Chromatography Forum

Hi
I have a refurbished Agilent GCMS 6890/5975. I am new to GCMS and ran an autotune, but realized I don't know what the various things on the report mean and that I also don't know what normal values are.

What values are really important and what should the normal values for those be?

For example the H2O is at 4%. Where I worked previously, 4% was not good and we would report it to the person who was in charge of the GCMS, but now that person is me. We also ran the autotune every day. I've tried to look for leaks and cannot get the 4% to go down. i tried to bake the oven at 250degC and that also did not do anything for lowering my water %. I do not know what to do so any help would be great. I do not have a transfer line though I did try to lower the temp on Aux 1 to see if that would help. It did -though I don't know why. Can anyone explain this to me?

I looked in the history of the instrument and no autotunes have been run on the instrument since September. I'm new to this company but it seems that the person before me never ran an autotune. I have seen wlsewhere ont he forum that autotune should be run monthly once the machine is set up properly, but I'm not sure that the machine is set up properly. Last week in my search for a leak, I found a loose nut between the inlet and the column. That made the O2, N2 values go down, but the water was still around 4%.

thank you for any help you can give me.

Sometimes if an instrument has been sitting for a while water can build up on the SS parts inside the MS and it takes a while for the water to be pumped away via vacuum. However, it doesn't sound like this is your issue as it seems it has been pumped down (under vacuum) since at least Sept. What type and quality of gas are you using? Best for GC/MS is to use UHP 5.0 (>99.999%) grade gases. They are more expensive but usually have no issues with water or other contaminants. The other question I have is about traps. Are you using any traps? Most Agilent systems come with a larger bulk hydrocarbon trap. It is also common practice to use at least a moisture trap as well. I have used these with a triple indicating trap downstream of the bulk traps so that you can tell by color change when the bulk traps have reach their capacity.

Without knowing more about your setup and assuming you have no leaks as you say, I think that the carrier gas may be the source of water.

I'm not sure what you mean by not having a transfer line? This is usually the metal rod attached to the MS that inserts in the GC oven and has a threaded end for the column nut and ferrule to seat on. What is supporting the column from the GC to the MS? What temp is being controlled by "Aux 1" and what temp is it at if there is no transfer line?

You can probably get away with some background water for a while until you figure it out but you will lose some sensitivity and you will have to clean the ion source more frequently as the water will oxidize the hot source. Once you get it figured out the 5975 is a very rugged and reliable instrument.

The autotune just adjusts the voltages to better focus the ions to the detector (as well as mass calibration I think). Once you do this it should be tuned. It theoretically shouldn't matter how long it has been since the last one as long as it tuned properly for you before you start running samples. Most EPA methods require that certain tune criteria are met every 12 hrs (using BFB or DFTPP) and that the tune is less than a month old. If the criteria are not met or it has been longer than a month you must perform a new tune.

Let me know if you have more specific questions or if you can answer some of mine.

Ty