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The following message was posted to: PharmPK
Dear Group,
It is a naive question. In an LCMS instrument, on nebulising the
spray, the
mobile phase will be evaporated. If the buffer used is not volatile, it
deposits on the interface, and degrades the sensitivity.
What factors determine, volatility of the buffers and ion pairs used?
What happens if the components produced are corrosive?
What precautions are to be taken, before trying out new ion pair or
buffer?
Thanks for the discussion.
Vinayak Nadiger
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The following message was posted to: PharmPK
Dear Vinayak Nadiger,
As general useful approach you can consider organic acids like formic
acid, trichloroacetic acid and acetic acid, volatile bases (the most
widely used I suppose is still NH4+) and relative salts (like
ammonium acetate, formiate, etc).
Best regards
Stefano
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A long time ago, in the days of thermospray, I knew of an incident
using perchloric acid with an amine modifier, which was often used as
an ion pairing reagent in HPLC. Fortunately no-one was around when
the dried residue destroyed the source!
Formic, acetic and trifluoroacetic acids are volatile. Ammonium
formate and acetate are volatile, but ammonium trifluoracetate can
hang around for a long time. Sodium salts are not volatile.
Sulfates, phosphates, citrates should be considered not volatile.
However, most sources these days are very tolerant of deposits of
buffers, and cleaning is simple. TFA is probably as far as you
should go towards ion pairing in electrospray; your HPLC might
benefit but ionisation efficiency will most likely be harmed by ion-
pairing.
Ted
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The following message was posted to: PharmPK
Thanks Ted and Stefano. I am sorry for not being very clear.
My question is What are the factors that make a particular buffer
volatile.
My understanding is, on heating, if buffer decomposes to volatile
components, then buffer is a volatile buffer.
Based on prior knowledge, if one chooses a new material as volatile
buffer,
how one tests whether that buffer is volatile or not?
Thanks
vinayak
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The following message was posted to: PharmPK
Dear Vinayak,
You may check the "volatility" of the buffer by simple gravimetric
analysis - by evaporating aliquot of the buffer (of required strength)
in an appropriate crucible at the temperatures you wish to use at the
"MS source" and compare the pre and post evaporation weights of the
crucible to check for any residue.
Kasiram
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The following message was posted to: PharmPK
Dear Vinayak,
To terst if a buffer is volatile or not I suggest you a "dry residue
analysis" of the buffer.
The easiest way to perform a "residue analysis" is to place a buffer
at a temperature where solvent is volatile (i.e. a temperature
similar to the MS source temperature), to eliminate the buffer by
volatilization and finally to weight the amount of non-volatile
residue. If all solution (buffer) components are volatile you don't
expect any dry residue.
Best regards
Stefano
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Hi Vinayak,
This seems like a bit of applied chemistry. I would consider three
properties. In the first place, are the components strong acids or
bases? Strong bases are hydroxides of permanently charged cations
such as Na+, K+, Ca2+, tetrabutylammonium+. Strong acids are not
exactly permanently charged but almost - sulfate, nitrate ...
Trifluoroacetic acid is almost in this category. The salts of strong
acids or strong basses are not going to be volatile. The difficult
acid is hydrochloric acid which is a strong acid in water, but a
relatively weak acid in the gas phase - salts of HCl can be volatile
(eg ammonium chloride) and often sublime.
The second rule is about multiple charges - salts of citric &
phosphoric acids are not likely to be volatile. Peptides are also
going to be a problem, which goes for zwitterions such as glycine -
not volatile. Also goes for multiple hydrogen bond donors as in
carbohydrates.
The third rule is about the molecular weight of the components - I
would not expect ammonium stearate to be very volatile except at very
high temperature.
If you want to prove that a buffer is volatile, then it depends on
the interface. A simple swab test is likely to show residues pretty
well, though you will have to think of ways to measure the components
- not easy.
I hope this helps a bit more.
Regards.
Ted
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The following message was posted to: PharmPK
Hi Ted, Thanks for this good piece of information.
So you are concluding salts of weak acid with weak base are going to
produce
volatile salts. What if we get a salt of weak acid and strong base
and vise
versa?
Also another case in point is fluoro compounds. I think all fluoro
compounds
are going to be volatile compounds , like various fluoro alkyl
carboxylic
acids.
But is it important that fluorine should be covalently bound ? What
happens
if it is ionic bond like in the case of Tetra butyl ammonium fluoride?
Whether free fluorine released will be very corrosive and spoil source?
Thanks for the group for the interest in the topic.
Vinayak Nadiger
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