Encyclopedia of Lipidomics

Living Edition
| Editors: Markus R. Wenk

New Plasma Ionisation Sources for Mass-Spectrometric Detection of Lipids

  • Sebastian Brandt
  • Robert Ahrends
  • Joachim FranzkeEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-7864-1_195-1

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.

Both the plasma jet and the inner early plasma start at the electrode nearby the outlet and develop outside and inside the capillary, respectively (Fig. 1). The ignition starts 0.95 μs after the displacement current occurring simultaneously to the switching of the high voltage from e.g., 0 to 3.5 kV. After both plasmas extinguished at about 1.05 μs, the coincident plasma ignites in-between the electrodes 1.15 μs after the displacement current.
Fig. 1

Temporal plasma appearance in the positive half cycle of a square wave voltage applied to a ring-ring shape plasma consisting of a glass capillary with two electrodes: (1) Time between the displacement current which indicated the positive slope of the applied square wave voltage. (2) Early plasma in between the electrodes and plasma jet beyond the capillary with a life time of 0.1 μs. (3) Coincident plasma which is coincident with the plasma current has a life time of 0.1 μs. (4) Most of the time of the square wave half cycle the plasma is off

The plasma jet and the inner plasma are plasmas usable for soft ionization, whereas the coincident plasma is capable to dissociate molecules. Therefore, it has to be avoided to introduce analytes into the coincident plasma. Since the inner plasma and the coincident plasma are in the same place but at different times, the analyte should not be introduced along with the plasma gas stream. Out of this reason analytes necessitating soft ionization avoiding fragmentation are introduced into the plasma jet and not fed in the capillary together with the discharge gas. The whole arrangement is shown in Fig. 2. On the left the plasma jet is aligned perpendicular to the axis of the MS inlet. A corona needle points with a small angle to the axis of the MS inlet. From the right, the analyte can be introduced by a thermospray to the point where the ionization takes place. Here the DBD ionization can be compared to the conventional atmospheric pressure chemical ionization (APCI). The analyte exits the thermospray under a high divergence in the big volume towards the MS. Therefore, the efficiency of both ionization techniques is weak; in case of APCI it is even weaker due to condensation of the analyte on the corona needle. Therefore, as shown in Fig. 3 another arrangement has been chosen to effectively guide the analyte to the area where the ionization takes place. Instead of a divergent thermospray with a nESI was used to inject the analyte into the plasma jet.
Fig. 2

Plasma jet of the Dielectric Barrier Discharge Ionization (DBDI) source [left to right],which is aligned perpendiculary to the axis of the mass spectrometer (MS) [top] and inlet corona needle [bottom] of the APCI source and thermospray

Fig. 3

A nESI applied as injection source for the DBDI source in front of a MS inlet

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.

Two different mass spectra are shown in Fig. 4 where a bovine liver total lipid extract is introduced to the setup of Fig. 3. Using solely the nESI for ionization, the signal of [cholesterol-17]+ is expected abundantly but could only be detected with low intensity (left). With additional activated plasma resulting in postionization (right), the cholesterol signal intensity could be increased in relative and absolute values resulting in the most abundant peak. By tuning the postionization, it is also possible to get increased signal values for different other lipid species (m/z above 700).
Fig. 4

Bovine liver total extract spectra ionized with nESI (left) and with nESI in combination with plasma jet as postionization source (right)

References

  1. Horvatic V, Vadla C, Franzke J. Discussion of fundamental processes in dielectric barrier discharges used for soft ionization, Spectrochimica Acta Part B. 2014;100:52.CrossRefGoogle Scholar
  2. Kogelschatz U, Eliasson B, Egli W, Dielectric-Barrier Discharges. Principle and Applications, J Phys IV. 1997;7:47.Google Scholar
  3. Teschke M, Kedzierski J, Finantu-Dinu EG, Korzec D, Engemann J. High-speed photographs of a dielectric barrier atmospheric pressure plasma jet, IEEE Trans Plasma Sci. 2005;33:310.CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  • Sebastian Brandt
    • 1
  • Robert Ahrends
    • 2
  • Joachim Franzke
    • 1
    Email author
  1. 1.MiniaturisationISASDortmundGermany
  2. 2.LipidomicsISASDortmundGermany