Seeing the Plasma in a Whole New Light: Advantages of DSOI ...
Transcript of 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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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
4 Seeing the plasma in a whole new light: advantages of DSOI technology
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.
5Seeing the plasma in a whole new light: advantages of DSOI technology
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
6 Seeing the plasma in a whole new light: advantages of 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.”
7Seeing the plasma in a whole new light: advantages of DSOI technology
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.
8Seeing the plasma in a whole new light: advantages of DSOI technology
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