Abstract:
Apparatus and method for sequential injection liquid-liquid extraction analysis. Under the control of a bidirectional precision pump, a stream-selection valve, and a microprocessor, a series of liquid zones is built up in a holding/mixing coil. The liquid zones are transferred from the holding/mixing coil to a separation cell. After phase separation into an extract and a raffinate, the extract is withdrawn from the separation cell and sent to a detector, which determines the amount of a component which was extracted from a sample by an extraction solvent. The principal advantages of this automated technology are elimination of the need for dynamic phase separation; on-line pre-extraction chemical conditioning; a substantial reduction in solvent, reagent, and sample usage; and a similar substantial reduction in waste generation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to instrumental chemical analysis. More particularly, the present invention relates to an automated instrumental apparatus and method for carrying out sequential injection liquid-liquid extraction. The principal advantages of this automated technology over the prior art are elimination of the need for dynamic phase separation; on-line pre-extraction chemical conditioning; a substantial reduction in solvent, reagent, and sample usage; and a similar substantial reduction in waste generation. 
     SUMMARY OF THE INVENTION 
     In general, the present invention in a first aspect provides an apparatus for sequential injection liquid-liquid extraction. The apparatus comprises (a) a bidirectional precision pump for controlling fluid flow; (b) a holding/mixing coil, for holding and mixing liquids, and carrying out liquid-liquid extraction; (c) a selection valve, for withdrawing, transferring, and injecting a plurality of fluids; (d) a separation cell, for separating an extract phase from a raffinate phase; (e) a detector, for detecting the quantity of a component which was extracted from a sample by an extraction solvent; and (f) a microprocessor, for controlling the selection valve and the bidirectional precision pump. 
     In a second aspect the invention provides a method for sequential injection liquid-liquid extraction. The method comprises (a) using a bidirectional precision pump under suction to transfer a sample through an inlet line to a holding/mixing coil, to purge the inlet line; (b) discharging the sample from the holding/mixing coil; (c) flushing the holding/mixing coil with a carrier solvent, to remove residual sample; (d) disposing a plurality of liquid zones in the holding/mixing coil, using a microprocessor to control the selection valve; (e) mixing the zones and providing efficient contact between an extraction solvent and the sample by passing the plurality of liquid zones through the holding/mixing coil; (f) reversing the flow through the holding/mixing coil, to provide further mixing of the zones and further liquid-liquid contact between the extraction solvent and the sample; (g) transferring the liquid zones from the holding/mixing coil to a separation cell; (h) holding the liquid zones in the separation cell, to separate an extract phase from a raffinate phase; (i) withdrawing the extract phase from the separation cell; (j) transferring the extract phase to a detector, for determining the quantity of a component extracted from the sample; and (k) determining the amount of the component extracted by measuring the response of the detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an automated sequential injection, solvent extraction, and chemical analysis system, made in accordance with the principles of the present invention. 
     FIG. 2-A is a schematic representation of a series of liquid zones in a holding/mixing coil. 
     FIG. 2-B is an enlarged portion of FIG. 2-A. 
     FIG. 3-A is a schematic representation of a portion of a separation cell containing a less-dense extract and a more-dense raffinate, made in accordance with the principles of the present invention. 
     FIG. 3-B is a schematic representation of a portion of a separation cell containing a more-dense extract and a less-dense raffinate. 
     FIG. 4 is a schematic representation of a separation cell, made in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     More specifically, reference is made to FIG. 1, in which is shown an analytical instrument for automated sequential injection liquid-liquid extraction, made in according with the principles of the present invention, and generally designated by the numeral  2 . 
     The sequential injection instrument  2  comprises a syringe pump  6 , a holding/mixing coil  8 , a stream selection valve  3 , a separation cell  10 , a detector  12 , and a microprocessor  7  which controls the selection valve  3  and the syringe pump  6 . 
     By operation of the selection valve  3 , a sample  4   a  to be analyzed is drawn from a sample container  4  through a sample line  4   b  under suction of the syringe pump  6 . The sample  4   a  is drawn through an inlet line  5  into the holding/mixing coil  8 , to purge the inlet line  5  with fresh sample  4   a.  The flow of sample  4   a  into the holding/mixing coil  8  is stopped when the inlet line  5  has been adequately flushed to remove any trace of a previous sample. 
     The selection valve  3  is switched to a waste line  14   a  leading to a waste reservor  14 . Excess sample  4   a  in the holding/mixing coil  8  is pushed out to the waste reservoir  14 , followed by an excess of carrier solvent  16   a,  previously drawn into the syringe pump  6  from a solvent container  16  through a solvent line  16   b.  The selection valve  3  is switched to the solvent line  16   b  connected to the solvent container  16  filled with a carrier solvent  16   a.  A sufficient quantity of the carrier solvent  16   a  is used to flush any residual sample  4   a  out of the holding/mixing coil  8 . The carrier solvent  16   a  always fills the syringe pump  6 . 
     Preparation of the separation cell  10  is effected by washing all lines  10   a,    10   b,    10   c  connected thereto with the carrier solvent  16   a  from the solvent container  16 . The cell  10  is then drained by drawing off the waste carrier solvent  16   a  through lines  10   b  and  5  into the holding/mixing coil  8  under suction of the syringe pump  6 . The selection valve  3  is then switched to line  14   a,  and the waste carrier solvent  16   a  is pushed out to the waste reservoir  14  under pressure from the syringe pump  6 , followed by sufficient excess carrier solvent  16   a  to adequately flush the holding/mixing coil  8 . After washing the separation cell  10 , lines  10   a,    10   b,  and  10   c  may be filled with carrier solvent  16   a  or air as required by a particular methodology. 
     After the separation cell  10  has been prepared, the selection valve  3  and syringe pump  6  are manipulated, under the control of the microprocessor  7 , to stack a series of liquid zones in the holding/mixing coil  8 . FIGS. 2-A and  2 -B show a typical profile of the holding/mixing coil  8  stacked with a plurality of liquid zones. A conditioning chemical  20   a,  drawn from a chemical container  20  through a chemical conditioner line  20   b,  is included in the stack of liquid zones, along with the carrier solvent  16   a,  sample  4   a,  and extraction solvent  18   a  drawn through line  18   b.    
     Referring again to FIG. 1, the selection valve  3  is switched to a port  3   a  open to the atmosphere, and the zone stack is withdrawn under pump  6  suction into the holding/mixing coil  8 . This movement results in mixing the zones, and in efficient contact of the sample  4   a  with the extraction solvent  18   a.    
     When the leading edge of the first zone reaches the end  8   a  of the holding/mixing coil  8 , flow is stopped, then reversed, pushing the zone stack shown in FIGS. 2-A and  2 -B back through the holding/mixing coil  8 . This operation results in further zonal mixing. It will be apparent to those skilled in the art that the forward-reverse mixing action can be repeated as many times as required to achieve any desired degree of agitation and any desired time of interphase contact. If only one mixing cycle is required, or at the end of the required number of mixing cycles, the zone stack is pushed out of the holding/mixing coil  8  through a port of the selection valve  3  connected to the inlet line  10   a  of the separation cell  10 . 
     After the zones have been transferred from the holding/mixing coil  8  to the separation cell  10 , the selection valve  3  is switched to a port connected to line  14   a  leading to the waste reservoir  14 . Additional carrier solvent  16   a  is conveyed to the waste reservoir  14  through the holding/mixing coil  8  to remove any remaining trace of either the sample  4   a  or the extraction solvent  18   a  which might have adhered to the walls of the holding/mixing coil  8 . 
     The immiscible zones are allowed to stand in the separation cell  10  as long as necessary to complete phase separation into an extract  22  and a raffinate  24 , as depicted in FIGS. 3-A and  3 -B. During this “static” separation period of time, other parts of the sequential injection instrument  2  can be processed; e.g., the holding/mixing coil  8  can be solvent-washed. 
     Reference is now made to FIG. 4, in which are shown further details of the separation cell  10 . 
     After phase separation is complete, and if the extract  22  is less dense than the raffinate  24 , as illustrated in FIG. 3-A, the selection valve  3  is switched to a port connecting the holding/mixing coil  8  to an upper portion  10   d  of the separation cell  10  through line  10   c,  the tip  10   e  of which is disposed just above the meniscus  23  defined by the two phases  22  and  24 . The extract  22  is withdrawn into the line  10   c.  The tip  10   e  of line  10   c  is then exposed to air drawn from a lateral port  10   f  in the upper portion  10   d  of the separation cell  10 . As suction continues, pulling the extract  22  into the holding/mixing coil  8 , air fills the line  10   c  behind the extract  22 , thereby minimizing or preventing dilution by solvent or other liquids. After all of the extract  22  withdrawn has been transferred to the holding/mixing coil  8 , and before any air enters the selection valve  3  port, flow is stopped. 
     Referring again to FIG. 1, the selection valve  3  is switched to a port connecting the holding/mixing coil  8  to the detector  12  through line  12   a,  and the extract  22  is conveyed to and through a flowcell (not shown) of the detector  12 , followed by a sufficient volume of the carrier solvent  18   a  to ensure that the flowcell has been contacted by all of the extract  22  which has been withdrawn, thereby generating a detectable and quantitative peak response. 
     The selection valve  3  (FIG. 1) is switched to the line  10   b  connecting the holding/mixing coil  8  to a lower portion  10   g  of the separation cell  10  (FIG.  4 ), and the raffinate  24  is withdrawn into the holding/mixing coil  8  through the inlet line  5  under syringe pump  6  suction. 
     Referring to FIG. 1, the selection valve  3  is switched to a port connecting the holding/mixing coil  8  to the waste reservoir  14  via lines  5  and  14   a,  and the raffinate  24  and any remaining extract  22  are jettisoned to the waste reservoir  14 . The holding/mixing coil  8  is then flushed out with the carrier solvent  16   a.    
     The selection valve  3  is then manipulated to flush all lines  10   a,    10   b,  and  10   c  connected to the separation cell  10  with the carrier solvent  16   a,  and to fill the separation cell  10  with the carrier solvent  16   a.  The carrier solvent  16   a  is then withdrawn from the separation cell  10  via line  10   b  into the holding/mixing coil  8 , and then sent to the waste reservoir  14  via line  14   a.  This procedure effectively washes the separation cell  10 , thereby preventing sample-to-sample carryover. 
     If the extract  22  is more dense than the raffinate  24 , as illustrated in FIG. 3-B, the above procedure is modified as follows. Following phase separation, the selection valve  3  is switched to a port connecting the holding/mixing coil  8  to the lower portion  10   g  of the separation cell  10  through line  10   b.  The extract  22  is withdrawn into the line  10   b  and transferred to the holding/mixing coil  8  through line  5 . 
     The selection valve  3  is switched to a port connecting the holding/mixing coil  8  to the detector  12  through line  12   a,  and the extract  22  is conveyed to and through the flowcell (not shown) in the detector  12 , followed by a sufficient quantity of the carrier solvent  18   a  to ensure that the flowcell has been contacted with all of the extract  22  which has been withdrawn, generating a detectable and quantitative peak response. 
     The selection valve  3  is switched to a port connecting the holding/mixing coil  8  to the lower portion  10   g  of the separation cell  10  through line  10   b.  The raffinate  24  is withdrawn into line  10   b  and transferred to the holding/mixing coil  8  through line  5 , thence to the waste reservoir  14  by switching the selection valve  3  to a port connecting the holding/mixing coil  8  to line  14   a.    
     The remaining procedure is similar to that described for the case in which the extract  22  is less dense than the raffinate  24 . 
     Reference is now made to FIG. 4, in which is shown a detailed representation of the separation cell  10 . 
     The separation cell  10  includes a tapered container  10   i  mounted in and to a first housing  26  by O-rings  28 , a first end cap  10   j,  an adjustment fitting  10   k,  and a second end cap  101 . 
     The adjustment fitting  10   k  is a very important part of the present invention. By means of the adjustment fitting  10   k,  which is slidably fitted into the end cap  10   j,  the line  10   c  can be raised or lowered to position the tip  10   e  just above the meniscus  23 . An adjustment nut  30  riding on threads  30   a  rides in a groove formed by a raised ring on the fitting  10   k  and tube union  10   o.  Only line  10   c  is so adjustable. Line  10   a  is fixed in place. 
     FIGS. 3-A and  3 -B actually depict two different dispositions of the tapered container  10   i,  depending on whether the raffinate  24  is heavier or lighter than the extract  22 , which is typically of smaller volume. 
     Referring now to FIGS. 3-A,  3 -B, and  4 , the tapered container  10   i  has first and second ends  10   m  and  10   n.  The container  10   i  is constructed and arranged to hold the extract  22  and raffinate  24 . The container  10   i  is preferably made of glass, and tapers from wide to narrow in a direction away from the first end  10   m  toward the second end  10   n.  The tapered container  10   i  is so arranged that the narrower second end  10   n  is pointed in a direction facilitating removal of the phase having the smaller volume—typically the extract. If the extract  22  is less dense than the raffinate  24  (FIG.  3 -A), the container  10   i  is disposed vertically with the first end  10   m  of the container  10   i  below the second end  10   n  of the container  10   i.  If the extract  22  is more dense than the raffinate  24  (FIG.  3 -B), the container  10   i  is disposed vertically with the first end  10   m  of the container  10   i  above the second end  10   n  of the container  10   i.  In either case, the container  10   i is oriented so that the extract  22  is disposed in the second end  10   n  of the container  10   i,  and the raffinate  24  is disposed in the first end  10   m  of the container  10   i.  It will be apparent to those skilled in the art that this arrangement optimizes and maximizes efficient withdrawal of the extract  22 . 
     The syringe pump  6  is connected to the holding/mixing coil  8  by a line  8   a  disposed in a first fitting  34 , to the solvent container  16  by line  16   b  disposed in a second fitting  38 , and to the selection valve  3  by the microprocessor  7 . 
     While certain embodiments and details have been described to illustrate the present invention, it will be apparent to those skilled in the art that many modifications are possible without departing from the scope and basic concept of the invention. For example, a selection valve with more ports would allow use of a plurality of extraction solvents, addition of different wash liquids, and addition of various standard solutions.