Accuracy volume measurement

Liquids have relatively low compressibility compared with gases and, thus, the mobile phase velocity is sensibly constant throughout the column. As a consequence, elution volumes measured at the column exit can be used to obtain retention volume data and, unless extreme accuracy is required for special applications, there is no need for the retention volume to be corrected for pressure effects. 

Loops are not calibrated accurately and a loop of nominally 20 p.l is unlikely to have this exact volume. This will not affect either the precision of measurement and, as long as the same loop is used for obtaining the quantitative calibration and for determining the unknowns , the accuracy of measurement. 

Gas pycnometry (gas displacement) Rapid Nondestructive Easy to use Expensive instrumentation Large sample size Accuracy limited by precision of volume measurements... 

Second, if the results of a quantitative analysis are to be reported to three or more significant figures, then volume measurements that enter directly into the calculation of the results should be made with volumetric glassware so that the accuracy of the analysis is not diminished when the calculation is performed. 

The accuracy of the injection volume measurement can be very important for quantitation, since the amount of analyte measured by the detector depends on the concentration of the analyte in the sample as well as the amount injected. In Section 12.8, a technique known as the internal standard technique will be discussed. Use of this technique negates the need for superior accuracy with the injection volume, as we will see. However, the internal standard is not always used. Very careful measurement of the volume with the syringe in that case is paramount for accurate quantitation. Of course, if a procedure calls only for identification (Section 12.7), then accuracy of injection volume is less important. See Workplace Scene 12.1 for an example of a purge-and-trap procedure for injecting a GC sample. 

The Half-micro-nitrometer.—The half-micro-nitrometer used to collect the nitrogen has a capacity of 8-10 c.c., a volume which corresponds to 20-30 mg. of substance. A completely satisfactory degree of accuracy in measuring the gas is attained by graduating in sub-divisions of 0-02 c.c. 

In earlier experiments the effect of branching on the second virial coefficient was not seriously considered because the accuracy of measurements were not sufficient at that time. With the refinements of modern instruments a much higher precision has now been achieved. Thus A2 can also now be measured with good accuracy and compared with theoretical expectations. The second virial coefficient results from the total volume exclusion of two macromolecules in contact [3,81]. Furthermore, this total excluded volume of a macromolecule can be expressed in terms of the excluded volume of the individual monomeric units. In the limit of good solvent behavior this concept leads to the expression [6,27] as shown in Eq. (24) ... 

One alternative is to compare the results of the method with results from an established reference method. This approach assumes that the uncertainty of the reference method is known. Second, accuracy can be assessed by analyzing a sample with known concentrations (e.g., a certified reference material) and comparing the measured value with the true value as supplied with the material. If such certified reference material is not available, a blank sample matrix of interest can be spiked with a known concentration by weight or volume. After extraction of the analyte from the matrix and injection into the analytical instrument, its recovery can be determined by comparing the response of the extract with the response of the reference material dissolved in a pure solvent. Because this accuracy assessment measures the effectiveness of sample preparation, care should be taken to mimic the actual sample preparation as closely as possible. 

For photometric measurement results, the ratio between the measurement accuracy of the photometer and the uncertainty of the upper standard used for its calibration is very important. Usually, this ratio should be of a minimum of 3. For physical standards used to calibrate photometric systems the ratio of 3 is most commonly followed. This rule generally applies also for weight and volume measurements performed in conjunction with the photometer. 

It became possible to significantly increase the accuracy of measurements of dissolved oxygen in the 1990s, because oceanographers started to use the 5-L PVC Niskin bottles instead of 1-L bottles (the larger volume of water allowed better protection of the portion of water sampled in the flask from the oxygen inside the bottle). 

Other common laboratory containers such as beakers, round bottom flasks, and Erlenmeyer flasks often have a limited graduated volume designated on their sides. These markings provide an approximate volume and cannot be used for quantitative work. The required accuracy of these containers is only 5% of volume. When there are no calibration lines on a flask, it still is possible to obtain an approximate volume measurement based on the stated volume In general, the stated volume will approximately fill any given nonvolumetric container to the junction of the neck and container (see Fig. 2.13). Thus, if you need about 500-mL of water, it is safe to fill a 500-mL flask up to the neck and you will have approximately the needed volume. 

A temperature of 0 K is called absolute zero . It coincides with the minimum molecular activity, i.e., thermal energy of matter. The thermodynamic temperature was formerly called absolute temperature . In practice, the International Temperature Scale of 1990 (ITS-90) [i] serves as the basis for high-accuracy temperature measurements. Up to 700 K, the most accurate measurements of thermodynamic temperature are the NBS/NIST results for Constant Volume Gas Thermometry (CVGT). Above 700 K, spectral radiometry is used to measure the ratio of radiances from a reference... 

These difficulties are inspiring the search for other alternative methods for the void volume measurements. The main question anyone should answer is, What should the required accuracy be For any prediction or thermodynamic-type work, the void volume values should be very accurate. For pure analytical purposes or for the comparison of different columns, the use of some markers is justifiable insofar as all experimental conditions are kept the same all the time. 

Note how simple the means are by which the weight of liberated oxygen was found, to 1 mg accuracy-only rough volume measurements (to 1 ml) needed to be made. 

The design and construction of flow systems is more complex than similar batch systems ° and requires larger volumes of reactant. It is often desirable to have automatic control of pressure, temperature and flow rates to increase the accuracy of measurement as well as the safety of operation. 

Weight measurements can be made with considerably greater precision and accuracy than can volume measurements. For example, 50 g or 100 g of an aqueous solution can be readily measured to 1 mg, which corresponds to 0.001 mL. This greater sensitivity makes it possible to choose sample sizes that lead to significantly smaller consumption of standard reagents. 

In 1964 the General Conference on Weights and Measures reestablished die name liter as a special name for the cubic decimeter. Between 1901 and 1964 the liter was slightly larger (1.000 028 dm ) when one uses high-accuracy volume data of that time, this fact must be kept in mind. 

Using a pipettor - check your technique (precision) by dispensing volumes of distilled water and weighing on a balance, assuming 1 mg = 1 gL = 1 mm3. For small volumes, measure several aliquots together, e.g. 10 x 15//L = 150 mg. Aim for accuracy of 1%. 

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