Development of a dielectric barrier discharge ion source for ambient mass spectrometry

Na Na 0 1 Mengxia Zhao 0 1 Sichun Zhang 0 1 Chengdui Yang 0 1 Xinrong Zhang 0 1 0 Address reprint requests to Professor Xinrong Zhang, Department of Chemistry, Tsinghua University , Beijing 100084, P.R. China 1 Department of Chemistry, Key Laboratory for Atomic and Molecular Nanosciences of the Education Ministry, Tsinghua University , Beijing, China A new ion source based on dielectric barrier discharge was developed as an alternative ionization source for ambient mass spectrometry. The dielectric barrier discharge ionization source, termed as DBDI herein, was composed of a copper sheet electrode, a discharge electrode, and a piece of glass slide in between as dielectric barrier as well as sample plate. Stable low-temperature plasma was formed between the tip of the discharge electrode and the surface of glass slide when an alternating voltage was applied between the electrodes. Analytes deposited on the surface of the glass slide were desorbed and ionized by the plasma and the ions were introduced to the mass spectrometer for mass analysis. The capability of this new ambient ion source was demonstrated with the analysis of 20 amino acids, which were deposited on the glass slide separately. Protonated molecular ions of [M H] were observed for all the amino acids except for L-arginine. This ion source was also used for a rapid discrimination of L-valine, L-proline, L-serine and L-alanine from their mixture. The limit of detection was 3.5 pmol for L-alanine using single-ion-monitoring (SIM). Relative standard deviation (RSD) was 5.78% for 17.5 nmol of L-alanine (n 5). With the advantages of small size, simple configuration and ease operation at ambient conditions, the dielectric barrier discharge ion source would potentially be coupled to portable mass spectrometers. (J Am Soc Mass Spectrom 2007, 18, 1859 -1862) 2007 American Society for Mass Spectrometry - Vbeen developed, including laser desorption/ arious desorption ionization (DI) sources have ionization (LDI) [1, 2], fast atom bombardment (FAB) [3, 4], and matrix-assisted laser desorption ionization (MALDI) [5, 6], etc. Recently, a new family of DI techniques has emerged for direct detection of samples on surfaces that allow ions to be generated under ambient conditions and then analyzed by MS, such as desorption electrospray ionization (DESI) or direct analysis in real time (DART) [711]. For DESI, sprayed charged droplets were directed at the ambient object of interest and release ions from the sample surface; for DART, the energetic agents generated by a gas discharge were used to desorb and ionize the analyte from samples. Despite these techniques being very elegant, the development of direct sampling and ambient ion sources with simple configurations are always desirable. The design of a new ambient ionization source, termed as dielectric barrier discharge ionization (DBDI), is based on the concept of dielectric barrier discharge (DBD). The DBD is obtained at atmospheric pressure with a dielectric layer between two electrodes applied with alternating voltages [12, 13]. The dielectric limits the average current density in the gas space, forming stable low-temperature plasma with large amount of high energetic electrons [14, 15]. The non-equilibrium plasma properties of DBD can be used to develop new ionization method. It is expected that the source based on DBD needs neither electrosprayed solvent to form desorbed ions as for DESI, nor a device with complex configuration as for DART. Reduced pressure is not required to maintain a stable discharge for DBDI compared with glow discharge ion sources [16 19]. In the present work, a DBDI source has been designed with advantages of small size and simple configuration for ambient desorption/ionization. Twenty amino acids deposited on a glass slide were analyzed individually to demonstrate the potentials of the present ion source. DBD power supplies were purchased from Beili Guoke Co. Ltd. (Beijing, China). Ions were mass analyzed using a commercial linear ion trap mass spectrometer (Finnigan LTQ, Thermo Electron Co., San Jose, CA). Data were processed using the instrument software interface (Xcalibur version 1.4 SR1). Mass spectrometry conditions were as follows: source voltage (applied to the spray capillary), 0 kV; tube lens voltage, 95 V; capillary temperature, 275 C; heated capillary voltage, 33 V; multipole rf amplitude (Vp-p), 400 V; and multiplier voltages 1 and 2, 1200 V. The ion injection time was set to 50 ms, and the number of microscans was set to one. All reagents were of analytical-reagent grade. Amino acids were purchased from Beijing Dingguo Biotechnology Co. Ltd. (Beijing, China). Helium (99.99%) and argon (99.99%) from Huayuan Gas (Beijing, China) were used as the carrier gases. Monosodium glutamate was the product of Henan Lotus Gourmet Powder Incorporate Ltd. (Henan, China). Water was deionized and further purified with a Milli-Q water purification system (Millipore, Milford, MA). Sample Preparation For each amino acid, a sample solution containing 40 nmol of the analyte was deposited on a piece of filter paper about 3 mm 3 mm. Twenty pieces of filter paper containing the individual amino acids were placed on the glass slide to form a 4 5 array for analysis. Results and Discussion Design of DBDI Source The schematic of the DBDI device is shown in Figure 1. A hollow stainless steel needle (20 mm long, 0.2 mm i.d.) was used as a discharge electrode. Helium or other gases flowed through the needle at a speed of 12 to 48 m/s. A copper sheet (25 mm 75 mm) was used as the counter electrode. A piece of glass slide (25.4 mm 76.2 mm 1.2 mm) was inserted between two electrodes and mounted on the surface of the copper sheet. The glass slide served as both the discharge barrier and the sample plate. The distance between the needle electrode tip and the surface of glass slide was 5 to 10 mm. The glass slide and copper sheet were mounted on a 3D moving stage, allowing them to be positioned at any chosen point with respect to the needle electrode. An alternating voltage of 3500 to 4500 V with a frequency of 20.3 kHz was applied between two electrodes, forming stable plasma between the tip of the needle electrode and glass slide. The analytes on the surface of glass slide were desorbed and ionized by the plasma. Then the produced ions were introduced to the mass spectrometer for mass analysis. Desorption and Ionization of Amino Acids The base peaks of [M H] were observed from all the amino acids except for L-arginine. The ions of [M H HCOOH] , [M H H2O] , or [M H NH3] were also observed, which were confirmed by collisioninduced dissociation (CID). Taking L-glutamic acid (Mr 147) as an example, ions of m/z 148 ([M H] ), 130 ([M H H2O] ), 102 ([M H HCOOH] ) and 84 ([M H HCOOH H2O] ) were obtained. Detail information of 20 amino acids is included in Supporting Information, which can be found in the electronic version of the article (Table S1). The observed (...truncated)