Pipette calibration example

In some cases, we apply an observation method that in a similar way leads to the introduction of a random error in the final outcome. For example, we always read 0.1 mL if we use a 10 mL measuring pipette, calibrated in tenths of a millilitre. Acmally we round off the observation to one decimal place without mentioning or realising that. 

In some cases, however, your instructor may have available a calibrated pipette for transferring nonlimiting reagents that may damage an automatic pipette. For example, a calibrated Pasteur pipette may be used with concentrated acids. 

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.]... 

Suspensions can be introduced in a variety of ways. Some examples are to manually use syringes or pipettes, pour from a fared beaker, or automate delivery using calibrated pipettes. Each method has its own set of limitations, although automated methods may show less variability. Mixing of the suspension sample will generate air bubbles therefore, the mixing time of suspension samples must be strictly uniform to reduce erroneous or biased results. 

The mathematics are even simpler when contributions to the uncertainty of a single quantity are combined. Here the sensitivity coefficient is 1, and the individual uncertainties are just squared and summed. For example, for the combination of the standard uncertainties of the effects on the volume delivered by a pipette discussed above, which are repeatability, calibration uncertainty, and the effect of temperature, the square of the combined uncertainty is simply the sum of the squares of each effect ... 

Sample loading pipette 40 pL of Interleukin IB sample solution in each of the eight reservoirs. In order to establish a calibration curve, interleukin concentrations ranging from 0.2 to 17.5 pM are used in the present example (IL-1B1,1 standards 1-8). The various solutions are then pumped through the micro-channels using 60... 

Calibration curves must be made from chemicals with the highest purity as possible. To avoid dilution errors a multi-level calibration curve (six points) based on three stock solutions is recommended. One must also be aware that low concentrations of for example, PAHs (2 ppm) may be adsorbed by the vials up to -90% (Pinto, Jose and Cordero, 1994). A calibrated and traceable balance or a traceable pipette must be used for accurate preparation and dilution of the standards. The calibration curve must cover the concentration range that is needed for the analysis. Both the slope and the intercept must be used to calculate the concentration in the sample, especially if the intercept is different from zero. 

Prepare the standard chromium solutions for calibration. Pipette respectively, for example, 0.1 ml, 0.2 ml, 0.4 ml, 0.5 ml and 0.7 ml of a concentrated standard chromium solution (such as 1000 ppm, commercially available) into 100 ml volumetric flasks. Dilute them to the mark with deionized water. The concentrations of the diluted standard solutions are 1 ppm, 2 ppm, 4 ppm, 5 ppm and 7 ppm, respectively. 

Using a clean and dry glass pipette (volumetric), transfer 1.0 mL of the 5000-ppm BTEX to a 10-mL volumetric flask which has been previously half-filled with the most suitable solvent that you chose earlier. Adjust to the calibration mark with this solvent and label as, for example, 500 ppm BTEX. This is what EPA methods call a primary dilution standard because it is the first dilution that the analyst prepares from a given source which has a higher concentration. 

The reason for systematic titration errors is that the equivalence point is indicated too early or too late. This happens when the transition point of the indicator does not exactly match the pH of the equivalence point of the titration (systematic errors caused by wrongly calibrated pipettes or burettes will not be discussed here). The transition point of an indicator gives the experimental endpoint of the titration. Because the term endpoint can also be applied in the sense of theoretical endpoint = equivalence point we shall use here the term transition point to be clear. The same can happen in case of instrumental methods of indication when these methods do not identify the equivalence point correctly, but systematically deviate from it. Color indicators are themselves acid-base systems Hl/1 (HI + H2O 1 + HsO ), the p a value of which is usually denoted as the pA) value, and it normally falls in the range of 2-12. There are bichromic and monochromic indicators. For example, a bichromic indicator may be red as an acid and blue as a base, and a monochromic may be colorless as an acid and violet as a base. In the case of bichromic indicators, the color changes when Chi = cr, that is at the buffer point of the indicator. Of course, the color change does not abruptly occur there, but it is smeared out in an interval (the so-called transition interval of an indicator), roughly in the... 

A Pasteur pipette may be supplied by your instructor for dropwise addition of a particular reagent to a reaction mixture. For example, concentrated sulfuric acid is often dispensed in this way. When sulfuric acid is transferred, take care to avoid getting the acid into the rubber or latex dropper bulb. It is best to avoid the rubber dropper bulb entirely by using one-piece transfer pipettes made entirely of polyethylene. These plastic pipettes are available in 1- or 2-mL sizes. They come from the manufacturers with approximate calibration marks stamped on them (Figure 55B). 

When preparing a dilution series, it is important to maintain aseptic conditions (Section 13.8). For example, dilution blanks must be flamed when opened to transfer sample (Fig. 13.2). Sterile 1.0 mL pipettes (0.1 mL graduations) or variable volume pipettors equipped with sterile plastic tips must be used to aseptically transfer liquid between dilution blanks. Sterile pipettes or pipette tips should be used only once, and then a new pipette or tip is used. Pipettors can be sterilized by wiping down their surfaces with 70% v/v ethanol and should be calibrated yearly. 

First we need to examine the assertion that in analytieal ealibration the concentrations are effectively error-free while the responses sueh as measurement results include error. While this is widely true, in some instanees it is at least questionable. In absorptiometry, for example, absorbanees are often repeatable to within 1 part in 500. Had the calibrators been prepared with small-volume automatic pipettes, random errors might easily amount to one part in 100. This clearly violates the usual assumptions so, prima facie, p-mode calibration should be used. Moreover, if the errors on both axes are quite small, there is little to choose between the two methods. In fact, the on-board calibration found in most modern instruments tends to use p-mode calibration (but fails to inform the... 

However, just because the results of a set of analyses are in close agreement does not necessarily mean that the values are correct. This is because of the possibility of determinate errors. Such errors have a definite cause (although it may be unknown to the analyst), and each type of determinate error is always in the same direction. For example, if an analytical chanist is using a pipette rated to deliver 25.00 mL of solution that through a manufacturing mistake actually delivered 25.35 mL, a determinate error would be introduced into the analysis in this case, it could readily be detected by calibrating the pipette. 

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