Alternating phased gradients

Alternative detection methods such as mass spectrometry, ELS or nitrogen detector are in most cases inappropriate due to the octanol saturated aqueous media. An alternative is the use of standard HPLC reversed phase gradient systems (Valko). These systems do not represent true octanol water partitions, but are also good lipophilicity parameters by themselves. 

Higher order binomial filters are efficient in applications where a relatively wide band, e.g. a strongly coupled multiplet, has to be suppressed (see Figure 13). A drawback of these filters is the introduction of a linear phase gradient which is usually easy to correct. Alternatively the spectra may be represented in the absolute value mode. 

Alternative LC methods in the separation of NPEO resulting in a separation based on EO number include the use of an alumina column using an ethylene oxide-n-hexane mixture as mobile phase [37], of a cyanosilica column and a mobile phase gradient of toluene and a 10 88 2 mixture of 0.5 mmol/1 sodium acetate in toluene, methanol, and water [38], and of a poly(vinyl alcohol) column and 10-55% acetonitrile in 30 mmol/1 aqueous ammonium acetate as mobile phase [39]. Ion-pair LC-MS, using 5 mmol/1 triethylamine in the mobile phase, was applied in the analysis of phenols and NPEC [33]. 

Eq. 11 shows that the flow rate of the mobile phase should be adjusted in the proportion of changing Vm, to keep the separation time constant. Alternatively, the gradient time can be adjusted to keep the ratio VJ(tc x F ) constant. Clearly, the speed of separation increases at a higher flow rate, but to achieve the desired decrease in the separation time, the gradient time should be decreased in inverse proportion. Fig. 3 illustrates proportional decrease in the elution times with increasing flow rate at the gradient time adjusted in this way, in agreement with Eq. 11. 

Mobile phase Gradient. A B 65 35 to 20 80 over 40 min. A was 10 mM ammonium acetate. B was MeCN MeOH 92.8 7.2 containing 7.2 mM ammonium acetate. (Alternatively, A 10 mM ammonium acetate in water containing 0.1% trifluoroacetic acid and B MeCN MeOH 92.8 7.2 containing 7.2 mM ammonium acetate and 0.1% trifluoroacetic acid with the same gradient.)... 

For LC, temperature is not as important as in GC because volatility is not important. The columns are usually metal, and they are operated at or near ambient temperatures, so the temperature-controlled oven used for GC is unnecessary. An LC mobile phase is a solvent such as water, methanol, or acetonitrile, and, if only a single solvent is used for analysis, the chromatography is said to be isocratic. Alternatively, mixtures of solvents can be employed. In fact, chromatography may start with one single solvent or mixture of solvents and gradually change to a different mix of solvents as analysis proceeds (gradient elution). 

Gradient-Accelerated Spectroscopy (GAS)—An Alternative to Phase Cycling... 

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