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History of Metrohm IC – Part 6

May 16, 2022

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This article is Part 6 of a series.

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Part 1Part 2Part 3Part 4

Even though combustion ion chromatography (CIC) is regarded as an up-and-coming sample preparation and analysis technique, the basic process behind CIC has already existed for many years. This blog post introduces the history of this technique, the working principle, and some applications for CIC.

The beginnings of combustion IC (CIC)

Our previous blogs about the history of Metrohm ion chromatography (Parts 1–3) outlined how IC has become one of the most used analytical techniques for the analysis of inorganic anions and cations in a wide variety of aqueous media.
 

History of Metrohm IC – Part 1

History of Metrohm IC – Part 2

History of Metrohm IC – Part 3


In the mid-1970s the impact of organic halogens and sulfur became a topic of intensified focus, as these compounds were shown to increase ozone destruction and adversely impact the environment [1]. In addition, they are corrosive and can threaten human health during water treatment processes [2,3].

Most organic halogens are not water-soluble, therefore decomposition is necessary as a first analytical step [4–6]. Using combustion as a sample preparation method to decompose such compounds and to enable subsequent sulfur determination [6] in a closed system (i.e., «combustion bomb») under pressurized oxygen atmosphere began in 1881. In 1955, Schöninger developed the first convenient handling of the combustion process—the so-called «oxygen flask» [7–9].

The basic principle of Schöninger-based analytical methods is to burn a certain amount of sample in oxygen-rich atmosphere. The resulting gases are bubbled through an absorber solution which is then transferred to the analytical instrument for measurement (commonly microcoloumetric titration) [2,5,7]. Between samples, the container must be cleaned extensively to avoid cross-contamination [7]. However, these methods did not have the possibility to be automated. Over time, the once dangerous procedure was modified to be much safer. Still, the manual process of preparing samples with extensive rinsing steps in between remained cumbersome and time-consuming.

Around the same time, pyrohydrolysis was established for analytical purposes by Warf [10,11] as «high temperature hydrolysis» to measure halogens, boron, and sulfur especially in geological samples [12]. As IC was already established as a highly sensitive technique for measuring halogens and sulfur, a combination of combustion with IC was introduced as a possibility for fast, accurate, and sensitive multi-element analysis. High sensitivities could already be achieved by combining the oxygen-bomb combustion method with IC [13], but pyrohydrolysis with combustion ovens enabled development of fully automated procedures [14].

The combustion process

The overall combustion process for major application fields such as AOX (adsorbable organic halogens), halogen, or sulfur determination in various matrices was improved upon, culminating in a complete inline connection of automated combustion ovens. In this automatic setup (Figure 1), the sample (liquid, solid, or gaseous) is introduced into the oven and subsequently combusted at high temperatures in a water/oxygen environment. The combustion gases are continuously fed through an absorber vessel. There, they are passed through an aqueous absorber solution where the volatile halogens and sulfur are captured and oxidized.

Classically, the absorber solution was analyzed using colorimetric titration in the case of AOX (e.g., ISO 9562:2004, DIN 38414-18:2019, or EPA 1650) or sulfur (e.g., ASTM C816-85 or [5]), or via potentiometric titration with ion-selective electrodes, e.g., for fluoride [5]. However, combining the combustion module with an IC revolutionized the field as detailed information about the analytes was now possible [15]. Halogens and sulfur are quantified individually, and additionally, analysts get fluoride results (DIN 38409-59) – a parameter which the classical techniques had trouble with.