Cees Gooijer

Room temperature phosphorescence in the liquid state as a tool in analytical chemistry

A wide-ranging overview of room temperature phosphorescence in the liquid state (RTPL1) is presented, with a focus on recent developments. RTPL techniques like micelle-stabilized (MS)-RTP, cyclodextrin-induced (CD)-RTP, and heavy atom-induced (HAI)-RTP are discussed. These techniques are mainly applied in the stand-alone format, but coupling with some separation techniques appears to be feasible. Applications of direct, sensitized and quenched phosphorescence are also discussed. As regards sensitized and quenched RTP, emphasis is on the coupling with liquid chromatography (LC) and capillary electrophoresis (CE), but stand-alone applications are also reported. Further, the application of RTPL in immunoassays and in RTP optosensing—the optical sensing of analytes based on RTP—is reviewed. Next to the application of RTPL in quantitative analysis, its use for the structural probing of protein conformations and for time-resolved microscopy of labelled biomolecules is discussed. Finally, an overview is presented of the various analytical techniques which are based on the closely related phenomenon of long-lived lanthanide luminescence. The paper closes with a short evaluation of the state-of-the-art in RTP and a discussion on future perspectives.

Liquid chromatography with atmospheric pressure chemical ionization and electrospray ionization mass spectrometry of flavonoids with triple-quadrupole and ion-trap instruments

With 15 flavonoids as test compounds, the analytical performance of four modes of LC-MS, multiple MS (MSn) and tandem MS operation (atmospheric pressure chemical ionization (APCI), electrospray ionization, positive and negative ionization) was compared for two mass spectrometers, a triple-quadrupole and an ion-trap instrument. Two organic modifiers, methanol and acetonitrile, and two buffers, ammonium acetate and ammonium formate, were used. In general, the use of APCI in the negative ion mode gave the best response, with the signal intensities and the mass-spectral characteristics not differing significantly between the two instruments. The best results were obtained when methanol-ammonium formate (pH 4.0) was used as LC eluent. Under optimum conditions full-scan limits of detection of 0.1-30 mg/l were achieved in the negative APCI mode. Here it needs to be emphasized that up to 2-order response differences were found both between analytes and between modes of ionization. This implies that one should be very cautious when interpreting data on the screening of real-life samples. The main fragmentations observed in the MSn spectra on the ion-trap, or the tandem MS spectra on the triple-quadrupole were generally the same. The advantage of the former approach is the added possibility to ascertain precursor-->product ion relationships.

Determination of isoflavone glucoside malonates in Trifolium pratense L. (red clover) extracts: quantification and stability studies.

Isoflavones, their glucosides and their glucoside malonates were determined in red clover leaf extracts using reversed-phase LC coupled to atmospheric pressure chemical ionisation mass spectrometry (APCI-MS), UV and fluorescence detectors and the stability of the malonates was investigated. Extracts can be stored at least 1-2 weeks at -20 degrees C without loss of malonates. In LC-separated fractions the malonates are most stable when stored at low temperature after evaporation to dryness. The concentrations of eight major isoflavones ranged from 0.04 to 5 mg/g leaves.

Flow injection determination of hydrogen peroxide by means of a solid-state-peroxyoxalate chemiluminescence reactor

Abstract A solid-state reactor for detection of hydrogen peroxide in aqueous samples by peroxyoxalate chemiluminescence is described. Bis(2,4,6-trichlorophenyl)oxalate in solid form is packed into a bed reactor, which eliminates mixing problems and facilitates the instrumental development. Perylene is added as a sensitizer to a water/acetonitrile (20:80) carrier stream into which the samples (200–600 μl) are injected. Detection limits of 6 × 10 −9 M H 2 O 2 (0.2 μg l −1 ) are obtained with both a commercial and a home-made luminescence detector. Calibration graphs are linear up to 10 −5 M. The r.s.d. for 2 × 10 −7 M (6.7 μg −1 ) hydrogen peroxide ( n = 10) is 2.8%. Sample throughput is ca. 120 h −1 .

Excited state intramolecular proton transfer in some tautomeric azo dyes and schiff bases containing an intramolecular hydrogen bond

Photophysical properties of several basically important aromatic azodyes (1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol) and Schiff bases (N-(2-hydroxy-1-naphthylmethylidene) aniline and N-(1-hydroxy-2-naphthylmethylidene) aniline) all containing an intramolecular hydrogen bond were studied by both steady-state and time-resolved fluorescence spectroscopy with temperatures down to 98 K. It was found that the fluorescence results from the quinone form (H-form) only. The enol form (A-form) undergoes rapid excited state intramolecular proton transfer (ESIPT) resulting in the excited H-form. The compounds have relatively low quantum yields at room temperature, which increase considerably at low temperatures. Lifetime data at the different temperatures indicate that a substitution by both acceptor or donor groups on the para position in the phenyl ring decreases the deactivation rate and hence results in increased lifetime.

Some aspects of room-temperature phosphorescence in liquid solutions

Experimental requirements for room-temperature phosphorescence measurements in liquids (RTPL) are discussed. Phosphorescence quantum yields and triplet lifetimes of some brominated naphthalenes and halogenated biphenyls at 77 K in 2-methyltetrahydrofuran and at room temperature in hexane are reported and compared. Surprisingly the naphthalenes show only little loss in quantum yields in going from 77 K to room temperature. Sensitized phosphorescence is discussed as a means of expanding the analytical potential of RTPL. Results with a model system of benzophenone as a donor (analyte) and 1,4-dibromonaphthalene as an acceptor are presented.

A solid-state chemiluminescence detector for hydrogen peroxide based on an immobilized luminophore : Application to Rain Water

Abstract The construction and functioning of a chemiluminescence detector for hydrogen peroxide is described. It is based on peroxyoxalate chemiluminescence and consists of a two-bed reactor packed with solid trichlorophenyloxalate (TCPO) and 3-aminofluoranthene immobilized on controlled pore glass beads. Optimal conditions (pH, solvent, TCPO purity) for flow-independent operation are discussed. Samples can be injected into a moving stream or directly into the monitor with a syringe so as to provide a manually operated field monitor. The detection limit is 1.5 × 10−8 M, and calibration graphs are linear over six orders of magnitude. The r.s.d. for the manual monitoring mode is ±3% for 17 μg l−1 hydrogen peroxide. A sample throughput of 100 h−1 is possible in the flow injection mode, and 40 samples h−1 for manual injection.

Liquid chromatography-Fourier-transform infrared spectrometry

Over the past years the coupling of liquid chromatography (LC) and Fourier-transform infrared spectrometry (FT-IR) has been pursued primarily to achieve specific detection and/or identification of sample constituents. Two approaches can be discerned in the combination of LC and FT-IR. The first and simpler approach is to use a flow cell through which the effluent from the LC column is passed while the IR spectra are continuously recorded. The second approach involves elimination of the LC solvent prior to IR detection using an interface which evaporates the eluent and deposits the analytes onto a substrate. This paper provides a general overview of flow-cell based IR detection and briefly discusses early solvent-elimination interfaces for LC-FT-IR. A more comprehensive description is given of interface systems which use spraying to induce rapid eluent evaporation, and which basically represent the state-of-the-art in LC-FT-IR. Finally, the interface systems suitable for reversed-phase LC are summarized and the perspectives of LC-FT-IR are discussed. The overview indicates that flow-cell LC-FT-IR has rather poor detection limits but can be useful for the specific and quantitative detection of major constituents of mixtures. Solvent-elimination techniques, on the other hand, provide much better sensitivity and enhanced spectral quality which is essential when unambiguous identification of low-level constituents is required.

Liquid-Phase and Evanescent-Wave Cavity Ring-Down Spectroscopy in Analytical Chemistry

Due to its simplicity, versatility, and straightforward interpretation into absolute concentrations, molecular absorbance detection is widely used in liquid-phase analytical chemistry. Because this method is inherently less sensitive than zero-background techniques such as fluorescence detection, alternative, more sensitive measurement principles are being explored. This review discusses one of these: cavity ring-down spectroscopy (CRDS). Advantages of this technique include its long measurement pathlength and its insensitivity to light-source-intensity fluctuations. CRDS is already a well-established technique in the gas phase, so we focus on two new modes: liquid-phase CRDS and evanescent-wave (EW)-CRDS. Applications of liquid-phase CRDS in analytical chemistry focus on improving the sensitivity of absorbance detection in liquid chromatography. Currently, EW-CRDS is still in early stages: It is used to study basic interactions between molecules and silica surfaces. However, in the future this method may be used to develop, for instance, biosensors with high specificity.

Fluorescence Rejection in Resonance Raman Spectroscopy Using a Picosecond-Gated Intensified Charge-Coupled Device Camera

A Raman instrument was assembled and tested that rejects typically 98–99% of background fluorescence. Use is made of short (picosecond) laser pulses and time-gated detection in order to record the Raman signals during the pulse while blocking most of the fluorescence. Our approach uses an ultrafast-gated intensified charge-coupled device (ICCD) camera as a simple and straightforward alternative to ps Kerr gating. The fluorescence rejection efficiency depends mainly on the fluorescence lifetime and on the closing speed of the gate (which is about 80 ps in our setup). A formula to calculate this rejection factor is presented. The gated intensifier can be operated at 80 MHz, so high repetition rates and low pulse energies can be used, thus minimizing photodegradation. For excitation we use a frequency-tripled or -doubled Ti: sapphire laser with a pulse width of 3 ps; it should not be shorter in view of the required spectral resolution. Other critical aspects tested include intensifier efficiency as a function of gate width, uniformity of the gate pulse across the spectrum, and spectral resolution in comparison with ungated detection. The total instrumental resolution is 7 cm−1 in the blue and 15 cm−1 in the ultraviolet (UV) region. The setup allows one to use resonance Raman spectroscopy (RRS) for extra sensitivity and selectivity, even in the case of strong background fluorescence. Excitation wavelengths in the visible or UV range no longer have to be avoided. The effectiveness of this setup is demonstrated on a test system: pyrene in the presence of toluene fluorescence (λexc = 257 nm). Furthermore, good time-gated RRS spectra are shown for a strongly fluorescent flavoprotein (λexc = 405 nm). Advantages and disadvantages of this approach for RRS are discussed.