Origins of headpace blank contamination

[fstabile]

Hi everybody,

We are trying to create a HeadSpace/GC method to quantify Ethanol and Acetone.
We have good parameters and result, but blanks have those peaks appearing also.
These peaks appear with no reproducable results (appears from area of 1000 to 10000)

We are using the highest purity DMSO available.
Empty vials don't have the acetone peak, but Ethanol still appears smaller.

Is there a way to determine if the headspace, GC, transfert lines are contaminated?

If it can help, the parameters are
HS: Oven 80°; Loop 95°; Transfert line 120°
GC: Oven 40°; Inlet 140°; Detector 250°(FID)
Split Ratio at 1:1

Thank you in advance

[Ron]

Do you have these solvents in the lab where the blanks are prepared? DMSO is very good at absorbing organic liquids from the air, and these will appear in the blank. To verify this, find a room without these solvents stored or used there, open a fresh bottle of DMSO, and fill a few vials to the usual level and see if you still have the peaks in the blank. At times I have prepared my blanks outside just to make sure I wasn't getting solvents from the laboratory atmosphere.

[fstabile]

We have those solvents in the lab, but they were not in use (no bottle opened or present close) at the time of the preparation.
We use a 'dedicated' fume hood. It is used for something else, but not at the same time than this analysis.

The main thing is that empty vials have the ethanol peak.

As an update: Since we are using an Agilent HS, we found on the website that we can 'Steam Clean' the lines. We'll try this also today.

[Peter Apps]

A fume hood may actually be worse for contamination than an open bench, because it sucks air in from the lab (and the labs next door etc) and passes it over your vials, solvents and samples.

Peter

[Schmitty]

Maybe it's a laminar flow hood so as not to draw in lab air?

[krickos]

The main thing is that empty vials have the ethanol peak.

As an update: Since we are using an Agilent HS, we found on the website that we can 'Steam Clean' the lines. We'll try this also today.

Well the Agilent config with your temp settings in loop/valve, t-line and potetially injektor can very much be the reason you have blank issues with ethanol. This phenomena I/we have seen plenty of times.

Bp of DMSO is what 189°C? with your settings you will over time contaminate sample loop/valve with DMSO residues, polar solvents like ethanol will attach to it, while less polar solvents/low boilers such as acetone and toluene is less prone to attach to contaminants.

Steam cleaning will likely help to clean out loop/valve in worst case loop has to be manually cleaned.

So a basic pointers with regard to Agilent headspace instruments:

ALWAYS, set temperatures in valve/loop, transfer line, injector above the boiling point of the higest boilier which includes the sample solvent. By doing this you will have considerble less downtime in long run.

ohh a second comment: Unless you really need it, avoid that low split ratio of 1:1 míght cause you even more troubles, try to stay at 1:5 or higher.
If you need trace analysis and really want to do like splitless or split ratio 1:1 you need to go for a high id column like 0,53mm id with higher flow rates in order to get decent peak shapes on to column. You can search Agilent homepage for headspace and splitless for more information.

[Bigbear]

I'm missing something what is your sample matrix DMSO?
In order to find where the contamination is comming from you need to "half split" your system. To test the gc use a gas tight syringe and directly ingecr a blank at the gc injection port.

[krickos]

I'm missing something what is your sample matrix DMSO?
In order to find where the contamination is comming from you need to "half split" your system. To test the gc use a gas tight syringe and directly ingecr a blank at the gc injection port.

Hi

Bigbears suggestion is good if the steam cleaning wont help you.

A related question? Anyone got an simple idea how to "half split" the system if you have a volatile inlet instead of a split/splitless inlet?