Correlations between HPLC and NMR properties of some
selected alkyl bonded phases
Citation for published version (APA):
Claessens, H. A., Ven, van de, L. J. M., Haan, de, J. W., Cramers, C. A. M. G., & Vonk, N. (1983). Correlations
between HPLC and NMR properties of some selected alkyl bonded phases. HRC & CC, Journal of High
Resolution Chromatography and Chromatography Communications, 6(8), 433-435.
https://doi.org/10.1002/jhrc.1240060806
DOI:
10.1002/jhrc.1240060806
Document status and date:
Published: 01/01/1983
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Short Communications
1041
3
Correlations between HPLC and NMR Properties of Some
Selected Alkyl Bonded Phases
H. A. Claessens*,
L.
J. M. van de Ven, J. W. de Haan, and C. A. CramersEindhoven University of Technology, Laboratory of Instrumental Analysis, P. 0. Box 513, 5600 MB Eindhoven, The Netherlands N. Vonk
Chrornpack Nederland B.V., P. 0. Box 3,4330 AA Middelburg, The Netherlands
Key Words:
Reversed-phase HPLC Solid state NMR
Surface characterization
1
Introduction
The silica alkyl bonded phases (SAPs) popular in HPLCencounter a number of problems in practice. The surface properties of SAPS are determined by source of the silica, the modifying reagent, the reaction conditions, etc., and are at least partly responsible for instances of non-reproducible capacity ratios
(k'),
selectivity (r), and efficiency (N) found with several SAPs. Differences in behavior are observed between productsfrom several sources, as well as batch to batch from one manufacturer. Packing, aging processes, and stability of columns cause problems in practice. Pertinent investigations are published regularly 11 -41. This state of affairs motivated our group to start an investigation into the subject in 1976 [5]; surface studies were done by the mull IR technique. The availability of CP-MAS-NMR prompted us to undertake further studies.Recently, information has been published regarding the charac- terization of silica-attached system by "Si-and 13C CP-MAS-NMR [6-111. It was shown that a number of structural features of SAP- binding can be deduced from "Si- and 13C chemical shifts [7,8,
lo].
Although quantitative interpretation of CP-MAS-NMR spectra is notoriously difficult[6],
it was shown that some of the problems can be overcome [9]. Here, preliminary solid-state NMR results of eight SAPs are correlated with HPLC properties(Table 1). Table 1
Survey of packing materials (SAP'S) under investigation
2 Experimental
Chromatographic test conditions: columns 25 cm X 0.46 cm internal diameter; flow rate 1 mllmin; injections 1 PI; detection 254 nm, 0.04 aufs.
Each column was tested three times:
Exp. 7: eluent MeOH/H20 = 60 : 4Ovlv; test mixture (a) consisting of phenol, p-cresol, anisol, 2,5-dimethylaniline, ethyl benzoate, and toluene in MeOH.
Exp. 2: eluent, anhydrous n-hexane; test mixture (b) consisting of 1-octene (dead time) and nitrobenzene in n-hexane.
Exp. 3: same as exp. 1. The k'values and selectivities were calculated from the chromatograms.
Between experiment 1 and 2, the columns were flushed with 100
ml
anhydrous MeOH, 100 ml anhydrous 2-propanol, and 200 ml anhydrous hexane. Between experiment 2 and 3 the same program was used in reversed mode.CP-MAS-NMR spectra were run on a Bruker CXPBOO spectro- meter at 75.48 MHz for 13C and 59.63 MHz for "Si. Proton
fi
pulses of 3 ps were used throughout. Contact times were 5 ms2
SAP Name
number
Batch Mean Specific
YO
Carbon Endcappingparticle area m2/g load
size Nucleosil C-8 Polygosil C-8 CP-Spher C-8 LiChrosorb RP-8 LiChrosorb RP-8 LiChrosorb RP-18 CP-Spher '2-18 Hypersil ODS 2111 909 1 E-68 1417 1459 2406 077 9/ 1 042 10 10 10 10 10 7.5 5 7.5 300 500 325 250 250 250 325 170 11 11 12 12 19.5 16 9.5 8.5 no no Yes no no no Yes no
Short Communications
(13C) and4ms(29Si),dec~uplingtimeswere50ms(13C) and 1Oms ("Si), while 1 s repetition times were used in both cases. Computer resolution was 4.9 Hz, chemical shifts were referred to TMS. Samples were spun in Andrew-type Delrin rotors at
4
kHz.3 Results and Discussion
At constant eluent composition, k' values will contain information about solute-surface interaction of the SAP's. The k' values of test mixture (a) were corrected for differences in specific area, % carbon load of the SAPs. The result is a consequent sequence of k' values for the C-8 SAPs (Table 2a). To get information about the residual silanol groups, k' values of nitrobenzene were determined in experiment 2 (Table 2b).
Table 2
a = sequence of k values of C-8 SAP's components of test mixture (a), except 2,5-dimethylaniline. b = sequence of k' values of C-8 SAP's of nitrobenzene.
c = sequence of residual silanols by CP-MAS-NMR. d = sequence of ligand coverage by CP-MAS-NMR.
e = sequence of ligand coverage by HPLC according to eq. (1).
a b C d e 2 5 5 2 2 1
4
4 1 1 5 33
54
4
1 14
5 3 2 2 3 3Functional groups were also measured by CP-MAS-NMR. The respectiye "Si- and 13C NMR spectra Nere run under identical conditions and yielded useful and consistent parameters for the compounds under study [12]. Typical spectra are very similar to those published earlier [7], see Figure 1. The following binding types could be unequivocally distinguished; their respective "Si ,NMR chemical shift ranges are summarized in the scheme below.
(X denotes "internal" silicon atoms
or
silicon in a neighboring silane group,R.=
CnH2,+, with n = 8 or 18).Type 1 Type
2
Type 36 = 12.7 ppm 6 = -9 to -18.5 ppm 6 = -48 to -66 ppm (XO) Si (CH3)2R (XO)2 Si (CH3) R (XO)3 SiR
(XO) Si (OH) (CH3)R
(XO) Si (OCH3) (CH3) R (X0)$3i (OCH3)R (X0)ZSi (0H)R (XO) Si (OH)*R (XO) Si
(OH)
(OCH3)R(XO) Si (OCH&R
Types 1, 2, and 3 clearly correspond to binding situations originating from tri-, di-, and monoalkylsilanes, respectively. More
434
VOL. 6, AUGUST 1983 Journal10413 SAP 1 f i -0 -50 -100 PPm Figure 1
Typical *'Si NMR spectra.
detailed assignments have to await the availability of appropriate model compounds [8]. The presence of -OCH3 groups was detected with 13C NMR. Distinction of type 1 from end-capping (with HMDS) is possible by 29Si NMR chemical shifts. From the ratios of integrated intensities of the signals in the "Si NMR spectra, sequences of coverage ("Si signal from silane divided by the total "Si signal) and residual silanol groups ("Si signal near -1 02 ppm divided by the total "Si signal) were deduced. Typicaltest chromatogramsofSAPs 1 and5areshown inFigure2. According to reversed-phase retention theory, k' values depend on ligand mass and silanols exposed to solutes. The k' values of nitrobenzene are assumed to provide information about the residual silanols, as do the k' values of apolar toluene about the ligand mass. The k' values of both nitrobenzene and toluene for each SAP were normalized to SAP no. 3. These two values correspond to the relative amount of residual silanols and the ligand mass of the SAPs, respectively. From these values the am6unt of ligand was calculated according to eq. (1).
L
Ligand coverage = ~
S + L
where:
S = relative residual silanols calculated from k' nitrobenzene for SAP i
L = relative ligand mass calculated from k' toluene for SAP i. A number of interesting correlations were observed. The sequence of concentrations of residual silanol groups corre- sponds well (one to one) with the order of k' values of nitroben- zene in hexane (Table 2b,c). Moreover, there is approximate agreement between the ligand coverage (determined by NMR and HPLC independently, vide supra) and the
K
values of the test compounds in methanollwater (Table 2a,d,e). The same is trueShort Communications 10413
SAP 5
3
1
5 + 6 to be correlated with a low residual silanol coverage, forC8aswell as for C18. Finally, the nature of silane surface bonding seems to play only a minor role in connection with the present results. Possible effects could, however, very well be masked by a dominating influence of either residual silanol or silane coverage. In experiment 3, behavior of the column differing from experiment 1 was observed with respect to
k’
values, plate height, and peak symmetry. A lack of any systematic trend in these deviations merits further investigation.Future research in these laboratories will include, inter aha:
0 Influence of organic modifiers on surface properties.
0 Improvement of qualitative and quantitative CP-MAS-NMR
0 Study of aging and packing processes. aspects.
h
a
1 2
1
0 Relation between surface morphology and retention; selec- tivity behavior. 0 5 10 SAP 1 5 4
h
0 5 1.0 (min) Figure 2Typical test chromatograms; see Experimental for conditions. 1 = phenol; 2 = p-cresol; 3 = anisole; 4 = 2,5-dimethylaniline; 5 = ethyl benzoate; 6 = toluene.
for relative selectivities taking CP-Spher C8 as a standard. When comparing C8 and C18 on the same silica base (i. e. LiChrosorb and CP-Spher) the latter contains more residual silanol groups and less silanes. Asymmetric LC peaks for 2,5-dimethylaniline appear
Journal of High Resolution Chromatography & Chromatography Communications
References
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9.
L. Ponzo, J. Chromatogr. Sci. 20 (1982) 14. [2] D. P. Lee, J. Chromatogr. Sci. 20 (1982) 203.[3] I? E. Barker, B. W. Hat?, and S. R. Holding, J. Chromatogr. 206
(1981) 27.
[4]
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Engelhard?, 0. Dreyer, and H. Schmidt, Chromatographia lS(1982) 1 1 .[5] J. L. M. van de Venne, “Nonpolar Chemically Bonded Stationary Phases in Liquid Chromatography”, Wibo, Helmond, The Nether- lands (1979).
G. E. Maciel and D. W. Sindorf, J. Am. Chem. SOC. 102 (1980) 7607. G. E. Maciei, D. W. Sindorf, and V. J. Bartuska, J. Chromatogr. 205 (1981) 438.
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