Tip# 115: Adduct formation in LC-MS (esp. ESI)
Almost
everything you analyze by Electrospray ionization mass spectrometry will create an adduct
with something in the system. Normally, hydrogen is the most common adduct formed (M+1),
but other chemicals, often in trace amounts may form adducts with your sample too.
Sometimes we can take advantage of this fact and introduce our own adduct into the system
(post column) to increase signal sensitivity or help us isolate one signal from another
(the addition of an adduct can sometimes increase the signal seen for one species, but not
the other).
One
of my favorite elements to form an adduct with is sodium (Na+). Two common forms are
sodium citrate and sodium acetate. Both have PKA’s between 3 and 6 so a variety of
buffered solutions can be prepared for use. However, in LC-MS it is very important that we
keep the concentration of sodium as low as possible so as to not clog the mass detector or
suppress ionization completely (and see nothing BUT Sodium for weeks …). My
suggestion is to prepare the buffers such that the solution is below 3 mM in
concentration. The lowest concentration should be used that yields reproducible results.
Only use the highest purity buffers (some manufacturer’s use names such as
“ultra” to describe them) when preparing these ‘doping’ solutions for
post-column addition and be sure and filter them through a 0.2 micron filter before use. A
syringe pump can be used to deliver the solution during the run. A low flow rate should be
used to infuse the adduct solution into the main inlet of the detector. Make sure you have
a simple way of controlling the pump through the system (e.g. ‘On’ /
‘Off’, contact closure) so the flow can be turned off when you are not acquiring
data.
Ammonium
(NH4) is another popular adduct to add to the system, Often in the form of ammonium
acetate. Whichever adduct you use in your system, always start off testing as low a
concentration as possible. Monitor the baseline carefully for noise and also to see if the
addition of the compound is suppressing or enhancing the signal generated for your
compound. Careful use of adducts in your system can provide you with another means to
selectively enhance the signal of some compounds without changing the original
chromatography method.
> Bill Letter, 11/17/2011
