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Waki, Y. Ido, S.
Akita, and Y. Yoshida, and T. Yoshida, Rapid Commun. Mass Spectrom.
Modern Mass Spectrometry Topics In Current Chemistry V 225
Copyright: John Wiley and Sons. Average of 82 laser shots. Reproduced from J. Sunner, E. Dratz, and T. Chen, Anal Chem , , 67 , Copyright: American Chemical Society. Regardless of its drawbacks in upper mass limit for the analysis of large molecules, the sensitivity was far from being satisfactory compared to hard ionization techniques in terms of testing LMW molecules. In fact, majority of research on SALDI-MS has been focusing on exploiting novel nanomaterial substrates, aiming at further broadening the mass range, improving the reproducibility, enhancing the sensitivity and extending the categories of compounds that were able to be analyzed.
So far, a variety of nanomaterials have been utilized in SALDI-MS, including carbon-based nanomaterials, metal-based nanomaterials, semiconductor-based nanomaterials, etc. Wei, J. Buriak, and G. Siuzdak Nature , , , Copyright: Nature Publishing Group. As a soft ionization technique, SALDI is expected to produce molecular or quasi-molecular ions in the final mass spectra. Since this requires the ionization process to be both effective and controllable, which means sufficient sample molecules could be ionized while further fragmentation should be mostly avoided.
While the original goal mentioned above has been successfully accomplished for years, the study on desorption and ionization mechanism in detail is still one of the most popular and controversial research areas of SALDI at present. It is mostly agreed that the substrate material has played a significant role of both activating and protecting the analyte molecules.
Energy input from the pulsed laser is largely absorbed by the substrate material, which is possibly followed by complicated energy transfer from the substrate material to the absorbed analyte molecules. As a result, both thermal and non-thermal desorption could be triggered, and for different modes of SALDI experiments, the specific desorption and ionization process greatly differs. The gray spheres and blue spheres represent substrate materials and analyte molecules, respectively. The mechanism for porous silicon surface as a SALDI substrate has been widely studied by researchers.
In general, the process can be subdivided into the following steps:. When no associated proton donor is present in the vicinity of analyte molecules, desorption might occur without ionization. Subsequently, the desorbed analyte molecule is ionized in the gas phase by collision with incoming ions. Since it is the active surface responsible for adsorption, desorption and ionization of analyte molecules that features the technique, the surface chemistry of substrate material is undoubtedly crucial for SALDI performance.
But it is rather difficult to draw a general conclusion due to the fact that the affinity between different classes of substrates and analytes is considerably versatile. Basically, the interaction between those two components has an impact on trapping and releasing the analyte molecules, as well as the electronic surface state of the substrate and energy transfer coefficiency. Another important aspect is the physical properties of the substrates which could alter desorption and ionization process directly, especially for the thermally activated pathway.
This is closely related to rapid temperature increase on the substrate surface. Those properties include optical absorption coefficiency, heat capacity and heat conductivity or heat diffusion rate. First, higher optical absorption coefficiency enables the substrate to absorb and generate more heat when certain amount of energy is provided by the laser source. Moreover, a lower heat capacity usually leads to larger temperature increase upon the same amount of heat. In addition, a lower hear conductivity helps the substrate to maintain a high temperature that will further result in a higher temperature peak.
Therefore, the thermal desorption and ionization could occur more rapidly and effectively. It contains a laser source which generates pulsed laser that excites the sample mixture. There is a sample stage that places the sample mixture of substrate materials and analytes. Zhang, Z. Li, C. Zhang, B. Feng, Z.
Zhou, Y. Bai, and H. Liu, Anal. Copyright: American Chemical Society, Porous silicon with large surface area could be used to trap certain analyte molecules for matrix-free desorption and ionization process. More interestingly, a large ultraviolet absorption coefficiency was found for this porous material, which also improved the ionization performance. Accurate mass measurements were obtained on WIN with a time-of-flight reflectron instrument, typically to within 10 ppm the limit of accuracy of this instrument in this mass range.
The inset spectrum represents post-source decay fragmentation measurements on WIN. These results are consistent with results from electrospray ionization tandem mass spectrometry experiments.
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Siuzdak, Nature , , , Copyright: Nature Publishing Group, Graphene is a type of popular carbon nanomaterial discovered in It has a large surface area that could effectively attach the analyte molecules. Polar compounds including amino acids, polyamines, anticancer drugs, and nucleosides can be successfully analyzed. In addition, nonpolar molecules can be analyzed with high resolution and sensitivity due to the hydrophobic nature of graphene itself. It has been demonstrated that the use of graphene as a substrate material avoids the fragmentation of analytes and provides good reproducibility and a high salt tolerance, underscoring the potential application of graphene as a matrix for MALDI-MS analysis of practical samples in complex sample matrixes.
It is also proved that the use of graphene as an adsorbent for the solid-phase extraction of squalene could improve greatly the detection limit. Detection limits were in the range of attomoles, but improvements are expected in the future. Reproduced from: S. Alimpiev, A. Grechnikov, J.
Sunner, A. Borodkov, V. Karavanskii, Y. Simanovsky, and S. Nikiforov, Anal. In the study of electrochemistry, it had always been a challenge to obtain immediate and continuous detection of electrochemical products due to the limited formation on the surface of the electrode, until the discovery of differential electrochemical mass spectrometry. Scientists initially tested the idea by combining porous membrane and mass spectrometry for product analysis in the study of oxygen generation from HClO 4 using porous electrode in In summary, the experiment demonstrated in not only showed continous sample detection in mass spectrometry but also the rates of formation, which distinguished itself from the technique performed previously in Hence, this method was called differential electrochemical mass spectrometry DEMS.
During the past couple decades, this technique has evolved from using classic electrode to rotating disc electrode RDE , which provides a more homogeneous and faster transport of reaction species to the surface of the electrode.
Described in basic terms, differential electrochemical mass spectrometry is a characterization technique that analyzes specimens using both the electrochemical half-cell experimentation and mass spectrometry. It uses non-wetting membrane to separate the aqueous electrolyte and gaseous electrolyte, which gaseous electrolyte will permeate through the membrane and will be ionized and detected in the mass spectrometer using continuous, two-stage vacuum system. This analytical method can detect gaseous or volatile electrochemical reactants, reaction products, and even reaction intermediates.
The instrument consists of three major components: electrochemical half-cell, PTFE polytetrafluoroethylene membrane interface, and quadrupole mass spectrometer QMS , which is a part of the vacuum system. In this section, each component will be explained and its functionality will be explored, and additional information will be provided at the end of this section.
The PTFE membrane is micro-porous membrane that separates the aqueous electrolyte from volatile electrolyte which will be drawn to the high vacuum portion. Using the high vacuum suction, the gaseous or volatile species will be allowed to permeate through the membrane using differential pressure, leaving the aqueous materials on the surface due to hydrophobic nature of the membrane.
The selection of the membrane material is very important to maintain both the hydrophobicity and proper diffusion of volatile species. The species permeated to QMS will be monitored and measured, and the kinetics of formation will be determined at the end. Depending on the operating condition, different vacuum pumps might be required.
Adapted from Aston, S. First major component of the DEMS instrument is the design of electrochemical cells. There are many different designs that have been developed for the past several decades, depending on the types of electrochemical reactions, the types and sizes of electrodes. However, only the classic cell will be discussed in this chapter.
aqiqozohegag.tk DEMS method was first demonstrated using the classical method. In the demonstration by Wolber and Heitbaum, the electrode was prepared by having small Pt particles deposited onto the membrane by painting a lacquer. It was later in other experimentations evolved to use sputtering electro-catalyst layer for a more homogenous surface. The aqueous cell electrolyte is shielded with an upside down glass body with vertical tunnel opening to the PTFE membrane.