Dual-channel detection

The assignment or recognition of peaks in different chromatograms may be aided if extra qualitative information about the solutes is obtained from the detector. The simplest way to obtain more information is to combine two detectors in series. Many combinations are possible, but some limitations arise (see for example ref. [586])  

The time delay between the two detectors should be minimal. 

The first detector should be non-destructive and should not contribute significantly to the band-broadening. 

The two detectors should have similar types of sensitivities (e.g. two concentration sensitive detectors or two mass flow sensitive detectors). 

Ideally, the degrees of sensitivity of the two detectors should also be comparable. Because of this last reason, it is not attractive to combine a hot wire detector with a flame ionization detector in GC (a combination that also conflicts with the third limitation above) or a differential refractometer with a UV spectrometer in LC. 

---

SM-FRET experiments are typically performed by using a dual-channel detection scheme. More specifically, one photo-excites the donor with CW radiation or a train of pulses, while simultaneously detecting the fluorescence photons from the donor and acceptor in a selective manner. The fraction of photons detected in the acceptor channel, over a given time averaging window of length Tw, provides a direct measure of the time-averaged FRET efficiency, which we will denote by E(Tw)- One may then define a time-averaged and TV-dependent donor-acceptor distance, which will be denoted by R)tw, such that... 

Hence, the use of dual-channel detection increases the possibilities for peak assignment in optimization processes. However, complications are caused by variations in the baseline [584] and the effect of solvent composition on the UV spectrum may now become even more serious than in the case of single-channel detection. This may occur for instance if one wavelength is selected on a rising flank in the UV spectrum, and the second one on a descending flank. In any case, the selection of the two most suitable wavelengths is one of the most critical factors. Unfortunately, the wavelength selection is usually quite arbitrary. 

Chen, Y. C. and Lo, J. G., Gas chromatography with flame ionization and flameless sulfur chemiluminescence detectors in series for dual channel detection of sulfur compounds, Chromatographia, 43, 522-526, 1996. 

More interesting is the use of two different detectors in parallel at the exit of a GC column—so called dual channel detection. The detectors chosen should have major differences in sensitivity for different classes of compounds. Both signals are recorded simultaneously producing parallel chromatograms like those shown in Figure 8.2. Identifications can be made... 

From the practical point of view, dual-channel phase detectors operated in quadrature appear to be the best hardware-detection choice. Unlike other techniques, phase detection is sensitive to RF frequency offset from resonance and to the RF phase which, on the one hand, makes the signals more complex to use (as well as and more sensitive to instrument instabilities) but, on the other hand, leads to a number of important advantages, such as ... 

The most recent generation of NDIR analyzers have evolved to satisfy the frequently harsh industrial environments encountered. These analyzers utilize solid-state sensors for the detection of infrared radialion. Most frequently used sensors are lead selenide (PhSc). thermopiles, or pyroelectric detectors. The gas analyzers generally are configured as single-path instruments, dual-beam with a reference palh. or dual-channel with a reference filter. 

Scalbert et al. (1989) described the chromatographic separation of cyanidin and delphinidin from methanolic extracts of oak heartwood using a C-18 Novapak column. The elution solvent was a mix of two solvents A and B that changed in composition from 0-100% B in a linear fashion over a period of 20 min. Solvent A was a mixture of 94 5 1 H20/methanol/H3P04, and solvent B was 99 1 methanol/H3P04, at a flow rate of 1 7mL/min. In this case a dual-channel spectrophotometer was used with two wavelengths selected for detection 280 and 530 nm. Under these conditions, and based on reference compounds, delphinidin eluted after 13.9 min and cyanidin after 14.8 min. 

These three examples illustrate technology developments over time (dual-channel detector, diode array detector, mass spectrometer). Note that while the overall methodology is very similar (methanolic extracts, methanol-based, acidified solvents used for HPLC, detection of eluted compounds), the exact conditions for successful separation need to be defined for each system. 

The HPLC chromatogram containing monomeric and dimeric lignols from spruce wood can be seen in Figure 2. Dual channel UV-detection was used during the HPLC analysis with one channel set at 280 nm to detect all lignin-related products and the other channel at 350 nm to monitor the presence of leucochromophores. Most... 

Sulfur dioxide 0-20 ppm 0.5 ppb Fluorescence dual-channel ratiometric phase detection or DOAS open path... 

The method development process will be aided if we are able to use sophisticated instrumentation (see also section 1.7.2). Automated injection and data handling will allow a number of experiments to be performed without the requirement of an analyst being present. Moreover, we have seen in chapter 5 (section 5.6) that the use of sophisticated detection techniques (dual-channel or multi-channel detectors) may be of help in the optimization process. 

Dual Detectors. Most dual detectors are run in parallel, the column effluent being split and run through both of them simultaneously. In GC the technique is known as dual channel GC usually, one of the detectors chosen is universal and the other is highly selective. Figure 6.6 shows the analysis of a commerical gasoline sample with dual detection by flame ionization (FID) and electron capture (ECD). The FID detects all the hydrocarbons, but the ECD is selective for the alkyl lead additives in gasoline and permits their detection without interference from the hydrocarbons. 

Pederson described a specific HPLC method for the determination of dipyridamole in serum [74]. The HPLC system used was a Waters model 600 liquid chromatograph equipped with a U6K injector, a pBondapak Ci8 column (30 cm x 39 mm) (10 pm), and a model 440 dual channel filter absorbance detector in conjunction with a Tarkan W + W 600 recorder. The mobile phase was a 75 25 mixture of methanol and a 0.02 M solution of sodium acetate (adjusted to pH 4 with acetic acid). The solvent flow rate of 2 mL/min was produced by an applied pressure of approximately 2000 p.s.i. Detection of the analyte was made at the UV absorption maximum of 280 nm. 

The various probe beams can be coupled into the same singlewavelength, dual-channel pulse-probe transient optical absorption set-up. A one-meter-long optical delay line is used to control the variable time delay between the electron and the probe pulses. Approximately half of the probe beam is deflected onto a reference photodiode while the other half of the beam is slightly focused into the sample, which is placed in front of the output window of the accelerator. Subsequently, the probe beam is then transported to the sample photodiode. (Alternatively, in some laboratories the probe and reference beams are transported into the detection room by long, low-OH silica optical fibers in order to reduce electronic noise pickup on the detector signal cables.)... 

Figure 3 Dual channel DRI and UVl Visible detection SEC traces for PMMA from catalytic chain transfer polymerisation...

UV-visible spectra were obtained using a Cary 118C spectrophotometer. Insertion probe mass spectra were obtained with a Dupont 21-49IB mass spectrometer, the FAB spectra with a VG MM-ZAB instrument, and the FD spectra with a JEOL DX-300 mass spectrometer. High pressure liquid chromatography measurements were made with a Waters instrument fitted with dual channel (405 and 546 nm) optical detection and using C18 analytical and semipreparative columns. 

Peak ratio from two detection wavelengths from a dual-channel IJV/Vis detector... 

Ladd et al. reported SPR sensor-based detection of hCG [52], exploiting a DNA-directed antibody immobilization method. The immobilization consisted of non-covalent attachment of streptavidin to a biotinylated SAM followed by binding of biotinylated oligonucleotides to available streptavidin binding sites. Antibodies chemically modified with oligonucleotides with a complementary sequence were finally attached to this surface via DNA hybridization. The detection limit for direct detection of hCG in buffer by a dual-channel SPR sensor with wavelength modulation was determined to be... 

An improvement introduced in the LPAS operation is a simultaneous calibration of the detection signal to increase the precision of tte measurement [40]. The photoacoustic signal of the sample is normalu by the signal of a reference standard solution of known absorbance. Both sample and reference solutions are measured simultaneously by the dual channel LPAS set-up (see Fig. 5). By this method, the influence of long term instability of laser energy and laser beam profile on the photoacoustic signal are eliminated and at the same time the spectrum is converted to absolute absorbance units. 

HabiboUahi, P., et al. Optical imaging with a cathepsin B activated probe for the enhanced detection of esophageal adenocarcinoma by dual channel fluorescent upper GI endoscopy. Theranostics 2(2), 227 (2012)... 

Multisizer 4). The principle of the symmetric dual-channel design is to have the noise levels for the output signals (Vdi and Vd2 as indicated in Fig. 3) in both gate branches identical, and hence, the noises can be canceled by using a differential amplification mechanism. However, ideal noise subtraction is not possible due to the real-world limitation in fabricating identical dual channels. Furthermore, this dual-channel method will not be able to detect particles when two particles pass the two apertures at the same time because the two signals with the similar amplitude will be subtracted by each other and canceled at the second stage of the differential amplifier. 

---