GC-FID baseline or contamination problem with biodiesel/FAME

[user8]

Hello,

I'm fairly new user to GC-FID and I'm having a bit of problem with my (new) Agilent GC-FID 6850 that I'm hoping someone can help me with. The problem consists of ‘noise’ or a possible contamination, after about 5-6 minutes on a 150C-250C gradient (10C/min) after the solvent peak and about 16 pA. The column is HP-88 (100 meters, 0.250 mm ID, max temp 250C) and inlet temp is 250C, and FID is at 300C. The carrier is helium, and with air and hydrogen for the FID. The issue appears to have started around the time that I injected the Supelco 37 FAME samples (10 mgs/mL, 1 uL injection, ranging 10:1 to 50:1 splits) and biodiesel transesterification reactions of plant oils.

I had initially tried the following, but without improvement:

o Baked column for 2 hours at 250C (column maximum)
o Changed the solvents to new bottles of methanol & hexane (HPLC/GC grade)
o Changed to new sample vials, glass pipettes, pipette bulbs
o Changed syringe needle
o Changed injection liner
o Changed gold seal with metal o-ring underneath liner
o Changed rubber septa above liner
o Cut off 6 inches at front and back of column & ensured column ends tightened into inlet & detector
o 4 runs with 5 uL injection with 100:1 split in order to wash out residue in split liner
o Washed inlet metal ports with hexane

I have then removed the syringe so there was no actual injection, but without improvement, so the issue is not with the syringe apparatus. Our helium line (carrier gas) is filtered, but the air and hydrogen are not (but have ordered filters). I disconnected the column from the FID, and the problem does not occur afterward, so I had think it must be with the column or inlet. Isotemp runs at 150C and 175C do not produce any problems, but runs at 200C and 225C have increasing ‘bumpy’ baselines, so perhaps something with high MW is on the column (or inlet) that is heating up and coming out.

I have finally rebaked at 250C for 2 additional hours, and cut off 30 additional inches at the front of the column, but still no improvement. I have not changed column to our spare HP-88 as I did not want to contaminate it if the problem was somewhere in the inlet apparatus that I had not cleaned or replaced. I also have not baked the inlet at 280C or 300C as we don’t have new o-rings (but have ordered them).

Chromatograms:

http://i1232.photobucket.com/albums/ff3 ... sample.jpg
http://i1232.photobucket.com/albums/ff3 ... yringe.jpg
http://i1232.photobucket.com/albums/ff3 ... yringe.jpg
http://i1232.photobucket.com/albums/ff3 ... yringe.jpg
http://i1232.photobucket.com/albums/ff3 ... yringe.jpg

Any suggestions would be greatly appreciated. Thanks!

[willnatalie]

It seems you have done quite a to alleviate this issue. At first glance, considering your column temp has a max of 250C I would lower the temperature of the FID to be with in the range of the column. That being said I would take out the jet to see if any silica residue is present, if not still consider dropping in a new one.

[Peter Apps]

Since you have eliminated a lot of possible problems with the troubleshooting so far there are some additional tests to try.

If the GC is in standby, with the column at say 60 C for a long period, and you then do a no injection programmed run, do the noise peaks get bigger ?. If they do they are getting onto the column with the carrier gas. This implies that they are coming from the inlet, or from the gas itself. If they stay the same size they are coming from the column itself, either from phase breakdown or ghost peaks..

If the inlet is cold and the GC in standby with the column at 60C for the same period and you do a no injection blank run, do the peaks get smaller ? If they do they are probably coming from the inlet.

When the GC is in standy is the inlet split or splitless ?, and do you have the gas saver running ?. There might be heavy muck in the split line that can diffuse back into the inlet under splitless flow, or with a low gas saver flow.

Do you have any packing (glass wool or whatever) in the liner, if so try an empty liner.

In my experience baking a column has limited benefit - if you do want to bake it then connect the detector end to the inlet and leave the other end free, that way you drive heavy muck back out of the inlet end (now downstream) rather than further into the column.

Peter

[Don Shelly]

Since you only seem to get this junk when you ramp, it makes me think that something is loading on your column when it is cool. Follow the troubleshooting tips provided by the previous post and let us know how it turns out.

[user8]

Hello all,

Thank you for all your suggestions. I still got the baseline problem when I put on a brand new HP-88 column, but not another different column. Quite confusing for me. An Agilent service engineer came by, and determined that the baseline issue is not contamination, but actually colummn bleed when the temperature is gets too high (apparently above 220C). In this case, this issue is resolved by simply not going above 215C, and which was confirmed by running various FAME standards.

Thanks again.

[Peter Apps]

Thanks for the feedback - can you do what you need to do with the lower limit on the programme temperature ?

Peter