Lanthanides detection

Fig. 6.5. Separation of the lanthanides. Column 5-/xm Nucleosil SCX. Eluent linear program from 18-70 mM HIBA over a 20-min period, pH 4.6. Flow rate 0.8 ml min . Sample 10 /il of a solution containing 10 pg ml of each lanthanide. Detection absorption at 600 nm after post-column reaction with Arsenazo I. (Reprinted with permission from ref [3], copyright 1979 American Chemical Society.)...

Fig. 6.14. Gradient cation-exchange (A) and anion-exchange (B) separation of the lanthanides. Detection post-column reaction using PAR, photometric detection at 520 nm, 0.5 AUFS. Sample 10 ppm each metal, 50 /il sample volume. Note Yb(III) and Lu(III) co-eluted for the anion-exchange separation. (Reproduced with permission from ref [29].)...

While there have been many non-isothermal studies of the decompositions of lanthanide oxalates, fewer detailed kinetic investigations have been reported. The anhydrous salts are difficult to prepare. La, Pr and Nd oxalates decompose [1097] to the oxide with intervention of a stable oxycarbonate, but no intermediate was detected during decomposition of the other lanthanide oxalates. The product CO disproportionates exten-... 

We have developed reverse-phase ion-pairing HPLC separations of substituted EDTA metal chelates of several transition metals (including Cd, Zn, Fb, and Hg) and several lanthanides (La, Ce, Eu, Dy, Er, Yb, Lu). Detection levels of these chelates are currently being assessed. A sensitive metal ion analysis employing an inherently fluorescent EDTA seems feasible. 

As in aqueous solution, the lanthanide contraction favors a change from nine-coordination for the light lanthanides to eight-coordination for the light lanthanides such that [Ln(DMF)8]3+ is the major species when Ln3+ = Ce3+-Nd3+, and that this becomes the only detected species when Ln3+ = Tb3+-Lu3+ in dimethylformamide perchlorate solution (11, 92, 93, 321-323). Thus, Nd3+ is characterized by AH° = -14.9 kJ mol-1, AS0 = -69.1 J K"1 mol-1, and AV° = - 9.8 cm3 mol-1 for the equilibrium shown in Eq. (25) (93). The molar volume of DMF is 72 cm3 mol- and it therefore appears that the substantially smaller magnitude of AV° is a consequence of significant... 

A variation on the theme of conventional assay uses both lanthanide-labeled and biotin-labeled single strands to form split probes for sequence of target strands (Figure 12).120 When both of these bind to DNA, the complex binds (via the biotin residue) to a surface functionalized with streptavidin, immobilizing the europium and allowing assay to be carried out. This approach is already very sensitive to DNA sequence, since both sequences must match to permit immobilization of the lanthanide, but can be made even more sensitive by using PCR (the polymerase chain reaction) to enhance the concentration of DNA strands. In this way, initial concentrations corresponding to as few as four million molecules can be detected. This compares very favorably with radioimmunoassay detection limits. 

Different lanthanide metals also produce different emission spectrums and different intensities of luminescence at their emission maximums. Therefore, the relative sensitivity of time-resolved fluorescence also is dependent on the particular lanthanide element complexed in the chelate. The most popular metals along with the order of brightness for lanthanide chelate fluorescence are europium(III) > terbium(III) > samarium(III) > dysprosium(III). For instance, Huhtinen et al. (2005) found that lanthanide chelate nanoparticles used in the detection of human prostate antigen produced relative signals for detection using europium, terbium, samarium, and dysprosium of approximately 1.0 0.67 0.16 0.01, respectively. The emission... 

Lanthanide chelates also can be used in FRET applications with other fluorescent probes and labels (Figure 9.51). In this application, the time-resolved (TR) nature of lanthanide luminescent measurements can be combined with the ability to tune the emission characteristics through energy transfer to an organic fluor (Comley, 2006). TR-FRET, as it is called, is a powerful method to develop rapid assays with low background fluorescence and high sensitivity, which can equal the detection capability of enzyme assays (Selvin, 2000). 

Figure 9.51 Time-resolved FRET assay systems involve energy transfer between the lanthanide chelate and an organic dye that are brought together as two labeled molecules bind to an analyte. In this illustration, an antibody labeled with a lanthanide chelate is used along with a Cy5-labeled antibody to detect a protein target in solution. Excitation of the lanthanide label results in energy transfer and excitation of the cyanine dye only if they are held within close enough proximity to allow efficient FRET to occur. Under these conditions, excitation of the lanthanide chelate results in cyanine dye emission, which will not occur if the labeled antibodies have not bound to a target.

Other fluorescent probes also may be used to label (strept)avidin molecules for detection of biotinylated targeting molecules. Chapter 9 reviews many additional fluorescent labels, such as quantum dots, lanthanide chelates, and cyanine dye derivatives, all of which may be used in similar protocols to create detection conjugates for (strept)avidin-biotin-based assays. 

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