Dwell time analysis

LC-MS/MS has dramatically changed the way bionalysis is conducted. Accurate and precise quantitation in the pg ml scale is nowadays possible however one has to be aware of certain issues which are specific to mass spectrometric detection such as matrix effects and metabolite crosstalk. With the current growing interest in the analysis of endogenous biomarkers in biological matrices, quantitative bioanalysis with MS has certainly the potential to contribute further in this field with the development of multicomponent assays. Modern triple quadrupole instruments have the feature to use very short dwell times (5-10 ms), allowing the simultaneous determination of more than 100 analytes within the timescale of an HPLC peak. Due to the selectivity of the MS detection the various analytes... 

Maximum (maximized) likelihood is a statistical term that refers to the probability of randomly drawing a particular sample from a population, maximized over the possible values of the population parameters. Selected entries from Methods in Enzymology [vol, page(s)] Theory, 210, 203 testing by simulations, 210, 225 computer applications for, 210, 233 fitting of sums of exponentials to dwell-time distributions, 207, 772 fluorescence data analysis, 210,... 

Some of these measurements can also be performed on compaction simulators, which are single-stroke presses specifically designed to evaluate individual materials and/or full formulations [ 12]. The simulation of short dwell times and of many different profiles for punch movement in real time are the advantages of this type of measurement. Recent work with a compaction simulator has even included a thermodynamic analysis of compaction [13,14]. 

MS Dwell Time Dwell time describes the time spent on a single step in a SRM or SIM analysis. Longer dwell time results in fewer data points but better signal-to-noise ratio, and should be optimized to produce acceptable data for each consideration. Common SRM dwell times in LC-MS would be 25-300 milliseconds (ms). 

To improve sensitivity, selected ion monitoring (SIM) mode may be used for the detection of routine YOCs and SVOCs and in low resolution dioxin/furan analysis (EPA Method 8280). In the SIM mode, only specific ions from the analyte s spectrum are scanned for the detector s dwelling time on each ion is increased resulting in higher sensitivity. A mass spectrometer operated in the SIM mode is approximately ten times as sensitive as one in the full scan mode. The SIM mode has limitations, such as the capacity to monitor only a limited number of ions and the need to monitor multiple ions for each compound to improve the degree of confidence in compound identification. That is why typically no more than 20 compounds can be analyzed simultaneously in the SIM mode. 

Fluorescence Correlation Spectroscopy and Fluorescence Burst Analysis. Several nanoscopic chemical imaging approaches work very well for measurements of chemical kinetics, interactions, and mobility in solution. Fluorescence correlation spectroscopy (FCS) measures the temporal fluctuations of fluorescent markers as molecules diffuse or flow in solution through a femtoliter focal volume.54 Their average diffusive dwell times reveal their diffusion coefficients, and additional faster fluctuations can reveal chemical reactions and their kinetics if the reaction provides fluorescence modulation. Cross-correlation of the fluorescence of two distinguishable fluorophore types can very effectively reveal chemical binding kinetics and equilibria at nanomolar concentrations. 

If we consider the case of ICP-MS, instruments typically achieve sensitivities of about 10,000 ion counts/sec/ppb thus, at the specified dwell time, a quadrupole mass spectrometer generates about 2000 counts at each mass for a 1-ppb solution. A TOF-MS with the same sensitivity generates 30,000 counts at each mass. In the scanned instrument, lower concentrations obviously yield fewer counts and require longer dwell times. Therefore, for analyses that require many elemental signals to be measured for either more sample information, internal standardization, or isobaric corrections the TOF-MS has the potential to deliver complete analysis with no loss in speed or sensitivity. 

Mass spectrometry Positive ion electrospray ionization mass spectrometry (ESl-MS) analysis was performed on a PE API 2000 triple quadrupole mass spectrometer (Sciex, Toronto, Canada). Spray voltage was set to 4.8 kV, and 30 V orifice voltage was applied. Samples were dissolved in a methanol-water (1 1, v/v) mixture containing 0.1% acetic acid, and 5 pL of sample was injected with a flow rate of 100 pL/min. The instmment was used in a Qj scan mode in the range of m/z 400-1700, with a step size of 0.3 amu and a dwell time of 0.5 ms. Other chimeric peptides in this study were purified and characterized in the same or a very similar way. 

It is, when possible, very convenient to be able to treat the kinetics of the chemical reaction separately from the translational diffusion in the fluctuation analysis. As mentioned, a rather broad range of chemical reactions fulfills the criteria for (8.3)-(8.5). In addition, for a reaction which under standard conditions does not fulfill these criteria, it is sometimes possible to modify the conditions. For instance, the dwell times can be retarded with respect to the chemical relaxation times by expanding the observation volume or by speeding up the reactions under study, for instance by using higher concentrations of unlabelled reactants. 

Experiments are controlled by the PC, which is connected to the boxcar averager by a GPIB line. Boxcar parameters such as the number of data points to collect, the dwell time, and the number of scans to average, are set by using the PC. Data sets are then transferred from the boxcar averager to the PC for storage. The PC is connected by ethemet to a workstation computer, where data analysis is performed. 

EDS offers two modes of examination stationary and scanning. In the stationary mode, the probe stays at one location until the collection of X-ray photons is complete. The dwell time of the probe for EDS analysis is determined by the number of X-ray photon counts received by the detector. Elemental detection relies on the signal-to-noise ratio. As discussed in Chapter 4, the... 

In digital signal analysis the values of functions are usually given at equidistant intervals. In NMR this interval is the dwell time or the sampling interval At of the measured response. With the time variable being... 

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