ELSD noisy baseline and low sensitivity

[bunnahabhain]

Hello everyone

Now I have a new (well, used, but new to me) ELSD detector: Varian LC-380 from 2008. First thing I tried is: water, 1 ml/min at 40° nebulizer, 80° evaporator and 1.4 slm gas flow. Noise is 2.6 mV at gain setting 1. This is far away from the specification 0.5 mV. Even without flow I have a noise level of about 0.7 and drift of -0.6 mV over 5 min.
Next things to try are another batch of water and pure acetonitrile (specifications are <0.3 mV noise).
About sensitivity: I tried to inject 10 µl 10 mg/L caffeine (equivalent to 100 ng). This should be easily detectable. I only see flat line. 50 mg/L is a peak I would say OK this is about my LOQ. Parameters used are acetonitrile/water 25/75 at 1 ml/min, 40°C /40°C /1.4 slm.
I'd appreciate any help in getting more familiar with this device.

Thanks a lot
Jörg

PS: I tried the cleaning procedure from the manual: 4 h acetone/water 50/50 at 2 ml/min and 2.8 slm flow. No change in noise.

[Vlad Orlovsky]

Sensitivity of ELSD is not very good. With totally non-volatile compounds (sodium ion) you can probably reach 20 ppm. For other compounds it might be even worse (100 ppm). Also ELSD is known for non-linearity at lower concentration. If you are trying to see how well your ELSD is working try to analyze sodium ion (will need ion-exchange or mixed-mode column). Also make sure that your system did not see TFA and ammonium formate/acetate. TFA is trapped everywhere and you will see a noise related to TFA/ammonia. We always have a separate system for TFA and ammonia buffers.

[entropiclabs]

ELSD's are definitely noisier than conventional UV detectors. You can expect anywhere from 0.1 up to 1mV peak-to-peak during standard runs. Most UV detectors will have between 5-20uV (uAU) noise, so at first it might take getting some used to.

The noise setting of 2.6mV you reported - during what time interval did you observe this? Is it from peak-to-peak? You might have a contaminant in your system. Most problems with ELSD are actually from two sources... the inlet source of nitrogen, or the HPLC system, not the detector itself. Is your nitrogen pure, and are you using a filter? What type of pumping system are you using? Do you have a pulse dampener? Are you using an autosampler, or manual injector?

Your nebulizer might also need cleaning/replacement. There is a square key-latch on the back of the unit. Rotate it anti-clockwise to unlock with a pair of needle nose pliers and open the unit up. Take a look at the glass adapter going to the nebulizer. It should be clear and clean. If it is fouled, you'll need to clean or replace it.

You might also want to try alternating from 1:1 acetone:water to 100% acetone over 20 minute intervals overnight. Good luck!

[bunnahabhain]

Entropiclabs: Thank you for these valuable hints! I'm in holidays next week but will certainly try out when I'm back in the lab.
Vlad: One question to an ELSD based application on sielc.com: What concentration did you use for sulfamic acid (http://www.sielc.com/Compound-Sulfamic-Acid.html)?

Thank you
Jörg