What does linear velocity (35 cm/sec) mean?

[Terry]

Hi, there

The USP <467> specifies a flow rate of 35 cm/sec for the determination of residual solvents.

Does this mean the linear velocity is always 35 cm/sec during the temperature ramp?

Terry

[zokitano]

Hi, there

The USP <467> specifies a flow rate of 35 cm/sec for the determination of residual solvents.

Does this mean the linear velocity is always 35 cm/sec during the temperature ramp?

Terry

I am interpreting (and working) this like: use constant linear velocity of the carrier gas at 35 cm/s throughout the chromatographic run (working with constant linear velocity of the carrier). When you set your PPC to work with constant linear velocity it compensates the variation of the viscosity of the carrier during the temperature ramp, increasing the column head pressure (when T raises) in order to maintain constant linear velocity of the carrier gas in the column.

Regards

[Peter Apps]

If you do not have a GC with constant velocity as an option, there is no need to rush out and buy one. Set the velocity by injecting a non-retained substance such as methane and adjusting the inlet pressure up or down with the column temperature at about the mid point of the programme, and then run at constant pressure.

Peter

[krickos]

Hi

I kind of disagree here, (EDIT Peter did not see Peters post until I posted this) though first I would like to emphazise that neither is wrong.

Firstly, this is a general pharmacopeia procedure intended to work on many different instrument setups. Having said that and just by reading these forum one can conclude that not everyone has GCs with the option to run constant flow and is stuck with constant pressure.

Practically in a temp gradient from 40-240°C for a 30m 0,32mm id column the initial flow of 35cm/s with constant pressure will drop to about 25cm/s still in the ptimum range of helium and frankly few residual solvents of intrest elutes above 160°C.

So basiclly you could do either way (constant pressure/flow) however if running constant flow in this case I normally set the velocity to 25-30cm/s in order to reduce the pressure strain on the colum slightly (especially when going up to 240°C). Have not broken a column this way but why strain the column more than necessary and it says approximately 35cm/s. If you are running an Agilent system try to have at least a 10ml/min split flow (ie minimum 2ml/min carrier to have the 1:5 ratio) as it usually dislike lower splits flows especially when running headspace injections.

[Terry]

Thanks for all your prompt reply.

As Krickos said, I am running an Agilent system, 6890N.

At 40°C, 35 cm/sec helium through a 30m, 0.32mm ID column has a flow rate of 2.2 ml/min; at 240 °C, helium flow rate would be 1.5 ml/min with the same linear velocity.

As far as I am concerned, carrier gas linear velocity is proportional to flow rate, and is independent of gas viscosity (oven temperature).

Constant linear velocity means constant flow, right?

Terry

[zokitano]

Thanks for all your prompt reply.

As Krickos said, I am running an Agilent system, 6890N.

At 40°C, 35 cm/sec helium through a 30m, 0.32mm ID column has a flow rate of 2.2 ml/min; at 240 °C, helium flow rate would be 1.5 ml/min with the same linear velocity.

As far as I am concerned, carrier gas linear velocity is proportional to flow rate, and is independent of gas viscosity (oven temperature).

Constant linear velocity means constant flow, right?

Terry

According to ideal gas equation PV=nRT, the gas volume is proportional to the temperature while keeping the pressure constant. So, if the
FLOW = VOLUME GAS / TIME and
VELOCITY = DISTANCE passed / TIME
it is evident that the parameters flow and velocity aren't equal. That's why while keeping the velocity constant, the gas flow changes (lowers) when temperature of the gas increases. During temperature increase gas volume increases but also does its viscosity (due to the higher kinetic energy of molecules and increased intermolecular interactions).

Regards

[HW Mueller]

Zokitano, it seems nobody said that flow rate equals velocity. For instance Terry mentioned a proportionality. The proportionality factor is the area of the capillary crossection (distance x area = volume, if you got your units straight out will come mL/sec). The proportionality factor is constant unless you want to correct for an expansion of the capillary due to a rise in temp.
Terry, this "As far as I am concerned," is a new scientific method?

[Peter Apps]

Terry, since you have a GC that does fancy things with the gas flow, get your money's worth out of it and set the linear velocity to what the USP dictates. You will get you analysis done a little bit quicker than with constant pressure (but I suppose that you are stuck with whatever temperature programme the USP dictates) but I doubt that you will see any difference in peak shapes or resolution.

Krickos, is it really possible to burst a silica column by the gas pressure in a GC ? I wonder what the burst pressure really is - anyone want to speculate ?

Peter

[Bruce Hamilton]

Polyimide-coated fused silica tubing can cope with pressures up to 1000 bar ( 14500psi ), according to SGE's old catalogue.

The prolem is more to do with bending stresses on the brittle, naturally-straight, silica tubing. Most coatings act by spreading the stresses along a length of the tubing.

I've used 0.1mm ID fused silica columns at 180 psi H2 and 380C with no tubing failures, and even pumped solvents through fused silica GC columns at HPLC pressures ( 3000 psi ) without obvious fractures, but it didn't unblock the column either

Please keep having fun,

Bruce Hamilton

[Terry]

Terry, this "As far as I am concerned," is a new scientific method?

Excuse me for my command of English. What I wanted to say is something like 'in my opinion but I am not sure', could you recomend a better expression? I will appreciate that.

The problem for me now is how to set the flow rate, while using constant flow mode. The linear velocity does decrease as the temp goes higher at this mode. So I can not get a constant velocity at neither constant flow nor constant pressure mode.

Terry

[HW Mueller]

Well, you now demonstrated that your English is good enough.

I worked with an old Siemens GC which regulated constant flow via physical means. This made sure that the flow was constant before it hit the injector, the rest was part of method development, a change of flow in the column due to temp. was then just part of the analysis. One does not have the same flow in all parts of the column anyway, this is also worked into the method.

[Terry]

HW, thanks for sharing your experience.

I would set the flow rate to 2.2 ml/min (i.e., a velocity of 35 cm/sec at the initial temp 40 °C), keep the flow constant during the temp ramp and never bother to think about a constant velocity.

Terry

[Peter Apps]

On the instrument that you have you can set the linear velocity, and the software calculates the volume flow, and adjusts it during the temperature programme to keep the linear velocity constant. You need to use this feature and not worry about setting and programming the volume flow rate.

Peter

[krickos]

Krickos, is it really possible to burst a silica column by the gas pressure in a GC ? I wonder what the burst pressure really is - anyone want to speculate ?
Peter

Peter, was a bit unclear there and more or less used a "backbone" recommendation. Broke could be either column and phase but hardly a risk in this case as discussed above. Seems to remeber that manufactures of columns like HP used to have some recommended pressure intervals for capillary columns. We run almost exclusively capillary columns here but also a few PLOT columns so "watch the pressure" tends to be included

Terry:
Some detailed info. If you use chemstation you swith the setting under columns in the "mode" window. Per default in chemstation you have above column, oven etc a plot of GC oven temperature versus time. To the left of the plot area you have a PLOT button, pushing enables you to add more info to the plot, you can simultanious plot column, flow/velocity/pressure to verify your settting or just for fun/info

[Terry]

On the instrument that you have you can set the linear velocity, and the software calculates the volume flow, and adjusts it during the temperature programme to keep the linear velocity constant. You need to use this feature and not worry about setting and programming the volume flow rate.

Unfortunately, there is no 'Constant Velocity' mode in the Chemstation. So I would have to chose either 'Constant Pressure' or 'Constant Flow'

Some detailed info. If you use chemstation you swith the setting under columns in the "mode" window. Per default in chemstation you have above column, oven etc a plot of GC oven temperature versus time. To the left of the plot area you have a PLOT button, pushing enables you to add more info to the plot, you can simultanious plot column, flow/velocity/pressure to verify your settting or just for fun/info

I have used this feature in B.02.01 Chemstation, it is fun. But in B.03.01, it is not available any more.

It seems very weird to see the plot when I am running constant flow mode as per USP <467>. The temp goes up, the velocity goes down, while the flow rate is constant. I am trying to figure out how to calculate Linear Velocity.

Terry