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Understanding Our Planet Through Chemistry

Laser Ablation, Inductively Coupled Plasma, Quadrupole Mass Spectrometry

A new technique that is highly sensitive for almost all elements, is laser ablation, inductively coupled plasma, quadrupole mass spectrometry (LA-ICP-QMS). To allow efficient, rapid, spatial sampling, a laser is used. As depicted below, the energy of the laser is focused onto a spot about 80 micrometers in diameter (slightly more than the diameter of a human hair) to vaporize and sputter material from small zones of the sample. The operating conditions of the laser range from 1 million to 1 trillion watts per square centimeter. This incredibly high energy density is created when the energy is packed into small bursts of 160 microseconds, which are then focused with a lens onto a very small spot.

Schematic diagram of LA-ICP-QMS technique.A laser ablation, induction coupled plasma, quadrupole mass spectrometer vaporizes a small spot on the sample. The vapor is then ionized in the plasma. The four charged rods (the quadrupole) then cause only the appropriate ions to arrive at the detector for counting; all others are lost. By changing the electric charge on the rods, different elements can be determined. [21k]

The vapor from the sample is then carried by a stream of argon gas into a 7,000°C argon plasma, where the vapor is ionized. These ions are then drawn into a quadrupole mass spectrometer (QMS). The QMS consists of two sets of electrically charged, machined rods. A radio-frequency signal is applied to both sets of rods. Under specific operating conditions, one unique, mass-to-charge ratio of ions will be directed down the opening between the four rods and exit to the detector. All other ions will be lost.

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