Solvents, HPLC

The efficiency of separation of solvent from solute varies with their nature and the rate of flow of liquid from the HPLC into the interface. Volatile solvents like hexane can be evaporated quickly and tend not to form large clusters, and therefore rates of flow of about 1 ml/min can be accepted from the HPLC apparatus. For less-volatile solvents like water, evaporation is slower, clusters are less easily broken down, and maximum flow rates are about 0.1-0.5 ml/min. Because separation of solvent from solute depends on relative volatilities and rates of diffusion, the greater the molecular mass difference between them, the better is the efficiency of separation. Generally, HPLC is used for substances that are nonvolatile or are thermally labile, as they would otherwise be analyzed by the practically simpler GC method the nonvolatile substances usually have molecular masses considerably larger than those of commonly used HPLC solvents, so separation is good. 

T.J. Baker, Chemists, HPLC Solvents Reference Manual, T.J.Baker Chemical Co., 1985. 

P.C. Sadek, The HPLC Solvent Guide, J. Wiley Sons, New York, 1996. ISBN 0471118559. 

Chloroform HPLC solvent hydrocarbon stabilized GR LiChrosolv for analysis Chromasolv ethanol, 0.5 to 1% amylene, 0.01 to 0.02% ethanol, 0.6 to 1.0% amylene ethanol, ca. 1% amylene Baker Baker Merck Merck RiEDEL-de Haen RiEDEL-de Haen... 

The addition of a discharge electrode and a filament to the thermospray source widens the range of compounds that may be studied and HPLC solvents that may be accommodated. Optimum ionization conditions for a particular compouud ueed to be determiued empirically and it is essential that switching between the possible ionization modes may be accomplished easily and quickly. 

The shape of the matrix peaks depends on the nature of the sample and also on the composition of the HPLC solvent system. For an HPLC column, a low level of detection requires that interfering peaks in the samples be minimal. 

HPLC solvents (PDMS-coated fibres are incompatible with hexane). PDMS fibres are more selective towards nonpolar compounds and polyacrylate fibres towards polar compounds such as acids, alcohols, phenols and aldehydes. Another feature of SPME fibre selectivity is discrimination towards high-MW volatiles. SPME has successfully been applied to the analysis of both polar and nonpolar analytes from solid, liquid or gas phases. Li and Weber [533] have addressed the issue of selectivity in SPME. 

Applications Chromatography is a preferred technique for additive analysis as it allows both separation of additives in a mixture and subsequent quantitation. Despite the developments in GC, this technique cannot separate many polymer additives. Even with its lower efficiency in comparison to GC, HPLC is today one of the cornerstones in a polymer additive laboratory. Judging by the number of publications in recent years, HPLC is first among analytical methods for additives (confirmation/identification/quantification). Most additives may be analysed by HPLC if they can be dissolved in an HPLC solvent and absorb UV light. Typical polymer/additive analyses are carried out using LPE followed by HPLC with UV or RI detection [605-611]. Verification of the identity of an analyte is then based on a combination of retention time, UV and RI evidence. RPLC is used most frequently for polymer/additive analysis, but normal-phase and SEC are also used. Consequently, techniques for additive analysis by HPLC are legion. 

The main characteristics of on-line SPME-HPLC(-MS) are shown in Table 7.18. Most of the SPME fibres are compatible with HPLC solvents. SPME combined with HPLC provides a means by which simple, rapid concentration of analytes can be achieved together with a means of introduction of the concentrated analytes to the HPLC system. This eliminates the need for larger injection volumes, and avoids derivatisation if the analytes were to be detected by GC. An advantage of the SPME method over LLE methods is the absence of a solvent peak in chromatograms obtained after extraction by SPME. SPME is not suitable for organic solutions. As SPME is a microextraction technique, coupling to ft, HPLC may be envisaged. 

A group of techniques employing differential selection of solute ions relies on nebulisation and ionisation of the eluent, with some discrimination of ion selection in favour of the solute. Main representatives are APCI [544] and thermospray [545]. In a thermospray interface a supersonic jet of vapour and small droplets is generated out of a heated vaporiser tube. Controlled, partial vaporisation of the HPLC solvent occurs before it enters the ion source. Ionisation of nonvolatile analytes takes place by means of solvent-mediated Cl reactions and ion evaporation processes. Most thermospray sources are fitted with a discharge electrode. When this is used, the technique is called plasmaspray (PSP) or discharge-assisted thermospray. In practice, many... 

Other properties of hplc solvents that we may need to consider include compressibility, viscosity, refractive index, vapour pressure, flash point, odour and toxicity. Most hplc textbooks contain tables of these properties. For instance, there is a useful table in the book edited by J. H. Knox. 

Prices are given in //dm3 (1986) for BDH HiPerSolv hplc solvents, except for ... 

Our efforts to concretely determine the relative stereochemistry of this dimer have been met by failure. We have made attempts to resolve several of the monomeric tetracyclic aminoaldehydes of type 100 by HPLC using chiral stationary phase, in order to know for sure the structure of the homodimer. The poor solubility of these compounds in typical HPLC solvents hampered these efforts to access enantiopure monomer. A few attempts at diastereomeric salt formation from compounds of type 101 using chiral carboxylic acids were also unsuccessful. Computational analysis corroborates the assumption that the homodimer should be formed preferentially. 

Figure 4.31(b) from Euston and Glatz, A new Hplc Solvent Delivery System, Techn. Note 88-2 (1988) by permission of Hewlett-Packard, Waldbronn, Germany. 

Common HPLC solvents with adequate purity are commercially available. Halogenated solvents may contain traces of acidic impurities that can react with stainless steel components of the HPLC system. Mixtures of halogenated solvents with water should not be stored for long periods, as they are likely to decompose. Mixtures of halogenated solvents with various ethers, e.g., diethyl ether, react to form products that are particularly corrosive to stainless steel. Halogenated solvents such as methylene chloride react with other organic solvents such as acetonitrile and, on standing, form crystalline products. 

Nylon (NYL) Aqueous and/or organic samples esp. hydrophilic resistant to chemicals and HPLC solvents... 

Algorithmic Methods Development. The recent development of statistically-based HPLC solvent optimization computer programs (3-9) have achieved useful behavior in experimental design by optimizing separations with respect to specific performance criteria. However, AI programming techniques were not applied in these programs. 

Typical UV Cutoff Wavelengths for Some Commonly Used HPLC Solvents and Buffer Components, Taken Primarily from Manufacturer Product Specifications... 

In direct APPI, the ionization process is activated by the photons emitted by the source and takes place if the ionization energies of the molecules are below 10 eV. This is the case of most analytes however, the ionization energy of most HPLC solvents is higher. This implies that the sample must be vaporized prior to detection. The molecular ion is first generated by an impact with photons ... 

---