Fatty acid profile analysis

[Ross C]

Hi all, I work in a lab performing Fatty acid profile analysis using a HP5890, can anyone help me with the problem breifly outlined below?

As part of laboratory quality control a standard containing known amounts of different fatty acids is analysed once a week. The standard is prepared in-house and consists of a mixture of different vegatable oil types. On analysing the standard all values are within specified tolerences apart from one which appears to be higher than what is expected. I cannot find out the reason why this is so. The standard has been made up fresh from a stock on several occasions and still the one fatty acid concentration is higher than usual. Initially I thought it may be due to contamination of the column, but running solvent blank gives rise to no peaks other than the solvent itself. I have replaced the septum and liner, checked the reagents / solvent wash etc but still cannnot find anything. Could it be possible for the detector response to have changed slightly for this one individual fatty acid? The instrument response factors were calculated over a year ago.

Thanks
Ross

[Don_Hilton]

Responses do change over time - but a sudden change of one analyte relative to others is not likely to be the detector. Let's start with the standard iteslf. You say the standard is made from a mixture of several different vegatable oil types. Have you changed to a new container of any of these vegatable oils? With other natrual products, you see changes in composition from crop to crop.

[chromatographer1]

Don is quite right.

Your oversight of mentioning WHICH fatty acid is a definite discouragement for constructive comments.

Unsaturated fatty acids are not stable and degrade with time, turning the sample yellow and eventually brown.

You should spend a few dollars and buy a certified std for calibration purposes, yes?

best wishes,

Rod

[Ross C]

Thanks for the reply's folks. Sorry i didn't provide more details. The fatty acid I have been having problems with is C16:0. and the standard material is over a year old (although stored in the fridge). I think the next steps I will be taking are purchasing some certified reference material to re-calibrate the response factors and using some commerciallly available 'ex-proficiencey test' material to use as a routine std.

Ross

[Don_Hilton]

And prepare to purchase standards at a regular interval to avoid woring with degraded materials. A year may be OK for a standard that has been opened, but it is worth checking periodically against fresh material to be sure of the lifetime of an opened standard.

[Consumer Products Guy]

See Nu-Chek Prep for fatty acid and blend standards. Are you using capillary with split injection, wide-bore capillary without split, or packed column? Are you using cyanopropyl silicone type column like SP-2330.EGS, or PEG? Are you injecting methyl esters "as is" or diluted in solvent? I'm assuming you are maing methyl esters, you apparently didn't detail. Type of GC (like 5890) doesn't matter. Are you using FID or TCD? To get good help, you need to provide details.