Each year, the challenges faced by environmental analysis experts increase. Presently, the focus is on highly diverse and partially persistent organic fluorine compounds (e.g. trifluoroacetic acid) as well as analysis of particularly toxic types of metals such as chromium(VI). Another current field of exploration is the ongoing analysis of toxic oxohalides like bromate and perchlorate.
There are multiple ways in which to achieve the lowest possible determination limits for the targeted trace parameters/analytes in the presence of high concentrations of chloride, nitrate, carbonate or sulfate often found in the complex environmental matrices. One such of these is by coupling chromatography systems with high-sensitivity, extremely specific mass detectors  – which is a long-established procedure in organic trace analysis.
The vastly more affordable conductivity or UV/Vis detectors can assist in many trace analysis tasks in the ion chromatography field. Larger injection volumes are sometimes required in order to achieve the required determination limits. These can only be supplied by optimized and intelligent sample preparation techniques.
Innumerable customers have experienced the value of Metrohm’s Inline Sample Preparation for decades, in their daily work in areas such as environmental analysis. For over twenty years, customers have benefitted from Inline Dialysis, which offers clear benefits compared to manual sample preparation techniques. In addition, Inline Ultra-Filtration and Inline Dilution have long been relied upon in combination with Metrohm ion chromatographs. These techniques offer the advantages of analyzing complex matrices, maximizing precision and accuracy of analysis results and reducing time spent and cost of consumables.
Table 1. Summary of the three main Inline Sample Preparation Techniques
|Fully automatic separation of high-molecular matrix components and particles for protecting the separating column using patented Metrohm Stopped Flow Dialysis.
||Fully automatic filtration (0.2 µm) of samples during sample introduction.
||Fully automatic dilution system can be based on logical software decisions, combinable with Inline Dialysis an Inline Dilution
Figure 1. Working principle of patented Stopped Flow Dialysis: During the whole sampling process, the charged sample is continiously passed through the sample side of the Dialysis Cell, to the left of the membrane. On the opposite side of the membrane, an acceptor solution is sitting idle in the dialysis cell and is being charged with the ions passing through the membrane due to the existing concentration gradient. This process is only stopped when an equilibrium is established, and thus the concentration of the acceptor solution matches that of the original sample. Finally the acceptor solution is injected directly and fully automatically into the IC.
Current Applications in Water and Environmental Analysis
The new ISO 19340:2017-10 used to determine dissolved perchlorate in water samples (by means of ion chromatography) was published in October 2017 . Public attention has recently turned to perchlorate, an ionic contaminant of food which can additionally be found in drinking water. Perchlorate can have harmful effects on health, by inhibiting iodine uptake in the thyroid gland. It poses a particular risk to those who have iodine deficiency or thyroid disease, as well as to newborns and young children .
Germany, unlike other countries, presently has no threshold values for perchlorate content in drinking water. Using ion chromatography, trace levels of perchlorate can be determined in drinking water next to higher concentrations of ions such as chloride, nitrate and sulfate. Detection sensitivity can be further improved, apart from injecting the sample directly, by using a special re-injection technique, as is described in ISO 19340:2017-10, Annex B.
The technique which is described here merely requires an additional injection valve in addition to the classical ion chromatograph, unlike comparable procedures which use two coupled chromatographs. . This, in many ways, is a simplification of the entire set-up.
Figure 2. Set-up of an IC system for re-injection analysis
To determine traces of TFA (trifluoroacetic acid), the environmental pollutant, re-injection analysis can also be used . It has been described as a non-relevant metabolite of the pesticide Flurtamone. The German Federal Environmental Agency has assigned a health protection benchmark (GOW) for Flurtamone; namely of 3 µg/L in drinking water . It is vital to study and monitor the quantity of TFA which had the potential to escape into the environment through the decomposition of coolants and other fluorine compounds.
This significant solvent is stable, mobile and highly water-soluble, all of which are properties which could lead to contamination of bank filtrates and aquifers. Reinjection analysis permits analysis of trifluoroacetic acid in different water samples in the lower µg/L range (e.g. for monitoring the GOW) with a simple IC system, which is simply extended with the use of an additional 6-port valve . Other procedures resulting in comparable trace analysis are typically associated with a far greater use of apparatus (e.g. an IC-MS coupling).
Figure 3. Chromatogram (re-injection) of a water sample prior to water treatment, result 4.8 µg/L TFA
Along with TFA, over 3000 other per- and polyfluorinated substances might appear as contaminants in water and other various environmental samples . A standard for determining organically bound fluorine after adsorption to active charcoal via combustion ion chromatography (CIC) is currently being developed as a project of the DIN Standards Committee NA-119-01-03. (Adsorbable organic fluorine compounds, AOF). The hope is that this will be providing a sum parameter allowing for the estimation of the total contamination of water samples with organic fluorine compounds.
The procedure, which has been developed in connection with AOX determination, incorporates the feeding of a 100 mL of a water sample though columns packed with active charcoal. The active charcoal, following a subsequent flushing stage optimized to remove inorganic components, is transferred into a quartz or ceramic boat and combusted in an oxygen stream with addition of water (a technique called hydropyrolysis).
The combustion gases are passed through an adsorption solution, which precedes its injection into an ion chromatography system. This technique, as well as allowing the determination of organic fluorine compounds, additionally permits the detection of adsorbable chlorine, iodine and bromine compounds. The CIC, or Metrohm combustion ion chromatography system, which is used for combustion and analysis, is a fully-automated combination of inline sample preparation and ion chromatography. This combination is then used within various fields for standard analysis of halogens and sulfur following combustion. The Metrohm chromatography software MagIC Net controls the whole system. A flame sensor is used which automatically controls and adjusts the passage of the sample boat through the combustion oven, in order to guarantee complete combustion.
Figure 4. Metrohm Combustion IC system for determining adsorbable organic fluorine compounds (AOF)
Chromium (V) is one ionic trace parameter which has garnered much attention within the department of drinking water analysis. The Federal Environmental Agency commissioned a report, some years ago, on the «Potential harmfulness of chromium» . This report then led to proposals of a new threshold value for chromium(VI) of 0.3 µg/L. The current Drinking Water Ordinance for total chromium is a threshold value of 50 µg/L.
For distinguishing chromium (V) from the less toxic chromium (III) anion exchange chromatography is excellently suited. After separation, detection is carried out by post-column derivatization with diphenyl carbazide, in order to achieve a high detection sensitivity, after which it is followed by VIS detection at 538 nm of the red color formed.
For many years this procedure has proved useful to chromate determination in leather extracts  and for determining hexavalent chromium in migration specimens from toys . It additionally permits detection thresholds in water sample analysis in the region of 0.02 µg/L chromium(VI). EPA 218.7  has a description of this method, amongst others, and provides a good cost-efficient alternative to IC-ICP-MS coupling.
The examples that have been detailed in this article demonstrate how ion chromatography methods with inline sampling preparation (TFA, perchlorate, AOF), or specialized detection techniques (chromium(VI)) can be used in modern trace analysis of ions in the environmental field.
In comparison to the expensive coupling techniques, the procedures are often a far more cost-efficient alternative. The high automation, robustness, and low consumption costs involved within these systems renders them ideal for routine analysis, in connection with large sample volumes.
- Andreas Seubert, Andrea Wille, IC-MS und IC-ICP-MS – Anwendungen und Perspektiven, Werkzeug für die Wasseranalytik, CLB Chemie in Labor und Biotechnik, 57th year, Number 09-10/2016, pp. 376-379
- ISO 19340:2017-10: Water quality – Determination of dissolved perchlorate – Method using ion chromatography (IC)
- German Federal Institute for Risk Assessment: Gesundheitliche Bewertung von Perchloratfunden in Lebensmitteln; Opinion No. 022/2013, June 28 2013; https://www.bfr.bund.de/cm/343/gesundheitliche-bewertung-von-perchloratfunden-in-lebensmitteln.pdf (Version 08.12.2015)
- Thomas Kolb, Andreas Röhrig: Umweltkontaminate TFA, LABO 7-8/2017, pp. 12-13
- German Federal Environmental Agency: Gesundheitliche Orientierungswerte (GOW) für nichtrelevante Metaboliten (nrM) von Wirkstoffen aus Pflanzenschutzmitteln (PSM); Update: 2017; 01.17.2017; (Version 04.04.2018)
- Metrohm Germany; Application Work AW IC DE8-0988-042017; 04.26.2017
- Per- und Polyfluorierte Chemikalien (PFC); 01.26.2016; https://www.umweltbundesamt.de/themen/chemikalien/chemikalien-reach/stoffgruppen/perpolyfluorierte-chemikalien-pfc#textpart-1 (Version 03.29.2018)
- German Federal Environment Agency: Bedeutung von Chrom im Trinkwasser; March 2014; (Version 03.29.2018)
- Claudia List-Romanelli: Chrom VI – Bestimmung in Leder mit Ionenchromatographie und gekoppelter In-line Dialyse, GIT Labor-Fachzeitschrift 2/2006, S.120-122
- Dominik Schwarzer und Peter Krebs: Chrom: Traum oder Albtraum? – Bestimmung von Chrom(VI) in Trinkwasser, Spielzeug und Leder; Deutsche Lebensmittel Rundschau DLR, 3/2015, S.112-115
- EPA 218.7: Determination of hexavalent chromium in drinking water by ion chromatography with post-column derivatization and UV-visable spectroscopic detection
This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.
For more information on this source, please visit Metrohm AG.