Abstract:
A spectrometer includes a light source, a fiber optic bundle with a first and second leg, where the first leg has more fibers than the second leg, a flow cell, focusing optics, a disperser, and a detector. The spectrometer may also include a mask and system electronics to control the spectrometer. The spectrometer allows for simultaneous dual analysis of a reference and sample beam and minimizes errors and attenuations in the signals. The elimination of mechanical moving parts and control of attenuation losses enables less sophisticated control electronics to be utilized.

Description:
FIELD OF THE INVENTION 
     This invention relates to optical instruments which provide analysis of materials, especially for chemical testing, such as blood analysis by means of spectrometry and, more particularly, to a dual beam spectrometer, without any moving parts, which enables simultaneous display and analysis of sample and reference beams with adjustments made for attenuation losses in the sample beam and for fluctuations in either beam. 
     The invention may be used in any spectropic system which performs chromatography. The preferred embodiment is used for high pressure liquid chromatography (HPLC). The improvements in spectrometry provided by the invention will be useful in other spectrometric instruments. 
     BACKGROUND OF THE INVENTION 
     Basically, a spectrometer is an optical device which uses a prism, diffraction grating or interferometer to separate light into its constituent parts. With a spectrometer, scientists and others are able to study matter by analyzing the spectrum produced by light passed through the matter. This has provided scientists and others with a powerful analytic tool. Unfortunately, limitations with spectrometers themselves have hindered the use of spectroscopy. 
     For example, many spectrometers utilize mechanically moving parts to control the transmission of light. These parts are subject to failure from repeated use and have inherent speed limitations. Additionally, spectrometers have traditionally lacked the ability to simultaneously display and analyze sample and reference beams. As a result, errors are often introduced by the sequential display and analysis. Even further, heretofore spectrometer have not effectively compensated for attenuation losses during display and analysis and have required complicated electronics and software to separate the desired signals from the noise in the detection systems. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved spectrometer. 
     It is another object of the present invention to provide a spectrometer which compensates for attenuation losses. 
     It is a further object of the present invention to provide a spectrometer which improves signal to noise ratios enabling less expensive control electronics and software to be utilized. 
     According to the present invention, these and other objects and advantages are achieved in a dual beam spectrometer, without any moving parts, which includes a light source, a fiber optic bundle which splits into a first and second leg, with the first leg having more fibers than the second leg, a transmission cell positioned at the end of the first leg, focusing optics positioned at the end of the second leg and on the opposing side of the transmission cell, a dispersing device for separating the sample and reference beams into their constituent parts, and a detector for detecting the constituent parts. 
     The elimination of mechanical moving parts and the simultaneous display and analysis of sample and reference beams allows for high speed and repeated analysis suitable for commercial industries. Attenuation losses in the sample beams are compensated for with the differentiation in the number of fibers in the first and second legs. Further compensation for attenuation losses can be obtained by making the overall length of the second leg longer than the first leg and by tapering the transmission cell. The addition of mask filters between the dispersing device and the detector compensates for wavelength dependent source intensities, thereby improving the signal to noise ratio and allowing for the use of less sophisticated and expensive electronic control circuitry and software for processing and analyzing the data. 
     The spectrometer may be housed in a casing made from low thermal expansion material. The housing helps to minimize long-term drift caused by changes in ambient temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, advantages and features of the invention and the best mode now known for practicing the invention will become more apparent from the accompanying drawings in which: 
     FIG.  1 ( a ) is an exploded, schematic view of a double beam spectrometer, in accordance with the present invention; 
     FIG.  1 ( b ) is an enlarged fragmentary view of the slits and the photodiode array of the spectrometer shown in FIG.  1 ( a ); and 
     FIG. 2 is a block diagram of the system electronics used in conjunction with the spectrometer shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A double beam spectrometer  10  in accordance with the present invention is illustrated in FIGS.  1 ( a ) and  1 ( b ). The spectrometer includes a light source  12 , a fiber optic bundle  14  which splits into a first and second leg  16  and  18 , a transmission or flow cell  19 , a focusing lens  44 , a dispersing device  22 , and a detector  24 . The spectrometer  10  has no moving parts and is designed to compensate for attenuation losses in the light in the first leg  16  and to improve the signal-to-noise ratio of the detected portions of the light, thereby requiring less sophisticated and expensive control electronics and software. 
     Referring more specifically to FIG.  1 ( a ), the light source  12  is preferably a deuterium or pulsed xenon source or an arc lamp, although other types of light sources could be used. For example, a super quiet L5257 manufactured by Hamamatsu could be used. The light source  12  provides a direct source of light to the fiber optic bundle  14  and is situated in the spectrometer for easy access. The light source  12  can be easily changed by the user with little or no alignment. 
     A focusing lens  26  may be positioned to focus light from the light source  26  onto one end of the fiber optic bundle  14 . Preferably, the focusing lens  26  is made from quartz or Suprasil. Suprasil lens are available from Spindler and Hayer in Germany. 
     The fiber optic bundle  14  is positioned to receive light from the light source  12 . Preferably, the fibers in the bundle  14  are disposed in a substantially random distribution. The random distribution of the fibers helps to avoid hot spots and light source wander inherent in source radiation. In this embodiment, Superguide G, manufactured by Fiberguide, is used for the fiber optic bundle  14 , although other types of fiber optic cables could be used without departing from the scope of the invention. 
     The fiber optic bundle  14  splits into the first and second leg  16  and  18  with the first leg  16  having more fibers than the second leg  18 . Preferably, the ratio of fibers in the first leg  16  to the second leg  18  is 10:1. The different ratio of fibers between the first and second legs  16  and  18  compensates for attenuation losses in the light in the first leg  16 . To further compensate for attenuation losses, the second leg  18  may be made longer than the first leg  16 . Preferably, the second leg  18  is approximately two times longer than the first leg  16 . 
     A focusing lens  27  may be positioned between the end of the first leg  16  and transmission cell  19 . The lens  27  helps to focus light into the cell  19  and may be made from material, such as quartz or Suprasil. 
     The transmission cell  19  is positioned to receive light from the end of the first leg  16 . Preferably, the transmission cell  19  is a flow cell, such as a QS-113 produced by Hellma, although any type of transmission cell may be used. To minimize the effects of changes in refractive indices, the flow cell  19  may be tapered. As with the light source  12 , the flow cell  19  is easily accessible within the spectrometer and can be replaced by the user with little or no alignment. 
     The focusing lens  44  is positioned on the opposing end of the transmission cell  19  from the end of the first leg  16  and focuses the sample beam onto one end of the first optical fiber  46 . One end of the second optical fiber  48  is positioned to receive the reference beam from the end of the second leg  18 . The other ends of the first and second fibers  46  and  48  are positioned in front of each of the vertically disposed sample and reference slits  52  and  53  in section  50 . Preferably, the focusing lens is made from quartz or Suprasil, and Superguide G made by Fiberguide, is used for the first and second optical fibers  46  and  48 , although other materials and fibers can be used without departing from the invention. 
     A casting  40  holds the dispersing device  22  and detector  24  and includes the section  50  with the slits  52  and  53 . Although the casting  40  only houses the dispersing device  22  and detector  24  in this particular embodiment, the casting  40  can be made to house some or all of the parts of the spectrometer  10 . The casting  40  may be made from ceramic or any other material with a low coefficient of thermal expansion to help minimize long term drift of the detector  24  as a result of changes in ambient temperature. 
     In this embodiment, the dispersing device  22  is a grating with a large number of narrow and substantially circular grooves placed side by side to diffract incident light into a spectrum. Preferably, a concave replicated holographic grating with 1,200 g/mm blazed at 250 nm is used. The device  22  disperses light at different angles depending on the wavelength of the light and the spacing of the grooves on the device  22 . Although a grating is shown, a prism or filters could also be used in place of the grating. A grating has been chosen because light is more uniformly dispersed by a grating than a prism. Additionally, gratings with mirror material will reflect all wavelengths, while no current prism material is known which will transmit all wavelengths. The grating may be coated with ALM g F 2  to maintain ultraviolet reflectivity. 
     In this embodiment, the detector  24  is a double-row PDA which has 512 by 2 pixels, such as the SQ-512Q available from Hamamatsu. Although a double-row PDA is shown, any type of detecting device may be used. The double-row PDA and the sample and reference slits  52  and  53  and are more clearly illustrated in the enlarged view of FIG.  1 ( b ). 
     The spectrometer may also include a pair of suitable mask filters  38  and  39  positioned between the dispersing device  22  and the detector  24 . Typically, the mask filters  38  and  39  are made from transmissive material and compensate for wavelength dependent source intensities and CCD inefficiencies. With the masks  38  and  39 , the analysis and display of the incident light is easier, allowing for the use of simpler and less expensive electronics and software. 
     In this particular embodiment, the spectrometer operates when the light source  12  is pulsed to produce a light which impinges on the focusing lens  26 . The focusing lens  26  focuses the light onto one end of the fiber optic bundle  14 . The light in the bundle  14  is guided down the first and second legs  16  and  18  splitting into a sample and a reference beam. A lens  27  located at the end of the first leg  16  focuses the sample beam emerging from the leg  16  onto the transmission cell  19 . 
     The difference in the number of optical fibers in the first and second legs  16  and  18 , the difference in the length of the first and second legs  16  and  18 , and the tapered flow cell  19  all help to compensate for attenuation losses in the sample beam. 
     The sample beam from the transmission cell  19  is focused by the lens  44  onto one end of the optical fiber  46 . The reference beams in the second leg  18  is transmitted to the one end of the second optical fiber  48 . The first and second optical fibers  46  and  48  guide the sample and reference beams to the sample and reference slits  52  and  53 . The beams pass through the slits  52  and  53  and are diffracted by the dispersing device  22 . 
     The dispersing device  22  separates the sample and reference beams into their constituent wavelength parts. Both the diffracted sample and reference beams pass through the mask filters  38  and  39  and strike the detector  24 . The mask filters  38  and  39  compensate for wavelength dependent source intensities. As a result, the signal to noise ratio is improved as much as 40%. The higher signal to noise ratio enables the spectrometer  10  to use less sophisticated control electronics and software. The device  24  accumulates charge for a predetermined amount of time and is then read out and analyzed as will be explained in greater detail with the description of FIG.  2 . 
     Referring to FIG. 2, the dual beam spectrometer in FIGS.  1 ( a ) and  1 ( b ) is operated by a digital processing unit, such as a microprocessor or central processing unit (CPU)  58 . The CPU  58  is connected to a PDA controller  60  and a control unit  62 . The PDA controller  60  is coupled to the detector  24  and the control unit  62  is coupled to the light source  12  (e.g., a xenon flash lamp), a light measuring device  64  and a power supply  66 . The CPU  58  may be connected to an LCD display  68 , keyboard  70  and/or analog output or digital output  72 . Although an LCD display device is shown, any type of display device may be used, such as PC monitor. A typical sequence of operations of the spectrometer  10  control system is as follows. 
     The microprocessor  58  controls the PDA controller  60  and control unit  62  according to the particular software program loaded into the spectrometer  10 , such as lab calculations by Galactic. In response to control signals from the CPU  58 , the PDA controller  60  controls the timing of the accumulation of data and controls when the data is retrieved from the detector  24 , which in this embodiment is a PDA array. The data read out by the PDA controller  60  is typically stored in a memory buffer in the PDA controller  60  before being sent to the microprocessor  58 . The CPU  58  stores and processes the received data according to the particular instructions in the software and transmits the processed data out via the analog output or digital output  72  or to the display  68 . 
     The control unit  62  also operates in response to control signals from the CPU  58  and from inputs from the light measuring device  64 , which measures the light output by the light source  12 . The control unit  62  adjusts the light output of the light source  12 , in response to these inputs. The control unit  62  is powered by the power supply  66 . 
     Having thus described the basic concept of the invention, it will be readily apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, it is not limiting. Various alterations, improvements and modifications will occur and are intended to those skilled in the art, those not expressly stated herein. These modifications, alterations and improvements are intended to be suggested hereby, and are within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims and equivalents thereto.