On-column preconcentration

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. 

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).

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... 

Hilhorst, M.J., Somsen, G.W., and de Jong, GH., Sensitivity enhancement in capillary electrochromatography by on-column preconcentration, Chromatographia, 53,190, 2001. 

On-column preconcentration procedures The word stacking defines any on-capillary mode of concentration or focusing analytes based on changes of electrophoretic velocity due to the electric field across concentration boundaries. Sample stacking can be performed in both hydrodynamic (e.g., gravity or pressure) and electrokinetic (e.g., voltages) injection modes. The sample solution is sandwiched between two portions of the CE separation buffer. When high... 

Copper oxide-covered electrodes in alkaline media have been anployed for detection of peptides in both HPLC [112,114] and CE [116]. Peptides that have been determined include small peptides such as Gly-Gly-amide [112,116], Gly-Gly [112,116], tri-, tetra-, penta-, and hexaglycine [114,116], aspartame [116] cyclic Leu-Trp-Asp-Pro-Val [116], cycfic Leu-Tyr-Asp-Pro-Val [116], and des-Tyr-leucine enkephalin [114]. Detection limits (withont on-column preconcentration) are poor, typically in the 5-100 pM range [114,116]. 

A simple approach to sensitivity enhancement is to utilize the preconcentrating ability of column chromatography. In on-column preconcentration, the sample is injected in a weak mobile phase so that it is retained at the head of the column and concentrated [30,31]. Analytes are eluted with a stronger mobile phase after the less retained solutes are washed out. The greater the capacity factor (k) for analyte in the sample solution, the greater the preconcentration that can... 

Eigure 7A illustrates successful detection of four neuropeptides at 100 pM to 1 nM using pre-column biuret derivatization, on-column preconcentration, and... 

Determination of Pb(II) ion by classical or reversed FIA consists of a preconcentration step either on columns packed with a chelating sorbent (PC-FIA) or on a mercury film, followed by spectrophotometric determination of the complex with 4-(2-pyridylazo)resorcinol (11, kmax 518 nm) in borate buffer solution RSD 3-6% at 0.01-1 pM. End analysis by ASV was also applied99. 

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. 

The ion -exchanger materials allow flow rates of 10 ml/min with minimum back pressure even at these flow rates, 95-100% of the transition metals are retained on the preconcentration column. Throughput is maximized without detriment to the extraction efficiency. 

Frommberger, M., Schmitt-Kopplin, R, Ping, G., Frisch, H., Schmid, M., Zhang, Y., Hartmann, A., and Kettrup, A., A simple and robust set-up for on-column sample preconcentration-nano-hquid chromatography-electrospray ionization mass spectrometry for the analysis of N-acylhomoserine lactones, Analytical and Bioanalytical Chemistry 378(4), 1014—1020, 2004. 

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. 

In a more recent work Ito [101] has described a simple and highly sensitive ion chromatographic method with ultraviolet detection for determining iodide in seawater. A high-capacity anion exchange resin with polystyrene-divinylbenzene matrix was used for both preconcentration and separation of iodide. Iodide in artificial seawater (salinity, 35 % ) was trapped quantitatively (98.8 0.6%) without peak broadening on a preconcentrator column and was separated with 0.35M sodium perchlorate+ 0.10M phosphate buffer (pH 6.1). On the other hand, the major anions in seawater, chloride and sulphate ions, were partially trapped (5-20%) and did not interfere in the determination of iodide. The detection limit for iodide was 0.2pg L 1 for 6mL of artificial seawater. This method was apphed to determination of iodide (ND-18.3pg L ) and total inorganic iodine (I +I03 -I, 50.0-52.7pg L 1) in seawater samples taken near Japan. 

I Ion exchange -resin column On-line preconcentration electrode Iodide selective electrode InM [328]... 

Jansen and co-workers [97] have described on-line TD-GC-FTIR. In the TD-GC-FTIR system a thermal desorption (TD) cold trap injector is used for the temperature-controlled outgassing of the samples with a maximum temperature of 350 °C. The volatile components are transferred to the cold trap by the carrier gas and preconcentrated. After completion of the outgassing process the cold trap is heated very quickly, causing on-column injection of the trapped components onto the gas chromatograph. The technique has recently been extended to include an ion-trap MS. 

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