PMT vs CMOS The paradigm shift in metal analyser detector technologies

The photomultiplier tube (PMT) used in some metal analysers, for high-end applications, is one of the few vacuum tube technologies still manufactured for industrial use.

However, an analyser based on PMTs suffers from inherent shortcomings. For example, it’s sharply limited in the number of elements it can detect. And its design makes it difficult or sometimes impossible to add new elements.

Fortunately, the solid-state revolution is finally catching up to this legacy application. Advanced complementary metal oxide semiconductor (CMOS) detectors made with proven integrated circuit technology have been developed for a new class of analysers. These equal or exceed every benchmark of PMT-based performance. Example: They render reliable results on a greatly expanded number of matrices, elements, and compounds, and can add new ones via simple software updates.

That’s good news for large foundries and primary producers of steel, aluminium or copper as well as secondary metal processors, aerospace and automotive companies, testing laboratories, and governmental or academic labs.

A new white paper Spectro Analytical Instruments details PMT issues as well as CMOS-based flexibility, sensitivity, stability, speed, and more.

Background: The principles of OES
The instruments discussed here are classed as arc spark optical emission spectrometry or arc spark OES analysers. Analysis begins when a metal sample is placed on the spark stand. The stand is internally flushed with argon gas to prevent contamination by elements in the air. An electrode several millimetres from the sample discharges a high-voltage impulse, or spark, which arcs to the metal. The spark vaporises or depletes some of the sample material, atomising and ionising it. This excited material emits energy, which becomes electromagnetic radiation, or light.

The latest revolution in metal analysis for process control and research

Specific spectral wavelengths of that light are characteristically emitted by specific elements. So each element has unique emission spectra, or analytical wavelengths. And the intensity of the light is directly proportional to the concentration level of a given element in the excited sample.

Emitted light reaches the optical system, and its wavelengths are separated by a diffraction grating. The light is directed onto a detector array and associated readout electronics, which provide data to allow the analyser’s software to quantify each light wavelength and intensity. Result: The user can identify and measure each element in the sample.
Note that the process involves numerous complications. The relevant wavelengths encompass the entire ultraviolet and infrared spectrum, from 120 nanometres (nm) to 780nm. Emission profiles are complex. Iron (Fe) alone possesses more than 4 000 different analytical emission lines.

Of course, each component of the OES process is important. But the part played by the detectors is especially critical.

The white paper discusses topics such as:
The trouble with tubes
The pros and cons of CCDs
The CMOS solution
Maximising flexibility
PMT: One detector, one line, no flexibility
CMOS+T: Full-Spectrum coverage for maximum flexibility
Achieving sensitivity and precision
Ensuring Stability
Accelerating speed of measurement
Enjoying industrial-strength durability
Drawing on dependable manufacturing sources

CMOS+T: Full-spectrum coverage for maximum flexibility
CMOS detectors are not limited to one detector, one element. So in the Spectrolab’s analyser, via a dedicated mirror, all of the thousands of pixels on the CMOS detector are exposed to all light lines emitted from the sample. Thus, in addition to efficient single-element focus, the system can capture full coverage of every wavelength on the entire relevant analytical spectrum simultaneously, from 120 to 780nm. This full-capture range exceeds anything possible with a PMT-based analyser.

A true revolution in high-end metal analysis

Spectro’s proprietary CMOS+T technology delivers flexibility that allows the instrument maker to design the optimal optical configuration for each customer, regardless of application. For example, users may specify any combination of the 10 standard primary metal producers’ matrices: Iron (Fe), aluminium (Al), copper (Cu), nickel (Ni), cobalt (Co), magnesium (Mg), titanium (Ti), tin (Sn), lead (Pb) or zinc (Zn).

Spectro Analytical Instruments possesses years of experience designing metal analysers using both PMT-based and CCD-based detectors. In fact, a previous model of their flagship metal analyser Spectrolab offered a hybrid PMT/CCD system, designed to maximise these technologies’ complementary capabilities.

In terms of sample throughput, Spectrolab S meets the metal market’s need for ultra-high-speed measurement. For example when analysing low alloy steel, it can deliver highly accurate measurements in less than 20 seconds

The continued development and optimisation of CMOS semiconductor detectors has transformed the paradigm. Today, using an all-CMOS detector array, coupled with Spectro’s proprietary CMOS+T technology, instruments such as the new Spectrolab’s analyser have proven to meet or exceed every aspect of both CCD- and PMT-based performance.

To view the full version of the white paper visit:

For further details contact Spectro Analytical South Africa on TEL: 011 979 4241 or visit