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Fluidic Logic Element Sample Switch for Correlation Chromatography Raymond Annino" Department of Chemistry, Canisius College, Buffalo, New York 14208
Marie-France Gonnord and Georges Guiochon Ecole Polytechnique, Laboratoire de Chimie Analytique Physique, Route de Saclay, 9 1728 Palaiseau Cedex, France
A gas stream switch has been fabricated from two bistable fluidic amplifiers. In contrast to the usual application of Coanda wall attachment devices which must operate into falrly low impedences, this system has been used as a gas chromatographic sample switch at column head pressures of 800 mbars. As demonstrated in this report, the ability of this switch to reproducibly inject varying amounts of sample on command makes it ideally suited for correlation chromatography. Further efforts are being directed to developing a switch which will work at higher pressures for use with high speed packed column chromatography, and also whlch wlll provlde smaller sample widths at faster switching speeds for capillary column applications of correlation chromatography.
T h e use of correlation chromatography as a means of increasing signal to noise ratios has been suggested by a number of investigators ( I , 2 ) . T h e origin of a number of difficulties which have been experienced with this technique has been established and some basic guidelines for the effective use of correlation chromatography have been published ( 3 , 4 ) . This previous work has been concerned primarily with the theoretical basis for the idea with some but not a great deal of supporting experimental data. One of t h e difficulties in obtaining laboratory evidence has been due to the lack of a suitable sampling valve for the chromatograph which would not only have the required switching speed but also a lifetime comparable t o t h e commonly used single injection devices. These requirements have been difficult t o meet. T h e duty cycle of a correlation chromatography sample valve is usually 500 to lo00 times greater than experienced in the normal mode of operation, and switching speeds must be a t least twice as fast as required for a comparable high speed injection procedure. Our experience (5-7) as well as t h a t of Cram and his coworkers (8,9) with fluidic wall attachment devices has shown t h a t they can be readily adapted to act as chromatographic injection valves. The advantages of these fluidic elements are that by virtue of their fast switching speeds they can deliver very small samples and, as they have no moving parts to wear, their lifetimes appear to be extremely long. This report is preliminary and describes our progress in the development of a suitable correlation chromatography injection system utilizing fluidic elements.
THEORY In correlation chromatography, the sample or carrier gas is alternately injected into the chromatograph a t the command of a pseudo random binary sequence (PRBS). There may be as many as 1000 injections made during t h e period of a retention time. This produces an output signal consisting of a number of overlapping peaks. Because of the special properties of the PRBS, it is quite simple to unscramble this output and obtain a chromatographic signal which theoret0003-2700/79/0351-0379$01 .OO/O
ically has an increased signal to noise ratio over the normal single injection chromatogram. T h e theory underlying this operation has been published as has a discussion of the mathematical operations involved in obtaining t h e correlogram from the output (I, 3 ) . Basically, an array of output signal values is memorized from the present time to one P R B S code period in the past. A single point on the correlogram is generated by multiplying each one of these values with the corresponding point of t h e PRBS (either +1or -l),summing these products and dividing by the total number of values. T h e next correlogram point is generated in the same manner after t h e P R B S is shifted one time unit. The process continues until one full code unit has been shifted through t h e experimental array. The correlogram is then presented as a plot of each of these correlation points vs. the time shift of the code which produced it. The above operation is a particularly simple task to accomplish electronically and thus the procedure would appear to offer increased detectability a t a minimum cost. The only basic hardware modification t h a t must be considered is t h e sample valve. T h e principal component of the sample injection system used in this research is the fluidic logic gate shown schematically in Figure 1. If the flow into the device is maintained in the turbulent region, the stream entering a t S will attach itself to a wall a t the junction of O1 and 02.A pressure signal (usually 10%-30% P,) momentarily applied t o a command port (C, or C,) will cause t h e stream to switch t o a channel opposite the active command port and remain in this position until a pressure pulse is applied to the opposite command port. Vents VI and V2 are provided to ensure sufficient volume flow and consequently proper stream switching even if ports O1 and O2 are blocked. Cram and his co-workers have published excellent papers describing various possible configuration of these devices as chromatographic logic and sampling systems, and the reader is referred to these reports for a more detailed presentation
(8,9 ) . EXPERIMENTAL The correlation chromatograph is shown schematically in Figure 2. The sample valve consists of two bistable fluidic logic elements (Corning 191454) and a control circuitry to switch carrier gas or sample gas into the column on command from the pseudo random switching code generated by the computer (HP21MX-30). A detail view of the logic gate housing and connections is shown in Figure 3. The sample valve consists of two previously described ( 5 ) single impulse units modified t o work into packed columns and connected t o each other through a check valve. Also, the original monostable amplifiers have been replaced with bistable amplifiers. Carrier gas is directed to the column and sample is diverted to O1 (sample) by momentarily activating electrovalve 4. Similarly, sample is directed to the column and carrier diverted to 0, (carrier) by momentarily activating electrovalve 4'. The various adjustments which must be made when initially setting up the apparatus can be generalized in terms of the G 1979 American Chemical Society
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Figure 1. Schematic representation of a bistable fluidic amplifier showing t h e supply gas attached to the wall of t h e 0,leg and splitting its flow between O2 and t h e vent, V,. S = Inlet for supply gas (carrier gas or sample gas), C , and C2 = control or command ports, V , and V, = vents, 0,and O2 = exits
Figure 3. Section of t h e fluidic gate housing not drawn to scale but illustrating the construction and location of t h e various elements of the system. (1) Pressure housing. The logic gate vents into this chamber, (2) Connection for restricter R, and pressure gauge, P,,, (3)Holder for logic gate, (4) Logic gate, bistable fluidic amplifier, (5) Rubber O-rings, (6) Gas entrance. This may b e carrier gas or sample gas depending on the application, (7) Command port. One is positioned behind the other, (8) Swagelok fitting. A '/,-inch 0.d. column with ends notched so that it c a n be mounted against the fluidic gate, fRs into this connection if the logic gate is powered by sample gas. If this is the carrier gas gate, connection is made between this point and the check valve input to the sample side of the system, (9) In the sample gas powered gate, t h e check valve is mounted here and carrier gas enters this port. In the carrier gas gate housing the pressure gauge, Po, is mounted at this point, (10) The exit for 0 1 is located behind 8 and is constructed in a manner similar to that shown for 7
Figure 2. Schematic diagram of correlation chromatograph. (1) Pressurized carrier gas source (nitrogen),(2) Pressurized sample source, (3)Pressure regulators, (4) and (4') Electrovalves (Clippard Minimatic EV-3), (5) Drivers for electrovalves, (6) Housings for fluidic gates, (7) Bistable fluidic amplifiers (Corning 191454),(8)Check valve, (9) Analytical column (3 ft X ' / 8 inch o.d., 17.8% bis(2-ethoxyethy1)sebacate on 80/100 mesh Chromosorb P), (10) FID detector (Varian 02-001953-00) requirements for the chromatographic column and the sample switch. Usually some known column inlet pressure is necessary to obtain the desired flow rate. Also the fluidic amplifiers used in this research will have a pressure drop of 800-1000 mbars if they are operated in the turbulent flow region. Finally, since one or the other of the two fluidic elements is always connected to the column, the total column flow of gas is available through either of the two fluidic devices. The procedure adopted in this research for apparatus setup involved connecting the carrier gas and increasing the input pressure, P,(carrier), to the fluidic amplifier while adjusting the vent restrictor, RJcarrier), until the desired column's input pressure, P,,, was obtained, and in addition, P, was 800-1000 mbars greater than Pev. Electrovalves 4 and 4' were activated while watching for a pressure change to O1 (carrier) to determine if indeed the stream was switching. The restrictor Ro,(C) was adjusted so that when carrier gas was exiting through 01,the pressure Pol(C)was the same as the pressure, Po,(C) when carrier was exiting through O&). The command pressure, Pc,was usually set a t 10-30% P,. The process was repeated with the sample, but, in addition, the FID output was monitored for sharp sample signal profiles and the absence of base-line upsets upon switching from sample to carrier and back. Pseudo-random code sequences were computer generated using the algorithms previously discussed ( 3 ) . These command signals were then transmitted to the electronic drivers which powered the electrovalves. Sample widths were limited to 50 ms or greater as the result of the combined time constants of the drivers, electrovalves, and the dead volume of the connecting tubing. Data acquisition was accomplished synchronously with a Hewlett-Packard 21-MX30 minicomputer equipped with a I ?-bit Analog-Digital interface subsystem operated with an external
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Figure 4. Reproducibility of sampling system. (a) Solid line = predicted output for multiple 100-ms injections commanded by a 51 1 PRBS using the experimentally determined 100-ms pulse response of the system a s the basis for the construction. Dotted line = experimental detector response to multiple 100-ms injections commanded by a 51 1 RPBS. (b) Solid line = correlogram calculated from t h e predicted detector response shown in a. Dotted line = correlogram calculated from the experimental detector output shown in a . Sample = methane doped air, Carrier gas = nitrogen. pacer at 1 kHz. To simplify programing, all data were acquired and stored on magnetic tape and correlations were performed at a later time and then presented on a Hewlett-Packard 9872 A plotter. For test purposes, methane doped air and 50-ppm concentrations of ethane, propane in nitrogen were used as samples and nitrogen was used as the carrier gas. To determine the reproducibility under the high speed conditions of correlation chromatography, the following method was used. A single sample was injected within a predetermined time interval and the resulting chromatographic signal was recorded and memorized. This peak profile was then used as the impulse response of the system for the particular sample pulse width. A synthetic detector signal was constructed using this impulse response as the building block. The number and spacing of these pulses was determined by a pseudo-random binary sequence. Using this same binary code as the pattern for actual sample inputs, an experimental curve was then generated and compared to the synthetic curve for accuracy of correspondence. An example of such a comparison is shown in Figure 4 for a 100-ms unit code.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
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Figure 5. Stability of sampling system over extended periods of time Experimental detector output measured over three periods of a 51 1 PRBS Unit code width = 100 m s Length of abscissa 1s equal to one PRBS period
Acceptable results were also obtained for 30-ms unit codes. The stability of the sample over longer periods of time is illustrated in Figure 3.
RESULTS A N D DISCUSSION In view of our suggestion t h a t correlation chromatography is possible with an unmodified chromatograph provided t h a t a sufficiently rapid sample valve is available ( 3 ) ,the stream switching arrangement depicted in Figure 1 seems unduly complicated. Indeed, preliminary experiments were conducted in this laboratory with a single bistable fluidic switch injecting small amounts of sample into the carrier gas. These efforts met with little success. T h e correlogram showed little or no relationship to single impulse chromatograms. On reflection, t h e reasons for this became quite obvious. Consider t h a t during the length of a correlation chromatography experiment, the pseudo-random code requests sample injection for 50% of the time. Thus, the detector base-line signal is really one for sample continuously present in the detector a t a concentration level of 50% of its original value. The output is then developed about this elevated base line. T h e retention time of the peaks is governed by the value of t h e equilibrium constant under these conditions a n d , although it may not be exactly the same as that obtained under normal single impulse chromatography conditions. the system will behave linearly as long as the concentration difference between the two streams is not too large ( 4 , I O ) , Le.. for trace analysis or difference chromatography. Thus retention time shift which may have been due to this effect was not the source of the problem we experienced in the analysis of the 50-ppm hydrocarbon mixtures used for test purposes. T h e absolute value of the FID elevated base-line signal, however, depends directly on the number of ionizable components entering the flame per second (coulombs/second = amperes) and thus is no longer flow insensitive as with pure carrier gas. With the fluidic switch single injection procedure, sample is injected in addition to the carrier gas flowing into the column. T h e accompanying flow changes which caused changes in base-line signal and possibly flow related retention time shifts destroyed the linearity of the system and thus made correlation impossible. I t should be noted a t this time t h a t this problem is also expected to occur with the normal sample loop type injectors unless great care is exercised in balancing the sample loop pressure t o t h e column head pressure. T h e stream switch shown in Figures 2 and 3 was developed to eliminate these difficulties.
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Figure 6 . Comparison of a single injection chromatogram with a correlogram run under the Same conditions Carriergas = n'trogen Sample = 50-ppm concentrationsOf ethane and propane I n nitrogen
(a) Chromatogram obtained from a single 50-ms injection of sample (b) Correlogram obtained from multiple 50-ms injections using a PRBS of 1023 On examination, the configuration shown in Figure 2 reveals only one check valve. The reason for this is not obvious. One would expect that another one is needed to prevent the carrier gas from shorting out through the sample logic gate. However, it was found that the addition of another check valve in the O2leg of the sample gate destroyed the sharp sample profiles which were obtained in its absence and also increased t h e minimum sample width which could be reproducibly injected into the column. This problem persisted even with a physically close coupled configuration of the two logic gates. I t was also noted t h a t P,(carrier) and P,,(carrier) had t o be operated a t higher pressures than P,(sample) and Pev(sample) to avoid the base-line shifts which occurred when switching from sample to carrier gas. In addition sharp increases in P,,(sample) were observed when carrier gas was switched into the column. These observations are consistent with the supposition that when the carrier gas was switched into the column, some of its flow is diverted back up through the sample logic gate while the rest flows into the column. The interesting consequence of this phenomenon is t h a t apparently the diverted carrier gas flow washes all remainder of sample from the connecting tubing and logic gate leg so that when sample is again switched to the column leg, a sharp profile is presented to the column. In absence of this washout, sample remains in the connecting tubing and is mixed with some of the carrier gas. T h e next sample injection forces this diluted sample plus pure sample into the column. This leads in some cases to what appears to be a double injection and a t other times to distorted peak profiles. Similarly the test which was devised to check reproducibility of the sampling system seems unduly complex. However, it was found early in the research that a set of conditions which apparently gave reproducible samples behaved anonymously under high speed conditions, Le., when the sampling interval was decreased below some minimum. Synthesizing the expected output under correlation conditions using the experimentally obtained single pulse chromatograms was found to be quickest way of identifying instrumental contributions to unsatisfactory correlograms. Finally, the results obtained with this system which are summarized in Figure 6 demonstrate the effectiveness of the correlation procedure for recovering signals which are buried in the noise of the detection system. Certainly 50-ppm concentrations of hydrocarbons do not normally constitute a detection problem but under the conditions of this experiment conducted in a laboratory where the progress of
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many other experiments contributed to a particularly dirty electrical environment, the improvement exhibited by the correlation mode of operation is rather remarkable.
ACKNOWLEDGMENT We are deeply indebted to Guy Preau for his technical advice and assistance.
LITERATURE CITED (1) Raymond Annino, "Signal and Resolution Enhancement Techniques in Chromatography'' in "Advances in Chromatography", Vol. 15, Marcel Dekker, New York, 1977, pp 33-63. (2) J. B. Phillips and M. F. Burke, J . Chromatogr. Sci., 14, 495 (1976).
(3) Raymond Annino and Eli Grushka, J . Chromatogr. Sci., 14, 265 (1976). (4) Raymond Annino and L. E. Bullock, Anal. Chem., 45, 1221 (1973). (5) Gyula Gaspar, Patrick Arpino, and Georges Guiochon, J . Chromatogr.
Sci., 15, 256 (1977). (6) Gyuia Gaspar, Raymond Annino. Claire Vidal-Madjar, and Georges Guiochon, Anal. Chem.. 50, 1512 (1978) (7) 9 u l a Gaspar, Jean-Piene Ollvo, and Georges Guiochon, Chromtcgraph!a, in press. (8) Robert L. Wade and Stuart P. Cram, Anal. Chem., 44, 131 (1972). (9) Stuart P. Cram and Stephen N. Chesier, J . Chromatogr., 99, 267 (1974). (10) Raymond Annino, Joseph Franko, and Harry Keller, Anal. Chem., 43, 107 (1971).
RECEIVED for review October 25, 1978. Accepted December 1, 1978.
Combination of Size Exclusion and Normal-Phase Partition Modes in High Performance Liquid Chromatography Sadao Mori" and Akira Yamakawa Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 5 14, Japan
Size exclusion chromatography (SEC) and normal-phase partition chromatography (NPPC) were performed on the same column packed with polystyrene gels used for SEC of small molecules. Chloroform was used as the mobile phase in SEC in order to measure the approximate numbers of components. Then, chloroform-n-hexane mixture was used for the complete Separation. The potential utility of the SEC-NPPC technique was illustrated by application of phthalate esters, alkylbenzenes, and ketones. The combined use of SEC and NPPC on the same column may extend their advantages and compensate their disadvantages.
Recent improvements in high performance size exclusion chromatography (SEC) (conventional GPC) have advanced t h e separation of low molecular weight materials ( I ) . Polystyrene gels with small pore and particle sizes permitted the complete separation of n-alkanes ( I ) , phthalate esters ( Z ) , alkylbenzenes ( 3 ) ,oligostyrenes ( 4 ) , and various other materials. In the separation of these materials by "true" SEC with polystyrene gels as packing materials, it is essentially necessary to use solvents as the mobile phase which minimize the interactions between t h e gels and solute molecules, Le., good solvents for uncross-linked polystyrenes ( 5 ) . When solvents having solubility parameters much different from that of the polystyrene gel are used as the mobile phase, the separation mode may be regarded as liquid-solid, Le., adsorption chromatography ( 5 ) . n-Hexane has a solubility parameter less t h a n t h a t for the polystyrene gel and the separation mode is normal-phase adsorption chromatography (henceforth referred to as NPAC) where the elution order of the homologous series is in the order of decreasing molecular weight. Chloroform and tetrahydrofuran are eluents often used for SEC with the polystyrene gel. The separation mode of the mixtures of chloroform and n-hexane is regarded as normal-phase partition chromatography (henceforth referred t o as N P P C ) , the elution order being the same as SEC and NPAC. Among advantages and disadvantages in SEC, NPAC, and NPPC, those for low molecular weight materials are itemized as follows from a comparison standpoint. Advantages in SEC 0003-2700/79/0351-0382$01 0010
are the following four factors: (a) As a molecule of the largest size elutes first from the SEC column in principle, the information on the size or molecular weight of the separated molecules can be obtained without difficulty. (b) As no solute elutes after the total permeation limit, it is suitable for the preliminary analysis of unknown samples. (c) Columns of 8-mm i.d. are normally used, which enables a large sample load and application of a refractive index detector. (d) Compared to the other separation modes, the separation time is short. The need for greatly increased column length in the case of insufficient separation and the high cost of columns are disadvantages in SEC. Advantages in NPAC and N P P C are (a) the capability of the increased resolution of two adjacent bands by selecting the mobile phase or the constituents and (b) less costly columns with small volume of packing materials. A lower sample load than SEC and the difficult prediction of the existence of retarded solutes in the case of unknown samples are the disadvantages in NPAC and N P P C . High performance liquid chromatography utilizes one of the four basic separation modes (adsorption, partition, ion exchange, and size exclusion) without combination of the other three modes. In SEC, for example, the information from the chromatogram is analyzed under the condition that adsorption or partition modes are not incorporated. On the other hand, the combined use of two separation modes will make it possible to extend their advantages and to compensate their disadvantages if the combination is adequate for the samples. Recently, coupled column chromatography using SEC and reversed-phase chromatography has been reported (6). In the method, SEC was used as the preliminary separation technique and some fractions which need more separation were transferred to other secondary columns(s1 for further separation. In our study, SEC and N P P C are performed on the same column packed with polystyrene gels used for SEC of low molecular weight materials. Chloroform was used in the first elution as SEC and then, the mobile phase was changed to the chloroform-n-hexane mixture as NPPC in order to obtain better separation. N P P C was performed on the SEC column (henceforth referred to as SEC-NPPC). The potential utility of the SEC-NPPC technique was illustrated by applications of three different homologous series. C 1979 American Chemical Society
