Chromatography Forum

Hi, I did a quick search but could not find any other topics regarding this.

I've been doing headspace analysis (in Scan) on soils and waters for a suite of alcohols. Lately I've been getting positives in my blanks and samples for MeOH. If the concentration in the blank is lower than what I get in the samples, then I usually blank-correct. But the blank can be a lot higher, so I cannot be sure what I'm getting in the samples are false positives or not.

The lab I work in uses MeOH for extractions so this could just be background lab contamination. But the problem seems to be getting worse. Other than lab contamination, any other ideas to what may be causing this?

Thanks.

Methanol?

Yes, indeed.

I performed headspace analysis in a building that had a LC lab and found I would get acetonitrile and methanol positives if someone kept a door open too long by accident.

Also when big trucks drove by the building I could get benzene and toluene positives as well as hydrocarbons in my analysis.

And be really careful about wearing too much after shave lotion !

Rodney George
consultant

Same problem here, but with methanol, ethanol, IPA, toluene.

I've been asking for a laminar flow hood for residual solvent sample preparation for a while. I'm not sure what I would do about putting an analytical balance in the hood, however.

Maybe it's the warm weather that's been causing my methanol (and to a lesser extent dichloromethane and chloroform) problems.

A Laminar flow cabinet could be the solution but that involves spending money which the higher ups don't want to do.

A simple solution to see if it is methanol in the lab atmosphere is to fill a vial outside, then run it to see if you still have the methanol. If you are collecting samples in the field it is a good idea to prepare your blank there as well.

Right, as this problem needs to be solved I am doing some investigative work. Last night I ran a series of blanks, each vialled up at different places in the lab (lab, upstairs, outside, supermarket car park etc.).

I ran standards at 50, 20 and 10 mg/L, then the blanks. What I found is that the methanol content started at about 6ppm, then gradually decreased as the run went along.

This suggests that the problem lies with the instrument. I don't think it is 'backflash' from high injection volume as this method has been used many times in the past without this problem. We had a MS-NoVent installed late last year. Could dead space be associated with this - methanol entering dead space then gradually being released?

Currently I'm stuck for ideas.

Hi

Well the headspace sampler could be the bad guy. Could you please post brand, parameters and temperatures used including the GC part?

Due to either contamination and/or less suiteble parameter/temperature settings, carry over of polar analytes may occur.
The problem may not always be seen directly, it can build up over time or show up due to another "poor" method being used prior to the current analysis.

There are a few post in the archives about this.

I have a problem with butanol from another process in the lab. I am trying to measure BTEX using an online auto thermal desorber. The sample is dried with a nafion membrane before collection on the TD trap and the butanol was getting into the dry purge air and then into the sample stream via the membrane. So for purging the nafion I switched from dried compressor air to cylinder air and the problem disappeared. Annoying because the butanol co-elutes with benzene on the BP1 column.

Pete

What instrument are you using for the analysis? I'm not sure if that will help, but sometimes there are quirks with certain instruments that someone on the forum will know about.

Does you inlst have a septum purge, and if so what is the septum purge flow?

What instrument are you using for the analysis? I'm not sure if that will help, but sometimes there are quirks with certain instruments that someone on the forum will know about.

Does you inlst have a septum purge, and if so what is the septum purge flow?

It is an Agilient 6890 GC with 5973 MSD and 7694 headspace sampler. There is a septum purge outlet but I'm not sure what the flow is (if any).

EDIT: total flow is 35.9 ml/min, split flow 30 ml/min, column flow 3ml/min, split ratio 1:10

Well the headspace sampler could be the bad guy. Could you please post brand, parameters and temperatures used including the GC part?

Due to either contamination and/or less suiteble parameter/temperature settings, carry over of polar analytes may occur.
The problem may not always be seen directly, it can build up over time or show up due to another "poor" method being used prior to the current analysis.

There are a few post in the archives about this.

The headspace oven is set at 85°C. GC oven temps: The first method I use starts at 70°C for 5 minutes than ramps up to 145°C at 10°C/min. The second method starts at 40°C.

Any clues to what to search for in the archive?

JPC

I assume there are no high methanol-containing standards run first, but if so, I'd run some blanks and samples first, then the standards, then the blanks and samples again ( different vials of same samples ).

As methanol decreases during the run, you need to also ensure that the column and headspace system have been properly heat treated to drive off residuals before the start of the run - but I would expect other alcohols to appear as well, if the instrument was the problem.

I'd still focus on sample preparation, or inadvertent contamination by methanol in a wash solution/standard, rather than nooks and crevices of your instrument.

Please keep having fun,

Bruce Hamilton