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- Posts: 36
- Joined: Tue Jun 10, 2008 6:49 pm
- Location: St. Louis
I'm working on the USP related compounds for glycerin for the first time. I was trying to convert the method over to columns/conditions we already have in my lab. I am having problems with peak shape, broad 2 minute wide glycerin and diethylene glycol peaks. Resolution is at most 4, but spec is 7.
I'm also occasionally seeing odd peaks.. almost like glycerin is splitting in the injector. I saw this with limit of ethylene glycol and diethylene glycol, adding a pressure pulse and speeding up the injection resolved those odd peaks.
My Conditions:
Restek 624 30m, 0.3-mm, 1.8um.
Carrier: nitrogen,
Column flow is set at 0.8mL/min with a pressure pulse of 16psi (to avoid backflash). I calibrated the septum purge for the 16psi.
Liner- restek cyclosplitter, pn 23312. (220C), Split 10
Samples are dissolved in water, 0.5uL injection volume. Washes are water.
Oven: 100C to 220C at 3.8C/min, hold 8 minutes.
Detector: FID at 240C
Original USP Method:
30m x 0.53mm x 3um
Carrier gas: Helium
Linear Velocity 38 cm/s
Liner: cup or spiral structure
Un specified sample preps, but I believe it's water. 0.5uL inj. volume
Oven 100-220C at 7.5C/min, hold 4 mins
I've tried adding a temperature hold, slow the gradient, increase split, and played with different pressure pulse settings (best results so far). One question I have, for the liner, restek says this part is for an agilent, but I have a varian. The basic dimensions are the same, but could the split location be an issue?
The column is fairly old. I can order a new, identical column, since we use it for other tests. Or, I could order the one specifically for this test. Alternatively, I could try switch out my carrier gas. The last two options are not ideal.. so, I'm wondering if one would be more promising than the other. Also, could my gas settings on my detector or gas quality be impacting the poor peak shape and tailing? Generally, I think of this as a column chemistry issue, but I'm no GC expert. Thanks!