Chromatography Forum

Dear all,

can somebody recommend how to solve this problem, I am using headpsace analysis to analyze samples in blood and I am using tert butyl alcohol.
I have run standards in sequence and area of ISTD is changing to double.
Also, I am using ZB 624 0.53, 0.32, 3.
Any recomendation?

thank you

A bit more detail about how your are running the analysis would help - equipment type, conditions, times, tempertures, etc.

Your sample size of the preparation, spiking methods, heating time, temperature, split or direct injection?

0.53mm ID

0.32mm ID

3µm film?

Is this correct? Two separate columns? joined together? or What, a precolumn uncoated?

Rodney George

How do you prepare the standards?. Is the same matrix?

A bit more detail about how your are running the analysis would help - equipment type, conditions, times, tempertures, etc.

Shimadzu GC-2010Plus with HT 200H, column ZB-624, 30m long,film thickness 3.00um, ID 0.53.
Inlet T: 150C
Detector: FID 250
Column: 45C isothermal 4 min
HSS conditions

1mL sample inject
Syringe T 70C
15 min heating and shaking
Oven T 65C
IST tert butyl alcohol.
Standard are made of solvent 99.98% etyl, methyl and ISTD.

I have good separation and good reproducibility.
Also I have changes the septa and I am using liner with wool is it OK?

thank you

The glass wool in the inlet has no purpose since your sample is already in the gas phase. It's only effect will be to cause peak tailing, so it is a good idea to remove it.

Next, check the tightness of your headspace vial caps.

Is this doubling effect consistent ?

Peter

The glass wool in the inlet has no purpose since your sample is already in the gas phase. It's only effect will be to cause peak tailing, so it is a good idea to remove it.

Next, check the tightness of your headspace vial caps.

Is this doubling effect consistent ?

Peter

I am using crimp cap vials.
Is it possible that tert butyl alchocol is not good ISTD? In all publications that I have read it is said that n-propanol is suitable for this kind of analysis and I am using tert bu?
Also, I am using 100uL ISTD in 20mL vials

Nearly all ethanol GC procedures use n-propanol as internal standard.

Of course USP <611> utilizes ACN, go figure.

I've not heard of t-butyl alcohol being used for ethanol.

The glass wool in the inlet has no purpose since your sample is already in the gas phase. It's only effect will be to cause peak tailing, so it is a good idea to remove it.

Next, check the tightness of your headspace vial caps.

Is this doubling effect consistent ?

Peter

I removed glass wool and yes my vials are crimped

You need to check that the vial caps are crimped consistently and leak tight. Crimp some vials using your usual technique. Then immerse them completely in hot water - if you see bubbles they are leaking.

You have still not told me whether the doubling effect is consistent.

Peter

Hi

An possible additional issue:
Syringe T 70C°
boiling point of tert butyl alcohol = 82°C.

So the ISTD can interact (can cause carry over) with syringe as it is colder than boiling point, I suggest you increase temperature to like 100°C.

You need to check that the vial caps are crimped consistently and leak tight. Crimp some vials using your usual technique. Then immerse them completely in hot water - if you see bubbles they are leaking.

You have still not told me whether the doubling effect is consistent.

Peter

no, the doubling effect is not consistent.

You need to check that the vial caps are crimped consistently and leak tight. Crimp some vials using your usual technique. Then immerse them completely in hot water - if you see bubbles they are leaking.

You have still not told me whether the doubling effect is consistent.

Peter

no, the doubling effect is not consistent.

I will try with water to see if vials are sealed properly.
Another thing, I have changed ISTD to n-pro and the still I have variation in area of ISTD it varies again not double but enough to have bad calibration. I have still 65 degree in oven, 70 syringe, removed glass wool, split 5, linear velocity 80 cm/s, column T 40 for 4 min, FID 325 and SPL 250.

I use n-propanol as the IS and T butanol as a surrogate with no problems.

Do you have a headspace sampler, or are you hand injecting? use a split One ml seems like a lot of sample. I use a 0.45 id 624 column and inject 100ul through a packed injector.
How do you introduce your internal standard? Blood is a tough matrix.
I make up an internal standard/surrogate solution in water ( containing 2% NaCl) , and add 1 ml of this to 1/2 ml of blood.

I use n-propanol as the IS and T butanol as a surrogate with no problems.

Do you have a headspace sampler, or are you hand injecting? use a split One ml seems like a lot of sample. I use a 0.45 id 624 column and inject 100ul through a packed injector.
How do you introduce your internal standard? Blood is a tough matrix.
I make up an internal standard/surrogate solution in water ( containing 2% NaCl) , and add 1 ml of this to 1/2 ml of blood.

I have sampler. i introduce ISTD with automatic pippete and yes I am adding the NaCl but solid.
You think that it is to much 1ml of sample?