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Emerging Biomedical Analysis
Published in Lawrence S. Chan, William C. Tang, Engineering-Medicine, 2019
Organic compounds that absorb at the laser wavelength, sublime readily and co-crystallize with the analyte are chosen as the matrix. The most commonly used materials are 2,5-dihydroxybenzoic acid (DHB); 4-hydroxy-3-methoxycinnamic acid (Ferulic acid); 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid) and a-cyano-4-hydroxycinnamic acid (CHC). The MALDI method has been successfully applied to protein, peptide and DNA analyses in biological research for many years. More recently, there has been a rapid increase in the interest in mass spectral imaging directly from biological tissue. Currently, MALDI is the method of choice in MS imaging applications due to its superior physical compatibility with imaging experimentation (Gessel et al. 2014). However, since the sample preparation for MALDI is time consuming and not compatible with commonly used chromatographic separation techniques, it is now generally replaced by ESI for analysis of biomaterials.
Molecular Weight of Polymers
Published in Charles E. Carraher, Carraher's Polymer Chemistry, 2017
In matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), the polymer is dissolved, along with a “matrix chemical,” and the solution deposited onto a sample probe. The solution is dried. MALDI MS depends on the sample having a strong UV absorption at the wavelength of the laser used. This helps minimize the fragmentation since it is the matrix UV-absorbing material that absorbs most of the laser energy. Often employed UV-matrix materials are 2,5-dihydroxybenzoic acid, sinapinic acid, picolinic acids, and alpha-cyano-4-hydroxycinnamic acid. The high energy of the laser allows both the matrix material and the test sample to be volatilized. Such techniques are referred to as “soft” since the test sample is not subjected to (much) ionizing radiation and hence little fragmentation occurs.
Mass Spectrometry Instrumentation
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
Yuan Su, Li-Rong Yu, Thomas P. Conrads, Timothy D. Veenstra
During the same time as ESI was being developed, significant advancements were being made in the laser desorption of biological molecules, especially after proper organic matrices were discovered for the desorption and ionization of proteins with masses of >10,000 Da (Karas et al., 1987; Tanaka et al., 1988). MALDI generates high-mass ions by using a pulsed laser beam to irradiate a solid mixture of an analyte with a suitable matrix (Ryzhov et al., 2000). The matrix consists of small and highly conjugated organic molecules that actively absorb energy in the UV region. The most commonly used matrices for proteins and peptides include α-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (DHB), and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid). For MALDI-MS analysis, a few microliters of a mixture is first deposited onto a MALDI target plate and dried to form a crystal. The MALDI target plate is then inserted into the ion source region and irradiated with a laser pulse (Figure 14.6). A nitrogen (N2) laser (337 nm) operating at 2–20 Hz is commonly used to irradiate the sample due to low cost and ease of operation; however, other lasers operating at different wavelengths can also be used according to the property of each analyte. The matrix molecules on irradiation, because of the high matrix-to-analyte concentration ratio, absorb most of the photon energy from the laser. This energy is then transferred to the analyte (i.e., peptides) in the sample, which is subsequently ejected from the target surface into the gas phase. These gas-phase ions are directly drawn into the mass analyzer, because the pressure in the mass analyzer is much lower than in the MALDI ion source region. Recently, however, MALDI sources operating at atmospheric pressure have demonstrated relatively high sensitivity for peptide mass fingerprinting (Dainese et al., 1997). By operating at atmospheric pressure, MALDI can be interfaced to most of the mass analyzers that have historically been reserved for ESI applications, such as ion traps and Q-TOF. Unlike ESI, MALDI typically produces singly charged ions regardless of whether the analyte is a peptide or a large protein. This propensity to produce large m/z species has made the coupling of MALDI with large m/z range mass analyzers, such as TOF spectrometers, a popular choice. For example, MALDI-TOF-MS is a powerful tool for detecting mass shifts between a parent monoclonal antibody and antibody–drug conjugate (ADC) (Safavy et al., 2003). Although MALDI-TOF has difficulty in resolving individual ADC species that correlate with different numbers of payload conjugation, the average drug-to-antibody ratio (DAR) can be determined based on the mass spectrum (Quiles et al., 2009). MALDI-TOF-MS is also used for glycomic analysis (Reiding et al., 2014) and clinical diagnostic microbiology (Croxatto et al., 2012) due to its high throughput.
Effects of dielectric barrier discharge (DBD) plasma on the drying kinetics, color, phenolic compounds, energy consumption and microstructure of lotus pollen
Published in Drying Technology, 2022
Jia-Bao Ni, Jia-Shu Zhang, Bhesh Bhandari, Hong-Wei Xiao, Chang-Jiang Ding, Wen-Jun Peng, Xiao-Ming Fang
Generally, thermal processing reduces phenolic acid contents to different degrees.[19] However, according to Table 2, after DBD plasma drying, the sinapic acid content tended to increase, Owing to its pleasant creamy odor, Vanillic acid also widely applied to fragrances and permitted as a food additive. Sinapic acid exhibits antioxidant, antimicrobial, anti-inflammatory, anticancer, and anti-anxiety activities. The results demonstrated that DBD plasma drying could increase the vanillic and sinapic acid contents, thereby improving the fragrance and pharmacological activity of lotus pollen. This may be due to the hydrolysis and oxidation of polyphenols during the drying process, which convert natural compounds into oligomers with higher antioxidant activity.[37] On the other hand, the increases in vanillic and sinapic acid contents could be attributed to the inactivation of PPO. Characteristics of the various biomolecules, including the enzymes, include possessing a dipole moment and a net charge. Thus, they are very sensitive to external electric fields, which causes different levels of molecular modification and conformational changes.[38] In addition to the stimulation of the electric field, the frequent reversals of alternating electric fields might also change the direction of the metal ions in the active center of the enzyme. This high-frequency reversal inevitably causes alternating stress, which causes the structural fatigue and ultimate inactivation of the enzyme. Sigh et al.[39] confirmed this hypothesis. They found that when the applied field intensity was 3 V/nm, the total dipole moment of soy protein isolate increased to 1000 Debye in approximately 200 ps, and the helix of soy protein isolate was rearranged in the direction of the applied electric field, but no preferentially arranged spirals are observed in the control group, which confirms that under an external electric field, the protein itself aligns in the direction of the electric field. However, some studies have shown that although EHD drying can inactivate PPO and POD enzymes, it still leads to a decrease in total phenol content.[37] These assumptions need to be verifiedby further studies.