Environmental Engineering Reference
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enough to feed directly into the ionization chamber of the mass spectrometer. If
packed columns are used, a jet separator can remove the carrier gas.
The interface between HPLC and MS is much more challenged and is the key
to success. HPLC systems use high pressure for needed separation efficiency and a
high load of liquid flow, and hence high gas load (For example, a common flow of
liquid at 1 mL/min, when converted to the gas phase, is 1 L/min). The MS part, on
the contrary, requires high vacuum and elevated temperate, and does not tolerate
high flow of introduced sample. Two interface technologies have become
available in the 1990s to address such problems in coupling LC with MS. Since
both technologies incorporate the interface and ionization, we will describe them
as the ion sources.
Commonly Used Ionization Methods in GC-MS and LC-MS
The four most commonly used ion sources are summarized in Table 12.1. Electron
impact (EI) ionization and chemical ionization (CI) are the two most common ion
sources for GC-MS, whereas electrospray ionization (ESI) and atmospheric
pressure chemical ionization (APCI) are the most common ones for LC-MS. With
some similarities in their device, many commercial GC-MS systems can be
configuredtohavebothEIandCI,andLC-MSsystemstohavebothESIand
APCI. All these ion sources listed in Table 12.1 can be configured to a quadrupole-
based mass analyzer.
There are two types of ionization techniques based on the energy level used to
ionize the analyte molecule—hard ionization and soft ionization. Hard ionization
commonly refers to electron impact (EI) ionization. In this EI mode, the ''reagent''
producing the ionic products is a beam of highly energetic (approximately 70 eV;
1eV¼ 23 kcal) electrons. Such electrons are produced by boiling electrons off a
narrow strip or coil of wire made of tungsten-rhenium alloy (Fig. 12.4). Note that
ionization energies for most organic compounds range from about 5-15 eV and the
bond dissociation energies are even smaller. Because of the high energy, the hard
ionization usually produces small pieces of fragment ions of the sample molecules,
thereby offering rich structural information. Since extensive mass spectrum database
has been developed, an added advantage of the EI mode GC-MS is the use of
Table 12.1 Most commonly used ionization methods in molecular mass spectrometry
Ionization methods
Ionization agent
Source pressure
Uses
Electron impact (EI)
70 eV electrons
10 4
10 6
torr
GC-MS
Chemical ionization (CI)
Gaseous ions
1 torr
GC-MS
Electrospray ionization (ESI)
Electric field
Atmospheric or slightly
LC-MS
reduced pressure
Atmospheric pressure
Corona
Atmospheric pressure
LC-MS
Chemical ionization (APCI)
Discharge
1 torr ¼ 1mmHg¼ 1=760 atmospheric pressure at sea level.
 
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