No external references

Because the electrical circuit is closed inside the sensor, no external reference electrode is necessary and the Severinghaus-type electrode can be used for measurement in either gaseous or liquid samples. It is important to remember, however, that the potential of the internal reference electrode must remain constant. In principle, it would be possible to use a liquid junction but it would add to the complexity of the design. Because the counterion resulting from the dissociation equilibrium is the only interfering ion, and because it is present in a very low concentration, it is possible to ascertain the constancy of the reference potential by careful choice of the internal electrolyte. Thus, for example, in the CO2 electrode the internal electrolyte is O.lMNaHCOs and 0.1 M NaCl and Ag/AgCl is used as an internal reference element. 

Ammonia has been employed mostly for cathodic reactions, but some oxidations [330,333] have been carried out in this medium, although the potential range in the anodic direction is quite small. The anodically limiting reaction is oxidation to nitrogen and protons [340] the cathodically limiting reaction is the transfer of electrons to the solvent, which occurs at about —2.3 V (versus Hg pool electrode) in a saturated solution of TBAI. In the elecltrolytic generation of solvated electrons the potential is determined by the surface concentration of electrons and no external reference electrode is needed. 

So, how can we tell the difference between an underfitted, good, or over-fitted model when we have no external reference points This problem is the basis of model validation. 

Fig. 23. The SNV corrected spectra of the same data in Fig. 21. Note that this is very similar to the results of MSC however, no external reference ( ideal spectrum) is required in order to calculate the correction.

The next step is to apply detrending. As with SNV, each spectrum is treated independently of the others in the training set so that there is no external reference. It is a relatively simple calculation A linear least squares regression is used to fit a quadratic polynomial to the responses in the spectrum. This curve is then subtracted from the spectrum to give the result. As mentioned earlier, the quadratic curvature component attempts to correct for the effects of particle size and sample packing (Fig. 24). 

Dilute solutions of iodine, e.g. 0.0001 M, may be titrated similarly with standard thiosulphate. The supporting electrolyte consists of 1.0 M hydrochloric acid and 0.004M potassium iodide. No external e.m.f. is required when an S.C.E. is employed as reference electrode. 

Two different and possibly complementary approaches have been explored. One utilizes a panel of quantifiable internal reference standards (QIRS), which are common proteins present widely in tissues in relatively consistent amounts.11,22 In this instance because the reference proteins are intrinsic to the tissue they are necessarily subjected to identical fixation and processing, and incur no additional handling or cost, other than synchronous performance of a second IHC assay (stain), such that the intensity of reaction for the QIRS and the test analyte can be compared by IA, allowing calculation of the amount of test analyte (protein) present on a formulaic standard curve basis. The other approach seeks to identify external reference materials and to introduce these into each step of tissue preparation for cases where IHC studies are anticipated in this instance the logistical issues of production, distribution, and inclusion of the reference standard into all phases of tissue processing also must be considered, along with attendant costs. 

Recent work [6 has been directed towards the simultaneous monitoring of potential and current noise, where the current noise signal is generated by coupling two nominally Identical electrodes with a zero resistance ammeter (ZRA), and the potential noise of the couple is monitored with respect to a reference electrode. In this manner no externally applied signal is required. 

When a non-centrosymmetric solvent is used, there is still hyper-Rayleigh scattering at zero solute concentration. The intercept is then determined by the number density of the pure solvent and the hyperpolarizability of the solvent. This provides a means of internal calibration, without the need for local field correction factors at optical frequencies. No dc field correction factors are necessary, since in HRS, unlike in EFISHG, no dc field is applied. By comparing intercept and slope, a hyperpolarizability value can be deduced for the solute from the one for the solvent. This is referred to as the internal reference method. Alternatively, or when the solvent is centrosymmetric, slopes can be compared directly. One slope is then for a reference molecule with an accurately known hyperpolarizability the other slope is for the unknown, with the hyperpolarizability to be determined. This is referred to as the external reference method. If the same solvent is used, then no field correction factor is necessary. When another solvent needs to be used, the different refractive index calls for a local field correction factor at optical frequencies. The usual Lorentz correction factors can be used. 

In another report, unlike CE/EC devices previously reported, not only the working electrode was integrated, but the reference and counter electrodes were also patterned (Pt/Ti) on the glass chip. No external wire electrodes were used. This method has minimized dispersion at the column exit [747]. The electrodes were also situated under a shelf (due to the cover plate) so that the detection volume was restricted and the dispersion was further reduced. The stability of the chip was found to be more than 2 months. LOD of dopamine and catechol were reported to be in the 4-5 pM range [747]. 

An external reference is a compound placed in a separate container from the sample. For liquid samples, an external reference compound is often placed as a neat (undiluted) liquid either in a small sealed capillary tube inside the sample tube or in the thin annulus formed by two precision coaxial tubes. In either case, the usual rapid sample rotation (Section 3.2) makes the reference signal appear as a sharp line superimposed on the spectrum of the sample. An external reference is advantageous in eliminating the possibility of intermolecular interactions or chemical reaction with the sample. Also, there are no problems with solubility of the reference in the sample solution. There is, however, a serious difficulty raised by the difference in bulk magnetic susceptibility between sample and reference. 

Fig. 3 The electrical double layer at a mercury drop shorted to a reference electrode and with no external applied potential.

Since the definition of E° in Eq. (96) refers to standard conditions, that is, conditions in which no external electrical field applies on the reactant or the product, it is more convenient to consider the act of electron transfer as a succession of three individual steps as outlined in Scheme 5, which is reminiscent of that in Scheme 3 established for the homogeneous analogous situation (Sec. II.C.l). In Scheme 5, Rq or Pq relates to R or P in the closest plane to the electrode where no electrical potential applies, that is, at the end of the diffuse layer, denoted Xq in Fig. 14d. Rd, and Pelectron transfer Xd, usually considered close to or slightly within the OHP, where an electrical potential Os (i.e., the electrical potential at the electron transfer site) applies. 

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