Solvent efficiency

The volatile solvents recoverable by the activated carbon system or any other system are nearly all organic, and many of them form flammable or explosive mixtures with air. Such mixtures may lie between upper and lower explosive limits. The activated carbon system can avoid the explosive range by staying well below the lowest percentage of vapor which is still explosive it functions well at very low concentrations. The system also recovers solvents efficiently even in the presence of water the recovery efficiency is high (98 percent and 99 percent are not unusual) it may be fully automatic. The annual maintenance charge rarely exceeds 5 percent of the cost of equipment. The recovery expense may be as low as 0.2 cent per pound in some installations it rarely exceeds 1 cent per pound. 

Narrow-bore columns are most useful for the analysis of polymers that are difficult to analyze in inexpensive solvents. However, if the appropriate equipment is available, good results can be obtained for a broad range of standard analyses. A comparison of an analysis of standards between an equivalent bank of conventional 7.8-mm and solvent efficient 4.6-mm columns is shown in Fig. 11.4. The columns used were Styragel HR 0.5, 1, 2, and 3 columns at 35°C with tetrahydrofuran (THF) as the solvent. The flow rate was 1 ml/min for the conventional columns (Fig. 11.4A) and 0.35 ml/min for the solvent-efficient 4.6-mm columns (Fig. 11.4B). If the correct equipment is available, the reduced solvent consumption of these solvent-efficient Styragel columns is of value to the environmentally conscious user. 

FIGURE 11.4 Comparison of chromatograms obtained on conventional (A) and solvent-efficient Styragel columns (B). In each case the column bank was a bank of Styragel HR 0.5, HR I, HR 2, and HR 3 columns at 3S°C with THF as the solvent. The sample is a mixture of polystyrene standards. With proper care and optimized instrumentation, good resolution can be obtained with solvent-efficient Styragel columns. (Courtesy of Waters Corp.)... 

Generally, conversion from one solvent to another is carried out at low flow rates. The commonly used flow rate for this conversion is 0.2 ml/min for standard columns and 0.1 ml/min for solvent-efficient columns. This minimizes any swelling/shrinking stress put on the column. The temperature of a solvent conversion is chosen to minimize any pressure stress on the column bank. As a general rule, the pressure per column should never exceed 3.5 MPa (500 psi) during solvent conversion. For example, the conversion of a column bank from toluene to trichlorobenzene (TCB) or o-dichlorobenzene (ODCB) is commonly carried out at 90°C. This minimizes the stress on the column due to the higher viscosity of the target solvents. 

Conventionally, analytical SEC columns have been produced with an internal diameter of 7.5 mm and column lengths of 300 and 600 mm. In recent years environmental and safety issues have led to concerns over the reduction of organic solvent consumption, which has resulted in the development of columns for organic SEC that are more solvent efficient (13). By reducing the internal diameter of the column, the volumetric flow rate must be reduced in order to maintain the same linear velocity through the column. This reduction is carried out in the ratio of the cross sectional areas (or internal diameters) of the two columns. Eor example, if a 7.5-mm i.d. column operates at 1.0 ml/min, then in order to maintain the same linear velocity through a 4.6-mm i.d. column the flow rate would be... 

Table 3.28 Comparison of microwave and reflux extraction solvent efficiencies for additives in LLDPE...

Notes A polar, aprotic solvent miscible in water and many organic solvents. Efficiently solvates cations. Can greatly enhance the rates of nucleophilic displacement reactions. 

Solvent efficiency. Synonymous with separation factor. 

In preparative GC it is particularly important that solvent efficiency (a) should be maximized as in analytical GC. A liquid substrate should be used that will give the maximum separation of peaks (high selectivity). The higher the a term the fewer theoretical plates (n) required and the larger samples that can be injected. 

S Nesheim, EM Stack, MW Trucksess, P Krogh, RM Eppley. Rapid solvent-efficient method for liquid chromatographic determination of ochratoxin A in com, barley, and kidney collaborative study. J AOAC Int 75(3) 481-487, 1992. 

Addition of aqueous potassium hydroxide solution to the organic solvent would be expected to provide ample water for reaction but would also decrease the boiling point of the system. The decrease in solvent efficiency of those systems that were aqueous by design might then be attributed to the decreased reaction temperature. To support or disprove this hypothesis, additional experiments were made to ascertain the solvent capability of the studied systems at the same temperature. 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

Used to introduce chromophores into alcohols and amines using pyridine as a solvent efficient means for the isolation of carbohydrates in complex mixtures Reference 3... 

Reduce waste through increased reagent and solvent efficiency. 

The solvent efficiency i.e. the extent to which two substances can be separated is expressed by relative retention, a, for the two components, a being defined by the equation ... 

The combined effects of solvent efficiency (i.e. the extent to which substances are separated on account of differences in their rates of migration with reference to a particular set of solvent and stationary phase) and column efficiency (i.e. ability to achieve prevention of remixing of the separated substances) is expressed in terms of the resolution, Rs, of the column ... 

The scale of an oligonucleotide synthesis is often 1 /amol or less (about 1000 times less than a typical peptide synthesis). At these small scales the instrument volumes (tubing, valves, etc.) become significant compared with the volume of the solid support. Most of the volume of reagents and solvents is used to flush the system and not the support. Therefore, minimizing the instrument system volumes maximizes the reagent and solvent efficiency during the synthesis. 

Mathematically, limiting value when b < , for example, lim = Vz-In Fig. 4 we have plotted common solvents. It appears that for (b > 10, v (solvent effects between materials like EtBr( a = 9.50) and DMSO (e = 48.9). This is contrary to practically any experimental evidence, which indicates, for instance, a sizable difference in solvent efficiency even between DMF ( = 36.7) and DMSO. This important shortcoming is shared by all the preceding theories and the inclusion of the solute polarizability and multipoles does not improve the picture. This fact has been recognized by... 

It is noteworthy that solvents such as nitrobenzene and benzonitrile, for which the EHS is practically nil, behave almost as select solvents (87). Inspection of the tables in Section I shows that very often benzene and toluene and, to a lesser extent, other more polar aromatic solvents display a solvation efficiency comparable to that of select solvents of much larger dipole moment. Benzene, for instance, has a rr value comparable to that of THF. The rationale is that some of the factors that favor the EHS may also contribute to the solvent efficiency in chemical reactivity and spectral shifts. 

To remove the solvents efficiently, it is then necessary to exceed the desolvation temperature of the solvates that may be produced during crystallization. 

---