Dialysis device

D.K. dk at no.email.thankstospam.net
Sun Nov 4 15:57:13 EST 2001


Lauri Lintott <llintott at shaw.ca> wrote:
>BL wrote:
>
>> In additional to soft dialysis tubings and those membranes mounted on a
>> frame, does anyone know of any other types of devices for dialysing a
>> small sample? I need to dialyse a small sample which I don't want to put
>> into a dialysis tubing or have to use a syringe to put into the membrane
>> device (for fear of shearing the sample). A rigid tube with a wide enough
>> opening (so that I can pipette in the sample) that has low adherence to
>> proteins would be ideal. thanks for any suggestions.
>
>When working with small volumes of protein or DNA I use ambions 'microcon'.
>(no personal affiliation).  They come in various mol wt cuttoffs.  Basically
>to change salt concentrations or buffers you dilute your sample with the
>buffer you are changing to, and then spin the sample.  

>Everything below the
 ^^^^^^^^^^
>mol weight cut off spins through the membrane 

This erroneous point of view appears to be widely held and is known to 
be a cause of many mistakes. I've seen a grad student who was 
seriously attempting to separate 50 kDa protein from 20 KDa contaminant 
this way (a completely hopeless and futile exercize). Truth is, cut-off 
value in both dialysis and ultrafiltration refers to a _retention_. As such, 
a cut off of 30 kDa does not mean that a 10 kDa molecule with go 
through with an efficiencty much higher that that of 30 kDa. All 
such a value says is that >95% of 30 kDa molecule will be 
retained (and less of 10 kDa molecule will be retained). It is prudent
to keep in mind, for example, that even salts and things like glycerol 
and sucrose are concentrated to some extent by 10 kDa cut-off 
Centricons. 

DK




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