Ion Chromatography - The Separation of Anions 3



Ion chromatography has many biological, medical and forensic applications an example of which is the anion analysis of saliva. The purpose of the analysis was to determine the nitrite, bromide, nitrate, sulphate and thiocyanate content of the saliva sample. The sample was diluted 1 to 10 and carefully filtered through a 0.45mm membrane to remove all solid material. Subsequently the sample was diluted a further ten times to measure the chloride and phosphate content which was present in the sample at a much higher concentration than the other ions. The electrical conductivity detector was used to monitor the separation and the sample volume used for both solutions was 100ml. The Metrohm anion SUPERSEP column, 10cm long and 4.5mm in diameter was employed for the separation with a mobile phase consisting of a solution of boric acid 4.5mMol/l, manite, 3.5nMol/l in a aqueous solution of 2%v/v methanol adjusted to a pH of 7.8 with TRIS. A chromatogram of the high concentration sample is shown in figure 37.
















(Metrohm Application Bulletin No. 154/2e)

Figure 37. The Separation of Anions Contained in a Saliva Sample


The analysis indicated that the ion content (excluding chloride and phosphate that overloaded the column) ranged from about 2ppm for bromide to 166ppm for thiocyanate. The separation of the most dilute sample separated under the same conditions is shown in figure 38.


It is seen that the two major components that were 'off chart' in the first chromatogram are now of reasonable and measurable size. It was determined that the chloride content was a little over 0,08% which was not particularly surprising. However, the phosphate content was almost as great at just under 0.06%, which seems high for a bodily fluid of a normal health person. Of course it would depend on the history of the individual from which the sample was taken as many soft drinks contain significant quantities of
phosphoric acid.

(Metrohm Application Bulletin No. 154/2e)

Figure 38 A Chromatogram of a More Dilute Sample of the Saliva.