated by high voltage under high vacuum, and a series of lenses shape and filter the ion beam. The ions in this beam are then filtered or separated based on their mass/charge prior to detection. The Plasma Lab has two ICP-MSes that have differing methods of mass separation.
Schematic of quadrupole mass filtering; source.
The simpler form of ICP uses a quadrupole, hence the designation ICP-Q-MS. A quadrupole has four electrically connected cylindrical rods that can be very quickly "tuned," by varying the applied high-frequency AC and DC, to filter the ion beam by mass/charge. An advantage of the quadrupole design is the ability to very quickly scan through the entire mass range. Quad ICPs are designed to be robust and relatively matrix tolerant.
Ion transmission in the ElementXR; source.
In ICP-SF-MS, a magnet and/or electric field is incorporated in the design ("SF" refers to sector field, a somewhat vague term that indicates that the mass spectrometer uses a magnetic field, an electrostatic field, or both). The ElementXR has a magnetic sector followed by an electric sector (electrostatic analyzer). Therefore, it is a double-focusing sector field ICP-MS with reverse Nier-Johnson geometry. ICP-SF-MS delivers higher sensitivity (and lower limits of detection) and mass resolving power compared to ICP-Q-MS.
The XR has a triple mode detection system (SEM, analog, and Faraday) that offers linear signal response over 12 orders of magnitude.
For more information, please check out our ICP Library on the bookshelf near the ICP-OES in EMS C416.
