Sample cycling technique

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Solutions. Correction for cXjj is most often made using a standard-sample bracketing technique (e g., Galy et al. 2001). In this protocol, standard and sample isotope ratios obtained by multiple measurement cycles are compared and the sample result expressed as a deviation from the standard. Cross contamination between the sample and the standard is avoided by washing the analytical instrumentation with dilute (usually about O.IN) HNO3 for several minutes between analyses. Introduction of Mg in dilute HNO3 (e.g., 0. IN) into the MC-ICPMS... 

However, zero-field experiments suffer from low sensitivity and can only be applied to materials with reasonably long relaxation times T] to allow for the time it makes to shutde the sample in and out of the field. This field cycling technique measures the evolution of the magnetization at zero-field with the sensitivity of high-field NMR. Zero-field NMR is particularly suited to the investigation of nuclei with quadrupolar moments, since it avoids orientation-dependent broadening. 

FIA is the simplest form of sample introduction into the mass spectrometer, and this injection format has been widely used in the analysis of combinatorial library samples. This technique offers the highest throughput combined with ease of use and facile automation. Richmond et al. [67-69] reported methods to minimize sample carryover for the FIA-MS analysis of combinatorial libraries. Samples were sorted before the analysis to maximize the molecular-weight difference between samples in the analysis queue and to minimize the conditions where consecutively measured wells contain samples similar to building blocks. Cycle times of less than a minute were reported with a carryover of 0.01%. A software appUcation was developed to automatically report the sample purity and calculate sample carryover by an automatic spectrum comparison method [70,71]. A quasi-molecular ion discovery feature was also implemented [72] in the automated data-processing program. Automated FIA-MS analysis and reporting were also used in the analysis of fractions from the purification of combinatorial libraries [73]. Whalen et al. developed software to allow automated FIA-MS analysis from 96-well plates [74]. The system optimizes the interface for mass spectrometry and MS/MS conditions, and reports the results in an unattended fashion. 

NOE experiment should be performed at 400, 300 and 200 MHz instruments in CDCI3 or CDCls-DMSOde solutions of the samples using frequency cycling technique [31]. 

One useful advantage of the method is that four to six peptides can be analyzed simultaneously, each cycle of degradation taking lh. This multiple sample sequencing technique is especially useful for screening the chromatographic elution profile of a... 

Finally degassing under a vacuum may be useful to eliminate the small quantities of paramagnetic oxygen dissolved in the sample the freeze-pump- thaw cycle technique appears to give the best results for eliminating oxygen in purified organic solutions. 

If one were to start with a greater amount of target DNA, a significant increase in fluorescence signal will appear more quickly, which would be reflected by a lower C, value. The curve of fluorescence intensity versus cycle number for standard samples of known concentrations can be used to generate data that can be used to determine the starting concentrations of unknown samples. This technique is termed real-time quantitative PCR, or qPCR. 

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

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