7 Replies Latest reply on May 10, 2018 12:16 PM by morlab

    SCD 355 loosing sensitivity over time


      Hi - we have an old SCD 355 coupled to a GC×GC instrument running sulfur speciation in petroleum samples.

      Over the last 5 years the sensitivity of the detector has decreased significantly, which in practice is seen as elevated background noise and thus a lower limit of detection.

      We have considered changing the reaction cell to regain the sensitivity - any experience on the effect of such a replacement?

      Looking forward to receiving your expert inputs :-) BR Karina

        • Re: SCD 355 loosing sensitivity over time

          Hi kake,

          Tags have been updated for better visibility.


          • Re: SCD 355 loosing sensitivity over time

            Hi David,

            When your SCD is losing signal the best place to start is to replace the upper ceramic tube.


            G3488-60008 Kit, Upper Ceramic Replacment (3 pkg)


            This tube often gets poisoned by column bleed which causes a loss of signal.   If you replace the tube and all your signal returns, this tube is being poisoned.   If the signal starts dropping again, your column is bleeding and needs to be conditioned or replaced.

            When you install a new column it must be conditioned by ramping it several times to max column temp at a slow rate before you connect it to the detector.

            There are times where the outer ceramic tube needs to be replaced, but most of the time it is just the upper ceramic tube.  Try this first.

              • Re: SCD 355 loosing sensitivity over time

                Hi danots,

                Thank you for your reply.

                We have tried exchanging the ceramic tubes (upper and outer) several times, but this does not do the trick. Also the columns have been replaced frequently, as we customize the application a lot being R&D. The SCD works in parallel to a FID on our GC×GC system, so we have an online verification of the column condition for each run.

                Now we are looking for suggestions for more radical actions to restore the sensitivity. Of course buying a new detector has also crossed our minds, but that is our last resort....

                  • Re: SCD 355 loosing sensitivity over time

                    Does changing the upper ceramic tube restore the signal for a day?

                    If yes, you are having a column bleed problem.   The column bleed can poison a upper tube in one day.

                    If changing the tube does not have any impact on the signal, you may have a problem with the reaction cell and it supporting elements.

                    Do you use air or Oxygen for the reaction cell/ozone generator gas?  Air often has water in it and this will cause the ozone generator to block the ozone restrictor preventing the reaction cell from getting any ozone.   Once this happens the signal will not return until the restrictor is replaced.


                    You can use this link, to find local support options.   https://www.agilent.com/home/more-countries?currPageURL=http://www.agilent.com/en/contact-us/page

                • Re: SCD 355 loosing sensitivity over time

                  Hi kake


                  I used SCD for more than 5 years . it is one of best selective detector for sulfur but only limitation there is losing signal by the time .


                  What is your signal reading in ozone on and ozone off position ?

                  Did you change you regulator or sampling system recently ?

                  How frequently you are using the analyzer with sulfur sample?

                  just Try following passivation procedure :

                  1. Connect the SCD with high conc of sulfur ( 30 ppm H2S/N2)
                  2. Increase the oven temperature to 100 C
                  3. Leave this condition over night
                  4. Now your regulator ,  line and system is totally passivated
                  5. Next day purge system with pure nitrogen for three cycles
                  6. decrease the temp
                  7. check your sample again and notice the difference in sulfur peak area comparing to before passivation area
                  • Re: SCD 355 loosing sensitivity over time

                    Hi Kake,

                    Due to age, I am marking this as assumed answered , feel free to jump back in and post if this is not the case. If any of the posts were helpful please mark as correct answer.



                    • Re: SCD 355 loosing sensitivity over time

                      Kake, the reaction cell is pretty easy to clean. The important thing is to make sure the room is dark when you are removing the PMT. I wrap the  capped PMT in a dark labcoat and store it in a drawer, then turn the lights back on to clean the reaction cell, use methanol or DI for cleaning.. You may also need to replace the PMT filter, especially if it is obvious that a lot of ozone corrosion has occured on the filter.  Also be careful with the post-ozone restrictor that is inside the reaction cell, it should not be bent.


                      If the previous poster is right and you have a post-ozone restrictor problem, you can remove the post ozone restrictor and measure the flow coming out the end of the restrictor. It is supposed to be between 20-30 mL/min. The post-ozone restrictor is a P/N that you can replace.


                      Lastly when was the last time you replaced your ozone generator? Other than signal issues, you can visually inspect your ozone generator to determine if it has failed (or failure is imminent). The tube inside the generator will change from gray to dark brown/orange as it fails.