Instrumentation nonidealities

Chemical and physical nonlinearities are caused by interactions among the components of a system. They include such effects as peak shifting and broadening as a function of the concentration of one or more components in the sample. Instrumental nonlinearities are caused by imperfections and/or nonideal behavior in the instrument. For example, some detectors show a... 

Band broadening arises from three principal mechanisms, one of which depends on the mean velocity of the carrier and two of which are independent of . The latter two represent nonidealities in instrument and sample. The total variance of an eluting peak is the sum of variances contributed by each bandbroadening mechanism. Expressed as plate height H, which is the total variance divided by column length L (12), the zone broadening is described by... 

As we discussed above, efficiency and selectivity are complementary descriptors dependent on the different sets of chromatographic parameters. Efficiency is more dependent on the quality of the column packing, particle size, flow rate, and instrumental optimization, while selectivity is more dependent on the stationary phase properties and the nature of the analytes themselves. However, efficiency is sometimes affected by nonideal interactions of the analyte with the stationary phase (i.e., peak tailing). 

Two types of errors of observation exist systematic and random. The total error associated with a measurement is a function of the systematic error, commonly called bias, and the random error. Systematic errors in environmental measurements can be divided into three general categories instrument errors (e g., nonideal functioning of an instrument) interference from envi-... 

There are three types of systematic errors (1) Instrumental errors are caused by nonideal instrument behavior, by faulty calibrations, or by use under inappropriate conditions. (2) Method errors arise from nonideal chemical or physical behavior of analytical systems. (3) Personal errors result from the carelessness, inattention, or personal limitations of the experimenter. 

At pH 7, where H = OH, the voltage from the electrode is zero. This is called the isopotential point (Fig. 4). In theory, this point is temperature-independent. The International Union for Physical and Applied Chemistry (lUPAC) (1) operational pH scale is defined as fhe pH relative to a standard buffer measured using a hydrogen electrode. In practice, a pH electrode is calibrated with standard buffers of pH 7.00 and pH 4 or 9 fo defermine the isoelectric point and slope, respectively. Conventional pH meters will read accurately over a range of 2.5-11 and beyond these ranges, accuracy cannot be assured. However, recently, instruments have become available that carry out calibration to allow correction for nonideal electrode behavior allowing accurate measurements between ranges of pH 1 and 13. 

Several SPM instrument manufacturers have corrected scanner nonideality by incorporating sensing devices in the piezoelectric elements. This modification allows real-time feedback control of the extension and eliminates many of the above problems. A number of different sensor types are... 

We will pursue the analysis, however, in the sense that it is more important to fit the initial portion of the breakthrough curve than the end (certainly so in design applications). One should also recognize that end-of-breakthrough data can be less reliable because they are more susceptible to influences of extraneous factors such as nonideal flow patterns, the sensitivity of the chemical analysis, and instrumental uncertainty. Continuing then... 

The transfer response error, when the measurement process has a nonideal transfer process, Uke a noncorrected nonlinear behavior of a length measuring instrument. 

Impedance interfaces often provide the facility for automatically switching between measurements of the sample to be measured and measurements of a low loss calibrated reference capacitor (Figure 3.2.10). An ideal reference capacitor would have a completely flat response (constant capacitance) across the entire frequency range. This ideal capacitor cannot be achieved in reality since there will always be some parallel resistance in the capacitor even though this can, in practice, be an extremely high value. However, the difference between an ideal and nonideal capacitor is sufficiently small for most purposes and the reference capacitor is a very useful tool that can be used to quantify errors due to cables and instrument measurement errors. The deviation of the capacitor from its ideal response due to cables... 

Several instrumental factors will effect nonidealities in the ILIT perturbation ... 

The laser flash method is used for thermal diffusivity measurements. In this technique, a laser is used to pulse one side of a test specimen uniformly. The temperature rise of the other side is measured using an infrared detector. This transient is then used to calculate the thermal diffusivity. While the technique is simple in concept, nonidealities such as heat loss from the front and back surfaces complicate the resulting data analysis, so that fairly complex models need to be used to extract the thermal dilfu-sivity. These calculations can be easily performed by the computers used to run the instruments. 

Figure 9.9 Nonideal instrument behavior a) hysteresis (b) deadband.

A challenging task faced by the analytical chemist when dealing with raw samples is extracting/isolating the analyte of interest from the sample matrix. It is fair to say that the majority of nonideal samples arrive in a format incompatible with most analytical instrumentation, and requires some degree of clean-up. Even well-defined samples (e.g. aqueous solutions) require basic filtration prior to analysis. Other desirable techniques may include liquid/liquid extraction and solid phase extraction. 

---