Resolution versus sensitivity

Calibration parameters are instrument parameters whose values do not vary with the type of experiment, such as peak widths, peak shapes, mass assignment, and resolution versus sensitivity. 

Resolution versus Sensitivity. Mass resolution is a compromise between ion intensity and peak width. In general, as the resolution is increased, the ion intensity decreases (see Section 13.3.2). 

Resolution versus Sensitivity. A quadrupole mass filter can be programmed to move through a series of RF and dc combinations. The Mathieu equation, which is used in higher mathematics, can be used to predict what parameters are necessary for ions to be stable in a quadrupole field. The Mathieu equations are solved for the acceleration of the ions in the X, Y, and Z planes. A selected mass is proportional to (dc x RF x inner radius)/(RF frequency). For a given internal quadrupole radius and radio frequency, a plot can be made of RF and dc values that predict when a given mass will be stable in a quadrupole field. This is called a stability diagram (Figure 13.3). RF and dc combinations follow the value shown... 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

The observed differences in the OH buildup rates at a given photolysis wavelength can be attributed to OH rotational levels having different parentages. For example, if higher-than-binary complexes yield mainly low N, this could be reflected in the product-state-selective measurements, even if the number of higher complexes is smaller than the number of binary complexes. In this case, there would be differences between results obtained with product state resolution versus those obtained by using a broadband probe. The latter would only be sensitive to the low N contribution in direct proportion to its fraction of the total product... 

FIGURE 8.5 UPLC versus HPLC for a compound and its hydroxyl metabolite. A threefold improvement in speed is observed while improving resolution and sensitivity. UPLC (1.7 pm particle size), RT 0.44 min, peak width 2s FWHM. HPLC (4pm particle size), RT 1.3-1.5 min, peak width 6 s FWHM. (From Wainhaus, S. et al., Amer. Drug Discov., 2, 6, 2007. With permission.)... 

FIGURE 5.25. Diffraction of light by a homeotropic liquid crystal in the spatially nommiform harmonic field. Below Relative modulation characteristics max versus spatial frequency u)s of the field, u max and 0 )max characterize the resolution and sensitivity of the liquid crystal layer, respectively. Curves 1-6 correspond to different anchoring energies Ow = V0dlK33 (1) 0.133 (2) 0.665 (3) 1.33 (4) 6.65 (5) 66.5 (6) 133. 

As the enzyme itself is usually the focus of interest, information on the behavior of that enzyme can be obtained by incubating the enzyme with a suitable substrate under appropriate conditions. A suitable substrate in this context is one which can be quantified by an available detection system (often absorbance or fluorescence spectroscopy, radiometry or electrochemistry), or one which yields a product that is similarly detectable. In addition, if separation of substrate from product is necessary before quantification (for example, in radioisotopic assays), this should be readily achievable. It is preferable, although not always possible, to measure the appearance of product, rather than the disappearance of substrate, because a zero baseline is theoretically possible in the former case, improving sensitivity and resolution. Even if a product (or substrate) is not directly amenable to an available detection method, it maybe possible to derivatize the product with a chemical species to form a detectable adduct, or to subject a product to a second enzymatic step (known as a coupled assay, discussed further later) to yield a detectable product. But, regardless of whether substrate, product, or an adduct of either is measured, the parameter we are interested in determining is the initial rate of change of concentration, which is determined from the initial slope of a concentration versus time plot. 

We have noted the noise-sensitivity problem of the simple inverse filter and introduced modifications to alleviate these difficulties. Modifications yielded different functional forms for y(co). The convenient single-step property of the basic method was nevertheless retained. This property contrasts with the need for possibly arbitrary stopping criteria when we use iterative methods, which are computationally more expensive. The iterative methods do, however, allow the user to control the signal-to-noise versus resolution tradeoff by stopping the process when the growth of spurious... 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

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