Probe for photoacoustic analysis

A system for photoacoustic detection of the concentration of an analyte in a sample, especially applicable for in situ chemical process monitoring of the sample, comprising:

• a hollow body having forward and rearward ends;
• a modulated light source;
• at least one fiber optic cable having one end operatively coupled to said modulated light source and extended through said body with an opposite end terminating within said body for conveying modulated light from said source through said body and into said sample along an optical axis when said forward end is placed in contact with said sample;
• a cap located at said forward end and having a window aligned with said optical axis to permit passage therethrough of modulated light into said sample;
• a hollow annulus sleeve portion of said cap projecting beyond said window and spaced laterally away from said optical axis;
• a piezoelectric transducer mounted within said sleeve for detecting an acoustic response of an analyte in said sample upon optical excitation thereof by said modulated light when said forward end is in contact therewith;
• and
• means for quantifying said acoustically detected response in order to obtain a value representative of the concentration of the analyte in the sample.

An apparatus for in situ detection of the concentration of an analyte in a sample, comprising:

• a hollow probe having forward and rearward ends;
• means for fiber optically directing modulated light from a modulated light source through said probe, beyond said forward end, and into said sample along an optical axis to optically excite a light absorbing analyte in said sample;
• and
• means for detecting an acoustic response of said optically excited sample, said detecting means extending through said probe beyond said forward end and spaced laterally away from said optical axis.

A method of detecting the concentration of an analyte in a sample comprising the steps of:

• placing a forward end of an elongated probe body into contact with the sample;
• directing modulated light along said body via at least one optical fiber, outwardly of said body at its forward end and into said sample along an optical axis;
• and
• acoustically detecting, via a transducer housed within said probe, a photo-generated response of said sample to said directed modulated light.

A method of preparing a sample having a desired concentration of analyte further comprising the steps of:

• a) placing a forward end of an elongated probe body into contact with a sample having a known concentrate of an analyte;
• b) directing modulated light along said body via at least one optical fiber, outwardly of said body at its forward end and into said sample along an optical axis;
• c) acoustically detecting, via a transducer housed within said probe, a photo-generated response of said sample to said directed modulated light;
• d) computing a reference normalization value representative of the relationship between said detected signal and said known sample concentration;
• e) directing a portion of said modulated light from said source into contact with a photodiode;
• f) generating at said photodiode an electrical signal representative of power and frequency characteristics of said modulated light;
• g) inputting said generated signal into said signal processor for comparison with said electrical signal generated in response to said acoustical detection, thereby to obtain, by comparison of said input signals, a normalization value representative of the relationship between said concentration of said analyte in said sample and said detected response;
• h) repeating steps a) through g) for additional samples having different, known analyte concentrations, thereby to obtain additional reference normalization values to establish a reference curve corresponding to the relationship between detected normalized values and actual concentration for a range of concentrations;
• and
• i) mixing the same analyte into another volume of the same sample, while using said probe and said photodiode to obtain a normalized value for said another volume, until said normalized value equals a value on said reference curve that corresponds to a desired concentration.

A method of preparing a second volume of a sample with an analyte equal in concentration to a first volume of the same sample with the same analyte, comprising the steps of:

• a) placing a forward end of a first elongated photoacoustic probe into the first volume of the sample with an analyte therein;
• b) optically exciting said sample via fiberoptic excitation means located at said forward end;
• c) detecting an acoustic response to said excitation via detection means located at said forward end;
• d) generating an electrical signal corresponding to said detected acoustic response;
• e) simultaneously performing steps a) through d) with a second photoacoustic probe on a second volume of the same sample having the same analyte;
• and
• f) adding more analyte to one of said volumes until the difference in value between said generated electric signals becomes zero.