Extracolumn band broadening detector

Detectors used in HPLC should have low internal volumes to minimize extracolumn band broadening in addition, they should be sensitive and should respond quickly to concentration changes. Few detectors fulfill all of these requirements. One of the oldest detectors used in HPLC is the refractive index detector, which detects subtle differences between the refractive index of the pure mobile phase and a mobile phase containing the solute. This detector is universal, i.e., it can detect any solute whose refractive index differs from that of the pure solvent. However, its sensitivity is poor, which practically precludes its use in trace analysis. Besides, refractive index detectors are very sensitive to changes in the composition of the mobile phase and to temperature flucmations. The former makes their use in gradient elution impractical the latter requires that the detector is thermostated to at least 0.01°C. 

Careful consideration must be given to the design of the detector flow cell as it forms an integral part of both the chromatographic and optical systems. A compromise between the need to miniaturize the cell volume to reduce extracolumn band broadening... 

HPLC detectors Like GC, HPLC has a wide variety of detectors, universal or specific, destructive or nondestructive, mass flow or concentration responsive, and with even more challenging requirements for interfacing to spectrometers in hyphenated techniques. Tubing of even smaller bore than described earlier in item 5 is necessary to connect the effluent end of the column to the detector to avoid extracolumn band-broadening effects. Pressures are lower, and high strength and density PEEK plastic may be used in place of stainless steel. 

For closed-cell detectors, including the photoionization detector, the electron-capture detector and the TCD, extracolumn band broadening can be excessive unless specially designed devices with small cell volume are used or the detector is operated at subambient pressure. At reduced column outlet pressure, the carrier-gas velocity in the detector is increased, and the cell is swept out more quickly. Extra gas, called makeup gas, can be introduced into the detector cell to sweep the cell more rapidly and reduce peak broadening and distortion. 

As shown in Figure 3.8, an increased injection volume can have a negative effect on the column efficiency. Therefore, the injection volume should be appropriately scaled down when an HPLC method is transferred to a UHPLC method or vice versa to achieve the same sensitivity and avoid overloading or detector saturation and extracolumn band-broadening. The scaling factor is mainly based on column dimension, as shown in Eq. (3.26). However, lower injection volumes than calculated can often be used on UHPLC to achieve the same sensitivity due to enhanced peak heights from use of the high-resolution columns and low carryover from the injector ... 

Considering a chromatographic process controlled by a partition equilibrium and neglecting extracolumn effects (i.e., band broadening caused by factors outside the column, e.g., tubings, detector etc.), several factors can contribute to the overall solute band broadening eddy diffusion, longitudinal diffusion, and resistance to mass transfer in mobile and stationary phase. 

Open-cell, flame-based detectors typically have response times of a few milliseconds or less if the capillary separation column is passed through the burner tip and positioned just below the base of the flame. Thus, At is small, and from the detector usually is negligible relative to other extracolumn sources of band broadening. While many HSGC studies have used the FID, few data are... 

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