lielium n a s not purged through the column, but the column n as allon ed to ztand for 2 days before the separation of mesh sizes was effected. The reiults from Procedure B showed that the 40- to 60-mesh support gained 27.47, and the 60- to SO-mesh support lost 27.770 pf the total amount of Carbowax 400 in the 2-meter column. Thus, it appears that the helium purging is not the dominant cause of liquid phase transfer or deactivation of absorption site.. In all the experiments, no meaqurable amount of Carbowax 400 was loit from outlet of the columns. This would be anticipated from considering the vapor pressure of Carbouax 400, 9 X 10-5 mm. H g a t 100' C. (If?), a value which would be appreciably le.; a t the temperature of the transfer experiments. -1Ithough it seems el ident that liquid phase transfer causes the adsorption sites on the uncoated support t o be rendered innocuous, and above series of transfer experiments do not permit any definite conclusions to be d r a n n concerning the dominant mechanism of transfer. They do, however, invite speculation as to the probable mechani m s involved. Three possible mechanisms of liquid phase transfer are: vapor-state transfer-Le., the liquid phase, at the temperature employed, has a sufficient vapor pressure to permit transfer through the vapor state of a significant amount of Carbowax 400; a liquid-state transfer-Le., the wetting properties of Carbonax 400 are sufficiently great to allow the Carbowax to creep from the coated support to the uncoated support n-hen they are in intimate contact for a wfficient period of time; and a twodimensional, surface transfer mech-
anism-i e., the Carbowax 400 molecules on the surface may have sufficient energy to become mobile, but not enough to overcome the adsorptive forces a t the gas-solid and/or gas-liquid interface Thus, a Carbowax 400 molecule can randomly migrate, in two dimensions, across a surface until i t becomes "trapped" by an active adsorption site. Further movement of the molecule would be restricted by the stronger adqorptive forces; hence, the most active sites would be preferentially filled by the migrating moleculeThe liquid-state transfer niechaniqm would qeem to be the dominant mechaniqm involved in the transfer of the Carbowax 400, when one considers that the amount of transfer wab independent of gas flow, but dependent on time. The two-dimensional, surface transfer mechanism could not account for the entire amount of tranifer involved. However, when 7.74% coated support ivas mixed u-ith the uncoated *upport, the amount trankferrd n-as only 30 mg ; i rough calculation indicates that 30 mg. of Carbowax 400 would form a layer four molecule. thick if deposited evenly over the uncoated Support For a liquid film thiq thin, it is difficult to explain the transfer by the liquid-state mechanism It would seem more probable that the vapor-phaqe or t\~o-dimensional,surface tranqfer mechanisms are operative, thus explaining the selective depoqition of Carbon ax 400 on the active ad-orption sites. All three transfer mechanisms may be operative, and i t is difficult on the basis of our experiments to didinguish between them. However, i t mould seem that the liquid-state transfer mechanism
accounts foi the bulk transfer, and t h e vapor-state and/or the two-dimensional surface transfer mechanism account for the observed selectivity in rendering t h e active adsorption sites innocuous. LITERATURE CITED
(1) Bsrnard, J. A,, Hughes, H. I\-, D., atitre 183, 250 (1959). ( 2 ) Rohenien, J., Purnell, J. H , J . C'hem. Soc. 360 (1861). (3) Carmen, P. C., "The Flow of Gases Through Porous Media," p. 8. Butterworths. London. 1956. (4) Dal kogare, S.', Chiu, J., ; ~ S I L . CHEM. 34, 890 (1962).
(5) Duvall, A. H., Tully, it'. F., J . Chrornntog. 11, 38 (1963). ( 6 ) James, A. T., Martin, .\. J. P., Biocheni. J . (London) 50, 679 r,19*52). ( i ),Jones. IT. -1.. ASAL. CHEJI. 33, 829 (1961). S)
I 1963. Accepted September 23, 1963. This research was supported by the -4dvanced Research Projects Agency. Contract S o . SD-100.
Principles of Low Pressure-Drop Packed Columns J. C. STERNBERG and R. E. POULSON' Beckman Instruments, Inc., Fullerton, Calif.
b The effect of pressure on the performance of chromatographic columns is analyzed. The local plate heights obtainable from rate theories of chromatography are related to the experimentally observed apparent plate heights through the expression of band spreading in terms of second moments. The Giddings pressure correction follows directly from this treatment without further kinetic assumptions. The appropriate choice of linear velocity expression for interrelating theory and experiment i s emphasized. The conclusions are applied to the problem of design of high efficiency packed columns with low pressure drop. 58
ANALYTICAL CHEMISTRY
W
H I L L T H E R E seeins to be no limit in principle to the plate height or total number of plates achievable n ith packed columns, the pressure drop required for very small particles or for very long columns presents a serious practical limitation. Even nithin the practical range of normal operating pressures, scrious Contradictions ehist in the literature regarding the influence of inlet-to-outlet pressure ratio on column lwformance. The preqent study n as undertaken in an attempt to gain a clearer understanding of the relationship between pressure drop and plate efficiency in packed columns. I t became apparent early in the study that inadequate
definitions ofsuch hasic concelkssslinear gas velocity and plate height w r e responsible for much confusion in the literature; fevi of the experimental results reported, for example. specify n-hich definition of linear velocity was used. and f e w r still give suffioiwt data to permit conversion of values given to othcr fornis suitable for makin; intercomparisons. In this paper an attempt is made t o clarify snine of these concept.< ant1 t o Iiroi-itie a scheme for more conr-enient x-isualization of the pertinent factors. 1 Present address, Laramie Petroleum Research Center, Bureau of Nines, U. S. Department of the Interior, Laramie,
Wyo.
The desirability and I equirements for Ion. pressure-drop packed columns will a1w he discussed.
Table I. Linear Velocity, u = F / ( V / I ) Linear velocity Superficial
SOME BASIC RELATIONSHIPS
~701ume/length
Unretained comp. Retained comp.
Sul)si.ril)t z denotes didance along the column from inlet (2 = 0) to outlet (? = L). Subscript a on pressure denote< atmospheric premrire; subscripts c. ri or s on temperature denote column, ambient, or sample 1 emperature, reslwctii ely. The pressure profile along t l v i.i~Iuiiinis given by Equation 2.
l‘hc i i,lationsliip betreen flow rate and ~ ~ J ~ I l m pressure Il drop is given by Equation 3.
AC is the column cross-sectional ares, L. is it,+ length, q‘, is the carrier gas viscositJ- a t column temperature, and
B,, is tlir ~)ermeahility,which will be discussed later. A p (= po
- p ~ is)the col-
(is the roliimn mean 1irc.ssure.
umn I r v s u r e drop, and p ,
~
Linear Velocity. A linear velocity (‘1‘:ihIo I can be defiiied ti.5 :i 1-oluine p(*r n i l i t time (flow rate) divided by a voliinit, per unit length (area). Sinw t h e flow rate depends upon prossurcs. both F and V L depend upon po..itioii z in tlie column; a linear vc~loi~ityiiiust therefore iiiclude a sl)c)c4fic.:ition of press,ire or position. T7:triou. linear velociti6.s can be defined dqmitlinp on the v o l ~me: T’, per unit l(~ngtli ior the area, -4) considered. IililIortant linear velocities used include tlic, sui ~rrficGdlinear r.elocity, u,,, the h e a r velocity of carrier gas, u,,, the liiitw velocity of an unretained com1)onent. I ( ~ arid ~ . the linear velocity of a rrtainril component, tiei. Earh linear v c h i t > -is tleaignated by t n o subscripts, the first referring to the applicable prcwure (or position in the column, from whic~hpressure can be found) and the s c ~ ~ n identifying tl the, type of linear velocity specified; column temperature, T,. is implied in all cs.ses. The linear vclocitiec are tabulated in Table I. Thus the superficial linear velocity, uZ8.empIom the entire volume per unit lciigth, or uoss-sectional area of the tulw itscalf. :lc; this velocity has been
u,,
=
~
;,so” -
71zu
d. =
e,but is instead the time average
Pm
velocity, equal t o L L Uuzu.ItLu dt; this value corresponds to the linear velocity of a n uiiretained component a t the average column Iressure, which is
i
= c F z c / A c ~
~ r = u fmFzc/Ace
+
( vo K % V L ) / L Pressure dependence
wide15 used. although it has no real physical significance. The total gas volume, I-,, nithni the column consists of the interparticle pore volume, I’,,, and the intraparticle pore volume. Tige. In d! namic considerations, the gas is considered to be moving only in the interparticle volume, V,,,,. while the intraparticle volume, V,,, is static. The dynamically important velocity, us,, through the volutne So, has been called the carrier gas velocity, the applicable volume in evaluating this velocity is thus Vom,which constitutes a fraction, E , of the total column volume, Tic = -4‘L, and another fraction, f m , of the total gas volume T7,. The carrier gas linear velocity was first employed in gas chromatography by Bohemen and Purnell (1). -in unretained component eyilores the entire gas volume, V,, as it moves down the column, so the linear velocity of such a component is the ratio of the volume flow rate to the total gas volume per unit length. -2 retained component, i, e~ploresboth the free gas volume, Ti,, and the liquid volume, TIL, which acts like a gas volume scaled up by the partition equilibrium constant. K,. for that component; the total volume accessible to such a component is thus V,+ K J L( =VR%’, the retention volume, corrected to atmosghei IC* pressure and column tenipeiature). Thus, the linear velocity of a retained component, is the ratio of the volume flon rate to the corrected retention volume per unit length. Manv workers in gas chromatngral~hy have ujed a quantity termed the “average linear velocitj ” or simply “average velocity,” 11hich is found hy dividinq the column Irngtli, L. 1), the retention time for an iuiretained (YIIIIponent. t,. This, quantitv, 7 i p z L sis not the ordinary position a\ erage velocity, which n-ould be a,
uzs= F,,/d,
v,,/ I, T7,/I>
Carrier gas
Volume Flow Rate and Pressure Drop. T h e volume flow rate, F,, ticpentls upon temperature, T,, a n d p r e w i r e p,; Equatior, 1 is es3entially :L.tatenlent of tlie ideal ga. equation.
Relationship
TVC/L
uzs= F,,/(VOR~/I.) 112pz
=
210pu
THE CHROMATOGRAPHIC PROCESS
Retention Time and Retention Volume. Retention times are found bjintegrating t h e appropriate velocity expresqions (up. for unretained components and u,, for retained componentq) along the length of the column. giving Equation 4 for the retention time for component i.
For an u n r e t a i n d wmponent. u . the partition equilibrium constant, K,. is equal to zero. Here upl is the linear velocity of component i a t the averagc column pressure, p , as discussed in the preceding section. Observed retention volumes are seen in Equation c5 to be the corrected values multiplied by the ratio of column average to atmospheric pressure.
p , the length-average pressure, arises from the averaging process shown in Eqiiation 6.
The familiar James-LIartin compressibilitv factor,j, is given by p~ p . P i s the ratio @,of column inlet to column out1)L let pir\sure. ’I’he time and volume relationships are coni eniently represented graphii+allyin Figure 1. In this graph, retention volume (I\ hich is carrier gas volume reduced to atmospheric pressure and column temperature) is plott (4 z s . colunin Ierigth. Retention volume a t fixed flon rate is also a direct measure of time of elution. The ga? and effecti\e liquid free volumes accumulate lineally dovc n the column; however, the carrier gas requii ed to sn eep out these volumes (and, hence, the retention volume) accumulates nonlinearly because of the greater number of moles of gas occui)>ing 3 given open volume a t a higher pressure. The sum of the pressurecorrected gas and liquid phase contributions to retention is shonn on the VOL 36, NO. 1, JANUARY 1964
59
*
2
O ~ o ~ S e c o nmoment d
6b=j
-I
'OdZ
0
Figure 2.
-02-
LZ (Z-7)+A d40L2/+L ~
Z-+
Band broadening
expression, although plate height is evaluated, as shown in Equation 11, through use of the experimental timeor volume-based plate number.
Z--+ Figure 1.
graph for two components. The corrected gas contribution to retention volume is the same for both components, and constitutes the entire retention volume for unretained components. The figure shows peaks for an unretained and two retained components at several retention volumes (hence, times), showing graphically holv the components move apart as they move down the column. The point a t which the z = L line is crossed corresponds to the observed retention volume (or time) for a given component; these values can be seen along the right hand edge of the figure. Band Spreading. EXPRESSIOKS FOR SPECIFYING. A typical chromatographic peak is displayed in Figure 2, which shows a plug sample as introduced and the broadened peak as eluted. T h e broadening of the peak as i t passes through the column limits t h e obtainable resolution, and is best expressed b y the second moment, or variance, of t h e distribution, which is found by weighting the height of each point by the square of its lever a r m about the center of mass of the peak, as shown in Equation 7 . m ffZL;
=
.
( z - i ) 2 . f ~ dz;
Som
(2
- Z)
. fL.dZ
=
0 (7)
The first subscript ( L ) on the second moment, cr*Lz, denotes that it is a lengthbased second moment; the second subscript (i), indicates the position of the center of mass of the peak at which the second moment has been evaluated. For a Gaussian peak. the second moment is the square of the standard deviation, u , which is half the peak width a t the inflection points (or 4 the base width). The second moment can be found for any shaped distribution, and is a property which accumulates additively as the peak pro-
60
e
ANALYTICAL CHEMISTRY
L
Transit of chromatographic peaks
ceeds down the column; furthermore, contributions of independent processes to the second moment as the peak passes a given point in the column are also additive, so that this parameter can be evaluated theoretically, summed over the column as shown by Equation 8, and compared with a value based on measurement of an experimental peak.
EXPERIMENTAL PLATENUMBERS AND PLATEHEIGHTS.For historical reasons arising from the analogies among chromatography, countercurrent extraction and distillation, second moments are commonly evaluated and expressed through the number of theoretical plates, N , and plate height, 8, defined in Equations 9 to 11. Plate numbers are usually measured from a time-based chromatogram, and are expressed as in Equation 9.
T ? represents the second moment of the peak on a time basis, and wb is the base Jvidth of the peak in time units. The appropriate correction for inlet 1-olume, VO,is made most conveniently when the plate number is expressed equivalently in terms of volume, as shown in Equation 10.
i 10) TIR i i the retention volume, corresponding to Fa, t R , where t~ is the true retention time. V,,,(= Fact,,,) is the apparent retention volume, n hich includes the carrier required to transport the center of mass of the peak from the inlet to the head of the column. Commonlv, plate number is also expressed in terms of length, and indeed, the plate height concept is based on this
B = L/hTv (11) where 8 represents the experimentally observed or apparent plate height. It is tempting to write an expression equivalent to Equation 10 for plate height on a length basis, as shown in Equation 12. This would lead to the plate height expression also shown in Equation 12. (12) THEORIESOF BAND BROADENING. Theoretical treatments of band broadening lead directly to a plate height or an expression for second moment in length. Equation 3 presents the van Deemtertype equation for plate height contribution as the center of mass of the sample peak passes a point z in the column.
I n the gas terms, D,; and u; both vary inversely n i t h pressure, so that any pressure effect on u;is compensated by an equivalent effect on D,, as shown in Equation 14. u;p; = uap.;
D,;p; = D,,p.
(14)
Thus any convenient pressure may be used, and atmospheric pressure is the simplest choice, as shonn in Equation 15.
The proper linear velovity expression to be used is subject to debate on theoretical grounds. The values of the pusition-independent constants, B. Cg. and CI, appearing in terms containing the velocity will, of course. be determined by the choice of linear velocity expression: the value of .i should n,It be affected unless a velocity which alters the functional relationship is used. If longitudinal diffusion occurred only in the moving portion of the gas stream, the carrier gas velocity would be appropriate for this term : however, the recently reported results of DeFord and coworkers (3) indicate that longitudinal diffusion must occur within the particles themselves as well. In mass transfer
spreading. which depmds on the velocity of the band certer relative t o a delayed portion of the hand, the velocity of an unretained component is probably appropriate. Many workers have used the “average velocity” found by dividing the column length by the retention time for a n unretained component. The velocity, u+. obtained in this way does affect the entire functional form of the plate height expression, since ext~aneous pressure dependences are arbitrarily introduced through its use. ‘The “constants” -4. B, C,, and C1 determined using u?, are meaningless quantities. I n the treatment which fol OTVS, the linear velocity of a n unretairted component a t atmospheric pressure will be used throughout. Correlation of Theory with Experiment. I n attempting t o compare t h e length-based theoretical espression for plate height or second moment with experimental plate numbers, certain contradictions arise. Earlier workers ( 1 1 ) have related local plate height, H,, to average plate height, R, by the usual averaging procedure
R has then been identified with the experimental appareni plate height. H. When the modified Iran Deemter espression of Equation 15 is introduced into Equation 16. the integration leads simply to R=
(A + -
nhere f l a p, 13 Po --
Uau
iq
+
unity and
+--.
PL
b Figure 3. Decompression spreading
The problem which exists can be illustrated graphically on a VR us. z plot, as shown in Figure 3. Here a hypothetically infinitely efficient column is considered, with the peak nidth on the basis of retention volume or time moving unchanged down the column. The decompression effect clearly produces a change in the length-based peak width. Thus, as the figure shons, ATv is infinite and N I L finite in this case, and they are clearly not equivalent; the relationships are shown in Equations 21.
U’LL C121aufza
fpa
(15)
is pa p,.
Giddings and co-
2 rrorkers (6), however, have given a theoretical treatment incorporating 11 hat thei- term a “decompression s p r ~ a d i n g ” effect pui portedly ignored i n the earlier treatme i t , leading to the different result
L2 # x v - @LO
(21)
The proper expressions for reconciling these definitions v d l noli- be considered. The relat,ionship between volume- and length-based second moments is given in Equation 22.
Introduction of Equations 5 and 22 into Equation 10 leads to the proper equivalent numbers of theoretical plates on length, volume, and time bases:
As was seen in Figure 3, even where no spreading occurred the second moment on a length basis increased. Equation 22 and Equation 23 suggest that the proper pressure-normalized second moment on a length lissis is
(7)’
upL;;
this pressure normalized value 1s invariant with position in the column, although the uncorrected length-based second moment itself varies inversely with the square of the pressure. Thus a second moment contribution U L O ~introduced a t the head of the column (2 = 0) n ill make a t any point in the column the same pressurenormalized contribution
(;)J.
&;
a t the tail of the column ( 5 = L ) this inlet spreading will add to the uncorrected length-based second moment U ~ L La contribution
Similarly, a small contribution to the length-based second moment cluZLi, introduced a t any point i during the passage of the peak through the column will yield a t the tail of the column a contribution to the uncorrected lengthbased second moment of
and f 2 = I
3 (P2 - 1) __2 (P3 - 1)
Thc treatment of Giddinm muntl general acceptance, although recent papers by DeFord ( 3 ) and Pretorius (8) suggest scme experimental verification of his results. A recent artemlit by Giddings (4)a t clarification may not significantly reduce the confusion.
the same contribution du2ri introduced Before the values of V Rand t~ can be evaluated, i t is necessary to correct the apparent retention volumes and times for the inlet sample volume as shown in Equations 10 and 24. The relationship between the effective inlet volume, VO, and the sample volume, T-s, is given by Equation 24, where p , is sample pressure.
at point z will give a pressure-normalized contribution of du2L-
(:)’
=
(F)’ ( d u * ~ ~ ) i
(27)
Such contributions to the second moment arise as local plate height terms a t each point i during the passage of the peak through the column; the local plate height. defined as VOL. 36, NO. 1, JANUARY 1964
61
is the quantity furnished directly b y theoretical treatments of the van Deeniter type. The final pressurenormalized second moment is found as tlie sum of the initial or inlet contribution and the integrated contributions from local plate height terms. Thus
Hence the experimentally observed plate height, i?, can be related to the theoretically derived local plate height. H;, by combining Equations 11, 23, and 29 to give
The kinetic approach employed by Giddings, while correct, is unnecessarily complex, although it is clearly possible t o obtain Equation 26 and the results which follow therefrom independentlv on the basis of kinetic postulates. N o s t important, these results lend further support t o the validity of the pressure correction: certainly with improving accuracy In esperiniental techniques this important correction can no longer be ignored. As pointed out by Giddings, the correction term f i ranges from unity n hen
correction never differs by more than 12l/2% from the results of the simpler Equation 17. Honever, this error is sufficient to obscure significant trends in othernise careful work. The fartor f2 can vary more significantly from equation 17, \\here f?= was equal to A.T h u j p,,,,
Using Equation 15 for Hi and Equation 2 to evaluate di in terms of dp;, Equation 30 integrates to
(3.1)
When
P approaches 2
unity, the
LOW PRESSURE-DROP PACKED C O L U M N S
I n the foregoing treatment t!ie performance of colurnns improves with increasing mean column pressure through the liquid mass transfer term, and is not otherwise significantly affected by inletto-outlet pressure ratio. Operation of a given column at elevated outlet pressure thus improves plate efficiency. but at the eypense of analysis time and of the inconvenience of sampling against high inlet pressure. I t is of greater practical value to seek means of dcsigning packed columns to make possible the combination of their sample capacity and favorahle I', ratio with R lower pressure drop. The specific permeability, Bo, determines how much prePsure drop is required to obtain a given superficial linear velocity (volume flow rate per unit area). The specific permeability as expre-ed by the Kozeny-Carman (2) equation. is proportional to the porosity. E . which is the fraction of the column volume open to moving gas, and inversely proportional to the square of the surface area per unit of free or interparticle pore volume in the column, as shonn in Equation 35.
PL
approaches the correct value, P However, as -' approaches infinity,
factor
fz.
where
PL
can be in error by as much as +331,'3%. The error i b in such a direction as to exaggerate the effect of inlet-to-outlet pressure ratio, contrihuting to the nidespread belief that large ratios of inlet to outlet pressure are seriou-ly damaging to column performance (12). I n fact, it is seen from
f2a
and
the form of the mean of the square of inlet and outlet pressures. The Equations 31 to 33 are in essential agreement nith the pressure corrections derived and discussed by Giddings (4, 6). The apparent difference in espressions for f 2 arises froin our use of linear velocity expressed a t atmospheric pressure, as contrasted ith the linear velocity a t column outlet Iiressure used by Giddings. If the B and C, constants are to be eiamined in detail, it must be recognized that the iresent derivation requires usc of gaseous diffusion coefficients evaluated a t atmospheric pressure, TT hile the earlier tlerivation utilizes gaseou;. diffusion coefficients evaluated a t the e\isting outlet pressure of the column. The correctionsfl a n d f p are here shonn to be a direct consequence of the relationship betneen volume- and length-based second moments. and to require no new assumptions beyond the identification of the theoretically obtained local plate duZLheight as R; = 2 , in agreement with dS both Giddings (5) and Golay (7). 62
ANALYTICAL CHEMISTRY
Bo
jca
f2
that the ratio P is itself
€/ks'oF
(35)
The porositv is defined as e\pressed in Equation 36. e
= V,,/V,
= fmV,/Vc
(36)
For an open tube, the specific permeability is found from Poisseuille's equation to be re2 8. I n ordinary packed columns, the specific permeability. shonn in Equation 37, increases with the square of particle diameter and is a very sensitive function of porositv.
PL
without influence; rather, plate height can be significantly improved by operation a t increased average column pressure. p , n hich was the apparent reason for the excellent column performance obtained in the cited work of Scott (12). Recent e~perimentalresults obtained bv the authors further confirm the validity of the Giddings pressure correction, fi. Through careful elimination of dead volumes, precise measurements made on the spreading of an unretained component in an uncoated capillarv column have shonn that the theoretical F (7) minimum plate height of T , '4 can be obtained only by ol)erating the column outlet a t elevated pressure, so P approaches unity. Operation at that 0 PL
atmospheric pressure gives the results predicted by Equations 31 through 33. with a minimum plate height about 6% greater than the ideal limiting value predicted by the Golav equation. A more complete report of the experimental results will hc pu1)lished a t a later date.
clp2/1000 (Packed column) ( 3 i h )
Bo = r C 2 / S(Open tube)
(37B)
For ordinary packed columns, the porosity is about 0.4, and Bo-dp2/1000, as conipared with rc2 8 for open tubes. Plate heights of packed and capillar,~ columns are found to be essentially equivalent when the particle diameter, dp,in the packed column is equal to the radius. rc, of the (%pillary (nith optimum conditions reached a t about the same superficial linear velocity in both cases); under these conditions tlie capillary column has ahoiit 100 times the perniealiility of the packer1 column. I t is, in fact, this advantage of capillarjcolumns 11 hich makes it possible for them to achieve very high over-all plate numbers: this is partly compensated by the more favorable ratio of liquid to gas volume in packed columns qiving th columnf, with a porosity of unity, c'an qive good plate heights, thcrP i b no basic reason n hy a n inrrease of Ilorosity of packed colunins should lead to impaired pe .formance. The problem of desirning a packed column N ith higher permeabilii y without sacrifice in plate heiehts th IS requires either
improvemrnt of efficiency with coarser packing particles or use of techniques of packing to give higher porosity TI ithout sacrifice of plate height. The recent work of Halasz and his associates a t the University of Frankfurt (9,10) using loosely packed capillary columns. points toward the fruitfulness of this area of investigation. By using columns of lcss than 0.020-inch inside diameter packed with particles to 1 / 2 the diameter of the column, Halasz and Heine (20) have obtained plate heights comparable to standard columns packed \\ith particles of the same dialmeter, but with porosities and permeabilities 10-fold higher The usefulness and fleyibility of this type of column iiiaT- n ell usher in a nen spiral of advance of column technology, leading to faster and better separations.
(3) DeFord, D. D., Loyd, R. J., Agers, B. O., A x ~ ~ LCHEM. . 35, 426 (1963). ( 4 ) Giddings, J. C., ANAL. CHEM. 35, 353 (1963). (5) Giddings, J. C., Ibicl., 34, 72% ( 1 9 6 9 . (6) Giddings, J. C., Seager, S. L., Studii, L. R., Stewart, G. H., Ibid., 32, SGT (1960). ( 7 ) Golay, 51. J. E., ',Gas ChromnD. H. Desty, ed., tography-1958,'' p. 36, Butterworths, London, 1958. (81 Haarhoff. P. C.. Pretorius. 1.. J.. S. Ajrican Chem. Init. 13, 97 (i960). (9) Halasz, I., Papers presented at International Symposium on Gas Chromatography, Houston, and Iiesenrt 11 Conference on Gas Chromstographj-, U.C.L.A., January 1963. (10) Hslasz, I., Heine, E., .Ynture 194, 971 (1962). (11) Littlewood, A . B., "Gas Chroinntography-1958," D. H. Desty, ed., p. 35, Butterworths Scientific Publications, London (1958). (12) Scott, R. P. W., ''Gas Chromatography--1958," D. H. Destp, ed.. p. 189, Rutterworths, London, 1958. \
,
LITERATURE CITED
(1) Bohemen, J., Purnell, J. H., J. Chenz. SOC.1961, 360. ( 2 ) Carman, P. C., "Flow of Gases Through Porous Media," p. 11, Academic Press, New Tork, 1956.
RECEIVEDfor review Julv 18, 1963. Accepted October 17, 1963. Presented a t the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy.
Inorganic Salts for Gas-Solid Chromatography JOHN A. FAVRE and LYLE R. KALLENBACH' Phillips Petroleum Co
., Research Division,
b Resolutions, as measured b y ability to separate terphenyl isomers, were determined for 44 solid phase inorganic salts including antimonates, borates, carbonates, hydroxides, metal halides, nitrates, oxides, and phosphates. Hiqh resolutisns and low retention times were obtained with some of the salts. Potassium carbonate, borate, antimonate, and phosphate were the best salts tested for separating the terphenyls. 'The addition o f KOH to K2COa packing significantly improved the resolution of the terphenyls. The solid phtrse salt packings can b e operated successfully up to 500" C.
L
OF yuitable organic liquid phaqes for column packings operating aboi e 300" C. has limited vparations a t higher temperatures. I-ze of inorganic molt :n salt mixtures proposed by Phillips ( 4 ) and separation of polyphenyls with snch liquid phabe columns by Hanneman, Spencer, and ,Johnson ( 1 ) led to an investigation of the inorganic salt>. 1:xcellent separations, obtained n i t h the eutectic mixture of L i S 0 3 , S a S 0 3 . and K S 0 3 a t temperatures below t k e melting point (1n.p. 150" C.), suggested possible use of solid pliase inorganic salts as column
ACK
Barflesville, Okla.
packings to improve resolution and decrease tailing of component peaks. This paper reports the results of a survey of 44 inorganic salts, containing a variety of anions and cations, as possible column packings. The two principal criteria used to select the salts were stability up to about 500" C., and a melting point above about 500" C. The salts were compared on the basis of their ability to separate a standard blend of 0-, m-, and p-terphenyl. EXPERIMENTAL
An F&M LIodel 500 linear programmed temperature gas chromatograph was used throughout the study. A 2-foot drying tube packed with molecular sieve was placed in the helium line to dry the carrier gas. The inorganic salts 15ere obtained from Raker Chemical Co., Fisher Scientific Co., Nallinckrodt Chemical Works, and Merck and Co., Inc. The terphenyls and tetrahydrofuran were obtained from Eastman Organic Chemicals. The column packings I\ ere prepared by placing 25 wt. % of the salt dissolved in deionized water on 35- to 50-mesh Johns-llanville Chroniosorb P. The solvent was evaporated on a hot plate a t temperatures up to 600" C., depending on the melting point of the salt. Although pretreating the LiCl packing a t temperatures above the LiCl
melting point caused improved resolution of the polyphenyls ( 3 ) . other salt packings showed decreased resolution after this treatment. As a result, only the LiCl packing was pretreated. The packings were screened to assure 35- to 50-mesh particle size and poured into a &foot tube. A hand vibrator was used to obtain uniform packing. The packed columns 15-ere usually conditioned in the instrument for 1 to 2 hours a t 400" C., depending on the properties of the packing. Ten microliters of a blend of 95% of tetrahydrofuran and 5% of an equimolar mixture of the three terpheiiyls was used for each run. The column temperature was programmed at t h e rate of 15" C. per minute over the range 250' to 400" C. The helium carrier gas was flow-controlled a t 45 cc. per mi nut e. RESULTS
The component peaks in adsorption chromatography frequently are not symmetrical and show raryiiig degrees of tailing. I n programmed temperature chromatography the earlier peaks may be broader than the later peaks. These difficulties are more apparent Present address, Department of Chemistry, University of Oklahoma. Sorman, Okla. VOL. 36, NO. 1, JANUARY 1964
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