Use in Series With Other Detectors

The most common dual arrangement with ED is in series with a UV detector. The order in which the detectors are placed will be dictated by the requirements of 

Special care should be taken when coupling detectors with PGEs. The Coulchem guard cell is designed to withstand back pressures of 6000 psi, but contamination of analytical PGEs can lead to rapid increases in resistance and back pressure. Consequently, it is reconunended that PGE cells are first in any series of cells. If the ED is operated in the coulometric mode, the production of new chemical species may have a marked effect on the spectral characteristics of the parent compound(s) and thus of the response. This may, as in the case of amoxicillin (Section 5.1), be exploited to produce a species with improved spectral characteristics, or it may lead to a loss of signal at subsequent detectors. 

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Modem amperometric detectors possess a number of useful features. These include rapid response time, low cell volume, ease of access to the electrode surface for cleaning, ability to be used in series with other detectors, good sensitivity with suitable analytes, in-built facilities for scanning the detection potential and minimal mnning costs. Since amperometric electrodes are small it is possible to incorporate more than one into a thin-layer electrode block. Most commercial cells usually contain two electrodes with their necessary connections. At the simplest level this allows the rapid connection of the second electrode when the first becomes contaminated. The electrode connections are simply transferred to the other electrode pin without having to dismantle the cell. A discussion of the use of two or more amperometric electrodes for differential analysis is given below (Section 7). 

A major advantage to this technique is that inorganics can be detected to low levels (1-2 pg) using a nondestructive detector. This means that the P1D can be connected in series with other detectors and is ideal for odor analysis. The sensitivity of the detector is directly related to the efficiency of ionization of the compound. The PID is about 5-10 times more sensitive to aliphatic hydrocarbons, 50-100 times more sensitive to ketones than FID, and 30 times more sensitive to sulfur compounds than flame photometric detection. Several reviews on the PID and its sensitivity have been published [94-97]. 

The PID (Fig. 8-12) can be used with any gas chromatograph and has a sensitivity at least equal to those of the FPD and HECD. The sample is not destroyed by PID analysis hence, the PID may be connected in series with other detectors or with a mass spectrometer for further analysis of the sample. 

Unlike most of the other spectrometric detectors described here, the DAD and FSFD are non-destructive and can be readily used in a true flow-through mode (unlike NMR, which generally requires slower flow rates or stop-flow conditions). Therefore, the chromatographic eluent from them can easily be coupled to another subsequent detector. The commercial FSFD described earlier in fact typically is ran in series with a DAD. 

A source of error in the HPLC method is the very short retention time of PEG in reversed-phase systems. This makes quantification by differential refractive index subject to interference by peaks from water, solvent impurities, and other unretained substances. This source of error may be eliminated by coupling the HPLC column to a GPC column, so that PEG is resolved from both the surfactant and from other impurities (36). Even with this modification, if phenol impurity were present in the initial alkylphenol, the resulting ethoxylated phenol could interfere with PEG determination by this method. Its presence is detected by using a UV detector in series with the RI detector. Another source of interference is the anion, such as acetate or lactate, added during the neutralization of the catalyst. For careful work, this should be removed by ion exchange prior to HPLC analysis (36). 

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