Thermal gradient microbore liquid chromatography with dual-wavelength absorbance detection

Abstract

and consequently that this process (which may involve the formation of carbon-carbon double bonds or of cross-linking between the polymer chains) or a similar one might be more common in the case of ion bombardment than in the case of atom bombardment. The exaggerated difference in time dependence behavior for ion beams, as compared to atom beams, observed for PTFE, as compared to PET, certainly suggests that particle-surface electronic interactions are a major contributor to this preferential sputtering of fluorine. CONCLUSIONSComparison of spectral time-dependence behavior for PET and PTFE has provided data concerning the nature of polymer degradation during particle bombardment. In particular, electronic particle-surface interactions have been shown to be a key contributing factor to degradation. In PET, this degradation is by straightforward physical degradation (random segmentation of polymer chains), which is accelerated by the use of charged primary particles. For PTFE, in addition to this process, an additional chemical degradation mechanism operates, which is, to a large extent, the consequence of electronic particle-surface interactions. This involves the preferential sputtering of fluorine and is manifested by the observation of a rise in peaks with a low fluorine to carbon ratio. At doses less than 1013 particles cmV2, this degradation mechanism may be nearly eliminated by the use of primary neutral particles rather than charged particles.Theoretical relationships are derived relating changes in the refractive index of the mobile phase in liquid chromatography to aperture limited absorbance measurements as applled to a single fiber-optic two-wavelength detector. The detection system was designed for remote sensing in thermal gradient microbore liquid chromatography (TG-pLC). TG-pLC was demonstrated with a reversed-phase separation of an unleaded gasoline sample for a temperature gradient of 25-150 OC over 30 min. The unique two-wavelength difference detection method, along with the single fiber-optic construction, virtually eliminated baseline drift associated with thermal Induced refractlve index ( R I ) aberrations. The detector provides a solution to R I aberrations not only for TEMLC but also for mobile-phase gradient liquid chromatography (MPG-LC) and other flow methods such as flow injection analyds (FIA). The advantages of TG-pLC are presented, including gradient separation capability for MLC, effective control of retention tlme comparable to MPG-LC, and separation efflclency over 72 000 theoretical plates/m using 5-pm packing material.

Detection Modes with High Temperature Liquid Chromatography—A Review

“…In these cases, it is due to variations in mobile phase temperature entering the detector, which induce modifications of the mobile phase physicochemical properties (viscosity, refractive index, and conductivity). The drift can be corrected by commercially available software (163).…”

Controlling the retention in capillary LC with solvents, temperature, and electric fields

“…When temperature programming is used with UV detection, a rising baseline is often observed due to refractive index changes, but might also in some instances be due to column bleeding [85]. The rising baseline can be corrected mathematically by a commercial software package [108] or by dual wavelength measurements and corrections [133]. To obtain improved limits of detection and increased linearity, the nebulizer of the ELSD has been modified for the low flow rates used in capillary LC [122].…”

Temperature programming in liquid chromatography

“…With temperature programming, a rising baseline may occur due to refractive index changes and can be mathematically corrected by a commercially available software package [57] or by dual wavelength measurements and corrections [58]. In a microcell equipped with optical fibers, no dramatic change in the signal-to-noise ratio was found when the temperature was raised to 1508C [59], while significant changes were found in another study [58].…”