There are inherent features both in design and the way in which analytical gas generators operate that mean they offer clear compelling reasons to switch from high-pressure gas cylinders
High pressure gas cylinders are a common site in many laboratories.
They have a familiarity and provide the gas that's required, so it can be said that the cliched phrase of 'If at isn't broke, don't fix it' could well apply.
Despite this, increasing numbers of analytical instrument users are choosing to supply their GC-FID, LC-MS and other instruments with gas using an analytical gas generator.
Driving this decision will be a combination of factors that broadly group into four areas: safety, cost, convenience and purity.
Safety concerns.
High pressure gas cylinders can provoke safety concerns in a number of ways and different scenarios - some with potentially fatal consequences.
The presence of high pressure gas cylinders in the laboratory has been likened to sharing the laboratory with a potential missile.
This stems from the behaviour of a cylinder that suddenly depressurises.
There is enough force released with a European L-sized cylinder to accelerate the cylinder to something like 108km/h in approximately ten seconds.
Cylinders this size weigh in at 90kg when empty, so there'll be enough momentum to cause some serve damage.
It is because of this potential 'missile scenario' that cylinders tend to be strapped down to something fixed.
Even if they're restrained should a large cylinder suddenly vent its content into the laboratory then there are potentially fatal consequences provided it's not an air cylinder.
If a high pressure cylinder of nitrogen suddenly vented to the atmosphere of the laboratory, then more than 9000litres of unbreathable gas would be released.
This would dramatically reduce the oxygen content of the air - presenting the possibility of asphyxiation.
The risk of oxygen displacement from the atmosphere is also associated with liquid gasses whose volume will increase 1000-fold when in the gas phase.
This mean that liquid nitrogen Dewars can also be hazardous.
If the gas suddenly venting was a potentially explosive gas, for instance hydrogen, the result could be much more dramatic.
Hydrogen will form a potentially explosive mixture at just 4% volume in the air.
These possibilities life threatening safety concerns associated with high-pressure gas cylinders.
However, there is still the potential for other non-fatal injuries.
There's the practise of rolling cylinders on their bottom edge and with it the possibility of trapping toes or feet.
With the 'smaller' cylinders there's potential for heavy lifting injuries if they're being placed on a bench top.
Counting the cost.
With high-pressure cylinders there are storage requirements that are dictated by safety concerns; things such as separating hydrogen cylinders and cylinders of oxidizing gases.
These often result in cylinders being some distance from where the gas is used and hence long gas lines.
While the longer gas lines result from the positioning of cylinders for safety concerns, the impact will be in the area of cost, convenience and purity.
With any gas line there's the potential for leaks and the longer the line, the greater the potential.
Hence there's a requirement to regularly leak-check the gas supply line - this increases costs and decreases connivance.
while leaks will allow gas to escape, they'll also allow impurities to enter the gas supply, which reduces purity and influences the accuracy of any analysis.
Analytical gas generator.
Analytical gas generators can remove the requirement for high-pressure gas cylinders for many analytical gas instrument users.
They are typical placed next to the instrument they're servicing.
This removes any need for extended gas lines and with them, associated problems affecting purity, cost and convenience.
There are inherent features both in design and the way in which generators operate that mean they offer clear compelling reasons to switch from high-pressure gas cylinders.
Increase safety.
High-pressure gas cylinders will contain gas at a pressure of approximately 200-300 times atmospheric pressure, and gas that is released to atmospheric pressure would have a volume in the region of 9000 litres.
Analytical gas generators operate at a fraction of this pressure and have very low volumes of stored gas within them.
A leading hydrogen generator will have just 50ml of stored gas, which will be at a maximum of around four times atmospheric pressure.
Hence, the missile concern is removed with a generator and there's no large volume of gas to suddenly vent and make the atmosphere potentially explosive or deficient of life-supporting oxygen.
Additional safety features can include leak detection auto shut-off devices.
Increased convenience.
High-pressure gas cylinders require regular replacements.
Gas cylinder running out part way through analysis, mean unplanned downtime.
A replacement cylinder has to be collected and the old one removed - which brings manual handling and safety concerns.
After the new cylinder has been connected, we've now got to restart the instrument, wait for stable baseline and possibly even recalibrate.
Then the samples can be run again.
Life is more convenient with a gas generator - there's no unplanned downtime.
Analytical gas generators only require simple, quick maintenance, which can be planned for - they don't unexpectedly run-out of gas halfway through analysis.
Increased purity.
Analytical gas generators provide a constant source of gas.
This removes the variances in purity between cylinders, helping to improve sensitive analysis.
Purity is also preserved because there is no chance for impurities to enter the gas pipes, which may happen as cylinders are switched and regulators changed over.
Hidden costs.
High-pressure gas cylinders can also prove to be costly; typical payment periods on analytical gas generators are short - sometimes less than one year.
The costs of using high-pressure cylinders is not just the costs of the gas itself but other charges, some of which can be seen and others which are hidden.
Cylinder rental and delivery charges are readily apparent, however, there are also hidden costs.
These must be included to reveal the true costs.
Hidden costs of high pressure cylinders are not seen with analytical gas generators.
There are no recurring costs with generators for activities such as ordering replacement cylinders; there are no storage costs for the spare and empty cylinders; the cost of lost productivity through the need to stop and replace cylinders is not seen either.
LC-MS.
Hollow fibre membrane nitrogen generators are often used in LC-MS applications.
The membrane simply and efficiently separates compressed air into nitrogen and an oxygen-rich permeate stream - which contains other unwanted impurities such as water vapour.
The membrane achieved this due to its selective permeation rates for different gases - oxygen and other unwanted constituents of the compressed air simply pass through the membrane at a much faster rate than nitrogen.
These generators, when connected to an existing compressed air supply will provide a constant supply of nitrogen with no moving parts inside the generator.
This means the generator is very quite while operating and there's practically nothing to wear out or need replacement.
The only consumable is a pre-filter that protects the membrane, which is changed in five minutes, in a planned manner every six months.
GC applications.
Hydrogen offers advantages for GC users when used as a carrier gas.
The Van Deemter curves illustrate the wide range of which high efficiency is obtained making hydrogen the best carrier gas for samples containing compounds that elute over a wide temperature range.
The risks associated with high-pressure gas cylinders have already been outlined - hence a gas generator is the smarter choice for hydrogen.
Some generators incorporate a hydrogen purifier, which is manufactured from palladium.
Palladium has unique properties, which permit only hydrogen to diffuse through it; this ensures the highest possible purity of hydrogen.
Gas generators can offer improvements in terms of safety, purity, convenience and cost so there is scope for traditionalists to, at least, re-consider the use of high-pressure gas cylinders.
