Chemistry Reference
In-Depth Information
of a molecule. Information from this fragmentation assists in the elucidation of the
compound's chemical structure and properties.
Modern mass spectrometers are simple, easy-to-use instruments with a much
smaller footprint than their predecessors and can be configured with a chromato-
graphic method development system to provide a wealth of useful information. The
basic components of an MS system are shown in Figure 3.11. Because a comprehen-
sive treatment of MS is outside the scope of this chapter, the reader is urged to con-
sult the many excellent detailed reviews of the technology that are available [24,25].
But single quadrupole mass spectrometers are becoming increasingly common in
the method development laboratory and is covered here in some detail.
Quadrupole MS uses radio frequency (rf) and direct current (dc) voltages for
the separation of ions, and are probably the most widespread mass spectrometers
because of their relatively low price and ease of operation. In a quadrupole mass
spectrometer, the rf and dc potentials are applied to four rods arranged in a square
array, as illustrated in Figure 3.12. Ions are scanned or filtered by varying the DC/
RF voltages across the quadrupole rods. Generally speaking, quadrupole analyzers
are used to determine the nominal mass of a compound. Nominal mass is often used
to confirm the identity of known compounds in method development.
In method development, MS is used in much the same way as the PDA: to iden-
tify and track peaks as selectivity changes, and to monitor for co-elution. But unlike
PDA, MS provides a positive identity, and can provide deconvoluted total ion chro-
matograms specific for a molecular weight when co-elution of partial resolution does
occur.
Of course, no detector response is universal. MS response is dependent on the
ability to ionize a compound, and not all compounds can be ionized under all condi-
tions. In similar respects, not all compounds have UV chromophores, so PDA detec-
tion is, of course, limited. However, it is very rare to have both no ionization and the
lack of a UV chromophore; therefore, it is increasingly common to use MS and PDA
in tandem during method development.
When it comes to MS detection, the low UHPLC system and dwell volume
increases peak concentrations with reduced chromatographic dispersion at lower
flow rates (no flow splitting), and the added resolution promotes increased source
ionization efficiencies, making UHPLC the ideal technology for an MS inlet in a
method development system. Higher UHPLC sensitivity also improves the quality
of the spectra obtained.
Mass Spectrometer
Sample
Introduction
Ion
Source
Mass
Analyzer
Data
Analysis
Detector
FIgure 3.11
The basic components of an MS system. (Reprinted from HPLC method
development for pharmaceuticals, Volume 8 of
Separation Science and Technology
, S. Ahuja,
Editor, Chapter 6, Contemporary liquid chromatographic systems for method development,
p. 167, 2007.)
