Column preconcentration

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

E. I. Vereda Alonso, L. P. Gil, M. T. Siles Cordero, A. Garcia de Torres and J. M. Cano Pavon, Automatic on-line column preconcentration system for determination of cadmium by electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom., 16, 2001, 293-295. 

Grudpan, K., Laiwraungrath, S., and Sooksamiti, R, Flow injection spectrophotomet-ric determination of uranium with in-valve ion-exchange column preconcentration and separation, Analyst, 120, 2107-2110, 1995. 

Dichlorbenil Crops Steam distillation HPLC with pre-column preconcentration [73]... 

Figure 10. Components of a liquid chromatograph used for the trace enrichment of aromatic amines by column preconcentration...

M. Schuster, M. Schwarzer, Selective determination of palladium by on-line column preconcentration and graphite furnace atomic absorption spectrometry, Anal. Chim. Acta, 328 (1996), ID 11. 

Figure 7.16A depicts a flexible SFE-HPLC coupled assembly developed by Ischi and Haerdi [106] that consists of three main parts [viz. the SFE system (Al), the interface (A2) and the HPLC system (A3)] each furnished with appropriate valves operating as shown in Fig. 7.16B. Thus, valve 5 in Fig. 7.16A is used to provide extraction with or without a modifier, via a tee connector on the other hand, valve 10 allows switching between static and dynamic extraction. The former is done by having the valve close the outlet of the extraction cell after the desired temperature is reached. By switching the valve back, the dynamic state is restored. Valve 13 enables trapping of the extracted analytes, either on a C, silica column placed in an oven for on-line preconcentration and insertion of non-polar or low-polar analytes into the chromatograph after elution or into a liquid phase to implement an off-line operation. When polar ionic analytes are to be preconcentrated, the eluent from the extractor is diverted to valve 18 and retained on the ion-exchange material packed in the column. Preconcentration of both non-polar, low-polar and polar ionic analytes can be accomplished by using both valves (13 and 18) [106],...

The volume that can be injected onto a packed microcapillary or nano-LC column is very limited, e.g., less than 0.1 pi for a 100-pm-lD column. This seriously compromises the achievable concentration detection limits, unless on-column preconcentration would be performed. However, for dilute sample solution, the injection volume is restricted by external peak broadening, and not by column loadability (typically -50-200 pg/g of porous packing material). Therefore, on-line SPE can be applied for sample preconcentration. 

The method for nitrite determination based on the diazotization-coupling reaction by column preconcentration and on the reduction of nitrate to nitrite using the Cd-Cu reductor column has been proposed for the determination of nitrate and nitrite in water and some fruit samples [6]. On-line monitoring of nitrite in fertilizer process streams, natural and waste water effluents based on the diazotization of nitrite in the sequential injection system with N-(l-naphthyl)etylenediammonium dichloride and the formation of a highly coloured dye has been described [7]. 

Figure 7.4 Microfluidic LC injector designs. (A) Double-T injector (B) on-column (head) preconcentrator (C) stand-alone preconcentrator (D) microfluidic arrangement for sample loading onto a particle packed on-column preconcentrator (E) enlarged view of a packed on-column preconcentrator with sample loading area highlighted (F) Rhodamine fluorescent dye loading at the front of the preconcentrator. (Adapted with permission from Ref. 33).

Lipid-Bound Macrocycles as New Immobilized Ligand Systems for Effective Separation of Metal Cations (Tsukube et al., 1991) (5) Application of Macrocyclic Ligands to Ion Chromatography (Lamb and Smith, 1991) (6) Chromatographic Separation of Metal Cations on Silica Gel Chemically Modified with a Polymeric Diaza-18-crown-6 Derivative (Basyuk, 1991) and (7) Column Preconcentration of Trace Metals from Sea Water with Macroporous Resins Impregnated with Lipophilic Macrocycles (Blain et al., 1991). 

Chen, H.-W., Fang, Z.-L., Combination of flow injection with capillary electrophoresis. Part 3. Online sorption column preconcentration capillary electrophoresis system. Anal. Chim. Acta 1997, 355(2-3), 135-143. 

Fang, Z. L., A sequential injection on-line column preconcentration system for determination of cadmium... 

Water, urine, mussel tissue, SRMs/CRMs Co Waters filter and acidify urine and mussel digest with HNO3/ HCl (bomb) [N/MT WDC] Direct solution by FI to on-line column preconcentration system, inject into transversely heated GFA (THGA), [FTAAS] [N/MT-SEP/ CONC-EAAS WDC-SEP/CONC- ETAAS] Anthemidis et al. (2002)... 

S. Hirata, Y. Umezaki, and M. Ikeda, Determination of Cadmium of ppb Level by Column Preconcentration-Atomic Absorption Spectrometry [in Japanese]. J. Flow Injection Anal., 3(1) (1986) 8. 

Reversed-phase separations using superheated water have not found any major applications so far, but a wide range of low molecular mass polar compounds have been separated at modest temperatures with relatively simple and rugged instrumentation. It has potential for wider use in laboratories that wish to reduce consumption of organic solvents. In addition, superheated water should be compatible with on-column preconcentration techniques for trace analysis... 

Pohl P., Prusisz B. On the applicability of Duolite GT-73 to column preconcentration of gold and palladium prior to determination by inductively coupled plasma-atomic emission spectrometry. Microchimica Acta 2005 150 159-165. 

In column preconcentration systems P is sometimes referred to as retention efficiency (% ), defined by Hartenstein et al.[12] as the analyte percentage in a sample recovered fiom a colunm following sorption and elution. In this book, only P will be used to describe the completeness of analyte transfer, since it may be used in a more general sense. 

Fig.2 (c)(d) Applications of the 8-channel multifunctional valve, c, merging zones application in which injected sample and reagent zones merge at point M. d. application for dual sorption column preconcentration with which a second valve is required to elute the two columns sequentially. V, V/, multifunctional valves V//, 2-way valve ... 

Despite the great potentials of using FI liquid-liquid extraction for achieving analyte preconcentrations for flame AAS. the number of applications are surprisingly few compared to sorption column preconcentration procedures. The performance of some published procedures 114,38,39] are shown in Table 3.2. Despite the additional enhancement effects from the organic solvents, the concentration efficiencies and consumptive indices generally tend to be inferior to those achievable by sorption procedures (cf. Sec. 

One of the characteri.stic features of FI column preconcentration procedures is that the analyte has to be sorbed on a column before elution into the detector. This collection process is often termed "sample loading" in preconcentration procedures, and should be differentiated from filling samples into a sample loop, sometimes also referred to as sample loading. The latter is a sequence for all FIA procedures excepting those which introduce samples based on timing, and should better be designated as sample filling". 

On-line columns packed with inert packing materials usually have very limited effect on the dispersion of an injected sample. When an active sorptive material is substituted in the column, the sorbent contributes significantly to the dispersion characteristics of the sorbed analyte. Despite the differences between FI column preconcentration and HPLC discussed in Chapter 1, the sorption and release of the analyte in FI on-line column is in principle a chromatographic process, and it may be used to predict and explain the dispersive behavior of the analyte within this sequence. 

Practical Considerations in the Design and Operation of FI Column Preconcentration Systems... 

The elution flow-rate is an important parameter in column preconcentration which is usually optimized for maximum sensitivity. However, the speed of elution is also a crucial factor for the efficiency of on-line preconcentration systems, because in most cases the eluent flow is connected directly (some after merging with reagent streams) with the detector. This is particularly important for detection systems which require a certain sample delivery rate for optimum response, e.g., the flame AA or ICP spectrometer, and will be discussed in more detail in section 4.6.3. 

Sorbents used successfully as packing materials for batch column preconcentration purposes are not always adaptable to FI column preconcentration systems. The special requirements for FI preconcentration sorbents may at least include the following ... 

---