AN-C-195
2024-04
Quantificazione dei cationi con prestazioni migliorate utilizzando IC microbore
Vantaggi della cromatografia ionica microbore per l'analisi dei cationi
Sommario
Le prestazioni analitiche con la cromatografia ionica (IC) sono in genere determinate dal rapporto segnale/rumore (S/N) che l'apparecchiatura analitica può raggiungere. Il rapporto S/N dipende fortemente dalle forme dei picchi cromatografici. Le forme dei picchi migliorano nei sistemi IC miniaturizzati con meno volume morto [1].
Il Microbore IC combina colonne di separazione da 2 mm, capillari microbore e un rilevatore di conduttività con volume cellulare ridotto per creare un sistema IC miniaturizzato con sensibilità ottimale [2]. Tali sistemi garantiscono tempi di ritenzione più brevi e consumano meno eluente, aumentando la produttività del campione e riducendo i costi delle analisi di routine quotidiane.
In questa nota applicativa, un sistema IC microbore (MB) è stato confrontato con un sistema IC standard bore (SB). Il sistema IC microbore ha mostrato una risoluzione migliorata e altezze di picco migliori (un fattore di circa il 30% in più per gli ioni di litio). L'IC microbore utilizza meno solventi e può comportare riduzioni dei costi fino al 75% rispetto all'utilizzo di sistemi di cromatografia ionica standard bore. L'utilizzo di sistemi MB ha il potenziale per migliorare le prestazioni di molte applicazioni IC tipiche.
Campione e preparazione del campione
Questo studio è stato condotto con ioni di metalli alcalini, ioni di metalli alcalino-terrosi e ammonio. Una soluzione standard mista (c(Li+) = 25 μg/L, c(Na+, NH4+) = 125 μg/L, c(K+, Mg2+, Ca2+) = 250 μg/L) è stata preparata da soluzioni madre da 1000 mg/L (Standard per IC, TraceCERT®, Sigma-Aldrich, Merck) mediante diluizione in acqua ultrapura.
Analisi
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).
Results
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% |
Conclusion
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.
References
- 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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