Page 260 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32

148 Mass Spectrometry

sector-based analyzers, quadrupole, and ion trap instru- is used rather than GC specifically because the sample
ments provide the requisite fast scanning capabilities. No molecules are relatively nonvolatile, and cannot be evapo-
change in the usual operating conditions of the GC is re- rated without decomposition. Therefore, other ionization
quired, and the mass spectrometer is usually operated at methods have been developed that produce ions directly
near the maximum allowable scan rate consistent with ex- from the solvent (electrospray ionization), or use additives
pected ion signal strength. within the solvent to cause a chemical-ionization-like re-
The usual graphical output of a GC/MS analysis is the action (for example, ammonium acetate buffer was used to
TIC trace, where TIC is an acronym for total ion current. cause protonation in the thermospray ionization source).
The output is graphically similar to the output of a single- Today LC/MS usually involves the use of the ESI
channel GC detector such as a flame-ionization detector source, described in detail in the next section. The com-
or an electron-capture detector. In GC/MS, the TIC trace position and flow of the solvent is constrained within
represents the sum of ion intensities across the scanned limits set by stable ESI source operation. A flow rate of
mass range, a single sum value for each scan number of the 1–10 µl/min of common LC solvents (methanol, ethanol,
GC/MS run. When compounds elute from the GC column, isopropanol, or acetonitrile) generates a stable ESI spray.
the number of ions formed increases, and the summed ESI interface designs used to accommodate higher LC
TIC value increases. The TIC trace is used to determine flow rates include pneumatically assisted electrospray, in
the retention times of compounds. Then a complete mass which a concurrent flow of warmed gas aids in confining
spectrum is assembled by adding together, and then aver- the spray and speeding desolvation, allowing flow rates
aging, mass spectra recorded during scans recorded across of 100–200 microliters/minute. Higher flow rate columns
the eluting peak. Finally, the averaged mass spectrum is can be coupled to an ESI source through a flow splitter.
searched against the library of electron ionization mass The effluent of lower flow rate capillary columns can be
spectra. GC/MS data can also be processed in many other augmented by additional liquid for a stable spray, or one
ways, with the reconstructed ion chromatogram a power- of the newer designs for a nanoelectrospray source can
ful means to extract additional information from the data be interfaced to the capillary LC column. Capillary LC
recorded. The reconstructed ion chromatogram (RIC) is columns with an integral spraying tip can also be used for
a postrun routine that plots intensities of mass-selected direct electrospray ionization at the end of the LC column,
ions vs scan number for a GC/MS run. Ions that “belong” with appropriate connection to a potential source. In all
together in the mass spectrum of an eluting compound of these interfaces, there is a direct connection between
will maximize in intensity at the same time (the retention the column and the ionization source, with minimized op-
time of the compound). Background ions have a steady or portunity for sample loss. Sensitivities are therefore max-
slowly changing intensity. Unresolved GC peaks can be imized, and are mitigated only by the convolution of the
discerned by slightly different intensity maxima in the RIC scanning speeds of the mass analyzer and the widths of
profiles. Data processing in other forms of chromatogra- the LC peaks, as in GC/MS.
phy/mass spectrometry is similarly accomplished, with
the TIC trace providing an overall snapshot of the sepa- c. Capillary electrophoresis. CE was developed in
ration, spectral averaging across the width of the eluting the 1990s, and was soon interfaced to mass spectrome-
peakgeneratingacharacteristicmassspectrumofthecom- try to meet the need for structure-specific identification
pound, and the use of reconstructed ion chromatograms of eluting compounds. Often, the sensitivity of the mass
to investigate the purity of the mass spectrum. spectrometric analysis is mentioned as a driving force for
CE/MS coupling, but until development of the latest gen-
b. Liquid chromatography. LC/MS requires an int- eration of electrospray ionization sources, laser-induced
erface and an ionization method that accommodates the fluorescence routinely provided lower limits of detection
polar solvent that carries the separated sample mix- than did mass spectrometry. The potential-driven move-
ture components through the (usually) reverse-phase col- ment of ions in solution is the basis for CE separations.
umn. Just as GC evolved from larger packed columns to The rate of movement of ions is determined by the sum of
smaller, higher resolution capillary columns, LC also pro- electrophoretic and electroosmotic flow. Both neutral and
gressed from columns with large flow rates of solvent to charged compounds move through the column, migrat-
smaller columns that operate with solvent flow rates of ing at different rates, and maintaining a high separations
a few milliliters per minute, and eventually to microbore resolution due to the shape of the flow gradient in the
columns with flow rates of 20–100 µl/min. Electron and small column. The flow rate through a CE column is a low
chemical ionization (vide infra) would require that the 1–2 microliters/minute. The sample loading on the CE
solvent be completely removed, and that stable gas-phase column is also low, with perhaps a few ng of sample at
neutral molecules of the sample be formed. However, LC most available for detection.