Anomalous Behavior of Some Aminoalkyl Polysiloxanes as Stationary Phases in Gas Chromatography J. R . Ashes and J. K. Haken Department of Polymer Science, The University of New South Wales, P.O. Box 1 , Kensington, N . S . W . 2033, Australia
An examination of several aminoalkyl polysiloxanes as stationary phases in gas chromatography shows that with these compounds containing both primary and secondary amino groups in the pendant groups, on column chemical reaction occurs with carbonyl compounds. The specificity compares favorably with other materials that have been reported as abstractors while the column stability and service life are enhanced. The polar character of the phases and the tendency to form hydrogen bonds are shown by the separation of isomeric alcohols.
Polysiloxanes as gas chromatographic stationary phases enjoy a unique position as a series of compounds which vary from essentially non-polar materials to those of considerable polar character and exhibit some donor and acceptor properties. Studies in these laboratories have been reported concerning the effect of molecular structure of various oxygenated compounds (1-3) on retention as observed with polysiloxane solvents possessing a common backbone structure where the effective character is due to the substituent groups. The phases may be characterized in terms of Rohrschneider or McReynolds constants and the effect of the substituent phenyl, chlorophenyl, cyanoalkyl, and fluoralkyl groups is evident in the values of the various constants and in the gas chromatographic performance. It was apparent that a polysiloxane was not available that exhibited a high S or S' value, Le., proton acceptance as indicated by pyridine, and a study of available commercial materials was initiated. The present work describes the examination of several aminoalkyl polysiloxanes where it is apparent that on column chemical reaction with carbonyl compounds occurs due to the formation of a carbon-nitrogen double bond resulting from elimination of a molecule of water from the initial addition products. The specificity of the reaction is shown to be greater than that of other reactions employed in abstractor columns though, as is general in reaction gas chromatography, the situation is not ideal. The column constants of the aminoalkylsiloxanes would suggest a suitability for the separation of compounds that exhibit hydrogen bonding. An examination of isomeric alcohols has been carried out using the most polar of'the materials and the results have been compared with those of Armeen SD, a material recommended for the separation ( 4 , 5 ) .The aminoalkylsiloxanes exhibit a high degree of selectivity and with some isomers the packed column performance is superior to that of Armeen SD in that the elution of branched chain species is further enhanced. (1) 1. D. Ailanand J. K . Haken, J. Chromatogr., 51, 415 (1970). (2) J. R. Ashes and J. K . Haken, J , Chromatogr., 60, 33 (1971). (3) J. K . Haken and V. Khemangkorn, J. Chromatogr, Sci.. 10, 41 (1972). ( 4 ) A. Zlatkis in "Gas Chromatography.'' E. Bayer, Ed, Elsevier Publishing Co. Amsterdam, 1961, p 11 7. ( 5 ) J. E. Zarembo and I. Lysyj. Ana/. Chem.. 31, 1833 (1959).
EXPERIMENTAL Gas Chromatography. The retention data were obtained on a modified F & M 8lOj29 Research Chromatograph with simultaneous flame ionization and thermal conductivity detection and fitted with an improved flow control system. Two 12-ft X ?h-in. o.d., aluminum columns were packed with 10% of each of the stationary phase on 60/80 mesh acid washed and silanized Celatom and operated isothermally a t 100 and 130 "C. A precolumn 6-in. X y4-in. o.d., was prepared using 20% of 26020 and was operated a t 100 and 130 "C. The detector temperatures were 220 "C, the injection temperature was 190 "C. Helium was used as the carrier gas with an inlet pressure of 45 psi and flow rate of 30 ml/min. With the thermal conductivity dtector, the bridge current was 150 mA. The sample size used was 0.1 el. Stationary Phases. The polysiloxanes used contained an ethylene diamine moiety with both primary and secondary amino groups in their structure. 26020 ( I ) is h:-p-aminoethyl-y-aminopropyl trimethoxysilane. DC530 (Dow Corning Corp., Midland, Mich.) is a copolymer of a low molecular weight dimethyl polysiloxane and 26020. Analysis shows the amine content of' DC530 to be approximately that of 26020. DC531 is a 50% solution of DC530 in 2-propanol and aliphatic solvents. DC535 (Dow Corning) or MS2560 (Midlands Silicones Ltd.. Barry, Glamorgan, Wales) are low molecular weight dimethyl polysiloxanes (D.P. = 100) containing approximately 2% of the ethylenediamine moiety. X-22-857 (Shin-Etsu Chemical Industry Co. Ltd., Japan) is of the same structure as DC535 but with approximately 21h7c substitution. McReynolds constants and other data concerning these compounds are shown in Table I.
DISCUSSION AND RESULTS During determination of the column constants, it was observed that no response for 2-pentanone was apparent a t the attenuation used for the other reference materials. By increasing the sample size by a factor of 20 and decreasing the attenuation a very small peak was observed which was used for measurement. It is apparent with almost all abstraction reactions, that completely quantitative removal is not achieved and if the attenuation can be reduced sufficiently a small peak due to minor residual ketone will be observed additional to any impurity that may have been present and eluted with 2-pentanone. From Table I, it is apparent that while the S' values, i.e., proton acceptance as indicated by pyridine are high, the Y values indicative of hydrogen bonding, i. e., hydrogen donor as shown with n-butanol, are also high. This effect is as expected due to hydrogen bonding of the type ( 0 - H . . . . NH-j. A proton may be donated by the hydroxyl group while the nitrogen atoms in the polyamine are common proton acceptors. The removal of 2-pentanone prompted examination of the aminoalkyl siloxanes as ketone and aldehyde abstractors in terms of criteria for an ideal subtractor column which have been detailed (6) as 1. Specific activity for a limited number of functional groups, i.e., preferably one, and inactive with other functional groups. 2. The column abstraction should be complete. 3. Compounds not abstracted should not suffer retardation or peak spreading. 4. The reagent selected must be suitable for use in a column such that it forms an integral part of the chromatoA N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
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The behavior of a variety of representative ketones was examined using the aminoalkyl siloxane materials at column temperatures of 100 and 130 "C with results as shown in Table 11. The straight chain methyl ketones were essentially removed on all four substrates while with the ethyl homologs (3-alkanones), a somewhat variable performance was observed. Removal of 3-pentanone and 3-heptanone was essentially complete, 3-octanone was partially removed, and 3-nonanone and 3-decanone were completely
Table I. McReynolds Constants and Data of Aminoalkyl
Polysiloxanes McReynolds Constants Compound
Viscosity (CS25"C)
X'
Y'
2'
26020 DC530 DC531 DC535 X-22-857
10.0 80.0 150.0 3000.0 80.0-100.0
247 63 30 24 31
700 269 155 159 166
393 129 119 70 67
U' 454 204 84 93 95
S' 433 116 62 70 78
removed.
Branched chain ketones were only partially removed and a consideration of some of the structures is of interest in relation to the percentage removal with the less effective 26020 precolumn. Several branched chain methyl a n d diketones are shown on the next page.
graphic equipment. 5 . The reagent should be stable and reproducible in use. 6. The reagent should have a high capacity for the functional class being s u b t r a c t e d .
Table I I. Percentage
Removal of Ketones by Aminoalkyl Polysiloxanes Aminoalkyl Stationary Phases Dk535
Ketone
Aliphatic 2-Propanone 2-Butanone 2-Pentanone 2-Hexanone 2-Heptanone 4-Methyl-2-pentanone 3,3-Dimethyl-2-butanone 5-Methyl-2-hexanone 3-Pentanone 3-Heptanone 3-Octanone 3-Nonanone 3-Decanone 4-Heptanone 2-,4-DimethyC3-pentanone 4-Octanone 5-Nonanone 2,6-Dirnethyl-4-heptanone 1 , l -Dimethoxy-3-butanone 4-Methyl-4-methoxy-2-pentanone Unsaturated Aliphatic 1-Butene-3-one 4-Methyl-3-pentene-2-one Acyclic
Cyclopentanone Cyclohexanone Cycloheptanone 4-Methylcyclohexanone 2-Methylcyclohexanone Turpenic Pulegone Aromatic Acetophenone Propiophenone Butyrophenone Diketones 2,3-Butanedione 2,4-Pentanedione 2,4-Hexanedione 2,5-Hexanedione Trifluoracetylacetone Hexafluoroacetylacetone Miscellaneous 4-Hydroxyl-4-methyl-2-pentanone
Methyl pyruvate
1132
X-22-85?
DC530
26020
26020 100'C
Precolumn 130°C
100°C
130°C
100'C
130°C
100°C
13OoC
100°C
130°C
100 100 100 100 100 67 46 100 86 92 59 100 100 76 58 81 100 43 100 94
96 91 98 100 100 59 22 98 65 64 16 86 99 56 0 56 91 0 42 79
97 97 100 100 100 71 51 100 85 97 68 100 100 89 22 93 100 29 100 96
100 100 100 100 100 70 16 100 72 93 58 100 100 81 0 84 98 19 100 100
100 100 100 100 100 95 51 100 86 97 68 100 100 92 0 100 100 14 100 100
100 100 100 100 100 91 16 100 75 93 58 100 100 93 0 100 100 0 100 100
100 100 100 100 100 100 58 100 100 100 100 100 100 100 50 100 100 71 100 100
100 100 100
100 100 100 100
100 100 50 100 100 100 85 100 100 100 34 100 100 36 100 100
100 82 100 100 100 100 100 100 100 63 100 100 76 100 100
100 100 100 100 100 100 73 100 100 100 100 100 100 100 30 100 100 66 96 100
100 89
100 90
100 67
100 70
100 86
90 100
100 100
100 100
100 100
90 100
100 100 100 100 100
100 100 100 100
100 100 100 100 100
100 100 100 100 100
100 100 100 100 100
100
100
100 100 100 100 100
100 100 100
100 100 100 100 100
100 100 100 100 100
100 100 100 100 100
100
50
100
100
100
100
100
100
100
100
100
100 100
100
96 99 100
100 100 100
100
100
100
100
100 100
100 100
100 100
100 100
100 100 100
100 100 100 100
100 100 100 100 100 44
100 100 100 100
100 100 100 100
100 100 100 100
100
100
100
80
71
100
100 100
100
100 100
100 100
100 100
100 100
100 100 100
100 100
100 99 100 95 99 84
100 100 100
100 100
99 36
99 60
100 100 100 100 98 24
100 100
100 100
100 100
100 97
100
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
100
50
100
100
100
100
100 100 100 100 100 93 100
100
Per cent removal at
100 "C
130 "C
46
22
67
59
100
98
58
0
43
0
c C
I
0
II
(2)
c-c-c-c-c
(3)
I II c-c-c-c-c-c
C
I
(5)
0
C
I1
c-c-c-c-c-c-c
I
Table I I I. Percentage Removal of Aldehydes and Miscellaneous Compounds by Aminoalkyl Polysiloxanes X-22-857
DC535 Ketone
26020
DC530
26020
Precolurnn
100°C
130°C
100°C
130 "C
100 "C
130 "C
100 "C
130 "C
100
100 100 100 100 100
100 100
100
100
100 100 100 100 100
100
100
100
00 00 00 00
100
00 00 00
100
100 100 100
100 100 100 100 100
100 100 100 100
0
0
0
0
0 0 0
0
0 0
0 12 00
0 27 100
78
64
26 8
14 11
32
0 45 27
16
49
35
89
18 19 95
100 100 100
100 98 76
100 O
C
130 "C
Aldehyde
n-C, to n-Cs Benzaldehyde Salicaldehyde Citral Furfural
100
100 100 100
100
100 100
00
100 100 100
Epoxides
1,2-Epoxyethane 1,2-Epoxypropane Glycidyl acrylate 1 -Chloro-2,3-epoxypropane Oxygenated
Tetrahydrofuran Dioxane
0 0
0 0
0 0
100 0
100 78
0
0
0
0
0 0 0 78
0 0
0
0
0
0
0
12 00
53
18
100 100
100
00 17 0
0
0
0
0
0 72 95
24 72
45
36 46 56
97
97
100
100 100 100
100
100 100 100
100 94 0
100
Alkyl Halides
Perchloroethylene Ethylene dichloride Ethyl bromide Ethyl iodide
0 0 0 10
0 0
25 20
29 89
96 100
Lactones
1,3-PropioIactones Butyrolactone 4-Valerolactone
100 100 100
100 100
100
The shape of the molecule is of major importance, the branched chains partially shielding the carbonyl from the reactive amino groups. The effect is apparent with the structures 1, 2, and 3 where increased reaction occurs, as the branched chain becomes more remote from the carbonyl group. Similarly a comparison of the reactivity of structures 2 and 5 and 1 and 4 (these latter are not strictly comparable) shows reduced activity with replacement of the methyl group. These steric effects are evident from a consideration of models of the compounds. The methyl groups and carbonyl structure exhibit some donor and acceptor character, respectively, and t h e location of these groups relative to one another in a molecule has a pronounced effect on the interactions t h a t are experienced with another compound, i e . , t h e stationary phase, which is in its close proximity. With the substances t h a t are not completely removed on the four columns, 26020 with the highest amino content is the most effective reagent, followed by DC530. Other ketones, i. e., acylic, terpenic, aromatic, and diketones as shown in Table I1 were essentially removed by
100 100 100
100 77
42
100 100
100
100 100
the columns as were 1-buten-3-one and hexafluoroacetyl acetone both anomalies in early studies (6). Table I11 shows homologous straight chain aldehydes, the aromatic aldehydes benzaldehyde and silicylaldehyde and also citral and furfural which were all completely removed by the five columns. The epoxides examined were largely unaffected while several lactones were substantially removed and are also shown in Table 111. The use of 26020 as a precolumn abstractor is shown in Tables I1 and I11 where complete removal of the aldehydes and lactones studied was apparent while the epoxides were not affected. The behavior of the ketones was very variable but generally greater abstraction was achieved a t the lower temperature due to the increase in residence time as shown in Table IV where different flow rates were used at 120 "C in order to obtain similar residence times as those obtained a t 100 "C and 30 ml/min. As the flow rate for the two diketones increased, the per cent abstraction decreased. (6) J K Haken, D K M Ho and M K Withers J Chromatogr 10,566 (1972)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, J U N E 1973
So
1133
Table I V . Comparison of the Various Abstractors Reagent
Sodium borohydride(7)
o-Dianisidine (8)
Benzidine (8)
Sernicarbazide(9)
Sodium rneta-bisulfite (6)
26020
26020
Precolurnn at 150 OC
Precolurnn at 150 O max.
Precolurnn 150 "C max.
Precolurnn 96-1 15 "C
Precolurnn 65 "C
Column 100-130 "C
Precolurnn 100-1 30 "C
6 1
unknown all examined
11
13 11
38
6
2
1
39 36 3
39
5
...
...
1 11 11
Conditions
Ketones examined Completely removed Partially removed Not affected Aldehydes examined Completely removed Partially removed Not removed Epoxides examined Completely removed Part i a1Iy Not affected Lactones examined Completely removed
5 1 14
5 1 4
... ...
14 14
11 11
...
14
...
... ...
unknown some some ...
2 ...
2
C
II
+
HJ-G
+
11 11
11 11
5
1
4 1
...
...
...
...
4
1 1
3
4 3 3
H.0
by the use of the thermal conductivity detector, the broad peak characteristic of water is readily observed with compounds that undergo this abstraction reaction. Several other materials have been reported as abstractors for ketones and aldehydes and their comparison with the aminoalkyl siloxanes is considered. Regnier and Huang (7) reported the use of various metal halides as abstractors for oxygen-containing functional groups. Using the injection port column liner as a precolumn, sodium borohydride was shown to react exclusively with carbonyl compounds. The precolumn was operated a t 150 "C but the abstractions were incomplete, the performance with ketones being very poor. Schiff base formation by reaction of aromatic amines in six-inch precolumns has been reported by Bierl and her coworkers (8). Complete removal of various aldehydes was reported using 5% o-dianisidine within the range 50-175 "C; ketones except cyclohexanone were not affected while epoxides exhibited variable behavior. Studies in this laboratory (6) confirm the reactivity but show a high bleed rate a t 175 "C. The more reactive amine, benzidine was used at the 20% level within the temperature range 100175 "C to abstract aldehydes, epoxides, and most ketones. As the temperature was increased, the removal of sterically hindered ketones was increased. Much less satisfactory specific performance with this material has been experienced in this laboratory, where also substantial column bleed was observed. Cronin (9) has reported an even higher rate and stated that the column was unusable above 150 "C. Bisulfite addition formation with butyraldehyde was reported by Kerr and Trotman-Dickenson (IO). Acetone was (7) F. E. Regnier and J. C. Huang, J. Chromatogr. So., 8, 267 (1970). (8) B. A. Bierl, M . Beroza, and W. F. Ashton, Mikrochim. Acta, 1969, 637. ( 9 ) D. A. Cronin, J. Chromatogr., 64, 25 (1972). (10) J. A. Kerr and A. F. Trotrnan-Dickenson, Nature, 182, 466 (1958).
1134
14
..
7
\
25
8
The mechanism of reaction of the abstraction columns is shown below where elimination of a molecule of water is indicated
/ /
36
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
5
5 , . .
3
not removed by the reagent and no mention was made of the examination of any other carbonyl compounds. Sodium metabisulfite 10% has been reported (6) as.a carbonyl abstractor using a 10-inch precolumn provided by a Beroza-N.I.L. carbon skeleton apparatus. An extensive study of the reation indicated that abstraction was much superior and more generally applicable to carbonyl compounds than the other procedures reported. A serious limitation was the low thermal stability of the bisulfite addition compound which restricted use of the precolumn to approximately 65 "C; however, with the analytical column operating at higher temperatures, appreciable deterioration of peak shape was observed with compounds boiling a t temperatures in excess of 150 "C. With the bisulfite column, several epoxides were removed as was the single lactone examined, i . e . , butyrolactone. Cronin (9) has reported the use of a 25-mm precolumn containing 40% semicarbazide with in situ formation of the semicarbazone of various carbonyl compounds as a subtractive procedure in gas chromatography. Aldehydes and most ketones were substantially removed, 2-methyl3-pentanone and 3-octanone being only partially removed. The working limits of the semicarbazide are quite restricted, below its melting points, i . e . , 96 "C, reactivity is low while above 115 "C, a series of degradation products are produced. After two hours at 115 "C, 2-alkanones were not removed by the semicarbazide column. Consideration of the experimental results and of Table IV shows that 26020 as a column packing effectively abstracts aldehydes, lactones, and most ketones. Epoxides remain largely unaffected and a specificity superior to the simpler aromatic amines is achieved. Partial abstraction of ketones that are sterically hindered occurs, this behavior being common to the other abstractors that have been reported. As a precolumn, 26020 is less effective with ketones although performance with aldehydes, lactones, and epoxides remains unchanged. Methyl vinyl ketone was not removed by bisulfite addition formation but was effectively removed by 26020 both as a column or precolumn packing. While 26020 is of very low viscosity, significant column bleed was not evident a t the highest operating temperature as some reaction of the trimethoxy groups in the molecules occurs due to cross-linking and possibly bonding to the support. Repeated use of 26020 did not show the rapid deterioration that has been reported with other abstractor columns. The columns used in this work were subjected to over 400 injections of carbonyl compounds, i.e., each of
1
A
B
4
12 1%
n
'
12 11
C
I
I
U
6
?
4
15 14
(MINUTES)
TIME
Figure 1. Chromatograms showing separation of isomeric alcohols using ( A ) 30-m capillary column coated with Armeen SD at 65 "C ( 4 ) ; ( B ) 20-ft x ' p i n . packed with 1 0 % Armeen SD at 75 "C: and ( C ) 24-ft X %-in. packed with 10% 26020 at 100 "C (1) Methanol: ( 2 ) ethanol: (3) 2-propanol: (4) tert-butyl alcohol; (5) 1-propanol: (6) 2-butanol: ( 7 ) 2-methyl-2-butanol; ( 8 ) isobutyl alcohol: ( 9 ) 2-methyl3-butanOl. (10) 1-butanol: (11) 3-pentanol: (12) 2-pentanol: (13) 2,2-dimethyl-l-propanol; (14) 3-methyl-1-butanol: (15) 2-methyl-1-butanol: (16) 1pentanol
Table V . Percentage Abstraction at Equivalent Residence Times on 26020 Column temperature Flow rate (ml min)
120 "C
100°C
120°C
20
30
40
64
36
30 2 7
23
63 35
27
80 59
4 2
61 33
76 69
4 2
30
2 4-Dimethyi-3-pentanone
Percentage removal Residencetime ( m i n )
12
30
2,6-Dirnethyl-4-heptanone
Percentage removal Residencetime ( m i n )
66
66
the compounds having been examined in triplicate and a t two temperatures, without deterioration. After storage for 3 months. L e . , with the usual caps, the columns were again used without loss of performance. Like all abstractors, the columns will eventually become depleted and the capacity may be determined analytically. Esters. ethers, and hydrocarbons were not removed by the aminoalkyl siloxanes although reaction with alkyl halides occurred as primary and secondary amino groups will form secondary and tertiary amino groups, respectively. The reaction was most apparent with simple alkyl halides, and was less effective with more complex halocarbons. The increasing order of reactivity, as expected, would appear to be chlorides, bromides, and iodides. The separation of isomeric alcohols using Armeen SD has been reported by Zlatkis ( 4 ) and Figure 1A shows a chromatogram from a 30-m capillary column. Armeen SD
is a mixture of normal primary amines with the approximate composition 20% hexadecyl, 17% octadecyl, 2670 octadecenyl, and 37% octadecadienyl amines manufactured by Armour Chemicals. Figures 1B and 1C show separations achieved using a 20-ft X y,-in. stainless steel column containing 10% Armeen SD and a 24-ft X ?/,-in. 0.d. column containing 10% 26020, each on acid washed DCMS treated Celatom. The principal difference between Figures 1A and liy is that the partial separation of 2- and 3-pentanol is lost as the packed column may be expected to be less efficient, while with 26020 (Figure le),it is apparent that the resolution is inferior hut the elution order is significantly varied. With a non-polar phase, i.e., SE-30, an elution order essentially determined by boiling point is observed, this being in agreement with the data of McReynolds ( 1 2 ) . The same elution order is observed with Armeen SD; however, with 26020 the elution of branched chain species is enhanced. Tertiary butanol, i e . , 2-methyl-2-propano1, and 2-propanol, are both eluted before methanol and ethanol which in admixture are not resolved. 2-Methyl-P-b~tanol and 2-butanol are eluted before isobutyl alcohol. Similar behavior is apparent with a highly polar phase, i.e., DEGS, except that isobutanol is eluted before the 3and 2-pentanol isomers. The aminoalkyl siloxane 26020 is worthy of further examination for the separation of isomeric alcohols. In addition to a significant selectivity, a higher operating tem(11) W 0 McReynolds, Gas Chromatographic Retention Data, Preston Technical Abstracts Co , Evanston, 111 , 1966
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1135
perature is possible and in this work, i.e., 130 "C, the material has been routinely used without significant column bleed while the mixed primary amines show some bleed a t 75 "C and are unsatisfactory a t 100 "C. Attempts a t determining the column constants were not successful because of the volatility of the mixed amines. For routine use as abstractors, the lower alkyl polysiloxanes, i.e., DC530, X-22-857, are preferred as the elution order and the net retentions are very similar to those obtained with SE-30 which is conveniently used as a reference column. Zarembo and Lysyj ( 5 ) used a 33% Armeen SD column, and n-butanol was eluted a t 88 "C after 60 minutes with very poor peak shape such that the separation of longer chain alcohols was not recommended while with 26020 hexanols are more rapidly eluted. Because of the reactive nature of 26020, bonding onto the support or cross-linking in the presence of the support, is possible and in these circumstances higher temperature operation is possible.
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A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 7, JUNE 1973
CONCLUSION The use of polyalkylaminosiloxanes as stationary phases show significant retardation of alcohols due to the occurrence of hydrogen bonding and removal of carbonyl compounds by on-column chemical reaction. As abstractors, the materials have higher operating temperatures and greater specificity than sodium metabisulfite which is much superior to the other materials reported. The limitations of the materials are apparent by the reactivity with alkyl halides and the partial reaction of one of the phases with several other oxygenated compounds. It is suggested that other abstractor columns are much less specific than indicated because of the restricted range of compounds that have been used.
Received for review November 6, 1972. Accepted December 18, 1972.
