GPC/SEC [August 24, 2004]
Posted: Sun Aug 29, 2004 8:39 pm
By Mike on Tuesday, August 24, 2004 - 09:05 am:
I am new to SEC, I have experience in reverse phase LC and GC/MS. Recently we have begun experimenting with a water soluble polyethyleneoxide dewatering aid, MW around 1000 to 3000, I need to analyze for this material in our process water.
I purchased a Waters Ultrahydrogel 120 column. The column comes with instructions to run an efficiency test using ethylene glycol in deionized water - you should get around 14,000 plates. I ran it using propylene glycol and get around 16,000. So far so good.
I also purchased from Waters a polypropylene glycol standard at MW=4000 recommended for use as an aqueous calibration standard. The recommendation is to run the polymer standard at around 0.1%. The problem is, this polymer does not appear to me to be water soluble. Even after heating and adding a few mls of MeOH, the polymer will not stay in solution. I have made the standard up as low as 500ppm with the same effect.
I understand that this particular column can handle around 25% or so of solvent, so I could make the standard in methanol or THF - where it does appear to be soluble. But, my question is this. If I make the standard in solvent, what prevents it from crashing out of solution in my column if I am running DI water or 0.1M NaNO3 as recommended as eluent?
Any help would be appreciated...
Thanks,
Mike
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By judd on Tuesday, August 24, 2004 - 01:02 pm:
Why not purchase a polyethylene glycol MW standard? I just checked the Phenomenex catalog (I have no affiliation) and found on pg 323 a series of 9 PEG standards from 92-19,250, which seems to be in your range. You may also want to correlate them to polyethylene oxide standard, however those appear to start at 25K and go up from there - on the large side for a Ultrahydrogel 120.
A thought: It sounds a bit like you're looking for a quantitative assay for the material in your processing water rather than an assay of the MW. Why not use the low MW PEO dewatering aid as your quantitative calibration standard and simply not bother too much with an expensive MW standard?
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By Chris Pohl on Friday, August 27, 2004 - 08:34 am:
Mike,
It's all a matter of what you mean by "not soluble". If polypropylene glycol is truly insoluble even at very low concentrations in your mobile phase, it will certainly adversely affect your chromatography to inject an analyte dissolved in a "good solvent" into a mobile phase in which it is largely insoluble since your analyte must be soluble in the mobile phase in SEC, otherwise you are actually doing a different mode of chromatography (either reversed phase or normal phase depending upon the specifics of your stationary phase and your analyte). But often, you can get away with the situation you describe because there is significant dilution in the chromatographic process which is sufficient to maintain solubility of your analyte under chromatographic conditions even though these conditions were not sufficient to solublize your analyte at the higher concentration conditions at which you prepared your standard. But in the latter case, the problem is easily correctable by simply including a modest amount of solvent in your mobile phase. Depending upon the solubility limit of polypropylene glycol in water (it has to have some solubility in water, the question is how much), you may be able to get away with as little as 5% solvent in your mobile phase.
I am new to SEC, I have experience in reverse phase LC and GC/MS. Recently we have begun experimenting with a water soluble polyethyleneoxide dewatering aid, MW around 1000 to 3000, I need to analyze for this material in our process water.
I purchased a Waters Ultrahydrogel 120 column. The column comes with instructions to run an efficiency test using ethylene glycol in deionized water - you should get around 14,000 plates. I ran it using propylene glycol and get around 16,000. So far so good.
I also purchased from Waters a polypropylene glycol standard at MW=4000 recommended for use as an aqueous calibration standard. The recommendation is to run the polymer standard at around 0.1%. The problem is, this polymer does not appear to me to be water soluble. Even after heating and adding a few mls of MeOH, the polymer will not stay in solution. I have made the standard up as low as 500ppm with the same effect.
I understand that this particular column can handle around 25% or so of solvent, so I could make the standard in methanol or THF - where it does appear to be soluble. But, my question is this. If I make the standard in solvent, what prevents it from crashing out of solution in my column if I am running DI water or 0.1M NaNO3 as recommended as eluent?
Any help would be appreciated...
Thanks,
Mike
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By judd on Tuesday, August 24, 2004 - 01:02 pm:
Why not purchase a polyethylene glycol MW standard? I just checked the Phenomenex catalog (I have no affiliation) and found on pg 323 a series of 9 PEG standards from 92-19,250, which seems to be in your range. You may also want to correlate them to polyethylene oxide standard, however those appear to start at 25K and go up from there - on the large side for a Ultrahydrogel 120.
A thought: It sounds a bit like you're looking for a quantitative assay for the material in your processing water rather than an assay of the MW. Why not use the low MW PEO dewatering aid as your quantitative calibration standard and simply not bother too much with an expensive MW standard?
-------------------------------------------------------------------------------------------------------
By Chris Pohl on Friday, August 27, 2004 - 08:34 am:
Mike,
It's all a matter of what you mean by "not soluble". If polypropylene glycol is truly insoluble even at very low concentrations in your mobile phase, it will certainly adversely affect your chromatography to inject an analyte dissolved in a "good solvent" into a mobile phase in which it is largely insoluble since your analyte must be soluble in the mobile phase in SEC, otherwise you are actually doing a different mode of chromatography (either reversed phase or normal phase depending upon the specifics of your stationary phase and your analyte). But often, you can get away with the situation you describe because there is significant dilution in the chromatographic process which is sufficient to maintain solubility of your analyte under chromatographic conditions even though these conditions were not sufficient to solublize your analyte at the higher concentration conditions at which you prepared your standard. But in the latter case, the problem is easily correctable by simply including a modest amount of solvent in your mobile phase. Depending upon the solubility limit of polypropylene glycol in water (it has to have some solubility in water, the question is how much), you may be able to get away with as little as 5% solvent in your mobile phase.