Mass Spectrometry

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The ICP-TIMS Isotope Lab houses three magnetic-sector based mass spectrometers, which are distinct primarily in their means of sample introduction.  The thermal ionization mass spectrometer (TIMS instrument) introduces the sample through evaporation and ionization off of a filament (commonly Re or Ta metal), whereas the two multi-collector inductively-coupled plasma mass spectrometers (MC-ICP-MS) introduce the sample through a nebulizer (to produce a very fine aerosol), followed by introduction into a hot Ar plasma (~ 8,000 oC) as the means for ionization.

Magnetic-sector based mass spectrometry is essential for providing “flat-topped” peaks for the masses, which is required for high-precision isotope ratios; the flat top removes any changes in the relative ion intensities if small amounts of drift occurs in the instrument during analysis.  In addition, simultaneous collection of all isotopes of interest is also important for high-precision analyses, since ion beams (particularly those generated by ICP sources) always have small variations in intensities due to small changes in source conditions during analysis.  These features distinguish magnetic sector-based instruments that have multiple collection from, for example, the common quadrupol-based ICP-MS instruments.  In addition, because the sources used in the mass spectrometers are either solid- or solution-based, we must measure absolute ratios, and this contrasts with gas-source mass spectrometers, which can inlet sample and standard gases under identical source conditions, allowing isotope compositions to be determined on a relative basis.

Our TIMS instrument is approaching three decades in age, but still highly capable. It is VG Instruments Sector 54, which has been significantly upgraded in electronics and detector systems. The TIMS has 7 movable Faraday collectors, a Daly multiplier, and a 16 sample turret.

We have two MC-ICP-MS instruments.  A Micromass IsoProbe was installed in 1999 and is equipped with a collision cell (hexapol) that provides very high sensitivity and is effective at removing a range of polyatomic interferences. In addition, the instrument has 9 movable Faraday collectors, four  movable channeltron multipliers, an ion-counting Daly multiplier, a wide-aperture retarding filter (WARP) for high-abundance sensitivity work. In 2013, accompanying an expansion of the mass spectrometry labs, we installed a second MC-ICP-MS instrument, a Nu Instruments Nu Plasma II, located next to the IsoProbe. The NP-II is a double-focusing instrument, and contains 16 Faraday collectors, zoon/variable dispersion ion optics, five ion counters, and a high-abundance sensitivity filter.

Finally, we have an ultra-fast (femtosecond; 10-15 s) laser ablation system (fs-LA) for in situ isotopic analysis. The fs-LA system is made by Photon Machines and operates at 263nm with a ~150 fs pulse width. The beam delivery system can operate in either image aperture mode or focus mode. In image aperture mode an image of a user selectable aperture is imaged on the sample with true laser focus occurring 10’s of μm below the sample. The laser spot size is selectable and can range between 9 and 78 μm. In focus mode the working laser spot size is 20-30 μm. A HelEx cell and a frame cell are available for use.

Figure 1. Our thermal ionization mass spectrometer (TIMS). Samples are loaded on the left in the 16-sample turret housing. Collector block is on the right side of the main bench. Electronics cabinet (mostly empty after many upgrades to more compact equipment) on right.

Figure 1. Our thermal ionization mass spectrometer (TIMS). Samples are loaded on the left in the 16-sample turret housing. Collector block is on the right side of the main bench. Electronics cabinet (mostly empty after many upgrades to more compact equipment) on right.

Figure 2. Our first MC-ICP-MS instrument on the right, which is equipped with a collisions cell to remove polyatomic interferences. Plasma box on left side of instrument, collector block on right side. Since the plasma is at ground in this instrument, the flight tube and collectors are at negative 6 kV (hence enclosed). The blue box on the left is our fs-LA system, and the encompassing superstructure holds the beam delivery system and ablation stage/cell. In the distance is our newest MC-ICP-MS instrument.

Figure 2. Our first MC-ICP-MS instrument on the right, which is equipped with a collisions cell to remove polyatomic interferences. Plasma box on left side of instrument, collector block on right side. Since the plasma is at ground in this instrument, the flight tube and collectors are at negative 6 kV (hence enclosed). The blue box on the left is our fs-LA system, and the encompassing superstructure holds the beam delivery system and ablation stage/cell. In the distance is our newest MC-ICP-MS instrument.

Figure 3. Our newest MC-ICP-MS instrument on the left. The torch box for the older MC-ICP-MS instrument is on the right (blue door). In lower right is part of the fs-LA system, which can feed either (or both!) MC-ICP-MS instruments.

Figure 3. Our newest MC-ICP-MS instrument on the left. The torch box for the older MC-ICP-MS instrument is on the right (blue door). In lower right is part of the fs-LA system, which can feed either (or both!) MC-ICP-MS instruments.

Figure 4. Close-up of the collector end of our newest MC-ICP-MS instrument. Note that for this instrument, the plasma box is at positive 6 kV, and the flight tube and collectors are at ground (and hence exposed).

Figure 4. Close-up of the collector end of our newest MC-ICP-MS instrument. Note that for this instrument, the plasma box is at positive 6 kV, and the flight tube and collectors are at ground (and hence exposed).