The method of carrying the light from the plasma to the detector impacts the results
When combining inductively coupled plasma (ICP) with optical emission spectroscopy (OES), the analysis depends on how the plasma is viewed in an ICP-OES device. The method that the spectrometer uses in plasma observation is typically axial-, radial-, or dual-view. Each method comes with some strengths and weaknesses, and some methods make a better fit with specific applications. This article reviews that information, includes a real-world example, and describes a new viewing technology.
Axial viewing
In a platform that uses axial viewing, the light ‘looks’ down the length of the plasma’s axis—that is, the direction of the light is parallel to the plasma. This approach delivers lots of light to the optical system, which supplies more information to the detector. “In many analyses, this is a benefit, leading to increased sensitivity in detecting trace-element emissions,” according to SPECTRO’s “Selecting your ICP-OES analyzer’s plasma interface: axial-view, radial-view, dual-view, or new MultiView.”
The large amount of light captured in this method, though, includes information from the sample of interest plus background. The background can be considerable. Also, matrix interferences, which originate in the cooler plasma tail, can degrade the precision and accuracy.
Still, this method works well in some applications, especially when sensitivity matters more than precision. This method provides the lowest detection limits—a factor of 10 better than radial-view platforms of the past and maybe 3–4 times better than today’s best radial-viewing platforms. That makes this approach appealing for trace-element analyses, especially in environmental and industrial applications.
On the downside, the more complex design of an axial-based instrument requires more maintenance and cleaning.
Radial viewing
With a radial-viewing instrument, the light is perpendicular to the longitudinal axis of the plasma. So, this method analyzes a slice of the plasma, and less light leads to less sensitivity. The less light, though, comes with less interference from background and matrices. That means that a radial system provides more precision than an axial one. According to the SPECTRO paper, “Users prefer radial models when seeking higher stability and especially higher matrix compatibility.”
A radial-based instrument is also easier to clean and maintain than an instrument with axial viewing.
In comparing these technologies, Dion Tsourides, director ICP—NAFTA at the Ametek Materials Analysis Division, says, “For anything below 100 parts per billion, run axial; above 100 parts per billion, you can go either way.”
Dual viewing
If a user needs the axial- and radial-viewing approach, a dual-view analyzer should be considered, because it allows both. This kind of instrument costs a little more, and the complication of providing dual techniques increases the maintenance and cleaning challenges. Nonetheless, when a user needs sensitivity, the axial mode can be used. To eliminate matrix interferences and expand the linear range to analyze higher concentration levels, switch to radial viewing.
The sample type that a lab must analyze impacts the choice of the best mode. “For oil, soil, and high-level work, use radial,” says Tsourides. “For trace analysis in, say, plant tissue, switch to axial.”
In North Carolina, Charlotte Water handles public water and wastewater for nearly one million customers. At more than 125 years old, this is the largest public water company in the Carolinas—performing about 235,000 laboratory analyses every year. In 2017, this utility pumped an average of 107 million gallons of drinking water per day and treated more than 79 million gallons of wastewater a day.
In the analyses, says Cody Gibson, laboratory analyst II at Charlotte Water, “We use ICP-OES for water and environmental samples, particularly industrial wastewater and minerals analysis.”
This lab includes a SPECTROBLUE TI, which has a twin interface that allows axial or radial viewing. “Most samples are analyzed in axial-view because we require high sensitivity and lower detection limits,” Gibson explains. “Minerals are viewed radially to minimize spectral interferences.” (The use of instrument names is for identification only and does not imply endorsement by Charlotte Water or the City of Charlotte.)
When asked what method is preferred, Gibson says, “We run a method including both axial and radial with our twin-interface SPECTROBLUE.” He adds, “We prefer this method because of the various matrices run by ICP-OES in our lab.”
The SPECTRO paper notes: “Some vendors recommend dual-view models to most or all of their customers, no matter what the application. This may be ill-advised.” As Tsourides says, “On all dual-view systems, one or both of the views—axial and/or radial—will be compromised analytically.”
The new view
At Pittcon 2015 in New Orleans, SPECTRO introduced its SPECTRO ARCOS ICP-OES analyzer. This MultiView instrument provides true axial and radial methods of observation. “It is the only system that can offer two truly optimized analytical systems in a single ICP spectrometer,” says Tsourides. With just 90 seconds of adjustments, a scientist can switch from axial to radial viewing or vice versa. “Where a lab has samples that aren’t always the same, then there’s a real benefit for MultiView,” Tsourides says.
In the past, platforms that provided axial- and radial-viewing options used a periscope. “This compromises performance, making it 3–5 times worse,” says Tsourides. “So, when you want the best performance from an instrument, this is not the best way, but it’s a convenient way.” This method also includes mirrors that must be maintained over time.
The SPECTRO ARCOS ICP-OES analyzer, on the other hand, doesn’t use a periscope. Consequently, it requires less maintenance and provides higher performance.
In addition, this platform includes a sealed optical system. So, it doesn’t need continuous argon purging to be ready to use. Instead, argon is recirculated and ready to go. “That can save $4,000 to $6,000 a year in operating costs,” Tsourides notes.
The take-home message is that the SPECTRO ARCOS ICP-OES analyzer gives scientists the best of both worlds—radial and axial viewing. Moreover, this platform provides optimized versions of both methods. Consequently, a scientist can easily try one method, and if it doesn’t provide the desired results, the instrument can quickly be switched to the other approach for comparison.
In this area of analytical instrumentation, many factors come into selecting the best platform. Most of the emphasis should go to how a platform will be used today and, possibly, tomorrow. In some cases, a small increase in upfront investment saves a lot down the road.
Mike May is a freelance writer and editor living in Texas. He can be reached at [email protected]