Method for detecting substance in liquid and sensor for detecting substance in liquid

A method for detecting substance in a liquid that uses a sensor for detecting a substance in a liquid that includes a sensing oscillator circuit including a sensing surface acoustic wave element with a sensing portion having an interdigital transducer and a reaction film arranged so as to cover the interdigital transducer and to react with a substance in a liquid, and a first amplifier element connected to the sensing surface acoustic wave element, a reference oscillator circuit including a reference surface acoustic wave element having an interdigital transducer but no reaction film that covers the interdigital transducer, and a second amplifier element connected to the reference surface acoustic wave element, and a recording unit that records an oscillator circuit output of the sensing oscillator circuit and an oscillator circuit output of the reference oscillator circuit, the method comprising:

• providing a number m of the sensing oscillator circuits, where m is an integer greater than or equal to 2, and a number n of the reference oscillator circuits, where n is an integer greater than or equal to 2;
• a first step of recording output frequencies of the m sensing oscillator circuits and output frequencies of the n reference oscillator circuits;
• a second step of measuring a frequency drift of each of the output frequencies of the m sensing oscillator circuits and each of the output frequencies of the n reference oscillator circuits;
• a third step of determining that, when the frequency drift measured in the second step falls within a predetermined range, the corresponding sensing oscillator circuit or the corresponding reference oscillator circuit is acceptable and, when the frequency drift falls outside the predetermined range, the corresponding sensing oscillator circuit or the corresponding reference oscillator circuit is unacceptable;
• a fourth step of selecting at least one of the sensing oscillator circuits having a frequency drift that is approximately the same as a desired frequency drift from among the sensing oscillator circuits determined to be acceptable in the third step and at least one of the reference oscillator circuits having a frequency drift that is approximately the same as a desired frequency drift from among the reference oscillator circuits determined to be acceptable in the third step;
• and
• a fifth step of bringing a liquid that includes a detection target substance into contact with the sensor for detecting a substance in the liquid and comparing an output frequency of the at least one of the sensing oscillator circuits selected the fourth step with an output frequency of the at least one of the reference oscillator circuits selected in the fourth step to thereby detect the detection target substance.