Hi Mahesha.

Probably the detection cell in your detector is obstructed. If you have experience in clean up the cell, you could make. You could remove the column and it  connect the HPLC pump direct in detector. Try low flow (about 0.1 or 0.2 mL/min) water, if maybe you used  high concentration of salts in mobile phase. You could use warm water (about 50ºC or 60ºC). If doesn't unobstructed try another HPLC solvents ( methanol or acetonitrile).

Regards

If you do hook the flow cell to the pump make sure you don't exceed the max pressure listed on the flow cell. For a DAD standard flow cell the max pressure is 12 MPa / 120 bar / 1740 PSI. Operating at or above this pressure will damage the flow cell.

The above message from Marcos Steola is correct--if you have been using mobile phase with buffers it is likely that there are salts clogging the cell or the tubing going to the cell. Get a new piece of capillary tubing to go from your column to the flow cell (since that is easy enough to try before doing anything with the flow cell.) If that piece of tubing isn't to blame then heat up some water to 50-60 C and pump it at 0.1-0.2 mL/min through the flow cell. I would go into the method settings for the pump and change the max pressure from the default (400 bar) down to like 60-70 bar, that way the pump will shut down before you approach the max pressure of the cell.

The volume of the flow cell is only about 10 uL so even at 100 uL/min you're purging the cell 10 times per minute. If the problem is salt and you're able to get any flow through you'll likely be able to remove the blockage if you let it pump for awhile. Monitor the pressure and as it drops bump the flow up to 0.15 or 0.2 mL/min. Once you're satisfied that things are looking better restore your configuration (put flow cell back in detector, set max pressure back to 400, reinstall column to flow cell etc) and see if you're able to run half your normal flow rate. Continue slowly ramping up the flow until things are normal. What is your normal flow rate and what back pressure does that usually give you?

If you have only been using organic solvents and not buffered aqueous solvent then the source of the clog is a bit more mysterious and it may be tough to find the right solvent to dissolve it.

Thank you all for your valuable comments.  They are really useful to me. I will follow the suggestions given. 

as per my view just connect the detector from the reverse direction without connecting column  and pass water for some time...followed by methanol

It is possible to replace the windows of the flow cell in the case of some leaks.

Any of these procedures should be done with care, the flow cell can definitely be damaged to the point where it must be replaced.

Gobardhan Bal's answer is the 1st thing to do and probably the easiest.  Use filtered water first.  What solvent were you using?  I like to use water ACN MeOH the IPA after the clog is gone.

Thanks Dr. Stefen for your answer. I was using ACN and IPA: n-hexane (1:1)...

Transition from 100% polar to 100% nonpolar and then back.

Dear Dr. Alex, I did not get your comment. Are you speaking about my mobile phase?

Ah I'm no Dr :) what I mean is that it may be useful when clearing the clog to flush the system first with 100% of the polar component of your mobile phase and then once you're able to get flow through the cell and things look OK you may want to run a gradient to 100% nonpolar and then change it back to your normal mobile phase conditions. Just to make sure you remove contaminants that may not be soluble in 100% polar conditions.

I agree with Alaa Khedr. Either the detector portion need to flush with cleaning solvent or also the cloumn frit at the detector end is to be inspected for clogging. 

Zoom in the baseline. is it very noisy? more than a usual chromatogram? yes, perhaps the clog is in the detector. No, it might the clog be in the linker tube. 

change the tube, and check the pressure. 

if the pressure would not decreased, check the content of the last mobile phase and samples content.

if it was a phase with high concentration salt wash your system forward and then backward with a weak acid (e.g. acetic acid solution) at low pressure. 

if your column was at high temperature in the final run, and your samples were lipophilic samples wash the detector at a warmer eluent e.g at 40 C.

use both polar acetonitrile and non-polar n-hexan solvents for washing. be ware of vaporization of solvents. 

you can open the detector as the final remedy but be careful it is a sensitive device. if you look at the light path you can see the residuals and particles in it (if presence any). you should soak it in polar solvent and ultrasound the separated tubes. Referring to a technical expert for the final step is recommended.   

I have actually used the Agilent 1200 HPLC-DAD system for several years. A few times in those years I got a clog in the DAD end of the instrument. In addition to Alaa Khedr's excellent suggestions, I would add to visually inspect the flow cell first before cleaning. The first instance our flow cell was clogged after several years of use, it was bad enough that we had to replace it.

Carefully backflush the DAD flow cell.  Some particle or rubbish is apparently blocking its exit. There should never be this much pressure increase, so there is a clog.  Such excessive back pressure ( plugged frit?) will destroy the DAD cell.  

I basically agree with Tom.

Sometimes a weak acid like acetic acid is enough for cleaning up the cell. Remember that most likely salts account for dirty or clogged PDA cells. In case the clogging is located in the tubes (entrance and exit of PDA cells are very thin), disassemble the cell and flush with a manual syringe some solvent like diluted acetic acid.

Best,