Using Two Detectors

If there is no possibility to cover the complete range with one detector, there may be the possibility to use two detectors which are placed in different distances from the sample. In the worst case the experiment has to be performed several times with different setups. 

If we were to use two detectors, one specific for only A and one specific for only B, the signals would appear, shifted In time, as shown In Figure 6a. The position of the Injection valve at the time these detector values were recorded Is also shown. Similarly, If we were to use on non-specific detector, the sum of the signals due to A and B would yield the detector signal labeled In Figure 6a as Combined Signal. 

Homing, S. R., Fange, O., Wieghaus, A., Malek, R., and Senko, M. W. (2004). Simultaneous acquisition of two ion signals using two detectors in a hybrid ion trap/fourier transform ICR mass spectrometer. In Proceedings of the 52nd ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN. 

Enantiomerically pure forms of 3-substituted-l,4-benzodiazepin-2-ones show significant differences in their pharmacological activity. Compounds (V) and (VIII) are the tranquilizers oxazepam and lorazepam while (VI) and (VII) are structural analogs of oxazepam. Discriminations are possible using two detectors in sequence after separation on a chiral column. 

As we saw in Section 3.4, quadrature phase detection discriminates between frequencies higher and lower than the pulse frequency, but it does not prevent foldover from frequencies higher than the Nyquist frequency. For a desired spectral width FT, there are two common methods for carrying out quadrature phase detection, as was indicated in Section 3.4. One method uses two detectors and samples each detector at FT points per second, thus acquiring 2 FT data in the form of FT complex numbers. The other (commonly called the Redfield method ) requires only a single detector and samples at 2 FT points per second while incrementing the phase of the receiver by 90° after each measurement. (In two-dimensional NMR studies, a variant of this method is usually called the rime-proportional phase incrementation, or TPPI, method.) Because these methods result in quite different treatment of folded resonances, we now consider these approaches in more detail. 

The Compton continuum, present in gamma energy spectra recorded either by a Nal(Tl) scintillator or by a Ge detector, is a nuisance that impedes the analysis of complex spectra. It is therefore desirable to eliminate or at least reduce that part of the spectrum relative to the gamma energy peak. One way to achieve this is to use two detectors and operate them in anticoincidence. Such an arrangement, known as the Compton-suppression spectrometer, is shown in Fig. 12.9. A large NaI(Tl) scintillator surrounds a Ge detector, and the two detectors are operated in anticoincidence. The energy spectrum of the central... 

A novel technique in UV detection was used by Mayer and Miller and Maclean, who measured proteins and peptides at 191-194 nm, thereby increasing the applicability of the UV absorption method. A number of workers have shown the potential of monitoring the eluent at two or more wavelengths simultaneously . This can be done by using two detectors in series, or by combining techniques in one detector. The extra information can be used for verification of tentative identification or for resolving partially merged peaks . 

Of course this analysis applies equally well to our spectrometers, in which case W represents the masks used and the unknown spectrum. The only restriction is that we cannot design masks with entries that are -1, so the results for chemical balance designs do not apply to the optical case. We could simulate a chemical balance by using two detectors and taking the difference of their readings (this is analyzed in Section 3.5.4. of ref. 2), but the results cannot be compared directly with those obtained for masks of O s and I s alone, since using two detectors produces an unfair advantage. 

Using two detectors simultaneously, one has to detect the 122 keV y quanta coming from the Mossbauer source as a start signal. This actually marks the birth of the 14.4 keV Mossbauer excited level (see the decay scheme inO Fig. 25.7). Then, with the other detector, the 14.4 keV y quantum is detected, and the elapsed time measured (stop signal). With this method, Mossbauer spectra can be recorded in any time interval after the nuclear decay of Co. Recording characteristic X-rays following the electron capture, part of the aftereffect events can be filtered off. Due to the coincidence technique, very low activities can only be used, and therefore these measurements are rather time consuming (several weeks or months). 

FTDI. The authors remarked that the instrumental advances and the associated open-source project recently reported can spread out the use of this universal detector for food analysis. Furthermore, the inherent portability of the C D system could make it an ideal instrument for on-site food testing. One example that combines creativity and the instrumental simplicity of this home-made conductivity detection is the analysis of monoalkyl carbonates in carbonated alcoholic beverages (sparkling wine, beer, and mixed drinks) [151]. By using two detectors, the authors not only demonstrated the presence of monoethyl carbonate (MFC) in the selected drinks but also proposed that the low pH values were responsible for the larger concentrations of MEC observed in lager beer and mixtures of rum and cola. In addition, the possibility to control not only a C D detector but also a series of valves using open-source software (Arduino) has been recently demonstrated [152]. 

Determination of Chemical Composition Distribution by Dual Detection Based on detection with independent concentration detectors, this approach provides information on the chemical composition of copolymers or polymer blends and the variation with the molar mass [26]. An independent detector signal is required for each comonomer in the macromolecule. Many results have been published for chemically heterogeneous samples using two detectors. Obviously, there is a practical limit (four signals have been reported so far), as there are not many detectors to choose from. In addition, the approach is limited by the contribution of each detector to band broadening. 

Fig. 4.39. A segmented linear quadmpole ion trap with mass-selective radial ejection capability. Applying higher potential to the front and back sections creates an axial trapping potential for ions in the center section. Radial trapping is again provided by the RF quadmpolar field. Ions are exiting through the slot in one of the four rods. The commercial design offers doubled sensitivity by using two detectors at opposite rods. Adapted from Ref. [109] by permission. Elsevier Science, 2002.

A compact detector system for this is the vertically integrated, or sandwich structure using two detectors - one mounted on top of the other. The top detector acts as a spectral filter, limiting the wavelengths that pass to the bottom detector. 

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