Chromatography Forum

Hello,

Here is my method:

isobutyl alcohol, bp=108C

Headspace:
oven loop transfer: 90C 105C 110C
vial eq time: 30min
pressure time: 0.20min
pressure eq time: 0.10min
loop fill time: 0.10min
loop eq time: 0.10min
inject time: 1.0min

Inlet:
5:1 split
temp: 140C

temp gradient: 40C to 240C

Col dimensions: 30m length, 320um dia, 1.8um thickness of film
Flow: 1.5 ml/min

Issue:

I am observing a little peak eluting before the standard peak at the base. This standard is USP and claims to be pure. So I think it's an instrument issue.

What could cause a false little peak like this to show up?

https://ibb.co/jgN3qc

<a href="https://ibb.co/jgN3qc"><img src="https://thumb.ibb.co/jgN3qc/2018_02_07_extra_peak.png" alt="2018_02_07_extra_peak" border="0"></a>

<a href="https://ibb.co/jgN3qc"><img src="https://preview.ibb.co/dHJbAc/2018_02_07_extra_peak.png" alt="2018_02_07_extra_peak" border="0"></a><br /><a target='_blank' href='https://imgbb.com/'>free picture hosting site</a><br />

Hi Joe

That's a teeny little peak compared to your butanol, and I suspect that if you integrate the two peaks you will find that it is well within the specs for USP purity.

Peter

What size is your loop?
You could increase your loop fill time, and decrease the inject time ( by maybe half).
If I remember sample loop head space techniques, you only need 3-5 loop volumes for the injection time.

Hi Peter, the area percent of the little peak is 1%.

Bigbear, the sample loop is 1.0 mL.

It seems like either the standard is not pure (degrading?) or about 1% of standard reaches the detector before the majority of sample (but how?).

Thanks,

I would have my suspicions of n-butyl alcohol in the iso-butyl alcohol standard. They are similar chemically.

Lookup on the internet both of the boiling points of these chemicals and devise a temperature gradient ramp that can 'tease' them apart.

MestizoJoe wrote:
Hi Peter, the area percent of the little peak is 1%.

Bigbear, the sample loop is 1.0 mL.

It seems like either the standard is not pure (degrading?) or about 1% of standard reaches the detector before the majority of sample (but how?).

Thanks,

Hi Joe

What is the actual purity spec in terms of mass fraction ?, I had a quick look on the net and couldn't find anything.

How does the blank look ? Maybe the extra peak is coming from elsewhere.

What is you temperature programme rate ? - if you go from 40 to 240 in the run time of the chromatogram any peak splitting at the inlet would have been focussed away before anything starts moving down the column.

A long shot; check that the column is not touching the oven wall anywhere.

Peter

It could also be from a pressure pulse when the valve switches. If so, cold trapping on the head of the column during injection then starting the ramp may help.