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None of the detectors previously described yield any information as to the nature of the eluted analytes. At most, they are selective for a certain category of compounds. Identification involves the use of an internal calibration based on retention times or requires the knowledge of retention indexes (see Section 2.10). When the chromatogram has peaks that are close together, a confusion of identity could occur.

To counteract this, several complementary detectors could be combined with each other (Figure 2.15), or we could select a detector able to provide structural information based on spectroscopic data or about the elemental composition of the analytes. The retention time and specific characteristics for each compound can then be determined. The mass detector is the best example, yet other combinations of techniques have been tested. Such is the case for atomic emission detection with a plasma torch: compounds at the column outlet flow into a plasma (see Chapter 14) where the temperature is sufficient to increase atomic or ion populations in the excited state. When returning from the excited state, these atoms or ions emit specific radiations, which help to identify them. Tests have been conducted to couple GC with an infrared spectrometer (GC‐IR). This pairing leads in principle to mid‐infrared spectra of the eluted analytes. An entire set of technological adaptations have made the existence of coupled methods possible. Therefore, potential applications are more numerous than for each of these when taken alone.

Figure 2.15 Three detectors connected in series. At the outlet of a capillary column, either in series or in parallel and depending upon whether any of them destroy the sample, several detectors can be installed. Here, three chromatograms of an injected mixture as obtained from each detector. Note that the sensitivity varies significantly from one detector to another.