Chemical Analysis. Francis Rouessac
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Название: Chemical Analysis
Автор: Francis Rouessac
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119701347
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This detector can function at more than 400°C and is not destructive, as the ionization is reversible and affects only a small fraction of the molecules of each compound.
2.7.3 Detectors Providing Structural Data
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.
2.8 OPTIMIZATION OF A SEPARATION
If we assume that the optimal stationary phase has been chosen, the length and internal diameter ID of a capillary column as well as its stationary phase film thickness (df) must also be taken into account. The conditions for a good separation of the analytes need to be found without increasing the analysis time. From a practical standpoint, in GC, we can only change the temperature and the carrier gas flow rate. In both cases, retention factors k and selectivities α are not much changed. For the carrier gas, we choose a flow rate such that its speed ū is close to the optimal value of the Van Deemter curve. For volatile compounds, we choose a column with a weak phase ratio (β < 100), hence with a thick film. Inversely, we choose a thin‐film column for less volatile compounds.
However, we must not forget that coupled GC‐MS does not necessarily require greatly optimized separation of chromatographic peaks (sufficient resolution), which is a significant time gain for the chromatographer. Moreover, in a series of analyses, if we can avoid the use of a temperature gradient, we can thus eliminate the time necessary to return to initial conditions in order to conduct the following analysis.
2D GC. Optimization sometimes involves dual chromatography on two different stationary phases (for example, polar and nonpolar) with a single set‐up including a carrier gas inlet shared by the two columns. The first chromatograph is used to isolate a peak, possibly corresponding to an unresolved mixture, which then goes through a second column for another separation.
2.9 FAST CHROMATOGRAPHY AND MICROCHROMATOGRAPHY
2.9.1 ‘Fast’ and ‘Ultra‐Fast’ Chromatography
In general, conventional chromatography is a slow method of analysis. For application reasons (such as control analysis, reaction monitoring, etc.), it may be beneficial to shorten analysis times. To reduce these times, use a shorter column, as retention times are proportional to column length (Table 2.1). Due to decreased efficiency, the phase ratio β must be increased by choosing a thinner stationary phase film (ID = 100 μm, film = 0.1 μm, β = 250). Hydrogen must be used as the carrier gas (see Van Deemter curve). The last factor that we can easily regulate is the oven temperature (the higher it is, the shorter the analysis). Be aware, however, of the first peaks appearing, since good separation requires a lower temperature. Therefore, the solution is a temperature gradient. Devices allow for ramps that can go up to 100°C/min. For volatile compounds and with various column designs, covered with a heat‐resistant outer sheath, the temperature can be increased more sharply (200°C/20 s). As a result, retention times are reduced significantly (Figure 2.16). This type of fast chromatography, sometimes called high‐speed GC, finds its main use in control analyses.
Table 2.1 Comparison of conventional, fast and ultra‐fast GC set‐ups.
| GC type | Ramp (°C/min) | Analysis time (min) | Peak width (s) | Column length (m) | Internal diameter (μm) |
|---|---|---|---|---|---|
| Conventional | Conventional oven (1–20) | ~30 | 5–10 | 15–100 | 250–320 |
| Fast chromatography | Conventional oven (20–100) | 5–10 | 0.5–5 | 5–15 | 100–250 |
| Ultra‐fast chromatography | Resistive heating (60–1,200) | ~1 | 0.05–0.2 | 2–5 | 50–100 |