Zhongjun Zhao

Microwave-induced plasma desorption/ionization source for ambient mass spectrometry.

A new ionization source based on microwave induced plasma was developed for ambient desorption/ionization. The microwave-induced plasma desorption/ionization source (MIPDI) was composed of a copper Surfatron microwave cavity where a fused-silica tube was centered axially. Stable nonlocal thermodynamic equilibrium plasma was generated in the quartz discharge tube when a microwave at a frequency of 2450 MHz was coupled to the microwave cavity. Analytes deposited on the surface of poly(tetrafluoroethylene) (PTFE) or quartz slide after hydrofluoric acid (HF) etching were desorbed and ionized by the plasma. The performance of the MIPDI technique was validated by the analysis of a variety of chemical substances, polymer compounds, and pharmaceutical drugs using argon or helium as the discharge gas. Protonated [M + H](+) or deprotonated [M - H](-) ions were observed in the positive or negative mode. MIPDI was also used for the analysis of compounds in a complex matrix without any sample preparation. MIPDI was also capable of analyzing liquid samples. The signal-to-noise ratio was 463 in the analysis of 9.2 ng of phenylalanine, and the limit of detection was 60 pg for phenylalanine. MIPDI could desorb and ionize analytes with a molecular weight of up to 1200, which was demonstrated by the analysis of polyethylene glycol 800 (PEG800). MIPDI has advantages of simple instrumentation, relatively high temperature, stability, and reproducibility.

Microfabricated glow discharge plasma (MFGDP) for ambient desorption/ionization mass spectrometry.

A novel ambient ionization technique for mass spectrometry, microfabricated glow discharge plasma (MFGDP), is reported. This device is made of a millimeter-sized ceramic cavity with two platinum electrodes positioned face-to-face. He or Ar plasma can be generated by a direct current voltage of several hundreds of volts requiring a total power below 4 W. The thermal plume temperature of the He plasma was measured and found to be between 25 and 80 °C at a normal discharge current. Gaseous, liquid, creamy, and solid samples with molecular weights up to 1.5 kDa could be examined in both positive and negative mode, giving limits of detection (LOD) at or below the fg/mm(2) level. The relative standard deviation (RSD) of manual sampling ranged from 10% to ~20%, while correlation coefficients of the working curve (R(2)) are all above 0.98 with the addition of internal standards. The ionization mechanisms are examed via both optical and mass spectrometry. Due to the low temperature characteristics of the microplasma, nonthermal momentum desorption is considered to dominate the desorption process.

Plasma enhanced label-free immunoassay for alpha-fetoprotein based on a U-bend fiber-optic LSPR biosensor

A simple, label-free and cost-effective localized surface plasmon resonance (LSPR) immunosensing method was developed for detection of alpha-fetoprotein (AFP). The U-bend fiber optic probe was firstly pretreated by microwave-induced H2O/Ar plasma to ensure better silanization, which could greatly improve the adsorbed amounts and uniformity of the gold nanoparticles (GNPs) on the fiber optic probe surface. Furthermore, according to the sucrose refractive index (RI) testing result, the absorbance sensitivity to RI change of this sensor was obviously improved due to the plasma pretreatment. Finally, on the basis of this U-bend strategy and plasma pretreated method, the fabricated biosensor displayed good analytical performance for detection of AFP, ranged from 5 to 200 ng mL−1 in both phosphate-buffered saline (PBS) and human serum, with different detection limits of 0.85 and 3.3 ng mL−1 respectively. Therefore, the present strategy definitely paves a way for wider applications of LSPR in clinical research and may eventually become a promising technique for protein detection.

Discovery of potential biomarkers in exhaled breath for diagnosis of type 2 diabetes mellitus based on GC-MS with metabolomics

The aim of the study was to apply gas chromatography-mass spectrometry (GC-MS) combined with a metabolomics approach to identify distinct metabolic signatures of type 2 diabetes mellitus (T2DM) and healthy controls from exhaled breath, which are characterized by a number of differentially expressed breath metabolites. In this study, breath samples of patients with type 2 diabetes mellitus (T2DM, n = 48) and healthy subjects (n = 39) were analyzed by GC-MS. Multivariate data analysis including principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) was successfully applied to discriminate the T2DM and healthy controls. Eight specific metabolites were identified and may be used as potential biomarkers for diagnosis of T2DM. Isopropanol and 2,3,4-trimethylhexane, 2,6,8-trimethyldecane, tridecane and undecane in combination might be the best biomarkers for the clinical diagnosis of T2DM with a sensitivity of 97.9% and a specificity of 100%. The study indicated that this breath metabolite profiling approach may be a promising non-invasive diagnostic tool for T2DM.

Performance evaluation of a newly designed DC microplasma for direct organic compound detection through molecular emission spectrometry

A low-power, portable dc microplasma source has been developed for use in molecular emission spectrometry. The device employs a 450 nL plasma chamber in which an atmospheric pressure dc glow discharge is generated with argon gas. The discharge ignites spontaneously when the system is operated at dc input power of 0.95–6.5 W and gas flows of 100–2500 mL min−1. Two sample introduction modes are used to enable direct analysis of gaseous and liquid samples. The detection of volatile organic compounds was achievable via the emissions from CN at 387.15 nm, CH at 431.41 nm and C2 at 516.53 nm. Under the optimized experimental conditions, the limit of detection (LOD) down to ppb (v/v) level can be achieved. These detection limits are competitive to or better than those of other microfabricated plasma devices. More importantly, the effect of organic compound structure on emission response is systematically studied. It was found for the first time that the ratio of C2/CH emission is closely associated with the ratio of hydrogen to carbon atoms (H/C) in a molecule, which might be potentially used for direct semi-qualitative analysis of organics. The dc microplasma detector possesses the advantages of simple construction, high sensitivity, low power consumption, long lifetime, and potential for portability in mass reduction and instrumentation.

A dielectric-barrier discharge enhanced plasma brush array at atmospheric pressure

This study developed a large volume cold atmospheric plasma brush array, which was enhanced by a dielectric barrier discharge by integrating a pair of DC glow discharge in parallel. A platinum sheet electrode was placed in the middle of the discharge chamber, which effectively reduced the breakdown voltage and working voltage. Emission spectroscopy diagnosis indicated that many excited argon atoms were distributed almost symmetrically in the lateral direction of the plasma. The concentration variations of reactive species relative to the gas flow rate and discharge current were also examined.

Ambient ionization and direct identification of volatile organic compounds with microwave-induced plasma mass spectrometry.

An innovative method of volatile organic compounds analysis by using microwave-induced plasma ionization (MIPI) source in combination with an ambient ion trap mass spectrometer is presented here. Using MIPI for direct sample vapor, analysis was achieved without any sample preparation or subsequent heating. The relative abundance of the target compounds can be obtained almost instantly within a few seconds. The ionization processes of different volatile compounds was optimized, and the limits of detection were identified in the range of 0.15-4.5 pptv or 0.73-8.80 pg ml(-1). The relative standard deviation (RSD) is in the range of 4-14%, while correlation coefficients of the working curves (R(2)) are better than 0.98. The new method possesses advantages of ease operation, time-saving, high sensitivity and inexpensive setup. In addition, the ionization processes of short n-alkane chains were investigated with the MIPI technique, and a unique [M + 13](+) was detected, which has not been reported in detail by any other related ionization techniques. An ionization mechanism was proposed on the basis of the experimental results obtained in this work and available information in literatures, in which the n-alkanes in the plasma environment possibly generate protonated cyclopentadiene [M - 5](+) or alkyl-substituted analogues as well as hydrous ions [M + 13](+) and [M + 13 + 18](+), as shown in Scheme 1 in the main text.

Exploration of Microplasma Probe Desorption/Ionization Mass Spectrometry (MPPDI-MS) for Biologically Related Analysis

To expand the applications of glow discharge microplasma into biological analysis, an innovative ambient ion source for mass spectrometry, microplasma probe desorption/ionization mass spectrometry (MPPDI-MS), has been developed and demonstrated. Electrodes and a sampling tube were creatively combined using a stainless steel syringe needle, and efficient methods of introduction for biological samples in solid, liquid, and gaseous phases like phospholipid and amino acids were specially designed. Based on the active species generated by glow discharge plasma, simplified protonated spectra were obtained without extra solvent spray assistance. The method is easy to operate and versatile and especially has the ability to distinguish the isomeric compounds of ketone and aldehyde. Quantitative results of this method for different biological samples in different phases were also performed well. It was proved that with further improvement, this sensitive and selective analysis using MPPDI-MS with minimal invasiveness will be an ingenious tool in disease diagnosis and single-cell detections in the future.

Microwave induced plasma desorption ionization (MIPDI) mass spectrometry for qualitative and quantitative analysis of preservatives in cosmetics

Ambient ion sources for mass spectrometry have been frequently reported in the past 10 years. The most attractive features of these ion sources are that they are fast and an easy way to analyze various samples. The microwave induced plasma desorption ionization source, the MIPDI source, is one among them. In this study, the qualitative and quantitative behavior of the MIPDI source has been demonstrated by analyzing the preservatives in cosmetics for the first time. The detection limit for the preservatives is as low as pg mm−2. The relative standard deviation of continuous analysis is 5.25%. Preservatives in commercial cosmetics samples were successfully detected including those in facial cream, sunscreen and moisturizer. The fast screening capability of the MIPDI source is proved. Five commercial samples were successfully classified into two groups within 5 minutes according to the added preservatives. The ability to quantitatively analyze the preservatives in commercial cosmetics was also investigated. The standard adding and calibration curve methods were used in the quantitation process. The results showed that the quantitative analysis accuracy of MIPDI-MS is −49% and −66% for a liquid state sample and solid state sample respectively, i.e. semi-quantitation is possible. A conceptual experiment was also conducted to validate the accurate quantitative analysis capability of MIPDI-MS. The concentration of caffeine in cosmetic matrices was quantified satisfactorily with no sample pretreatment, with an RSD value of 5.5% and accuracy of 3.6%. The approaches established in this work indicate that the MIPDI source is a promising tool in future applications where rapid qualitative and quantitative analysis is needed.

Matrix-Assisted Plasma Atomization Emission Spectrometry for Surface Sampling Elemental Analysis

An innovative technology has been developed involving a simple and sensitive optical spectrometric method termed matrix-assisted plasma atomization emission spectrometry (MAPAES) for surface sampling elemental analysis using a piece of filter paper (FP) for sample introduction. MAPAES was carried out by direct interaction of the plasma tail plume with the matrix surface. The FP absorbs energy from the plasma source and releases combustion heating to the analytes originally present on its surface, thus to promote the atomization and excitation process. The matrix-assisted plasma atomization excitation phenomenon was observed for multiple elements. The FP matrix served as the partial energy producer and also the sample substrate to adsorb sample solution. Qualitative and quantitative determinations of metal ions were achieved by atomic emission measurements for elements Ba, Cu, Eu, In, Mn, Ni, Rh and Y. The detection limits were down to pg level with linear correlation coefficients better than 0.99. The proposed MAPAES provides a new way for atomic spectrometry which offers advantages of fast analysis speed, little sample consumption, less sample pretreatment, small size, and cost-effective.