Automatic Pipetting Systems

Implementation of microanalytical devices presents some issues mostly related to the scale of the volumes. In fact, successive reduction in the sample volume may compromise analysis either because the measurement limit of the analytical method is exceeded or because the sample is no longer representative of the bulk specimen. Another drawback for microchip devices is microvolume evaporation of both sample and reagent from the microchip, compromising quantitative determination or inducing unwanted hydrodynamic flows. This problem has been addressed by designing pipetting systems that automatically replace fluid lost by evaporation or by enclosing the chip in a controlled... 

The tritium measurement protocol Is relatively simplistic and consists of neutralization of the sample, single plate distillation, removal of an appropriate size aliquot (usually eight mL.) via reproducible automatic pipets and the addition of 15 mL. of dark adapted scintillation cocktail under Incandescent lighting conditions. After shaking to ensure a uniform gel, the sample is allowed to settle and dark adapt for up to twenty but no more than sixty minutes. The liquid scintillation unit efficiency Is determined dally on a previously prepared standard and background measurements are determined at least dally. Quench corrections are not applied to the system due to the lack of an external standards ratio capability and an effort to minimize the amount of hazardous waste which would be generated if an Internal standards approach were adopted. 

The authors applied this concept to both gas/liquid (see Figure 3.75) and liquid/ liquid systems (see Figure 3.76). This set-up consisted in the core of a tubular reactor with an interdigital micro mixer as dispersion unit (compare Figure 3.77). The peripheral equipment consisted of an automated pipetting robot, a fraction collector and a gas-chromatograph equipped with an automatic injector. 

SOPHAS M (Figure 13.10) is an automatic modular solid-phase synthesizer based on a robotic system. Synthesis can be carried out in a variety of reaction vessels, such as 96-well microtiter plates, tubes, or vials. The vessels are mowed on the 1- or 1.2-m-length workbench in aluminum carriers (12 mm x 86 mm) by a robotic arm. The content of the reaction vessels is isolated from the atmosphere by a pierceable double seal. There are four independent pipetting probes on the synthesizer. Each probe has three independent channels. The channels allow the synthesizer to simultaneously aspirate and add washing solvents and nitrogen. 

After the operator has selected the desired method menu of the relevant samples and has started the instrument, all subsequent steps are fully automated. Since 1987 it is also possible to effect a direct identification of the sample so that there are no longer any problems in respect of a dialogue with a central EDP system. The samples are taken from the sample vessel by means of disposable single use pipette tips that are used for one sample only and exchanged via a computer-monitored pipetting unit. This method excludes the possibility of a carry-over between samples. In accordance with the preset conditions, the required slides are automatically moved to the sample dosage unit (see Fig. 23). Samples of 11 pi serum or plasma will be sufficient for kinetic measurements (enzymes), 10 pi of sample for all other tests. As soon as application of the sample has been completed, the slide is moved to the appropriate incubation chamber by means of the slide rotor (see Fig. 23). The chemical reactions take place in these chambers. This is followed by measurement either by reflectometer (end point or kinetic) or a potentiometric measurement unit. 

In many cases, the identity of the analyte will be known nonetheless, it is highly desirable that this be confirmed to avoid the possibility that an interfering compound fortuitously has, for example, the same GC or HPLC retention time as that of the desired analyte. Indeed, many protocols that are now advocated use mass spectrometric systems so that this control is automatically incorporated. Samples may be spiked with internal standards to simplify calculation and eliminate small errors in pipetting and injection, or surrogate standards may be employed where, for example, incomplete extraction of the analyte is unavoidable. When MS is used as the detection system, analytes labeled with suitable isotopes have been widely used for PAHs, fully deuterated standards, and for PCBs and agrochemicals, Relabeled compounds. For partially labeled standards of analytes, care must be exercised in their choice if it is intended to analyze for metabolites of a substrate in which the label may have been lost. 

Once the spinning band has been tested and rotates freely, place 1.0 mL of 2-methylpentane and 1.0 mL of cyclohexane in the vial (to be delivered with a Pasteur pipet or an automatic delivery pipet). Reassemble the system and lower the column into the sand bath or copper-tube block. The beveled edge on the air condenser should be rotated 180° from the collection arm. 

---