Method and apparatus for liquid addition and aspiration in automated immunoassay techniques

Apparatus and method for adding liquid to, aspirating liquid from, a rack of reaction tubes wherein a rack and carriage-dependent probes are made to move relative each other in the horizontal and vertical planes. The method calls for performing the first half of a wash sequence on tube located at x row, y column, then completing the sequence on tube at (x-1) row, y column, and returning to the tube at x row, (y+1) column to initiate the first half of the sequence. This "backtracking" of probes permits an incubation-soak period without cumulatively extending the total rack processing time.

TECHNICAL FIELD 
This invention relates generally to liquid addition and aspiration in 
immunoassay techniques, and more specifically to methods and apparatus 
which enable the performance of the necessary washing cycles in a 
relatively short period of time while meeting the assay specifications for 
removal of unbound components. 
BACKGROUND OF THE INVENTION 
Several common immunoassay techniques utilize solid support-antibody 
complexes to accomplish the detection of specific antigens or antibodies 
in the sample serum. See, e.g., the immunoassay methods described in U.S. 
Pat. No. 4,376,110, the relevant portions of which are hereby incorporated 
by reference into the present application. Typically, the solid support is 
a plastic bead. After the sample serum and antibody-solid support complex 
have been incubated for a period of time in the presence of a conjugate, a 
wash step is required to remove unbound chemical species from the solid 
support prior to the introduction of reagents; which are used in 
subsequent reactions which reactions result in the formation of a 
measurable characteristic, e.g., fluorescence. Immunoassay techniques, 
however, are extremely sensitive. The presence of trace amounts of unbound 
components can dramatically affect the subsequent reactions and thereby 
alter the assay results. When the immunoassay technique is performed 
manually, the bead can be soaked for a period of time to remove all traces 
of unreacted materials. The washing step can also be repeated several 
times to accomplish the desired result. A more completed description of 
the requirements and suggested techniques for this wash step can be found 
in the product literature which accompanies Tandem-E Immunoassay Kits 
marketed by Hybritech, Inc., San Diego, Ca. The relevant portions of that 
product literature are hereby incorporated by reference into this 
application. 
In the automation of immunoassay techniques, an extended soaking period is 
not possible if the instrument is to be capable of completing one rack of 
reaction tubes before the end of an incubation period for a second rack of 
tubes. Without soaking, however, the twenty-five parts per million 
specification of the assay protocol is difficult to meet. 
The instant invention makes possible the automation of the wash step 
providing methods and apparatus which are capable of removing unbound 
components and serum from reaction tubes in a suitably short period of 
time while simultaneously accomplishing the twenty-five parts per million 
specification. 
It is therefore an object of this invention to provide apparatus and 
methods which make possible the automation of the routine operation of 
removing unreacted components from the solid support and reaction tube 
used in immunoassay techniques. 
It is a further object of this invention to provide a simple system for 
performing the wash step on a rack of reaction tubes in a short cycle 
time, thereby enabling sufficient through-put to meet the demands of 
clinical use. 
It is another object of this invention to remove unreacted components from 
the solid support and tubes with detergent solutions without having to 
remove the rack of tubes from the instrument to soak. 
These, and other objects of this invention, shall become apparent to those 
skilled in the art with reference to this specification and the 
accompanying figures to which it refers. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus and method for adding liquid 
to, and aspirating liquids from, tubes which comprises means for receiving 
and positioning a rack of reaction tubes, said rack having n rows in the 
x-direction and m columns in the y-direction arranged in a rectangular 
matrix; a carriage; at least one liquid probe depended from said carriage 
and selectively moveable along said probe longitudinal axis relative to 
said carriage; means for selectively moving said carriage in the 
x-direction or y-direction; means for selectively registering the liquid 
probe with a row of tubes in said rack; control means for actuating said 
rack positioning means to move said rack discrete distances in the 
y-direction, substantially equivalent to one column of tubes, relative to 
said probe, and for selecting movement of said carriage in the x-direction 
or y-direction; wherein said control means, for each of y=1 to m columns 
is set to first deliver liquid from the probe to a tube located in x row, 
y column, then to aspirate through the probe previously-added liquid in a 
tube located at (x-1) row, y column, and then to return said carriage, 
rack and probe to register said probe with the tube in x row, (y+1) 
column, wherein x=1 to n rows.

DETAILED DESCRIPTION OF THE INVENTION 
The methods and apparatus which are the subject of this invention are only 
part of the overall instrument which permits automated performance of 
immunoassay techniques. Other features of the preferred embodiment of this 
invention are described and claimed in several copending applications, 
including my applications for METHOD AND APATUS FOR AUTOMATED, 
MULTI-SEQUENTIAL IMMUNOASSAYS (Ser. No. 757,676), and for METHOD AND 
APATUS FOR DILUTION AND MEASUREMENT (Ser. No. 757,785), as well as 
applications assigned to a common assignee entitled TUBE TRAP APATUS 
(Ser. No. 757,646), PROBE WASH STATION (Ser. No. 757,742), and IMPROVED 
TUBE TRAP APATUS (Ser. No. 850,941). The relevant portions of all of 
these applications are hereby incorporated by reference into this 
application. 
The preferred embodiment of this invention comprises the reaction processor 
module of the Hybritech Immunochemistry Analyzer. Typically, "sandwich" 
type immunoassays will be performed on this instrument, but the 
performance of other immunoassay techniques are intended to be within the 
scope of this invention. 
GENERALIZED IMMUNOSSAY TECHNIQUE 
According to the protocol developed for the Immunochemistry Analyzer, the 
instrument transfers sample fluids from a tray of sample cups to reaction 
tubes which are held in a rectangular rack. Each reaction tube contains a 
specific monoclonal antibody which has been bound, in this preferred 
embodiment, to a solid bead. A more complete description of sandwich 
immunoassay techniques is found, e.g., in U.S. Pat. No. 4,376,110, 
relevant portions of which are hereby incorporated by reference. First, 
the sample serum is introduced to the reaction tube. Then, a reagent is 
added to the reaction tube. The resulting rack of tubes is then removed 
from the sample processor module of the Immunochemistry Analyzer to 
incubate (antigenantibody reaction) for some period of time. In the 
sandwich immunoassay technique, this first step of processing and 
incubating results in the formation of an antigen "sandwich." The antigen 
to be detected and determined by the immunoassay is separated from the 
solution and immobilized by becoming bound between the solid-supported 
antibody and the reagent which is typically a second antibody which also 
immunochemically binds to the antigen to be detected. 
When the rack of reaction tubes is returned to the Immunochemistry Analyzer 
after a suitable antibodyantigen incubation time related to the particular 
immunoassay to be performed, and is loaded into the reaction processor 
module, all traces (usually 25 ppm specified) of unreacted components in 
the reaction tubes must be removed from the solid support. This wash step 
is followed by the addition of a second reagent, in this preferred 
embodiment a substrate, which will react in the presence of the 
solid-bound enzyme "sandwich" to form a chromophore. This chromophore then 
can be spectrophotometrically-analyzed to determine the presence of, and 
the concentration of, the target antigen in the sample serum. The 
formation of, and quantitative measurement of concentration of, this 
chromophore (or other equivalently "labeled" compound) is particularly 
sensitive to the presence of unreacted components. Since the concentration 
of antigen to be determined is usually &lt;10.sup.-4 M, common assay 
specifications require that unreacted components be present at levels 
below twenty-five parts per million prior to the addition of substrate. It 
will be recognized by those skilled in the art that although this 
embodiment utilizes spectrophotometry other signal-producing reactions may 
be used in the immunoassay technique, e.g. fluorescence and radioisotopes. 
Each technique, however, has similar specifications for the removal 
unreacted species, independent of the signal detected. 
Another important factor in the automation of immunoassays involves time. 
To be able to use an automatic instrument in a clinical environment, the 
instrument must be capable of nearly simultaneous, or sequential 
performance of the same, or different, immunoassay techniques. Since the 
length of the incubation period is an important parameter in the 
technique, and since its length must be reliably reproducible, the 
automatic immunoassay instrument should be able to accomplish the unbound 
component washing step quickly to permit other racks of tubes to be 
similarly washed within the constraints of the overall cycle time 
limitations. Thus, it is desirable to provide a wash and substrate 
addition cycle whose cycle length is comparable to the antibody-antigen 
incubation period. This matched cycling increases the attractiveness of 
the automated procedure in a clinical environment. This invention 
recognizes that reproducible and efficient cycle times can be achieved by 
providing multiple liquid probes which are offset from each other by 
columns and by permitting rapid position change from row to row. To 
optimally achieve the necessary cycle time without introducing 
unacceptable delay, the present invention focusses cycle times upon each 
individual tube, rather than a row, column or rack of tubes. In this way, 
efficiency and reproducibility are both addressed. 
When the unbound component wash step of the immunoassay protocol is 
practiced manually upon a small number of tubes, the wash step usually 
involves a soaking period to dislocate unreacted components which have 
been absorbed on the solid surface. This soaking period, however, could 
not be easily accommodated in an automated process as a result of the time 
constraints described above. However, we found, that without allowing for 
some soaking, the twenty-five parts per million specification for unbound 
components proved difficult to satisfy on a consistent basis. 
By way of this background, the importance of the method and apparatus of 
the instant invention can be more fully described with reference to a 
particularly preferred embodiment. 
Reaction Processor Module 
The Immunochemistry Analyzer 10 is shown in FIG. 1. The analyzer has a 
sample processing module 12 and a reaction processor module 14. Sample 
cups held in rack 16 are loaded into the sample module 12 where the serum 
is transferred from cups in rack 16 to reaction tubes held in rack 18a. In 
the preferred embodiment, each of the reaction tubes contains a solid 
sphere to which is bound one member of the immunological pair used in the 
"sandwich" technique. After the diluted serum is placed in the reaction 
tubes and conjugate reagent has been added, the entire rack 18a is removed 
from the instrument and undergoes a carefully timed antibody-antigen 
incubation period, the length of which is determined by the particular 
assay protocol. 
At the end of the antibody-antigen incubation period, the operator returns 
the rack 18b containing reaction tubes to the reaction processor module 
14. As seen through the partially broken away front face of the unit 10, 
carriage 20 is positioned above the rack 18b to effect horizontal 
(x-direction) and vertical (z-direction) movement of the probes relative 
to the rack 18b and tubes therein. In this preferred embodiment, a 
personal computer 22, e.g. an IBM PC-XT, is programmed to control the 
movement of the carriage 20 and to time the various steps of the assay 
protocol, both on and off of the instrument 10. 
The reaction processor module of the preferred embodiment is shown in 
greater detail in FIG. 2. The reaction tube rack 18b consists of a 
rectangular array of rows (in the x-direction) and columns (in the 
y-direction) of apertures for receiving reaction tubes. In the preferred 
embodiment, carriage 20 moves across the rows and has no component of 
movement in the "column" vector. However, those skilled in the art will 
appreciate that an apparatus which has a y-direction carriage is intended 
to be within the scope of this invention. In this preferred embodiment, 
movement of the rack 18b in the y-direction, relative to the carriage 20, 
is accomplished by a table 30 underneath the rack 18b. The table 30 is 
motor-driven with a single axis of movement in the "column" or 
y-direction. The y-direction movement of this platform is controlled by 
table drive belts 32, table motor 34 and an automatic controller which in 
this preferred embodiment comprises an IBM personal computer. The 
controller can control the table 30 movement in discrete units equivalent 
to the width of one row relative to the carriage. The controller also 
controls the movement of the carriage 20 in discrete movements equivalent 
to the width of one column. Thus, the controlled movement of the carriage 
20 and the table 30, permits the probe assembies which are attached to the 
carriage 20, to register the liquid probes with every tube contained 
within the rectangular matrix. 
The carriage 20 is conveyed in the x-direction by carriage drive belt 36, 
driven by a carriage motor 38. Guide rods 40a and 40b insure accurate 
movement of the carriage in the x-direction to permit proper registration 
of the probes with the tube mouths. 
Vertical movement of the first probe is accomplished by a first probe motor 
42 which drives jack screw 44. A guide rod 46 fixes the position of a 
first probe assembly 48 in the horizontal plane, to permit registration of 
the optics-aspirate probe 49 with the reaction tubes. In one of the final 
steps of the immunoassay protocol probe 49 aspirates quenched, substrate 
fluid from the tubes into an optics module for quantitative, optical 
determination of analyte concentration. Second probe motor 52 controls 
vertical movement of probes 50 and 54 which are depended from a second 
probe assembly 56. The probe motor 52 drives a jack screw (shown in dashed 
lines) fixed to the bottom of the carriage 20. Guide 58 fixes the 
horizontal position of the probe assembler 56. The direction of the jack 
screw drives determines whether the probe assemblies ascend or descend. 
Probes 50 add substrate reagent and quench reagent. Probes 54 add 
detergent solution to, and aspirate the same from, the tubes. 
Referring now to FIG. 3, second probe assembly 56 and its associated 
mechanisms can be seen here clearly. Guide 58 and jack screw 60 are shown 
in relation to be second probe motor 52 which drives jack screw 60. 
FIG. 3 shows a reaction tube 62 into which a spherical support 64 is placed 
according to the immunoassay protocol. Typically, the surface of the solid 
support will have bound to it one member of the immunological 
antigen-antibody pair, dependent upon the assay protocol being practiced. 
Depending from the first probe assembly 48 is the optics-aspirate probe 
49, shown in the retracted position. This probe is offset by one row from 
the probes 66 and 68, but in the same column. Substrate dispensing probe 
66 and quench/diluent dispensing probe 68 occupy one tube position on 
first probe assembly 56. In a tube 70, identified as being in the "second" 
position in rack 18b, detergent dispensing probe 72 and aspirating probe 
74 are depended from the second probe assembly 56 at a second position, in 
the same row as the probes 66 and 68, but in a different column. It is 
also shown in FIG. 3 that the probes and their respective assemblies can 
be withdrawn from the tubes to permit movement of either the carriage 20 
or the rack 18b, or both, without damage to the probes, their supporting 
assemblies or the tubes. 
Referring to the top plan view of FIG. 4, the relationship between the rack 
18b, carriage 20 and the probe assemblies 48 and 56 is shown. Carriage 20 
causes movement in the x-direction, along the rows of the rack 18b. Table 
30, underneath rack 18b causes the rack 18b to be moved in the 
y-direction, along the columns of the rack. It will be recognized by those 
skilled in the art that the important aspect of this invention is the 
relative movement of the probes and the rack. While this embodiment is 
described with reference to a carriage having x-direction movement and a 
table accomplishing y-direction movement, two carriages can be used, or 
the table can effect x-direction movement. Each of these various 
configurations is intended to be within the scope of this invention and 
the claims appended hereto. 
Referring now to FIG. 4, the wash cycle according to the present invention 
will be described. Row 1, consisting of five positions 1 through 5 and row 
2, consisting of positions 6 through 11. This wash cycle commences after 
the antibody-antigen incubation period which occurs off the machine has 
ended and the rack 18b has been loaded into the reaction processor module. 
Once the automatic controller has confirmed the identity of the rack 18b 
and the type of assay to be performed, the wash cycle begins when the wash 
probes are inserted into the first tube in the first row (position 1). 
For each individual tube, the first part of the wash cycle consists of 
aspirating the reagents and sample serum from the tube through probe 74, 
adding detergent through probe 72 and aspirating detergent through probe 
74. These aspiration and addition steps are repeated again with the final 
step in the first half of the wash cycle being to add detergent to the 
tube through probe 72. This detergent addition corresponds to the "soak 
period" discussed above. The probe assembly 56 then extracts the wash 
probes 72 and 74 from the first tube. The rack table drive is then 
activated to position the wash probes over a preceding row of tubes to 
which detergent has already been added and which have been permitted to 
soak. Since the description relates to a first row of tubes, the 
description of the action on the preceding row is deferred until the 
second row. It must be recognized that the first row is a special case, 
different from the succeeding rows. After this sequence of registering 
with a tube position in the preceding row, but the same column, the wash 
probes register with the second position in the first row. The reagents 
and sample serum are aspirated, and then detergent is added and aspirated 
twice. Finally, the detergent probe 72 injects detergent fluid. According 
to the configuration of FIG. 4, the entire wash cycle has been completed 
in positions 1, 2, 3 and 4. The tube in position 5 will be completed after 
detergent has been dispensed into tube 10 and the table activated to 
register position 5 with the aspirating probe 74. Substrate has been added 
to positions 1, 2, 3 and 4 through substrate probe 66 which trails behind 
the wash probes 72 and 74 by one position in the same row. 
FIG. 5 shows the path of the probes and indicates when the specific steps 
of the cycle take place in a specific embodiment having at least 5 columns 
of tubes. Commencing with the tube in position 1, the above described 
series of detergent addition-aspiration steps occur, leaving tube 1 with a 
volume of detergent in which to soak (W.sub.1) The wash probes 72 and 74 
are then removed from the tube in the first position. Since the first row 
is a special case, in that there are no proceeding rows, the travel of the 
probes and the steps they perform are represented by dashed lines. W.sub.2 
-W.sub.5 are intended to represent completed first half wash cycles in 
each of the tubes in positions 2 through 5. It should be noted that the 
probe assembly and controller recognize a further position in the first 
row to the left of tube position 5. For completeness of describing the 
probes 72 and 74 travel, after the first half of the wash cycle has been 
performed upon position 5, the probes 72 and 74 move one further position 
so that the substrate 66/quench 68 probes also are inserted into each of 
the five positions across the row. In the first half of the wash cycle, no 
substrate is added to the tubes, but once the reaction tube has been 
"dried" according to the second half of the wash cycle, substrate is added 
to initiate the chromophore-forming reaction. 
After detergent or wash fluid has been added to each of the tubes in 
positions 1 through 5, and the probes 72 and 74 have moved through the 
sixth position, the rack table is activated to move the rack one row's 
distance forward relative to the probes 72 and 74. Thus, the probes 72 and 
74 are registered with a tube in the second row, in position 6, as 
indicated by the diagonal line and arrow. The probes 72 and 74 then go 
through the first half of the wash cycle in this position (W.sub.6). After 
the probes 72 and 74 are removed from the tube in position 6, the rack 
table is again activated to move the rack one row's distance rearward 
relative to the probes 72 and 74. This time the movement of the rack is in 
the reverse, positioning probes 72 and 74 above the tube in position one. 
This tube one has been soaking in the detergent dispensed in the first 
pass through row one. However, in this second half of the wash cycle, the 
detergent previously dispensed is aspirated through probe 74 and one final 
aliquot of detergent is dispensed through probe 72. The final step in the 
wash cycle is to vacuum dry the tube in position one by aspirating through 
probe 74 (D.sub.1, hereinafter D represents the second half of the wash 
cycle). Thereafter, the rack table is again activated to cause forward 
relative movement and the probes 72 and 74 are positioned over tube 7 for 
the first half of the wash cycle (W.sub.7). When the rack again shifts to 
permit step (D.sub.2) in position 2, the substrate probe 66, offset by one 
column from probes 72-174, is inserted into tube 1 and substrate reagent 
is added to the now dry tube one. 
Thus, this invention permits multiple sequences to be performed in a single 
pass across the rack, yet permits incubation periods without extending the 
cycle time to a clinically impractical length. 
FIG. 6 shows how the various fluids are delivered and aspirated on the 
reaction processing module. In the upper left of the figure, wash fluid is 
stored in a wash reservoir 100 whose liquid level is sensed by a wash 
liquid level sensor 101. Wash fluid flows from wash reservoir 100 to probe 
wash pump 102. A wash line solenoid-valve 104 is used to control flow in 
the wash fluid line. Wash fluid, also referred to in the specification as 
detergent, is dispensed from the detergent probe 72, depended from second 
probe assembly 56. In this embodiment, detergent is forcefully ejected 
from probe 72 to insure sufficient agitation and mixing to accomplish the 
wash steps. The wash aspirator probe 74, also depended from second probe 
assembly 56, is connected through a solenoid activated valve 106 to a 
vacuum pump 108 through waste reservoirs 190a and 109b. These reservoirs 
are used to store fluids aspirated from the various reaction tubes. Each 
reservoir has a control valve 110 which determines where the liquid will 
be stored. 
Substrate reagent, used in chromophore formation, is stored in substrate 
reservoir 111. Since precise amounts (e.g., 200 ml.) of substrate must be 
dispensed, a substrate syringe 112 is used to withdraw fluid from the 
reservoir 111. Solenoid-activated valves 114 and 116 open and close to 
permit or to restrict flow of substrate reagent to and from the reservoir 
111 and to and from the substrate probe 66. A substrate motor 118 is used 
to aspirate substrate from the reservoir 111 into the syringe 112 and to 
deliver substrate reagent from the syringe 112 to the probe 66 which is 
depended from the second probe assembly 56. 
Quench reagent, which is added to the substrate reagent after a suitable 
enzyme substrate incubation period, is stored in quench reservoir 120. 
Liquid level in reservoir 120 is maintained by a level sensor 121. Quench 
reagent is used in the chromophore detection sequence and is also used to 
wash the aspirator probe 49 free of contaminants. In the detection 
sequence, precise amounts of quench reagent are required to insure 
reproducible results, particularly as compared to standards. Less precise 
delivery of quench reagent is required for the aspirator probe 49 wash. To 
accommodate both of these operating modes, the quench delivery system 
includes a three way valve and pump 122. This valve 122 communicates 
alternatively with the quench probe 68, a quench syringe 124 and a probe 
wash 125. In FIG. 6, this valve-pump 122 is positioned to communicate with 
the syringe 124 and probe 68. Syringe 124 is driven by a quench syringe 
motor 126. 
The final component of the reaction processor module seen in FIG. 6 is the 
optics section. This part of the module is intended to quantitatively 
measure the signal generated in the reaction tubes. While the 
specification makes reference to a chromophore, implying 
spectrophotometric detection, it will be appreciated by those skilled in 
the art that other species of detectable signal are possible without 
varying from the scope of the present invention. For example, it is 
contemplated that fluorometric determinations could be made in the optics 
section. Referring to FIG. 6, an optics aspirator probe 49 is depended 
from the first probe assembly 48. Quenchedsubstrate is withdrawn from the 
subject reaction tube through a flow cell 128 by the action of a 
peristaltic pump 130. The spectrophotometric or fluorometric measurements 
are made on the fluid as it flows through the optics flow cell 128. Fluid 
leaving the peristaltic pump 130 passes through solenoid activated valve 
132 and is briefly stored in an enlarged section 133. After measurement in 
the optics flow cell 128, the direction of the peristaltic pump 130 is 
reserved and the fluid is made to flow back into the reaction tubes 
through the aspirator probe 49. The inside of the aspirator probe 49, the 
flow cell 128 and perstaltic pump 130 are washed with quench reagent when 
valve 134 is open and valve 132 is closed. This prevents 
cross-contamination of sample results which leads to error in the 
technique. 
The reaction tube rack of the present invention is shown in a perspective 
view in FIG. 7. The rack 18 is comprised of a lower half 140 and an upper 
half 142 which contains apertures 144 to receive the tubes. A handle 146 
is provided at either end of the rack 18. One side panel 148 of the rack 
18 also contains apertures 150 which are used to optically identify a rack 
of tubes. In this embodiment a reflective button is inserted in the 
aperture 150 and is optically detected by the instrument each time the 
rack 18 is loaded into the reaction processor module. 
FIG. 8 shows the wash-aspirate 74 and detergentadding probe 72 in a 
reaction tube 62 which is held in place by the rack 18. The distal end 140 
of the aspirate-probe 74 has an outside diameter which approaches that of 
the tube 62 mouth inside diameter. This large diameter permits intense 
aspiration, leaving a substantially dry bead at the end of the second half 
of the wash cycle. The wash probe 74 is connected to a shaft which moves 
vertically in conjunction with the probe assembly 58 (not shown in this 
figure). While the "basket" end 140 of the probe 74 is useful in drying 
the solid support bead at the end of the wash cycle, its large size makes 
accurate insertion of the probe essential. In order to stabilize the 
reaction tube mouth prior to insertion of the wash-aspirate probe 74, a 
tube trap 142 is provided to focus the tube mouth to coincide with the 
probe longitudinal axis. The tube trap 142 is lowered onto the tube mouth 
by cooperation with a bale 144 which is sized to mate with the outgoing 
basket 140. See our application Ser. No. 757,646. Other embodiments of the 
tube trap are described in our copending application Ser. No. 850,941, the 
relevant portions of which are incorporated herein by reference. 
In summary, the present invention permits the introduction of an incubation 
period in an automated sequence wherein the incubation period is dependent 
upon a single tube position rather than upon an entire row, column or rack 
of tubes. This simple provision is made possible by arranging the probes 
to perform multiple sequences during a single pass of the probes through a 
rack of tubes. Thus, for example an eighty-five second wash-soak pause per 
tube is made possible without unduly extending the wash cycle time. The 
present invention introduces backtracking of the probes by a single row to 
permit completion of a wash cycle in one row while commencing the 
operation in the next row. 
While the subject invention has been described with reference to a 
preferred embodiment, it will be appreciated by those skilled in the art 
that modifications and variations may be made which are within the scope 
of the invention and the claims appended hereto.