A multicomponent mobile phase for ion chromatography applied to the separation of anions from the residue of low explosives

A multicomponent mobile phase utilizing ion-exchange, ion-exclusion, and ion-pairing principles for the rapid isocratic separation of anions in low explosives residue by ion chromatography (IC) has been developed. The notable feature of this system is that an ion-pairing reagent and an ion-exclusion reagent are combined in the same mobile phase. Contrary to expectation, these reagents act independently of each other in solution. The stock mobile-phase composition consisted of boric acid, D-gluconic acid, lithium hydroxide, and glycerol. Tetrapropylammonium hydroxide, an ion-interaction reagent was used to achieve pH 8.5. Acetonitrile (ACN) was added to enhance resolution and octanesulfonic acid, an ion-exclusion reagent, was added to adjust the retention time of perchlorate. Separation of a mixture of anions common to low explosives residue was achieved in less than 16 min using a Waters IC-Pak Anion HR column. Optimization studies were performed by changing the concentration of the ACN and by altering the pH or the type of ion-interaction or -exclusion agents. Simulated case studies were performed using postblast residues from pipe bombs. The results show this method to be a valid and reproducible procedure for forensic casework analysis. The practical significance of this system is that a reduction in the analysis time and an improvement in efficiency of late-eluting peaks can be achieved without resorting to gradient elution techniques. For the analysis of anions detected in explosives residue, the Waters IC-Pak Anion HR column has proven to be a suitable replacement for the Vydac 300IC405 column, which has been discontinued by the manufacturer.