Chromatography Forum

hi everyone,
i'm working with two Shimadzu GC-14A gas chromatographs, the injections are made manually through a septum. A week ago i couldn't see any peak in both chromatographs, so i decided to clean the inlet of the injector. It was FULL of septa pieces, i removed them and although the flow of the carrier gas measured it was the same with the one before the cleaning procedure, the retention times of the probes injected were much much lower.
That applies in both chromatographs.
I wonder if the septa pieces served as a filter without knowing it and if they did why i mesured the same flow before and after the cleaning?
The other unlike, for me, possibility is that both of my packed ss columns with polymers are destroyed (aged maybe) by some reason.
can the septa pieces affect the retention time?????
can anyone help?

additional information:
flow rate 20 ml/min
injector temperature 150 celcius
working months 7
column temperature 30-50 celcius


Thanks

Stella

What type (phase) of column You are using, and what samples are analyzed??
Maybe analytes are adsorbed by septa material....

What type (phase) of column You are using, and what samples are analyzed??
Maybe analytes are adsorbed by septa material....

i'm performing igc experiments, so i pack a stainless steel column with my a polymer coated on chromosorb. The substances i inject are solvents with known properties(alkanes, ketones, alcohols etc etc)

Hi Stella

The inlet full of septum chips acts as an extra packed column, and the silicones that the septa are made of are very retentive. So the retention times having got shorter while the carrier gas flow rate stayed the same is perfectly normal. You need to use syringe needles with coned or domed ends to stop the needle cutting bits out of the septum. It wouild be a good idea to do you inlet maintenance on a cycle of days rather than months.

Peter

Stella - I think you just learned first-hand why cleanliness and routine "maintenance" improves chromatography. It is not uncommon over time to have septum pieces build up in an inlet or in an inlet liner.

Hi Stella

The inlet full of septum chips acts as an extra packed column, and the silicones that the septa are made of are very retentive. So the retention times having got shorter while the carrier gas flow rate stayed the same is perfectly normal. You need to use syringe needles with coned or domed ends to stop the needle cutting bits out of the septum. It wouild be a good idea to do you inlet maintenance on a cycle of days rather than months.

Peter

dear Peter,
i' m not sure if the septa chips are retentive, because if they are i lose an unknown quantity of the probe injected in every single injection i make, irrespectively if the inlet is full or empty. The thing is that a got good reproducible results for many months and suddenly they are not reproducible and if i had to make a plot of RTlnVn vs the number of carbon atoms of the injected alkanes the regression coefficient wouldn;t be more than 0.91 while my previous results gave 0.9998. That happened for months, so now i'm really sceptic about the new results.

Thanks

I don't know your toy, so the following may be unhelpful.

Firstly, I'm a strong advocate of "if it ain't broken, don't fix it". In my experience, injectors can last many, many months, or just a few days, before maintenance is required. The duration between servicing depends on sample cleanliness, septum robustness, analyte sensitivity to system activity, column sensitivity to junk, and many other factors.

The preventative maintenance programme should be based on experience. Given that you had a relatively long hassle-free period, I suspect you have inadvertently affected something, most probably introducing either activity or a injector leak during service. Once it's fixed, wait for the next failure before setting preventative maintenence times.

The septa coring is an issue, and I'd switch to coned needles if you aren't already using them. I would also look closely at the septa you are using, and ensure they are the most suitable, with regard to injector temperature, needle type, and resistance to coring. I prefer either multi layer septa or really good quality, low bleed, core resitant, single layer types. Cheap septa will tear and core easily.

I would first ensure that the system isn't leaking around the septa or column connection at injector operating temeperature. If you don't have a leak detector, Snoop ( detergent and water ) can be used - just ensure the system is pressurised, and rinse the Snoop off with distilled water. Then I would condition the system ( column and injector ) at near recommnded isothermal maximum temperature, at least for several hours. I would then inject some blanks, and once the baseline was settled, I'd inject, six to ten times, a test mixture of alkanes, alcohols, and/or your samples, followed by a couple of blanks. I know some many injections manually are a bore, but you need to ascertain the cause of your problems.

I would look at the data for trends, any random RT variation is most likely caused by injector/syringe leaks. A trend of increasing peak sharpness and symmetry with injections is likely to caused by activity, a trend of all peaks increasing area with injection is a sign that there is residual activity, a trend of the higher boiling compounds % area increasing is a sign of discrimination or activity, and you should either keep injecting the mixture, or allow the system to condition for much longer.

Please keep having fun,

Bruce Hamilton

I don't know your toy, so the following may be unhelpful.

Firstly, I'm a strong advocate of "if it ain't broken, don't fix it". In my experience, injectors can last many, many months, or just a few days, before maintenance is required. The duration between servicing depends on sample cleanliness, septum robustness, analyte sensitivity to system activity, column sensitivity to junk, and many other factors.

The preventative maintenance programme should be based on experience. Given that you had a relatively long hassle-free period, I suspect you have inadvertently affected something, most probably introducing either activity or a injector leak during service. Once it's fixed, wait for the next failure before setting preventative maintenence times.

The septa coring is an issue, and I'd switch to coned needles if you aren't already using them. I would also look closely at the septa you are using, and ensure they are the most suitable, with regard to injector temperature, needle type, and resistance to coring. I prefer either multi layer septa or really good quality, low bleed, core resitant, single layer types. Cheap septa will tear and core easily.

I would first ensure that the system isn't leaking around the septa or column connection at injector operating temeperature. If you don't have a leak detector, Snoop ( detergent and water ) can be used - just ensure the system is pressurised, and rinse the Snoop off with distilled water. Then I would condition the system ( column and injector ) at near recommnded isothermal maximum temperature, at least for several hours. I would then inject some blanks, and once the baseline was settled, I'd inject, six to ten times, a test mixture of alkanes, alcohols, and/or your samples, followed by a couple of blanks. I know some many injections manually are a bore, but you need to ascertain the cause of your problems.

I would look at the data for trends, any random RT variation is most likely caused by injector/syringe leaks. A trend of increasing peak sharpness and symmetry with injections is likely to caused by activity, a trend of all peaks increasing area with injection is a sign that there is residual activity, a trend of the higher boiling compounds % area increasing is a sign of discrimination or activity, and you should either keep injecting the mixture, or allow the system to condition for much longer.

Please keep having fun,

Bruce Hamilton

well...
of course the septa i use are not cheap at all and their maximum working inlet temperature is 300 C, of course i'm familiar with detergent and water( i'm a woman, i wash dishes) and i check all the possible connections prone to leakage before the experiment starts, of course i condition my column overnight (either or else working is conditioning), of course i check if my baseline is settled, of course i use syringes with coned needles, of course....
thanks anyway

Dear Stella,

There is no reason to take offense. The responses are adequate to the information You provided.

If You would describe in detail in first post - i use this in this way, have tried this and this an this, did all the following experiments - people would not suggest possibilities which come to mind first naturally. They simply don't know if You ruled out these possibilities or not. Some beginners post questions which have solutions as simple as mentioned above....

Speaking about information. Do You inject air samples or liquid samples, how much?

You are speaking about "loosing" part of analytes. Did change of septa solve that, did removal of septa particles solve that? There is possibility, that some part of analyte is "blown back" when withdrawing needle if septa is worn and not sealing well - that will not be detected by flow measurement as it is "momentarily" event.