How to achieve better separation using linear velocity

Discussions about GC and other "gas phase" separation techniques.

8 posts Page 1 of 1
Hello, can someone help me with this question? I am working with a Shimadzu GC-2014 with FID. It has installed an SH-Polar Wax column 30m length, 0.32mm ID, film thickness 0.25µm. The Carrier Gas is Nitrogen. I am using split mode with 200°C temp, split ratio 85.0. I have column flow 4.10ml/min that corresponds to a linear velocity of 58.6cm, I am using Pressure as the flow control mode with a pressure of 109.2 kPa. I have injected lemon tea and got, I think a good chromatogram. I have used the Pressure Flow Calculator from Agilent and after putting all the data it tells me that the optimum velocity range is between 8 and 16 cm/s. I used 12 cm/s as the velocity, the flow dropped to 0.582ml/min and the resulting chromatogram has peaks with lower height, but it seems to have achieved a better separation, is this normal. What I am doing wrong. I have expected peaks with greater height, but not.
Life is a game, and in a game you can win or loose, but it is still a game.
I assume that the 200 °C is your oven temperature.

Restek's flow calculator says your optimum flow with N2 carrier should be about 0.65 mL/min. That's a head pressure of 42.08 kPa. That just means that the separation will be most efficient at that flow rate. You don't have to use that setting, especially if the separation of your analytes that elute most closely to each other is easy. If time of analysis is more important to you, you can go with a larger flow.

If you have a difficult separation to make under these conditions, you'll more readily separate the pair at the optimum flow rate. In the end, you just need to have a method that gets the job done.
What you see is normal; because the lower flow rate moves everything through the column at a slower rate the peaks with nitrogen are wider, and for a given area will not be as tall. They will be a bit better separated, but your analysis will have taken longer. The "classical" optimum flow rate gives you the maximum resolution between peaks, there is another higher flow that gives the maximum resolution in a given time.

Peter
Peter Apps
rb6banjo wrote:
I assume that the 200 °C is your oven temperature.

Restek's flow calculator says your optimum flow with N2 carrier should be about 0.65 mL/min. That's a head pressure of 42.08 kPa. That just means that the separation will be most efficient at that flow rate. You don't have to use that setting, especially if the separation of your analytes that elute most closely to each other is easy. If time of analysis is more important to you, you can go with a larger flow.

If you have a difficult separation to make under these conditions, you'll more readily separate the pair at the optimum flow rate. In the end, you just need to have a method that gets the job done.


Thank you for your answer. The 200°C is the temperature of the injection port. I have used an Oven Temperature Program: 55°C for 1 minute, 10°C/min to 240°C for 1 minute. I have checked the Restek flow calculator as you said. I understand that the separation could be better enough with the pressure control.

We are analyzing essential oils and thought that a better resolution could give us more information. I want to share the chromatograms but I don't know how to attach an image in this forum. I understand that the image should be on a server, could it be Google Photos? I will try it.
Life is a game, and in a game you can win or loose, but it is still a game.
Peter Apps wrote:
What you see is normal; because the lower flow rate moves everything through the column at a slower rate the peaks with nitrogen are wider, and for a given area will not be as tall. They will be a bit better separated, but your analysis will have taken longer. The "classical" optimum flow rate gives you the maximum resolution between peaks, there is another higher flow that gives the maximum resolution in a given time.

Peter


Thank you for your answer, I understand your point. It is important to have a better separation If we have bad resolution in a pair of peaks.
Life is a game, and in a game you can win or loose, but it is still a game.
Linear velocity and flow rate are telling you the same thing. I think that "back in the day", it was easier to calculate linear velocity than measure the flow rate. You also can't measure the flow rate if your flame detector is actually on. You can, however, adjust the inlet pressure while the detector is on.

If you have a 30 m column, and something unretained (I typically use methane on my film-type columns) comes out at 2 minutes at 40 °C (oven temperature), then your linear velocity is:

3,000 cm/120 s = 25 cm/s

If you want the best possible resolution, you'd like to operate at the optimum flow rate over the entire chromatographic run. This means you must operate at constant flow and not the traditional constant pressure. Because the viscosity of gases increases with temperature, you have to increase the inlet pressure as the temperature of the oven increases to maintain the same flow rate. Most modern GC's will allow you to operate at constant pressure or constant flow.
Capillary GC is remarkably robust to not having the optimum flow rate, but the other side of that coin is that if you have a poor separation, changing flow rate will not fix it unless your conditions are way off.

Flow rates that are too slow are worse than having them too fast, because resolution deteriorates faster as flow decreases than as it increases, and slower flow rates waste a lot of time to get a poorer separation.

The operation of the inlet, and the selection of column stationary phase have much. much more influence od separation than carrier gas flow rate.
Peter Apps
I agree with Peter. While the flow-rate/efficiency details are interesting, in the end, it's all about solving your separation problem. Whether or not you are at the optimum flow rate is sort of irrelevant. If you've made your best effort to get the separation you need while adjusting the flow rate and oven temperature and it's still not good enough, changing to a different stationary phase is your best option.
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