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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.
Best practice for electrodes in Karl Fischer titration

Best practice for electrodes in Karl Fischer titration

Have you ever asked yourself why you need an electrode for the endpoint detection in Karl Fischer (KF) titration? Theoretically, the endpoint of a Karl Fischer titration could be determined based on the color change of the reagent. However, if accuracy and reproducibility are important, endpoint detection with a double Pt electrode is a much better choice.

As the indicator electrode detects the endpoint, you can imagine that the results depend highly on the condition of the electrode. In coulometry, an additional electrode (generator electrode) is used to generate the iodine needed for the titration. Both electrode types (i.e. indicator and generator electrode) need to be kept in good shape to guarantee the correct results. It goes without saying that cleaning, storage, maintenance, and checks of the KF electrodes are important factors for success. This blog post takes a closer look at these topics.

Did you catch our series about frequently asked questions in Karl Fischer titration? Find them here!

Cleaning

Indicator electrode

Double  Pt-wire or double Pt-ring electrodes can be easily cleaned with an abrasive cleaning agent like aluminum oxide powder or toothpaste. After cleaning, rinse the electrode well with water and let it dry before mounting it in a titration cell. Check out our video below for more tips and tricks about the proper cleaning procedure for Karl Fischer titration indicator electrodes.

Take special care not to bend the Pt pins of the double Pt-wire electrode. Bending the pins can lead to tiny cracks in the glass body of the electrode. Over time, reagent can flow into the electrode and lead to corrosion (short circuit). If this happens, the electrode is beyond repair and needs replacement. Alternatively, a double Pt-ring electrode can be used instead. Problems with bent pins are then a thing of the past.
Generator electrode
Without diaphragm
Rinse generator electrodes without diaphragms with water, or if the contaminant is not water soluble, then rinse with a suitable organic solvent. If the anode or the cathode of the generator electrode shows discoloration or deposits that cannot be removed with rinsing, the electrode can then be cleaned with concentrated nitric acid (65%). Be aware that nitric acid is a strong oxidizing agent and must be handled carefully according to relevant safety regulations and instructions. Remember to first mount the green protection cap on the connector to avoid corrosion caused by fumes of nitric acid. Afterwards, rinse the electrode with water and finally with methanol.
With diaphragm
To remove salt-like residues, the generator electrode with diaphragm can be rinsed with water. Oily contamination can be rinsed off with an organic solvent (e.g. hexane). Sticky residues on the diaphragm can be removed in the following way: 

  1. Mount the green protection cap on the connector of the electrode.
  2. Place the electrode in an upright position (e.g. in an Erlenmeyer flask) and add a few milliliters of concentrated nitric acid (65%) in the cathode chamber. Let the acid flow through the diaphragm.
  3. Fill the cathode chamber with water and let it flow through the diaphragm to remove the nitric acid. Repeat this step two or three times. A simple way to see whether another rinsing step is required is by performing a quick check of the pH value at the cathode using pH indication paper.
  4. Finally, fill the cathode chamber with methanol and let it flow out.

Now the generator electrode is as good as new and ready for use in a titration cell again.

Maintenance

Except for the generator electrode with diaphragm, KF electrodes are maintenance free. However, the catholyte filled in the generator electrode with diaphragm can decompose over time. To avoid any influence of the decomposition products on the results, exchange the catholyte on a regular basis according to the manufacturer’s recommendations.

Storage

Unlike pH electrodes, KF electrodes do not contain a glass membrane that could potentially dry out. Therefore, no special solution is required in which to store KF electrodes. If you use the electrodes frequently, it is recommended to keep the electrodes mounted in the titration cell and immersed in the KF reagent. Alternatively, all KF electrodes (indicator and generator electrodes) can be stored dry.

What to check for

It is recommended to check the complete titration setup instead of only the electrode(s).

Volumetry

Carry out a threefold titer determination using either a liquid or a solid water standard suitable for volumetry and calculate the mean value of the titer. Then, determine the water content of a water standard (also via triplicate determination). Make sure that you do not use the same standard as for the titer determination but use a different batch of the standard or even a completely different standard. Calculate the water content and compare it to the certified water content of the standard.

If the recovery is determined to be in the range of 97–103%, the titration system (including the electrode) is working fine. Finding values outside this range means that there is something wrong with the titration system or with the determination procedure. Results of the sample analysis would very likely also deviate from the real water content. Therefore, it is important to find the reason for values that are too high or too low. Sometimes the reason for deviations is just an air bubble in the dosing cylinder or due to an exhausted molecular sieve. However, if you do not find the reason, do not hesitate to contact your local Metrohm agency.

Coulometry

Water standards with lower water contents (0.1%) are available to properly check the health of coulometric titration systems. Carry out a water content determination in triplicate with such a standard. Calculate the recovery with the obtained results and the certified water content of the standard.

A recovery value in the range between 97–103% means that everything is fine with the system and that the electrodes work as expected. As with volumetry, in coulometry it is important to find the reason for any deviating recovery values. Make sure that you find and eliminate the problem to obtain correct results for your samples.

What you should avoid

  • Do not use solvents that contain ketones or aldehydes (e.g. denatured ethanol) to clean KF electrodes or any KF accessories.
  • Do not treat KF electrodes in an ultrasonic bath. This might destroy the electrode.
  • For drying, use a maximum temperature of 50 °C. Higher temperatures might damage the electrode.
  • Do not bend the Pt pins of the double Pt-wire electrode.

Summary

As you can see, keeping your KF electrodes in good shape is actually very simple. Regular cleaning helps to avoid erroneous results and ensures that your Karl Fischer electrodes will work for a long time.

Best practice for electrodes in titration

Treat your sensors right!
Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.
NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 2

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 2

Differences between gasoline, diesel, and jet fuel

Gasoline is a fuel made from crude oil and other petroleum-based liquids, containing carbon numbers generally between 4 and 12, and exhibiting boiling points of up to 120 °C. Gasoline is primarily used as a fuel for vehicles. Petroleum refineries and blending facilities produce motor gasoline for sale at gas (or petrol) stations. Most of the gasoline that petroleum refineries produce is unfinished gasoline. This unfinished product requires blending with other liquids to control parameters such as octane rating and volatility to make gasoline meet the basic requirements for fuel that is suitable for use in spark ignition engines.

Diesel fuel is refined from crude oil at petroleum refineries. «Diesel» is the common term for the petroleum distillate fuel oil sold for use in motor vehicles that use the compression ignition engine, invented by the German engineer Rudolf Diesel (1858–1913). He patented his original design in 1892. One of the fuels that Rudolf Diesel originally considered for his engine was vegetable seed oil, an idea that eventually contributed to the biodiesel production process of today. Prior to 2006, most diesel fuel contained high quantities of sulfur. Sulfur emissions from combusting diesel fuel leads to air pollution that is quite harmful to human health. Therefore, the U.S. Environmental Protection Agency issued requirements to reduce the sulfur content of diesel fuel to be as low as 15 mg/L. Diesel fuel contains components with a carbon number range from 8 to 21 (though mainly between 16–20) and is the fraction that boils between 200 °C and 350 °C.

Jet fuels (or aviation fuels) are one of the basic products used by aircraft. Jet fuel is comprised of refined petroleum products with carbon numbers between 10 to 16 (although they can range from 6 to 16), and it boils between 150 °C and 275 °C. This type of fuel is heavily regulated by national and international bodies. There are two main types of jet fuel: Jet A and Jet B. The main difference between the two is the freezing point. Jet B is usually used for military operations and locations with inclement weather. Jet A is mainly used to fuel commercial airplanes.

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of gasoline, diesel, and jet fuel

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, available turnkey solutions for gasoline, diesel, and jet fuel are outlined which have been developed according the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation). Afterward is a discussion about the return on investment (ROI) of using NIRS as an alternative to the CFR Engine.

Did you miss Part 1 in this series about NIRS as a QC tool for the petrochemical industry? Check it out below!

Read our previous blog posts to learn more about NIRS as a secondary technique.

NIRS expedites and simplifies fuel quality control

 Without high quality fuels (e.g., gasoline, diesel, and jet fuel), our daily lives would look much different. At the end of the production process as well as at various steps in the distribution chain, the quality of the product needs to be determined. Typically, key quality parameters such as RON/MON (research and motor octane numbers), cetane index, and flash point are determined in the laboratory by chemical and physical methods. These methods not only incur high running costs but they are also quite time consuming.

NIRS on the other hand requires neither chemicals nor sample preparation. This technique can even be used by non-technical people (no chemistry degree necessary) and it provides results in less than a minute. Furthermore, multiple chemical and physical parameters can be determined simultaneously. The combined benefits of this technology make NIRS the ideal solution for many daily QA/QC measurements or ad-hoc at-line analysis.

Metrohm offers the NIRS DS2500 Petro Analyzer for quality control and routine analysis of fuels and is compliant with ASTM D6122. Resistant to dust, moisture, and vibrations, this instrument is not only suitable for laboratory use, but also use in direct production environments. Learn more on our website

Turnkey solutions: available pre-calibrations for gasoline, diesel, and jet fuel

Table 1 lists all constituents covered by the pre-calibrations for these different fuels. Click on the fuel type in the table to learn more about its pre-calibrations offered by Metrohm.

Table 1. Pre-calibrations available for a variety of key quality parameters in gasoline, diesel, and jet fuel.
Fuel type Parameters Range SECV
Gasoline RON 81–100 0.68 0.958
MON 81–88 0.53 0.889
Anti-Knock Index 85–94 0.45 0.948
Aromatics 20–45% 0.011 0.959
Benzene 0.15–0.70 % 0.0004 0.902
Density 0.74–0.76 g/cm3 0.0024 g/cm3 0.797
Olefins 0–25 % 0.013 0.909
Oxygen 0.2–2.0 % 0.00045 0.994
Diesel Cetane index 46–77 0.62 0.987
Cetane number 45–60 0.942 0.942
Density 0.82–0.89 g/cm3 0.0021 g/cm3 0.968
CFPP -22–(+19) °C 2.8 °C 0.963
T95 325–410 °C 7.04 °C 0.799
Flash Point 56–120 °C 2.7 °C 0.97
Viscosity 2–5.5 cSt 0.15 0.91
Kerosene / Jet Fuel Cetane index 36–50 1.1 0.871
API gravity 38–48 ° 0.56 ° 0.931
Aromatics 10–25 % 0.01 0.851
T10 158–200 °C 4.1 °C 0.801
T20 165–205 °C 3.1 °C 0.88
T50 180–220 °C 4.1 °C 0.789
Density 0.78–0.83 g/cm3 0.003 g/cm3 0.936
Flash Point 38–65 °C 4.3 °C 0.62
Freeze Point -65–(-40) °C 3. 5°C 0.576
Hydrogen 13.2–14.2 % 0.0005 0.934
Saturates 75–90 % 0.009 0.888
Viscosity at 20 °C 3–7 cSt 0.33 cSt 0.804

Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Analyzers in our free brochure.

Application example: quality control of diesel with the NIRS DS2500 Petro Analyzer

The cetane index (ASTM D613), flash point (ASTM D56), cold filter plugging point (CFPP) (ASTM D6371), D95 (ISO 3405), and viscosity at 40 °C (ISO 3104) are among some of the key parameters to determine the quality of diesel. The primary test methods for these parameters are labor intensive and challenging due to the need for multiple analytical methods.

In this turnkey solution, diesel samples were measured in transmission mode with a NIRS DS2500 Petro Analyzer over the full wavelength range (400–2500 nm). The built-in temperature-controlled sample chamber was set to 40 °C to provide a stable sample environment. For convenience reasons, disposable vials with a pathlength of 8 mm were used (Figure 1), which made a cleaning procedure unnecessary.

Figure 1. Quality control of diesel fuel as performed by the Metrohm NIRS DS2500 Petro Analyzer.

The obtained Vis-NIR spectra (Figure 1) were used to create prediction models for the determination of key diesel parameters. The quality of the prediction models was evaluated using correlation diagrams which display the correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 2).

Figure 2. Correlation plots and figures of merit (FOM) for the different constituents tested in diesel.

This solution demonstrates that NIRS is excellently suited for the analysis of multiple parameters in diesel fuel, providing results in less than one minute without the need for sample preparation or any chemical reagents.

Want to learn more? Download our free Application Note.

Return on investment: CFR Engine vs. NIRS

Gasoline requires intensive checks on several quality parameters which must be within certain specifications before commercialization. These parameters which can also be controlled by NIRS analysis include the research octane number (ASTM D2699) and motor octane number (ASTM D2700), also known as RON/MON.

The importance of measuring these values precisely is not only to comply with regulations, but also because of the further potential to save costs for manufacturers. As an example, RON values exceeding the stated requirements will still be accepted by the market, but these products will then include a higher amount of lucrative long-chain organic molecules. This so-called «RON giveaway» is estimated at approximately 0.5 RON per barrel, resulting in $2.25 million USD/month in lost revenue for a production process of 100,000 barrels per day.

Figure 3. CFR® F1/F2 Octane Rating Unit Combination Research & Motor Method. (Source: CFR Engines Inc.)

The Combination Cooperative Fuel Research (CFR) octane rating engine (model F1/F2) is used to determine the octane quality of gasoline and fuel blending components. This unit is recognized and approved by ASTM D2699 and D2700. The engine is equipped with a heavy-duty crankcase, variable compression cylinder, carburetor with adjustable fuel to air ratio, and knock measurement equipment (Figure 3).

Ready-to-use NIRS systems are also available for monitoring several gasoline quality parameters which cover varied ranges and their respective precisions (Table 1). Additionally, the manufacturers of NIRS analyzers usually offer application support to extend these ranges or improve upon the precision.

An overview of estimated costs for the analysis of RON and MON with a CFR Engine compared to the Metrohm NIRS DS2500 Petro Analyzer is shown in Table 2. The full payback is achieved within two years if considering only 50% of the primary analysis method (CFR Engine) is replaced by NIRS. This calculation is based on 2000 analyses per year (1000 RON + 1000 MON), with total running costs of approximately $32.50 per analysis (chemicals, maintenance, and labor).

Table 2. Cost of ownership CFR engine vs. DS2500 Petro Analyzer.
Total analyses RON + MON per year 2000 2000
Cost of operator per hour $25.00 $25.00
Cost of Analyzer CFR Engine NIRS DS2500 Petro Analyzer
Analyzer $500,000.00 $55,000.00
Total initial costs $0.001 $55,000.00
Running costs consumables / chemicals / maintenance
Chemicals per year (ASTM D2699/D2700) $20,000.00 $0.00
Maintenance cost per year $20,000.00 $1,500.00
Chemicals plus maintenance cost per analysis $20.00 $0.75
Total running costs per year $40,000.00 $1,500.00
Time spent per analysis  30 minutes < 1 minute
Labor cost of 1000 analyses of RON (ASTM D2699) $12,500.00 $416.50
Labor cost of 1000 analyses of MON (ASTM D2700) $12,500.00 $416.50
Labor cost per analysis $12.50 $0.42
Total labor costs per year $25,000.00 $833.00
Total running costs per year $65,000.00 $2,333.00
1Assumption that the instrument had previously been purchased, and therefore this cost is not included in the ROI calculation.

More information about the analysis of RON/MON and other parameters in gasoline can be found in our free Application Notes below.

In this example, RON/MON analysis was used to show cost savings and ROI when using NIRS to supplement a primary method. However, when expanding this to consider other key quality parameters such as the ones indicated in Table 1, the financial incentives for such an investment are even more compelling.

Summary

Near-infrared spectroscopy is very well suited for the analysis of key quality parameters in gasoline, diesel, and jet fuel. Available pre-calibrations are developed and validated in accordance with the ASTM guidelines. Positive aspects of using NIRS as an alternative technology are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

Future installments in this series

This blog article was dedicated to the topic of gasoline, diesel, and jet fuel and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. Future installments will be dedicated to other important applications in this industry. These topics will include:

 

  • Pyrolysis gasoline (Pygas)
  • Lubricants
  • ASTM Norms

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
Staircase or linear scans: two options for reliable electrochemical experiments

Staircase or linear scans: two options for reliable electrochemical experiments

Electrochemical experiments are performed by delivering and controlling a potential or current signal to the electrochemical cell/device under test and measuring its response through a potentiostat/galvanostat (PGSTAT). Here, two different options for performing different electrochemical experiments are discussed: linear and staircase scans, as well as some applications where one may be preferred over another.

From analog to digital

Before modern digital electronics were widely available, PGSTATs worked with analog electronics, and therefore delivered analog signals. Analog boards were expensive and time-consuming to produce and test. Moreover, controlling the equipment from a computer is done via digital communication and requires digital electronics.

An analog signal is continuous, and it has virtually infinite resolution. On the other hand, a digital signal is written in discrete bits (0 and 1), so it is not continuous.

Linear scans

To better explain an analog signal, consider a linear sweep voltammetry (LSV) in potentiostatic mode, performed with an analog scan. Here, the applied voltage E versus time plot consists of a straight line between the initial and end potentials. The potential interval between two consecutive data points and the scan rate define the interval time—the slope of the E versus time plot (Figure 1).

Figure 1. A typical linear scan from an initial and an end potential. The interval time and the measurement time are also shown here.

The current resulting from the application of the LSV is measured at the end of the interval time. The measurement time defines the duration of the sampling. The current is composed of a capacitive part ic (given by the charging of the double layer), and a faradaic part if.

A double layer is formed when a potential is applied to an electrode. Then, current flows to the electrode, which becomes charged. Ions from the bulk solution migrate to the surface to balance this charge. Therefore, a layer of ions at the interface between the electrode and the electrolyte builds up, forming the equivalent of a capacitor.

Learn more about the principles and characterization of capacitors in our free Application Note.

The faradaic current is the result of the electrochemical reactions occurring at the working electrode | electrode and counter electrode | electrolyte interfaces, and it changes with the scan rate either linearly or with the square root of the scan rate, depending on the type of electron transfer.

The capacitive current resulting from a linear scan iC,ls is a constant value given by the product of the double layer capacitance Cdl and the scan rate.

Find out more about the differences between linear and staircase cyclic voltammetry on a commercial capacitor in our free Application Note.

Staircase scans

When modern digital electronics became more commercially available and economically feasible, PGSTAT manufacturers adopted them, together with personal computers, to control PGSTATs. This allowed researchers to create more complex procedures as well as to perform data analysis via software. Metrohm Autolab was the first company to deliver computer controlled PGSTATs to the market back in 1989.

In the case of a digital scan, the applied potential versus time plot between an initial and end potential has the typical «staircase» shape of a digitized signal. The interval time tint defines the duration of each step, while the step potential Estep defines the potential difference between two consecutive steps (Figure 2).

Figure 2. A typical staircase scan profile. The step potential Estep is shown along with the interval time and the measurement time.

The resulting current is measured at the end of the step. The measurement time defines the sampling time.

In a digital scan, the step potential  Estep results in a capacitive current iC,ss which rises almost immediately up to the maximum value allowed by the current range, lim(CR), and then it decays exponentially as time constant t = RC. After the decay of the capacitive current, the faradaic current if is predominant.

The current is measured at the end of the step in order to remove the capacitive current and collect only the faradaic one (Figure 3).

Figure 3. Potential step (in blue) and current profile for a staircase scan. The current profile is divided into capacitive current iC,ss (black line) and faradaic current if (purple line). Total current itot is shown in dark red. The measurement (sampling) time is also shown here. For clarity, the decay of the capacitive current and the decay of the faradaic current are not in scale.

Application examples: staircase or linear scan?

Capacitors

Some electrochemical processes can result in a capacitive current, having a characteristic time comparable to the charging of the double layer. In such cases a digital scan would neglect such capacitive currents and all of the information contained within them.

This is the case of highly capacitive cells, such as capacitors and supercapacitors.

In Figure 4, cyclic voltammograms at different scan rates of a commercial 1 µF capacitor are shown. The diagram on the left shows the results from the digital scan, and on the right are the results from the analog scan [1].

Figure 4. Cyclic voltammograms of a 1 µF capacitor at different scan rates. Left: the cyclic voltammogram resulting from a digital scan. Right: the cyclic voltammogram resulting from an analog scan.

It is pretty clear that the digital scan results do not contain any information about the charge/discharge of the capacitor, while the analog scan results have the expected shape of a capacitor’s cyclic voltammograms.

Adsorption/desorption processes

Another application example includes cells in which fast adsorption/desorption of species at the electrode surface occurs in a short time, like the adsorption/desorption of hydrogen as part of the electrochemical behavior of platinum submerged in an aqueous solution of sulfuric acid (Figure 5).

Figure 5. The cyclic voltammogram of a Pt working electrode immersed in a 0.5 mol/L H2SO4 aqueous solution. The reference is a Ag/AgCl 3M KCl electrode, while the counter electrode is a Pt electrode.

Here, the fast adsorption/desorption of hydrogen occurs at time scales that are similar to the charging of the double layer in the capacitor example. Therefore, the linear scan is the preferred option, compared to the digital staircase scan which is unable to capture the hydrogen adsorption/desorption (Figure 6) [2].

Figure 6Top: linear cyclic voltammograms of a Pt working electrode immersed in a 0.5 mol/L H2SO4 aqueous solution at different scan rates. Bottom: staircase cyclic voltammograms of the same setup at the same scan rates.

Redox reactions

Another example of experiments in which the capacitive current should not be neglected are redox reactions in which the electron transfer is very fast. In these situations, the charge transfer resistance is very small and the cyclic voltammogram results in redox peaks which are symmetrical over the potential axes. Examples include redox reactions on species adsorbed on the working electrode surface [3].

VIONIC: the future of electrochemistry

The most recent generation of PGSTATs, such as VIONIC, are equipped with electronics that allow to users perform analog scans without the drawbacks mentioned earlier. This gives researchers the opportunity to choose the type of scan according to the type of systems being studied, the materials, and the importance of the capacitive current for the outcomes of the research.  

Learn more about VIONIC

Maximum experimental possibilities, intelligent software, and the most complete data.

References

[1] Locati, C. Comparison between linear and staircase cyclic voltammetry on a commercial capacitor, Metrohm AG: Herisau, Switzerland, 2021. AN-EC-026

[2] Locati, C. Study of the hydrogen region at platinum electrodes with linear scan cyclic voltammetry – How VIONIC powered by INTELLO can be used to characterize processes at the Pt-electrolyte interface, Metrohm AG: Herisau, Switzerland, 2021. AN-EC-025

[3] Chi, Q.; Zhang, J.; Andersen, J. E. T.; Ulstrup, J. Ordered Assembly and Controlled Electron Transfer of the Blue Copper Protein Azurin at Gold (111) Single-Crystal Substrates. J. Phys. Chem. B 2001, 105 (20), 4669–4679. https://doi.org/10.1021/jp0105589

Post written by Dr. Corrado Locati, Application Specialist at Metrohm Autolab, Utrecht, The Netherlands.

Frequently asked questions for beverage analysis with ion chromatography

Frequently asked questions for beverage analysis with ion chromatography

A brief overview of beverage analysis with ion chromatography

The analysis of beverages is extremely important for the general health of the population. Why is this so? Our bodies are composed of about 60% water, depending on several factors like weight and sex. Hydration is one of our basic physiological needs, as noted in Maslow’s hierarchy of needs. When we are dehydrated, a number of problems occur, from irritability to confusion leading to severe kidney problems and even low blood volume shock in extreme cases. Therefore it is incredibly important for standards to be set by regulatory agencies regarding the contents of the beverages we choose to drink, whether this is water, milk, coffee, juice, soft drinks, beer, wine, or any other number of items. Reliable beverage analysis is critical for many reasons: product monitoring and quality control, general content determination, and to avoid health issues.

Depending on the regulations regarding the concentration limit for a compound as well as on the complexity of the analysis and the detection limit of the determination, different instruments and analysis techniques can be applied for the analysis. For single analyte determination, mostly in a higher concentration range (e.g., sugar determination in wine) instruments like titrators, refractometers, or enzymatic kits can be applied for analysis. For analyte qualification and quantification in complex matrices (e.g., lactose determination in dairy products) instruments like ion chromatographs (IC), high-performance liquid chromatographs (HPLC), gas chromatographs with mass spectrometers (GC-MS), or hyphenated liquid chromatographic techniques (LC-MS/MS) are necessary.

Ion chromatography is a simple and robust analysis technique that is able to measure several components in beverages with relative ease compared to these other technologies.

Typically, conductivity detection is used for IC analysis. Other options are available including UV/VIS and amperometric detectors for more specialized analyses (e.g., carbohydrate analysis). Find out more on our website.

When analyzing complex beverage matrices like milk, coffee, or wine, sample preparation steps are normally required to protect the instrument (e.g., from contamination or blockages due to particles). Performing these steps manually is a very time-consuming and costly process that is also prone to human errors. Metrohm offers a time-saving solution for this with «MISP»: Metrohm Inline Sample Preparation specifically developed for difficult sample matrices. Several options are available including Inline Ultrafiltration, Inline Dialysis, Inline Dilution, and much more.

Watch our LabCast video below about to learn more about the benefits of using Inline Ultrafiltration in IC.

With fully automated sample preparation, analysts can be sure that every sample is treated in the exact same manner, leaving time for other more important tasks. Not only does this increase sample throughput, but it also improves accuracy and reproducibility of analyses and results. Discover the variety of Metrohm Inline Sample Preparation techniques on our website.

FAQ about beverage analysis with IC

Now that you know a bit more about the capabilities of ion chromatography for quality control in beverage analysis, it’s time to answer some frequently asked questions in this field. Dr. Gabriele Zierfels, Senior Product Specialist Ion Chromatography at Metrohm, has given a webinar hosted by New Food Magazine discussing how IC can help modern quality control labs from the beverage industry comply with official quality and labelling standards and make their daily routine analytics more efficient, which you can watch on-demand for free.

The webinar begins with an overview of the latest analytical techniques used by the beverage industry to comply with quality standards and labelling requirements such as EU regulation 1169/2011 and US regulation 21CFR101. Then the focus shifts to the versatility of ion chromatography for beverage testing and how it can help modern QC labs increase the efficiency of their daily routine analytics, which is exemplified in the main part of the webinar by numerous application examples.

Here we answer the top five questions asked by participants regarding beverage analysis with ion chromatography after the webinar.

1. What are the differences between HPLC and ion chromatography (IC) when it comes to beverage analysis? What are the main benefits of using IC?

High-performance liquid chromatography (HPLC) is typically used to separate complex mixtures with large organic (nonpolar) molecules by utilizing their affinities for different solvents and interactions with modified stationary phases. Many analytes required for food and beverage testing are either ions or polar molecules, some of which cannot be measured using reversed-phase HPLC.

IC on the other hand is a simple and robust analysis technique which allows determination of similar chemical substances in a single chromatographic run. With IC, ionic or polar analytes can be determined in very complex matrices with superior sensitivity and reproducibility using analytical separation columns made of ion exchange resins. The analytes undergo chemical/electrostatic interactions with the column resin. Due to such interactions these analytes are retained stronger than on reversed-phase columns. This allows excellent separation from the matrix components.

Check out the benefits of using IC over HPLC in our video.

2. How easy or difficult is it to switch between applications with a single instrument setup (e.g., analyzing different beverages like coffee and juices)?

Switching between different sample types can be simple, but every sample requires preparation before injection into the chromatographic system. In most cases this means sample dilution or filtration. This procedure can be done manually (which is time consuming) or completely unattended utilizing automated Metrohm Inline Sample Preparation (MISP) techniques. Therefore, several different sample types (e.g., tea, coffee, or juices) can be analyzed one after another for the same analyte profile, such as sugar content. The sample matrices can vary widely, as Metrohm offers various MISP techniques to get the cleanest possible extract for injection and subsequent separation and quantification of the target analytes.

Visit our website to download free IC Application Notes for the analysis of a variety of analytes in multiple beverage types.

3. How robust is IC when it comes to analytes that require stabilization, for example, sulfite?

Determination of samples containing analytes that must be stabilized prior to analysis (e.g. sulfite) is even more robust when using IC for the task. Even if samples have been stabilized, the detection can be disturbed by electrode fouling in the amperometric detector. To avoid this process (which is common in Direct Current mode), a short automatic cleaning method was applied between the sample analyses for stabilization of the signal and results that lasts for up to three weeks. This means no manual polishing steps and no disposable accessories are required.

To learn more about simplified sulfite analysis with Metrohm ion chromatography, download our free White Paper, check out our previous blog post, and download our free article featured in LC/GC’s The Column.

4. If a series of different samples are analyzed for the same parameter, will the instrument automatically calculate the dilution factor for each individual sample, or does it need to be predetermined for each sample?

When working with the logical Inline Dilution setup, samples containing analytes in different concentration ranges can be determined automatically with correct results. Because every single sample can contain varying concentrations of analytes, the software calculates the dilution factors individually for each sample. The summary report then gives the correct results from the first and second determinations.

5. Are the results traceable?

In short, yes. The MagIC Net software has been developed by Metrohm to intelligently operate the instruments and provide full traceability of results. All parameters of the system components e.g. the series number or the separation column are documented thanks to the intelligent chip technology integrated into various working parts of the instrument. This also permits the monitoring of the inline sample preparation and automation steps, improving the reliability of the analysis. The results are traceable for repeatability and audit control, fulfilling GLP and FDA standards.

Read more about the MagIC Net software and its capabilities on the Metrohm website.

Thirsty for more knowledge?

Download our free application ebook made in cooperation with with SelectScience:

Ion chromatography for food & beverage analysis

Post written by Dr. Alyson Lanciki (Scientific Editor) and Dr. Gabriele Zierfels (Senior Product Specialist Ion Chromatography) 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.