Dialysis unit is a module for online sample preparation in ion chromatography permitting the use of automatic sample dialysis directly before sample injection
Quite often with problematic ion chromatography applications, the matrix of the sample makes it difficult to accurately quantify the species of interest with the standard ion chromatography set-up, and some form of sample preparation then becomes necessary.
The sample preparation may be as straightforward as simply diluting the sample with deionised water or can involve injection of the sample through a solid phase extraction cartridge to remove the interference.
In the case of more difficult forms of sample matrices it may be necessary to add additional dedicated sample preparation modules to the standard ion chromatography configuration.
What is ion chromatography?.
Chromatography is a method for separating mixtures of substances using two phases, one of which is stationary and the other mobile moving in a particular direction.
Chromatography techniques are divided up according to the physical states of the two participating phases.
The term ion exchange chromatography or ion chromatography (IC) is a subdivision of high performance liquid chromatography (HPLC).
A general definition of ion chromatography can be applied as follows: "ion chromatography includes all rapid liquid chromatography separations of ions in columns coupled online with detection and quantification in a flow-through detector".
A stoichiometric chemical reaction occurs between ions in a solution and a solid substance carrying functional groups that can fix ions as a result of electrostatic forces.
For anion chromatography these are quaternary ammonium groups and for cation chromatography sulphonic acid groups.
In theory ions with the same charge can be exchanged completely reversibly between the two phases.
The process of ion exchange leads to a condition of equilibrium, the side to which the equilibrium lies depends on the affinity of the participating ions to the functional groups of the stationary phases.
Different types of sample preparation techniques.
Dilution of the sample is performed when the concentration of the analytes of interest either exceed the working capacity of the separation column chosen, or there are sample matrix effects that can often be minimised by a dilution usually with water but eluent can also be used.
Filtration of the sample.
It is recommended to filter all samples prior to injection with 0.45mm filters to ensure that any particulate material from the samples doesn't make its way onto the injection valve or the analytical column where it could cause blockages and considerably reduce the lifetime of the column(s).
Solid phase extraction cartridges.
Passage of the sample through one or more solid phase extraction cartridges prior to injection will often retain selectively certain species within the homogeneous sample.
Quite often the retained species are substances that would interfere with the chromatography had they not been previously removed.
There are a number of different cartridges whose suitability depends upon the type of chemistry undertaken.
For anion analysis, the sample can be treated with a cation exchanger in the H+ form that removes divalent cations that can mask any fast eluting anions.
This type of exchange cartridge removes carbonate/bicarbonate and is also useful for the removal of cations from samples being determined by ion exclusion chromatography.
Another option is the use of a cation exchanger in the Ag+ form for the removal of any halides present in the sample.
Similarly for cation analysis, one can employ an anion exchanger in the OH- form to remove any interfering anions present in the sample.
Another common type is the non-polar exchange cartridge (reversed phase) that often utilises C18 groups to remove organic substances that would otherwise interfere with the chromatography.
Digestion of the sample.
If digestion techniques are to be employed then analyte content should be changed as little as possible and any organic matter present should ideally be destroyed completely.
One can obtain analytical inaccuracies due to an exaggerated blank value as a result of the chemicals used during the digestion.
Different types of digestion include wet, microwave and UV, the suitability of each depends on both the sample matrix and the analytes of interest being determined.
Instrumental sample preparation modules.
Often with more complex sample matrices, one has to add additional dedicated sample preparation modules to the standard ion chromatography configuration.
There are a number of different instrument options available within the Metrohm range depending on the type of sample treatment required prior to analysis.
Metrohm has actually been a pioneer of inline sample preparation modules with the release of the 754 IC dialysis unit in 1997, since then the technology has been optimised and considerably improved so that today the Metrohm IC portfolio contains many different sample preparation instruments.
The Metrohm 833 dialysis unit is a module for online sample preparation in ion chromatography permitting the use of automatic sample dialysis directly before sample injection.
It consists of a dual channel peristaltic pump for conveying the sample and acceptor solutions and the actual dialysis cell in which the ions from the flowing sample solution are enriched in the resting acceptor solution.
The 833 dialysis unit is strongly recommended by Metrohm for demanding applications that contain strongly loaded samples.
Dialysis is the diffusion of ions from a sample solution into an acceptor solution to achieve a concentration equilibrium.
The sample solution is constantly being renewed, resulting in no depletion of the sample.
The ions pass through the membrane without hindrance but larger sample particles - the sample matrix - are transported past the membrane to waste reducing matrix effects to an absolute minimum.
Calibration of the standards is easily done, requiring no extra outlay.
The reported dialysis recovery rates have been found to be in excess of 98% using the patented stopped flow method.
838 advanced IC ultra-filtration sample processor (2.838.0210).
The aim of sample filtration is to protect the separation columns from contamination and blockage from particulates that may be present in the sample.
The 838 IC ultra-filtration sample processor combines inline filtration with automatic sample injection through the use of an ultra-filtration cell.
It is eminently suitable for those samples with a light to medium load such as surface waters and digestion solutions.
The samples are placed onto the sample carousel before being processed automatically.
Sample filtration and introduction to the injection valve is achieved by means of an integrated double channel peristaltic pump meaning that it is possible to aspirate slightly viscous samples.
The sample is conveyed by one channel of the pump through the ultra-filtration cell passing the membrane.
At the same time the filtrate is aspirated off from the rear of this membrane and transferred to the sample loop by the second channel of the pump.
Only a small fraction of the sample is removed as filtrate so the contaminants remain mainly in the sample stream, preventing the regenerated cellulose membrane from becoming blocked too quickly.
833 advanced IC liquid handling sample preparation unit (2.833.0030).
Inline sample preparation is rapidly becoming the method of choice for eliminating difficult sample matrices in ion chromatography, says Metrohm, which has developed the 833 advanced IC liquid handling sample preparation unit based upon its Metrohm Suppressor Module (MSM) for difficult anion analyses such as those found in concentrated alkaline solutions.
The modules consists of a reactor block that houses the cation exchangers with a control unit that contains a two channel peristaltic pump that conveys the regenerant and rinse solutions.
The matrix elimination occurs inline while the regeneration and rinsing of the packed bed suppressor occur simultaneously offline.
A fresh suppressor channel is used for each new analysis and, because the rinse and regeneration occurs after each determination, the capacity of the 833 is unlimited.
The sample solution is transferred to the 833 module from an autosampler via a loop injection and rinsed with deionised water.
The sample cations are exchanged against protons (H+).
If sodium hydroxide constitutes the sample matrix, water is formed by neutralisation.
The sample solution then passes onto the preconcentration column where the trace anions to be determined are retained and then eluted by the eluent flowing in a counter flow direction.
The analyte anions are then separated on the analytical column before quantification using chemical suppression with conductivity detection.
Metrohm inline sample preparation (Misp).
With Metrohm it is possible to perform the time consuming sample preparation inline using the 838 advanced IC sample processor equipped with Misp technology.
The 838 comes in a number of different variants that mean it is possible to dialyse, ultra-filtrate, or even dilute the sample automatically inline.
Located on the side of the tower is the relevant sample preparation technology for example an ultra-filtration cell or an injection valve which is utilised for dilution along with Dosino liquid handling technology.
Dialysis - an overview.
Dialysis is a successful method for the separation of low molecular substances from high molecular ones by means of a semi-permeable membrane and is used on patients with kidney deficiencies.
Low molecular substances in the blood refer to ions that disturb the electrolyte balance.
As the kidneys can not function correctly, the concentrations of these ions increases - impairing the metabolic functions.
The concentrations of these ions must be reduced at frequent intervals, and this is achieved by continuous flow dialysis.
An acceptor solution of a low ionic strength (usually deionised water) is pumped along the semi-permeable membrane with the blood flowing past on the other side.
As the ions pass through the membrane, virtually unhindered, they diffuse from the high molecular strength blood into the low ionic strength acceptor solution.
The acceptor solution is permanently renewed in its continuous flow ensuring a steep concentration gradient with relatively large efficiency.
Deliberate care is taken to ensure that no concentration equilibrium can be established between the two solutions.
Metrohm developed the patented stopped flow method for dialysis where the sample solution is continuously pumped past a semi-permeable membrane but the acceptor solution lies at rest, and here lies the inherent difference from continuous flow dialysis.
This ensures that equilibrium is attained between the sample solution and the acceptor solution usually in less than ten minutes.
Once the equilibrium has been set up, the dialysed acceptor solution is transferred to the sample loop and injected onto the separation column.
Applications of dialysis.
The benefit of dialysis is that there are no complicated, time consuming sample preparation steps such as digestion that can potentially destroy the analytes of interest.
This is particularly applicable to foodstuffs and other complex matrices that carry high organic loads such as waste waters or soil eluates.
In the food industry the ionic contents of milk and other diary products can easily be determined using an ion chromatography system incorporating a dialysis module.
It is no longer necessary to separate the proteins from the milk using Carrez precipitation ensuring that the sample preparation is reduced to a simple dilution step.
Other difficult matrices include the analysis of fruit juices that contain fruit pulp, cutting oil emulsions and inks; with dialysis these types of samples no longer represent a problem for the ion chromatography user.
Method for analysis of milk.
Samples The modular system used for the determination of anions present in samples of milk comprised the Metrohm modules 818 advanced IC pump, 819 advanced IC detector, 820 advanced IC separation centre, 833 IC suppressor module, 830 IC interface and 838 advanced IC dialysis sample processor.
The milk sample was diluted 1:5 with deionised water and placed in the sample vials upon the rack of the 838 advanced IC sample processor.
The subsequent dialysis of the sample and injection of the dialysed sample onto the separation column was fully automated and the response for the peaks recorded using a mobile phase eluent of sodium carbonate/sodium bicarbonate.
The calculation was carried out automatically using integration software IC Net 2.3 against a previously prepared calibration plot.
Conclusion of sample preparation techniques for ion chromatography.
Ion chromatography as an analytical technique has seen an enormous surge in popularity due partly to the simplicity of the method as well as other factors such as market forces driving down the expenditure costs of the initial instrumentation and improved reliability and power.
For a sample in a homogeneous, ionic form then very little sample preparation is required and quantified results can be obtained within a matter of minutes.
As in any industry, the consumer places ever more stringent demands and requirements upon the manufacturer and the world of ion chromatography is no different.
Because of the ease of use at which ion chromatography as a method can be manipulated, the end user today wishes to analyse ions within increasingly complicated sample matrices which until recently would not have been possible.
Metrohm has developed a range of instrumental sample preparation modules that can be added to standard ion chromatography configurations to allow quantification even with the most difficult sample matrices.
The analysis is fully automated so once the sample is loaded, the analyst is then free to carry out other functions within the laboratory affording an increase in efficiency of the employed manpower.
Even for those samples requiring sample preparation by dialysis or ultra-filtration, still only a relatively small volume of sample is required.
This coupled with the low running costs of ion chromatography using Metrohm instruments really does mean that ion chromatography is the method of choice for the analyst even with the most difficult and problematic sample matrices, saus the company.
References.
Fundamentals of Analytical Chemistry, (1992) 6th edition, DA Skoog, DM West and FJ Holler, Saunders College Publishing, ISBN 0-03-075397-X.
Principles and Practice of Analytical Chemistry, (1992) 3rd edition, FW Fifield and D Kealey, Blackie Academic and Professional, ISBN 0-216-92920-2.
The Essential Guide to Analytical Chemistry, (1999) 2nd edition, G Schwedt, John Wiley and Sons, ISBN 0-471-97412-9.
