Problems with acetic measure in gc

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hidracina
hidracina's picture
Problems with acetic measure in gc

Hello,

I´m working with a gc 8000 and I need to measure acetic but the gc doesn´t detect it.

I could measure it, but without a retention time constant: it goes from 4.8 to 5.2 minutes
I had a baseline ON, but when I measured this sample I needed to do another one and clean the system because there was a broad peak in 3 minutes. I had to do this, three other four times until I have a new constant baseline.

The injector is at 260, the detector is at 240; and the oven is at 170.

Could you tell me the retention time of the acetic in this conditions? Could you tell me the peak shape? Because I have one which is totally asimetric, and it looks like a a shoulder.

Other problem I have is the level loss, the flame goes out without any reason, because there´s H2 flow and it seems everything is stable.

Thank you.

Dr. Analytical
Dr. Analytical's picture
Please provide some more

Please provide some more information about your system:
1. What column are you using - phase, length, diameter, film thickness?
2. What is your carrier gas, and what is the flow or pressure?
3. What solvent is the acetic acid dissolved in?
4. Are you using split or splitless injection?  What are the settings?

Asymmetric peaks in GC usually mean you are using the wrong stationary phase for your analyte.  If you have a polar analyte (acetic acid is polar) and are using a non-polar phase (DB-1, DB-5, etc.), the analyte does not mix well with the stationary phase.

The solvent and injection conditions can also cause peak shape problems.

Please send us more information and then we can try to help you solve the problem.

hidracina
hidracina's picture
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  1. What column are you using - phase, length, diameter, film thickness?

 
GC 8000 Top 220-230 Vac version. Requires 220-230Vac single phase. Until next week I can´t tell which is the length, diameter…

2. What is your carrier gas, and what is the flow or pressure?
 
Nitrogen, with a Pressure 99,9 kPa, the flow is 80 ml/min

3. What solvent is the acetic acid dissolved in?
 
In distillate water.

4. Are you using split or splitless injection? 
 
Splitless injection

The stationary phase is: polyethylene glycol 20 000 dalton

Thank you.

hidracina
hidracina's picture
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Hello, the column data is:

PERMABOND- CW20M – 0.5 µm.
25m * 0,32 mm ID.
 T= 220°C the maximum for permanent use; maximum 240 for curves a few minutes.

This is the oven programme:

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STEP

RATE

 ISO TEMP

ISO TIME (min)

1

--

60

1

2

40

200

1

3

70

60

--

The detector is at 240°C, the injector at 260°C and the H2 flow is 25.

Dr. Analytical
Dr. Analytical's picture
Your flow is too high for a 0

Your flow is too high for a 0.32 mm column.  Is this the total flow into the system, or the column flow?  The column flow should be about 1 - 3 mL/min.

Do you need to analyze low concentrations?  That is the only reason to use splitless injection.  What is your injection volume?

First, try a split injection of a high concentration standard, and adjust the split ratio (from 1:100 to 1:10) to see what your peak shape looks like.

Using water in splitless mode is not recommended.  However, if you want to try you should reduce your injection volume to 0.5 uL and reduce the purge on time (the time after injection when you change to split mode) to 0.5 min. or less. The injector temperature should also be reduced.

Try these experiments and report back.