Reproducibility, instrument reading

The major problems arise from accepting an instrument reading as reliable without determining its calibration and the reproducibility of its response and in not obtaining representative tests. Plant personnel must have some experience with and knowledge of the resolution of these problems. 

There are several sources of irreproducibility in kinetics experimentation, but two of the most common are individual error and unsuspected contamination of the materials or reaction vessel used in the experiments. An individual may use the wrong reagent, record an instrument reading improperly, make a manipulative error in the use of the apparatus, or plot a point incorrectly on a graph. Any of these mistakes can lead to an erroneous rate constant. The probability of an individual s repeating the same error in two successive independent experiments is small. Consequently, every effort should be made to make sure that the runs are truly independent, by starting with fresh samples, weighing these out individually, etc. Since trace impurity effects also have a tendency to be time-variable, it is wise to check for reproducibility, not only between runs over short time spans, but also between runs performed weeks or months apart. 

The loot-mean-square deviation for several identical instrument readings makes no more than 5% [5]. The error in determination of the AO content including the error by reproducibility of results was within 10%. The method involves no model chemical reaction, and measurement time makes 10-15 min. 

Manually operated spectiophotometers are particularly suitable for this type of w ork (we use either a Zeiss PMQ II or an Optica CF 4 instrument) Most automatic instiuments do not reproduce density readings accurately enough for determination ot pK, but can help in shortening the time taken to find the anal>tical wavelength. 

In many years of experience in the NIR applications department at Technicon Instruments, there was about an hour and a half available to teach both theory and practice of calibration to each group of new users the rest of the training time was spent teaching the students how to set the instrument up, prepare samples, take reproducible readings,... 

Frequently, absolute accuracy cannot be established owing to the lack of a suitable calibration standard. For this reason, precision or repeatability can be more important than accuracy Because the measurement is being made continuously or repeatedly, we are more interested in changes in the reading than its absolute value. Precision is defined as the ability of an analyzer to produce the same output each time the same quantity of the component or property is being measured. The terms stability, reliability, and reproducibility are sometimes used synonymously with repeatability. However, the term reliability is also used to describe the instrument s "up time."... 

Figure 5. Result of the GC/MS/MS(MRM) analysis of a metal fragment extracted with dichloromethane proving the presence of sarin, (a) mlz 125 - mlz 81, (b) mlz 125 mlz 99, and (c) mlz 125 (unfragmented). Data recorded at DSTL, Porton Down (UK) on a Finnigan MAT TSQ 700 GC/MS instrument (Finnigan MAT, UK) (27). (Reproduced from Black, R. M., Clarke, R. J., Read, R. W. and Reid, M. T. J., J. Chromatogr., A, 662, 301-331 (1994) by permission of Elsevier Science)...

In practice it is the International Practical Temperature Scale of1968 (IPTS-68) which is used for calibration of scientific and industrial instruments-t This scale has been so chosen that temperatures measured on it closely approximate ideal-gas temperatures the differences are within the limits of present accuracy of measurement. The IPTS-68 is based on assigned values of temperature for a number of reproducible equilibrium states (defining fixed points) and on standard instruments calibrated at these temperatures. Interpolation between the fixed-point temperatures is provided by formulas that establish the relation between readings of the standard instruments and values of the international practical temperature. The defining fixed points are specified phase-equilibrium states of pure substances, t a given in Table 1.2. 

Standard Curve Atomize portions of the Standard Dilutions as described under Procedure (below) until readings for the series are reproducible, adjusting the instrument so that the solution containing 10 p,g/mL gives a full-scale reading. Prepare a standard curve by plotting the absorbance against the concentration. 

Procedure Assemble the aerating apparatus as shown in Fig. 16, with bottles c and d empty and stopcock b in the bypass position. Connect the apparatus to the absorption cell (f) in the instrument, and adjust the air or nitrogen flow rate so that in the following procedure, maximum absorption and reproducibility are obtained without excessive foaming in the test solution. Obtain a baseline reading at 253.6 nm, following the manufacturer s instructions for operating the instrument. 

Reproducibility of Couples.—When thermocouples are employed in the laboratory for scientific purposes, although desirable, it is not of serious importance that the calibration or temperature-electromotive force relation of couples of the same type be exactly similar. However, in the industrial plant this question of reproducibility is of considerable moment. The indicating instruments are usually graduated in degrees of temperature and the graduation applies for one definite temperature-electromotive force relation only. If the temperature electromotive force relations of various couples of the same type are not similar, corrections must be applied to the readings of the indicator, and these corrections will be different for each couple. When several couples are operated with one indicator and when the process is such as to require a frequent renewal of couples, the applying of these corrections becomes very troublesome. 

Once reproducible readings have been produced, do not touch the scale knob again until all measurements have been completed. Do zero the instrument after every reading. 

In general, AOTF-based instruments are rugged and fast, capable of numerous readings per second. As long as there is no radio frequency interference, the wavelength accuracy and reproducibility is excellent. While individual instruments vary, the nominal spectral resolution is around 10 nm. This is quite sufficient for process purposes, but if greater resolution is needed, a FT-NIR is required. 

---