Analytical Chemistry Chemical Cytometry Quantitates Superoxide

Figure 7. The measured sorption time, τmeas, is proportional to the
inverse saturation vapor pressure. The nonzero y intercept of 13.8 s
is the flow cell fill time, which when subtracted from the measured
sorption time gives the true sorption time, τd. The sorption time of
undecane is nearly 2 orders of magnitude larger than that of toluene
despite their equilibrium response curves being nearly identical as in
Figure 2.
analyte activity as demonstrated in Figure 6. A plot of this
average time versus the inverse analyte saturation vapor pressure
results in a straight line with a nonzero y intercept, as seen in
Figure 7. Of course, at infinite saturation vapor pressure the
sorption time τd must be zero since the diffusive flux is infinite.
Therefore, this finite intercept of 13.8 s corresponds to the fill
time of the flow cell and can be used to produce the corrected
sorption times, τd, in Figure 7. This measured flow cell fill time
is in good agreement with the theoretical prediction, using Vf/F.
Figure 7 shows a nearly perfect proportionality between the
sorption time, τd, and the analyte saturation vapor pressure, P*.
Equation 7b shows that other analyte parameters, such as the
diffusivity and the � parameter, also determine this sorption time,
but apparently these combined factors are more-or-less constant
for the analytes we tested. Also, the saturation vapor pressure
varies strongly from analyte to analyte. If finer resolution was
desired for differentiation between similarly volatile analytes, these
other analyte parameters could be taken into account. The
homologous series of aromatic analytes, all of which have nearly
identical Flory parameters, and thus identical equilibrium chemiresistor
responses as a function of activity, are now easily distinguished.
For toluene, p-xylene, and mesitylene the sorption times
are 3.7, 10.0, and 37.0 s, respectively, which are in good agreement
with their relative reciprocal saturation vapor pressures, 3.45, 11.4,
and 39.2 (×10 -2 Torr -1 ). These sorption time differences are
large compared to the errors associated with our measurements,
even though the sorption time of toluene is much faster
than the fill time of 13.8 s. The measurement of faster sorption
times is possible, but would require either higher flow rates
and a smaller flow cell volume, so that the fill time is faster, or
thicker sensors, to increase the sorption time. Of course,
increased sorption time comes at the cost of increased sensor
response time, so this would only be desireable if high-volatility
analytes are to be measured.
DISCUSSION
In the pulsed flow experiments the determination of the
sorption time is straightforward, but would require a test unit with
an engineered flow system. This mechanical requirement is
antithetical to the inexpensive and simple chemiresistor concept.
6974 Analytical Chemistry, Vol. 82, No. 16, August 15, 2010
Other approaches are possible, but are yet untested in the context
of vapor sorption, though analogous approaches have been used
to determine photoluminescence lifetimes. In fact, luminescence
is a useful language in which to describe the other possibilities.
First, a luminescence lifetime can be measured by suddenly
subjecting a material such as a phosphor to steady illumination.
The rise time of the photoemission is the luminescent lifetime.
This approach is analogous to the sorption kinetics reported above.
If instead the steady illumination is suddenly shut off, the emission
decay gives the lifetime. This is analogous to analyzing the
desorption kinetics shown in Figure 1. A third approach is to excite
a phosphor with amplitude-modulated light. The photoemission
will also be amplitude modulated, but shifted in phase, and the
lifetime can be determined from the phase shift and the modulation
frequency. This is analogous to modulating the activity of
the analyte, which is not really practical. The fourth approach is
to excite a phosphor with light that randomly fluctuates in
intensity. The photoemission then also fluctuates in intensity, but
these fluctuations are time correlated. The associated correlation
time can be determined from the photoemission time autocorrelation
function, which is easily computed with a simple shift
register device. This fourth method is applicable to a sensor in
an environment where there is nonstationary air movement that
causes fluctuations in the delivered analyte activity. An engineered
mechanical system needed to deliver a flow pulse can potentially
be supplanted by an electronic chip that computes the correlation
time of the sensor response.
Of course, we cannot expect the activity fluctuations delivered
to the sensor to be purely random. Instead, these activity
fluctuations will themselves have a correlation time (which is an
interesting issue in and of itself). Therefore, the measured
response fluctuations will in general be a convolution of both the
activity fluctuations and the sorption kinetics. Extracting the true
sorption kinetics will require two sensors that have widely
disparate response kinetics (e.g., a thick and a thin polymer film).
The thin sensor will respond rapidly, relative to the correlation
time of the activity fluctuations, to changes in analyte activity. This
sensor will thus measure the correlation time of the activity
fluctuations. The thick sensor will have a sorption time long
compared to this correlation time, and its response will be largely
determined by the sorption kinetics. The thin sensor can be used
to correct this measured sorption time to obtain the true sorption
time. Therefore, by standard time correlation techniques, discrimination
based on sorption kinetics can be developed without
an engineered flow system. This approach will be the subject of
our future research.
CONCLUSIONS
We have used a field-structured chemiresistor to demonstrate
that response kinetics can be used to discriminate between
analytes, even between those that have identical chemical affinities
for the polymer phase of the sensor. To do this, we have used
the analyte-independent transduction curve (conductance versus
polymer swelling) to transform the time-dependent sensor conductance
into a time-dependent polymer swelling. From these
swelling data we can determine the measured sorption time, which
is a combination of the true sorption kinetics and the fill time of
the flow cell. The true sorption kinetics is obtained by correcting