Chromatography Forum

Dear Chromatographers,

With the current resources that are available to me for analysing waste solvent, I am analysing various solvent mixtures with a run time of 57 minutes (a 22 component standard).

Carrier Gas : Nitrogen (I know its not the best and have suggested moving to helium)

Initial: 0.3ml/min for 31mins then
Ramp1: 1.0ml/min/min to 0.4ml/min and hold for the rest of the run.

Column: Elite-624 - 30m*0.25mm*1.4mm

Detector: FID

Oven: Initial: 38oC for 26mins
Ramp 1: 0.50oC/min to 42oC and hold for 1min
Ramp 2: 8.0oC/min to 150oC and hold for 5mins
Ramp 3: 25oC/min to 175oC and hold for 2.5mins

Instrument: Perkin Elmer Clarus 500
I tried several approaches but this was the best I could do to get a decent separation.

My question is:

Our company is a waste disposal company and we dispose chemical waste (either recovery or incinerate) so we get all kinds of waste (solvents with paints, oils, etc, etc.)

1) If we were to make the best use of our GC, what changes should I make to get the best out of my GC.

I am open to ideas and suggestions. If anyone is already doing this kind of stuff, what parameters are you using for the analysis.

2) If I was to create 1 method for all the solvents (>40), how should I go about doing this. The solvents are chlorinated and non-chlorinated.

3) Should I stick to a capillary column or move onto a packed column?

Thank you all in advance for your ideas and suggestions.

Salma

Hi Salma

Question 3 is the easiest - capillaries give the best separations in the shortest time.

The 0.3 ml/min for the first 31 min is too slow. It gives you a linear flow velocity of only 10 cm/s, which is to the left of the optimum on the Van Deemter curve. This gives the worst of all possible worlds; you are getting less than optimum resolution, and wasting time at the same time. Your volume flow rate must be at least 0.45 ml/min (so the rest of the run is too slow as well). You might be able to set a linear flow rate on the GC - if so set it to 15 cm/s, with constant flow throughout the run. Check the linear flow velocity by injecting methane and measuring the retention time. Until you have the flow rate set properly there is not much point in trying to optimise the temperature programme.

Have you checked the applications on the column suppliers' websites/catalogues for a column and a set of conditions that will achieve the separation that you are looking for ?


Peter

I know helium must be expensive.

Try using Hydrogen as carrier, the best of all worlds.

You will have flexibility in flow rates and you will speed up your analysis considerably.

best wishes,

Rod

Hi Peter,

Thanks for the response. My GC has a Column Carrier Gas Calculator and here are the calculations its giving:

Carrier gas Flow
N2 0.24 - 0.53 (Velocity 8-16)

He 0.72 - 1.87 (Velocity 20-40)

H2 0.99 - 2.12 (Velocity 30-55)

Basically, I choose the Carrier gas and put in the column info and it gives me the suggested velocities and from the flows that I enter, it gives me the actual velocities.

By the way, I was trying to post a chromatogram but dont know how...can anyone help?

Salma

Hi Salma,

If you can, change your carrier gas to hydrogen or helium.

Whatever gas you use have a look at this to see the effect of linear velocity on the efficiency of your separation for different carrier gases http://www.chem.agilent.com/cag/cabu/carriergas.htm. The lowest point on the curve is the best efficiency. Also note that as you slip below the optimum flow rate how quickly you lose efficiency compared to going the other way.

You can see that the carrier velocity that you were using for nitrogen was too slow. As Peter said, you need to be at about 0.5ml/min at 38°C to give you 15cm/sec .

In practise we usually work at 1.5x the optimum linear velocity for any given carrier - it is better to be too fast than too slow.

You will find that disposable lighters have been designed to take a 10ul syringe needle to enable you to inject an inert gas to check your linear velocity

Regards,

Ralph

Hi GOM,

Thanks for the reply and suggestions. I will work on that. However, the web address you have posted is not accessible....Its been moved (DARN!)

Also, I had started with a flow of 0.5ml/min but I was having problems separating 2 of my peaks (MEK and Ethyl Acetate). I would have really liked to post a chromatogram, but I dont know how.

Salma

Hi Salma

For instructions on how to post chromatograms, go to the liquid chromatography part of the forum and there is a sticky at the top of the list with how to do it.

Did you really get better resolution between MEK and ethyl acetate with a lower than optimum flow rate ?

Peter

Hi Salma,

http://www.chem.agilent.com/cag/cabu/carriergas.htm

should work if you cut and paste it. If it still doesn't then try putting "van Deemter curve" into Google

Regards,

Ralph

Thank you Ralph.

Peter, thanks, I knew I had seen it somewhere.

As for improved resolution, the flow rate that I used is within the GC's suggested flow rates, so yes the separation was slightly better, but I need to investigate the slightly higher rates further............Thanks and you know what, I have been able to come this far with your help so thank you for all your suggestions coz they have been great.

I will post the chromatogram on Monday as I am not working this week end.

Salma

Hi Salma

Separation and resolution are not the same thing. You can separate peaks more widely by running at low temperatures and low flow rates, but because the peaks get wider the resolution is actually decreased. You can see this as the valley between two adjacent peaks becoming higher in relation to peak height, and by the difference between the retention times becoming a smaller multiple of the peak width (in other words fewer peaks would fit between them). What you are aiming for is to get the valley as low as possible and as many peak widths as pssible between the retention times, in as short a run time as possible. The Van Deemter curve gives the flow rates for maximum resolution per unit time (which is what you want), not for maximum separation (which is probably what you have acheived by running the carrier gas too slowly).

Peter

Ok, here goes:



I think I have done it. What do you think about the chromatogram?

Also, If I ran a sample, how do you suggest quantifying the unknown peaks, especially because chlorinated solvent have much lower response factors than non-chlorinated, so how should I go about quantifying the unknown peaks?

Salma

If your samples only have those peaks, then it looks like you've achieved good separation . Thanks for sharing the chromatogram, it's very helpful.

I'd be trying to gain some more time back at the beginning, probably by increasing initial temperature, if you don't want to change other parameters.

That method should be able to come back to about 30 - 40 mins, especially if you use hydrogen and drive slightly up the Van Deemter curve. That's the advantage of hydrogen, the increased velocity doesn't adversely affect the plate count as much.

The obvious issue is how do your samples look?. With a 624 column, I'd be concerned about separation of some common industrial solvents, eg Acetonitrile/acetone, THF/chloroform/sec-BuOH, IBA/Benzene/EGMME,
MIBK/Pyridine, but if you know they aren't present, and you have resolved all the peaks of interest then your only problem remaining is the responses of halogen containing solvents, and possibly some of the lower alcohols ( low response ) and aromatics ( high response ).

The best way forward for halogens would be to invest in an ECD and split the column effluent between FID and ECD, but in the absence of that, you may just have to calibrate using standard mixtures that bracket your sample composition extremes. It really depends on how accurate you have to know the composition.

Please keep having fun,

Bruce Hamilton

Thanks for the response Bruce,

I tried to start at slightly higher temperature but then the MEK and Ethyl Acetate were not separating properly. I even tried to hold it longer at about 45 degrees but still couldnt achieve the separation. I was only able to achieve it after starting at 38oC and holding it for 26minutes and then slowly ramping it. As for using higher flow, I used slightly higher flow than the optimum but was getting poor reproducibility. On this, I managed to get RSD of less than 0.6%.

I will eventually change my carrier to H2 as I know that I will achieve shorter analyses times. When I started my present position, the Company was already using N2 as Carrier.

Because we deal with chemical waste, the material we receive could have anything really, so I cannot say that the above are the only components but the above Chromatorgram is that of a standard that I prepared.

Thanks. Also Bruce, have you any suggestions about an alternative column, rather than and alternative detector?

Thanks

Salma

First, try the method out on real world samples, see how it performs. If it's OK, then you can then look to optimise it.

With regard to another column, it's hard to consider one until we know what's in your real world samples, so instead, let's use some knowledge you've gained to your possible advantage.

You've discovered that the delta retention times of compounds changes with your temperature programme, so why not try quite a radically different oven programme, and review how the peaks rearrange themselves?. As many solvents may change slightly, you may find that you can resolve/identify compounds just by using two temperature programmes on the same column. I assume your instrument has only one detector, so can't use two columns?>

I'd suggest a gradient of xx/1min -> 3.5C/min , and watching where peaks move to. If it doesn't work you could them explore the two obvious options, reduce sample quantity onto column or change to another column polarity, but that really requires some clear information about the typical samples you will be analysing.

If you have some other columns lying around, you could try them. A non-polar column probably will not offer sufficient change, but you could try.

Please keep having fun,

Bruce Hamilton

Hi Salma

This is quite a tricky mixture that you have to separate ! - and the real samples are probably going to be even more difficult because the peaks will be radically different sizes.

Could you run the same standard with the gas flow at 0.5 ml/min and post the chromatogram please.

I doubt that any single column will be able to resolve all the components in all your real samples, so (as Bruce said) you need to be testing different columns. A GC with multiple detectors would allow you to run on two (or more) columns at once, with the constraint that they would all have to have the same temperature programme. It is straightforward to connect two or more columns of the same dimensions to one inlet - the injected sample then splits to the different columns and you get different separations and peak elution orders. Both chromatograms will have some overlapping peaks, your aim is to have each compound as a clean peak on at least one chromatogram. Of course, you can do exactly the same thing with a single column and detector set up by changing columns, but your sample throughput is going to be very slow.

For quantitation you need to calibrate the response of different solvents separately, especially if you are looking at mixtures of chlorinateds and hydrocarbons. You certainly cannot take the peak % area as the % of that component in the mixture (unless you have a mixture of similar compounds).

Peter