Liquid photometer using surface tension to contain sample

[0002] This invention relates to the fields of photometry, spectrophotometry, fluorometry, spectrofluorometry and the like and their use in optically quantitating and or characterizing liquids and solutions.

[0003] This invention relates to the field of spectrophotometers and related instruments. More particularly the invention relates to ultra low volume instruments working in the volume range of 2 microliters or less. Such devices are particularly useful in quantitation of biotechnology samples including nucleic acids or proteins where it is desirable to keep sample loss and/or cross-contamination to a minimum.

[0004] The prior art contains examples of attempts to supply low volume spectrophotometers. World Precision Instruments of Sarasota, Fla. offers parts from which an instrument handling less than 20 microliters can be built for around $3000. This uses a fiber optic dipping probe with a tip diameter of 1.5 mm (DipTip®), their miniature fiber optic spectrometer and F-O-Lite H light source. With a deuterium lights source (D2Lux) a UV spectrophotometer can be constructed.

[0005] U.S. Pat. No. 4,643,580 to Gross et al. discloses a photometer head in which there is a housing for receiving and supporting small test volumes. A fiber optic transmitter and receiver are spaced within the housing so that a drop can be suspended between the two ends.

[0006] McMillan, in U.S. Pat. No. 4,910,402, discloses apparatus in which a syringe drops liquid into the gap between two fixed fibers and an IR pulse from a LED laser is fed through the droplet. The output signal is analyzed as a function of the interaction of the radiation with the liquid of the drop.

[0007] Ocean Optics, of Dunedin, Fla. 34698 supplies a SpectroPipetter for microliter-volume samples using a sample volume of about 2 microliters. The optics not only carry light to and from the samples but act as a plunger to load the sample. The tip of the pipette includes the sample cell.

[0008] Liquids, mixtures, solutions and reacting mixtures are often characterized using optical techniques such as photometry, spectrophotometry, fluorometry, or spectrofluorometry. In order to characterize samples of these liquids, the liquid is usually contained in a vessel referred to as a cell or cuvette two or more of whose sides are of optical quality and permit the passage of those wavelengths needed to characterize the liquid contained therein. When dealing with very small sample volumes of say from 1 to 2 microliters, it is difficult to create cells or cuvettes small enough to to be filled and permit the industry standard 1 cm optical path to be used. It is also difficult and/or time consuming to clean these cells or cuvettes for use with another sample. In the case of photometry or spectrophotometry, the value most commonly sought is the sample absorbance A defined by

[0009] A=−log T

[0010] Where T is the transmittance, or

[0011] A=log(I/I₀)

[0012] where I₀is the level of light transmitted through a blank sample (one containing all components except the one being measured or one whose absorbance is known to be negligible and with optical properties identical to those of the sample being measured), and I the level of light transmitted through the sample being measured. Most commonly the absorbance value is measured in a cell or cuvette with a 1 cm path length. However, Lambert's Law states that for a collimated (all rays approximately parallel) beam of light passing through a homogeneous solution of uniform concentration the absorbance is proportional to the path length through the solution. For two path lengths X and Y,

[0013] (Absorbance x)/(Absorbance y)=(Pathlength x)/(Pathllength y)

[0014] Thus it is reasonable that absorbance can be measured with path lengths other than 1 cm and corrected for path length to the equivalent value for a 1 cm path which can be more easily compared to data from other spectrophotometers. The sample path lengths in the range of 0.2 to 2 mm used in this invention can be used to generate absorbance values that can be easily corrected to the 1 cm path equivalent.

[0015] Although liquids confined by surface tension and an optical surface are well known, e.g. raindrops on the window, establishing a collimated optical light path of known length through such confined liquids has been perceived as difficult. The recent advent of small spectrometers designed to be used with fiber optics has made it possible to consider spectrophotometric geometries not readily possible before.

[0016] It is the object of the instant invention to make use of the surface tension of a microliter or submicroliter sample of liquid to provide suficient means to confine it within the analysis region of an optical analysis instrument and to carry out the requisite measurement.

[0017] The invention is an optical instrument for photometric, spectrophotometric, fluorometric or spectrofluorometric analysis of liquids contained between two substantially parallel surfaces on anvils spaced apart a known distance, wherein the sample liquid is confined by the surfaces and the surface tension of the liquid. Each of these surfaces contain an optical fiber mounted coaxially with each and perpendicular to the parallel confining surfaces The shape and nature of the surfaces serve to confine the liquid so as to center the confined droplet in the optical path of the optical fibers embedded in the surfaces. An apparatus supporting the surfaces permits the surfaces to be controllably separated to allow loading of the sample and cleaning of the surfaces after sample analysis.

[0018] For some applications, the optical fibers can be replaced by miniature sources like light emitting diodes (LEDs) and detectors or detectors with optical filters. The LEDs with their characteristically small emitting area would replace the source fiber and small solid state detectors with associated filters like those used in color charge coupled devices (CCDs) for imaging would replace the receiving fiber and spectrometer.

[0019] FIG. 1 is a drawing of a cutaway section of the optical path of the invention.

[0020] FIG. 2 is a drawing of a cutaway section of the optical path of the invention with additional fluorometric source optical fibers shown.

[0021] FIG. 3ais a perspective drawing of a process by which the sample is loaded into the optical path of the invention.

[0022] FIG. 3bshows the sample between the two anvils in the measurement position.

[0023] FIG. 3cshows the sample compressed between the two anvils in the short path or sample compression position.

[0024] FIG. 3dshows the sample column drawn by opening the anvils from the sample compression position to the sample measurement position.

[0025] FIG. 4ais a perspective drawing of apparatus of the invention in its open position with the source and spectrometers.

[0026] FIG. 4bshows the apparatus in its closed position as it would be for sample compression or analysis.

[0027] FIGS. 5aand 5bshow a system with four spectrophotometer systems operating simultaneously for use on robotic platforms in both the loading, 5a, and measuring, 5b, configurations respectively.

[0028] FIG. 6ais a perspective view of the open instrument of the invention showing two pipetter tips loading two parts of a mixture each on either side of the measurement apparatus.

[0029] FIG. 6bshows the instrument of FIG. 6aclosed.

[0030] FIGS. 7aand 7bshow perspective views of two path lengths provided by the apparatus of the invention so that a differential absorbance path is established.

[0031] FIG. 8 shows a perspective view of a measurement made in the instrument of the invention holding the sample between two transparent sheets.

[0032] FIG. 9 shows a perspective view of a measurement made in the instrument of the invention holding the sample within a transparent container with flexible walls.

[0033] The liquid sample shown in FIG. 1 is contained by its surface tension between surfaces 2 and 7 also shown in FIG. 1. Light 3 from the system source coming through the fiber contained in and flush with surface 7 radiates upward through the liquid sample and is collected by the larger fiber or light pipe 6 in the upper surface 2 and sent on to the analysis photometer or spectrometer for absorbance measurements. For making fluorescence measurements, the excitation light can also be brought to the sample through fibers surrounding the collection fiber 6 as is shown in 83, FIG. 2, This reduces the need for high excitation wavelength rejection on the part of the spectrometer collecting the fluorescence light from the sample through collection fiber 6.

[0034] Samples are loaded onto the lower source with a pipetting means 40, FIG. 3asuch as a 2 microliter Pipetteman® from the Ranin® Corporation of Woburn, Mass., part of the tip of which is shown in 40. The droplet 5 when emptied from the pipette, if of sufficient volume, will spread to cover the lower anvil surface 7, FIG. 1 which is typically the end of an industry standard SMA fiber optic connector 10, FIG. 3a(found as connectors on the ends of optical patch cords like p/n P-400-2-UV-VIS from Ocean Optics inc. of Dunedin, Fla.) until it encounters the sharp edge shown as 4, FIG. 1. For most SMA connectors the approximate 2 mm end diameter can be effectively covered with 2 microliters of water or a water-based solutions.

[0035] The upper second surface is brought into the measurement position 18, FIG. 3band then into close proximity 20 to the lower surface 7, FIG. 3c, making contact with the deposited droplet wetting the entire confining surface before returning to the sample measurement position and drawing up the sample measurement column 30, FIG. 3dshown in 25. By applying blank samples, samples missing the component being analyzed, the difference in transmitted light intensity can be used to characterize the sample according to

[0036] A=−log (I/I₀)

[0037] where I₀is level of transmitted light with the component being analyzed absent, and I is the level of light transmitted through the sample and A is the absorbance value which can be related to the concentration of the component being analyzed by Beer's law which states that for solutions 1 and 2 that $\frac{\left(\mathrm{Absorbance}1\right)}{\left(\mathrm{Absorbance}2\right)}=\frac{\left(\mathrm{Concentration}1\right)}{\left(\mathrm{Concentration}2\right)}$

[0038] Thus, when compared with a blank sample, the concentration can be directly determined from the absorbance A.

[0039] The SMA optical fiber connectors can be held in place by apparatus shown in FIGS. 4aand 4b. The light from the system source 74 (Ocean Optics inc. p/n DT-1000, a combined deuterium arc and quartz halogen incandescent lamp, alternatively a xenon flashlamp can be used) is coupled through a bifurcated optical fiber assembly (Ocean Optics inc. p/n BIF-100-2-UV-VIS) to the apparatus 50 with the second fiber of the bifurcated assembly going to reference slave spectrometer 72 (Ocean Optics p/n SD2000). The SMA connectors 53 are mounted to the apparatus by means of threaded couplers 52 (World Precision Instruments p/n 13370) which are threaded into the apparatus. The swing arm 56 carrying the upper optical fiber 53 can be lowered to align the source with the detection fiber as is shown in 60, FIG. 4b. The swing arm spacing is controlled by pin 62 resting on the plunger 67 of solenoid 68 (Lucas Ledex of Vandalia, Ohio p/n 174534-033) shown mounted below the apparatus. The other end of the solenoid plunger 67 rests on spring plunger 58 (Manhattan Supply of Plainview, N.Y. p/n 82412032). The sample can be compressed as is shown in 20, FIG. 3cby pushing on the swing arm 56 so as to push the solenoid plunger to the limit of its travel or by actuating the solenoid electrically and pulling the plunger to its stop. The swing arm is held in both its up and its down position by magnet 64 (Edmund Scientific of Barrington, N.J. p/n J35-105) attracted to either post 66 or allen head cap screw 54. With the swing arm in its raised position, the sample detection surfaces may be cleaned by wiping the sample from both the lower and upper SMA connector ends before reloading.

[0040] Light transmitted through the sample is collected by the upper fiber and coupled to the detection master spectrometer 70 (Ocean Optics p/n SD2000). The fiber is confined to minimize flexure of the fiber which would cause unnecessary variability in its transmission and thus unnecessary variation in measured absorbances. The swing arm is precision pivoted on a shaft turning in two preloaded ball bearings 66. Note that the instrument is opened by turning through an arc sufficient to permit loading and cleaning. See FIG. 4. Preferably it is opened at least 90 degrees as seen in FIG. 4 and more preferably the arc approaches 160 degrees as seen in FIG. 6a.

[0041] In order to accomplish illumination of the sample for fluorescence as shown in 83, FIG. 2, the upper fiber assembly would need to be custom made so as to surround the collection fiber with illumination fibers. These would be bundled and illuminated with the source wavelength compatible for use with the sample.

[0042] As seen in FIGS. 5aand 5b, two or more of the photometric systems can be grouped in unitary form to test multiple samples simultaneously. Such a multiple parallel photometer system can be employed with a multi-pipette robot system such as the MultiPROBE II made by Packard Instrument Company of Meriden, Conn. In FIG. 5, the four signal fibers 6, FIG. 1 each feed a fiber optic spectrometer and the spectra are taken simultaneously. The four source fibers, not shown, are illuminated from a single source which may be referenced with a single reference spectrometer as with the single channel system. In FIG. 5 the apparatus is shown closed 100 and open for loading 110. Opening and closing are controlled by a rotary actuator 108 such as p/n 195191-031 made by Lucas Ledex of Vandalia, Ohio and cam 106. Spring plunger 104 such as p/n 3408A35 sold by McMaster-Carr of New Brunswick, N.J. controls the position and provides the spring force against which the compression overtravel is accomplished for initial spreading of the sample.

[0043] Samples for photometric analysis can also be loaded into the sample handling apparatus in two parts, one on each of the opposing surfaces. This is especially useful where the sample of interest is a two part reacting mixture where the one or more of the beginning absorbance, the ending absorbance, and the rate of reaction are of interest and can be measured photometrically of fluorometrically. Samples of this sort can be loaded into the open spectrophotometer as shown in FIG. 6a, where two pipetter tips 205 are shown loading the two parts of the mixture, each on either side of the measurement apparatus 200. When closed as shown in FIG. 6b, measurement can be made from onset of any reaction. The exact time of sample mixing can be determined from photometric or fluorometric measurement through the apparatus optical fibers.

[0044] Samples can also be measured with a differential absorbance path as shown in FIGS. 7aand 7b. Here sample absorbance can be measured by changing the optical path over which the absorbance is measured, measuring the sample at each of one or more path lengths, where the difference in path length combined with the difference in transmitted intensity can be used to calculate the sample absorbance. This can be of significant value if the sample is highly absorbing and the accuracy of the path difference for a small path difference can be better determined than the absolute full optical path. Measurements are taken as shown in FIG. 7a, where sample 310 is shown with a relatively long path P₁and FIG. 7bwith a relatively short path length P₂between the moveable anvils carrying the facing surfaces with one or more path differences ΔP with the absorbance at the shorter path P₂being subtracted from the absorbance of one or more of the longer paths to arrive at the absorbance of the sample.

[0045] Samples can also be contained between two thin sheets of optically transparent material like Teflon™ or polyethylene films. As shown in FIG. 8, the same sort of column drawn between the two parts of the sample apparatus can be drawn between the two thin optical sheets 400 where the anvils of the apparatus are wetted to the film surfaces to minimize reflection at the interface and aid in pulling the measurement column. This would be of significant use where samples are corrosive or dangerous to handle for safety reasons and containment of the sample is preferred. The two containing sheets 400 allow the two parts 2 and 7 of the sample apparatus to draw the sample 9 into column 410. Differential measurement of the sort discussed above would be of significant value in this sort of measurement as the effects of the interfaces could be minimized with differential measurement. The two sheets 400 can be replaced by a small vessel 412, see FIG. 9, with flexible walls 414 such that the sample contained can be pulled into a measurement column. The column is pulled by pushing the anvils of the apparatus into the film until contact is made by both films, then drawing the measurement column. The compliance of the film will cause it to remain in contact with the anvils. Wetting the anvils before making contact will assist in maintaining contact and in minimizing reflection at the interface between the film and the optical fiber imbedded in the anvil. Once the measurement column is pulled, absorbance can be measured as a difference in absorbance between two path lengths.