Resolution between acetaldehyde and methanol

Discussions about GC and other "gas phase" separation techniques.

8 posts Page 1 of 1
Hello all,

I have a problem of resolution. The test method is Organic impurities of Alcohol (USP) by GC. The resolution for system suitability requirement is NLT 1.5 between acetaldehyde and methanol but I can't achieve it without changing chromatographic conditions below.

Column: DB-624 (0.32-mm × 30-m fused-silica capillary; bonded with a 1.8-µm layer of phase G43)
split ratio: 20:1
Linear velocity: 35 cm/s
Carrier gas: Helium
Injection volume: 1.0 µL
Injection port temp: 200°C
Detector temp: 280°C
Column temp: 0-12 min : 40°C

Is there a way to improve the resolution without changing the conditions above?
Hello,

We run this occasionally using the DB-624 and the same conditions you mention. The methanol peaks tends to tail which will lower the resolution. We are able to get the needed resolution so I would suggest making sure your inlet is clean and has a new liner. USP allows you to make changes to their methods, see USP<621>. You can increase the column length by 70% which may get you the resolution you need.

Dave
dlbenach wrote:
USP allows you to make changes to their methods, see USP<621>.


Not if you had the pointy-haired boss I had the last few years !!! He didn't accept the written word of USP or FDA ORA, and I always argued that it was stated very clearly using the 26 letters we all learned in kindergarten....
dlbenach wrote:
Hello,

We run this occasionally using the DB-624 and the same conditions you mention. The methanol peaks tends to tail which will lower the resolution. We are able to get the needed resolution so I would suggest making sure your inlet is clean and has a new liner. USP allows you to make changes to their methods, see USP<621>. You can increase the column length by 70% which may get you the resolution you need.

Dave



That's right. The methanol peak was tailed. Does the liner type affect peak tailing? We use the liner (Agilent, cat# 5183-4711; split, single taper, glass wool, deactivated).
Yes, the liner can affect peak tailing. In our runs both peaks come off very early, ~2min. That can mean that tailing is being caused somewhere at the inlet which includes the liner. You may want to try a run without glass wool in the liner. We use an Agilent 5062-3587 Inlet liner, splitless, single taper, glass wool, deactivated. It would not be my first choice but it was how our method was validated so that's what we use. It has glass wool but it is at the bottom of the liner.

Dave
The low boiling point probably means you don't need a laminar cup split liner, but Restek makes a liner that will hold the glass wool at the point in the middle of the liner where the tip of the needle will be, which when doing split injections can help. Also if the analytes absorb onto the glass wool you can switch to one of the glass cyclo liners for better evaporation without the tailing caused by glass wool.

With methanol and acetaldehyde being so low boiling you probably don't even need glass wool, unless the samples are dirty and you have to prevent high boiling analytes from getting onto the column, if they are clean maybe just use a double taper glass liner.

Also for split, make sure the column is not installed too far or not far enough into the inlet, that can cause tailing as well as a rough cut on the column end. Use a magnifier to make sure the column end is cut smooth and square.

To get a starting column depth measurement I place a used gold seal on the column on top of the ferrule to see how far up into the inlet the column will be when installed. It is an easy way to visualize what it will look like inside the inlet. Start with the column tip about 2mm above the seal. If you see tailing move it up about 1mm in steps until the tailing goes away.
The past is there to guide us into the future, not to dwell in.
Hi,

Without changing the conditions you've stated, column installation at the inlet is going to be key. I assume you're using an Agilent machine. For the split/splitless inlet, you want to aim for 4-6mm of the column above the ferrule when installing at the inlet. I work in a rum lab so we have to measure acetaldehyde and methanol in all of our samples. Under these conditions, you need a 60m DB-624 if you really want to resolve those 2 (mine is p/n 122-1364). My choice liner is 5183-4647. I doubt this will be a difference maker for you though.

Now, if you want to really improve things with a 30m 624, switch to hydrogen, keep the same split ratio, but run at a higher linear velocity (65cm/s). The methanol will still tail a bit, but you will get good resolution between the 2 compounds and you will still be able to achieve near baseline resolution of 2&3 methylbutanol (if you test for those too). Both my columns are 0.25mmID as well (the 30m is p/n 122-1334). Hope this helps!
I do realize the issue with changing method conditions, however lowering the initial temperature to 35°C or 30°C will likely improve the separation of the acetaldehyde and methanol. I have had the task of separating even more volatile compounds and lowering the temperature was the best way to afford separation.
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