Constructing a Calibration Curve

Corrected absorbance (after subtracting average blank) 

Inspect your data and use judgment before mindlessly asking a computer to draw a calibration curve  

All three points at 25 pig lie slightly below the straight line through the remaining data. Many repetitions of this experiment show that these points are consistently below the straight line. Therefore the linear range for this determination extends from 0 to 20 pig, but not to 25 pig. 

In view of these observations, we discard the 0.392 absorbance value and we do not use the three points at 25 pig for the least-squares straight line. We could use a nonlinear calibration curve that extends to 25 pig, but we will not do so in this book. 

Suppose that the measured absorbance of an unknown sample is 0.373. How many micrograms of protein does it contain, and what uncertainty is associated with the answer  

---

Repeat the boiling point determination with the following pure liquids (a) carbon tetrachloride, A.R. (77°) (6) ethylene dibromide (132°) or chlorobenzene (132°) (c) aniline, A.R. (184-6°) and (d) nitrobenzene, A.R. (211°). An air condenser should be used for (c) and (d). Correct the observed boiling points for any appreciable deviation from the normal pressure of 760 mm. Compare the observed boiling points with the values given in parentheses and construct a calibration curve for the thermometer. Compare the latter with the curve obtained from melting point determinations (Section 111,1). 

Construct a calibration curve for the electrode, and report (a) the range of concentrations in which a linear response is observed, (b) the equation for the calibration curve in this range, and (c) the concentration of penicillin in a sample that yields a potential of 142 mV. 

Calibrate the system. Use narrowly dispersed molecular weight standards of the polymer of interest to construct a calibration curve of log molecular weight versus elution volume (Eig. 3.2). If a more sophisticated software system is available, a broad molecular weight standard may be used to calibrate the system. 

Construct a calibration curve using appropriate volumes of the standard antimony solution treated in the same way as for the sample solution. 

Construct a calibration curve (for details, see Sections 17.14 and 17.16) using... 

Construct a calibration curve using the synthetic standard solution add the standard copper solution immediately before the reagent. 

Construct a calibration curve using standard ammonium chloride solution (1-100 mg L-1 Cl-) and deduce the chloride ion concentration of the test solution with its aid. 

Construct a calibration curve using a standard sodium chloride solution containing 10 fig Cl mL 1 cover the range 0-50 fig as above. Plot absorbance against micrograms of chloride ion. 

Prepare a series of standards from potassium dihydrogenphosphate covering the range 0-2 mg phosphorus per 100 mL and containing the same concentration of acid, ammonium vanadate, and ammonium molybdate as the previous solution. Construct a calibration curve and use it to calculate the concentration of phosphorus in the sample. 

Construct a calibration curve by plotting the peak area against the standard concentration to obtain a least-squares regression line. 

The peak of 2-TFBA Me-ester usually appears at a retention time around 2.4 min. Construct a calibration curve by plotting the natural logarithm of the peak area counts against the natural logarithm of the standard concentration to obtain a least-squares regression line. 

In fact, either visual or photoelectric colorimeters may be satisfactorily employed as turbidimeters. However, the use of the blue filter normally enhances the sensitivity appreciably. It has been observed that the light transmitted by a turbid solution does not normally obey the Beer-Lambert Law accurately and precisely. Therefore, as an usual practice it is advisable to construct a calibration curve by employing several standard solutions. The concentration of the unknown solution may be read off directly from the above calibration curve as is done in the case of colorimetric assays. 

External standardization is obtained by constructing a calibration curve, i.e., from plotting measured intensities versus rising concentration of the target compound. Calibration curves are generally linear over a wide range of concentrations. When concentration approaches the detection limit (Chap. 5.2.3) the graph deviates from... 

We should appreciate that a superior (although much more time-consuming) approach would be to start by constructing a calibration curve of /um (as y ) against Canaiyte (as x ) with known standards. The construction of a calibration curve is always a superior approach the methodology in the worked example... 

An even better approach when employing the above simple relationship is to construct a calibration curve of /lim (as y ) against Canaiyte (as jc ) with a series of known standards. (Note that we must ensure that the flow rate V remains constant, for example, by the use of a pump or small constant-head tank.)... 

As always, a superior method would have been to construct a calibration curve, and read off the values of Canaiyte since we are more likely to discern non-faradaic currents in this way. It is also advisable to start any series of analyses by varying the flow rate over as wide a range as possible in order to observe those flow regimes which are reproducible and can therefore be employed, and those which should be avoided. 

Quantification of radioactivity is possible by densitometry (scanning) of the developed film. It should be taken into consideration that the optical density of the film is not linear proportional to the amount of radioactivity, especially at lower radioactivity. Dot defined amounts of radioactivity onto a part of the gel or membrane and use the obtained darkening to construct a calibration curve. 

This variation in ionisation can spontaneously occur when the matrix naturally contains one or more alkaline elements. To avoid these random errors, a buffer of potassium or sodium salt is systematically added to the solutions. An alternative is to construct a calibration curve using a matrix that is very close to that of the analyte. 

Another systematic error arises from an uncalibrated buret. The manufacturer s tolerance for a Class A 50-mL buret is 0.05 mL. When you think you have delivered 29.43 mL, the real volume could be anywhere from 29.38 to 29.48 mL and still be within tolerance. One way to correct for an error of this type is to construct a calibration curve, such as that in Figure 3-3, by the procedure on page 38. To do this, deliver distilled water from the buret into a flask and weigh it. Determine the volume of water from its mass by using Table 2-7. Figure 3-3 tells us to apply a correction factor of —0.03 mL to the measured value of 29.43 mL. The actual volume delivered is 29.43 — 0.03 = 29.40 mL. 

We adopt the following procedure for constructing a calibration curve ... 

Different groundwaters have different concentrations of many anions, so there is no way to construct a calibration curve for this analysis that would apply to more than one specific groundwater. Hence, the method of standard addition is required. When we add a small volume of concentrated standard to an existing unknown, we do not change the concentration of the matrix very much. 

A challenge is to manufacture glucose monitors in such a reproducible manner that they do not require calibration. A user expects to add a drop of blood to the test strip and get a reliable reading without first constructing a calibration curve from known concentrations of glucose in blood. Each lot of test strips is highly reproducible and calibrated at the factory. 

The volume inside this tensimeter varies with pressure. Therefore, in situations which require a knowledge of the number of moles of gas, it is necessary to construct a calibration curve relating the internal volume of the tensimeter to the mercury level. The procedure for calibrating a manometer and attached volume has been described in Section 5.3.H. 

Unlike laboratory analysis, the majority of immunoassay kits do not produce results that are expressed as single concentrations. Results may be expressed semi-quantitatively as the ranges of concentrations within which the contaminant concentration falls (for example, greater than 0.5mg/kg and below 2mg/kg), or as comparisons to a standard value, such as less than 1 mg/kg or greater than 1 mg/kg. Immunoassay kits produce quantitative data only when configured with a series of reference standards used to construct a calibration curve. 

---