Dear All
I have two GC's machines 6890N, the first give me the GC response by count >10000 and up to 1200000 and the second give me the response by pA that is usually less than 500.
How to make the two machines obtained the same response (counts) or guide me to convert the pA to counts.
Thanks in advance.
Ill ask the "is it pluged in" questions first. First,do the instruments have the same detectors. Certainly,you will get different responses from an ECD and a FID or TCD. Second,what are you using to collect the data. Chemstations,Waters Empower,and Fishers Chromelion should all collect digital data,with some exceptions. All of those can be configured to use an external A/D convertor box.
If your using the analog output,either through an agilent integrator,a strip chart recorder or a D/A convertor,then check the settings in the signal menu. The range settings effects the 1v and 10v outputs. The attenuation setting affects the current loop output to the integrator. Both of those are defined as 2^n,where n is the number you enter into the settings. So increasing those numbers by 1 halves the signal that it controls. If you have one of those set to 0 and one set to 10,then one signal will be 1024 times the other.
Also check the wireing to make sure its connected properly and not loose.
Next,whether digital are analog,the check the automatic liquid samplers. Fiirst,(and this is another "is it plugged in" moment,and Ive done this enough,that its one of the first things I check) open the door to the autosampler,place your finger under the set screw that holds the pluger and move that part up. Often you dont get it screwed down on the plunger right for whatever reason,and you had nothing actually injected. Next, remove the syringes and make sure they can pull up some liquid. A clogged syringe (or one that didnt have the plunger screwed down) will make the instrument have a tiny response,basicly whatever manages to get into the injector from the tip of the needle and thats it. Hold the syringes up to the light and look at the spot where the end of the needle is cemented into the glass. Rotate it 360 degrees in the light,and look for a shiney flat area inside the glass. Thats a sign that the syringe clogged,and there is now an internal crack in the glass. Slide the pluger up and down and if its a non gas tight,it should be smooth,if its a gas tight,smooth,but have a little resistance. If in doubt,replace the syringe.
Next check the settings on the injector menu. Make sure your injecting the same amounts.
Now go to the inlet menus. Make sure the splits are set the same,if split mode. The mode should be the same as well. (split,splitless,pulsed split,etc) If your using a split/splitless injector,Id use them in split mode for now,but if your doing splitless make sure the purge times are the same. If one instrument is set for 300:1 split and the other is at 5:1 (or splitless) thats going to make a huge difference,as it will in splitless mode if your septa purge on one is 1 minute on one,and 0.01minutes on the other. Make sure the pressures are the same as well. Look at what kind of gas its set for. That makes a big difference because if thats not right,none of the pressure or flow settings mean anything. Then look at your column settigns. THe dimensions should be correct,as should the carrier gas type. Check the pressure mode is the same,(constant,ramped,etc) and that the flows are the same. Check that they calculate the same velocities (assuming the columns are the same). If the columns are different,then you may not be looking at the same peaks. For a .320mm capilarly column,30cm/s is a reasonable place to be at for now. Now look at your detectors. For an fid,your typical settings for a capilary column are H2:30,Air:300,makeup:30. The typical baseline current you should see on a lit FID is around 5-20. for FPD,TCD,ECD, NPD etc consult your manual. (be carefull if NPD,the new BLOS beads will be instantly destroyed if you set the voltage over about 2-2.5V ,or you do an adjust offset proceedure with on an instrument with an older firmware revision,consult your manuals before messing with it) There are enough variations with these that its not practical to go over them all. Fiinally,go back to the signal menu and look at the "zero" setting. Set it to 0.
Once you have gone over these,your instruments should be setup the same. Now if you dont get the same numbers,its probably a problem with the instrument itself. Turn off the flows,and open the inlets. Check that the liner is correct,not cracked and inspect it for buildup of "crud". Look down in the inlet with a flashlight. There is a seal that will either be silver,gold,or have that sort of "thin film coated" rainbow look. If its black,it needs replacement. If that looks good,(or when you fix it) put it back together. it cant hurt to change the septum now as well. Since your having a problem with A vs B,you could swap the liner and septum between the two instruments and see if it changes the result. Next if there is a second identical detector on the back,swap the outlet of the column to it (or the front if its already on the back) ,set it up the same and try it. Then you can try removing the columns,swap them between instruments and reinstall them. If the problem moves,your column is bad. If both work now,then it was installed improperly. (Agilent makes this nice poster on installing columns,and you can find instructions on their website)
If at this point,you swapped the column front to back and it did not fix the problem,and the problem did not move when you move the column,then you may have missed something. Go over everything again.
Next,your going to need to look at the baseline current of the two detectors. (if you didnt already when you were setting things) Are they both reasonable. If the one that works is at 10 and the one that does not is at 50,then your problem is probably a dirty detector. I prefer to simply replace the teflon insulators rather than cleaning them. THey are cheap and they are damn near impossible to get clean enough. I clean the detector in a solvent like toluene,then methanol. Then place the collector in HCl for a few minutes,and then rinse it,and clean the buildup from the center with a wire brush,then with the fine green abrasive paper often used to clean mass specs.Once its nice and clean,I clean it in methanol again. The jet gets cleaned in methanol,and a fine cleaning wire is used in the orafice. If anything looks wrong with the jet,I replace it. Then reinstall the column.
If that did not fix it,its looking like a electronics problem. Try swapping the electrometers,and the controller boards,and finally if necessary the main boards. If thats not it,you probably missed somethign. Start back at the beginning.
Essentially,the rule is,follow the path,from sample,to detector,to output.
