An improved ion chromatography system coupled with a melter for high-resolution ionic species reconstruction in Antarctic firn cores

Abstract

We improved an on-line multi-ion chromatography (IC) system combined with a custom firn core melter (IC-melter). Five anions (F−, CH3SO3−, Cl−, NO3−, SO42−) and five cations (Na+, NH4+, K+, Mg2+, Ca2+) are simultaneously determined every 1.3 min; high-resolution ion data, with a depth interval of approximately 1.8 cm, can thus be obtained from Antarctic firn cores using the IC-melter. The IC-melter provides a processing speed of 1.1–1.7 h per ~0.7–0.8 m firn core. The depth resolution was designed to capture seasonal variations of ions based on the accumulation rate of Styx Glacier (Northern Victoria Land, Antarctica), where a firn core used herein was obtained, and variations of the firn core density. The analytical conditions (eluent concentration and flow rate, run time, peak separation, and sensitivity) of the multi-IC system were optimized to achieve the research goals. Cations and anions were separated through 4-min isocratic elution (CH3SO3H eluent) and 5-min isocratic elution (KOH eluent), respectively. The isocratic elution method for anion analysis was selected, rather than the gradient elution method, due to the exceptionally low ionic strength of the meltwater and easy operation of the multi-IC system. All ionic species showed calibration curves with determinant coefficients >0.997, standard errors of estimated values <0.015, and relative standard deviation values <4.17% for anions and <2.42% for cations at levels of 5–25 µg L−1. The method detection limits (MDLs) for assessed ions were comparable to detection limits (DLs) previously reported for on-line IC-melter systems, except the limit of SO42− (~3.0 µg L−1). In particular, the MDLs of fluoride ion (F−) and methanesulfonate ion (CH3SO3−) were 0.03 and 0.3 µg L−1, respectively; these species were successfully measured in an Antarctic firn core for the first time, using the improved on-line IC-melter. The relative errors for ions other than Na+ and Cl− were generally < 13.3% at a level of approximately 50 µg L−1. The expanded measurement uncertainties (k = 2 at the 95% confidence level) were ~0.13 µg L−1 and ~1.59 µg L−1 at levels of ~1.0 µg L−1 for F− and ~5.0 µg L−1 for CH3SO3−, respectively. The measured ions from parallel two firn core stick samples showed reproducibility values <29.3%. Pearson’s r values between ions obtained from the IC-melter and conventional method were > 0.67. In this study, as an application of the IC-melter, high-resolution ion species data from the firn core (depth interval: ~20.11–22.85 m) are shortly presented.