Spectroscopic method for quantitatively determining the change of concentration of a light or other radiation absorbing compound in a medium which is interrogated through an intervening medium

A method of determining a change in concentration of at least one subject substance in one region of a plurality of regions of a medium, the one region being adaptable to both absorb and scatter radiation, substantially all of the change in concentration of the at least one subject substance in the medium occurring in the one region of the plurality of regions, said method comprising:

• introducing into and propagating through the medium first interrogating radiation which is penetrative of the medium;
• detecting the first interrogating radiation which has propagated through the medium and determining a total differential pathlength traversed by the first interrogating radiation in the medium;
• identifying at least one reference substance, the at least one reference substance possessing a sufficiently distinct specific absorption coefficient to the first interrogating radiation and a known concentration in each region of the plurality of regions of the medium;
• introducing into and propagating through the medium second interrogating radiation which is penetrative of the medium, detecting and spectrometrically analyzing the second interrogating radiation which has propagated through the medium, and determining an intensity of a spectral feature of the at least one reference substance;
• introducing into and propagating through the medium third interrogating radiation which is penetrative of the medium;
• detecting and spectrometrically analyzing the third interrogating radiation which has propagated through the medium and determining a change in an intensity of a spectral feature of the at least one subject substance;
• determining a partial differential pathlength of the one region by simultaneously solving first and second equations, the first and second equations respectively being expressed as follows:
• ##EQU6## where a.sub.ref is the intensity of the spectral feature of the at least one reference substance;
• C.sub.j is the known concentration of the at least one reference substance in region j of the plurality of regions;
• .di-elect cons..sub.ref is the absorption coefficient of the at least one reference substance;
• Z.sub.j is a partial differential pathlength in the region j;
• and
• Z.sub.T is the total differential pathlength;
• n is a total number of regions in the plurality of regions;
• and
• obtaining the change in concentration of the at least one subject substance in the one region of the plurality of regions of the medium by solving a third equation, the third equation being expressed as follows:
• C=A/(.di-elect cons..multidot.Z)
• where
• A is the change in the intensity of the spectral feature of the at least one subject substance;
• C is the change in concentration of the at least one subject substance in the one region of the plurality of regions of the medium;
• .di-elect cons. is an absorption coefficient of the at least subject substance;
• and
• Z is the partial differential pathlength of the one region.

An apparatus for determining a change in concentration of at least one subject substance in one region of a plurality of regions of a medium, the one region being adaptable to both absorb and scatter radiation, substantially all of the change in concentration of the at least one subject substance in the medium occurring in the one region of the plurality of regions, the plurality of regions each comprising at least one reference substance possessing a distinct specific absorption coefficient to a specific radiation and a known concentration, said apparatus comprising:

• a first light source for introducing into and propagating through the medium first interrogating radiation which is penetrative of the medium;
• a first photodetecting unit for detecting the first interrogating radiation which has propagated through the medium to measure an intensity of a spectral feature of the at least one subject substance or the at least one reference substance;
• a second light source for introducing into and propagating through the medium second interrogating radiation which is penetrative of the medium;
• a second photodetecting unit for detecting the second interrogating radiation which has propagated through the medium to measure a total differential pathlength traversed by the second interrogating radiation in the medium;
• a first processing unit for receiving the intensity of the spectral feature of the at least one subject substance or at least one reference substance from said first photodetecting unit, for receiving the total differential pathlength from said second photodetecting unit, and for determining the partial differential pathlength of the one region by solving first and second equations, the first and second equations being expressed as follows:
• ##EQU7## where a.sub.ref is said intensity of said spectral feature of the at least one reference substance;
• C.sub.j is the known concentration of the at least one reference substance in region j of the plurality of regions;
• .di-elect cons..sub.ref is the absorption coefficient of said at least one reference substance;
• Z.sub.j is a partial differential pathlength in the region j;
• and
• Z.sub.T is the total differential pathlength;
• n is a total number of regions in the plurality of regions;
• and
• a second processing unit for determining the change in concentration of the at least one substance in the one region of the plurality of regions of the medium by a third equation, the third equation being expressed as follows:
• C=A/(.di-elect cons..multidot.Z)
• where
• A is the change in the intensity of the spectral feature of the at least one subject substance;
• C is the change in concentration of the at least one subject substance in the one region of the plurality of regions of the medium;
• .di-elect cons. is an absorption coefficient of the at least subject substance;
• and
• Z is the partial differential pathlength of the one region.