Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 316

Filament
& heater

Ion accelerating lenses

- - -

Form GC

To mass filter

+ + +

Repelling
lens

Anode

Vacuum system

+

Analyte molecules Electron beam Molecule fragments (ions)

Figure 12.4 Schematic diagram of an electron impact (EI) ion source used in GC-MS

commercially available mass spectrum library. The library allows for automated
search by comparing the mass spectrum of the unknown to that in the electronic
library.
The soft ionization used in GC-MS and particularly LC-MS has a variety of
techniques. One of the most common soft ionization techniques called chemical
ionization (CI), for example, uses a stream of gas such as CH 4 ,NH 3 ,isobutenein
both GC-MS and LC-MS. Since lower energy is used to bombard the analyte
molecule than the electron impact ionization, fragmentation is minimized in
soft ionization mode. Mass spectrum is usually simpler with few peaks. This
frequently allows intact molecule to be present and detected on the mass spectrum
with a soft ionization mode. It is, therefore, particularly useful for characterizing
mixtures, although structural information is not as detailed as that acquired in the
EI mode.
The electrospray ionization (ESI), first described in 1984, has now become one
of the most important techniques for analyzing large molecules (100,000 Da or
more). The eluent from HPLC is pumped through a stainless steel capillary needle at
a rate of a few

L/min. The needle is maintained at several kilovolts with respect to a
cylindrical electrode that surrounds the needle. As shown in Figure 12.5, analyte
solution is sprayed (nebulized) into a chamber at atmospheric pressure in the
presence of a strong electrostatic field and heated drying gas. The resulting charged
spray of fine droplets then passes through a desolvating capillary, where evaporation
of the solvent and attachment of charge to the analyte solvent take place. As the
droplets become smaller as a consequence of evaporation of the solvent, their
charge density becomes greater and desorption of ions into the ambient gas occurs.
These ions are then passed through a capillary sampling orifice into the mass
analyzer.
The ESI enables mass spectra to be obtained from highly polar and ionic
compounds. Another complementary ionization technique using atmospheric-
pressure chemical ionization (APCI) allows spectra to be obtained from nonpolar

m