New Plasma Ionisation Sources for Mass-Spectrometric Detection of Lipids
Lipidomics is a fast growing discipline and aims for the comprehensive and quantitative analysis of all lipid compounds in a given biological system. Current lipidomics studies are predominantly based on mass spectrometry (MS) and either use a direct infusion of lipid extracts (i.e., shot-gun-lipidomics) or rely on liquid chromatography (LC)-based separation of lipids prior to MS detection. In the vast amount of studies, electrospray ionization (ESI) or nanoelectrospray ionization (nESI) are applied for ionization purposes. However, nonpolar lipids such as cholesterol and its derivatives most often display detection difficulties. In order to analyze such lipids with the majority of other lipid classes, alternative ionization strategies are mandatory.
Such an alternative might be dielectric barrier-discharges (DBD). DBD can be applied for analytical applications as dissociative source for optical emission spectrometry (OES) as well as for ambient-ionization. In the latter case, it has attracted much attention in fields like food safety, biological analysis, MS for reaction monitoring and imaging forensic identification. It is applied as combined desorption and ionization source as well as solely for application as ionization source with different sample introductions.
Formed between two electrodes, separated by one or two dielectric barriers, and powered by alternating voltage with appropriate high frequency and amplitude, DBDs are nonequilibrium gas discharges. The most attractive characteristics of these discharges are stable operation, possibility to operate at atmospheric pressure with different discharge gases, low power consumption, and comparably easy setup in simple and miniaturized form resulting in usage for a variety of analytical applications. Possible applications can be the generation of various radicals and ionic species that enable molecular dissociation, excitation, and ionization of the analytical targets or as source for soft ionization.
As described in Horvatic et al. (2014), there are various designs of DBD ionization sources. Most of the applied and characterized DBDs are maintained in a glass capillary. There are different configurations changing the appearance as the plasma is restricted inside or departing the capillary so-called plasma jet or afterglow. Due to Kogelschatz for DBDs, at least one dielectric layer has to be in between the electrodes (Kogelschatz et al. 1997). A DBD with just one dielectric layer is formed by a pin-ring-shape or a tube-ring-shape. Therefore, in most cases an alternating high voltage is applied to the ring electrode, which is wrapped around the capillary, while the electrode inside the capillary (pin or tube) is grounded. This polarity is preferred in order to avoid a direct plasma in-between the unshielded electrode to possible grounded surfaces in comparison to a polarity setup if the pin or tube electrode would be contacted to the alternating high voltage.
Another configuration is a DBD with two dielectric barriers. For this purpose, a dielectric tube guides the plasma gas (e.g., helium or argon) whereupon two conductive ring electrodes with a certain distance are wrapped around the dielectric tube. This configuration was first reported by Teschke et al. (2005), when a 20 kHz high voltage square wave was applied to the electrodes, a cold plasma jet is generated at ambient air.
The mechanism of a DBD is related to a polarization of the dielectric barrier glass wall of the capillary resulting in charge accumulation at the inner surface of the capillary. The amount of these accumulated charges by polarization/charging is proportional to the change per time of the voltage. Therefore, instead of the use of an ordinary sinusoidal generator, a square wave generator would be of great interest. Combined with a smaller dimension of the capillary not only a pin-ring shape but also a ring-ring-shaped DBD can be operated more reproducible and in case of using helium as plasma gas it can be operated in a homogeneous plasma mode, which is preferable for soft ionization.
Due to spatio-temporal emission measurements, it could be shown that the ring-ring shape plasma consists of several plasmas as plasma jet, inner early plasma, and coincident plasma. The emission of the plasma jet can only be measured during the positive cycle of the applied voltage. The emission of the inner as well as the coincident plasma is nearly an order of magnitude more intense than the plasma in the negative cycle of the alternating voltage. Since the emission of the plasma jet and the inner early plasma correlates with the efficiency of the ionization process, only the positive half-cycle will be regarded in the following. Applying a square wave voltage at a frequency of 20 kHz, the duration of the positive half-cycle is 25 μs.
When the plasma jet is off the analyte will be ionized by the electrospray itself whenever polar molecules are present in the analyte. In case of molecules with no or low polarity, these will not be ionized by the nESI, but these can be postionized by the plasma jet when it is on.
- Kogelschatz U, Eliasson B, Egli W, Dielectric-Barrier Discharges. Principle and Applications, J Phys IV. 1997;7:47.Google Scholar