Ghost peaks elimination

Quadrature images Any imbalances between the two channels of a quadrature detection system cause ghost peaks, which appear as symmetrically located artifact peaks on opposite sides of the spectrometer frequency. They can be eliminated by an appropriate phase-cycling procedure, e.g., CYCLOPS. 

Mounts et al. proposed a binary gradient of chloroform/THF (1/1) on the one hand and methanol/NH4OH/C (92/7/1) on the other hand (79). According to Abidi et al., incorporation of pure THF to a mobile phase consisting of C/M and ammonium hydroxide enhances the separation of PC from PA (80). However, impurities were often found in THF, causing severe ghost peak interference with PL analytes. The use of tertiary butyl methyl ether (tBME) instead of THF eliminated these problems. 

At any time it is possible that unexpected positive or negative peaks will occur in a chromatogram. If these peaks cannot be reproduced they are called ghost peaks It is necessary to eliminate them, although this can be tedious and time-consuming. Reasons for ghost peaks include bubbles in the detector, impurities in the mobile... 

The purity of mobile phase A (MPA, the weaker initial mobile phase) is critical since several milliliters of MPA are used to equilibrate the column and any trace contaminants in MPA are concentrated and elute as ghost peaks in the blank chromatogram. One successful strategy is to use high-purity buffer (>99.995%) and to eliminate the buffer filtration step to minimize potential for contamination. 

Baker [577] used an FFAP coliurm for direct injection of dilute aqueous solutions of acids (FFAP is a reaction product of polyethylene glycol 20,000 and 2-nitrophthalic acid developed by Varian Aerograph). The acetic acid peak was not clear and the ability of this column to separate normal and iso fatty acids was not reported. Van Huyssteen [608] successfully used a Chromosorb 101 column coated with 3% FFAP for separation of volatile acids by direct injection of synthetic aqueous solutions and anaerobic digester samples, which were first centrifuged and acidified to pHl-2 with hydrochloric acid. His column afiected complete separation of the Cj-Cs straight and branched short-chain fatty acids from synthetic aqueous solutions, but less sharpened peaks were obtained from anaerobic digester samples. The response with acetic acid approximated that of the other acids additional peaks, probably alcohols, appeared between the acid peaks. To eliminate ghosting l-2pL water was injected between samples. 

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