Chromatography Forum

Hi
The cation supressor on my DX-120 is leaking from the centre seam. This apparently may be caused by a high system backpessure. Anyone know how I can check the value of this?
Thanks
Tim

Hi Tim

The source of high backpressure can be due to

A) Higher than recommended backpressure to the cell. The recommended backpressure is 40 psi.

B) High backpressure from the cell.

C) High backpressure from the connecting tubings or the waste line.

Typically A) is the common cause of the leakage problem.

To test the contribution from the backpressure coil do the following:

1) Disconnect the lines in and out of the eluent ports of the leaking suppressor and connect them togather using a coupler.

2) Disconnect the lines in and out of the regen ports of the suppressor. Ensure that the line from the regen out port to the waste is a clear 1/8" tubing. This line can cause backpressure if the ID is < 0.02" and can cause suppressor leakage.

3) Turn on the pump at the application flow rate and measure the total system pressure. Let us call this X psi.

4) With the pump running disconnect the line out of the cell out port on the cell.

5) Measure the system pressure. Let us call this Y psi.

6) X-Y should be less than 40 psi. IF this is not the case then replace or trim this tubing to achieve 40 psi.

Now to test the contribution from the cell,

7) Disconnect the line (from the suppressor) to the cell in port.

8) Measure the pressure. Let us call this Z psi.

9) Y-Z should be less than 100 psi at 2 ml/min or less than 50 psi at 1 ml/min.

10) If the pressure of the cell exceeds the cited numbers in step 9 then replace the cell.

Note the contributions from any connecting tubings. Occassionally when the connections are overtightened this can cause high backpressure. Ensure that the tubing connections are all finger tight with a quarter turn with the wrench.

For more info the CSRS manual should contain all the trouble shooting information. This is available at Dionex.com at the following link. http://www1.dionex.com/en-us/webdocs/ma ... RA_V14.pdf

Thanks very much for the reply. I have checked the system backpressure following your instructions and it is about 10psi (also the same for the anion column). Is this too low do you think?
Also, I have noticed that during normal running of the machine the system pressure is a lot higher for the cations (1600psi) compared to the anions (980psi). Should there be such a large difference between them?
The flow rate is the same for both (1ml/min).

Thanks again
Tim

Hi Tim

I am not sure what you mean by the system backpressure of 10 psi. Is this the measured pressure of the coil ? The recommended backpressure to the cell is 40 psi.

Once the excessive backpressure in your system is removed the leaking suppressor unit can be reinstalled and in most cases would resume normal operation.

As far as the column pressures are concerned these depend on the type of the column and the test conditions. The column manual and the test chromatogram should display the expected pressure of the column. Please refer to these or you can visit dionex.com for more info.


Thanks,

Kannan
(kannan.srinivasan@dionex.com)

There is no problem with the suppressor itself?

Hi Kannan
Thanks for the help.
Yes, I meant the measured backpressure of the coil, it is 10psi. At least it is not too high but is it too low?
After Christmas I am going to try one of our old cation supressors in the machine, if it starts leaking too then I know we have a backpressure problem.
We have another problem with our machine. A sample of 100ppm sodium nitrite was injected and ever since we have had problems with measuring anions. We are now getting double peaks for nitrate, phosphate and sulphate. Numerous acid washes etc have not solved the problem. Any idea?
Many thanks
Tim

Tim,

The problem you report following an injection of 100 ppm nitrite is rather unusual as the column should be easily capable of dealing with such concentrations. Generally, double peaks is indicative of a void at the top of the column, contamination of the top of the column bed or contamination of the column inlet frit. Most of the time, the problem is due to particulate contamination of the column inlet frit and replacement of the inlet frit will correct matters. If this doesn't work, try reversing the column flow, making sure that you send the column effluent to waste, bypassing the suppressor and the cell. If neither of these corrective actions work, you might be able to correct matters with an acid regeneration step (rinsing the column was 0.1 M MSA).