Air displacement pipets

DIN 12650 Error Limits for Single Channel Air Displacement Pipets ... 

Figure 2.3 Schematic cross-sections of (a) air displacement pipet and (b) positive displacement pipet.

Inasmuch as the right conditions for crystallization cannot be predicted, a large number of conditions (precipitant, pH, temperature, protein concentration, additives, etc.) need to be screened to produce a crystal suitable for data collection. To minimize the amount of precious protein material used in these preliminary experiments and avoid the large amount of manual labor involved, automation is becoming very common.8 When setups are done by hand, the pipetting is usually done using standard air-displacement pipettes, which are extremely inaccurate under 1 pi, making this the minimal practical protein volume per experiment. All of the various robotic systems can use sample volumes as low as 50 nl, and some as little as 1 nl. 

Procedure. Transfer 0.6 to 0.8 g of the dry sample weighed to 0.1 mg to a 100 ml beaker. Add 50 ml of 25 percent acetic acid soln and stir to dissolve. Transfer to a 250 ml volumetric flask and dilute to the mark. Attach the cylinder of oxygen-free carbon dioxide to the titration flask as shown in Fig 1 and allow the gas to flow for 5 minutes thru gas humidifier (H) to displace the air. Continue the flow of gas during the-entire titration procedure, pipet a 25 ml aliquot of the solution of the sample into the titration flask (Fig 2). Add 30 ml of 20% Na acetate soln and 20.00 ml of 0.2N titanous chloride soln and swirl the flask for about 20 secs. 

Hon et al. [34] describe a simple piece of equipment for the determination of down to 80 pg/1 of mercury by AAS using a static cold vapour procedure. In this method [35], the sample was digested with the sulfuric acid, a measured portion pipetted into the reduction vessel, and the vessel immediately capped. The reductant, comprising 1% stannous chloride, was introduced. The evolved elemental mercury in the headspace was then introduced into the absorption cell by water displacement. Maximum sensitivity is obtained when the volume of the displaced air is equal to the internal volume of the absorption cell, and the mercury solution is 9 M in sulfuric acid. The peak absorbance at 253.7 nm exhibited a marked decline for hydrochloric acid concentrations above 1.5 M and for nitric acid concentrations above 3 M. The calibration graph obtained for mercury(II) in 9M sulfuric acid is linear from 0 to 17ng/ml, and the sensitivity is 0.08 ng/ml. A windowless absorption cell can also be used with a narrower linear calibration range. 

Dilute DNA solution to 1-2 ug/ml. in sterile TE and fill the injection pipet by dipping the end in the diluted DNA solution. Allow capillary action to fill the pipet to several millimeters above the tip. Attach the injection pipet to a second micromanipulator via its instrument tube, and connect the instrument tube to a glass 50-mL syringe filled with air. Insert the injection pipet tip into the injection chamber at a 5-10° angle. Demonstrate that the injector is not clogged by displacing an egg with a stream of DNA solution. 

The setup and amplification section of a protocol also contains specific recommendations for the prevention of carryover of aerosolized DNA into the new reaction. Dedicated hoods or dead-air boxes are recommended in this step of the procedure. All pipets should be of the positive-displacement type. They should be kept in a dedicated setup hood and should never have previously been used to pipet amplified target. No amplified DNA should ever be brought into this area. During the reaction setup, either dUTP and UNG or isopsoralens may be added. 

Absorption bulbs are modifications of the original Hemple pipets, and some people still use that name. The principle is to provide as much surface area as possible for the gas to come in contact with. One design is shown on the left of Figure 42-6. Several glass tubes are placed in one side of the bulb. This is the side connected to the manifold. The other side is to collect the displaced absorption solution when the gas enters. A small rubber bag is attached to the top of this side to hold the air being pushed out when the liquid fills this chamber. A bag is provided for each absorption bulb and one is shown as R in Figure 42-3. The bag allows liquid to transfer and absorption to take place, without contamination of the solution from the outside air. 

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