Current-efficient suppressors and method of use

A method and apparatus for increasing the current efficiency of suppressor and suppress-like pretreatment devices is disclosed for the purpose of suppressing a high concentration of eluent without the detrimental effects of excess heat generation. The method and apparatus may be used in ion chromatography.

BACKGROUND OF THE INVENTION 
The present application relates to a current-efficient device and method 
for reducing the concentration of matrix ions of opposite charge to ions 
to be analyzed, and specifically for use of an ion chromatography 
suppressor or to a pretreatment device. 
Ion chromatography is a known technique for the analysis of ions which 
typically includes a chromatographic separation stage using an eluent 
containing an electrolyte, and an eluent suppression stage, followed by 
detection, typically by an electrical conductivity detector. In the 
chromatographic separation stage, ions of an injected sample are eluted 
through a separation column using an electrolyte as the eluent. In the 
suppression stage, electrical conductivity of the electrolyte is 
suppressed but not that of the separated ions so that the latter may be 
determined by a conductivity cell. This technique is described in detail 
in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019; and 3,926,559. 
Suppression or stripping of the electrolyte is described in the above prior 
art references by an ion exchange resin bed. A different form of 
suppressor column is described and published in U.S. Pat. No. 4,474,664, 
in which a charged ion exchange membrane in the form of a fiber or sheet 
is used in place of the resin bed. The sample and eluent are passed on one 
side of the membrane with a flowing regenerant on the other side, the 
membrane partitioning the regenerant from the effluent of chromatographic 
separation. The membrane passes ions of the same charge as the 
exchangeable ions of the membrane to convert the electrolyte of the eluent 
to weakly ionized form, followed by detection of the ions. 
Another membrane suppressor device is disclosed in U.S. Pat. No. 4,751,004. 
There, a hollow fiber suppressor is packed with polymer beads to reduce 
band spreading. There is a suggestion that such packing may be used with 
other membrane forms. Furthermore, there is a suggestion that the function 
of the fiber suppressor is improved by using ion exchange packing beads. 
No theory is set forth as to why such particles would function in an 
improved manner. 
Another suppression system is disclosed in U.S. Pat. No. 4,459,357. There, 
the effluent from a chromatographic column is passed through an open flow 
channel defined by flat membranes on both sides of the channel. On the 
opposite sides of both membranes are open channels through which 
regenerant solution is passed. As with the fiber suppressor, the flat 
membranes pass ions of the same charge as the exchangeable ions of the 
membrane. An electric field is passed between electrodes on opposite sides 
of the effluent channel to increase the mobility of the ion exchange. One 
problem with this electrodialytic membrane suppressor system is that very 
high voltages (50-500 volts DC) are required. As the liquid stream becomes 
deionized, electrical resistance increases, resulting in substantial heat 
production. Such heat is detrimental to effective detection because it 
greatly increases noise and decreases sensitivity. 
In U.S. Pat. No. 4,403,039, another form of electrodialytic suppressor is 
disclosed in which the ion exchange membranes are in the form of 
concentric tubes. One of the electrodes is at the center of the innermost 
tube. One problem with this form of suppressor is limited exchange 
capacity. Although the electrical field enhances ion mobility, the device 
is still dependent on diffusion of ions in the bulk solution to the 
membrane. 
Another form of suppressor is described in U.S. Pat. No. 4,999,098. In this 
apparatus, the suppressor includes at least one regenerant compartment and 
one chromatographic effluent compartment separated by an ion exchange 
membrane sheet. The sheet allows transmembrane passage of ions of the same 
charge as its exchangeable ions. Ion exchange screens are used in the 
regenerant and effluent compartments. Flow from the effluent compartment 
is directed to a detector, such as an electrical conductivity detector, 
for detecting the resolved ionic species. The screens provide ion exchange 
sites and serve to provide site-to-site transfer paths across the effluent 
flow channel so that suppression capacity is no longer limited by 
diffusion of ions in the bulk solution to the membrane. A sandwich 
suppressor is also disclosed including a second membrane sheet opposite to 
the first membrane sheet and defining a second regenerant compartment. 
Spaced electrodes are disclosed in communication with both regenerant 
chambers along the length of the suppressor. By applying an electrical 
potential across the electrodes, there is an increase in the suppression 
capacity of the device. The patent discloses a typical regenerant solution 
(acid or base) flowing in the regenerant flow channels and supplied from a 
regenerant delivery source. In a typical anion analysis system, sodium 
hydroxide is the electrolyte developing reagent and sulfuric acid is the 
regenerant. The patent also discloses the possibility of using water to 
replace the regenerant solution in the electrodialytic mode. 
U.S. Pat. No. 5,045,204 discloses an electrodialytic device using an ion 
exchange membrane separating two flowing solutions in flow-through 
channels for generating a high purity chromatography eluent (e.g., NaOH). 
Water is electrolyzed in a product channel to provide the source of 
hydroxide ion for sodium which diffuses across the membrane. The patent 
discloses a mode of eliminating hydrogen gas generated in the product 
channel. 
U.S. Pat. No. 5,248,426 discloses a suppressor of the general type 
described in U.S. Pat. No. 4,999,098 in an ion chromatography system in 
which the effluent from the detector is recycled to the flow channel(s) in 
the suppressor adjacent the sample stream flow channel. 
U.S. Pat. No. 5,597,481 disclosed a suppressor-type device of the foregoing 
type used in sample pretreatment to reduce or suppress matrix ions in the 
eluent of opposite charge to the analyte ions and then to analyze the 
analytes in their conductive forms. Using existing suppressor devices, ion 
exchange interactions and hydrophobic interaction of the analyte, 
particularly in the eluent flow channel, affects recovery of certain 
analytes such as oligonucleotides and oligosaccharides. In order to 
improve recovery, high concentrations of eluents coupled with solvents are 
generally used. Similarly, in order to elute certain highly charged 
multifunctional analytes from the chromatographic column, high 
concentrations of eluents are normally used. High concentrations of 
eluents, however, are not easily suppressed. 
In all of the disclosed approaches, currents higher than theoretically 
predicted are required for achieving quantitative suppression. Under high 
eluent concentration conditions, this high current translates into heat 
generation and high background noise. Therefore, there is a need for a 
suppressor that would enable suppression of a wide range of eluent 
concentration and operate near the current-efficient faradaic regime. 
There is a need to increase the current efficiency of suppressors and 
suppressor-like pretreatment devices to permit suppression of a high 
concentration of eluent without the detrimental effects of excess heat 
generation. Similarly, in sample preparation applications it would be 
useful to have a suppressor that would enable good recovery of analytes 
and suppress high concentrations of eluent or mobile phase. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, methods and apparatus are 
provided of improved current efficiency. In one embodiment, an aqueous 
sample stream including analyte ions of one charge and matrix ions of 
opposite charge flows through a sample stream flow channel, while flowing 
an aqueous stream through an ion receiving flow channel separated 
therefrom by a first ion exchange membrane, and passing a current between 
the channels to reduce the concentration of the matrix ions. The sample 
stream flow channel has an upstream sample stream portion containing the 
matrix ions and an adjacent downstream portion in which the matrix ions 
have been suppressed. The upstream portion has an electrical resistance no 
greater than about 0.9 times that of the downstream portion. The ion 
receiving flow channel includes stationary flow-through first packing of 
ion exchange material. Neutral or low capacity packing may be disposed in 
the sample stream flow channel. 
In another embodiment, a second ion exchange membrane adjacent to the 
sample stream flow channel is used defining an ion source flow channel 
through which another aqueous stream flows. The first membrane has a net 
charge of no greater than about 0.9 times the net charge of the second 
membrane. The current is passed between first and second electrodes in 
electrical communication with the ion source flow channel and ion 
receiving stream flow channel. 
In another embodiment, the downstream portion has a net charge of no 
greater than about 0.9 times the net charge of the upstream portion. 
In a further embodiment, current is passed at a first amperage between the 
upstream sample stream portion and an adjacent upstream ion receiving 
stream portion using first and second electrodes, and a second current is 
passed at a second lower amperage between the downstream sample stream 
portion and an adjacent downstream ion receiving stream portion using 
third and fourth electrodes. 
In other embodiments, the current is maintained at a substantially constant 
voltage along the length of the sample stream flow channel. Also, the 
analyte ions exiting from the sample stream flow channel are detected. For 
pretreating a sample prior to analysis, the analyte ions exiting from the 
sample stream flow channel are detected, normally after separating the 
analyte ions exiting from the sample stream flow channel. For ion 
chromatography, the analyte ions in the sample stream are 
chromatographically separated prior to flowing through the ion receiving 
flow channel. Further, a portion of the sample stream is recycled to the 
ion receiving stream flow channel and ion source flow channel, if present. 
The invention also relates to apparatus for performing the above method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The system of the present invention is useful for determining a large 
number of ionic analyte so long as the ions are solely anions or solely 
cations. Suitable samples include surface waters and other liquids such as 
industrial chemical waste, body fluids, beverages such as fruits, wines 
and drinking water. 
The present invention is directed to a method and apparatus for treating an 
aqueous sample stream including analyte ions of one charge and matrix ions 
of opposite charge. In one application, the treatment is in a suppressor 
for ion chromatography and the matrix ions are the electrolyte ions in the 
eluent of opposite charge to the analyte ions. In another application, the 
method and apparatus is used for pretreating an aqueous sample stream 
prior to analysis, preferably including separation on a chromatography 
column. In this instance, the matrix ions typically are compounds of high 
ionic strength in the sample stream (e.g., commercial sodium hydroxide) 
which can obscure the sample peaks by large interfering peaks of the 
sample matrix ions. Such matrix ions can severely change chromatography 
because the sample matrix ion is of such high concentration it becomes the 
major eluting ion, temporarily overriding the eluent. A typical minimum 
concentration to warrant pretreatment is when the matrix ion is at least 
ten times the molar ionic concentration of the chromatographic eluent. 
Such a system to which the present improvement in current efficiencies is 
applicable is set forth in Stillian, et al., U.S. Pat. No. 5,597,481, 
incorporated herein by reference. 
As used herein, the term "matrix ion" refers to either the electrolyte in 
an eluent used for chromatography which is suppressed or whose 
concentration is reduced to non-interfering levels after separation and 
prior to detection, or to matrix ions in a sample stream whose 
concentration is significantly reduced prior to separation and/or 
detection. Since, in either case, the matrix ions are suppressed in the 
device, the term "suppressor" will be used generically to include a 
suppressor for ion chromatography and a pre-treatment device including the 
modifications of the present invention. 
For the analysis of anions, the matrix ions typically are a base (e.g., 
sodium hydroxide or other alkyl metal hydroxides). Other matrix compounds 
include sodium carbonate, ammonium hydroxide, means over alkyl ammonium 
hydroxide. For cation analysis, the matrix ions typically are an acid such 
as a common mineral or organic acid (e.g., sulfuric acid, phosphoric acid 
or methane sulfonic acid). 
The term "packing" refers to stationary flow-through solid material 
disposed in a flow channel of the suppressor. It can be a screen or a 
porous monolithic matrix, a resin particle bed or other form. It can be 
strongly charged, weakly charged or of neutral charge, as will be 
explained. The term packing is alternatively called "bridging means." 
During suppression, the conductivity and noise caused by matrix ions in an 
analysis stream is reduced. The present invention serves to increase the 
current efficiency of the suppressors described above. Various embodiments 
of such current efficient suppressors will be described herein. 
In one embodiment, a suppressor of increased current efficiency will be 
described with respect to a chromatography system of the type using an 
electrochemical suppressor with detector effluent recycle as described in 
Stillian, et al., U.S. Pat. No. 5,248,426, incorporated herein by 
reference. 
The specific purpose of the suppressor stage in ion chromatography is to 
reduce the conductivity and noise of the analysis stream background while 
enhancing the conductivity of the analytes (i.e., increasing the 
signal/noise ratio), while maintaining chromatographic efficiency. Thus, 
the following parameters bear upon the performance of the suppressor: (1) 
dynamic capacity of suppression, measured as .mu.Eq./min of eluent for 
each device; and (2) background conductivity measured as .mu.S/cm per 
device. 
Referring to FIG. 1, a simplified schematic apparatus for performing the 
present invention is illustrated using a recycle stream from the detector 
to the suppressor. The system includes a chromatographic separator, 
typically in the form of a chromatographic column 10 which is packed with 
a chromatographic separation medium. In one embodiment referred to above, 
such medium is in the form of ion-exchange resin. In another embodiment, 
the separation medium is a porous hydrophobic chromatographic resin with 
essentially no permanently attached ion-exchange sites. This other system 
is used for mobile phase ion chromatography (MPIC) as described in U.S. 
Pat. No. 4,265,634. An ion exchange site-forming compound, including 
hydrophobic portion and an ion-exchange site, is passed through the column 
and is reversibly adsorbed to the resin to create ion-exchange sites. 
Arranged in series with column 10 is a suppressor 11 serving to suppress 
the conductivity of the electrolyte of the eluent from column 10 but not 
the conductivity of the separated ions. The conductivity of the separated 
ions is usually enhanced in the suppression process. 
The effluent from suppressor 11 is directed to a detector, preferably in 
the form of flow-through conductivity cell 12, for detecting all the 
resolved ionic species therefrom. A suitable sample is supplied through 
sample injection valve 13 which is passed through the apparatus in the 
solution of eluent from eluent source or reservoir 14 drawn by pump 15, 
and then passed through the sample injection valve 13. The chromatography 
effluent solution leaving column 10 is directed to suppressor 11 wherein 
the electrolyte is converted to a weakly conducting form. The 
chromatography effluent with separated ionic species is then treated by 
suppressor 11 and passed through conductivity cell 12. 
In conductivity cell 12, the presence of ionic species produces an 
electrical signal proportional to the amount of ionic material. Such 
signal is typically directed from the cell 12 to a conductivity meter, not 
shown, thus permitting detection of the concentration of separated ionic 
species. 
The effluent from conductivity cell 12, referred to herein as the detector 
effluent, is directed to at least one flow-through detector effluent 
channel in ion-exchange membrane device 17. The membrane device will be 
described in detail hereinafter. As illustrated, the detector effluent 
flows through a splitter valve or tee 19 which separates the detector 
effluent into two different conduits 20 and 21 to supply the detector 
effluent to flow-through channels on opposite sides of the two membranes 
of the suppressor adjacent the central sample stream flow channel and then 
to waste through conduit 22. In one alternative, the detector effluent 
flows through such channels sequentially and then to waste. The 
chromatography effluent flows from chromatographic column 10 to membrane 
device 17 through conduit 23, and from the membrane device to the 
conductivity detector through conduit 24. 
Sandwich Suppressor Device 
Referring to FIGS. 2-5, a device is illustrated in the form of a sandwich 
suppressor device including a central sample stream flow channel defined 
on both sides by ion-exchange membranes to the exterior of which are an 
ion receiving flow channel and an ion source flow channel, respectively. 
Referring specifically to FIGS. 2, 3 and 4, membrane device 17 is 
illustrated which includes a central sample stream flow channel 31 flanked 
by such ion receiving and ion source flow channels. Membrane device 17 
includes means defining a sample stream flow channel in the form of a 
sample stream compartment, partially bounded by sample stream gasket 30 
defining a central cavity. To minimize dead space in the cavity, it is 
preferable to form both ends of the flow channels in a peak or V-shape. 
Stationary flow-through packing, preferably bridging means in the form of 
sample stream screen 32, may be disposed in the cavity. Ion exchange 
membrane sheets 34 and 36 are mounted to extend along opposite sides of 
screen 32 and, together with gasket 30, define the outer perimeter of the 
sample stream flow channel. Openings 36a and 36b are provided for sample 
stream inlet and outlets to the sample stream flow channel. 
Gaskets 38 and 40 are mounted to the facing surfaces of ion exchange 
membrane sheets 34 and 36, respectively, and define an ion receiving flow 
channel 35 and an ion source flow channel 37, respectively. In one 
embodiment, packing or bridging means are provided in ion exchange flow 
channels 35 and 37, in the form of ion exchange screens 41 and 43, 
respectively. Openings 40a and 40b are provided for inlet and outlet 
detector effluent flow-through gasket 40. To simplify connections with the 
external flow lines, it is preferable to form the chromatography effluent 
flow channel slightly longer than the flanking regenerant flow channels. 
As illustrated, spaced electrode means in the form of flat plate electrodes 
42 and 44, are placed on the exterior sides of gaskets 38 and 40, 
respectively, extending substantially across the length and width of the 
chambers in the gaskets. An electrical potential is applied across the 
electrode means. Electrode 42 includes openings 42a and 42b to permit the 
inlet and outlet flow of detector effluent solution to ion receiving flow 
channel 35 Similarly, electrode 44 includes inlet and outlet openings 44a 
and 44b, respectively, for detector effluent liquid flow and ion source 
flow channel 37 and gasket 40, and also defining inlet and outlet openings 
44c and 44d for the chromatography effluent flow channel defined by gasket 
30. 
External support blocks 46 and 48 are formed of a rigid nonconductive 
material, such as polymethylmethacrylate, or polyether-ether ketone (PEEK) 
and serve to provide structural support for the remainder of membrane 
device 17. Referring to FIG. 3, fittings 50 and 52 are provided for 
detector effluent inlet and outlet lines 54 and 56, respectively. 
Similarly, fittings 58 and 60 are provided for detector effluent inlet and 
outlet lines 62 and 64, respectively. Fittings 66 and 68 are provided for 
chromatography effluent inlet and outlet lines 70 and 69, respectively. 
The fittings may be mounted to the support blocks by any conventional 
means such as mating screw threads. 
The above assembled sheets and gaskets are mounted under pressure supplied 
by bolts 71 to form liquid-tight seals. Also, by use of such pressure in 
combination with appropriate sizing of the screen (or other bridging means 
described below) in comparison to the flow channel dimensions, the screen 
extends substantially the entire distance across the flow channels and 
contacts the membranes, resulting in significantly improved ion transport 
and efficiency. It is preferable for maximum membrane transfer efficiency 
to connect the lines to the chromatography effluent and detector effluent 
flow channels for countercurrent flow. 
Ion-exchange membrane sheets 34 and 36 may be of a type such as disclosed 
in U.S. Pat. No. 4,486,312. In particular, such sheets may be 
cation-exchange or anion-exchange membranes with polyethylene, 
polypropylene, polyethylene-vinylacetate-based substrates. Other suitable 
substrates include poly-vinylchloride or polyfluorocarbon-based materials. 
The substrate polymer is solvent and acid or base resistant. Such 
substrates are first grafted with suitable monomer for later 
functionalizing. Applicable monomers include styrene and alkylstyrenes 
such as 4-methylstyrene, vinylbenzylchloride or vinylsulfonates, 
vinylpyridine and alkylvinylpyridines. As an example, to form a 
cation-exchange membrane, the sheets grated with styrene monomers are 
functionalized suitably with chlorosulfonic acid, sulfuric acid, or other 
SO.sub.2 or SO.sub.3 sources. To form an anion-exchange membrane, the 
sheets grafted with vinylbenzylchloride monomers are functionalized with 
alkyl tertiary amines such as trimethylamine or tertiary alkanolamines, 
such as dimethylethanolamine. Particularly effective membranes are no more 
than 10 mil thick, and preferably no more than 2-4 mil when wet. Suitable 
polyethylene substrate membranes of the foregoing type are provided by RAI 
Research Corp., Hauppauge, N.Y. (the cation exchange membrane provided 
under designation R5010 (0.008 inch thick) and the anion-exchange membrane 
under designation R4015 (0.004 inch thick)). Other cation exchange 
membranes supplied by the same company which are fluorocarbon based 
include R1010 (0.002 inch thick) and R4010 (0.004 inch thick). 
Sample stream screen 32 may be formed integral with chromatography effluent 
gasket 30 or may be inserted independently into the effluent flow channel. 
A screen integral with the surrounding gasket material may be formed by 
cutting a gasket from plastic sheet to include the desired flow path and 
pressing this gasket into a rectangular piece of screen such that only the 
flow path is not covered by the gasketing material. 
Ion exchange packing in the form of screens 41 and 43 may be constructed in 
the same manner as set forth with respect to screen 32. 
For a flat sheet suppressor, the packing preferably includes continuous 
portions which extend substantially the entire distance across flow 
channels 31, 35 and 37 transverse to flow. In the embodiment of FIGS. 2 
and 3, this distance extends between membrane sheets 34 and 36 and between 
the same membrane sheets and electrodes 42-44, respectively. In an 
alternate embodiment of FIG. 6 described below, only one membrane is used 
which separates ion receiving flow channel 35 from sample stream flow 
channel 31. The packing defines a continuous convoluted flow-through 
passageway in the flow channel in which it is disposed along substantially 
the entire length of the membrane. This creates turbulence and thus 
increases the efficiency of mixing and transfer of the ions across the 
membrane as described below. The physical configuration of the screen may 
vary so long as its bridging function and turbulence-producing function is 
accomplished. Thus, the screen may vary so long as its bridging function 
and turbulence-producing function is accomplished. Thus, the screen may be 
provided with a weaving pattern either perpendicular or diagonal to the 
direction of flow. Also, the fibers may be smooth or contain protrusions 
such as bumps. 
A major function of the flow-through packing 41 and 43 in ion exchange form 
is to provide a site-to-site path for ions in the direction transverse to 
the flow channel to increase the efficiency of ionic transfer across the 
ion-exchange membrane as more fully described below. Such packing in the 
form of a screen may be functionalized for this purpose in a manner 
analogous to the functionalization of the ion-exchange membranes set forth 
above. Suitable screens may be formed of the same base polymers grafted 
with the same functionalizing monomers as those set out above for the 
membranes. 
Good chromatographic efficiency of the screen embodiment of the 
flow-through ion-exchange packing may be achieved using a relatively small 
mesh (measured after functionalization), e.g., on the order of 110 .mu. 
mesh size or less with relatively thin fibers, e.g., on the order of 0.004 
inch in diameter. An open area in flow channels 35 and 37 on the order of 
5% to 70% (preferably, on the order of 8%) provides excellent 
efficiencies. A suitable proportion of grafting monomer to grafting 
polymer substrate is on the order of 5%-50% (preferably about 25% to 35%). 
In order to obtain maximum efficiency, flow channels 35 and 37 should be 
fairly narrow, e.g., on the order of 0.5 cm, with the weave pattern 
oriented diagonally to the direction of flow. As the exposed membrane 
surface area increases, suppression capacity increases. However, practical 
limits are prescribed by known principles of chromatography. For example, 
to minimize band broadening, a minimum volume is desired. To maximize the 
dynamic capacity, screens 41-43 may be functionalized to relatively high 
ion exchange capacity, e.g., 2 meg/g. 
In the embodiments of FIGS. 2 and 3, an electrical potential from a direct 
current source (not shown) is applied between electrodes 42 and 44 from 
any suitable source. The electrodes are formed of highly conductive 
material which is inert to the solutions being passed through the membrane 
suppressor. Platinum is a preferred form of electrode for this purpose. 
In one mode of operation of the suppressor device 17, effluent from 
chromatographic column 10 is directed through sample stream flow channel 
31 bounded on both sides by ion-exchange membranes 34 and 36 partitioning 
the detector effluent from the chromatography effluent. The detector 
effluent flows from the conductivity cell through channels 35 and 37. The 
membrane is preferentially permeable to ions of the same charge as the 
exchangeable ions of the membrane and resists permeation of ions of 
opposite charge. The exchangeable ions of the membrane are in the ion form 
necessary to convert the developing reagent of the eluent to a weakly 
ionized form. For maximum capacity, the detector effluent flow is 
countercurrent to the sample stream flow. The chromatography effluent from 
chromatographic column 10 is passed through the sample stream flow channel 
and contacts both membranes. The membranes are simultaneously contacted on 
their outer sides with the detector effluent flowing in the opposite 
direction through the detector effluent flow channel so that membrane 
forms a selective permeability partition between the detector effluent and 
the sample stream from the chromatography column. Ions extracted from the 
same stream at the active ion-exchange sites of the membranes are diffused 
through the membranes and are exchanged with ions of the detector 
effluent, and thus diffused ultimately into the detector effluent. 
Application of a potential across the electrodes increases the mobility of 
the ions across the membrane. The resolved ionic species in the effluent 
leaving the suppressor device are detected, as with a conductivity 
detector. 
FIG. 4 schematically illustrates the electrochemical operation of the 
present invention for a particular system, using a sandwich suppressor 
with screens 32, 41 and 43 in flow channels 31, 35 and 37, respectively, 
and applying an electrical potential between spaced electrodes. The system 
illustrated is for anion analysis and includes sodium hydroxide as the 
electrolyte of the effluent to be converted into weakly ionized form 
(H.sub.2 O) in the suppressor. Thereafter, the solution passes through the 
conductivity cell and is recycled to flow channels 35 and 37. The 
ion-exchange membrane sheets allow the positively charged sodium and 
hydronium ions to permeate across the membrane together. 
A suitable ion-exchange membrane for this purpose is a sulphonated 
polyethylene sheet. Hydroxide ions tend not to permeate the membrane sheet 
because of Donnan Exclusion forces. Thus, the sodium hydroxide stream is 
converted to deionized water in the chromatography effluent flow channel 
and the sodium ions permeate the membrane sheet and are dispersed in the 
negatively-charged detector effluent flow channel as NaOH and thus 
ultimately routed to waste through the detector effluent outlet lines. 
Applying a potential across electrodes 42 and 44 increases the kinetics of 
ion flow across the membrane and thereby increases capacity and, thus, the 
suppression efficiency of the suppressor device. 
In the illustrated embodiment, the positively charged sodium ions of the 
electrolyte in channel 31 electromigrate under the influence of the 
electric field, across the negatively charged membrane 34 into the 
detector effluent channel. The hydronium ions generated at the anode by 
electrolysis of water, flow from the positively-charged flow channel 37 
across membrane 36 into flow channel 31 to form water with hydroxide ions 
therein. The sodium ions, being attracted to the negative electrode, are 
more rapidly removed leading to a substantial increase in the capacity of 
the suppressor device. 
In operation of the system of FIG. 4, in flow channel 37, hydronium ion is 
generated at the anode according to the following equation and passes 
through membrane 36 
EQU H.sub.2 O-2e.sup.- .fwdarw.2H.sup.+ +1/2O.sub.2 (1) 
In flow channel 31, the sodium ion passes through membrane 34 under the 
influence of the electric field. Hydroxide is converted to water according 
to the following equation: 
EQU OH.sup.- +H.sup.+ .fwdarw.H.sub.2 O (2) 
In flow channel 35, the sodium ion is converted to NaOH with hydroxide ion 
produced at the cathode by the following equation: 
EQU 2H.sub.2 O+2e.sup.- .fwdarw.2OH.sup.- +H.sub.2 (3) 
High capacity ion exchange screens 41 and 43 substantially increase the 
capacity of the suppressor device to remove ions from the chromatography 
effluent sample stream. The threads of the screen preferably extend 
substantially across the flow channels transverse to flow to contact both 
membranes. 
The functionalized screens 41 and 43 include exchangeable ions of the same 
charge as those of the membranes. In this manner, the screen provides a 
direct site-to-site contact between the membrane walls for the ions to be 
diffused through the membranes. It has been found that the capacity of the 
system is significantly increased by the use of such functionalized 
screens. 
Referring again to FIG. 4, sample stream flow channel 31 may include weakly 
charged packing such as lightly sulfonated packing (screens) of low 
capacity. In a preferred embodiment the capacity of such packing would be 
less than 0.1 meq/g. In a more preferred embodiment the capacity of such 
packing would be less than 0.01 meq/g. The sample stream flow channel 31 
in an alternative embodiment may include neutral packing material (screen) 
of substantially no ion exchange capacity. 
The reduction in the ion exchange capacity for the packing (screens) is 
achieved by reducing the extent of functionalization; for example, 
optimizing the sulfonation conditions for a cation exchange material by 
time or temperature or both. Optimizing the graft level is another means 
for reducing the capacity of the packing material. 
The potential to be applied to the electrodes in the above system may be 
relatively low due to the presence of the functionalized screens 41 and 
43. Thus, capacity is substantially improved with a voltage of about 
1.5-20 VDC, preferably about 2-8 VDC. 
While the above sandwich suppressor embodiment includes a center sample 
stream flow channel 31 separated by two membranes from two coextensive 
flow channels 35 and 37, the system is also applicable to the use of a 
single ion receiving stream flow channel separated from the sample stream 
flow channel by a single membrane. 
Referring to FIGS. 5 and 6, another embodiment of suppressor 70 is 
illustrated using a single membrane. Suppressor 70 includes upper rigid 
support block 72 with sample stream flow channel wall 73 and lower support 
block 74 with ion receiving flow channel wall 75, separated by an 
ion-exchange membrane 76 of the type described above. 
The chromatography effluent flows into the suppressor device through 
effluent inlet 78, fitting 80 and flows along a sample stream flow channel 
defined by wall 73, through screen 94 and then through fitting 82 and out 
sample stream outlet line 84. Similarly, detector effluent solution flows 
from inlet line 86 through fitting 88 across the ion receiving flow 
channel defined by wall 75, out fitting 90 and through ion receiving flow 
channel outlet 92 to waste. The device of FIGS. 5 and 6 is used in the 
overall system of FIG. 1 in place of the device of FIGS. 2-5. 
The liquid flows through the channels formed by the spacing among the 
projections. The dimensions of the projections and spacing is selected to 
provide the desired frequency of contacts with the flowing ions to 
increase their mobility across the membrane and to create sufficient 
turbulence for increased mixing efficiency. 
Suitable eluent solutions for ion chromatography of anions include alkali 
hydroxides, such as sodium hydroxide, alkali carbonates and bicarbonates, 
such as sodium carbonate, alkali borates, such as sodium borate, 
combinations of the above, and the eluent systems of the aforementioned 
patents. 
The system of the present invention is also applicable to the analysis of 
cations (e.g., lithium, sodium, ammonium, potassium, magnesium, and 
calcium). In this instance, the electrolyte of the eluent is typically an 
acid which does not damage the membrane. Methane sulfonic acid has been 
found to be inert to the membrane under electrolytic conditions. Other 
acids such as nitric acid and hydrochloric acid produce electrochemical 
by-products that may damage the membrane and are, thus, not generally 
preferred for that typical membrane. 
In cation analysis, the flow of the electrolyte ion is from the cathode 
toward the anode, rather than the reverse as in anion analysis and the ion 
exchange screens and membranes are aminated and permeable to anions. Thus, 
in the negatively charged ion source flow channel, water is converted to 
hydroxide ion and hydrogen gas. The hydroxide ion passes through the 
adjacent membrane into the sample stream flow channel and combines with 
hydrogen ion (or an amine or other basic organic molecule group) to form 
weakly ionized electrolyte. The negatively-charged transmembrane ion 
travels through the second membrane into the positively-charged ion 
receiving flow channel under influence of the anode to form an acid which 
passes to waste. In summary, for cation analysis, the electrical charges 
of the analyte, eluent reagent, and membranes are reversed for cation 
analysis and anion analysis. 
In a single membrane suppressor, gases are generated in the chromatography 
effluent which can interfere with detection in the conductivity cell. For 
example, for ion analysis, oxygen is generated in the detector effluent 
flow channel. One way to remove the oxygen is to pass the effluent from 
the sample stream flow channel through a gas diffusion removal device, 
using a gas diffusion membrane, prior to reaching the conductivity cell. 
One such device is disclosed in U.S. Pat. No. 5,045,204. In another 
embodiment, a gas diffusion membrane forms a wall defining the opposite 
side of the chromatography effluent flow channel from the ion exchange 
membrane. An inert gas stream such as nitrogen, may be flowed in a channel 
bounded on one side by the gas diffusion membrane, preferably 
countercurrent to the chromatography effluent flow. In this manner, the 
solution leaving the chromatography effluent flow channel is degassed 
prior to reaching the conductivity cell. In either event, a suitable gas 
diffusion membrane is a gas diffusion membrane such as one sold under the 
trademark Accural.RTM. or Celgard.RTM.. 
The above system illustrates an ion exchange screen as the preferred 
flow-through ion exchange packing. However, it should be understood that 
other ion exchange packing may also be employed for the sandwich 
suppressor or other relatively flat suppressor. For example, ion exchange 
particles may be packed in the flow channels for this purpose. Here, it 
would be preferable to include some mode to keep the ion exchange 
particles in the device by using a porous polymeric support that has 
smaller pores than the resin being used, such as sintered polyethylene 
available from General Polymeric. 
Referring to FIG. 7, a schematic cross-sectional view of a tubular form of 
the electrodialytic suppressor of the present invention is illustrated. In 
this instance, it is assumed that the sample stream channel is the lumen 
of the innermost tube. The device includes anode 122 (in the form of a rod 
or wire, e.g., formed of platinum, gold, carbon or stainless steel), 
cation exchange membrane 124, and outer wall 126, which may be formed of a 
conductive material to serve as the cathode. In one embodiment, high 
capacity flow-through ion exchange packing in the form of a bed of ion 
exchange resin particles is disposed in the ion receiving flow channel 
with low capacity or neutral resin or open space in the sample stream 
channel. This system is comparable in general function to the one 
illustrated in FIG. 4. Alternatively, the ion receiving flow channel may 
be the lumen of the inner tube. In this instance, the polarities of the 
electrodes are reversed. Membrane 124 may be formed of stretched or 
unstretched tubular ion exchange membranes, e.g., Nafion 811X from 
Perma-Pure Products, J.S. Outer wall 126 may be formed of an 18 gauge 
stainless steel (SS) tubular case. 
FIG. 8 illustrates a tubular type of dual-membrane suppressor of similar 
function to the sandwich membrane suppressor. It is generally constructed 
by inserting a length of suitably inert wire inner electrode 128 into a 
length of tubular inner membrane 130 which is itself inserted inside a 
length of somewhat larger diameter tubular outer membrane 132 and 
enclosing the whole assembly in stainless steel tube 134 of appropriate 
dimensions. The outer tube itself functions as the electrode, connections 
being made at the ends to allow access to the flow channels between the 
inner electrode and inner membrane, between the two membranes (annulus) 
and between the outer membrane and stainless steel case. High capacity 
flow through ion exchange packing in the form of a bed of ion exchange 
resin particles is disposed in the ion receiving flow channel with neutral 
or low capacity or open space in the sample stream channel. 
The power requirements for this system are dependent to some extent upon 
the flow rate through the system and the concentration of electrolyte 
solution. For this purpose, a suitable flow rate or chromatography 
effluent is about 0.01 to 10 mls/min. and, preferably, 0.25 to 2 mls/min. 
The concentration of eluent varies between about 5 and 500 mM. Suitable 
power supply requirements are about 2 to 12 V at 0.001 to 2 A. Suitable 
power requirements are 2 to 12 volts at 0.001 to 2 A. This applies to both 
the flat membrane suppressor and tubular membrane assembly. 
Other alternative configurations (not shown) of the suppressor can be used 
in accordance with the present invention. For example, referring to the 
suppressor of FIGS. 2-4, the positions of screens 41 and 43 may be 
reversed with the positions of electrodes 42 and 44, respectively. 
Specifically, in such alternative configurations, electrodes 42 and 44 
extend along, and are pressed flush against, ion exchange membranes 34 and 
36, respectively. The electrodes are in contact with the solution flowing 
through the outside flow channels 35 and 37. In this instance, the 
electrodes include openings to permit ion transport across the ion 
exchange membranes between the outside flow channels 35 and 37 and the 
sample stream flow channel. Such openings may be formed in a number of 
known ways, e.g., by punching of spaced holes (typically from 0.010" to 
0.250" across), or by forming the electrodes of a woven screen, or by 
notching an inert foil electrode so that the electrode forms a zig-zag or 
serpentine pattern along the length of the chamber. For example, platinum 
wire bent into a zig-zag pattern can be used, however, platinum or 
platinum plated foil is preferable to prevent excessive resistive heating. 
In yet another embodiment (not shown), a "hybrid" suppressor may be formed 
in which the electrode and screen is in the configuration illustrated in 
FIGS. 2-4 for one of the outside flow channels while in the opposite 
outside flow channel the electrode and screen are reversed in the manner 
described in the previous paragraph. An effective hybrid configuration for 
an ion analysis is formed in which an anode with spaced openings is flush 
against the ion exchange membrane and the cathode (the compartment to the 
left of FIG. 3) is in the configuration illustrated in FIGS. 2-4. The same 
configuration is preferred for cation analysis. 
According to the present invention, suppressor current efficiency is 
increased. In such devices, the upstream portion of the device includes a 
concentration of the matrix ions which decreases in the downstream portion 
of the channel to a level where the matrix ion is "suppressed", i.e., 
present at a level which does not significantly interfere with subsequent 
analysis. Thus, it is preferable to provide a maximum current in the 
upstream portion in which suppression of the matrix ion takes place. In 
the downstream portion in which suppression is substantially complete, the 
current does not provide sufficient beneficial effect to counter-balance 
its negative effects such as generation of high background noise and heat 
caused by the application of the electrical field. Thus, one of the 
objectives of the present invention is to provide a lower electrical 
resistance in the upstream portion in which the matrix ions are present 
and being suppressed in comparison to the downstream portion. 
It is preferable that the upstream portion of the sample stream flow 
channel in which matrix ion is present and suppression occurs have an 
electrical resistance no greater than about 0.9 times that of the 
downstream portion in which suppression is substantially complete. 
Suppression is considered substantially complete at a distance along the 
sample stream flow channel when the matrix ion concentration has been 
reduced by at least 95% from the concentration at the beginning of the 
flow channel. This typically occurs at a distance of about 20 to 80% (more 
typically about 40 to 60%) along the length of the sample stream flow 
channel. Preferably, the electrical resistance ratio of the upstream and 
downstream portions is no greater than about 0.7 to 0.9, and most 
preferably no greater than about 0.7. 
The resistance of the upstream and downstream portions is determined as 
follows. A suppressor of the type sold by Dionex Corporation under the 
ASRS name is fitted with two anodes and two cathodes such that the 
electrodes flank the upstream and downstream portions of the ion source 
and receiving channels, respectively. When powered and monitored for 
suppression with 100 mM of NaOH at a flow rate of 1 mL/min, this unit 
provides average upstream and downstream portion resistances. For example, 
using 100 mA settings for the two zones the measured resistances were 
approximately 40.2 and 35.3 ohms, respectively, suggesting that the 
upstream portion was more resistive than the downstream portion. In 
contrast, the devices of the present invention have the upstream portion 
less resistive than the downstream portion. For example, in the above 
example when the current to the upstream portion was 100 mA and the 
downstream portion was 75 mA, the device resistances were approximately 
40.2 and 45.46 for the upstream and the downstream portions, respectively. 
The current efficiency went up from 80% to 92% in the above example. 
An alternative means for demarking the upstream and downstream zones is by 
disassembling the suppressor unit and visually examining the upstream and 
downstream zones in the eluent channel. The ion exchange material in the 
eluent form shows a lighter coloration in comparison to the downstream 
portion, which is in the suppressed form. For example, an ASRS suppressor 
of the prior art with 100 mM NaOH at 1 mL/min and run at 500 mA and 4 V 
shows 50% of the eluent channel in the eluent form. Similarly, a CSRS 
suppressor of the prior art run with 22 mN H.sub.2 SO.sub.4 at 100 mA and 
approximately 3.6 V shows the eluent zone to be 50% of the eluent channel. 
The suppressed form is less resistive than the eluent form, hence in both 
of the above devices the upstream portion is more resistive than the 
downstream portion, hence the devices are not current efficient. 
In the above system, one way to increase current efficiency is leave the 
sample stream flow channel open without packing or to use packing which is 
of neutral charge or of low capacity relative to the packing of high 
capacity ion exchange material in the ion receiving flow channel and, for 
a two membrane suppressor, in the ion source channel. While the above 
description refers to the stationary flow-through packing of ion exchange 
material in the form of a high capacity charged screen, other forms of 
packing may also be employed as described above. Such other packing forms 
of ion exchange material include packed beds of ion exchange resin or 
monolithic materials of charged material with sufficient porosity for the 
flow of an aqueous liquid stream through them. The packing in the ion 
receiving channel has a substantially higher capacity than ion exchange 
packing in the sample flow channel, if present. Thus, if a charged packing 
is used in the sample stream flow channel, it preferably is of low 
capacity, with a capacity of substantially less than that of the packing 
in the ion receiving flow channel. Suitably, the ratio of total capacities 
of the packing in the sample stream flow channel to that in the ion 
receiving stream flow channel is no greater than about 0.9, and preferably 
no greater than about 0.7 to 0.5, and more preferably no greater than 
about 0.1. 
A suitable low capacity packing in the sample stream flow channel has a 
capacity less than about 0.1 meq/g and preferably less than about 0.01 
meq/g. This difference in the capacity of the sample stream flow channel 
and the ion receiving flow channel and ion source flow channel (if 
present) will be referred to herein as the "packing principle." This 
principle can be used in combination with all of the other embodiments of 
current efficiency described hereinafter. 
The current efficiency by this approach is substantially increased in 
comparison to having fully charged packing in the sample stream flow 
channel as described in the prior art, e.g., in U.S. Pat. No. 5,248,426. 
The mechanism of suppression in the cited prior art is described as 
follows. The electrochemically-generated hydronium ions at the anode are 
transported across the ion exchange bridging means towards the cathode and 
migrate into the eluent channel. For each H+ ion transported into the 
eluent channel, either a Na+ ion or a H+ ion is transported across the 
channel towards the cathode and forms either sodium hydroxide or water at 
the cathode. The current used in forming water is the excess current and 
is not used for suppression. Since the ion exchange means in the hydronium 
form is less resistive than the sodium form, transport of hydronium, 
particularly across the eluent channel, is preferred and hence the device 
is not current efficient. In these devices transport of hydronium into the 
channel does not guarantee transport of sodium out of the eluent channel. 
Thus, the presence of functionalized screen drives up the current 
requirement for suppression and an excess of hydronium ions is required to 
ensure complete suppression of the eluent. 
By reducing the capacity of the eluent screen, the present invention forces 
the current to be carried by the eluent alone in the eluent channel. Thus 
transport of hydronium into the eluent channel guarantees transport of 
sodium out of the eluent channel, and formation of water by transport of 
hydronium across the eluent channel and to the cathode is minimized. Thus 
suppression is guaranteed with near faradaic efficiency. 
The reduction in capacity for screens is achieved by using either a neutral 
unfunctionalized screen or by reducing the extent of functionalization; 
for example, optimizing the sulfonation conditions for a cation exchange 
material by time or temperature or both. Optimizing the graft level is 
another means for reducing the capacity of the packing material. 
The current required to suppress a given concentration of eluent with 100% 
faradaic efficiency can be calculated from 
EQU I=FCV/60 
wherein 
I is current mA 
F is Faraday's constant (coulombs/equiv.) 
C is the concentration of eluent in M 
V is the flow rate in mL/min. 
Now, the current required to suppress 20 mM of NaOH eluent with 100% 
faradaic efficiency can be calculated as approximately 32 mA. 
In FIG. 1, when a neutral screen or no screen is used in the eluent 
channel, then 100% of the current is carried by the eluent and the device 
is expected to show 100% faradaic current efficiency. One way to compare 
current efficiency to capacity is to test the suppressor using an aqueous 
stream of 100 mM NaOH at 1 mL/min. Current efficiency was determined by 
optimizing the current required for suppression (with a variance of .+-.10 
mA). The results are set forth in the following table. 
TABLE 1 
______________________________________ 
Estimated Current 
Eluent screen capacity 
Efficiency (%) 
Volts 
______________________________________ 
0.925 meq/g 67 3.71 
(prior art suppressor) 
0.005 meq/g 92 5.2 
(device of present invention) 
Neutral screen - 0 meq/g 
95 5.0 
(device of present invention) 
______________________________________ 
In the case where the neutral screen is replaced by a low capacity, lightly 
functionalized screen bulk of the current is still transported by the 
eluent with a small percentage of the current being wasted in formation of 
water due to transport of excess H.sup.+ across the screen. The voltage 
applied across the above disclosed suppressor devices is comparable to the 
voltage generated in an SRS device (4-9 V) as disclosed in U.S. Pat. No. 
5,246,426. 
In the sandwich suppressor described above, high capacity ion exchange 
packing of the type described regarding the packing in the ion receiving 
flow channel also is used in the ion source flow channel. This is a 
preferable form of the sandwich suppressor because the high capacity 
packing allows efficient transport of the electrochemically generated 
hydronium from the anode (in the case of anion analysis) to the membrane 
interface. Similarly, fast transport of sodium from the eluent channel to 
the cathode becomes possible in the presence of high capacity ion exchange 
packing. The presence of packing in the channels keeps the voltage drop 
across the entire device within acceptable limits. In the absence of ion 
exchange packing, it is not possible to use the suppressed effluent (e.g. 
water) as the regenerant (as used in the recycle mode). Additionally, the 
device voltage becomes very high. 
Referring to FIGS. 5 and 6, another embodiment of suppressor 70 is 
illustrated using a single regenerant flow channel. Suppressor 70 includes 
upper rigid support block 72 with sample stream flow channel wall 73 and 
lower support block 74 with ion receiving flow channel wall 75, separated 
by an ion-exchange membrane 76 of the type described above. 
The chromatography effluent flows into the suppressor device through 
effluent inlet 78, fitting 80 and flows along the sample stream flow 
channel defined by wall 73, through screen 94 and then through fitting 82 
and out chromatography effluent outlet line 84. Similarly, detector 
effluent solution flows from inlet line 86 through fitting 88 across the 
ion receiving flow channel defined by wall 75, out fitting 90 and through 
detector effluent outlet 92 to waste. The device of FIGS. 5 and 6 is used 
in the overall system of FIG. 1 in place of the device of FIGS. 2-4. 
The liquid flows through the channels formed by the spacing among the 
projections. The dimensions of the projections and spacing is selected to 
provide the desired frequency of contacts with the flowing ions to 
increase their mobility across the membrane and to create sufficient 
turbulence for increased mixing efficiency. 
As illustrated, the detector effluent is recycled to the ion receiving flow 
channel, and if present, to the ion source flow channel. While this is 
efficient for the reasons set forth above, it should be understood that 
this system is applicable to suppression without such detector recycle as 
illustrated in U.S. Pat. No. 4,999,098, incorporated herein by reference. 
As set out above, electrochemical suppressor device of the type described 
above can be used in a pretreatment device prior to analysis of the 
analyte. One pretreatment device and system is illustrated in U.S. Pat. 
No. 5,597,481, incorporated herein by reference. This pretreatment device 
is substantially the same as the foregoing suppressor. The matrix ion 
which is removed in the pretreatment device is of opposite charge to the 
analyte ion. The improvements in current efficiency described herein are 
also applicable to the pretreatment device as set forth in the patent. 
Another embodiment of increasing current efficiency according to the 
invention is applicable to two membrane suppressors. Here, the net charge 
of one of the membranes is greater than the charge of the other one. In 
one embodiment, the membrane separating the ion source channel from the 
sample stream flow channel has a net charge no greater than about 0.9 
times the net charge of the membrane separating the sample stream flow 
channel from the ion receiving channel. Preferably, this ratio is no 
greater than 0.7 times, and more preferably no greater than 0.5 times. 
A preferred means for altering the net charge of the two membranes is by 
using two different functionalities, a strongly ionized functionality in 
combination with a weakly ionized functionality. One sandwich suppressor 
of the general type illustrated in FIG. 4 suitable for anion analysis 
includes a strong cation exchange membrane 36 (sulfonated membrane) 
forming the eluent channel in the anodic side and a weak cation exchange 
membrane 34 (carboxylated membrane) forming the eluent channel in the 
cathodic side. This combination of functionality is a convenient means of 
accomplishing different net charges on the two membrane surfaces. 
Hydronium that is generated at the anode is efficiently transported across 
the strong cation exchange membrane and into the eluent channel by high 
capacity ion exchange packing in the ion source flow channel. The weak 
cation exchange membrane readily allows transport of eluent cations (such 
as sodium ions in the case of sodium hydroxide eluent) across the membrane 
towards the cathode, while retarding the transport of hydronium ions. The 
weak cation exchange membrane is less resistive in the sodium form 
relative to the hydronium form, hence transport of sodium is preferred 
over transport of hydronium. Therefore, by minimizing transport of 
hydronium the wastage current is minimized and current efficiency 
improved. 
Wastage current is minimized across the lower (suppressed) section of the 
suppressor (Rw) and good current efficiency is achieved as the eluent 
carries the bulk of the current in this device. For cation analysis 
membrane 34 has a strong base functionality and membrane 36 could have 
either a weak base functionality or a hydroxide selective base 
functionality. In both cases the wastage current is minimized. 
Based on the foregoing, it is preferred to have the membrane with the 
higher net charge close to the ion source channel; for example, anode that 
generates hydronium in the case of anion analysis (and hydroxide in the 
case of cation analysis). The present suppressor has significantly less 
current wastage in the lower section. 
In the above suppressor, neutral packing, low capacity packing, or high 
capacity packing may be eliminated in the sample stream flow channel. 
Conversely, it is preferred to use high capacity packing in the ion 
receiving flow channel and ion source flow channel. 
In its above embodiment, the packing principle may also be employed. Thus, 
the ion receiving flow channel and ion source flow channel (if present) 
include high capacity packing with the sample stream flow channel 
including neutral or low capacity stream packing. 
In another embodiment, not shown, increased resistance of the downstream 
portion is achieved by including means, coupled electrically in series 
with the lower portion for increasing electrical resistance of the 
downstream portion. One such means is the use of an electrical resistor in 
the downstream portion but not in the upstream portion or using one or 
more resistors such that the downstream portion has a higher resistance 
than the upstream portion. 
The resistance R of a conductor is defined as 
##EQU1## 
where .rho. is the resistivity of the conductor in ohm-cm L is the length 
of the conductor in cm 
A is the cross-sectional area of the conductor in cm.sup.2. 
Thus, the resistance is directly proportional to the length and inversely 
proportional to the cross-sectional area of the conductor. In the 
suppressor example, the cross-sectional area of the downstream section 
could be reduced to increase resistance. The increased resistance of the 
lower section forces the current to be carried by the eluent in the upper 
section of the eluent channel/suppressor and hence good current efficiency 
is achieved. 
For example, if R.sub.e is the resistance of the eluent (unsuppressed) 
section of the suppressor and R.sub.w is the resistance of the region 
where the eluent is completely in the suppressed form (water in above 
example), then the total resistance R.sub.t 
##EQU2## 
As R.sub.w increases, R.sub.t approaches R.sub.e and hence current 
efficiency is improved. 
Another means of increasing the resistance of R.sub.w is by attaching a 
resistance in series with R.sub.w. One way to accomplish this is to use 
two spacers (e.g., lightly functionalized screens such as lightly 
sulfonated screens) in series with the lower section of the suppressor but 
not in the upper section of the suppressor. Such an arrangement increases 
the resistance in the lower section of the suppressor and forces the 
current to pass through the upper section of the suppressor. A resistor in 
place of the spacers will also accomplish increased resistance of the 
lower section relative to the upper section of the suppressor. 
In another embodiment, an additional pair of electrodes is employed. Thus, 
for a one-membrane device, one pair of electrodes is positioned in 
electrical communication with the upstream portion of the sample stream 
flow channel and ion receiving flow channel, respectively. The second pair 
of electrodes is positioned in the downstream portion of the same 
channels. Each of these electrode pairs is connected to an independent 
power source. Wastage current is minimized and current efficiency is 
improved by supplying a lower current to the downstream portion relative 
to the upstream portion. Thus, in use, the electrical current in the 
upstream portion exceeds the electrical current in the downstream portion. 
Suitable ratios are from 10:1 to 2:1. For a two-membrane device, the 
electrode pairs are disposed in the ion receiving flow channels and the 
ion source flow channel, respectively. 
In another embodiment of current efficiency, the downstream portion of the 
sample stream flow channel in which the matrix ion is suppressed has a net 
charge of no greater than about 0.9 times, preferably no greater than 
about 0.3, and most preferably the net charge of the upstream is no 
greater than about 0.1. The packing in the sample stream flow channel 
could be altered in this preferred embodiment to have a higher resistance 
on the downstream portion relative to the upstream portion. By increasing 
the resistance in the downstream portion, less hydronium is wasted in the 
formation of water, hence current efficiency is improved. In a preferred 
embodiment a carboxylate functionalized eluent screen accomplishes the 
above. The carboxylate functionality in the hydronium form is highly 
resistive compared to the eluent form. Similarly, a partially 
functionalized screen with approximately the upper half of the screen in 
the functionalized form and the lower half in the neutral or lightly 
functionalized form would accomplish the above. For example, an eluent 
screen with full sulfonation on the upper half and room temperature 
sulfonation or no sulfonation in the lower half of the screen. Similarly, 
combinations of the above-discussed functionalities could be used to 
improve current efficiency. 
In another embodiment of the invention, a constant voltage is applied from 
a constant voltage power source. The advantage of a constant voltage mode 
is that the current required for suppression would be self regulated or 
adjusted to the eluent strength or concentration. Hence, unlike constant 
current mode, which requires prior knowledge of the eluent strength or 
concentration, constant voltage would correct for variations in eluent 
strength caused by, for example, variations in the eluent flow rate. 
In gradient applications, a constant voltage mode for current efficient 
suppressors would allow for self regulation of the current required for 
suppression. Under a constant current mode this would require setting the 
current to suppress the highest eluent concentration, hence under low 
eluent conditions (usually during the beginning of the run) high currents 
are imposed on the device. Higher currents result in higher heat and gas 
formation and, in turn, higher noise and baseline perturbations. These 
effects also limit the maximum operable concentration under constant 
current mode. Constant voltage mode overcomes the above limitation and the 
current is self adjusted during gradients, hence a higher concentration 
range could be suppressed. 
Any of the foregoing methods and apparatus for increasing current 
efficiency can be used in combination with two or more approaches. Thus, 
for example, the packing principle can be used in combination with the two 
membranes of different net charge. 
In order to illustrate the present invention, the following examples of its 
practice are provided. 
EXAMPLE 1 
In Example 1, a sandwich suppressor device, as illustrated in FIG. 2 
suitable for anion analysis, is constructed for use in the system of FIG. 
1. 
The cation-exchange screens 41 and 43 are formed as follows. The base 
screen is of a polyethylene monofilament type supplied by Tetko, Inc. Such 
screen is immersed in a solution of 30% styrene w/w in methylene chloride 
solvent. Grafting occurs by irradiation with gamma rays at a dose of 
10,000 rads/hour for about 48-120 hours at 80.degree.-90.degree. F. under 
nitrogen atmosphere. The screen is then soaked in 10% w/w chlorosulfonic 
acid in methylene chloride for 4 hours at about 40.degree. C. The screen 
is then immersed in 1M KOH at 55.degree. C. for 30 minutes. 
The substrates for the ion exchange membranes 34 and 36 are film or sheet 
type made of PTFE (Teflon). The substrate polymer is solvent and acid or 
base resistant. Such film is first grafted with styrene monomer and then 
functionalized to form a cation-exchange membrane. Membrane 
functionalization, by sulfonation, is performed in the same manner as 
functionalizing the screens in the previous paragraph. 
The gasket is formed of an inert, chemical resistant material suitable for 
providing a liquid seal for the flow channel it defines. 
The overall hardware includes external support blocks made of a rigid 
nonconductive material (PEEK) serving to house the screens, membranes and 
electrodes. It also provides structural support for the suppressor. The 
top has four fittings (one pair for the eluent inlet and eluent outlet and 
other pair for regenerant inlet and regenerant outlet, respectively). The 
blocks are pressed together by conventional means, such as screws, to 
obtain a liquid-tight seal. 
The sub-assemblies are formed as follows. A screen with surrounding gasket 
material is formed by cutting a gasket from plastic film that includes the 
desired flow path and pressing this gasket into the screen such that only 
the flow path is not covered by the gasket material. For each gasket, two 
rectangles of ultra-low molecular weight polyethylene (Parafilm "M", 
American National Can Company) are cut with the appropriate dimensions of 
the flow channel also cut out. The screen is sandwiched between the 
Parafilm gaskets, and the stack is pressed to 10,000-20,000 psi at ambient 
temperature. One eluent screen/gasket assembly and two regenerant ones 
made with sulfonated screen and Parafilm are required per suppressor. The 
screen mesh (the size of the screen opening) for the central screen 32 are 
140 .mu.m for the outside screens 41 and 43. 
Two rectangles of cation-exchange membrane are cut to match the inlets and 
outlets of the flow path profile and the overall dimension of the screens. 
3 mil thick polytetrafluorethylene (Teflon) base membrane is used. 
An anode and a cathode made of conductive, chemically platinum foil, 0.025 
mm thick (Johnson Matthey Electronics), with measurements of 1.0 by 12.0 
cm were used. 
The system is in the form of a chromatographic column arranged in series 
with the suppressor. The solution leaving the column is directed to the 
suppressor wherein the electrolyte is converted to a weakly conducting 
form. The effluent was then directed to a detector in the form of a 
flow-through conductivity cell for detecting all the resolved ionic 
species. The effluent after passing through the conductivity cell is 
redirected to the inlet port of the outside channels in which the detector 
cell effluent is electrolyzed supplying hydronium ions (H.sup.+) for 
neutralization reaction. 
The suppressor used was a commercially available Dionex 4 mm ASRS which was 
modified by removing the functionalized screens 32 in the sample stream 
flow channel and replacing it with a neutral polyethylene screen. 
A direct current power supply from Pharmacia was used in the constant 
current mode. The suppressor was tested for current efficiency by pumping 
in 25, 50, 100 and 200 mM sodium hydroxide solutions at a flow rate of 1 
ml/min. The conductivity of the effluent from the suppressor was monitored 
using a conductivity cell. The current applied to the suppressor was 
varied over a range of 5-20 mA near the current efficient regime and the 
background level of the effluent was monitored. Good suppression occurred, 
with the device showing the lowest resistance and lowest background 
(average background conductance was approximately 2.35 .mu.Siemens/cm in 
the range of 25-200 mM of sodium hydroxide) near the current 
efficient-regime. When the current applied was plotted against the eluent 
concentration, excellent fit was observed as shown in FIG. 9. A slope of 
0.61 mM/mA was obtained, which is very close to the faradaic value of 0.62 
mM/mA. 
EXAMPLE 2 
Anion Separations Using a Current Efficient Suppressor-constant Current 
Mode 
The suppressor was similar to the one described in Example 1. A direct 
current power supply from Pharmacia was used in the constant current mode. 
The analytical column was a Dionex AS4A-SC (4.times.250 mm) column and the 
eluent used was 1.8 mM sodium carbonate/1.7 mM sodium bicarbonate at a 
flow rate of 2 ml/min. Excellent separation and detection of a test 
mixture comprising 7 anions was achieved at an applied current of 18 mA, 3 
V, as shown in FIG. 10. Peaks labeled 1-7 are Fluoride, Chloride, Nitrite, 
Bromide, Nitrate, Phosphate and Sulfate. A background level of 15-16 
.mu.S/cm indicated complete suppression of the eluent to carbonic acid 
using the neutral screen suppressor. 
EXAMPLE 3 
Anion Separations Using a Current Efficient Suppressor-constant Voltage 
Mode 
A direct current power supply from Hoeffer Scientific was used in the 
constant voltage mode. The suppressor used was a commercially available 
Dionex 2 mm ASRS device that was modified by removing the functionalized 
eluent screen and replacing it with a neutral polyethylene screen as shown 
schematically in FIG. 2. All other conditions were similar to Example 2. 
The analytical column was a Dionex AS11 (4.times.250 mm) column and the 
eluent used was 10 mM sodium hydroxide at a flow rate of 1 ml/min. 
Excellent separation and detection of a test mixture comprising 4 anions 
was achieved at an applied voltage of 7 V as shown in FIG. 11. Peaks 
labeled 1-4 are Fluoride, Chloride, Sulfate and Nitrate. The current 
generated was approximately 16 mA, which is very close to the theoretical 
faradaic current of 16 mA. 
EXAMPLE 4 
Anion Separations Using a Current Efficient Suppressor-constant Voltage 
Mode 
A direct current power supply from Hoeffer Scientific was used in the 
constant voltage mode. All other conditions were similar to Example 2. The 
analytical column was a Dionex AS10 (4.times.250 mm) column and the eluent 
used was 85 mM sodium hydroxide at a flow rate of 1 ml/min. Excellent 
separation and detection of a test mixture comprising 4 anions was 
achieved at an applied voltage of 7 V as shown in FIG. 12. Peaks labeled 
1-4 are Fluoride, Chloride, Sulfate and Nitrate. The current generated was 
approximately 137 mA which is very close to the theoretical current. 
EXAMPLE 5 
Anion Separations Using a 10.5-V Battery 
A DC power source of 10.5 V was made by arranging 7 1.5 V cells in series. 
All other conditions were similar to Example 3. A sample comprising 3 
anions were well resolved as shown in FIG. 13. Peaks labeled 1-3 are 
Fluoride, Chloride and Sulfate. 
EXAMPLE 6 
Gradient Separations Using a Current Efficient Suppressor Powered by a 
Universal AC-DC Adapter 
A universal AC-DC adapter set at 10.5 V DC was used to power the 
suppressor, all other conditions are similar to Example 3. The eluents 
used were E1: 50 mM NaOH and E2: 200 mM NaOH. The gradient used was 100% 
E1 at 0 min to 38% E1, 62% E2 at 31 min. The advantage of constant voltage 
is that the current is adjusted by the influx of the gradient. Hence, the 
current is self-regulating in the eluent channel. In contrast to the above 
approach, constant-current devices supply a huge excess of current 
(particularly at the beginning of the gradient), which may translate at 
noise and heat and be detrimental to the device lifetime. Separation of a 
text mixture comprising 5 anions is shown in FIG. 14. Peaks labeled 1-5 
are Fluoride, Chloride, Sulfate, Phosphate and Nitrate. No trap column was 
used in this and the increase in the background is attributed to 
increasing levels of carbonate in the eluent. 
EXAMPLE 7 
Cation Separations Using a Current Efficient Suppressor 
A direct current power supply from Hoeffer Scientific was used in the 
constant-voltage mode. The suppressor used was a commercially available 
Dionex 4 mm CSRS device that was modified by removing the functionalized 
eluent screen and replacing it with a neutral polyethylene screen as shown 
schematically in FIGS. 1-4. All other conditions were similar to Example 
2. The analytical column was a Dionex CS12A (4.times.250 mm) column and 
the eluent used was 18 mM methanesulfonic acid (MSA) at a flow rate of 1 
ml/min. Good separation and detection of a test mixture comprising 4 
cations was achieved at an applied voltage of 5 V as shown in FIG. 15. The 
current generated was 29 mA, which is indistinguishable from the 
theoretical value of 29 mA. Peaks labeled 1-4 are Lithium, Sodium, 
Ammonium and Potassium. 
EXAMPLE 8 
Recovery Studies on an Oligonucleotide Standard 
A commercially available Dionex 2 mm ASRS device was compared with the 
suppressor of Example 3. The positions marked as A and B are positions 
marked on the ASRS as eluent in and eluent out. Similarly, positions C and 
D are regenerant in and out. Both suppressors were operated under external 
water mode. A direct current power supply from Pharmacia was used in the 
constant-current mode. Recovery of a target oligonucleotide a (GT).sub.10 
20-mer, was attempted using both suppressors. The eluent used was sodium 
salt of triflouro acetic acid (Gradient: 0.13 M-0.35 M) with and without 
added acetonitrile (16%). The results of the recovery studies, shown in 
Table 1, clearly show the advantage of having a neutral screen instead of 
a functionalized screen. 
TABLE 1 
______________________________________ 
Recovery studies ASRS-Cation exchange eluent vs. Neutral eluent 
screen 
Eluent screen-Functionality 
Acetonitrile (% v/v) 
% Recovery 
______________________________________ 
Cation exchange 0 84 
Neutral 0 94 
Cation exchange 16 91 
Neutral 16 103 
______________________________________ 
EXAMPLE 9 
Cations in Acid Analysis 
The device of Example 7 is used as a sample preparation device useful in 
analyzing cations in acid. The conductive acid component is reduced to the 
suppressed low or non-conducting form (from HCl to water) and the sample 
cations converted to their base form (from NaCl to NaOH). The sample ions 
could be diverted to a preconcentrator and then analyzed using an IC 
system and a suppressor of Example 7. The above is a current-efficient 
suppressor for sample preparation application. 
EXAMPLE 10 
Anions in Base Analysis 
The device of Example 2 is used as a sample preparation device useful in 
analyzing anions in base. The conductive base component is reduced to the 
suppressed low or non-conducting form (from NaOH to water) and the sample 
anions are converted to their acid form (from NaCl to HCl). The sample 
ions could be diverted to a preconcentrator and then analyzed using an IC 
system and a suppressor of Example 2. The above is a current-efficient 
suppressor for sample preparation applications. 
EXAMPLE 11 
Low Capacity Functionalized Eluent Screen 
The device was similar to Example 1 except the eluent screen was replaced 
with a functionalized screen which had a very low capacity. The base 
screen is made of polyethylene monofilament type supplied by Tetko, Inc. 
This screen is immersed in a solution of 30% styrene w/w in methylene 
chloride solvent. Grafting occurs by irradiation with gamma rays at a dose 
of 10,000 rads/hour for about 48-120 hours at 80-90.degree. F. under 
nitrogen atmosphere. The screen is functionalized by soaking in 
concentrated sulfuric acid for 1 hour at room temperature. Then the screen 
was washed with dilute acid followed by base and water, and then fitted in 
place of the neutral screen. The capacity of this screen was measured to 
be 0.005 meq/g. The above suppressor was tested using a Dionex AS10 column 
and 100 mM sodium hydroxide eluent under external water mode. DI water was 
pumped at a flow rate of 3 mL/min through the electrode chambers while 
eluent was flowing through the sample stream channel at 1 mL/min. 
Excellent separation of a test mixture comprising of 5 anions was achieved 
at an applied voltage of 5V as shown in FIG. 16. Peaks labeled 1-6 
corresponding to Fluoride, Carbonate, Chloride, Sulfate, Phosphate and 
Nitrate. The typical noise in this chromatogram was less than 3 nS 
cm.sup.-1. The current generated was 169 mA, which is approximately 95% 
current-efficient. There is some wastage of the current in the formation 
of water by transport of hydronium ion across the eluent channel. However, 
most of the current in this device is still carried by the eluent.