Internal energy distributions of five classes of ion sources based on dielectric barrier discharge (DBD) were measured, with results suggesting that one particular type of DBD can minimize fragmentation of ions with labile bonds.
A study led by Marcos Bouza of the University of Jaén in Spain sought the most reliable ambient source for forming intact ions for the purpose of mass spectrometry (MS) molecule analysis, measuring multiple kinds of dielectric barrier discharges to assess sensitivity and ion fragmentation (1).
The research by Bouza and seven co-authors across Australia, Germany, and Spain, published in the Journal of the American Society for Mass Spectrometry, looked at the four primary classes of DBD-based ion sources: DBD ionization (DBDI), low-temperature plasma (LTP), flexible microtube plasma (FµTP), and active capillary plasma ionization (ACaPI) (1). The study additionally examined atmospheric pressure chemical ionization (APCI) with para-substituted benzylammonium thermometer ions.
DBD-based ion sources for mass spectrometry encompass various classes, including DBDI, LTP, FµTP, ACaPI, and APCI. DBDI employs a dielectric barrier discharge to generate ions, providing enhanced ionization efficiency. LTP operates at lower temperatures, enabling the analysis of thermally labile compounds and reducing matrix effects. FµTP employs a flexible microtube electrode to generate plasma, offering improved ionization stability and sensitivity. ACaPI utilizes a capillary as an active electrode, leading to efficient ionization and desolvation. APCI introduces reactant ions to the sample through corona discharge, enabling the ionization of non-polar and thermally stable compounds. These different DBD-based ion sources offer unique advantages and are employed based on specific analytical requirements in mass spectrometry applications.
According to the study, ambient ionization MS is favored for its speed and simplicity in providing direct analysis of small molecules in a wide variety of samples. Complementing laser or spray ionization methods, the plasma-based DBD approach is known to tolerate complex, heterogeneous mixtures (1). DBDs, as defined by the researchers, are formed by a high-voltage alternating waveform applied between two electrodes separated by some sort of insulator, with several factors facilitating their generation near room temperature and at ambient pressures.
One pitfall of ambient ion sources is the potential for in-source fragmentation, which the study claims can increase the spectral complexity of analytes and hinder sensitivity and, therefore, interpretation. The researchers posit that characterizing the extent of ion heating during formation and transfer is essential to minimizing fragmentation, and tested DBDI, LTP, FµTP, and ACaPI with benzylammonium thermometer ions, comparing each source to each other and to APCI (1).
The researchers expressed some level of surprise at the results of this experiment. Internal energy deposition in the four DBD-based sources was characterized as being “softer” than APCI (1). The average energy deposited by ACaPI, meanwhile, being measured at 90.6 kJ/mol, was approximately 40 kJ/mol (almost 30%) lower than DBDI, LTP, FµTP, or APCI, all which registered within a range of 130.2 to 134.1 kJ/mol. ACaPI was also found to exhibit a slightly higher extent of energy than electrospray ionization (80.8 kJ/mol).
Further refinement was attempted by the research team positioning the plasma jets of the DBDI, LTP, and FµTP sources on axis with the capillary entrance to the mass spectrometer in use (1). This, they found, reduced the extent of the internal energy depositions in each by up to 20 kJ/mol – LTP’s decreased by 8.5%, DBDI’s by 13.0%, and FµTP’s by 16.0% – but sensitivity was not as high as a result. Additionally, neither sample introduction nor DBD plasma conditions were proven to have strong correlations to internal energy distributions.
Because ACaPI directs gaseous analytes toward the capillary inlet of the mass spectrometer through the center of a DBD halo-plasma, as the authors explained, interactions between plasma and vaporized molecules are limited, reducing the extent of energy deposited (1). Therefore, the team concluded that an active capillary-based DBD is the preferred choice when it comes to minimal ion fragmentation.
(1) Bouza, M.; Ahmed, E.; Rocío-Bautista, P.; et al. Ion Heating in Advanced Dielectric Barrier Discharge Ion Sources for Ambient Mass Spectrometry. J. Am. Soc. Mass Spectrom. 2023, 34 (6), 1145–1152. DOI: 10.1021/jasms.3c00087