ELSD Gas Flow Rate Exceeding Limit

[rycar]

Hi all,

I have run into a problem with my Agilent 1260 ELSD where I am receiving an error that the evaporator gas flow rate is exceeding the limit (Error 21). This seems to indicate that the mass flow controller has gone bad so Agilent is recommending an exchange for the ELSD.

However, this error only occurred when I handed the instrument over to R&D to run a new method for a couple of weeks and I just took it back and switched it back over to our method. Talking to R&D, they never saw this problem while operating it and they ran the ELSD using pretty much the same conditions as we do (Evaporator & Nebulizer @30C and Gas Flow Rate @1.60SLM). The main difference with their method is they used a buffered mobile phase of 10mM Ammonium Formate in 95:5 ACN:H2O while we do not (we use 80:20 MeOH:H2O with 0.1% FA each). It sat for a few days between them using it and us taking it back over to run our method. I checked the inlet gas pressure and it is normal, 60 psi, so I don't expect that changed at all over the course of operation.

I think the ELSD may be toast and we will have to do the exchange, but I would like to know what happened to it to cause it to go bad so that it doesn't happen again. Any ideas?

[Multidimensional]

You have identified a few of the possible problems that may have led to the failure.

Use of "Ammonium Formate in 95:5 ACN:H2O". ACN is very poor at maintaining buffers in solution and that is too high a concentration of ACN, esp for use with a very delicate detector as an ELSD (or CAD).

Possible improper shutdown procedure. The ELSD requires that pure, clean solvent be flowed through it after use (no water and no buffers) with high gas flow to dry out the inside. Then flush with flow off, just gas one for a while before turning off for the day. Failure to do so, esp after use with any buffer or additive, may result in clogging, contamination or damage.

The inlet pressure needed to operate the detector is a MINIMUM of 60 psi, which is what you mentioned your pressure was, so it may be too low. *A low inlet pressure will trigger the exact error message you have. We run ours at 70 psi min using a two stage regulator.

Operating temp may be too low. Evap temps with aqueous phases usually need to be at least 60C (depending on liquid and of course, gas flow rates). Use with mostly organic phases, such as 95% ACN usually need to be at least 40C fo good nebulization, but in ALL cases, proper method optimization must occur monitoring S/N ratios of a std/sample (not peak areas which are misleading as they do not account for baseline noise)!) under true method conditions to find teh best temperatures and gas flow rates for your specific method

We would check the gas inlet pressure first with a secondary gauge for low pressure, then evaluate for contamination after that. If the supply pressure is 65+ psi and stable, there may be damage to the ELSD's Control Board or the pressure sensor. Pressure sensors may be damaged from contamination.

All ELSD and CAD systems are highly prone to damage and high maintenance/operating costs. This is by design. They are best used only when no other detection system is available. More general info here: https://hplctips.blogspot.com/2017/12/e ... arged.html

[rycar]

Thanks for the input and advice.

Just to check, you recommended increasing the inlet pressure up to 70psi but the Agilent 1260 ELSD manual recommends not to go above 67psi and for initial setup sets it to 51psi. Could that be a unique Agilent range?

Also, if 95:5 ACN:H2O is too much ACN, is there an upper threshold of ACN you recommend?

[Multidimensional]

Yes, 67psi is in fact what our unit is set to (I said 70 before, but it is actually 67 psi). Be sure to use clean nitrogen gas, FILTERED, high purity is best (as it will always be filtered) for the inlet gas. Any particulate in the gas or gas tubing supply can damage the pressure sensor inside the ELSD plus add to baseline noise.

Agilent sells several different versions of the ELSD (not made by them as these are all OEM products). Their are at least two versions of the 1260 (I) version and two of the newer 1260 (II) version. The specs in the manuals are different so the values we use for ours (a type II) may be different that the one you have (you probably have a G4218 which is slightly different). *Higher gas inlet pressures allow for a wider range of gas FLOW values, needed during optimization. We usually set it to a high value, then there is no need to adjust the inlet pressure after that, just the flow rate.

"Also, if 95:5 ACN:H2O is too much ACN, is there an upper threshold of ACN you recommend?" : For RP, ~95% ACN in general is a reasonable max, but my statement was in context with using ACN with BUFFERS, not pure water. The max ACN used with buffers must not result in precipitation or partial obstruction of the lines or valves used. When little (i.e. 5%) or no water is present, ACN is not suitable for use with many buffers. Even 10 mM can cause problems. This is a fundamental HPLC concept so I would encourage you to review some of the classic texts on method development for more information. Solubility is so very important in HPLC analysis, not just for the samples but for the mobile phase too.