Product
Review
ICPMS -Beyond Quadrupole (Anal. Chem. 1996, 68, 46 A-51 A), New designs emerge to tems the market was dominated by quadrupole Although conventional quacombat interferences instruments. drupole ICPMS systems are still promimanufacturers now offer alternatives and reduce costs. nent, designed to help overcome polyatomic ion Since it wasfirstcommercialized in 1983, inductively coupled plasma mass spectrometry (ICPMS) has become the method of choice for fast, trace-level elemental analysis. It is much more sensitive than flame atomic absorption and ICP atomic emission spectrometry, offering multielement detection limits below 1 ng/L for solutions and in the parts-per-billion range for solids. In addition, it has the ability to provide isotopic information. But despite its advantages, ICPMS has traditionally suffered from two limitations polyatomic ion interferences and high cost. When Analytical Chemistryfirstreviewed commercially available ICPMS sysBritt E. Erickson
interferences and, in some cases, keep costs down. Quadrupoles with collision-cell technology, high-resolution magneticsector instruments (now in their second generation), multicollector systems for high-precision isotope ratio determinations and the recently introduced time-offlight (TOF) systems have all made their wav into the market Selected quadruoole masrnetic-sector and TOF ICPMS systems sold in the United States and Eurnne are presented in Tables 1—S The ICPMS market in Asia encomoasses several commanufacturers offer ICPMS systems in addif t th e pr senteri in the tabl r>
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5000 sector instruments are often, but not always, capable of separating analyte peaks from polyatomic peaks. Although they offer high resolving power and exceptionally low detection limits (parts-per-quadrillion levels for most elements) magnetic-sector systems come with a hefty mice tag—roughly twice that of a conventional quadrupole ICPMS system Even so these high-resolution instniTTIPTlts PTP
beginning cant percentapp of the total ICPMS market sharp saysMrTaren High-resolution ICPMS owes much of its success to the semiconductor industry, which requires high purity, trace metal-free starting materials. Taylor predicts, however, that this limited market will soon become saturated. "[High-resolution ICPMS systems] don't wear out. At a quarter- to a half-a-million dollars, you plan on using them for at least 10 years," he says. Growth in other market segments particularly geochemistry however continues to bring in sales of magnetic-sector ICPMS instruments According to the manufacturers the magnetic-sector ICPMS market has increased every year since 1994 High-resolution sector instruments are not as amenable to routine work as a quadrupole or TOF ICPMS system, says Taylor. In addition, "They require a greater deal of expertise on the part of the operator," he says. "The operator becomes more intimately involved in the maintenance,
conditioning, and tuning of the instrument than with a quadrupole, which essentially can be run by a technician." Even so, sector ICPMS systems have found their way into production environments. Magnetic-sector ICPMS is commonly used for high-precision isotope ratio determinations. Single detector instruments, however, typically yield a precision in the 0.050.5% range, which is insufficient for many applications, such as geochronology. One way of improving precision is to have multiple detectors, says McLaren. That is just what some manufacturers have done with
According to the manufacturers, the magnetic-sector ICPMS market has increased every year since 1994. magnetic-sector ICPMS systems. These multiple detector instruments, also referred to as multicollector (MC) systems, use a separate detector for each isotope being measured. Detectors typically include an array of analog devices, such as Faraday cups, for high concentrations and pulse-counting electron multipliers for low concentrations. MC systems have found a niche in the geological and nuclear areas for accurate and precise isotope ratio measurements, and they are now even being marketed to the bioclinical industry by some manufacturers for determination of traditionally difficult elements such as iron calcium selenium chromium and sulfur as well as those elements commonly analyzed by thermal ionization MS such as uranium neodymium strontium thorium lead However they are niiite an investment with p r i r p c starting around
Time of flight In a conventional quadrupole ICPMS system, all ions generated in the plasma are sorted by the quadrupole mass filter. Only one species gets through to the detector,
and the settings have to be changed for each element. In contrast, ions of all m/z from the plasma are sent into the TOF mass spectrometer, which offers simultaneous, multielement detection. As a result, TOF is much faster at reading the entire mass spectrum. In addition, the entire spectrum can be measured from small sample volumes. TOF ICPMS systems are about the same price as conventional quadrupoles, with list prices starting around $180,000. Currently only two manufacturers GBC Scientific and LECO compete in the TOF ICPMS market TOF ICPMS instruments have the potential to offer an inexpensive alternative to multicollector magnetic-sector systems for high-precision isotope ratio work. "One of the difficulties in using a conventional quadrupole or single collector magneticsector instrument is that variations in the stability of the ion beam result in reduced precision for isotope ratio measurements when going from one mass to another" says Taylor. "In a multicollector magneticsector system ion beams are collected simultaneously so you get the optimum precision " Likewise high precision in principle be obtained with a TOF ICPMS system because of its simultaneous nature although it has onlv been shown to yield 0 02% RSD (W7Ag/miAe) in practicp "TOF ICPMS is relatively new technology, so you are going to see a lot of development and a lot of changes as we go along," predicts one TOF ICPMS manufacturer. "Some say TOF will be the death of the quadrupole, but I think they are mistaken. Every instrument has its place and particular use. A quadrupole will always have better single element sensitivity than the multielement, simultaneous TOF." Britt Erickson is an assistant editor of Analytical Chemistry. Upcoming product reviews for 2 0 0 0 March 1: Fluorometers April 1: Surface plasmon resonance May 1: Electrochemical detectors for LC If your company manufactures any of these instruments, please let us know. E-mail (
[email protected]) or call us (202-8724570) at least three months before the listed date of publication.
Analytical Chemistry News & Features, December 1, 1999 8 1 5 A
