Stepped plunger

FIGURE 5.16 Template scheme (top view) for solid phase sample application (SPSA) and process of performance (cross section of steps a to e) 1 — base of the device, 2 — glass plate, 3 — adsorbent layer, 4 — sample, 5 — top of the device, 6 — plunger to compress. Step a Template placed onto the preparative plate Step b Marking by means of a thin needle Step c Scraped out channel on the preparative plate Step d Filling in of the prepared mixture of sample and deactivated adsorbent Step e Compression by means of a plunger. (From Botz, L., Nyiredy, Sz., and Sticher, O., J. Planar Chromatogr, 3, 10-14, 1990. With permission.)... 

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. 

The first step in the wetted path in the HPLC is the solvent reservoir holding freshly filtered (and possibly degassed or deoxygenated) solvent. Most systems use a porous fritted stone (5-30-/an filter) as a solvent line sinker. The tubing to the solvent inlet is wide-diameter Teflon . In the solvent inlet line we may have another frit and a sapphire ball/stainless steel check valve. The wetted surfaces in the pumping chamber are all stainless steel except for the plunger... 

Two factors are driving the market for precise, very-low-flow HPLC pumping systems extremely limited sample sizes in biotechnology and the electrospray and nanospray interfaces that are concentration and flow-rate dependent. It is very difficult to get precise flow and gradient formation from pumps that have a 5- to 10-/iL plunger displacement, even using 3200-step stepper motor drives. This has forced manufacturers to resurrect a very old concept from the earliest days of HPLC, the syringe pump. 

Figure 13.44 represents the various stages of the compression molding cycle from the point of view of the plunger force needed to close the mold at a constant rate. In the first region, t < the force increases rapidly as the preform is squeezed and heated. At tf, the polymer is presumably in the molten state and, as such, is forced to flow into the cavity and fill it. Filling terminates at tc, when compression of the polymer melt takes place, to compensate for the volume contraction that results from the polymerization reaction. The bulk of the chemical reaction occurs after tc. We now comment on each of the steps of the compression molding process. 

At the large industrial scale the reverse flow step for elution is, however, a relatively heavy operation. It first requires one to stop the flow and wait for solid phase settlement and then to adjust the plunger on the top of packed bed. Additionally, when initiating another step the bed must be reexpanded again, requiring some time. Difficulties are related to the expansion of resins after several cycles they form aggregates and time needed for the perfect bed expansion equilibrium becomes longer. 

The headspace over tobacco can be sampled and analyzed using a solid-phase micro-extraction (SPME) technique. The apparatus used for SPME is shown in Fig. 1. The basic extraction device consists of a length of fused-silica fiber, coated with a suitable polymeric adsorbent, which is attached to the steel plunger contained in a protective holder. The steps that are taken to sample a vapor are depicted in Fig. 1. The sample is first placed in a small headspace vial and allowed to come to equilibrium with the air in the vial (1). The needle of the syringe containing the fiber is then made to pierce the cap, and the plunger pressed to expose the fiber to... 

Place a syringe barrel (without plunger, fitted with a 25-gauge needle) at one end of every channel for use as a reservoir. These reservoirs will remain in place throughout steps 5-15. 

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