Polar or Nonpolar – That is the Question
Polarity is one of the fundamental properties of molecules. It determines, for example, the substances in which a molecule can dissolve. This, in turn, impacts the choice of research methods and instruments, ultimately influencing the results and the conclusions drawn from them. While it might seem like a straightforward process – starting with identifying polarity and ending with research results – the analysis must often be repeated multiple times for compounds of varying polarities, such as lipids, to ensure that each analyte is properly ionized. In this article, you’ll learn more about the concept of molecular polarity, lipids specifically, and the challenges encountered in studying them.
The Polarity of Molecules
When several atoms bond together to form a molecule or a group of atoms, differences in electronegativity among the atoms can lead to shifts in charge distribution. A classic example of a polar molecule is water: H2O. The oxygen atom has a higher electronegativity (the electronegativity values of elements can be found in the periodic table) than the two hydrogen atoms and therefore attracts their electrons. As a result, the oxygen atom develops a partial negative charge, while the hydrogen atoms carry partial positive charges. This creates a permanent electric dipole moment, making the entire molecule polar. In contrast, nonpolar molecules lack such a permanent dipole moment and exhibit little to no partial charge differences. It’s important to note that the transition from polar to nonpolar is gradual and depends on the electronegativity differences between the bonding partners.
Like Dissolves Like
When polar substances are combined with polar solvents, the differently charged parts of the molecules attract each other, breaking down previously stable structures. In nonpolar substances, where no permanent dipole moment exists, the constant movement of electrons around the nucleus generates brief charge shifts. This allows nonpolar solvents to interact with these structures through electromagnetic attraction and repulsion, breaking down the chains and creating a homogeneous mixture.
Polarity and Solvent Determine Ionization
The choice of solvent – for example water for polar substances or acetonitril for nonpolar substances – greatly influences the ionization method used in mass spectrometry. In the article ‚By Ionization Only‘, you can learn more about standard ionization methods for both polar and nonpolar substances. While polar mixtures are typically sprayed as small droplets and subsequently ionized by electrospray ionization (ESI), in LC-MS nonpolar mixtures are usually vaporized and ionized via atmospheric pressure chemical ionization (APCI), or photo ionization (APPI). In the lab, this means that different ion sources are needed for different substances, requiring either multiple instruments or the reconfiguration of sources by lab personnel.
Lipids: Both Polar and Nonpolar
Research on lipids is especially significant in the field of metabolomics. [Learn more about lipids and metabolomics in recently published blog articles.] A unique aspect of this class of compounds is that there are dozens of different types of lipids, some of them are polar, others nonpolar. For a comprehensive study of lipids, one would either need two different ion sources – one for polar substances and another for nonpolar ones – or utilize SICRIT®: a single source uniquely designed to ionize both polar and nonpolar substances.
- SICRIT® is capable of covering the ionization range of APCI without major sensitivity drawbacks.
- SICRIT® can nearly cover the entire ionization range of EI (excluding residual gases like N2, O2, etc.) without sensitivity issues.
- SICRIT® can cover a large portion of the ESI ionization range up to an m/z ratio of 2000 (excluding large biomolecules like proteins).
SICRIT® and Lipids
In recent application notes, we explore lipid analysis using our LC module. In the app note ‚LC-SICRIT®-HRMS Analysis of Non-Polar Lipids featuring Shimadzu Nexera LC and 9030 LC-MS QToF‘, we demonstrate SICRIT®’s ability to ionize eight different triacylglycerides with minimal molecular fragmentation, enabling substance identification via Shimadzu software
In the app note ‚Lipidomics Decoded: Targeted Assignment of Polar Lipids using the SICRIT® LC-Module‘, we show that SICRIT® can also ionize a wide range of polar lipids using a standard ESI workflow, with identification facilitated by an MS standard library.
Moreover, with a thermal desorption unit, SICRIT® opens up the possibility of creating a person’s lipidomic profile, offering forensic science a new approach to individual identification.
For SICRIT®, polar or nonpolar is no longer a question.
If you would like to learn more about SICRIT® in the analysis of lipids within the field of metabolomics, we encourage you to sign up for our free webinar.