Adjustable peak detection limits

Magnuson et al. [105] described the use of CE with a hydrodynamically adjusted EOF with hydride generation for arsenic speciation and ICP-MS detection. Hydrodynamic pressure was applied in the opposite direction to the EOF so that large quantities of the analyte could be injected without significant peak broadening. Four arsenic species were separated with detection limits in the range 6-58 ppt. 

Under the assumption that the low frequency component is a stationary Gaussian random process, let us proceed with the specification of a detection algorithm. The detection limit indicates the performance of the detection algorithm. Detection of peaks of interest in the adjusted chromatogram involves removal of the low frequency component to the extent possible and perhaps the search in time for the desired peak. 

As seen in previous sections, the response to a potential step is a pulse of current, which decreases with time as the electroactive species near the electrode surface is consumed and consists of a faradaic, /f, and a capacitive contribution, Iq. The advantage of most pulse techniques results from the measurement of the current flow near the end of the pulse when the faradaic current has decayed, often to a diffusion-limited value but when the capacitive current is insignificant. Pulse widths, tp, are adjusted to satisfy this condition and the additional condition that time has not been allowed for natural convection effects to influence the response. There is a greatly improved signal-to-noise ratio (sensitivity) compared to steady state techniques and in many cases, greater selectivity. Detection limits are of the order of 10 M. Furthermore, for analytical purposes, most current-voltage profiles from the pulse techniques are faster to interpret than those of dc voltammograms, because they are peak-shaped rather than the typical step curve of conventional voltammet-ric methods. 

An optimization procedure for the separation of epinephrine bitartrate, L-DOPA, 3,4-dihydroxyphenylacetic acid, notepinephrine-HCl and dopamine-HCl (with 3,4-dihydroxybenzylamine-HCl as internal standard), was described by He et al. [1075]. A C,8 column was used in conjunction with an electrochemical detector (-1-0.6 V vs, Ag/AgCl). A window diagram of relative retention times for adjacent eluting solute pairs (i.e., lR2/tRi) resulted in three acceptable solvent composition windows. The optimal solvent conditions were found to be 2.5/97.5 acetonitrile/water (0.23% sodium acetate with 0.02% EDTA and 0.066% sodium heptanesulfonate adjusted to pH 3.9 with monochloroacetic acid). Elution was complete in <30 min and all peaks were well resolved. Detection limits for dog or human plasma samples were reported as 10pg/mL for epinephrine and norepinephrine. 

As depicted in Table 3.17, column efficiency increases as column diameter decreases. Sharper peaks yield improved detection limits. However, as column diameter decreases, so does sample capacity. Column temperature conditions and linear velocity of the carrier gas can usually be adjusted to have a more favorable time of analysis. In Figure 3.32 these parameters are placed into perspective in a pyramidal format as a function of the inner diameter of a capillary column. 

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