During the development of the Tennant20, occupational safety, user-friendliness, and quality were top priorities. 

In the coating process using plasma-enhanced chemical vapor deposition (PE-CVD), osmium tetroxide (OsO4) is separated into osmium and oxygen within the plasma, and metallic osmium is deposited onto the sample. 

As the Tennant20 is a closed system, users do not come into contact with free osmium tetroxide during normal operation. The device operates under vacuum, so in the event of a leak, ambient air enters the system and prevents the release of OsO₄ gas. 

Users are guided through all work steps by the operating menu. After inserting the samples and closing the chamber, the coating process runs fully automatically. This minimizes the risk of user error. 

The Tennant20 uses the same ampoules filled with osmium tetroxide crystals as those used for heavy metal contrasting of TEM samples. The ampoules are loaded into a metal cylinder, broken open inside the cylinder and safely stored under vacuum. 

Multiple safety valves prevent unauthorized inflow of osmium tetroxide gas into the reaction chamber. This makes it safe and easy to load and remove samples when the reaction chamber is open. 

After the coating process, unused osmium tetroxide gas is directed from the reaction chamber into the oil of the rotary pump, where it is neutralized. An activated carbon filter attached to the pump outlet further neutralizes any remaining traces before the gas is released into the exhaust air. Contaminated oil and filters can be conveniently disposed of as heavy metal waste. 

Before opening the reaction chamber to remove the samples, the chamber is repeatedly evacuated and ventilated to remove even the smallest residues of osmium tetroxide. Afterwards, the reaction chamber is flooded with ambient air and can be opened. 

 

Osmium Tetroxide Detection Limit and Legal Thresholds: 

To test the safety of the device, an independent Japanese test laboratory carried out osmium tetroxide concentration measurements at various locations on the device and under various operating conditions. 

The measured emission values were determined by mass spectrometry using ICP-MS. 

Detection limit of the instrument: 0.02 ppb*/0,2 µg/m³
* ppb (parts per billion) 

The legal limit values were determined by the American Conference of Governmental Industrial Hygienists (ACGIH). 

Limit value – time-weighted average (TWA): 0,2 ppb 

Limit value – short-term exposure limit (STEL): 0,6 ppb 

** The exposure limit value is for an 8-hour working day and a 40-hour working week.
*** The exposure limit value for a short period of time (approximately 15 minutes) without adverse health effects. 
 

The concentrations under normal working conditions were all below the detection limit (0.02 ppb) of the measurement method. It was confirmed that the measurement results were below the ACGIH limits (TLV-TWA: 0.2 ppb; STEL: 0.6 ppb). 

The Osmium-coating system is often, but not necessarily, installed under a fume hood in which open osmium tetroxide can be used.  

 

General Information on Osmium Tetroxide and Pure Osmium Metal: 

Osmium tetroxide (OsO₄) is a chemical compound frequently used in electron microscopy for the fixation and heavy metal staining of samples. It primarily oxidizes lipids and contrasts them selectively. OsO₄ is a volatile, toxic chemical compound that reacts strongly with organic substances. Without proper safety precautions, it can cause severe damage, especially upon contact with skin or eyes, or through inhalation or ingestion. 

Osmium as a pure metal, with a density of 22,59 g/cm³, is the densest naturally occurring element on Earth. Among all the platinum metals, osmium has the highest melting point (approx. 3130°C) and the lowest vapor pressure. Its bulk modulus of 462 GPa is among the highest of all known elements and compounds, making osmium less compressible than diamond, which has a bulk modulus of 443 GPa. 

These properties explain the stabilizing effect of ultra-thin osmium layers on fragile samples.