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