More than one killer app can be found for ICP-OES
To analyze a sample, scientists need a method to interact with it and then record the results. One of the simplest methods—and one of the oldest—comes from just illuminating it, and seeing what is visible to the naked eye. To interact more directly with a sample, scientists move beyond just making it bright enough to see. For example, an inductively coupled plasma (ICP) can be used to push some of a sample’s electrons to higher shells—exciting them—and that results in an ion-emitting electromagnetic radiation as the electron drops back into its usual spot. That stimulus can be analyzed with optical emission spectroscopy (OES), and ICP-OES can be used in numerous ways.
The applications of ICP-OES cover a wide variety of research areas, one of which is food science. Here, ICP-OES can be used to analyze contaminants, such as arsenic in food or metals in wine. Dangerous chemicals can sometimes bind to proteins at trace levels, and ICP-OES can analyze this, too. Likewise, the method can measure trace elements in soil samples, including the forensic analysis of soil samples from crime scenes. In modern farm fields, ICP-OES can be used to find out if the soil lacks any crucial nutrients; if so, the proper kinds and amounts of fertilizer can be added. Consequently, this technology can be incorporated in precision agriculture—ensuring that crops receive the right inputs in the proper places.
In fact, the applications of ICP-OES run as far and wide as chemicals, and that encompasses about everything. A mineralogist can use ICP-OES to analyze a gem, and an automotive expert can apply this method to motor oil to determine wear in engine parts—indicated by traces of metal or other contaminants or components. At Portland State University’s geology laboratory, laboratory technician Alexandra Franco gives testimony to the wide range of sample types to which ICP-OES may be applied, saying, “We use the ICP-OES to analyze mostly water samples, digested soil, and digested rock samples.”
To get to these important analytical results, however, ICP-OES must be built in the most effective way. A key part of that entails the optics.
When an argon-based ICP excites elements in the sample, features in the light can be extracted to identify the components. Diffraction gratings separate the emitted light in spectral lines that a detector quantifies. The results produce an elemental fingerprint of a sample.
In most ICP-OES platforms, echelle gratings provide the diffraction. These create a two-dimensional array of results that consist of short segments of the spectrum—lined up one over the other from infrared (IR) to ultraviolet (UV). That step-like appearance explains the name “echelle optics,” which comes from echelle, or ladder, in French.
These gratings employ a low groove density and high angle of incidence from the light to provide higher diffraction orders, which puts more spacing between the lines at the detector. Echelle gratings have been used in various ways since the late 1890s and even more so by the 1920s.
Despite the heritage of these gratings, they still require quite a few optical components to use with ICP-OES. In particular, most platforms need several other components—mirrors, prisms, and, possibly, cross-dispersers. This can really cut down the light that goes from a sample to a detector. In some cases, every optical surface can remove as much as 15% of the light, and maybe more in the UV range.
Alternatively, some ICP-OES platforms replace echelle gratings with optimized Rowland circle alignment (ORCA) technology. Instead of the two-dimensional detectors used with echelle gratings, ORCA technology includes an array of charge-coupled devices (CCDs). In addition, this technology reduces the light lost by using curved mirrors. These put as much light as possible on the CCD detectors.
Overall, ICP-OES with ORCA technology provides high sensitivity and similar resolution of a wide spectral range. The technology also remains stable. This combination of technology can be found in a variety of platforms from SPECTRO Analytical Instruments, including the ARCOS, GENESIS, and SPECTROBLUE ICP-OES analyzers.
The ARCOS, for example, covers wavelengths of 130–770 nanometers, and it can capture a sample’s complete spectrum in just 4 seconds. This platform also includes 32 line array detectors.
Scientists from Rutgers University used the SPECTRO GENESIS ICP-OES to study crayfish in the Hudson River.1 In particular, the researchers studied this organism for the presence of platinum-group metals (PGMs), including palladium, platinum, and rhodium. They explained: “In this study, PGMs’ effects on bioaccumulation and histopathological changes were investigated using Orconectes virilis, a native Hudson River crayfish, as a model.” The researchers exposed the crayfish to a series of PGM concentrations—from 0 to as much as 10 parts per million—for 10 days. Then, they used the SPECTRO GENESIS to analyze the metals in samples, including exoskeletal, hepatic, and nervous system tissues. The results showed statistically significant increases in PGMs in all of the tissues, and some even included visual evidence of structural damage. For instance, the scientists pointed out that the “exoskeleton exhibited visible bands in the exocuticle indicating demineralization,” and “brain and ganglia demonstrated extensive vacuolization.” This work gives just one glimpse into some of the uses of this technology and its capabilities.
Comparing the optics
The straight light paths in an ORCA-based ICP-OES platform do require a bigger instrument. In labs with very limited space, this can be an issue. In most cases, the benefits of the ORCA approach offset this space requirement.
In most echelle-based platforms, it takes eight reflective surfaces to get the light from the source to the detector. With the ARCOS in the UV range, only two optical surfaces—in addition to the gratings—are used in the light path. That explains some of the reasons this device is as much as five times more sensitive than a platform that uses echelle optics.
To collect more chemical data from samples at the most sensitive levels, scientists seek the platforms that make the best fit in a lab. An ORCA-based ICP-OES system could be just the thing to add more options.
- Wren, M.; Gagnon, ZE. A histopathological study of Hudson River crayfish, Orconectes virilis, exposed to platinum group metals. J. Environ. Sci. Health A Tox. Hazard Subst. Environ. Eng. 2014, 49, 135–45.
Mike May is a freelance writer and editor living in Texas. He can be reached at [email protected]