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History of Metrohm IC – Part 3

History of Metrohm IC – Part 3

Part 3 of this series on the history of ion chromatography development at Metrohm focuses on the near past, from the mid 2000s until a few years ago. Here, sequential suppression was introduced, making analysis even more sensitive with the removal of baseline disturbances from the chromatogram. In the rest of this blog post, I cover the 4th and 5th instrument generations, presenting professional, flexible, intelligent ion chromatography from Metrohm to the world.

Have you read the other parts in this series? If not, find them here to understand the history of IC development at Metrohm over the past few decades.

«An IC system so smart that it can make logical decisions on its own? For example, diluting samples automatically, if the concentration of your target analyte is too high and results would fall outside the calibrated range?»

Dr. Markus Läubli, R&D Ion Chromatography, Metrohm AG

«This is exactly what the 850 Professional IC and MagIC Net™ software can do. In fact, our Professional IC system takes care of the liquid handling & sample preparation with hardly any work required from the user!»

Dr. Andrea Wille, Manager Competence Center Ion Chromatography, Metrohm AG

2005: Sequential suppression is introduced

Sequential suppression was introduced in 2005 to overcome issues that arise from using chemical suppression alone.

In chemical suppression (using the packed bed Metrohm Suppressor Module, MSM), the dissociated carbonic acid from carbon dioxide attributes a background conductivity of approximately 15 µS/cm. This yields in relatively large water dips as well as system peaks (from carbonate). Depending on the carbonate concentration, the system peak may interefere with other peaks of interest in the chromatogram.

Furthermore, the pH in a peak changes due to the increasing concentration of H+, as e.g. chloride is eluted as HCl. This pH change induces a decreasing baseline as the hydrogen carbonate—carbonic acid equilibrium is pushed towards development of carbonic acid. The effect is schematically illustrated in Figure 1.

Here, the calculated baseline is marked with the straight red line, but the real baseline shows small negative deviations under the analyte peaks. This negative peak area is not taken into account for the quantification of the respective analyte. This and other effects result in a deviation from the linearity of the calibration curve. In most cases it is therefore recommended to apply a quadratic curve fit.

Figure 1. Chromatogram with chemical suppression. The blue area is not taken in to account in the quantification. Negative peaks: real baseline due to pH change.

Download our free poster: Sequential suppression for conductivity detection in ion chromatography. The poster describes how different suppressors (MSM and MCS) work and mentions possible applications. 

Sequential suppression for anions

The term «sequential suppression» represents the combination of chemical suppression and CO2 suppression. The Metrohm CO2 Suppressor (MCS) removes CO2 from the eluent (mobile phase) after chemical suppression, but before detection. This shifts the equilibrium from hydrogen carbonate towards dissolved CO2. Applying sequential suppression therefore reduces the background conductivity to < 1 µS/cm, corresponding to ultrapure water itself.

As an effect of sequential suppression, the water dip as well as the system peak (carbonate peak) is reduced dramatically. The former allows easier integration of the early eluting peaks (Fig. 2), e.g. fluoride. The latter reduces the interference and disturbance of peaks of interest. Using the MCS in combination with the MSM, there are no negative baseline peaks present in the chromatogram, and the linearity is improved. Nevertheless, it is still recommended to apply a quadratic curve fit when calibrating a concentration range of one or more orders of magnitude.

Figure 2. Overlay of a chromatogram of standard anions with chemical suppression (MSM alone, blue) and a chromatogram of the same standard, but while applying sequential suppression (MSM + MCS, red). The water dip (1, injection peak) and the system peak (2, carbonate peak) are no longer present with sequential suppression.

Here you can find a selection of free application notes for download using sequential suppression for both anions and cations.

4th generation: Intelligent ion chromatography – 2007

The fourth generation of Metrohm ion chromatography was introduced in 2007, bringing with it a higher level of detection data handling and finally adding intelligence to the IC instruments.

After the introduction of the 850 Professional IC series in 2007, the respective compact versions (881 Compact IC pro and 882 Compact IC plus) were launched in 2009 (Figure 3), offering IC systems for all kinds of laboratories and sample throughput needs. The 883 Basic IC plus followed shortly after this as well in 2009 (Fig. 3).

Figure 3. New additions to the Metrohm IC family (left to right): The 850 Professional IC, 881 Compact IC pro, 882 Compact IC plus, and 883 Basic IC plus.

Aside from general improvements on the hardware modules, the conductivity detector was switched from analog to digital. The previous iteration consisted of a stand-alone detector block and an electronic unit, which was able to cover the full signal range of conductivity for IC. However, it was required to select a dedicated measuring range and an optimal full-scale (e.g., 20 µS/V) for the best signal quality.

The new IC Conductivity Detector for 850 Professional IC instruments consisted of only the «detector block» itself. The complete electronics were now integrated within the thermostated detector block. Besides the digital data acquisition capability, this significantly improves the signal stability which yields in an extremely low noise level. The digital detector could now handle the full conductivity range without the need for any range or full-scale settings.

MagIC Net, the new fully in-house developed software for both hardware control and data handling, brought many enhanced features and capabilities to the world of Professional IC (Figure 4). Here, «Intelligent IC» was born. Intelligent IC stands for the automatic recognition of most of the hardware components, e.g., the high pressure pump, the separation column, etc. This information is stored in every determination, allowing users full system traceability for each analysis.

Figure 4. MagIC Net software for the full hardware control and data handling of Metrohm IC instruments.

MagIC Net also brought forth many special control functions enabling sophisticated Inline Sample Preparation and automatic calibration techniques. Logical decisions are available, allowing analysts to perform logical dilutions for example. Here, the logical decision-making software decides whether an analysis is a standard, a QC standard, or a sample. After the chromatographic run, the results can be tested for concentrations out of the calibration range. When such outliers are found, MagIC Net calculates new dilution factors and automatically re-runs the samples with the new values. At the end, perfect results are available for all analytes without manual redilution and re-injection.

5th generation: Modular flexibility arrives – 2013

The fifth generation of Metrohm ion chromatography arrived with an upgrade to the Professional IC system allowing even more application capabilities. The 940 Professional IC Vario and the 930 Compact IC Flex were introduced in 2013 (Figure 5).

Figure 5. The Metrohm 940 Professional IC Vario (left) and the 930 Compact IC Flex (right), developed with flexibility in mind.

These instruments were followed in quick succession by the 942 Extension Modules Vario as well as the stand-alone 945 Professional Detector with conductivity and/or amperometric detection options to further broaden application suitability.

The 941 Eluent Production Module, also introduced in 2013, enabled the continuous preparation of all types of eluents via dilution of concentrated mobile phase constituents. Commercial as well as homemade concentrates may be applied. Therefore, the eluent production is not reduced to standard or costly eluents.

Figure 6. Ultimate modularity for the laboratory – mix and match modules: the Metrohm 940 Professional IC Vario TWO/ChS set up for AnCat analysis, containing 2 IC Conductivity Detectors, sitting atop two 942 Extension Modules Vario LQH and a 941 Eluent Production Module.

Intelligent IC: Not only limited to the laboratory

After the introduction of the new MagIC Net software for IC analysis, an updated version of the Metrohm IC process analyzer from Metrohm Process Analytics was also developed and launched. In 2016, the Process IC ONE and Process IC TWO were introduced, only differing in the amount of measurement channels and detectors (Figure 7). These process analyzers were built using the 940 Professional IC Vario series with the same functionality for the laboratory, in a rugged housing suitable for harsh industrial conditions.

The use of various MISP techniques (Metrohm Inline Sample Preparation) such as Inline Ultrafiltration and Inline Dilution, along with nine configurable wet part modules for further sample conditioning, integrated eluent production, and the possibility to connect one system to up to 20 process points for time-saving sequential analysis at multiple areas inside of a plant further expanded the application capabilities beyond what any lab instrument could offer. The use of liquid level sensors and integrated alarms for leakages and out-of-specification data results in maximum analyzer uptime due to reduced maintenance intervals.

Figure 7. The Metrohm Process IC TWO configured for AnCat analysis, with optional PURELAB® flex 5/6 from ELGA®, a pressureless inline ultrapure water feed.

Are you interested in ion chromatography applications for industrial process analysis and optimization? Did you know that you can also monitor the air quality indoors as well as in the environment with these products? Check out our selection of FREE Process Application Notes (PANs) for IC:

What’s next?

After the mid 2010s, more focus was given to the development of hyphenated techniques to support IC as part of a comprehensive analytical solution for more difficult sample matrices and analytes. In the next installment, I will discuss TitrIC, VoltIC, Combustion IC (CIC), and more, as well as what is on the horizon for the process analysis world. Stay tuned, and don’t forget to subscribe to the blog!

Visit our website

to find out more about Metrohm Inline Sample Preparation (MISP)

Post written by Dr. Markus Läubli, Manager Marketing Support IC at Metrohm International Headquarters, Herisau, Switzerland.

Trace metal analysis with solid-state electrodes – Part 3

Trace metal analysis with solid-state electrodes – Part 3

In Part 2 of this series on trace metal analysis with solid-state electrodes, we introduced the scTRACE Gold electrode. The third part of this series explains even more applications which can be performed with this electrode, but this time after modifying the gold micro-wire with a thin layer of another metal.

Catch up on the series «Trace metal analysis with solid-state electrodes» here:

Why modify the electrode material?

As explained for the Bi drop electrode in Part 1 of this series, stripping voltammetry is a two-step measurement.

In the first step, the analyte is deposited on the working electrode. In the case of anodic stripping voltammetry (ASV), the analyte is reduced and forms an alloy with the electrode material (Fig. 1). In the case of adsorptive stripping voltammetry (AdSV), the analyte forms a complex which is adsorbed to the working electrode.

In the subsequent stripping step, the deposit is brought back into solution, giving the analytical signal which is proportional to the deposited amount of analyte. In the case of ASV, the electrochemical reaction is the re-oxidation of the analyte during an anodic scan (Fig. 2). In the case of AdSV, the adsorbed metal complex is reduced during a cathodic scan.

Figure 1. Anodic stripping voltammetry (ASV) with Ag film modified scTrace Gold electrode – deposition of lead (solution stirred).

Figure 2. Anodic stripping voltammetry (ASV) with Ag film modified scTrace Gold electrode – stripping of lead (solution not stirred).

Both steps, deposition as well as stripping, are subject to the principles of kinetics and thermodynamics. Without going into detail, the result is simply that some analytes cannot be determined with certain electrode materials. One way to solve this problem is to modify an existing working electrode with a different material that is more suitable.

Applications

Lead in drinking water

Most of the lead which is present in surface and ground water is of anthropogenic origin, resulting from the leaching of contaminated soils. Lead in tap water, however, often originates from the household plumbing system. Pipes from lead metal were popular in some countries until the 1970s. Although Pb is barely soluble in water, it slowly dissolves in the presence of oxygen. As a result, the allowed limit for lead in tap water can be easily exceeded by a significant amount. Now lead pipes for municipal water transport are forbidden, but there are still houses with old installations intact. The WHO (World Health Organization) recommends a limit value for lead in drinking water of 10 µg/L. In the European Union, the upcoming limit is as low as 5 µg/L.

For the determination of lead, the scTRACE Gold electrode is modified with a silver film. The film is plated ex situ from a separate plating solution. Once plated, it can be used for multiple determinations. When the film is depleted, it can be removed, and then a fresh film is plated again. A side effect of the silver film is that the scTRACE Gold electrode lasts longer, since aging processes mainly affect the renewable silver film. In a repeatability study, determining 10 µg/L Pb with 3 different electrodes on 4 different days (total number of determinations = 10) the average recovery of Pb was 96% with a relative standard deviation of 5%.

Using the 884 Professional VA it is possible to measure lead concentrations in water down to 0.4 µg/L, allowing a simple and reliable determination of even the future limit in the European Union.

With the 946 Portable VA Analyzer, the limit of detection is only slightly higher: 0.6 µg/L. However, the mobile use offers the possibility for close monitoring of individual installations without the need to preserve samples and send them in to a central lab. Furthermore, the concerned resident gets an immediate result on the spot.

Free Application Note download: AN-V-214 Lead in drinking water – Straightforward determination by voltammetry using a gold microwire electrode.

If you want to learn more about our voltammetry product lines, lab as well as mobile, check out our website!

Nickel and cobalt in drinking water

Similar to lead, nickel concentrations present in water sources can be increased by human influence as well. Plumbing fixtures and faucets are often plated with a thin layer of nickel for protection against corrosion, even if the finish is made of chromium. Furthermore, nickel is part of many alloys from stainless steel to nickel brasses and bronze. Nickel steel alloy cookware or nickel pigmented dishes can also cause increased nickel levels. The maximum allowed level in drinking water in the European Union is 20 µg/L whereas the WHO recommends a limit of 70 µg/L.

For the voltammetric determination of nickel and cobalt, an ex situ plated bismuth film on the scTRACE Gold electrode is used as working electrode. Nickel as well as cobalt are determined in the form of their DMG (dimethylglyoxime) complex. This method had already proven its reliability with the mercury electrode, and therefore this application can now be transferred to a mercury-free electrode. With a detection limit of 1 µg/L with the 946 Portable Analyzer, and even lower at 0.2 µg/L with the 884 Professional VA lab instrument, the method is surely sufficient to monitor the compliance with legal requirements. The recovery for 1 µg/L Ni in a standard solution is about 99% (mean of 10 determinations) with a relative standard deviation of 5%.

Free Application Note download: AN-V-217 Nickel, cobalt in drinking water – Straightforward determination by voltammetry using a gold microwire electrode.

Chromium(VI) in drinking water

The problem of chromium(VI) in drinking water was brought to the attention of the general public with the movie «Erin Brockovich» in 2000, starring Julia Roberts. The plot is based on a true story, which happened in the small community of Hinkley, California, where the local energy provider contaminated the groundwater with the carcinogenic hexavalent chromium. The company attempted to cover up the incident, but an increased number of tumors and other health problems among the residents could finally be traced back to the contaminated drinking water.

Contamination with Cr(VI) in the environment is usually the result of improper handling of various industrial processes, especially abandoned waste dumped from galvanic chromium plating. The WHO recommends a maximum limit of 50 µg/L total chromium for drinking water

After modifying the scTRACE Gold electrode with an ex situ plated mercury film, Chromium(VI) can be determined as a complex with DTPA (diethylenetriaminepentaacetic acid). The recovery of a standard containing 30 µg/L Cr(VI) is 115% (mean of 3 determinations) with a relative standard deviation of 2%. Using the 946 Portable VA Analyzer it possible to determine concentrations down to 2 µg/L Cr(VI), allowing the on-site determination, providing immediate results without delay.

Free Application Note download: AN-V-230 Chromium(VI) in drinking water – Sensitive determination on the mercury film modified scTRACE Gold (DTPA method).

Summary

Talking about all of the applications that are possible with the scTRACE Gold electrode would go beyond the scope of this blog. The table here gives an overview of several elements for which methods with the scTRACE Gold are currently available from Metrohm. Your local Metrohm representative can assist in case of questions regarding the determination of one of the elements, or analysis in a specific matrix.

Figure 3. The scTRACE Gold electrode from Metrohm is suitable for trace analysis of several elements in water.

Overview: Applications with the scTRACE Gold
Element Application document
As(total) Application Note V-210
As(III) Application Note V-211
Hg Application Note V-212
Cu Application Note V-213
Pb Application Note V-214
Zn Application Note V-215
Tl Application Note V-228
Fe Application Note V-216
Ni, Co Application Note V-217
Bi Application Note V-218
Sb(III) Application Note V-229
Cr(VI) Application Note V-230

What’s next?

In Part 4 of this series, I will discuss the use of screen-printed electrodes (SPEs) which had already been introduced in the blog post «Virus detection using screen-printed electrodes». However, this time the focus will be on the determination of heavy metals using these disposable electrodes.

Post written by Barbara ZumbrägelProduct Manager VA/CVS at Metrohm International Headquarters, Herisau, Switzerland.

Forewarned is Forearmed: Error and risk minimization in process analysis – Part 3

Forewarned is Forearmed: Error and risk minimization in process analysis – Part 3

In the course of life, each of us learns to trust our gut feelings or our experiences to avoid situations that seem dangerous or risky. You quite literally sense potential dangers with an uneasy feeling. Who hasn’t painfully learned that touching a hot stove top isn’t a good idea? Or who voluntarily goes outside during a tornado?

While humans can rely on their intuition and learned patterns to avoid dangers or use protective strategies, this is far more complicated with electronic systems or machines. All components of a system must be in a permanently safe state. Failures and malfunctions of individual components can have devastating consequences for production processes and the safety of the operators.

An example of this is the Seveso disaster in 1976, in which highly toxic dioxin TCDD escaped as a result of an uncontrolled reaction, and sustainably poisoned flora and fauna. With regard to other major chemical accidents, the European Seveso III Directive then came into force in 2012 to control major accident hazards to prevent major accidents.

Have you read Part 1 and Part 2 of our «Advantages of PAT (Process Analytical Technology)» series? If not, find them here!

Recognize, master, and avoid errors

Process engineering systems that are operated continuously contain countless components that can wear out or fail during their life cycle. However, if the measuring, control, or regulating circuit is affected, failures can cause immense damage. Under no circumstances should humans nor the environment be exposed to any kind of danger. For this reason, the functional safety of the components must be guaranteed, and their risk and hazard potential must be analyzed in detail.

The service life of mechanical components can be evaluated by observing mechanical wear and tear. However, the aging behavior of electronic components is difficult to assess. A unit of measure that makes risk reduction and thus functional safety quantifiable is the so-called «Safety Integrity Level» (SIL). 

The following procedure is followed:

  1.   Risk analysis
  2.   Realization of risk reduction
  3.   Evidence that the realized risk reduction corresponds at least to the required risk reduction

«Process analysis systems are part of the entire safety cycle of a manufacturing plant and therefore only one component whose risk of malfunctions and failures must be considered in an assessment.»

Risk assessmentA process is considered safe if the current risk has been reduced below the level of the tolerable risk. If safety is ensured by technical measures, one speaks of functional safety.

Significance for process analysis systems

Errors can happen anywhere, and can never be completely excluded. To minimize possible errors, it is therefore necessary to estimate the risk of occurrence and the damage to be expected from it as part of a risk analysis. A distinction must be made here between systematic and random errors.

Systematic errors are potentially avoidable and are caused, for example, by software errors or configuration deficiencies. Accordingly, they already exist during or prior to commissioning.

In contrast, random errors are potentially difficult to avoid because they occur arbitrarily. Nevertheless, the error rate or failure probability can be determined statistically and experimentally.

Random errors usually result from the hardware and occur during operation. Ultimately, systematic errors should be avoided, and random errors should be mastered to ensure trouble-free functionality.

Process analysis systems are the link between manual laboratory analysis and the industrial process. In applications where continuous and fully automatic monitoring of critical parameters is required, process analyzers are indispensable. Due to the different analysis conditions in the laboratory and directly in the process, there are some challenges when transferring the measurement technology from the lab to the process. The decisive factors are the working and environmental conditions (e.g., high temperatures, corrosive atmospheres, moisture, dust, or potentially explosive environments) which the process analyzers have to meet regarding their design, construction materials, and reliability of the components. The analyzer automatically and continuously transmits system and diagnostic data to prevent hardware or software components from failing through preventive measures. This significantly reduces the chance of random errors occurring.

General process analyzer setup

a) Analyzer Setup

Process analyzers have been specially developed for use in harsh and aggressive industrial environments. The IP66 protected housing is divided into two parts, and consists of separate wet and electronic parts. The electronics part contains all components relevant to control and operate the process analyzer. Modular components like burettes, valves, pumps, sampling systems, titration vessels, and electrodes can be found in the analyzer wet part. Representative samples can thus be taken from the process measuring point several meters away. The analysis procedure, the methods to be used, and method calculations are freely programmable.

A touchscreen with intuitive menu navigation allows easy operation, so that production processes can be optimized at any time. The course of the measurement is graphically represented and documented over the entire determination, so that the analysis process is completely controlled. The measurement results can be generated 24/7 and allow close and fully automatic monitoring of the process. Limits, alarms, or results are reliably transferred to the process control system.

When operating the analyzer, there is a risk that software errors can lead to failures. In order to recognize this with foresight, the system delivers self-diagnostic procedures as soon as it is powered on and also during operation. This includes, e.g., checking pumps and burettes, checking for leaks, or checking the communication between the I/O controller, the human interface, and the respective analysis module.

b) Sensors

The central component of a process analyzer is the measurement technique in use. In the case of sensors or electrodes, there are several requirements such as chemical resistance, ease of maintenance, robustness, or precision which they must meet. The safety-related risk arises from the possibility if measurement sensors fail due to aging, or if they become damaged and subsequently deliver incorrect measurement results.

Failure of the electrode, contamination, or damage must be reported immediately. With online analysis systems, the analysis is performed in an external measuring cell. In addition, recurring calibration and conditioning routines are predefined and are performed automatically. The status of the electrode is continuously monitored by the system.

Between measurements, the electrode is immersed in a membrane-friendly storage solution that prevents drying out and at the same time regenerates the swelling layer. The electrode is therefore always ready for use and does not have to be removed from the process for maintenance. This enables reliable process control even under harsh industrial conditions.

c) Analysis

Process analyzers must be able to handle samples for analysis over a wide concentration range (from % down to trace levels) without causing carry-over or cross-sensitivity issues. In many cases, different samples from several measuring points are determined in parallel in one system using different analysis techniques. The sample preparation (e.g., filtering, diluting, or wet chemical digestion) must be just as reliable and smooth as the fully automatic transfer of results to the process control system so that a quick response is possible.

Potential dangers for the entire system can be caused by incorrect measurement results. In order to minimize the risk, a detector is used to notify the system of the presence of sample in the vessel. The testing of the initial potential of the analysis or titration curves / color development in photometric measurements are diagnostic data that are continuously recorded and interpreted. Results can be verified by reference analysis or their plausibility can be clarified using standard and check solutions.

Detect errors before they arise

The risk assessment procedures that are carried out in the context of a SIL classification for process engineering plants are ultimately based on mathematical calculations. However, in the 24/7 operation of a plant, random errors can never be completely excluded. Residual risk always remains. Therefore, the importance of preventive maintenance activities is growing immensely in order to avoid hardware and software failures during operation.

A regular check of the process analyzer and its diagnostic data is the basic requirement for permanent, trouble-free operation. With tailor-made maintenance and service concepts, the analyzer is supported by certified service engineers over the entire life cycle. Regular maintenance plans, application support, calibration, or performance certificates, repairs, and original spare parts as well as proper commissioning are just a few examples.

Advantages of preventive maintenance from Metrohm Process Analytics

  • Preservation of your investment
  • Minimized risk of failure
  • Reliable measurement results
  • Calculable costs
  • Original spare parts
  • Fast repair
  • Remote Support

In addition, transparent communication between the process control system and the analyzer is also relevant in the context of digitalization. The collection of performance data from the analyzer to assess the state of the control system is only one component. The continuous monitoring of relevant system components enables conclusions to be drawn about any necessary maintenance work, which ideally should be carried out at regular intervals. The question arises as to how the collected data is interpreted and how quickly it is necessary to intervene. Software care packages help to test the software according to the manufacturer’s specifications, to perform data backup and software maintenance.

«Remote support is particularly important in times when you cannot always be on site.»

In real emergency situations in which rapid error analysis is required, manufacturers can easily support the operator remotely using remote maintenance solutions. The system availability is increased, expensive failures and downtimes are avoided, and the optimal performance of the analyzer is ensured.

Read what our customers have to say!

We have supported customers even in the most unlikely of places⁠—from the production floor to the desert and even on active ships!

Post written by Dr. Kerstin Dreblow, Product Manager Wet Chemical Process Analyzers, Deutsche Metrohm Prozessanalytik (Germany).

History of Metrohm IC – Part 2

History of Metrohm IC – Part 2

In the second part of our series behind the development of high quality ion chromatography instrumentation at Metrohm, I will cover the mid 1990s until the mid 2000s. During this time, Metrohm focused on modular IC, lowering background suppression, as well as bringing further robust detection methods on to the market.

Did you miss Part 1? Click here to read the first part of our series on the history of ion chromatography at Metrohm:

«The 1990‘s. People start to care about the environment. Authorities impose quantitative limits on the presence of many substances, most of which must be detected down to trace levels. Metrohm builds the perfect tool for this: the 761 Compact IC.»

Dr. Helwig Schäfer, retired Head of R&D Ion Chromatography, Metrohm AG

2nd generation: The modular IC system – 1996

While the Labograph was soon replaced by integrators (initially with integrators and later on by PC-based integration tools), the conductivity detector stood unbeaten for a long period. Improvements to the system setup, as well as additional liquid handling tools and automation capabilities yielded the second generation of Metrohm IC: the modular system.

At the same time, the initial patents on chemical suppression were about to expire, allowing the possibility to begin the development of the Metrohm Suppressor Module.

Metrohm Suppressor Module (MSM)

The idea for the MSM is based on the suppression column as described in the paper by Small, Stevens, and Baumann [1]. Its main purpose is to remove the eluent conductivity after the separation and prior to the conductivity detection. Thus, the eluent needs to be convertible to water by ion exchange.

In the case of anion chromatography, sodium hydroxide is an example of such a candidate. By replacing sodium by a proton through ion exchange, the eluent is converted to water alone. The authors applied a suppressor column of opposite charge (compared to the analytical column) after the analytical column [1].

The Metrohm Suppressor Module.

As with all things, suppressor columns do have a couple of disadvantages. They have to be externally regenerated on occasion. Depending on the amount of cations already bound to the suppressor column, its separation and ion-exclusion behavior is modified. This leads to changes in retention times of the ions, especially regarding the carbonate peak, which shifts quite strongly and interferes with other peaks of interest. On the other hand, one of the most positive points of suppressor columns is their ruggedness.

Metrohm was looking for solutions to the disadvantages without compromising the ruggedness of this column-based approach.

To overcome the shifting retention time over the usage of suppressor columns, the dimensions of the column were reduced dramatically. This yielded in a small cartridge-like compartment. The exchanger capacity needed to stay high enough for running, minimally, one single chromatogram. Under the precondition that only one chromatogram is suppressed with a single suppressor compartment, in this way all determinations have exactly the same conditions and no retention time shifts can occur.

Now it was required to regenerate the suppressor compartment prior to the next sample injection. Here, the idea of a rotating unit with three compartments was born. 

All three compartments are connected to a liquid stream: i.e. unit 1 suppresses the eluent conductivity in the analytical stream, unit 2 is being regenerated with acid, and unit 3 is rinsed (acid-free) with ultrapure water or with the detector effluent (now known as STREAM). Prior to each injection, the MSM rotor is switched by one position. In this way, each injection uses its own freshly regenerated and rinsed suppressor unit.

The final suppression setup was launched as the 753 Suppressor Module in 1996 together with the modular system consisting of the 732 Conductivity Detector, 709 IC Pump, 733 IC Separation Center, and the 766 IC Sample Processor plus further liquid handling modules. Together with IC Net, the PC-based data acquisition and handling software, full automation of the ion chromatographic system was available

The Metrohm 753 Suppressor Module. 
Modular IC at Metrohm, circa 1996.

While modular IC was extremely flexible and opened up possibilities for a high grade of automation opportunities, it also was quite complex for straightforward, everyday applications.

This routine IC required for general users was introduced in 1999 as the first all-in-one ion chromatograph – the 761 Compact IC. It was the ideal instrument to run standard applications on directly due to the integration of all basic components required for IC analysis. These included: IC pump, injector, Metrohm Suppressor Module with peristaltic pump for regeneration (when required) and rinsing and the conductivity detector. The 761 Compact IC was the first instrument available in only a metal-free version.

The Metrohm 761 Compact IC. 

IC with built-in amperometric detection

The initial 641 VA Detector and its successor the 791 Amperometric Detector were electronic high-performance instruments requiring a quite high level of knowledge in electrochemistry. Handling and maintenance were not easy tasks, however, analysts which were familiar with these products were extremely happy.

The Metrohm 641 VA Detector and its successor, the 791 Amperometric Detector.

By then, setting voltages manually, as well as compensating the background with potentiometers was outdated. Therefore, Metrohm introduced the 817 Bioscan in 2001.

The Metrohm 817 Bioscan.

It was based on the concept of Compact IC. The 817 Bioscan consisted of the amperometric detector used mainly for Pulsed Amperometric Detection (PAD) applications, a built-in column oven, the 812 Valve Unit (injector), and the 709 IC pump. This was Metrohm’s entry to the analysis of sugars.

The 791 Amperometric Detector (introduced in 1998 as the successor of the of the 641 VA Detector), was still dedicated for use as the ideal detector for applications applying DC amperometric detection.

3rd generation: Advanced Modular IC – 2003

In 2003, Metrohm introduced the «Advanced Modular IC» system, featuring the same modularity and remote control concept as the previous «Modular IC», but with improved capabilities added to the individual modules. Both the data acquisition and remote control were still managed by the IC Net software.

Around the same time period, the 811 Online IC was developed, as a more suitable instrument for the harsh environmental conditions of industrial production processes. Weighing in at approximately 450 kg, this heavyweight was built with a top-of-the-line Metrohm modular IC system and was controlled by IC Net software, coming in two versions: single channel as well as a dual channel version to measure both anions and cations. This process analyzer was combined with a modular wet part setup, which allowed the use of various modules (e.g., 10-way sampling valve or tubing pump), so the IC could be fully customized to meet customer requirements for any application.

The 811 Online IC (2001) and its successor, the 821 Compact Online IC (2002).

Due to the success of the 811 Online IC, in 2002 a smaller version was introduced: the 821 Compact Online IC. It was commonly referred to as the «little brother» due to its lighter weight and reduced size.

In 2005, the 861 Advanced Compact IC was introduced to the laboratory world, and in the same year the 844 UV/VIS Compact IC was placed on the market. This was both the first Metrohm UV/VIS IC as well as the first all-in-one UV/VIS ion chromatograph. It was dedicated to direct as well as post-column reaction applications with photometric detection. The 844 UV/VIS Compact IC was complementary to the Bischoff Lambda 1010, used in modular systems as an optional optical detector.

The Metrohm 844 UV/VIS Compact IC (front view).
The Metrohm 844 UV/VIS Compact IC (inside view).

What’s next?

In Part 3, I will continue into the later 2000s and beyond, covering the evolution of sequential suppression (the combination of chemical suppression and CO2 suppression) in addition to the 4th and 5th generations of Metrohm ion chromatography.

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Download our free White Paper for more information about suppression

When HPLC fails: IC in food, water, and pharmaceutical analysis

Reference

[1] Small, H.; Stevens, T.S.; W.C. Baumann. Novel ion exchange chromatographic method using conductimetric detection. Anal. Chem. 1975, 47 (11), 1801–1809. https://doi.org/10.1021/ac60361a017

Post written by Dr. Markus Läubli, Manager Marketing Support IC at Metrohm International Headquarters, Herisau, Switzerland.

Frequently asked questions in Karl Fischer titration – Part 2

Frequently asked questions in Karl Fischer titration – Part 2

Since I started working at Metrohm more than 15 years ago, I have received many questions about Karl Fischer titration. Some of those questions have been asked repeatedly from several people in different locations around the world. Therefore, I have chosen 20 of the most frequent questions received over the years concerning Karl Fischer equipment and arranged them into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Part 1 covered instrument preparation and handling, and Part 2 will now focus on titration troubleshooting and the KF oven technique.

Summary of questions in the FAQ (click to go directly to each question):

Titration troubleshooting

1.  If the drift value is 0, does this mean that the titration cell is over-titrated?

A drift of zero can be a sign that the cell might be over-titrated. In combination with the mV signal (lower than end-point criteria) and the color of the working medium (darker yellow than usual), it is a clear indicator for over-titration. However, volumetric titrations sometimes exhibit a zero drift for a short time without being over-titrated. If you have a real excess of iodine in the titration cell, the result of the next determination will most likely be erroneous. Therefore, over-titration should be avoided. There are various possible reasons for over-titration, like the sample itself (e.g., oxidizing agents which generate iodine from the working medium), the electrode (coating or invisible depositions on the Pt pins/rings), the reagent, and method parameters (e.g., the titration is rate too high), to name just a few.

2.  Should I discard the Karl Fischer reagent immediately if it turns brown?

Different factors can cause over-titration, however, the reagent is not always the reason behind this issue. The indicator electrode can also be the reason for overshooting the endpoint. In this case, regular cleaning of the electrode can prevent over-titration (see also questions 7 to 9 from Part 1 in this series on cleaning).

A low stirring speed also increases the risk of over-titration, so make sure the solution is well mixed. Depending on the type of reagent, the parameters of the titration need to be adjusted. Especially if you use two-component reagents, I recommend decreasing the speed of the titrant addition to avoid over-titration. Over-titration has an influence on the result, especially if the degree of over-titration changes from one determination to the next. So over-titration should always be avoided to guarantee correct results.

3.  What is drift correction, and when should I use it?

I recommend using the drift correction in coulometric KF titration only. You can also use it in volumetric titration, but here the drift level is normally not as stable as for coulometric titrations. This can result in variations in the results. A stabilization time can reduce such an effect. However, compared to the absolute water amounts in volumetry, the influence of drift is usually negligible.

4.  My results are negative. What does a negative water content mean?

Negative values do occur if you have a high start drift and a sample with a very low water content. In this case, the value for drift correction can be higher than the absolute water content of the sample, resulting in a negative water content.

If possible, use a larger sample size to increase the amount of water added to the titration cell with the sample. Furthermore, you should try to reduce the drift value in general. Perhaps the molecular sieve or the septum need to be replaced. You can also use a stabilizing time to make sure the drift is stable before analyzing the sample.

Karl Fischer oven

5.  My samples are not soluble. What can I do?

In case the sample does not dissolve in KF reagents and additional solvents do not increase the solubility of the sample, then gas extraction or the oven technique could be the perfect solution.

The sample is weighed in a headspace vial and closed with a septum cap. Then the vial is placed in the oven and heated to a predefined temperature, leading the sample to release its water. At the same time, a double hollow needle pierces through the septum. A dry carrier gas, usually nitrogen or dried air, flows into the sample vial. Taking the water of the sample with it, the carrier gas flows into the titration cell where the water content determination takes place.

6.  Can all types of samples be analyzed with the oven method?

Many samples can be analyzed with the oven. Whether an application actually works for a sample strongly depends on the sample itself. Of course, there are samples that are not suitable for the oven method, e.g., samples that decompose before releasing the water or that release their water at higher temperatures than the maximum oven temperature.

7.  How do I find the optimal oven temperature for water extraction?

Depending on the instrument used, you can run a temperature gradient of 2 °C/min. This means it is possible to heat a sample from 50 to 250 °C within 100 minutes. The software will then display a curve of water release against temperature (see graph).

From such a curve, the optimal temperature can be determined. Different peaks may show blank, adherent water, different kinds of bound water, or even decomposition of the sample.

This example curve shows the water release of a sample as it has been heated between 130 and 200 °C. At higher temperatures, the drift decreases to a stable and low level.

Generally, you should choose a temperature after the last water release peak (where the drift returns to the base level) but approximately 20 °C below decomposition temperature. Decomposition can be recognized by increasing drift, smoke, or a color change of the sample. In this example, there are no signs of decomposition up to an oven temperature of 250 °C. Therefore, the optimal oven temperature for this sample is 230 °C (250 °C – 20 °C).

In case the instrument you use does not offer the option to run a temperature gradient, you can manually increase the temperature and measure the sample at different temperatures. In an Excel spreadsheet, you can display the curve plotting released water against temperature. If there is a plateau (i.e., a temperature range where you find reproducible water contents), you have found the optimal oven temperature.

8.  What is the highest possible water content that can be measured with a Karl Fischer oven?

Very often, the oven is used in combination with a coulometric titrator. The coulometric titration cell used in an oven system is filled with 150 mL of reagent. Theoretically, this amount of reagent allows for the determination of 1500 mg of water. However, this amount is too high to be determined in one titration and it would lead to very long titration times and negative effects on the results. We recommend that the water content of a single sample (in a vial) should not be higher than 10 mg, ideally around 1000–2000 µg water. For samples with water contents in the higher percentage range, you should consider the combination with a volumetric titrator.

9.  What is the maximum sample size that can be used with the oven? If I use too much sample, will the needle be blocked?

The standard vial for the oven method has a volume of approximately 9 mL. However, we do not recommend filling the vial completely. Do not fill more than 5–6 mL of sample in a vial. We offer the possibility to customize our oven systems, allowing you to use your own vials. Please contact your local Metrohm agency for more information on customized oven systems.

For liquid samples, we recommend using a long needle to lead the gas through the sample. Solid samples and especially samples that melt during analysis require a short needle. The tip of the needle is positioned above the sample material to avoid needle blockage.

Additionally, you should use a «relative blank value», i.e., taking only the remaining air volume into account for blank subtraction. You can find more information about the relative blank and how to calculate it in Application Note AN-K-048.

10.  What is the detection limit of the oven method, and how much sample is required to analyze a sample with 10 ppm (mg/L) water content?

We recommend having at least 50 µg of water in the sample, if analyzed with coulometry. However, if conditions are absolutely perfect (i.e., very low and stable drift plus perfect blank determination), it is possible to determine even lower water contents, down to 20 µg of absolute water. For a sample with a water content of < 10 ppm (mg/L), this would correspond to a sample size of at least 2 g.

11.  How do I verify an oven method?

For the verification of an oven system, you can use a certified water standard for oven systems. With such a standard, you can check the reproducibility and the recovery. There are a few types of standards available for different temperature ranges.

I hope this collected information helps you to answer some of your most burning KF questions. If you have further unanswered questions, do not hesitate to contact your local Metrohm distributor or check out our selection of webinars.

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Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.