GC/FID TPH (C10-C40) Baseline Shift Problem

[qazedc]

Hi,

i hope that writing right section.
i have a situation about analyses of TPH (Total Petroleum Hydrocarbons) using standard of "Mineral oil standard mixture Type A and B for EN ISO 9377-2" with GC-FID.

The conditions are:
column, DA-5ms, 60m x0,25mm x 0,25um Oven Temp: 40C hold 2min. and 15C/min. to 310C hold 15min.
injection 1ul and split ratio 1/2 Temp: 250C
Detector Temp: 300C
Carrier Gas: Helium

The view of the chromatogram should be below like a hill but,




The problem is baseline do not down after reach high temperature



Thank you in advance.

[tkubowicz]

Hello

2 questions:

1. Is it the same method for second chromatogram? As you can see solvent peak is after about 5 minutes. It comes out much faster on first one.
Are you sure there is nothing wrong with method (column flow or mode - const pressure or const flow)

2. What is your blank profile? no injection run or solvent injection?

Regards

Tomasz Kubowicz

[Peter Apps]

Welcome to the forum

Your detector temperature needs to be higher than the maximum of the column temperature programme - so increase it to 320C

You probably have either column bleed or contaminants coming from the inlet. When did you last do inlet maintenance, and do you have an oxygen and moisture scrubber on your carrier gas line ?

Peter

[qazedc]

First of all, Thx for the replies.

This section for Tkubowicz
Answer of the 2 questions (by tkubowicz)
1. No they are not same method. First regular chromatogram is taken from agilent app note (screenshot). I added it cos it is the typical chromatogram of the total petroleum hydrocarbons.
Flow rate is 1.3ml/min. And const flow. (also tried before const. pressure)
2. Blank is hexane with N-decane (to see C-10 start point)
The chromatogram added as a problem (with red arrow) has 100ppm concentration.

column has a certificate that let the temp to 325C.
_________________________________________________


This section for Peter
Thx Peter,
Yes right about detector temp. (my mistake sorry)
Agilent GC Service just controlled inlet and it was clean. Also column is almost new (installed 2 months before).
Yes i have oxygen/moisture trap installed for He (carrier) but dont know how often should it replace.

[Peter Apps]

What inlet septa are you using, what is the septum purge flow rate ?

What happens if you increase the split ratio to 20:1 instead of 2:1 ?

Peter

[qazedc]

Firstly there was an article written by Davide Balbo but it was erased. Do not have idea why he did İt .

_________________________

To Peter
Septa is (5183-4757): http://www.agilent.com/store/en_US/Prod ... /5183-4757
Liner is (5183-4647): http://www.agilent.com/store/productDet ... =5183-4647#
Split flow: 2.6ml/min.

I tried 1/20 and 1/25 split ratio but nothing was change.
Thx much.

[Hornet]

Firstly there was an article written by Davide Balbo but it was erased. Do not have idea why he did İt .

Hi, i deleted it because i missed to read the second chromatogram is a 100 ppm standard, so everything i wrote wasn't answering your question. Plus i was in hurry yesterday and couldn't repost.

I try to tell you how i run this method and what i would change to your instrument conditions to give a better signal, given that your GC is free of leaks and other issues.

I run this method using splitless injection:
splitless time for 1 min then open the valve
liner is single gooseneck with wool
injection volume 2 ul
T injector = 280°C

the main change i would do is the column. The trick is to raise the hydrocarbon hump by shortening the gap between the first eluted peak of interest and the last, for example C10-C40 (unless you NEED to separe them to quantitate single peaks).
Low thermal mass application for TPH from Agilent is a good example of what i'm talking about; it's a different technique but the trick is the same.
My column is 15m and i can go as low as 10 mg/L with the lowest calibration point.
A 60m column will "dilute" your hump with time, it will make hydrocarbon profile hard to integrate or barely visible like your chromatogram.

The baseline rise will always be there, just make sure to acquire a baseline profile everyday and to make your software subtract it from the following chromatogram.
A good baseline is clean from peaks, and when it gets to the latest pleateau it should remain flat, usually it take 1 to 2 blank solvent injection to obtain it.
If everything has gone well, you'll inject your sample and when it's time to integrate you'll just have a flat chromatogram with just the peaks (for diesel) or hump (for oil) ready to be integrated.

In order to optimize splitless injection, since you are using hexane which is good too (i prefer heptane), make your first overn temp ~10°C less than the solvent boiling point to perfectly focus the solvent on the first part of the column. This way your solvent peak will be narrow and won't interfere with C10 integration (which could happen with a short column).

I would try the injector setup i wrote before and this oven program:

60°C for 1 min
20°C/min to 325°C (or whatever is your max allowed temp)

FID has to be at the same temperature as the highest of the overn.

Good work and hope it helps.

Davide

[qazedc]

Hi, Davide

Firstly, Thx to you that your writing before and now.

I can not cut the column or change it. Have to do other analyses on the GC. I have tried the things that you have written before. I understood that baseline will go flat after reach high temperature (by the way i will change the oxy/moisture trap. The last time changed, seems 6 years a go).

I have tried on the method which use subtract the blank for quantative results.
But the problem is now: the baseline is not stable for subtract the blank as it is shown below in the picture.
The integration range between 10 min. and 30 min.
All the 4 injections are same blank (hexane with N-decane)




i have conditioned column at 250C for over an hour then started sequence. but blank baselines have different altitude.

If i fix the baseline altitude problem, the calibration correlation can be acceptable.

Thank you for your interest.

[Hornet]

You have some problem in the instrument, maybe the FID isn't perfectly conditioned, what's the make-up flow?
What's the FID temperature during those runs? If you raised it recently it needs to equilibrate.

Then of course change the oxygen/moisture trap and retry.

[tkubowicz]

Hello

I would check FID. If you have this big "bump" when oven temperature is going up perhaps there is huge leak somewhere in FID...
Check jet and seal that is underneath FID block (white seal).


Regards

Tomasz Kubowicz

[Peter Apps]

A six year old trap is definitely past its useful lifetime, so you need to change that before you heat the column again. Let's hope that exposing the column to bleed from the exhausted trap has not damaged the column permanently.

With such a long column the hydrocarbon hump will be extremely broad, chanig columns with an FID is quick and easy, I recommend that you use the short columns that David uses.

Peter

[qazedc]

Hi,
to Davide, Tkubowicz and Peter

FID jet replaced a week a go.
Flow rates are: H2 40ml/min. - Air 450ml/min. - He (makeup) 30ml/min.
FID temp was not changed and conditioned more than a hour.
There are double inlet, column and detector (FID/ECD) installed in GC. And FID is the back detector.

It is not easy to reach and change the column. If it was like ECD as front section, it might be more easy.

Davide could you offer a column which useful for TPH with FID and could you tell your LOD and LOQ.
If there might a new GC in the company, i may offer it.

I also think about to try see 2ug/ml for TPH.
if i inject 5ul and split ratio 1:2 concentration between (2ug-20ug-50ug-100ug-200ug-500ug/ml) to see 2ug/ml. Column is HP-5ms 60m-0,25mm-0,25um. but afraid of to harm it. Is it too harm and risky for the column and FID?

Thx to all of you for your support.

[tkubowicz]

Hello

It is not easy to reach and change the column. If it was like ECD as front section, it might be more easy.

If you don't want to remove column and check inlet/detector or if column is installed properly, how do you want to find a problem?

If you'd ask in car service to replace brakes pads because you think there is something wrong, would you be happy to hear: sorry but there is no easy access to it so we'd rather not touch it

Regards

Tomasz Kubowicz

[qazedc]

Hi Tkubowicz

I did not want mean that i dont want to change the column, inlet or etc. Already did many times. Seems misunderstanding.
I just tried to mean that if ECD was at back section instead of FID, would be easy for me. FID/ECD in one GC. I hope can explain myself properly this time .

Thanks anyway.

[Hornet]

Hi,
to Davide, Tkubowicz and Peter

FID jet replaced a week a go.
Flow rates are: H2 40ml/min. - Air 450ml/min. - He (makeup) 30ml/min.
FID temp was not changed and conditioned more than a hour.


Davide could you offer a column which useful for TPH with FID and could you tell your LOD and LOQ.
If there might a new GC in the company, i may offer it.

I also think about to try see 2ug/ml for TPH.
if i inject 5ul and split ratio 1:2 concentration between (2ug-20ug-50ug-100ug-200ug-500ug/ml) to see 2ug/ml. Column is HP-5ms 60m-0,25mm-0,25um. but afraid of to harm it. Is it too harm and risky for the column and FID?

Thx to all of you for your support.

FID flow rated are ok, the problem is elsewhere, a leak for example.

I'm currently using a Phenomenex ZB5HT 15m x 0.25 x 0.25, which works up to 400°C, but i guess any 15m HT column will be ok. For C40 i only need 340°C as last temperature, which i keep for 8 minutes to be sure everything cleans out.
At these temperature make sure to use proper ferrules, standard vespel ferrule aren't the best choice. In the past i ran graphite ferrule which worked fine but needed to be tighten everyday. I'm now using Restek encapsulated graphite ferrules, they don't unscrew as the temperature lowers and they are good at high temperatures. Siltite ferrules work fine too but don't use them if you plan on swapping columns often.

About the LOQ, for every method i alway use the lowest calibration point as my LOQ, i don't even care about calculating LOD.
My LOQ is 10 mg/L for C10-C40 mineral oil A and B mixture (diesel + motor oil) with splitless injection and i really pushed the method a lot to reach this level.

There is noway you are getting a 2 mg/L calibration point with a split injection, even with a 15m column.
You could try pulsed splitless if your instrument has this option (mine doesn't have it, it's an old varian 3800 without even EFC), 15m AND 5 ul volume, but be ready to clean your inlet very often because 5ul is a lot of sample and it is going to take a lot of dirt.
PTV is the best option for a low LOQ.

Davide