Michael P. Balogh
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The number of atmospheric-pressure sampling techniques grew exponentially in the past few years. We have not seen such a concerted
effort at innovation since the early 1990s, when electrospray was becoming the commercial prospect it is today. Back then,
techniques like particle-beam mass spectrometry (MS), atmospheric-pressure chemical ionization (APCI), fast-atom bombardment,
and others vied for equal merit largely because analytical practitioners lacked experience using them.
Currently, nearly a dozen incipient technologies exist — MS techniques that are only now coming into view. Typically, when
they emerge, such technologies stimulate interest among practitioners. Nevertheless, their commercialization is not yet widespread
and the underlying functional principles of their newly filed patent applications are not yet completely understood. Yet if
the past is prologue, it is safe to say that in coming years most incipient technologies will not withstand the rigors of
commercialization. Which will survive? Which will find extensive, robust usage as did electrospray ionization (ESI) and APCI?
Which are fated to become footnotes in MS practice, like particle-beam MS, a worthy technique that never quite achieved becoming
an established utility for various reasons?
In most cases, new techniques today are developed and commercialized with an eye toward compelling, market needs such as food
safety and (now) homeland security. In those areas, a need to reduce sampling time and simplify sample preparation couples
with a need to achieve first-order results at the hands of users with relatively scant training.
Surface Sampling and IonizationThe techniques for surface sampling and ionization at atmospheric pressure are attractive alternatives to liquid chromatography
(LC)–MS. A Chemical &Engineering News feature was devoted to the topic (Oct 8 issue, Taking Mass Spec into the Open, http://pubs.acs.org/cen/coverstory/85/8541cover.html) and the American Society of Mass Spectrometry (ASMS) organized the annual fall workshop "The Art of Open Air Ionization
on Surfaces" (November 8 and 9, 2007, Philadelphia, Pennsylvania).These techniques provide reasonable expectations that the
demand for a quantitative response with a broad range of compounds (or, at least, target compounds) can be met.
Dr. Gary Van Berkel and his coworkers in the Organic and Biological Mass Spectrometry group at Oak Ridge National Laboratory
(Oak Ridge, Tennessee) have devoted much time investigating surface sampling and ionization techniques. Their website is a
source of valuable information on the topic ( http://www.ornl.gov/sci/csd/Research_areas/obms_group.html).
Figure 1: Surface sampling and chemical imaging under ambient conditions.
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Figure 1 encapsulates some aspects of this discussion, associating related techniques by their fundamental properties including
the overlapping combinations. Often, as the work evolves, the true nature of the mixed modes at work is revealed. For instance,
more than 30 years after the introduction of APCI, there are a number of approaches using a variety of materials and means
of desolvating the liquid flow owing to a complexity of underlying fundamental properties (though we suspect they are a mixture
of nucleate boiling and Leidenfrost effects). Nevertheless, our relative ignorance does not inhibit our success using APCI
for neutral and nonpolar analytes in place of ESI.
Desorption
Desorption electrospray ionization (DESI): http://www.prosolia.com/
Graham Cooks' group (Purdue University, West Lafayette, Indiana). First publication, Science 306, 471 (2004).
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