2430
Anal. Chem. 1902, 5 4 , 2438-2443
Comparison of Methods Used for the Determination of Void Volume in Reversed-Phase Liquid Chromatography Ante M. Krstulovle," Henrl Colln, and Georges Guiochon Ecole Polytechnlque, Laboratolre de Chimie Analytique Physique, 9 1 128 Palalseau Cedex, France
Several methods are used indlscrlmlnateiy for the determlnation of the vold volume ( V , ) in reversed-phase liquld chromatography, and there Is a conslderable controversy regarding their relative merlts in terms of physlcal meanlng and method-to-method reproduclbillty. The static method by weighing glves the maximal column poroslty which is an upper llmlt of V,. Calculations based on lnjectlons of deuterated forms of both solvent components result In V , values which parallel closely the maxlmal poroslty. The results of injections of sodium nltrate and deuterated water are below the maxlmal poroslty and exhibit a dependence on solvent composition. Llnearization of retention data for homologous serles offers an alternatlve approach for assessment of V,. A new convergence crlterlon Is proposed for testlng the suitability of lndlvlduai homologous serles for V , calcuiatlon. When only convergent series are used, they give approxlmately the same V , values. Although thls approach Is tlme-consumlng, It seems to be a good way to estimate the vold volume.
The exact knowledge of the column void volume ( Vo)is of paramount importance not only for the optimization of separation conditions and characterization of solute bands but also for exploitation of the thermodynamic information contained in the capacity factor ( k ? . Contrary to the situation in gas chromatography (GC), there is no generally accepted method for precise determination of the void volume in reversed-phase liquid chromatography (RPLC). Unlike in GC, the void volume in RPLC may be a function of the mobile phase composition and the molecular size of the solute used for its determination (1-3). Some investigators believe that only the total column porosity has a true physical meaning, free from any assumptions inherent in other void volume measurements (4,5). Chromatographic phenomena involving unsorbed solutes have been discussed extensively by Horvgth and Lin (6). Depending on the experimental conditions, V , can vary between the interparticulate column volume and the sum of the inter- and intraparticulate column volumes. Moreover, methods for the measurement of Vowhich involve injections of deuterated solvent components are closely related to the distribution equilibria of the components of the mobile phase in RPLC (3, 7). Thus, the composition and volume of the stationary phase vary with a change in solvent polarity and surface properties of the adsorbent, causing a concomitant change in Vo (1). Several experimental methods have been used indiscriminately for the determination of Vo: static methods (1); dynamic methods which involve injections of the mobile phase components (I,7) or isotopically labeled compounds (such as deuterated water, methanol, and acetonitrile (1,3, 7)) as well as radiolabeled components of the eluent (8); injection of "nonretained" organic and inorganic salts (1, 2), or organic compounds (such as acetone (9),uracil (lo), or N,N-dimethylformamide (11));measurements of retention values at different temperatures (12);and mathematical methods based 0003-2700/82/0354-2438$01.25/0
on the linearization of retention data for homologous series
(1,13-15). Reported in this work is a critical evaluation of the existing methods for V, determination. An improved procedure for linearization of retention data of homologous series based on a convergence test is proposed and compared with other methods.
EXPERIMENTAL SECTION Apparatus. The liquid chromatographic system consisted of a Waters Model 6000A pump (Waters Associates, Milford, MA), connected to a Rheodyne Model 7125 injection valve with a 10 pL sample loop, a Model R 401 differential refractometer (Waters Associates), or a Waters Model 440 UV absorbance detector. The chromatographic columns were thermostated through a column jacket, using a Haake Model D3 constant temperature circulator (Haake, Karlsruhe, F.R.G.). The chromatographic measurements were performed at 25.0 "C, 40.0 "C, and 56.6 "C with a temperature precision of *O.l "C. Reagents and Chemicals. All chemicals were of reagent grade quality and they were used without further purification. Members of various homologous series were purchased from different sources. Pro analysis methanol was obtained from Merck (Merck, Darmstadt, F.R.G.) and HPLC grade acetonitrile from Carlo Erba (Carlo Erba, Milan, Italy). Water used for the preparation of mobile phases was doubly distilled. The packing materials LiChrosorb RP 18 (5 pm average particle size) and LiChrosorb RP 8 (10 pm average particle size) were purchased from Merck. The RP 14 packing material (10 pm average particle size) was kindly supplied by L. de Galan (Delft University, Holland). The 10 cm or 15 cm long stainless steel columns were home packed using a balanced-density slurry method. The slurry solvent was a mixture of dibromoethane-acetonitrile (1.4:1, v/v) and the pumping solvent was pure methanol. Prepacked stainless steel columns used in the course of this study were as follows: Lichrosorb RP 2 (250 mm X 4.0 mm i.d., 10 pm) was purchased from Merck, Zorbax ODS (250 mm X 5.0 mm i.d., 10 pm) from E. I. du Pont de Nemours and Co. (Wilmington, DE), and Ultrasphere-ODS (150 mm X 4.6 mm i.d. (pm) from Altex Scientific, Inc. (Berkeley, CA)). Preparation of Chromatographic Mobile Phases. Hydroorganic solvents were carefully prepared by mixing appropriate volumes of doubly distilled water and organic modifier, delivered by a buret. Sufficient quantities of each solvent mixture were prepared for chromatographing all solutes under study using identical mobile phase compositions. Prior to their use, solvents were always degassed by filtration through membrane filters type HA (0.22 pm average pore size, Millipore Corp., Bedford, MA). For the determination of void volume using injection of salts (sodium nitrate and sodium benzenesulfonate),the mobile phases were prepared by using 0.1 M NaBr and the organic modifier. Chromatographic Conditions. All chromatographic experiments were carried out at a nominal flow rate of 1.00 mL/min. Exact values of the volumetric flow were measured three times a day using a volumetric flask. The average flow variation observed over a period of 2 months was of the order of *0.4%. The length of the connecting tubing between the injector and the column inlet, and the detector and the column outlet, was minimized (approximately 40 pL) and its volume carefully measured in order to correct for the extracolumn residence. Total column porosity (emax) was measured by weight difference, using a procedure described in the literature (6). Acetonitrile and carbon tetrachloride were chosen as solvents because of the large 0 1982 American Chemlcal Soclety
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14,DECEMBER 1982 2439 nL I ,
I
2. 4
Emax.
i
i
2. 2
R.I.
.-
e
2
1
0
X 10
20
30
40
50
60
70
80
90
MIOH
a
Flgure 2. Variation of the vold volume determined by injections of lo3 M solutions of sodium nitrate (A)and sodium benzenesulfonate (0). 0
2
I
3
TIME ( m i n )
Figure 1. Chromatogram of a mlxture of six n-alcohols. Chromatographic conditions: column, LiChrosorb RP 18 (150 mm X 4.6 mm i.d., 5 pm average particle, size); mobile phase, 60% THF-40% water (v/v); nominal flow rate, 1.0 mL/min; column temperature, 20.0 f 0.1 O C , detection R.I., 8X. Solute identity: 1, methanol; 2, ethanol; 3, propanol; 4, butanol; 5, pentanol; 6, hexanol. e,,, maxlmai column porosity determined by the static weighing method.
difference in their densities. In addition,,e values were obtained by measuring the difference between the mass of the column filled with a given solvent and the mass of the dry column. The mass of the dry column was obtained by purging the column with pentane and subsequent ovemight drying at 60 O C under a stream of helium. The drying w4wcontinued until a constant mass reading was obtained. Linearization of Homologous Series. The general mathematical treatment used in this approach is based on the assumption that there exists a linear relationship between the logarithm of k’values and the carbon numbers n, of the successive members of homologous series. We have developed a linearization method which tests the convergence of retention data and makes possible the correlation of Vovalues obtained from the plots of log k’vs. n, for different homologous series. V , was chosen as that value which gave the best agreement between the slopes for the series investigated: alkanes, alkylbenzenes, methyl esters, chloroalkanes, and alcohols. Peak asymmetry factors were obtained for all peaks, and for those exceeding the value of 1.1 (at 10% of total height), the centers of gravity were calculated in order to obtain the correct retention data. All retention data were expressed in volume units and averaged from, at least, three consecutive measurements. The determination of Vo by using homologous series requires extremely precise and accurate retention data. This topic will be examined in a forthcloming publication (16).
RESULT’S A N D DISCUSSION Maximal Column ]Porosity. The values of the maximal porosity (emax) are indlicated in all figures by dotted lines. It has been suggested that the maximal column porosity may be the only meaningful measure of the void volume (4, 5 ) . With solvents which solvate the stationary phase to a
Chromatographic conditions: column, LiChrosorb RP 18 (150 mm X 4.6 mm i.d., 5 pm average particle size); mobile phase, mixtures of 0.1 M NaBr and methanol; nominal flow rate, 1.0 mL/min; column temperature, 25 f 0.1 O C ; detection, UV, 254 nm (0.05 aufs). significant extent (such as tetrahydrofuran, THF), the use of this measure may lead to negative k’values, due to the negative slope of the excess isotherm (preferential adsorption of the organic component of the mobile phase, rather than the solute) (4). This was found to be the case with alcohols chromatographed with a 60% THF-40% water eluent, as shown in Figure 1. Analogous results were obtained with the Vovalue calculated from the retention volumes of the radiolabeled solvent components (8). In addition, it was also observed that if the maximal porosity was used for Vodetermination, the plots of log k’vs. n, and log k’vs. 1/T were not h e a r in both methanol-water and acetonitrilewater mixtures (16). This is not, however, a valid argument against this estimation of V,. It must also be noted that with such a definition of V,, the composition of the mobile phase (the total volume of liquid in the column) is not the same as that of the liquid actually pumped through the column, and additional experiments are needed for its determination. Injection of Salts. The elution behavior of ionic solutes was studied with sodium nitrate and sodium benzenesulfonate. The solutes were detected spectrophotometrically at 254 nm. In the absence of ionic species in the eluent, the calculated void volumes were significantly below the total column porosity (elution volume = 0.4Vm,), indicating that the solutes were most likely excluded. This phenomenon may be attributed to the existence of alike charges at the internal surface of the packing material, which prevented the salts from exploring fully the intraparticulate void volume (1). However, when water was substituted with a 0.1 M NaBr solution, higher VOvalues were obtained. Figure 2 illustrates the general behavior of the two salts chromatographed with mixtures of aqueous salt-methanol. It is interesting to note that there is a gradual increase in the elution volume of NaN03 of approximately 12% when going from 100% methanol to 100% salt solution. This effect may be due to the diminished solvation of the hydrocarbon chains with decreasing methanol concentration, which may result in increased intraparticulate
2440 1.3
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
“
mL
4
0
10
20
30
40
50
60
70
80
90
100
Flgure 3. Void volume measurements obtained by llnearizatlon of homologous series (maximization of correlation coefflclent and minimization of the dispersion of the values of the slopes) and injection of D,O, as a functlon of the composition of methanol-water eluents. Chromatographic conditions: column, LiChrosorb RP 18 (100 mm X 4.0 mm id., 5 hm average particle slze); mobile phase, methanolwater mlxtures (v/v); detection, RI, 8X; nominal flow rate, 1.0 mL/min; column temperature, 25.0 f 0.1 OC. Symbols: esters, A; alkylbenzenes, +; alkanes, +; chloroalkanes, 0 ;alcohols, V; D,O, 0 ; overall V , determlned from all serles chromatographedunder identlcal conditions, ; maximal column porosity, -e-.; iinearizatlon resuits for individual series enclosed in rectangles.
+
porosity and, thus, increased void volume. Other investigators observed a decrease in void volume with decreasing methanol concentration when ethanol was used as a void volume marker, and they attributed this effect to the reduced surface area of the stationary phase due to the collapse of alkyl chains (17). However, our results obtained with ethanol were not analogous to those reported by Scott et al. (17) for the RP 18 phase in pure water. Contrary to their findings, we have observed that ethanol was retained considerably under these conditions. The results obtained with sodium nitrate chromatographed using salt solutions of low organic modifier concentrations were below the maximal porosity and within the Vo range obtained with the homologous series. Injections of sodium benzenesulfonate gave widely different V, values, depending upon the composition of the eluent. A t methanol concentrations below 50%, the calculated Vo values begin to exceed significantly the maximal column porosity. This indicates that the salt is retained in solutions of low organic modifier concentration. Thus, this solute appears to be unsuitable for the determination of void volume. Analogous conclusions were reached by other investigators (2). Injections of Isotopically Labeled Compounds. The elution behavior of D 2 0 on a R P 18 column and methanolwater mixtures is illustrated in Figure 3. It is interesting to note that, with both methanol-water and acetonitrile-water mixtures, the curves exhibited a minimum at approximately 40-50% of organic modifier and were close to the maximal porosity. These observations are in agreement with the literature data (I). Analogous behavior of DzO was also observed with the RP 14 and RP 18 columns, in both acetonitrile-water and methanol-water mixtures. In addition, we have also examined a variant of the method suggested by Knox et al. (8). Instead of the radiolabeled
components of the eluent, we have used the deuterated compounds. The overall Vo was calculated by using the formula
Vo = $AVA+ $BVV
(1)
where $A and $B are the volume fractions of the two solvent components (water and organic modifier), and VAand VB are the retention volumes of their deuterated counterparts. It is assumed that the partial molar volumes of components A and B are independent of composition and adsorption (1). This method was not tested with methanol-water mixtures because the possibility of isotopic exchange. Linearization of Homologous Series. By careful examination of the experimental data obtained with all the series and solvents mentioned in the experimental section and several chromatographic columns (LiChrosorb R P 18, LiChrosorb RP 14, LiChrosorb R P 8, Ultrasphere-ODS and Zorbax-ODS),we have observed, in some cases, a considerable dispersion of the Vo values calculated both by maximization of the correlation coefficients of the log k’vs. n, plots and by use of Berendsen’s method ( I ) . This is illustrated in Figure 3; the Vo values obtained for all series in a given solvent are enclosed in rectangles. This situation was encountered especially when a limited number of predominantly low members of homologous series was used, in which case the scatter may have been due to the imprecision of the measurements of retention data for insufficiently retained homologues ( I 6 ) , and/or the existence of a curvature for the low homologues. The same effects were observed when pure organic solvents were used as eluents. It is difficult to conclude from these results that the individual series give identical Vo values when chromatographed under identical conditions, as observed by Berendsen et al. ( I ) . Moreover, it is obvious that the dispersion of V, estimates obtained with different series in a given solvent increases with decreasing number of experimental points. In mixtures of low organic modifier content, the retention times of higher homologues of the more hydrophobic series (particularly the alkanes and alkylbenzenes) become impractical to measure. Thus, for the series under study, the utility of the higher homologues diminishes rapidly with the decreasing content of organic modifier. Other series would be needed for determinations of Vo over the entire range of methanol and acetonitrile concentrations. Furthermore, the overall Vo values (denoted by the symbol +) were obtained by minimizing the difference among the slopes of all individual series chromatographed under the same conditions. In most instances, the overall V, values were within or in close proximity to the rectangles. Except for some points obtained with a minimal number of homologues in mobile phases of low organic modifier content, most Vo values assessed by different methods (injections of D20 and sodium nitrate) were below the total porosity line. Although extreme care was exercised in collecting and processing the experimental data, the results, as shown in Figure 3, were not particularly revealing with respect to several fundamental questions which had to be answered: (1) Do individual series give identical V, values when chromatographed under the same conditions? (2) Do these values vary from solvent to solvent? (3) How do other methods (injection of DzO, static method by weighing, etc.) compare to the linearization procedure? In order to test our results further, we have applied an additional procedure to determine if, and under what conditions, the different series “converged”. Contrary to some other findings which showed that, with reasonable precautions, good estimates of Vo could be obtained even with few homologues, we have observed in some cases a critical dependence of the value of Vo on both the number and choice of homologues used. Therefore, we have developed a convergence test which consisted of calculating the V, values
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982 2. 05
F
2441
l t
1. 91 4
y @
'
8
1.65
9
5%
1.78
t
1
1.51 5 7 NUMBER OF HOMOL13GUES
3
.B
B
9
'
0
10
207
30
48
t50
60
70 1
80
90
100 Z MeOH
2.85
Flgure 5. Void volume measurements obtained by linearization of convergent series only and injections of D,O. Ail chromatographic conditions and symbols are the same as those given in Figure 3.
1. 91 4
E
g 2 2
8
1.78
1.65
1. 51
5
3
7
NUMRER OF HOMOLOGUES
C 2. 45
4
I
?
1
.7
t
L
#
I Y MsCN
2. 30
2.15
2 3 Q 9
8
2.88
1.85
1. 70
1.55
3
5
7
I, the V , values change significantly with a change in the number of homologues examined. When the higher homologues were removed, the effect was much reduced, indicating that the scatter was more pronounced with the lower homologues. Conversely, the corresponding correlation coefficients remained surprisingly high and constant. Thus excellent correlation coefficients (as high as 0.999 95) cannot be used as a criterion for convergence of retention data. From Table I it is evident that the choice of homologues is of critical importance for obtaining self-consistent data. Therefore, we have rejected all the results which did not fall within the 5% range in the convergence test. An example of a satisfactory convergence is illustrated with esters chromatographed in 100% acetonitrile on a R P 18 column. The results are illustrated in Figure 4A,B. In order to demonstrate that the extent of convergence did not depend upon the linearization procedure used, we have calculated the V0values by two different methods. The results obtained by maximization of the regression coefficients of log k'vs. n, plots for esters are shown in Figure 4A, and those obtained by the method of Berendsen et al. (I) are shown in Figure 4B. Analogous results were obtained by both methods since either treatment gave the same point of convergence (1.78 mL). A typical example of a nonconvergent series is illustrated with esters chromatographed on a R P 1 column with a 70/30 acetonitrile/water mixture (Figure 4C). Insufficient retention of the members of this homologous series contributed significantly to the imprecision of results. Upon rejection of all series which did not exhibit sufficient convergence, considerably less scattered results were obtained, as shown in Figures 5 and 6. It is obvious that by using only the convergent series, the results obtained with different series
1 11
NUMBER OF HOMOLOGUES
Flgure 4. (A) Convergence test for esters: vold volume determined as a function of the number of homologues used in the iinearizatlon procedure obtained by maximization of the correlation coefflcient,(solid
line) lower homologues eiiminated, (dotted line) hlgher homologues eiiminated; column, LiChrosorb RP 18 (150 mm X 4.6 mm id., 5 pm average particle size); eluent, 100% CH3CN. All other chromatographic conditions and syiribols are the same as those given in Figure 3. (6) Convergence tesl for esters: void volume determined as a function of the number of homologues used in the linearization procedure of Berendsen et al (ref 1); chromatographic conditions same as in (A). Symbols are the same as those given in Figure 3. (C) Convergence test for esters: void volume determined as a function of the number of homologues used in the linearization procedure by Berendsen et ai. (ref 1); column, LiChrosorb RP 2 (250 mm X 4.00 mm i.d., 10 p m average particle size); mobile phase, 70% acetonitrile-30 YO water; all other chromatographic conditions same as in Figure 3. for the entire series, as well as its segments obtained by sequential elimination of either the lower or the higher homologues from the calculations. This is illustrated with the chloroalkanes chromatographed on a UP 18 column with a 80/20 acetonitrile/wakr mixture (Table I). The results were obtained by maximizing the correlation coefficient. It is striking that in certain cases, such as the one shown in Table
2442
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table I. Variation of the Void Volume and the Correlation Coefficient Determined by Linearization of Retention Data of Chloroalkanesa no. of homologues
homologues chromatographed
coefficient of correlation
8 7 6 5 4 3
c,, c,, c , , c,, c,, c,, ClO c,, c , , c,, c,, c,, Cl, c5, c6, c,, c8, c,, c,, c , , c,, (37, c,, c,, Cl, c,, c,, Cl,
7 6 5 4 3
c,, c,, c,, c,, c7, c,, c, c3, c4, c 5 > c6, c 7 , c8 (33, c,, c,, c,, c7 c3, c4, c5, c 6
V,, mL
(4
c7,
c7,
c9>
c10
(39,
(33,
0.999 0.999 0.999 0.999 0.999 0.999
995 994 994 997 994 999
0.842 0.852
0.999 0.999 0.999 0.999 0.999
998 998 998 999 999
0.832 0.828 0.824 0.832 0.828
0.880 0.994 0.968 1.280
(B)
c,, c,
Part A, sequential elimination of lower homologues. Part B, sequential elimination of higher homologues. Chromatographic conditions: column, LiChrosorb RP 18 (150 mm X 4.6 mm i.d., 5 pm average particle size); eluent, 80/20 mixture of acetonitrile/water; flow rate, 1.00 mL/min; column temperature, 25 f 0.1 "C. a
chromatographed under identical conditions afford approximately the same Vo value. However, because of the impractically long retention of the series examined in solvents with low modifier content, it is impossible to determine the extent of variation of Vo over the entire range of solvent compositions. The overall void volume obtained by linear regression of different series, followed by subsequent minimization of the difference among the slopes (log a ) of log 12' vs. n, plots, does not always fall within the range of the individual Vovalues for all the series chromatographed with a particular solvent. This is especially evident in Figure 6 (80% acetonitrile). This apparent anomaly is due to the relatively flat curve of the percent deviation of the slope (log a ) vs. Vo, in which case the mathematical search for a minimum becomes more difficult and has a questionable meaning. Effect of Temperature on the Void Volume. The effect of temperature on the void volume does not seem to have received a considerable amount of attention. It is to be expected that the temperature will play a certain role, particularly with solvents which solvate the stationary phase to a large extent (e.g., THF). In order to investigate the effect of temperature on the void volume determined by injection of DzO, we have conducted a series of experiments a t 25.0 "C, 40.0 "C and 56.6 "C and used several methanol-water and acetonitrilewater mixtures. The results are given in Table 11. Prior to the evaluation of experimental results, one must account for the thermal expansion of the column tubing and the solvent. The effect of the first factor was found to be negligible, since the total cm3, using the value of 5.33 change in volume was 2.0 X Kcm/(cm "C) for the coefficient of dilatation of stainless steel 304. The second effect, the solvent expansion with temperature, must be corrected for in Vo calculations. Indeed, if the measured dead time at the temperature T i s toT, the measured dead volume ( V 0 , m e - d ) is given by the following expression: VTO,measd
=
where DTo is the flow rate of the pumping system, measured a t a certain reference temperature To (usually ambient). However, because of the solvent expansion, the actual flow rate at temperature T is given by (18)
(3) where 12, is the thermal expansion coefficient for a particular
Table 11. Effect of Temperature on Void Volume Measurements Obtained by Injection of D,Oa temp, "c 25.0 40.0 56.6
60% CH,OH40% H,O VO,measd
VO
V0,measd
vo
1.771 1.774
1.771 1.797
1.819 1.799 1.763
1.819 1.826 1.821
60% CH,CN40% H,O 25.0 40.0 56.6 a
80% CH30H20% H,O
1.628 1.624 1.611
1.628 1.650 1.666
80% CH,CN20% H,O 1.699 1.704 1.709
Column: LiChrosorb RP 18 (150 mm
1.699 1.736 1.777 X 4.6
mm i.d.,
5 pm average particle size).
composition of hydroorganic solvent. Then, the true dead volume a t temperature T is
VoT =
pO,measd[l
+ k e ( T - To)]
(4)
It is evident from the data in Table I1 that Voincreases with temperature in acetonitrile-water mixtures. This can be explained in terms of diminished solvation of the stationary phase at higher temperatures. This effect was less pronounced with methanol-water mixtures where the surface excess amount is expected to be lower than in acetonitrile ( 3 ) . Analogous conclusions were reached in the study of the temperature effects on Voobtained by linearization of homologous series of n-alcohols and esters. It should be noted that, unlike the Vodetermined with D20,the total porosity is independent of temperature, provided the structure of the bonded phase remains unchanged.
CONCLUSIONS The determination of the void volume in RPLC systems, a sine qua non condition for the calculation of k'values and the assessment of thermodynamic parameters, remains at present a controversial problem. Vodepends on the thermodynamic model chosen to describe the adsorption of the mobile phase components on the stationary phase (19, 20). Consequently, one cannot define a single Vovalue. It is certain that the use of homologous series requires tedious and extremely precise data, as well as sophisticated data processing, which makes this method impractical. It is interesting to note, however, that careful use
Anal. Chem. 1982, 5 4 , 2443-2447
of retention data of convergent homologous series gives very similar void volumes which are, moreover, in rather close agreement with those obtained by using injections of D20. Although this is not a thermodynamic proof for the use of convergent homologom series, it is however a remarkable result. It should also be' noted that the V ovalues determined with all homologous seiriies under study give linear log k ' vs. n, plots of the same slope. According to our results, this is not the case when Vois taken as the total volume of liquid in the column. ACKPJO WLEDGMENT A.M.K. gratefully acknowledges the scientific exchange agreement for financial support. LITERATURE CITED (1) Berendsen, G. E.; Schoenmakers, P. J.; de Galan, L.; Vigh, G.; VargaPuchony, 2. J . Llq. Chromatogr. 1980, 3 , 1669-1686. (2) Wells, M. J. M.; Clark, C . R. Anal. Chem. 1981, 53, 1341-1345. (3) Slaats. E. H.; Markovski, W.; Fekete, J.; Poppe, H. J . Chromatogr. 1981, 207, 299-323. (4) Riedo, F.; KovLts, E. s;?.J . Chromatogr. 1982, 239, 1-28. (5) HorvLth, Cs.; Melander, W. R. "Book of Abstracts", 183rd National Meeting, of the American Chemical Society, Las Vegas, NV, March
2443
28-April 2, 1982; American Chemical Society: Washington, DC, 1982; ANYL 072. (6) Horvlth, Cs.; Lin, H.J. J . Chromatogr. 1976, 726, 401-420. (7) McCormick, R. M.; Karger, B. L. Anal. Chem. 1980, 5 2 , 2249-2257. (8) Knox, J. H.; Kawszan, R., Kennedy, G. J. Symp. Faraday SOC. 1980, 15, 113-125. (9) Johnson, J. K., Jr.; Cernosek, S. F., Jr.; Gutierrez-Cernosek, R. M. J . Chromatogr. 1979, 777, 277-31 1. (10) Karger, B. L.; a n t , J. R.; Hartkopf, A.; Weiner, P. H. J . Chromatogr. 1976, 128, 65-78. (11) Unger, S. H.; Feuerman, T. F. J . Chromatogr. 1979, 176, 426-429. (12) NeMhart, B.; Krlnge, K. P.; Brockmann, W. J . Liq. Chromatogr. 1981, 4 , 1875-1886. (13) Vigh, G. Y.; Varga-Puchony, Z . J . Chromatogr. 1980, 796, 1-9. (14) Haken, J. K.; Wainwright, M. S.;Smith, R. J. J . Chromatogr. 1977, 733,1-6. (15) Wainwright, M. S.;Haken, J. K. J . Chromatogr. Chromatogr. Rev. 1980, 784, 1-20. (16) Colin, H.; KrstulovlE, A. M.; Guiochon, G., manuscript in preparation. (17) Scott, R. P. W.; Simpson, C. F. J . Chromatogr. 1980, 797, 11-20. (18) Colin, H.; Dlez-Masa, J. C.; Gulochon, G.; Czajkowska, T.; Miedziak, I. J . Chromatogr. 1978, 767, 41-65. (19) KovLts, E. sz., prlvate communication. (20) Poppe, H., private communication
RECEIVED for review April 22,1982. Accepted September 7, 1982.
Capillary Gals Chromatography/Fourier Transform Mass Spect romet ry Robert L. Whlte and Charles L. Wllklns" Department of Chemistty, Universi@ of California -Riverside,
Riverside, California 9252 1
The coupling of capillary gas chromatography to two different Fourier transform mass spectrometers (FTMS) is descrlbed. By use of a jet separator gas chromatography/mass spectrometry (GC/MS) Interface, It is shown that the technlque Is capable of rapid scannilng as well as high-resolution mass analysis when combined with support-coated open tubular (SCOT) Capillary columns. With the appropriate software, It Is possible to peak swlitch at high resolution over arbitrarily wlde mass ranges (a feat which cannot be accomplished by conventional mass spectrometry). I n additlon, with no changes in instrument,atlon, FTMS can be operated in a chemical lonlzatlon (Cli) mode, for GC/MS.
There is little doubt that the combination of gas chromatography with mass spectrometry is one of the most useful tools available to analytical chemists for complex mixture analysis. Current literature reflects the usefulness of this versatile technique in many areas of research including agricultural ( I ) , biomedical (2), energy (3), environmental (4, 5), and industrial (6) applications. Because GC/MS is such an important application of mass spectrometry, it follows that characterization of new analytical mass spectrometric techniques should include ain evaluation of t,he GC/MS combination. This paper deals with the evaluation of GC/MS using SCOT capillary columns and Fourier transform mass spectrometry. In recent years Fourier transform mass spectrometry (FTMS) has become a new and viable analytical mass spectrometric technique (7-25). Previous studies have shown that this type of instrument is capable of rapid scanning and 0003-2700/82/0354-2443$0 1.25/0
ultrahigh mass resolution and would be expected to interface well with capillary gas chromatography (11). Interfacing a packed column gas chromatograph to an ion cyclotron resonance spectrometer has been reported as a means of determining momentum transfer collision frequencies but was not advocated for routine GC/MS applications (16). However, our preliminary experiments demonstrated that GC/FTMS is feasible and that SCOT capillary column peak profiles can easily be obtained (13). In other work, it was determined that exact mass assignments could be made on fragment ions using low-resolution (ca. 1000 full width at half height (fwhh)) FTMS spectra and that, if isobaric peaks are suspected, high resolving power may be employed to distinguish them (12). Thus, three different GC/FTMS operation modes are available: low-resolution wide mass range scanning and both low- and high-resolution selected ion monitoring. All of these modes employ rapid scan times (e1s) and as a result are compatible with narrow capillary column peak widths (5-10 9).
EXPERIMENTAL SECTION Two Fourier transform mass spectrometers were used in this work. One of these instruments was constructed at the University of Nebraska and has been described in detail previously (13). This system was constructed for use with a Varian V-7300 electromagnet and was interfaced to a dual column Perkin-Elmer Sigma I1 gas chromatograph. The other FTMS was manufactured by Nicolet Analytical Instruments (FT/MS-1000) and uses a horizontal bore superconducting magnet. This instrument was interfaced with a Varian Vista 6000 gas chromatograph via a probe inlet vacuum lock. Both instruments operated using 0.0254-m cubic trapped ion analyzer cells. The electromagnet-based instrument was operated at a magnetic field strength of 1.2 T whereas the FT/MS-1000 field strength was 1.9 T. Both in@ 1982 American Chemical Society
