Analytical performance with ion chromatography (IC) is typically determined by the signal-to-noise (S/N) ratio that the analytical equipment can reach. The S/N ratio strongly depends on chromatographic peak shapes. Peak shapes improve in miniaturized IC systems with less dead volume [1].
Microbore IC combines 2 mm separation columns, microbore capillaries, and a conductivity detector with reduced cell volume to create a miniaturized IC system with optimal sensitivity [2]. Such systems provide shorter retention times and consume less eluent, increasing sample throughput and reducing the costs of daily routine analytics.
In this Application Note, a microbore IC system (MB) was compared to a standard bore IC system (SB). The microbore IC system showed improved resolution and better peak heights (a factor of ~30% more for lithium ions). Microbore IC uses less solvents and can result in cost reductions of up to 75% compared to using standard bore ion chromatography systems. Using MB systems has the potential to improve the performance of many typical IC applications.
This study was conducted with alkali metal ions, alkaline earth metal ions, and ammonium. A mixed standard solution (c(Li+) = 25 μg/L, c(Na+, NH4+) = 125 μg/L, c(K+, Mg2+, Ca2+) = 250 μg/L) was prepared from 1000 mg/L stock solutions (Standards for IC, TraceCERT®, Sigma-Aldrich, Merck) by dilution in ultrapure water.
A microbore IC system comprised of a 930 Compact IC Flex Oven/DEG/MB together with an IC conductivity detector MB (Figure 1) was compared to its respective standard bore IC system configuration (930 Compact IC Flex Oven/DEG).
The MB setup from Metrohm has a reduced dead volume with shorter capillaries and smaller capillary inner diameters (0.18 mm) wherever possible.
The microbore conductivity detector has a small inner cell volume (0.3 μL) and a low noise level (<0.1 nS). Furthermore, it even tolerates challenging eluents such as methanesulfonic acid (MSA). Microbore columns, which have a 2 mm inner diameter and associated reduced eluent flow rates, lead to better S/N. This increases sensitivity even further and lowers limits of detection.
The mixed cation standard solution was injected using a 5 μL loop and then separated on a 2 mm version of the Metrosep C 6 column on both tested IC systems. The conductivity was directly recorded (non-suppressed cation analysis, Table 1).
| Column | Metrosep C 6 - 150/2.0 |
|---|---|
| Eluent (from Merck concentrate Sigma-Aldrich, Merck 19399) | c(HNO3) = 1.7 mmol/L c(DPA) = 1.7 mmol/L |
| Flow rate | 0.25 mL/min |
| Temperature | 30 °C |
| Injection volume | 5 µL |
| Detection | Direct conductivity |
For performance comparison reasons, the retention times, resolution, peak heights, and repeatability were evaluated with MagIC Net software (version 4.1).
Overall performance was improved when using the MB system for analysis. Retention times were shorter with the MB system (approximately 0.2 minutes in this case) than with the SB system (Figure 2).
Resolution with the MB system was ~115% better than with the SB system (Table 2). Peak heights were higher, with most improvement shown for the early-eluting peaks (lithium, sodium, ammonium) on the MB system (Table 3). The noise was comparable for both tested IC setups.
Minimal improvement effects were observed for later eluting peaks (e.g., potassium, magnesium, and calcium). For all other relevant parameters, MB and SB showed similar results (e.g., repeatability).
| Resolution | MB | SB |
|---|---|---|
| Lithium | 5.6 | 5.6 |
| Sodium | 3.0 | 2.6 |
| Ammonium | 7.9 | 7.3 |
| Potassium | 6.0 | 5.8 |
| Peak height [µS/cm] | MB | SB | Improvement factor |
|---|---|---|---|
| Lithium | 1.16 | 0.88 | 131% |
| Sodium | 1.14 | 1.01 | 113% |
| Ammonium | 1.23 | 1.13 | 108% |
| Potassium | 0.71 | 0.70 | 100% |
The MB system combines microbore capillaries, a conductivity detector with reduced cell volume, and a 2 mm separation column—all of which lead to improved peak shapes and shorter retention times. This enables increased sensitivity and lower limits of detection. Lower flow rates reduce eluent consumption and overall running costs.
Non-suppressed MB systems in combination with 2 mm columns deliver significant improvements with respect to resolution and sensitivity. For sequentially suppressed IC systems (SES) including a microbore CO2 suppressor (MCS) with reduced dead volume, the main improvement is shorter retention times. This is helpful with low flow rates, and especially in combination with gradient applications as changes in the eluent composition will quickly impact the analysis and the effect will not be delayed by unnecessary dead volume.
MB systems can be used with 2 mm as well as 4 mm separation columns. These systems are suitable for all IC applications.
- Diederich, V.; Riess, A. K. Best Practice for Separation Columns in Ion Chromatography (IC) – Part 2. Analyze This – The Metrohm Blog, 2021.
- Metrohm AG. Metrohm Microbore Ion Chromatography – Maximize the Efficiency of Your Ion Chromatography!, 2023.
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