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 4^th and 5^th 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.

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.

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.

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.

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.

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.

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.

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

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

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.

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. 

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.

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 4^th and 5^th generations of Metrohm ion chromatography.

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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.

Ion chromatography (IC) has been a part of the Metrohm portfolio of analytical chemical instrumentation since 1987, and in that span of 33 years, several new and exciting developments have been introduced challenging the limits of what IC can do. From simple setups for academic laboratories, to hyphenated techniques (e.g., IC-ICP-MS) broadening the capabilities of chemical analysis – we’ve done it! This week, I would like to begin to unveil the history of this analytical method at Metrohm and how it has changed over the intervening decades.

The Metrohm 636 Titroprocessor.

You can see how this is measured in the video below. Other detection techniques can be used as well, but typically are applicable only in special cases.

The modernized, compact, and intelligent Metrohm IC Conductivity Detector.

The Metrohm 641 VA Detector.

In 1984, a test was run on an initial setup consisting of a single-piston HPLC pump, a 6-port injector, commercially available IC separation columns, a conductivity detector, and a chart recorder (586 Labograph). This test proved that the 1 mg/L limit could be reached, and thus the project of developing an official Metrohm conductivity detector began.

At that time, chemical suppression introduced by Small, Stevens, and Baumann [1] was patented and not available. However, non-suppressed conductivity detection described by Gjerde, Schmuckler, and Fritz [2] was seen as a viable alternative. When measurement of low concentrations of ions in solution was necessary, the very small chromatographic peaks plus the high conductivity background from the mobile phase (eluent) created a challenge, and special requirements for the conductivity detector had to be taken into account. The most critical of these was the temperature coefficient of the conductivity, which is typically around 2%/°C. This requires maintaining an extremely stable temperature during the measurement.

During the initial development phase it was found that, aside from bulk measurement, platinum was not the best material for electrodes in a flow-through cell. However, stainless steel worked perfectly. The measuring cell still needed to be insulated, however, insulation alone was not sufficient. Active thermostating was required to achieve a temperature stability of better than 0.01 °C. That stability was measured with a thermocouple, and recorded on the Labograph. Later on, with more sophisticated tools the stability was determined to be better than 0.001 °C.

Even after all of this hard work, the initial system baseline stability was still not good enough. As it turned out, several components of the IC system needed to be thermally stabilized. Additionally, the different brand of HPLC pump was not optimal for the development of the Metrohm ion chromatograph.

The first decision was to put the conductivity detector project to the side, and start building an ion chromatograph. Thus, the first Metrohm IC (the 690 Ion Chromatograph) was developed. The 690 IC consisted of: a foam polymer housing for perfect thermal insulation, the electronic and detector block, as well as a pulse dampener, a sample injector, and separation column. All capillary connections consisted of HPLC capillaries at the time (made from stainless steel). The inadequate HPLC pump was replaced and upgraded with a Metrohm IC Pump, and the Labograph was almost immediately followed by an integrator, which completed the IC system.

Despite the general consensus in the 1980s that ion chromatography was only robust while using metal-free instruments, Metrohm was able to run anion, cation, and ion-exclusion chromatography on stainless steel-based systems. Even determinations of heavy metals were performed without issues.

Conductivity detection with «electronic suppression»

A drawback of non-suppressed IC is the relatively high inherent baseline noise, due to high conductivity levels from the mobile phase. Parameters which add to this baseline noise include temperature induced fluctuations, pump noise, and electronic noise.

Internal view of the Metrohm 690 IC. The conductivity detector is highlighted.

The thermostated detector block consisted of an aluminum block for thermostating, a built-in measuring cell, and an electronic preamplifier. This preamplifier guaranteed that the measured analog conductivity signal was insensitive to external fields when guided to the main electronics.

The Auto Zero function measured the actual conductivity at initialization of the function and was subtracted from the signal throughout the chromatogram. This can be called background compensation. The «electronic suppression» designation is given due to an electronic setup which additionally reduced the electronic noise. The idea behind this is as simple as it was effective. The electronics were set to measure the actual conductivity signal as well as the measured background conductivity through two parallel paths with identical electronic components. Subtraction of the two signals was done just prior to the output to the external A/D converter. Under an assumption that the same components should add the same noise and exhibit similar thermal behavior, both signals are influenced in the same manner. Therefore, the noise level was minimized even further.

Additionally, the apparent noise level was improved using the optimal output window (called «Full-scale») in units of [µS/cm]. The Metrohm Application Note AN-C-032 describes this effect. At that time, this noise level of approximately 2 nS/cm was similar to or better than analyses performed with chemical suppression.

Separation column developments

At market launch in late 1987, Metrohm offered a total of six IC separation columns: two suitable for anions, one for monovalent cations, one for divalent cations, and one for organic acids (ion-exclusion). At that time, the group of Prof. Dr. Schomburg (Institut für Kohlenforschung, Mühlheim/Ruhr, DE) studied the preparation of HPLC phases by coating polymer materials on to e.g. silica. One of the phases used was poly(butadiene/maleic acid) on a silica material, which was found to be able to separate mono- and divalent cations in a single isocratic run. Metrohm acquired the technology and started column production in Herisau, Switzerland.

The so-called «Schomburg column» or later «Super-Sep Cation column» was the very first column on the market allowing the simultaneous separation of alkali and alkaline earth metal cations. Even the current Metrosep C 4 and Metrosep C 6 columns’ roots date back to the Schomburg column.

Data handling capabilities

In the first months on the market, only the Labograph (a chart recorder) was available for the new IC. This was of course not really acceptable. Nevertheless, results achieved by cutting out and physically weighing the peaks were quite correct. The first integrator (Shimadzu C-R5A) was a tabletop integrator with LCD display (2 lines), storage capabilities (2 chromatograms in the instrument, and 5 chromatograms per external card), and a thermo-printer for documentation.

Top: Metrohm 690 Ion Chromatograph with Labograph on the left, and separation columns in the foreground.

Bottom: Metrohm 690 Ion Chromatograph with the Shimadzu C-R5A tabletop integrator on the left.

The 690 IC featuring the 714 IC-Metrodata, ushering scientists into a new era of peak integration possibilities.

What’s next?

Stay tuned for the next installment in this series, covering the 1990s and early 2000s. During this time, Metrohm developed modular IC, the Metrohm Suppressor Module (MSM), as well as some outstanding separation columns. Subscribe to the blog below so you don’t miss out!

References

[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

[2]  Gjerde, D. T.; Fritz, J. S.; Schmuckler, G. Anion Chromatography with Low-Conductivity Eluents. J. Chromatogr. A 1979, 186, 509–519. https://doi.org/10.1016/S0021-9673(00)95271-3

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