b Figure 2. Inversion of the NMR signal
I
I
I RED
., A
,
- c A 3 A2 .
r-
IBiACK attenuated by a factor l/& Noise components with a higher frequency are progressively removed. For optimum noise filtering, the time constant, RC, should be about equal to the time spent in traversing the resonance. At a sweep rate of 1 Hzlsecond, a peak which is inhomogeneously broadened to 0.5 Hz is traversed in 0.5 second, and the optimum setting of the bandwidth switch would be vc = 1/2n X 0.5 = 0.32 Hz. Because this setting is not available, the nearest position is selected, which is 0.4 Hz. If the cutoff frequency selected
HEATH RECORDER
EUW-20A
is too low (too much filtering), the signal is removed along with the noise. The system was satisfactory except for the rather unreliable paper drive of the servo recorder. Inversion of the signal may also be useful for ESR and optical spectra. RECEIVED for review February 3, 1967. Accepted March 27, 1967.
Improved Gas Chromatography Packings with Fluidized Drying R . F. Kruppa, R. S. Ilenly, and D. L. Smead Applied Science Luborarories, Inc., P . 0 . Box 440, State College, Pa. 16801
THEPREPARATION of column packings for gas chromatography is a critical step in the use of that analytical tool. The two ieading methods of preparation, slurry (1) and fitration (solution-coating) (2), have been frequently described, with the latter method preferred for low-loaded-i.e., 3 Z-packings. Regardless of which method is employed, the final drying of the packing is accomplished in several different ways (1,3,4). Parcher and Urone (5) described the use of a fluidized drying technique, claiming the advantages to be: “(1) a better assurance of a uniform coating. . ., (2) a distinct saving in time, (3) less fragmentalion. . .,” and “(4) expulsion of the fine particles and light impurities.” If their conclusions (l), (3), and (4) are correct, then the efficiency of a column prepared from such a packing should be better than one prepared from a packing made by the most commonly used method-namely, tray drying without stirring. I n addition, the assurance of a more uniform coating should result in greater reproducibility of packings from batch to batch. This paper reports a simple, durable fluidizer which was designed, constructed, and tested specifically for drying of
packings made by the filtration technique, although the slurry method can also be employed if the procedure of Kaiser (6) is followed. Comparisons with packings dried in a tray without stirring were made by measuring column efficiencies and packing reproducibility. EXPERIMENTAL
Fluidized Bed Drying. If a gas is passed upward through a porous plate covered with a finely divided solid, the particles are suspended and intimately mixed in the gas stream. The suspended particles exhibit fluid characteristics during this process and the phenomenon is referred to as fluidization. Among all the known drying methods, fluidization is the most rapid and effective means of drying a damp solid. T o achieve maximum drying speed and efficiency, the drying medium (in this case nitrogen) should be preheated. With this technique, optimum heat transfer (resulting in optimum drying efficiency) is achieved by convection from the fluidizing gas to the solid particles as opposed to radiation and conduction from the walls of the containing vessel. In addition, the bed temperature is uniform and closely controllable (7). Gas velocity during fluidized drying is critical if particle attrition is to be kept to a minimum (8). With excessive gas
(1) Howard Purnell, ‘‘Gat, Chromatography,’’ Wiley, New York, 1962, p. 240. (2) E. C . Horning, E. A. Moscatelli, and C. C. Sweeley, Chem. Ind. (London),1959,751.
(3) W.J. Zubyk and A. Z . Conner, ANAL.CHEM., 32,912(1960). (4) H. H. Wotiz and S. C. Chattoraj, Zbid.,36, 1467 (1964). ( 5 ) J. F. Parcher and P. Urone, J . Gas Chromatog., 2, 184 (1964).
(6) Rudolf Kaiser, “Gas Phase Chromatography,” Vol. I, Butterworths, Washington, D. C.,1963,p. 56. (7)V. Vanecek, M. Markvart, and R. Drbohlav, “Fluidized Bed Drying,” Leonard Hill, London (1966),p. 168. (8) lbid.,p. 167. VOL. 3 9 , NO. 7, JUNE 1967
a
a51
Table I. 3% QF-1 nn 100/120 Gas-Chrorn Q Packings; &foot Columns Uncoated Theoretical plates Drying conditions support lot (cholesterol) A 4 m PF-H A 4700 PF-H A 3900 SF-PF-H B 3900 SF-PF-H A 3600 SF-PF B 3700 SF-PF A 3700 PF B 3700 PF A 2500 Tray-dried B 2700 Tray-dried B 2400 SF-Tray-dried PF-coated packing fluidized, SF-uncoated support fluidized, and H-heat used during fluidization. Column operating parameters: column 230" C, Rash heater 275"-300" C, detector 250" C. Flow rates 30 to 40 ml/minute with nitrogen pressures from 12 to 16 psig.
velocity, the fluidized bed is highly expanded and severe particle attrition occurs because of violent turbulence. When the gas velocity is just enough to expand the particles into a boiling bed, minimum particle attrition and good intermixing occur. At proper gas flow rates, the particles can be observed moving gently throughout the bed giving the impression of a gently boiling liquid. me Fluidizer. The unit (Figure 1) is made of chromeplated brass and consists of a base section with a 0.25-inch gas inlet, a porous bronze plate, a barrel, and a n SO-mesh screen cap. The screen cap prevents packing from blowing out the top during the first minute of drying if the initial gas flow rate is too high. The porous plate is placed on a flange in the base section and the barrel is screwed into place. The base section has a built-in spiral insert (silver-soldered in place) which provides large heat transfer surface area for preheating the fluidizing gas. A 6-mm thermometer well is also installed in the base section (at the gas outlet just below the porous plate flange) to permit measurement of the preheated gas temperature. Procedure. All packings were prepared in 50-gram batches by the filtration technique. Approximately 250 ml of solvent was used to dissolve the phase in a beaker, the support was added with gentle stirring, and the resulting slurry was carefully poured into a Buchner funnel fitted with a filter flask. Excess solution was removed by vacuum filtration and the damp packing was split into two parts. The first part was tray-dried in a thin layer without stirring by means of infrared bulbs; this took 30 to 40 minutes. The second part was gently dried in the fluidizer unit with about 1 psig of nitrogen. When preheated gas was to be used the fluidizer was placed on a hot plate controlled by a Variac and allowed to warm up for 15 minutes. A setting of 40 volts achieved a fluidizing gas temperature of 60"C. Drying time was 4 to 5 minutes when preheated gas was used and 10 to 15 minutes without preheat. The packings were evaluated in a Barber-Colman Model 5000 equipped with a flame ionization detector, using nitrogen as carrier gas. All columns were 6-foot glass U-tubes filled by adding the packing in 0.5-gram increments and vibrating the bottom until all settling ceased (5 to 10 seconds per increment).
RESULTS AND DISCUSSION The initial studies with QF-I packings (shown in Table I) indicated that maximum efficiency columns were produced when some form of heat was used during fluidization of the packings. Attempts to fluidize the support to remove "fines" 852
ANALITICAL CHUUSTW
Figure 1. Fluidizer unit with parts breakdown before coating did not result in any improvement of the final packing. No explanation is offered for this behavior. The packings dried hy fluidization with preheated gas produced columns which averaged 1700 theoretical plates better than columns made from tray-dried packings, as shown in Table I. No attempt was made to study the effects of drying temperature; 60" C was arbitrarily chosen because it was below the boiling point of the solvent and achieved with an even 40-volt Variac setting. When heat was not used in the fluidization process a decrease of 500 in average theoretical plates was noted, possibly because of wide variations in the drying temperature during the process, hut this is not known. Since conversion to fluidized drying in this laboratory, 6foot columns exhibiting 4003 plates or more for cholesterol have been frequently obtained with 3 % loadings of SE-30, QF-1, XE-60, and JXR on Gas-Chrom Q. Before adoption of this technique, efficiencies of this magnitude were never encountered. A low-loaded polyester packing was selected for the next part of the study. Similar improvement in column efficiencies was expected and confirmed by the results shown in Table 11. The separation of the trimethylsilyl derivatives of a mixture of pregnanediol (PD), 5ndrosterone (A), dehydroepiandrosterone (DHEA), etiocholanolone (E), and pregnanetriol (PT) was selected for this packing evaluation. This mixture was a fortunate choice since the relative retention, (E/PT), decreases from approximately 1.10 to 0.9 as phase loading is lowered below 3%. The relative retention for the critical pair ( E m showed less than 1%variation from hatch to batch of fluidized packing, while the tray-dried packings exhibited almost 5 variation. Closely reproducible results with all five components were achieved on the fluidized packings, as is shown in the bottom part ofTable 11. Except for the pressures, which had to be varied between 14 and 16 psig, the operating conditions of all columns prepared from fluidized packings were identical. The fluidized packings
averaged 1200 more {.heoretical plates than the tray-dried packings and showed much less variation in efficiencies. Craig has shown th;it the relative retention of fatty acid methyl esters on polyester columns is a function of the weight of liquid pha:je coated on the support (9). Because it was desired to stud:y the fluidization technique on highloaded polyester packings, a 10% EGSS-X packing was selected and evaluated with a mixture of several fatty acid methyl esters. The relative retention of methyl linolenatemethyl arachidate IS sensitive to changes in phase concentration and was chosen for the study of reproducibility with this packing. As can be seen from Table 111, the variation among the fluidized dried packings was one part in 120 while the tray-dried packings varied three parts in 120. As before, column efficiency was improved (but to a lesser extent) by using fluidized drying. The fluidized packings averaged 400 plates more than the traydried ones. A study of packings made with a viscous, sticky material [diethylene glycol succinate (DEGS)] was then undertaken, with the results shown in Table IV. Efficiencies of the packings dried by fluidization averaged 500 plates better than the tray-dried pacltings. The relative retention (methyl linolenate-methyl arachidate) showed a n equal variation of one part in 120 for both drying methods. Parcher and Urone did not report improved column efficiencies in their work. We prepared duplicate batches of 2 0 z 1,2,3-tris(2-cyiinoethoxy)propane (trispropane) on Gas-Chrom RA (a firebrick support), and compared traydrying and fluidization drying as before. Trispropane, like tritolyl phosphate (the phase they used), is a low viscosity, highly penetrating liquid and was judged to be a suitable substitute. No improvc:ment of column efficiency was noted on any of the tests. An explanation for this behavior may well be that the high adsorptivity of firebrick coupled with the low viscosity of the liquid phase results in a coating which cannot be improved by fluidization. No attempt was made to check phase loading reproducibility. CONCLUSIONS
Table 11. 3 % EGSS-X on 100/120 Gas-Chrom Q; 6-foot Columns Test Relative retention E/PT packing Jkoretical plates DHEA ~. lot PF-H Tray-dried PF-H Tray-dried K L
4ooo 3600 3600 3800
M N
3200 2200 2800 1800
1.08 1.09 1.09 1.08
1.09 1.10 1.07 1.05
Fluidized Packing Retention Data Test Absolute oack- retention ing time PD lot (minutes) K
PD
3.80 3.78 3.75 3.85
L M N
1.00 1.00 1.00 1.00
Relative retention A PT E 2.14 2.15 2.16 2.14
2.84 2.84 2.86 2.85
3.08 3.09 3.11 3.07
DHEA 3.86 3.87 3.89 3.84
Column operating parameters: column 200" C, flash heater C, and detector 250" C. Flow rates 40 to 45 ml/ minute at pressures between 13 and 16 psig nitrogen. 245"-255"
Table 111. 10% EGSS-X on 1001120 Gas-Chrom P; 6-foot Columns Test packing lot C D E
Theoretical plates methyl linoleate PF-H Tray-dried 3100 3000 3200 3Ooo
F
2800 2500 2700 2300
Relative retention linolenate-arachidate PF-H Tray-dried 1.20 1.21 1.18 1.19
1.19 1.19 1.18 1.18
Column operating parameters: column 185" C, flash heater 300" C, and detector 250" C. Flow rates 40 to 50 ml/minute with nitrogen pressures from 7.5 to 10 psig.
The use of fluidized drying for preparation of column packings has proved to be superior to the conventional traydrying procedure for solid polyester and silicone phases. Columns of greater efficiency can be produced from fluidized dried packings and greater reproducibility from batch t o batch is possible. The degree of improvement is greatest with low loaded packings and it is here that fluidized drying demonstrates its greatest advantage. The technique is rapid and the gentle treatment of the fragile diatomaceous earth particles keeps attrition 1.0 a minimum. One can actually see this. The removal of fine particles from the support by fluidization prior to coating did not improve the final packing nor can this be explained. The efficiency of columns produced from highly adsorptive supports loaded with low viscosity, highly penetrating phases in the range 10% and above is not enhanced by fluidization. The use of this technique in mixing multiphase packings is a distinct application in addition to the one reported in this work. The coating of Teflon supports by this method, using gas cooled to 0°C with a n ice bath is being studied by this laboratory. Finally, the preconditioning of packings by this method appears t o be feasible.
We are grateful t o George Fleming of Nittany Scientific Services who constructed the final unit after assisting in its design.
(9) B. M. Craig, "Gas Chromatography, Third Symposium," Academic Press, N. Y.,1962, pp. 37-56.
RECEIVED for review February 7, 1967. Accepted April 11, 1967.
Table IV. 18%Hi-Eff 1BP (DEGS) on 100/120 Gas-Chrom P; 6-foot Columns Test packing lot G
H I J
Theoretical plates methyl linoleate PF-H Tray-dried 2400 2400 2300 2400
2100 1700 1900 2000
Column operating parameters: column 185" C, flash heater 250" C, and detector 300" C. Flow rates 55-65 ml/minute with nitrogen pressures from 8 to 12 psig.
ACKNOWLEDGMENT
VOL 39, NO. 7 , JUNE 1967
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