Peak time determination

Of the six parameters shown in Figure 13.18, the most important are peak height and return time. The peak height is related, directly or indirectly, to the analyte s concentration and is used for quantitative work. The sensitivity of the method, therefore, is also determined by the peak height. The return time determines the frequency with which samples maybe injected. Figure 13.19 shows that when a second sample is injected at a time T after injecting the first sample. 

By the integrating the current over the time for each peak we determine the number of charge carriers which equals the number of traps N, (under the condition that all traps were occupied at the starting temperature) ... 

Computer Techniques McLafferty (Ref 63) has pointed out that the usefulness of elemental composition information increases exponentially with increasing mass, since the number of elemental combinations with the same integral mass becomes larger. There are compilations of exact masses and elemental compositions available (Refs 12a, 13 18a). Spectral interpretation will be simplified in important ways if elemental compositions of all but, the smallest peaks are determined. Deriving the elemental compositions of several peaks in a spectrum is extremely laborious and time-consuming. However, with the availability of digital computers such tasks are readily performed. A modern data acquisition and reduction system with a dedicated online computer can determine peak centroids and areas for all peaks, locate reference peaks, interpolate between them to determine the exact masses of the unknown peaks, and find within minutes elemental compositions of all ions in a spectrum (Refs 28b 28c)... 

Total theoretical peak capacity for the ID and 2D LC/MS analyses of the yeast ribosomal protein sample was calculated as 240 and 700, respectively. Individual separation peak capacities were calculated by dividing the total separation time by the average peak width at baseline, and the 2D peak capacity determined as the product of the peak capacity of the two dimensions. These theoretical calculations rely on optimal use of the two-dimensional separation space, which in turn is dependent upon the lack of correlation between the component retention times in the two separation modes. Thus, the maximum use of the theoretical peak capacity is not only dependent on the selection of chromatographic modes based on different physicochemical... 

Tominaga et al. [682,683] studied the effect of ascorbic acid on the response of these metals in seawater obtained by graphite-furnace atomic absorption spectrometry from standpoint of variation of peak times and the sensitivity. Matrix interferences from seawater in the determination of lead, magnesium, vanadium, and molybdenum were suppressed by addition of 10% (w/v) ascorbic acid solution to the sample in the furnace. Matrix effects on the determination of cobalt and copper could not be removed in this way. These workers propose a direct method for the determination of lead, manganese, vanadium, and molybdenum in seawater. 

The effect of NO exposure time on the time at which the N2 and N2O signals attain a maximum is shown in Fig. 16. It is seen that the model of NO reduction predicts that N2 formation peaks about 0.5 s after the peak in the N2O formation and that the peak times for both products decline by about 0.5 s as the NO exposure time is increased from 5 to 30 s. These trends are in good agreement with the data. It should be noted that since a product analysis could be taken only once every 0.5 s, it was not possible to determine product peak positions with an accuracy of better than 0.5 s. Consequently, both the predicted difference between... 

The precision of the assay for nonreduced samples was demonstrated by the evaluation of six independent sample preparations on a single day (repeatability) and the analysis of independent sample preparations on three separate days by two different analysts (intermediate precision). The RSD values for the migration time were 0.9%. The RSD values for peak area percent of the main peak and the minor peaks in the profile were 0.6 and 12.6%, respectively. The higher variability observed with the minor peaks was determined to be primarily related to the sample heating during preparation for the analysis. These results demonstrate that the use of uncoated fused-silica capillaries in combination with a sieving matrix can provide adequate precision and analyte recovery. 

From this comparison it follows that the observation of the structural relaxation by standard relaxation techniques in general might be hampered by contributions of other dynamic processes. It is also noteworthy that the structural relaxation time at a given temperature is slower than the characteristic time determined for the a-relaxation by spectroscopic techniques [105]. An isolation of the structural relaxation and its direct microscopic study is only possible through investigation of the dynamic structure factor at the interchain peak - and NSE is essential for this purpose. 

The first two points are best dealt with as part of the process for developing vahdated analytical methods. Vafidation should include testing the robustness of a method in repeated use over a period of time determining the precision and accuracy and study of potential interferences. As an example, it would be expected that in the capillary GC—TEA method for organic explosives, a peak should be at least three times the basefine noise to be counted as a real signal, and that the relative retention time should be within 1.0% of the standard for volatile compounds and within 0.5% for the rest. The relative retention time is simply the ratio of the analyte s retention time compared with that of an internal standard. Use of relative retention times significantly improves the repeatabdity of GC analysis... 

We will use the means and 95% confidence intervals for both and K2 to determine the time, and distance through s = Ut, when Cpeak = 0.0005 g/m. These conditions are listed in Table E9.2.1, with the times determined through iteration on equation (E9.2.3). Table E9.2.1 shows that the peak value of concentration is no longer sensitive to longitudinal dispersion coefficient after roughly 3 days, because the peak is widely spread. The time when the water treatment plants downstream of the spill could turn on the water intake, however, would likely be sensitive to longitudinal dispersion coefficient. 

The manner in which the RF pulse is applied is critical to NMR analysis. A very simplified pulse sequence is a combination of RF pulses, signals, and intervening periods of recovery, as illustrated in Figure 6.80. The main components of the pnlse sequence are the repetition time, TR, which is the time from the application of one RF pulse to the application of the next RF pulse (measured in milliseconds) and the echo time, TE. The repetition time determines the amount of relaxation that is allowed to occur between the end of one RF pulse and the application of the next. Therefore, the repetition time determines the amount of Ti relaxation that has occnrred. The echo time is the time from the application of the RF pnlse to the peak of the signal induced in the coil (also measured in milliseconds). The TE determines how much decay of transverse magnetization is allowed to occur before the signal is read. Therefore, TE controls the amount of T2 relaxation that has occnrred. 

This system resolved the aniline peak (retention time (rt) = 2.67 min) from the benzidine peak (rt = 2.27 min) as can be seen in Figure 2. Other potential interferences were selected for study by looking at the expected fragments from the reduction of various dyes. Reduced dye samples were spiked with aniline (rt = 2.67 min), -aminophenol (rt = 1.97 min), -phenylenediamine (rt = 1.93 min) and -nitroaniline (rt = 3 16 min). None of these materials interfered with the detection of the benzidine peak. To determine if other types of dyes might interfere with the analysis, two sets of filters were spiked at low and high levels separately with C.I. Direct Red 28 (13 7 yg and 137 yg), C.I. Direct Blue 53 formulation (o-tolidine-based) (21.2 yg and 212 yg) and C.I. Direct Blue 8 formulation (o-dianisidine-based)(23.3 yg and 233 yg). 

The smaller peaks in the mass spectra showed random fluctuations in the number of ions formed in the interval between consecutive pulses. The fluctuations were smoothed by performing several experiments at the same conditions and averaging peak heights from spectra taken at the same time interval after the flash. The time dependence of individual peaks was determined by ratioing the peak height of an m/z signal of interest to the peak height at m/z = 20, due to Ne+ from deliberately added Ne. The Ne, of course, does not participate in the reaction and its concentration is time invariant. 

Error Analysis. The difficulties encountered in resolving peak profiles from each other, and at the same time determining the background scatter, call to question the extent of the error involved in this type of analysis, given that the final resolution appears realistic in the light of known information about the structure of the material. The problem of error in profile resolution has been considered (14) in terms of the equatorial trace of a viscose rayon fibre specimen which is similar to Fortisan, Figure 5 ... 

FIG. 8-27 The optimum settings produce minimum-IAE load response. (a) The proportional band primarily affects damping and peak deviation. (b) Integral time determines overshoot. 

Development of the method involved the installation of a system in an existing mass spectrometry laboratory and working with chemists for 3 months to determine specific needs and to develop a consistent, reliable procedure. The instrument was moved to an open-access laboratory and chemists were trained in its use. A key to making this approach a success is the fact that instrument downtime was kept to a minimum. Understandably, maintenance is done at off-peak times, and support mechanisms are put in place so problems are immediately addressed. Training and education was highlighted as a key factor for the successful implementation of this LC/MS system to optimize performance and to reduce the possibility of instrument contamination. 

After the noise level has been calculated then the difference between two adjacent data points will be compared to the noise level. If the difference is greater than the noise then the program will assume that a peak has begun. At this point an integral is begun to determine the area under the peak. The end point of the peak is determined in the reverse manner. At the end of each peak the starting, center, and ending times of the peak are printed as well as the area under the peak. 

FIGURE 41. Part (a) is a 6Li—29Si HMQC correlation for 35 (0.7 M in THF-ds) measured at 294 K [sweep width FI (29Si) 27.46 ppm at 79.5 MHz, ref. TMS sweep width F2 (6Li) 3.05 ppm at 58.6 MHz, 64 increments, acquisition time 5.71 s evolution time determined by 6Li relaxation]. The 6Li signal at —1.20 ppm is not shown as it yields no cross-peaks—it is assigned to the solvent-separated external 6Li cation. Parts (b) and (c) are traces through the cross-peaks. Reprinted from Reference 298. Copyright 1996, with permission from Elsevier Science... 

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