Research and Development in Liquid Chromatography and Mass Spectrometry







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Anal. Chem. 2016, 88, 3435–3439. Establishing Atmospheric Pressure Chemical Ionization Efficiency Scale

Tetraalkylammonium ions, amines, guanidines, pyridines, anilines, phosphines (phosphanes), esters, carboxylic acids, acridine, pyrene, anthracene

H2O : MeCN

APCI-MS in positive mode (APCI+ mode)

For the first time an ionization efficiency scale for APCI source has been created. The scale spans over 5 logIE units and includes 40 compounds with a wide range of chemical and physical properties. The results of the experiments show that for most of the compounds the ionization efficiency order in the APCI source is surprisingly similar to that in the ESI source. Most of the compounds that are best ionized in the APCI source are not small volatile molecules. Large tetraalkylammonium cations are a prominent example. At the same time low-polarity hydrocarbons pyrene and anthracene are ionized in the APCI source but not in the ESI source. These results strongly imply that in APCI several ionization mechanisms operate in parallel and mechanism(s) not relying on evaporation of neutral molecules from droplets has significantly higher influence than commonly assumed.

APCI Ionization Efficiency scale (PDF)

J. Am. Soc. Mass. Spectrom. 2016 DOI: 10.1007/s13361-016-1384-2 Ionization Efficiency of Doubly Charged Ions Formed from Polyprotic Acids in Electrospray Negative Mode

Polyprotic acids (disulfonic acids, sulfo-carboxylic acids, etc)

H2O : MeCN
H2O : Methanol

ESI-MS in negative mode

The ability of polyprotic acids to give doubly charged ions in negative mode electrospray was studied and related to physicochemical properties of the acids. It was discovered that the compound has to be strongly acidic (low pK a1 and pK a2) and to have high hydrophobicity (logP ow) to become multiply charged. Ability to give multiply charged ions in ESI/MS cannot be directly predicted from the solution phase acidities. A quantitative model to predict the charge state of the analyte in ESI/MS is proposed and validated for small anions. Results indicate that acidity of the analyte, its octanol-water partition coefficient, and charge delocalization are important factors that influence ionization efficiencies as well as charge states of the analytes. The pH of the solvent was also found to be an important factor influencing the ionization efficiency of doubly charged ions.


Anal. Chem. 2015, 87, 2623−2630

Absolute acidity values

H2O : MeCN
H2O : Methanol

Any reversed phase LC method

This work introduces a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) and liquid chromatography mass spectrometry (LC-MS) mobile phases. The new approach is based on the recently introduced unified pH (pHabs) scale, which enables direct comparison of acidities of solutions made in different solvents, based on chemical potential of the proton in the solutions. This work represents the first experimental realization of the pHabs concept using differential potentiometric measurement for comparison of the chemical potentials of the proton in different solutions (connected by a salt bridge), together with earlier published reference points for obtaining the pHabs values (referenced to the gas phase) or pHabsH2O values (referenced to the aqueous solution). pHabs values for a number of common LC and LC-MS mobile phases have been determined.

Tables of unified pH values (acidities) of common LC and LC-MS mobile phases (PDF)

Tutorial review on validation of liquid chromatography–mass spectrometry methods: Parts I and II. A. Kruve, R. Rebane, K. Kipper, M.-L. Oldekop, H. Evard, K. Herodes, P. Ravio, I. Leito.

Anal. Chim. Acta 2015, 870, 29-44

Anal. Chim. Acta 2015, 870, 8-28



Any LC-MS method

This two-part tutorial review intends to give an overview of the state of the art of method validation in liquid chromatography mass spectrometry (LC–MS), especially with electrospray ionisation (LC-ESI-MS), and discuss specific issues that arise with MS (and MS-MS) detection (i.e. LC-MS-MS) in LC (as opposed to the “conventional” detectors).

The review addresses and compares all the major validation guidelines published by international organizations: ICH, IUPAC, AOAC, FDA, EMA (EMEA), Eurachem, SANCO, NordVal, European Commission Decision 2002/657/EC. With every performance characteristic the tutorial review briefly compares the recommendations of the guidelines.


Journal of Food Composition and Analysis 2015, 41, 221–225

Pesticides (thiabendazole, aldicarb, imazalil, methomyl and methiocarb)

Fruits and vegetables

electrospray mass spectrometry (ESI-MS) with paper spray ionization

A systematic proof of concept study of paper spray mass spectrometry method for pesticide detection as a screening method is presented. Two sampling approaches – wiping the surface with paper and applying the sample homogenate directly on the paper – were used. The wiping method was more extensively studied for imazalil and thiabendazole originally present in oranges. For homogenized samples three matrices – oranges, tomatoes and grapes – and five pesticides of different chemical nature and polarity – thiabendazole, aldicarb, imazalil, methomyl and methiocarb – were chosen. It has been shown that limits of detection below maximum residue levels can be achieved for both methods (e.g. imazalil and thiabendazole detection limits were found to be lower than 5 mg/kg). The methods are therefore suitable for fast screening of samples.


J. Agric. Food Chem. 2014, 62, 5259−5268

Perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), and perfluorooctanesulfonic

acid (PFOS)


LC-MS/MS (liquid chromatograpy, electrospray mass spectrometry (ESI-MS) in negative mode)

Perfluoroalkyl acids (PFAAs) are extensively used in industry for different purposes and are harmful for living organisms. This work focuses on chromatographic analysis of three PFAAs in fish. The analytes, are perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), and perfluorooctanesulfonic acid (PFOS). Fluorinated alcohols are used as eluent components, and their possible advantages as eluent modifiers in LC-MS analysis of PFAAs, alternative retention mechanism and enhanced ionization efficiency, are examined. The analyzed fish samples originating from Estonian fresh and marine waters had low contents of PFAAs.


Anal. Chem. 2014, 86, 4822−4830. Negative Electrospray Ionization via Deprotonation: Predicting the Ionization Efficiency

Phenols (incl nitrophenols, alkylphenols, pentafluorophenoil, pentachlorophenol, pentabromophenol), carboxylic acids (e.g. benzoic acid, substituted benzoic acids, sorbic acid, salicylic acid, 4-aminobutanoic acid)

0.1% aqueous ammonia solution and acetonitrile in 20:80 volume ratio

electrospray mass spectrometry (ESI-MS) in negative mode

ESI ionization efficiency scale has been established via deprotonation of substituted phenols and benzoic acids. Electrospray ionization efficiency correlates with charge delocalization in anions [quantified by the weightedaverage positive sigma (WAPS) parameter rooted in the COSMO

theory] and with the pKa values of the acids. The higher the WAPS value and the lower the pKa the is the ionization efficiencyin the negative-ion ESI/MS. A predictive model was established and successfully validated using a test set of acids belonging neither to phenols nor to benzoic acids.

Electrospray ionization efficiency scale, including also pKa values, WAPS values, etc.


Scheme of ESI ionization efficiency measurement (PDF)

J. Am. Soc. Mass Spectrom. 2012, 23, 2051-2054. Enhanced Nebulization Efficiency of Electrospray Mass Spectrometry: Improved Sensitivity and Detection Limit

In principle, any
(experiments have been done with Carbendazim, Imazalil, Thiabendazole and Methiocarb)

Onion, Garlic, Apple

with novel ion source design

New, more efficient ESI ion source has been created. The essence of the idea is to modify the nebulizer (the key component of an ESI source) by adding an additional capillary directing the nebulization gas right into the stream of solution (the thin innermost capillary on the image). The prototype of this ion source has been built (see the image) and experiments have shown that this design offers significantly enhanced ionization efficiency compared with the classic nebulizer design and leads to improved sensitivity (by three to 10 times) and decreases the detection limit, on an average 10 times.

Image of the ion source

Journal of Chromatography A, 2011, 1218, 8175– 8180. Fluoroalcohols as novel buffer components for basic buffer solutions for liquid chromatography electrospray ionization mass spectrometry: Retention mechanisms

2-Tertbutylphenol (pKa = 10.28), 2,4-Dinitrophenol (pKa = 5.15), 2,6-Dimethylpyridine (pKa = 6.65), Pyrrolidine (pKa = 11.27), 2-Nitroaniline (pKa = 17.9), 2-Nitrophenol (pKa = 7.17), 2-Methylpyridine (pKa = 5.97), Diethylamine (pKa = 11.02), 3-Nitroaniline (pKa = 17.9), 2,3,5,6-Tetrafluorophenol (pKa = 5.53), 2-Methoxypyridine (pKa = 6.47), Piperidine (pKa = 11.12), 4-Chloro-2-nitroaniline (pKa = 17.1), 2,3,4,5,6-Pentafluorophenol (pKa = 5.41), Diisopropylamine (pKa = 11.05)

(in brackets: pKa values in water)

NA (model systems were used)


Two fluoroalcohols – 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HFTB) – offer interesting possibilities to adjust retention behavior of different analytes and expand the currently rather limited range of ESI-compatible buffer systems for basic mobile phases. The fluoroalcohols did not suppress the ionization of the analytes and for several analytes ionization enhancement was observed.

All trends in retention of the acidic and basic analytes can be interpreted by the following model: the neutral fluoroalcohols are quite strongly retained by the stationary phase whereas their anions are less retained, thus their amount on the stationary phase is dependent on mobile phase pH; the anions of the fluoroalcohols form ion pairs in the mobile phase with the basic analytes; the fluoroalcohols on the stationary phase surface compete with acidic analytes thereby hindering their retention; the fluoroalcohols on the stationary phase bind basic analytes thereby favoring their retention.


Rapid Commun. Mass Spectrom. 2011, 25, 1159–1168. Accounting for matrix effects of pesticide residue liquid chromatography/electrospray ionisation mass spectrometric determination by treatment of background mass spectra with chemometric tools

Pesticides: thiabendazole, carbendazime, methomyl, aldicarb, imazalil and methiocarb

garlic and onion

Sample preparation: buffered QuEChERS;

Determination: LC-ESI-MS

Data treatment: PCA and PLS regression

The utility of background ions in MS1 scan spectra was assessed to account for and/or predict matrix effect in LC-ESI-MS analysis using MS2 signals. It was found that a number of ion intensities related to matrix effect are detectable from the MS1 scan spectra and could be used in a partial leastsquares (PLS) model to calculate the actual concentration of the analyte in the sample. The accuracy of the PLS method was considerably higher compared to the classical solvent calibration.


Electrospray Ionization Efficiency Scale of Organic Compounds. Talk given at 14th Nordic MS Conference (Uppsala, 2010)

Anilines, pyridines, guanidines, amines, esters,phosphazenes,DBU, benzoic acid, methiocarb, methomyl, aldicarb, piperidine, pyrrolidine, 1- naphtylamine,acridine, EMIM+, HexMIM+, tetraalkylammonium cations

MeCN : 0.1% formic acid (80 : 20)

Electrospray Ionization efficiency measurement in the ESI ion source

The talk gives an overview of the current status of the research of ESI ionization efficiency. This topic of a very broad importance to the LC-ESI-MS analysis and its many specific issues (e.g. ionization suppression by the the matrix effects, quantitation problems, etc)

Full PDF printout of the slides of the talk

Anal. Chem. 2010, 82, 2865-2872. Electrospray Ionization Efficiency Scale of Organic Compounds

See also Poster on ESI ionization efficiency presented at the ACS Spring meeting 2011 in Anaheim

Anilines, pyridines, guanidines, amines, esters,phosphazenes,DBU, benzoic acid, methiocarb, methomyl, aldicarb, piperidine, pyrrolidine, 1- naphtylamine,acridine, EMIM+, HexMIM+, tetraalkylammonium cations

MeCN : 0.1% formic acid (80 : 20)

Electrospray Ionization efficiency measurement in the ESI ion source

The scale of logRIE values containing 62 different compounds and spanning for 6 logIE units-a million-fold difference in ionization efficiencies-has been established. The higher the ionization efficiency of a given compound, the better it is detectable by ESI-MS (ionization via monoprotonation). Initial correlations between logIE values of the compounds and their molecular parameters have been carried out using a linear regression model. The parameters that are most influential in predicting the ionization in ESI source are the pKa value of the compound in water and the logarithm of molecular volume of the compound.

ESI Ionization efficiency scale

J. AOAC Intl. 2010, 93, 306-314. Electrospray Ionization Matrix Effect as an Uncertainty Source in HPLC/ESI-MS Pesticide Residue Analysis

Pesticides: thiabendazole, aldicarb, imazalil and methiocarb

Tomato, cucumber and sweet corn

Sample preparation: buffered QuEChERS;

Determination: LC-ESI-MS

This work presents an empirical approach—the matrix effect graph approach—for estimating the uncertainty due to the matrix effect in HPLC/ESI-MS analysis of pesticide residues in fruits and vegetables. The root mean square of the relative residuals on the matrix effect graph are used as the estimate of relative uncertainty of the sample peak areas caused by the matrix effect. The approach was validated with tomato, cucumber, and sweet corn matrixes at the 0.5 mg/kg concentration level.


Anal. Chim. Acta 2009, 651, 75-80. Combating matrix effects in LC/ESI/MS: The extrapolative dilution approach

Pesticides: methomyl, thiabendazole, aldicarb, imazalil and methiocarb

tomato, cucumber, apple, rye, garlic

Sample preparation: QuEChERS

Determination: LC-ESI-MS

Matrix effect is investigated as the function of dilution. It is demonstrated that in some cases dilution can eliminate matrix effect, but often it is just reduced. A new quantitation method based on consecutive dilutions of the sample and extrapolation of the analyte content to the infinite dilution, i.e. to matrix-free solution (where ionization suppression is absent) was proposed.


The method was validated for LC/ESI/MS analysis of five pesticides in five matrices at two concentration levels (0.5 and 5.0 mg kg−1).


Journal of Chromatography A, 2009, 1216, 5949–5954. Simultaneous determination of fluoroquinolones, sulfonamides and tetracyclines in sewage sludge …

three fluoroquinolones: ciprofloxacin, norfloxacin, ofloxacin; two tetracyclines: tetracycline, doxycycline; two

sulfonamides: sulfadimethoxine, sulfamethoxazole

Sewage sludge

Sample preparation: Pressurized liquid extraction (PLE) followed by SPE for purification of the extracts

Determination: LC ESI MS in SRM mode

The best recovery of FQs and TCs was obtained by using hydrophilic–lipophilic balance cartridges, recoveries ranged 59% for norfloxacin to 82% for ofloxacin and 95% for doxycycline; for SAs strong cation-exchange cartridges were more efficient, recoveries were 96% for sulfamethoxazole and 43% for sulfadimethoxine. Limit of quantification ranged from 0.1 ng/g for SAs to 160 ng/g for tetracycline. Method precision for TCs was 5.06% and 1.12%, and for SAs 0.43% and 2.01%. FQs precision ranged from 0.77% to 1.89%.


J. Chrom. A 2008, 1187, 58-66. Matrix effects in pesticide multi-residue analysis by liquid chromatography-mass spectrometry

Polar pesticides: aldicarb sulphoxide, aldicarb sulphone, demeton-S-methyl sulphoxide, carbendazim, methomyl, thiabendazole, methiocarb sulphoxide, methiocarb sulphone, aldicarb, imazalil, thiodicarb, phorate sulphoxide, phorate sulphone, methiocarb

15 different fruits and vegetables: tomato, sweet pepper, orange, raspberries, banana, cucumber, lemon, blackcurrant, peach, grape, apple, grapefruit, pear, red currant and leek

Sample preparation: Luke method (AOAC 985.22), QuEChERS and matrix solid phase dispersion (MSPD)

Determination: LC ESI MS

Three sample preparation methods: Luke method (AOAC 985.22), QuEChERS (quick, easy, cheap, effective, rugged and safe) and matrix solid phase dispersion (MSPD) were applied to different fruits and vegetables for analysis of 14 pesticide residues by high-performance liquid chromatography with electrospray ionization – mass spectrometry (HPLC/ESI/MS). Matrix effect, recovery and process efficiency of the sample preparation methods applied to different fruits and vegetables were compared. The Luke method was found to produce least matrix effect. On an average the best recoveries were obtained with the QuEChERS method. MSPD gave unsatisfactory recoveries for some basic pesticide residues. Comparison of matrix effects for different apple varieties showed high variability for some residues. It was demonstrated that the amount of co-extracting compounds that cause ionization suppression of aldicarb depends on the apple variety as well as on the sample preparation method employed.


J. Agric. Food Chem., 2008, 56, 10716–10720. Evaluation of the Botanical Origin of Estonian Uni- and Polyfloral Honeys by Amino Acid Content

α-alanine, β-alanine, asparagine, γ-aminobutyric acid, glutamine, glycine, histidine, ornithine, phenylalanine, proline, serine, and tryptophan

Honey from Estonia (seven types of unifloral honeys and polyfloral honeys)

HPLC-UV with precolumn derivatization with diethyl ethoxymethylenemalonate

The main amino acids found in Estonian honeys were proline and phenylalanine. The resulting data have been analyzed by t test and principal component analysis (PCA). t Test revealed that some amino acids (α-alanine, β-alanine, asparagine, γ-aminobutyric acid, glutamine, glycine, histidine, ornithine, phenylalanine, proline, serine, and tryptophan) are more potent for assigning honey botanical origin than others. PCA enabled differentiation of some honey types by their botanical origin. In the space of the two first principal components, heather honeys form a cluster that is clearly separable from, for example, polyfloral honeys. It is concluded that analysis of the free amino acid profile may serve as a useful tool to assess the botanical origin of Estonian honeys.


Rapid Comm. MS 2008, 22, 379-384. Towards the electrospray ionization mass spectrometry ionization efficiency scale of organic compounds

Esters and aromatic amines: diphenyl phthalate, dimethyl phthalate, dimethyl glutarate, dimethyl succinate, dimethyl malonate, phenyl benzoate, methyl benzoate, 2-nitroaniline, 2,4-dinitroaniline and diphenylamine

MeCN : 0.1% formic acid (80 : 20)

ESI Ionization efficiency measurement in the ESI ion source

An approach that allows setting up under predefined ionization conditions a rugged self-consistent quantitative experimental scale of ESI ionization 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 (B1 and B2) by infusing a solution containing both compounds at known concentrations (C1 and C2) and measuring the mass-spectrometric responses of the protonated forms of the compounds (R1 and R2). The RIE of B1 and B2 is expressed as logRIE(B1, B2) = log[(R1×C2)/(C1×R2)]. The relative way of measurement leads to cancellation of many of the factors affecting ionization efficiency (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 ionization efficiency scale containing 10 compounds (esters and aromatic amines) and spanning over 4 logRIE units has been compiled. The consistency of the ESI ionization efficiency scale (the consistency standard deviation of the scale is s = 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).

ESI Ionization efficiency scale

J. Chrom. A, 2007, 1160, 227-234. “Fast peaks” in chromatograms of Sudan dyes

Sudan dyes (Sudan I, II, III and IV)


HPLC with UV-Vis and MS detection (LC-MS)

It is demonstrated that if the sample is kept in darkness before analysis, only one chromatographic peak appears for Sudan III or Sudan IV. In normal lighting conditions two peaks are observed. This light-induced effect can lead to under- or overestimation of the Sudan III and Sudan IV content and false-positive detection of Sudan I and Sudan II, if proper care is not taken.

Compound structures, Chromatograms, mass and UV-Vis spectra of the "fast" and "slow" peaks (PDF)

Food Chemistry 2007, 100, 1713-1721. The occurrence of volatile N-nitrosamines in Estonian meat products


Raw, fried, grilled, smoked, pickled, and canned meat products (e.g. smoked fish, fried fish, smoked ham, etc)

GC-MS with Chemical ionization (Ammonia as reagent gas)

Total concentrations of NAs in 386 studied samples of meat ranged from non-detectable to 30 lg/kg. The highest levels of NAs were found in samples of fried meat. Relatively high level was found in grilled meat, in smoked pork, in half-smoked sausage, and in ham. With the addition of sodium nitrite, one can observe roughly linear increase in concentration of NAs in fried and raw meat. About 73% of NAs are concentrated in fat of baked mutton. In fried pork, the concentration of NAs in fat exceeds the concentration in lean 6 times. Apparently, the temperature and time of cooking, nitrite concentration, and storage conditions of meat have a significant effect on the concentration of NAs.

Abstract and data Tables (PDF)

Food Chem. 2006, 96, 325-333 Volatile N-Nitrosamines in various fish products


Various samples of fish, and oil

GC-MS with Chemical ionization (Ammonia as reagent gas)

The level of five N-nitrosoamines (N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodibutylamine, N-nitrosopiperidine, and N-nitrosopyrrolidine) was determined in 294 various samples of fish, and in 77 samples of oil during 2001–2005. For the sample cleaning the two-step solid-phase extraction with Extrelut and Florisil sorbents was used. Nitrosoamines were separated by gas chromatography and detected by positive ion chemical ionization using ammonia as reagent gas. The HP 6890 Plus GC/HP 5973 MSD was used in the selected ion monitoring mode with pulsed splitless injection. In this work, the limit of detection and the limit of quantitation of NA were approximately 0.10 and 0.35 μg/kg, respectively with about 85%. The sum of the average of five NAs content in cold-smoked fish was found to be 1.92 μg/kg, in hot-smoked fish – 4.36 μg/kg, in fried fish – 8.29 μg/kg, in pickled fish – 5.37 μg/kg, in salted fish – 3.16 μg/kg, in salted/dried fish – 3.81 μg/kg, and in the fresh fish it was not detected.


J. Chrom. A 2005, 1126, 55-63 Uncertainty in liquid chromatographic analysis of pharmaceutical product: Influence of various uncertainty sources

Active substances of drugs on the example of Simvastatin

Pharmaceutical preparations (Tablets)

HPLC with UV-Vis detection

ISO GUM measurement uncertainty estimation procedure was developed. In quantification of uncertainty components several practical approaches for including difficult-to-estimate uncertainty sources (such as uncertainty due to peak integration, uncertainty due to nonlinearity of the calibration curve, etc.) have been presented. Two different calibration methods – single-point (1P) and five-point (5P) – were used. The uncertainty estimate for 1P is only slightly larger than with 5P calibration.

Abstract and Electronic supplementary material (PDF)



Should you have any questions regarding the data, the used experimental, data treatment or computational methods, etc, please do not hesitate to contact Ivo Leito (e-mail:!     Proposals for collaboration are also most welcome!

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See also other research topics at UT Chair of Analytical chemistry


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