A method of sensing presence or absence of a specified gas molecule in a target gas, the method comprising:
- providing, in a chamber, a target gas that may contain up to H candidate gas molecules present with unknown concentrations ch, numbered h=1, . . . , H (H≧1), first and second interdigitated electrodes, connected to at least one of (i) a voltage source having a controllable voltage difference and (ii) a controllable current source, and a path Q that includes a plurality of carbon nanotubes (CNTs) and that extends between the first and second interdigitated electrodes, where the path Q is loaded with at least one loading chemical and is exposed to the target gas;
- providing, within the chamber, an environment having a sequence of values EPp (p=1, . . . , P;P≧1) of at least one environmental parameter, chosen from among temperature, relative humidity, pH and gauge pressure;
- measuring, between the first and second interdigitated electrodes, in a presence of the target gas, an electrical parameter value (“response value”), denoted V(EPp;{ch};meas), of at least one of electrical current, electrical conductance, voltage difference and electrical resistance, where the target gas experiences an environment having at least one of the environmental parameter values EPp;
- providing reference response values V({EPp};{ch(ref)};ref) for a sequence of reference values of known concentrations, {ch(ref)}={ch=1(ref), . . . , ch=H(ref)}, for the path Q exposed to an environment having the environmental parameter value or values {EPp};
- approximating the sequence of electrical parameter values V({EPp};
- {ch(ref)};ref) as linear combinations,
- ≈V(EPp;{ch};approx)=a0+a1c1+. . . +aHcH, ≈V(EPp;{ch(ref)};ref)
- of the concentration values ch of the candidate gas molecules, where a0, . . . , aH are known response coefficients that may depend upon one or more values EPp of at least one of the environmental parameters that the candidate gas molecules experience, and a0 represents a response coefficient when none of the candidate gas molecules is present;
- estimating an error value ε({ch}) for the candidate gas molecule concentrations ch, the error value being expressed as
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- where wp is a non-negative weight value that may vary with estimated relative importance of the environmental parameter values EPp and q is a positive number;
- determining or providing concentration values ch=ch(min) (h=1, . . . , H) in the approximate response values V({EPp};
- {ch};approx) that provide a minimum error value, ε({ch};min)=ε({ch=ch(min)}), for the error value ε({ch});
- comparing the minimum error value ε({ch};min) with a positive threshold error value ε(min.thr);
- when ε({ch};min)≦ε(min;thr), interpreting this condition as indicating that at least one of the H candidate gas molecules, designated as h=h0, is present in the target gas, and that the concentration value of at least one candidate gas molecule that is present in the target gas is substantially equal to ch=h0(min);
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- when ε(ch;min)>ε(min;thr) for all h=1, . . . , H, interpreting this condition as indicating that none of the H candidate gas molecules is likely to be present in the target gas.