Sensitive, Quantitative Metal Analysis
Atomic absorption (AA) has been known since the 19th century, but it was not until the 1950s, thanks to efforts by Alan Walsh at Australia’s CSIRO research center, that use of AA spectrometers became routine for metals analysis.
A true Beer’s law absorbance technique, AA measures ultraviolet light absorbed by hot, atomized metals. The absorbance wavelength is unique to every metal, but the signal intensity varies by concentration. Instrumentation is straightforward, consisting of a light source, atomizer and detector. Atomizers are traditionally high-temperature flames, but graphite furnaces and various plasma sources are also used. Light sources include hollow cathode lamps (most common) and diode lasers. Detectors are most usually photomultiplier tubes.
AA handles many sample types, but operation requires that metal analytes exist as freestanding atoms in the gas state, rather than as ions in solution or solid metals or salts. Solution samples must first be dried, a process known as “desolvation,” before they are atomized and pumped with UV light. AA could be considered for nearly any application that requires the identification and/or quantification of metals. For example:
• environmental analysis
• quality control for contaminant, ingredient, or trace metal in foods, drugs, personal care products, paper, materials, and other products
• forensics, archeology, mining, agriculture
• natural science experimentation
Interesting developments in AA include the introduction of the single xenon arc lamp, which permits a single light source or instrument to address all metals accessible by AA. Xenon lamps also provide up to ten times the sensitivity of conventional AA. Similarly, the introduction of solid-state charge-coupled device detectors, improved background detection, direct analysis of solids (without atomization), and the ability to detect nonmetals has extended the capabilities of modern AA spectrophotometers.
Chuck Schneider, business unit manager for PerkinElmer’s (Shelton, Conn.) inorganic analysis products, breaks AA instrumentation down into three categories: flame, graphite furnace, and dedicated analyzers. Perkin- Elmer further delineates these into entry-level systems and higher-end systems with more extensive automation, software, data handling, and the ability to switch back and forth from flame to graphite furnace operation.
Graphite furnace AA spectrometers are significantly slower than flame-atomizer instruments, but they provide several benefits. Because they concentrate the cloud of atomized metals, graphite furnace instruments require less sample than flame AA spectrophotometers— 20 microliters vs. up to five milliliters. Sensitivity (ppb vs. ppm) is also higher in graphite furnace models.
On the business side, AA can be divided into instruments serving developed countries and those in developing nations. “More so than other inorganic analytic techniques, AA is very much divided into the primary instrument, consumable sales into that market, and service,” Schneider explains. “There is a very large installed base of AA systems around the world, so the service component is quite large.” The Chinese drug regulatory agency, for example, has recently ordered one hundred PerkinElmer AA instruments, Schneider says.
Yong Xie, product manager for AA instruments at Aurora Biomed (Vancouver, B.C.), notes higher demand for AA in the environmental (particularly for heavy metal analysis) and biomedical industries. The former is driven, he says, by government mandates to measure ever-smaller concentrations of metallic contaminants. AA is not typically thought of as a “life science” technique, but that is changing. Xie notes emerging applications in the pharmaceutical industry, particularly for measuring the function of potassium ion channels—physiologic structures in cells that allow ions to enter and leave—that are critical for cardiovascular health.
What customers look for
PerkinElmer has recently completed a large survey of inorganic analysis customers and found that the top two factors entering into purchase decisions are customer service (including the salesperson’s knowledge and service support) and ease of use and setup for hardware and software. The third factor is the vendor’s reputation. Price is “fifth or sixth on the list,” Mr. Schneider says. “AA has got to be dead simple to use, because instruments are used by a lot of different people who may not have specific training in the technique. Years ago a lab might have had five people operating six or seven instruments. The number of techniques has remained the same, but the number of analysts might be down to two. Lab workers need to be generalists.”
With more or less the same hardware technology accessible to all manufacturers, Yong Xie believes that some vendors err in focusing on the hardware and automation alone and not enough on the ease of use—a factor noted in the PerkinElmer study. “The computer industry has made huge progress in both hardware and software,” he says, as have advances in automation, autosampling, and unattended operation. Since these advances won’t help with detection limits or other fundamental performance factors, “they are best applied to enhancing the user experience, to provide the most convenient environment for operating the instrument and achieving desired objectives.”