Safe handling of gases used in Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy requires two main gases, namely, air as oxidant and a fuel gas such as acetylene or nitrous oxide. Safe handling of gas supplies is of utmost importance for efficient operation of instrument and averting laboratory mishaps.
The following practices are recommended for safe handling and storage of gas cylinders:
- Fasten gas cylinders securely to the work bench or a permanent wall
- Always use cylinder trolleys for cylinder transportation. Never roll on floors or ramps.
- Locate cylinders preferably outside the laboratory in a well ventilated housing protected from direct sunlight and rain
- Use approved colour-coded gas pipelines
- Use only approved regulators and hose connections. Left-handed thread fittings are used for fuel tank connections whereas right-hand fittings are used for oxidant and support gas connections.
AAS Gases handling practices
Compressed air is necessary as oxidant for flame operation. It is advisable not to use oxygen in place of zero air due to risk of flashback. Use oil-less noise free air compressor. On line filter is strongly recommended to remove entrained water, oil, water aerosols and solid particulates from compressed air lines.
Acetylene is the most commonly used fuel for flame operations. The gas is dissolved in acetone so cylinder must always be kept upright to prevent acetone from entering pipelines and hoses. At low tank pressure the acetone content in the gas increases and can result in an unstable flame. Gas cylinders should be changed when pressure falls to 600 kpA (85 psig) otherwise tubing and valves within burner gas control system can also get damaged due to acetone carryover. Two major precautions should be adopted when using acetylene gas. Firstly always use stainless steel tubing and fittings. Copper tubing and fittings should never be used as acetylene reacts with copper to form explosive acetylides. Secondly line pressure from gas tank to the instrument should not exceed 103kpA (15 psig). At higher pressures acetylene can spontaneously explode or decompose. Use of a filter and in-line flash arrestor is recommended for removal of particulates and acetone droplets and also to prevent flame flash back.
Nitrous oxide/acetylene flame temperature can reach up to 2800°C which is necessary for determination of elements that form refractory oxides and to overcome chemical interferences that may occur in flames at lower temperatures.
Nitrous oxide is supplied in a liquid state, initially at a pressure of about 5000 pkA (750 psig). As it is in a liquid state the pressure gauge does not give a true indication of the gas remaining in the tank until the pressure starts falling rapidly as the gas begins to deplete.
Nitrous oxide flame requires high flow rates of 14 – 30 L per minute. The expanding gas causes cooling of the cylinder pressure regulator which can lead to regulator diaphragm freezing. This can cause erratic flame behavior or even flashback. It becomes necessary to heat the regulator using either the built-in heater or an external heat source such as an electrical resistance heating tape. Lines carrying nitrous oxide must be kept free of grease, oil or other organic material that may cause spontaneous combustion.
Argon being inert is required in graphite furnace analysis to prevent combustion of graphite tubes at high temperatures. It is used as external and internal gas streams when using graphite furnace or as a carrier gas with mercury/hydride systems. Both liquid and gaseous tanks are available. Liquid argon is generally less expensive but cannot be stored for extended periods. If liquid argon is used the tank should be fitted with an overpressure regulator which will vent the tank which is necessary to keep the liquid argon cold enough to remain in liquid state. Gaseous argon tanks do not require venting and consequently can be stored for extended periods without loss.
At the end of the day close the valves tightly and relieve pressure in gas lines before turning off the exhaust hood.