Advantages of atmospheric pressure photoionization mass spectrometry in support of drug discovery

The performance of the atmospheric pressure photoionization (APPI) technique was evaluated against five sets of standards and drug-like compounds and compared to atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). The APPI technique was first used to analyze a set of 86 drug standards with diverse structures and polarities with a 100% detection rate. More detailed studies were then performed for another three sets of both drug standards and proprietary drug candidates. All 60 test compounds in these three sets were detected by APPI with an overall higher ionization efficiency than either APCI or ESI. Most of the non-polar compounds in these three sets were not ionized by APCI or ESI. Analysis of a final set of 201 Wyeth proprietary drug candidates by APPI, APCI and ESI provided an additional comparison of the ionization techniques. The detection rates in positive ion mode were 94% for APPI, 84% for APCI, and 84% for ESI. Combining positive and negative ion mode detection, APPI detected 98% of the compounds, while APCI and ESI detected 91%, respectively. This analysis shows that APPI is a valuable tool for day-to-day usage in a pharmaceutical company setting because it is able to successfully ionize more compounds, with greater structural diversity, than the other two ionization techniques. Consequently, APPI could be considered a more universal ionization method, and therefore has great potential in high-throughput drug discovery especially for open access liquid chromatography/mass spectrometry (LC/MS) applications.     

Liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry: a highly sensitive method for the analysis of organic explosives

Gas chromatography/mass spectrometry (GC/MS) is applied to the analysis of volatile and thermally stable compounds, while liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (LC/APCI‐MS) and liquid chromatography/electrospray ionization mass spectrometry (LC/ESI‐MS) are preferred for the analysis of compounds with solution acid‐base chemistry. Because organic explosives are compounds with low polarity and some of them are thermally labile, they have not been very well analyzed by GC/MS, LC/APCI‐MS and LC/ESI‐MS. Herein, we demonstrate liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry (LC/NI‐APPI‐MS) as a novel and highly sensitive method for their analysis. Using LC/NI‐APPI‐MS, limits of quantification (LOQs) of nitroaromatics and nitramines down to the middle pg range have been achieved in full MS scan mode, which are approximately one order to two orders magnitude lower than those previously reported using GC/MS or LC/APCI‐MS. The calibration dynamic ranges achieved by LC/NI‐APPI‐MS are also wider than those using GC/MS and LC/APCI‐MS. The reproducibility of LC/NI‐APPI‐MS is also very reliable, with the intraday and interday variabilities by coefficient of variation (CV) of 0.2–3.4% and 0.6–1.9% for 2,4,6‐trinitrotoluene (2,4,6‐TNT). Copyright © 2008 John Wiley & Sons, Ltd.     

Comparison of APPI, APCI and ESI for the LC-MS/MS analysis of bezafibrate, cyclophosphamide, enalapril, methotrexate and orlistat in municipal wastewater

The applicability of three different ionization techniques: atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) was tested for the liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of five target pharmaceuticals (cyclophosphamide, methotrexate, bezafibrate, enalapril and orlistat) in wastewater samples. Performance was compared both by flow injection analysis (FIA) and on-column analysis in deionized water and wastewater samples. A column switching technique for the on-line extraction and analysis of water samples was used. For both FIA and on-column analysis, signal intensity and signal-to-noise (S/N) ratio of the target analytes in the three sources were studied. Limits of detection and matrix effects during the analysis of wastewater samples were also investigated. ESI generated significantly larger peak areas and higher S/N ratios than APCI and APPI in FIA and in on-column analysis. ESI was proved to be the most suitable ionization method as it enabled the detection of the five target compounds, whereas APCI and APPI ionized only four compounds.     

Phosphate buffers in capillary electrophoresis/mass spectrometry using atmospheric pressure photoionization and electrospray ionization

Capillary electrophoresis (CE) has been combined with atmospheric pressure photoionization (APPI) and electrospray ionization (ESI) for mass spectrometric (MS) detection. Separation conditions using potassium phosphate buffer and ammonium formate buffer have been compared for analysis of eleven pharmaceutical bases. The results showed improvements in separation efficiency and peak symmetry when phosphate buffer was used. The low flow in CE may enable utilization of these advances with MS detection. Compared with ESI, the APPI technique provided a cluster-free background. The enhanced signal-to-noise ratio in the total ion current (TIC) and the reduced spectral background indicated that the APPI process is less affected by non-volatile salts in the CE buffers. This results in a wider range of choice of CE buffers in CE/MS analysis when APPI is the ionization method.     

Identification and characterization of impurities in an insecticide,bifenthrin technical

The HPLC‐DAD and GC/MS methods were successfully used for the identification and characterization of the impurities in an agrochemical insecticide, bifenthrin technical. Three impurities ranging from 0.175%–0.541% were detected by the HPLC‐DAD method. The LC/MS technique with ESI or APCI source failed to detect the impurities detected by HPLC‐DAD, due to lack of ionization in ESI or APCI. The three impurities were enriched by prep‐HPLC, and then their structures were elucidated based on the GC/EIMS and CIMS data. The EI mass spectra of bifenthrin and its impurities displayed molecular ion and provided structure indicative fragment ions; the CIMS data further confirmed their molecular weight. The identity of the impurity 1 was further confirmed by the synthesis of the authentic sample followed by NMR and GC/MS data.     

Anisole, a new dopant for atmospheric pressure photoionization mass spectrometry of low proton affinity, low ionization energy compounds

Atmospheric pressure photoionization (APPI) is a novel method of ionization in liquid chromatography/mass spectrometry (LC/MS). It was originally developed in order to broaden the range of LC/MS ionizable compounds towards less polar compounds that cannot be analyzed by electrospray (ESI) and atmospheric pressure chemical ionization (APCI). Studies done thus far have shown that non-polar compounds that earlier were not ionizable in LC/MS can indeed be ionized by the use of APPI. However, the best ionization efficiency for low polarity samples has been achieved with low proton affinity (PA) solvents that are not suitable in reversed-phase LC (RP-LC). Here it is demonstrated that the signals for analytes with low proton affinities in acetonitrile can be increased 100-fold by using anisole as the dopant for APPI, which takes the sensitivity to the same level achieved in the analysis of high PA analytes.     

Dimerization of ionized 4‐(methyl mercapto)‐phenol during ESI,APCI and APPI mass spectrometry

A novel ion/molecule reaction was observed to occur under electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI) conditions, leading to dimerization of ionized 4‐(methyl mercapto)‐phenol followed by fast H^· loss. The reaction is particularly favored during ESI, which suggests that this ion/molecule reaction can occur both in the solution inside the ESI‐charged droplets and in the gas‐phase environment of most other atmospheric pressure ionization techniques. The dimerization reaction is inherent to the electrolytic process during ESI, whereas it is more by ion/molecule chemistry in nature during APCI and APPI. From the tandem mass spectrometry (MS/MS) data, accurate mass measurements, hydrogen/deuterium (H/D) exchange experiments and density functional theory (DFT) calculations, two methyl sulfonium ions appear to be the most likely products of this electrophilic aromatic substitution reaction. The possible occurrence of this unexpected reaction complicates mass spectral data interpretation and can be misleading in terms of structural assignment as reported herein for 4‐(methyl mercapto)‐phenol. Copyright © 2009 John Wiley & Sons, Ltd.     

LC‐MS analysis of estradiol in human serum and endometrial tissue: Comparison of electrospray ionization,atmospheric pressure chemical ionization and atmospheric pressure photoionization

Accurate measurement of estradiol (E2) is important in clinical diagnostics and research. High sensitivity methods are critical for specimens with E2 concentrations at low picomolar levels, such as serum of men, postmenopausal women and children. Achieving the required assay performance with LC–MS is challenging due to the non‐polar structure and low proton affinity of E2. Previous studies suggest that ionization has a major role for the performance of E2 measurement, but comparisons of different ionization techniques for the analysis of clinical samples are not available. In this study, female serum and endometrium tissue samples were used to compare electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) in both polarities. APPI was found to have the most potential for E2 analysis, with a quantification limit of 1 fmol on‐column. APCI and ESI could be employed in negative polarity, although being slightly less sensitive than APPI. In the presence of biological background, ESI was found to be highly susceptible to ion suppression, while APCI and APPI were largely unaffected by the sample matrix. Irrespective of the ionization technique, background interferences were observed when using the multiple reaction monitoring transitions commonly employed for E2 (m/z 271 > 159; m/z 255 > 145). These unidentified interferences were most severe in serum samples, varied in intensity between ionization techniques and required efficient chromatographic separation in order to achieve specificity for E2. Copyright © 2013 John Wiley & Sons, Ltd.     

New perspective on the determination of flame retardants in sewage sludge by using ultrahigh pressure liquid chromatography–tandem mass spectrometry with different ion sources

Analysis of 11 polybrominated diphenyl ethers (PBDEs), tetrabromobisphenol A bis 2,3-dibromopropylether (TBBPA-bis), tetrachlorobisphenol A (TCBPA), tetrabromobisphenol A (TBBPA) and hexabromocyclododecanes (HBCDs) was optimized by ultrahigh pressure liquid chromatography/tandem mass spectrometry (UPLC–MS/MS) operating in negative ion (NI) mode. Electrospray ionization (ESI), atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization (APCI) sources were tested and for PBDEs APCI gave higher sensitivity than APPI while for TBBPA-bis APCI and APPI showed similar performance. ESI was the best option for TCBPA, TBBPA and HBCDs. Detection limits were between 20 and 59 fg for the compounds analyzed by ESI, 0.10 and 0.72 pg for PBDEs and 6 pg for TBBPA-bis. The matrix effect of sewage sludge extract was also tested showing negligible ion suppression for APCI and an increase of the background level of all investigated pollutants leading to a worsening of the limits of quantification by a factor between 1.2 and 3.3. The UPLC-APCI/MS/MS method for PBDEs, after pressurized liquid extraction (PLE), was validated by comparison with the concentration values from the NIST 1944 standard reference material. The advantages of the methods include low detection limits, PBDE congeners specificity using selected multiple reaction monitoring (MRM) transitions, and the absence of thermal degradation of higher PBDE congeners, especially BDE-209. The methods were applied for the determination of the above reported flame retardants in sewage sludge in order to get more information about the degradation on PBDEs (in particular BDE-209) during municipal wastewater treatments.     

Determination of carotenoids by liquid chromatography/mass spectrometry: effect of several dopants

Various carotenoids were analyzed by ultra-high-pressure liquid chromatography with tandem mass spectrometry detection (UHPLC-MS/MS). Three different techniques to ionize the carotenoids were compared: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). For all of the carotenoids tested, it was possible to obtain characteristic transitions for their unequivocal identification using each ionization technique. APCI was shown to be a more powerful technique to ionize the carotenoids than ESI or APPI. Transitions to differentiate carotenoids that coelute were determined to distinguish antheraxanthin from astaxanthin and lutein from zeaxanthin. In addition, four dopants were evaluated to improve ionization and enhance the carotenoid signal strength in APPI. These dopants were acetone, toluene, anisole, and chlorobenzene. Carotenoids improved their response in almost all cases when a dopant was used. The use of dopants allowed the enhancement of the carotenoid signals strength up to 178-fold.     

Comparison of atmospheric pressure photoionization and electrospray ionization mass spectrometry for the analysis of UV filters

A comparison was made between the electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) tandem mass spectrometric (MS/MS) responses of eleven ultraviolet (UV) filters. Four of the target compounds were favourably ionized in negative ion mode, and the other seven compounds in positive ion mode. For nine of the compounds APPI generated a similar response to that of ESI, but the APPI signal‐to‐noise (S/N) ratios were 1.3–60 times higher. The two most polar of the UV filter compounds (PBSA and BP‐4) were more efficiently ionized by ESI, offering higher signal intensities and lower detection limits. APPI was, however, less susceptible to ion suppression than ESI when real samples were injected. In order to optimize the APPI conditions different dopant solvents were examined to enhance the efficiency of the photoionization process. Among the evaluated dopants, toluene was selected as the best compromise. At a toluene flow rate of 10% of the solvent flow rates the ionization response increased by a factor of 40–50 over the use of no dopant for the compounds in positive ion mode and by more than 300 for the compounds in negative ion mode. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of ionization techniques for mass spectrometric detection of contact allergenic hydroperoxides formed by autoxidation of fragrance terpenes

Hydroperoxides formed by autoxidation of common fragrance terpenes are strong allergens and known to cause allergic contact dermatitis (ACD), a common skin disease caused by low molecular weight chemicals. Until now, no suitable methods for chemical analyses of monoterpene hydroperoxides have been available. Their thermolability prohibits the use of gas chromatography and their low UV-absorption properties do not promote sensitive analytical methods by liquid chromatography based on UV detection. In our study, we have investigated different liquid chromatography/mass spectrometry (LC/MS) ionization techniques, electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI), for detection of hydroperoxides from linalool and limonene.Flow injection analysis was used to evaluate the three different techniques to ionize the monoterpene hydroperoxides, linalool hydroperoxide and limonene hydroperoxide, by estimating the signal efficacy under experimental conditions for positive and negative ionization modes. The intensities for the species [M+H]+ and [M+H-H2O]+ in positive ionization mode and [M-H]- and [M-H-H2O]- in negative ionization mode were monitored. It was demonstrated that the mobile phase composition and instrumental parameters have major influences on the ionization efficiency of these compounds. ESI and APCI were both found to be appropriate as ionization techniques for detection of the two hydroperoxides. However, APPI was less suitable as ionization technique for the investigated hydroperoxides.     

On-line strategies for determining trace levels of nitroaromatic explosives and related compounds in water

We report the development and tests of several systems for the simultaneous determination of 18 energetic compounds and related congeners in untreated water samples. In these systems a Restricted Access Material trap or liquid-chromatography precolumn (with a C(18) or porous graphitic carbon, PGC, stationary phase) followed by a PGC analytical column are used for sample clean-up, enrichment and separation of the trace level analytes, which are then analyzed by mass spectrometry (MS). The relative merits of two MS ionization interfaces (atmospheric pressure chemical ionization, APCI, and atmospheric pressure photoionization, APPI) were also compared for the MS identification and quantification of these analytes. APCI was found to be superior in cases where both alternatives are applicable. A major drawback when applying APPI is that no signal is obtained for the cyclic nitramines and nitrate esters. Using APCI, a wide spectrum of unstable compounds can be determined in a single analysis, and the feasibility of using large volume samples (up to 100 mL) in combination with the sensitivity of the MS detection system provide method detection limits ranging from 2.5 pg/mL (for 2,4-dinitrotoluene and 2,6-diamino-6-nitrotoluene) to 563 pg/mL (for pentaerythritol tetranitrate, PETN), with repeatability ranging from 2 to 7%. Other chemometric parameters such as robustness, selectivity, repeatability, and intermediate precision were also evaluated in the validation of the extraction methods for use in water analysis. Tests with untreated groundwater and drinking water samples, spiked with 20 ng of the analytes, yielded results similar to those obtained with high purity water samples.     

Comparison of multiple API techniques for the simultaneous detection of microconstituents in water by on‐line SPE‐LC‐MS/MS

This study described a fully automated method using on‐line solid phase extraction of large volume injections coupled with high performance liquid chromatography (HPLC) and tandem mass spectrometry (MS/MS) to simultaneously detect a group of recalcitrant microconstituents (pharmaceuticals and personal care products, steroid hormones and sterols) in aqueous matrices. Samples (1 mL to 20 mL) were loaded to the preconcentration column at 1 mL/min, and the column was washed with 1000 μL of 25% methanol in LC/MS water to remove polar and ionic interferences before LC‐MS/MS analysis. Three different atmospheric pressure ionization (API) techniques, including photoionization (APPI) with four different dopants (acetone, anisole, chlorobenzene and toluene), heated electrospray ionization (HESI) and atmospheric pressure chemical ionization (APCI), were evaluated on the basis of method detection limits (MDLs) and recoveries from different aqueous matrixes. Results indicated that APPI with toluene as dopant was the most sensitive ionization method for the majority of the analytes. When using 5 mL of sample, MDLs for pharmaceuticals and personal care products, including carbamazepine, DEET, caffeine, naproxen, acetaminophen and primidone, were between 0.3 ng/L and 15 ng/L. MDLs of hormones, including testosterone, equilenin, progesterone, equilin, 17β‐estradiol, 17α‐ethynylestradiol, estrone, androsterone, mestranol and estriol, were between 1.2 ng/L and 37 ng/L. The combination of APPI with dopant allowed the detection of two difficult to ionize fecal related sterols, such as coprostan‐3‐ol and coprostan‐3‐one with MDLs of 5.4 ng/L and 11 ng/L, respectively. Calculated MDLs are more than adequate for analysis of wastewater using 1 to 5 mL sample size and for surface waters using up to 20 mL sample size. Copyright © 2012 John Wiley & Sons, Ltd.     

Fourier transform ion cyclotron resonance mass spectrometry using atmospheric pressure photoionization for high-resolution analyses of corticosteroids

Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) was coupled with atmospheric pressure photoionization (APPI) for the first time and used for the analysis of several corticosteroids.1 The analytes showed excellent response using APPI when compared with both electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). APPI has the advantage of requiring less heat for desolvation, resulting in less thermal degradation of the analytes and higher signal-to-noise than APCI. In terms of ultimate sensitivity, APPI is more efficient than either ESI or APCI for the analysis of corticosteroids. With some compounds, the high-resolution capability of FTICRMS was necessary to obtain an accurate mass due to contributions of the M(+.) (13)C isotope in the [M+H](+) ion peak.     

Atmospheric pressure photoionization. II. Dual source ionization

In this paper we describe results based on the combination of atmospheric pressure photoionization (APPI) with atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). The main purpose of combining more than one ionizer is to extend the range of compounds that can be simultaneously analyzed. Three modes of operation are presented; use of either ionizer, simultaneous use of two ionizers, and rapid switching between ionizers during a single chromatographic run. The dual ionizer configurations only minimally affect the performance of either ionizer relative to the standard single-ionizer sources. However, it is observed that the operation of both ionizers together does not typically give the sum signal from either source operating alone. For APCI/APPI the signal can range from less than that of either source alone to the sum of the two individual sources. For ESI/APPI, we observed large suppressions of the ESI multiply-charged signal of proteins when the APPI source was on. These behaviors are presumed to be due to the interaction of the initially formed ions by both sources and attests to the importance of ion-molecule reactions that occur during and after the primary ionization events. We give examples of compounds that are preferentially ionized by either APPI, APCI or ESI and present thermochemical arguments based on molecular structure and functionality to explain this behavior. The dual source is also shown to be able to operate in negative ion mode opening up the potential to conduct wide ranging chemical analyses.     

Atmospheric pressure photoionization for coupling liquid-chromatography to mass spectrometry: A review

This review presents the state-of-the-art techniques that couple liquid chromatography (LC) and mass spectrometry (MS) via atmospheric pressure photoionization (APPI). The different ionization mechanisms are discussed as well as the influence of the mobile phase composition, the nature of the dopant, etc. A comparison with other ionization sources, such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), is reported, and the combination of APPI with these sources is also discussed. Several applications, covering the time period of 2005-2008, for the analysis of drugs, lipids, natural compounds, pesticides, synthetic organics, petroleum derivatives, and other substances are presented.     

Atmospheric-pressure laser ionization: a novel ionization method for liquid chromatography/mass spectrometry

We report on the development of a new laser-ionization (LI) source operating at atmospheric pressure (AP) for liquid chromatography/mass spectrometry (LC/MS) applications. APLI is introduced as a powerful addition to existing AP ionization techniques, in particular atmospheric-pressure chemical ionization (APCI), electrospray ionization (ESI), and atmospheric pressure photoionization (APPI). Replacing the one-step VUV approach in APPI with step-wise two-photon ionization strongly enhances the selectivity of the ionization process. Furthermore, the photon flux during an ionization event is drastically increased over that of APPI, leading to very low detection limits. In addition, the APLI mechanism generally operates primarily directly on the analyte. This allows for very efficient ionization even of non-polar compounds such as polycyclic aromatic hydrocarbons (PAHs). The APLI source was characterized with a MicroMass Q-Tof Ultima II analyzer. Both the effluent of an HPLC column containing a number of PAHs (benzo[a]pyrene, fluoranthene, anthracene, fluorene) and samples from direct syringe injection were analyzed with respect to selectivity and sensitivity of the overall system. The liquid phase was vaporized by a conventional APCI inlet (AP probe) with the corona needle removed. Ionization was performed through selective resonance-enhanced multi-photon ionization schemes using a high-repetition-rate fixed-frequency excimer laser operating at 248 nm. Detection limits well within the low-fmol regime are readily obtained for various aromatic hydrocarbons that exhibit long-lived electronic states at the energy level of the first photon. Only molecular ions are generated at the low laser fluxes employed ( approximately 1 MW/cm(2)). The design and performance of the laser-ionization source are presented along with results of the analysis of aromatic hydrocarbons.     

Comparison of electrospray ionization,atmospheric pressure chemical ionization,and atmospheric pressure photoionization for the analysis of dinitropyrene and aminonitropyrene LC-MS/MS

The only relevant source for human exposure to dinitropyrenes is diesel engine emissions. Due to this specificity, dinitropyrenes may be used as biomarkers for monitoring human exposure to diesel engine emissions. Only few analytical methods have been described for the quantitation of dinitropyrenes and their metabolites, aminonitropyrenes, and diaminopyrenes. Therefore, for dinitropyrenes, aminonitropyrenes, and diaminopyrenes were selected as model compounds for the development of a sensitive HPLC-MS/MS method (high performance liquid chromatography coupled to triple quadrupole mass spectrometry) was to quantify polyaromatic amines and nitroarenes in biological matrices was developed optimal methods by comparing electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) sources. Dinitropyrene was not effectively ionized and diaminopyrene yielded mainly [M(.)](+) ions by electrospray ionization. With APCI and APPI, precursor ions of diaminopyrene and aminonitropyrene were [M + H](+) and [M(.)](-) for dinitropyrene. Precursor ions with [M - 30(.)](-) for dinitropyrene and [M - 30 + H](+) for aminonitropyrene were observed. Reversed and normal phase HPLC-MS/MS with ESI, APCI and APPI were optimized separately with respect to unequivocal analyte identification and sensitivity. Normal phase HPLC coupled to APPI-MS/MS gave the highest precision and sensitivity for aminonitropyrene (6%/0.2 pg on column) and dinitropyrene (9%/0.5 pg on column). The limit of detection in spiked rat plasma was 5 pg/100 microL for aminonitropyrene (accuracy 82%) and 10 pg/100 microL for dinitropyrene (accuracy 105%). In plasma of rats treated with dinitropyrene by oral administration, no detectable levels of dinitropyrene but higher aminonitropyrene levels compared with intratracheal instillation were observed. These findings clearly demonstrate that dinitropyrene was absorbed after oral and intratracheal application and that a reduction of nitro groups occurs to a high extent in the reductive environment of the intestine. To our knowledge, this is the first time that aminonitropyrene was observed in plasma after intratracheal or oral administration directly demonstrating the reductive metabolism of dinitropyrene in vivo.     

Effect of eluent on the ionization process in liquid chromatography-mass spectrometry

The most widely used ionization techniques in liquid chromatography-mass spectrometry (LC-MS) are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). All three provide user friendly coupling of LC to MS. Achieving optimal LC-MS conditions is not always easy, however, owing to the complexity of ionization processes and the many parameters affecting mass spectrometric sensitivity and chromatographic performance. The selection of eluent composition requires particular attention since a solvent that is optimal for analyte ionization often does not provide acceptable retention and resolution in LC. Compromises must then be made between ionization and chromatographic separation efficiencies. The review presents an overview of studies concerning the effect of eluent composition on the ionization efficiency of ESI, APCI and APPI in LC-MS. Solvent characteristics are discussed in the light of ionization theories, and selected analytical applications are described. The aim is to provide practical background information for the development and optimization of LC-MS methods.