Overviewing ICP-OES Analyzers’ Plasma Interfaces

Inductively coupled plasma optical emission spectrometry (ICP-OES) instruments have become the analyzers of choice for various industrial, environmental, and research tasks. In the ICP-OES method, when a sample is excited within an analyzer’s high-temperature argon plasma, given spectral wavelengths are characteristically emitted by specific elements. Emitted light reaching an optical system is resolved into these separate wavelengths by diffraction gratings. The light is finally directed onto a detector array that quantifies light intensities at these wavelengths. Thus, users can identify and measure each element in the sample.

One key differentiator is how spectrometers handle optical plasma observation. Understanding axial, radial, and dual views is critical to deciding which instrument to purchase for which analysis. Let’s overview each.*

AXIAL VIEW

An axial-view system “looks” from end to end of the plasma’s axis. It observes all phenomena in the excitation channel.

Usually, observation is accomplished via an optical interface next to the plasma. The interface releases argon gas to cool itself and deflect parts of the plasma from its opening, through which light passes into the optical chamber.

This axial-view design allows a large amount of light into the optical system and thus makes a relatively large volume of information available to process. This is a crucial benefit for many analyses, leading to maximum sensitivity in detecting trace element emissions.

However, all that light can contain more than emissions from elements of interest. It may also include background emissions. The light may be influenced by matrix interferences such as the easily ionized element (EIE) effect. These can degrade analytical accuracy.

Finally, axial-view systems can feature a horizontal plasma torch. This can increase challenges when measuring samples with high amounts of total dissolved solids (TDS) or organic solutions.

RADIAL VIEW

A radial-view system looks across the plasma. It sees only a relatively narrow cross-section of light rather than light from the whole length of the excitation channel.

With less light to process, a radial system can’t match the sensitivity of an axial system in detecting trace elements.

However, a radial view reduces or eliminates certain background emissions and matrix interferences by observing less light. (Its higher tolerance for challenging matrices is also due to using a vertical plasma torch.) So, it suffers less from noise than axial systems and usually offers higher analytical precision.

DUAL-VIEW

Offering both radial and axial plasma observations, a dual-view system is designed to achieve both benefits. However, almost all current models must compromise this goal, favoring one view.

Build details differ among major manufacturers:

• Axially optimized dual-view provides direct axial observation of the plasma while utilizing periscopes or other means to achieve an added radial view. They thus offer full axial-view sensitivity.
• Radially optimized dual-view favors radial observation. They can provide full radial-view precision with minimal background emissions or matrix interferences. Their axial mode can also deliver good but not exceptional sensitivity.
• Vertical-torch dual-view provides a vertical plasma torch and a direct radial view, plus an axial view supplied via several mirrors in a periscope optic mounted just above the plasma. This approach may even furnish simultaneous observation of the plasma, both axially and radially, in a single measurement. The design has both advantages as well as compromises.

MultiView

The sequential MultiView approach eliminates the use of added mirrors or periscopes — and transcends almost all dual-view disadvantages. Instead, a user who typically uses radial mode, for example, can measure a sample that needs extreme sensitivity simply by shifting the direction of the plasma torch into axial orientation. This relatively simple mechanical changeover takes about 90 seconds.

MultiView technology is available in the only ICP-OES instrument that offers plasma views without bias and compromise: one version of the top-of-the-line SPECTRO ARCOS analyzer. The instrument provides full radial precision, full axial sensitivity, unmatched precision, dynamic range, and matrix compatibility. It’s the equivalent of buying two dedicated, fully optimized analyzers in one.

DUAL SIDE-ON INTERFACE (DSOI)

While “dual” usually means offering radial and axial views, SPECTRO’s new DSOI technology refers to a single view — but one that’s effectively doubled. This design uses a vertical plasma torch, observed via a new variety of direct path radial-view technology.

In a DSOI system, an optical interface on one side of the plasma captures the amount of emitted light normal for radial observation and conveys it into the optical system. But the concave mirror of a second interface on the other side captures additional emission light. This extra light — and added spectral information — is also reflected in the optics.

Compared to conventional radial-view analyzers, the system’s sensitivity for most elements is effectively doubled. This generally enhances the sensitivity on average by a factor of two. DSOI technology provides high stability, freedom from matrix effects, high matrix tolerance, and high linear dynamic range.

Introduced in the midrange SPECTROGREEN analyzer and optimized for environmental analyses, the design allows for an optimized blend of analysis speed, robustness, uptime, ease of use, and competitive operating costs.

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*Dive deeper into the technology, advantages, disadvantages, and applications of each ICP-OES analyzer plasma interface. Get  the white paper, “Comparing ICP-OES Analyzers’ Plasma Interfaces: Axial, Radial, Dual, MultiView, and New Dual Side-On.” Click for an instant, direct download. No registration is required!