Page 74 - MTW JANUARY 2022 Digital Magazine

TECH INNOVATION

Innovations and Advancements in

Optical Emission Spectrometry

By Mukund Pant

ptical Emission Spectrometry – or Arc-Spark Optical /
Atomic Emission Spectrometry, to use it’s full and proper
Oname – is well over a century old. As far as its basics are
concerned, the technology as about as stable as it gets. Over time
though, the instruments used for this have not only evolved and
changed but done so dramatically. Indeed, the last five years have
arguably seen the most critical advances in the field – and these
advances have spanned across virtually every system that these
instruments are comprised of.

From single to dual – a massive leap in resolution
First up, there are the optics. As the name itself suggests, optics
lie at the heart of the Optical Emission Spectrometer (OES). For
over a century since the first instrument was made, designers of
OES relied upon a single and very large optical chamber in which
light was received, diffracted and then the individual elements
were analysed. This ‘single optics’ design however has now
become virtually obsolete, replaced almost entirely by the more
sophisticated ‘dual optics’ design.
The quest for every optics engineer is to enhance optical
resolution. Optical resolution depends on a number of parameters,
but most critical are the gratings’ groove densities and the focal
lengths. For a particular groove density, the only way to enhance Metavision-10008X features full spectral coverage from 120
to 800 nm with capability to analyze 60+ elements across
resolution would be to increase the focal length and for decades, all bases including C, S, P, N, O & H down to 1 ppm; Rapid
this was the path designers took. So much so, that it became analysis in 10 Seconds!
almost a given that the higher the focal length, the better the
spectrometer. As a result, every single modern OES is designed today with
With advances in grating manufacturing technology though, dual (or more) optics chamber. In these designs, an exceptionally
optics designers today have the option of using gratings with high-resolution UV optics using high groove-density gratings
50%-100% higher groove densities! The higher groove density is accompanied by a marginally lower-resolution Visible optics
results in a much larger spectral spread and brings into play chamber (which also uses a high groove-density grating). So,
the option of using a shorter focal length to generate the same if you want to understand how modern or state-of-the-art
resolution or using the same focal length to deliver much greater the technology of your spectrometer is, the optical setup is an
resolution. As with everything though, this does come at a certain excellent place to start!
cost. In this case, the cost is that the spectral spread becomes
so wide as to eliminate the option of using a single optical Neither purged nor a vacuum – hermetically sealed!
chamber. So wide and high-resolution is the dispersion that it is Another big change on the optics has been the continued move
geometrically impossible to accommodate in a single chamber. away from the inefficiency of vacuum optics. The early days of
Nonetheless, so significant are the benefits of higher resolution this shift saw designers move towards a purged optical chamber
on the ultra-violet (UV) and deep ultra-violet (DUV) sides of the – where Argon gas was constantly purged through the optics
spectrum that the use of a dual-optics design is well worth the chamber to ensure inertness. Purging however is also not perfect
investment. and ‘as near as possible to perfect’ really is what any good

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