Abstract:
This invention discloses an Excitation Emission Matrix (EEM) fluorescence spectrometer system that uses an LED array to cause excitation emission matrix fluorescence that is imaged onto a spectrograph for sample identification and analysis. By using the LED array, the spectrograph requires only high spectroscopic resolution of about 1 to 5 nm in the fluorescence emission range and low excitation light resolution of about 14 to 73 nm in the excitation range. While individual LED optical excitation spectra may contain wavelength regions that overlap, as long as the various LEDs have different excitation wavelengths and intensities, spectral overlap does not preclude data analysis. The invention provides an EEM spectroscopy system that is not a high resolution such as a laser or lamp based excitation system, but instead uses a lower resolution, lower power LED system. The invention also provides results that are comparable to existing systems with lower component cost and lower power requirements while also optically stable, small, and easy to use.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention is concerned with single measurement Excitation Emission Matrix (EEM) spectroscopy using an array of light emitting diodes (LED). The array of LEDs is focused into a sample cuvette, creating spacially separated excitation spots.  
           [0003]    2. Description of the Related Prior Art  
           [0004]    Fluorescence excitation emission matrix (EEM) spectroscopy has long been known as a powerful method for complex mixture analysis. (See Hershberger, L. W.; Callis, J. B.; Christian, G. D.; Anal. Chem., 1981, 53, 971-975; Skoropinski, B.; Callis, J. B.; Danielson, J. D. S.; Christian G. D.; 1986, Anal. Chem., 58, 2831-2839; and Jalkian, R.; Denton, B.; Proc. SPIE 1989, 1054, 91-102.)  
           [0005]    The ability to easily collect full emission spectra for several excitation wavelengths, has generally required expensive and complex instrumentation. The multi-way characteristics of EEM data enable the extraction of the salient chemical features. As a consequence, mathematical resolution of analytes is possible, even in the presence of unknown interferences, which is known as the second-order advantage. (See Booksh, K. S.; Kowalski, B. R.; Anal. Chem., 1994, 66, A782-A791.) The full range of molecules and dyes which fluoresce under long wavelength UV (370 nm) through near infra-red (NIR) wavelength (980 nm) excitation can be used with this inexpensive and simple to construct EEM system. The potential applications of upper UV, visible, and NIR fluorescence include fluorescence of dyes, larger PAHs (anthracene, chrysene, benzo[a]pyrene, perylene, etc.), humic materials, (See Baker, A.; Environ. Sci. Tech., 2002, 36, 7, 1377-1382; and Del Castillo, C. E.; Coble, P. G.; Morell, J. M.; Lopez, J. M.; Corredor, J. E.; Mar. Chem., 1999, 66, 35-51.) chlorophylls from plants (See Moberg, L.; Robertsson, G.; Karlberg, B.; Talanta, 2001, 54, 161-170.) and algae (See Henrion, R.; Henrion, G.; Bohme, M.; Behrendt, H.; Fresen. J. Anal. Chem., 1997, 357, 522-526.) and NIR fluorescence from bacteriochlorophyll (See Albrecht-Buehler, G.; Exp. Cell Res., 1996, 236, 43-50) in certain bacteria. Others applications include environmental dye tracers (See. Smart, P. L.; Laidlaw, I. M. S.; Water Resour. Res., 1977, 13 (1), 15-33; and Lyons, R. G.; J. Hydrol., 1993, 152 (1-4), 13-29) and porphyrin fluorescence. (See Ricchelli. F.; Gobbo, S.; J. Photochem. Photobiol. B, 1995, 29 (1), 65-70.) Fluorescence of porphyrins and derivatives can be used for detection of certain cancers using patient sera analysis (See Aiken, J. H.; Huie, C. W.; Terzian, J. A.; Anal. Lett., 1994, 27 (3), 511-521.), and as markers for heavy metal poisoning through urine analysis. (See Bowers, M. A.; Aicher, L. D.; Davis, H. A.; Woods, J. S.; J. Lab. Clin. Med., 1992, 120 (2), 272-281; and Ng, J. C.; Qi, L. X.; Moore, M. R.; Cell. Mol. Biol., 2002, 48 (1), 111-123.)  
           [0006]    The application of LED array excitation is well suited to fluorescence in situ hybridization (FISH) of cell and bacterial suspensions. The true advantage, in this arena, with the LED-EEM system is for multiplexed fluorescence in-situ hybridization (M-FISH) (See Henegariu, O.; Bray-Ward, P.; Ward, D. C.; Nat. Biotechnol., 2000, 18 (3), 345-348.), where many dyes (5-10) are used to simultaneously detect various cell or bacterial types using specific oligo-nucleotides. Furthermore, the possibility of multiplexing FISH-based genome analysis is very attractive. (See Fauth, C.; Speicher, M. R.; Cytogenet. Cell Genet., 2001, 93 (1-2), 1-10; Speel, E. J. M.; Histochem. Cell Biol., 1999, 112 (2), 89-113; and Szuhai, K.; Bezrookove, V.; Wiegant, J.; Vrolijk, J.; Dirks, R. W. Rosenberg, C.; Raap, A. K.; Tanke, H. J.; Gene. Chromosom. Canc., 2000, 28 (1), 92-97.) The availability of full emission spectra at multiple excitation wavelengths will allow better characterization of FISH dyes. This can be achieved using readily available fluorophores, without resorting to expensive designer dyes which absorb maximally at a common wavelength (typically 488 nm—argon ion laser) and emit in well separated regions for simplified detection. Future advances in LED technology may result in lower UV wavelengths being made available, thus extending the range of applications.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of this invention is to provide an Excitation Emission Matrix (EEM) fluorescence spectrometer system for resolving and identifying a wide variety of components and closely related chemical species.  
           [0008]    Another object of this invention is to use a light emitting diode (LED) light source array in order to provide the emission matrix fluorescence excititation needed for EEM spectroscopy.  
           [0009]    Another object of this invention is to provide an Excitation Emission Matrix (EEM) fluorescence spectrometer system which requires only high spectroscopic resolution of about 1 to 5 nm in the fluorescence emission range and low resolution of about 14 to 73 nm in the excitation range.  
           [0010]    Another object of this invention is to provide an EEM fluorescence spectrometer system in which the LEDs have different excitation wavelengths as well as intensities and spectral overlap does not preclude data analysis.  
           [0011]    Another object of this invention is to provide low spectroscopic resolution optical excitation elements (LED) used for the analysis of complex chemical samples when several are combined and used in concert for excitation emission matrix spectroscopy.  
           [0012]    Another object of the invention is to provide a combination of partially selective instrumental elements (LEDs) whose results are comparable to other EEM systems.  
           [0013]    Another object of the invention is to provide an LED array EEM spectroscopic system whose results are comparable to existing systems yet with lower component cost and lower power requirements while also optically stable, small, and easy to use.  
           [0014]    Another object of the invention is to provide a EEM spectroscopy system that does not require a high resolution excitation system such as a laser or lamp, but instead uses a lower resolution, lower power LED system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block diagram showing the system of the invention  
         [0016]    [0016]FIG. 2 is a block diagram showing the mounting device for holding and aligning the LED emission source array  
         [0017]    [0017]FIG. 3 is a diagram of an implementation of an instrument for LED EEM spectroscopy which using optical fiber  
         [0018]    [0018]FIG. 4 is the LED output intensities as a function of wavelength, a) 370 nm, b) 430 nm, c) 470 nm, d) 505 nm, e) 530 nm, f) 590 nm, and g) 636 nm  
         [0019]    [0019]FIG. 5 is Fluorescence spectra of 100 ppb fluorescein in ethanol collected simultaneously at LED wavelengths a) 370 nm, b) 430 nm, c) 470 nm, d) 505 nm, e) 530 nm, f) 590 nm, and g) 636 nm  
         [0020]    [0020]FIG. 6 a  is a PARAFAC model of fluorescein in ethanol using 3 factors: for resolved excitation spectra. I=fluorescein monoanion factor, II=fluorescein dianion factor, and III=instrumental background factor  
         [0021]    [0021]FIG. 6 b  is a PARAFAC model of fluorescein in ethanol using 3 factors: for resolved emission spectra. I=fluorescein monoanion factor, II=fluorescein dianion factor, and III=instrumental background factor 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    This invention represents a new instrument for single measurement EEM spectroscopy which is based upon an array of light emitting diodes (LED). The array of LEDs is focused into a sample cuvette, creating spatially separated excitation spots. LED-induced fluorescence from analytes in solution is collected at right angles and delivered to the entrance of a spectrograph with a CCD camera for detection. The broad emission spectrum of LEDs permits continuous coverage over a large excitation range with a limited number of LEDs, allowing excitation of all analytes with absorption within the LED&#39;s excitation range. Each analyte may be uniquely excited by each LED, thereby conserving the multi-way characteristics of the data common to all EEM methods. In this regard, LED-EEM spectroscopy can be viewed from a sensors standpoint: multiple partially selective instrumental elements. These individual components, when combined, provide a more complete spectroscopic picture of the chemical puzzle. The success of LED-EEM spectroscopy extends the possibilities: it is not always necessary to have high excitation wavelength resolution. In fact the instrument performs well even with overlap of the excitation wavelength regions. Mathematically, unique excitation information (differing amounts of emission at each excitation channel) are generated for even heavily overlapped excitation ranges. This instrument was designed for the single measurement collection of multi-dimensional fluorescence data. Multi-dimensional fluorescence excitation emission matrix (EEM) data, emission spectra measured at multiple excitation wavelengths, provide the analyst with a wealth of analytical information. The additional information makes possible the qualitative and quantitative analysis of complex mixtures. Furthermore, using LEDs permits the construction of a simplified instrument, without significant compromises to the analytical information required for analysis of complex samples. A variety of samples can be measured using the LED wavelengths currently available: upper UV through visible and NIR: 350 nm to 980 nm.  
         [0023]    Theory  
         [0024]    In traditional EEM spectroscopy, the excitation resolution is often quite high; there is no redundancy in the excitation wavelengths from channel to channel. In contrast, excitation wavelength regions can overlap when using LEDs, due to their broad wavelength ranges. This is especially true when trying to maximize excitation spectral coverage. The theoretical basis for LED-EEM fluorescence spectroscopy is based upon the variation in both the LED optical output function (generally an approximately Gaussian intensity distribution) and the molar absorptivity (ε) over the excitation wavelengths of the LED. The fluorescence intensities at any given emission wavelength (FL) due to LED A and LED B for a single analyte are given by Equations 4 and 5,  
               FL   LEDA     ∝       ∑     λ   =   1     Λ            I     LEDA   λ            ɛ   λ                 (   4   )                 FL   LEDB     ∝       ∑     λ   =   1     Λ            I     LEDB   λ            ɛ   λ                 (   5   )                               
 
         [0025]    where ε λ I LEDAλ , and I LEDBλ  represent the molar absorptivities and LED intensities at the LED excitation wavelengths, λ=1 to Λ, respectively. The wavelength range, 1 to Λ is given by the wavelength distributions of LED A and LED B. The fluorescence emission spectra recorded on two LED channels A and B will be linearly independent so long as the molar absorptivities are different at the LED excitation wavelengths with regard to experimental noise. The excitation overlap in LED-EEM spectroscopy need only be reduced as required to resolve analytes of interest. As we will show, even closely related analytes are distinguishable using a general purpose LED array which included some spectrally overlapped excitation wavelengths. A more specific selection of LEDs can be used to better resolve a particular sample. This is analogous to moving or selecting a different grating in an excitation spectrograph.  
         [0026]    Application  
         [0027]    The novel light source used in this instrument will allow a cost-effective EEM instrument to be produced. The cost of the traditional excitation source used in single measurement EEM spectroscopy, a lamp and spectrograph, is prohibitive: generally more than $10,000. The LED excitation array is very cost effective and so an instrument can now easily and inexpensively be developed and marketed to utilize the multi-way advantages offered by EEM spectroscopy. Future advances in LED technology may result in lower UV wavelengths being made available, thus extending the range of applications.  
         [0028]    [0028]FIG. 1 is a block diagram of the LED array instrument as used for fluorescence excitation emission matrix (EEM) spectroscopy. This diagram also includes the light pathway through the system culminating in a focused spot at the spectrograph entrance slit ( 165 ). In reference to FIG. 1, the LED array ( 120 ) consists of seven individual LEDs of varying peak output wavelengths: 370 nm, 430 nm, 470 nm, 505 nm, 525 nm, 590 nm and 636 nm. The LEDs are arranged linearly and wired to be illuminated by a DC voltage between 1.9 V and 3.8 V. A summary of the LED specifications used to construct the array is given in Table 1. The array was attached to a linear translating stage ( 105 ) using two optical posts ( 107 ). This allowed precise positioning of the array image, within the cuvette ( 150 ), and thus on the imaging spectrograph ( 170 ) (257i, Oriel, Stratford, Conn.) entrance plane ( 165 ) and charge coupled device (CCD) ( 180 ) (Instaspec V, Oriel, Stratford, Conn.). Although many types of commercial spectrographs and CCD cameras could be used to demonstrate LED EEM spectroscopy, these models were chosen for their performance characteristics.  
                                                                   TABLE 1                           LED Array Electrical and Optical Specifications            Nominal   Actual   Bandpass   Viewing               LED λ   LED λ   FWHM a     Angle   Forward   Optical       (nm)   (nm)   (nm)   (degrees)   Voltage (V)   Power (μW) b                      370   379.4   15   10   3.83   759       430   434   73   15   3.55   223       470   473.6   22   15   2.84   78       505   514.5   47   15   2.97   108       530   533.2   32   15   2.40   56       590   593   14   8   1.95   236       636   633.5   17   8   1.85   533                                  
 
         [0029]    Light emitted from the LED array ( 120 ) is passed through adjustable iris ( 130 ) set at a 40 mm opening to limit off axis rays and stray light. The LED array image is focused into the liquid sample using a 50 mm diameter, 50 mm focal length lens ( 140 ) placed 430 mm from the LED array. This distance results in an appropriate demagnification of the LED array image. The resulting reduction in the size allowed the image to enter the quartz sample cuvette (1 cm×1 cm×5 cm) ( 150 ) and generate fluorescence from an appropriate solution. The sample is placed in a quartz fluorescence cuvette ( 150 ), which is held in a metal cuvette holder. LED light entering the cuvette ( 150 ) may generate fluorescence at each LED spot according to which sample is being interrogated. A 25 mm diameter, 50 mm focal length lens ( 160 ) is used to collect fluorescence at right angles to the LED excitation spots and image the fluorescence onto the entrance image plane of the spectrograph. The fluorescence collection lens ( 160 ) was 190 mm from the sample cuvette. This distance also resulted in a reduction in the size of the LED array image which was imaged through the spectrograph slit ( 165 ).  
         [0030]    In FIG. 1, the detection system  170  and  180  consisted of a ¼ m imaging spectrograph  170  fitted with a removable 50 μm fixed slit and a  300  line/mm grating blazed at 500 nm resulting in a 1 nm emission bandpass. Mounted at the exit pane was a 1024×256 pixel CCD camera ( 180 ) cooled to −55° C. The integration time used throughout this work was 60 s, unless otherwise noted, and the dark current was subtracted in real time by the acquisition software. The CCD was binned by 10 pixels in the vertical direction, defining each excitation channel. Binning was chosen by observing the LED scattering spots measured by the CCD and selecting the ten best pixels to collect data from each excitation spot. Seven 10 pixel sections, corresponding to each LED, of the CCD image were hardware binned in the vertical dimension producing 7×1024 LED-EEMs.  
         [0031]    [0031]FIG. 2 depicts the mounting system for the LED array. In reference to FIG. 2, the 5 mm diameter LEDs were mounted and epoxied into holes ( 230 ) which had been drilled out of an aluminum plate ( 210 ) to accommodate each LED. The plate ( 210 ) had additional holes ( 220 ) to mount the plate to a vertical translator using optical posts ( 107 ). In FIG. 2, a side view ( 240 ) of the mounting plate ( 110 ) is shown. The seven LEDs ( 250 ) are spaced evenly at approximately 7 mm center to center, which resulted in a 57 mm long linear array, from the top LED to the bottom LED. The LEDs were not all driven at the optimum voltages; the system was optimized with respect to the blue excitation spectrum due to the lower excitation intensities of those LEDs. The incident excitation powers, at the sample, were between 0.4% and 5% of the total optical power measured at the LED. These losses were worse in off-axis positions due to aberrations and vignetting. The excitation bandpass of the LED system is unconventional and must be considered on a per channel basis. For our system the minimum bandpass was 14 nm and the maximum was 73 nm. The wavelength output distribution of the LED defines the bandpass for that channel.  
         [0032]    The general concept of employing broad wavelength LED excitation sources resulting in low resolution excitation dimension EEM data could be implemented in a variety of fashions in addition to the lens based version described above. An alternative implementation of the invention that uses optical fiber rather than a lens to reduce the size of the array image is shown in FIG. 3. In this implementation, the optical outputs ( 320 ) of the LED ( 300 ) in the mounting plate ( 310 ) are collimated and then refocused into individual optical fibers ( 330 ). The collimation and refocusing are accomplished using simple two lens systems ( 322 ) comprised of a collimating ( 324 ) and focusing lens ( 326 ) for each LED. The optical fibers are then brought together at separation distances (≈0.5 mm) sufficient to prevent distal cone overlap in the solution ( 340 ). A second array of fibers ( 350 ) is used to collect fluorescence emission and deliver it to the entrance plane ( 360 ) of the spectrometer.  
         [0033]    In order to understand the performance of the LED array system, the optical output of each LED must be known in terms of both total power (Table 1) and wavelength distribution. The wavelength distribution will define the range and types of molecules which can be measured with a selected set of LEDs. Furthermore, the LEDs must be chosen so that each wavelength range contains at least some unique excitation wavelengths. The broad emission spectrum of LEDs permits continuous coverage over a large excitation range with a limited number of LEDs, allowing excitation of all analytes with absorption within the LED&#39;s excitation range. From a sensors standpoint, the LED array can be viewed as a collection of partially selective instrumental elements. These individual components, when combined, provide a more complete spectroscopic picture of the chemical puzzle.  
         [0034]    [0034]FIG. 4 contains the optical output of each LED as a function of wavelength, measured using a commercial miniature spectrometer (S2000, Ocean Optics, Dunedin, Fla.). This miniature spectrometer was used solely for the purpose of instrument development and is not part of the invention described herein. Different LEDs of the same nominal wavelength, from the same manufacturer, had peak wavelengths which varied by less than 1%. Considering the large wavelength range of each LED, these represent relatively minor wavelength variations. LEDs with varying wavelengths were chosen, but complete wavelength separation was not a requirement. This can be seen in FIG. 4 with significant spectral overlap between curves B and C, curves B and D, curves C and D, and finally curves D and E.  
         [0035]    [0035]FIG. 5 shows the raw spectroscopic output of the LED array instrument invention. Referring to FIG. 5, the fluorescence of 200 ppb fluorescein in ethanol at the LED excitation wavelengths is shown in FIG. 5 a - 5   g . As can be seen, there is variation in the emission peaks as a function of excitation wavelength indicating the presence of more than one component. The fluorescence spectrum is broader at lower excitation wavelengths with a maximum at 520 nm and shoulder around 550 nm. At higher excitation wavelengths the shoulder disappears resulting a single maximum at 520 nm.  
         [0036]    Upon data processing using the well known technique, parallel factor analysis (PARAFAC) (See R. Bro, Chemometr. Intell. Lab., 1997, 38, 149), using three factors, the emission of both anionic forms of fluorescein were resolvable as given in FIGS. 6 a    6   b . FIGS. 6 a  and  6   b  show the results of data processing with spectroscopic output collected using the invention. There are many multidimensional data processing algorithms which may be employed for analysis of EEM data. These include least squares (See G. D. Christian, E. R. Davidson, and I. M. Warner (1977) Anal Chem 19: 2152-2159), rank annihilation factor analysis (RAFA) (See A. Lorber, Analytica Chimica Acta (1984) 164: 293-297), and the generalized rank annihilation method (GRAM) (See Sanchez E, Kowalski BR (1986) Anal Chem 58: 498-499). PARAFAC was chosen for its flexibility and capability for the analysis of complex mixtures. It is clear from the resolved components that two distinct species are fluorescing. In FIG. 6 a , the monoanion (I) was maximally excited at 430 nm versus the maximum excitation of the dianion (II) at 505 nm. Similarly, in FIG. 6 b , the emission maximum for the dianion (II) was 520 nm versus two maxima for the monoanion (I) at 515 nm and 535 nm. The third factor, while somewhat elevated, is dominated by noise and describes the instrumental background. These two forms of fluorescein have been observed (by spectral subtraction) and studied in the literature; the spectra obtained using the invention are in agreement with the published data (See Klonis and Sawyer, Photochem. Photobiol., 2000, 72 (2), 179). It is well known that only the dianion form of fluorescein is present in basic solutions. A 200 ppb solution of fluorescein in basic ethanol showed the emission spectra were independent of excitation wavelength, confirming that the two factors represent two fluorescent forms of fluorescein present in neutral environments. The contributions of the mono- and dianion forms of fluorescein were resolved apriori, as they cannot be obtained individually and exist in solution only. This example demonstrates the power of the LED-EEM system combined with PARAFAC analysis because the highly overlapped fluorescence spectra were resolvable. This led to the visualization of components (anionic forms of fluorescein) at neutral pH which have not been measurable before this invention.  
         [0037]    To further characterize and test the instrument, two to six component mixtures of dyes in ethanol were measured and analyzed. The ability of the system to resolve a wide variety of components and closely related species is well demonstrated using these two to six component samples. The dyes were chosen to illustrate the range of molecules detectable using an LED array EEM instrument, but also to show the ability to resolve overlapping spectra. The full emission wavelength range was filled with analyte emission, and several components were significantly overlapped: rhodamine B and fluorescein, and 9,10-bis(phenylethynyl)-anthracene and fluorescein. However, the slight differences in the LED-excitation and emission spectra allow resolution and quantitation of both rhodamine B and safranin O. The effect of the broad LED wavelength range can be seen in the resolved excitation profiles of the six components. The excitation spectra of the dyes are weighted with respect to the LEDs with the broadest excitation ranges (470 nm, 73 nm FWHM and 505 nm, 47 nm FWHM).  
         [0038]    The data of merit for the analysis of both the single component standards and multi-component mixtures are listed in Table 2. Table 2 lists the results of data analysis of the spectroscopic output collected using the invention. The PARAFAC resolved concentration profiles were used to build the calibration curves for the quantitative analysis described in Table 2. The total fluorescein concentration should be proportional  
                                             TABLE 2                           Figures of Merit for Quantitation of the Dye Standards and Unknowns                Concentration   Sensitivity   LOD   RMSEC a     RMSEP b             Analyte   Range (ppb)   (counts/ppb)   (ppb)   (ppb)   (ppb)   r 2                 Diphenyl anthracene   62.5-500.0   1.25E+02   3.68   0.30   1.67   0.999       9,10-bis(phenylethynyl) anthracene   12.5-100.0   2.13E+03   0.53   0.10   0.18   0.996       Fluorescein (monoanion)   25.0-200.0   1.21E+03   1.57   0.30   3.99   0.991       Rhodamine B   12.5-100.0   2.80E+03   0.76   0.23   0.60   0.980       Safranin O   50.0-400.0   1.18E+03   2.53   0.39   1.30   0.996       Nile Red   25.0-200.0   2.16E+03   0.72   0.20   2.13   0.996                                  
 
         [0039]    to both the monoanion and dianion tautomers. However, fluorescein was quantified using only the predicted fluorescence of the monoanion due to the better linearity and sensitivity for this species. The root mean squared error of calibration (RMSEC) and correlation coefficient (r 2 ) show that the instrument is capable of precise calibration with root mean squared error of prediction (RMSEC) better than 4 ppb and r 2  values greater than 0.98. The LOD were less than 4 ppb with the best cases being in the mid ppt range. The quantitative capabilities of the instrument are good, as given by the RMSEP. The RMSEP were less than 4 ppb for all analytes and were in the sub ppb range for 9,10-bis(phenylethynyl)-anthracene and rhodamine B.  
         [0040]    Although this invention has been described in relation to an exemplary embodiment thereof, it will be understood by those skilled in the art that still other variations and modifications can be affected in the preferred embodiment without detracting from the scope and spirit of the invention as described in the claims.