Mass change

While electrospray is used for molecules of all molecular masses, it has had an especially marked impact on the measurement of accurate molecular mass for proteins. Traditionally, direct measurement of molecular mass on proteins has been difficult, with the obtained values accurate to only tens or even hundreds of Daltons. The advent of electrospray means that molecular masses of 20,000 Da and more can be measured with unprecedented accuracy (Figure 40.6). This level of accuracy means that it is also possible to identify post-translational modifications of proteins (e.g., glycosylation, acetylation, methylation, hydroxylation, etc.) and to detect mass changes associated with substitution or deletion of a single amino acid. 

Bulk-wave piezoelectric quartz crystal sensors indirecdy measure mass changes of the coating on the surface of the sensing device. This change in mass causes changes in the resonant frequency of the device, and measurements ate based on frequency differences. 

Because the (-) sign has no significance, except to indicate the direction of mass change, use 500 (xj). 

Assume values of yi and solve the equated mass change for values of x. 

Intergranular Oxidation Intergranular penetration of oxide can be a serious problem particularly when thin-walled components are used in load-bearing applications such as cooled turbine blades. Mass change data often do not adequately reflect the extent of this type of attack, and examination... 

Table 7.28 Mass changes during exposure of nickel and nickel alloys to carbon dioxide" ...

Liquid type and composition Test condition Category of attack Mass change (g) Change in diameter cm ... 

Very precise studies of this sort can be made by continuous observation of mass changes, as in the use of micro-balances, such as used and described by Gulbransen Such data have quantitative significance only when the exact composition (metal content) of the scale is known or can be determined and when there has been no loss of loose scale during or after the test. Fundamental studies of the initial stages of corrosion when films of a few mono-layers are formed have made use of an ellipsometer to follow the increase of thickness of corrosion products without disturbing the specimen. ... 

Carbide decompositions yield no volatile product and, therefore, many of the more convenient experimental techniques based on gas evolution or mass change cannot be applied. This is a probable reason for the relative lack of information about the kinetics of reaction of these and other compounds which are correctly classifed under this heading, such as borides, silicides, etc. 

The electrochemical quartz crystal microbalance (EQCM) is a powerful tool for elucidating interfacial reactions based on the simultaneous measurement of electrochemical parameters and mass changes at electrode surfaces. The microbalance is based on a quartz crystal wafer, which is sandwiched between two electrodes, used to induce an electric field (Figure 2-20). The field produces a mechanical oscillation... 

Such approximation is valid when the thickness of the polymeric layer is small compared to die thickness of die crystal, and the measured frequency change is small with respect to the resonant frequency of the unloaded crystal. Mass changes up to 0.05% of die crystal mass commonly meet this approximation. In die absence of molecular specificity, EQCM cannot be used for molecular-level characterization of surfaces. Electrochemical quartz crystal microbalance devices also hold promise for the task of affinity-based chemical sensing, as they allow simultaneous measurements of both tile mass and die current. The principles and capabilities of EQCM have been reviewed (67,68). The combination of EQCM widi scanning electrochemical microscopy has also been reported recently for studying die dissolution and etching of various thin films (69). The recent development of a multichannel quartz crystal microbalance (70), based on arrays of resonators, should further enhance die scope and power of EQCM. 

The above methods measure ion transport rates as ionic conductivities. By varying the parameters of the experiment, it is often possible to indirectly identify the mobile ion(s),173 and in some cases to estimate individual ion mobilities or diffusion coefficients.144 Because of the uncertainty in identifying and quantifying mobile ions in this way, EQCM studies that provide the (net) mass change accompanying an electrochemical process36 have played an important complementary role. 

The results from EQCM studies on conducting polymer films can be ambiguous because the measured mass change results from a combination of independent ion transport, coupled ion transport (i.e., salt transport), and solvent transport. In addition, changes in the viscoelasticity of the films can cause apparent mass changes. The latter problem can be minimized by checking the frequency response of the EQCM,174 while the various mass transport components can be separated by careful data analysis.175,176... 

Table 5.13 Mass changes associated with common biotransformation processes...

FIGURE 9.11 Relative mass change as a result of water absorption and loss in a solid piece of recombinant resilin prepared from a 20% protein solution in phosphate-buffered sahne (PBS), cross-linked using peroxidase (see [29] supplementary material). The fuUy swollen sample is designated 100. (Data courtesy of Shekibi, Y., Naim, K., Bastow, T.J., and HiU, A.J., The states of water in a protein based hydrogel. Internal CSIRO... 

Changes in thermal stability and mass due to the formation of CdS nanoparticles in LB films were examined [180]. The LB films were formed onto gold-coated quartz oscillators from monolayers of arachidic acid or nonacosa-10,12-diynoic acid on CdCH containing subphases. The films were exposed to H2S gas until the mass change indicated complete conversion of Cd to CdS. The thermal stability of the H2S-treated films was reduced, with significant mass loss initiating at 55°C, compared to minimal mass loss in the untreated films up to at least 80°C under mild vacuum. The average CdS-particle size... 

Am — (4.00260gmol ) - (4.032979gmol ) = 0.03038gmol Equation lets us calculate an energy change from a mass change ... 

When the energy change accompanying a process is known, Equation can be used to calculate how much the mass changes during the process. Example shows how to do this for a chemical reaction. 

Substitute this mass change into Equation to find the energy released ... 

Solutions of c acetate buffer, pH 5.0, were injected into the flow cell and passed over the PGIP surface at the flow rate of 10 il/min. The interaction was followed in real time at different analyte concentrations. The binding was monitored as a mass change in the vicinity of the sensor surfiice, reflecting the progress of the interaction. 

The electrochemical quartz crystal microbalance (EQCM) is a very useful technique for detecting small mass changes at the electrode surface that accompany electrochemical processes. In 1880, Jacques and Pierre Curie discovered that when stress was applied to some crystals, such as quartz, it resulted in an electrical potential across the... 

Any mass changes that occur on the working elechode are reflected in changes in the frequency. The quantitative relationship is given by the Sauerbrey equation ... 

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