Patent Publication Number: US-7224448-B2

Title: Apparatus and methods for evaluating an optical property of a liquid sample

Description:
BACKGROUND OF THE INVENTION 
   There are many use environments, the fields of medical research and pharmaceutical development being examples, where it is necessary to accurately acquire fluid samples with volumes that may be as small as a few nanoliters. In these same fields, it is also often desirable to measure optical characteristics of the acquired fluid samples. Such optical characteristics include, for example, the ability of a sample to absorb light. 
   For instance, UV-Visible Spectrophotometry may be used to characterize the chemical composition of a liquid sample (in solution or suspension phase) using the absorbed spectra of the sample. The light absorbance of a sample depends on the pathlength L of light passing through the sample, as well as on the concentration of light absorbers (e.g., biomolecules, cells, etc) in a sample solution and the wavelength (λ) of light being used to characterize the sample. The wavelengths of UV-Visible light span from 200 nm to 800 nm, while ultraviolet wavelengths range from 200 to 400 nm. 
   UV-Visible spectrophotometry provides a convenient analysis technique to determine the concentration, purity, and integrity of a biological sample without requiring additional sample preparation other than acquiring a sample. UV-Visible Spectrophotometry measurements depend on the light source (UV lamp), the sample and sampling technique. Most biological samples absorb electromagnetic radiation at wavelengths ranging from 200 nm to 800 nm, mostly 230, 260 and 280 nm. For a DNA or RNA sample in aqueous phase, one unit of absorbance 1 Å measured at a λ 260 nm and a pathlength of 10 mm is equal to 50/(40) ng/μl concentration. 
   Most biological samples are highly concentrated for downstream processing (such as microarray spotting or protein sample preparation for mass spectrometers). The absorbance of such samples can be above the saturation limit for typical spectrophotometers if the pathlength is about 10 mm. While the sample concentration range can be extended by diluting the sample, diluting sample requires additional laboratory work and can result in errors. Other approaches are needed to extend the sample concentration range that can be evaluated by the instrument. 
   Sampling techniques used in conventional UV-Visible Spectrophotometers include utilizing a cuvette with an optical window and fixed optical pathlength that holds a sample in a semi-closed way, direct measurement of liquid sample in a sample container (e.g., a well) along with a real-time pathlength measurement, and using a cuvetteless sample held in semi-free space between optical fibers which define a light path from a light source to a detector. 
   The cuvette-based sampling technique is widely used in conventional UV-Visible spectrophotometers. Generally, a sample is pipetted into a cuvette that has either a 10 mm or 2 mm path length. This technique is very limited for most biological samples since cuvettes typically used generally require a minimum 10 μl sample, which is problematic for valuable biological samples which may be present in limiting quantities, such as samples of protein or nucleic acids. A cuvette made of quartz or silica is expensive so it is typically reused after cleaning and drying. Further, adding 10 μl of sample with a pipette into a cuvette sometimes produces an air-bubble interface in the light path that can cause measurement error or void measurements. Additionally, a pathlength of 2 mm or 10 mm limits the sample concentration that may be measured to 1000 ng/μl for DNA/RNA sample due to the limited dynamic range of absorbance of most spectrophotometers. 
   Direct UV-Visible spectrophotometry measurement of liquid samples also suffers from limitations, such as the need to determine pathlength and adjust sample concentration. Pathlength depends on the sample well dimensions and sample volume. The determination of pathlength requires use of instruments such as level detectors or position sensors. For a pathlength ranging from 2 mm to 10 mm or above, the workable range of sample concentration for a spectrophotometer measurement becomes limited. For an example, for a DNA sample, if the pathlength is 10 mm, one unit of absorbance is equal to 50 ng/ul concentration (OD), and the upper limit of detection is typically 250 ng/μl if the upper limit absorbance of the spectrophotometer is 5. In this case, sample dilution is required for a sample concentration greater than 250 ng/μl. Sample dilution for multiple well plate measurements can be a complex laboratory operation. 
   Cuvetteless sampling also suffers from drawbacks. For example, in cuvetteless sampling, typically a narrow beam of light is directed to a sample stage that consists of a 1-2 μl liquid droplet suspended between two multi-mode optical fibers, one source-side fiber which provides light from a light source to the droplet and a detection-side fiber that guides light from the droplet to appropriate detection optics. The close proximity between the source-side and detection-side fibers allows enough of the light cone emanating from the source-side fiber to be collected by the detection-side fiber after passing through a liquid sample. 
   Cuvetteless instruments typically require a clamping surface that can be wetted with sample to avoid an air-bubble interface. Carry-over contamination is not completely removed with a simple wiping-off of the clamping surface. Adding a small amount of sample (1 μl) to the center of the clamping surface is also a complicated lab technique. 
   In summary, existing sampling techniques used in the conventional UV-Visible Spectrophotometers generally require too much sample, provide insufficient confidence in the sample application technique, may result in carry-over contamination, and may require pathlength determination and/or dilution of sample, over a range of solution concentrations. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention provides an apparatus for acquiring and holding a small volume of a liquid sample (e.g., less than about 5 μl, or about 2 μl or less) whose optical properties may be detected, monitored and/or quantitated without determining an optical pathlength for the sample and/or apparatus in which the sample is placed. 
   In one embodiment, the apparatus includes a body having a first opening located at a first end, a second opening located at a second end, two planar inner surfaces, and two planar outer surfaces, where the two planar outer surfaces are substantially parallel to the two planar inner surfaces. 
   An inner space within the body connects the first opening and the second opening and provides a passage from the first opening to the second opening. The two inner surfaces form two sides of a portion of the passage. In one aspect, the two surfaces are parallel for at least a portion of their length. In another aspect, the passage defined by the space between the two parallel portions of the body constitutes a measurement area of the device. The passageway may be square, rectangular or polygonal. In one aspect, the pathlength of a light passing through a measurement area of the apparatus is predetermined. In another aspect, the pathlength is less than about 10 mm, less than about 5 mm, less than about 2 mm, or about 1 mm or less. 
   At least a portion of the body is made of material semi-transparent or transparent to electromagnetic radiation in some wavelength range that is detectable by a detection system being used. In one aspect, at least one of the first opening and second opening have a dimension sufficiently small to enable a liquid to enter and be held within the passage by capillary forces sufficient to hold the liquid sample against opposing forces such as gravity or other forces such as small pressure changes or gentle thermal expansion. 
   In another aspect, the invention provides an adaptor for providing a substantially gastight connection to a pipette/pipettor (e.g., such as a Pipetman®, a Gilson®, Rainin®, Eppendorf® or Finnipipette® pipette) or a fluid-dispensing device. In one aspect, the adaptor is configured in the shape of a pipette tip. 
   In another embodiment, the invention provides a holder comprising a housing capable of receiving the apparatus body. In one aspect, the holder housing has two openings that are substantially aligned to define a light transmission path for electromagnetic radiation when the hollow body is held in the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
       FIGS. 1   a ,  1   b ,  1   c  and  1   d  are views of a schematic representation of an embodiment of the apparatus of this invention; 
       FIGS. 2   a ,  2   b  and  2   c  are views of a schematic representation of an embodiment of an adaptor of this invention; 
       FIGS. 3   a ,  3   b ,  3   c ,  3   d  and  3   e  are views of a schematic representation of another embodiment of the apparatus of this invention; 
       FIGS. 4   a ,  4   b  and  4   c  are views of a schematic representation of yet another embodiment of the apparatus of this invention; and 
       FIG. 5  is a block diagram illustrating the light absorbance in the pathlength defined by an apparatus of the invention in a holder housing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Before describing the present invention in detail, it is to be understood that this invention is not limited to specific apparatuses, method steps, or equipment, as such may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. Methods described herein may be carried out in any order of the recited steps that is logically possible. Furthermore, where a range of values is provided, it is understood that every intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. Also, it is contemplated that any optional feature of the inventive embodiments and aspects described herein may be set forth and claimed independently, or in combination with any one or more of the features described herein, or may be specifically excluded. 
   Unless defined otherwise below, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Still, certain terms are defined herein for the sake of clarity. 
   The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates, which may need to be independently confirmed. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. 
   It must be noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a biopolymer” includes more than one biopolymer, and the like. 
   It will also be appreciated that throughout the present application, that words such as “upper”, “lower” are used in a relative sense only. 
   The following definitions are provided for specific terms that are used in the following written description. 
   A “biopolymer” is a polymer of one or more types of repeating units. Biopolymers are typically found in biological systems and particularly include polysaccharides (such as carbohydrates), peptides (which term is used to include polypeptides and proteins, such as antibodies or antigen-binding proteins), glycans, proteoglycans, lipids, sphingolipids, and polynucleotides as well as their analogs such as those compounds composed of or containing amino acid analogs or non-amino acid groups, or nucleotide analogs or non-nucleotide groups. Biopolymers may be heterogeneous in backbone composition thereby containing any possible combination of polymer units linked together such as peptide-nucleic acids (which have amino acids linked to nucleic acids and have enhanced stability). 
   Polynucleotides include single or multiple stranded configurations, where one or more of the strands may or may not be completely aligned with another. A “nucleotide” refers to a sub-unit of a nucleic acid and has a phosphate group, a 5 carbon sugar and a nitrogen containing base, as well as functional analogs (whether synthetic or naturally occurring) of such sub-units which in the polymer form (as a polynucleotide) can hybridize with naturally occurring polynucleotides in a sequence specific manner analogous to that of two naturally occurring polynucleotides. Biopolymers include DNA (including cDNA), RNA, oligonucleotides, PNA, LNA and other polynucleotides as described in U.S. Pat. No. 5,948,902 and references cited therein, regardless of the source. 
   “Communicating information” refers to transmitting the data representing that information as signals (e.g., electrical, optical, radio, magnetic, etc) over a suitable communication channel (e.g., a private or public network). 
   As used herein, a component of a system which is “in communication with” or “communicates with” another component of a system receives input from that component and/or provides an output to that component to implement a system function. A component which is “in communication with” or which “communicates with” another component may be, but is not necessarily, physically connected to the other component. For example, there may be a structural, functional, mechanical, optical, or fluidic relationship between two or more components or elements, or some combination thereof. As such, the fact that one component is said to communicate with a second component is not intended to exclude the possibility that additional components may be present between, and/or operatively associated or engaged with, the first and second components. 
   The term “assessing” and “evaluating” are used interchangeably to refer to any form of measurement, and includes determining if an element is present or not. The terms “determining,” “measuring,” and “assessing,” and “assaying” are used interchangeably and include both quantitative and qualitative determinations. Assessing may be relative or absolute. “Assessing the presence of” includes determining the amount of something present, as well as determining whether it is present or absent. 
   The term “using” has its conventional meaning, and, as such, means employing, e.g. putting into service, a method or composition to attain an end. 
   An apparatus for holding small volume liquid samples with a predetermined pathlength is described hereinbelow. 
   In one embodiment, the apparatus includes a body having a first opening located at a first end, a second opening located at a second end, two planar inner surfaces, and two planar outer surfaces, where the two planar outer surfaces are substantially parallel to the two planar inner surfaces (such that each inner surface has a corresponding planar outer surface to which it is substantially parallel). 
   The body connects the first opening and the second opening and comprises a passage from the first opening to the second opening whose walls are formed by the inner surfaces of the hollow body. In one aspect, the two inner surfaces are parallel for at least a portion of their length, forming a measurement area that is polygonal in cross-section (e.g., square or rectangular). The measurement area of the device comprises a portion of a first inner surface and its corresponding outer surface, a portion of a second inner surface and its corresponding outer surface, and a passage between them for holding a liquid sample by capillary action. In one aspect, the distance from a first inner surface to a second parallel inner surface is less than about 10 mm, less than about 5 mm, less than about 3 mm, 2 mm or less. The distance, in certain aspects, ranges from about 0.1 to 2 mm. 
   An embodiment of the apparatus  110  of this invention is shown in  FIGS. 1   a ,  1   b ,  1   c  and  1   d . In the embodiment shown in  FIGS. 1   a - 1   d , the apparatus comprises a body  110  that has two open ends  120 ,  130  with corresponding openings  125 ,  135 . The body  110  of the embodiment shown in  FIGS. 1   a - 1   d  has a square shaped flow channel  150 , having at least planar inner  157 ,  167  and outer surfaces  155 ,  165 , where the planar outer surfaces  155 ,  165  are substantially parallel to the planar inner surfaces  157 ,  167 . 
   The body  110  of the embodiment shown in  FIGS. 1   a - 1   d  comprises a dimension sufficiently small to hold a liquid sample within the passageway by capillary action, despite opposing forces such as gravity. 
   In one aspect, at least one inner surface  157 ,  167  and its corresponding parallel outer surface  155 ,  165  is at least partially transparent. In another aspect, at least two inner surfaces  157 ,  167  and their corresponding parallel outer surfaces  155 ,  165  are at least partially transparent. An “at least partially transparent” material, as used herein, refers to a material that transmits sufficient light that may be detected by a detection device in an optical instrument (e.g., such as a spectrophotometer). In one aspect, an “at least partially transparent material has at least about 50% transmittance of electromagnetic radiation. 
   In a further aspect, first  157  and second  167  at least partially transparent inner surfaces are sufficiently parallel to each other and dimensioned to hold a liquid sample between them in a measurement area by capillary action, such that light from a light source may pass through the first inner surface  157  (and its corresponding outer surface  155 ), the liquid sample, and the second inner surface  167  (and its corresponding outer surface  165 ). 
   Materials used to form the at least partially transparent portion(s) of the body may vary and may include any at least partially transparent material, for example, a polymeric material such as polyimide, polycarbonate, polystyrene, polyolefin, fluoropolymer, polyester, a nonaromatic hydrocarbon, polyvinylidene chloride, polyhalocarbon, such as polycholortrifluoroethylene. Polyolefins may include polyethylenes, polymethylpentenes and polypropylenes, and fluoropolymers may include polyvinyl fluorides. Other materials glass, quartz, silica, silicon rubber, such as crosslinked dimethyldisiloxane, or materials used in optical crystals, such as sapphire or garnet (e.g., undoped Yttrium Aluminum Garnet). In certain aspects, the material transmits light with a range of about 200-1100 nm, from about 180-1000 nm, and/or transmits light of a wavelength greater than about 900 nm. The apparatus of this invention can be manufactured by casting or molding or other methods routine in the art. 
   In certain aspects, materials and dimensions are selected to ensure that a measured signal relating to a sample within the measurement area of the body remains within the limit of the linear range for measurements by a particular detection device with which the apparatus of this invention is used (e.g., such as a spectrophotometer, photometer, spectrofluorometer, and the like). 
   In one aspect, the body comprises an outer coating or clad  160 . In certain aspects, the outer coating reduces stray light (light other than from a light source being using by the optical detector) during optical measurement. In one aspect, the coating comprises a UV absorber. 
   However, in another aspect, at least a portion of the body is not coated to provide an optical window or aperture  140 . In one aspect, a clad is stripped at one section to form the optical window  160 . In another aspect, to reduce surface scattering during the optical measurement, the outer surface of the aperture window is smooth. Portions of the surface may be removed to create the desired smooth surface (e.g., by laser machining) or materials may be added to create a smooth surface (e.g., an at least partially transparent coating may be provided). 
   In embodiments in which the optical window-containing portion of the body is directly dipped into a liquid sample, a portion of the surface area of the body  110  may, in one embodiment, be coated by a hydrophobic coating to eliminate/prevent any liquid sample residue remaining on the outer surface of the body  110 . In one aspect, the coating is less than about 1 μm in thickness. In another aspect, the coating is transparent or semi-transparent to electromagnetic radiation. An exemplary embodiment of a hydrophobic coating material comprises a siloxane, for example, the coating may be polydimethylsiloxane silicon rubber, PTFE (e.g., Teflon), a polyacrylate, and the like but this invention is not limited to only these exemplary embodiments. 
   In one aspect, the passage connecting the first and second openings comprises a channel. Channel dimensions may range from about 50 μm×50 μm to about 2 mm×2 mm, and in one aspect, the passage holds a minimum channel volume of about 25 nl. In certain aspects, the passage comprises varying channel dimensions at one or more sections through the length of the body. However, in one aspect, the channel dimensions within the measurement area do not vary. 
   In some embodiments, the length of the body  110  ranges from about 2 mm to about 100 mm. In one embodiment, an optical window  140  is positioned approximately 0-10 mm above the bottom end  130 , and has an aperture width ranging from about 1 mm to about 10 mm. In one embodiment, but not a limitation of this invention, the outer coating  160 , which covers a portion of the body excluding the optical window, is round, with diameter ranging from about 0.3 mm to approximately 3 mm. The outer coating  160  can be, but is not limited to, a polymeric material such as polyamide. The outer coating  160  can also be an at least partially UV transparent material. 
   In one aspect, at least a portion of the body  110  is comprised of a material capable of allowing transmission of electromagnetic radiation of sufficient intensity to enable performance of an optical measurement (e.g., the material is a semi-transparent or a transparent material). In one embodiment, at least the optical window is comprised of a semi-transparent or a transparent material. The material is also capable of maintaining a liquid sample within the measurement area of the device by capillary action. 
   In one embodiment, the invention provides an adaptor  190  ( FIG. 2   a ) that can be used to connect the body to a pipette, (a pipette as used herein, unless otherwise specified, refers to that aspiration causing portion of a pipette e.g., such as a Pipetman®, a Gilson®, Rainin®, Eppendorf® or Finnipipette® pipette, also referred to as pipettor), a fluid-delivery device or to an interface to such a device (e.g., to a pipette tip). In one aspect, the adaptor comprises a first opening and a second opening and walls defining a lumen through which a fluid (liquid or air may pass). In another aspect, the first opening is dimensioned to receive a portion of the apparatus body while a second opening is dimensioned to receive an end of a pipette, a pipettor, a fluid-delivery device or to an interface to such a device. In a further aspect, the first opening is polygonal (e.g., square or rectangular) while the second opening is round or elliptical or oval. The lumen may comprise a varying internal diameter for at least part of its length to further conform to the dimensions of a tapering end of a pipette. 
   An example of such an adaptor for use with a pipette is shown in  FIGS. 2   a - 2   c . In operation, a pipette may be used for aspirating the sample into the body  110 .  FIGS. 2   a - 2   c  show an embodiment  190  of the adaptor with one end  170  having an opening  175  capable of providing a substantially gastight connection to one end of the body ( 110 ,  FIG. 1   c ) and another end  180  having an opening  185  capable of connecting to a conventional pipette (e.g., such as a Pipetman®, a Gilson®, Rainin®, Eppendorf® or Finnipipette® pipette, also referred to as pipettor). In one embodiment, the adaptor comprises a pipette tip that forms a substantially gas-tight connection with a body  110  configured, e.g., as shown in  FIGS. 1   a ,  1   b  and  1   c . The adaptor can be made any suitable material because it will generally not contact liquid sample and will not be in light path. 
     FIGS. 3   a - 3   e  show another embodiment  210  of an apparatus of this invention in which a body  210  comprises at least two sections. In this embodiment, the body  210  has two ends  220 ,  230 , each end having an opening  295 ,  250 . The first end  220  with opening  295  is capable of connecting to a conventional pipette for aspiration and dispensing, while the second end  230  with opening  250  is capable of being dipped into a liquid well for aspirating liquid. In one embodiment, the body  210  also has two passageways, shown in the Figure as flow channel sections  240  and  260 , each having different dimensions. Both of the flow channel sections  240 ,  260  have parallel inner and outer surfaces which are substantially planar, forming a flow channel  250 ,  270 , with different dimensions L 1  and L 2  flow channel length H 1  and H 2 , and flow width (aperture width) W 1  and W 2 . In one aspect, the flow channels are rectangular. In another aspect, at least one of the flow channels has an aspect ratio less than 1. 
   In one aspect, the two flow channels are joined by a taper transition area  280 , that has internal rectangular flow channel. In one embodiment, the upper section  210 , which includes the first end  220  with opening  295 , generally has a round taper shape  295  in order to fit a conventional pipette. In one aspect, the openings  250  are co-centered and the flow channels share the same longitudinal axis. In another aspect, at least one of the flow channels comprises dimensions that are suitable for holding a liquid sample within the flow channel by capillary action. 
   In the embodiment shown in  FIGS. 3   a - 3   e , the two planar flow sections  240  and  260  are designed to minimize sample volume. An of flow channel  240  is a channel with dimensions 0.2 mm(L 1 )×4 mm(H 1 )×2 mm(W 1 ), to produce a 1.6 μl channel volume. Another example of a flow channel  260  is a channel with dimensions of 0.5 mm(L 2 )×3 mm(H 2 )×3 mm(W 2 ) to provide a 4.5 μl channel volume. The total volume of two channels  250  and  270  including the transition section is less than 8 μl. 
   The dimensions of sections of the body may vary and are not limiting features of the invention. However, in certain aspects, L 1  ranges from about 0.05 to about 5 mm, L 2  ranges from about 0.05 to about 10 mm, H 1  ranges from about 0.25 to about 50 mm, H 2  ranges from about 0.25 to about 50 mm, W 1  ranges from about 0.25 to 25 mm, and W 2  ranges from about 0.25 to about 25 mm. 
   In one aspect, flow channel  150  ( FIG. 1   c ) or two flow channel sections  240  and  260  ( FIG. 3   a ) define an optical path comprising a substantially predetermined pathlength for transmission of electromagnetic radiation. 
   In another embodiment, shown in  FIGS. 4   a ,  4   b  and  4   c , the invention further provides a holder comprising a housing  310  capable of receiving a body  110  or  210  and of holding the body ( 110  or  210 ). The housing shown in  FIG. 4  has two co-axial side openings  325 ,  335  with an axis of each of the openings perpendicular to the housing axis, the two openings being substantially aligned. In one aspect, the center of the optical window of the body is co-centered with the axes of the openings and the surface of an optical window is perpendicular to excitation light from a source light in an instrument in which the apparatus of this invention is used (e.g., such as a spectrophotomer). The openings  325 ,  335  and the body  110  define a transmission path for electromagnetic radiation when the body  110  is held in the housing  310  and are adapted to receive electromagnetic radiation. Since when the body  110  is held in the housing  310 , the center of the optical window of the body is co-centered with the axes of the openings and the planar surface of the optical window is substantially perpendicular to the longitudinal axis of a beam of excitation electromagnetic radiation, the housing  310  does not require focusing optics. 
   In another embodiment, the openings  325 ,  335  are capable of receiving portions (e.g., such as ends) of optical waveguides such as fiber optic connectors. In that embodiment, which is shown in  FIG. 4   a , both source-side and detection-side optical fibers  320  and  330 , respectively, are provided. 
   The center of the optical window of the body may be co-centered with the axes of a collimated source of electromagnetic radiation or with the longitudinal axes of optical waveguides. For example, a planar square surface of an optical window may be provided which is perpendicular to excitation light from a light source. The bottom face of the housing is not necessary closed, but in one aspect, a closed bottom reduces the stray light (e.g., non-source light) getting into the sample pathlength in order to improve the sensitivity of optical measurement. In certain aspects, an adaptor  340  may be used to interface the top face of the housing with the body of the apparatus to reduce stray light. 
   In one embodiment, the housing  310  can seat the body  110  (or at least a measurement area of the body) at a position that aligns the optical window of the body  110  with a light path defined by source-side and detection side optical fibers, such that sufficient light from the source-side fiber passes through the window to the detection-side optical fiber to be detected by a detector and distinguished from background signal (e.g., produced by a blank). 
   In certain embodiments in which a body  210  comprises at least two optical windows, the housing  310  can seat the body  210  at two positions (shown by double arrow line in  FIG. 4   c ) to align one of the two optical windows to the openings at each position. The apparatus may be positioned within the housing by manually pressing frictional or mechanical detents or by providing an automatic and/or motor-assisted element that can move in an appropriate direction (e.g., see,  360  in  FIG. 4   a ), for example. Such frictional or mechanical detents and/or motor-assisted elements comprise exemplary representations of a securing component. The securing component positions the measurement region including at least one optical window in order to provide a transmission path. 
   In one aspect, during operation, the apparatus of this invention is fitted to a conventional pipette (a pipette or pipettor, as used herein, unless otherwise specified, refers to that aspiration-causing portion of a pipette or pipettor, such as a Pipetman®, a Gilson®, Rainin®, Eppendorf® or Finnipipette® pipette) by a substantially gas-tight fitting, either directly, as in embodiment  210 , or indirectly, e.g., using an adaptor  190 . 
   In one aspect, the pipette/pipettor is used to aspirate a liquid sample, for example, a biological sample comprising a biopolymer such as a nucleic acid, peptide, polypeptide and/or protein, into the body ( 110  or  210 ) of the apparatus. After a sufficient amount of sample (e.g., 1-2 μl) is aspirated into the body ( 110  or  210 ) to fill a measurement area (e.g., such as an entire optical window channel region), the body is placed into a housing  310 , as shown in  FIGS. 4   a ,  4   c . The body can be moved in an axial direction in the housing  310  to provide one more predetermined pathlength measurements. In one aspect, due to the dimensions of the passage defined by the inner walls of the body, capillary force will hold the liquid sample in the measurement area. Both vertical placement of the tip and capillary force may be used to substantially eliminate the likelihood of air bubbles being generated. After completion of optical measurement, the body ( 110  or  210 ) may be pulled out of the housing and discarded. 
     FIG. 5  illustrates the measurement of an optical property of a sample held in the measurement area of an apparatus according to the invention. Referring to  FIG. 5 , the total signal (A total ) (e.g., such as absorbance) would be
   A   total =2 A   body wall   +A   sample   &gt;A   sample   
Where  2 A body wall  refers to the signal contributed by a first and second parallel walls of the apparatus body which define the measurement area (each wall comprising an inner and outer side) and A sample  refers to the signal contributed by a sample held between the walls by capillary action.
 
   Since a blank measurement is generally required before the sample measurement, the actual measurement reading would still be A sample  (i.e., the signal from the body walls would be subtracted). Hence, the optical signal produced by a wall of the apparatus will not affect the measurement of an optical signal from the sample. 
   The invention also provides methods for detecting, monitoring (e.g., determing a change in) and/or quantitating an optical property of a sample. 
   In one aspect, the method comprises placing the measurement area of an apparatus according to the invention in a positional relationship to a light source and detector of an optical detection device (e.g., a spectrophotometer, photometer, spectrofluorometer, and the like) such that a light path is provided from the light source through the measurement area, to the optical detection device. In certain aspects, the light path is at least partially defined by an optical waveguide, for example a source-side optical fiber and/or a detection-side optical fiber. In certain other aspects, other optical elements may be used to further define the light path. In one aspect, a sample, such as a liquid sample, is held in the measurement area by capillary force and the detector detects, monitors and/or quantitatively identifies an optical property of the sample (e.g., such as absorption, emission, or scattering of light). In one aspect, the concentration of a component (e.g., a nucleic acid, polypeptide, peptide, or protein) in a sample can be determined by comparing light transmission by a sample without the component to light transmission by a sample with the component. A standard curve may be used in certain cases to correlate optical properties (e.g., such as absorbance) with characteristics of the sample (e.g., such as concentration of a biomolecule within the sample). 
   In one embodiment, a liquid sample is placed within the measurement area of the apparatus by interfacing the apparatus body with a pipette or pipettor, either directly or indirectly using an adaptor as described above, and aspirating a sample from a sample source into the measurement area. In certain aspects, the apparatus may be placed into a sample holder-receiving area of an optical device (such as a spectrophotometer) or into a cartridge for receiving such a sample holder, which may be placed in the device. In one aspect, the ejector of a pipette/pipettor can be used to place the apparatus into the sample holder-receiving area or cartridge. 
   Although embodiments of the invention have been described with respect to applications to specific liquid samples (analytes) and specific optical equipment, it should be noted that these are not limitations of this invention and are only presented for exemplary purposes. 
   Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.