Art ides Anal. Chem. 1994,66, 3036-3041
Pressure-Tunable Selectivity for High-speed Gas Chromatography Michael Akardt and Richard Sacks’ Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48 109
A high-speed GC instrument using a cryofocusing inlet system and pressure-tunable selectivityis described. Tunable selectivity is achieved by the use of the tandem combinationof a nonpolar and a polar capillary separation column. Selectivity is tuned by the adjustment of the carrier gas pressure at the midpoint between the columns. When this pressure is changed, differential changes occur in the carrier gas flow rates for the two columns. This results in changes in the relative residence times of all components on the two columns and thus changes the contributionof each column to the overall separation. Relatively short columns are used for high-speedseparations. This results in minimal gas compression effects and has allowed the development of a linear tuning model and a reliable tuning procedure. A column bifurcation system is described which obtains independent holdup time measurements for the tandem columns. From these measurements, chromatogramsobtained from the tandem column system can be used to estimate capacity factor values for the individual columns. Design features and a statistical analysis of the tuning procedure are presented.
Recently, there has been considerable work reported which focuses on obtaining drastic reductions, often 2 orders of magnitude, in the analysis time of gas chromatography (GC).l-13 Most of this work has involved the use of relatively short capillary columns coupled with efficient inlet systems. The use of short columns, relative to conventional practice, results in a significant loss in resolution and zone capacity. Thus, the use of high-speed G C has been limited to relatively simple mixtures. The use of microbore (100-wm-diameter) columns results in a greater rate of production of theoretical p l a t e ~ . ~ -However, l~ resolution and zone capacity are still less than values obtained with much longer 0.25-mm-diameter f Present address: Chromatofast, Inc., 912 N. Main St., Suite 14, Ann Arbor, MI 48104. (1) Laming, L.; Sacks, R.; Mouradian, R.; Levine, S.; Foulk, J. Anal. Cfiem. 1988, 60, 1994-1996. (2) Mouradian, R.; Lcvine, S.;Sacks, R. J. Cfiromatogr.Sci. 1990,28,643-648. (3) Annino, R.;Leone, J. J . Cfiromatogr.Sci. 1982, 20, 19-26. (4) Rankin, C.; Sacks, R. LC-GC 1991, 9, 428434. ( 5 ) Peters, A.; Sacks, R. J . Cfiromatogr.Sci. 1991, 29, 403-409. (6) Peters, A.; Sacks, R. J. Cfiromatogr.Sci. 1992, 30, 187-191. (7) Klemp, M.; Akard, M.; Sacks, R. Anal. Cfiem. 1993, 65, 2516-2521. (8) Gaspar, G.; Arpino, P.; Guiochon, G. J. Cfiromatogr.Sci. 1977, IS,256-261. (9) Phillips, J.; Luu, D.; Lee, R. J . Cfiromatogr.Sci. 1986, 24, 396-399. (10) Liu, Z.; Zhang, M.; Phillips, J. J. Cfiromarogr. Sci. 1990, 28, 567-571. ( I I ) Liu, Z.; Phillips, J. J. Microcolumn Sep. 1989, I , 249-256. (12) Tijss.cn, R.; Van den Hoed, N.; Van Kreveld, M. E. Anal. Cfiem. 1987, 59, 1007-101 5 . (13) Gonnard, Guichon, G.; Onuska. F. Anal. Cfiem. 1983, 55, 21 15-2120.
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Analytical ChemlstIy, Val. 66, No. 19, October 1, 1994
capillary columns. In addition, pressure drops are significantly greater for microbore columns and maximum sample size is significantly reduced. If high-speed GC is to be applied to more complex mixtures, greater control of relative peak positions within the chromatograms is essential. Any single stationary phase at a specified temperature will result in a specific selectivity. By the use of mixed stationary phases, a more continuous range of selectivities can be obtained.l”-lg Several mixed-phase columns are available commercially for specific applications, often involving U S . regulatory-driven procedures. The phase mixture is formulated to give the best separation of the worstcase (critical) pair of compounds in a specifiedmixture. Similar results are achieved by varying the length ratio of a tandem (series-coupled) combination of columns using different stationary p h a s e ~ . l ~For - ~ ~the real samples encountered in many environmental applications, only a small subset of these compounds will be present. Thus, for field and screening applications as well as process monitoring applications, the required zone capacity may be greatly reduced, and continuously adjustable selectivity would allow for significant reductions in analysis times. Continuously adjustable selectivity can be achieved by combining two or more columns of different selectivity and adjusting the pressure at the junction point between the As the pressure is changed, the gas flow velocities change differentially in the two columns. This changes the relative residence times of all components in the two columns and thus changes the relative contributions that the two columns make to the overall selectivity. In the present study, pressure-tunable selectivity is developed for high-speed GC using relatively short capillary (14) Maier,; Karpathy, 0. J . Cfiromatogr.1962, 8, 308-318. (15) Laub, R.; Purnell, J. J . Am. Cfiem.Soc. 1975, 97, 3585-3590. (16) Laub, R.; Purnell, J. J. Am. Cfiem. Soc. 1976, 98, 30-39. (17) Laub, R.; Purnell, J. J. Cfiromatogr.1975, 112, 71-79. (18) Laub. R.; Purnell, J. Anal. Cfiem. 1976, 48, 799-803. (19) Purnell, J.; Wattan, M. Anal. Cfiem. 1991, 63, 1261-1264. (20) Jones, J.; Purnell, J. Anal. Cfiem. 1990,62, 2300-2306. (21) Sandra, P.; David, F.; Proot, M.; Diricks, G.; Verstappc, M.; Verzele, M. J.
High Resolur. Cfiromatogr. Cfiromatogr.Commun. 1985, 8, 782-797. (22) Kaiser, R.; Reider, R. J. High Resolut. Cfiromarogr.Cfiromatogr.Commun. 1979, 2, 416-422. (23) MatisovB, E.; KovaciwvB, E.; Garaj, J.; Kraus, G. Cfiromatogrupfiia1989, 27, 494-498. (24) Deans, D.; Scott, I. Anal. Cfiem. 1973.45, 1137-1141. (25) Benick& E.; Krupclk, J.; Kuljovsky, P.; Repka, D.; Garaj, J. Mikrocfiim.Acta 1990,3,1-10.
0003-2700/94/0366-3036$04.50/0
@ 1994 American Chemical Soclety
Flgure 1. Diagram of high-speed GC system with pressure-tunable selectivity and cryofocusingsample collection and injectlon. See text and Table 1 for component descriptions and specifications. Table 1. Instrumentation SpecHlcations for the Tunable Selectlvlty System
Gas Chromatograph Varian Model 3700 with two flame ionization detectors Bifurcation four SGE pneumatic on-off microvalves in 'L" pneumatic valves configuration with 50-mm stem electric valves four Valcor solenoid valves, Model H55P18DlA three Chromfit all-glass splitter 0.25-0.25425 mm connection connected with two 15-cm pieces of 0.25-mm-i.d. columns with 0.25-pm DB-5 370 cm of 0.25-mm i.d. with 0.25-pm DB-5 columns 410 cm of 0.25." i.d. with 0.15-pm DB-WAX R1-R6: deactivated fused silica tubing, 0.1-mm id.; restrictors R1,14 cm; R 2 , l l cm; R1,40 cm; %, 15 cm; Rs, 10 cm; R6, 10 cm R7,4 m, 0.25-mm i.d. Data Collection data translation 2801 12-bit AID board AID computer Dell Systems 200 AT-286
columns. Gas compression effects are relatively small for the short columns used, and a simple, linear tuning procedure has been developed. A bifurcating column switching apparatus is described which allows independent holdup time measurements for a tandem column pair. This allows tuning data to be related directly to the capacity factors of the compounds on the individual columns.
EXPERIMENTAL SECTION Apparatus. Figure 1 shows the high-speed tunableselectivity G C system developed for this work. Component specifications are found in Table 1. The system uses a reverseflow cryofocusing sample collection and inlet system which has been described in detail.7 In the figure, points labeled G1-G3 are carrier gas supplies. The 0.30-mm4.d. metal trap tube T is cooled by a continuous flow of cold nitrogen gas. After sample collection, the tube is resistively heated by the current pulse from a capacitive discharge power supply.' This revaporizes the sample as a narrow plug. Components labeled V1-V4 are pneumatically actuated gas microvalves. Computercontrolled solenoid valves are used to actuate the pneumatic valves. Components labeled R I - R ~are fused silica capillary flow restrictors. P is a vacuum pump; F1 and F2 are flame ionization detectors, and C1 and C2 are the series-coupled
separation columns. Arrows show carrier gas flow directions for the separation and analysis operating mode which is illustrated in Figure 1. For sample collection, valve VI is opened and V2 is closed. This connects the cryofocusing tube to the vacuum pump, resulting in a flow of sample gas from inlet point S through restrictors R2 and R3. Sample vapor is cryofocused on the right-hand end of the trap tube. Sample collection may continue for several seconds to several minutes. After sample collection is complete, valve V2 is opened and a carrier gas flow from GIpurges restrictors R2 and R3, as shown in Figure 1. Toinject the sample, valve V1 is closed, isolating thevacuum pump from the system. The gas flow direction through the trap tube is then reversed, and the tube is heated. The sample isintroducedinto theseparationcolumnasa 5-10ms (standard deviation) wide plug. Columns C I and C2 are nonpolar DB-5 and polar DB-Wax stationary phases, respectively. During a separation, valve V4 is open and V3 is closed. Carrier gas flow through Rg then diverts all sample components to column C2 after elution from column C1. By adjusting the pressure at G2, the selectivity of the tandem combination c 1 - C ~ is changed. Restrictor R7 and FID F2 as well as the valve combination V3-V4 are used to obtain independent holdup time measurements for the two columns. When V3 is open and V4 closed, the effluent from C1 is diverted through R7 and detected at F2. This configuration is used only during holdup time measurements. The instrument components were assembled in a Varian 3700 gas chromatograph equipped with two FIDs. The FID signals were amplified by a high-speed electrometer-amplifier which was constructed in-house. The electrometer was operated with a 5-ms time constant. Data were collected on a 286 PC equipped with a 12-bit analog-to-digital converter. System components and data acquisition were controlled with Labtech Notebook software. Data analysis was performed using software developed in-house. Procedures and Materials. All carrier gas supplies were purified with water vapor, oxygen, and hydrocarbon traps. All compounds used in this study were reagent grade. Samples were introduced from Tedlar gas sampling bags. Liquid sample mixtures were injected into the bag with a microsyringe and diluted with compressed air. While the dilution air contained considerable water vapor, this had no apparent effect on the experiments. Two hours was allowed for equilibration. In order to collect a sample in the cryofocusing tube, restrictor R2 is inserted through the septum of the gas sampling bag. This is accomplished by piercing the septum with a 20-gauge needle and passing the 0.40-mm-0.d. fused silica restrictor tube through the metal needle. The needle is then withdrawn, and the fused silica tube remains in the septum. In order to measure holdup times for the two columns, methane was generated in the metal trap tube by deliberately overheating the tube in order to thermally decompose the sample. A 2-fluorotoluene sample was used for holdup time measurements since a clean methane peak is obtained from thermal decomposition. If valves V3 and V4 are toggled before the methane peak elutes from C1, then the entire methane peak appears at F2. If the valves are toggled after elution of themethane peak, theentire peakappears at FI. Theswitching AnaljdicalChemistry, Vol. 66, No. 19, October 1, 1994
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time required to obtain half of the methane peak at F1 and half at F2 defines the holdup time for column CI. From this value, and the total holdup time from the tandem combination of C I and C2, the holdup time for C2 is obtained.
11
tR
= fm(k
A
z 0
+ 1)
B
4 1
RESULTS AND DISCUSSION Linear Tuning Model. Increasing the pressure at G2 (see Figure 1) reduces the pressure drop along C I . This increases sample component residence time on C1 and decreases residence time on C2, which increases the influence of C1 on the selectivity of the overall separation. For the tandem combination of columns 1 and 2, the overall retention time tR is given by (1)
5
7
9
11
Time
where 2, is the overall gas holdup time and k is an overall capacity factor for the samplecomponent of interest. A similar expressiondescribes the retention time for each of the columns.
I
13
15
17
(8)
Figure 2. Chromatograms used for holdup time measurements, (a, top) chromatogram from FID F,; (b, bottom) chromatogramfrom FID F2.
From eq 6 , a plot of k vs tm2/tmgives a straight line with slope equal to k2 - kl and intercept value equal to kl. Note that, in conventional chromatography using a single column, the value of k is constant under isothermal conditions. If two components in a mixture coelute, in general, it is difficult or not possible to change the column selectivity so that the compounds can be separated. By use of tandem columns of different selectivity, the overall k values can be changed by changing the value of tm2/tm. This results in a change of selectivity and the ability to separate components which cannot be separated on either column alone. Holdup Time Measurements. In order to use eq 6 , it is necessary to obtain values of tm and tm2. Figure 2 shows chromatograms obtained by thermally cracking a 2-fluorotoluene sample during injection from the metal cold trap tube. This procedure has been d e ~ c r i b e d . ~By ~ . ~deliberately ~ overheating the trap tube during sample injection, methane is produced as a decomposition product. This in situ method of methane generation is convenient for holdup time measurements, since methane cannot be collected in the trap tube at the temperatures used in this study. For these chromatograms, 2-fluorotoluene vapor in dry air was sampled from a Tedlar bag using a trapping temperature
of +5 OC. The sample was injected from the trap at a temperature of over 400 OC. This is much greater than the injection temperature usually used and results in thermal cracking of the sample. The column oven temperature was 40 OC. The upper chromatogram was obtained at FID F1and the lower chromatogram at FID F2. Peak labeled 1 is from the 2-fluorotoluene; while peaks labeled A-C are the decomposition products methane, fluorobenzene, and toluene, respectively. Initially, valve V3 was open and V4 closed. At 3.74 s after injection from the cold trap both valves were toggled. This is about equal to the holdup time for column C I , and the methane peak is split between column C2 and restrictor R7. If the valves are toggled after 3.74 s, most or all of the methane peak appears at FID F2, while if the valves are toggled before 3.74 s, most or all of the methane peak appears at FID FI. The overall holdup time from the C1-C2 tandem combination was found to be 6.07 s. The difference, equal to 2.33 s, gives the holdup time for column C2. Holdup times measured using this procedure show standard deviations of about f0.03 s. While the precision of holdup time measurements is satisfactory, accuracy may be limited by software delays and by valve actuation time. While holdup times for a tandem column system can also be measured by on-line detection between the two columns, the method described here has the advantage of very low dead volume. Figure 3 shows holdup time versus tuning pressure (G2) plots. Each point represents a single measurement at the indicated pressure. Ordinate values on the right side of the figure are expressed as the holdup time fraction tm2/tm. Plot A shows the overall holdup time tm. Plots B and C show the holdup times tml and t,2 for columns CI and C2, respectively. Plot D shows the holdup time fraction tm2/tm. As expected, tml increases with increasing tuning pressure, while tm2 decreases with increasing tuning pressure. Both plots are nonlinear because of gas compression effects.26 Plot A for overall holdup time is also nonlinear and shows a very shallow minimum at a tuning pressure of about 175 kPa. Note that the overall holdup time increases substantially at the higher
(26) Maurer, T.; Engewald, W.; Steinborn,A. J. Chromafogr.1990, 517, 77-86. (27) Akard, M.; Sacks, R. J. Chromarogr. Sci. 1993, 31, 297-304.
(28) Ewels, B.; Sacks, R. Anal. Chem. 1985, 57, 277&2779. (29) Klemp, M.; Sacks, R. J . Chromarogr. Sci. 1991, 29, 507-510.
tR1
= tml(k,+ 1)
(2)
+ l)
(3)
tRZ = tmz(k2
Equations 1-3 are related by eqs 4 and 5 . 'R
= t R I + tR2
(4)
tm
= t m l + tm2
(5)
Equation 1 can be solved for k , and after substitution of eqs 2-5, eq 6 is obtained. k = (tR - fm)/fm = ki
(tmJtm)(kZ
- ki)
(6)
3038 Analytical Chemistry, Vol. 66,No. 19, October 1, 1994
1-
o.6 0.5
-- -
- 0.4
8-
8 -
G
- 0.3
3 z 64
- 0.2
4 -
180
180
200
(a)
tI -t J
I I
0
6
4
a'
3
220
Tuning Pressure (kPa) Flgure 3. Holdup time versus tuning pressure: (A) overall holdup time tm; (6) holdup time tml for column Cl; (C) holdup time fm2for column C2; (D) holdup time fraction fm2/fm
tuning pressures. This results in a significant increase in the analysis time for relatively nonpolar compounds. This increase coupled with reduced column efficiency for both columns at high tuning pressures limits the useful range of the pressuretunable system. All studies reported here used t m z / t m values in the range 0.20-0.52. Other ranges can be obtained by adjusting the actual column length ratio. Plot D is nearly linear with a linear regression correlation coefficient of 0.997. Because of this high linear correlation, the time fraction tm2/tm is easily calculated from the pressure data. This allows for the straightforward utilization of eq 6 . The individual capacity factors, kl and k2, can be determined from the overall k and the holdup time fraction t , 2 / t , by use of eq 6 . Selectivity Tuning. Figure 4 shows chromatograms of an eight-component mixture obtained at several values of tuning pressure Gz. Components are listed in Table 2. Note that the last component elutes in under 35 s in all cases. For all chromatograms, the sample vapor mixture was collected at a trapping temperature of -100 OC and injected at a temperature of 120 OC. The sample collection time was 12.0 s. The oven temperature was 40 "C for both columns. Chromatograms a-f were obtained with t,z/t,values of 0.52, 0.48, 0.44,0.34, 0.27, and 0.24, respectively. When the tuning pressure is change, both the overall holdup time tm and the overall capacity factor k also change. The result is that all retention times show nonlinear changes with tuning pressure and thus with t,z/t,. For components which elute with very low capacity factors, most of the change in retention time with changing tuning pressure is the result of the change in overall holdup time (see eq 1). For components with larger capacity factors, the additional change in overall capacity factors results in a variety of patterns of changing retention time with changing tuning pressure. Methylcyclohexane (peak E), which has a significantly smaller capacity factor on the polar column than on the nonpolar column, shows a monotonic but nonlinear increase in retention time with decreasing value of t , z / t , (increasing tuning pressure). For 1-butanol and 2-methyl- 1-butanol (peaks G and H), retention times first decrease and then increase with decreasing tm2/t,. These polar compounds have larger capacity factors on the polar column. These relative shifts provide the basis for selectivity tuning. Note that, for chromatogram c, which
01
"
6
"
10
"
I4
"
18
"
22
"
28
"
"
30
34
Time (a)
Figure 4. Chromatogramsof an eight-componentmlxtureusingdifferent values of 0.52 tuning pressures resulting in holdup time fraction k2/f,,, (a), 0.48 (b), 0.44 (c), 0.34 (d), 0.27 (e) and 0.24 (f). Peaks labeled A-H are ldentlfled In Table 2. Table 2. Measured and Extrapolated Values of k, for Components In the Text Mixture label compound k , measured kl from Figure 6 A
B C
D E F G
H -
n-hexane 2,3-dimethylpentane ethyl ethanoate methyl propanoate meth ylcyclohexane valeraldehyde 1-butanol 2-methyl-1-butanol
0.35 f 0.002 0.39 f 0.006 0.84 f 0.007 0.79 f 0.007 1.40 f 0.012 1.09 f 0.012 0.79 f 0.007 1.59 f 0.01 1
0.33 f 0.004 0.35 i 0.008 0.80 f 0.006 0.75 f 0.005 1.35 i 0.007 1.03 i 0.009 0.77 & 0.015 1.58 f 0.021
corresponds to a t,z/tm value of 0.44, the separation is complete. For all other cases, one or more overlapping pairs is observed. The nonlinear nature of overall retention times with tuning pressure would make selectivity tuning difficult and data intensive. However, if overall capacity factors are plotted versus tuning pressure, linear plots should be obtained. Figure 5 shows plots of overall capacity factor versus tuning pressure for four of the compounds in the test mixture. Error bars shown on the figure are 2avalues. Relative standard deviations for capacity factor values typically are in the range of 0.1OS%, and thus the error bars are barely discernible on the figure. This points out thevery high retention time and holdup time measurement precision that is obtained with the highspeed GC system. Linear regression correlation coefficients for the plots are all in the range 0.995-0.998. The standard error in the y intercept (kl value) typically is in the range 0.5-2.0%, and the standard error in the slope (k2- kl value) typically is in the range of 2-5% except for a few compounds Analytical Chemisfry, Vol. 66,No. 19, Ocfober 1, 1994
3039
n
r
1
140
160
180
200
Tuning Pressure (kPs) Figure 5. Total capacity factor versus tuning pressure for four compounds from Table 2. Linear regression correlation coefficients are 0.995 (C), 0.997 (E), 0.998 (G), and 0.998 (H).
P
0
0.2
0.4
0.6
0.8
I
t m 2 / tm
Flgure 0. Computergenerated plots of overall capacity factor versus holdup time fraction tm2/fmfor the eight components from Table 2.
having relatively small slopes where larger standard errors are observed. Again, this points out that relatively accurate k l and k2 values as well as the optimal tuning pressure can be obtained from the tuning procedure. This provides significantly more qualitative information related to peak identification than can be obtained from a single separation column. Figure 6 shows computer-generated plots of overall capacity factor versus tm2/tm for all eight compounds in the test mixture. These plots are based on linear regression slope and intercept values for plots like those in Figure 5 along with the calibration data obtained from plot D in Figure 3. Note that the slopes of the plots in Figure 6 are proportional to the difference in capacity factors for the twocolumns. A positive slope indicates a larger capacity factor on the polar DB-Wax column, while a negative slope indicates a larger capacity factor on the nonpolar DB-5 column. Note that the nonpolar hydrocarbon compounds (peaks A, B, and E) show pronounced negative slopes while the two alcohols (peaks G and H) show large positive slopes. The overall capacity factors for the aldehyde and the esters (peaks C, D, and F) are quite similar on the two columns. These plots are based on extrapolations from empirical data obtained over a limited range of tuning pressures. The overall linearity of this tuning model was checked by comparing the intercept values (t,2/tm = 0) with measured values of 3040
t
AnalyticalChemistty, Vol. 66, No. 19, October 1, 1994
Figure 7. Relativeretention(a) w W w diagram for the eightampanent mixture of Table 2. Relative retention values were computed from the plots In Figure 6.
capacity factors obtained using only the DB-5 column (see eq 6). Values for the eight compounds in the text mixture are given in Table 2. In all cases, reasonable agreement is observed between the measured kl values and the values obtained from extrapolation of the plots in Figure 6. Note that, for ethyl ethanoate (peak C) and valeraldehyde (peak F), reasonable agreement in the measured and extrapolated k values is observed despite very poor correlation coefficients (0.478 and 0.357, respectively) for the corresponding plots in Figure 6. The poor correlation coefficients are the result of considerable curvature in the plots of k versus tuning pressure. The origin of this curvature, which has only been observed for some compounds containing carbonyl oxygen, is unclear. Note that while reasonable agreement between pairs of k values is seen in Table 2, all values derived from extrapolation of plots in Figure 6 are statistically smaller than the corresponding measured values. The origin of this determinant error is under investigation. Thus, in addition to obtaining the optimal separation quality, the tuning procedure results in the acquisition of two independent capacity factor values, which facilitates peak identification. Alternatively, if kl and k2 are known for all components in the mixture, the optimal tuning pressure can be determined without the need for any empirical data from the tandem column combination. Note that when plots in Figure 6 for different compounds cross, use of the corresponding holdup time fraction t m 2 / t m will result in the coelution of the components. In order to obtain the optimal value of tm2/tmand thus tuning pressure, a window diagram was constructed from the plots in Figure 6 . The resulting window diagram is shown in Figure 7. This plot shows the relative retention of the worst case (critical pair) versus fm2/t,. Relative retention values of 1.Ocorrespond to crossings of plots from Figure 6 and thus correspond to coelution of the corresponding components. Large values of relative retention correspond to the most useful values of tm2/ t,. Note that the tunable range for the present system extends from about 0.2 to 0.52. The window diagram clearly shows that the DB-5 column alone (tml/tm = 0) even if significantly longer would not be adequate for a complete separation. However, the relative retention value of about 1.18 for the DB-Wax column alone
responding values of measured critical pair resolution and identifies the critical pair peaks. Note that, for all four time fractions considered, the measured and computed relative 1.13 f 0.0002 1.11 0.82 f 0.01 B,C 0.480 retention values are in very good agreement. Also note that 1.21 & 0.012 1.21 1.34 0.02 B,C 0.437 1.13 f 0.003 1.12 1.08 f 0.03 E,F 0.371 while the relative retention values for the 0.480 and the 0.371 1.09 & 0.004 1.08 0.76 & 0.03 E,F 0.341 holdup time fractions are very similar, the corresponding resolution values are quite different. Again this points out the shortcomings of the use of relative retention in the construction of window diagrams for components with very (tm2/tm = 1) should be adequate for a complete separation. However, inspection of Figure 6 shows that the plots for small capacity factor values. components A and C are very close together for the tm2/tm In conventional GC, a single value of retention time is = 1 case and these components are not separated on the DBobtained for each separated component in the mixture. Wax column alone. This points out an important shortcoming Severely overlapping peaks may go undetected, and unamin the use of relative retention as a measure of separation biguous peak assignments often are not possible. Both quality. Laub and Purnell18 have pointed out that relative problems are greatly reduced with the tandem column system. retention window diagrams are most satisfactory for the case By obtaining overall k values for two values of tm2/tm (twowhere the critical point components have large capacity factors. column system), in most cases, unambiguous peakassignments For high-speed GC, relatively small capacity factor values will be possible and the coincidental overlap of two peaks at often are required, and in general, values between 1 and 2 any given value of tm2/tm will be detected. In addition, it produce the greatest rate of theoretical plate p r o d u ~ t i o n . ~ ~ should be possible to compute the value of tm2/tm that gives For compounds with small capacity factors, relative retention the best overall separation. For every column added to the values may significantly overestimate the quality of the tandem ensemble, an additional set of retention values will be separation. This will be discussed in detail in a future obtained. This will further enhance the qualitative information publication. obtained from the system as well as improving the selectivity. However, independent tuning of the several column length In order to determine the validity of the tuning procedure ratios will be more complex. described here, measured and computed relative retention values were compared for several values of tm2/tm. The results are summarized in Table 3. This table also contains corReceived for review October 18, 1993. Accepted May 18, 1994.' ~~
Table 3. Measured and Computed Values of Relatlve Retention tm2/tm peaks a measured a computed resolution
(30) Guichon, G. Anal. Chem. 1978, 50, 1812-1821
* Abstract published in Adunnce ACS Absrracis, July 1, 1994.
Analytical Chemistry, Vol. 66, No. 19, October 1, 1994
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