Calibration glassware

To reduce the source of bias, it is important to have an authentic reliable reference standard, a properly maintained and calibrated balance, knowledge of the purity and form of the standard, calibrated glassware, calibrated pipettes, analyte-free matrix, and pooled and individual lots. Poor calibration, incorrectly or poorly prepared standards, interaction between analytes and container, and incomplete reactions will lead to bias. Some further precautions can be taken to minimize the possibility of errors, such as preparing QC samples in bulk (pools) and separately from the calibrators, which are prepared fresh everyday. 

Buoyancy corrections are usually significant in glassware calibration. 

Solutions Aqueous Triton X-100 —0.1%, 0.05 M nitric acid, and 1 M nitric acid for cleaning tubes and glassware. Calibration standards are made up to concentration of 8, 16, and 24 jig/liter. 

Volumetric procedures incorporate several important sources of systematic error. Chief amongst these are the drainage errors in the use of volumetric glassware, calibration errors in the glassware, and indicator errors. Perhaps the commonest error in routine volumetric analysis is to fail to allow enough time for a pipette to drain properly, or a meniscus level in a burette to stabilize. Pipette drainage errors have a systematic as well as a random effect the volume delivered is invariably less than it should be. The temperature at which an experiment is performed has two effects. Volumetric equipment is conventionally calibrated at 20°C, but the temperature in... 

Glassware designed to contain a specific volume of solution when filled to its calibration mark. 

Three important precautions are needed when working with pipets and volumetric flasks. First, the volume delivered by a pipet or contained by a volumetric flask assumes that the glassware is clean. Dirt and grease on the inner glass surface prevents liquids from draining evenly, leaving droplets of the liquid on the container s walls. For a pipet this means that the delivered volume is less than the calibrated volume, whereas drops of liquid above the calibration mark mean that a volumetric flask contains more than its calibrated volume. Commercially available cleaning solutions can be used to clean pipets and volumetric flasks. 

Second, when filling a pipet or volumetric flask, set the liquid s level exactly at the calibration mark. The liquid s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware s calibration mark (Figure 2.6). The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors. If your eye level is above the calibration mark the pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be underfilled if your eye level is below the calibration mark. 

Determinate measurement errors can be minimized by calibration. A pipet can be calibrated, for example, by determining the mass of water that it delivers and using the density of water to calculate the actual volume delivered by the pipet. Although glassware and instrumentation can be calibrated, it is never safe to assume that the calibration will remain unchanged during an analysis. Many instruments, in particular, drift out of calibration over time. This complication can be minimized by frequent recalibration. 

Personal Errors Finally, analytical work is always subject to a variety of personal errors, which can include the ability to see a change in the color of an indicator used to signal the end point of a titration biases, such as consistently overestimating or underestimating the value on an instrument s readout scale failing to calibrate glassware and instrumentation and misinterpreting procedural directions. Personal errors can be minimized with proper care. 

To ensure that S eas is determined accurately, we calibrate the equipment or instrument used to obtain the signal. Balances are calibrated using standard weights. When necessary, we can also correct for the buoyancy of air. Volumetric glassware can be calibrated by measuring the mass of water contained or delivered and using the density of water to calculate the true volume. Most instruments have calibration standards suggested by the manufacturer. 

A hydrolysis unit is constructed using the commercially available glassware shown in Figure 2. The collection funnel should be calibrated and marked for volumes of 35 and 85 mL. All 20/40-glass joints of the hydrolysis unit should be assembled with Teflon sleeves and secured with spring clamps to prevent loss of the volatile EMA and HEMA analytes. 

Initially, this method utilized 5-mL conical centrifuge tubes as the collection device for final elution of the extract from the Cig tubes. In practice, these tubes were found to be very difficult to clean and in few instances were the cause of cross-contamination when low-concentration samples were extracted following samples with very high concentrations. Since no commercial graduated tubes were available, disposable culture tubes are used as the receiver. These tubes are individually calibrated before use. A solvent blank sample may be processed through the method from extraction to quantification to determine if contamination from glassware occurs. 

HPLC column, Merck, LiChrospher 60 RP Select B, 5 itm, 150 x 4-mm i.d. Autosampler vials, 1.8-mL Assorted laboratory glassware Autopipettes, calibrated... 

Describe individual items of equipment and how they are connected in sufficient detail to enable unambiguous set-up. List minimum performance requirements and verification requirements, cross-referenced to the calibration section and any relevant instrument manuals. Number for later reference. For glassware, include grade where applicable (bear in mind that use of a particular grade may require justification and that proof of compliance may be required). Include environmental requirements (fume cupboards, etc.). 

The term calibration is often used when in fact what is meant is verification . Calibration of an instrument or a piece of equipment (e.g. glassware) involves making a comparison of a measured quantity against a reference value. For example, to calibrate a spectrophotometer response, the appropriate reference material is selected and the spectrophotometer response to it, under the specified conditions, is measured. Then, the measured value is compared to the value quoted in the literature. Either a correction is made to the results from subsequent measurements or an adjustment is made to the instrument. 

What is essential in establishing traceability is that the measurand is specified unambiguously. This may be, e.g. in terms of extractable cadmium from soil by using a named acid mix or the concentration of a metal in a particular oxidation state, e.g. Fe(n) or Fe(m). The units used to report the result should also be known and acceptable SI units are preferred. The method used will be validated and if used in accordance with the written procedures should produce results that are fit for purpose . The class of glassware to be used will be specified in the method procedure, e.g. Class A pipettes and volumetric flasks, as these are manufactured to a specified tolerance. Instruments will be regularly calibrated and their performance verified daily. In terms of the chemicals used, these will... 

Instrument/Equipment Effects Examples include the calibration and precision of an analytical balance, the specified tolerance for volumetric glassware and a temperature controller that maintains a mean temperature which is different (within specification) from its indicated value. 

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