Table 2.2 McReynolds constants (∆I) for several stationary phases normalized to squalane.
| Stationary phase | Benzene X′ | 1‐butanol Y′ | 2‐pentanone Z′ | Nitropropane U′ | Pyridine S′ |
|---|---|---|---|---|---|
| Squalane | 0 | 0 | 0 | 0 | 0 |
| SPB‐Octyl | 3 | 14 | 11 | 12 | 11 |
| SE‐30 (OV‐1) | 16 | 55 | 44 | 65 | 42 |
| Carbowax 20M | 322 | 536 | 368 | 572 | 510 |
| OV‐210 | 146 | 238 | 358 | 468 | 310 |
| Kovats index for the five reference compounds above (X′, Y′, Z′, U′, S′) on squalane | |||||
| Isqualane | 653 | 590 | 627 | 652 | 699 |
The sum of the five calculated values, using Eq. (2.7), has been used to define the overall polarity of the phase under study.
Each of the test compounds yields specific information regarding the stationary phase: benzene for the inductive effect, pyridine for H+ proton accepting, butanol for hydrogen bonding, nitropropane for dipolar interactions, etc.
These constants, which are related to molecular structures, allow an appreciation of the interactive forces between stationary phase and solute as a function of compound class. A high index suggests that the stationary phase strongly retains the compounds that contain the corresponding organic functions. This generally leads to improved selectivity for this type of compound. Thus, to separate an aromatic hydrocarbon from a mixture of ketones, we would select a stationary phase whose McReynolds constant for benzene is sufficiently different from that for butanone. These differences in retention indexes are provided by manufacturers for use by chromatographers (Table 2.2). McReynolds constants have replaced Rohrschneider constants, which were based upon the same principle but used certain different reference compounds.
KEY POINTS OF THE CHAPTER
1 As long as settings remain unchanged, GC chromatographs are able to reproduce the retention time of a compound to the nearest second, in the case of several successive injections. This can only be obtained by perfect control of all parameters: temperatures, flow rates, pressures, and carrier gas purity.
2 The range of use of GC depends on the volatility of compounds. The upper limit of this range is reached if molecular weight exceeds the 500 Da boundary, or if hydrogen bonds or dipole–dipole interactions are created between compounds.
3 At equal volatility, compounds elute from the column by following the order of their distribution coefficients in the stationary phase. The carrier gas does not participate in concentration equilibria. The two main factors determining the behaviour of an analyte are its volatility and analyte‐stationary phase interactions.
4 The carrier gas must be free from oxygen in order not to alter the analytes, which are weakened when they are brought to a high temperature in the instrument’s oven. In general, hydrogen is chosen as the carrier gas. It enables faster analyses than nitrogen or helium, without altering the efficiency (N) of the separation.
5 An abundance of capillary columns for GC is on the market, either for general use or for specific separations. They are classified according to their retention index and their polarity, as defined by their McReynolds constants. For the GC‐MS technique, we choose grafted, cross‐linked, and low‐bleed columns, which are able to interact with the molecules that require separation.
6 A new column is accompanied by a document specifying its efficiency (N), with the conditions of acquisition of that value, as well as its retention indexes for five test compounds, with different, universally used chemical properties.
7 Detectors are either general, such as FID, which is by far the most common, owing to its sensitivity and its linearity, or they are adapted to a category of compounds or are even specific to a single compound, thus simplifying the chromatograms when the matrix is complex and enabling a better quantification of the analytes in question.
8 The Kovats index of a compound is calculated from the retention times of adjacent n‐alkanes. It is of interest because it depends only on the stationary phase and not on other characteristics of the column or apparatus. Index tables help to identify compounds by comparison of their retention indexes, without worrying about retention times, which are variable.
PROBLEMS
1 Show that for a capillary column, the average flow can be calculated from the following formula: (where ū represents the average linear velocity in a column of internal diameter ID).
2 A comparative study of the evolution of the retention factors of n‐undecane and n‐tridecane, as a function of the temperature of the GC column, at a constant carrier gas flow rate of 3 ml/min, gave the following results:n‐decane: logk1 = −6.58 + 2,450 ∙ T −1n‐tridecane: logk2 = −7.91 + 3,010 ∙ T −1Justify the general form: logk = −A + B/TAt what temperature T1 would these two solutes coelute? Which of the two would elute first if we work at a temperature T below T1? Same question if T is over T1?At what temperature T2 will the separation factor be equal to 2?Knowing that the column phase ratio is equal to 250, calculate the Nernst distribution factors K1 and K2 of the two solutes when we work at 150°C.
3 We propose determining the maximum efficiency of a capillary column, with the following characteristics:L = 12 m, ID = 200 μm, stationary phase: methyl‐phenyl polysiloxane, df = 0.33 μmOperating conditions: carrier gas: H2; injector temperature: 250°C; oven temp.: 100°C; FID temp.: 250°C; split flow rate: 30 ml/min.We conduct several injections of an n‐undecane solution in pentane (injected volume: 0.5 μl) while changing the carrier gas flow rate. Calculation software gives the Golay curve equation:H = 5.44/u + 0.004u where H is in mm and u in mm/s.For what value of u does the height equivalent of a theoretical plate go through a minimum? What is the value of Hmin ?Calculate the maximum efficiency value Nmax.Between what values of u can we work, if we tolerate efficiency being greater