Self referencing LED detection system for spectroscopy applications

Abstract

A light emitting diode (LED) based detection system is employed for spectroscopy based applications. LEDs are used as monochromatic light sources for applications at specific and pre-defined wavelengths. Spectrographic information is generated using LEDs of different wavelengths ranging from 260 nm to 1400 nm. Multiple wavelength information is generated by coupling light from each LED into an intensity and mode mixing fiber bundle. A dual beam approach of using a reference and a sample photodiode ensures automatic drift correction. Interference filters at the LED input fiber reduce the spectral bandwidth of the monochromatic light emission to a useful 10 nm bandwidth by cutting off the LEDs trailing emission distribution allowing for absorbance measurements similar to typical spectrometers.

1 . A method for spectroscopically analyzing a sample in a cell comprising:
 (a) energizing an LED to emit radiation at a selected wavelength;   (b) transmitting the radiation to the cell via a fiber optic element and optically coupling the radiation to the sample;   (c) transmitting radiation from the sample to a sample photodetector via a fiber optic element;   (d) transmitting the radiation from the LED via a fiber optic element to a reference photodetector; and   (e) comparing the radiation detected by said sample photodetector and said reference photodetector.   
   
   
       2 . The method of  claim 1  further comprising filtering said radiation of step (a) to reduce the bandwidth. 
   
   
       3 . The method of  claim 1  wherein filtering said radiation comprises coating said element. 
   
   
       4 . The method of  claim 1  wherein step (a) further comprises emitting a train of pulses with the selected wavelength. 
   
   
       5 . The method of  claim 1  wherein the step of comparing further comprises generating a signal indicative thereof and further comprising using the signal to determine the presence of the sample. 
   
   
       6 . The method of  claim 1  wherein the cell is a liquid wavelength capillary cell. 
   
   
       7 . A method for spectroscopically analyzing a sample in a cell comprising:
 (a) energizing a plurality of LEDs wherein each LED emits radiation at a selected wavelength;   (b) transmitting the radiation to the cell and optically coupling the radiation to the sample;   (c) transmitting the radiation from the sample via a fiber optic element to a sample photodetector;   (d) transmitting the radiation from each LED via a fiber optic element to a reference photodetector; and   (e) comparing the radiation simultaneously detected by said sample photodetector and said reference photodetector.   
   
   
       8 . The method of  claim 1  wherein step (a) comprises sequentially energizing said LEDs. 
   
   
       9 . A method for spectroscopically analyzing samples comprising:
 (a) placing a plurality of samples in a plurality of cells;   (b) energizing a plurality of LEDs to emit radiation at a plurality of selected wavelengths;   (c) transmitting radiation at each selected wavelength via a fiber optic element to each said cell and coupling said radiation to each said sample;   (d) transmitting radiation from each said cell sample via a fiber optic element to corresponding photodetector;   (e) transmitting radiation at each selected wavelength to a reference photodetector; and   (f) comparing radiation detected at each sample photodetector with the radiation detected at said reference photodetector.   
   
   
       10 . The method of  claim 9  wherein step (b) further comprises pulsing said LEDs. 
   
   
       11 . The method of  claim 1  wherein step (c) further comprising mixing the radiation from each LED. 
   
   
       12 . The method of  claim 10  wherein the step of pulsing the LEDs comprises producing a sequence of intervals of no emitted radiation and further comprising comparing radiation detected at said reference detector during emission to radiation detected during a no emission interval to compensate for stray radiation. 
   
   
       13 . An instrument for spectroscopically analyzing a sample comprising:
 a first LED which emits radiation at a first wavelength;   a second LED which emits radiation at a second wavelength;   a first transmitter channel comprising a fiber optic element to transmit radiation from the first LED to a first optical output port;   a second transmitter channel comprising a fiber optic element to transmit radiation from the second LED to a second optical output port;   a reference photodetector;   a first reference channel comprising a fiber optic element to transmit radiation from the first LED to the reference photodetector;   a second reference channel comprising a fiber optic element to transmit radiation from the second LED to the reference photodetector;   a first sample photodetector in optical communication with a first input optical port;   a second sample photodetector in optical communication with a second input optical port; and   a microcontroller in communication with said first LED and second LED and said reference, first sample and second sample photodetectors so that said LEDs are energized and first and second signals indicative of radiation detected by said first and second sample photodetectors and a reference signal indicative of radiation detected by said reference photodetector are generated.   
   
   
       14 . The instrument of  claim 13  further comprising a filter which filters radiation to at least one transmitter channel. 
   
   
       15 . The instrument of  claim 13  further comprising a driver for each LED, said driver being responsive to signals generated by said microcontroller. 
   
   
       16 . The instrument of  claim 16  further comprising a converter in communication with said photodetectors and said microcontroller, wherein the intensities of the radiation emitted by the LEDs are scaled. 
   
   
       17 . An instrument for spectroscopically analyzing a sample comprising:
 a first LED which emits radiation at a first wavelength;   a second LED which emits radiation at a second wavelength;   an optical mixer;   a first transmitter channel comprising a fiber optic element to transmit radiation from the first LED to said optical mixer;   a second transmitter channel comprising a fiber optic element to transmit radiation from the second LED to said optical mixer;   a reference photodetector;   a reference channel comprising a fiber optic element to transmit radiation from said optical mixer to said reference photodetector;   an output channel comprising a fiber optic element to transmit radiation from said optical mixer to an output port;   a sample photodetector in optical communication with an input port;   a second input optical port; and   a microcontroller in communication with said first LED and second LED and said reference and sample photodetectors so that said LEDs are energized to emit a train of radiation pulses and a sample signal indicative of radiation detected by said sample photodetector and a reference signal indicative of radiation detected by said reference photodetector are generated.   
   
   
       18 . The instrument of  claim 17  further comprising a driver for each LED, said driver being responsive to signals generated by said microcontroller. 
   
   
       19 . The instrument of  claim 18  further comprising a converter in communication with said photodetectors and said microcontroller, wherein the intensities of the radiation emitted by the LEDs are scaled. 
   
   
       20 . The instrument of  claim 19  further comprising a plurality of output ports and a plurality of output channels comprising a fiber optic element to transmit radiation from said optical mixer to an output port.