Optical Emission Spectrometers (OES) make use of detectors that convert incident light (optical signal) into electrical impulses that can then be measured quantitatively and compared using a database, to give the required output – the elemental concentration of the element’s being measured.
After its invention in 1930, Photomultiplier Tubes (PMTs) emerged as the detectors of choice for OES makers. PMTs were seen as the cutting-edge of technology – as they replaced the use of photographic plates that were the norm till that time. Using PMTs, instruments could automate measurement. PMTs rapidly took over the market and soon virtually every OES used these detectors. PMTs also improved over the decades and reached their peak by the late-1980s. Thereafter however, as with every technology, they stagnated – and a newer more modern technology emerged.
CCD, though invented in 1969, started being regularly used in industry only towards the end of the 1980s. The introduction of CCDs to spectrometry, indeed, marked the beginning of the end for PMT-based Spectrometers as well as a lot of other devices. However, it is only now, with the increasing maturity of the new incumbent, that the tolling of the bells has become loud and clear enough to be heard.
The Charge-Coupled Device (CCD) linear array detector technology had started seeing applications in optical fields well before, but by the 1990s, CCD technology had advanced to the point of making them viable for usage in high-precision applications like spectrometry. Given their advantages – smaller, lighter, dense number of pixels on each detector (a PMT is a single-pixel detector), CCDs offered all the advantages an R&D design engineer could ask for – apart from the highest levels of sensitivity. This however was improving rapidly too. By the turn of the millennium, every renowned OES maker had started evaluating CCD detectors and introducing models based on them. This pace has not slackened; CCD detectors have advanced so rapidly, they’re used in all manner of high-end applications, including space applications, imaging from satellites etc. and of course, the highest-end applications in spectrometry.
As OES manufacturers, we evaluated all manner of detectors – and chose the CCD as our detector of choice. As technology evolved further, CMOS, which had previously lagged CCDs on performance despite offering cost and scalability advantages began to gather steam. By the mid-2010s, the latest generation of CMOS detectors began to match and even better CCDs on some aspects. Compared to PMTs, these detectors were superior on just about every parameter. As leaders, we were the first to evaluate and then adopt these CMOS detectors – launching our first CMOS-equipped OES back in 2016! Our pioneering work has today been mirrored by virtually everyone else. Today, CMOS/CCD detectors are accepted globally as the very best detectors for all manner of spectrometer applications, even – and indeed, particularly – at the very highest end!
We continue, however, to face questions from customers who carry legacy views on the benefits of PMTs – or those who are fall prey to the marketing ploys of a handful of competing firms that have still not been able to optimize CMOS/CCD technology in their own instruments and therefore stick to PMTs detectors. This document aims to dispel some misconceptions and educate users on the differences between the detectors and their capabilities.
In simple terms, a PMT is a single-pixel detector; each PMT can therefore detect just one wavelength – the one it is specifically aligned for. A near-obsolete legacy device, the PMT is a remnant of the vacuum tube era. Each PMT captures emitted photons of light and its photocathode layer then converts these photons to electricity. Dynodes are then used to multiply this charge multi-fold making it readable for the instrument. PMTs are of different specifications and types – and therefore prices too. PMTs used in the Deep UV section of a spectrometer for example are multiple times as expensive as those used in the Visible section. Channel PMTs (also called CPMs) are just another variant of PMTs that narrow, curved semi-conductive channel to perform the same functions as a classical dynode chain. CPMs are therefore identical in nature to PMTs and offer the same pros and cons. PMTs are, fundamentally, large (as compared to other solid state devices), single-pixel detectors that offer excellent signal amplification and near-instant response times (given they’re analog devices, as it were). The Pros and Cons of PMTs can be summarised as below:
Pros of PMTs / CPMs | Cons of PMTs / CPMs |
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CMOS (Complementary Metal Oxide Semiconductor) and CCD (Charge-Coupled Device) detectors are solid state detectors that convert incident photons into electrical signals. The fundamental advantage of these detectors stems from the fact that – as solid state devices, they are extremely compact and each detector has thousands of pixels – each of which is, in effect, a detector as each pixel’s output can be individually measured. Hence, technically, a 2048-pixel detector covers 2,048 separate wavelengths. Given the extremely compact size of these detectors, therefore, a spectrometer using these detectors can cover every wavelength of light, without making the compromises that PMT detectors mandate.
The CCD was the first to emerge and linear CCD detectors were first used in spectrometers back in the late 1980s. At this time, these were considered fit only for basic and entry-level models, given higher noise, low speed and lower sensitivity as compared to PMT detectors. With the influx of more persons and ideas into this field, the rapid advance in solid state devices has seen them progress and improve in leaps and bounds. By the mid-nineties, CCD detectors began to offer far better performance – and emerged on mid-range OES, even offering low level Nitrogen analysis. By the 2010s, CCD and CMOS detectors surpassed PMT devices even in their core strengths – sensitivity, noise levels and speed. Today, the latest CCD/CMOS devices offer better performance on virtually every parameter relevant to a spectrometer! Indeed, CMOS detectors now enable even lower detection limits than PMTs and allow for all manner of top-end features including Time-Resolved Spectroscopy (TRS), Single-Spark Analysis and a whole range of superior features and analytical upsides.
Note: While CMOS detectors have replaced CCD detectors virtually across the board, the term “CCD” remains ubiquitously used, by manufacturers and customers alike, to refer to all such solid-state devices.
Output Parameters – Elements covered, detection limits, feature-set etc. |
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Parameter #1 |
Elemental coverage: |
CPM / PMT Spectrometers |
A PMT OES typically offers a maximum of 40 elements as delivered to multi-calibration customers as it runs up against the physical Normally a PMT OES has ~50 PMTs. The maximum is typically about 100 in very large units. Now, even a single element requires at least |
Metavision CMOS/CCD |
Even the most compact CMOS/CCD detectors have at least 2,048 pixels – each of which can detect a distinct wavelength. The CMOS/CCD The most compact CMOS/CCD OES (MPS’s MOSS) today covers 35+ elements as standard. The top-end models offer well over 60 |
Parameter #2 |
Analytical range: |
CPM / PMT Spectrometers |
PMT OES had historically been considered superior for low-level trace analysis and high-purity metals analysis. This was on account of their |
Metavision CMOS/CCD |
Advances in speed and sensitivity have led CMOS/CCD detectors being capable of Time-Resolved Spectroscopy (TRS) and Single-spark Some examples are analysis of Se, Te and Bi all down to sub-ppm levels in Pure Copper and the analysis of C, O and N down to single-ppm Metavision CMOS/CCD OES today deliver pure metals analysis of 99.997+% purity with ease! |
Parameter #3 |
Wavelength Range / span |
CPM / PMT Spectrometers |
Unlike CMOS/CCD OES, PMT OES makers mention range as representative of the range within which they can place a PMT. The range of a While PMT OES models claim coverage of the full range, these OES have severe limitations and therefore struggle to even utilize the |
Metavision CMOS/CCD |
120–800 and even more. Using CMOS/CCD detectors, the entire working spectrum is covered in the specified range and any elements that |
Parameter #4 |
Feature-set |
CPM / PMT Spectrometers |
PMT OES offer similar features in terms of soluble-insoluble analysis. Other features vary by manufacturer. |
Metavision CMOS/CCD |
Today, Metavision CMOS/CCD OES deliver a full feature set including soluble-insoluble analysis for inclusions, melt addition programs to |
Parameter #5 |
Precision and accuracy |
CPM / PMT Spectrometers |
PMT detectors are large – and limited to a single wavelength per detector. This results in twin issues:
As a result, PMT OES make design compromises around the inherent limitations of PMTs and give inferior performance to CMOS/CCD OES. |
Metavision CMOS/CCD |
Metavision CMOS/CCD OES offer better performance across the range of analysis as compared with PMT OES. This is down to our ability The result is that for every element, in every part of the range, a Metavision CMOS/CCD OES optimizes the lines used and therefore |
Flexibility and Scalability |
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Parameter #1 |
Upgradeability – Addition of lines post-purchase: |
CPM / PMT Spectrometers |
Normally not possible & extremely expensive if done at all. A detector needs to be added per line as each PMT detector can only detect one line. Even a single element addition requires at least one Beyond a certain point, line addition becomes impossible as the optics chamber can no longer accommodate more detectors. At this time, |
Metavision CMOS/CCD |
Can be easily and economically done at Site Requires no hardware addition and no opening of instrument. Low investment in terms of cost and time. MPA accomplishes these |
Parameter #2 |
Upgradeability – Addition of programs post-purchase: |
CPM / PMT Spectrometers |
Is prohibitively expensive. Since hardware addition is needed and that too within the Optics Chamber, the instrument has to be sent back to the manufacturer for |
Metavision CMOS/CCD |
Can be easily and economically done at Site As described earlier, no hardware addition is required. As such, additional matrices, bases and elements can be added easily and |
Ease of Use – Operating, daily tasks and routine maintenance |
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Parameter #1 |
Profiling |
CPM / PMT Spectrometers |
Needs very regular profiling. |
Metavision CMOS/CCD |
Is self/automatic profiling. |
Parameter #2 |
Warming-up time |
CPM / PMT Spectrometers |
Takes several hours (some models even take a full day from cold start). |
Metavision CMOS/CCD |
Within 45 minutes from a cold start; top-end models can take as little as 15 minutes |
Parameter #3 |
Analysis times |
CPM / PMT Spectrometers |
PMT OES typically take longer to deliver results. Analysis times range from 30-60 seconds for most PMT models |
Metavision CMOS/CCD |
Metavision CMOS/CCD OES leverage the strengths of solid-state devices and deliver rapid analysis results within 10 seconds for the needs |
Parameter #4 |
Stability |
CPM / PMT Spectrometers |
PMT OES typically require far more frequent re-standardisation. |
Metavision CMOS/CCD |
Metavision CMOS/CCD OES are remarkably stable over extended periods of time – lasting for several days without the need for restandardisation. |
Parameter #5 |
Re-standardisation |
CPM / PMT Spectrometers |
PMT OES typically offer only multi-point re-standardisation; this is on account of these being extremely dated instruments rather than their |
Metavision CMOS/CCD |
Metavision CMOS/CCD OES offer the facility of single-sample re-standardisation which saves large amounts of time for users. For users |
Parameter #6 |
Size |
CPM / PMT Spectrometers |
Is comparatively far larger; even the smaller, low-spec and low-end PMT OES are significantly larger than the larger CMOS/CCD OES models |
Metavision CMOS/CCD |
Even the largest Metavision CMOS/CCD OES, which are floor-standing, are far more compact than any PMT OES, given that they do not |
Lifetime costs and Risks |
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Parameter #1 |
Acquisition costs |
CPM / PMT Spectrometers |
PMT OES are always more expensive than any comparable CMOS/CCD model. These models require more detectors, more electronics |
Metavision CMOS/CCD |
Metavision CMOS/CCD OES offer exceptional economy in every class. From offering Oxygen (10 ppm) and soluble-insoluble analysis etc. |
Parameter #2 |
Electricity consumption |
CPM / PMT Spectrometers |
Typical PMT OES consume over 2 KW as they are much more power-hungry and cost a lot more to maintain. |
Metavision CMOS/CCD |
CMOS/CCD OES are very economical. For example, Metavision OES consume just 50 W in standby and 120 W during analyses |
Parameter #3 |
Risk – caused by Vacuum Pump |
CPM / PMT Spectrometers |
PMT OES need a high vacuum pump. This is deleterious for the optics, as in case of oil suck-back, the optics get destroyed. This also |
Metavision CMOS/CCD |
CMOS/CCD OES do not need a vacuum pump. These use sealed Argon-filled optics and therefore are risk-free. |
Parameter #4 |
Expense on spares |
CPM / PMT Spectrometers |
PMT detectors are more prone to failure, requiring replacements. PMTs are expensive – especially UV region PMTs. Added to this, there |
Metavision CMOS/CCD |
Very little; in fact, negligible. CMOS/CCD detectors are used in all manner of applications from space probes to spectrometers and then |
Parameter #5 |
Obsolescence |
CPM / PMT Spectrometers |
PMTs are getting obsolete; CMOS/CCDs are the present and future of OES. Just 2-3 firms globally continue on PMTs and each of them |
Metavision CMOS/CCD |
CMOS/CCD technology is a core research area and performance levels are continuing to improve rapidly. As such, CMOS/CCD detectors |
Of course! As with any field, this is merely the beginning. For more, do refer to: