Abstract:
A system and method for monitoring the concentration of a medium in at least one container using photothermal spectroscopy. The medium can be a gas, such as oxygen or carbon dioxide, or a solid or liquid. The system and method each employs an energy emitting device, such as a laser or any other suitable type of light emitting device, which is adapted to emit a first energy signal toward a location in the container. The first energy signal has a wavelength that is substantially equal to a wavelength at which the medium absorbs the first energy signal so that absorption of the first energy signal changes a refractive index of a portion of the medium. The system and method each also employs a second energy emitting device, adapted to emit a second energy signal toward the portion of the medium while the refractive index of the portion is changed by the first energy signal, and a detector, adapted to detect a portion of the second energy signal that passes through the portion of the medium. The system and method each further employs a signal analyzer, adapted to analyze the detected portion of the second energy signal to determine an amount of a sample in the container based on a concentration of the medium in the container. In particular, the signal analyzer can analyze the detection portion of the second energy signal to determine whether the sample includes an organism which consumes or emits the medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Related subject matter is disclosed in a U.S. Patent Application of Nicholas R. Bachur, Jr. et al. entitled “System and Method for Optically Monitoring the Concentration of a Gas, or the Pressure, in a Sample Vial to Detect Sample Growth” (Attorney Docket No. P-5026), the entire contents of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a system and method for optically monitoring the concentration of a gas in a sample vial using photothermal spectroscopy to detect the presence of sample growth. More particularly, the present invention relates to a system and method employing an excitation source, such as a diode laser, which is used to excite gas, such as carbon dioxide, in a sample vial to create a gas lens in the sample vial, and an interrogation source, such as another diode laser, which emits light through the gas lens to a detector which measures the degree that the light is refracted by the gas lens to determine the concentration of the gas in the sample vial, which is representative of microorganism growth in the sample vial.  
           [0004]    2. Description of the Related Art  
           [0005]    Many medical diagnoses require that a fluid sample, such as a blood sample, be taken from a patient, cultured in a growth medium, and then examined for the presence of a pathogen believed to be causing the patient&#39;s illness. The growth medium provides nutrients that allow the pathogen, such as a bacteria, virus, mycobacteria, mammalian cells or the like, to multiply to a sufficient number so that their presence can be detected.  
           [0006]    In some cases, the pathogen can multiply to a large enough number so that it can be detected visually. For example, a portion of the culture can be placed on a microscope slide, and visually examined to detect for the presence of a pathogen of interest.  
           [0007]    Alternatively, the presence of a pathogen or other organism can be detected indirectly by detecting for the presence of byproducts given off by the microorganism during its growth. For example, certain microorganisms such as mammalian cells, insect cells, bacteria, viruses, mycobacteria and fungi consume oxygen during their growth and life cycle. As the number of microorganisms increases in the sample culture, they naturally consume more oxygen. Furthermore, these oxygen consuming organisms typically release carbon dioxide as a metabolic byproduct. Accordingly, as the number of organisms present increases, the volume of carbon dioxide that they collectively release likewise increases.  
           [0008]    Several methods exist for detecting the presence of carbon dioxide in a sample to determine whether organisms are present in the sample. For example, an instrument known as the Bactec® 9050 manufactured by Becton Dickinson and Company detects for the change in color of an indicator to determine whether carbon dioxide is present in a sample. That is, each sample is collected in a respective sample vial containing an indicator medium having a chemical that reacts in the presence of carbon dioxide to change color. A light sensor detects the color of the indicator medium in the sample vial when the sample vial is loaded into the instrument. If the sample contains an organism which emits carbon dioxide, the reflected or fluorescent intensity of the indicator medium will change in response to the presence of carbon dioxide. The light sensor will therefore detect this change in intensity, and the instrument will thus indicate to an operator that an organism is present in the sample contained in the sample vial. Other examples of instruments for detecting the presence of organisms in a sample by detecting for the change in carbon dioxide in the sample are described in U.S. Pat. Nos. 4,945,060, 5,164,796, 5,094,955 and 5,217,876, the entire contents of each of these patents are incorporated herein by reference.  
           [0009]    Alternatively, instead of detecting for the presence of carbon dioxide to detect the presence of an oxygen consuming microorganism, it is possible to detect for a depletion in the concentration of oxygen in the sample of interest. In such a system, the sample vial includes an indicator whose color or fluorescence changes as the concentration of oxygen in the vial changes. This change in color or fluorescence can be detected by an instrument, which can provide an indication to a technician that oxygen in the sample is being depleted by an oxygen consuming organism within the sample. An instrument employing this oxygen detecting technique is described in U.S. Pat. No. 5.567,598, the entire contents of which are incorporated herein by reference.  
           [0010]    The presence of oxygen consuming organisms can also be detected by detecting for a change in pressure in a sealed sample vial containing the sample of interest. That is, as oxygen in a closed sample vial is depleted by oxygen consuming organisms, the pressure in the sealed sample vial will change. The pressure will further change in the sample vial as the organisms emit carbon dioxide. Therefore, the presence of such organisms can be detected by monitoring for a change in pressure in the closed sample vial. Instruments that are capable of detecting changes in pressure in the sample vial are described in U.S. Pat. Nos. 4,152,213, 5,310,658, 5,856,175 and 5,863,752, the entire contents of each of these patents are incorporated herein by reference.  
           [0011]    It is noted that the techniques described above each detect for the presence of oxygen or carbon dioxide in a sample vial by detecting the change in a state or condition of an indicator other than the oxygen or carbon dioxide itself For example, certain of the techniques detect for a change in color of an indicator, while others detect for a physical change, such as the movement of a diaphragm which indicates a change in pressure. These techniques can therefore be susceptible to erroneous results if, for example, the indicators themselves are inaccurate.  
           [0012]    Accordingly, to avoid such errors, detection probes or sensors can be inserted directly into the sample vial to detect for the presence of carbon dioxide or oxygen directly. An instrument for detecting for the presence of carbon dioxide in a sample directly is described in U.S. Pat. No. 4,971,900, the entire contents of which are incorporated herein by reference. This probe technique, however, is an invasive technique which requires that a sensor or probe be inserted directly into the sample vial containing the sample. This technique can prove hazardous because the probes can become contaminated with the organism present in the sample. Moreover, when the probes are being inserted into or removed from the vial, the potentially hazardous organisms can escape into the atmosphere, thus endangering the technician or others in the general vicinity of the instrument.  
           [0013]    Techniques have therefore been developed which are capable of detecting the presence of, for example, carbon dioxide without the need for detecting a change in the condition of an indicator, and without the use of an invasive detector or probe. In one technique, infrared light is irradiated through the sample vial containing the sample of interest. The infrared light passing through the sample vial is detected by an infrared detector. Because carbon dioxide absorbs infrared light within a certain wavelength range, if any carbon dioxide is present in the sample vial, infrared light within that particular wavelength range will be absorbed by the carbon dioxide and thus not be detected by the infrared detector. The signals from the infrared detector are analyzed to determine whether any of the infrared light being emitted into the sample vial is absorbed and thus not detected by the infrared detector. If any absorption has occurred, the instrument provides an indication that carbon dioxide is present in the sample vial, and thus, a carbon dioxide producing organism is likely present. Examples of instruments which perform this type of technique are described in U.S. Pat. Nos. 5,155,019, 5,482,842 and 5,427,920, the entire contents of each are incorporated by reference herein.  
           [0014]    The infrared light detecting technique has advantages over the technique described above which uses an invasive detector or probe, because the technique reduces the possibility of contamination. Furthermore, because the infrared light technique directly detects for the presence of carbon dioxide instead of detecting for a change in an indicator, more accurate results can be attained. However, the infrared light technique has certain disadvantages. For example, carbon dioxide absorbs infrared light within a somewhat wide range of wavelength, which can also be absorbed by other gases. Hence, if gases in the vial other than carbon dioxide absorb some of the infrared light, the instrument may provide a false indication that carbon dioxide is present Accordingly, the accuracy of the infrared light technique described in the patents referenced above is somewhat limited.  
           [0015]    A need therefore exists for an improved non-invasive system and method for detecting for the presence of oxygen or carbon dioxide in a culture sample, to thus detect for the presence of an oxygen consuming or carbon dioxide producing organism in the sample.  
         SUMMARY OF THE INVENTION  
         [0016]    An object of the present invention is to provide an improved system and method for optically monitoring the concentration of a gas in a sample vial to detect the presence of sample growth.  
           [0017]    Another object of the present invention is to provide a system and method employing photothermal spectroscopy to monitor the concentration of a gas, such as oxygen or carbon dioxide, or the concentration of a liquid or solid, in the sample vial to thus detect for microorganism growth in the sample vial based on the monitored concentration.  
           [0018]    A further object of the present invention is to provide a system and method capable of housing and incubating multiple sample vials containing respective samples, and optically monitoring the concentration of a medium in each of the sample vials by photothermal spectroscopy to detect the presence of sample growth in the vials based on the respective monitored concentrations.  
           [0019]    These and other objects are substantially achieved by providing a system and method for monitoring the concentration of a medium in at least one container. The medium can be a gas, such as oxygen or carbon dioxide, or a solid or liquid. The system and method each employs an energy emitting device, such as a laser or any other suitable type of light emitting device, which is adapted to emit a first energy signal toward a location in the container. The first energy signal has a wavelength that is substantially equal to a wavelength at which the medium absorbs the first energy signal so that absorption of the first energy signal changes a refractive index of a portion of the medium.  
           [0020]    The system and method each also employs a second energy emitting device, adapted to emit a second energy signal toward the portion of the medium while the refractive index of the portion or portion of an adjoining medium is changed by the first energy signal, and a detector, adapted to detect a portion of the second energy signal that passes through the portion of the medium or adjoining medium. The system and method each further employs a signal analyzer, adapted to analyze the detected portion of the second energy signal to determine an amount of a sample in the container based on a concentration of the medium in the container. In particular, the signal analyzer can analyze the detection portion of the second energy signal to determine whether the sample includes an organism which consumes or emits the medium.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    These and other objects, advantages and novel features of the invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a block diagram of a system employing multiple incubation and measurement instruments according to an embodiment of the present invention, which each uses photothermal spectroscopy to monitor the concentration of a gas, such as oxygen or carbon dioxide, in sample vials, to thus detect for microorganism growth in the sample vials;  
         [0023]    [0023]FIG. 2 is a detailed view of an instrument employed in the system shown in FIG. 1;  
         [0024]    [0024]FIG. 3 is a top view of the instrument shown in FIG. 2;  
         [0025]    [0025]FIG. 4 is a detailed view of an example of a movable monitoring assembly employed in the instrument shown in FIGS.  1 - 3  which uses photothermal spectroscopy techniques to monitor the concentration of a gas in the sample vials;  
         [0026]    [0026]FIG. 5 is another view of the monitoring assembly shown in FIG. 4;  
         [0027]    [0027]FIG. 6 is a detailed view of an example of a sensor head assembly employed in the monitoring assembly shown in FIGS. 4 and 5;  
         [0028]    [0028]FIG. 7 is a detailed view illustrating the sensor head assembly shown in FIG. 6 retracted into the sensor head housing of the movable monitoring assembly shown in FIGS. 4 and 5;  
         [0029]    [0029]FIG. 8 is a detailed view showing the sensor head assembly shown in FIG. 6 extended from another end of the sensor head housing of the monitoring assembly shown in FIGS. 4 and 5;  
         [0030]    [0030]FIG. 9 is a conceptual block diagram of an example of electronic components used by the monitoring assembly to monitor the concentration of one or more gasses in the sample vials;  
         [0031]    [0031]FIG. 10 is a block diagram of an example of another type of monitoring assembly that can be employed in the instrument shown in FIGS. 2 and 3;  
         [0032]    [0032]FIG. 11 is another block diagram of the monitoring assembly shown in FIG. 10;  
         [0033]    [0033]FIG. 12 is a top view of the instrument shown in FIG. 2 including the monitoring assembly shown in FIGS. 10 and 11;  
         [0034]    [0034]FIG. 13 is a side view of another type of instrument employing another embodiment of a detector assembly which uses infrared laser spectrography and dual wavelength modulation techniques to monitor the concentration of a gas or the pressure in sample vials according to another embodiment of the present invention;  
         [0035]    [0035]FIG. 14 is a front view of the instrument shown in FIG. 13;  
         [0036]    [0036]FIG. 15 is a detailed perspective view of the carousel and detector assembly arrangement in the instrument shown in FIGS. 13 and 14;  
         [0037]    [0037]FIG. 16 is a side view of the carousel and detector head arrangement shown in FIG. 15; and  
         [0038]    [0038]FIG. 17 is a detailed view of the detector assembly arrangement as shown in FIGS. 15 and 16.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    A system  100  for detecting growth of microorganisms in sample cultures according to an embodiment of the present invention is shown in FIG. 1. As illustrated, the system  100  includes a plurality of incubation and measurement modules  102  that are connected to a central computer  104 . The central computer  104  can control the incubation temperatures and times, as well as the timing of the measurements performed by the modules  102 , and can collect and classify the data readings obtained by the modules  102 . The system  100  can also include a data output device, such as a printer  106 , that can be controlled by the central computer  104  to print data readings obtained by the incubation and measurements modules  102 .  
         [0040]    Further details of the incubation and measurement modules  102  are shown in FIGS. 2 and 3. As illustrated, each incubation and measurement module  102  in this example includes a housing  108  and two shelves  110  that can be slid into and out of the housing  108  in a direction along arrow A. Each shelf  110  includes a plurality of openings  112 , each of which is adapted to receive a sample vial  114 . The openings  112  are arranged in a plurality of rows and columns as shown, and each shelf  110  can have any practical number of openings. For example, the openings  112  can be arranged in nine rows, with nine columns in each row, thus totaling  81  openings  112  per shelf  110 .  
         [0041]    When a sample culture is to be analyzed by the incubation and measurement module  102 , the sample culture is placed in a sample vial  114 , and the sample vial  114  is loaded into a respective opening  112  in the incubation and measurement module  102 . The sample vial  114  is a closed sample vial in this example. The incubation and measurement module  102  can further include a keyboard, a barcode reader, or any other suitable interface that enables a technician to enter information pertaining to the sample into a database stored in a memory in the incubation and measurement module  102 , in the central computer  104 , or both. The information can include, for example, patient information, sample type, the row and column of the opening  112  into which the sample vial  114  is being loaded, and so on.  
         [0042]    Each incubation and measurement module  102  further includes a movable monitoring assembly  116  which is capable of monitoring the contents of a medium in the sample vials  114  through the use of a technique known as photothermal spectroscopy. The principles of photothermal spectroscopy are generally described in a publication by Stephen E. Bialkowski entitled “Photothermal Spectroscopy Methods for Chemical Analysis”, John Wiley &amp; Sons, Inc., 1996, the entire content of which is incorporated by reference herein.  
         [0043]    Details of the movable monitoring assembly  116  will now be described with reference to FIGS.  4 - 9 . As shown, the monitoring assembly  116  includes a sensor head housing  118  which houses a sensor assembly  120  that includes a plurality of lasers  122  and  124  and a detector  126 , the details of which are described below.  
         [0044]    Sensor head housing  118  is movably mounted to a vertical shaft  128  and can be moved in the “Y” direction along the vertical shaft  128  by, for example, a motor and pulley arrangement (not shown) or any other type of arrangement as can be appreciated by one skilled in the art. As further illustrated, vertical shaft  128  is movably mounted to a horizontal shaft  130 , and can be moved along the “Z” direction along horizontal shaft  130  by a motor or pulley assembly (not shown) or any other type of arrangement as can be appreciated by one skilled in the art. As shown in FIG. 3, monitoring assembly  116  can be mounted in a module  102  between the two shelves  110  so that the sensor assembly  120  can take readings from sample vials  114  in both shelves  110  as described in more detail below.  
         [0045]    As further shown in more detail in FIGS. 7 and 8, the sensor assembly  120  is movably mounted in sensor head housing  118  so that sensor assembly  120  can be retracted into the sensor head housing  118  along the “X” axis by a motor and gear or pulley arrangement (not shown), or by any other type of arrangement as can be appreciated by one skilled in the art. As shown in FIG. 8, the sensor assembly  120  can be further moved along the “X” direction to extend out of the other side of sensor head housing  118 .  
         [0046]    The operation of monitoring assembly  116  will now be described with reference to FIGS.  3 - 9 . As discussed briefly above, central controller  104  (see FIG. 1) or a controller (not shown) in module  102  can control movement of the sensor head housing  118  and vertical shaft  128 , as well as the extension of the sensor assembly  120 , so that one pair of lasers  122  and  124  and one detector  126  are positioned on opposite sides of the neck of a sample vial  114  of interest loaded into an opening  112  in one of the shelves  110 , as shown conceptually in FIG. 9. In this example, laser  122  is a diode laser that emits photons (an excitation laser beam) having a wavelength at or about 2004 nanometers, which is in the absorbance wavelength band for carbon dioxide. The laser  122  is energized by a reference voltage source  132  and an alternating voltage source  134  as shown, under the control of, for example, central controller  104  or a controller in module  102 .  
         [0047]    When a photon emitted from laser  122  strikes a carbon dioxide molecule and is absorbed by the carbon dioxide molecule, thermal energy is released into the surrounding atmosphere in the vial  114 . The thermal energy heats the gas molecules in the vicinity of the carbon dioxide molecule, and the local air density is reduced. As this density is reduced, the refractive index nD of the atmosphere in the vial  114  consequently is reduced. This localized reduction in the refractive index nD of the atmosphere in the neck of vial  114  creates a gas lens  136  that is capable of refracting light.  
         [0048]    While laser  122  is energized to form the gas lens  136  in the neck of vial  114 , second laser  124  is energized by a voltage source  138  under the control of, for example, central controller  104  or a controller in module  102 . In this example, laser  124  is a diode laser that emits photons (an interrogation laser beam) at a wavelength at or about 650 nanometers. The photons are directed toward the intersection of the excitation laser beam emitted from excitation laser  122  and the gas contained in the neck of vial  114  as shown. As the interrogation laser beam emitted from laser  124  strikes the gas lens  136  formed by the photothermal effect described above, the interrogation laser beam is defocused or refracted by the gas lens  136 . It is noted that as the concentration of carbon dioxide in the gas mixture increases, the gas lens  136  formed will have a greater refractive effect on the interrogation laser beam. That is, a small concentration of carbon dioxide will form a lens  136  having a smaller refractive effect and thus, the interrogation laser beam will be refracted less. However, a large concentration of carbon dioxide will form a lens  136  having a larger refractive effect and thus, the interrogation laser beam will be refracted more. Improvements to the interrogation beam path not shown in the figures such as optical lenses or spatial filtering may be applied to increase the magnitude of the gas lens&#39; refractive effect.  
         [0049]    As further shown in FIG. 9, detector  126 , which in this example is a silicon photodetector, receives the refracted interrogation laser beam that has passed through gas lens  136 . The photocurrent output by detector  126  is input to a transimpedance amplifier  140 , and the output of amplifier  140  is input to a lock-in amplifier  142 . A reference voltage provided by reference voltage source  132 , which is representative of the modulation waveform of the excitation laser  122 , is also input to the lock-in amplifier  142 , which is capable of accommodating relatively poor signal to noise ratios for the detected refracted interrogation laser beam. The output of the lock-in amplifier  142  can be provided to a display  144 , that displays a waveform which is representative of the concentration of carbon dioxide in the vial  114 . The waveform can be further analyzed to determine the amount of sample growth in the vial  114 , which in this example is proportional to the concentration of carbon dioxide in the vial  114 . Also, it is noted that other signal processing techniques, such as those employing discriminator circuits, ratio detectors, phase shift demodulation, and so on, can be used to process the photocurrent output by detector  126 .  
         [0050]    The sensor head housing  118  can then be moved along the “Y” direction to take readings from that column of sample vials  114  in the manner described above. The sensor assembly  120  can then be retracted into the sensor head housing  118  as shown in FIG. 7 so that the vertical shaft  128  and sensor head housing  118  can be moved in the “Z” direction to be aligned with another column of sample vials  114 . The reading process can then be repeated to take readings from that column of sample vials  114 , and the retracting, moving and reading process can be repeated for all columns of sample vials  114 .  
         [0051]    Once readings have been taken from all of the sample vials  114  in all of the columns in shelf  110 , the central controller  104 , for example, or a controller in the module  102 , can control the monitoring assembly  116  to take readings from the sample vials  114  in the opposite shelf  110  in a similar manner. In this event, the central controller  104  or other controller controls the monitoring assembly  116  to position the sensor head housing  118  for reading a column of sample vials  114  in that shelf  110 . Once the sensor head housing  118  has been properly positioned, the sensor head housing  118  is controlled to extend the sensor assembly  120  from the opposite end of the sensor head housing  118  as shown in FIG. 8. The central controller  102  or other controller then controls the sensor head housing  118  to move along the vertical shaft  128  so that the sensor assembly  120  can take readings from all the sample vials  114  in that column in a manner similar to that described above.  
         [0052]    Once all of the sample vials  114  in that column have been read, the sensor head housing  118  is controlled to retract the sensor assembly  120  as shown in FIG. 7. The central controller  104  or other controller then controls movement of the vertical shaft  128  and sensor head housing  118  in the “Z” direction along horizontal shaft  130  until the sensor head housing  118  is positioned to read another column of sample vials  114 . The sensor head housing  118  is then moved in the “Y” direction along vertical shaft  128  as appropriate to take readings from the sample vials  114  that occupy column of sample vials  114 . The process is then repeated until readings have been taken from the sample vials  114  in all of the columns of that second shelf  110 . Once all of the readings have been taken, or in contemporaneously, the data can be processed, displayed and analyzed in a manner discussed above.  
         [0053]    The photothermal spectroscopy technique described above has several advantages over other types of spectroscopy techniques used to monitor gas metabolites as a representation of sample growth. For example, the photothermal spectroscopy technique described above has a sensitivity of 1000 to 10,000 times that of gas absorption spectroscopy. Also, with the photothermal spectroscopy technique, the sensitivity of the measurements tends to increase as the volume of sample gas decreases. On the contrary, with the gas absorption spectroscopy technique, the sensitivity of the measurements increase with increased gas volume and decrease with decreased gas volume. Accordingly, the photothermal spectroscopy technique is more desirable for use in monitoring samples that produce small volumes of metabolites.  
         [0054]    In addition, the photothermal spectroscopy technique described above can be used to interrogate liquids and solids using the excitation laser  122  and interrogation laser  124 , and the detector  126 , and the material being interrogated need not be transparent. Furthermore, the excitation laser  122  need not be a laser, but rather, can be any type of light source that is capable of emitting light having a wavelength confined to the absorption band of the gas being analyzed (e.g., carbon dioxide). However, the light source must not emit light that has a wavelength within the absorption bands of other gases that may be present in the atmosphere in which the gas being analyzed is present. In other words, for the above example, the excitation light source  122  must not emit light having a wavelength that is within the absorption band of any gases other than carbon dioxide that may also be present in the neck of vial  114 . For example, the light source  122  can be an LED that produces light throughout the absorption spectrum of the gas under analysis (e.g., carbon dioxide) can be used. Alternatively, a strobe lamp can be used in conjunction with narrow band absorption filters which confine the wavelength of the light entering the neck of the sample vial  114  to the absorption band of carbon dioxide or any other gas or medium being analyzed. Also, a tunable bandpass filter can be applied to the path of the excitation light emitted from excitation light source  122  to permit rapid scanning of many different types of analytes over a wide wavelength range.  
         [0055]    It is also noted that the interrogation laser  124  can be an inexpensive type of laser, such as the type used in laser pointers. Furthermore, the detector  126  can be a photodiode detector using an inexpensive silicon material capable of detecting light in the visual wavelength range rather than, for example, expensive Aluminum Gallium Arsenide Phosphide detectors that are typically used to detect light in the mid-infrared (mid-IR) range.  
         [0056]    It is further noted that the photothermal spectroscopy techniques described above can be used to analyze molecules other than carbon dioxide. For example, the monitoring assembly  116  can be used to monitor the concentration of oxygen (O 2 ), NH 3 , H 2 S, CH 4  or SO 2 , in the head space above a liquid growth medium in the sample vials  114  to detect a microorganisms&#39; metabolic activity. However, to detect these different molecules, the excitation laser or light source  122  needs to be configured to emit light within the absorption band of the molecules of interest. For example, to detect NH 3 , a laser  122  that emits light in the 1997 nanometer band is used. To detect H 2 S, a laser  122  that emits light in the 1570 nanometer band is used, to detect CH 4 , a laser  122  that emits light in the 1650 nanometer band is used, and to detect SO 2  , a laser that emits light in the 7280 nanometer band is used. The monitoring assembly  116  can also be used to monitor the concentration of other molecules such as glucose, creatine kinase-MB, and so on.  
         [0057]    In addition, the monitoring assembly  116  can have different configurations. For example, as shown in FIGS.  10 - 12 , the incubation and measurement module  102  can be configured to include a plurality of monitoring assemblies  150 , which are positioned in the incubation and measurement modules  102  to obtain readings from the sample vials  114 . In the example shown in FIGS. 10 and 11, each monitoring assembly  150  is configured to obtain measurements from sample vials  114  inserted in two rows of openings  112 . However, the monitoring assembly  150  can be configured to obtain readings from sample vial  114  in any number of rows of openings  112  as desired.  
         [0058]    The monitoring assembly  150  includes a movable assembly  152  which, in this example, is slidably coupled to a rail assembly  154  which is fixedly coupled to the top portion of shelf  110 . A motor and pulley assembly  156  comprising a motor  158 , such as a D.C. servo motor, and a pulley arrangement  160  that is driven by the motor  158 , is coupled to the rail assembly  154  and movable assembly  152 . The motor  158  is controlled by, for example, central computer  104  or a computer in incubation and measurement module  102  to drive the pulley arrangement  160  which, in response, drives movable assembly  152  to slide along rail assembly  154  in a sample vial reading direction indicated by arrow B in FIG. 10.  
         [0059]    Moveable assembly  152  also includes a sensor  164  which includes a light emitting device  166  and a detector  168  positioned on opposite sides of a rail  170  of rail assembly  154 . As the motor and pulley assembly  156  drives the moveable assembly  152  along rail assembly  154 , the sensor  164  detects the openings  172  in the rail  170 , and provides a signal indicative of this detection to central computer  104  or a computer in the incubation and measurement module  102 . The central computer  104  or a computer in the incubation and measurement module  102  uses this detection signal to monitor the position of the moveable assembly  152  along rail assembly  154 . Also, because each opening  172  corresponds to a respective column of openings  112  in the shelf  110 , the computer can determine which sample vials  114  are being read by the detectors in the moveable assembly  152  of monitoring assembly  150 .  
         [0060]    Moveable assembly  152  further includes a plurality of sensor assemblies  174 , the number of which corresponds to the number of rows of sample vials  114  that the monitoring assembly  150  is configured to read. That is, if the monitoring assembly  150  is configured to read two rows of sample vials  114 , the movable assembly  118  will include two sensor assemblies  174 . For illustration purposes, FIGS. 10 and 11 show only one sensor assembly  174 . Like sensor assembly  118  described above, each sensor assembly  174  includes an excitation laser or light emitting device  122 , and an interrogation laser or light emitting device  124 , as described above. The laser  122  can be coupled to a laser assembly  176 , which includes a cooling and heating device  178  that can cool or heat the laser  122  to tune the frequency of the light being emitted by the laser  122  . In other words, because the laser  122  emits light having a single frequency, central computer  104  or another controller can control the cooling and heating device  178  to change these frequencies, thus enabling the laser  122  to scan a range of frequencies. The laser assembly  176  further includes a heat sink  180  that can dissipate heat from the cooling and heating device  178 , and thus aid in controlling the temperature of the laser  122 . A similarly configured cooling and heating device  178  and heat sink  180  can be employed in the laser assembly  120  described above (see FIGS.  4 - 8 ) to heat and cool laser  122  in that laser assembly as desired.  
         [0061]    As further illustrated, each sensor assembly  174  includes a detector  126  that is mounted to receive the laser light being emitted by laser  124  that has passed through the gas lens  136  that has been created by the light from the excitation laser  122  as described above. The lasers  122  and  124 , laser assembly  176 , and detector  126  are coupled to a laser and detector mounting bracket  182 , that is further coupled to a movable mounting bracket  184 . The movable mounting bracket  184  is coupled along slide rails  186  to a fixed mounting bracket  188 , which is coupled to rail assembly  154  for movement along rail assembly  154  by motor and pulley assembly  156 . A motor  190  is coupled to movable mounting bracket  184  and is controlled by central computer  104  or a computer in the incubation and measurement module  102  to move the movable mounting bracket  184  in a direction along arrow C as shown in FIG. 11. The motor  190  can thus position lasers  122  and  124  and detector  126  at the appropriate location along the neck of sample vial  114  to obtain the most accurate readings. Also, as can be appreciated from the above description, by moving the fixed mounting bracket  188  along rail assembly  154 , the motor and pulley assembly  156  translates the entire movable assembly  152  including the lasers  122  and  124  and detector  126  along the direction B in FIG.  10 . This movement thus positions the lasers  122  and  124  and detector  126  at the necks of the sample vials  114  in the row of sample vials  114 , so that readings can be taken from all the sample vials  114  in the row.  
         [0062]    It is further noted that the techniques described above are not limited to use with a particular type of sample vial. Rather, sample vial  114  can by any of the various types of culture vessels capable of containing the growth media. The sample vials  114  also can use various types of growth media to allow for detection and observation of the growth of mammalian cells, insect cells, bacteria, virus, mycobacteria, fungi, and other organisms which produce or consume gases as part of their growth and life cycle. The sample vials  114  can include a gas permeable membrane, slug, aliquot, or target which permits the optical interrogation of the gas signal and excludes intervening liquids or solids.  
         [0063]    The above carbon dioxide, oxygen and other gas detection techniques can also be used to test if materials which are designed to be sterile are indeed free of contamination or infection with any of the organisms listed above. Examples of materials which may be tested includes processed foods, biological preparations such as banked human blood, mammalian cell lines and prepared injectables.  
         [0064]    The photothermal spectroscopy described above for the detection of carbon dioxide and oxygen, as well as other gases, can also be used to enhance growth detection, provide presumptive speciation, and to separate background metabolism such as that caused by blood cells from bacterial or other cells. The techniques described above could also be used to determine the quantity of oxygen, carbon dioxide gas or other gases flushed into sealed containers as a preservative or stabilizer to maintain a product&#39;s shelf life or quality, or to detect immediate gas concentrations within a gas stream used, for example, in a production supply line.  
         [0065]    In the arrangements discussed above, the light emitting devices and sensors move with respect to the containers. However, it is noted that the apparatus can be configured so that the containers are housed in a rotor, drum, conveyor or the like and controlled to move past the light emitting devices and sensors which remain stationary. In this arrangement, the containers are thus sensed as they move between the light emitting devices and sensors, and the readings obtained representing the contents of the containers are evaluated in the manners described above.  
         [0066]    That is, as shown in FIGS.  13 - 17 , an instrument  200  can employ a stationary monitoring assembly as will now be described. Specifically, instrument  200  includes a housing  202  and a door  204  that is coupled to the housing  202  by a hinge  206  and a piston arrangement  208  to provide access to the interior chamber of the housing  202 . As discussed above with regard to a module  102 , instrument  200  can act as an incubation chamber to incubate the samples stored in the sample vials  114 .  
         [0067]    As shown in FIGS.  14 - 16 , sample vials  114  are loaded into openings  210  of a carousel  212 . The carousel  212  is rotatably mounted to a carousel mount  214 , which are both housed in the interior chamber of the housing  202 . The carousel is operable by a motor  216  under the control of a controller (not shown), such as the type of controller described above, to rotate in a clockwise or counter clockwise direction, as desired. The instrument  200  further includes a control panel  218  which enables an operator to set the parameters of the instrument  200 , such as the incubation temperature, speed of rotation of the carousel  212 , and so on.  
         [0068]    As further shown in FIGS.  14 - 17 , instrument  200  includes a stationary monitoring assembly  220  that is mounted to the carousel mount  214  and is used to monitor the samples in the sample vials  114  in the manner similar to that described above. However, instead of the monitoring assembly  220  moving with respect to the sample vials  114 , the carousel  212  rotates the sample vials  114  past the respective sets of lasers  122  and  124  and detector  126  so that the lasers  122  and  124  can emit laser light as described above through the respective necks of the sample vials  114 . The lasers  122 ,  124  and detector  126  are coupled to the type of circuitry shown, for example, in FIG. 9, and described above. Accordingly, as the carousel  212  is rotated to move the sample vials  114  past their respective lasers  122  and  124  and detector  126 , the photothermal spectroscopy techniques described above are performed to monitor the concentration of a gas, such as oxygen or carbon dioxide, or the concentration of a liquid or solid, in the sample vial to thus detect for microorganism growth in the sample vial based on the monitored concentration.  
         [0069]    Although only a few exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.