# Question about stationary and mobile phases

1. Nov 21, 2014

### Latsabb

We have been learning about HPLC and GC machines, and discussing the various types of columns, and I felt that everything was pretty clear and obvious. However, something from yesterday made me a bit confused, so I am hoping for some clarity. If you have a polar stationary phase, and you want the mobile phase to travel through the column faster, you want to up the polarity of the mobile phase. But why is that? It seemed to me like it would be the opposite, as two polar phases would "grip" each other, and actually make it travel slower. I thought that this would most certainly be the case in something like an open tube column, where a polar mobile phase would have much more interaction with a polar stationary phase.

Can someone shed some light on this, please? I am having trouble wrapping my head around it. Thanks.

2. Nov 21, 2014

### Bystander

The "mobile phase" in the instance you're discussing is probably the carrier. Higher polarity increases mobility of the polar components of the sample you're trying to separate.

3. Nov 21, 2014

### Latsabb

Yes, the mobile phase is the carrier liquid for the analyte. And I understand that a higher polarity mobile phase travels faster through a column with a polar stationary phase, but I am curious as to why. Typically polar things draw each other closer, and provide a force of attraction, which would make me think that the liquid would travel slower due to more "grip" from the stationary phase. That clearly isnt how it works, but I would like to know why.

4. Nov 21, 2014

### Bystander

The mobile carrier is flowing at a constant rate whether you have sample on the column or not; when a sample is injected at the head of the column the rates at which sample components move through the column are a function of their polarity, for the polar case we're discussing, the most polar being held most strongly on the stationary phase (to the point of permanent attachment if the carrier is not polar enough to dissolve it), and the least polar being freely soluble and only slightly held on the stationary phase can be swept through at nearly the same flow rate as the "mobile"/carrier phase.

Help?

5. Nov 21, 2014

### Latsabb

Ah! Yes, this helps quite a bit. So the reduction in retention time is not due to the flow rate being changed, but rather the interaction between the analyte and the stationary phase. And the reason a higher polarity mobile phase speeds it up is that it needs the polarity to "tug" the analyte along, or else the analyte would stay in/with the stationary phase. Thank you very much! This makes much more sense now.