Unlike other analytical methods that necessitate that the surface be sprayed with electrically charged solvents, or that require solvent extraction, DART leaves the sample surface undisturbed.
Analysis of low-volatility, condensed-phase chemicals on surfaces
has been an extremely difficult
and long-standing objective
in environmental monitoring.
When target analytes possess picotorr
vapor pressures, the problem
of monitoring becomes formidable.
As such, any noncontact sampling
at atmospheric pressure that
does not require solvents or wipes
would be a technological breakthrough.
In addition, the absence
of sample preparation would
allow extremely rapid analysis
in time-critical situations.
Today, such a technology is
now available. It is known as
Direct Analysis in Real Time1
or DART™ (JEOL USA, Inc.,
Peabody, MA).
The U.S. Army has been testing
and developing DART since
2002 for the detection of chemical
warfare agents on surfaces.
Fast, safe, and accurate detection
of chemical agents is critical
for protection, security, and
decision-making. Sample preparation
is seldom a requirement with
DART, since the contaminated surface
is simply analyzed directly using
a plume of gaseous Rydberg atoms.
Unlike other analytical methods
that necessitate that the surface be
sprayed with electrically charged
solvents,2 or that require solvent
extraction, DART leaves the sample
surface undisturbed. This is a forensically
worthy advantage. Recent
findings of chemical warfare agent
detection on militarily relevant surfaces
in Homeland Security is a new approach to Warfighter
safety and
counter-terrorism.
Experimental
The DART source
The use of Rydberg atoms as a
replacement for the radioactive
source in chemical agent monitors
was conceived by Dr. James A. Laramée
in 2001. A working prototype
was developed by Drs. Cody and Laramée, which was publicly disclosed on
April 14, 2003 in two patent applications
as the first ambient mass spectrometric
method.3
DART/AccuTOF™ system
The experimental apparatus consisted
of three practically identical
DART/AccuTOF (JEOL USA) systems,
which were used in unison to
give multiple checks. The DART
source consists of a tube divided into
three chambers through which a gas, typically helium, flows. A needle in
the first chamber is electrified to a
few kilovolts to form a plasma that
consists of helium cations, electrons,
and excited-state species, which are
the working reagent in the DART
method. The second and third chambers
contain electrodes to remove
most of the ionic species from the gas
stream. Detailed operating parameters
have been described elsewhere.1
The gas flow and electrode potentials
have a broad range of operating values
that are not critical to the
information fidelity of the experiment.
Many combinations of settings
gave similar sensitivity.
Mass spectra were recorded on
a modified AccuTOF time-of-flight mass spectrometer (model
JMS-100TLC) operating at 6000
mass resolution (FWHM definition).
The sampling orifice was
set to 60 V relative to ground in
order to attract the analyte ions
into the spectrometer. Higher-orifice
voltages were used when
fragmentation of the molecular
cation was desired.
Chemicals
Chemical Agent Standard Reference
Materials (CASARM) were used:
O-ethyl N,N-dimethylphosphoramidocyanidate
(GA), O-isopropyl
methylphosphonofluoridate (GB),
bis-(2-chloroethyl) sulfide (HD), and
O-ethyl S-2-diisopropylaminoethyl
methylphosphonothiolate (VX).
The Chemical Weapons Convention
allows development, production,
acquisition, stockpile, or possession of these chemicals and their transfer,
directly or indirectly, only to treaty
declared facilities and private facilities
with bailment agreements.4
Results and discussion
A meaningful chemical warfare agent
detector must fulfill three criteria in
order to be useful. First, the technology
must be able to detect actual agents
and not just simulants; although simulants
are popular in academic laboratories,
they give meaningless evaluations
of detection capability. Second,
trace-level concentrations must be
detectable regardless of their physical
state. Third, chemical warfare agents
must be detectable regardless of the
nature of the sample surface that holds
them. The DART technology was
tested with these constraints in mind.
Exact mass measurements of the major
peaks in the mass spectrum were
made in order to assign correct and
unambiguous
elemental compositions.
The average mass accuracy of the measurements
was ±0.00004 Da. By way of
contrast, the instrument manufacturer
claims a mass accuracy of ±0.002 Da
and recently reported peaks in agreement
with predicted values of less
than or equal to 0.007 Da with even
larger deviations observed at masses below 200 and above 600 Da.5 This
instrument behavior has not been
observed on any of the authors’ three
DART/AccuTOF systems, which
they attribute
to careful operation
and setup, both of which appear to
be essential for obtaining meaningful
exact mass measurements.
Figure 1 - High-fidelity mass spectra were obtained by DART for 800 ng of VX on aluminum, concrete, and a bird feather. VX appearsas the MH+ molecular cation at m/z 268.150 Da. Other militarily relevant surfaces were examined with equal success.
VX was spiked onto various sample
surfaces collected from an urban
environment. Eight hundred nanograms
of VX (in isopropyl alcohol)
was applied to 20 surfaces and
analyzed.
None of the surfaces tested
were treated or cleaned up in any
way. These included aluminum, roadway
asphalt, automobile tail light,
birch tree bark, cardboard, two types
of concrete (foreign and domestic),
glass, holly leaf, bird feather, waxed
paper cup, paper towel, clothing fabric,
stainless steel, rusted steel, plastic,
tire tread rubber, roofing tile,
and a Viton® (DuPont Performance
Elastomers L.L.C., Wilmington,
DE) O-ring. Difficult sample surfaces
were readily analyzed, such as porous
concrete, electrical conductors, and
biological samples. The protonated
molecular cation at m/z 268.150
appeared within seconds and with little
fragmentation (Figure 1). Only a
small peak at m/z 128.144 corresponding
to the diisopropylaminoethyl
cation fragment (C8H18N) was seen.
This soft ionization is an advantage
of the DART ion source over other
methods for the detection and identification
of chemical warfare agents,
since the mass spectra remain uncluttered
by extraneous peaks. Ion intensities
were typically greater than
several
hundred thousand counts,
with signal-to-noise ratios in excess
of 100,000.