Mass-sensitive applications

Mass Sensitive Applications. Uses of piezoelectric materials in mass-sensitive oscillators encompass several configurations (Fig. 2). Mechanical oscillations propagated perpendicularly between the parallel faces of a thin quartz crystal... 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

Coupled LC-LC can separate high-boiling petroleum residues into groups of saturates, olefins, aromatics and polar compounds. However, the lack of a suitable mass-sensitive, universal detector in LC makes quantitation difficult SFC-SFC is more suitable for this purpose. Applications of multidimensional HPLC in food analysis are dominated by off-line techniques. MDHPLC has been exploited in trace component analysis (e.g. vitamin assays), in which an adequate separation for quantitation cannot be achieved on a single column [972]. LC-LC-GC-FID was used for the selective isolation of some key components among the irradiation-induced olefinic degradation products in food, e.g. dienes and trienes [946],... 

LOD for 1,2-xylene at the SAW chemosensor was as low as 0.1 pL L 1 at the appreciable resonant frequency change of 6 Hz. In contrast, LOD for 1,2-xylene at the QCM chemosensor, coated with a similar film, was rather high and equal to 4 pL L 1 at the resonant frequency change of merely 0.7 Hz. Apparently, 40-fold lower LOD for the SAW chemosensor indicates its prospective application for mass-sensitive gas sensing. 

Z. Xie, Q. Liao, X. Xu, M. Yao, J. Wan, D. Liu, Rapid and sensitive determination of donepezil in human plasma by liquid chromatography/tandem mass spectrometry application to a pharmacokinetic study, Rapid Commun. Mass Spectrom. 20 (2006) 3193-3198. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

Charlton, T.S., de Nys, R., Netting, A., Kumar, N., Hentzer, M., Givskov, M., and Kjelleberg, S., A novel and sensitive method for the quantification of JV-3-oxoacyl homoserine lactones using gas chromotography-mass spectrometry application to a model bacterial biofilm, Environ. Microbiol., 2, 530, 2000. 

The internal diameter of a column will affect the sample load, the peak dilution, and the flow rate. The larger the inner diameter, the greater is the loading capacity and the higher is the flow rate. However, peak dilution increases with internal diameter, and therefore mass sensitivity decreases. Most analytical columns range from 2 to 5 mm in diameter. Narrow-bore (or smaller) columns with diameters of 2 mm or less are used for applications where high sensitivity is required, the amount of sample is limited, or solvent purchase and disposal costs are significant. 

Sweedler et al.224 have exploited the mass sensitivity of pCoil NMR probes in the development of micromixer-based time-resolved NMR, demonstrating the application of the method to the conformation of the protein ubiquitin. To probe changes in protein conformation based on the elapsed time after a change in solvent composition, two capillary flows are mixed rapidly followed by the acquisition of... 

The high mass sensitivity of ETSM sensors renders them particularly suited for the analysis of monolayer and submonolayer films. In fact, the earliest applications of the ETSM involved studying the electrochemical deposition of monolayers, including the formation of metal oxides [207], electrosorption of halides [208], and the underpotential deposition of metal atoms [209-213]. In some cases, the electrovalency (i.e., the ratio of moles of electrons transferred at the electrode to moles of adsorbate deposited) was found to vary with adsorbing species the adsorption of iodide onto gold, for example, occurs with complete charge transfer from the halide to the electrode, whereas the adsorption of bro-... 

Surface mass changes can result from sorptive interactions (i.e., adsorption or absorption) or chemical reactions between analyte and coating, and can be used for sensing applications in bodi liquid and gas phases. Although the absolute mass sensitivity of the uncoated sensor depends on the nature of the piezoelectric substrate, device dimensions, frequoicy of operation, and the acoustic mode that is utilized, a linear dependence is predicted in all cases. This allows a very general description of the working relationship between mass-loading and frequency shift, A/ , for AW devices to be written ... 

Yet another significant challenge to the successful use of AW sensors is the isolation of their sensitivities to numerous different perturbations, so that only a single, desired interaction is observed. As an example, AW sensors are sensitive to enviixmmental variables such as temperature, pressure, and gas or liquid flow rate in mass-sensing applications, excessive response to these variables can be a serious problem. Controlling the AW sensor environment is the focus of Section 6.4. 

MIP sensor elements are also suitable for the analysis of multicomponent samples. The cost-effective, miniaturised, non-covalent MIP sensor arrays, when combined with computational data evaluation, make weak artificial recognition phenomena highly applicable for smart sensors. In comparison to gas or liquid chromatography, the results with mass-sensitive MIP sensors are faster and cheaper to obtain [32]. For effective on-line monitoring, the ideal MIP sensor or actuator should allow reversible analyte enrichment without dependencies on intermediate washing procedures (with organic solvents, for example). 

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