Chromatography Forum

Who took my pentachlorophenol?

GCMS tuning solution which contains pentachlorophenol (PCP), DFTPP, Benzidine, and 4,4'-DDT @ ~ 50µg/mL has been used as a tune check for many years. Recently, pentachlorophenol has gone missing (just isn't there) in the check on two instruments, but it's there on a third. I find it odd that this compound would disappear from two instruments at the same time.

GC columns are < 1yr old/in service, but I will change one out today and re-check regardless.


GC parameters:
HP6890 with a 5973 Mass spec
2mm focus liner packed (by me) with glass wool.
60m .25x.25 DB-5 column
Injection Port: 300C
Initial Temperature: 40C
Initial Hold: 1.0 minute
Ramp Rate #1: 6C/min
Final Temperature: 300C
Final Hold: 20 minutes
Carrier gas: Helium
Transfer Line: 300C
Detector Temperature 230°C
Quad temperature 150°C

Any ideas? Has anyone seen this before?

I have seen this where the penta peak is so wide as to not be observable. A column change and a clean inlet fixed it. Normally seen after fish extracts or other nasty samples.

I would suspect the inlet rather than the column. When was the last time you changed the bottom seal and the split vent line?
In the past I've also cleaned the injector body. I have spares and swap them and clean the dirty ones with the alumina powder and Q tip, rinse with water.

Inlet is most common cause, sometimes also need to remove at least one loop of column, sometimes two.

Split line on 6890s often gets crud in it and need to be replaced, simply install a new piece of 1/8" copper line. Also pick up a 32 caliber bronze bore brush and rod used for gun cleaning, use it to scrub out the inlet when very dirty. I was actually told about that by an Agilent engineer almost 20 years ago. The bronze doesn't dig into the stainless steel and it allows you to clean it easily without removing the inlet weldment, just remove the bottom seal and run it up and down through the inlet a few times, then clean with swabs and solvent until they come out clean (can also use real pipe cleaners). The pipe cleaners also work to clean out the split vent port easily.

We have in the past used a brush or pipe cleaners mounted in a dremel tool with chrome polish to actually polish the inside of the port, best done with the port out of the instrument. Get a mirror polish on the inside and it will stay clean for a long time. Probably not good on the newer coated inlets on the 7890s but on the old 5890 and 6890 ports it worked well with trouble samples.

James_Ball wrote:
Split line on 6890s often gets crud in it and need to be replaced, simply install a new piece of 1/8" copper line.

Can you elaborate about the impact of this on analyte breakdown in the inlet? In my view, once the analytes are in the split line, they weren't going to the column anyway. But I might be wrong or missing something.

Problem solved ladies and gents! What was killing me was that I did full inlet maintenance each time I ran the standard. I clipped the column and changed everything... except the glass wool in the liner. I had been packing split precision liners with wool that had been labeled as deactivated (baked) when it was not. As soon as I used the pre-packed split precision liners from Agilent, PCP was a crisp and pretty peak. Stuck the glass wool in the oven at 400°C, packed a liner and PCP was there. Glass wool. Go figure.

Thanks so much for the ideas,

Stunt.

Rndirk wrote:

James_Ball wrote:
Split line on 6890s often gets crud in it and need to be replaced, simply install a new piece of 1/8" copper line.

Can you elaborate about the impact of this on analyte breakdown in the inlet? In my view, once the analytes are in the split line, they weren't going to the column anyway. But I might be wrong or missing something.

It happens mostly in splitless mode. If you think about gas phase, when the sample is volatilized and everything is a gas, the molecules move around quite a bit expanding throughout the liner. Some will make it as far as the split line then back to the column inlet in the short amount of time until the split vent opens, which can be as long as 1 minute. 250C+ molecules are moving quite fast and can travel a long way in a minute. I never believed it at first, but when changing the split line became the answer to breakdown problems, I had to admit it was possible.

Stunt wrote:
Problem solved ladies and gents! What was killing me was that I did full inlet maintenance each time I ran the standard. I clipped the column and changed everything... except the glass wool in the liner. I had been packing split precision liners with wool that had been labeled as deactivated (baked) when it was not. As soon as I used the pre-packed split precision liners from Agilent, PCP was a crisp and pretty peak. Stuck the glass wool in the oven at 400°C, packed a liner and PCP was there. Glass wool. Go figure.

Thanks so much for the ideas,

Stunt.

Glad you found the problem! Glass wool can cause many problems and even fresh glass wool can cause analyte loss if it is broken into very small pieces when inserting it. Each break can possibly create an active site to adsorb an analyte molecule.

James_Ball wrote:

Rndirk wrote:

James_Ball wrote:
Split line on 6890s often gets crud in it and need to be replaced, simply install a new piece of 1/8" copper line.

Can you elaborate about the impact of this on analyte breakdown in the inlet? In my view, once the analytes are in the split line, they weren't going to the column anyway. But I might be wrong or missing something.

It happens mostly in splitless mode. If you think about gas phase, when the sample is volatilized and everything is a gas, the molecules move around quite a bit expanding throughout the liner. Some will make it as far as the split line then back to the column inlet in the short amount of time until the split vent opens, which can be as long as 1 minute. 250C+ molecules are moving quite fast and can travel a long way in a minute. I never believed it at first, but when changing the split line became the answer to breakdown problems, I had to admit it was possible.

I have to say this just fixed quite a stubborn sensitivity loss in our PAH analysis of waste waters on a 7890 GC with an MMI inlet. The method uses large volume injection and a post column backflush - both of which contribute to getting the split line contaminated over time more quickly than 1µL injections and no backflush (I realize now). The difference in sensitivity after changing the split line (which was visually dirty, especially the part of the line the closest to the inlet) is a factor 5-10x for the heavier PAHs.

Thanks!

Rndirk wrote:

James_Ball wrote:

Rndirk wrote:

Can you elaborate about the impact of this on analyte breakdown in the inlet? In my view, once the analytes are in the split line, they weren't going to the column anyway. But I might be wrong or missing something.

It happens mostly in splitless mode. If you think about gas phase, when the sample is volatilized and everything is a gas, the molecules move around quite a bit expanding throughout the liner. Some will make it as far as the split line then back to the column inlet in the short amount of time until the split vent opens, which can be as long as 1 minute. 250C+ molecules are moving quite fast and can travel a long way in a minute. I never believed it at first, but when changing the split line became the answer to breakdown problems, I had to admit it was possible.

I have to say this just fixed quite a stubborn sensitivity loss in our PAH analysis of waste waters on a 7890 GC with an MMI inlet. The method uses large volume injection and a post column backflush - both of which contribute to getting the split line contaminated over time more quickly than 1µL injections and no backflush (I realize now). The difference in sensitivity after changing the split line (which was visually dirty, especially the part of the line the closest to the inlet) is a factor 5-10x for the heavier PAHs.

Thanks!

Glad it worked

I have started using the MMI, and just wondering how well it can be cleaned when stubborn crud gets in the bottom, since you can't just replace a gold seal in them.

We have been doing 25µL injections for 3 years now on our 7890GC-7000C. It's not running 24/7 but let's say an average of 15 injections/day (including calibration/QC).

If I do inlet cleaning, I always make sure to look at the bottom of the inlet with a flashlight. I never really see a difference before and after cleaning it with some solvent and the metal brush that came with the instrument, can't remember seeing black stuff there or anything. Still looks like a shining metal surface.

Nice, I have the same instrument configuration and am beginning to work on Pesticides in Hemp, probably one of the dirtiest matrix I have tried next to the turkey fat, which I will never do again.