LITERATURE CITED
(1) Bombaugh, K. J., Thomason, W. E., AKAL.CHEM.35, 1452 (1963). (2) Jones, W. L., Kieselbach, R., Ibid., 30, 1590 (1958). ( 3 ) Littlewood, A. B., ,J.Gas Chromatog. 1 , 6 (May 1963). (4)Parker, K. D., Fontan, C. R., Yee,
J. L., Kirk, P. L., ANAL.CHEM.34, 1234 (1962). (5) Phillips, C. S. G., Second Symposium on Gas Chromatography, Amsterdam, May 22, 1958. (6) Smith, E. D., Gosnell, A. B., ANAL CHEM.34,646 (1962). ( 7 ) Smith, E . D., Johnson, J. L., Zbid., 33, 1204 (1963).
(8) Crone, P., University of Colorado,
Boulder, Colo., private communication to E. D. Smith, 1963. (9) Urone, P , , Kathik, R. J., ANAL. CHEM.35,767 (1963). RECEIVED for review December 16, 1963. Accepted May 27,1964. Presented at the Southwest Regional Meeting, ACS, Houston, Texas, December 1963.
Studies of the Liquid Phase Mass-Transfer Term in Gas Chromatography JAMES
K. BARR'
and
DONALD T.
SAWYER
Department of Chemistry, University of California, Riverside, Calif. 92502 The functional dependence of the liquid phase mass-transfer term in gas chromatography has been studied as a function of liquid loading with helium, nitrogen, and argon a s carrier gases. Carrier gas velocities up to 300 cm. per second have been used in these studies. The long stainding assumption that C I is independent of carrier gas has been shown to b e incorrect. With 3-pentanone as the solute, Carbowax400 as the solvent, and helium as the carrier gas the expeicted k / ( l k)* dependence of CI has been verified. However, the use of argon and nitrogen under the same conditions has shown essentially no k dependence for C iexcept a t extremely low loadings. A limiting rate of solute transfer across the gas liquid interface qualitatively accounts for theunusually large lband broadening observed for argon or nitrogen as the carrier gas. The effective depth of the liquid phase appears to be essentially independent of loading when Chromosorb W is used as a support material.
+
I
N GAS C H R O M A T O G R ~ ~ P H I C mass-trans-
fer formulations .the liquid phase mass-transfer contribution is generally the most important at higher carrier velocities and represents the limit,ing factor in the resolut,ion of adjacent elution bands. The band-broadening contributions to t,he t,heoret'ical plate height' are reasonably well understood except for the liquid. phase variance. Purnell (25) summari:zes two forms of the liquid phase t,erni, C i , in which a homogeneous film and a droplet distribution of the liquid are proposed. He suggests t'hat the actual Ci contribution may be some combination of these two models. Giddings, more realistically, has derived expressions for C l which depend on the geometric shape of the Present address, Engineering Physics Laboratory, E. I. du Pont de Xemours and Co., Inc., Wilmington, Del. 19898
pores in t,he solid support and has proposed eight separate expressions which depend on t'he surface st,ructure of the support ( 5 , 6, 8, 24). These expressions range from the uniform liquid-film model to a model in which any known porr distribution and size are allowed. However, no experimental evidence or opinion is given for a preference of one model over the others. More recently Perrett and Purnell ( M j ,in a review art'icle, have stated that the form of the expression used t.0 calculate Ci is unkriolvn. Finally, Dal Nogare and Chiu ( 3 ) have presented experimental evidence that the capacityratio (kj dependence of the C l term coincides with t,he uniform thickness model or the random-size pore model of Giddings (14). In mass-transfer theory, the number of sorpt,ions or desorpt,ions of a solute entering or leaving the liquid phase is inversely proportional to the theoretical plate height. In addition, t,he transfer of the solute across the gas-liquid interface and the desorption step, in the case of surface adsorption, are assumed to be kinetically faster than the limiting diffusion in the gas and liquid phases. This implies that the liquid phaAe masstransfer contribution to plate height will depend only on the partition coefficient of the solute in the liquid phase, the interdiffusion coefficient of that solut'e in the liquid phase, and the amount of the liquid present. Therefore, no rat,e constants appear explicitly in this formulation. Several suggestions exist' in the literature that the form of the rate equation also may contain a kinetically controlled gas liquid interfacial resistance to masstransfer term. Although no experimental evidence was given, Giddings (10) first proposed that simple diffusion steps alone are unlikely without a series of kinetic complications. Later papers by Giddings (11, 1%)expanded this concept t o include all possible rate steps
under chromatographic conditions ( 5 , 9, 13, 15). Recently, a series of measurements using helium carrier gas and glass beads as the support material has been made (16); the authors conclude that there appears to be no interfacial contribution under these conditions. Khan (28) has derived a term for the interfacial contribution to band-broadening in which interfacial transfer rate constants appear explicitly. The present, discussion summarizes research directed a t isolating the liquid phase mass-transfer term from the other terms, studying the functional dependence of Ci on column parameters, deciding which of the many liquid distribution models is realistic under chromatographic conditions, and determining if interfacial transfer really contributes to plat'e height. Several considerations have been important in the accomplishment of this goal. To st.ay in the Henry's law range and obtain a constant partition coefficient, extremely small sample sizes have been used. The practical Ilroblem of band spreading due t'o the finite rate of vaporization of a liquid sample has been avoided by introducing the sample as a vapor of minimum volume. To avoid longitudinal diffusion corrections, all measurements have been performed a t high carrier gas velocit,ies where liquid phase mass transfer and interfacial mass transfer should predominate. Finally, an extremely wide range of immobile phase loadings has been used to study the functional dependence of t'he Ci term upon the capacity factor, k. EXPERIMENTAL
Reproducible amounts of vapor were introduced into the chromatographic column by means of a borosilicate glass vacuum sampling system (26) in conjunct,ion with a gas Pampling valve (24). The sampling system consisted of two separate parts. The first way connected directly to the vacuum pump through VOL. 36, NO. 9, A U G U S T 1964
1753
a dry-ice trap and contained a mercury manometer to measure the atmospheric pressure. This half also was used to evacuate t'he sample loop in the sampling valve. The other half of the system was more elaborate and consisted of three 500-ml. storage bulbs, a 1-liter mixing bulb, a pressure bulb to introduce permanent gases, another mercury manometer, three sampling tubes to vaporize organic liquids, and a trap to condense the vapors when flushing the system. All the above components were connected to a 1-inch manifold, which was evacuated through a dry-ice trap to the vacuum pump. Organic liquids were introduced into the vacuum system by first freezing them in the sample tube, pumping off the air above the sample, and then, after liquefying, pumping the sample intermittently to expel dissolved gases. The vacuum-pump valve to the manifold was then closed and the sample tube valve barely opened to introduce the desired amount of vapor. Throughout the work, the moles of 3-pentanone used were varied from 8 X to 40 X depending on the carrier velocity. The amount of vapor injected into the chromatograph was calculated from the pressure of the organic vapor used (the difference in manometer readings) and the volume and temperature of the sample loop. The vacuum system had to be covered with aluminum foil t'o prevent' photolysis of the ketone vapor stored in the sampling system. Before covering the system, noticeable band broadening was detected after 1 hour. Although no photolysis occurred after covering, the system was flushed every 2 hours and fresh sample introduced to ensure that no reactions ( 4 ) were occurring. The stainless steel sampling valve was constructed from the design of Pratt and Purnell (24) and connected directly to the column. Hydrocarbons could not be used with this valve because t'hey dissolved in the rubber O-rings. The study was limited to polar solutes for this reason. The size of the rubber Orings was dependent entirely on the part,icular valve and various sizes were necessary within the same valve to prevent leaks a t the high inlet pre-7 bwres used. The valve was operated a t 70" C. and the carrier gas inlet tubing to the valve also was heated to the same temperature. h microswitch actuated by rotation of the valve handle triggered a capacitor discharge which put a blip on the recorder when operating a t low carrier velocities or act'uated the timing circuit of t,he oscilloscope when operating a t high velocities. The position of the microswitch was set to function as the sample feed band contacted the head of the column. The feed bands were calculated to vary from 3 to 7 pl. a t velocities above 50 em. per second and the dead volume between the sample valve and tlie column head was less t,lian O.lY0 of the total dead volume. The solid support was 100- to 200mesh Chronio.orb W obt'ained from n'ilkens Instrument and Research, Inc., Ralnut Creek, Calif. The material !vas resieired and treated with hexa-
1754
ANALYTICAL CHEMISTRY
methyldisilazane as described previously (1, 27, 68). The liquid phase, Carbowax-400, also was obtained from Wilkens Instrument and Research, Inc. I t was heated to 110" C. for 8 hours and stored under a desiccant. Solut'ions of the liquid phase were prepared determinately using reagent grade acetone. illiquots of the acetone solutions of Carbowax-400 and weighed amounts of solid support were mixed and stirred over a steam bath to slowly evaporate the acet,one. The coated support was vacuum-dried a t room ternperat,ure and packed by gentle tamping into ',/8-inch X 3-foot columns. h 300-mesh brass screen was soldered over the ends of the column to seal the packing material instead of using the usual glass wool. The columns were coiled to a 5-inch diamet,er and placed in the oven of a Wilkens Model 600 B gas chromatograph. Ijy using a 60 cubic-foot-perminute blower an essentially constant temperat,ure of 27' C. was maintained across the column. The solute used for the entire study was 3-pentanone obtained from Matheson Coleman & Bell. The material was purified by distillation prior to introducing it into the vacuum system. The hydrogen flame detector from a Rilkens Model 600 13 gas chromatograph was used to detect the elution peaks. The response time was determined with an oscilloscope to be about 65 mseconds for the combined flame head, connecting cables and electromet,er with a 107-ohm input. Fortunately, t,he response time did not appear to be a limiting factor in detection a t the velocit'ies used. Flow rates were measured a t the column outlet a t room temperature by a calibrated soap-bubble flowmeter using a Standard electric timer to measure elapsed time. Velocities were calculated using a porosity of 0.42 as given by Bohemen and Purnell ( 2 ) . The measuring resistor in the electrometer was kept at lo7 ohrns except at relatively low carrier velocities where the added response t'ime of the lO9-ohm resistor was allowable. For low carrier gas velocities, a Leeds and Northrup I-mv., 1-second (full scale) Model H recorder was used. For short retention times, a Model 535 .A Tektronix oscilloscope equipped with a Polaroid camera was used. A 60-cycle rejection filter of the standard design was necessary a t each of the inputs to the scope preamplifier. The microswitch on the sampling valve was set to trigger the variable delay in the "13" circuit of the scope; the sweep of the "h" mode (single sweep) was delayed by "13." The sweep rate was adjusted to fill the screen as much as possible with the elution band. Retent,ion times were calculated from the sum of the "13" delay time and t'he position of the recorded elution band a t maximum height. I n all cases, the number of theoretical plates, N, was determined with the formula -V = 5.545(tJwl 2)2 where t~ is the retention time and w1I2 is the width a t half height in time units. Dead-volume measurements were
made using methane and were similar for coated and uncoated supports. Measurements of Gas Diffusion Coefficients. Interdiffusion coefficients for 3-pentanone were measured in the gas phase in argon, nitrogen, and helium at 27" C. using a BarberColman gas chromatograph, Series 5000, with a hydrogen flame detector. A '/8-inch X 16-foot unpacked column was used a t low carrier gas velocities. The solute was introduced a? the liquid by syringe into a greatly modified injection port operated at 140' C. The injection port volume and the volume of the connecting tubing to the column were reduced so that the dead volume before reaching the column was approximately 25 ~ 1 . The liquid volume injected was 0.10 p1. It was necessary to introduce a small, continuous amount of the solute into the carrier gas by a side-arm containing 3pentanone in order to obtain spmmetrical peaks. Elution peaks were recorded on a Leeds and Sorthrup, Model H. 1-mv. recorder. The longitudinal diffusion plate height, H D , was measured from the recorded elution band in the same manner as previously described. The value of the diffusion coefficient, D,, was calculated using the formula (2)
where u, is the outlet velocity of the carrier gas, ro is the inside radius of the column, and y is assumed to be unity for unpacked columns. RESULTS
In the extended rate equation,
H = '4
+ B / u , + C g t ~4, Ciu,f
three terms contribute to band width a t high flow rates, -4, C L ,and C,. The isolat,ion of C i from t,he ot,hers is a prerequisite to the study. The method of Perrett and Purnell (22),which has been used wxeasfully with hydrocarbons as the solute and immobile phases, involves the subtract,ion of gas phase terms by using two carrier gases and measuring H values at, various velocities for each gas. By choosing outlet velocities in the ratio of the gas phase diffusion coefficients of the solute in the two carrier gases, t'he corresponding difference in H will be proportional to the liquid phase mass-transfer term. However, the assumpt>ion is made that' there are no other contributions to band broadening. The values of the gas phase diffusion coefficient for 3-pentanone in three different carrier gases a t 27' C. have been determined and are tabulat.ed in Table I. The carrier gas velocit'ies used are tabulated. Figure 1 indicates the theoretical plat,e heights for the elution of 3-pent'anone as a function of carrier gas velocity with five different loadings of Carbowax400. Data are given for argon and
Kx)
200
300
100
200
300
0 0
200
300
50r
0 0
Figure 1 . HETP data as function of outlet velocity for helium and argon carrier gases Loading of columns is indicated as percentage, by weight, of Carbowax400 on silazaned 100- to 120 mesh-Chromosorb W
helium carrier gases and an arbitrary tie line is shown connmecting the curves for the two carrier gases a t a velocity ratio equal to t,he ratio of the gas phase diffusion coefficients of 3-pentanone in the two gases (Table I). The difference in EI values ( A H ) a t the intersection of the tie lines has been used to calculate C I by the following forinula:
Aff
=
Extensive investigation of the C, term ha5 led to a summary of data and conclusions ( 7 , 8 , d S ) . The value of C, may be calculated directly using an empirical expreqsion and an experimentally determined value of Do. I n this way, the terms of the rate equation may be arranged as follows :
Table I . Interdiffusion Coefficients of 3-Pentanone in Three Carrier Gases a t 27" C.
Carrier
uo,
gas He
cm./sec.
Ar
0 994
s*
122 1 14
D o , cm.z//sec. 0 33 i 0 01 0 095 f 0 006 0 074 f 0 002
C,(l.i, - GZ)
Table 11 summarizes .the Ci values obtained by this methoi.1. Obviously CZ cannot br negative and the positive \.slues are Iras than w3uld be expected. To resol\-e the issue, an alteriiate method of isolatiiig Ci has been devised.
The outlet velocity is given by u,and f is the James and Martin pressure factor (17):
f =
-[
where p , and p , are the inlet and outlet column pressures. respectively. The best function obtained experimentally (23) for C, is
1
3 (P,/P,)* - 1 2 (P,//P,)3 - 1
VOL. 36, NO. 9, AUGUST 1964
1755
0.3r Table II. Values of C i Calculated Using Perrett and Purnell (22) Method on Carbowax-400 Columns a t 27" C. with 3-Pentanone as Solute
HELIUM
103, 0ec.
5%
h
I V Y
-
1.1 3.8 0.3 -2.1 -12.7
20 5 1
0.2 0.05
/
t
0.2-
cix
Loading,
I
0.05%
where d, is t'he diameter of the support particles; t,he numerical constants are assumed to apply generally to any system. This expression has been employed to calculate C, for the three carrier gases by using the values of D o given in Table I. The value of H is measured experimentally and Blu,, which is small but easily estimated ( B = 2 D 0 ) , is then subtract,ed. By plotting the liquid phase mawtransfer plate height', Hi, us. ucJsa straight line of slope Ci should result with an intercept of .l. Figure 2 indicates the results of such a treatment for t,he data from Figure 1. In this way separate values of Ci can be calculated for each carrier gas and if no adsorpt,ion effect is present, the terms should agree. Straight' lines have been obtained for all of the columns with the three carrier gases except for the 20Yc loading using argon or nitrogen, where a slight convex shape ha$ made the choice of slope difficult (see Figure 2 ) . I n the case of argon and nitrogen at 207, loading, the slope was chosen a t the lower velocities. Table 111 summarizes the values of the rat,e terms calculated by this method. The intercept of the H i us. uJ curve represents the velocity independent contribution to the plate height and i!: listed in Table I11 as the -1 term. The magnitude of this term varies between two and three particle diameters for the 207, column. .is lower loadings are used, the calculated value of C, becomes more critical to the position of the curve in Figure 2 and thus C, completely determines the value of d. In addition, the value of =I is not consisknt for different carrier gases within specific column loadings of less than 20%. I t seems
0
IO
20
30
40
50
60
70
80
90
01 0
IO
20
30
40
50
60
70
80
90
/ 0 0
/ 20
40
30
Po
H I as
safe to conclude that the experimental value of this 1 term is approximately 2 d, and the errors in the measurement of H and calculation of C, represent the reason for the wide fluctuation in the
Table 111.
Liquid Phase Mass-Transfer Coefficients for 3-Pentanone in Five Carbowax-400 Columns a t 27" C. Using Three Carrier Gases Loading, Helium Argon Sitrogen 70 k d x 103 A x 108 A x 10s 2 1 0025 2 9 1 63 0 030 42 0 035 20 0
1 0 2 0 05
1756
10 2 5 0 56 0 14
0 035
-0 057
-0 145 0 000
ANALYTICAL CHEMISTRY
2 75 5 45 5 0
0 48
NITROGEN
cI
0 0 0 0
075
039 130 160
ci
2 1 2 1 6 3
13 0
I
I
IO
Figure 2. Variation of carrier gases
cf
I
0135 0000
2 2 1 9
'
50
60
70
00
90
function of u,f far three
value of -4 for loadings less t,han 20%. If these values of :I are real, then the parallel formulation of the rat,e equation as presented by Giddings ('7, 8) is incorrect,. Having once obtained values of Ct in helium, argon, and nitrogen, the dependence of CL upon k can be ascertained. .1 plot, of Cl us. k for the three carrier gases is shown in Figure 3. For helium, a maximum at k = 1 s h o w that the major k dependence is k (1 k)*. Also, the elusive d j 2 term is not proportional to Vi2, as assumed by the liquid film model, but appears to remain relat,ively independent of the amount of liquid on the support except at estremely low loadings. This is more
+
141
Table IV. Retention Volume as a Function of Loading and Carrier Gas
70 Loading, Carbowax-
400 20 5 1
0.2 0.05 0.00
t
OO
l
1
IO
20
I
40
30
k
Figure 3. Variation of liquid phase mass-transfer coefficient, CI, as function of capacity ratio, k, for three carrier gases
clearly illustrated by Figure 4 where C l is plotted 1's. k / ( l kj2. The st,raight line obtained for helium has a slope whore value apparently equals 2/3 d i 2 ,D I , where d l is understood to mean t,he awrage dept,h of liquid in the support pores ( 8 ) . Thus, the quantity d i 2 /D1 appears to be independent of loading from 20 to 0.2'3& by weight. The slope of the helium plot, is 18.5 x l o + second. By assuming D l = IOw5 sq. em. per second, a reasonable value for most liquids, di can be estimated to be 5.3 X lo-* em. It is surprising that the value for d i remains so constant but lends support to the dominance of capillary forces over adsorbed forces in the distribution models proposed by Giddings (6, 14). Using a surface %rea for Chromosorb W of 1.0 sq. met,er per gram the calculated film thickness for Carbowax, assuming, a uniform film, varies from 2.4 X 10-5 t,o 2.4 X lo-' cm. for loadings ran.ging from 20 to 0.2y6,. Obviously, most of the Carbowax is located within the pores of the solid and this distribution may be observed chromatographically. Only the 0.05y0 rolumn appears to deviate slightly to give lower ;:,late heights; the amount of liquid phase is so low that t'here apparently is f,oo little liquid to give the previous pcre depth and Ci decreases proportionally. Therefore, at, least for helium, the model (8)that' relate:; Ci to the variable pore depth and size seems to be appropriat,e
+
where di is the mean pore depth and q lies between and depending on the geometry of the smallest pores. The argon data, as well as the nitrogen data in part, show essentially no k dependence, except a t very low loadings (see Figure 4). Here, to suggest that C1 is determined by mechanisms other than diffusion in the liquid phase is contrary to nearly all accepted theories for the liquid mass-transfer contribution. The value of C"1 using argon remains relatively constant as shown in Figure 4.
12 '-
1
VR(A~)/ VR(H~) 1.06 1.10 1.07 1.37 2.04 1.00
VR(H~) 171.0 38.9 9.90 1 80 0.541 4.20
The reason for the deviation to greater C l contribution a t 0.2, and 0.05y0 is not immediately apparent. -in additional interesting observation is the effect of carrier gas on the retention volume of 3-pentanone at various loadings of liquid phase. Table IV summarizes these data and indicates that the solute is retained significantly longer when argon is used. This is the opposite effect to what would be expected if adsorption by the support material were contributing to the retention volume (surface adsorption effects tend to be more prevalent with helium than with argon). CONCLUSIONS
Perrett and Purnell (23) have suggested that the k dependence of Cl is of the order (1 k ) / k . although this form is not chromatographically meaningful. Using argon as the carrier gas, the k function of Cl appears to folloii qualitatively Purnell's (1 k ) / k dependence. It seems possible that the C1 tern) contains more than one term and one whose magnitude is sensitive to the
+
+
0.05%
\
I
v -
I
v -
0
2
4
6
8
IO
12
14
I
16
I
18
1
I
I
20
22
24
V(l+k)* x 10' Figure 4.
(1
Variation of liquid phase mars-transfer term, C1, as function of
+ k ) 2 for three carrier gases
VOL. 36, NO. 9, AUGUST 1964
k/
1757
choice of carrier gas, among other experimental variables. Referring again to the helium curve of Figure 4, not all the plate height is accounted for in the k / ( l dependence. The residual, obtained as the y-axis intercept, amounts to 1.20 x 10-3 second. This value may represent the k independent part of the overall liquid mass transfer or it niay be due to a choice of an expression for C, which is too small. The k independent' term appears to be dominant in the case of argon and nitrogen as carrier gases. Such a proposition is not new (6, 10I S , 16, 18-20) but sc far never has been observed experiment'ally. The liquid surface area available for partition processes for any loading appears to be very much less than the surface area of the support,, because t'he liquid is distributed in deep, narrow pores. ;Iny phenomena which limit the rate of t'ransfer across the gas liquid boundary probably would affect the liquid mass-transfer part' of t,he total plate height. Surface area studies, the latest of which is given by Slowinski ( $ I ) , show that there is a decrease in the surface t,ension of various organic liquids when in contact with permanent gases, such as nitrogen and argon. Especially noticeable in studies a t high pressure is the fact that the surface tension of a liquid is directly proportional to the pressyre. There seems to be general agreement among workers in this field that lowering of liquid surface tensions by pressurized gases can be attributed qualitatively t,o adsorption of the gas a t the liquid surface as an incomplete monolayer. The conditions used in chromatography are sufficiently similar that it, seems reasonable to suppose that t'he carrier gas is adsorbed on the liquid surface, especially a t the high pressures used here to achieve high carrier-gas velocities. This may account in part for the dependence of the retention volume upon carrier gas (Table IV). Kahn (18) ha,s proposed two t'erms for the total C i cont,ribution, one of which is the usual van Deeinter t'erm with rate const,ants stated explicitly and t8he other an interfacial resistance to mass-transfer term.
+
where kl is the sorption rat,e const8ant and ICs is t,he desorption rate const,ant (both in velocity units). a2'ul is the ratio of cross section of the liquid to that of the gas, d, is the cffcctive depth of liquid phase, and u i < t h e y surfare area of liquid per unit column lrngth. In this ( k l k n ) normally is formulation (a2,:nl) equivalent to the capacity factor, 12. 1758
ANALYTICAL CHEMISTRY
tribution with lightly loaded columns For the case of using helium as the and may be predominant for the cases carrier gas, reference to Figure 3 indiof argon and nitrogen. Therefore, the cates that the Khan expression reduces trend toward a maximum in Ci when to the van Deemt'er expression and that using these carrier gases for low-loaded the interfacial resistance to mass transfer is negligible. However, when argon or columns would indicate that this term is important and accounts for the major nitrogen is used as a carrier gas a maxicontribution to Ci a t extremely low mum for C i at k = 1 is not observed in loadings. Figure 3. This implies that the van is Ad3orption of the solute on the supDeemter term, based on k ! ( l port is not considered a likely process adequate. If ( a n / a l (kl/ICz) ) does not because it would be expected to be less change proportionally with k , then the in argon than in helium a t low loadings. maximum for C iwould occur a t values The opposite is observed by 2. large of IC less than unit'\- and where (a2iai) magnitude (see Table IV and Figure 3). ( k l / k z ) is unity. Thus (a2,1al) k 2 j k l Also, the Martin effect (18, 19) has not appears t'o be larger. relative to helium, been considered because the solute-solwhen argon or nitrogen is used and vent combination which has been apparently causes the maximum for C I studied is alike chemically and is misto occur a t lower values of k . This cible a t all concentrations. would be the case if the desorption rate I t should be noted that the best overconstant, k2, is reduced by an adsorbed or static layer of carrier gas a t the interall theoretical plate heights have been face of t'he two phases. The rate of obtained using helium as the carrier gas with the 0 05y0 Carbowax column (see diffusion through t'his third phase should Figure 1). K i t h this combination, the depend on the thickness and densit'y of the interface. Only a relatively small value of low-loaded columns (27) for general use is again indicated because fraction of the molecules need undergo the usually dominant liquid phase masst,his kinetically controlled t'ransfer to transfer term 1' at a minimum. Plate cause considerable band broadening. heights of about three particle diamAt the high carrier gas velocities used eters are observed in this case and in this study, interfacial effects would be represent efficiencies which are signifimore noticeable t,han a t the velocit'ies cantly greater than are usually realized usually employed. Such a possibility is with conventionally loaded columns. further supported by the data in Table ddditional research is needed to resolve IV. The k values shown in Figure 3 some of the anomalies obserted for the for argon a t low velocities should be increased by the factor ( V A ~ ) / V ~ ( R ~ ) )interfacial band-broadening processes. Such studies are currently in progress given in Table IT. This increased with other solute-solvent systems and retention for argon is believed to be the carrier gases, and will be reported in the result of a decrease in IC,, which would future. tend to displace the maximum of the argon curve (Figure 3) away from the helium maxirnum. Although this correction for the argon curve is not of LITERATURE CITED sufficient magnitude to give a maximum (1) Bohemen, J., Langer, S.H., Perrett, coincident with the helium curve, the R . H., Purnell, J. H., J . Chem. Soc. shift is in the right direction. Finally, 1960, p. 2444. the difference between the actual and ( 2 ) Bohemen, J.. Purnell, J. H., I b i d . , 1961, p , 360. the effective values of t'he ratio ( a d a l ) 13) Dal Sonare. 3.. Chiu. J.. ANAL. a t low loadings is not known and could CHEY.34,'s90 i1962). further account, for the displacement of ( 4 ) Fischer, L. C., ?*lains, G. J., J . Phys. Chenz. 68, 188 (1964). the argon maximum. ( 5 ) Giddings, - J. C., Ax.4~.CHEM.33, 962 The relatively larger value for (up'' (1961) a l ) ( k l / k z ) (indicated by the data in (6) Ibid., 34, 458 (1962). (7) Ibid., p. 1186. Table IV), then, appears to fit the data 18'1 Ibzd.. 35. 439 11963). for argon and nitrogen in Figure 3, al(9j1bid.j p. ' 1 ~ ~ 9 . though no maximum is observed and t'he (10) Giddings, J. C., J . Chem. Phys. value of C l for the 0.057, column is 169 (19.37). (11) Zbid., 31, 1462 (1959). higher than expect,ed. This latter ob(12) Giddings, J. C., J . Chromatog. servation may give some experimerital 44 (1969). support to the interfacial term of Khan. (13) Ibid., 3, 443 (1960). (14) Ibid., 5 , 46 (1961). Thus, if k2, the desorpt'ion rate constant', (15) Giddinns. J. C.. J . Phus. Chern. decreases because of an absorbed layer 184 (19647. of argon on the liquid surface, then the (16) Giddings, J. C., llallik, K . L., Eikelberger, lI,, .%SAL. CHEM. 34, 1026 second term of tmhe Khan expression f 1962 - - -~ / . would be expected to increase. This (17) James, A , T., llartin, .4. J. p. increase would be enhanced if u , the Riochem. J . 50, 679 (19.52). (18) Khan, XI, A , >.\-atwe 186,,800 (19160). surface area of the liquid phase, also is (19) llartin, R. L., . ~ K A L . CHEM. 33, being decreased at extremely low load347 (1961). ings. Thus, the interfacial term of (20) I b i d . , 35, l l i fl96.7). (21) llasterto Khan should have its maximum con-
+
\
,
I
~
\
,
Slowinski, E . J., J . Phys. Chem. 67, 615 (1963). (22) Perrett, R. H., Purnell, J. H., -4NAL. CHEM. 34, 1336 (1962). (23) Ibid. 35, 430 (1963). (24) Pratt, G. L., Purnell, J. H., Ibid., 32, 1213 (1960). (25) Purnell, J. H., “Gas Chrornatography,” pp. 147-150, Wiley, New York, 1962.
(26) Purnell, J. H., Department of Physical Chemistry, University of Cambridge, England, private communication, 1962. 127’1 Sawver. D. T.. Barr. J. K.. ANAL. ‘ CHEY.34,‘1052 (1962). ’ (28) Ibid., p. 1518. RECEIVEDfor review March 11, 1964. Accepted May 25, 1964. Work submitted
by J. K. Barr in partial fulfillment of the requirements for the degree of Doctor of Philosophy to the faculty of the Vniversity of California, Riverside, June 1964. Division of Analytical Chemistry, 147th Meeting, ACS, Philadelphia, Pa., April 1964. Work supported by the U. S. Atomic Energy Commission under Con: tract N o . AT(11-1)-34, Project No. 45.
Determination of Residua I Monome rs in Latex by Gas Chromatography L. BETH WILKINSON, CHARLIE W. NORMAN, and JOHN P. BUETTNER Central laboratory, Texas Division, The Dow Chemical Co., Freeport, Tex.
b A gas chromatographic method for the determination of residual monomer in several latex systems is described. The latex is dissolved in a suitable solvent and the solvent-sample solution chromatographed for residual monomer on a standard thermal conductivity instrument. In most cases a solvent can be chosen which dissolves the latex completely to asisure release of any entrained monomer, and converts water to a more suitably chromatographed component. One column (E6000 on Celite) is adequate for the determination of 13 monomers in eight latex systems studied. A presection prevents coagulated latex from plugging the column. The limits of monomer detection are principally a function of the retention time of the monomer. They vary from 100 p.p.m. by weight for vinyl c:hloride to 500 p.p.m. by weight for ni-butyl acrylate. Relative standard dleviation is no greater than 10%. and meaningful method of determining residual or unreacted monomer in lat,ex has been sought for some time. Research, particularly in the field of paint formulations, is rapidly expanding the number of chemical compouncls under test as latex paint componenk In addit’ion to the (’ominon problem of odor formation, monomer content is closely related to shclf st,ability and mechanical stability. Hydrolpis of the monomeric esters msy altrr. the pH of the lat,ex, markedly reducing the effectiveness of other additives in t h e lat,ex system. For these reasons. dptermination of residual rnonoinrr is generally required now as a standard specification. RELIABLE
Until recent years noninstrumental methods were used for this type of analysis. Two chemical procedures that have been employed are t’he bromination-steam distillation ( 5 ) and the morpholine reaction methods ( 1 ) . Neither is applicable to the wide spectrum of latex formulations currently encountered. The bromination technique is time-consuming, largely because of the distillation step, and the choice of solvents is restricted by the requirement of compatibility with the halogenation reagent. The morpholine procedure is limited in its scope, in that only alpha-, beta-unsaturated carbonyls with no beta-position substitution react. However, the methacrylate radical is an apparent exception to this generalization. h brief, but excellent, comment on other aspects of this method has been published (6). Correlation between the results obtained from each method and the physical behavior of the latexes when subjected to aging tests has never been satisfactory. Later, a polarographic analysis ( 3 ) utilizing the waves of the acrylate esters (-1.9 to -2.0 volts) was investigated for use with that particular class of compounds. Values obtained in this way, however, represented the content of all acrylate moieties present, including hydrolysis products and t,he like; thus the method lacks necessary specificity. A mass spectrometric procedure ( 7 ) for use with acrylate, methacrylate, or nitrile latexes also received some attention during prior work in our laboratory on this problem. .in internal standard t’echnique using 1,4-dioxane in a benzene-ethanol solution of the sample was developed. The dioxane peak a t m,!e 88, together with the acrylate-methacrylate peaks a t m/e
55 and 69 or the acrylonitrile monomer peak a t m!e 53, was chosen for quantitative work. This method also lacked wide applicability. The adaptation of gas chromatography to this analytical problem has been limited. Tweet and Miller (6) describe its use in analyzing a rapidly distilled organic layer from ethyl acrylate-styrene emulsions. Extractability from water is a necessary qualification of this procedure. The direct injection of latex samples from a hypodermic syringe into a conventional column entry port has not been feasible because of t,he physical nature of the latex. The adhesiveness of the suspended particles allows them to coat the walls of the syringe barrel, causing the plunger to bind and preventing reproducible sample entry. A recent innovation designed to avoid these problems ( 4 ) employs disposable glass capillary t’ubes from which the sample is swept into a vaporizer by the carrier gas. I t therefore becomes apparent that a gas chromatographic technique that would permit an accurate and reproducible sample to be injected directly into a partitioning column would greatly expand the usefulness of gas chromatography in the latex field. The use of commercially available instruments without adaptation would then become possible. The method presented in this paper provides for complete solution of the sample in a suitable solvent and direct, introduction of an aliquot of this solution into the chromatograph. I t is very specific for the monomers present in finished latex. The analytical time required is brief-less than 15 minutesand the period to remove solvent from the column is about the same. Column conditions are varied to acconimodate VOL. 36, NO. 9, AUGUST 1964
1759
