4.3 Summary
Mass spectrometry detection greatly expands the applications of ICP system in the analysis of metals. Not only can more elements be analyzed, as compared to FAAS, FAES, and ICP-AES, but isotopic data can be collected. A variety of sample introduction techniques and mass filters make the ICP-MS a diverse instrument. But increased capabilities and lower detection limits come with a relatively high price tag. For example, a basic ICP-AES costs in the range of $70-80 thousand while a basic ICP-MS with reaction cell-quadrupole technology costs around $150 thousand; high-resolution instruments can cost as high as $600 thousand or more, and some are not even commercially available.
Mass spectrometry is in a constant and rapid state of development. In this Etextbook, we have focused on the basic mass analyzers, such as the quadrupole, ion trap, time of flight and magnetic sector (double focusing) designs. Recent technological advances have allowed for the development of two upcoming instruments; a new magnetic sector mass analyzer (Walder, et al., Journal of Analytical Atomic Spectrometry, 1992, 7, 571-575) and orbital trap (Makarov, Analytical Chemistry, 2000, 72(6), p.1156). The new magnetic sector instrument relays on a new sector design referred to as the Mattauch-Herzog geometry. Although the resolution is relative low (~500) for a high-end instrument, this mass filter allows for the monitoring of multiple masses at the exact same time by using a multi-collector-Faraday based detector. The advantage of this system is that high accuracy in isotopic ratios that can be obtained. The orbital trap instrument is an electrostatic ion trap capable of resolution values (Rs) approaching 200 000. While these instruments carry a high price tag, they greatly increase the normal capabilities of the instrument.
Additional recent breakthroughs in mass spectrometry include the drastic lowering of detection limits. A new technique referred to as nanostructured initiator MS (NIMS) is being used in research-grade instruments to measure biological metabolites. Utilization of these systems with a laser-based systems produces detection limits are easily at the attomole (10-18) amounts. Specific molecules in the yoctomole (10-24) levels have even been detected (Nature, 2007, 449, 1003). These systems make the ppb and ppt detection limits discussed in this Etextbook seem trivial. It is likely that similar detection limits will soon be achieved for ICP-based instruments.
A summary of resolution and price for commercially available instruments is given in the following table.
Table 4.2 Summary of Mass Filter Features. Source: Personal Communiqué David Koppenaal, Thermal Scientific & EMSL, Pacific National Laboratory.
| Type of Mass Filter | Resolution | Detection Limit | Approximate Instrument Price |
Routine Mass Filters Coupled with ICP |
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| Single Quadrupole with a collision / reaction cell (CRC) | 250-500 |
low ppb – high ppt |
$150 000 - $200 000 |
| Ion Trap (no longer available) | 1 000 – 10 000 |
ppb |
$400 000 - $500 000 |
| Time of Flight | 3000 – 10 000 |
high ppt |
$300 000 - $400 000 |
| Double Focusing | 10 000 – 20 000 |
mid to high ppt |
$750 000 - $1 000 000 |
| Fourier Transform | 200 000 – 1 000 000 |
ppb |
Not commercially available |
New Mass Filters |
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| Magnetic Sector / Multi-collector with the Mattauch-Herzog Geometry | ~500 |
high ppb |
$350 000 - $400 000 |
| Orbital Trap (Electrostatic Ion Trap) | 150 000 – 200 000 |
ppb |
$600 000 (currently only available with HPLC; possibly soon to be available with ICP) |
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