Line pair selection

A two-line method of emulsion calibration has been described by Churchill it makes use of a pair of spectral lines whose intensity ratio is known. The spectral line pair selected must possess certain characteristics, as follows ... 

In many cases this results in relative errors of 1% and lower. This is possible by simultaneous measurements of line and background intensities, leaving more room for improvements by internal standardization. This, however, was found to be very critical with respect to line pair selection. 

The choice of analytical lines and calibrations was dictated primarily by the requirements for quality control of purified rare earth elements. The line pairs selected covered the determination of the two rare earths of lesser and of greater atomic number than the matrix element. Because yttrium normally falls between different pairs of rare earths depending upon specific separation procedures used, calibrations were also established for the determination of yttrium in each of the lanthanide elements. In addition, line pairs were also selected for the determination of all other naturally occurring rare earths in yttrium because this element is often used as a carrier for other reu e earths. 

Figure 2.6. The tetrahedral structures of ice (a), (fc) are planes through sheets of selected oxygen nuclei (open circles), hydrogen nuclei (shotm in the insert as solid circles) are not shown in the main drawing. The insert illustrates the overlap of oxygen line pairs and the hydrogen nuclei, thus forming the hydrogen bonds (dotted lines)...

The principles of the selection of line pairs in the case of ICP-AES have been described well and in detail by Fassel et al. [404]. 

In 1925 Gerlach introduced the concept of the internal standard and listed the criteria that should be used for the selection of an internal standard with his homologous line pair concept. An internal standard can be a weak line of a major constituent of the sample, but more likely will be a substance, not already a constituent of the sample, that is added to all samples and standards at a constant concentration level. In practice, an internal standard, whose concentration is constant, produces a spectral emission line whose... 

With the aid of an internal standard, fluctuations in sample delivery and, to a certain extent, changes in excitation conditions can be compensated. To gain the full benefit of internal standardization, the intensities for the analytical and internal standard lines (and in trace determinations that of the spectral background near the analytical line) should be measured simultaneously. The selection of line pairs for ICP-AES has been investigated by Fassel et al. [248] and the background correction by Meyers and Tracy [249]. With internal standardization relative standard deviations below 0.1 % can be obtained. 

The selected line pairs are summarized in table 37D.7. The analyte lines differ from many of those previously suggested. This is not surprising because many of the earlier selected analysis lines represented a compromise between maximal power of detection and minimal spectral interference from other impurities in less pure samples. As purer base materials and better spectral data became available during the intervening years, continuing critical re-examinations of the earlier suggestions frequently led to a different choice of analysis line. 

Samples and standards, in the form of the ignited oxide powder, are mixed with graphite powder which acts as a spectroscopic buffer, and prevents the formation of molten globules which tend to be ejected from the electrode cup. This mixture is loaded into an undercut graphite cup electrode. Direct-current arc excitation is employed, and the spectra of samples and standards are recorded on suitable photographic plates. Intensity ratios of selected line pairs are determined photometrically. The concentration of the element sought is read from an analytical curve that relates the logarithm of the concentration to the... 

The spectrograph must have sufficient resolving power and linear dispersion to resolve the analytical lines from adjacent lines in the spectrum. A large grating instrument having a reciprocal linear dispersion of about 2.5 A per mm was found to be satisfactory. The exposure conditions are outlined in table 37D.6. Table 37D.7 lists the selected line pairs, their concentration ranges, and other photometric parameters. 

Nischwilz V, Berthele A, Michalke B. Rapid size fractionation of metal species in paired human serum and cerebrospinal fluid samples using ultrafiltration with off-line element selective detection. J Anal At Spec 2010 25 1130-7. 

In the determination of the rotational constants a pair of rotational lines is selected whose separation is dependent on the rotational constant of the... 

In fact this line of investigation, via direct chemical experiments was almost completely abandoned, and in the late 60 s it was proposed that RNA could be a possible precursor and this was termed the RNA-world [40j. RNA would be both the information carrier and the catalyst, and a fair amount of research, including the discovery of ribozymes and enzymes made of RNA provided a possible route to the last ancestor. Despite these advances, however, the fundamental question still remains What were the chemical steps that allowed the emergence of chemical complexity that could eventually lead to the onset of life, with or without the RNA world Implicit within this question are further questions such as - how was DNA, with four based pairs, selected as a information carrier, why are only 20 amino acids utilised by proteins and what mechanism lead to the chiral world Was there chemical evolution before bio-chemical evolution Was the origin of life inorganic ... 

After selecting the MEX command (selection 5) the comma delimited input consists of groups of basic events that are pairwise mutually exclusive. To extend the group to the next and subsequent lines, the first field must be blank. Up to 100 names per group are allowed with a new group starting when a non-blank character is encountered in the first field. The number of pairs that are mutually exclusive is n /[(n-2) 2 ], where n is the number of basic events in the group. An example for the plant blackout fault tree is DG-A-M,DG-B-M,DG-C-M (where means enter). 

In the internal standard method the intensity of the unknown line is measured relative to that of an internal standard line. The internal line may be a weak line of the main constituent. Alternatively, it may be a strong line of an element known not to be present in the sample and furnished by adding a fixed small amount of a compound of the element in question to the sample. The ratios of the intensities of these lines — the unknown line and the internal standard line — will be unaffected by the exposure and development conditions. This method will provide lines of suitable wavelength and intensity by variations of the added element and the amount added, due regard being paid to the relative volatility of the selected internal standard element. It is important to use as internal standard pairs only those lines of which the relative intensities are insensitive to variations in excitation conditions. The line selected as standard should have a wavelength close to that of the unknown and should, if possible, have roughly the same intensity. 

Several other approaches to solve the quantitation problem have been proposed. Hoppe et al. [2] determined y/ by calibrating it against constructs with known FRET efficiency. We and others [3, 6] have used data from a cell before and after acceptor photobleaching to relate the FRET-induced sensitized emission in the S channel to the loss of donor emission in the D channel by factors termed or G, respectively. For the CFP/YFP pair this works very well on confocal microscopes with a 514-nm Argon ion laser line, but on wide-held systems, selective acceptor photobleaching reportedly causes problems [ 14]. F inally, G can also be determined by comparison of several constructs that differ in FRET efficiency, a bit analogous to the Yellow Cameleon calibration described above [10,14],... 

Quasi-Newton methods start out by using two points xP and jfl spanning the interval of jc, points at which the first derivatives of fix) are of opposite sign. The zero of Equation (5.9) is predicted by Equation (5.10), and the derivative of the function is then evaluated at the new point. The two points retained for the next step are jc and either xP or xP. This choice is made so that the pair of derivatives / ( ), and either/ (jc ) or/ ( ), have opposite signs to maintain the bracket on jc. This variation is called regula falsi or the method of false position. In Figure 5.3, for the (k + l)st search, x and xP would be selected as the end points of the secant line. 

---