Relative standard deviation. See

Quantification Precision can be quantified by suitable dispersion characteristics. It is proposed to characterize precision by standard deviation, see Eqs. (4.12)-(4.14) and relative standard deviation, see Eq. (4.15) (Fleming et al. [1996b] Prichard et al. [2001]). Because of some uncertainty, the characterization of analytical proceedings in their hierarchy (see Fig. 7.1) and of analytical results, respectively, will be considered in detail. 

Precision is the closeness of agreement between independent test results obtained under stipulated conditions. The precision tells us by how much we can expect the results of repeated measurements to vary. The precision of a set of measurement results will depend on the magnitude of the random errors affecting the measurement process. Precision is normally expressed as a standard deviation or relative standard deviation (see Section 6.1.3). 

CZE = capillary zone electrophoresis EC = electrochemical detector GC = gas chromatography HCD = Hall conductivity detector HPLC = high performance liquid chromatography IDMS = isotope dilution mass spectrometry MS = mass spectrometry RSD = relative standard deviation SEE = supercritical fluid extraction SPE = solid phase extraction UV = ultraviolet absorbance detection... 

From the Lambert—Beer law it is also possible to link relative standard deviation (see Section 22.2) on the measurement of concentration C to the relative standard deviation made on the transmittance T (curve 4). 

Notice that a result of this type, in order to be interpretable, must comprise three numbers the mean, the (relative) standard deviation, and the number of measurements that went into the calculation. All calculations are done using the full precision available, and only the final result is rounded to an appropriate precision. The calculator must be able to handle >4 significant digits in the standard deviation. (See file SYS SUITAB.xls.)... 

Chromatographic system (See Chromatography <621 >.) The liquid chromatograph is equipped with a 230 nm detector and a 4.6 mm x 30 cm column that contains packing L7. The flow rate is about 2 mL/min. Chromatograph the Resolution solution and the Standard preparation, and record the peak responses as directed under Procedure the resolution, R, between the dibutyl phthalate and miconazole peaks is not less than 5, the tailing factor for the miconazole peak is not more than 1.3, and the relative standard deviation for replicate injections of the Standard preparation is not more than 2%. The relative retention times are about 0.7 for dibutyl phthalate and 1 for miconazole. 

Coefficient of variation The ratio of the standard deviation to the mean often expressed as a percentage (see also Relative standard deviation). 

Thus we have 16 determinations (without the difference values) from which to calculate the mean in samples 1 and 2, and four in sample 3. Sample 3 (NBS SRM 124c) has a known value of 84.22%, with which we can compare our value (from four laboratories) of 82.6%. Our mean is somewhat low. The median, higher in each case, may be a better estimate of what the copper content actually is in each sample. The standard deviation and relative standard deviation are shown (see footnote b, Table IV). The relative standard deviation decreases from sample 1 to sample 2 and from sample 2 to sample 3. This may reflect greater inhomogeneity in sample 1 than sample 2 and in sample 2 than sample 3 it may also simply reflect the fact that we have fewer laboratories reporting on sample 3. Since the relative standard deviation on copper is so low, some of it may indeed arise from the materials themselves. On the other hand, the relative standard deviations for other elements (Table IV) do not show this pattern. In fact, they seem to be more closely related to the amount present, implying that the determinations become less precise as the amount of the element decreases (viz., Sn 1 + 2 Zn 1, 2, + 3 Ag 2 + 3 Si 1 + 2). 

Chromatographic System (See Chromatography, Appendix IIIA.) Set up the system with reference to High-Performance Liquid Chromatography. The chromatograph has a 254-nm detector and a 15-cm x 4.6-mm column that contains 5-to 10-mm porous microparticles of silica bonded to octylsilane (Zorbax 8, or equivalent). Set the flow rate to about 2 mL/ min. Chromatograph three replicate injections of the Standard Preparation, and record the peak responses as directed under Procedure. The relative standard deviation is not more than 2.0%, and the resolution factor between nitrilotriacetic acid and Calcium Disodium EDTA is not less than 4.0. 

Chromatograph the System Suitability Preparation, and measure the peak areas as directed under Procedure. The resolution, R, between the all-trans retinyl acetate and the all-trans retinyl palmitate peaks is not less than 10 (see System Suitability in High-Performance Liquid Chromatography under Chromatography, Appendix IIA), and the relative standard deviation for replicate injections is not more than 3.0%. 

---