Dear Klaus,


Sugars (and honeys) are notoriously difficult to analyse for 2H (and 18O)
and I suspect every lab has its own SOP for this task, which in turn will
depend on what answer is required; an absolute true value or a relative yet
stable value, perfectly suitable to detect difference in a reliable fashion
provided everything has been treated the same way (PIT; see paper by Bowen,
Cerling, Ehleringer in RCM).

If your needs can be meet by settling for the latter, life will become a bit
more tolerable.

Personally, I would not dilute or dissolve the honey samples in water for 3
reasons: 1. you have no control over the rate of H-exchange since no two
honeys are the same; 2. you will never get rid of all the added water to the
same degree in all samples (getting sugar, even fully derivatised sugar
water free is a nightmare); 3. there is also the risk of O- (or OH)
exchange.

I appreciate this leaves you on a sticky wicket when it comes to dispensing
samples in the silver capsules but try freezing some honey spread on a glass
slide with liquid N2.

With regards to H-exchange with lab air (humidity), I have to admit I am not
aware of any equation that would enable you to predict the extend of this
effect even assuming the 2H composition of the humidity in the lab and the
exchange rates for each functional group with an exchangeable H were known.
However, total exchange experiments have been carried under controlled (and,
shall we say 'extreme' conditions; i.e. not normal working conditions) but
this would be the way to go if you want to report true d2H values.

So, for reasons of practicality, we stick to the principal of identical
treatment, which works quite well. For example, for cellulose samples
covering a 2H-range of 80 o/oo and analysed on the basis of PIT we observed
only a small constant off-set of 9 o/oo when comparing results between our
lab (precipitation and, hence humidity -50 o/oo) and a lab where
precipitation is -110 o/oo.


Probably not much help but perhaps food for thought.


Best regards,

Wolfram



> -----Original Message-----
> From: Stable Isotope Geochemistry 
> [mailto:[log in to unmask]] On Behalf Of Klaus Meylahn
> Sent: 11 April 2006 09:37
> To: [log in to unmask]
> Subject: dD and dO in lab air humidity, exchange, sugar
> 
> 
> Dear All,
> we measure dD and dO in honey-samples.
> We fear, that there is an influence to the values of dD and 
> dO comming from 1. the water we use to dilute the honey 
> before pipetting it into the Ag- capsules for 
> TCEA-measurement 2. the lab air humidity, which may influence 
> the dD- and the dO-values of 
> the dries samples (70°C over night).
> 
> Does anyone know, what range of values for dD and dO we can 
> expect (20°C, 
> 20-50% Humidity, 1000 hPa). Can we calculate it from 
> temperature, relative 
> humidity and pressure?
> 
> To which extent will the dD- and the dO-values be influenced 
> by exchance- reactions with H and O in the sugar-molecules. 
> And what time does it need? Is the role of the deliquescence 
> of the dried sugar greater or lower than 
> that of the exchange-reactions?
> 
> Thank you, Klaus Meylahn
> 
> -------------------------------------
> Dr. Klaus Meylahn
> LAVES Lebensmittelinstitut Oldenburg 
> http://www.laves.niedersachsen.de/master/C4259035_N4270905_L20
_D0_I826.html