Use of mobile phase 18-crown-6 to improve peak resolution between mono- and divalent metal and amine cations in ion chromatography.
It is difficult to quantify NH4+ by ion chromatography in the presence of high concentrations of Na+ due to peak overlap. The Dionex IonPac CS15 column, which contains phosphonate, carboxylate, and 18-crown-6 functional groups, was originally developed to overcome this problem. We have found that the addition of 18-crown-6 to the eluent promotes improved peak resolution between Na+ and NH4+ even at concentrations as high as 60,000 to 1 using this column. Its use also improves the separation of alkali and alkaline earth metal and amine cations. Mobile phase 18-crown-6 increased the retention times of CH3NH3+, NH4+, and K+, and decreased the retention time of Sr2+. The retention times of Li+, Na+, Mg2+, Ca2+, (CH3)2NH2+, and (CH3)3NH+ were not affected. This method makes possible the direct analysis of ammonia from nitrogenase, the enzyme responsible for biological nitrogen fixation. The resolution of the NH4+ peak from the Na+ and Mg2+ peaks improved from zero resolution to values of 6.19 and 5.65, respectively. This technique considerably reduces the analysis time of NH4+ in the presence of high concentrations of Mg2+ and Na+ over traditional indophenol measurements.
Department of Chemistry and Biochemistry, Brigham Young University, C100 Benson Science Building, P.O. Box 25700, Provo, UT 84602, USA.
Dean A
DA
Richens
David
D
Simpson
Sarah
S
Peterson
Arlo
A
McGinn
John D
JD
Lamb
D000588:Amines / Q000302:isolation & purification*; D002412:Cations / Q000302:isolation & purification*; D002853:Chromatography, Liquid / Q000379:methods*; D043844:Crown Ethers / Q000737:chemistry*; D004563:Electrochemistry / Q000379:methods; D008670:Metals / Q000302:isolation & purification*
D000588:Amines; D002412:Cations; D043844:Crown Ethers; D008670:Metals; C015762:18-crown-6