Stable standard deviation

Statistical control applies to all parts of the analytical system - sampling process, the calibration, the blank, and the measurement. Statistical control is attained by the quality control of the entire system and Involves maintenance of realistic tolerances for all critical operations. A system of control charts is the best way to demonstrate attainment of statistical control and to evaluate the appropriate standard deviations. In the simplest form, the results of measurement of a stable check sample, obtained over a period of time, are plotted. Statistical control is demonstrated when the values are randomly distributed around their average value." Control limits are often taken as 2 or 3 standard deviation units of these replicates. Dr. Taylor also adds, "Even the ranges of duplicate measurements of the actual samples tested can be plotted in a similar manner to demonstrate a stable standard deviation. In either case, the statistics of the control charts are the best descriptors of the variability of the measurement process."... 

Sutton Micrometeorology, McGraw-Hill, 1953, p, 286) developed a solution to the above difficulty by defining dispersion coefficients, O, Gy, and O, defined as the standard deviation of the concentrations in the downwind, crosswind, and vertical x, y, z) directions, respectively, The dispersion coefficients are a function of atmospheric conditions and the distance downwind from the release. The atmospheric conditions are classified into six stability classes (A through F) for continuous releases and three stability classes (unstable, neutral, and stable) for instantaneous releases. The stability classes depend on wind speed and the amount of sunlight, as shown in Table 26-28,... 

One can apply a similar approach to samples drawn from a process over time to determine whether a process is in control (stable) or out of control (unstable). For both kinds of control chart, it may be desirable to obtain estimates of the mean and standard deviation over a range of concentrations. The precision of an HPLC method is frequently lower at concentrations much higher or lower than the midrange of measurement. The act of drawing the control chart often helps to identify variability in the method and, given that variability in the method is less than that of the process, the control chart can help to identify variability in the process. Trends can be observed as sequences of points above or below the mean, as a non-zero slope of the least squares fit of the mean vs. batch number, or by means of autocorrelation.106... 

When Er was more than five standard deviations away from the center of the Normal distribution representing AEx, the computed variance was fairly small and reasonably stable, and decreased as Et was moved further away from the center of AEr. This might be considered an empirical determination of the point of demarcation of the small-signal case. 

The shelf temperature and the controlled operation pressure must be controlled in such a way, that TKt in the laboratory plant is stable and measured with a standard deviation less than = 0.5 °C. 

Tice data fluctuate and the standard deviation of the measurements becomes larger than approx. 0.3-0.5 °C. The monitored pc, 7 sh and Tco are stable, but the pressure rise measurements change and vary as shown in Fig. 2.18.2. Most likely the valve between chamber and condenser does not close reproducibly. If the drive of the valve has been checked externally as far as possible, the run cannot be finalized automatically and hand operation should be used, if the time course of the program has been already established. 

For example, an 11-month-old sensor based on 50 mol% octyl-triEOS exhibits more than fourfold greater sensitivity in comparison to an equivalent sensor based on pure TEOS (Figure 6.7). Over an 11-month time period, the sensitivity of a pure TEOS-based sensor drops by more than 400%, whereas a sensor based on 50 mol% octyl-triEOS remains stable (relative standard deviation ca. 4%). Integrated in fibre optics, optical sensors based on these materials are commercialized by the US... 

Stable labelled isotopes are spiked into samples before extraction and the ratio of unlabelled compound and stable labelled isotope was used to quantitate the unlabelled compound. Analysis is by high-resolution gas chromatography-mass spectrometry. Fifteen standard water samples and ten standard soil samples containing 2,4-D at known concentrations were analysed. Compound concentrations ranged from 100 to lOOOOug per kg for soil samples. Average recoveries were over 84% and method precision, given as relative standard deviation, was better than 19%. 

NaCl being smaller than the best precision. Values of these parameters obtained from fits of data with Radiatom I, with the corresponding reduced standard deviations of the respective fits are presented in table 6 all parameters therein convey the same significance as in preceding tables 1-3. Comparison of values of the same parameters in the separate fits indicates that only small changes occur hence for corresponding parameters these fits are reasonably stable. Because the... 

Foster et al. have developed a method for determining technetium in dissolved nuclear fuel solutions. Tetrapropylammonium pertechnetate is doubly extracted from a basic medium into chloroform and the colored technetium (V) thiocyanate complex is formed in the chloroform phase by the addition of sulfuric acid, potassium thiocyanate and tetrapropylammonium hydroxide. The colored complex absorbs at 513 nm, has a molar extinction coefficient of 46,000 and is stable for several hours. Of more than 50 metals studied, none impairs measurements at ratios less than 100 to 1 mol with respect to technetium. Most anions do not disturb the determination of technetiiun. The standard deviation for a single determination is 0.09 fig over the range of 1 to 20 fig of technetium. 

As with the determination of the intralaboratory repeatability, the intralaboratory reproducibility was determined by analyzing a cleaned sediment extract and a 3-pM 2,3,7,8TCDD standard on 10 separate days and by multiple persons. The reproducibility for the 3-pM 2,3,7,8-TCDD standard was found to be 13.8%, whereas the reproducibility for the cleaned sediment extraet was shown to be 19.9%. Since the observed reproducibilities are in the range of relative standard deviations for two sediment extracts analyzed in 10-fold on the same day (intralaboratory repeatability), the DR CALUX bioassay can be evaluated as a stable and robust bioanalytical tool. 

Sample means x are plotted on the x chart. Assume that random samples of n items are collected and examined from a stable process with a process mean p and standard deviation o. Then x can be considered as a random variable following a normal distribution with mean pv, and standard deviation o where... 

On line additions of aqueous standard solutions for the calibration of LA-ICP-MS including a comparison of wet and dry plasma conditions are discussed by O Connor et al.ls For solution calibration of standard solutions the authors used a 100 (xl PFA nebulizer together with a cyclonic spray chamber or a MCN-6000 sample introduction system with desolvator, to study the wet and dry plasma, respectively. A polypropylene Y piece was applied to mix the laser ablated material and the nebulized standard solutions. The authors found that the on line addition of water is the preferred mode of operation for quantification by LA-ICP-MS, i.e., wet plasma is more stable (improved standard deviation of sensitivity ratios). 

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