doi:10.1016/j.chroma.2005.08.086 
Copyright © 2005 Elsevier B.V. All rights reserved.
Chelates of cobalt(III) and iron(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol as test probes for the characterization of chromatographic effects on a reversed-phase liquid chromatography stationary phase
Sławomir Oszwałdowski
, 
Department of Analytical Chemistry, Faculty of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland
Received 12 May 2005;
revised 27 August 2005;
accepted 29 August 2005.
Available online 19 September 2005.
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Abstract
The chromatographic effects on a reversed-phase liquid chromatography (RPLC) phase were established with the use of chelates system: cobalt(III) and iron(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol as the test probes. Both chelates have the same octahedral structure, M:L = 1:2, the former chelate is cationic and can be used to probe the ion-exchange phenomena on a RPLC phase, whereas the latter is not charged and can be used as reference molecule with respect to the charged one. Based on well-established LC phenomena referred in the literature, the suitability of the chelate system for examining some chromatographic effects was tested. It was concluded that the proposed test method is sensitive towards the ion-exchange phenomena on the LC phase and the hydrogen bonding between the solvent eluent and the LC phase. In addition, chromatographic effects due to the presence of ultrasonic field or due to the presence of aromatic amines in the eluent were observed with the help of the proposed test method. Based on molecular computation, the properties of chelates were compared with properties of quaternary amines, the probes most frequently used for testing LC phases, and the possible differences in an interaction of the mentioned compounds with a surface of a RPLC phase were indicated.
Keywords: RPLC stationary phase; Probing; Chelates of Co(III) and Fe(II); 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol; Chromatographic effects
Fig. 1. Retention of CoL2+ FeL20 and L0 species as a function of mobile phase composition. The correlation of log k of these species with the solubility parameter (δT); (a) Zorbax, SB-C18 column, eluent: organic phase (ACN, MeOH):water containing 5 × 10−4 mol l−1 NaClO4. (b) Zorbax, Eclipse XDB-C18, eluent: organic phase (ACN, MeOH):water, without IP reagent. Solubility parameters for MeOH, ACN and water: 15.85, 13.15 and 25.52 cal1/2 cm−3/2 were taken from ref. [8].
Fig. 2. Influence of pH on retention of CoL2+ and FeL20 chelates—two different columns with the same eluent (MeOH:water, 80/20, v/v, containing 5 × 10−4 mol l−1 NaClO4). Labels, (A and C) denote CoL2+, (B and D) FeL20; columns: Nucleosil, 100-5 C18 e.c. (A and B), Zorbax, SB-C18 (C and D).
Fig. 3. The relationship between log k of (CoL2+, FeL20) and log[NaCl] being in the eluent. The solid lines denote retention of CoL2+ at different pH, whereas dashed lines denote retention of FeL20 at pH 7.4 and 4.5 (bottom) or L0 at pH 2.1 (upper). The label Δ[Na+] denotes the concentration of NaCl in eluent necessary to start the ion-exchange process, and this reflects the column ion-exchange capacity at different pH.
Fig. 4. Influence of triethylamine (TEA) on k of CoL2+, FeL20 and L0 species examined in the present work. Column: Zorbax SB-C18; eluent: ACN:H2O, 80:20, v/v containing NaClO4 (5 × 10−4 mol l−1) and TEA. The pH of the eluent is shown in the figure.
Fig. 5. (a) Influence of R4N+Br− salt on the retention of CoL2+ and FeL20 chelates and L0 ligand. (b) Relationship between k of CoL2+ and log PW/DCE of R4N+Br− salts (PW/DCE = partition coefficient: water-1,2-dichloroethane). Concentration of R4N+Br− in the eluent 1 × 10−3 mol l−1. Column: Zorbax SB-C18, eluent: ACN/water 70/30, v/v (in the absence of R4N+ salt, 70/30, v/v ACN/water with NaClO4, 5 × 10−4 mol l−1 was used).
Fig. 6. Influence of sulphonate salts on retention of CoL2+ and FeL20. Column: Nucleosil 100-5 C18 e.c., eluent: ACN:water 80:20, v/v and RSO3−Na+ (1 × 10−3 mol l−1), (in absence of RSO3−Na+ salt, 80:20, v/v ACN:water with NaClO4, 1 × 10−3 mol l−1 was used).
Fig. 7. The application of CoL2+ chelate to the determination of degree of interaction of methanol with RPLC stationary phase (Zorbax, Eclipse XDB-C18). Upper, (a) the strong interaction of CoL2+ with stationary phase is observed using the eluent 20:80, v/v, water:organic phase (ACN + MeOH) with the content of MeOH up to 20% (v/v). At the MeOH concentration (>20%) the interaction of CoL2+ with stationary phase is replaced by the interaction of MeOH with stationary phase. It should be noted that k of FeL20 and L0 were insensitive about changes in eluent composition, which indicates that CoL2+ is a specific test probe. (b) Comparison of retention of CoL2+ using two different eluents, which indicates that the specific interaction between organic solvent and stationary phase (20:80, v/v water:organic phase eluent) is replaced by water–stationary phase interaction when eluent 35:65, v/v water:organic phase (ACN + MeOH) was applied.
Fig. 8. Modification of RPLC phase by aromatic amine (DMA—dimethylaniline). Solid lines, Nucleosil 100-5 C18 e.c.; dashed lines, Zorbax SB-C18. Eluent: MeOH/water = 80/20, v/v; NaClO4, 4 × 10−3 mol l−1.
Fig. 9. Influence of ultrasonic field on retention of CoL2+/FeL20 chelates reflected by log k = log[Na+] curve (see Section 3.4). Column: Nucleosil 100-5 C18 e.c., eluent: MeOH:water, 80/20 containing NaCl, pH 4.5. Descriptions: “no-ultr” denotes k of chelate in the absence of ultrasonic field and description ‘ultr’ in the presence of the field.
Table 1.
The basic properties of Co(III)(5-Br-PADAP) chelate determined by spectrophotometric measurement
a Chelate once formed is then stable in 3 mol l
−1 HCl
[17], 3 mol l
−1 H
2SO
4 or 1.8 mol l
−1 HNO
3 [16].
b The cobalt(III) ion in chelate is not prone to a reduction in the presence of reducing agents up to pH 9
[12].
c In 50:50 MeOH/water solution. The value of
λmax was stable in different solvents: ACN/water, from 20% to 100% of ACN or in MeOH/water mixture, in the same proportions.
d The Bent-French method.
Table 2.
Comparison of k of CoL2+, FeL20 and L species on different RP-LC columns using pure solvents (ACN or MeOH) as eluenta
a (Column cleaning) column was purged by 0.05 mol l
−1 H
2SO
4 for 30 min and next it was left stand in 50:50 MeOH:water for 1 day, otherwise the column behaves as if the separation was performed with the use of acidic eluent.
Table 3.
The influence of TMA+ interaction with Cl− on the TMA+ parameters
a (mm+, Hyperchem), the angle 108.5° was determined for the free TMA
+ cation.
b The charge of N atom was determined (PM3) at the appropriate TMA
+ geometry. Both AM1 and PM3 (Hyperchem) were indicated an increase in N charge due to dipper distortion of TMA
+ geometry; values obtained by PM3/Hyperchem are close to these obtained by AM1/Ampac.
c Energy of molecule at the appropriate geometry (mm+, Hyperchem). The (Δ
E) denotes energy of the molecule at its distorted geometry minus energy of molecule at its optimal geometry.
d Restrained distance.
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