On the influence of column temperature on the isothermal retention indices of structurally different solutes on a poly(dimethylsiloxane) capillary column

Journal of Chromatography A, 1365 (2014) 204–211

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On the influence of column temperature on the isothermal retention indices of structurally different solutes on a poly(dimethylsiloxane) capillary column J.M. Santiuste a , J.E. Quintanilla-López b , R. Becerra a , R. Lebrón-Aguilar a,∗ a b

Instituto de Química-Física “Rocasolano” (CSIC), Serrano 119, 28006 Madrid, Spain Instituto de Química Orgánica General (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 16 July 2014 Received in revised form 4 September 2014 Accepted 5 September 2014 Available online 16 September 2014 Keywords: Gas chromatography Isothermal retention indices Poly(dimethylsiloxane) Temperature dependence Homologous series Carbon atom number

The dependence of isothermal retention indices (I) on column temperature over a wide temperature range has been studied for solutes belonging to nine chemical functions on a capillary column coated with poly(100% dimethyl siloxane). I values for some solutes are reported for the first time on capillary columns. I values increased with increasing column temperature, with the exception of the linear alcohols and the esters, which decreased with increasing temperature, and of cyclobutanol, 2-butanone, 2-pentanone, 1butylamine and 1-pentylamine that showed a well-defined minimum in the 358–377 K range. Moreover, a minimum at the higher temperature range for longer and less polar solutes such as 1-nonanol was observed for the first time. The three trends of I vs. T were perfectly described by the extended model (I = a + bT−1 + c lnT). On the other hand, the dependence of I on the carbon atom number (z) of the solute was linear and with a slope of similar magnitude for all homologous series studied, except for the alicyclic compounds. For the latter, higher slope values and worse correlations were obtained, owing to their larger surface area and to the different conformations that they adopt in order to minimize the ring strain. In addition, due to its higher chain stiffness, an important influence of the column temperature on these slopes was observed. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The retention index (I), devised by Kováts [1] only a few years after the introduction of gas chromatography by James and Martin [2], is the most widely used parameter for describing the retention of a solute on a chromatographic stationary phase (SP) at a given temperature. After more than 50 years, this subject still elicits much interest, as proven by the thousands of papers and reviews published [3]. The I values are usually obtained by interpolation of the loga ) between rithms of the adjusted retention times of the analyte (tRi  ) and after those of the two reference n-alkanes eluting before (tRz  ) it: (tR(z+n)

 I = 100 z +

 − log t  log tRi Rz

log

 tR(z+n)

− log



 tRz

∗ Corresponding author. Tel.: +34 917459544; fax: +34 915642431. E-mail address: [email protected] (R. Lebrón-Aguilar). http://dx.doi.org/10.1016/j.chroma.2014.09.020 0021-9673/© 2014 Elsevier B.V. All rights reserved.

(1)

where the subscripts z and z + n refer to the linear alkanes of z and z + n carbon atoms eluted before and after the analyte i, respectively. The hold-up time (tM ), needed for the application of Eq. (1), is difficult to correctly estimate [4–8]. Fortunately, retention indices can be obtained without the hold-up time [9] by using the so-called LQG method, which allows to calculate I values with a precision similar to that of traditional methods but with a 10 times higher accuracy [9]. The dependence of I on temperature has been studied by many researchers [10–19]. Initially, the linear model for large temperature intervals, especially for nonpolar solutes on nonpolar SPs, predominated, but later on the Antoine-type hyperbolic equation was adopted: I =˛+

ˇ  +T

(2)

where ˛, ˇ and  are empirical parameters with a thermodynamic meaning [20]. However, this equation could not account for a minimum observed in the I vs. T curves for some polar solutes on apolar columns [10,13,14]. Twelve years ago, Ciazynska-Halarewicz,

J.M. Santiuste et al. / J. Chromatogr. A 1365 (2014) 204–211

Kowalska, Görgényi and Héberger [12–16] modelled the retention on the basis of the activated complex transition state, under kinetic and thermodynamic approaches. This led to the so-called extended model for the temperature dependence of the retention index: I =a+

b + c ln T T

(3)

where a, b and c are adjustable parameters [13]. Since the LQG method provides very accurate I values, their variation with column temperature and the existence of minima can be accurately studied. In this paper the isothermal retention indices estimated by the LQG method of 67 solutes belonging to 9 different chemical functions in a wide temperature range on a poly(dimethylsiloxane) stationary phase are presented. These data were used to study the dependence of I on column temperature, as well as on the carbon atoms number of the solute.

205

at least in triplicate and with retention times expressed in thousandths of a minute. In order to simplify the notation, temperatures have been rounded up throughout the manuscript. Additional I data of arenes (benzene, toluene, ethylbenzene, propylbenzene and butylbenzene) at 323–423 K recently reported [19] and of cycloalkanes of z = 4–8 at 300, 322, 340 and 359 K [21], z = 5–7 at 323 K [22], and z = 5–8 at 353 and 403 K [23] were included in Section 3. 2.4. Mathematical treatment

2. Experimental

As indicated in Section 1, the retention indices were calculated with the LQG method because it provides very accurate I data [9]. Briefly, the experimental retention times of all alkanes eluted in the same chromatogram were used to estimate the parameters A, B, C and D of Eq. (4) [4,5], and then, the I values of all substances in this chromatogram were calculated with Eq. (5). The hold-up time is not needed for these calculations [9].

2.1. Apparatus

tR = A + exp(B + Cz D )

 D

[ln(tR − A)] − B C

(4)

A HP-5890 Series II (Agilent, Palo Alto, CA, USA) gas chromatograph with split/splitless injection system, flame ionization detector (FID), back-pressure regulator and WIKA Tronic 891.13.500 pressure transducer (Alexander Wiegland, Klingenberg, Germany) with numeric monitor PM-2900 (Félix Mateo, Barcelona, Spain) was used. Data acquisition and processing was carried out using Clarity Lite chromatographic software (Data Apex Ltd., Prague, Czech Republic). A poly(100% dimethylsiloxane) fused-silica WCOT column (TRB1) from Teknokroma (San Cugat del Vallés, Barcelona, Spain) with 0.25 mm of internal diameter, 0.25 ␮m of film thickness and 30 m long, was used.

I = 100

2.2. Solutes

3.1. Isothermal retention indices

Linear alkanes (from z = 5 to 12), and linear, cyclic and aromatic compounds belonging to the following chemical families were used: chlorocompounds (linear 1-chloroalkanes from z = 4 to 9 plus chlorobenzene), alcohols (linear 1-alkanols from z = 4 to 9 plus cycloalkanols from z = 4 to 8), ketones (linear 2-alkanones from z = 4 to 10 and cycloalkanones from z = 4 to 8 plus acetophenone), esters (methyl pentanoate, ethyl butanoate and butyl acetate, plus propyl alkanoates from z = 2 to 4, 6 and 7), aldehydes (linear alkanals from z = 4 to 9 plus benzaldehyde), amides (N,N-dimethylformamide, N,N-diethylformamide and N,N-diethylacetamide), nitriles (linear alkanenitriles from z = 4 to 8 plus benzonitrile), nitrocompounds (linear 1-nitroalkanes from z = 3 to 6 plus nitrobenzene), and amines (linear 1-alkylamines from z = 4 to 8 plus aniline and N,Ndimethylaniline). The solutes were from Merck (Darmstadt, Germany), Carlo Erba Reagenti (Rodano, Milano, Italy), and Sigma–Aldrich Co. (St. Louis, MO, USA) with a purity of 98–99.5%, sufficient for gas chromatographic use.

Isothermal retention indices for the 67 solutes on the TRB-1 column at the 333–423 K temperature range are listed in Table 1. They were obtained by the LQG method and are the average values of at least three injections. The mean of the standard deviations of I values was 0.2 i.u. (index units), except for the amides and the short-chain amines for which it was higher, 0.7 i.u. (see Table S1 in Supplementary Data section for individual standard deviations). These I values were compared with literature values from the NIST database [24]. Table 2 summarizes the number of I determinations for solutes belonging to the nine chemical functions studied on capillary and packed columns coated with poly(100% dimethyl siloxane) found in the NIST database (SE-30, OV-1, OV-101, DB-1, BP-1, HP-1, Ultra-1, RTX-1, AT-1, CPSIL-5CB, DC-200, DC-400, E-301, and SF-96). Thus, 806 (capillary columns) and 249 (packed colums) data were reported, with a ratio of 76/24 in capillary/packed columns, respectively. Ketones and alcohols were the solutes with the most I values, and amides those with the least. Moreover, 1-chloroheptane, 1-chlorooctane, 1-chlorononane, N,N-diethylformamide, N,N-dimethylacetamide, cyclobutanol, heptanenitrile and octanenitrile have never been determined on either type of column, and cyclopentanol and 1-octylamine have never been obtained on capillary columns. Therefore, our I determinations of these compounds at ten temperatures will be a useful contribution to the NIST database of isothermal retention indices. In order to study the concordance between data, the four solutes belonging to different chemical functions with the most determinations in the database were selected. Fig. 1 shows our experimental I values and those from the NIST database for 1-butanol, 2-pentanone, cyclohexanone and chlorobenzene on capillary and packed columns coated with

2.3. Chromatograms Several mixtures of the above-mentioned solutes along with the linear alkanes were chromatographed isothermally from 333.15 to 423.15 K at increments of 10 K (±0.1 K) using nitrogen as carrier gas. The alkanes were chosen in order to bracket each one of the solutes of the mixtures of the nine chemical families studied. About 0.05–0.1 ␮L of these mixtures were injected in a split ratio of 100:1. Symmetric peaks were obtained for all analytes at all the temperatures, except for amides at the lower temperatures and for 1-alkylamines, that showed tailing. Chromatograms were recorded

(5)

A FORTRAN programme written by the authors was used for these calculations, although any commercial mathematical package including non-linear regressions would be equally valid. Microsoft Office Excel (Microsoft Corp. Redmond. WA, USA), Microcal Origin 6.0 (OriginLab Corp. Northampton, MA, USA) and Statgraphics Centurion XV (StatPoint Technologies, Warrenton, VA, USA) were used for data handling, basic calculations, and linear and non-linear regressions. 3. Results and discussion

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Table 1 Retention indices at 333–423 K of solutes studied on the TRB-1 column. Solute

Retention indices 333 K

343 K

353 K

363 K

373 K

383 K

393 K

403 K

413 K

423 K

1-Chlorobutane 1-Chloropentane 1-Chlorohexane 1-Chloroheptane 1-Chlorooctane 1-Chlorononane Chlorobenzene

639.9 741.9 842.9 944.0 1044.7 1145.2 829.7

640.8 742.9 844.0 945.2 1045.9 1146.5 832.8

641.8 744.0 845.2 946.4 1047.2 1147.8 836.0

642.9 745.2 846.4 947.7 1048.6 1149.2 839.2

643.8 746.3 847.7 949.0 1049.9 1150.6 842.6

645.3 747.5 849.0 950.5 1051.4 1152.1 846.0

646.3 748.9 850.5 951.9 1052.9 1153.7 849.3

647.4 750.2 852.0 953.5 1054.5 1155.3 853.2

648.6 751.5 853.4 955.0 1056.1 1157.0 856.7

649.9 753.1 854.8 956.5 1057.6 1158.6 860.8

1-Butanol 1-Pentanol 1-Hexanol 1-Heptanol 1-Octanol 1-Nonanol Cyclobutanol Cyclopentanol Cyclohexanol Cycloheptanol Cyclooctanol

647.8 751.6 853.9 955.8 1057.2 1158.4 667.2 766.1 864.2 991.0 1107.1

646.2 750.1 852.6 954.3 1055.8 1156.9 666.5 766.5 865.7 993.4 1110.6

644.3 748.4 851.0 952.9 1054.4 1155.7 665.5 766.8 867.1 996.0 1114.1

643.1 747.3 850.0 952.0 1053.6 1154.8 665.3 767.6 869.1 999.3 1118.2

641.8 746.3 849.0 951.2 1052.8 1154.0 664.9 768.5 871.1 1002.2 1122.2

641.1 745.7 848.6 950.8 1052.4 1153.7 665.2 769.8 873.6 1005.6 1126.7

640.3 745.1 848.1 950.4 1052.1 1153.4 665.4 771.5 876.1 1009.4 1131.4

640.0 744.8 847.9 950.3 1052.1 1153.5 666.0 773.6 878.9 1013.3 1136.2

639.6 744.6 847.8 950.2 1052.1 1153.6 667.0 775.1 882.0 1017.4 1141.3

639.3 744.8 848.1 950.5 1052.3 1153.9 667.9 777.0 885.0 1021.6 1146.6

2-Butanone 2-Pentanone 2-Hexanone 2-Heptanone 2-Octanone 2-Nonanone 2-Decanone Acetophenone Cyclobutanone Cyclopentanone Cyclohexanone Cycloheptanone Cyclooctanone

577.0 667.9 769.3 869.8 970.0 1070.7 1171.1 1027.8 637.6 762.4 863.2 975.5 1076.3

576.6 667.6 769.3 869.8 970.0 1070.8 1171.2 1030.8 638.7 764.4 866.2 979.1 1080.7

576.3 667.5 769.4 869.9 970.2 1071.0 1171.4 1033.9 639.7 766.3 869.3 983.0 1085.2

576.1 667.5 769.6 870.1 970.5 1071.3 1171.7 1037.1 640.9 768.3 872.4 987.0 1090.0

576.0 667.6 769.8 870.5 970.9 1071.7 1172.2 1040.5 642.2 770.6 875.9 991.0 1094.8

576.1 667.9 770.1 870.8 971.4 1072.2 1172.7 1044.0 643.6 772.8 879.4 995.4 1099.9

576.1 668.1 770.5 871.3 971.9 1072.8 1173.4 1047.5 645.0 775.3 883.1 999.9 1105.1

576.2 668.5 771.2 871.9 972.5 1073.5 1174.1 1051.4 646.6 778.1 887.0 1004.5 1110.6

576.6 669.1 771.7 872.5 973.2 1074.2 1174.8 1055.3 648.2 780.7 891.0 1009.4 1116.4

576.8 669.7 772.3 873.4 974.0 1075.0 1175.7 1059.3 650.3 783.7 895.2 1014.3 1122.2

Propyl acetate Propyl propanoate Propyl butanoate Propyl hexanoate Propyl heptanoate Methyl pentanoate Ethyl butanoate Butyl acetate

697.2 793.2 882.1 1079.4 1178.4 807.7 784.7 797.3

695.9 792.1 881.2 1078.6 1177.6 806.9 783.6 796.1

694.4 790.8 880.3 1077.9 1177.0 806.1 782.4 794.8

693.3 789.7 879.4 1077.2 1176.4 805.4 781.4 793.7

691.7 788.7 878.6 1076.6 1175.8 804.7 780.1 792.4

690.5 787.7 877.9 1076.1 1175.4 804.0 779.1 791.3

689.8 786.8 877.3 1075.5 1175.0 803.6 778.5 790.7

688.9 786.0 876.6 1075.1 1174.6 803.1 777.7 789.6

687.6 785.2 876.0 1074.7 1174.3 802.8 777.0 788.7

687.0 784.3 875.4 1074.4 1174.1 802.3 776.2 788.1

Butanal Pentanal Hexanal Heptanal Octanal Nonanal Benzaldehyde

573.2 676.5 778.5 879.6 980.6 1081.3 927.1

573.0 676.8 778.9 880.0 981.1 1081.7 930.0

573.3 677.0 779.2 880.4 981.6 1082.3 933.0

573.7 677.7 779.7 881.0 982.2 1082.9 936.2

574.1 677.9 780.2 881.7 982.9 1083.9 939.5

574.6 678.7 781.1 882.6 983.8 1084.7 943.1

575.0 679.2 781.8 883.3 984.7 1085.7 946.8

575.6 680.1 782.7 884.3 985.8 1086.8 950.6

576.2 680.7 783.5 885.2 986.8 1087.9 954.6

577.1 681.5 784.4 886.4 987.9 1088.9 958.6

746.0 901.9 835.5

746.2 902.4 836.1

747.0 903.7 836.8

748.0 904.8 837.4

749.1 906.6 838.4

750.5 908.5 839.9

752.1 910.6 841.2

753.6 912.2 842.7

754.9 914.4 844.2

756.7 917.0 846.2

Butanenitrile Pentanenitrile Hexanenitrile Heptanenitrile Octanenitrile Benzonitrile

640.0 742.2 844.2 944.8 1045.6 944.6

640.0 742.3 844.3 945.2 1046.2 947.0

640.1 742.6 844.8 945.7 1046.8 949.7

640.3 743.2 845.4 946.5 1047.7 952.7

640.6 743.9 846.2 947.4 1048.7 955.9

641.4 744.9 847.3 948.6 1050.0 959.4

642.1 745.7 848.3 949.7 1051.3 963.0

642.9 747.0 849.7 951.1 1052.7 966.8

644.0 748.4 851.0 952.5 1054.2 970.6

644.9 749.7 852.2 953.9 1055.7 974.7

1-Nitropropane 1-Nitrobutane 1-Nitropentane 1-Nitrohexane Nitrobenzene

706.7 805.5 906.7 1007.4 1041.0

707.4 806.4 907.8 1008.5 1045.0

708.1 807.4 908.9 1009.8 1049.0

708.9 808.5 910.2 1011.2 1053.4

710.0 809.8 911.6 1012.8 1057.8

711.0 811.2 913.2 1014.4 1062.5

712.4 812.8 914.8 1016.2 1067.2

713.7 814.5 916.7 1018.1 1072.2

715.2 816.2 918.5 1020.1 1077.4

716.6 817.9 920.3 1022.2 1082.4

1-Butylamine 1-Pentylamine 1-Hexylamine 1-Heptylamine 1-Octylamine Aniline N,N-Dimethylaniline

632.3 731.6 832.1 933.0 1033.9 945.0 1057.0

631.4 731.3 831.9 933.0 1034.1 946.8 1060.5

630.5 730.9 832.0 933.1 1034.1 948.6 1064.1

630.0 731.0 832.2 933.4 1034.3 950.8 1067.6

629.8 731.3 832.5 933.9 1034.7 953.0 1071.4

629.9 731.4 832.9 934.2 1035.1 955.7 1075.0

630.2 731.8 833.4 934.9 1035.8 958.6 1079.0

630.7 732.6 834.1 935.4 1036.5 961.7 1082.9

631.2 733.4 834.8 936.3 1037.4 965.0 1086.9

632.1 734.2 835.6 936.9 1038.3 968.6 1091.2

N,N-Dimethylformamide N,N-Diethylformamide N,N-Dimethylacetamide

J.M. Santiuste et al. / J. Chromatogr. A 1365 (2014) 204–211

a

710

720

b

700

690

Retention index

207

680

670

660 650 640 630 620 260

Retention index

940

300

340

380

420

460

280

900

c

915

875

890

850

865

360

400

440

480

d

825

840

800

815 320

320

775 340

360

380

400

420

440

Temperature (K)

280

320

360

400

440

480

Temperature (K)

Fig. 1. Comparison between the retention indices from the NIST database on capillary () and packed columns (), and our experimental values (䊉) at different temperatures on the TRB-1 column for: (a) 1-butanol, (b) 2-pentanone, (c) cyclohexanone and (d) chlorobenzene.

poly(dimethylsiloxane). A fair agreement was observed, especially for data obtained on capillary columns. This agrees with a previous report [25] based on statistical comparison of I values recorded in the NIST database. Values on packed columns from the NIST database showed a higher dispersion, probably because in these columns it is more difficult to reduce the residual adsorption than in capillary ones. It can be seen in Table 1 that the presence of an electronegative group in the molecule slows down its chromatographic elution, that is, increases its retention index. In general, an increase (Ialkane ) of at least 170 i.u. relative to the linear alkane of the same number of carbon atoms (z) was observed. Aldehydes, 2-ketones and esters showed the lower increase, while tertiary amides and nitrocompounds gave the higher Ialkane values. In Fig. 2.a, these increments at 373 K for compounds with five carbon atoms and the different functional groups studied (1-chloropentane, 1-pentanol, 2-pentanone, propyl acetate, pentanal, N,N-diethylformamide, pentanenitrile, 1-nitropentane and 1-pentylamine) are plotted for easy comparison. Likewise, the position of the functional group into the hydrocarbon chain showed small differences. In the case of esters, the Ialkane value slightly decreases as the length of the hydrocarbon chain of the alcohol part decreases, except when it is a methyl group, for which a slight increase of I value was observed. This effect, also reported by other authors [26,27], is shown in Fig. 2.b for esters with six carbon atoms. When the linear hydrocarbon chain was substituted by an aromatic ring of equal z, an increase in the I value was observed, except for the 1-chlorohexane/chlorobenzene pair (Fig. 2c). The

higher Ialkane values for 2-octanone, heptanal, heptanenitrile, 1-nitrohexane and 1-hexylamine compared with acetophenone, benzaldehyde, benzonitrile, nitrobenzene and aniline, respectively, are due to the increase of dipolarizability/polarizability of these molecules as a result of conjugation with the phenyl ring. In the case of chlorobenzene, the orbital overlap is less effective as a consequence of the different size of Cl C orbitals. Moreover, if the hydrocarbon chain was forming an unsaturated ring, an increase of the I value relative to the equivalent linear homologous compound was observed, due to their greater surface area that increases dispersion forces and therefore the I value. In Fig. 2.d. it can be seen that the Ialkane values for some linear/cyclic alkanols and linear/cyclic alkanones increase with the size of the ring. 3.2. Dependence of the retention indices on column temperature Apart from the increase in I values caused by the substitution of the linear chain by a cyclic or aromatic moiety, column temperature also affects them as can be seen in Table 1. Thus, I values of the 1-chloroalkanes, amides and 1-nitroalkanes increase moderately with increasing temperature from 10 to 15 i.u. within the temperature range studied. Alkanals, alkanenitriles, linear 1-alkanols and alkanoates show a variation of only 4–10 i.u., positive for the first two families and negative for the last two ones. Specifically, the I values of linear alcohols decrease by ca. 5–9 i.u. with increasing temperature, and I values of the cyclic alcohols increase between 10 and 40 i.u. from cyclopentanol to cyclooctanol in the same temperature range. Cyclobutanol is an exception, because its I values hardly

208

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Fig. 2. Effect on the retention index at 373 K of the type and position of the functional group attached to the hydrocarbon chain: (a) solutes with z = 5 belonging to nine chemical functions; (b) esters with z = 6; (c) six chemical functions with both alkyl and phenyl substituents; (d) linear and cyclic alcohols and ketones with z = 4, 6 and 8.

vary (∼3 i.u.), with a minimum at 372 K. Furthermore, on closer examination, a minimum for 1-nonanol at 400 K was also observed, but it is less defined because the data above this temperature are scarce. For linear amines the I values increase very little, between 3 and 4 i.u., unlike the first terms of the series, 1-butylamine and 1-pentylamine, which show a minimum at 376 and 359 K, respectively. Similarly, the retention indices of the linear 2-ketones rarely increase with column temperature, by no more than 5 i.u., except for 2-butanone and 2-pentanone, which show a minimum at 377 and 358 K, respectively. By contrast, the I values of cyclic ketones increase significantly (12–46 i.u.). Finally, for the compounds with an aromatic ring (chlorobenzene, acetophenone, etc.) the increment of I with temperature is high, around 31 i.u., apart from aniline, for which it is lower (24 i.u.), and nitrobenzene, for which it is higher (41 i.u.). Some illustrative examples of the three different behaviours observed for the I data with temperature are represented in Fig. 3. As indicated in Section 1, a minimum in the plot of I against T has already been reported by other researchers for the early members of homologous series of polar solutes on apolar stationary phases [13–15]: acetone, 2-butanone, 2-pentanone, 2-hexanone, ethanal, propanal, butanal, nitromethane, nitroethane, acetonitrile and propanenitrile. In agreement with this, we have found a distinct minimum for 2-butanone, 2-pentanone, 1-butylamine, 1-pentylamine and cyclobutanol. Furthermore, a minimum for less polar solutes, such as 1-nonanol, has also been obtained. For a given

class of analytes, it has been established that the increase of the length of the carbon chain induces a shift of the minimum in the I vs. T plot towards lower temperatures [14,15]. This can be clearly seen in linear ketones and alkylamines, and also in the linear alcohols (Table 1). Even though the Antoine-type equation (Eq. (2)) usually describes well the variation of isothermal retention indices with temperature, its hyperbolic character cannot explain the observed minima. Therefore, the I values from Table 1 were fitted to the extended model (Eq. (3)) that better describes all types of I variation with the column temperature. Table 3 lists the parameters a, b, c of the Eq. (3) for all the solutes, as well as the goodness-of-fit statistics. The a-parameter was negative for all solutes, except 2-decanone (a = 0) and esters (positive values). The other two parameters (b and c) were positive in all cases. Very good regression models with adjusted correlation coefficients (Radj ) higher than 0.992 (average value >0.9992), and standard errors of the estimate (sy ) lower than 0.20 (average value 0.08) were obtained. Moreover, the residual plots did not show any significant trend, which reinforces the statistical reliability of the models built for describing the I dependence on column temperature, even when minima are observed. 3.3. Variation of the retention index with the carbon atom number A very good linear relationship between I and z was found for all series of compounds, with adjusted correlation coefficients

J.M. Santiuste et al. / J. Chromatogr. A 1365 (2014) 204–211 Table 2 Number of retention index determinations for solutes of nine chemical functions on capillary and packed columns coated with stationary phases similar to TRB-1 taken from the NIST database [24].

Chlorocompounds 1-Chloroalkanes Chlorobenzene Alcohols 1-Alkanols Cyclic alcohols Ketones 2-Alkanones Cyclic alkanones Acetophenone Esters Propyl alkanoates Methyl pentanoate Ethyl butanoate Butyl acetate Aldehydes Alkanals Benzaldehyde Amides Amides Nitriles Alkanenitriles Benzonitrile Nitrocompounds 1-Nitroalkanes Nitrobenzene Amines 1-Alkylamines Aniline N,N-Dimethylaniline Sum

Packed

Retention index

Column type Capillary

a

650

645

640

38 19

0 10

178 13

69 7

173 48 2

31 4 15

36 1 9 30

22 6 8 10

700

76 6

6 11

690

5

1

59 7

5 0

56 7

12 9

22 13 8

19 3 1

806

249

635

630

Retention index

Compound class

209

340

360

380

400

420

340

360

380

400

420

b

680

670

660

Temperature (K) Fig. 3. Variation of isothermal retention indices of some illustrative solutes with temperature: (), 1-butanol; (), cyclobutanone; (), 1-butylamine; (䊉), propyl acetate; (), pentanal; and () cyclobutanol.

120

110

m

higher than 0.9998 and standard errors of the estimate less than 3.7 except for cyclic alcohols and cyclic ketones, in which the fit was slightly worse, with adjusted correlation coefficients between 0.998 and 0.9991, and standard errors of the estimate between 7.3 and 11.0. The slope values (m, see Table S2 in Supplementary Data section) for the different homologous series of compounds in the 333–423 K range are plotted in Fig. 4, in which several trends emerge. Firstly, if only the m data at a single temperature are considered, e.g., 423 K (full circles in Fig. 4), a value around 100 i.u. was obtained. This indicates that for most solutes the dispersion interactions with the SP decrease by the same magnitude as for linear alkanes, although the functional group has a slight but meaningful effect on retention. However, for the alicyclic compounds higher slope values were observed. This behaviour was the same for all temperatures studied (void circles in Fig. 4), even for cycloalkanes (triangles in Fig. 4), but not when the ring was aromatic, as for the alkylbenzenes (squares in Fig. 4). Therefore, the difference should be related to their cyclic and non-aromatic character. Structurally, the alicyclic compounds have a larger surface area able to interact with the SP than the linear ones. The addition of a methylene group to enlarge the cycloalkane ring produces a higher increase of its surface area than in the linear homologue. Consequently, as the cycle size enlarges its retention increases considerably, and hence m values become higher. Moreover, as indicated at the beginning of this section, the linear relationship between I and z in the cycloalcohols and cycloketones (in cycloalkanes too) was worse than in their linear analogues. The main difference between cyclic and noncyclic compounds is the lack of complete rotation about the carbon to carbon ring bond. This forces cyclic compounds to adopt different conformations in

100

90

1-

lo ch

r

lk oa

es ol s ols nes nes ates nals triles nes ines nes nes e a a an k a n k a n no no n o al k a eni l l alk ylam loalk enz a a lka alka alka n b ro 1clo 2-a lka -nit 1-alk cyc lkyl lo l c y a cy 1 a cy op pr

Fig. 4. Slopes (m) of the plots of retention index vs. the number of carbon atoms in homologous series with different functional groups: (䊉) 423 K and () 333–413 K. Data for cycloalkanes () and alkylbenzenes () have been taken from the literature [19,21–23] to help in discussion.

210

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Table 3 Retention index variation with column temperature on the TRB-1 column. Solute

I = a + bT−1 + c lnT a

b

c

Radj

sy

1-Chlorobutane 1-Chloropentane 1-Chlorohexane 1-Chloroheptane 1-Chlorooctane 1-Chlorononane Chlorobenzene

−237 −363 −265 −223 −118 −77 −1770

34,311 45,218 44,256 46,575 45,793 48,428 100,048

133 167 168 177 177 185 396

0.9995 0.9998 0.9999 1.0000a 1.0000a 1.0000a 0.9999

0.11 0.07 0.06 0.02 0.04 0.03 0.16

1-Butanol 1-Pentanol 1-Hexanol 1-Heptanol 1-Octanol 1-Nonanol Cyclobutanol Cyclopentanol Cyclohexanol Cycloheptanol Cyclooctanol

−1024 −1039 −1010 −878 −817 −811 −1860 −2137 −2443 −2935 −3321

101,571 105,910 108,646 106,077 107,647 112,315 135,889 142,491 151,515 172,073 187,755

235 254 265 261 267 281 365 426 491 587 665

0.9990 0.9989 0.9987 0.9992 0.9996 0.9995 0.9915 0.9987 0.9999 0.9999 1.0000a

0.13 0.11 0.11 0.08 0.05 0.05 0.13 0.20 0.09 0.13 0.09

2-Butanone 2-Pentanone 2-Hexanone 2-Heptanone 2-Octanone 2-Nonanone 2-Decanone Acetophenone Cyclobutanone Cyclopentanone Cyclohexanone Cycloheptanone Cyclooctanone

−304 −518 −283 −228 −76 −45 0 −1931 −1002 −1637 −2378 −2689 −3224

47,872 61,736 52,919 54,802 51,244 54,643 57,287 118,764 72,333 102,013 133,469 147,278 172,506

127 172 154 161 154 164 172 448 245 360 489 555 651

0.9936 0.9985 0.9984 0.9987 0.9999 0.9999 0.9998 1.0000a 0.9995 0.9998 1.0000a 1.0000a 1.0000a

0.04 0.03 0.06 0.06 0.01 0.02 0.03 0.09 0.13 0.14 0.09 0.11 0.12

451 692 751 777 805 509 482 596

26,923 17,131 15,884 22,984 25,795 23,220 27,623 23,131

29 9 14 40 51 40 38 23

0.9985 0.9998 0.9999 0.9997 0.9996 0.9993 0.9990 0.9991

0.19 0.05 0.04 0.04 0.04 0.07 0.13 0.13

Butanal Pentanal Hexanal Heptanal Octanal Nonanal Benzaldehyde

−390 −291 −302 −348 −317 −193 −2254

46,487 45,727 50,575 57,534 60,581 58,678 130,659

142 143 160 182 192 189 480

0.9969 0.9980 0.9994 0.9997 0.9998 0.9998 1.0000a

0.11 0.11 0.07 0.05 0.05 0.05 0.06

N,N-Dimethylformamide N,N-Diethylformamide N,N-Dimethylacetamide

−1355 −1737 −1386

99,141 122,639 106,308

310 391 328

0.9991 0.9993 0.9993

0.16 0.19 0.14

Butanenitrile Pentanenitrile Hexanenitrile Heptanenitrile Octanenitrile Benzonitrile

−1031 −1237 −1041 −931 −809 −2599

83,902 97,061 91,057 89,235 86,855 151,974

244 290 278 277 274 532

0.9991 0.9996 0.9995 0.9999 0.9999 1.0000a

0.08 0.07 0.09 0.05 0.04 0.04

1-Nitropropane 1-Nitrobutane 1-Nitropentane 1-Nitrohexane Nitrobenzene

−1018 −1011 −966 −964 −2752

80,328 81,824 83,355 87,220 150,659

255 270 279 294 575

0.9998 0.9999 0.9999 1.0000a 1.0000a

0.07 0.07 0.06 0.02 0.06

1-Butylamine 1-Pentylamine 1-Hexylamine 1-Heptylamine 1-Octylamine Aniline N,N-Dimethylaniline

−1638 −1028 −495 −173 −387 −2625 −1608

123,034 94,763 67,086 54,557 71,246 162,272 99,479

327 256 194 162 208 531 407

0.9958 0.9970 0.9993 0.9982 0.9988 0.9998 1.0000a

0.08 0.09 0.05 0.08 0.07 0.14 0.10

Propyl acetate Propyl propanoate Propyl butanoate Propyl hexanoate Propyl heptanoate Methyl pentanoate Ethyl butanoate Butyl acetate

a

Value higher than 0.99996.

order to minimize the ring strain (angle, torsional and steric strain). As a result, the variation of the contact surface area along the series for alicyclic compounds is not regular, causing small deviations of their retention with z, which leads to a worse correlation between I and z. Secondly, if each homologous series is considered separately, their m values increase slightly in the 333–423 K range. This increment (m) ranges from 0.7 for 1-chloroalkanes, 1-alkanols, 2-alkanones or alkanals, to 1.6 for 1-nitroalkanes. However, alicyclic compounds were again an exception to this behaviour (Fig. 4), since a higher increase of the m value with increasing temperature was observed (m = 8.4 and 9.7 for cycloalkanones and cycloalkanols, respectively). This quite high variation of m can be explained by the fact that increasing column temperature decreases the SP cohesion, making easier the solute insertion. Obviously, the difficulty of the solute transfer from the gas phase to the SP is far more evident for alicyclic compounds due to their higher stiffness and surface area, and this difficulty increases with increasing the solute carbon atom number. Consequently, the higher the ring size, the higher the temperature effect on its retention, and therefore, the increase of I values with z becomes steeper as temperature increases.

4. Conclusions Isothermal retention indices for 67 solutes on a TRB-1 column at the 333–423 K temperature range have been obtained. They were calculated with the LQG method, which provides very accurate I values without the need of hold-up time. Comparison of these I values with those taken from the NIST database yields satisfactory results, especially for capillary columns. Moreover, the 670 I values in poly(100% dimethyl siloxane) SPs reported in this work provide an important contribution to increase the isothermal retention index data in the NIST database, in which cyclopentanol and 1-octylamine have no I data obtained on capillary columns, and 1-chloroheptane, 1-chlorooctane, 1-chlorononane, N,N-diethylformamide, N,N-dimethylacetamide, cyclobutanol, heptanenitrile and octanenitrile have no previous I determination at all. The large amount of data obtained has allowed us to study the column temperature effect on I. Overall, three different behaviours have been observed for the variation of I with temperature in the 333–423 K range. First and as a general rule, I increases with increasing temperature, but the opposite trend was found for linear alcohols and esters. Secondly, cyclobutanol and the first terms of the linear ketones (2-butanone and 2-pentanone) and amines (1-butylamine and 1-pentylamine) show a well-defined minimum between 358 and 377 K. Thirdly, other compounds such as 1-nonanol show a curvature at the higher temperatures, this being the first time that a minimum has been detected in the I vs. T plot for a solute that is not an early member of a homologous series. All these types of variation with the column temperature were perfectly described by the extended model. On the other hand, linear relationships were found between the retention index and the number of carbon atoms in the solute. A slope value around 100 was obtained, except for cycloalkanols and cycloalkanones for which higher values were observed (ca. 15–40%). Moreover, a significant increase of these slopes with increasing temperature was noticed for these alicyclic compounds. They are more affected by the easier solute insertion in the SP as temperature increases, due to their higher stiffness and surface area, this being more evident with increasing cycle size. As a result, for alicyclic solutes the slope of the I vs. z plot becomes steeper as temperature increases.

J.M. Santiuste et al. / J. Chromatogr. A 1365 (2014) 204–211

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