Baseline width

The second important parameter is the chromatographic peak s width at the baseline, w. As shown in Figure 12.7, baseline width is determined by the intersection with the baseline of tangent lines drawn through the inflection points on either side of the chromatographic peak. Baseline width is measured in units of time or volume, depending on whether the retention time or retention volume is of interest. 

Measurement of the column s void time, and the retention time, and baseline width, w, for a solute. 

In a chromatographic analysis of lemon oil a peak for limonene has a retention time of 8.36 min with a baseline width of 0.96 min. y-Terpinene elutes at 9.54 min, with a baseline width of 0.64 min. What is the resolution between the two peaks ... 

The increase in a solute s baseline width as it moves from the point of injection to the detector. 

A chromatographic analysis for the chlorinated pesticide Dieldrin gives a peak with a retention time of 8.68 min and a baseline width of 0.29 min. How many theoretical plates are involved in this separation Given that the column used in this analysis is 2.0 meters long, what is the height of a theoretical plate ... 

The preceding eqiiations are accurate to within about 10 percent for feed injections that do not exceed 40 percent of the final peak width. For large, rec tangiilar feed injections, the baseline width of the response peak is approximated by ... 

In practice it is easier to measure baseline width or the width at one half of the peak height, so Vis generally calculated using one of the alternative formulae ... 

For example, in the ideal model, the baseline width of the profile obtained with a Langmuir isotherm (W, ) is... 

Figures 5(a) and (b) illustrate the chromatograms obtained at u with columns 1 and 7, respectively, of a commercial petroleum°] roduct ("white gas"). Tlie temperature was programmed but the rate was virtually identical in each case. There are approximately a third again as many peaks visible in the latter than in the former and, furthermore, all are obviously considerably sharper (smaller baseline widths). The last-eluting compound provides an example the peak is barely discernible in Figure 5(a) at an elution time of 30 min, but is clearly seen at 36 min in 5(b). In addition, several very small peaks are visible penultimate to this, but are not detected in 5(a). Overall, then, the column of reduced internal diameter and film thickness is very much superior as expected on the basis of efficiency. Nevertheless, whether the results obtained justify the practical difficulties associated with its fabrication and use remains a question open largely to subjective judgment.

The three steps of an MS/MS experiment are performed by DIT using an approach substantially different from that employed in 3D IT or linear ITs (Ding, 2004). In those cases, the precursor ion isolation is performed by applying one or more dipole excitation waveforms, with a maximum isolation resolution of -1300 (expressed as the isolation mass divided by the baseline width of the isolation window). In the case of DIT, ion isolation is performed by sequential forward and reverse scans, so as to eject all ions with m/z values lower and higher than that of interest, respectively. This method can provide precursor ion isolation with a resolution >3500. 

The baseline widths related to the sample and reagent zones are normally estimated by sequentially replacing the sample and reagents by a coloured solution (dye approach [26]), so that chemical reactions are not involved. Eq. 5.11 provides only an estimate of P because the zone of the product formed depends on the chemical reactions involved, the related equilibria and kinetics and the molecular diffusion coefficients. So, its shape (thus the value of wp) is dependent on the specific analysis involved [98]. A complete overlap between sample and reagent zones is attained for P = 1, and this can be achieved by increasing n in order to get a (ws + wr) value equal to 2wp. This has seldom been exploited, however, because a higher value of n means a lower sampling rate. 

FIGURE 5.19 Recorded peaks for the reagent (R) and sample (S) solutions in a typical sequential injection system. M = monitored signal IP = iso-dispersion point wD, w wr, = baseline widths of the overlapped zone, sample zone and reagent zone respectively. Note that wD is obtained by extrapolation. For details, see Ref. [97]. Figure adapted with permission from "T. Guebeli, G.D. Christian,. Ruzicka, Fundamentals of sinusoidal flow sequential injection spectrophotometry, Anal. Chem. 63 (1991) 2407". Copyright 1991, American Chemical Society. 

N can be measured from the peak profile (Fig. 7). Assuming the peak to be Gaussian, the baseline width of the peak as measured by drawing tangents from the curve to the baseline is equivalent to four standard deviations. Therefore ... 

During the first transition (CO-O2), a rapid desorption of CO2 is observed as soon as the gas is switched to O2. The CO2 is fonned by the surface reaction of preadsorbed CO and O2 via the well-known Langmuir-Hinshelwood mechanism. The exact amoimt of CO2 liberated, and the peak baseline width, are given in Table 8. After t2, the catalyst is subjected to the second transition (O2-CO), and a second CO2 peak is observed. We suggest that during t2, all the preabsorbed CO reacts, and is replaced on the surface by O2. After t2, CO2 is formed by tlie siuface reaction of preadsorbed O2 with CO. 

Table 8. Peak areas, and baseline widths for the CO/O2 reaction observed by...

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