Pipets pipetting exercise

Preventive Strategy. The firm should have performed a proficiency training exercise for new analysts in which the unclear pipetting step was explained. If each successive analyst found the explanation inadequate, then a method revision should have been made with a statement explaining that the clarification did not affect the original validation. 

Citrate, Oxalate, Phosphate, or Tartrate Dilute 5 mL of sample to 50 mL with recently boiled and cooled water. Add 6 N ammonium hydroxide or 3 N hydrochloric acid to 4 mL of this solution, if necessary, to bring the pH to between 7.3 and 7.7. Add 1 mL of calcium chloride TS, and heat in a boiling water bath for 5 min. The solution remains clear. Cyanide (Caution Because of the extremely poisonous nature of potassium cyanide, conduct this test in a fume hood, and exercise great care to prevent skin contact and the inhalation of particles or vapors of solutions of the material. Do not pipet solutions by mouth.)... 

Table 4.3 Worksheet for exercise on measurement uncertainty in pipetting...

The practical exercise with the pipette shows how the uncertainty due to one component of a test method could be measured. In this case we expect measurements made with both the balance and the thermometer to be both accurate and precise. Most of the uncertainty in the measurement of the volume of repeated pipettings of water would arise from the pipetting technique. 

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. 

The exercise will take only a few minutes, but it will make you absolutely confident of preventing pipetting errors related to the function of the pipet. 

To get used to the pipets, perform some simple exercises in making dilution series in microtiter plates. The following materials are needed ... 

Handling, Storage, and Precautions the dispersion is a solid and may be handled in the air. The mineral oil may he removed from the dispersion by stirring with pentane, then allowing the hydride to settle. The pentane/mineral oil supernatant may he pipetted off, but care should be exercised to quench carefully any hydride in the supernatant with a small amount of an alcohol before disposal. The dry powder should only be handled in an inert atmosphere. 

Exercise care when using laboratory glassware. Do not use chipped pipets, volumetric flasks, beakers or any glassware with sharp edges exposed in order to avoid the possibility of cuts or abrasions. 

Purpose. This exercise illustrates the general procedures that are used to determine a partition coefficient at the microscale level. Experience in weighing milligram quantities of materials on an electronic balance, the use of automatic delivery pipets for accurately dispensing microUter quantities of liquids, the transfer of microHter volumes of solutions with the Pasteur filter pipet, and the use of a Vortex mixer, are techniques encountered in this experiment. 

Pipet 15 mL of methyl ethyl ketone into the test tube and place the latter just up to the level of its contents in a hot water or steam bath. Heat the solvent w mixture, stirring up and down with the wire stirrer, until a homogeneous solution is obtained. Exercise care to avoid loss of solvent by prolonged boiling. 

---