Degassing of rinse fluids

[Jensvd]

Our Shimadzu LC-30 systems have a DGU-20A5R and DGU-20A3r degassing units. The solvent lines going through the degasser are, off course, the mobile phase lines. However, Shimadzu also routes the 3 rinse fluids lines that go the the autosampler (SIL-30AC) through the DGU-20A3r degassing unit. We have never thought anything of it: that's how the HPLC's were installed long ago. However, now the degasser of the rinse fluids is broken and to be honest: we don't see a difference. That's when I realized: do rinse fluids for internal and external rinse in the autosampler even need to be degassed? Or was it just an up sell for an extra degasser?

[DR]

If the extra channels are available, why not degas the needle and seal wash?
It's not really necessary but may occasionally prevent cavitation of one of them.

I suspect that degassers are designed for high pressure mixing binary systems with line switching, so that's 4 channels needed just for mobile phase. If your system is a low pressure mixing binary system, you would only need 2 degasser channels for it, so they run the wash solutions through the available channels. As these degassers typically only have one vacuum pump, they have to either run a line through each channel or plug them...

[Multidimensional]

This information is specific to the Shimadzu model SIL-30AC AutoSampler (different types of A/S use different flow paths).
To answer your question about the separate vacuum degasser module, Shimadzu DGU-20A3R, used specifically for the A/I's 'Rinsing Ports', it is optional, but strongly advised. Degassing of the rinsing fluids IS recommended to maintain proper flow of the rinsing fluids used through the system. These solutions contain dissolved gas inside which may cause the pump to not deliver the fluid to the injector. This liquid wash is needed to remove contaminants and carry-over inside the valve and tubing. For this reason, vacuum degassing of the separate rinse channels in this A/I will improve the reliability of the A/S rinse feature (required for normal operation, as contamination of the flow path may result if not used). When problems occur within the "rinse system" of this HPLC module, you may NOT NOTICE it a first as the rinse pump is not able to alert you to problems delivering the fluid through the A/I. Over-time, the failing system may result in ghost peaks and other problems noted during analysis. If you operate in a regulated environment OR use the HPLC to provide "services", then you may be required to have the broken degasser replaced/serviced to continue to use the HPLC. If you are not using the HPLC in any such role, and the validity of the data is not of concern, then you could continue to use the system with the separate rinse degasser removed from the flow path (do not leave it connected inline if it is broken. Doing so will contaminate the flow path of your autoinjector and column from the accumulating debris inside the broken degasser).

Please be sure to have your regular DGU-20A5R degasser professionally serviced (or replaced) at least every five years. These vacuum degassers have a max life of five years and introduce contamination into the HPLC flow path once damaged. They require service/replacement just like any other module in an HPLC system and you should always budget for such expenses just as you do for routine maintenance and repair services.

[Jensvd]

Thank you both for the answers.
We are aware of the function of the degassers for the mobile phases and we service them regularly. But yes, specifically for this model autosampler, I was baffled by the need of degassing of the rinse fluids because i thought: "the rinse fluids are never compressed to high pressure by a pump, they are just for rinsing and are supplied with a small pump". For use of internal rinse however, the low-pressure valve switches the rinse R0 line towards the sample metering pump, to internally rinse the low pressure flow path and/or sample loop and internal parts of the needle. I can assume that due to temperature changes, rinse fluid that is not degassed can contain bubbles, which if left in the sample loop or the flow line to the metering pump can cause issues. But here's why I never realized that: we don't perform internal rinsing. Due to the flow through needle design, the internal flow is always constantly "purged" by the high pressure mobile phase. The needle is only rinsed externally.
That's also the reason why the rinse fluid on or LC-2030 I-series system's is not degassed: they don't have an internal rinse!

[Multidimensional]

"I can assume that due to temperature changes, rinse fluid that is not degassed can contain bubbles" -

Note: NOT due to temperature changes alone. All solutions, esp organic, contain gasses which when compressed (or just introduced to the pump) may result in bubble formation inside the flow path. Those "bubbles" will not only disturb the flow rate and pump operation, but increase error in the injection volume (by adding bubbles to the sample). This is why degassing of all solutions used for the A/I's wash lines are advised to be degassed. Again, it is optional, but for the best performance degassing these fluids is recommended.

[H.Thomas]

A small correction: gases don't dissolve "especially in the aqueous phase". The solubility of gases is higher, the less polar a solvent is. Therefore solubility in methanol or ACN is about 1 order of magnitude higher than in water.

Formation of gas bubbles can occur when mixing aqueous and organic solvents (low pressure gradient mixer).

See e.g., S.R. Bakalyar, M.P.T. Bradley, and R. Hoganen, The role of dissolved gases in high-performance liquid chromatography, J. Chromatogr. 158, 277 (1978)

[Multidimensional]

"H.Thomas", thanks for spotting my typo. It has been corrected and was supposed to show organic, not 'aqueous'.