Calibration of pipettes

In many cases, the classical volumetric pipette can be substituted by plunger-type pipettes or by dispensers, provided that these are regularly calibrated. This is in particular convenient for dispensing concentrated acids. For work where high precision is required, calibration of pipettes is needed. 

Calibration of Pipettes.—Pipettes are calibrated by weighing the water which they deliver. In carrying out the calibration, however, several precautions must be observed if an accuracy of o os per cent, is to be obtained. In the first place, it must be seen that the glass of the pipette is free from... 

When the correct solvent for recrystallisation is not known a procedure similar to that given on pp. 15-16 should be followed, but on the semi-micro scale not more than 10 mg. of the solid should be placed in the tapered-end test-tube (Fig. 29(B)) and about o i ml. of the solvent should be added from the calibrated dropping-pipette (Fig. 30(B)). If the compound dissolves readily in the cold, the solvent is unsuitable, but the solution should not be discarded. [In this case recourse should be had to the use of mixed solvents (p. 18). For example if the substance is very soluble in ethanol, water should be added from a calibrated pipette with shaking to determine whether crystallisation will now take place, indicated by a cloudiness or by the separation of solid.]... 

Calibration of apparatus and application of corrections. All instruments (weights, flasks, burettes, pipettes, etc.) should be calibrated, and the appropriate corrections applied to the original measurements. In some cases where an error cannot be eliminated, it is possible to apply a correction for the effect that it produces thus an impurity in a weighed precipitate may be determined and its weight deducted. 

Calcium, D. of - continued in limestone or dolomite, (fl) 813 in presence of barium, (ti) 333 with CDTA, (ti) 333 with lead by EDTA, (ti) 333 with magnesium by EDTA, 328 by EGTA, (ti) 331 by flame emission, (aa) 804 Calcium oxalate, thermal analysis 498 Calcon 318 Calculators 133 Calibration of apparatus, 87 of burettes, 88 of graduated flasks, 88 of pipettes, 88 of weights, 74... 

In a similar manner, the calibration of glassware, such as volumetric flasks, pipettes, burettes, measuring cylindres are duly carried out by specific methods recommended by Indian Standards Institution (ISI), British Standards Institution (BSI), National Physical Laboratory (NPL), United States Pharmacopoeia (USP) at specified temperatures (See Chapter 2). 

Volume measurements include the use of pipettes, volumetric flasks, measuring cylinders, and burettes. Each has different calibration uncertainties that are detailed in the manufacturer s information supplied with the... 

The manufacturer s calibration information must now be accounted for. You might be lucky and have a 10-mL pipette that is indeed 10.00 mL, but the manufacturer is only guarantees the pipette to be not less than 9.98 mL and not more than 10.02 mL. As the pipette is used throughout many experiments, then this possible systematic effect will not dissipate with repeated measurements and must be accounted for. There are two alternatives. The first is to calibrate the pipette in the laboratory. The fill-and-weigh experiments that would give the standard deviation of the measurement will also, from the mean, give an estimate of the volume of the pipette (see spreadsheet 6.1). 

There are pros and cons to both types of pipettes. The decision to use one or the other depends on the reliability, and repeatability, of your laboratory technique as well as the nature of the liquid you are pipetting. All non-blow-out pipettes are calibrated with water. Thus if your liquid has different surface tension or viscosity characteristics than water, your measurements will not be accurate. On the other hand, not everyone will exert an equal amount of blow-out force. Thus different people may deliver different volumes using the same equipment. 

Instead of using a separate vial with buffer for the calibration of the EVOM, it is also possible to pipette some buffer into the interjacent slots between the wells of a Transwell plate and measure the buffer resistance there. This guarantees also comparable conditions (temperature, etc.). 

The pipettes are all based on air displacement with a simple plunger and are provided with non-wettable plastic (usually polypropylene) disposable tips to contain the solution, preventing any contamination of the pipette itself. Most micropipettes have a double action plunger system, i.e. calibration and overshoot positions, which ensures that the sample is completely dispensed. There are many manufacturers offering a complete range of volumes in addition some have available a selection of pipettes with adjustable volumes. 

The sample dilution required will depend on the concentration of Pb present in the gasoline. This should be chosen to fall within the range defined by the calibration standards. Pipette the chosen volume of gasoline into 100 ml volumetric flasks in duplicate. To this add approximately 50 ml of MIBK and 0.2ml (200pi) of a solution of iodine in toluene (see note 2). Swirl the mixture and allow to stand for two minutes. Add 5 ml of a 1% v/v solution of Aliquat 336 (see note 1), swirl to react and dilute to the mark with MIBK. 

A more recently developed force measurement technique, coined the liquid siu-face force apparatus (LSFA), brings a drop made from a micropipette close to a flat liquid/liquid interface [29-32]. A piezo electric drive is used to change the position of the micropipette while the deflection of the pipette and the radius of the drop are recorded with piezo motion. The drop radius and thus the film thickness between the two liquid/liquid interfaces are recorded using interferometry. The method requires a calibration of the interferometer, where the drop must come into contact with the other liquid interface. The distance resolution of the film is about 1 nm at a 50-nm separation and 5 nm at a separation of 10 nm. This is a very robust technique where the authors have proposed attaching a particle to the end of the pipette instead of a drop [29]. In comparing this method to AFM, the only drawback of the LSFA is the weaker distance resolution. It is important point out that both methods required a contact point for distance calibration. 

A calibration schedule details the calibration of balances, volumetric glassware, automatic pipettes, thermometers, pH and conductivity meters, wavelength and photometric scales etc. The schedule consists of periodic external checks, employing a suitably accredited calibration service, supported by more regular in-house performance checks. 

Good Accuracy. Graduated pipettes and burettes should deliver within 1% of the nominal marked volume. There are several different types of calibration of graduated pipettes ... 

Touch the tip against the inside of the vessel - this removes some of the solution held in the tip. The final portion of solution remaining in the tip should not be expelled, because the calibration of the pipette will have allowed for it. 

A volumetric transfer pipette (Figure 1-1, A) is calibrated to deliver accurately a fixed volume of a dilute aqueous solution. The reliability of the calibration of the volumetric pipette decreases with a decrease in size, and therefore special micropipettes have been developed. 

Certain variables— water quality, calibration of analytical balances, calibration of volumetric glassware and pipettes, stability of electrical power, and the temperature of heating baths, refrigerators, freezers, and centrifuges—should be monitored on a laboratorywide basis because they wiU affect many of the methods in the laboratory (see Chapter 1). In addition, there wiU be certain variables that relate more directly to individual analytical methods, and these require... 

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