Seeing the Plasma in a Whole New Light: Advantages of DSOI

Seeing the Plasma in a Whole

New Light: Advantages of DSOI

Technology

Introduction

Manufacturers of inductively coupled plasma

optical emission spectrometers keep refining

their designs. In particular, ongoing innovations

implemented in recent ICP-OES models seek to

optimize their plasma viewing technologies to

improve performance, efficiency, and ease of use.

This paper compares two recent developments.

Vertical-torch dual-view plasma observation

has demonstrated benefits such as reduced

interferences and higher matrix compatibility.

However, its design also suffers from issues

of contamination plus higher requirements for

A WHITE PAPER FROMSPECTRO ANALYTICAL INSTRUMENTS

maintenance, adding complexity and cost that can

limit its usefulness.

By contrast, newer dual side-on interface (DSOI)

technology — as seen in the SPECTROGREEN

ICP-OES analyzer — puts the challenge of plasma

viewing in a whole new light. It can provide

sensitivity, freedom from interference, and

matrix compatibility while avoiding the earlier

design’s drawbacks. This marks a revolutionary

improvement. DSOI enables radial-view-based

analyzers to deliver previously unachievable levels

of performance and ease in a wide variety of

applications.

When results matter

2 Seeing the plasma in a whole new light: advantages of DSOI technology

Background Most ICP-OES analyzers work by exciting

atoms and ions within a high-temperature

argon plasma. This causes the emission

of different wavelengths, or spectral lines

— with characteristic lines being emitted

by specific elements. The emitted light

is viewed via an optical interface, and

is resolved into these separate lines by

diffraction gratings or other devices. Next,

the light is directed onto a detector array.

The different light intensities that reach the

array are quantified for each wavelength.

Thus, the analyzer enables users to

identify and, after calibration, measure the

concentration of each element present in

the sample.

One important distinction: how a given

analyzer’s optical interface views the light

emitted from the plasma.

An axial-view system observes light from

end to end down the plasma’s whole

central channel. A radial-view system (see

Fig. 1) looks at a slice of light across the

width of the plasma. A dual-view system

manages to observe light both axially and

radially. Note that when collecting spectral

information, it’s often the case that the

more light, the better. So for example, axial

views provide inherently higher sensitivity

than radial views. (For more information

on plasma observation technologies, see

this SPECTRO white paper: “Comparing ICP-

OES Analyzers’ Plasma Views: Axial, Radial, Dual,

MultiView, and New Dual Side-On Interface”. This

1. Multiple mirrors2. Thermal stress3. Plasma contamination4. Plasma 5. Viewing volume6. Induction coil7. Sample in8. Light path9. Transfer optic

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1. Plasma2. Transfer optic3. Viewing volume4. Induction coil5. Sample in6. Radial light path

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1. Plasma2. Transfer optic3. Dual side-on light path4. Viewing volume5. DSOI single mirror6. Induction coil7. Sample in

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Fig. 1: Radial Plasma Observation

3Seeing the plasma in a whole new light: advantages of DSOI technology

paper focuses on two recent variations of

a dual-view design that users are applying

for many types of ICP-OES spectrometer

analyses.)

Vertical-torch dual-view: certain advantagesA vertical-torch dual-view system (Fig. 2) takes a

primary radial view across a plasma from a

vertically mounted torch, supplemented by

a sequential axial measurement taken down

the plasma’s central axis. This axial “second

look” is supplied via several mirrors,

arranged in a periscope optic mounted just

above the plasma.

Thus, the system achieves observation of

the plasma both axially and radially, in a

single analysis. In addition, such a system

may feature a specialized interference filter

that can allow the user to combine the two

views into one simultaneous measurement

by “blocking out” wavelengths above or

below 500 nanometers (nm) for the radial

observation, and above 500 nm for the axial

observation. This system possesses several

benefits.

In axial-only designs, especially when

attempting to analyze alkali and alkali

earth metals, the easily ionizable element (EIE)

effect degrades accuracy. By contrast,

the vertical-torch dual-view instrument’s

primary radial-view observation allows

it to compensate for EIE effects. Plus, its


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Fig. 2: Vertical-Torch Dual-View Plasma Observation


added axial capability contributes enough

extra sensitivity to measure down to trace

concentrations of elements such as toxic

metals (lead, cadmium, mercury, chromium,

etc.).

Vertical-torch dual-view: significant compromisesAlmost all dual-view designs share a bias

toward either axial-view or radial-view

performance. Only one view can utilize

the direct, primary light path; the other is

usually compromised in some fashion.

The vertical-torch dual-view design

mentioned above (see Fig. 2) illustrates

some of these compromises.

Its axial periscope optic contains three

mirrors. Since light is lost at each step in the

transmission path, three added reflections

diminish light throughput substantially.

Results: reduced light transmission in the

200 nm range — reduced even more below

185 nm, since this light path is purged less

effectively. Unfortunately, this limitation

strikes at what should be an axial-view

strength: sensitivity.

Also, remember that in this design, the axial

periscope interface is located just above the

plasma. This can pose significant drawbacks.

Particularly with samples containing high

total dissolved solids (TDS) or organic

materials, interface contamination can fall

back down into the plasma. Thus analysis

accuracy may be negatively affected.

And again, this design positions a relatively

delicate optical interface just above a

superhot plasma. It’s no surprise that

the resulting thermal stresses may inflict

significant component wear. So users must

be prepared for the added trouble and

expense of more frequent maintenance and

replacement.

The DSOI solutionOne newly developed plasma viewing

technology captures the advantages

of vertical-torch dual-view systems —

without their troubling disadvantages. Its

revolutionary design may represent the

simplest, most successful ICP-OES solution

in this category yet.


The dual side-on interface (DSOI) approach

(see Fig. 3 and Fig. 4) was pioneered by

SPECTRO Analytical Instruments in the

recently introduced SPECTROGREEN ICP-

OES analyzer.

“Dual” usually means providing both radial

and axial views. But in DSOI, it refers only to

a radial view: one that’s effectively doubled.

That is, DSOI uses a high-stability, vertical

torch whose plasma is observed via a unique

direct-light-path radial-view technology

which, unlike any previous design, deploys

two radial optical interfaces.

Thus, light emitted from the plasma in

both directions reaches the optical system

for analysis, using only a single additional

reflection.

The benefits of DSOIThe extra spectral information contained

in that additional light substantially boosts

sensitivity. So an analyzer equipped with

DSOI can yield twice the sensitivity of

conventional radial systems — and match

that of vertical-torch dual-view models.

(Both can achieve about half the sensitivity

of a specialized axial-view-only instrument.)

In cases such as the alkali elements, the

difference is even greater. Sensitivities

for sodium (Na) and potassium (K) are

enhanced by up to 5 times!

Additionally, a DSOI analyzer like

SPECTROGREEN provides high stability and

freedom from matrix effects, since it “blanks

out” interference-prone sections of the

plasma. It furnishes high matrix tolerance

and high linear dynamic range. So it’s ideal

for trace element analysis of higher-matrix

samples. Altogether, DSOI helps deliver

good performance at parts per billion

(ppb) to parts per million (ppm) elemental

concentration levels.

DSOI Advantages • High sensitivity

• No axial-view EIE effects

• No contamination of the

optical interface

• No thermal stress on

optical components

• One analysis for all required

wavelengths

• Fast analysis times

• Minimal maintenance

intervention


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Fig. 3: Plasma Observation with DSOI

The interface on one side of the plasma

captures the amount of emitted light normal

for radial observation, transmitting it into

the optical system. But the concave mirror

of a second interface on the other side

captures additional emissions. This added

light — with all its extra spectral information

— is reflected into the optics as well.

Fig. 4: Dual Side-On Interface (DSOI) Technology


Finally, where a vertical-torch dual-view

design places its optics directly above the

high-temperature plasma, a DSOI system

positions its optical interfaces safely to each

side. So it entirely avoids any accompanying

problems of contamination and thermal

stress. Thus, DSOI technology enables an

analyzer to maximize its durability, ensuring

significantly less need for the trouble and

expense of extra maintenance or repair.

User reactions have singled out DSOI

systems for their sensitivity, stability, and

lack of interferences. These make the

technology ideal for a wide range of routine

analyses in the following fields:

• Environmental

• Consumer product safety

• Pharmaceuticals

• Chemicals/petrochemicals

• Cosmetics

• Foods

• Agronomy

Note that one other technology avoids all

dual-view limitations — even the minor light

loss arising from a DSOI system’s single

added mirror. The SPECTRO Multi-View

system, available only in the top-of-the-line

SPECRO ARCOS analyzer, can shift viewing

orientation completely via a 90-second

mechanical changeover. So it’s the only

ICP-OES instrument that provides both

axial and radial plasma views without bias

or compromise. Again, see “Comparing ICP-

OES Analyzers’ Plasma Views: Axial, Radial, Dual,

MultiView, and New Dual Side-On Interface.”


CONCLUSION Although vertical-torch dual-view plasma observation systems

offer several advantages over earlier designs, they can come with

significant added complexities, trouble, and expense. On the other

hand, with few compromises or added costs, revolutionary new DSOI

technology can equip an elemental analyzer to deliver substantially

improved analysis for a wide array of applications.


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Seeing the Plasma in a Whole New Light: Advantages of DSOI