Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)

Take Home Point:

Trace elemental analysis on small areas of solid samples  

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What It Provides:

Provides concentration tables of low level components and impurities present from localized areas on a sample.  Can also provide depth profiles and elemental imaging.

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In a Nutshell:

 

Laser Light In/Ions Out

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LA-ICP-MS is a specialized variation of ICP-MS that differs in the method of introducing the sample into the ICP-MS instrument.  In conventional ICP-MS solid samples must be dissolved prior to introduction into the instrument.  This process is destructive, often involving the use of hot acids or a microwave oven and is not practical for small samples or for the analysis of specific locations/features on a sample.  LA-ICP-MS circumvents these limitations by instead using a laser to produce an aerosol plume that is swept into the ICP-MS instrument where it is subsequently ionized by the ICP and analyzed by the MS.  The use of a laser not only allows for solid samples to be analyzed directly and non-destructively (accept for the small, ablated area), but also allows for small and/or specific areas to be identified and analyzed.  These tools are also commonly used for depth profiling as well as elemental imaging in a wide variety of applications not possible with conventional ICP-MS.   

 

While IR lasers were used initially, UV lasers (193nm, 216nm and 266nm) are most commonly used in current LA-ICP-MS instruments.  Generally, smaller wavelength (i.e. 193nm) and shorter pulse (fs rather than ns) laser give better results (e.g. less thermal damage, sharper ablation craters, the ability to efficiently analyze opaque and clear samples, less matrix effects). 

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In LA-ICP-MS samples are placed in an enclosed Sample Chamber.  A set of optics is used to focus the laser light onto the sample.  The chamber is equipped with a camera allowing viewing of the sample surface and location of specific analytical areas.  Each laser pulse creates an analytical plume as particles are ejected from the sample surface.  A sweep gas (He or Ar) is used to transport the Analytical Plume into the ICP torch where it is mixed with an Ar buffer gas (to maintain plasma stability) before entering the plasma undergoing decomposition, atomization and ionization.  The ion/gas mixture then flows through a set of cones that removes much of the Ar gas before the ions enter the vacuum of the mass spectrometer for mass selection and detection. 

 

Different mass spectrometers have been used for LA-ICP-MS applications.  Quadrupoles are the most common and least expensive and are used for accurate quantification.  Magnetic Sector tools have better sensitivities making them better suited for small spot imaging and for some geological applications.