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Ionisation Techniques
Two atmospheric pressure ionisation techniques are available.
Atmospheric Pressure Chemical Ionisation
Atmospheric pressure chemical ionisation (APcI) generally produces protonated or
deprotonated molecular ions from the sample via a proton transfer (positive ions) or
proton abstraction (negative ions) mechanism. The sample is vaporised in a heated
nebuliser before emerging into a plasma consisting of solvent ions formed within the
atmospheric source by a corona discharge. Proton transfer or abstraction then takes
place between the solvent ions and the sample. Eluent flows up to 2 ml/min can be
accommodated without splitting the flow.
Electrospray
Electrospray (ESI) ionisation takes place as a result of imparting a strong electrical
field to the eluent flow as it emerges from the nebuliser. producing an aerosol of
charged droplets. These undergo a reduction in size by solvent evaporation until they
have attained a sufficient charge density to allow sample ions to be ejected from the
surface of the droplet (“ion evaporation”).
A characteristic of ESI spectra is that ions may be singly or multiply charged. Since
the mass spectrometer filters ions according to their mass-to-charge ratio, compounds
of high molecular weight can be determined if multiply charged ions are formed.
Eluent flows up to 1 ml/min can be accommodated although it is often preferable with
electrospray ionisation to split the flow such that 100 to 200 µl/min of eluent enters
the mass spectrometer.
Nanoflow Electrospray
The optional nanoflow interface allows electrospray ionisation to be performed in the
flow rate range 5 to 1000 nanolitres per minute.
For a given sample concentration, the ion currents observed in nanoflow are
comparable to those seen in normal flow rate electrospray. Great sensitivity gains are
therefore observed when similar scan parameters are used, due to the great reductions
in sample consumption.
Sample Inlet
Sample is introduced from a suitable liquid pumping system along with the nebulising
gas to either the APcI probe or the electrospray probe. For nanoflow electrospray,
metal coated glass capillaries allow the lowest flow rates to be obtained while fused
silica capillaries are used for flow injection analyses or for coupling to nano-HPLC.
Instrument Description
Page 17
Quattro Ultima
User's Guide
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