Duck-billing

Figure 3.8 Alternative designs for cold on-column injectors, h, Injector with a duck bill valve (Hewlett-Packard), B, an injector with provision for secondary cooling of the column inlet (Carlo-Erba), and C, a temperature- programmable on-column injector with its own oven isolated from the column oven (Varian Associates).

The diets can be offered in mash or pelleted form, and pellets are generally recommended to reduce wastage and build-up of caked feed on the ducks bills. Alternatively, dry mash can be mixed with water and offered as a wet mash. Scott and Dean (1999) recommended a pellet of 4.0 mm diameter and 7.9 mm maximum length for newly hatched ducklings and for Pekin-type ducks from 2 weeks of age onwards a pellet of 4.8 mm diameter and of 12.7mm maximum length. 

On-column injection into columns of 320 pm or less is more difficult and not easily automated. This usually requires the use of a fused silica needle of sufficiently small outer diameter or a needle capable of entry into the analytical column. The column is usually placed into a specially designed injection port fitted with duck bill or isolation-valve-type septa. A second approach which has shown great success is the use of a precolumn of a wide-bore capillary (530 pm ID) which is connected to the analytical... 

Oval - irregular shapes, form, not round, uneven pieces, uniform shape - duck bill, saddle, size - pre-cut... 

Fig. 2 Schematic diagram of a cross section of a cold on-column injector with a duck-bill valve. 1 Cool tower needle guide, 2A disk septum for automated injections, 2B isolation valve for manual injections, 3 frit, 4 carrier gas, 5 oven wall, 6 ferrule, 7 capillary column, 8 column nut.

Figure 27. Schematic of a cool on-column injector a) Cool tower, needle guide b) Duck bill valve (isolation valve) c) Spring d) Insert e) Cryogenic cooling (optional) f) Heater block g) Column h) Septum...

In on-column injection, the sample is injected in a solvent directly onto the column. The sample is concentrated by "cold trapping" or a "solvent effect" at the head of the column, so must be injected at a column temperature near the boiling point of the solvent. The column inlet does not have a septum but rather a "duck-bill" valve, made of a soft elastomer. This consists simply of two plastic surfaces which are pressed together by the pressure of the carrier gas in the injection port. During injection, a needle of fused silica is merely slipped between the two surfaces and is guided into the top of the WCOT column. (Some commercial systems use a more complex solenoid valve). 

The positive cutoff must be timed with the cycle of the machine. It must open at the instant the dispense button is depressed and is set to close a few milliseconds after the dispense button is released. The designs range from a simple scissor arrangement to the manipulation of valves pinch valves, collapsible tubes and duck bill valves. The best are those which operate without contacting the mixed materials. 

Alternative injector septum solution Merlin MicroseaF using a septum less seal formed like a duck bill and sealed by the inside carrier pressure and spring load. 

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