The four basic analytical techniques commonly used for elemental analysis are atomic absorption (AA), inductively coupled plasma-mass spectrometry ICP-MS (ICP-MS), ICP-optical emission spectroscopy (ICP-OES), and X-ray fluorescence (XRF). Each analytical technique is capable of recording the presence and abundance of individual elements across the bulk of the Periodic Table. These are by no means new—some elemental analysis techniques have been in existence for decades, in fact—but development is ongoing. From new hardware to new application areas, researchers continue to discover ways to probe the elemental composition of matter.
The 4100 MP-AES microwave plasma atomic emission spectrometer from Agilent Technologies (Palo Alto, CA) employs a new elemental analysis technique and offers improved performance over flame AA, according to Kentaro Suzuki, the company’s ICP-MS Marketing Manager. It also provides a cost-of-ownership advantage by creating plasma from nitrogen rather than argon, thereby saving on gas expenses (it can even be coupled to a nitrogen generator that extracts nitrogen from room air), making the technology amenable to use in remote locations such as mines.
PerkinElmer (Shelton, CT) Optima 8x00 series ICP-OES instruments are also designed specifically to minimize argon consumption, said Charles Schneider, the company’s Inorganic Product Planning Manager. While a typical ICP-OES consumes 15−18 L/min of argon, the 8x00 uses as little as 9 L, due to a new, more efficient Flat Plate™ plasma-generation system. This can save laboratories thousands of dollars per year in argon.
PerkinElmer also updated its atomic absorption instrument line with the PinAAcle, an eight-lamp system that replaces traditional mirror-based optics with fiber optics that reduce the instrument footprint, according to Schneider. Also new is the company’s NexION ICP-MS, an orthogonal quadrupole MS with a “universal” reaction and collision cell design that gives researchers a choice in terms of how to eliminate potential interferences
Elemental analysis techniques in the pharmaceuticals industry
At Microbac Laboratories in Wilson, NC, a contract testing lab that serves such industries as food, environmental, pharmaceuticals, and cosmetics, elemental analysis probably accounts for 10% of the company’s business, said Kim Baughman, Director of Development. Microbac runs a wide variety of assays from quantifying lead in soil, to cadmium and lead in food, to titanium in sunscreens, using a suite of instruments from PerkinElmer, including an AAnalyst 800 AA, Optima 5300 ICP, and NexION ICP/MS.
Baughman explained that ICP-MS and ICP-OES are playing expanded roles in the pharmaceuticals arena, as new standards under consideration by the United States Pharmacopeia propose to replace a century-old colorimetric assay, in which heavy metals are precipitated by conversion into metal sulfides, with the newer ICP-based approaches.
According to Baughman, customers in the pharmaceuticals industry have begun gearing up for the change: “Companies are starting to move in that direction, because once [the change] becomes official, even though there will be a phase-in period, it’s a lot of work for companies to switch all their methods.”
Applications in the life science lab and in food safety testing
Despite their age, elemental analysis techniques are finding use in a variety of new applications and industries. For instance, ICP-MS is being used in Japan to quantify radioactive iodine from soil samples near the Fukushima nuclear power plant. It is also finding use in life science laboratories, where it is used as a “companion piece” to traditional organic molecular methods such as LC/MS and GC/MS to quantify metals in applications like proteomics and metabolomics. “This is probably one of the more intriguing and fastest growing areas for ICP-MS today,” said Suzuki.
Elemental analysis also figures prominently in environmental and food safety applications. According to Fergus Keenan, Field Marketing Manager for Trace Chemical Analysis at Thermo Fisher Scientific (Waltham, MA), when measuring elements like arsenic and lead, for instance in soil or (as occurred recently) in fruit juices, it is important not only to know the total abundance; molecular form matters also. “If there’s lots of arsenobetaine in your food sample, that’s not a major worry,” he said, because arsenobetaine is relatively harmless. “But if it’s inorganic arsenic, As(III) or As(V), then we want to be concerned, because that would be quite toxic.”
To help address this issue, Thermo offers a new ICP-MS application called elemental speciation, in which the MS detector (an X-series 2 ICP-MS) is coupled to an ion chromatography system (Dionex ICS-5000) on the front end to distinguish different molecular forms of, among other things, arsenic-containing compounds. “The beauty of using an ICP-MS as the detector is that it will only see those compounds that have that metal in it,” maintained Keenan, “so it simplifies the chromatogram.”
Trace element analysis
One particularly interesting application of elemental analysis techniques is in museum laboratories. For instance, the extreme sensitivity of ICP-MS makes it amenable to trace element analysis. A material may be identified by its bulk composition—to grade steel, for instance—but its minor constituents can serve as a kind of geologic signature. Karen Trentelman, Senior Scientist at the Getty Conservation Institute (Los Angeles, CA) used that approach in her museum laboratory to pinpoint the likely origin of a lead-tin-based red pigment found on a collection of Egyptian mummies called red-shroud mummies.
However, ICP-MS is not normally Trentelman’s go-to technology; it is too destructive. “In the cultural heritage field, our primary goal is to get as much information as we can without having to take any samples or damage the work of art in any way,” she stated.
Her lab thus reserves ICP-MS for cases in which the extreme detail that technique can provide is required, and instead turns most often to XRF, a nondestructive technique that measures elemental abundance, from about sodium to uranium, by bombarding a sample with X-rays and recording the characteristic X-rays that it produces in response.
Wavelength-dispersive and energy-dispersive X-ray fluorescence
In wavelength-dispersive X-ray fluorescence (WD-XRF), explained Ravi Yellepeddi, Marketing Director for Bulk Elemental Analysis at Thermo Fisher Scientific, the different X-ray wavelengths returning from a sample, which are signatures for each element, are separated and measured using a series of optics (crystals), like white light with a prism. In energy-dispersive X-ray fluorescence (EDXRF), a solid-state detector measures the complete energy spectrum instead.
WD-XRF is generally more powerful, precise, and covers a wider dynamic range; ED-XRF is generally simpler and amenable to use in either a lab or the field. But WD-XRF traditionally also requires much brighter X-ray sources, which have to be cooled with water. As a result, WD instruments tend to be larger and more expensive than their ED counterparts. Bridging that gap to some degree, Thermo Fisher Scientific has developed WD-XRF instruments that couple the lower X-ray power of some ED-XRF instruments with the improved performance of WD-XRF; according to Yellepeddi, “There are mostly air-cooled, very compact, and reliable systems even for routine elemental analysis.”
Trentelman’s lab has two types of EDXRF. The Tracer (Bruker AXS, Madison, WI) is a portable, handheld unit that looks like a Star Trek Phaser coupled to a Blackberry. That form factor, Trentelman explained, means that the Tracer can be brought into the gallery for a quick analysis, or taken into the field to study objects in situ.
However, handheld XRF is not extremely sensitive (perhaps 10 ppm or so), nor does it offer good spatial resolution, with spot sizes on the order of millimeters in diameter, according to Trentelman. “If you are looking at a giant bronze sculpture, that may be fine. But if you’re trying to look at individual components in, say, an illuminated manuscript, that’s not going to help you,” she advised.
In such cases, the lab uses a Bruker AXS ARTAX 800, a benchtop microXRF unit with an articulated arm and optics that allow the system to image regions of about 2.5-cm square with spots as small as 65 μm in diameter. The result is a pixel-by-pixel map of elemental composition over the scanned area.
In one study, Trentelman’s team used the ARTAX to image apples in a tree in the 15th-century painting Bathsheba Bathing. The apples were just 2 mm in diameter, yet the analysis could clearly distinguish the mercury-containing vermillion paint used to highlight the apple’s contours, a sickle-shaped feature just 280 microns across. “XRF mapping is an incredibly powerful technique that’s starting to be used much more frequently,” said Trentelman, “because of the development of tools to be able to do it.”
Total-reflection X-ray fluorescence
Development of new XRF techniques continues. Total-reflection X-ray fluorescence (TXRF) is a relatively new benchtop XRF variant that is capable of measuring materials down into the ppb range, according to Michael Beauchaine of Bruker AXS. The improved sensitivity of the TXRF-based S2 Picofox makes it attractive in fields not normally associated with XRF, such as nanoparticles, pharmaceuticals, clinical, and food and beverage analysis, Beauchaine stated, because “these particular areas demand much lower detection limits than traditional energy-dispersive or wavelength-dispersive XRF can achieve.”
Sometimes, though, it is not sufficient to scan a surface and see what bounces back. Many materials are layered with several coatings, and to study them, researchers must dig into the layers underneath.
Trentelman and her team, for instance, study the “stratigraphy” of paintings—the composition of their paint layers—using scanning electron microscopy energy-dispersive spectroscopy (SEM-EDS), basically XRF using the SEM’s electron beam instead of an X-ray source, on microscopic cross-sections.
Another approach is glow discharge OES-based depth profiling. Glow discharge OES is similar to ICP-OES in that it is an emission technique that employs an argon plasma, but it analyzes solid samples rather than liquids, and uses a low-temperature plasma to bombard the sample, causing it to “sputter” material that then emits light from the sample’s elements upon collision with an electron or ion in the discharge. The technique is often used to study conductive materials without sample preparation, for instance in steel foundries, said Philippe Hunault, General Manager in the Elemental Analysis Division at Horiba Jobin Yvon (Edison, NJ). The company offers a system based on a pulsed radio frequency source that can handle nonconductive materials as well.
“This new innovation opens the market of the glass industry, the photovoltaic, and semiconductor industries,” Hunault asserted—applications that previously were too fragile (that is, had too low-melting-point coatings) for glow discharge work. Yet glow discharge OES can also peel away the layers of almost any material, such as the coatings on cutting tools, for instance, by rapidly and continuously bombarding the sample until the underlying metal substrate is reached.
According to Hunault, this application of glow discharge OES is not particularly new. But it also is not well-known. As more and more industries discover novel applications for elemental analysis, expect this technique, like other related approaches, to gain wider adoption in the years ahead.
Jeffrey Perkel holds a Ph.D. in Cell and Molecular Biology from the University of Pennsylvania, and did postdoctoral work at the University of Pennsylvania and Harvard Medical School; e-mail: [email protected].