Ionization of small, large, volatile, and nonvolatile compounds with charge states nearly identical to electrospray ionization
are produced from a solid matrix or solution with high sensitivity utilizing the vacuum inherent with any mass spectrometer.
With the proper matrix, analytes can be analyzed from ambient conditions or by direct introduction into vacuum. The ion source
is simply the inlet to the mass analyzer. The new ionization methods have been interfaced with liquid chromatography, used
for imaging tissue at atmospheric pressure or in vacuum, demonstrated for high-throughput analyses, applied for fast defect
analysis, and shown to be compatible with electron transfer dissociation and ion mobility spectrometry–mass spectrometry.
With the proper matrix, even large proteins are converted to gas-phase multiply charged ions without application of an external
source of energy other than sub-atmospheric pressure. This latter method has great potential for extending mass spectrometry
to areas such as clinical analysis where cost, robustness, and simplicity are important assets.
In matrix-assisted laser desorption–ionization (MALDI), predominantly singly charged gas-phase molecular ions are produced
from analytes incorporated into a matrix. However, laser ablation of a common MALDI matrix at atmospheric pressure produced
ions with higher numbers of charges common with electrospray ionization (ESI). Fundamental studies directed at understanding
how multiply charged ions are produced by a process that is nearly identical to MALDI, with the main difference being that
ion extraction voltage is removed or lowered, led to a series of new ionization methods given the general terms inlet and vacuum ionization. The laser was found to not be necessary for matrix-assisted ionization (MAI). Ionization occurs when the analyte incorporated in a small molecule matrix is introduced into a heated inlet tube
linking atmospheric pressure and the vacuum of the mass spectrometer. MAI has now been extended to operate in vacuum using
laser ablation similar to MALDI, but producing ions with ESI-like charge states. Here, the laser is also not a requirement.
Matrices have been discovered that efficiently lift solid-phase molecules into the gas phase as ions without any external
energy source when exposed to subatmospheric pressure. The development of these novel ionization processes for use in mass
spectrometry (MS) is discussed. These methods are simple to use, safe, robust, and sensitive.
A Brief Evolution of Ionization Methods for Use in Mass Spectrometry
The inventions of ESI (1) and MALDI (2,3) in the 1980s led to important advances in science because they provided, for the
first time, a means of characterizing minute quantities of nonvolatile compounds even in complex mixtures using MS. Before
the advent of ESI and MALDI, methods such as field desorption, plasma desorption, fast atom bombardment, laser desorption,
and thermospray ionization made inroads into converting nonvolatile compounds into gas-phase ions (4–19). ESI and MALDI quickly
replaced other ionization methods used with nonvolatile compounds, each having strengths that complement the other. ESI produces
multiply charged ions from compounds in solution having multiple basic sites, is compatible with liquid separation methods,
and is capable of analyzing large molecules on mass-range limited mass spectrometers. MALDI replaces the solvent in ESI with
a solid small-molecule matrix and produces predominatly singly charged ions upon laser ablation of the matrix. MALDI operates
from surfaces and has excellent sensitivity, and singly charged ions simplify analysis of complex materials such as polymers.
However, MALDI is incompatible with liquid chromatography (LC) except under certain conditions (20). Because of deficiencies
in ionization of low-polarity compounds using ESI, atmospheric-pressure chemical ionization (APCI) has gained in popularity
as an alternative LC–MS ionization method (21,22).
In the past decade, numerous innovative new sampling methods have been developed under the umbrella of "ambient ionization"
(23–31). The mechanisms by which ions are formed in these methods are either gas-phase ion–molecule reactions as in APCI (32)
or charged droplets as in ESI (33,34). Desorption electrospray ionization (DESI) (35,36) appears to have elements of both.
The ambient ionization methods minimize sample preparation (31) and are associated with direct ionization in which chromatography
is not used. For complex samples, high resolving power MS or ion mobility spectrometry (IMS) enhances (37) or replaces (38–40)
the chromatographic separation, but ion suppression issues are more pronounced with the direct methods. Progress has been
made in coupling MS with IMS in which gas-phase ions are separated in time before MS analysis (41–43). Commercial IMS-MS instruments
are now available providing rapid and high sensitivity analysis. The most prominent IMS-MS instrument uses traveling wave
ion mobility spectrometry (TWIMS) technology (44). Improvements in TWIMS have enabled the separation of isomeric species (39,45,46),
which is unavailable in MS alone.
Within ambient ionization there are ESI-based approaches capable of ionizing nonvolatile compounds such as DESI, laser ablation
ESI (LAESI), and matrix-assisted laser desorption ESI (MALDESI), as well as those that ionize vaporizable compounds using
APCI such as atmospheric solids analysis probe (ASAP) (25) and direct analysis in real time (DART) (24). A number of ambient
methods are suitable for MS imaging (47). These approaches create a molecular image from the surface of interest (48). Ambient
ionization methods also show promise in the characterization of proteins, but because these approaches operate on mass spectrometers
with limited mass range, they must produce multiply charged ions in sufficient abundance for imaging (49). These atmospheric-pressure
imaging approaches currently suffer either from limited ionization sensitivity, low spatial resolution, or both.