Hi All,

I am relatively new to GC and have encountered a problem with falling baselines.

When I do a blank injection (no solvent) the baseline rises w/ the temp ramp until about 1/2 way thru and then drops continually even below the starting baseline point. Before this I observed the "normal" baseline rise throughout the temp rampup.

Equipment/Conditions:
PE Sigma 300, Supelco packed column, flow is 20 mL/min, temp program is 150, hold 4 min, then ramp to 260 at 4 C/min, hold 4. Injector temp is 270, detector (ECD) is 350.

If I monitor the baseline at 150 it is OK, also at 250. Can any one offer suggestions? TIA

Russell

Russell,

I will assume that by "blank" injection you mean no injection, just pushing the start button. The baseline is OK isothermally because this instrument will autozero the baseline at injection (unless A/Z is turned off) thereby removing any normal baseline offset.

What were you doing differently "before this"? Do you mean that when you inject a sample you don't see the baseline deviation? Also, what is the stationary phase in your column and at what coating percent?

Have you checked the column inlet and outlet fittings for leaks? What is the makeup gas flow, and are you using N2 or He carrier gas? Are the gases properly filtered and of sufficient purity? How does the injection port liner or the beginning of the column look -- contaminated or clean?

Sorry for questions answering your question, but the additional information will help a lot to find the problem.

John

John,

I believe I found the problem. I switched what I believe to be the anode for the ECD. I let the system go at initial temp overnite and did another "blank" run this morning. The chromatograph looks alot more acceptable! I will clean the dirty anode w/ hexane and bake it at 250 for a couple of hours. Just to reiterate the previous situation. The baseline was falling sharply (3-4 mV) w/ temp increase.

Carrier gas is Ar/CH4 and the makeup flow is 40 mL/min. I ck'd the flow rate thru-out the temp ramp-up and it is constant at 60 mL/min. The column is a Supelco, w/ 1.5% SP-2250. The assay is run to determine PCB level in our product. One thing I've noticed about the injection port liner is that it accumulates bits of septa. Otherwise it is clean. Thank you for your response. I've enjoyed your input to other folks questions.

Russell

Russell

Regarding accumulating bits of septa, this is usually a result of tightening the injection port cap too much. You just want to tighten it enough to seal. No need to "force" it.

Jim

Thanks for that suggestion. You are absolutely correct, I do "torque" the inj. port cap down pretty good. Just loosened it. Again thanks. Any suggestions on how to thoroughly clean an ECD? Already soaked the anode in hexane and baked at 250 for a couple of hours. Am getting small negative peaks throughout baseline after sample/contaminant peaks. TIA

Russell

Russell,

Be Careful! When removing the anode of an ECD, you are exposing yourself to the ⁶³Ni foil contained in the detector. This is a b emitter that is regulated by the NRC. Unless you have a license for your lab to handle this material outside of the detector, you shouldn't open it up. Part of your routine for the ECD should include performing a "wipe test" to sample the surfaces around the detector for radioactive material.

So, how to clean it? First, you should run the ECD at an elevated temperature of 300 °C or higher at all times in order to keep the detector clean. When contaminated, disconnect the column and cap off the column connection. Run makeup gas at the normal flow rate and heat the ECD to its maximum rated temperature (usually 350 or 400) for several hours while monitoring the baseline. When the baseline stabilizes the detector has finished baking out.

Be sure that the carrier and makeup gases are of high purity and are filtered for oxygen as well as hydrocarbons. The ECD is quite sensitive to O₂ contamination of even 5 ppm, which will result in noise and elevated baselines.

If these don't do the trick, and you can't disassemble the detector for the reasons above, you should send it back to the manufacturer for more extensive cleaning/reconditioning.

The negative peaks you see are probably due to hydrocarbons in the inlet being separated and eluted from the column. The ECD will respond in this way to large hydrocarbon peaks. Remove and clean the inlet liner and repack with new glass wool if appropriate. Handle the liner with gloves on. Also, use an appropriate inlet seal. Some of the cheaper ones will emit plasticizers or contaminants that will appear as negative peaks.

After reassembling the inlet, don't connect the column yet. Set the carrier pressure to 2-4 psig, the split flow to 50, and bake the inlet at 350 for a few hours. Cool to room temperature and then connect the column.

Also, if so equipped, the inlet split-vent filter may need to be repacked. This filter traps material after the split point to prevent a buildup in the downstream pneumatics. It can become saturated and can release material backwards into the splitter when performing splitless injection.

Hope this helps,


John