Select Page
Best practice for separation columns in ion chromatography (IC) – Part 2

Best practice for separation columns in ion chromatography (IC) – Part 2

The second part of this blog series about best practice for IC separation columns focuses on application related topics that have an impact on the column suitability and stability. First, there is the proper choice of the column that best suits the intended application. Then we turn to the operating parameters which can be modified in order to optimize the separation between analytes, and what the respective effects and possibilities are.

Choice of column length and diameter

Metrohm offers a broad range of columns that contain different stationary phases, have different lengths and/or inner diameters. The choice of the stationary phase has a great impact on the selectivity between the individual analytes on the one hand, as well as the stability against different sample matrices on the other hand. Instead, the column length has no impact on the selectivity, but rather on the separation efficiency between the individual peaks.

Find out more about Metrohm’s wide selection of separation columns for ion chromatography in our Column Catalog.
Effects of column length

In the following chromatograms (Figure 1), the effect of the column length on the separation efficiency for the Metrosep A Supp 17 column series is shown. Whenever choosing a column length, you should take the complexity of the intended separation and the presence of matrix components that could disturb the ions of interest into account.

Figure 1. Effect of column length on the retention times of the standard anions on the Metrosep A Supp 17 column (1: fluoride, 2: chloride, 3: nitrite, 4: bromide, 5: nitrate, 6: sulfate, 7: phosphate). Click image to enlarge.
Effects of column diameter

In addition to providing different lengths of IC separation columns, Metrohm also offers most columns in both in 4 mm inner diameter and 2 mm inner diameter (known as «microbore») versions. With regard to this, there are several criteria to distinguish:

  • If you use online systems in a continuous mode (i.e. systems which run unattended for several days in a row such as the Metrohm Process Analytics MARGA system – Monitor for AeRosols and Gases in Ambient air), we recommend using 2 mm IC columns. Due to the reduced flowrate for microbore columns (only 25% of the flowrate for 4 mm columns), the eluent and the regenerant solutions last much longer, which increases the time the instrument can be left unattended.
  • There are applications that require hyphenated techniques such as IC-MS for higher analyte selectivity and sensitivity. In this case, the use of 2 mm columns is ideal. The low flowrate is optimal for the electrospray process, and thus no flow splitter is required before entering the mass spectrometer.
  • Sometimes, only a limited amount of sample is available for injection. In these situations, 2 mm columns are preferred. This is because less dilution/diffusion occurs during the separation process and therefore higher signals are obtained.
  • On the other hand, if your sample contains a high load of matrix components, then selecting a suitable 4 mm IC columns will be a better choice because of the higher capacity available to separate the desired analytes from the matrix.
Find out more about MARGA and its capabilities for continuous air quality monitoring in our blog post.

Optimizing the analyte separation

Next to the column itself, several other parameters can be modified to optimize the selectivity of the separation. These parameters include temperature, eluent components and strength, and organic modifiers.

Effects of modifying the temperature

One of the simplest ways to fine tune the separation selectivity in IC is by modifying the temperature of the analysis. This is accomplished by using the integrated column oven in the instrument (if available). Multiple effects can be observed, for instance in anion analysis. As an example, the impact of the temperature on the selectivity is shown in the chromatogram overlay (Figure 2) for the Metrosep A Supp 17 column line.

Figure 2. Effect of temperature variation on the retention times of a suite of standard anions on the Metrosep A Supp 17 column (1: fluoride, 2: chloride, 3: nitrite, 4: bromide, 5: nitrate, 6: sulfate, 7: phosphate). Click image to enlarge.
  • The monovalent ions such as fluoride, chloride, nitrite, bromide, and nitrate are all accelerated with increasing temperature, indicating that fewer interactions with the stationary phase happen.
  • The behavior of multivalent ions such as phosphate or sulfate is more complicated to describe and will vary with each stationary phase. In general, multivalent ions are retarded more at higher temperatures, which causes the retention times to increase, as can be seen for sulfate. Phosphate on the other hand behaves differently, because of the temperature induced change of the eluent pH in a range close to the pKa value of phosphate. Due to this pH change, the effective charge of the phosphate ion changes as well (in this example, the effective charge is reduced with increasing temperature).
  • The peak shape of the polarizable ions such as nitrite, bromide, and in particular nitrate, is significantly improved at higher temperatures. The reason for this behavior is the reduction of secondary interactions with the stationary phase.
Effects of modifying the eluent composition and strength

Eluent composition and strength can be used to change the elution order of several analytes while using the same separation column. In cation chromatography, a retention model was developed by P.R. Haddad and P.E. Jackson, which allows researchers to predict retention times when changing the eluent composition [1].

Considering that the column remains identical in each determination, no change of ion exchange equilibrium and column capacity is to be expected. Therefore, when changing only the eluent concentration, the following correlation can be used:
Where:

  • k’ is the retention factor of the analyte of interest
  • c is a constant
  • x is the charge of the analyte
  • y is the charge of the eluent
  • Ey+M is the concentration of the eluent in the mobile phase
If nitric acid is used as the eluent, y = 1, and the model can be simplified to:
Applying this formula to practical situations in the laboratory means the following: with increasing the eluent strength, alkaline earth metals are accelerated much faster (x = 2) in comparison with alkali metals (x = 1), and thus it is possible to elute magnesium before potassium. This effect is called electroselectivity.

Multivalent metal ions are capable of forming complexes with dedicated complexing agents. Therefore, selectivities can be modified by adding complexing agents to the eluent. As an example, dipicolinic acid (DPA) is often used to complex calcium, which leads to a reduction of the effective charge of calcium. As a consequence, the retention time of calcium is reduced and calcium elutes before magnesium in the chromatogram (Figure 3).

The retention of monovalent cations can be influenced by the addition of crown ether to the mobile phase.

Figure 3. Effect of DPA concentration in the eluent on the retention times of several cations measured using the Metrosep C 6 column.
Anion systems are more complex regarding the retention time model, although the same electroselectivity effect can be observed to some extent for anions. However, when changing the eluent strength, the eluent pH also frequently changes, leading to different deprotonation equilibria of multivalent anions (e.g. phosphate). This influences the effective charge of the analyte, and by doing so, also influences its retention in a similar way as previously described for the effects of changing temperature.

In some cases, the use of a small amount of an organic modifier such as methanol, acetonitrile, or acetone in the eluent can make sense:

  • If bacterial contamination has been an issue before, the addition of 5% methanol to the eluent can help prevent future bacterial growth.
  • When samples containing a lot of organic solvent(s) need to be injected and no sample pretreatment such as extraction or matrix elimination (MiPCT-ME) is possible, it is recommended to add a suitable organic modifier to the eluent to ensure that the organic solvent(s) can be properly flushed out of the chromatographic column.
  • When using IC-MS, it is also recommended to add an organic modifier to the eluent to improve the electrospray process.

Be aware that the addition of organic modifiers will also affect the separation selectivities. For the standard anions, the effect is similar to that observed with increased temperatures: the peak shapes of the polarizable ions such as nitrite, bromide, and nitrate are improved.

Organic acids on the other hand may react very differently compared to the standard anions, and their reaction also strongly depends on the type of organic modifier used. Sample chromatograms that show the effect of the organic modifier on retention of analytes are shown in the manual for the Metrosep A Supp 10 column.

Download the Metrosep A Supp 10 column manual here to see example chromatograms showing the effects of organic modifier on analyte retention time.
For more information about column care, check out our blog post for different tips and tricks.

The History of Metrohm IC

Metrohm ion chromatography: bringing top quality and exceptional analytical performance to the lab since 1987. 
Reference

[1Haddad, P. R.; Jackson, P. E. Ion Chromatography: Principles and Applications; Journal of chromatography library; Elsevier; Distributors for the U.S. and Canada, Elsevier Science Pub. Co: Amsterdam, Netherlands; New York: New York, NY, USA, 1990.

Post written by Dr. Vincent Diederich (Jr. Product Manager IC Columns) and Dr. Anne Katharina Riess (Head of Column Division) at Metrohm International Headquarters, Herisau, Switzerland.
Spectroelectrochemistry: shedding light on the unknown

Spectroelectrochemistry: shedding light on the unknown

The combination of two well-known analytical techniques, electrochemistry and spectroscopy, gives rise to spectroelectrochemistry (SEC), an established scientific methodology. This hybrid technology combines the advantages of each technique, offering the best of both worlds [1]. The word «spectroelectrochemistry» is the result of combining these two terms as two pieces of a puzzle that fit perfectly together.

In this article, written for both beginners in the field as well as more experienced readers, we focus on introducing this technique from its beginnings to its advantages in research, and then discuss new systems and solutions that will make it easier to work on the multitude of applications that spectroelectrochemistry can offer. 

Shedding light, in the literal sense of the phrase, on electrochemical knowledge and procedures. Spectroelectrochemistry offers analysts more information by being able to record both an optical and an electrochemical signal at the same time to obtain new data.

This is a multi-response method—it studies the process of electrochemical reactions with simultaneous optical monitoring. Spectroelectrochemistry provides two individual signals from a single experiment, which is a very powerful feature to obtain critical information about the studied system. Moreover, the autovalidated character of spectroelectrochemistry confirms the results obtained by two different routes. Find out more about this topic by downloading our free Application Note below.

Spectroelectrochemistry allows researchers to collect molecular, kinetic, and thermodynamic information from the reactants, intermediates, and/or products involved in electron transfer processes. Thus, it is possible to perform spectroelectrochemical studies on a broad range of molecules and different processes including: biological complexes, polymerization reactions, nanomaterial characterization, analyte detection, corrosion mechanisms, electrocatalysis, environmental processes, characterization of memory devices, and much more!

Ultimately, different kinds of information is obtained depending on the spectral range used. UV-VIS spectroscopy provides molecular information related to the electronic levels of the molecules, the NIR region provides data associated with the vibrational levels, and the Raman spectrum provides very specific information about the structure and composition of the sample due to the fingerprinting characteristics of this technique.

Electromagnetic Spectrum
Diagram of the electromagnetic spectrum.

The beginnings of spectroelectrochemistry

Theodore Kuwana, Ph.D. in chemistry and specialist in spectroelectrochemistry, bioelectroanalytical chemistry and modified electrodes.

This analytical technique was developed in the 1960’s, and became popularized by Professor Theodore Kuwana and other researchers [2]. They had begun to work with transparent electrodes to study a simultaneous process—measuring the charge and absorbance (at the same time) when a beam of light passes through the electrode. As a result, they developed transparent electrodes and this marked the beginning of concurrent electrochemical and UV-VIS absorption measurements.

These so-called «optically transparent electrodes» (OTEs) were developed to carry out the combination of optical and electrochemical experiments. Some of the most commonly used OTEs began as oxide doped with antimony on glass, then developed into different thin films of gold or platinum on quartz, followed by germanium electrodes for IR wavelengths, as well as pure gold and platinum micromeshes (where the holes provide the required transparency to light). However, not all spectroelectrochemical configurations require transparent electrodes. For more information, download the electrode reference flyers from Metrohm DropSens to properly work with the different spectroelectrochemical techniques.

A selection of Metrohm DropSens references. Left: C013 for in-situ surface Raman spectroscopy. Right: P10 optically transparent and specially designed for spectroelectrochemical applications.

Download the free flyers for these electrodes below for further information.

The first published paper on spectroelectrochemistry [2], in which Dr. Kuwana participated, describes the use of tin oxide-coated glass surfaces (optically transparent electrodes) for following the absorbance changes of different electroactive species during electrolysis. Since then, the number of works and investigations based on this technique have grown steadily.

Spectroelectrochemistry publications have increased significantly since its discovery in the 1960’s (results of searching «Spectroelectrochem*» as a term in Scopus as of June 2021).

Take a look for yourself to see how things have changed in the last several decades!

An array of spectroelectrochemical techniques to choose from

The following graphic is classified according to the combination of different electrochemical and spectroscopic methods. The general classification is based on the spectroscopic technique: ultraviolet (UV), visible (Vis), photoluminescence (PL), infrared (IR), Raman, X-ray, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR).

Spectroelectrochemistry (SEC) is the combination of spectroscopy and electrochemistry.

In recent years, significant advances have occurred regarding the design, development, and possibilities offered by instruments for working with spectroelectrochemical techniques. Also, the assemblies and the connections between products and accessories that facilitate the use of this equipment have improved in recent decades, contributing to make research and experiments in this field easier and more affordable.

The evolution of spectroelectrochemical instrumentation

Traditionally, the configuration for spectroelectrochemical analysis consists of two instruments: one spectroscopic instrument and the other for electrochemical analysis. Both instruments are connected independently to the same spectroelectrochemical cell and are generally not usually synchronized. In addition, each instrument is controlled by a different (and specific) software in each case, so two programs are also needed to interpret each signal and yet another external software for the processing and analysis of the data obtained by the first two programs. Finally, it must be considered that synchronization is not guaranteed, making the performance of experiments and tests with this configuration slow, complex, and costly.

This detached spectroelectrochemical setup displays the complexity of various software and programs used, showing that different systems are not able to obtain actual synchronized electrochemical measurements and data (click to enlarge).

Metrohm DropSens took this opportunity to create something that did not exist before—a revolution in the state-of-the-art of spectroelectrochemistry: the SPELEC line of instruments which are fully integrated, synchronized solutions that offer much more versatility to researchers. The devices include all of the components needed to work with spectroelectrochemical techniques in a simple way and in a single system with a (bi)potentiostat/galvanostat, the light source, and the spectrometer (depending on the selected spectral range).

Find out more about SPELEC, the next-generation tool for spectroelectrochemical research on our website.

The SPELEC systems from Metrohm DropSens consist of one device and one software—a fully integrated, easy to use, practical setup for researchers.

These designs and configurations simplify the work, processes, and spectroelectrochemical measurements as well because only a single system and a single software are needed. In the case of the SPELEC solution, its advanced dedicated software (DropView SPELEC) is a specific program that controls the instrument, obtains the electrochemical and spectroscopic signals simultaneously, and also allows users to process and analyze the data together in a single step. It’s really that simple!

The future of spectroelectrochemistry: SPELEC systems and software

One instrument and one software: Metrohm DropSens SPELEC has everything you need for your spectroelectrochemical experiments while saving laboratory space and valuable time. SPELEC instruments offer the combinations of electrochemistry and UV-Vis, Vis-NIR, or even Raman spectroscopy in a single measurement with several different instrument options available (see below). Everything is integrated which allows more tests in less time, multiple spectra, a full range of accessories, and research flexibility with the different configurations available.

Several options are available depending on the spectral range needed:

SPELEC: 200–900 nm (UV-VIS)
SPELEC NIR: 900–2200 nm
SPELEC RAMAN: 785 nm laser
(other wavelengths available upon request)
SPELEC 1050: 350–1050 nm (VIS-NIR)

DropView SPELEC is a dedicated and intuitive software that facilitates measurement, data handling, and processing. With this program, you can display electrochemical curves and spectra in real time and follow your experiments in counts, counts minus dark, absorbance, transmittance, reflectance, or Raman shift. As far as data processing is concerned, DropView SPELEC offers a wide variety of functions including graph overlay, peak integration and measurement, 3D plotting, spectral movie, and more.

Testimonial from the University of Burgos on the integrated SPELEC system from Metrohm DropSens.

SPELEC instruments are very versatile, and although they are dedicated spectroelectrochemical instruments, they can also be used for electrochemical and spectroscopic experiments. They can be used with any type of electrodes (e.g., screen-printed electrodes, conventional electrodes, etc.) and with different spectroelectrochemical cells. Optical and electrochemical information is obtained in real time/operando/dynamic configuration. The main advantages of spectroelectrochemical techniques can be summarized as follows:

  • they simultaneously provide information obtained by two different techniques (electrochemistry and spectroscopy) in a single experiment
  • qualitative studies and quantitative analyses can be performed
  • high selectivity and sensitivity
  • spectroelectrochemistry is used in a wide variety of different fields due to its great versatility
  • new configurations facilitate the performance of spectroelectrochemical experiments, saving time, samples, costs, etc.

Learn more about the next-generation tool for spectroelectrochemical research on our website!

SEC analysis techniques: suitable for multiple applications

The characteristics of spectroelectrochemistry allow the constant development of new and broad applications in several different fields. Read on below to discover the capabilities of this technique.

Materials science: characterization of specific properties of carbon materials, quantum dots, composites, nanoparticles, Janus materials, polymers, as well as stability studies, determination of photochemical properties, development of new materials, etc.

For more information, download our free related Application Note below.

Sensing: selective and sensitive detection, rapid quantification of a huge variety of analytes, diagnostic tool, development of new methodologies and sensors, etc. [3].

Organic and inorganic chemistry: study of the properties and structure of different compounds, analysis of kinetic reactions, determination of electron transfer capacity, etc. [4].

Corrosion: evaluation of protective films as corrosion inhibitors, determination of electrode stability and reversibility, monitoring of layer and sublattice generation, improvement of protective properties of coating materials, etc.

Energy storage: monitoring of exchange and discharge cycles, determination of oxidation/reduction levels, characterization of new electrolytes for batteries, understanding of doping and splitting processes in solar cells, etc.

Electrocatalysis: characterization and comparison of the electrocatalytic activity of different catalysts, identification of intermediate species and their structural changes, elucidation of the reaction mechanism, etc. [5].

Life sciences: study of biological processes, characterization of molecules used in biotechnology, biochemistry or medicine, determination of antioxidant activity, etc.

Environment: identification and quantification of pesticides, dyes, and pollutants, monitoring of degradation and filtration processes, etc. [6]

Download our free related Application Note for more information.

Others: characterization of new materials for memory devices, comparison of minerals, identification of pigments, oils, and pastes, etc.

Learn even more about spectroelectrochemistry (SEC) and what it can do for your research by downloading our free brochure.

Contact us

to discuss how spectroelectrochemistry can boost your research.

References

[1] Kaim, W.; Fiedler, J. Spectroelectrochemistry: The Best of Two Worlds. Chem. Soc. Rev. 2009, 38 (12), 3373. doi:10.1039/b504286k

[2] Kuwana, T.; Darlington, R. K.; Leedy, D. W. Electrochemical Studies Using Conducting Glass Indicator Electrodes. Anal. Chem. 1964, 36 (10), 2023–2025. doi:10.1021/ac60216a003

[3] Martín-Yerga, D.; Pérez-Junquera, A.; González-García, M. B.; Perales-Rondon, J. V.; Heras, A.; Colina, A.; Hernández-Santos, D.; Fanjul-Bolado, P. Quantitative Raman Spectroelectrochemistry Using Silver Screen-Printed Electrodes. Electrochimica Acta 2018, 264, 183–190. doi:10.1016/j.electacta.2018.01.060

[4] Perez-Estebanez, M.; Cheuquepan, W.; Cuevas-Vicario, J. V.; Hernandez, S.; Heras, A.; Colina, A. Double Fingerprint Characterization of Uracil and 5-Fluorouracil. Electrochimica Acta 2021, 388, 138615. doi:10.1016/j.electacta.2021.138615

[5] Rivera-Gavidia, L. M.; Luis-Sunga, M.; Bousa, M.; Vales, V.; Kalbac, M.; Arévalo, M. C.; Pastor, E.; García, G. S- and N-Doped Graphene-Based Catalysts for the Oxygen Evolution Reaction. Electrochimica Acta 2020, 340, 135975. doi:10.1016/j.electacta.2020.135975

[6] Ibáñez, D.; González-García, M. B.; Hernández-Santos, D.; Fanjul-Bolado, P. Detection of Dithiocarbamate, Chloronicotinyl and Organophosphate Pesticides by Electrochemical Activation of SERS Features of Screen-Printed Electrodes. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2021, 248, 119174. doi:10.1016/j.saa.2020.119174

Post written by Dr. David Ibáñez Martínez (Product Specialist Spectroelectrochemistry) and Belén Castedo González (Marketing and Communication) at Metrohm DropSens, Oviedo, Spain.

Best practice for separation columns in ion chromatography (IC) – Part 2

Best practice for separation columns in ion chromatography (IC) – Part 1

The high performance ion chromatography (IC) separation column is often referred to as the «heart» of the ion chromatograph. The reason for this denomination is straightforward: the column is responsible for the separation of the analytes of interest from each other as well as from interfering sample matrix ions. The unique separation capabilities of IC columns allow the determination of multiple analytes within one run. In this blog series, we will share what is required to ensure the proper operation of an IC column and how to maximize the column lifetime.

Standard operating conditions

To begin with, the standard operating conditions should be considered, such as the eluent (mobile phase) composition, the eluent flow rate, the column oven temperature, and the detection method. These standard conditions are specific for each individual column type and correspond to the conditions that work best with the application the column is intended for. Every analytical column sold by Metrohm is delivered with a certificate of analysis (CoA) which is recorded under standard operating conditions. In the following table, you can find the standard operating conditions for three different Metrohm IC columns.

Column Metrosep C 6 – 150/4.0 Metrosep A Supp 17 – 150/4.0 Metrosep Carb 2 – 150/4.0
Eluent composition 1.7 mmol/L nitric acid

1.7 mmol/L dipicolinic acid

5.0 mmol/L sodium carbonate

0.2 mmol/L sodium bicarbonate

100 mmol/L sodium hydroxide

10 mmol/L sodium acetate

Flow rate 0.9 mL/min 0.6 mL/min 0.5 mL/min
Oven Temperature 30 °C 25 °C 30 °C
Detection Non-suppressed conductivity Suppressed conductivity Amperometric detection
Analytes Lithium, sodium, ammonium, potassium, calcium, magnesium Fluoride, chloride, nitrite, bromide, nitrate, sulfate, phosphate Inositol, arabitol, sorbitol, glucose, xylose, fructose, lactose, sucrose

Equilibration

Next to the standard operating conditions, the start-up parameters play an important role for the lifetime of the separation column. High mechanical and thermal stress is a frequent reason for decreased column lifetime. It is therefore recommended to slowly increase the eluent flow rate to the column and to avoid thermal and pressure shocks. For more information regarding specifics, please refer to the recommended equilibration conditions in the corresponding column leaflet.

Operating limits

Based on the different stationary phase designs that are required to achieve different selectivities and guarantee a wide range of IC applications, the chemical and physical properties of the ion exchangers can vary. Therefore, different operating limits are recommended for various column types and set according to stress tests performed during the development of a product. Before beginning to optimize an application, please refer to the respective column leaflet for the corresponding operating limits to achieve optimal results and guarantee a long column lifetime.

The chemistry of the ion exchanger defines the limits of operating temperature, eluent pH, and organic modifiers that can be present in the eluent. These values are valid for every dimension of a column type (e.g., the Metrosep C 6). Exceeding these limits can strongly affect column performance and, in the worst case, lead to irreversible damage. Tolerated flow rates and maximum pressure correlate with physical properties and of course column dimension—therefore these limits are set for every column dimension (e.g., Metrosep C 6 – 100/4.0 vs. Metrosep C 6 – 250/2.0).

Are you searching for a specific IC column for your research? Then check out the Metrohm Column Finder!

Information leaflet

In addition to the CoA, a lot of necessary valuable information can be found in the respective column leaflet. Instructions about equilibration, regeneration, operating limits, and much more can be found in the column leaflet which is provided for every IC column type offered by Metrohm.

You can download an example of a column leaflet here, where the information is provided in several different languages (DE, EN, FR, and ES).

Storage

Depending on the operation conditions and the properties of the ion exchanger material, different storage conditions are recommended for different column types. These conditions (storage temperature and storage eluent) are described on the respective column leaflet and should be strictly followed.

Please note that storing a new column for long time periods as well as frequently switching between storage and operation can actually be more stressful to the column than operating it 24/7! It is therefore not recommended to stock a column for a long time without using it.

Column guards

To protect your separation column from sample contamination and to extend its lifetime, it is critical to use a guard column. The guard column should be exchanged at regular intervals—a general rule of thumb is that about four guard columns will be used over the lifetime of an IC column.

Guard columns are offered in two designs: «on column» (e.g. Metrosep A Supp 17 Guard/4.0, on the left) and as a separate guard that is connected to the IC column with a capillary (e.g. Metrosep A Supp 17 S-Guard/4.0, on the right). For microbore separation columns, the corresponding microbore column guards are recommended (e.g. Metrosep A Supp 10 Guard/2.0).

By default, a guard column with the same material as the column should be used. However, for special applications, combining different ion exchange materials by using a different guard column can help to optimize the separation. One such example is in the case of sulfate and sulfite, as shown below.

Pulsation absorber

With the exception of the Eco IC product line, all Metrohm ion chromatography instruments are equipped with a pulsation absorber. For the Eco IC, it is strongly recommended to add the pulsation absorber as an option. As mentioned earlier, IC columns do not like repeated mechanical stress, especially those based on polyvinyl alcohol or polymethacrylate stationary phases. Therefore, the pulsation absorber is a useful tool to protect the column from possible pressure fluctuations in the system and enhance the column lifetime.

Find out more about the full line of Metrohm ion chromatography products and accessories on our website!

Chemical quality

In ion chromatography, the ratio between the volume of sample and the volume of eluent that flows through the column is very small, usually in the range of 1:1000. Therefore the quality of the chemical reagents used in the eluent preparation plays a crucial role for the column lifetime. To guarantee optimal performance, use chemicals denominated as «for IC» whenever possible as they are tested particularly on impurities (e.g. metals) that could harm the chromatographic column.

For dilution of the eluent components, ultrapure water is typically used in ion chromatography. To ensure good chromatographic results, the ultrapure water should have a specific resistance greater than 18 MΩ · cm and be free from particles. The ultrapure water is filtered through a 0.45 µm filter and treated with UV. Modern ultrapure water sources for laboratory use guarantee this level of water quality (Type I).

Sample preparation

With the Metrohm Inline Sample Preparation (MISP) options, Metrohm provides a large number of sample preparation techniques that are beneficial to the separation column, as well as analysts. Instead of loading the full sample onto the column, these techniques ensure the reduction of several sample matrix effects, thus avoiding potential harm to the column.

One of the most prominent MISP techniques is Inline Ultrafiltration (illustrated here), which efficiently removes particles from the sample in a fully automated way, before they ever reach the column. In that way, column blockage from dirty samples can be avoided without any manual effort.

iColumn features

All Metrosep columns offered by Metrohm are equipped with an intelligent chip containing useful information about various column operation conditions (e.g., equilibration, standard operation conditions, operation limits, and more) and tracks certain parameters (e.g., set to work, number of injections, number of working hours, and maximum working values such as pressure and flow rate) over the column lifetime. It is beneficial to attach the column chip to the chip reader as illustrated here for proper monitoring as well as support.

Column shelf life

How many injections are possible on a specific column? Unfortunately, it is not possible to deliver an exact answer to this question. This is because the column lifetime strongly depends on the sample matrix and elution conditions. Due to the multitude of different applications and samples that can be covered with a single IC column, it is not possible to predict the column life for every application and sample type.

During column development, several endurance tests are performed under standard conditions with a guard column using appropriate standards. Under these conditions, the column must withstand at least 2000 injections.

The History of Metrohm IC

Metrohm ion chromatography: bringing top quality and exceptional analytical performance to the lab since 1987. 
Post written by Dr. Vincent Diederich (Jr. Product Manager IC Columns) and Dr. Anne Katharina Riess (Head of Column Division) at Metrohm International Headquarters, Herisau, Switzerland.
Does counter electrode (CE) size matter?

Does counter electrode (CE) size matter?

To begin, let’s go back to the year 1950 when metallurgists and chemists tried to shine a light on a fascinating electrochemical phenomenon originally discovered in the 17th century by the chemist Sir Humphry Davy [1].

Sir Humphry Davy (1778–1829) was credited with many discoveries in the field of electrochemistry.

If you dip a wire made of iron (or as electrochemists say: an iron electrode) into diluted sulfuric acid (which is considered the electrolyte), it instantly starts to dissolve—it corrodes. If you then insert another electrode which does not corrode (e.g. platinum), and connect the iron electrode to the negative pole of a current source, and the platinum wire (electrode) to the positive pole, the iron dissolution will slow down or even stop, depending on the voltage applied.

On the other hand, if you connect the iron electrode to the positive pole and raise the voltage from very low values to higher ones, the dissolution grows exponentially with the increasing voltage.

However, above a certain current limit (and depending on the electrode area, electrolyte composition, and temperature), the current suddenly drops to very low values, and the iron electrode stops dissolving. This phenomenon was detected by Michael Faraday, and he called it «passivation». This phenomenon has been subject of controversy and disputes until the 1950s when a better understanding was possible with the invention of the modern potentiostat (Figure 1).

Figure 1. Basic line diagram of a potentiostat/galvanostat.

In experiments where the ohmic drop may be high (e.g., in large-scale electrolytic or galvanic cells or in experiments involving nonaqueous solutions with low conductivities), a three-electrode electrochemical cell is preferable. In this arrangement, the current is passed between the working electrode (WE) and a counter (or auxiliary) electrode (CE).

Learn more about the ohmic drop in our free Application Notes.

The counter electrode can be made of any available electrode material because its electrochemical properties do not affect the behavior of the working electrode of interest. It is best to choose an inert electrode so that it does not produce any substances by electrolysis that will reach the working electrode surface and cause interfering reactions there (e.g., platinum or carbon). Because the current flows between the WE and the CE, the total surface area of the CE (source/sink of electrons) must be larger than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical processes under investigation.

Sometimes it is placed in a compartment separated from the working electrode by a sintered-glass disk or other separator (Figure 2). Bulk electrolysis experiments typically require much longer times than electroanalytical experiments, so separation of the counter electrode is required.

Figure 2. Gastight electrochemical cell for CO2 reduction study (click to enlarge).

The potential of the working electrode is monitored relative to a separate reference electrode (RE), positioned with its tip nearby (with a Luggin capillary as shown in Figure 3). The potentiostat used to control the potential difference between the working electrode and the reference electrode has a very high input impedance so that a negligible current flows through the reference electrode. Consequently, the potential of the reference electrode will remain constant and equal to its open-circuit value. This three-electrode arrangement is used in most electrochemical experiments.

The counter electrode is used to close the current circuit in the electrochemical cell. It is usually made of an inert material (e.g., Pt, Au, graphite, or glassy carbon) and it hosts a redox reaction which occurs at the CE surface that balances the redox reaction at the surface of the WE. The products of this reaction can diffuse to the WE and interfere with the redox reaction occurring at that site. However, in electroanalytical experiments such as cyclic voltammetry (CV), the time scale of the experiment is too short for this diffusion to be able to cause significant interferences. Therefore, in most cases there is no need to place the CE in a separate compartment, as shown in the electrochemical cell in Figure 3.

Figure 3. Metrohm Autolab 1 liter corrosion cell labelled with electrodes.

The potential profile in an actual cell depends on the electrode shapes, geometry, solution conductance, and more. If the reference electrode is placed anywhere except precisely at the electrode surface, some fraction of an uncompensated potential, iRu (due to the uncompensated resistance, Ru) will be included in the measured potential. Even when the tip of the reference electrode is designed for very close placement to the working electrode by use of a fine tip (Luggin capillary, also known as a Luggin-Haber capillary), some uncompensated resistance usually remains. Modern electrochemical instrumentation includes circuitry for electronic compensation of the iRu term [2].

As mentioned earlier, the electrochemical reaction of interest takes place on the working electrode, and the electron transfer there will generate the measured current which flows between the WE and CE. As a general rule for accurate current measurements and an unhindered flow of electrons in the cell, the counter electrode should be three times larger than the working electrode. The kinetics of solvent electrolysis are generally slow, so the only way to ensure the CE reaction can sustain the required cell current is to significantly increase the surface area (e.g., with Pt gauze or a Pt rod/sheet).

However, in poorly conductive media (or high currents), when positioning the counter electrode in the cell (i.e., for the cell geometry), it is worth considering where the field lines between the WE and CE will go, and whether the WE will experience a uniform field. It is generally recommended to move the counter electrode away from the working electrode, or even to put it behind a separator, unless you are certain that the WE reaction is insensitive to pH. The CE reaction sustains the current flow, but it is often unknown exactly what kind of reactions are occurring at the counter electrode. For most aqueous electrochemistry experiments this is likely to be solvent electrolysis, which leads to a change in the pH.

When the application requires, the potential of the counter electrode can be monitored during the experiment with the S2 electrode of VIONIC. Find out more in our free Application Note.

Alternative counter electrode materials

Platinum is expensive, and it is too costly to use for large CE areas. Suitable alternative electrode materials include nickel-based alloys or carbon. Be aware that nickel-based alloys may passivate, and carbon will be oxidized at high potentials of the CE. Therefore, those materials should have a sufficient area to avoid strong polarization.

Platinized titanium is a good choice when larger counter electrode areas are required. Platinized titanium is produced either in the form of sheets or mesh grids. Mesh grids (Figure 4) have very desirable properties: a large active area compared to the geometric area, and the electrolyte is able to flow through the counter electrode.

Figure 4. Example of wire mesh grid suitable for use as a counter electrode.

 Also, at the counter electrode, a voltage drop occurs across the metal-electrolyte interface. To reduce this drop, it may be useful to use platinum covered with platinum black to increase the roughness, and consequently, the active surface area.

Final notes

Size matters in typical electrochemical measurements. Larger counter electrodes create an unconstrained current flow between WE and the CE, leading to more stable and insightful experiments. It is equally important that the overall cell configuration provides the required current density distribution. Thus, a counter electrode of the same size as the working electrode, mounted parallel to it, may optimize the current distribution without significant adverse effects from the size of the counter electrode.

The reaction occurring at the CE should be fast so that the potential drop between the counter electrode and the electrolyte does not limit the polarization that can be applied.

The products of the reaction occurring at the CE should not contaminate the solution. In practice, there will always be an electrochemical reaction at the counter electrode, and the products of that reaction should be harmless or able to be easily removed. Inert electrodes such as platinum or graphite are often used, in which case the reaction products are usually gases (oxygen or chlorine when anodic, or hydrogen when cathodic) that can be removed by bubbling air or nitrogen past the counter electrode (although there may also be a pH change at the CE).

Curious about electrochemistry?

Metrohm has you covered.

References and suggested further reading

[1] Knight, D. Humphry Davy: Science and Power (Volume 2 of Cambridge Science Biographies); Cambridge University Press: Cambridge, UK, 1998.

[2] Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2001, 2nd ed. Russian Journal of Electrochemistry, 2002, 38, 1364–1365. doi:10.1023/A:1021637209564

[3] Yang, W., Dastafkan, K., Jia, C., et al. Design of Electrocatalysts and Electrochemical Cells for Carbon Dioxide Reduction Reactions. Adv. Mater. Technol. 2018, 3, 1700377. doi:10.1002/admt.201700377

[4] Cottis, R. A. 2.30 – Electrochemical Methods. In Shreir’s Corrosion; Cottis, B., Graham, M., Lindsay, R., Lyon, S., Richardson, T., Scantlebury, D., Stott, H., Eds.; University of Manchester: Manchester, UK, 2010; Vol. 2, pp 1341–1373. doi:10.1016/B978-044452787-5.00068-8

[5] Vanýsek, P. Impact of electrode geometry, depth of immersion, and size on impedance measurements. Canadian Journal of Chemistry 1997, 75(11), 1635–1642. doi:10.1139/v97-194

Post written by Martijn van Dijk, Area Manager Electrochemistry at Metrohm Autolab, Utrecht, The Netherlands.

Fast and fundamental: influences on reliable electrochemical measurements

Fast and fundamental: influences on reliable electrochemical measurements

The ultimate goal of any researcher is to contribute to the progress of society by pioneering exploration beyond the known limits. Depending on the research type and application field, one way to fulfill this is to collect reliable experimental data on rapidly occurring processes (less than 1 ms).

Having insight into the fundamentals of these reaction mechanisms can ultimately lead to the discovery of new materials or the improvement of current solutions. In electrochemical research, reaction mechanisms and intermediates are investigated by measuring the kinetics and dynamics of the electrochemical processes happening at the surface of the electrode on a sub-ms timescale.

This article provides a short overview of the factors that have a direct influence on fast and ultra-fast electrochemical measurements from an experimental setup perspective.

Considering the following factors in the experimental design and execution is the first condition to obtain reliable experimental results for such measurements.

Additional challenges which researchers must be aware of when experimenting with «transient electrochemistry», i.e. doing electrochemical measurements at very low time scales, is presented in the featured article from E. Maisonhaute et al. [1].

Main factors that influence the reliability of fast electrochemical experimental results

The primary components of an electrochemical experimental setup are:

  • The electrochemical cell including the electrodes and electrolyte
  • The electrochemical instrument, i.e., the potentiostat/galvanostat (PGSTAT)

To perform reliable electrochemical experiments in general, and fast electrochemical measurements in particular, the specifications of the complete work system must be considered and the optimal settings must be used for all of the individual parts of the experimental setup.

Time constant of the electrochemical cell

The electrochemical cell and its specifications must be taken into account as it is an important element of the experimental setup.

Transient electrochemical experiments are not meaningful unless the cell time constant is small relative to the timescale of the measurement, regardless of the high-frequency characteristics of the control circuitry.

The cell time constant RuCdl (s) depends directly on the uncompensated resistance Ru (Ω) (i.e. the resistance of the electrolyte between the reference and the working electrode) and the double-layer capacitance Cdl (F) of the electrode [2].

As a consequence, when the potential is stepped or scanned rapidly, the true measured potential Etrue (V) lags behind the applied potential Eappl (V), according to the following equation:

Where RuCdl (s) is the time constant of the cell and t (s) is the time at which the measurement is taken.

Figure 1. Theoretical and true waveform applied to a real electrochemical cell [1].

For fast scan rates (i.e. when 𝑡 is much smaller than RuCdl ), the exponential term approaches 1 and significant errors in 𝐸true with respect to 𝐸appl can arise. For slow scan rates (i.e. when 𝑡 is much larger than RuCdl), the exponential approaches 0 and the errors become negligible.

The time constant of the cell can be reduced in three ways:

  • Reduce Ru via increasing the conductivity of the electrolyte by either increasing concentration of supporting electrolyte or decreasing viscosity
  • Reduce the size of the working electrode (e.g., by using microelectrodes) so that Cdl will be minimized
  • Move the reference electrode as close as possible to the working electrode (e.g., by using a Luggin capillary) so that Ru will be minimized

The electrochemical instrument: potentiostat/galvanostat (PGSTAT)

The potentiostat/galvanostat (PGSTAT) is used to accurately control the applied signal (potential or current) and measure the response (current or potential, respectively) from the electrochemical cell. The accurate control of the applied signals is achieved by using a control loop (or feedback loop) circuit.

When fast electrochemical measurements are executed, the following specifications will have a direct influence on the results and must be considered.

Bandwidth of the control loop of the PGSTAT

In general terms, bandwidth can be described as the parameter that defines how fast the instrument is able to react to any changes in the signal.

In electrochemical terms, the bandwidth is the frequency beyond which the performance of the system is degraded.

The bandwidth of the control loop of the PGSTAT (i.e. bandwidth of the instrument) indicates how fast the applied signal is controlled through the feedback loop.

Higher bandwidth means that the instrument uses a faster control loop (faster feedback). As a result, the applied signal will reach the desired set point faster, and in ideal circumstances the output signal will be identical to the theoretical waveform. However, depending on the properties of the electrochemical cell connected to the instrument, the applied signal might overshoot. In extreme cases, the instrument feedback loop might get out of control causing the potentiostat to oscillate. This is more likely when high-capacitance electrochemical cells are connected to the PGSTAT.

When a Lower bandwidth is used, the overall stability of the PGSTAT increases by reducing the speed of the control loop. In this case, the consequence is that at very high measurement speeds, the output of the applied signal may be slightly less accurate due to a slower slew rate. Nevertheless, when measuring fast transients is not within the scope of the experiment, using the instrument with a lower bandwidth setting is recommended for highly accurate experimental results.

Figure 2. Schematic representation of the applied signal when Low bandwidth (Low speed) and High bandwidth (High speed) settings are used compared with the theoretical response.

Therefore, it is important to choose the control loop bandwidth settings according to the type of the measurement. For ultra-high speed measurements, a higher bandwidth setting must be used with the following considerations:

  • The higher the bandwidth, the higher the noise and the probability that the control loop will go out of control and oscillate.
  • When working with a High bandwidth setting, it is necessary to pay special attention and use adequate cell shielding and electrode connectors. The use of a Faraday cage is recommended in these cases.
  • The use of a high impedance reference electrode (RE) (e.g., double junction reference electrode, a salt bridge with frit) in combination with a High bandwidth of the control loop might lead to instability of the PGSTAT and even to oscillations.
Bandwidth of the current sensor (current range)

The measurement of the current response of an electrochemical cell (in potentiostatic mode) and the control of the applied current value (in galvanostatic mode) is executed with specially designed current sensors. In order to achieve the best sensitivity and resolution for the measurement, individual current sensors are used depending on the magnitude of the measured (or applied) current.

Each current sensor circuit (which corresponds to a current range) has a specific bandwidth or response time. Therefore for the most accurate results (especially important for fast, time resolved experiments), the current range must be selected so that the bandwidth of the current sensor will not be the limiting factor for the time response (speed) of the measurement.

In general, the lower the measured currents, the lower the bandwidth of the current sensor.

Data sampling interval vs the timescale of the investigated transient signal

The measured electrochemical response can have a complex shape with components at many frequencies. The highest frequency component of the measured or applied signal determines the bandwidth of that signal. The bandwidth of the signal should not be higher than the bandwidth of the measuring device.

If the highest frequency component of the signal is fSIGNAL, then according to the Nyquist Theorem [3] the sampling rate fSAMPLE must be at least 2 fSIGNAL (i.e. two times higher than the highest frequency component of the signal).

Figure 3. Effect of the sampling frequency of an ideal sinusoidal signal [3]. Shown here are the theoretical signal (dashed line), sample points, and resulting measured signal (orange line).

In other words, the data sampling interval must be lower than the timescale in which the time resolved (transient) measurement from the investigated electrochemical process is expected to occur. There is a practical correlation between the sampling interval and instrument bandwidth. When the sampling interval is:

  • higher than 100 μs: the 10 kHz (High Stability) bandwidth should be selected.
  • between 10–100 μs: the 100 kHz (Fast) bandwidth should be selected.
  • smaller than 10 μs: the 1 MHz bandwidth (Ultra-Fast) should be selected.

Summary

To measure reliable experimental data, all elements of the experimental setup must be considered with their own specifications and limitations. The overview above highlights the main factors and parameters which can have a direct influence on fast electrochemical measurements.

Fast measurements start here!

Visit our website to learn more about the variety of potentiostats/galvanostats from Metrohm Autolab.

References

[1] Maisonhaute, E.; et al. Transient electrochemistry: beyond simply temporal resolution, Chem.Commun., 2016, 52, 251—263. doi:10.1039/C5CC07953E

[2] Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2001, 2nd ed. Russian Journal of Electrochemistry, 2002, 38, 1364–1365. doi:10.1023/A:1021637209564

[3] Keim, R. The Nyquist–Shannon Theorem: Understanding Sampled Systems. All About Circuits, May 26, 2020. https://www.allaboutcircuits.com/technical-articles/nyquist-shannon-theorem-understanding-sampled-systems/ 

Post written by Dr. Iosif Fromondi, Product Manager and Head of Marketing and Sales Support at Metrohm Autolab, Utrecht, The Netherlands.