Advertisement
avatar: Brainz
Brainz
7 posts
Replied to Brainz's post on July 16th, 2017

Hey guys,

A couple of questions;

So, I have an agilent GC 7890A, TCD detector and He carrier gas and I intend to run hexane, decane and acetone with it.

1. is the Agilent GC meant to be turned off?. Or it should be left on with the He carrier gas running?

2. Is there a standard to what temperature a detector should be set to?. Mine currently is at 270oc.

3. Can any carrier gas or detecter run any sample or there is also a standard for those.

4. how long should  a sample run for?

avatar: ECS
ECS
6 posts
Replied to Brainz's post on July 17th, 2017

Hello Brainz,

Your questions are fundamental GC operation procedures.  You're clearly a novice.  I would recommend taking a GC course from Agilent before you make a catastrophic mistake with your instrument.

avatar: Dan Ste.Marie
Dan Ste.Marie
359 posts
Replied to ECS post on July 17th, 2017

Brainz,

You have some good questions that are asked by everyone as they are first starting into GC analyses.  I will try to give you some simple answers here and make a recommendation as to a good reference for you on GC techniques.

 

1) Turn it off or leave it on?  This entirely depends upon presonnel preference and workload.  If you will be running many samples daily I would leave it on do as to not have to wait for it to come to temperature and stabilize.  If you are running just a few samples sporaidcally, then it makes sense to shut it down when it is not being used.  Depending on the sopecific analysis and parameters, it might make it tougher to meet calibration check criteria if you do this.

For the huge area between these two extremes is where you personally have to decide what is best for you.  Many users will have standby methods which minimize gas use during idle periods but maintain temperatures so as to be ready to go quickly.

2) Different detectors have different considerations for operating temperatures.  A few like the TCD, have responses that are temperature dependent and will influence your optimal operating temperature.  Typically the minimum operating temperature for your detector is set to at least 20C higher than your final oven temperature.

3) Carrier gas choice is based on detector requirements and economic considerations.  Order of carrier gas efficiency is as follows: Hydrogen, Helium, Nitrogen, Argon with hydrogen being the best.  Hydrogen gas allows for the highest nuber of theroretical plates across the widest range of linear flows than any other carrier gas (look up Van Deemter plots).

4) How long your analysis is depends on many factors, but the most basic answer is as long as it needs to in order to have all peaks of interest as well as any sample matrix elute from the column.

 

My suggestion to you is to research your analysis online to get examples of what others have done before (Restek, Supelco, Agilent, Thermo, etc all have databases of reference chromatograms available online.  I would also suggest you read up a little on GC techniques:

If you contact me I can provide a list of older Agilent pyublications that are excellent primers for all manufacurers and models of GCs.

Dan

avatar: Brainz
Brainz
7 posts
Replied to Dan Ste.Marie's post on July 18th, 2017

Thanks Dan for your elaborate response. I was kind of hoping you'd be online to respond and here you are. So grateful for your kind gesture.

 

I would like to attach a typical chromatogram from my analysis for you to see, but i'm not sure if that is possible here.

The challenge is in my typical chromatogram, I see peaks (ghost peaks maybe) consistently at 1.7 and 3.7 retention time, which i think shouldnt be there. I have done everything possible to get rid of it (by changing septum, liner, syringe, ferrule and gold seal) to no avail. The % area of my chromatograms aren't representative of my sample or so i think.

Secondly, my typical efffluent sample consist of Hexane & Decane (Oil composition) and water. To achieve  a homogenous mixture I have mixed it with Acetone. Recently I have used Iso-propanol to give a better mix.    

I am trying to analyse the components in the mixture so as to determine the %weight of each component, which in turn can give me an idea of the volume since i know their densities.

I am keen to learn more and wouldn't mind the help you can render me. Please advice me on how best to contact you. 

 

Thanks alot again.

Brainz.

 

avatar: Dan Ste.Marie
Dan Ste.Marie
359 posts
Replied to Brainz's post on July 18th, 2017
Contact me through my website www.rj2.biz Dan
avatar: Brainz
Brainz
7 posts
Replied to Dan Ste.Marie's post on July 19th, 2017

I sent you an email Dan.

 

Quote Dan Ste.Marie

Contact me through my website www.rj2.biz Dan

 

avatar: Brainz
Brainz
7 posts
Replied to Brainz's post on 5 mins ago

Hello guys,

Just wondering, So I have 30m x 530um x 40um (HP Plot Q) and 30m x 320um x 0.25 um (HP-5) capillary columns.

Which is better to use?, as my typical mixture would consist of the following compounds (Acetone, CO2, Hexane, Decane and a little water).

Or the choice of column Spec doesn't matter?