Chromatography Forum

We're trying to develop a method to analyze acetone, butanol, ethanol, butyric acid and acetic acid. We are using a SRI 6810 GC splitless injection with 10 mL/min nitrogen carrier gas flow and uses Peaksimple software. The column we are using is a Restek MXT-WAX Polyethylene glycol nominal dimensions: 30.0 m x 530 µm x 1.00 µm.

At a different facility we were using an Agilent 6850C GC system with split injection ration of 50:1, variable nitrogen carrier gas flow and uses Chemstation software. The column there is an Agilent 19091F-115E HP-FFAP Polyethylene Glycol TPA capillary, nominal dimensions: 50.0 m x 320 µm x 0.50 µm. We were able to get good results using this setup.

With the SRI 6810 we are able to get good acetone, butanol and ethanol peaks, but the acetic and butyric acid proves to be more problematic. We also protonate the sample with 0.2 M HCl and we're worried about column deterioration as it is a splitless injection without an inlet liner. Are there any suggestions or different techniques that we could use to quantitatively analyze butyric and acetic acid?

Current method for SRI 6810:
The ramp oven temperature is initially set at 45 oC and held for 5.0 minutes. The temperature is then raised to 190 oC at rate of 30 oC/min and held for 2 minutes (ramp duration = 5.8 minutes). The total running time is 20 minutes. The nitrogen gas flow is fixed at 10 mL/min.

Current method for Agilent 6850C:
The ramp oven temperature is initially set at 80 oC and held for 0.5 minutes. The temperature is then raised to 200 oC at rate of 10 oC/min and (ramp duration = 7.5 minutes). The total running time is 20 minutes. The nitrogen gas flow starts from 0.8 ml/min and held for 8 minutes. It is then increased to 1.0 ml/min at an acceleration rate of 1.0 ml/min2 and held for 5 minutes. The flow rate is then raised to 1.3 ml/min with the same acceleration rate of 1.0 ml/min2. The total running time is 20 minutes.

Thanks in advance.

Can you change the SRI 6810 to run with a split injection and the same type of column and similar temperature ramps and flow as the Agilent 6850? If the flow controller on the SRI 6810 cannot be programmed, a value close to 1 mL/min may work well for you.

And, the acid: Yes it will degrade the column. As an alternative, you could derivatize samples, but that adds time and cost. And the cost of derivatizing a number of samples can add up quickly - to the point where it is less expensive to just change a column every now and again.

I am used to an environment where getting it done yesterday is desired, but today is acceptable. My philosophy: go with something that works and then make it prettier and fancier as time allows.

Adding an inlet liner would be a good first step. Have you actually tested whether acidifying the sample is necessary ?

Peter