8355 SCD detector low response

I have my GC/SCD for analyzing butane samples. After I did maintenance on my pump for the SCD, I lost my signal. All I did was change and top off the oil. I was able to get a little response but the repeatability is not good. I use to get around 7000 area counts with my 10 ppm isopropyl mercaptan standard, but now I only get 2000. I tried changing both inner and outer tubes. I checked the reaction cell and the flow in the restrictor and it was good. I also just use oxygen for the ozone generator. Is there a recommended way for doing maintenance? Do I have to change everything when I do any maintenance? 

  • I am also facing similar issue. after PM, the response came down. replaced the pump oil, reaction tube. Base line is around 20. Response became 10 times lower. Did not find solution.

  • Hi,

    I only have very old experiences of the Sievers SCD, do not blame me if I am wrong


    pump maintenance:
    ... does the vacuum pressure remain the same after maintenance (as before)?
    ... is the pump out filter (oil mist filter) fine or could there be a minor blockage?
    I always wiped out the reaction cell (PMT through/filter) during every pump maintenance. Unfortunately the PMT has to be removed & the PMT can "age" quickly due to external light exposure (minimize exposure). Please read the manual carefully!


    ceramic tubes:
    * did you let the detector (ceramic tubes) cool down sufficiently before ventilating the pump?
        I had once heard that the ceramics can have a negative surface effect if ventilated incorrectly.
    * When changing the tubes, new ferrueles must also be used.

    outside burner :
    * Flows to the burner: have you checked or really measured the flows?

    * At that time I had checked for leaks from the burner by holding diluted liquid sulphur (mercaptan) to all screw connections. When the signal increased there was a leakage.


    regards, M.

  • Hello,


    pump oil mist filter:
    the pump used to be equipped with 2 filters.
    * The long l-shaped cylinder = ozone barrier (this should be changed regularly).
    * (experience on GC MS) the cubic oilmist filter can be overlooked as a PM part & the oil separator can soak up with oil if excessive gas ballast is used.
    For simple / short-term tests: remove the cube-shaped Oilmist filter & apply a suction hood to the pump.
    a) does the vacuum change?
    b) is the SCD sensitivity better?

    * vacuum pressure value: a very important value that I wrote down regularly. When the value has been determined, a sticker on the display can be helpful (then everyone should remember to always look at it), because poor vacuum explains a problem with the sensitivity.


    * the drop test: if possible do not let anything drip on it, so that there is no later overload of the SCD.

    It can be enough to open a small "smelly" sulfur vial & hold it on all screw connections, since many screw connections are under vacuum.


    * tighten / swaging all connectors:
    (Unless otherwise specified in the manual) Do not overtighten the connectors. Hand tight & maximum one 1/16 (- 1/8) turn. When the burner got hot, some seals may still be leaking; But the same applies here: gentle tightening (1/16 turn).
    But before I tight seals I always observed the vaccum value and used the "stinking" vial / drop.


    * wiping cleaning:
    sorry I wasn't clear: the PMT is only being removed & not cleaned! Cleaning is OK for: buner cell & filter disc with lint-free cloth & solvent.


    * ceramic tubes
    even if these parts cost something, I always swapped them in pairs, because the time until everything was conditioned took too long ;-)


    I cannot give taht many general recommendations because it was all 20 years ago.

    I suggest: asking Agilent here again what there is to be learned from experience/empirical values.
    best might be: based on these current tests and our own experience write a new message here with final question & actual SCD status.



  • Hi mhardacker, 

    Thanks for responding. 

    For the pump maintenance - we always change to a new mist filter so there hopefully should be no issues for blockage. I also noticed blockage with the traps even though it is new, but we check the vacuum to make sure it was the same after the maintenance. I will try wiping the PMT. I never did that before after doing a maintenance.

    As for shutting down, I follow the manual - it does not say how long you should wait between steps. Do you have any suggestions? See pic below. What is your recommendation, should I always change both tubes once I take the SCD down? Is there other steps to make sure it works the same when I bring it back on?

    For the burner - I had my coworker check the flow with the flow meter and it looked good. Thanks for the advise on the leak check. I will try that and see. 


  • Sorry, I tried to capture the section of the shutdown procedure I follow. It is page 56 from the Agilent 8355 Sulfur and 8255 Nitrogen Chemiluminescence Detectors manual - online pdf. 

  • Thanks for the input. I will try these out to see if I get a better response on the peak. Also, would you de-coke the tubes to activate the sensitivity? It seems like when I do de-coke, it doesn't make a change. 

  • I've found a few useful tips and tricks when working on these model detectors, when it comes to shutdown and maintenance procedures:

    • Close off the gas taps when the system is shut down and switched off. It helps protect from any small air slugs/leaks that may come through, more so for the older 355 models with the Dual Plasma Controller but could also work for the 8355 with inbuilt EPC
    • Leave the pump running for about 15 minutes once the base & burner temperatures are switched off. This allows sudfficient time for the burner to cool before removal, and is roughly the same time recommended for the older model burners. The GC firmware has an inbuilt feature that it will keep the pump running until 100mL of oxidiser gas (usually air) has been pumped through the burner, once hydrogen flow has been switched off. This should prevent any moisture buildup on the ceramics, I have also been in the habit of switching off the temperatures during this stage.

    (If using controlling SW on the PC, you can program a maintenance method with the reduced flows/temperatures, loading this is suffcient to shut down the system - the firmware will keep the pump running until the oxidiser gas purge has completed)

    • Take a note of the burner and cell pressure before and after maintenance, to make sure thee is no dramatic change in pressure after the pump maintenance. There may be a small shift due to oil level and quality, but unless there is a leak in the system the pressure should be very close to the original level.


    The detector can take a long time to warm up and equilibrate after maintenance, so be sure to leave the system at full method parameters for 2-4 hours (ideally overnight) before running samples. The inner ceramic especially can take a number of injections to warm up/activate properly so don't expect your first injection to be at the same level. I usually wait for 6-10 samples before getting too concerned.


    Pump maintenance parts depend on type of samples, how often the system is run etc. The rough starting recommendations are to change the ozone trap every 6 months (to prevent O3 accumulation), and the filters within the mist filter every 3 months - but again this may not be necessary quite this often, wiping out excess oil in the filter is important. Also check the oil level/colour weekly and top up if necessary.


    The reaction cell does not have to be cleaned every pump maintenance, but it is a good idea if this hasn't been done for a while (and with your current low response issue). The PMT will have to be removed for this step, ensure you reduce the ambient room light as much as possible before opening up the detector (room lights off, curtains down if present), cover the end of the PMT with the supplied black cap, once in place wrap the PMT in alfoil and store within a box in a drawer or similar to further protect from ambient light.


    The outer ceramic does not need to be changed at the same time as the smaller upper ceramic - this may be more common on the older burners as you had to fully remove the burner from the system to change the tubes. With the new design the burner can stay installed on the GC to change the upper ceramic drastically reducing the maintenance time. Changing the larger outer ceramic should only come after the upper ceramic has been changed and more likely issues for low sensitivity have been addressed (supply/column leaks, correct volume injected, pump oil etc). Bad column leaks can kill the large ceramics quite quickly, if you suspect this to be the case then also change the large ceramic.


    Decoking is not really necessary with new ceramics, it's mostly used to bake off the tubes after a period of use. Sufficient warmup time is enough to activate the sensitivity.


    So for your current low response issue here is a summary of what I would be looking at:

    • Check pressure of burner/reaction cell, verify oil is at correct level, no leaks from mist filter/ozone trap, ensure fittings are tight (black transfer line to top of burner should only be finger tight and maybe 1/8 of a turn with the spanner, any further and the tube can be compressed).
    • Make sure that the system has had plenty of warmup time and run enough samples to confirm the low responses
    • Check depth of the column in the burner, trim/reset ends if required. Flip the upper ceramic (swapping ends) to see if you get a quick response jump back. You can also run inlet pressure tests in case a small leak has developed around this point - steps will vary depending on the inlet you have.
    • If still low, replace upper ceramic with a new tube, warmup and run a set of injections to check response.
    • If still low with new upper ceramic, replace both ceramics. As system needs to be fully shut down at this point, good idea at this point to remove PMT to clean the reaction cell (noting the minimisation of ambient light above), and the reaction cell lens. Best method is methanol on a kimwipe/lint-free cloth.


    These steps should cover the majority of issues from low SCD response, deeper issues like PMT failure etc are much less likely.

  • With these pumps you will normally see a small amount of oil leaking over time, but if it's leaking very quickly and dropping the level (or the pressure is quite different to normal) then you may need to reseat/tighten everything. It's best to have the pump shut down when adding oil, as it will be hot and could potentially spit out. The oil level will drop slightly once it's been running initially for a while (as it reaches steady vacuum level), and then should stabilise. Used oil will go from clear to dark, this is the sign to change it.


    You have several options with how you leave the system, depending on when you run and how many samples the system runs. The biggest factor with the time it takes for the system to stabilise is the temperature of the burner - as the setting is normally 800 degrees it takes several hours to thermally equilibrate fully. Many customers will leave the system on full analysis method all the time, some will turn off the ozone power/flow to save some air/oxygen (depending on their ozone generation gas). There is also the option to turn off the hydrogen flows to save H2 gas as well, leaving the system in a decoking state between runs. The customers who leave the SCD on full analysis method would be running a decoke perhaps every couple of weeks to remove any accumulation from the ceramics.

  • Hi ryanmanser, 

    Thanks so much for the input. when you mean no leaks from the mist filter/ozone trap, do you mean I should have no oil anywhere? I seem to get a build up on the top of the pump which is just below the mist filter. Can you just top off the vacuum oil when it is running? It seems like we have to change the oil because the color is dark but it seems to drop once it has been running for a couple of weeks. Can you change the filters in the mist filter while it is running or would you do a complete shutdown?

    If you are just running samples in the morning, do you recommend to leave it in a sleep method - just like the one on page 55 of the NCD/SCD user manual (resource conservation)? Is it ok to have the SCD on normal parameters the whole time? Currently we just have the GC in a shutdown mode.

  • I do have a preventative measure to this problem, but you will need an extra vacuum pump.


     A great way to ruin a perfectly running SCD is to disrupt the burner, and by willingly turning off the vacuum (for maintenance) to the system it will disrupt the burner. When I started working with SCD's I banged my head with vacuum pump maintenance too, then I had the idea to install a 3-way valve on the vacuum line to supply a secondary vacuum to the system.  Now when we do vacuum pump maintenance, we hook a second pump up to the system so our SCD system never looses vacuum.


    My advice would be to get a second vacuum pump to have as a backup if you do not already have one.

    Purchase a 3-way valve from a fitting vendor of your choice, I like Swagelok.

    The valve and fitting we are using is B-45XF8, and it will need three hose adapters Part No.  B-8-HC-1-8

    Next maintenance cycle when you have to shut it down , cut the line and install a three way valve and make sure to use hose clamps.


    When doing vacuum pump maintenance with this setup, I have a few steps I like to follow.

    1. Before any work is done, record the cell and burner pressure for reference.

    2. Turn off the ozone if your spare pump has no trap.

    3. Connect the spare pump to the 3-way valve, do not switch the valve yet

    4.Turn on the spare pump, let it warm up for 1 minute.

    5. After the spare pump has warmed up, turn the 3-way valve so the system is being supplied by the spare pump

    6. Compare the systems cell and burner pressure to see they are the same using the spare pump, if drastically different swap back to the main pump. Fix the connections on the spare pump (O-rings, clamps, etc...) and try again.

    7. If the system pressure is acceptable with the spare pump, go forward with maintenance on the main pump.

    8. After maintenance, reconnect the main pump and turn it on. Let it run for at least a minute.

    9. Turn the 3-way valve so the main pump is supplying vacuum to the SCD system, verify that the cell and burner pressures are where they are suppose to be. 

    10.  If the pressures match or are close to the recorded values, turn the ozone back on and the job is complete. You can proceed running in about 15 minutes.

    11. IF the pressures are drastically off, swap the system back to the spare pump and fix all your connections on your main pump.

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