Dear Yarong,
As Uli has pointed out already, cholesterol is susceptible to autoxidation. In your case matters might be helped along by the fact that cholesterol has been dissolved in DCM. DCM has a C-H acidic properties, not as strong as chloroform but seemingly strong enough to mount an electrophilic attack on the double bond at C-5 since the shift in d13C-values you have observed seems to indicate a process involving a C-C bond.
One suggestion I would have is to dissolve some of your solid cholesterol (assuming you have some left and it has been stored safely, e.g. in an evacuated desiccator or under Argon) in n-hexane (degassed with Argon) and monitor its d13C-value over time. Under these conditions I would expect its d13C to remain stable.
Best,
Wolfram
****************************
Dr W Meier-Augenstein, CChem, FRSC
Extension: 2624
E-mail: [log in to unmask]
URL: http://www.scri.ac.uk/staff/wolframmeieraugenstein
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Yarong Li
Sent: 26 August 2009 00:22
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Variation of d13C-value for cholesterol
Hello all,
Thanks for Dr Wolfram's correction.
I try to use pure free cholesterol (not a derivative) as a QC material for compound specific isotope analysis. The cholesterol has been certificated by EA-IRMS analysis and used for calibration verification in GC-IRMS analysis.
The cholesterol was original dissolved in DCM and stored in glass vials at 4ºC.
Regards,
Yarong
Email: [log in to unmask]
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Wolfram Meier-Augenstein
Sent: Tuesday, 25 August 2009 4:23 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Variation of d13C-value for cholesterol
Dear Yarong Li,
Your enquiry begs a number of questions on points not mentioned in your posting to the list but required to think of possible explanations.
What solvent is your cholesterol QC material dissolved in?
Is this a solution of free cholesterol or a derivative, e.g. O-acetyl or O-TMS?
Are you using your cholesterol as QC material for compound specific isotope analysis?
Best,
Wolfram
****************************************
"Facts do not cease to exist because they are ignored."
Aldous Huxley, "Proper Studies", 1927
****************************************
Dr W Meier-Augenstein, CChem, FRSC
Principal Scientist - Stable Isotopes
SCRI
Invergowrie
Dundee, DD2 5DA
Tel: +44 (0)1382 560025
Fax: +44 (0)1382 568544
E-mail: [log in to unmask]
URL (SCRI): http://www.scri.ac.uk/staff/wolframmeieraugenstein
Senior Lecturer (Associate Professor) - Stable Isotope Forensics Centre for Anatomy & Human Identification University of Dundee http://www.lifesci.dundee.ac.uk/CAHId/
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-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Yarong Li
Sent: 25 August 2009 01:10
To: [log in to unmask]
Subject: [ISOGEOCHEM] Variation of d13/d12 ratio for cholesterol
Hello all,
I am still a learner to stable isotope ratio and try to use cholesterol
as a QC sample. For the same solution, I found its d13/d12 ratio varied
along the storage time (from -25.5 to - 30.5 in 5 months).
Does anyone have an explanation for this? How to address this issue if
because the sample preparation or storage?
Thanks,
Yarong Li
Environmental Forensic and Analytical Science Section Department of
Environment and Climate Change (DECC)
Email: [log in to unmask]
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