|
Many of today's analytical practitioners, their primary training notwithstanding, separate compounds using liquid chromatography–mass
spectrometry (LC–MS). In so doing, they depend upon solvents to transport their samples into a mass spectrometer's source,
where the samples are (usually) ionized from solution. Yet, the solvent that is so essential to this process also presents
a downside. Besides limiting by dilution the absolute amount of sample provided for analysis, the solvent plays a significant
role in mediating the kind of ions you can see and the kinds of materials the instrument can ionize. So, understandably, mass
spectrometrists have long sought a way to rid themselves of the solvent and its effects so that they could ionize more species.
One attempt at doing this in the gas phase was atmospheric pressure chemical ionization (APCI), invented in the 1970s by Horning's
group (1). But in the last 25 years, APCI, like other alternatives to electrospray ionization (ESI), seemingly has been left
in the wake of ESI's overwhelming popularity.
At CoSMoS, in July 2006, Chuck McEwen, a scientist at DuPont's Corporate Center for Analytical Sciences Wilmington, Delaware,
offered insight into his work, insight that proves quite interesting.
Michael P. Balogh
|
The diversity of analytical requirements he faces in his laboratory impelled McEwen to rethink the APCI approach to mass analysis.
He calls one recently published outcome (2) the atmospheric pressure solids analysis probe, or ASAP, method. The acronym "ASAP"
is appropriate, because ASAP is a rapid method of analyzing volatile and semivolatile compounds in solids, liquids, polymers,
and biological tissue, which gives it a fairly wide utility. A second, almost obvious approach aimed at overcoming the solvent
problem directs a gas chromatography (GC) outflow into an unmodified LC–MS (ESI) instrument (3).
Today's Emerging AlternativesIn recent years, several direct ionization methods have emerged that this column has addressed (4). Among them is desorptive
ESI, or DESI, developed in Graham Cooks' laboratory at Purdue University (West Lafayette, Indiana) (5). In its simplest sense,
DESI is an electrospray stream of charged droplets devoid of analyte that, at atmospheric conditions, is directed onto the
surface to be analyzed. The droplets hit and ionize both volatile and nonvolatile materials that generally are analyzable
by electrospray.
Figure 1
|
A recent extension of DESI technology from the same group is called desorptive APCI (DAPCI). In DAPCI, a charge is imposed
on a needle, not directly on the liquid, as done in ESI. Liquid is still the vehicle, and charged droplets hit the sample
surface where they ionize volatile materials. The compounds amenable to this technique are the same as those you would likely
see in APCI.
Direct ionization real-time (commercialized as DART, by JEOL, Peabody, Massachusetts) has been promoted as an alternative
to DESI (6). In this approach, metastable neutrals carried in a heated gas stream are thought to perform the ionization. McEwen
said he does not see much difference between the published results of the direct ionization real-time method and his own ASAP.
However, if you buy the direct ionization realtime method, you also must buy the company's source, whereas ASAP requires no
additional source.
So how simple is ASAP, you might ask. Actually, it is pretty simple. The method requires access to the source, for instance
using the instrument's photoionization port. Through a hole, you insert a probe holding a melting point (mp) capillary tube,
the sample on its closed-end. Thus, you introduce the sample into the source and also into the heated gas flow from an APCI
probe (or electrospray probe if your instrument uses heated nebulizing gas).
![[Get Copyright Permissions]](http://license.icopyright.net/images/icopy-w.gif){kind=small}
