During
the last decade, the application of sensors for the detection and determination
of various substances has gained an increasing popularity not only in the field
of analytical chemistry but also in our daily life. Most sensor systems such as exhaust gas sensors for automobiles are
based on single sensors, which are as selective as possible for the analyte of
interest. The problems of interfering cross-reactive analytes and the lack of
specific sensors for many analytes have ended up in the development of
so-called sensor-arrays. Thereby, several analytes can be simultaneously
quantified by the multivariate data analysis of the signal patterns of several
cross-reactive sensors. Yet, this approach is also limited since the number of
sensors in the array has to exceed the number of cross-reacting analytes.
In
this work, a new approach is presented, which allows multi-analyte
quantifications on the basis of single-sensor systems. Thereby, differences of
interaction kinetics of the analytes and sensor are exploited using
time-resolved measurements and time-resolved data analyses. This time-resolved
evaluation of sensor signals together with suitable sensor materials combines
the sensory principle with the chromatographic principle of separating analytes
in space or time. The main objectives of this work can be subsumed into two
focuses concerning the measurement principle and the data analysis.
The
first focus is the introduction of time-resolved measurements in the field of
chemical sensing. In this work the time-resolved measurements are based on the microporous
polymer Makrolon as sensitive sensor coating, which allows a kinetic separation
of the analytes during the sorption and desorption on the basis of the size of
analytes. Multi-analyte determinations using single sensors are successfully performed
for three different setups and for many multicomponent mixtures of the low
alcohols and the refrigerants R22 and R134a.
The
second focus concerns the multivariate data analysis of the data. It is
demonstrated that a highest possible scanning rate of the time-resolved sensor
responses is desirable resulting in a high number of variables. It is shown
that wide-spread data analysis methods cannot cope with the amount of variables
and with the nonlinear relationship between the sensor responses and the
concentrations of the analytes. Thus, three different algorithms are innovated
and optimized in this study to find a calibration with the highest possible generalization
ability. These algorithms perform a simultaneous calibration and variable
selection exploiting a data set limited in size to a maximum extend. One algorithm
is based on many parallel runs of genetic algorithms combined with neural
networks, one algorithm bases on many parallel runs of growing neural networks
and the third algorithm uses several runs of the growing neural networks in a
loop. All three algorithms show by far better calibrations than all common
methods of multivariate calibration and than simple non-optimized neural
networks for all data sets investigated. Additionally, the variable selection
of these algorithms allows an insight into the relationship between the
time-resolved sensor responses and the concentrations of the analytes. The
variable selections also suggest optimizations in terms of shorter measurements
for several data sets. All three algorithms successfully solve the problems of
too many variables for too few samples and the problems caused by the
nonlinearities present in the data with practically no input needed by the
analyst.
Together,
both main focuses of this work impressively demonstrate how the combination of
an advanced measurement principle and of an intelligent data analysis can
improve the results of measurements at reduced hardware costs. Thereby the
principle of single-sensor setups or few-sensor setups is not only limited to a
size-selective recognition but can be extended to many analyte discriminating
principles such as temperature-resolved measurements leaving room for many
further investigations.