Chromatography Forum

Hellow,

I am praveen . I wan to know that what is ELSD detectores , what is therir principle and for which compound it should uses.

Regards

Praveen

ELSD - Evaporative Light Scattering Detector

Very useful to detect compounds that do not have UV-VIS absorbing Chromophores and/or difficult to derivatize through absorption technique.

Mass sensitive detectors - and can offer good sensitivity in this regard.

ELSD is often used as an alternative to RI for compound without or woith a poor chromophore.

ELSD Applications: http://www.ingenieria-analitica.com/Llo ... a_424A.pdf

You should note however that ELSDs may not exhibit a linear response oer will have a very narrow linear range for most compounds. In my experience, a first order quadratic based calibration is normally required for successful quantitation.

Note: you will also need a plentiful Nitrogen supply to operate an ELSD (sometimes up to 1 Large cylinder/day).

ELSD is a general detector，it is a alternative when some substance haven't UV absorption，certainly you can also use the CAD ,which sensitivity is better than ELSD.

A CAD is certainly more sensitive (although I'm not sure how more sensitive). But the CAD is also more expensive. So in case you don't need high sensitivity, the ELSD will be better.

I worked with ELSD before for analyzing Glucosamine formulations - and it worked wonders - (Alltech ELSD)

However, prevention is better than cure. If the drift tube gets dirty - it can show peaks all over the place. In other words - one should be cautious in choosing a proper mobilephase, modifiers etc.

I did not work with a CAD before but I heard from some of my colleagues that they can provide a little bit of sensitivity over the ELSD but can create more potential problems, specially maintenance.

SofTA has some nice information on ELSDs:
http://www.softacorporation.com/operation.htm

How to choose between different ELSDs:
http://www.softacorporation.com/how_to_ ... tector.htm

JGK:
You mentioned ELSD requires lots of Nitrogen to run, can this be replaced by compressed air?? What is the common psi being used? A validation was performed using Nitrogen with 2 sharp peaks observed but when I tried performing the same method with compressed air (with higher psi), spilt peaks observed and the baseline is much noisier compared to the run using Nitrogen. Is compressed oil not a good alternative or the spilt peaks is due to the increased gas pressure??

mohan_2008:
In regards to cleaning the drift tube, is sonicating the part in HPLC grade water for 15 minutes sufficient?? Could you please elaborate on your comment on "one should cautious in choosing a proper mobile phase, modifiers"

Thanks in advance!

JGK:
You mentioned ELSD requires lots of Nitrogen to run, can this be replaced by compressed air?? What is the common psi being used? A validation was performed using Nitrogen with 2 sharp peaks observed but when I tried performing the same method with compressed air (with higher psi), spilt peaks observed and the baseline is much noisier compared to the run using Nitrogen. Is compressed oil not a good alternative or the spilt peaks is due to the increased gas pressure??

The 2 models i've used (Alltech and Waters) both required nitrogen gas for use and this is what we used at the specified pressure < 60 psi (what is the point of spending $20 - 30, 000 on an instrument and not using it as directed?).

I don't have any experience of using an ELSD running on compressed air so I'm on uncertain ground. However, as the detector needs to evaporate the nebulised mobile phase, air may not be the best gas to conduct heat from the heated tube containing the gas stream.

baseline is much noisier compared to the run using Nitrogen. Is compressed oil not a good alternative or the spilt peaks is due to the increased gas pressure??

An ELSD is going to show a response to any particle passing thorough the light source. If your gas supply has particles, oils, or even water vapor it will increase the baseline signal and noise. Adding a filter to the gas line might help. However, use of an inert gas is recommended by all manufacturers.

The quality of the mobile phase solvents and modifiers can also affect the baseline signal and level by introducing non-volatile components. For the best results, use the highest grade available. Generally LC/MS grade has the lowest residue on evaporation. If you are not working near the detection limit of the detector, you might not see an effect from the higher quality and can save $ by using a slightly lower quality. It is possible for analytes or contaminates to build up over time, especially with lower quality gas, solvents and modifiers, and create noise. An annual cleaning and recalibration is a good practice.

Please refer to your owners manual or detector manufacture for cleaning procedures. In most cases, the drift tube can not be removed for cleaning and is cleaned while still in place.

Thank you all for your replies.

I've tried both ELSDs and CADs and if found the CAD worth the extra money if covering a wide range of concentrations. Its linear range is much wider than ELSD but it does take a little extra effort to keep clean. One potential issue is that CADs require nitrogen gas and can't just use any inert gas to nebulize. So in the long run, it could cost significantly more than compressed filtered house air.