Solvent and flow rate

The extent to which exchange occurs and the speed with which equilibrium is reached depend very much on the nature of the solvent. For the majority of surfactants, 80% methanol is a good solvent for both the sorption (uptake) and elution (recovery) steps. However, the rate of elution can be markedly affected by the concentration of alcohol in the eluent (see the end of section 4.5.4). Methanol has the added advantage that it is easily removed by evaporation. Ethanol and propan-2-ol (isopropanol) are also satisfactory, and some variation in concentration can be tolerated. Not all surfactants are readily soluble in methanol, and in such cases other solvents such as acetone or chloroform may be added. An example is the di(hydrogenated tallow)methylamine used in some soften-in-the-wash detergent powders, which requires the addition of up to 50% chloroform. 

Fairly obviously, the sorption of ions is most efficient, i.e. most closely approaches the theoretical maximum, when each resin particle has time to take up as many ions as it is capable of retaining, i.e. when the flow is very slow. In practice, however, there is usually a time constraint on analyses, and a good compromise is to use a flow rate of 1 ml/min/cm of column cross-section. This permits practical exchange capacities not 

Usually supplied as the free base Strongly acidic cation Sulphonate 

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There are only two references to continuous US-assisted digestion, both by the same authors [11,12], who optimized the composition of the liquid phase, sonication time, temperature, volume of solvent and flow-rate. On the other hand, they failed to consider the potential effects of other important variables such as the shape of the container, the volume and nature of transmitting liquid, particle size and, especially, the position of the container in the ultrasonic bath. 

Referring to Figure 2, by considering solute mass balances over n, (n — 1),. .. 2, 1 units in turn and eliminating intermediate solute mass fractions and flow rates, the amount of solute associated with the leached sohd may be calculated in terms of the composition of the sohd and solvent streams fed to the system. The resulting equation is (2)... 

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. 

In the development of a SE-HPLC method the variables that may be manipulated and optimized are the column (matrix type, particle and pore size, and physical dimension), buffer system (type and ionic strength), pH, and solubility additives (e.g., organic solvents, detergents). Once a column and mobile phase system have been selected the system parameters of protein load (amount of material and volume) and flow rate should also be optimized. A beneficial approach to the development of a SE-HPLC method is to optimize the multiple variables by the use of statistical experimental design. Also, information about the physical and chemical properties such as pH or ionic strength, solubility, and especially conditions that promote aggregation can be applied to the development of a SE-HPLC assay. Typical problems encountered during the development of a SE-HPLC assay are protein insolubility and column stationary phase... 

A uniform deposit of analyte(s) on the belt is required and it is possible to do this with a range of mobile phases and flow rates by a very careful balancing of the rate of solvent deposition, the speed at which the belt moves and the amount of heat supplied by the infrared evaporator. 

P 53] Before operation, a start-up time of about 10 min was applied to stabilize pressure in the chip micro reactor ([R 6]) [20]. As a result, a stable flow pattern was achieved. The reactant solutions were filled into vials. Slugs from the reactant solutions were introduced sequentially into the micro chip reactor with the autosampler and propelled through the chip with methanol as driving solvent. The flow rates were set to 1 pi min The slug volume was reduced to 2.5 pi. 

Figure 3.55. Correlation of dispersed phase fractional holdup values with aqueous (L ) and solvent (O ) flow rates.

The proper elution and wash solvent composition and the volume and flow rate through the cartridges must be determined. The SPE steps are critical to the separation and cleanup of the sample extract. Listed brands for Cg and silica gel cartridges should be used, if possible. 

Some kinds of chromatography require relatively little optimization. In gel permeation chromatography, for example, once the pore size of the support and number of columns is selected, it is only rarely necessary to examine in depth factors such as solvent composition, temperature, and flow rate. Optimization of affinity chromatography is similarly straightforward. In RPLC or IEC, however, retention is a complex and sensitive function of mobile phase composition column type, efficiency, and length flow rate gradient rate and temperature. 

For a given range of feed flow rate, L, it is assumed that the fractional holdup, h, and the solvent phase flow rate, G, can be correlated in the form... 

Fig. 3.11. Positive-ion SRM ion current profiles for 1 (m/z 443—415 black trace), 2 (mJz 443 - 415, red trace), and 3 (m/z 345-285, blue trace) obtained during development lane scans of replicate development lanes of the RP C2 TLC separation of a mixture (50 ng each) of rhodamines 6G (1), B (2), and 123 (3) at surface scan rates of (a) 19, (b) 44, and (c) 190 jum/s using a DESI solvent (methanol) flow rate of 0.5 //Emin. Dwell time was 100 ms for each transition. Signal levels were normalized to the signal in panel (c). Chromatographic resolution, R, calculated from the data is shown in each respective panel. Reprinted with permission from G. J. Van Berkel et al. [89].

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