Chemical Analysis. Francis Rouessac

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      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.

      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 than 0.95 Nmax?

      4 The table below contains values of the retention factor k for four refinery gases, studied at three different temperatures on the same capillary column (length L = 30 cm, internal diameter = 250 μm), whose stationary phase is of the SE‐30 type. The chromatograph is equipped with a cryogenic accessory.k valuesCompoundB.P. (°C)−35°C25°C40°Cethene−1040.2490.1020.0833ethane−890.4080.1480.117propene−471.8990.4320.324propane−422.1230.4810.352Can the polarity or nonpolarity of the SE‐30 phase be deduced from the elution order of the compounds?Calculate the selectivity factor α for the propene–propane pair at the three temperatures indicated.For a given compound, why does k decrease in response to an increase in temperature?What is the number of theoretical plates of the column for propane at 40°C, if it is known that at this temperature the resolution factor for the propene–propane pair is 2? Calculate the corresponding HETP.What would be the minimum theoretical value of HETP for propane at 40°C?

      5  In a GC analysis series, we seek to determine the influence of the column’s length on several parameters of the chromatogram. All experiments are conducted under the same temperature and carrier gas flow rate conditions.L (m)a = √LtR (min)tR/LRR/a153.72.05307.52.916015.34.15Complete the table.To the nearest measurement uncertainties, what simple relationship can we consider between retention times and column length?To the nearest measurement uncertainties, what simple relationship can we consider between the resolution and the square root of the column length?If we assume that the two peaks used to determine the resolution have practically the same full width at half‐maximum (FWHM), deduce from the previous questions a relationship between the FWHM and the column length as well as a relationship between the theoretical efficiency of the column and its length.The two peaks used to calculate R have retention times respectively of 8.3 min and 9.7 min with the column length equal to 60 m. Calculate the FWHM value of these peaks (same approximation as in question d), as well as the theoretical efficiency of the column for the solute whose retention time is 8.3 min.Calculate the theoretical efficiency of 15 and 30 m columns for the same solute.

      6 The chromatographic analysis of an unleaded gas is conducted under the following conditions:Column: L = 150 m, ID = 0.25 mm, df = 0.25 μm, nonpolar stationary phase ‘PDH 150’. Injector, 100:1 split ratio, T = 200°C; FID detector, T = 200°C; oven T = 35°C; carrier gas, He; u = 20 cm/s; column head pressure, 2.5 bar; injected volume = 1 μL.In the following table, the variations in the free enthalpy of dissolution are shown, at the experimental temperature, for the solutes observed on the chromatogram.Solute2,3‐dimethyl pentane2,4‐dimethyl pentane2‐methyl hexane3‐methyl hexanebenzeneΔG0308 (kJ/mol)−17.79−16.80−17.73−18.05v17.27The retention times of the various solutes are given according to peak No.Peak No.1011121314tR (min)48.0055.1763.9064.5968.28Calculate the hold‐up time tM associated with this analysis. Since methane is not retained by the stationary phase, determine its free enthalpy variation at the temperature of the experiment.Assign each peak to its corresponding solute and justify your answer.From the СКАЧАТЬ