Comparative validation of amperometric and optical dissolved oxygen sensors

A comprehensive comparative validation for two different types of dissolved oxygen (DO) analyzers, amperometric and optical, together with estimation of measurement uncertainty is presented in the recently published article I. Helm, G. Karina, L. Jalukse, T. Pagano, I. Leito, Environmental Monitoring and Assessment 2018, 190, 313.

A number of performance characteristics were evaluated including drift, intermediate precision, accuracy of temperature compensation, accuracy of reading (under different measurement conditions), linearity, flow dependence of the reading, repeatability (reading stability), and matrix effects of dissolved salts. The matrix effects on readings in real samples were evaluated by analyzing the dependence of the reading on salt concentration (at saturation concentration of DO). The analyzers were also assessed in DO measurements of a number of natural waters. The uncertainty contributions of the main influencing parameters were estimated under different experimental conditions. It was found that the uncertainties of results for both analyzers are quite similar but the contributions of the uncertainty sources are different.

The results imply that the optical analyzer might not be as robust as is commonly assumed, however, it has better reading stability, lower stirring speed dependence, and typically requires less maintenance. On the other hand, the amperometric analyzer has a faster response and wider linear range.

(Photo by Lauri Jalukse: measurements of dissolved oxygen concentration with amperometric and optical analyzers at Jordan spring, Karksi-Nuia, Estonia)

 

What can we learn from mass spectrometry about charged droplets?

Charged droplets occur everywhere in the world. They are created by the oceans (known as sea spray aerosols), near waterfalls and in thunderstorm clouds. Such droplets are expected to play significant role in environmental processes. Similar droplets are also created in electrospray ionization (ESI) source.

Mari Ojakivi joined Mass Spectrometry lab three years ago to conduct her bachelor thesis with us. Mari started studying how different acids, salts, and bases influence the ionization of some amines in charged water droplets. Soon, some extremely interesting results were revealed that allowed to make much wider conclusions about charged droplets.

It became possible to pinpoint, that protonation of the amines is strongly dependent on the type of additives present in the droplets and is virtually independent of the pH of the solution used for “preparing” the droplets. In “normal” solutions the protonation is determined solely by the pH of the solution. This led us to conclude that some of the additives change something about the droplets that other additives do not affect. It turned out, that the factor determining the protonation is the cation present near the surface of the charged droplets. Cations, such as hydronium ion, which are strong acids protonate the compounds, while weak acids, such as ammonium cation, do not. If both types of cations are present in the solution, the protonation is determined by the ion that has higher affinity for the droplets surface. The support for this model was found from the molecular dynamics simulations carried out in Prof. Konermann’s group.

Why is the protonation in charged droplets at all important? Protonation is one of the fundamental properties of compounds; it may catalyze reactions, break up or induce complexation, change conformation of the macromolecules, etc. Therefore, it can be assumed, that the reactions and processes taking place in charged droplets also depend on the protonation.

The results were published in ChemistrySelect . 

MALDI‐FT‐ICR‐MS for Archaeological Lipid Residue Analysis: Cover Paper of JMS!

JMS_v52_i10_CoverThe Analytical chemistry group at UT recently received a very pleasant and well-deserved recognition: the paper MALDI‐FT‐ICR‐MS for Archaeological Lipid Residue Analysis J. Mass Spectrom. 2017, 52, 689-700 led by research fellow Dr Ester Oras was selected by the editorial board as the cover article for the Oct 2017 issue of the Journal of Mass Spectrometry!

Ester_OrasEster’s research demonstrates that tiny (and to a large extent degraded) food remains on ceramic potsherds, dating back many hundreds of years, can still tell interesting stories about the food practices of our ancestors. The key to these results is clever usage of high-resolution FT-MS with MALDI ion source.

The developed methodology is expected to lay foundation to further studies of ancient food practices in Europe.

 

(Photo on the left: cover of the Oct 2017 issue of the Journal of Mass Spectrometry; photo on the right: Ester Oras)

 

Sponge Spray – New Approach for Direct Sampling and Analysis by MS

DSC_0294Mass spectrometry is currently probably the No 1 technique for determining trace-level components in complex (especially biomed-related) mixtures. The key issues in such applications are sample preparation, sample introduction to MS and ionization of components of interest (analytes). Big efforts are continually made to improve any of them.

In his recent development – Sponge spray – Max Hecht, an AMS graduate, now PhD student at UT attempts improvements in all of the above issues. The elegant approach proposed by Max utilizes a volumetric sampling device – a hydrophilic sponge, which after absorbing a predetermined amount of sample (e.g. blood or urine), can be directly used for sample introduction to MS and ionizing the analytes.

The seriousness of the work has been demonstrated by the fact that it was accepted for publication by Analytical Chemistry, the top journal in the field. The published article Sponge Spray – Reaching New Dimensions of Direct Sampling and Analysis by MS is now available from the journal website.

Further developments of this approach may lead e.g. to fast medical diagnosis MS methods that, contrary to the current situation with MS in medicine, could be applied as “bed-side” diagnosis tools in hospitals.

(Photo: Max Hecht working with the sponge spray ion source)

 

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

 

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)

 

Metrology in chemistry in a nutshell

Random_and_Systematic_Effects_TimelineIn a recent edition of the premier journal devoted to quality and metrology in chemistry Accreditation and Quality Assurance Ivo Leito has attempted to express in very simple terms the essence of Metrology in Chemistry. In the article Accred. Qual. Assur. 2015, 20, 229–231 he arrived at three main recommendations:

1. Whenever possible, comparisons with reference values should be carried out. The reference values can be realized in different ways: Certified reference materials (CRMs), Laboratory reference materials (LRMs), Measurements with reference methods, etc.

2. Data on stable samples should be collected over long time periods (e.g. as the X chart), in order to evaluate as many sources of variability in the analysis method, as possible. The longer the time period, the more systematic effects will become random and thus easier to evaluate (more on this topic can be found in a recent review on bias).

3. “Do not stop there!”, meaning that the above mentioned activities should run in a lab on a continuous basis.

As a conclusion, it can be said that constant improvement is the key to reliable analytical results.

 

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).