Re: ORGLIST: physical properties testing

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From: Richard Prankerd (Richard.Prankerd$##$vcp.monash.edu.au)
Date: Wed Mar 05 2008 - 16:42:48 EST


Younan:
 
There are several methods for looking at your problem, with varying
degrees of information content:
 
1. The definitive method for detemination of the location of water is
single crystal X-ray diffraction. You are best to contact a good quality
chemistry department which has an X-ray research group for this. The
process will take about 2-3 weeks depending on the complexity of the
problem. Alternatively, powder X-ray patterns will change if crystal
water is removed from the sample (e.g., by heating the solid, especially
on a hot stage XRPD instrument, or by exhaustive desiccation under
vacuum over phosphorous pentoxide), however the patterns for a
crystalline solid with adsorbed surface moisture will change very little
if at all;
2. You can get an indication of the location of water molecules using
differential scanning calorimetry, especially in combination with
thermogravimetric analysis. In general, the loss of water from surface
adsorption occurs at relatively low temperatures (70-110 deg C, although
I have seen it at temperatures down to 50 C), whereas crystal water is
lost at higher temperatures (up to 180 deg C). More critically, crystal
water is lost over a narrower temperature range (say, 5-10 deg C at
most), while surface water is lost over a broader range. Water loss is
seen by an endotherm in the DSC for both kinds of water (broad for
surface water, narrower and sharper for crystal water) and a
stoichiometric mass loss in TGA for crystal water, but a
non-stoichiometric mass loss for surface water.
3. Karl Fisher titration can tell you if water is present. If the amount
of water in the sample is stoichiometrically related to the principal
material, then it is most likely to be crystal water. If this is the
case, then expose the sample to a range of different humidities and
recheck the water content. If it increases fractionally with each change
in humidity, then it is likely to be surface water, but if the
stoichiometry is maintained at different humidities, then it is crystal
water.
4. Hot stage microscopy can be performed under a blanket of silicone
oil. This will allow you to see loss of water as droplets (below 100 C)
or bubbles (above 100 C). While heating under polarised light with a
crossed polariser analyser, you should see no change in the appearance
of the crystalline solid if the lost water is surface water. However, if
the water is crystal water, then you are very likely to see changes in
appearance, especially in the birefringence.
 
I can do this type of work using methods 2, 3 and 4, but not 1.

Rgds
Richard
 

Richard J. Prankerd, PhD
Senior Lecturer
Victorian College of Pharmacy, Monash University
381 Royal Pde., Parkville VIC 3052

Phone: INT+613-9903-9003
Phax: INT+613-9903-9583
Phax: INT+613-9903-9560
http://www.vcp.monash.edu.au/departments/pharmaceutics/staff/rjprankerd.
html
Drugs need to be designed with delivery components in mind - Takeru
Higuchi

-----Original Message-----
From: everybody-bounces$##$orglist.net
[mailto:everybody-bounces$##$orglist.net] On Behalf Of
Younan.Shih$##$scinopharm.com.tw
Sent: Wednesday, 5 March 2008 9:21 PM
To: everybody$##$orglist.net
Subject: ORGLIST: physical properties testing

Hello everybody

Can anyone help me in suggesting an analytical method which can
distinguish whether a water molecule is bonded in the crystal lattice or
just absorbed on the surface?

Or, if you can recommend any contract lab that can perform this kind of
analysis.

With best regards

Younan Shih


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