High Precision Sampling for Chromatographic Separations Barry E. Bowen and Stuart P. Cram' Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
John E. Leitner Shoreline Dredging and Constr. Co., Jacksonville, Fla. 32202
Robert L. Wade Winton-Hill Technical Laboratories, The Procter and Gamble Company, Cincinnati, Ohio 45237
The precision of several chromatographic sampling valves of original design is shown to approach 0.05% for unretained solutes. Hybrid-fluidic, high pressure, and commercial valves have been characterized by measuring the precision of their column input profiles and statistical moments. A computer-based data acquisition and control system was developed for use with high precision algorithms.
High precision sampling in gas and liquid chromatography is a prerequisite to the development of new chromatographic techniques for the measurement of fundamental parameters in chromatographic systems and for quantitative analyses. High precision gas chromatography (1-4), high resolution separations (5, 6 ) , automated and computerized chromatographic systems (7-9), qualitative identification from retention data (IO), studies of column properties ( ] I ) , and extracolumn contributions to band broadening (12-16), all require a reproducible injection peak profile. Furthermore, the input profile should be of known functional form in order to obtain the maximum resolution and column efficiency, and to measure the transfer functions of the column. ITo w h o m all correspondence should b e addressed. J. E. Oberholtzer and L. B. Rogers, Anal. Chem., 41, 1234 (1969). R. A. Culp, C. H. Lochmuller, A. K. Moreland, R. S. Swingle, and L. B. Rogers, J. Chromatogr. Sci., 9, 6 (1971). L. J. Lorenz, R. A. Culp, and L. B. Rogers, Anal. Chem., 42, 979 (1970). D. Macnaughton and L. B. Rogers, Anal. Chem., 43, 822 (1971). C.A.M.G. Cramers, "Some Problems Encountered in High Resolution Gas Chromatography," Technische Hogeschool, Eindhoven, The Netherlands, 1967. J. Baudisch. H. D. Papendick, and V. Schloeder, Chromatographia, 3, 469 (1970). S. P. Cram and J. E. Leitner, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 6-1 0, 1972, Paper No. 337. R. G. Thurman, K. A. Mueller, and M. F. Burke, J. Chromatogr. Sci., 9 , 77 (1971). T. H. Glenn and S. P. Cram, J. Chromatogr. Sci., 8, 46 (1970). G. Schomburg, F. Weeke, 8 . Weimann, and E. Ziegler, J. Chromatogr. Sci., 9, 735 (1971). S. P. Cram and S. N. Chesler, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 6-10, 1972, Paper No. 24. J. C. Sternberg. "Advances in Chromatography," J. C. Giddings and R. A. Keller, Ed., Vol. 2, Marcel Dekker, New York, N.Y., 1966, p 205. J. W. Ashley, G. P. Hildebrand, and C. N. Reilley, Anal. Chem., 36, 1369 (1964). C. N. Reilley, G. P. Hildebrand, and J. W. Ashley, Anal. Chem., 34, 1198 (1962). T. H. Glenn and S. P. Cram. Anal. Chem., submitted for publication. V. Maynard and E. Grushka, 162nd National ACS Meeting, Washington, Sept. 12-17, 1971, Paper No. ANAL 098.
Methods of chromatographic sampling have been reviewed (17-24) and the errors associated with sampling and injection have been treated elsewhere. For example, Hamilton ( 2 5 ) ,Condon and Ettre (26), Pitt ( 2 7 ) ,and others (28, 29) have considered the practical aspects of sampling, such as the rate of sample injection, the effect of sample volume and mode of introduction on the retention time, prevolatilization losses, syringe discharge and withdrawal effects, and temperature gradients across a syringe as being among the factors limiting the reproducibility of syringe injections. Goedert and Guiochon (30) have shown that the maximum error in the sample size can be only 4.1 X if a total analysis error of 1 X (at the 95% confidence level) is desired, and that a t least a 10% measurement error may result if the sampling precision is *2%. A number of new analytical techniques and studies have appeared recently as a direct result of new developments in sampling and the studies of a few groups which have critically evaluated this step. Pseudorandom sampling has been used with cross-correlation techniques to enhance the signal-to-noise ratio of small signals (31, 32). Reilley, Hildebrand, and Ashley (14) suggested a periodic introduction of samples into a column with subsequent Fourier Analysis of the waveform appearing a t the detector. Phase modulation techniques have been developed (33) as well as rapid repeated injections to enhance peak deconvolution and t o decrease excessive peak tailing (4). The high precision studies of Rogers et al. (1-4) have been culminated by demonstrating signal-to-noise enhancement for gas chromatographic analyses by ensemble averaging
R. L. Wade and S. P. Cram, Anal. Chem., 44, 131 (1972). R. S. Juvet and S. Dal Nogare, Anal. Chem., 40 (5), 33R (1968). R. S. Juvet andS. P,Cram, Anal. Chem., 42 (5). 1R (1970). S. P. Cram and R. S. Juvet, Anal. Chem., 44 (5), 1R (1972). G. LeMoan, Chim. Anal. (Paris), 47, 341 (1965). S. Dal Nogare and R. S. Juvet, "Gas-Liquid Chromatography,'' Interscience, New York. N.Y., 1962, p 165. A. B. Littewood, "Gas Chromatography," Academic Press, New York, N.Y., 1970, p383. "Modern Practice of Liquid Chromatography." J . J. Kirkland, Ed., Wiley-lnterscience, New York, N.Y., 1971, pp 65-69, 173-76, 327-28. C. H. Hamilton, "Instrumentation in Gas Chromatography." J. Krugers, Ed.. Centrex Publ. Co., Eindhoven. The Netherlands, 1968, p 33. R. D. Condon and L. S. Ettre, "Instrumentation in Gas Chromatography," J. Krugers. Ed., Centrex Publ. Co., Eindhoven, The Netherlands, 1968, p 87. P. Pitt, Chromatographia, 1, 252 (1968). J. W. Todd and C. G. Courneya, U.S. Patent 3,393,551 (July 23, 1968). G. R. Harvey, U.S. Patent 3,368,385 (Feb. 13, 1968). M.Goedert and G. Guiochon. J. Chronatogr. Sci., 7, 323 (1969). H. C. Smit, Chromatographia, 3, 515 (1970). R. Annino and L. E. Bullock, "Advances in Chromatography 1973." A. Zlatkis, Ed., University of Houston, Houston, T e x u , 1973, p 67. D. Obst, J. Chronatogr., 32, 8 (1968).
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-
SAMPLE IN
50”
150“ 0”
0“
COLUMN
SUPPORT
Figure 1. Hybrid-fluidic valve shown from the front with away view of the valve assembly
TUBE
a cut-
Valve is in the normal position for carrier gas flow onto the column
Reilley et al. (14) developed a general relationship between the sample input profile and the shape of the response curve. They employed a Laplace Transform method to calculate the shape of the output peak for any input function modified by a Gaussian operator (the column) and by extracolumn contributions t o band broadening of defined form. This work was extended by Sternberg (12) in an elegant treatment of the second moment contributions of the extracolumn contributions and their effects were applied to wall-coated open tubular column separations and ultra-high speed chromatography. Recently, the relationships between peak shape, peak symmetry, retention time, sample volume, and column type were measured in order to study the effects of overloading the column, although the precision and significance of this work is directly dependent upon the type and mode of sampling used (6). However, the exact nature of the transfer function of the column itself has not been previously determined and is the subject of a continuing investigation in this laboratory. The prerequisites for such a study are (a) a high precision sampling system with a well defined input function which can be treated mathematically, ( b ) a high precision, low noise digital data acquisition system, and (c) an accurate method of determining the instrumental contributions to band broadening and a definition of their functional form so that raw data may be corrected for these effects. The first of these requirements has been completed recently and is reported elsewhere ( 7 , 11, 15, 34, 35). Sampling valves are developing rapidly as a peripheral device for gas and liquid chromatographs. They can be readily automated, have the potential of high precision, and have a great deal of versatility. Most sampling valves may be classified as the sliding seal type ( 3 6 ) ,rotary (37), and diaphragm seals (38, 39). Two papers have reported the design development of several fluidic logic systems (17, 40). All of these devices have inherent limitations and none of them operate as an ideal sampling system. Ober(34) S.N. Chesler and S. P. Cram, Anal. Chem., 43,1922 (1971). (35) S. P. Cram, S. N . Chesler, and D. B. Cottrell, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 6-10, 1972, Paper No. 376. (36) R. J. Harris, U S . Patent 3,385,113 (May 28, 1968). (37) E. L. Karas U.S. Patent 3,339,582 (Sept. 5, 1967) (38) E. L. Szonntagh, U.S. Patent3,386,472 (June4. 1968). (39) W. H. Topham, British Patent 1,111,443 (April 24, 1968). (40) S. P. Cram and S. N. Chesler. Anal. Chem., submitted for publication.
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holtzer and Rogers ( 1 ) measured the band width and the precision in the elution time for three different types of automated sampling valves. Their results indicated that the valve which had the most reproducible timing did not necessarily give the narrowest injection profile and vice versa. Two of their valves also showed a memory effect. Although they did not report their peak profile shapes, their precision of determining peak parameters will be compared to three valves here. Two of the valves reported in this work are of original design. The third is commercially available and was originally designed for high pressure liquid chromatography (41). Peak profiles for all of the valves were measured on a chromatographic system where the instrumental effects on the peak shape were minimized and were accurately known, and with a computer-based data acquisition and control system for high precision and speed.
EXPERIMENTAL Sampling Valves. A cut away, cross sectional view of the “hybrid-fluidic valve” is shown in Figure 1 in the normal position. T h e push-pull solenoids are Guardian type 14 with a n 11-ohm coil. The carrier gas and inlet tubes were 0.040 in. i.d. stainless steel tubing mounted in a block attached to the armatures of the solenoids. The centerline distance between the tubes is 0.188 in. which is also t h e total travel distance of the solenoid armatures. The column is mounted in a brass bellows beneath the inlet tubes so t h a t i t is normally aligned with t h e He carrier gas inlet. In this manner, there are no sliding seals in the valve and the dead volume is negligible. However, t h e coaxial alignment of t h e column with t h e carrier and sample inlets is critical in order t o achieve good reproducibility and t o prevent sample bleed into t h e carrier gas line. T h e inlet tubes and the column are separated by about one-half the inside diameter of the tubes or 0.02 in. so t h a t the dynamic lines of sample gas flow intersect t h e column axis only when the valve is switched for sampling. This minimizes the “end effect” on the laminar flow through the tubes and allows flow of gas into t h e column only from the inlet capillary with which t h e column is aligned. T h u s the separation distance and the alignment are critical and the carrier gas flow rate must be 10 to 20 times the column carrier gas flow rate. The sample gas and carrier gas are allowed to escape through a vent valve (Nupro, Model SS-2MA) which is mounted in the base of the flexible bellows assembly. This valve allows control of the rate of venting of sample and carrier gases with a column of different pneumatic resistance. In order to change t h e sample size, t h e valve gate pulse width, the sample gas flow rate, or the delay time in s a m pling from t h e exponential dilution flask can be varied. The “high pressure valve” evaluated in this study is also shown in a cut-away. cross sectional view in Figure 2. This valve was designed to maintain a pressure differential of 2000 psi, to have minimum dead volume, to be amenable to complete automation for high precision and minimum injection time, and t o avoid interruption of the carrier gas flow while in the sampling position so t h a t pneumatic equilibrium is maintained on t h e column. The stainless steel plunger was lapped, polished, and flashchrome plated (0.0001-0.0002 in., Master-Chrome. Inc.). The sample volume was 1.0 ~1 and was defined by a n annular groove 0.015 in. X 0.005 in. on the plunger. These dimensions were chosen to provide the minimum surface areas to the transfer surface and thus reduce sample wiping or gouging of the seals when the valve was activated. T h e shaft sealing nut compresses t h e tension spring and can be adjusted for the pressure differential desired. One unique feature of this high pressure valve is the capability of constant tension adjustment on increasing temperatures afforded by the spring and sample transfer chamber moving in tandem t o offset the thermal expansion of t h e high pressure Teflon (Du Pont) seal. Capillary columns or 48-in. packed columns may be placed within 0.094 in. of the plunger shaft to reduce the dead volume. This chamber represents a mixing volume of 22 ~1 a n d yet allows columns to be readily changed without worrying about alignment. The valve body was 304 stainless steel a n d was heated with a 200-watt cartridge heater (Model HS3725, Hotwatt). The travel distance between the sample chamber a n d the injection port was 0.726 in. This is sufficiently long t o provide a large (41) Hamilton Co.. Whittier. Calif., Bull. 744 (1969).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
SOLENOID
r
1
He lN
a
1 1 0 ~ AC
I
t
S H A F T S E A L I N G N~T~JJ TENSION
SPRING
SAMPLE CHAMBER TEFLON
COLUMN S U P P O R T T U B E
I
IlOV AC
COLUMN
SEAL
INJECTION PORT
Figure 2. High pressure sampling valve shown f r o m the front with a cut-away view of the valve assembly
Valve is in the sample injection position. Sample inlet ports are perpendicular to the plane of the drawing
SCOPE
sealing surface with the Teflon seal a n d yet short enough t o retain the pulling power of t h e solenoids for high speed operation. Fiftypound solenoids (No. 447.1, Dormeyer Industries) were used to actuate the valve under computer control. In order t o change sample size, a new shaft with a different sized annulus must be installed, t h e sample pressure increased, or t h e sampling time out of t h e exponential dilution flask varied. T h e third valve studied was a pneumatically operrated valve designed for gas a n d liquid chromatography which is cnmmercial-
ly available from t h e Hamilton Company. This valve is also designed around a n annulus cut on t h e sliding shaft. A 1-p1 sample volume was used for t h e work reported here. T h e valve was actuated by a pneumatic piston in t h e valve which was driven by air pressure controlled by two 3-way electric switching valves (Model 25031-3-10-21-36-61, Humphrey Products), The switching pressure was nominally 120 psi of N a , Pneumatic System. A well regulated, stabilized flow system designed for these studies is shown in Figure 3. T h e balanced
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Table I. Flow Rates Used to Measure Sampling Valve Characteristics Valve
He, ml/min
Hybrid-fluidic, 1 0 0 m s e c Hybrid-fluidic, 50 m s e c High pressure Hamilton, 100 “Ca Hamilton, 30 “C
770 770 114 113 113
CHI, ml/min
HI, ml/min
Air, ml/rnin
76.8 76.8
54.0
...
41.6 98.3 95.6
263 263 468 568 589
182 116
54.0
Temperature refers to column oven temperature. Valve by conduction to a temperature less than that of the oven.
D EFVRI C OE M CCO PU DE
9 i IOP
was heated
2
no
VALVE GATE 2
B €M 3 BMl OB r J 9 & z L l
Figure 4. D e v i c e selector logic for computer control of the sampling valves
I
VALVE
GATE I
I
I
>+ I I I
I
,
IOP 4
I .-i
V A L V E GATE 2
1
IOOQ
‘I
Figure 6. Driver circuit for automating the solenoid actuated sampling valves
pressure regulators (Veriflow Corp., Model PN-41200649) were chosen over standard regulators because they are virtually independent of changes in the inlet pressure: the outlet pressure is changed less t h a n 0.02 psi for a 100-psi drop in inlet pressure. The regulators were followed with l.5-foot, 5A Molecular Sieve traps and a standard single stage. pressure regulator (Veriflow Corp.. Model 413004511. These regulators offer precise control when the inlet pressure is held constant. Thus they were used a s a second stage of regulation. An extra fine constant upstream flow controller (Veriflow Corp., Model PN-42300080) was used to regulate t h e flow t o better than f 2 R over a change in ambient temperature of *lo “F. The flow diverters (Varian Aerograph, Model 51-0001462188
00) enable the carrier gas flow rate (or CH4 sample) to he measured at the column inlet because the micrometering valves (Veriflow Corp., Model PN-43000285) represented a negligible restriction in the line. The 650-ml exponential dilution flask was of standard design and was independently heated. Three-way switching valves were also included for gas sampling a n d t o take the dilution flask out of t h e sample line. The pneumatic system was not temperature regulated although t h e room temperature was buffered from fluctuations so t h a t it varied less t h a n i2 “C throughout these studies. Gas Chromatograph. A Varian Model 2100 gas chromatograph with a linear temperature programmer and dual flame ionization detector was used in this work. The sampling valves were mounted above the injection ports and nominally operated a t room temperature. The other temperatures were as follows: injection port, 80 “C; column oven, 96 “C: and detector, 85 “C. The volumetric flow rates used in the system are given in Table I. A Barber-Colm a n Model 5044 electrometer was used in order to get a Ion noise and wide dynamic range input for the digital d a t a acquisition system. Computerized Data Acquisition and Control System. Figure 3 also illustrates the computer interface used in these studies. The details of the hardware and software are presented elsewhere (421 hut it is important to point nut here that the system does include a 10-bit ADC (Digital Equipment Corp.. Model A8111 with sample and hold amplifier (Digital Equipment Corp., lZlodel A400), a programmable gain amplifier, and a programmable clock. The computer is a PDP-8/L (Digital Equipment Corp.1 with 8K of core. A four-tape magnetic tape cassette system (Model 4096, Tri-Data Corp.), a high-speed paper tape reader (Mark V. Datascan Corp.1, ASR-33 Teletype (Teletype Corp.). and a 15inch display scope (Model l735D, ITTi were the principal peripherals used in this work. The device selector for the sampling valves in Figure 4 was controlled by the device code and the input/output pulses (IOP) which were brought out from the buffered memory buffer (BMB) register of the computer. The device code is used to enable NAND gates 3, 4. and 5 upon receipt of a 614X instruction, where X is an I O P 2 , 4, or 6 pulse. The valve gate pulse to switch the valve to the sampling position is generated by the IOP2 pulse which clocks flip flop No. 1 IFF1) to a logical “1” at the Q output. Then a 6146 command enables S A N D gate 5 and resets FF1 and FF2. T h e next instruction is an IOP4 which puts a logical “0” on the clocked input of FF2 and thereby fires the second solenoid to return it to its original position. The timing sequence for act. uation of the valve is shown in Figure 5. The duration of the valve gate pulses is variable in the sdftware. The time for the valve gate pulse No. 1 for t h e “hybridfluidic valve” determines the width of the sample profile entering the column. For t h e “high pressure valve” and the Hamilton valve,” the pulse width determines the amount ot’ time during which the sampling volume is flushed with carrier gas in the sample injection position. The valve gate pulse width No. 2 corresponds to the time required t o build u p the field in the solenoid, bring the column hack into alignment with the He inlet line on the hybrid-fluidic valve or return the other valves to their normal position, and ensure that the valves seat and do not ring or recoil. At the end of the valve gate pulse S o . 2, the flip-flops are again reset and the clock rate is changed for the start of d a t a acquisition. The solenoids are fired and switched by the driver circuit in Figure 6. The positive going input pulse from the device selector is driven through the emitter coupled transistors in a Darlington configuration in order t o increase the current gain at the output and to reduce the loading on the device selector by the high impedance input. The R F interference filter (Filtron 96051 serves t o keep the electrical noise, generated by switching the inductive load, out of the digital logic system. The diode around the solenoid acts as a low impedance path to allow the magnetic field to collapse when it is cut off without dumping the current in the coils through the 255877. The 18-ohm resistor is a damping resistor.
RESULTS A N D DISCUSSION F i g u r e s 7-10 a r e t h e v a l v e s a m p l e i n j e c t i o n profiles for a 5 0 - m s e c v a l v e g a t e pulse w i d t h on t h e h y b r i d - f l u i d i c (42) J. E, L.eitner tion.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 13, NOVEMBER 1973
and S. P. Cram, Anal. Chem.. submitted
for
publica-
Figure 7. Valve injection profile obtained with a 5 0 - m s e c valve gate pulse to the hybrid-fluidic valve Marker dots denote integration limits at 1 2 0, 1 0 5, and f0.2% of the peak height Time base of display I S 0-2000 msec
Figure 9. Experimental valve injection profile for the high pressure valve with integration limits at f 2 . 0 , f 0 . 5 , and f 0 . 2 % of the peak height. T i m e base of display is 0-2000 m s e c
Figure 8. Valve injection profile obtained with a 100-msec valve gate pulse to the hybrid-fluidic valve
Figure 10. Experimental valve injection profile for the pneumatically operated Hamilton valve at 100 "C
Marker dots denote integration limits at f2.0,f0.5,and f0.2% of the peak height. Time base of display is 0-2000 msec
Limits of integration are at 12. -0.5, and -0.2% of the peak height. Last data point on the tail of the peak is 1.25% of the peak amplitude. Time base of display is 0-2000 msec
valve, the hybrid-fluidic valve with a 100-msec gating pulse, the high pressure valve, and the Hamilton valve operated a t 100 "C, respectively. All of the data were taken with a n ADC conversion rate of 500 Hz and are displayed on the same time scale. The amplitude of the peaks and their locations have been normalized for display purposes. The marker-indicator dots denote the limits of integration and are located a t the point where the signal is f 2 . 0 , f0.5, and *0.2% of the maximum peak amplitude. The description and error analysis of this technique has been described elsewhere ( 3 4 ) . Figures 7 and 8 are virtually identical in peak shape so t h a t valve gate pulse widths down t o 50 msec can be expected to introduce unifcrm input profiles to the column. The sample shown in Figure 8 is approximately twice as large as the one in Figure 7 , but was divided by two for display purposes and thus appears to be the same size as Figure 7. The narrow injection profile in Figure 9 can be explained in terms of the short effective injection time of the high pressure valve. Only during the last 2.1% of travel of the annulus will the sample volume of the valve come into alignment with the column. Thus the effective injection time could be as fast as 200 psec. Figure 10 shows a peak shape similar to those in Figures 7 and 8, except that it has a much longer tail due to a small amount of leakage and/or the presence of' diffusion chambers in the valve. Thus the values of the higher order moments are considerably larger for this valve.
The sample size was adjusted so that the range in sample sizes was less than a factor of 2.5 for the three different valves in order to avoid introducing additional variables. The ratio of the peak areas for the hybrid-fluidic valve was 2.038 when the valve pulse width ratio was 2.00 so that it is nearly linear. The deviation from 2.00 was probably due to some recoil and ringing of the inlet tubes which will introduce increasingly larger errors as the valve gate pulse width is decreased. The difference in sample sizes of the Hamilton valve a t two different temperatures is only 3% and can be explained as a change in the leakage rate through the Teflon (Du Pont) seals and the travel time of the sample shaft as it is affected by frictional forces. The high pressure valve minimizes these problems because it has a constant tension spring and thus can be operated over much wider temperature ranges. Table I1 summarizes the characteristics of each of the peak profiles in terms of their statistical moments and their precision for a t least 10 determinations. A relative precision of &0.1-0.2% in the peak area is particularly encouraging when one considers the errors associated with locating the peak start and finish (limits of integration) and lack of regulation on the sample gas supply. The method of peak truncation previously described by Chesler and Cram (34) was used to minimize the first of these errors, and because the relative errors at the f 2 % integration limits can be easily determined. Thermostating the pressure m d flow regulators would also increase the precision of the pneumatic control system.
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Table 111. Comparison of the Precision of Several Automated Sampling Valves with Unretained Solutes Valve
Hybrid-fluidic High pressure Hamilton Seiscor (7) Carle ( 7 ) Kieselbach (7)
Precision in elution time, %
f0.05-0.08 f0.64 f1.36 f0.22 f1.08-1.17 11.00-2.00
As the order of the moment ( n ) increases, we would expect to realize an increase in the errors because of the uncertainty in the time of each data point raised to the nth power. However, the data taken for the hybrid-fluidic valve with a pulse width of 100 msec indicates that the precision is constant to within 0.1% for all of the peak parameters and as much as 28 times better than some of the other valves. The nearly Gaussian character of this valve is also significant and unique (skew = 0.71 us 0.00 for a Gaussian and excess = 3.50 us. 3.00 for a Gaussian). It should be pointed out that valve gate pulse widths as short as 9 msec have been used with the hybrid-fluidic valve. However, the best precision for this value was obtained at 100 msec and the increased pulse width is not detrimental for most chromatographic experiments. Thus the hybrid-fluidic valve is the valve of choice for the best precision and for generating near Gaussian input profiles. The precision of the high pressure valve for the zeroth and first moments is comparable t o that of the hybridfluidic valve. The fact that the former gives narrower pulse profiles and that columns can be changed much more readily, currently makes it the valve of choice for implementation in the laboratory. Although the excess for this valve profile is markedly increased, the skew is not. The larger error in the first moment is due to variation in the sealing pressure, caused by frictional heating when the valve is fired. This time represents a longer travel time than the first valve and does require sliding seals. The Hamilton valve was originally designed for sampling in liquid chromatography a t pressures to 5000 psig. Any valve of this nature can be expected to vary in performance with temperature changes. The relatively good precision makes it well suited for liquid chromatography. The most serious disadvantage is the tailing of the input profile which arises from leaks in the Teflon seals (because they can't be too tight with a pneumatic drive), diffusion chambers, and the perturbation in the band shape caused by the carrier gas pressure surge following shut off during the sampling time. Because precision sampling data are very scarce in the chromatographic literature, only the relative precisiop in the elution time can be compared directly. By defining our elution time as the time a t which data acquisition is started to the center of gravity of the peak, and the per cent relative standard deviation as a measure of precision, then several valves can be compared as shown in Table 111. The dependence of the moments and their precision on the location of the peak limits is shown in Table IV for the hybrid-fluidic valve with a 100-msec valve gate pulse width. Peak sensing by thresholding and the first derivative method are subject to intolerable errors when making moment calculations and in the presence of noise. Thus the method of limits in terms of a fraction of the peak 2190
ANALYTICAL CHEMISTRY, VOL.
45, NO. 13, NOVEMBER 1973
Table IV. Effects of Limits of Integration on the Mean Value and Precision of the Statistical Moment Calculationsa Integration limits Moment
PO PI,
sec
1 1 2 , sec2 ~ 3 sec3 , ~
4
sec4 ,
Skew Excess
f2.0%
*
2145 f 0.15% 0.5132 f 0.08% 1.388X IO-' f 0.07% 1 . 1 0 1 x 10-3 f 0.15% 0.6273 X f 0.16% 0.7144f 0.12% 3.502f 0.08%
f0.2%
*0.5%
2171 0.16% 0.5189f 0.08% 1.656X lo-'* 0.30% 2.687 X f 1.22% 1.672 X f 1.52% 1.261 f 0.79% 6.097f 0.95%
(I Data was taken with the hybrid-fluidic valve at a vaive gate pulse width of 100 msec. same conditions as reported in Table I I .
amplitude is used here and has been previously justified ( 3 4 ) . I t is of particular interest to note that the precision of the peak area and mean are about constant for the limits tested. This is most encouraging because these are the moments of general interest. As expected, the precision then decreases by as much as a factor of 50 as the order of the moment increases and as the limits are extended to lower amplitudes. The peak area naturally increases as the limits are extended to include more of the peak area. At &0.2% limits, the peak area is 2% larger than a t limits of f2.0%. This error is nontrivial and not readily apparent from the valve profile in Figure 8. The high order moments also increase in a regular manner by as much as 1.9 x IO3% for the fourth moment) with the extended limits while the reproducibility decreases by a factor of about 50 for the same reasons. These data clearly show the importance of specifying the limits of integration as they affect both the absolute value of the moments and their precision.
2189 f 0.16% 0.5273 f 0.12% 2.538 X IO-* f 1.95% 11.99X f 5.52% 12.63X f 7.44% 2.996f 2.76% 19.48 f 3.86%
All calculations were made on the same data set and under the
ACKNOWLEDGMENT The authors gratefully acknowledge the construction of the valves by Art Grant, Frank Ottinger, and Chester Eastman of the University of Florida. Received for review July 17, 1972. Accepted June 12, 1973. Paper presented in part a t the 1973 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 5-8, 1973, Paper No. 126. Certain commercial materials and equipment are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. The financial support of the National Science Foundation under Grant No. GP-14'754 and the National Institutes of Health under Grant No. GM-17203 is gratefully acknowledged.
Performance of a Reduced Volume Thermal Conductivity Detector R. E. Pecsar, R . B. DeLew, and K. R. lwao Varian Aerograph, 2700 Mitchell Drive, Walnut Creek, Calif. 94598
By reducing the internal volume of the thermal conductivity detector, the range of applicability can be significantly increased. The minimum detectable quantity is reduced by an order of magnitude from that normally achievable. Such a detector is compatible with l/a-in. diameter and capillary columns, which are intrinsically more efficient, because of the decreased contribution to extra-column band-broadening. This increased sensitivity can only be realized by careful control Of the detector temperature and flow rate. Dual differential flow control, as well as fully proportional temperature control, are employed to achieve the maximum signal t0 noise ratio. The efficacy of such a proportionally controlled detector is demonstrated by the low thermal gradients across the cell and the highly stable output signal. When operated with an %-in. column, hydrocarbons in the 1-5 ppm range can readily be quantitated. Comparison is made of programmed temperature capillary column separations with
both the thermal conductivity and flame ionization detectors. Resolution in both cases is equivalent, the sensitivity of the thermal conductivity detector being within 100fold of the ionization detector.
The thermal conductivity detector (TCD) has been the principal detection means in gas chromatography since the inception in 1956, Prior to the introduction of the ionization detectors in 1959-60, it was the only practical device, since that time, this predominant position has in large measure been reduced but even today approximately 40% of all gas chromatographs soid are equipped with the
TCD. A number of reasons exist why the popularity of this detector continues. The setup and operation is very straightforward requiring no auxiliary gas flows. The operating parameters which need tuning or exact control are usually preset by the instrument design and do not neces-
A N A L Y T I C A L CHEMISTRY, VOL.
45, NO. 13, NOVEMBER 1973
2191
