Dealing with solvent peaks

There are two fundamentally different ways in which we can deal with solvent peaks. 

By modifying the optimization criteria such that an optimum separation of the solutes from the solvent signal is achieved. This latter method, which is the relevant one in the context of this chapter, has not received much interest in the literature so far. Solvent peaks are usually highly non-symmetrical, so that neither Rs nor S can be used. 

Snyder and J.J.Kirkland, An Introduction to Modern Liquid Chromatography, Second edition, Wiley, New York, 1979. 

Vanroelen and D.L.Massart, Fresenius Zeitschr.Anal.Chem. 273(1975) 1. 

Laub in Th.Kuwana (ed.), Physical Methods in Modern Chemical Analysis, Vol.3, Academic Press, New York, 1983, pp.249-341. 

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An exception to this is when weighting factors are only used to deal with a solvent peak in the chromatogram (see section 4.6.3). 

SAXSAVAXS/RAMAN is especially useful when dealing with chemically induced phase transitions. The example shown in Figure 2(e) is the polymerisation of solvent styrene into polystyrene in which polyethylene is in solution. Polyethylene is soluble in styrene but insoluble in polystyrene. RAMAN allows the determination of the reaction kinetics of polystyrene formation and monitors the crystallisation of the polyethylene. The SAXS monitors the liquid-liquid phase separation followed by the liquid-solid phase transition, whilst the WAXS also observes the liquid solid phase by monitoring the appearance of peaks due to the crystallisation of polyethylene. These are very valuable parameters when trying to define any new manufacturing process. ... 

Within the last decade, hyphenation of LC-MS, LC-NMR, and LC-MS-NMR have become available. Because MS is destructive (in contrast to NMR) and requires far less sample than NMR, a splitter is incorporated, on-line, to direct the bulk of the sample to the less sensitive technique. In addition to the advantage of having MS and NMR information for the same chromatographic peak, the combination of these two techniques with different sensitivities must deal with other issues, such as the effect of deuterated solvents on the MS, the limitation of source of ionization on the MS compatible with low flow rates, and the timing which depends on the slower NMR technique. There is still room for improvement for LC-MS-NMR and the next decade will define the areas where this hyphenated technique is best suited. 

This spectrum was run in a different solvent, DMSO (dimethylsulfoxide), hence the brown solvent peaks are in a different region and have a different form. Again, we will deal with these in Chapter 13. 

Mobile phase/sample solvent mismatch is a common problem. As seen in the methanol example in Figure 1.16, peak shape can be grossly affected. Papers have been published dealing with the effect that tte injection solvent has on peak shape in acetonitrile mobile phases as well. Hoffman et al. [887] used benzyl alcohol, tryptophan, cimetidine, and phenylalanine as test solutes. A Cig column was used... 

The sample solvent and/or the sample medium can have an impact on retention time, peak shape and peak area. Change one of the following parameters in the sample solution as appropriate pH value, organic component, matrix, constitution of the sample, air content in sample solvent. When dealing with samples from a production plant, it is a good idea to be in close contact with the colleagues working... 

Figure 19 [33,34] shows two NMR spectra of poly(VDF-co-HFP) copolymers. The first one (top) deals with the spectrum of poly( VDF-co-HFP) copolymer treated with DBU and the bisphenol AF in DMAC, while the other one (bottom) represents the same sample but precipitated twice from an appropriate solvent for free phenol or any unreacted phenolate (acetonitrile). The F NMR spectra of the washed polymer (bottom) clearly shows the presence of the geminal trifiuoromethyl groups. So, after precipitation in acetonitrile of all the phenol and phenolate that did not react with the copolymer, the peak at - 55 ppm was still noted. It proves that a part of the bisphenol-AF enabled the crosslinking of the copolymer. Under those conditions, about 40% of the phenolate were incorporated based on the internal p-fiuoroanisole standard. 

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