Abstract:
A column used for gas chromatography (GC) is fabricated from an open tubular light guiding capillary. The light guiding capillary is suitable for transmission of optical frequencies employed in commercially available hand held laser range finders.

Description:
BACKGROUND 
       [0001]    Gas chromatographs (GC&#39;s) employ the use of open tubular capillary columns in order to affect a separation of chemical constituents contained in a sample mixture. The history of the development of modern capillary columns can be found in an article by S. R. Lipsky entitled “The Fused Silica Glass Capillary Column for Gas Chromatography—The Anatomy of a Revolution” Journal of Chromatography Library, Volume 32, 1985, pages 257-282. Since this time, the “anatomy” of GC columns has remained largely unchanged. 
         [0002]    Accuracy, and more importantly, precision of the flow rate through a capillary column is necessary for repeatable retention times, critical to component identification. The small bore and high pressure drop of these capillary columns result in small volumetric flow rates, which are difficult to precisely control when closing the control loop using a flow sensing element. Since this is the case, a calculated flow is generally used instead by closing the control loop using a pressure sensing element. This is commonly referred to as “Electronic Pressure Control” or EPC. The relationship of flow to pressure is given by the well known Poiseuille equation using prior knowledge of the gas viscosity, column dimensions, inlet and outlet pressures. 
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         [0000]    where: 
         [0003]    P i  inlet pressure 
         [0004]    P o  outlet pressure 
         [0005]    L is the length of the column 
         [0006]    η is the viscosity of the gas 
         [0007]    r is the column internal radius 
         [0008]    As can be seen in Equation 1 above, the column radius term is raised to the fourth power. It is thus desirable that the actual radius is known to a high degree of accuracy in order to avoid multiplicative errors. This is especially important to achieve the highest analytical reproducibility column to column and instrument to instrument. The stated internal diameter (2×radius) as reported from a column manufacturer is a “nominal” one and subject to inaccuracy. 
         [0009]    Methods are known in the art for more accurately determining the column radius (and thus the diameter) by measuring column flow rate accurately in accordance with Equation 1, using a high enough pressure drop which results in a large enough flow to accurately be measured. Once the radius is known, lower pressures can then be used to set the column flow through calculation also in relation to Equation 1. 
         [0010]    Another known technique for measuring the column internal diameter involves measuring the retention time of an “un-retained” peak to determine average linear velocity. This technique does not require an accurate flowmeter, as time can be measured accurately. Regardless of which of these techniques is used for determining column ID, it is necessary to know in advance the accurate column length in order to make the determination. 
         [0011]    Capillary column lengths are subject to an initial error from the manufacturer as well as operator induced uncertainties. As a matter of routine practice, columns are trimmed in length in order to re-establish adequate performance. The column end inserted into a detector and more particularly the end inserted into a chromatograph inlet need to be trimmed when fouled with non-volatile sample matrix, particles from septa or ferrules, oxidation from excessive heat and oxygen exposure. Trimming GC columns in this manner necessitates the keeping of a log book which notes the total remaining column length so that proper values can be entered into the EPC control algorithm. In addition, the logbook must associate this information with each particular column so that columns may be disposed of when they reach a lower practical length. Even if a logbook is maintained, the error in cut lengths can stack up, eventually resulting in a column with an inaccurate known length. Measuring the column length by the use of a ruler is impractical for all but the shortest GC columns, as often columns are in excess of 15 meters so that adequate separation can be accomplished. It is also impractical to unwind these columns from their supplied cages as doing so can damage the fragile column. 
       SUMMARY 
       [0012]    For applications that require knowing accurate column dimensions, the present invention provides a device and method for accurately measuring a GC column length, without the need for maintaining a log book or physically dismounting a column from its cage. The system disclosed easily measures the length of a capillary column for purposes of determining a minimal practical length. 
         [0013]    A column used for gas chromatography (GC) is fabricated from a light guiding open tubular capillary. The light guiding capillary is suitable for transmission of optical frequencies employed in commercially available hand held digital range finders. A laser rangefinder adapter and method are also disclosed, which allow measuring a GC column length using a standard off-the-shelf range finder. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a functional block diagram of the system in use according to the invention. 
           [0015]      FIG. 2  illustrates a cross-section of the capillary according to the invention. 
           [0016]      FIG. 3  is a process flowchart according to the invention. 
           [0017]      FIG. 4  shows shot-to-shot precision and accuracy data for a 20 meter length of light guiding capillary. 
           [0018]      FIG. 5  shows data sets for replicate measurements of multiple length light guiding capillaries. 
           [0019]      FIG. 6  shows data obtained from a light guiding capillary from a differing lot number. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates a functional block diagram of the system in use according to the invention. A range finder  110  is positioned proximate a light guiding capillary  100 . The range finder  110 , e.g. Bosch DLR165K in the illustrative example, has an operable wavelength of 635 nm and a range from 5 centimeters to 50 meters in open air. A column adapter  120  which can be fabricated from injection molded rubber such as silicone, neoprene and like materials has a column adapter port  102  in light communication with the laser output of rangefinder  110 . In addition, the column adapter  120  has an adapter port  104  in light communication with a detector input of rangefinder  110 . The adapter  120  may also include a capillary guide means such as short glass capillaries sealed at one end or clear plastic tubes sealed at one end located within adapter ports  102 ,  104 . These can act to keep light guiding capillary  100  in proper axial alignment and distance from the input and output of rangefinder  110 , as well as acting as a lens element for proper dispersion at the detector end. 
         [0021]    The adapter  120  is preferably made of a material which is optically opaque at the intended wavelength of use in order to eliminate backscattered laser light from directly entering the receiving end of rangefinder  110 . Adapter  120  may be constructed as a rectangular tubular member. It is to be understood that laser rangefinder  110  can be a unit purchased directly as is available for example in home improvement stores, or it can be an OEM device with a built in calibration factor as will be described. The OEM device may also include either separately, or as an integral part of the design, the adapter  120 . 
         [0022]    The rangefinder  110  of  FIG. 1  can be an off-the-shelf device. In general, laser rangefinders act by sending a pulse of laser radiation to a distant object and measure the time for the pulse of radiation to arrive back at a detector. Prior knowledge of the speed of light in air along with the propagation time allows for the device to calculate an accurate distance. For the DLR165K example, the open air measurements are accurate to within +/−1.5 mm. Optical wavelengths in the visible or invisible spectrum may be used, provided the substrate material   150  of  FIG. 1  is capable of transmission. 
         [0023]    The off-the-shelf rangefinder  110  of  FIG. 1  can be configured to transmit laser radiation through a silica tubular member such as light guiding capillary  100  using the adapter   120 . In this case, the rangefinder will report a length which can be corrected with a proper multiplier. If for example the propagation delay were equivalent to air, a capillary of e.g. 20 meters in length would be reported as being 10 meters long. This is due to the fact that the light travels through the tube directly to the detector rather than reflecting off a distant object and covering the same distance twice. A correction factor of 2.000 in this example would allow measuring column lengths twice the rated limit of the rangefinder. In reality, the silica matrix of the light guiding capillary  100  causes a longer propagation delay than that of air, and so the effective range is less than twice. 
         [0024]    For the examples illustrated in the Figures, a correction factor of 1.350 applied to the rangefinder readings result in the actual column length. Thus a rangefinder limited to an output of 50 meters will be capable of measuring a column length of 67.5 meters. Rangefinders providing longer distance measurements will allow measuring columns longer than this. 
         [0025]      FIG. 2  illustrates a cross-section of a GC capillary column fabricated in accordance with an embodiment of the present invention. A fused silica light guiding capillary tube  125  such as is used in evanescent wave detection, e.g. Poly Micro LTSP250350, is coated internally with a cross bonded stationary phase  160 , in accordance with coating techniques commonly employed in the art. 
         [0026]    In this non-limiting illustrative embodiment, a substrate layer  150  is fused silica. An outer second layer of doped silica cladding  140  is positioned over the substrate  150  and a protective outer coating  130  of polyimide, aluminum or other known materials for protecting fused silica capillaries. A necessary requirement for efficient total internal reflection (TIF) is that the refractive index of silica substrate  150  be higher than that of the surrounding layers   140  and  160 . Doped silica layer  140  can therefore comprise a fluorine doped silica. 
         [0027]    Capillary tubes are drawn from tubular quartz “preforms” and fluorine doped preforms are commonly available for fabrication into capillaries. Quartz tubular preforms of this nature can be coated externally as well as internally with layers of lower index materials. If the stationary phase  160  has a higher index than substrate  150 , a second doped silica layer (not illustrated) may be placed between layers  150  and  160 . It is understood that additional layers or surface treatments may also be incorporated between the stationary phase  160  and the adjacent underlying layer for proper surface chemistry consistent with efficient chromatography and cross linking and bonding of stationary phase  160 . 
         [0028]      FIG. 3  is a process flowchart according to the invention. In step  100 , the range finder emits a light pulse along the length of the column at a first time. In step  102 , the light pulse is received by the range finder at a second time. In step  104 , a correction factor is applied to determine how long the column is. In step  106 , the length of the column is recorded. The correction factor applied in step  104  may be known in advance, or it can be measured with an initial known column length. 
         [0029]      FIG. 4  illustrates accuracy and precision data for a 20 meter length of a light guiding capillary. These data represent information gathered from multiple measurements from a single cut capillary attached to a laser range finder. The average correction factor was used to calculate an actual length. As can be seen in the data, a high degree of precision is possible. 
         [0030]      FIG. 5  illustrates data obtained when measuring multiple known lengths of a light guiding capillary. Seven differing lengths were measured in replicate with a maximum absolute error less than 2 cm for a 20 meter capillary. 
         [0031]      FIG. 6  illustrates data obtained from a separate lot number and manufacturing date of capillary tubing ordered as a 10 meter length. The “as received” length was first measured in accordance with the method used in  FIG. 5 . This gave an average determination of 10.83 meters. When uncoiled and measured, the actual length was found to be 10.83 meters. This capillary was then cut to a length of 10 meters and measured four times using the rangefinder method. The maximum absolute error was 1 cm.