HELP!!! SPME

[rb6banjo]

The more you dilute the sample, the less you will be able to see acetaldehyde. We use static headspace for AA and DMS in beer. If you want it using SPME you might try the carboxen/pdms (black) fiber. I find it to be a little better for really light materials.

[xeim]

The more you dilute the sample, the less you will be able to see acetaldehyde. We use static headspace for AA and DMS in beer. If you want it using SPME you might try the carboxen/pdms (black) fiber. I find it to be a little better for really light materials.

Is it possible to not dilute the beer samples at all? I don't have car/pdms.

[rb6banjo]

I never dilute the beer if it's somewhere close to 5% (v/v) EtOH.

[MSCHemist]

Keep in mind acetaldehyde is a small 2 carbon molecule. It won't bind to the fiber as tightly as larger compounds and the ethanol is going to crowd out a lot of analytes. Also it's small size gives it a low repsonse to begin with so your LOD is going to be pretty high for acetaldehyde.

[xeim]

I never dilute the beer if it's somewhere close to 5% (v/v) EtOH.

Thanks for the post. It's better to adjust the EtOH concentrations in the samples until 5% v/v? I'm afraid that excessive dilution won't allow FID detecting the aroma compounds.

[xeim]

Keep in mind acetaldehyde is a small 2 carbon molecule. It won't bind to the fiber as tightly as larger compounds and the ethanol is going to crowd out a lot of analytes. Also it's small size gives it a low repsonse to begin with so your LOD is going to be pretty high for acetaldehyde.

Thank you. What is the approximate LOD for AA using the SPME?

[MSCHemist]

We still did pretty good though we were a bit overgenerous with the LOQ. Our level 1 was 20 ppb.

[xeim]

We still did pretty good though we were a bit overgenerous with the LOQ. Our level 1 was 20 ppb.

You did that with SPME and FID?

[MSCHemist]

No MSD-scan Though we probably could have done better on some compounds using the SIM/SCAN and using the SIM mode on the more trace compounds.

[xeim]

Could you tell me please how many times i need to dilute the sample consisting 40%v/v EtOH in order to avoid fiber swelling?

[MSCHemist]

Could you tell me please how many times i need to dilute the sample consisting 40%v/v EtOH in order to avoid fiber swelling?

You're going to have to experimentally determine that. run a dilution series until you get the optimal combination of less SPME swelling and detectablilty of your analytes.

[BZK]

Hi Xeim, measuring of low boiling aroma compounds with only FID is a problematic task.
I do this with MSD because into the range of acetaldehyde elution could be present methanol or some light hydrocarbons. First of all you need, as recommended from other colleague, to decrease the inlet diameter to the minimun possible. In my case I using a self made on-column arrangement and inject the fiber directly into 0.53 mm id precolumn. In this way without splitting or cryotrapping it's possible to see also permanent gases such as oxigen, argon or nitrogen. With DB-5ms, 60m, the elution order is permanent gases togheter followed from propane, isobutane, butane, SO2, acetaldehyde, methanol and so on. But without MSD and with optimized injection system you need anyway a very polar and long column to discriminate from non polar compounds very often present on acetaldehyde range and also to increase separation between acetadehyde and methanol.
I hope to be helpful to you.

Robertino