coordinates with the silver ion itself. Therefore, the steric hindrance of the double bond caused by an alkyl group a t the double bond carbon atom is not strong enough to prevent the complex formation to a considerable degree. On the contrary, the whole PdCl2 molecule is combined with an alkene. This bulky grouping very likely prevents complexation of the alkenes with the screened double bond. The effect of the steric hindrance of the double bond is more significantly expressed with the complexes containing PdC12. Therefore, a wider range of the stability con-
stants can be seen with these complexes, and it is responsible for a wider range of the retention data of the individual alkenes. The higher selectivity of the chromatographic separation of alkenes by PdC12-NMA can be explained on the basis of the structure differences between the alkenePdC12 and Ag+ complexes. Received for review June 18, 1973. Accepted January 14, 1974.
Gas Chromatographic Determination at the Parts-per-Million Level of Aliphatic Amines in Aqueous Solution Antonio Di Corcia and Roberto Samperi lstituto di Chimica Analitica, Universita di Roma, 00785 Roma, Italy
Modifications of Sterling FT-G and Vulcan, which are two well-known examples of graphitized carbon blacks (GCB) with suitable amounts of KOH and polyethylene glycols (PEG), e.g., PEG-2OM and PEG-1500, allow the linear elution of free aliphatic amines from C1 to C16 to be performed. In particular, because of the use of such packing materials, the quantitative determination of C 1 - G aliphatic amines in aqueous solution is made possible even at the sub-ppm level. At these high sensitivities, the sole factor affecting in some measure the quality of the chromatographic profile is due to the water disturbance. By varying the liquid-to-solid ratio, gas-liquid-solid (GLS) columns have been evaluated in terms of selectivity and elution time.
Gas chromatographic determination of strong organic bases is often unsatisfactory although this method is recommended and widely used. Errors accrue from loss of sample, ghosting phenomena and badly tailed elution peaks. The source of these unwelcome effects is always traced to abnormally strong solid-gas interactions, which take place either by using gas-liquid (GLC) or gas-solid chromatography (GSC). Therefore, efforts for improving the gas chromatographic method for the analysis of aliphatic amines are generally confined to the search for means of eliminating strong adsorption effects. In GLC, strong adsorption is the result of hydrogenbonding to the protonized silanol groups of the commercially available support materials, which are siliceous in nature. Among the numerous remedies offered to reduce the influence of the support, the treating of the substrates with an alkali hydroxide actually appears to be the most effective. Alkali coating enhances the performance of gasliquid columns; yet it is common enough to find large irregularities when quantitative determinations of basic nitrogen-containing compounds are attempted (1-4). With exception (j),works reported in literature concerning R. A . Simonaitis and G. C. Guvernator I l l , J . Gas Chromafogr.. 5 , 49 (1967). L . D Metcalfe and A . A . Schmitz, J. Gas Chromatogr.. 1, 15 (1964) J. J. Cincottaand R Feinland, Anai. Chem.. 3 4 , 774 (1962) S. Hantzsch, J. Gas Chromatogr.. 6, 228 (1968) G. R . Umbreit, R . E. Nygren. and A . J . Testa, J. Chromatogr.. 43, 25 (1969)
GLC of unaltered amines deal only with qualitative aspects or a t the best with quantitative determinations of concentrated solutions. In GSC, peak tailing, which sharply increases as the sample size of a basic eluate is decreased, is accounted for by the formation of some kind of chemical complex between the adsorbate and traces of oxygen complexes (6) or metal transition salts (7) contaminating the surfaces of the commonly used adsorbing materials. Thermal treatment a t 1000 "C of graphitized carbon black (GCB) with a stream of hydrogen was found very effective in removing either chemical (6) or geometrical irregularities (8), thus reducing greatly the peak tailing for hydrogen-bonding compounds (9). However, hydrogentreated carbon surfaces pick up carbon-oxygen complexes on exposure to water (10). This precludes the determination of amines in aqueous solution. The usual way of preempting surface heterogeneities is to add a suitable amount of a nonvolatile liquid phase itself containing basic nitrogen, so as to make high-energy sites unavailable for adsorption of eluates. Hollis (11) reported that linear elution for aliphatic amines was obtained by coating porous polymers with either polyethylene imine (PEI) or tetraethylene pentamine (TEPA). Also, the effect of increasing the PEI loading was to gradually reduce retention times of amines (7). TEPA modified GCB enabled us to achieve a well-defined elution peak for few nanograms of methylamine (12). In this work, by varying the liquid-to-solid ratio, gasliquid-solid (GLS) columns were evaluated in their ability to separate the first terms of aliphatic amines. It was pointed out that a large range of selectivity could be made available by changing the surface concentration of the liquid. However, because of the volatility of TEPA, aliphatic amines containing more than three carbon atoms could hardly be eluted. (6) A . Di Corciaand R Samperi, J . Chrornafogr., 77, 277 (1973). (7) J. R . Lindsay Smith and D. J. Waddington, Ana/ Chem.. 40, 522 (1968). (8) A . Di Corcia and R . Samperi, J. Phys. Chem., 77, 1301 (1973) (9) A . Di Corcia and F. Bruner, Anai. Chem.. 43, 1634 (1971) (10) R . Nelson Smith, J. Duffield, R. A . Pierotti, and J. Mooi, J. Phys. Chem.. 60, 495 (1956). (11) 0. L . Hollis,Ana/. Chem., 38, 309 (1966). (12) A. Di Corcia. D. Fritz, and F . Bruner, Anal. Chem 42, 1500 (1970) A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
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"t1
I2L
I
' 'i-
iio Figure 1. Plots of the capacity ratio ( k ) for butylamine (---) and V S . the relaseparation factors (cu) for some amine pairs (-) tive amount of PEG-20M added to 0.3% KOH modified Sterling FT-G for
a , Propylamine/isopropylamine; 0, isobutyiamine/sec-butylamine; 0 . diethylamine/ferf-butylamine; 0 , ethylamine/trimethylamine; A. terfbutylamine/propylamine;A ,trimethylamine/dimethylamine
In the present paper, an improved chromatographic method is reported for the trace amounts determination of aliphatic amines from CI to ClS using GCB modified with appropriate amounts of KOH and polyethylene glycols. Particular attention was paid to the analysis of aqueous solutions of the first members of aliphatic amines a t the ppm level. By varying the liquid-to-solid ratio the chromatographic behavior of this kind of GLS columns is briefly discussed.
EXPERIMENTAL Two graphitized carbon blacks were used in the present worknamely, graphitized sterling FT (Sterling FT-G) and Vulcan. These two materials (60-80 mesh) were provided by Supelco Inc., Bellefonte, Pa., as Carbopak A and Carbopak B: respectively. Measurements of specific surface areas, based on nitrogen adsorption and assigning for the cross-sectional area of r\;2 a value of about 16 A2, gave respectively values of about 15 and 110 m2/g. Column packings were prepared by dissolving weighed samples of KOH and polyethylene glycol in methanol and adding the SOlution to a known weight of GCB. Packings were dried with use of a heat lamp and no special attempt was made to avoid pick up of COZ. While drying. stirring of these materials must he avoided, as it may cause some crushing of graphitized carbon particles. This precaution is particularly important when working with Carbopak A, whose mechanical strength is rather low, when compared with the solid media commonly used as supporting materials for gas-liquid columns. In any case, dried materials were resieved to maintain the proper mesh range. The following packing materials were used: 1) Sterling FT-G + 0.3% + 1.3% PEG-20M; 2) Sterling FT-G + 0.270 KOH 0.5% PEG-1500; and 3) Vulcan + 0.8% KOH + 4% PEG-2OM. With the materials prepared in this fashion. columns made from glass tubing (4- or 2-mm i.d.) were packed by moderately vibrating with aid of a vibrator. The first three centimeters of the inlet side of each column were left empty to use as an expansion chamber for injected liquid samples. To avoid peak ghosting, the glass wool at the inlet side of the column was eliminated (131. (13) D. A . M. Geddes and M . N. Gilmour, J. Chromatogr. Sci.. 8, 394
+
(1970).
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ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
Columns of PEG-ZOM coated GCB were conditioned for 24 hr at 220 "C; when PEG-1500 was used, the conditioning temperature of the column was brought down to 150 " C .Before using these columns, they were preconditioned by injecting 10 p1 of water (3 pl for a 2-mm i.d. column) about thirty times in rapid succession, a t the temperatures reported above. This procedure is also necessary when a column has been left unused for some time. Helium completely free of COz was employed as carrier gas to avoid reaction with KOH. We noticed that even the presence of traces of COz in the carrier gas a t the best resulted in slow but continuous modification of the characteristics of retention and separation of the packing material, while at the worst, tailing in the peaks for amines appeared after a prolonged use of the column. Anyway, CO2-contaminated columns could be completely restored by water injections, as described above. Some considerations about this effect suggest the hypothesis that the role played by water is that of destroying the chemical compound formed between CO2 and KOH through a reaction of hydrolysis. The apparatus used was a Carlo Erba gas chromatograph model GI (Milan, Italy) equipped with a flame ionization detector and constructed so as to allow the sample to he injected directly into the column packing. The chromatographic apparatus was connected to a Leeds and Korthrup Speedomax Model G recorder operating with a 1-mV, full-scale response. At the maximum sensitivity of the amplifier system (1 x 1) about 1.5 pA give a fullscale response of the recorder. The artificial mixtures of aliphatic amines considered were prepared from various commercial sources. A 10-p1SGE (Melbourne, Australia) syringe was used.
DISCUSSION OF RESULTS For each column packing investigated, calibration curves ranging from fifty to a few nanograms of methylamine, ethylamine, and propylamine were prepared. Correct straight-line curves passing through the origin were obtained for each set of experiments, thus making evident the absence of anomalous effects of adsorption. It was previously reported (6) that about 0.1 pg of propylamine eluted on uncoated GCB could not be detected in spite of the use of a very sensitive detector, thus indicating a sort of "salt formation" taking place between the eluate and surface acidic sites. As a first attempt, KOH was added to the graphitized carbon. This modification improved the performance of the column, though peaks for amines emerging from the column were still tailed. This demonstrated that the inorganic base was not capable of neutralizing every polar adsorbing site. Peak tailing could be definitively suppressed by further addition of an appropriate amount of a medium-polar, high-boiling organic compound, such as PEG-20M. Generally, coating the solid surface by a liquid also produces the effect of modifying the chromatographic features of the solid substrate to a greater or lesser extent depending upon the liquid-to-solid ratio. The behavior of the system investigated as the relative quantity of PEG-20M is increased using a constant amount of KOK (0.3% w/w) added to the Sterling FT-G is shown in Figure 1. The quantities plotted are the capacity ratio (dotted line) a t 68 "C. which is proportional to the retention time, for an amine of reference, e . g . , butylamine, and separation factors (solid line) a t 49 "C for some pairs of amines arbitrarily considered us. percentage of liquid, based on the liquid-to-solid ratio. First of all, because of the low degree of polarity of the liquid used, symmetrical peaks for amines were obtained only for relative amounts of PEG-20M higher than 1.11.2% deposited on 0.3% KOH modified GCB. The capacity ratio reported for butylamine decreases sharply as small amounts of polymeric macromolecules are added to the carbon surface, until about 1.2% liquid is present. The same behavior was observed with alkanes as eluates and, therefore, it is not included here. It has been shown (14, 15) that when a high-area adsorbent is progres-
sively coated with a liquid, a minimum in the retention volume-coverage curves appears which corresponds to the formation of a monolayer of the preadsorbed solvent on the solid. In our instance, therefore, a t the concentrations below 1.2% liquid, the composite substrate contains submonolayer amounts of solvent. The rapid decrease of the capacity ratio is explained by considering that the first effect of submonolayer concentrations of a nonvolatile liquid on a solid substrate is to reduce the specific area available to a gaseous adsorbate. At concentrations higher than 1.2% liquid, the liquid surface adsorption takes place and the capacity ratio of butylamine remains almost unchanged. On the other hand, considering that specific interactions can be established between amine molecules and the terminal hydroxyl groups of the preadsorbed solvent, one would expect (14, 15) an increase in the retention of butylamine. An explanation can be put forth by assuming that on the carbon surface, the functional groups of the liquid interact preferentially with KOH. As a consequence, specific interactions between the liquid and basic eluates are in great measure hindered. Separation factors as well as retention times are greatly influenced by the degree of surface coverage of the adsorbing surface by the nonvolatile liquid. This modification in the chromatographic process arises from the combined effects of the chemical and physical properties of both the solid and the liquid phase. Curves of separation factors for some pairs of amines of interest as the relative quantity of PEG-2OM on the surface of Sterling FT-G is increased are useful for illustrating the working mechanism of liquid-modified solids. At low degrees of surface coverage, say up to about 0.4% PEG-POM, the influence of the carbon surface in the elution process is still predominant. Generally, advantage can be taken of this situation in separating molecules which differ mainly in geometrical structure and polarizability. As the relative quantity of liquid is increased, however, the separating process is more and more influenced by specific interactions occurring between the liquid and eluates. In such a way, separation factors tend t o reach the values attainable in the pure solution process. It is noteworthy that the influence of the solid surface carbon atoms is still partially preserved even when the surface coverage of PEG-2OM is more than its monolayer capacity.
Definitively, the effect of adding a liquid to a solid substrate with a relatively large specific surface is manifold: it neutralizes surface active sites, decreases retention times, and modifies partitioning properties of the adsorbing medium. Also, it has been shown ( 1 5 ) that when the liquid is deposited on a solid medium a t the rate of one or a few monolayers, its thermal stability is enhanced to a greater or lesser extent depending upon the liquid-to-solid ratio. The analytical potential of a suitably modified GCB's surface has been exploited to perform the analysis of aliphatic amines. We paid particular attention to making a single chromatographic column capable of eluting not only low-boiling aliphatic amines, but also high-boiling ones. This aim was reached by making use of Sterling FT-G 0.3% KOH coated with a relative quantity of PEG-20M equal to 1.3%. As an example, Figure 2 is a chromatogram showing the elution a t 220 "C of a methanol solution of some free aliphatic amines, such as decylamine, dodecylamine, tetradecyclamine, and hexadecylamine, including cyclododecylamine. As can be seen, well-shaped peaks
+
( 1 4 ) A . Di Corcia, Ana/. Chem.. 45, 492 (1973). (15) A . Di Corcia, A . Liberti, and R . Samperi, Ana/. Chem.. 45, 1228 (1973).
3
1, 3
I
0
I 10
I
I
I
20
sa
40 time(min)
Chromatogram showing the elution of a methanol solution containing some high-boiling amines Figure 2.
1, Decylamine; 2. dodecylamine; 3, cyclododecylamine; 4 , tetradecylam0.3% ine; 5, hexadecylamine. Column, 1.4 X 2 m m ; Sterling FT-G KOH 1.3% PEG-POM; temperature, 220 "C; linear carrier gas velocity; 8.3 c m j s e c : sample size, 1.0 @I containing about 80 ng of each component
+
+
without tailing were obtained for all the eluted amines. No particular care was devoted to chromatograph the compounds under consideration, except that the solution was injected directly into the packing to avoid condensation in the inlet part of the column, which resulted in broadened peaks for the higher molecular weight eluates. As shown in Figure 3a, by using the same column packing, we were able to perform the linear elution at 49 "C of a very dilute (-0.8 ppm) aqueous mixture containing C I - C ~ aliphatic amines. The chromatogram reported makes it clear that, because of the use of a suitably modified GCB's surface, even at sub-ppm concentrations, quantitative determinations of the first members of aliphatic amines can be performed. By comparing this chromatogram with that obtained by injecting the same amount of pure water (Figure 3b), it appears that the sole difficulty in obtaining quantitative data for such a diluted amine solution arises in practice from spurious peaks due to water. Different sources can be responsible for these signals. First, a t these sensitivities, water emerging from the column is detected by the flame ionization detector probably through slight variations in the temperature of the flame. Next, water may contain traces of detectable organic impurities. Finally, despite the water conditioning step, a slight decomposition of the preadsorbed nonvolatile liquid still occurs. Anyway, in this instance, these interferences do not affect appreciably the chromatographic profile and the precision of the method, which, a t this level of concentration, was estimated to be good to within 8-1070.Also, water interferences can be attenuated simply by making the water sample smaller. For the system under consideration, a critical factor is the use of small water sample sizes. Under the experimental conditions used, if the sample size is made larger than about 1.5 p1 the column overloads and its chromatographic features appear to be modified in proportion as the sample size is increased. In particular, we observed that retention times of amines increase and separation factors are modified in a way that the degree of polarity of the column seems to be increased. This anomalous behavior ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, JULY 1974
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27
I!
a
i
5
7 6
F
I 110 J'
~
.
I
3
6
8
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t m e ,m
ni
Figure 4. Chromatogram showing the separation of C1-C4 ahphatic amines in water solution Sample components as in Figure 3 Column, 1 4 m X 2 mm Sterling 0 2% KOH 0 5% PEG-1500, temperature, 75 "C,linear carrier FT-G gas velocity 11 c m j s e c , sample size, 1 pI containing about 2 ppm of each component
+
+
from methylamine to hexadecylamine. On the other hand, this packing material does not appear to be particularly useful for fractionating the lower members of aliphatic amines. This is so, mainly because the selectivity power of the carbon surface is considerably attenuated by the presence of the preadsorbed dense monolayer of macromoletricules. Also, in the instance of the dimethylamine methylamine pair, low values of the capacity ratio are responsible for the poor resolution obtained. The plots reported here confirm that when working with liquid-modified GCB, a surface coverage of about 80-90% is generally best in order to achieve highly selective chromatographic columns ( 1 5 ) .But, GCB coated with a quantity of PEG-20M lower than 1.2% was found useless, for decreasing the amount of the liquid resulted in tailed peaks for primary amines. Therefore, with a view of attaining a completely deactivated carbon surface having an optimum liquid-to-solid ratio, we decided to replace PEG20M with PEG-1500. A closely packed monolayer of PEG1500 was observed to take place when the liquid was added a t the rate of about 0.7%. Also, symmetrical peaks for amines were obtained even though 30-40% of the carbon surface area was left uncoated by the liquid. The reason for this is that, surface concentration being equal, PEG-1500 is more effective in neutralizing residual active sites than PEG-BOM, as the former has a higher number of hydroxyl groups per unit weight than the latter. More likely, because of a better orientation of the terminal hydroxyl groups for the lower molecular weight polymer, the minimum relative quantity of the inorganic deactivating agent to be added could be brought down to 0.2%. In Figure 4 is shown a chromatogram of the elution of the amine mixture considerably improved by using Sterling FT-G modified with 0.2% KOH and 0.5% PEG1500. As can be seen, by substituting the appropriate modifying liquid an almost base-line separation of the lower members of aliphatic amines can be obtained in a relatively short analysis time. Under the experimental condition used, the water disturbance affects in some measure the quality of the analysis, thus hindering correct determinations of amines a t the sub-ppm level. When analyzing amines from methylamine to propylamine, this limitation can be quite easily eliminated by decreasing the column temperature a t about 50 "C. In this instance, the chromatographic profile is improved as decreasing of the temperature causes the water disturbance to be partially attenuated and the relative distance of the elution peaks from the spurious peaks to be changed. It is interesting to note that ethylamine is retained
+
I
I
I
I
1
0
2
4
5
8
i
70 time ( m i n )
Figure 3 ( a ) . Chromatogram showing the separation of C,-C4 aliphatic amines in water solution 1, Methylamine; 2, dimethylamine: 3, trimethylamine; 4, ethylamine; 5 , isopropylamine; 6, propylamine; 7 , teit-butylamine; 8 , diethylamine; 9, sec-butylamine; 10, isobutylamine; 11, butylamine. Column, 2 m X 4 mm: Sterling FT-G 0.3% KOH 1.3% PEG-20M; temperature, 49 "C; linear carrier gas velocity, 5.3 cm/sec; sample size, 1.4 pI containing about 0.8 ppm of each component
+
+
( b ) . Chromatographic profile obtained by injection of 1.4 pl of p u r e water under the experimental conditions reported above
can be explained considering that a t the temperature of the experiment, water is retained by the column for a time comparable with those of amines considered. Then, when relatively large amounts of water are injected with amines, the picture of the adsorption on the layer of PEG20M is that of an amine molecule surrounded by water molecules. As a consequence, the chromatographic process of adsorption is altered by lateral interactions via hydrogen bonds which take place between water and amines. After a period of time corresponding to the elution of all water molecules, however, the original chromatographic characteristics of the column are completely restored, and anomalous effects are no longer observed if the sample size is made lower than the critical amount. Chromatograms reported above make evident the usePEG-2OM system fulness of the Sterling FT-G + KOH in performing linear elution of traces of aliphatic amines
+
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
Figure 5. Chromatogram showing the separation phatic amines in water solution
of C1-C4
ali-
+
Sample components as in Figure 3. Column, 1.4 m X 4 mm; Vulcan 0.8% KOH i- 4% PEG-20M; temperature. 91 "C:linear carrier gas velocity, 7 cm/sec; sample size, 4.5 PI containing about 2 pprn of each component
longer than trimethylamine on the 1.3% PEG-20M + Sterling FT-G adsorbing system, whereas this elution order is reversed when 0.5% PEG-1500 Sterling FT-G is used. At first glance, considering that ethylamine is more polar than trimethylamine, this effect could be accounted for by the fact that the first column contains a higher relative amount of KOH than the second one. However, only slight changes were observed in the retention volumes of amines by moderately increasing the relative amount of inorganic base. On the other hand, if the surface density of the terminal hydroxyl groups of the deactivating organic agent is taken into account, then one would expect an effect opposite to that observed, as the former adsorbing material has a considerably lower surface concentration of functional groups than the latter. Two considerations can actually account for the opposite behavior shown by the two columns. One is that uncoated GCB definitely retains trimethylamine more than ethylamine, as the former has a higher polarizability than the latter. Second is that even though only a small fraction of the graphitized carbon surface areas is left uncovered by the liquid, yet the separation process is considera-
+
bly influenced by the adsorbing solid (15). Therefore, bearing in mind that PEG-1500 covers only partially the carbon surface whereas this is completely shielded by a monolayer of liquid when PEG-2OM is used, the considerations made above appear consistent with the different behavior shown by the two chromatographic columns under comparison. Definitively, it has to be pointed out once more that as to the characteristics of selectivity of a GLS column, neither the nature of the liquid nor its quantity are factors as important as is the surface concentration of the liquid. A suitably modified surface of Vulcan, which is another well-known example of GCB with a surface area about seven times higher than that of the Sterling FT-G, was found useful for eluting trace amounts of the lower members of the aliphatic amines with a good selectivity power. The modification was carried out by adding on the carbon surface 0.8% KOH and 4% PEG-2OM and the relative chromatogram showing the elution of C I - C ~ amines is shown in Figure 5. The performance of this adsorbing system is similar to that obtained by the use of Sterling FT-G modified with PEG-1500. This is so because the surface coverage is roughly equal in the two cases and the two adsorbing media have the same properties per unit surface area. Nevertheless, when possible Vulcan is preferable to Sterling FT-G as the former has a higher mechanical resistance than the latter. Besides, it was observed that the higher surface area carbon black offers a somewhat higher column efficiency. I t is known that the degree of surface heterogeneity of the GCB's series increases as the surface area increases. In this contest, it appears surprising to note that the quantit y of KOH per unit surface area needed for deactivating Sterling FT-G is about three times higher than that needed for deactivating Vulcan. The same anomalous behavior was previously noted using HsPOa-modified GCB's surfaces for the analysis of C2-C5 free acids (16). The reason for this effect does not appear very clear to us, though in our opinion variations in the degree of wettability of these solid media should be taken into account to explain away the effect observed. Received for review September 24, 1973. Accepted January 21, 1974.
(16) A . Di Corcia and R. Samperi. Anal. Chem.. 46, 140 (1974)
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