Abstract:
A photoacoustic spectrometer cell includes a specimen chamber having a specimen port, an optical window in optical communication with the chamber, a microphone in acoustic communication with the chamber, and a push-on closure for closing the port. At least one of the closure and the port have a groove adapted to relieve pressure in the chamber during closing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/250,216 filed Nov. 30, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a photoacoustic spectrometer and, in particular, to a photoacoustic spectrometer for measuring the characteristics of living plants.  
           [0003]    The photosynthesis process encounters two groups of biochemistry reactions, one is light reaction and the other is dark reaction. In light reaction, absorbed light energy is used to split water molecules, producing protons and electrons and forming oxygen molecules. The electrons are transferred between a series of molecules that form an electron transferring train. With the electron translocation, high-energy molecules are formed to energize dark reaction that consumes carbon dioxide molecules and protons to synthesize sugars.  
           [0004]    After light is absorbed by leaves, the major potion of absorbed light energy is converted to heat, at the same time, most of the remaining absorbed light is used by the photosynthesis process. A minor potion of absorbed light is re-radiated as fluorescence. Measurements of CO 2  (consumed), O 2  (evolved) and fluorescence (re-radiated) are three major methods used in photosynthesis study of leaves in vivo. CO 2  gas exchange and fluorescence techniques have become traditional methods for photosynthesis research. However, it is hard to obtain more detailed information by using both of the techniques because there are other electron bypass ways where the electrons are not ultimately consumed by CO 2  reduction, for example, photorespiration or Mehler reaction.  
           [0005]    The major advantage of the photoacoustic (PA) technique is that it can sense the signal generated by either photothermal or photobaric effects. If a photosythetically active sample is illuminated with periodical light pulses, both its oxygen evolution and thermal release will be modulated at the same frequency as the light source, which are both PA signals and can be sensed by a microphone. With a lock-in amplifier processing signals from the microphone, only the signal modulated at a determinated frequency and having a certain phase angle can be amplified. With this method, oxygen evolution from the sample can be distinguished from existing ambient oxygen within a chamber.  
           [0006]    U.S. Pat. No. 4,533,252 to Cahen et al. and U.S. Pat. No. 6,006,585 to Forester are incorporated herein by reference.  
         SUMMARY OF THE INVENTION  
         [0007]    A photoacoustic spectrometer cell includes a specimen chamber having a specimen port, an optical window in optical communication with the chamber, a microphone in acoustic communication with the chamber, and a push-on closure for closing the port. At least one of the closure and the port have a groove adapted to relieve pressure in the chamber during closing.  
           [0008]    A photoacoustic spectrometer cell includes a specimen chamber having a specimen port, an optical window in optical communication with the chamber, a microphone in acoustic communication with the chamber, and a push-on closure for closing the port. At least one of the closure and the port have a groove adapted to relieve pressure in the chamber during closing. The spectrometer also includes a light source adapted to communicate with the window, and a controller. The controller is adapted to operate the light source concurrently at a first frequency and a second frequency and to process a signal from the microphone with respect to said first frequency and with respect to said second frequency. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a schematic block diagram of a spectrometer according to the invention.  
         [0010]    [0010]FIG. 2 is a schematic diagram of a PA spectrometer cell according to the invention.  
         [0011]    [0011]FIG. 3 is a perspective view of a PA spectrometer cell according to the invention.  
         [0012]    [0012]FIG. 4 is a perspective view with portions cut away of a PA spectrometer cell body according to the invention.  
         [0013]    [0013]FIG. 5 is a top plan view of a PA spectrometer cell body according to the invention.  
         [0014]    [0014]FIG. 6 is a perspective view of a PA spectrometer cell closure according to the invention.  
         [0015]    [0015]FIG. 7 is a top plan view of a PA spectrometer cell closure according to the invention.  
         [0016]    [0016]FIG. 8 is a bottom plan view of a PA spectrometer cell closure according to the invention.  
         [0017]    [0017]FIG. 9 is a light source and light pipe according to the invention.  
         [0018]    [0018]FIG. 10 is a perspective view of a PA spectrometer cell body in a vibration reducing unit according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    Referring to FIG. 1, a photoacoustic spectrometer  10 , includes a photoacoustic spectrometer cell  12  having a body  13 , a chamber  14 , an optical window  16 , an acoustic passage  18  and a microphone  20 . The body  13  may be constructed, for example, from metal, high density plastic, or other strong, durable, material. The window  16  may be, for example, sapphire, glass or other durable material transparent to the wavelength of interest.  
         [0020]    Light sources  22 ,  24  are in optical communication with a light pipe  26  for illuminating the window  16 . The light sources  22 ,  24  may be, for example LEDs or other easily modulated light sources. The light pipe  26  may be, for example, glass or plastic, but a lens system can used instead.  
         [0021]    The light sources  22 ,  24  are driven by a modulated driver  28  and a non-modulated driver  30 , respectively. The drivers  28 ,  30  may be, for example, electrically controlled switching/modulating devices such as solid state switches or waveform synthesizers. The drivers  28 ,  30  may be connected to an unshown power supply as a source of power for the light sources  22 ,  24 .  
         [0022]    The microphone  20  provides a signal to pre-amplifiers  32 ,  34 , which amplify the microphone signal. The pre-amplifiers  32 ,  34  may also include bandpass filters for respective frequencies of interest.  
         [0023]    The amplified microphone signals are provided to respective lock-in amplifiers  36 ,  38 . The amplifiers  32 ,  34  are lock-in amplifiers as are well-known in the art. The amplifiers receive synchronizing signals from the modulated driver  28 .  
         [0024]    A controller  40  controls the operation of the spectrometer  10  by providing control signals (e.g., to control light level and modulation) to the drivers  28 ,  30  and control signals to the lock-in amplifiers  36 ,  38  (e.g., phase and time constant). The controller  40  also processes that signals from the amplifiers  36 ,  38  to provide the desired measurements. The controller  40  may be, for example, a general purpose computer such as a laptop computer or a specialized instrument such as the combination of a programmable controller, and a display and/or a data capture device.  
         [0025]    Referring to FIG. 2, the cell  12  can be advantageously enclosed in an environmental enclosure  42  that permits controlling the ambient gas about the cell  12  with a gas inlet  44  and a gas outlet  46 . The inlet  44  has a valve  48  and a filter  50 . The outlet  46  has a valve  52 . The inlet  44  can be connected to an unshown gas source.  
         [0026]    The chamber  14  is closed by a closure  54  applied to the body  13 . The closure  54  has an optical window  16 ′ in optical communication with the chamber  14  similar to the window  16 . An optional gas permeable member  56  provides a path for ambient gas into the chamber  14 . A gasket retaining member  58  retains a gasket  60  on the closure  54 . The gasket  60  provides a seal between the body  13  and the closure  54  and, may also, serve to frictionally retain the closure  54  on the body  13 .  
         [0027]    In the embodiment shown, the body  13  is provided with a beveled edge  62  that assists in aligning the closure  54  for insertion into body  54 . Pressure relief grooves  64  are provided in the body  13  to help avoid a piston/cylinder compression effect when inserting the closure  54 . Such compression effect could otherwise cause the closure  54  to pop off the body  13 .  
         [0028]    Referring to FIGS. 3, 4 and  5 , the body  13  may, for example include mounting holes  66 . The bottom of the chamber  14  may also include, for example, a ledge  68  to support a round disk (unshown) cut from, for example, a plant leaf. A relief groove  70  provides a gas path around the disk.  
         [0029]    Referring to FIGS. 6, 7 and  8 , the gasket  60  may be, for example, an elastomer o-ring and the retaining member  58  can include a groove for retaining the o-ring on the closure  54 . Similar to the body  13 , the closure  54  can be constructed, for example, from metal, high density plastic, or other strong, durable, material.  
         [0030]    The gas permeable member  56  can be included if it is desired to control the gas constituents within the chamber  14 , otherwise, a non-permeable member can be used. The components of the closure  54  can be, for example, assembled with screws  72 .  
         [0031]    The light sources  22 ,  24  may be advantageously composed of an array of many LEDs with, for example, half being the light source  22  and half being the light source  24 , all evenly dispersed. The light pipe  26  can then be advantageously formed, for example, from a frustoconical piece of glass or plastic that focus the light onto the window  16 .  
         [0032]    Referring to FIG. 10, the body  13  may be shock mounted for portable use. The body is mounted to a plate  74  with screws  76 . The plate  74  is mounted to the baseplate  78  by spongy material  80 . U-shaped members  82  provide limits to the movement of the plate  74 . Screws  84  and springs  86  provide adjustment for the members  82 .  
         [0033]    In operation, a sample is placed in the chamber  14  and the closure  54  pushed on the body  13 . Constant light is applied by the source  24  and modulated light is applied by the source  22 . The source  22  may be advantageously modulated at two frequencies concurrently. The first frequency may be, a low frequency, e.g., 1-100 Hz and the second frequency a high frequency, e.g., 100-10,000 Hz. The frequencies may be, for example, 3 Hz and 480 Hz.  
         [0034]    The microphone  20  provides a signal in response to the applied light and the lock-in amplifiers  36 ,  38  then provide a respective signal corresponding to the high frequency and the low frequency. Control of the operation is by the controller  40 . The controller  40  processes that signals from the amplifiers  36 ,  38  to provide the output of the spectrometer.  
         [0035]    The spectrometer  10  may be used to provide dual-frequency-operation. The spectrometer  10  may employ a gas-permeable PA cell. It can be used with a special optical focusing design for ultra strong light obtained from an LED array. This makes it convenient the PA technique to be used in the field. This invention provides an ideal device for use in the fields of plant physiology, ecology, agronomy, crop screening and environmental stress monitoring.  
         [0036]    Operating in a dual-frequency mode, makes the device work more effectively, measurements of oxygen evolution and energy storage can be conducted simultaneously. This is not only faster, but also the data is more consistent.  
         [0037]    The easily removable closure  54 , making replacement of samples easy and fast. Two types of closures are available, one with a gas-permeable material and the other without. Depending on experimental requirements, it is easy to make the photoacoustic cell either gas-permeable or not.  
         [0038]    Experiments with a gas-permeable photoacoustic cell can provide more information about the photosynthesis process. If the outer housing is flushed with gas that has a high CO 2  content, photorespiration will be suppressed. While, if the outer housing is flushed with gas that has a low O 2  content, Mehler reaction will not occur. Using this novel instrument, we can evaluate the photosynthetic electron pathway by measuring light response curves under different gas combinations.  
         [0039]    The novel light focusing system makes it more convenient to use an LED array as a light source for photoacoustic measurements of photosynthetic tissues in the field. Advantages of using an LED as a light source are: (1) it draws a much lower current than traditional light sources; (2) it is modulated electrically rather than mechanically since mechanic light chopper is difficult in carrying out measurements in the field; (3) it causes no worry about UV or IR comparing traditional light sources that must be equipped with optical filters to purify their spectrum output.  
         [0040]    The whole system can be built in a small instrument case about 9″×4″×5″, not including the power supply (e.g., batteries) and the computer.  
         [0041]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.