How to better separate peaks in GC FID?

04 April 2018 4 574 Report

Hello,

I am creating a FAME mix (70 FA) with various fatty acids. But I am having difficulties to efficiently separate two fatty acids. C17 anteiso and C16:4n1 are coeluting (peak with a slight shoulder). I know their retention because I also injected pure standards of these fatty acids.

They elute around 32 minutes nearly at the end of the first plateau.

My setup is:

· GCFID-Model : TraceGC Ultra 100, Thermoscientific

· Column: DB-23, Agilent Technologies, Canada

· Dimensions: 120m x 0.25mm x 0.15µm

· Ramp protocol :

§ Initial : 80°C, hold 1min

§ Ramp 1 : from 80 to 140°C, rate : 30°C/min, hold 0min

§ Ramp 2 : from 140 to 200°C, rate : 3.5°C/min, hold 15min

§ Ramp 3 : from 200 to 215°C, rate : 0.5°C/min, hold 0min

§ Ramp 4 : from 215 to 225°C, rate : 20°C/min, hold 15min

· Carrier gas : Helium, Ultra high purity (Helium UN1046)

· Column flow : Constant flow

· Injection Volume : 2µl

· Washing solution : Toluene, methanol, chloroform, hexane, in this order (2 x after each injection)

· Inlet : Split mode

§ 230°C

§ Split flow : 11 ml/min

§ Split ratio : 7

· Detector : FID

§ 280°C

§ Air flow : 450 ml/min

§ Hydrogen flow : 40 ml/min

§ Makeup Flow : 30 ml/min

· Run time : 80.64 min

The first plateau helped me to efficiently separate fatty acids in the C18:1n etc area which was difficult in the beginning.

Does anyone have an idea on how to better separate the two peaks I am talking about?

Thanks in advance

Charles H Hocart

You have a long column and carefully programmed temperature ramp but sometimes it is just not enough. You may need to consider a different column chemistry. Consider the SGE BPX70 or the Agilent HP88 columns. I

Muhammad Asim Khan

Dear, Felix Christen

Retention times of standard fatty acids are important in same developed method. If they are eluting at different times then its easy.

First you can make mixtures of standard fatty acids in different ratios and try to get their retention times and separate peaks.

Decrease flow rate, but it will increase run time. you can increase ramping temperature to decrease run time.

Can you tell me the boiling points of the fatty acids?

Felix Christen

Thanks everybody for your answers.

Changing columns is not an option for the moment because I need to rapidly analyze some samples. But we still have a Hp-88 in stock so I will give it a try later this summer.

The supelco documents seem quite helpful, especially to decrease sample variability, which I recently detected.

Finally by testing a dozen of different ramp protocols I was able to separate all peaks with the 120m DB23 column! I increased the initial ramp to 40°C per minute. And added another plateau at 197 degrees for 17 minutes followed by a 5°C/min ramp to 200°C and a 4 minutes plateau. Followed by a 0.4°C/min ramp to 215°C and final ramp at 20°C/min to 225, finishing of with a 5 min plateau to ensure complete emptying of the column.

Thanks again for your input!!

Have a nice day

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