Characterization of Gas Chromatographic Liquid Phases Using McReynolds Constants An Experiment for Instrumental Analysis Laboratory Steven R. Erskine, John C. Lennox,' and Donald R. Bobbm University of Arkansas, Fayetteville, AR 72701
Numerous exneriments are available in the literature illustrating the quantitative capabilities of gas chromatogranhv. However. exneriments denictine the aualitative asnects bf GC are usual1;limited to simple retent'ion time comparisons even thouah this method is inherentlv sensitive to the experimental c h t i o n s . This paper desrAbes a laboratory exveriment that is desirned to aid in the develn~mentof an understanding of the f&damental processes involved in gas chromatographic separations by introducing the student to the Kovats retention index system. Such an understanding should be invaluable in both qualitative analytical applications and in the establishment of criteria for optimal selection of gas chromatographic stationary phases. Although the simplest qualitative descriptor available from gas chromatography is the retention time, t ,,in practice the adjusted retention time, t,, is generally applied. Here. t~ is the retention time for a svecies with a distrihution coefficient that is effectively zerd(i.e., spends essentialIv no time in the stationarv ~ h a s e )and . it is a measure of the interstitial or void volumeoi the c&mn itself. This parameter may be measured as the elution time of any unretained solute (air, methane, etc.). Since t ; is highly dependent upon such experimental conditions as column temperature and flow rate, it is often not used in deference to the relative retention time, the ratio of the adjusted retention time of that solute to some other standard solute added to the sample mixture. Such a method of internal standardization is much less suscevtihle to error due to slight variations in exeprimental conditions, but i t is not readzy applicable as a qualitative descrivtor of a comnound for universal a v ~ l i c a ;on due to the difficulty in applying one standard t o all chromatographic situations. Due to the interrelationship between vapor pressure, distribution coefficient, and structure, for a homologous series of compounds, the adjusted retention time will exponentially increase throughout the numbers of that series. As such, a plot of the logarithm of the adjusted retention time versus carbon number will exhibit very reasonable linearity for any such homologous series on any gjven column material, and although the actual values of t , will be dependent upon column conditions. the linearitv of this overall relationshin will hr R cnnstanr (the figure).'l'his linear relarion has led to the develovment of t h ~ -Kwats retenrion index ( 1 ) . which relates t; f i r a given solute to the adjusted retention times of the normal alkanes eluting just prior to, and just following that particular solute species. The retention index, I, is defined by
Here, ti, is the adjustedretention time for the sample; t;, and t,,, are the adjustedretention times for the n-alkane eluting
in 5
I
I
6
7
I 8
I 9
Alkane Carbon No. Plot of alkane carbon number versus adjusted retentiontime. 0, Squalane; X, Carbowax 20M.
just before, and just after the sample, respectively; and n is the number of carbon atoms in the n-alkane eluting a t t,,,. Since variations in column temperature, stationary phase loading, and carrier gas flow rate will affect the absolute value of the adjusted retention times for the n-alkanes and the solute fairly uniformly, the retention index is a parameter that is much less sensitive to such changes in experimental conditions thereby permitting its widespread use as a qualitative descriptor of the solute's chromatographic properties on that stationarv nhase. Althoueh less sovhisticated than GC-MS for unaml;i&ous identiii&tion, thi; reproducihilitv, coupled with literature v a l u r of retention indices for a variety oi compounds can serve as a valuable complement to such a qualitative analysis (2,3). The McReynolds constant for a particular solute on a given stationary phase is defined as the difference between 'Present address: Burroughs Welicome Co., 3030 Cornwallis Road. Research Triangle Park, NC 27709. Volume 63 Number 12 December 1986
1099
Table 1. Selected McReynolds Constants (4) Stationary
Benzene
Phase
(Inductive, r)
Squalane Apiemn L
0
32 15 109 322
SE-30
Versamid 930 Carbowax 20M
1-Butanol (H-bonding)
,
.
Nitropropane (Dipolar)
-p
p
42 41 209
368
510
64 211 572
313 536
Experimental In a few, relatively simple steps, enough data can he obtained to permit observation of the relationship between lne t i and nudber of carbon atoms for a homolo&us series. 1; these experiments, the series of n-alkanes Cg to Cg was nsed, although the linearity was also established for an n-alcohol series. Determination of McReynolds constants for benzene and hutanol permits comparison of these constants with the literature values (4). Any simple gas chromatograph employing a flame ionization or thermal conductivity detector can be nsed. For these exneriments. the instrument was a Varian model 90-P eas chrornatographe(l~lippedwith a thermal rvnductivity detector. Columni of 5 9 Carbowax 2034 or Sounlane on Chromosorb G, 6 ft X li4in. were operated a t 6 0 ; ~and 75 mllmin. Three samples were prepared for analvsis containing the n alkane series, butanbl and benzene, ;espectively. Sample volumes of 3 6 pL injected into the chromatograph resulted in the reduced data that is presented in the figure. All sample adjusted retention times were obtained by subtraction of the retention time for either air or methane as per eq 1. Retention indices can be obtained graphically from Figure 1 or by linear regression of n-alkane carbon number versus log t, to permit calculation of I from the slope and intercept of this data. A less ideal method is to apply eq 2, which simply uses the retention times of the n-alkanes immediate-
Pyridine 0
Table 2. Experimentally Deterrnlned Retention Indices Compound
(3)
-
Journal of Chemical Education
0 15 44 144
32
The McReynolds constant is then a comparison of the retention indices (carbon numbers) of that solute on two phases, and it permits comparison of that phase to the nonpolar reference nhase soualane. Since a solute's retention index on sqnalane is primarily reflective of its partial pressure with respect to the n-alkanes, a difference in its retention index on some other column material will he indicative of some specific solute interaction with that stationarv phase. This is illustrated in the data of Table 1 by com&rison of the McReynolds constants for 1-butanol on SE-30 (silicone polymer) and Carbowax 20 M (polyethylene glycol). It should he emphasized though that these interactions are not solely related to the relative polarities of the solutes and stationary phase, but reflect a variety of such interactions: di~ole-dinole. . . hvdroeen " " bonding with solute or stationhrv phase as donors or acceptors, inductive effects, etc. To aid in an assessment of these types of interactions, McReynolds has selected several reference compounds which are representative of specific classes of interactions, thus permitting empirical determination of the extent of such an interaction with a particular stationary phase. These compounds are presented in Table 1 along with the McReynolds constant for several representative stationary phases.
1100
(Proton Acceptor)
0
0 22 53
its retention index on that particular phase (P) and its retention index on a nonpolar reference phase (INP),nsnally taken as squalane (C30 hydrocarbon) (4). , p
Solute (Classof Interaction) ?-Pentanone (Dipolar.Carbonyl)
1s p " a
650 567
Benzene
I-Butanol
lP'"A! 20M
932 1106
282 (322)' 519 (536)
'Values in parenthews are from ref. 4.
ly bracketing the compound of interest. T o verify eq 2 for other homologous series (e.g., n-alcohols), i t is suggested that the lowest member of the series be Cg or greater to ensure linearity (5). Results and Dlscusslon Students performing this laboratory experiment were easily able to obtain meaningful results. A summary of some representative results are presented in the figure and in Table 2. I t should be noted that the values determined by the student in this exercise and those in ref. 4 are very similar, even though the latter were obtained a t 120 "C. This underscores the fact that one of the primary advantages to the retention index system is its small temperature dependence (5).If necessary, several additional experiments will allow one to calibrate for the affect of temperature as I tends tovary linearly with temperatnre for small AT and as hyperbolic function for ranees ereater than 100 "C (6). .. One detriment to the experiment is the time required when one must eouilibrate the second column. This resulted in the exercise occupying two laboratory periods. However, if a dual-column instrument is available. this would no longer Iw a consldrrnti(m. This experimenr lends itself to tu80interesting variations. First, the temperature dependence uf I, though small, tends to be characteristic of a particular substance on a particular stationary phase. t here fore, a determination of d l l d can ~ be used to expand the qualitative aspects of the exercise (7). Second, for linear temperatnre programmed operation, the temperature of elution of a species can be substituted in eq 2 for the adiusted retention time (. 8) ...therebv introducine the student to temperatnre programmed GC. In summary, the exercise outlined here is easy to implement and coalesces a variety of basic GC concepts into one laboratorv experiment. The exercise is general enough that it can be Garied to meet specific needs. w
-
-
Literature Cited ill ELtre, L. S.Anol. Chrm. I964,36,31A. (21 Rrhrschneider. i.. J. Ckromotagr. 1966.22.6. 13) Lewis. J. S. "Compilation of Gaa Chromatographic Data"; American Society for Tesfing and Materials: Philadelphia. 1963. 14) McReynoidr. W. 0. J. Chromalogr.Sci.
1910,R,685.
~ i d d i ~J.gC~;KKIIII,R.A., ~, E~~.;DD~~LT: New York. 1965:Vol1.p 2 % 16) Hoigne, J.; Widmer, H.; Gaomsnn, T. J. Chromologr. 1963,11,459. (7) MiLra, G. D.:Mohan, G.:Sinha,A. J. Chromotogr. 1974.91.633. 18) Haheood.H. W.: Harris. W. E. AnolChem. 1160.32.450.
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