Spectrum bias

Lachs MS, Nchamkin I, Edelstein PH, Goldman J, Feinstein AR, Schwartz JS. Spectrum bias in the evaluation of diagnostic tests lessons from the rapid dipstick test for urinary tract infection. Ann Intern Med 1992 117 135-140. 

At the other extreme of the stereoselectivity spectrum of the Bucherer-Bergs reaction, the steric bias is sometimes not powerful enough to exert any selectivity at all, as exemplified by the conversion of 37 — 38. " Amino acid 38 was produced as a 1 1 mixture of two diastereomers. 

Figure 15-13. (a) Pholocurrenl action spectra at room temperature for a thick photodiode (ITO/780 nm MEH-PPV/A1) under illumination through the ITO electrode, under forward bias (dash dot) and reverse bias (solid line), with the room temperature absorption spectrum (dashed line) shown for comparison. (b) Pholocurrenl action spectra at room temperature for a thin photodiode (ITO/120 nm MEH-PPV/A1) under illumination through the ITO electrode, under forward bias (dash dot line) and reverse bias (solid line), with the room temperature absorption spectrum (dashed line) shuwn for comparison (reproduced by permission of the American Physical Society from Ref. 176)). 

Figure 15-12. Spectrally resolved pliotocurrent of a ITO/PPV/Mg photodiode at dilTcrcnt bias after correction for dark current, light source, and monochromator response, and normalization to the same peak value. The broken line is the normalized absorption spectrum of PPV (reproduced by permission of the Institute of Physics from Ref. 143)).

Figure 16-42. Semi log ploi of current density (0) and luminance (O) of an ITO/Oocl-OPV5 (ISO nm)/Oocl-OPV5-CN" (45 nm)/AI double-layer device as a function of bias voltage. Inset double-layer electroluminescence spectrum.

Second generation COMT inhibitors were developed by three laboratories in the late 1980s. Apart from CGP 28014, nitrocatechol is the key structure of the majority of these molecules (Fig. 3). The current COMT inhibitors can be classified as follows (i) mainly peripherally acting nitrocatechol-type compounds (entacapone, nitecapone, BIA 3-202), (ii) broad-spectrum nitrocatechols having activity both in peripheral tissues and the brain (tolcapone, Ro 41-0960, dinitrocatechol, vinylphenylk-etone), and (iii) atypical compounds, pyridine derivatives (CGP 28014,3-hydroxy-4-pyridone and its derivatives), some of which are not COMT inhibitors in vitro but inhibit catechol O-methylation by some other mechanism. The common features of the most new compounds are excellent potency, low toxicity and activity through oral administration. Their biochemical properties have been fairly well characterized. Most of these compounds have an excellent selectivity in that they do not affect any other enzymes studied [2,3]. 

Figure 5.49. (a) STM image (unfiltered) of the initially sodium-contaminated Pt(l 1 l)-(2x2)-0 adlattice (b) corresponding Fourier transform spectrum (c) Fourier-filtered STM image of the overlapping Pt(l 1 l)-(2x2)-0 and Pt(111)-(12x12)-Na adlayers (bias Ut = 80 mV, tunelling current I, = 10 nA, total scan size 319 A).78 Reprinted with permission from Elsevier Science. 

The signal-to-noise ratio can be increased by treating the data so as to bias the spectrum in favor of the signals and against the noise. This can be done by multiplying the FIDs by the proper apodization functions. What would happen if the spectrum is recorded without apodization ... 

The first term measures the difference between the data and the fit, KF. The second term is a Tikhonov regularization and its amplitude is controlled by the parameter a. The effect of this regularization term is to select a solution with a small 2-norm 11 F 2 and as a result a solution that is smooth and without sharp spikes. However, it may cause a bias to the result. When a is chosen such that the two terms are comparable, the bias is minimized and the result is stable in the presence of noise. When a is much smaller, the resulting spectrum F can become unstable. 

The very high resolution for the ESR spectrum of cob(II)alamin in the enzyme system is undoubtedly due to the fact that all the coenzyme molecules are bound in an identical environment at the enzyme active site. This results in a homogeneous cobalt-benzimidazole geometry, because both identical binding sites, solvent, and solute molecules can no longer approach the Bia-molecule closely. In addition, the enzyme bound cob(II)alamin molecules are more isolated from one another and thus relaxation due to spin-spin interactions is less effective in broadening spectral lines. 

As you are no doubt aware, integrals are one of the key parameters in the interpretation of proton spectra and are pivotal in quantification. They measure the area under a peak and this is directly proportional to the number of protons (in the case of proton NMR) in that environment. Most software will automatically try to identify the peaks in your spectrum and integrate them for you. If you need to do it yourself, then it is a fairly trivial matter of defining the start and end point of the integrals of interest. The only complication is that you may need to tweak the slope and bias of the integral. This should be unnecessary if you have got the phase and baseline of your spectrum correct. If you find that you need to adjust slope and bias, we suggest that you go back and try to sort out baseline and phase a bit better. 

In order to ameliorate the sharply sloping background obtained in an STS spectrum, the data are often presented as di,/dFh vs. Vb, i.e. the data are either numerically differentiated after collection or Vb has a small modulation applied on top of the ramp, and the differential di,/d Vb is measured directly as a function of Vb. The ripples due to the presence of LDOS are now manifest as clear peaks in the differential plot. dt,/dFb vs. Vb curves are often referred to as conductance plots and directly reflect the spatial distribution of the surface electronic states they may be used to identify the energy of a state and its associated width. If V is the bias potential at which the onset of a ripple in the ijV plot occurs, or the onset of the corresponding peak in the dt/dF plot, then the energy of the localised surface state is e0 x F. Some caution must be exercised in interpreting the differential plots, however, since... 

Standard data reduction, i.e. bias and flat field correction, has been performed with Iraf. The Iraf task APEXTRACT/APALL was used to extract the spectra, with interactively selected background sampling, in order to avoid contamination for the star spectrum. The wavelength calibration has been done using daily He, Ne, HgCd arcs, and, in order to improve the calibration, wavelengths values for the transitions used were taken from http //physics.nist.gov/. 

Fig. 3 Energy diagram for an M-A-M diode showing elastic and inelastic tunneling processes (top). The HOMO (n) and LUMO (71 ) orbital energies and a few vibrational levels are indicated. Applied bias energy (eV) is just sufficient to allow inelastic tunneling with excitation of the first vibrational level, eV = hv. Also shown (bottom) are the I(V) curve, conductance- / curve, and the IETS spectrum that would result from both elastic processes and the first inelastic channel. (Reproduced by permission of the American Chemical Society from [19])...

The invariance of IETS in an M-A-M junction vs an M-I-A-M device is exceptionally well demonstrated by the work of Reed [30], Figure 7 shows the Au-alkanedithiol-Au structure he used to create a single barrier tunnel diode. The IET spectra obtained from this device were stable and repeatable upon successive bias sweeps. The spectrum at 4.2 K is characterized by three pronounced peaks in the 0-200 mV region at 33,133, and 158 mV. From comparison with previously reported IR, Raman, and high-resolution electron energy-loss (HREEL) spectra of... 

Fig. 9 OMT bands for NiOEP, associated with transient reduction (1.78 V) and transient oxidation (—1.18 V). Data obtained from a single molecule in a UHV STM. The ultraviolet photoelectron spectrum is also shown, with the energy origin shifted (by the work function of the sample, as discussed in [25]) in order to allow direct comparison. The highest occupied molecular orbital, n, and the lowest unoccupied molecular orbital, %, are shown at their correct energy, relative to the Fermi level of the substrate. As in previous diagrams,

T(E) spectrum. When the Fermi level EF is located between the D-HOMO and the A-LUMO resonances, a large rectification effect is observed where T(EF) reaches almost 104. At a low 100 mV bias voltage and in a forward polarity, the tunnel current intensity reached around 1 nA. The T(E) spectrum of Fig. 2b was calculated using the ESQC technique associated with a semiempirical description of the tunnel junction [110]. The full valence MO structure of the junction is taken into account in the calculation. 

Figure 15.10 Normal-incidence electroreflectance spectrum of Ag(100) in 0.5 M NaF solutions for various bias potentials. Reprinted from Ref. 10 with...

The Csv symmetry of 14 was reflected in its five-line 13C NMR spectrum. X-ray crystallographic analysis revealed further that the all-equatorial conformation is adopted in the solid state (Fig. 3-1). This bias persists as well in solution and may be a consequence of more favorable dipole-dipole interactions. The parallelism between 14 and its isomer 24, whose ground-state conformer also projects all three C-0 bonds in the equatorial plane (Fig. 3-2), is striking.8... 

Although mass bias effects relative to total ion intensity may be corrected using a working curve (Fig. 12), anomalous mass bias in natural samples due to the presence of other elements cannot be corrected. Figure 13 shows the effects of anomalous mass bias in a 400 ppb Fe ultra pure standard that has been doped with varying concentrations (up to 75 ppb) of Mg, Al, or La. Carlson et al. (2001) noted a similar effect of Al on the isotope composition of Mg standard solutions. These matrix elements were chosen because they would not produce isobars on the Fe mass spectrum (Fig. 13D) and for their spread in atomic mass Mg and Al both have masses less than Fe and La is greater. Additionally Mg and Al are major elements (7 and 3" most... 

During real-time operation, statistical/-tests can be used to determine whether the slope and bias correction calculated for a particular unknown spectrum are within an expected range [108]. If not, then a warning can be issued indicating that the current sample is not appropriate to apply to the model. 

At one end of the spectrum, the event may be a simple dosage problem which could be an error on the part of the prescriber or an unanticipated hypersensitivity for that particular patient. At the other end of the spectrum, is an uncommon, serious adverse reaction not revealed in premarketing clinical trials. Somewhere between those two extremes are more or less serious adverse events which are not entirely unexpected but appear to be more common than is accepted for comparable products in the same therapeutic category. This maybe a real increase in frequency or may be due to patient selection bias. The later has arisen with new products which claim a lower incidence of certain adverse reactions which encourages doctors to precribe them preferentially for patients who have suffered such reactions with older products. 

---