hydrolyzed C18 chains coming off the column as particles

[Omeed]

We know at low pH the bonded phase can hyrolyze and come off. Is it possible that hydrolyzed and separated C18 chains coaggulate into small white particles in a 100% methanol mobile phase and then pass through the column frit?

[tom jupille]

I suppose it's possible, but I don't think it's very probable. A more likely scenario would be fracturing of the silica to form fines.

What is the symptom that you are seeing?

[Omeed]

irregular shape white particle in the column efflent. They are not soluble in water, acid, or base. They dissolve in DCM and chloroform. NMR shows they might be hydrocarbons. pH of the mobile phase is close to the lower pH limit of the column. Column is performing as usual, no deterioration yet.

[tom jupille]

Okay, I may have to retract my statement, but I'm still surprised. The reason I would put it as low probability is that C18 is extremely hydrophobic (obviously!) and even if the silyl ether linkage that holds it to the silica cleaves, I would expect that it would still stick to the stationary phase (consider all the trouble we have washing traces of ion-pair reagents like octane sulfonate off!)

[Omeed]

Unless we apply a strong solvent to the column. But in this case, the strongest we have is 100% methanol at the end of the gradient. So, do you think the "loose C18 chains" will be washed off the column by methanol?

[Scott F]

I know of a similar issue but related to SPE cartridges, still a C18 alkyl chain bonded onto an inorganic silica particle. After months of investigation it turned out to be related to residual reagent remaining folowing the end-capping process. As the end-capping is proprietary the manufacturer would not share the chemical identity but adapted the cartridge washing process in order to 'condition' the tubes prior to use.

Out of interest, what column are you having a problems with?

Scott

[Omeed]

I understand your point about methanolnot being strong enough to pull the loose HCs from the attached HC chains on the column.Actually, I never observed white particles coming out of the column. It is when they are in the waste container that we see small white particles, grow larger by time. The waste container has mobile phases from several different analyses but it is mainly aqueous. Could it be that the loose HC chains (if we assume sre washed off the column with 100% methanol) or the residual endcappiing reagent, , aggregate to avoid the undesirable aqueous environment?

[prepcolumn]

I understand your point about methanolnot being strong enough to pull the loose HCs from the attached HC chains on the column.Actually, I never observed white particles coming out of the column. It is when they are in the waste container that we see small white particles, grow larger by time. The waste container has mobile phases from several different analyses but it is mainly aqueous. Could it be that the loose HC chains (if we assume sre washed off the column with 100% methanol) or the residual endcappiing reagent, , aggregate to avoid the undesirable aqueous environment?

Omeed

Let me make a simple calculation..I dont know the dimensions of your column but in a 250*4.6 mm column, total particle amount is 3-5 g. And let's assume the carbon load is 20% (c18 column), which means you may have maximum 1 g of (chloro or another counter ion)octadecane in your column. As you stated there is no change in column performance, this shows that you still didn't loose even 20% (again as a general approach) your bonded phase which leads us that the maximum 200 mg HC may be unbondeded. Is this amount overlap with the particle amount in waste container?

Besides they have (both HC chains of octadecyl salts and end capping reagents) melting point below 20 degrees celcius which makes you tough to see them as white solid particles.

Good luck