Cells

Hellma Cells are manufactured from different types of glass. The most important criterion for the choice of a particular type of glass ist the spectral range for which the cell is intended. Basically, we differ between two ranges of material: 

Quartz Glass and Optical Glass

Quartz consists exclusively of silicon dioxid (SiO₂)and shows some remarkable properties:

• Quartz displays a high UV transmission, in highly purified synthetic Quartz down to well below 200 nm
• The thermal expansion of quartz is extremly low
• Quartz is chamically very resistant and maintains its shape, even at high temperatures up to approximately 1000°C

Common criteria for all types of Optical Glass are:

• the cells made from it are used in the visible range of the spectrum
• the glasses Hellma uses are characterised by relatively low values of refractive index and dispersion
• the display good resistance to chemicals

Application and Handling of Cells

The Hellma precision cells are manufactured from glass and quartz and possess all the benefits of this material. Hellma generally recommends that cells are cleaned and dried immediately after use and returned to their storage cases. Do not keep the cells in the open in a corrosive atmosphere, and do not leave the polished windows in contact with liquid over long periods of time. Both conditions could lead to formation of deposits or stains and could render the cells unusable.

To avoid scratches on the precision-polished windows, the cells should never come into contact with objects made of hard materials like glass or metal.

• Care is required when inserting the cells into a metal cell holder.
• When filling the cells with liquids using a pipette, never touch the polished windows with the pipette.
• Never use metal tweezers or pliers for carrying or holding the cells.

Handling Cells with Stoppers

Use caution when handling cells with stoppers: Cells that contain liquid and are sealed with stoppers may crack as a result of increased inner pressure. The most common cause of such a pressure increase is the expansion of the liquid within the cell due to an increase in temperature.

A temperature increase can be caused by:

• heat conducted from an exterior source
• a chemical reaction within the liquid
• radiation absorption within the liquid

You can avoid the destruction of the cell by too much pressure in the following ways:

• Fill the cell just high enough so that the light beam can pass through the liquid. The liquid can expand into the remaining cell volume when its temperature increases.
• Should you fill the cell to the brim, only put the stopper on loosely so that the extra liquid can escape.
• Do not try to force the stopper into place, as this will inevitably result in damage to the cell.
• Use stoppers with a pressure release capillary.

Please note that high pressure may destroy some other kinds of cells as well. This occurs if the liquid contained is subjected to extreme changes in temperature. For
example, cells for anaerobic measurements may be affected. On the one hand it is possible to cool an empty cell down to just few Kelvin without destroying it, but the same cell, even if it is not sealed will burst, if filled with water and brought to a temperture a few degrees below the freezing point. The reason for this is the fact that water does not only expand upwards when it freezes, but in all directions equally which can cause the cell to burst.

Ultrasonic Machinig:

Many cell typed, especially flow-trough cells, require borings and cavities with highly complex shapes. With ultrasonic machining, which resembles the spark erosion procedure used in metall working, Hellma possesses a technology that allows to produce high-precision borings and cavities of 0,5 nm to 60 nm in the brittle glass material. The tools required for manufacturing borings of almost any imaginable cross-sectional shape are made in Hellma´s own machine-too, department.

Fusion Technology:

the process of direct fusion, which was developed by Hellma, and is almost always used nowadays ideally fulfills these requirements. However, it also presupposes that the surfaces to be fused together are polished and possess a flatness tolerance of less than 1 µm. Since no adhesives whatsoever are employed for fusing the glass parts, the seams display the same high resistance against chemicals and increased temperatures as the solid glass itself. Sinter glass fusing is only used in the few exceptional cases where direct fusion is impossible for technical manufacturing reasons.

Antireflection Coating:

When radiation passes through the cell, part of it is reflected by the exterior surfaces. The transmission is reduced by this reflected portion by approximately 8%. Vacuum evaporation of thin layers of a suitable material can reduce these reflections, thereby giving a higher transmission for the cell.
The standard is a multilayer antireflection coating that reduces reflection over a broad spectral range. Over a spectral range of between 440 nm and 650 nm the reduced reflectance is at most 0,4%. This coating is highly adhesive and resistant to both abrasion and climatic influences.

Mirror Coating:

For some purposes (e.g. fluorescence measurements) mirror coated cells are used. Both the rear window to the incident light and the left adjacent window are mirror-coated on the outside. The standard mirror coating consists of a layer of vacuum-evaporated aluminium that has a degree of reflection of mor than 80% over a spectral range between 250 nm and 2500 nm. This coating is covered with a protective layer which is extremly hard and durable. Additionally, the mirror coated window are protected against external scratches by a layer of black lacquer.
On request we can also supply metal coatings with different reflective properties and with the coated surfaces in other positions.

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