1932
Anal. Chem. 1982, 5 4 , 1932-1938
Comparison of Liquid Segmented with Nonsegmented Flow Systems in Postcolumn Reactors for Liquid Chromatography A.
H. M. 1. Scholten, U. A. Th.
Brinkman, and R. W. Frel"
Free Unlversity, Department of Analytical Chemistry, de Boelelaan 1083,
1081 HV Amsterdam, The Netherlands
The band broadenlng In segmented-flow reactlon systems was Investlgated. I t showed that band broadenlng can be efflclently suppressed In solvent- and alr-segmented systems. The malor contrlbutlon to band broadenlng orlglnates from phase separators. Proper constructlon of phase separators has been dlscussecl. Flnally relatlonshlps have been developed whlch enable one to calculate the proper changeover condltlon from nonsegmented (open tubular and packed bed) to segmented reactor systems. The concluslon Is that In general segmented-flow systems are to be preferred even for rather short reactlon tlmes.
liquid and a fraction & of the carrier stream. For good operation of the system, it is essential to control the flow rate (@3 + &), since sucking the flow 42 with the pump through the flow cell would cause base line instability and unacceptably high noise. A Rheodyne (Berkeley, CA) six-port injection valve, various PTFE capillaries (Omnifit,Biolab, Cambridge, Great Britain), a PyeUnicam variable-wavelengthUV detector (Philips, Eindhoven, The Netherlands), and a fast Moseley (Pasadena, CA) Model 2DR-2AM recorder were used. The tee piece which was used for addition of the segmentation liquid was a commercial part (A-10% Technicon,Tarrytown,NY); the phase separator routinely used (cf. below) was a similar tee piece with 1.52 mm i.d. All solvents were of analytical grade quality.
Postcolumn reaction detectors are being more widely accepted as valuable alternative detection modes in liquid chromatography ( I ) . Three types of reactor are currently in use: (1)tubular segmented reactors; (2) tubular nonsegmented reactors; (3) packed-bed reactors. The selection of the proper type of reactor for a specific application depends on the reaction kinetics and conditions and is a matter of much debate in the literature. The theory of band broadening in postcolumn reactors has been treated by several groups of workers. Nonsegmented flow conditions in coiled tubular reactors were studied by Tijssen (2, 3), Deelder and co-workers (4, 5 ) , and Huber et al. (6). Tijssen recommends the use of narrow-bore tubing (20-80 pm i.d.) for reaction times of up to 20 min. Deelder and his group and Huber et al. prefer the use of a packed-bed reactor for reaction times of up to 10 min; only in the case of very short reaction times of a few seconds is their choice is an open capillary of typically 0.25 mm i.d. For reaction times of over 10 min, Deelder and co-workers recommend air-segmented reactors. As for flow phenomena in open tubes, one can also consult the work of Hofmann and Halasz (7,8), who report on the behavior of chromatographic peaks in (deformed) open capillaries. Dispersion in air-segmented systems has been treated in a semiempirical manner by Snyder and Adler (9-11). Experimental data on band broadening in segmented-flow systems are given in a paper by Deelder et al. (4). These authors state that band broadening in a segmented system is primarily due to the phase separator and tee pieces and report values of U: = 7000-9000 pL2. In our opinion, their conclusion is essentially true. In the present paper we shall demonstrate, however, that the utilization of properly designed miniaturized phase separators-with geometric dead volumes of 10-50 pL-considerably reduces band broadening and thereby significantly expands the feasibility of (solvent- and air-) segmented flow systems. For this work, suitable theoretical relationships have been developed which permit a valid comparison of segmented flow and other types of reactors and which permit one to select the proper reactor system. EXPERIMENTAL SECTION Figure 1shows a schematic representation of the experimental setup. A Technicon (Tarrytown, NY) AuhAnalyzer I1 pump with standard Tygon tubing delivered the carrier stream at a flow-rate of b1and the segmentation liquid at a flow rate of @4 and also controlled the flow going to waste, i.e., all of the segmentation 0003-2700/82/0354-1932$01.25/0
RESULTS AND DISCUSSION Band Broadening of Phase Separators. Initially, a series of experiments was carried out with pure water as carrier stream and hexane as segmentation liquid (see Table I). Sodium nitrate dissolved in the carrier stream invariably was the test compound. In Table I, band broadening has been expressed in volume units (u, in microliters) rather than in time units (ut in seconds),since it is often overlooked that with high flow rates even small ut values can give rise to high uv values as u, = q5uv From the band broadening data of Table I it is evident that, except with the injection volume of 19 fiL, band broadening in the segmented flow system is smaller than that in a, otherwise identical, nonsegmented flow system. Results of experiments not included in Table I show that, when using somewhat larger flow rates of & = 1.2 mL/min and & = 1.0 mL/min, this phenomenon was also observed with the 19-pL injection volume. As will be shown in the next paragraphs, this at first sight rather astonishing result is due to the insertion of a phase-separation step in the segmented system. In this step a fraction of the original carrier stream is sucked to waste (see Experimental Section) which causes a partial loss (typically 10-40%) of the carrier stream and, thus, the injected volume. As a consequence, compared to the peak volume measured in the nonsegmented system, after segmentation and desegmentation in the phase separator a smaller peak volume is measured in the detector with the segmented system. A theoretical discussion is as follows. The total variances of a tubular segmented reactor system (uzvw) and a tubular nonsegmented ( ( T ~ , , ~system, . ~ ~ ) in which the injection valve is coupled to the detector with the connective capillary, are given by eq 1 and 2, respectively, where u2v,aeg
=
+ uzv,con + u2v,reactor + u 2 v , c e ~
g2v,inj
u2v,n-seg
=
uzv,inj
+
n2v,con
+ u2v,cell
(1)
(2)
the terms on the right-hand side of the equations give the variance contribution due to the injection system, connective capillaries, segmented reactor proper (in eq 1)and detector, respectively. Even if equal injection volumes are used, the total variances, c ? " , ~ and u2vp.seg,cannot be compared directly since the variance contributions due to injection are not the same in eq 1 and 2. For the variance of a plug injection of volume Vi one can write a2,,inj
0 1982 American Chemical Soclety
= Vt/K
(3)
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Table I. Influence of Injection Volume, Vi, and Flow Rates, Nonsegmented (n-seg.)Flow Systemsa
a
@*,
on Peak Height, h , in Segmented (seg.) and hseglhn-seg AseglAn-seg hsedhn-seg
$29
$1,
and
pL h (exptl) + 0.71 19 22.7 76 0.64 0.72 177 67 30.5 0.80 96 40.5 208 t 0.88 0.67 19 20.8 81 0.69 0.78 67 28.8 194 0.86 222 96 37.5 + 0.88 0.4:4 19 21.1 53 0.45 0.61 150 67 24.1 0.68 176 96 29.9 t 0.88 19 19.3 118 67 34.3 245 96 45.7 260 0.63 0.4:9 19 22.5 80 0.67 + 67 28.8 190 0.79 96 38.1 212 0.84 + 0.63 19 21.2 119 67 33.3 240 96 45.8 252 Carrier stream, water; segmentation liquid, hexane; test compound, sodium nitrate. seg.
non-seg. mL/min 0.88
@,
mL/min
GI segm liquid
eluent
PUmD
Vi, p L
ov,
(exptl) 0.76 0.64 0.71 0.74 0.66 0.70 0.49 0.43 0.44
(eq 15) 0.62 0.89 0.94 0.60 0.88 0.93 0.43 0.77 0.86
0.78 0.70 0.72
0.56 0.89 0.94
@2/@l
0.81 0.76 0.50
0.78
waste
,,,.,,
segmentation s y s t e m (reactor)
1 ni
1933
C connective
detector
Y
capillaries
Flgure 1. Experimental setup of the Segmentation system.
Here, K is a proportioinality constant, which is determined experimentally by plotting C T ~ , , vs. ~. V .: In the present work, a value of K = 5 was found, which satisfactorily agrees with literature data (12,13). If, in a segmented system, a fraction $3/$1 of the carrier stream and, thus, of the injected volume, goes to waste, the variance contribution t h e to injection must be written as
500
1000
1500
2000
- q5 2500
Figure 2. Dependence of total variance, g2y,see of a segmented flow system on injection volume, according to eq 5.
UV detectors)-or from eq 7 using the experimentally determined values of of Table I. Calculation yielded u2v,inj = (vi- $3 Vi) rather similar results of 150 f 20 pL2 (eq 5) and 130 jtL2 (eq (4) 5 7). The calculated variance for the reactor, u2,,re.BBCtor, is solely due to the phase separator and/or tee piece, since it has Since $3 = - d2 (cf. :Figure 1) combination of eq 1 and 4 previously been demonstrated (14) that no noticeable band now yields broadening occurs in the reaction capillaries if segmentation is used. This implies that the band broadening contribution $22 v,:] of the phase separator is rather low compared to earlier obQ2v,,eg = - + g2v,con + g2v,reactor + g2v,ce11 (5) $12 5 servations of 7000-9000 pL2reported for an air-segmentation system (4). The validity of eq 5 can be read from Figwe 2, in which $vgeg. Table I1 summarizes results obtained when using various has been plotted as a function of Vi2/5. In all cases, the carrier-stream compositions and/or segmentation liquids. experimentally found values of the slope, r , and the slopes Since in this study the same flow rates were used throughout, calculated from the actual flow-rate data, d22/$12,agree to at instead of 0;values are given. For carrier streams containing within 0.03. a low ((50%) percentage of organic modifier, (it is seen to be Substitution of the variance of a nonsegmented system typically 2.0 f 0.2 s; it increases to about 2.4 f 0.2 s for high (> 50%) proportions of modifier. When such high percentages of organic modifier were used, small droplets of organic solvent occasionally were present in the aqueous stream after phase into eq 5 gives separation. In order to prevent these droplets from passing through the detector cell or, in other words, to create an efficient phase separation, we inserted a small plug of PTFE wool in the phase separator just prior to the branching point, as is shown in Figure 3A. This had no noticeable effect on The variance contribution of the reactor proper, gzVJmctor, can either peak shape or band broadening. The phase separator shown in Figure 3B was also tested with and without the plug be calculated from either eq 5-using the intercept read from Figure 2 and assuming a 250-300 pL2 contribution due to 2v,con of PTFE wool; results were similar to those obtained with the device shown in Figure 3A. For the rest, one can read from (20 cm capillary tubing of0.25 mm i.d.) plus (commercial
1
1
2
1934
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Table 11. The Influence of Mobile Phases and Segmentation Liquids on Band Broadeninga
I UdS)
at
carrier stream water
segmentation liquid hexane ( p = 0.66) chloroform (p = 1.48) hexane-chloroform (3:2) (p = 0.98) hexane chloroform hexane-chloroform (3:2) hexane
water-methanol 1:l water-methanol 9:l
0.6h
2.0
1:1 a Capillary, 1 m X nitrate.
5,
I
1.8
20
2.5 2.7 1.9
2.0 2.7
hexane
I
I
1.8
2.0
7:3 3 :7 water-acetonitrile 7 :3
x
A
C 350p2
D 60Opl'
m
2.3 2.4 0.8 mm i d . ; test compound, sodium
Table I1 that segmentation can be done with solvents lighter and heavier than the mobile phase, and even in equal-density solvent systems. In such cases, insertion of a plug of PTFE wool is mandatory for creating efficient phase separation. As indicated in the previous paragraph, experiments were routinely carried out with the phase separator depicted in Figure 3A,B. Their variance contribution of 150 pL2 was smaller than that of the devices shown in Figure 3C,D,F (350-1000 pL2). The phase separator designated E in Figure 3, which has a capillary port of 0.8 mm i.d. showed excellent performance with water as carrier stream and hexane as segmentation liquid (6: = 200 pL2); however, with aqueous/organic carrier streams, the phase separator of Figure 3A showed better base line stability and less noise. Other types of phase separator are discussed in ref 15-17. Dependence of Peak Height on Segmentation Conditions. From Table I one can read that peak heights decreased if (1) a smaller fraction of the carrier stream passed through the detector or (2) a smaller injection volume was used. This is in agreement with literature data (7). A relationship which shows the dependence of peak height, h, an injection volume, Vi, and flow rates (& and &) can be derived as follows. Table I indicates that the theoretical relationship Aseg/An-aeg
= 42/61
A = &ha where h is the peak height and can be written as
CT
E 2GCp12
II
F
1CGOp12
Flgure 3. Deslgn of phase separators tested with measured variances (in pL2). Other values relate to dlmenslons and are in mm: A-D (all glass; the inserts are PTFE capillaries), the segmented flow enters horlzontally from right to left, after desegmentation the organic stream goes to waste (upward for the lighter organic solvents, downward for the heavier ones) and the aqueous phase flows through the detector cell; E (I, glass; 11, PTFE; ports 0.8 mm i.d.) and F (I, nylon; 11, PTFE; ports 1.6 mm Ld.), the segmented stream enters via port a, the aqueous stream leaves via b, and the organic one via c to waste.
with r, the radius and L, the length of the connective capillary,
D, the diffusion coefficient of the test solute, and 4 the flow rate. C T can be ~ replaced ~ by ~2v,pbsep, as has been shown in the previous section. u2v,seg can then be expressed as U'v,seg
(8)
is well borne out by the experimental results. Here, Aaegand An.segare the areas of peaks recorded in a tubular segmented and nonsegmented system, respectively. With
I
=
cpZ2Vi2 ?rr,4Lt 42 + 'J'v,phase-sep
-- +q1,2
5
240,
(12)
If, for convenience, the term xr$Lt/24D, is written as kl and ff2v,phme.sep as k2,eq 12 can be written as
(9)
the band broadening, eq 8
Likewise, for the nonsegmented system, in which the injection valve is coupled to the detector via the connective capillary, the variance according to eq 6, can be written as
Vi2
g2v,n-seg
The variances shown on the right-hand side of this equation are given by eq 5 and 6, respectively, and hsegand hn.segare the height of the peaks recorded in a segmented and nonsegmented system, respectively. In eq 5 the usually low variance contribution of the detector cell (u2v,ceu) can be neglected. The variance contribution due to connective capillary (u2v,con)can be written as
+ k141 5
(14)
Substitution of eq 13 and 14 into eq 10 results in
-=( hseg
hn-seg
Vi242' + 5k14142~
+ 5k14i2d2 + 5k24i2
)'2
(15)
vi%22
The usefulness of eq 1 5 w h i c h relates loss of peak height with injection volume and flow rate in a comparison of a segmented with a nonsegmented tubular reactor-can be read from the data in Table I. The experimental and calculated values of hsep/hn.seg clearly show a similar trend. For small injection
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
volumes of, Le., 19 pL, the factor Vi2+z2generally can be neglected (in both the numerator and denominator of eq 15), which now simplifies eq 15 to
1935
rinmn
t-
06
c6-
with k3 = kz/kl. In practice, is a constant (often having a value of about 0.8; that is, eq 16 shows that, for a given flow system, h,/h,., is dependent on a single parameter, only. For high values of Vi the ratio approaches a limiting value of 1. Comparison of Segmented and Nonsegmented Raactors. Tubular Reactor. The various reaction systems can be compared on the basis of their respective variances. The use of a segmented flow system is preferable to that of a nonsegmented one if hseg/hn.w> 1, or (cf. eq 10)
+',
dZ2g2v,n-seg
> d12g2v,Beg
(17)
For a nonsegmented tubular reactor, the variance of the reaction capillary ( u ~ ~ , ~ ~is ,given , , . ~by)
provided the diffusion term may be neglected; here r is the radius and L the length of the reaction capillary and D, the diffusion coefficient. The proportionality factor, K , the value of which depends ( 2 , 4 )on the flow profile in the capillary is given by K
= 5.6(DnS~'/~)~.~~
(19)
for 200 > DnSc1/2> 10, and
K = l
(20)
for DnSc'/2 < 10. In eq 19 and 20 Dn and Sc are dimensionless Dean and Schmidt numbers (see Glossary). Substituting eq 18 into eq 6 yields
0
1
2
3
When neglecting the last term on the right-hand side of eq 22 (the variance of a good flow cell is about 50 pL2) and rearranging it, one arrives at Usually, the second term on the right-hand side of eq 23 is large compared to 1.25ar,4Lt&, so that eq 23 can be written as
In Figure 4 a plot of r vs. L is shown according to eq 24 (with the > replaced by an = sign) for different K values. The region above the curves represents conditions in which a segmented tubular reactor should be used. The region below the curves indicates the preference for a nonsegmented tubular reactor. More concretely, from Figure 4 it can be concluded that, for tubular reactors with reaction capillaries having r > 0.2 mm, segmentation has to be preferred virtually irrespective of length. For capillaries with r < 0.07 mm, on the other hand, a nonsegmented tubular reactor normally should be the choice.
5
6
7
8
9
10
hnm.-,
Flgure 4. Plot of the radius of the reaction capillary, r , vs. the length of the capillary, L , for various K values (see eq 24): D, = 1.5 X mm2 s-', 4 , = 17 pL s-l, k , = 150 pL2.
The following can be considered a representative example for the potential of a segmented system. Insertion of suitable numerical values-viz. p = (0.7-1.1) X g mm-3 and 17 = (0.5-1.0) X p; kz = 150 pL2; D, = (0.5-2.5) X loM3 mm2 s-'; = 10-20 p L s-l; K = 0.25 (average value for capillaries with 6-cm coil diameter and 0.34.8 mm id.)-into eq 24 yields
r4L > 0.15-2.27 mm5
(25)
or
L, 1 m L, 10 m Combining eq 5, 11, 17, and 21, using the assumption dZ = 0.8'' and neglecting cr2v,inj(for explanation, see below) results in
4
r > 0.11-0.22 mm r > 0.06-0.12 mm
Taking r4L > 0.65 mm5- which corresponds to the experimental conditions cited above (cf. Figure 4)-this means that for a postcolumn reactor of length L = 1 or 10 m, segmented flow should be preferred to nonsegmented flow for reaction capillaries with an inner diameter larger than 0.32 and 0.18 mm, respectively. Experiments have shown (14) that h,/h,., = 0.95 for a 1 m X 0.35 mm i.d. reaction capillary and increases to 1.8 with a 1 m x 0.50 mm i.d. reaction capillary. This agress very satisfactorily with the above conclusions. By substituting
t, = L / a = Lrr2/$
(26)
with t , the residence time in the reaction capillary and Q the mean linear flow velocity, in eq 24 conclusions can be obtained in terms of residence time, as is shown by the following equation
A graphical representation is shown in Figure 5 where t , is plotted as a function of r, for various values of K; the numerical values for &, D,, and k2 and the interpretation are the same as those for Figure 4. The conclusions self-evidently are equivalent to those drawn above. We add that one should realize that preference for small-bore capillaries and, thus, nonsegmented systems, implies the use of rather high pressures since-at constant residence time-pressure drop in-
1938 tr
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
injection volumes are used. From the arbitrary assumption that uZv,injcan be neglected if its contributions to the total variance is less than 10% of u2v,cap,n.seg (cf. eq 21 and 18), i.e., if
In sac
41
I
the limiting injection volumes can be calculated. r,0.125mm 0.4 mm
50
-
40
-
L, 1m 10 m l m 10 m
7rL < 20rL < 70pL < 220rL
Vi
0.02-0.71 mm2 s
(28)
or
r, 0.15 mm r, 0.25 mm r, 0.40 mm
t , > 0.9-31 s t , > 0.3-11 s t , > 0.1-4 s
where results are shown for inner radii typical of commercially available capillaries. These data show, e.g., that for a reaction capillary of r = 0.25 mm depending on the experimental conditions, the switch-over point from a nonsegmented to a segmented system is somewhere between 0.3 and 11 s residence time. For our experimental conditions (cf. Figures 4 and 5), with reaction capillaries of r = 0.15 or 0.25 mm, segmentation has to be preferred for reaction times longer than 7 and 2 s, respectively. In an earlier paper (14)a comparison was made-at a reaction time of 19 s-between (A) nonsegmented and segmented flow through a capillary of r = 0.25 mm and (B) nonsegmented flow in a capillary with r = 0.15 mm and segmented flow in a capillary with r = 0.40 mm. In the former experiment band broadening was distinctly higher, and resolution less good, in the nonsegmented system. In the latter experiment, resolution was the same in the segmented and nonsegmented system; band broadening was slightly smaller (80 vs. 90 pL) in the segmented system. These results fit in nicely with the above data. In the comparison of segmented and nonsegmented systems (see eq 22) we have neglected u2v,hj;therefore, one has to keep in mind that eq 24 and 27 will be valid only if relatively small
=
(31)
with 4 the flow rate through the reactor, t, the residence time in the reactor, and L, the length of the reactor. For the plate height, H, of a bed reactor packed with small-particle glass beads, the same equation can be used as for an analytical column. That is, the plate height due to convective mixing according to Hiby (see for instance ref 5) can be used:
In this equation X, and X2 are geometry parameters, d, is the particle diameter, u the mean linear flow velocity, and D , the diffusion coefficient of the test solute. In practice, one often selects a specific internal diameter, d,, for the packed bed reactor, which can be written as follows (5): (33) with t the internal porosity factor of the column. Combining eq 30-33 yields
Vi' 5
u2v,n.seg =-
M p + + 0.25ta4trd,2 1 + X2d,(taD,/4~dp)1/2
Combining eq 12,17, and 34, using the assumption & = 0.841, and neglecting (as was discussed earlier) u2v,conand u2v,ce~~, one can derive that tubular segmented reactors should be preferred to packed bed reactors (provided the injection volumes are sufficiently small) if
Rearranging eq 35 results in 6.2 kz 1 +&dr(t~Dm/44~dp)"'
t,>--
dI2 ea4
w,
(36)
Flow rates through a packed-bed reactor normally are of the order of 20 p L s-l. If a reactor of d, 4.6 mm is packed with particles having d, 15 pm, insertion of k z , 150 pL2, and D,,
ANALYTICAL CHEMISTRY, VOL. 54,
1.5 x mm2 s-l, into eq 36 yields t, > 50 s. That is, segmentation is advisable for reaction times of over 50 s, which corresponds to a bed reactor length of 150 mm. If the particle size is reduced from 16 to 10 pm, the critical residence time becomes 88 s, which corresponds to a 270-mm bed reactor length. As an alternative to the plate-height eq 30 Snyder and Kirkland (19) have suggested H / d p = ( u ~ , / D , ) ~ /+ ~ O.lud,/D,
(37)
Combining eq 17, 31, 33, and 37 and using 42 = 0.84, gives tr
’
6.2k2
md,24[ (44d,4/7r~d,~D,)~/~ + 0.1(44dP2/dr2D,)] (38)
Using eq 38 instead of 36-and selecting the same reactor dimensions as quoted above-segmented-iflow systems are now seen to be preferable to packed-bed reactors already for reaction times of over 1!3 s (dp 15 pm) and 36 s (dp 10 pm), respectively. The corresponding bed reactor lengths are 60 and 110 mm. In our laboratory, 100 mm X 4.6 mnn i.d. reactors were packed with 10- and 15-pm particles. The variance contribution due to these reactors was u2v,n.seg = 340 pL2 (ut = 1.1 s) and u2v,n.seg= 450 pL2 (ut = 1.45 s), respectively; the residence time under these conditions was about 40 s. Combining the reaction time of 40 s (which is larger than the 19 and 36 s calculated from eq 38) and u2v,n.Beg of about 400 pL2 (a value larger than the 150 pL2 contribution from the phase separator) indicates that, in our case, the use of eq 37 and 38 is more appropriate than that of eq 30 and 36. This is in agreement with Tijssen ( 2 ) , who stated that the empirical eq 30 IS probably too optimistic and that eq 37 shows a better fit with experimental data for d, < 20 ym. CONCLUSION In the present paper it has been demonstrated that with appropriately designed phase separators (and tee pieces) segmented-flow systems can become highly competitive with nonsegmented systems as postcolumn reactors even for short-i.e., less than 0.5-1 min-reaction times. The reported results, admittedly, relate to solvent-segmented systems only; it has experimentally been verified, however, that the various conclusions also hold true in the case of air segmentation (14, 20)* Decisions on the use o f a tubular segmented instead of a tubular nonsegmented reactor can be based on eq 27 (Figure 5) or eq 24 (Figure 4). The choice between a tubular segmented and a packed-bed reactor should be made by consulting eq 36 or, preferably, 38. Inspection of Figure 5 confirms the data previously reported by Tijssen ( 2 ) who stated that a good performance can be obtained by utilizing nonsegmented flow in narrow-bore capillaries of typically 20-80 pm i.d. It is likely, however, that fear of technical and experimental difficulties will prevent such narrow-bore reactors to become widely accepted in the immediate future and that the use of commercially available 0.3-0.8 mm i.d. capillaries will generally be preferred. Provided this is true, one can state that solvent (or gas)-segmented tubular reactors should be used for reaction times of over 15-20 s (cf. experiments discussed above and ref 14). For shorter reaction times, both according to ref 5 and 6 and the results presented in this paper, packed-bed reactors should be preferred, with nonsegmented tubular reactors coming in only for extremely rapid reactions ( t R of, typically, 0-5 s). For reasons of convenience, however, many workers will prefer the use of a tubular (segmented or nonsegmented) reactor to that of a packed-bed reactor (even for residence times of 5-20 s.
NO. 12, OCTOBER 1982
1937
For the rest, one should realize that there are situations in which the conditions, and not the time, of reaction determine the choice of reactor type. Generally speaking, the use of aggressive chemicals or high temperature will somewhat limit one’s choice in reactor design. Other examples are photochemical reactions (14),where packed-bed reactors are excluded, and extraction detection systems (Z), which make the use of a solvent-segmented system mandatory. Packedbed reactors, on the other hand, become of interest when the bed itself participates in the reaction, as e.g., in an enzyme (22) or a solid-phase reagent (23) reactor. ACKNOWLEDGMENT We thank I. D. Cockshott (I.C.I., Macclesfield, England) for sending us the phase separators depicted in Figure 3E,F (24) and C. de Jonge for typing the manuscript. GLOSSARY area of a peak in a segmented system, mm2 Aseg area of a peak in a nonsegmented system, mm2 An-seg helix diameter of the coil, mm dC internal diameter of a capillary, mm dt particle diameter, mm dP internal diameter of a bed reactor, mm dr diffusion coefficient, mm2 s-l Dm Dn Dean number, t internal porosity factor, 0.4 peak height of a signal in a segmented system, mm hseg peak height of a signal in a non-segmented system, hn-seg mm H plate height, mm dynamic viscosity (loW3P), g mm-l s-l f rrt4Lt/24Drn, mm3 s kl k2 u2v hase-sep? mm6 kz/xl, mm3 k3 K velocity profile factor L length of a reaction capillary, mm length of a bed reactor, mm Lr length of the connecting capillary, mm Lt x curvature ratio, dc/dt geometry factor, 10 A1 geometry factor, 18 A2 flow rate before segmentation, mm3 s-l 91 flow rate through flow cell after segmentation, mm3 42 S-1
43 44 Re
r rt P
sc u2v,cap.n.u2
u;:::::; :zv8inj 2V’n-seg
v,packed-bed u2v,phase-sep u2v,reactor u2seg fJ
t1
a
Vi
- +2 mm3 s-l flow rate of the segmentation liquid, mm3 Reynolds number, pud,/v internal radius of the capillary, mm internal radius of the connecting capillary, mm density, g mm-3 Schmidt number, v/pDm variance of reaction capillary of a nonsegmented system, mm6 variance of a connective capillary, mm6 variance of the flow cell, mm6 variance due to the injected plug, mm6 variance of a nonsegmented system, mm6 variance of the packed-bed reactor proper, mm6
variance of the phase-separatorltee piece combination, mm6 variance of the reactor in a segmented system, mm6 variance of a segmented system, mm6 residence or reaction time, s mean linear velocity mm s-l injection volume, mm3 LITERATURE CITED
Frei, R. W.; Scholten. A. H. M. T. J . Chromatogr. Sci. 1979, 77, 152. Tijssen, R. Anal. Chim. Acta 1980, 774, 71. Tijssen, R. Thesis, Delfl University of Technology, 1979. Deelder, R. S.;Kroll, M. G. F.; Beeren, A. J. B.; van den Berg, J. H. M. J . Chromatogr. 1978, 749, 669. (5) van den Berg, J. H. M., Deelder. R. S.;Egberink, N. G. M. Anal. C h h . Acta 1980, 1 7 4 , 91. (6) Huber, J. F. K.; Jonker, K. M.; Poppe, H. Anal. Chem. 1980, 52,2 . (1) (2) (3) (4)
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1938
(7) Hofmann, K.; Halasz, I. J . Chromatogr. 1979, 173, 211. (8) Hofmann, K.; Halasz, I. J . Chromatogr. lg80, 199, 3. (9) Synder, L. R.; Adler, H. J. Anal. Chem. 1976,48, 1017. (10) Snyder, L. R.; Adler, H. J. Anal. Chem. 1976,48, 1022. (11) Snyder, L. R. J. Chromatogr. 1976, 125, 287. (12) Huber, J. F. K.; Mulsman, J. A. R. J.; Meljers, C. A. M. J. Chromatogr. 1971, 62, 79. (13) Coq, B.; Cretler, G.; Rocca, J. L.; Porthault, M. J . Chromatogr. Sci. 1981, 19, 1. (14) Scholten, A. H. M. T.; Brlnkman, U. A. Th.; Frei, R. W. J. Chromatogr. 1981,205, 229. (15) Karlberg, B.; Thelander, S. Anal. Chim. Acta 1978, 9 8 , 1. (16) Bergamln, H.; Medeiros, I . X.; Reis, B. F.; Zagatto, E. A. G. Anal. Chlm. Acta 1978, 101, 9. (17) Nord, L.; Karlberg, B. Anal. Chim. Acta 1980, 718, 285.
(18) Karlberg, B.; Thelander, S. Anal. Chim. Acta 1980, 714, 129. Introduction to Modern Liquid Chromatography"; Wlley: New York, 1979. (20) Brlnkman, U. A. Th.; Welllng, P. L. M.; de Vries, G.; Scholten, A. H. M. T.; Frei, R. W. J. Chromatogr. 1981,217, 463. (21) Lawrence, J. F.; Brlnkman, U. A. Th.; Frei, R. W. J. Chromatogr. 1979, 185, 473. (22) Schlabach, T. D.; Regnler, F. J. Chromatogr. 1978, 158, 349. (23) Studebaker, J. F. J . Chromatogr. 1979, 185, 497. (24) Cockshott, I. D.; Payne, R.; Copsey, P. 8.research disclosure, Nov 1979,p 639.
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Received for review May 6,1981.Resubmitted May 18,1982. Accepted June 7, 1982.
Molten Organic Salt Phase for Gas-Liquid Chromatography Frank Pacholec, Hal T. Butler, and Colln
F. Poole"
Department of Chemistty, Wayne State University, Detroit, Michlgan 48202
The molten organlc salt ethylammonlum nltrate Is shown to be sultable for use as a statlonary phase In gas-liquid chromatography. I t has a usable temperature range of ~ 4 0 - 1 2 0 OC. I t Is shown to behave as a polar llquld wlth a stronger lnteractlon than Carbowax 20M for test compounds having large dlpole or hydrogen-bondlng functlonal groups. The chromatographicproperties of the molten salt are illustrated for the separatlon of alcohols and monofunctional benzene derlvatlves. Amlnes are not eluted from the column withln the accessible operatlng temperature range for the stationary phase.
Separations by gas-liquid chromatography occur due to differences in the residence time of the solutes in the stationary phase. All solutes have the same residence time in the mobile phase. Consequently, retention, selectivity, and resolution are adjusted by changing the stationary phase or the experimental operating conditions, as appropriate, in gas-liquid chromatography. This realization has led to the description of numerous liquid phases for general use in gas-liquid chromatography (I). Indeed, the expression "stationary phase pollution" has been coined to describe the substantial redundancy that exists in the market place for the sale of liquid phases having essentially identical properties as well as the tendency of some vendors to repackage common liquid phases under their own brand name (2-4). In recent years, this situation has eased somewhat due to the general acceptance of the Rohrschneider/McReynolds schemes for stationary phase characterization and their widespread publication in the scientific and trade literature (1). In light of the large number of stationary phases already available, there would seem to be only two reasons for the introduction of further phases. Firstly, a need exists for thermally stable polar reference stationary phases. As most common liquid phases are polymeric materials defined in terms of an average molecular weight, they have properties which may vary from batch to batch. Further composition changes may occur due to the selective loss of low-molecular-weight oligomers during column conditioning. This problem has been addressed by Kovats, who prepared a synthetic hydrocarbon (24,24-diethyl-19,29-dioctadecyl-
hepatetracontane, CS7Hl7,Jfor use as a nonpolar reference stationary phase (5, 6). We have made similar attempts to prepare polar reference stationary phases having a mphenylene oxide backbone substituted with polar functional groups (7). These phases are characterized by a clearly defiied chemical structure having physical and chromatographic properties independent of the method of synthesis. The second reason for introducing a new stationary phase would be to provide some unique selectivity advantage over available materials. For example, the groups of Bayer (8)and Verzele (9) have described the preparation of stationary phases with chiral centers for the separation of optical isomers. In general, selectivity is mainly a property of the polar interactions of the stationary phase with the solute and is governed by the type and concentration of functional groups present in the phase. The majority of polar stationary phases contain cyano, nitro, amine, or amide functional groups which influence retention largely through dipole and hydrogen bonding interactions. In this paper we wish to describe our work using a molten organic salt as a novel stationary phase containing ionic groups. This material was investigated as its ionic character is likely to promote selective interactions with polar solutes which differ in both magnitude and type from those obtained with conventional phases. The use of salt systems as stationary phases in gas chromatography is not a new concept. Saturated solutions of silver nitrate in ethylene glycol, glycerol, and benzyl cyanide have been used as selective phases for the separation of unsaturated compounds by charge transfer interactions (10-12). Fused inorganic salt systems have been used to separate mixtures of various inorganic compounds (1,13,14). In these systems, separations were achieved by selective complexation of the solutes within the molten salt stationary phase. These inorganic melts are generally unsuitable for the separation of organic compounds due to the poor solubility of the organic solutes in the inorganic salt systems. It is very likely that organic solutes would be much more soluble in a molten organic salt so that partitioning between the stationary and mobile phases would be possible. This partitioning, in conjunction with any complexation/ionic interactions which may occur between the organic solute and the molten salt stationary phase should lead to some intriguing separation possibilities. To test this hypothesis, ethylammonium nitrate
0003-2700/82/0354-1938$01.25/00 1982 American Chemical Society
