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Measurement principle

Our mercury analyzers rely on the atomic absorption spectrometry (AAS).

This method is well proven for decades and used worldwide. Due to constant improvements and optimizing our instruments reach a maximum of analytical performance and measurement accuracy. Detecion limits, reproducibility and precision of measurements were raised to highest operational standards. Specially developed calibrators guarantee a very reliable quality assurance.

Atomic absorption

Atomic Absorption Principle:

The small violet balls represent light photons which are emitted from a UV source and pass through the measuring cell axially. Some of them are initially absorbed by the mercury atoms (big red). After an extremely short period of time they are emitted again, namely omnidirectional. Hence the original light beam is attenuated.

As analytical detection method we use the cold vapour atomic absorption spectrometry (CVAAS) to get maximum analytical performance and measurement accuracy for mercury concentrations ultratrace levels.

The absorption of light caused by atoms in gaseous state was studied already in the year 1859 by Bunsen & Kirchhoff in Heidelberg. Much later, in 1952, Sir Alan Walsh developed the Atomic absorption spectroscopy method of chemical analysis.

Sketch of a flame atomic absorption spectrometer as designed by Sir Alan Walsh

 

However, mercury being an element with very special physical and chemical properties, it cannot be measured very sensitively by flame photometry. A more successful way to precisely analyze traces of mercury was proposed by Poluektov, Vitkun and Zelyukova in 1963, it is characterized by measuring mercury at room temperature without any prior atomizer (cold vapour atomic absorption spectrometry, CVAAS).

Setup of our detectors

The mercury concentration is measured at a wavelength of 253.7 nm in a 230 mm optical cell entirely made of fused silica.

Mercury Instruments

Schematic diagram of the AAS measuring-cell unit

 

The atomic absorption method, in contrast to the atomic fluorescence method, is not prone to interferences caused by the quenching effect and does not require expensive noble gases as carriers.

All of our mercury analyzers use a high-frequency driven electrodeless Hg low pressure discharge lamp (EDL) as UV light source. This lamp generates emission lines of an extremely narrow bandwidth which are congruent with the absorption lines of the Hg atoms. Nonspecific absorption is thus minimized.

To compensate lamp drifts the reference beam stabilization method is applied in our analyzers.

Benefits of electrodeless lamps compared
to common low pressure Hg vapor lamps with electrodes

EDL
  • Considerably longer lifetime
  • High stability of light intensity

The electrodes in mercury lamps limit useful life
for multiple reasons:

  • The metal on the electrodes evaporates,
    causing bulb wall blackening, and leading to lumen depreciation
    and elevated lamp temperatures.
  • Hot spots can form on the filaments
    accelerating evaporation and leading to
    filament failure.
  • The glass to metal seals required
    to mount the electrodes weaken the bulb wall
    leading to outgassing and failure.

The optical bench: "The Soul of the Machine"

The lamp unit containing the UV lamp (EDL), together with the optical cell and the UV detector form the
optical bench : "The Soul of the Machine". It's what makes all our instruments come alive.

The soul of the machine

The GoldTrap: "The Heart of Gold"

The red mercury atoms are selectively captured on the surface of the gold trap, whereas the rest of the sample gas molecules (blue spheres) are passing the measuring apparatus unobstructed. After a short sampling phase a valve changes from sample to clean air and the gold trap is heated immediately. The liberated mercury is carried through the measuring cell where its mass is quantitatively determined by atomic absorption (see above). This method is also called amalgam technique because mercury forms an amalgam with the gold surface.

For enhancement of sensitivity and parallel total elimination of interference by matrix-components we use the specially developed GoldTrap. During a sampling phase mercury in the sample gas is adsorbed and concentrated in the GoldTrap. Subsequent very rapid heating abruptly evaporates all of the mercury within the GoldTrap. This technique makes mercury measurements at ultratrace levels in complex sample matrices possible.

GoldTrap

 

"Heart & Soul"

One Heart and One Soul: The specially devised GoldTrap and the optical bench form the foundation of the quality and accuracy of our successful analyzers to measure mercury at trace and ultratrace levels.