Ion Chromatography (IC)

Contributors: Catherine H. Stephens, David Thickett

Overview

Ion chromatography (or ion exclusion chromatography or IC) is a derivative of liquid chromatography (LC). It uses columns with different affinities to sets of ions, so they are separated passing through the column and then most commonly conductivity measurement to generate a series of peaks with the position characteristic of the ion and the area proportional to the concentration.

Details

Ion chromatography has been used in heritage science to measure a variety of ions. Most commonly salt and organic acid anions and salt cations for pollution or soluble salt analyses in stone, brick, mortar, ceramic. Ions in corrosion products have also been measured including metal cations with UV/vis detectors. Low molecular weight sugars from paper degradation have been measured with pulsed ampometric detectors for environmental dosimetry and Oddy tests. Detection limits are of the order of 10ppt, depending on the ion and conditions used. Many systems do not detect carbonates, although systems are available that will. Ion chromatography is normally run fully quantitatively using certified standard solutions.

Samples

The sample must be a liquid and many systems can only tolerate low concentrations of organic solvents. The sample is destroyed. It is an invasive technique as samples have to be removed. Many researchers remove salts from glass by swabbing the surface with a water/IMS mixture. Analyses can be run on samples down to 5microlitres of solution. A typical sample runs for 10-30minutes, but several dilution standards are required for quantification. Samples are generally dissolved in 18.2MΩ cm-1 water and passed through 50µm filters (CEN BS16455 2014).

Applications

Ion chromatography has been extensively used for airborne pollution analysis, its sensitivity to and quantification of formic acid providing an important adjunct to many MS based methods (Gibson et al 1997). It has been used to quantify soluble salt loads from deposited particulates. It is frequently used to quantify soluble salts in porous materials such as stone, brick, mortar and ceramic (CEN BS16455 2014). It is also used to help identify (particularly organic acid) corrosion products and salts. Its tolerance to strong acid means acid solutions of corrosion products can be readily identified. Low molecular weight sugar columns and pulsed amperometric detectors developed industrially have been used to measure deterioration of paper, both after exposure in an environment and from modified Oddy tests (Breituing and Wiggins 2014, Wiggins and Breiting 2014, Vulpi et al 2020).

Budgetary Considerations

An average single column system costs in order of £15-20k. Annual cost is around £3k, depending on number of samples. Commercial analysis is circa £30 per sample and takes on average 10 days.

Case Studies

• The organic acid concentration in metal cabinets containing Japanese lacquer boxes with lead/tin inlays was determined using Palmes diffusion tubes with potassium hydroxide sorbent exposed for 14 days (Gibson et al 1998). After expsosure the tube was extracted with 18.2 MΩ cm-1 water and passed through a 50µm filter. The solution was analysed on a Dionex 600 ion chromatograph with AS14 column and 18mM sodium carbonate/5mM sodium carbonate eluent. The concentrations of acetic and formic acid in the solution were converted to airborne concentrations. The concentrations measured where significantly over the threshold known to cause lead corrosion. Further tests applying activated charcoal cloth with magnets to the interior of the cabinets showed a large surface area with all 6 internal surfaces covered was required to reduce the concentrations to safe levels in all cabinets.
  
• Polychrome limestone fragments from the Thetford tombs were showing damage from salt activity in storage. Examination of the grain sizes indicated a 3mm diameter sample would be significantly representative. Samples were drilled into break surfaces, with the first 2mm sampled separately and then to a depth of 8mm. The powders were dried at 110°C and weighed and extracted and analysed as previously. Cations were also quantified using a CS12 column with 10mM methane sulfonic acid eluent. The content of salt ions per mass of stone was calculated from the solution concentrations. These figures were input into the ECOS thermodynamic model to determine under which conditions of temperature and RH phase and volume changes would occur. This information was used to improve the storage environment (Thickett 2018).
  
• An unusual pale blue corrosion product was observed on a number of Egyptian copper alloy objects. X-ray diffraction and FTIR analysis generated good patterns, but they did not match any reported compounds at the time. Tiny samples were removed and solubilised in dilute nitric acid. When analysed with ion chromatography, the extracts showed strong acetate and sodium peaks. Further analysis with atomic absorption spectroscopy and thermogravimetric analysis determined a consistent stoichiometry of sodium copper carbonate acetate (Thickett and Odlyha 2000).
  
• Whatmans number 1 filter paper was exposed in three potential basement stores for print collections. After exposure for three and twelve months, the pieces were extracted with water as previously and analysed with a Dionex 600 ion chromatograph with Carbopac PA1 column, sodium hydroxide eluent and pulsed ampometric detection. The amounts of glucose, xylose and arabinose were used to rank the three rooms in order of aggressiveness towards paper collections.

References

• BS EN 16455 Conservation of cultural heritage. Extraction and determination of soluble salts in natural stone and related materials used in and from cultural heritage 2014
  
• Gibson, L.T., Cooksey, B.G., Littlejohn, D, and N.H. Tennent, 1997. A diffusion tube sampler for the determination of acetic acid and formic acid vapours in museum cabinets, Analytica Chimica Acta, 341, 11-19
• D. Thickett, and M. Odlyha, Note on the Identification of an Unusual Pale Blue Corrosion Product from Egyptian Copper Alloy Artefacts, Studies in Conservation, 45, (2000), 63-67
• E.M. Breitung and M. Wiggins, Evaluating storage materials: alternatives to the Oddy test, (Library of Congress 2014) https://www.loc.gov/preservation/outreach/tops/breitung/index.html.
• F. Volpi, C.H. Stephens, A. Potthast, E.M. Breitung, Development of a rapid and semi-quantitative protocol for assessing the suitability of commercial materials used to store or exhibit cellulose-based artworks, Eur. Phys. J. Plus, (2020), manuscript submitted Nov. 2020.