2. Historical Perspective
The chemistry and physics of ions in ambient air were the subject
of intense experimental scrutiny between 1880-1910 and subsequently became
a recurring, but minor, theme within the expanding field of mass
spectrometry. Short periods of active exploration, motivated by interest
in specific problems, were separated by long intervals of apparent
disinterest. Related work (Figure 3) on bimolecular reactions, drift tube
mass spectrometry, ionization detectors and the ionization chemistry of
air comprised a rich blend of background information and supporting
details for IMS. In a technical sense, the prerequisites for the creation
of modern analytical IMS (e.g. atmospheric pressure ionization processes
in air and ion separation in a bath gas) were gradually developed over
nearly seventy years prior to the introduction of IMS as a analytical
device in the late 1960s. In another sense, the specific critical
motivations and essential impulses and associations that resulted in the
first configurations of modern analytical IMS are obscure and anecdotal.
The two components to this development were: A) the connection between
particular gaseous ionic species and the chemical composition of airborne
vapors, and B) the recognition that ion separations based on mobilities
were achievable at atmospheric pressure. The history of these two
separate subjects and their coalescence as pertinent to IMS during the
1960s provides a remarkable story of technical ingenuity and creativity
that has not yet been fully chronicled. These technical and conceptual
foundations for IMS are described in the following sections.
3. Ions in Gases at Atmospheric Pressure
Air is generally regarded as an electrical insulator provided that
electrical breakdown (i.e. ionization of air) does not occur through arcs,
sparks or corona discharges. However, the realization that air becomes an
electrical conductor in close proximity to a radioactive material was a
fundamental development in all branches or facets of studies on
atmospheric ionizations. Changes in the electrical conductivity of air
were described and investigated by physicists4,5 in the 1880s and the
observed changes were quickly attributed to the existence of ions in the
gas phase. The gas phase ion were created through interactions between
constituents of air and charged particles released from the radioactive
material. A unifying thread for ionization of air came from Roentgen's
discovery that x-rays also caused gases to become electrically
conducting4. These studies involved simple ionization cells where only
bulk properties of conductivity were measured. In 1897 Thompson and
Rutherford5 explored these events by measuring the velocities of the rays
of positive polarity. They found that the speed of these rays was
proportional to the electric field strength, thus portending the
relationship between ion speed, field strength and mobility (eq. 1). Much
later, the secondary rays were seen as streamers from the primary rays and
these were attributed to collisions between ions and neutral molecules6.
The premise that ions could exist in air, and the widespread acceptance of
a chemical interpretation, should be considered remarkable in view of the
primitive state of these pioneers' materials and instrumentation.
Nonetheless, the existence of ions produced in air by x-rays or
radioactive emission was well established in the early 1900s and was
empirically undeniable if not fully rationalized. Missing was a thorough
discussion of the identity of the ions created in air and the tools to
characterize such ions.
A French investigator, Paul Langevin, made expansive contributions
through two remarkable articles7,8 in both the experimental and the
theoretical aspects of gas phase ions in air at ambient pressure. In
1903, he demonstrated experimentally that ionized air was a mixture of
several chemical species and in 1905 he promulgated the mathematical basis
and chemical models for ion mobilities. His pioneering efforts involved
placing gas phase ions in a weak electric field and determining the
velocities of individual ions, i.e. the essential steps of analytical IMS.
In terms of modern journal articles, the depth and breadth of Langevin's
reports were startling and included the construction of a functional
instrument, somewhat similar to drift tubes of current ion mobility
spectrometers, the collection of elementary measurements, the formulation
of a detailed theory on ion mobility and the refinement of the mathematics
for his model on mobility. His formulas were suitable for the simple ions
studied and apparently, very little additional attention was given to such
studies. Instead, investigators were attracted to the opportunities of
nascent high vacuum ion analysis (mass spectrometry); atmospheric pressure
methods were rapidly abandoned in the first of such cycles repeated to the
present.
Interest in measurements of ions reappeared in 19269 and 193110
and later in the 1950s. Studies11-15 on collisional based ion-molecule
reactions were carried out in the 1920s albeit under moderate vacuum using
mass spectrometers as summarized in Table 1. Such studies and their
modern antecedents have been treated comprehensively in a text that
provides a relevant background for current gas phase ion-molecule
investigations16. A major interest in this subject in the period
following World War II was in the effects of nuclear blasts on ionization
of air. Few of these studies involved ion mobility directly and the
pressures were not always at ambient levels. In the 1960s, a new
generation of studies highlighted a then growing interest in tropospheric
and stratospheric chemistry17,18 and a few investigations of ion-molecule
reactions actually involved the measurement of ionic mobilities in air at
atmospheric pressure.
A pivotal development in vapor monitoring technology and in the
creation of modern analytical IMS was the observation that simple
ionization devices responded to ultra-low vapor concentrations of
atmospheric pollutants. Particularly noteworthy was Lovelock's discovery
of the effects of industrially-related organic vapors and the response or
behavior of a simple radioactive-based sensor19. Lovelock had been
engaged20 in medical studies associated with the speculation that breezes
could be implicated with human influenzas or illness. For these studies,
he developed a device in which slight perturbations of wind direction or
speed were registered as changes in the ion current at a collector plate.
This vapor anemometer (shown schematically in Figure 4) was dependent upon
the degree to which cross winds deflected slow moving large positive ions
from the region between a radioactive source to a flat plate collector or
detector. The displacement of these ions was related to the direction of
the cross current flow of air and proportional to the strength of the
breeze. Lovelock also found that his anemometer responded to certain
airborne vapors, especially halocarbons, which reacted with the charge
created by the radioactive source and wildly altered measured currents.
Versions of this technology became widely distributed in time as the
electron capture detector (ECD) for gas chromatographs and eventually
revolutionized chemical measurements for certain substances, including
pesticides. The ECD was configured also as an atmospheric sensor for
halogenated tracers21-23 and thus served as type of a precedent for IMS.
The ECD and IMS share common principles of gas phase ionization chemistry
at atmospheric pressure but there is no suggestion that Lovelock's studies
directly influenced the modern pioneers in IMS. Nonetheless, those active
in ionization detector technology including IMS pioneers must have been
generally aware of his contributions.
A related, but clearly separate development, was the study of ions
in flames and in corona discharges at elevated pressures using mass
spectrometry24-26. Shahin first reported ion clusters of hydrated
protons26 using a mass spectrometer where ions were generated in air at
atmospheric pressure and chemical ionization mass spectrometry (CI-MS) was
described by Munson during this period27. Interpretation of early IMS
results relied strongly on findings from CI-MS studies28 in the absence of
direct ion mass analysis. Although GC-IMS-MS combinations were described
in some of the first papers on IMS29, such instrumentation was not
routinely available for use. As with Lovelock's findings, there is no
evidence that the inspiration for the creation of modern analytical IMS
was specifically influenced by the work of Shahin or Munson. Rather, the
founders of IMS were contemporaries of a large number of workers who had
been studying ion-molecule processes at elevated pressures. This
perspective should not lessen in any way the significance of those
pioneering IMS inventors and simply illustrates the context from which
their developments arose.
In summary, modern analytical IMS was created between 1965 and
1970 from a wealth of well-established and contemporary studies on
ion-molecule chemistry at atmospheric or elevated pressure with mass
spectrometers and from ionization detectors for airborne vapor monitoring.
Unfortunately, the specific influence of any of these on the thinking of
the founders of IMS is not part of the historical record. A final and
essential component to the development of analytical IMS was the
adaptation of drift tubes from drift tube-mass spectrometers for
convenient operation under practical, i.e. ambient pressures. The link
between drift tube-MS and IMS is known and these studies proved
indispensable for the creation of IMS, as described in the following
section.
4. Drift Tube Studies
Prior to the mid-1960s, mobility measurements on gas phase ions
had been the domain of physics laboratories equipped with drift tubes that
were operated in slight to medium vacuums. Langevin may be credited with
having published the first extensive discussion of gaseous ionic
mobilities, but advances in mobility theory awaited more refined
instrumentation capable of separating ions and measuring their mass. In
the mid-1960s, several research groups described their findings on
refinements to Langevin's theory for small inorganic ions for which highly
accurate mobility measurements were sought30,31. A combination of a drift
tube to sort ions in a weak electric field with a mass spectrometer to
gain exact mass determinations provided the experimental confidence to
allow the development of refined models on ion mobilities. The work of
Crompton et al. in Australia32 and Albritton and McDaniel in Georgia33
illustrate this line of study. The interest of these investigators and
others was exclusively in developing the means to accurately measure ion
mobilities and application of drift tube technology for chemical analyses
was irrelevant. Moreover, their use of moderate vacuums in the drift tube
was impractical for the creation of analytical monitoring technology.
However, the concept of combining a drift tube with an ECD-type ion source
and operating drift tube and source alike in air at atmospheric pressure
transpired with those seeking to add selectivity to general ionization
detectors.
5. Modern Ion Mobility Spectrometry and Related Devices
A number of devices based upon some degree of ion separation were
described between 1965 and 1972 and IMS was created in this milieu or at
least within this context. In the development of each of these detectors,
mostly ECD related, attempts were made to add some further specificity to
the underlying principles of the ECD. The single unifying theme for these
precedents, or technical competitors, to IMS was a connection to military
establishments in the US and Europe, specifically the UK. The designs of
these detectors followed directed applications such as the detection of
humans or human activity in the jungles of Vietnam or the detection of
chemical warfare agent vapors including nerve gases and blister agent.
Several of these detectors, none of which was highly publicized, were
tested under actual field conditions or were actually deployed. Examples
include the Condensation Nuclei Personnel detector34 and the M-8A1
detector system with the M-43A1 ionization detector cell34. Contemporary
with these two analyzers were European devices from England35 and
Germany36. The DICE35 (acronym for Detection by Ion Combination Effect)
was developed at the Chemical Defence Establishment at Porton Down (UK)
and was based on principles of ion mobility or diffusion. The
Mini-ionization detector built by Honeywell36 (Germany) was a modified
flow-through ECD, in which ions moved through a stream of gas against an
electric field. In Finland, a device was developed where ions traversed a
tortuous path through electric fields, much the same as the M-8A1, and
this led to development of an IMS based agent detector37.
Each of these rudimentary ionization detectors came to be
recognized as potentially valuable but aspects of performance were
limited. In the mainstream of efforts to add specificity to general
sensors, the original concept of ion separation at atmospheric pressure
originated with a private research company known as Franklin GNO (GNO for
gnosticism or knowledge). Part of their interests culminated with the
production of an ion mobility spectrometer38 (or plasma chromatograph in
the original vocabulary) in essentially a final modern configuration.
This instrument bore strong resemblances (Figure 5) to the drift tube-MS
designs of McDaniel who was a consultant to Franklin GNO on this
project39. Unfortunately, the origin of the pivotal suggestion to couple
a radioactive ionization source with a drift tube operating at atmospheric
pressure (rather than in a vacuum customary with drift tube mass
spectrometry) was not well documented within the small group of
investigators at Franklin GNO39. Nonetheless, an extensive patent record
ensued from the developments of this group and some of these are cited in
Table 2. In retrospect, the utilization of the term "plasma chromatgraph"
may have encouraged comparisons to chromatographic technologies and
resulted in serious misunderstandings that have been rectified only with
time. A major episode in the development of IMS began when a Beta-VI
plasma chromatograph from Franklin GNO (soon PCP, Inc.) was acquired by
F.W. Karasek after his move from Philips Oil Company to the University of
Waterloo in 1969. Karasek began a period of intense study and evaluation
of the response of IMS to organic compounds and a partial listing of the
titles from Karasek's group between 1970 and 1973 is shown in Table 3.
There was considerable diversity within Karasek's studies in the early
1970s in areas such as on the elementary response
characteristics42,43,45,47, searches for a comparison to, or adaptation
of, existing technologies29,47,48,51, and some enthusiastic pondering
about the significance of IMS as an analytical instrument29,49. The
absence of an IMS/MS in his laboratory certainly hampered his progress
toward establishing a foundation for IMS principles. Fortunately,
elementary mechanisms of the ECD had been addressed by Wentworth52-55 and
some supporting information was available to rationalize IMS response.
As part of an effort to place IMS within a technical point of
reference, parallels between IMS and chemical ionization-mass spectrometry
(CI-MS) were made and IMS seemed to these early workers as a type of
chemical ionization time-of-flight (TOF) mass spectrometer40. While
certain functional similarities did exist between TOF-MS and IMS, these
comparisons proved as unprofitable as comparisons to chromatographic
methods by creating unattainable expectations for IMS performance. By
1975, a general disenchantment with IMS arose when users, seeking small,
simple alternatives to mass spectrometers, found the response by IMS to
mixtures confusing or unsatisfactory. The decline in the number of
publications on IMS in refereed journals, conference presentations and
other published literature, from 1970-1980 mirrored the disenchantment
with IMS as suggested by Figure 6. This was coupled with a conspicuous
absence of an underlying theory of ionization chemistry (still currently
under active development) and with the IMS drift tubes that exhibited
severe memory effects. Ion mobility spectrometry was viewed generally as
a technology too complicated for practical uses and the field was
seemingly abandoned to the original researcher, F.W. Karasek, who
continued investigations until 1980.
As implied above, the assessment and promotion of ion mobility
spectrometers as a type of chromatograph might be viewed as a source of
the ensuing frustration and disenchantment with IMS. This is best
epitomized in comments by Metro and Keller56 that IMS "...was not what we
have come to believe a chromatographic method should be." Indeed, ion
mobility spectrometry is not now, and was not then, a chromatographic
method and such comparisons, in retrospect, seem ill-advised. In contrast
to such sentiments, IMS had become recognized as useful in a number of
narrow technological niches for chemical analysis of vapors from
electronics packaging57,58 and in the detection of toxic halocarbon
compounds59. Still, there were no compelling reasons for the use of IMS
in general chemical measurements since the contemporary configurations
suffered from a number of impediments, including limited or narrow linear
range, prolonged memory effects and serious matrix interferences.
Activity in IMS as evident to the outside observer by 1979 was virtually
nonexistent. However, military establishments in the US, UK and Canada
realized that the principal objections to IMS were associated with
technology or engineering. The principles of IMS were fundamentally
attractive due to proven low detection limits, reasonable selectivity and
potential for miniaturization (small size, light weight and low power).
The ionization chemistry was favorably matched with the substances of
military concern, namely the high proton affinity of nerve agents, i.e.
organo-phosphonate esters.
Throughout the 1970s, development programs in military
establishments in the US and UK built a base of experience and exploration
with IMS. Their findings ultimately demonstrated that detection limits
for warfare gases were excellent and that selectivity and resolution were
adequate for the operation of portable ion mobility spectrometers in
hostile outdoor environments. The ion mobility spectrometer was viewed as
being part of a new generation of vapor sensors in which the mobility
measurement added a degree of confidence to the high sensitivity of
non-specific ionization detectors. The assertions that ionization
chemistry could be controlled and that limitations in IMS were simply
mechanical were crucial in the development of portable ion mobility
spectrometers. A photograph of the product of this development program is
shown in Figure 7. Not obvious in this photograph is the ease of
operation and interpretation of response provided to soldiers in the
hand-held configuration. Not shown also are the engineering solutions to
constraints of clean drift gas flow, high voltages and small signals
characteristic of IMS; not intrinsically compatible with field
instrumentation. These hand-held ion mobility spectrometers laid the
foundation for IMS as a field analyzer and proved that the miniaturization
of ion mobility spectrometer was realistic. In a sense, the long sought
goal of IMS as a practical, sensitive, and selective analyzer was
fulfilled by these military analyzers. However, not all IMS research
programs during the 1980s were associated with military establishments.
6. Recent Trends in Ion Mobility Spectrometry
In the early 1980s, several research and development programs were
begun at universities, government laboratories and small companies,
principally in the US, UK, and Canada. The number of investigators has
grown during the last decade, so that about forty such programs currently
exist worldwide. The interests of this diverse group cannot be summarized
in a sentence or paragraph. Moreover, the motivation for considering IMS
as a viable technology arises from different requirements. Most are
attracted to IMS studies, instrumentation, technology and applications by
those features which first fired the imaginations of Karasek and
co-workers at Franklin GNO. These were the astonishing detection limits
and remarkable simplicity and reliability of hardware and electronics.
Subsequently, after 1980, IMS has undergone a revolution in
technical reputation and practical applications as summarized in several
review articles1-3,60 and evident in the increase in number of IMS
publications (Figure 6). In particular, the advantages of IMS in size,
power and weight have motivated evaluations of IMS as an environmental
monitor for in-field operations60-61. In 1992, several broad categories
in the development of IMS exist and these involve IMS as A) a selective
sensor in which the chemistry of ionization can be tailored or manipulated
for the detection of a specific compound or classes of compounds; B) a
more general analyzer for samples of predictable composition; and C) a
detector for chromatographs (see Chapter 7) where complications of
ionization chemistry are remedied by the preseparation inlet. More
fundamental issues currently under study are: A) alternate ionization
schemes; B) the behavior of ions in air at atmospheric pressure; C)
predictive and interpretive models for response of the IMS; D) models for
mobilities of organic molecules; and E) processing of information from
mobility spectra. In addition, improvements in instrument design and
construction have stimulated thoughts and experimentation for IMS
applications in environmental, industrial and medical sensing. These are
occurring in the context of demands for highly sensitive and specific
environmental monitors. This is coincident with a trend for analytical
instrumentation toward operation as field or process analyzers where
samples are immediately accessible.
In summary, more than twenty years have elapsed since the
inception of modern analytical ion mobility spectrometry and the field of
study has become a rich mixture of basic exploration and applied activity.
In a sense, the development of IMS may seem to some to have been
meandering and sleepy, possibly even bordering (during the late 1970s) on
technical oblivion. Thus, the changes since 1980 may seem all that more
remarkable. The last major effort62 to summarize IMS technology and
principles occurred almost a decade ago and has become outdated by the
recent advances in the field. An objective in this book is to provide a
clear and thorough presentation of the principles of ion mobility
spectrometry technology for novices, to summarize the very recent
developments for specialists, and to assess technical capabilities of
contemporary IMS technology for prospective users.
References
last modified July 10, 1997