New Article Published: Analytical Chemistry

Think Negative: Finding the Best Electrospray Ionization/Mass Spectrometry Mode for Your Analyte

Previously our group has developed extensive ionization efficiency scales in ESI positive and negative mode. Thus far, the comparison between the two modes has only been qualitative. Due to use of different anchor compounds the scales were not quantitatively comparable. To solve this problem and to enable direct quantitative comparison of the two ESI modes we searched for an anchor compound ionizing to the similar extent in both modes. To find such a compound we combined mass spectrometry with laser induced fluorescence measurements (to find out the solvent composition and pH in the ESI droplets), NMR and UV-Vis (to characterize the potential anchoring compounds ionization degree in corresponding solvent). Trans-3(3-pyridyl)acrylic acid was found to be a suitable anchoring compound, if analysed in mobile phase with pH 4.00.

The link between two ESI modes ionization efficiency scales enables the user to choose the most optimal ESI mode for analysis for the analyte in question.

We also compared ionization efficiencies of 33 compounds ionizing in both modes and found that, contrary to general practice, negative mode allows higher ionization efficiencies for 46% of the compounds. For 18% positive mode ESI provides better ionization efficiencies and for 36% the results obtained in both modes are comparable. However, not all compounds can be ionized with ESI negative mode, and some unfortunately also not with ESI at all.

Published in: Piia Liigand; Karl Kaupmees; Kristjan Haav; Jaanus Liigand; Ivo Leito; Marion Girod; Rodolphe Antoine; Anneli Kruve; Anal. Chem. 
DOI: 10.1021/acs.analchem.7b00096

 

What is the best LC-MS ion source? How to determine Limit of detection of a method?

Asko_Laaniste_Hanno_EvardThese very important (and up to now not completely solved) questions got a lot clearer on Aug 31, 2016 as PhD dissertations addressing these topics were defended at UT Institute of Chemistry.

Asko Laaniste (left on the photo) in his thesis titled “Comparison and optimisation of novel mass spectrometry ionisation sources” and in the recent paper ESI outcompetes other ion sources in LC-MS trace analysis Anal. Bioanal. Chem. 2019 has carried out an extensive experimental comparison of 4 different LC-MS ion sources operated altogether in 7 different modes in the analysis of 41 different pesticides. The obtained large pool of data was used for comparing the sources in terms of matrix effects, limit of detection (LoD), repeatability, linearity, signal to noise ratio (S/N) and sensitivity.

Asko demonstrated that for low levels of analytes in most cases the conventional ESI is the ion source of choice (provided the analytes are ionizable with ESI), while dopant-assisted APPI is a good alternative if low detection limits are not required and if compounds not ionizable with ESI are determined.

This is currently the most comprehensive comparison of this type available and Asko’s thesis (and the forthcoming publication) could serve as a “desk manual“ for LC-MS practitioners on choosing ion source for LC-MS analysis.

The central question of Hanno Evard’s thesis “Estimating limit of detection for mass spectrometric analysis methods” and in the corresponding two-part tutorial review Tutorial on estimating the limit of detection using LC-MS analysis (Anal. Chim. Acta 2016, 942, 23-39, Anal. Chim. Acta 2016, 942, 40-49) was: what is the best way of evaluating detection limit (LoD) of an analytical method? There are around ten widespread approaches for LoD in the literature (plus less well known ones) and the LoD values obtained using different approaches can differ by up to 10 times.

Hanno (right on the photo) carried out comprehensive analysis of the literature approaches and combined that with extensive experiments. As a result he was able to propose and convincingly justify one approach, which has merits over others and should be used for evaluation of LoD.

Hanno Evard is an alumnus of the Applied Measurement Science programme – the predecessor programme of EACH.

Our warmest congratulations to Asko and Hanno!

Establishing Atmospheric Pressure Chemical Ionization Efficiency Scale

UT100412AT462The series of works from the UT Analytical chemistry group on measuring and predicting ionization efficiency in the electrospray (ESI) ion source of MS and LC-MS has reached a new milestone: for the first time an ionization efficiency scale for the atmospheric pressure chemical ionization (APCI) source has been established.

The work led by Dr Riin Rebane (photo on the left) resulted in APCI ionization efficiency scale containing 40 compounds with widely ranging chemical and physical properties and spanning 5 orders of magnitude of ionization efficiency. Analysis of the resulting data challenges the common knowledge about APCI as ionization method. Contrary to the common knowledge, ionization efficiency order in the APCI source is surprisingly similar to that in the ESI source and most of the compounds that are best ionized in the APCI source are not small volatile molecules. Large tetraalkylammonium cations are a prominent example. These findings suggest that the atmospheric pressure chemical ionization mechanism can be more complex than generally assumed and most probably several ionization mechanisms operate in parallel and a mechanism not relying on evaporation of neutral molecules from droplets has significantly higher influence than commonly assumed.

See the original publication Anal. Chem. 2016, 88, 3435-3439 for more information.

(Photo: Andres Tennus)

 

Tutorial Review on LC-MS Method Validation

Graphical_AbstractThe LC-MS group at the UT Institute of Chemistry were recently invited by the journal Analytica Chimica Acta to write a tutorial review on the topic of validation of liquid chromatography mass spectrometry (LC-MS) methods. This work has now been completed. The 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 was eventually split in two parts (because of its large volume):

(as an April joke from Elsevier, part II appears page-wise before part I)

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.

The Part I briefly introduces the principles of operation of LC–MS (emphasizing the aspects important from the validation point of view, in particular the ionization process and ionization suppression/enhancement); reviews the main validation guideline documents and discusses in detail the following performance parameters: selectivity/specificity/identity, ruggedness/robustness, limit of detection, limit of quantification, decision limit and detection capability. The Part II starts with briefly introducing the main quantitation methods and then addresses the performance related to quantification: linearity of signal, sensitivity, precision, trueness, accuracy, stability and measurement uncertainty. The last section of Part II is devoted to practical considerations in validation and a possible step by step validation plan specifically suitable for LC-MS-MS is presented.

With every method performance characteristic its essence and terminology are addressed, the current status of treating it is reviewed and recommendations are given, how to determine it, specifically in the case of LC–MS methods. In many cases the published guidelines remain too general for practicing analyst. This tutorial review gives more specific advice based on the best available practice.

Based on the recommended approaches presented in this tutorial review an LC-MS validation software ValChrom is currently under development by the UT team. The software development is supported by the EU Regional Development Fund (Development of software for validation of chromatographic methods, Project No. 3.2.1201.13-0020).