Product
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Elements of
ICPMS Manufacturers offer lower prices, high sensitivities, and a wide array of accessories for a host of applications
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In many applications, inductively coupled plasma mass spectrometry (ICPMS) has emerged as the "gold standard" for fast, trace-element analysis. Commercial instruments offer multielement detection limits below 1 ng/L for solutions and into the low part-per-billion range for solids. Applications range from analyzing complex matrices, such as environmental or geological samples, to determining the purity of process chemicals in the semiconductor industry. Currently eight manufacturers compete in the North American market. Despite prices that start well above $100,000, ICPMS instruments are being purchased by many laboratories. Because the instruments are largely under computer control, most of these systems can be run overnight with an autosampler to increase productivity. In addition, the growing number of accessories allows one instrument to handle a wide assortment of samples. Finally, the size of the instruments has also decreased over the years, reaching the benchtop category in 1994 with the introduction of Hewlett Packard's HP 4500. A typical ICPMS instrument has six key sections: a sample introduction system, an Ar plasma ion source, a set of
Analytical Chemistry News & Features, January 1, 1996
cones for sampling ions from the plasma, ion lenses for selecting and focusing ions, a mass discriminator, and a sensitive detection system. Samples introduced into the Ar plasma are decomposed primarily into neutral atoms along with singlycharged atoms and electrons. Because of the energy of the Ar plasma, most elements in the periodic table can be ionized. The excited ions produce photons, which are the basis of ICP optical emission spectroscopy. However, the photons are a problem for ICPMS instruments because the detectors respond to light. To avoid this problem, manufacturers place the detector off-axis from the torch or incorporate a photon stop inside the instrument. According to one marketing expert in the field, "Those [manufacturers] who get the most ions out of the plasma and into the detector usually win in this market." The human factor
Despite all the software power that comes standard with ICPMS instruments, the method still requires some skill in acquiring and interpreting data. "People who get an ICPMS instrument and put out poor data often haven't done their home-
work," says James McLaren of the National Research Council of Canada (Ottawa, Ontario). McLaren, a seasoned user of ICPMS instruments and Analytical Chemistry's technical expert for this review, added, "Failure to take into account spectroscopic overlaps is the largest source of inaccuracies in [reporting data from] ICPMS." Those overlaps result from polyatomic species that form in the plasma torch. For example, ultratrace concentrations of 56 + Fe can easily be swamped by the presence of 40Ar160+. Measurements of other elements at very low concentrations can be obscured by polyatomic ion interferences from even more "exotic species," says McLaren. To get around some of these interferences, analysts can go to a higher resolution instrument to separate analyte and polyatomic peaks, select different isotopes, or change sample preparation methods to remove polyatomicforming species such as water or chloride ions. McLaren and several technical experts from commercial manufacturers also warn that just relying on automated features can lead to inaccurate data. "Companies do not currently supply cookbook methods," cautions McLaren, "[analysts] need
to take the time to read the literature or take additional training."
graded sensitivity. For example, Brown says that the best compromise between sensitivity and resolution on Fisons' Genesis instrument falls at a resolution of 0.8 Surveying the landscape amu at 10% peak height. Commercial ICP instruments with quadrupole mass discriminators have been But what about separating analyte available for more than a decade and in peaks from polyatomic ion interferences? that time have improved significantly, Brown points out that most elements of enMcLaren says. Representative commercial vironmental concern often occur at high instruments being sold in the United enough concentrations that polyatomic inStates are listed in Table 1. For more infor- terferences are of limited concern. As a mation on individual products, send an eresult, quadrupole systems are sufficient mail message with one of the keywords for most environmental applications. listed at the bottom of the table in the Other manufacturers of quadrupole insubject line to our reflector service at struments point out that design
[email protected]. ments and the choice of proper operating conditions circumvent some polyatomic McLaren divides the current class of ion interferences. For example, Perkin products broadly into "medium-resolution" instruments, represented by systems Elmer's ELAN 6000, Hewlett Packard's HP 4500, and Fisons' PlasmaQuad III all with quadrupole mass spectrometers, claim to detect difficult-to-determine K, Ca, and the newer "high-resolution" systems, and Fe at part-per-trillion levels. These based on magnetic-sector mass spectrometers. The good news is that stiff com- manufacturers' technical experts attribute such detection limits to unique designs petition has led to significant price drops for both types of instruments in the past that allow the instruments to generate a "cool plasma" (low-power plasma and high few years. nebulizer flow). Under these conditions, Quadrupole ICPMS instruments have + approximately unit mass resolution. Peter Ar concentrations in the central channel of the plasma drop, and this change leads Brown, ICPMS product manager at to a concommitant decrease in unwanted Fisons Instruments, warns that claims for argon polyatomic ions. For example, higher resolution come at the cost of deAnalytical Chemistry News & Features, January 1, 1996 4 7 A
Product
Review
Table 1. Summary of representative products
INA = Information not available at press time USN = ultrasonic nebulizer DIN = direct-injection nebulizer
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RP = resolving power MCN = microconcentric nebulizer ETV = electrothermal vaporization
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Table 1. (cont.) Summary of representative products
Ar16O+ concentrations fall from almost part-per-billion background levels to levels of a few parts per trillion. Magnetic-sector ICPMS systems offer increased resolution at prices starting at about $400,000. These instruments provide resolving powers of > 5000, separating element ion peaks from polyatomic ions and offering exceptionally low detection limits. For example, JEOL's JMSPLASMAX2 detects Fe in ultrapure water at low picogram-per-milliliter concentration and Fisons' PlasmaTrace 2 is designed to measure part-per-quadrillion levels for most elements. Finnigan MATs ELEMENT lists resolving power as high as 7500. However, warns McLaren, the same trade-off between resolution and sensitivity seen for quadrupole instruments is also valid for magnetic-sector ICPMS. Sector instruments offer high or low resolution by changing slit widths in the ion optics which, he says, is a nice feature. But a sector instrument operated at resolutions much above 5000 probably doesn't offer detection limits as good as those for a quadrupole instrument. In some applications, magnetic-sector instruments are the only way to get results, says Finnigan MATs Chuck Douthitt, an MS specialist. The semiconductor industry, which requires high purities for starting materials such as HF, is a key market for these high-resolution instruments. Other markets include geology, nuclear industry, and reference material analysis laboratories. Detection limits meet the dynamic range
INA = Information not available at press time USN = ultrasonic nebulizer DIN = direct-injection nebulizer
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RP = resolving power MCN = microconcentric nebulizer ETV = electrothermal vaporization
Analytical Chemistry News & Features, January 1, 1996
Although ICPMS manufacturers brag about low detection limits, most experts were careful to qualify numbers. Actual detection limits depend on the element, the sample matrix, the sample preparation, and the condition of the instrument. Bert van der Hoeff, ICPMS product manager for Thermo Jarrell Ash Corp., points out that ICPMS instruments can suffer from "memory effects." For example, says van der Hoeff, "lithium and mercury have detection limits around 1 part per trillion, but these limits are not easy to reproduce because, after running high levels of these elements, they show a physical memory effect in the sample introduction system." It can take a long time to clear the unwanted ions from the instrument. "You need good cleaning of the instrument, especially the sample introduction area, to return to the lowest possible detection limits."
In addition, low detection limits may not always be desirable. Brown says that Fisons' Genesis system exhibits detection limits on the order of 25-50 pptr, whereas the PlasmaQuad offers detection limits of 10-100 parts per quadrillion. However, for an environmental laboratory monitoring for regulatory compliance at part-perbillion levels, the higher senstivity (lower detection limits) may be wasted. The low detection limits also present a challenge to the dynamic range of the detection system. Real-world samples come in a wide variety of analyte concentrations. To handle ion currents that may reflect sample concentrations ranging from parts per trillion to parts per million, commercial instruments use either dual-mode electron multipliers or multiple detectors. In either configuration, pulsecounting electron multipliers handle the lowest concentrations (or highest sensitivities). For example, many manufacturers have adopted discrete dynode electron multiplier detectors as their sensitive detectors. These detectors amplify the impact of an ion by triggering a growing cascade of electrons and measure the signal as an electrical pulse. High-concentration determinations rely on an analogue device such as a faraday detector or the analogue side of a dual-mode detector. Combined, these approaches typically provide linear concentration responses over a range of 8-9 orders of magnitude. Commercial manufacturers offer several variations on and modifications of these approaches. Perkin Elmer's ELAN 6000 and Fisons' PlasmaQuad III provide dual mode in a single detector that determines high and low concentrations in a single run and detects transient peaks. Varian's UltraMass runs a "reconnaissance scan" before analysis to ensure that a high ion current will not harm the detector. With the addition of an autosampler and an autodilutor, the UltraMass can then automatically dilute samples that show high concentrations of target elements and reanalyze. Thermo Jarrell Ash takes a different strategy with Quadrion by adding an optical emission spectrometer accessory to create an instrument that prescreens samples for concentration range. This hybrid system, known as POEMS (plasma optical emission MS), combines the charge-injection device optical detector (see Anal. Chem. 1994, 66, 105 A) with the MS detector for simultaneous analysis of trace and major elements. POEMS also offers the possibility of screening samples followed by automated
dilution. Or the analyst can set up criteria by which to select either optical or mass analysis based on a particular element or concentration range.
longer lifetimes than the standard all-nickel cones; however, the platinum coating significantly increases the cost. It is also possible to increase the orifice diameters and couple that with a larger vacuum pumping system to inSample introduction Just as important as the "back-end" detec- crease the instrument's tolerance to dissolved solids. According to van der Hoeff, tion is the way in which the sample is insuch a strategy is used for Thermo Jartroduced. One of the joys of commercial rell Ash's Quadrion system, and that ups ICPMS systems is that they offer a surprising range of sampling accessories. All the tolerance to total dissolved solids to ~ 1.0%. Don Potter, Hewlett Packard's instruments come with a conventional product manager for ICPMS, adds that nebulizer for liquid samples. they have found that larger torch injector ICPMS instruments have a low tolerance for high levels of dissolved solids. As tubes and a lower sample usage rate also increase the instrument's tolerance to a rule, problems develop when total disdissolved solids. solved solids in samples exceed 0.1-0.5%. (This value depends on the matrix.) The Adding an attachment for electrotherproblem manifests itself in the orifices mal vaporization (ETV) opens up ICPMS leading into the mass spectrometer. Ions to analysis of small-volume liquid samples. According to McLaren, typical sample volumes range from 10 to 20 uL. But the real advantage of ETV, McLaren finds, is that water and other matrix materials are removed before analyte introduction into the ICPMS; thus, interferences from oxygen-containing polyatomics such as ArO+ are reduced. Direct introduction of solids offers a different set of problems. "Running solids versus liquids is the great divide [among ICPMS spectroscopists]," claims Douthitt. For those who use solids, laser ablation has emerged as a key technique for sample introduction. In addition to analytical data, laser ablation can provide spatial information on solid samples, says McLaren. The laser systems have applications in enter the mass spectrometer through two such diverse realms as geochemistry, cesequential orifices, the sampler and skimramics, and polymers. However, he adds, mer cones. In effect, these are controlled the laser ablation generally produces semileaks against which the vacuum pumps quantitative results because of the lack of must work to maintain a good vacuum to standards with similar composition. minimize unwanted background ions. The sizes of these orifices also affect the ratio of Other solid-sampling methods are also analyte ions to oxide interferences. It is available. Cetac offers a spark ablation acthese orifices that clog when high levels of cessory for electrically conducting solsolids are introduced. ids, and Finnigan MAT's two ICPMS instruments can swap their plasma torches "Calibration stability is very difficult to for a glow discharge source. Finally, buymaintain when solids are being deposited ers can choose either a 27- or a 40-MHz on the sampler or skimmer during a run," says McLaren. One way to limit the prob- plasma torch powered by generators that are either held to a fixed frequency (cryslem is to use flow injection to introduce samples; this slows the rate of deposition be- tal controlled) or allowed to oscillate (free running). The free-running torch is more cause smaller samples are introduced. robust in varying matrices such as organNevertheless, along with purchasing highgrade Ar gas for the torch, McLaren says ics, says Chuck Schneider, technical marketing manager with Perkin Elmer. that he budgets a "few thousand dollars" each year to replace the sampler and skim- Other manufacturers say the differences are small. Otherwise, the experts agree mer cones. The cones have a finite lifetime because of erosion and chemical corro- that there isn't a significant performance difference between the torches. sion. Some manufacturers offer platinumtipped sampler and skimmer cones for Alan Newman
Actual detection limits depend on the element, the matrix, sample preparation, and the condition of the instrument.
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