8355 SCD detector low response

Question

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?

ryanmanser · Answer

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.

mhardacker · Answer

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.

8355 SCD detector low response

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