Facilities & Procedures
Inductively coupled plasma (quadrupole) mass spectrometry (ICP-MS)
Instrumental structure of ICP-MS (Courtesy of Agilent Technologies)
In ICP-MS, a sample in the form of an aerosol is introduced into high temperature argon plasma by a carrier gas. Aerosol particles are generated either by nebulizing a sample solution or by ablating a solid sample with a laser. The temperature of the centerline of argon plasma reaches ~6000 to 10000 K and the sample is vaporized, atomized, and ionized (mostly singly charged positive ions with small amounts of doubly charged ions). The ions are extracted from the plasma into a high vacuum (~ 10-5 torr) region and focused by the ion lens system. Ions are discriminated from one another according to their mass/charge ratio by the quadrupole mass analyzer and detected by an electron multiplier.
The UCD ICPMS center has two ICP quadrupole mass spectrometers (Agilent Techologies 7500a and 7500ce). The 7500ce instrument has an additional octopole component called a reaction/collision cell between the ion lens system and quadrupole (the reaction/collision cell is not shown in the above figure). When hydrogen gas is used to react with argon oxides and argon dimers to reduce their interference with elements such as Fe and Se. When helium gas is used, it decelerates compounds such as chlorine oxide and argon chloride through collision, reducing their interference with V and As. The reaction/collision cell ICP-MS is a useful tool for reducing interferences that cannot be minimized using standard methods.
We utilize a Nu Plasma HR (Nu032) multiple-collection high-resolution double-focusing mass spectrometer ICP system (pictured above) capable of determining isotope ratios with external precision on the order of 10-20 ppm. This instrumentation better constrains isotopic proportions in natural samples than by quadrupole ICPMS or single-collector HR-ICPMS systems. MC-ICPMS (multiple collection inductively coupled plasma mass spectrometry) precision is similar to that achieved by TIMS (thermal ionization mass spectrometry) but has a tremendous speed advantage over TIMS for analyses can be performed within 20 minutes per sample (depending on the analytes) as compared to hours per sample typically required for TIMS. In addition, due to the ionization efficiency of the high temperature ICP ion source, MC-ICPMS can be used to study both traditional and nontraditional isotope systems for a wide range of applications. A compiled list of elements measured by MC-ICPMS techinques and estimate of peer reviewed publications since 1992 are included in the following figure from Douthitt, 2008: