Head Space Sampler: Pressure/loop Vs Balanced-pressure

[Terry]

Hey guys,

I am selecting a Head Space (HS) sampler for our chemistry lab.

As you know, there are two kinds of HS sampler: Balanced-pressure and pressure/loop.

I have only used the presse/loop type, and have no idea about the balanced-pressure.

Would you please shed some light on the pros and cons for each type?

Thanks

Terry

[krickos]

Hi Terry

Well was a long time ago I used balanced pressure (ie Perkin Elmer Hs-40)
and not that much either but as I recall:

Using the pressure-balanced technique,
the headspace sample is introduced onto the
column without using a gas syringe, avoiding
fractionation due to pressure changes in the
syringe; and, since the system is closed, there
is no loss of headspace during transfer. The
technique does not require multiport valves,
minimizing the number of components in
contact with delicate samples. (nicked that part from the web)

Downsides, well memory is vague but always found Agilents systems (pressured loop) with EPC controls of all gases more flexible.
Also have a bit of a stomach feeling that the HS-40 in general had a slightly worse precision that the Agilent counter part.

Upside with a fixed loop (normally 1 or 3 ml) is that you do not have to consider flow controll in the same way ie breifly put "just" make sure that the loop is filled with sample and run column at flow/pressure as you desire.
Down side with loop, well fixed sample size is one. An other is mentioned above ie caution (correct temperatures) has to be taken so that high boling analytes/sample solvents do not condesate in valve/loop causing contamination and potential loss of sensitivity over time. Loop/valve system may also require "steam cleaning" or physical removal/cleaning of loop at intervals depending on use.

[Ron]

Bottom line, valve and loop, heated syringe, and balanced pressure all work. Fewer people use balanced pressure because it is only made by one company, while there are multiple manufacturers of both valve and loop and heated syringe.

I would consider price and local support strongly in the decision making process.

[chromatographer1]

Both systems will work and will work well for general analysis work.

The reason that only one company has the balanced pressure system is becaused it is patented. And PE had the other design patented as well but choose not to prosecute companies that use the older and less inert loop design.

Generally, for the new user, the PE design is less difficult to use and to develop a method. That is a plus. The negative is that the design has more parts. However, there is a great likelihood that the valve loop design will require more maintenance than the pressure balanced timed injection design.

To be fair, PE has made an effort over decades to make the best headspace analyzer. Both Tekmar and Agilent make good units. Price and service and of course, your specific application are more important factors than a design choice per se. Trace and reactive analytes? Chose PE. Otherwise, pick any of the major three as well as several other good companies. And my opinion on the top three may not be accurate today.

Rodney George
consultant

and not connected to any HS company

[Terry]

Hello everyone,

Thank you for all your professional input. I will let you know when we make the decision.

Regards

Terry

[LindseyPyron]

Sorry, but I had to answer thsi one....EST Analytical makes a system that does BOTH loop and Pressure balanced injection as well as dynamic sampling to a built in trap for improved sensitivity.

Check out www.estanalytical.com