Chromatography Forum

A guard column is part of the separation system, together with the analytical column. Good manufacturers do QC also for the guard column. Cheap guards are not tested. That makes the difference.

A guard column is part of the separation system, together with the analytical column. Good manufacturers do QC also for the guard column. Cheap guards are not tested. That makes the difference.

We asked the Phenomenex people. They informed that QC was done without the guard column. But we can't run the samples without guard column. So the QA is not serving any purpose.

The column length is 300mm and the guard column is 1/6th of column length i.e. 50 mm.
Both are made of the same material.

Does your analyte have a retention time of about 18 minutes without using a guard column?

Check the back pressure of 1.) system without any column, 2.) column without guard column, 3.) column with guard column and 4.) guard column without column. Post the values here.

1. System without any column- 131 psi
2. Column without guard column- 537 psi
3. Column with guard column- 682 psi
4. Guard column without column- 334 psi

1: Backpressure from System = 131 psi
2: Backpressure from Column = 537 -131 = 406 psi
4: Backpressure from Guard column = 334 - 131 = 203 psi

Therefore the calculated pressure with column and guard column should be theoretically: 131 + 406 + 203 = 740 psi
Your measured value 3) = 682 psi is about 9 % below of the calculated backpressure. In absolute terms of (740 - 682) 58 psi difference, this difference from the calculated value (about 4 bar absolute) is absolutely negligible.

you also need to consider ID of both the guard and the column, sometimes actual guard ID is 4 mm vs. 4.6 mm for example. This might contribute to additional back pressure coming from the guard being connected.

Yes Vlad this is surely correct.

The column length is 300mm and the guard column is 1/6th of column length i.e. 50 mm.
Both are made of the same material.

The pressure is 696 psi which is less than the max. 1000 psi.

I had assumed that the ID is the same based on this post. Of course the ID will have a great influence, especially when 5 cm guard columns are used.

Also the total backpresserue < 1000 psi seems to be quite low. To be honest, I have never performed a hplc-separation at such a low backpressure. Nevertheless I'm usually performing reversed-phase lc-seperations and as far as I have read this thread, no exact technique was described by the thread-opener.

Yes Vlad this is surely correct.

The column length is 300mm and the guard column is 1/6th of column length i.e. 50 mm.
Both are made of the same material.

The pressure is 696 psi which is less than the max. 1000 psi.

I had assumed that the ID is the same based on this post. Of course the ID will have a great influence, especially when 5 cm guard columns are used.

Also the total backpresserue < 1000 psi seems to be quite low. To be honest, I have never performed a hplc-separation at such a low backpressure. Nevertheless I'm usually performing reversed-phase lc-seperations and as far as I have read this thread, no exact technique was described by the thread-opener.

Thanks Klaus I. for the information regarding backpressure.

Yes, the analyte has a retention time of 18.7 without the guard column.
I am performing normal phase hplc using Rezex column and separation is based on ion exclusion.