ELS detector's imprecision and increasing response factors

[spinta]

Hi,

I have got a new toy in my lab, namely ELS-detector (evaporating light scattering). I have found that the precision and linearity are not in the same level than with a UV-detector. For example the RSD for six consecutive injections of pure standard is about 6.5 % and showing an increasing trend (carry-over is not the cause since the blank is ok). Also an increasing trend of response factors (peak area / mass) is observed with the linearity test.

Has anybody noticed same kind of problems / features with ELS-detectors?

Thanks in advance!

Sami

[DR]

Is it a "regular" ELSD or a Corona Discharge unit?

Are you running isocratically? Are your RTs consistent?

We need more details.

[spinta]

I suppose it is regular detector (Sedere Sedex 85). I have a gradient run, using Waters HILIC column with 10 mM NH4COOH (eluent A) and ACN (eluent B). Retention times are ok, RSD for those is about 0.3 %. The peak characteristics (form, tailing factor, plates) are all fine, so I think the reason is on the detector, not in the chromatography process.

Sami

[AA]

Area count precision is always worse on an ELSD. However, 6.5 % is on the high side. I would expect (assuming the autosampler and pump are working correctly) area counts of about 3% RSD. ELSD's are NOT linear (you can not change the laws of physics), fit the data to a quadratic equation. To troubleshoot the issue, put a UV detector in-line before the ELSD and inject a compound that will give response in both. I would expect <0.5% area from the UV and <3.0 from the ELSD.