Pulse dampening

Refractive index detectors. These bulk property detectors are based on the change of refractive index of the eluant from the column with respect to pure mobile phase. Although they are widely used, the refractive index detectors suffer from several disadvantages — lack of high sensitivity, lack of suitability for gradient elution, and the need for strict temperature control ( + 0.001 °C) to operate at their highest sensitivity. A pulseless pump, or a reciprocating pump equipped with a pulse dampener, must also be employed. The effect of these limitations may to some extent be overcome by the use of differential systems in which the column eluant is compared with a reference flow of pure mobile phase. The two chief types of RI detector are as follows. 

To minimize pulses in the reagent flow, for the reasons already mentioned, a syringe pump is normally used for the reagent supply. Alternatively, if a piston pump is used, then an efficient pulse dampener should be inserted between the pump and the mixing T. 

Figure 2, Block diagram of a liquid chromatograph. A, solvent reservoir B, filter C, pump D, pulse dampener (optional) E, pre-column (used only in liquid-liquid chromatography) F, pressure gauge G, infector H, column I, detector J, fraction collector K, recorder or oth readout device.

FIGURE 14.18 Flow diagram of split flow capillary LC system. 1. Solvent reservoirs. 2. Model 5000 syringe pump (Varian, Walnut Creek, California). 3. Static mixer. 4. Injection port. 5. Column. 6. Detector. 7. Pressure transducer. 8. Pulse dampener. 9. Purge valve. 10. U-flow controlling device. 11. Waste. 

Electrochemical detectors are more sensitive to flow pulsation than UV-detectors and usually an efficient pulse dampener is required to minimize the contribution to baseline noise at high sensitivity. 

Fig. 113 Pump, pulse dampener, and pressure-gauge unit.

Concentration of 500-L samples was usually performed at a flow rate of 150 mL/min and 500 lb/in.2. A piston-type pulse dampener (Hydrodyne) was used to minimize breakdown of resin particles. Flow rates did not exceed 250 mL/min (0.2-cm/s linear flow velocity). 

Samples were eluted in the reverse direction by using the Milton-Roy pump with the pulse dampener removed. The eluant flow (50-75 mL/min at 200-300 lb/in.2) was monitored at 254 nm by using an Altex 153 detector with a biochemical flow cell. Elution with each solvent was continued until the detector response returned to base line. All columns were eluted with acetonitrile this solvent was preceded by 4.5 M NaCl/0.04 M HC1 and 0.04 M HC1 elutions on the MP-1 column and by 4.5 M NaCl and distilled water elutions on the MP-50 column. The aqueous column effluents were adjusted to pH 2 (MP-1) or pH 11 (MP-50) and then extracted three times with dichloromethane. The acetonitrile column effluents were saturated with NaCl to separate the water, which was extracted twice more with acetonitrile. Fifty percent aliquots of the processed organic solvents from each respective column were concentrated in Kudema-Danish evaporators to a final volume of about 10 mL (any remaining water was removed as the low-boiling azeotrope in the process) to give 25,000 1... 

Figure 1. Schematic of resin concentration system. Filter is a replaceable Carborundum K-27 3.0-pM filter in a 90 X 270-mm stainless steel holder. The pump is a MUton-Roy FR 141-144 Teflon diaphragm type capable of 1500 lb/in.2 of pressure and a maximum flow rate of 40 L/h. A hydrodyne pulse dampener precedes the columns. All resins are packed in 49-mm i.d. stainless steel columns the XAD-2 column was 204 mm long, and the others were 102 mm long. Column effluent flow was split between the...

For chemiluminescence measurements, a postcolumn reactor with a pulse-dampening filter was added to the HPLC apparatus. A 3-cm piece of narrow-bore tubing joined the pulse dampener to a Valeo l-/tl T chemiluminescent reagent with the chromatographic eluent. A C8 ECONOSPHERE (250 X 4.6-mm ID) column was used. Modifications were made with the... 

Reactant feeds are generated by vaporizing liquid flows from HPLC pumps with manometric pulse dampeners or high-pressure syringe pumps and mixing this vapor with gas components metered through mass flow controllers. This reaction feed is then divided between the 48 channels equally by using flow restrictors such as silica capillaries or micromachined channels (Fig. 3.12). The capillaries feed into the inlet stand-offs of the reactor modules. 

The next step was the electronically compensated pump. All pumps speed the motor as resistance increases to maintain a constant solvent slow. These pumps also add a major plunger speed-up during refill and repressurization. With this modification, a pump with a single pump head and a pulse dampener could give 90% of the performance of a two-headed pump for 50% of the cost. An overall dramatic price reduction for the dual-pump HPLC system resulted. 

If the detector passes the static test, turn on the flow and watch the baseline. Noisy baseline at this point is probably coming from before the detector. Realize that a reciprocating pump is noisy and it now lacks pulse dampening from the column. While we re here, shoot a sample response should be instantaneous, straight up and down. I have not tried it, but you might be able to quantitate lamp strength by shooting a known standard over a time period and measure the deflection. 

Pulse Dampeners—Device used to control pump pulsing. Usually a tight coil of metal tubing in a metal container that acts as a baffle and counters pulsing by a spring recoil effect. 

Isocratic elution is commonly used for the elution of analytes from the column. In isocratic elution, the mobile phase is kept constant throughout the analysis. The mobile phase can be a single solvent or a solution of two or more miscible solvents. The major requirements of isocratic pumps are accuracy and smoothness of flow. Because the pump delivers only one solvent system, simple, inexpensive pulse dampeners and rudimentary flow or pressure feedback control circuits can be used. The basic setup of an isocratic system is illustrated in Figure 3.10. 

Pump pulsations. 6. Incorporate pulse dampener into system. 

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