An approach that allows setting up under predefined ionization conditions a rugged self-consistent quantitative experimental scale of electrospray ionization (ESI) efficiencies of organic compounds is presented. By ESI ionization efficiency (IE) we mean the efficiency of generating gas-phase ions from analyte molecules or ions in the ESI source. The approach is based on measurement of relative ionization efficiency (RIE) of two compounds (B 1 and B 2 ) by infusing a solution containing both compounds at known concentrations (C 1 and C 2 ) and measuring the mass-spectrometric responses of the protonated forms of the compounds (R 1 and R 2 ). The RIE of B 1 and B 2 is expressed asThe relative way of measurement leads to cancellation of many of the factors affecting IE (ESI source design, voltages in the source and ion transport system, solvent composition, flow rates and temperatures of the nebulizing and drying gases). Using this approach an ESI IE scale containing ten compounds (esters and aromatic amines) and spanning over 4 logRIE units has been compiled. The consistency of the scale (the consistency standard deviation of the scale is s ¼ 0.16 logRIE units) was assured by making measurements using different concentration ratios (at least 6-fold concentration ratio range) of the compounds and by making circular validation measurements (the logRIE of any two compounds was checked by measuring both against a third compound). Copyright # 2008 John Wiley & Sons, Ltd.The electrospray ionization (ESI) method 1,2 is one of the most widely used ionization methods in contemporary mass spectrometry (MS). Its compatibility with liquid chromatography (LC), soft nature and virtual absence of restrictions on the molecular mass of the compounds have enabled its successful application to a vast number of different analytes. 3,4 The ESI method has played a key role in the spectacular success of LCMS during the recent years. Besides practical applications research has been carried out on elucidating the ESI mechanism 5-12 and on the principles that govern the ESI ionization efficiency of different analytes. 6,9 By ESI ionization efficiency we mean the efficiency of generating gas-phase ions from analyte molecules or ions in the ESI source. ESI is not an efficient method for generation of gas-phase ions. Only a small fraction of analyte molecules are ionized in the ion source and only a part of the resulting gas-phase ions are successfully transmitted to the mass analyzer and eventually detected. 7 The actual process of generating gas-phase ions in the ESI source is complex. A multitude of processes in the liquid phase as well as in the gas phase and on the gas-liquid boundary must be considered. Furthermore, solvents, additives and impurities may cause ionization suppression or an enhancement effect 8,11 that is frequently observed when introducing a solution containing different compounds into an ESI source (also known as the matrix effect). Several models for relating the response of an analyte ion in the mass spectrum to the a...

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The aim of this work was to evaluate the contributions of the main chromophores to the total UV absorbance of the spent dialysate and to assess removal dynamics of these solutes during optical on-line dialysis dose monitoring. High performance chromatography was used to separate and quantify UV-absorbing solutes in the spent dialysate sampled at the start and at the end of dialysis sessions. Chromatograms were monitored at 210, 254 and 280 nm routinely and full absorption spectra were registered between 200 and 400 nm. Nearly 95% of UV absorbance originates from solutes with high removal ratio, such as uric acid. The contributions of different solute groups vary at different wavelengths and there are dynamical changes in contributions during the single dialysis session. However, large standard deviation of the average contribution values within a series of sessions indicates remarkable differences between individual treatments. A noteworthy contribution of Paracetamol and its metabolites to the total UV absorbance was determined at all three wavelengths. Contribution of slowly dialyzed uremic solutes, such as indoxyl sulfate, was negligible.

The present study contributes new information on the removal of uremic retention solutes during hemodialysis and on the origin of the optical dialysis adequacy monitoring signal.