Detector attributes

Figure Cl.5.9. Vibrationally resolved dispersed fluorescence spectra of two different single molecules of terrylene in polyetliylene. The excitation wavelengtli for each molecule is indicated and tlie spectra are plotted as the difference between excitation and emitted wavenumber. Each molecule s spectmm was recorded on a CCD detector at two different settings of tire spectrograph grating to examine two different regions of tlie emission spectmm. Type 1 and type 2 spectra were tentatively attributed to terrylene molecules in very different local environments, although tlie possibility tliat type 2 spectra arise from a chemical impurity could not be mled out. Furtlier details are given in Tchenio [105-1071.

The fire death rate in the United States is decreasing, dropping from a rate of 76 per million in the 1940s, when most constmction and decorative products were made of natural materials, to 29 per million in the 1980s, by which time, PVC had replaced natural materials in numerous appHcations (189). This downward trend can be attributed in large part to improved building codes and the broader use of sprinkler systems and smoke detectors. However, the increased use of more fire-resistant materials, such as PVC, deserves part of the credit for this improvement. 

Ideally, an observatory would install perfect detectors in the focal plane of its instruments. What makes a perfect detector The attributes of an ideal detector and the performance achieved by today s technology are given in Table 1. Optical and infrared detectors are nearly ideal in several ways ... 

Delta function response - Over most of the wavelengths of interest, optical and infrared detectors produce one photoelectron for every detected photon, which provides a one-to-one correspondence between detected photons and photoelectrons. This means that the detector response is exactly linear to the intensity incident on the detector - an attribute that allows astronomers to precisely remove sky background and electronic bias to accurately measure the intensity of the astronomical object. 

Table 1. Attributes of an ideal detector and performance achieved by the state-of-the-art detectors in astronomy.

Other detector imperfections - Detectors also exhibit a wide range of other features that make these devices less than perfect. A major attribute that describes performance is cosmetic quality. Due to defects in the material or fabrication errors, some pixels of an array can exhibit... 

A very important attribute of light sensitive materials is their absorption depth. The absorption depth is the length of material that will absorb 63.2% of the radiation (1/e of the energy is not absorbed). After two absorption depths, 87% of the light has been absorbed, and after three absorption depths, 95% has been absorbed. To make an efficient detector of light, the material thickness should be several times the absorption depth. 

With proper design, fabrication and clocking, most of these negative aspects can be overcome, but there are usually some bad columns due to blocked columns or hot pixels. There are many positive aspects of charge transfer, giving the CCD some very good and unique attributes vis-a-vis an infrared detector ... 

A more general discussion of their attributes will, hopefully, provide an insight into some of the ways in which the mass spectrometer may be used to advantage as a detector. 

The major attribute that distinguishes planar techniques from column chromatography is that in the former separation and detection are discontinuous ( offline )- In column chromatography analytes are carried through the entire column and monitored at the end, usually by flow-through detectors measuring changes in some physical characteristics of the effluent (optical... 

When a model for a CUORICINO detector (see Section 15.3.2) was formulated and the pulses simulated by the model were compared with those detected by the front-end electronics, it was evident that a large difference of about a factor 3 in the pulse rise time existed. This discrepancy was mainly attributed to the uncertainty in the values of carrier-phonon decoupling parameter. For the thermistor heat capacity, a linear dependence on temperature was assumed down to the lowest temperatures. As we shall see, this assumption was wrong. 

The flow cell is the most important component of a flow injection manifold for CL measurements since maximum radiation should be generated while the solution is flowing in front of the detector. Other attributes of the flow cell are the small dead volume of the cell to allow fast and effective washing between injections... 

Fig. 11.16. Representation of three tandem mass spectrometry (MS/MS) scan modes illustrated for a triple quadrupole instrument configuration. The top panel shows the attributes of the popular and prevalent product ion CID experiment. The first mass filter is held at a constant m/z value transmitting only ions of a single mlz value into the collision region. Conversion of a portion of translational energy into internal energy in the collision event results in excitation of the mass-selected ions, followed by unimolecular dissociation. The spectrum of product ions is recorded by scanning the second mass filter (commonly referred to as Q3 ). The center panel illustrates the precursor ion CID experiment. Ions of all mlz values are transmitted sequentially into the collision region as the first analyzer (Ql) is scanned. Only dissociation processes that generate product ions of a specific mlz ratio are transmitted by Q3 to the detector. The lower panel shows the constant neutral loss CID experiment. Both mass analyzers are scanned simultaneously, at the same rate, and at a constant mlz offset. The mlz offset is selected on the basis of known neutral elimination products (e.g., H20, NH3, CH3COOH, etc.) that may be particularly diagnostic of one or more compound classes that may be present in a sample mixture. The utility of the two compound class-specific scans (precursor ion and neutral loss) is illustrated in Fig. 11.17.

Typically, NIR optical fiber response times (/95%) for potassium solutions less than 5 x 10 M were less than 5.0 sec however, longer response times were observed as the concentration increased. For concentrations above a 1 x 10 3A7 K+,a typical t % was 1.25 min. The longer fluorescence response of the optical fiber detector at high metal concentrations is attributed to the thickness of the polymer matrix and the diffusion coefficient of potassium (10 10 cm2/sec). 

An HPLC detector is often a modified spectrophotometer equipped with a small flow cell, which monitors the concentration (or mass) of eluting analytes.Common detectors in the pharmaceutical laboratory are listed in Table 2 with their respective attributes and sensitivity levels. A recent survey found that 85% of pharmaceutical applications use absorbance detectors such as UVA/ is or photodiode array detectors (PDA). These two detectors are covered in more detail in this section. 

TABLE 2 Common HPLC Detectors and Their Attributes... 

Peak purity tests are used to demonstrate that an observed chromatographic peak is attributable to a single component. Mass spectrometry is the most sensitive and accurate technique to use for peak purity evaluation because of the specific information derived from the analysis. However, a good number of HPLC methods use mobile phase conditions that are incompatible with mass spectrometry detection. In this case, PDA spectrophotometers using peak purity algorithms may be used to support the specificity of the method. Almost all commercially available diode array detectors are equipped with proprietary software that will perform these calculations. Although this technique is more universal in application to HPLC methods, the data provided is neither particularly... 

---