disulfide was examined for purity chromatographically and was observed to be 95 to 99% pure. At the same time, it was evident that peak 20 was not being produced in the instrument from the disulfide because none was present in these purity runs. I t is suspected that the diallyl disulfide disproportionates to a small extent upon standing, since both the trisulfide and monosulfide were observed in appro\imately equimolar quantities in the standard. The purified diallyl disulfide was used for the previously mentioned trisulfide synthesis, and the main peak observed after this reaction compared with peak 20 in both retention time and 9 value. Peak 21. Peak 21 was tentatively identified as an unsymmetrical, branched, 11 carbon disulfide. Here the 9 value of 3.2 was the main factor in the identification, for it was assumed that this was a sulfur compound. With the 3.2 value, it corresponded to a saturated disulfide, and from the log plot of n saturated disulfides it was estimated to consist of a total of 11 carbons. DISCUSSION
Katural products will frequently yield a sequence of peaks in which there will be some flame peaks and some
electron capture peaks which may not exactly line up in time. Even though the peaks are within a fraction of a minute from alignment, they will necessarily represent entirely different compounds. This has been repeatedly proved by injecting larger samples until the missing flame peak appeared. The similarity of garlic to onion is interesting in that no ethyl sulfides have been detected in either, contrary to Kertheim (11) and Semmler ( 9 ) , who reported diethyl disulfide in garlic. Whereas methyl and propyl sulfides are the principal constituents in onion, (unpublished data), garlic sulfides are mainly methyl and allyl, as reported also by Xatsukawa et al. (8). Of the 18 peaks seen from garlic on the electron capture channel, nine are represented as the mono-, di- and trisulfides of methyl-methyl, methylallyl, and allyl-allyl. The only other peak identified, an eatremely minor component, is methyl propyl trisulfide. ACKNOWLEDGMENT
The authors thank J. F. Carson and F. F. Wong, USDh, Albany, Calif., for their helpful advice and for supplying certain standards. Credit is also due E. M. Taft and J. E. Booker, Wilkens Instrument &: Research, Inc., for
preparative scale runs, and Philip Hinstridge, F M C , Siagara Division, Richmond, Calif., for micro sulfur analyses. LITERATURE CITED
(1) Amy, J. W., Dimick, K. P., 14th
Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1963. (2) Carson, J. F., Wong, F. F., J . Agr. Food Chem. 9, 140 (1961). (3) Carson, J. F., Wong, F. F., J . Org. Chem. 2 4 , 175 (1959). (4) Cavillito, C. J., Bailey, J. H., J . Am. Chem. Soc. 66, 1950 (1944). (5) Fujiwara, Yoshiniura, M.,Tsuno, S., J . Riochem. ( J a p a n ) 4 2 , 591 (1955). (6) Hartmann, H., Oaks, I). XI,,Dimick, K. P., 145th Meeting, ACS, Sew York, K. Y., September 1963. (7) Lovelock, J. F., ASAL.CHEM.3 5 , 474 i1963). \ - - - - ,
(8) Matsukawa, T., et al., Science 118, 325 (1953). 19) Semmler. F. W.,Arch. Pharm. 2 3 0 . ' 424 (1892)'. (10) Stoll. A , . Seebeck. E.. Helv. Chim.
Smyth, C. P., 436 (1950). RECEIVED for review February 13, 1964. Accepted April 30, 1964. Presented at the Second International Symposium on Gas Chromatography, Houston, Texas, March 23-26, 1964.
Applications of Isotopic Exchange in Gas Chromatography JACOB TADMOR Radiochemistry Department, Israel Atomic Energy Commission, Soreq Reseaich Establishment, Yavne (Israel)
b Isotopic exchange in gas chromatography was studied and applied to labelling of inorganic compounds, analytical determination of inorganic compounds separated by gas chromatography, and a study of the interaction between the solid stationary phase and the solute in gas liquid chromatography. By gas chromatography isotopic exchange, the use of a radioactivity detector is made possible, without the introduction of radioactive metal compounds as starting materials. GeClr, SnCIr, AsC13, and FeC13 were labelled and determined by using C136labelled Sil-0-Cel as the solid stationary phase. Gas liquid chromatography experiments showed that even after coating the radioisotope labelled solid stationary phase with a thin liquid layer, isotope exchange still occurs between the C P sorbed on the solid phase and the gaseous inorganic compound sample, indicating that the solid support is
not always inert. For a given surface area of the solid support both the isotopic exchange and the number of theoretical plates increase with the decrease of its specific porosity and tortuosity coefficient. More highly polar liquids reduced the isotopic exchange to a greater extent than liquids of low polarity.
T
H E S1:PARATION POTEXTIALITIES Of
the gas chromatography (GC) method and the dependence of the heterogeneous isotope exchange on diffusion (21) make this method attractive for the study of heterogeneous isotope exchange reactions and their application to: labelling of inorganic compounds; analytical determination of inorganic compounds separated by G C ; study of the interaction between the solid .tatlonary p h a v and the solute in gab liquid cht~oniatogialihy (GLC) ; and
study of the column efficiency, as a function of different column variables. The procedure is based on labelling the stationary phase with a radioactive ion common to the metal compounds to be studied, followed by GC separation and isotopic exchange of the compounds, and identification of the components of the effluent by a radioactivity detector. The interest of the method lies in the fact that by a single operation both GC separation of pure compounds and their labelling with radioisotopes is obtained. Thus the method makes it possible to label and also determine inorganic compounds, or to study different GC variables, without the need for radioactive metal com1)ounds a' starting materials. The potentialities of GC in labelling compounds by i,-otopir exchange was indicated by Srhniidt et al. (18) for organic compound-, and by the author (82) for inorganic compound-. VOL. 36, N O . 8 , JULY 1964
e
1565
EXPERIMENTAL
The GC apparatus consisted of a glass column and furnace, described elsewhere (21). The stationary phase consisted of Sil-0-Cel insulating fire brick (Johns-Manville) and -4lundum (Korton Co.) of different mesh sizes, either uncoated or coated with a liquid layer. For isotoiie exchange studies between the chlorine of a metal chloride and that sorbed on the uncoat'ed solid phase, the latter was labelled with C13fiby either of the following methods: .\ solution of hydrochloric acid labelled with C136 (Radiochemical Centre, .Imersham) was injected into the chromatographic column containing the solid phase. The unsorbed hydrochloric acid was eluted using nitrogen as carrier gas, a t the temperature to be used in the subsequent experiments. The elution was followed with a continuous beta flow counter or by collecting fractions in ethanol and counting with a liquid beta counter; or -4 diluted solution of hydrochloric was added to the acid labelled with solid in small portions while heating and gently stirring the slurry. The material was then dried for several hours a t l l O o , sieved again to the initial mesh size, and introduced as a thin layer into a large diameter glass column. The unsorbed hydrochloric acid was eluted as above and the labelled solid support was then introduced into the column. The sample of met'al chloride was injected into the column and the isotopic exchange between the chlorine of the metal chlorides and the C136 retained on the stationary phase, as well as concentration profiles, were determined by counting the activity of the eluted matmerialwith a 20th Century Elect,ronic liquid beta counter (type M6) connected to an iltomic Scaler, Model 1091. Counting of the activity of the effluent with a liquid counter was made possible by washout of the gaseous effluent with a solvent stream ( 2 1 ) . In effluent fractions which showed high activities of chlorine the relevant cation of the metal chloride was identified by neutron activation analysis and by comparison of the retention times with those found in the GC of compounds labelled with radioactive cations. Samples of hC13 (British Drug House, BDH) GeCI4 (Johnson Matthey) SnC14, and FeC13 (BDH) were used in the experiments. The hsC& and GeC14 were injected into t8he column in the form of the compounds themselves or as a solution in xylene (-lgan, Tel-Aviv) or alcohol (.lssis, Ramat Gan, Israel), while FeC13 was injected only in the form of its solution in xylene or alcohol. Nitrogen was used as carrier gas a t flow rates of 5 to 40 ml./minute. The temperature of the experiments varied between 60" and 300" C. Some experiments were performed to study isotope exchange between the iron of a FeC13 sample and that of a FeS (Baker) solid stationary phase, or that contained in Sil-0-Cel insulating brick (0.78%). The solid phase was ~
1566
ANALYTICAL CHEMISTRY
I
I
temperature
1000 I
I
I
I
I
I
FeCL3
.-
C
0 c
e
c
c Q) u c 0
0
0
2
3
4
14
16
18
20
Time (minutes) Figure 1. Separation of germanium, tin, arsenic, and iron by GC of their chlorides and their determination by Cl36 labelling Column ( 1 60 cm.): Sil-0-Cel gas flow: 10 ml./minute
brick (-30+50 mesh); temperature:
labelled with radioactive Fe59 by irradiation in the neutron flux of the IRR-1 reactor. The temperature used in these experiments varied between 250" and 420'. The number of theroretical plates, n, in the column, was calculated from the relation n = (4d/zu)*,where d is the distance in millimeters on the time axis, from injection of the sample up to appearance of the maximum of its peak, and w is the distance in millimeters on the time axis, between the points of intersection of this axis with tangents to the slopes of the effluent peak. More details on the experimental procedure are given elsewhere ( 2 3 ) .
'
RESULTS A N D DISCUSSION
The experiments reveal that within the limits of experimental error the two methods described above for labelling the solid stationary phase with C136 give the same isotopic exchange of the metal halides, Labelling of Inorganic Compounds. Samples of 1.4 fig. t o 5 mg. of AsC13or GeC14 either alone or dissolved in 10 pl. of xylene, were injected into the chromatographic column containing Sil-0-Cel labelled with C136. The GC experimental conditions were identical with those described in Figure 2 of reference (20). Well defined peaks of the chlorides of different specific activities were obtained. The retention times of the chlorides were identical with those obtained previously (20) where As76 and Ge7' labelled halides were used: 30 seconds for GeCll and 3 minutes for AsC19.
100'
to
300'; carrier
The specific activity of the metal chlorides was computed assuming a recovery yield of 65% for GeC14 and 78y0 for AsC13, as determined in previous experiments. I t is assumed that the unrecovered metal chlorides are hydrolyzed, and that the hydrolysis products do not participate in the isotope exchange process. Two preliminary experiments were performed in order to confirm that the eluted activity is that of C136 labelled metal halides and not of a pulse of HC136eluted on injection of xylene or by the overpressure caused by injection of the sample. In one experiment 10 p l . of xylene, and in a second experiment 10 pl. of air only, were injected into the column under the same experimental conditions as used for the metal halides. No C136 activity was detected in the effluent obtained in these experiments. Further confirmation of the presence of germanium and arsenic in the effluent fractions corresponding to high 0 activity peaks, was obtained by neutron activation analysis of the relevant fractions. Labelling of FeCh with radioactive FeSg was investigated by GC of the compound on neutron irradiated FeS or Sil-0-Cel containing 0,78Y0 Fe, as the solid stationary phase. The solid was irradiated for one hour in the IRR-1 in a neutron flux of 2 X 10'3 neutrons/ second. Samples of 2 mg. of FeC& in a solution of 20 PI. of xylene or ethyl alcohol were used in the experiments. The activity of fractions of the effluent was counted with a well type scintillation gamma detector. Isotope
exchange between the iron of FeC13 and that of the solid stationary phase was calculated from these results and concmtration profiles were drawn. No activity was detected in the effluent when FebgSwas used as the solid stationary phase. When irradiated Sil-0-Cel firebrick was used as the solid stationary phase, Fe59C13of low specific activity in a well defined peak was obtained. It is expected that Fej9C13 of higher specific activity could be obtained by the use of longer columns, larger amounts of solid support, and longer irradiation of the solid stationary phase in the neutron flux. Analytical Determination of Inorganic Compounds Separated by GC. Amounts of the order of gram of BrE2 labelled organic materials have been detected by Evans and Killard ( 9 ) using a radioactivity detector connected to a GC apparatus. The high sensitivity of the radioactivity detector, together with fast separation and labelling of inorganic compounds by isotope exchange in GC, make this a useful method for separating and determining inorganic compounds in a mixture. For example, by labelling inorganic chlorides with ClS6, amounts of the order of gram can be detected by GC. I n a typical experiment the method was applied to a mixture of Ak3c13, GeCl4, FeC13, and SnC14, using a C136 labelled Sil-0-Cel insulating firebrick column. The solid stationary phase was labelled by addition of a HC136solution to the solid in small portions, and elution of the unsorbed hydrochloric acid a t 300'. In the first experiment performed a t 100' using a column of 160 cm. in length and nitrogen as carrier gas a t a flow rate of 10 ml.jminute, no FeC13 was eluted from the column. The temperature of the column in the subsequent experiments was therefore kept a t 100' up to the end of elution of GeC14, SnC14. and ,IsC13, and was then raised to 300"; a t this temperature FeC13was removed from the column. Figure 1 shows the GC separation of the chlorides and their determination by Cld6labelling. The peaks were identified by comparing the retention times with standards. Separation between GeC14, SnC14,and ;\sC13 is seen to be incomplete under the conditions of the experiment. The following changes were therefore made: GLC with a thin liquid stationary phase was used instead of gas solid chromatography (GSC). The ClT6 labelled Sil-0-Cel was coated with 1% 1%. w. nitrobenzene. I t was expected that the liquid film would improve the separation between the chlorides, while labelling of the chlorides by isotope
Time (minutes) Figure 2. Separation of germanium, tin, and arsenic by GLC of their chlorides and their determination by C P labelling Column (240 cm.): Sil-0-Cel brick c o a t e d with 1 % corrier g a s now: 5 mi. /minute.
exchange with the C136 of the solid phase would still occur by diffusion of the chlorides through the thin liquid phase. .Ilonger column (240 cm.) and a lower carrier gas flow rate (5 ml./minute) was used in the GLC experiments. The new experimental conditions gave good separation between the chlorides, as seen in Figure 2, while isotope exchange occurred as before. The reproducibility of the quantitative determination of the chlorides was checked by weighing the recorder paper corresponding to each peak. The height and area of the peaks were found to decrease in consecutive experiments performed on the same column because of depletion of Cl36 in the column. Experiments showed that this could be corrected either by calculation of the depletion of Cl36 in the column, or by addition of a known amount of a standard, and normalization. The success in converting almost any inorganic substance into its chloride, reported recently ( I d ) , stimulates further work on the development of the G C isotopic exchange method as a general method for the separation and determination of inorganic materials. Interaction between Solid Stationary P h a s e and Solute in GLC, and Column Efficiency. T h e experiments described in the previous section showed t h a t even after coating the labelled solid stationary phase with a thin liquid layer, isotope exchange still occurred between the ClS6sorbed on the solid phase and the gaseous inorganic compound. Albsence of C136 in the effluent prior to injection of the inorganic chloride, points to the lack of diffusion of HClS6from the solid phase into the liquid or gaseous phase. The isotope exchange between the inorganic chloride and the C136 sorbed on the solid phase shows that the solid support in GLC is not always inert, as assumed by some authors ( 2 , f7),but interacts
w./w.
nitrobenzene; temperature; 100' C.
with the solute sample by adsorbing it, thus influencing its retention time. James and Martin ( I S ) , and Eggertsen and Knight ( 7 ) also found a strong interaction between the solid support and the gaseous sample in GLC, shown by changes in the retention time of the sample as a function of the thickness of the liquid layer. The interaction between the solute sample in GLC and the radioisotope sorbed on the solid support can be studied by the isotopic exchange between the two phases, using a radioactivity counter, a highly sensitive detector. Thus even weak interactionse.g., adsorption-which might remain undetected by other methods can be detected by the GC isotopic exchange method. I n our experiments the interaction of the solid support and the gaseous sample was studied as a function of the particle size, surface area, and porosity of the solid support, and the amount and polarity of the liquid phase layer. The porosity of the different solid supports used in the experiments was measured by Innes' method (12) while their surface area was measured by a modified B E T method (8,16). Table I shows the specific porosity and surface area of the different solid supports.
Table I. Specific Porosity and Surface Area of Solid Supports
Solid material
Surface Porosity area (ml./ (meters2/ Mesh size gram) gram)
Sil-0-Cel -10+12 Sil-0-Cel -30+50 Sil-0-Cel -70+100 Sil-0-Cel - 100+ 140 Alundum -100+140
1.11 0.91 0.51 0.42 0.05
VOl. 36, NO. 8, JULY 1964
2.02.8
4.1
5,O 0.86
1567
as shown in Van Deemter's relation for the H E T P (4). r\ similar influence of the decrease of solid particle size on column efficiency was also found by other authors (4, 5 ) .
Particle Size (US. sieve) -7OtlOO -1OOt140
-10+12-30+50
8
I
I
I
I
I
Influence of Porosity of Support.
To investigate the influence of the porosity of the solid support on column efficiency and isotopic exchange, a number of experiments were performed, in which the columns contained solid supports of identical particle size but different specific porosity. For this purpose Sil-0-Cel of - 10+12 mesh particle size was synthesized by sticking together - 100+140 mesh Sil-0-Cel with crystal cement, and then sieving the synthesized material to -10+12 mesh size. The specific porosity of this new material was measured and found to be 0.50 ml./gram and the surface area 2.9 meters*/gram. Comparison of these data with those given in Table I for the natural - 10+12 mesh Sil-0-Cel shows that in spite of its lower porosity, the synthesized Sil-0-Cel has a higher specific surface area than that of the natural material. This difference might be explained by the fact which was also visually observed that some of the synthesized - 10+12 mesh particles had individual particles of - 100+ 140 mesh size adhering to their surface. The isotopic exchange properties of the synthesized material were compared with those of the natural Sil-0Cel of -10+12 and -100+140 mesh particle size. The comparison was made in static experiments performed in the apparatus shown in Figure 5.
GeCl,
SnC1,
1 0
I
I
1
I
20 30 Surface area (m?
IO
50
40
Figure 3. GC isotopic exchange as function of particle size and surface area of solid support Column:
5 grams of uncoated Sil-0-Cel
brick
After labelling the solid support with C138,it was coated with various amounts of liquid materials. The following liquid coatings were used (numbers in parentheses indicate the dipole moment of the material in Debye units) : n-decane (0) (BDH), n-butanol (1.17) (Baker), glycol (2.28) (Fluka), and nitrobenzene (4.27) (May and Baker). The inorganic chlorides used in the experiments were (Debye units again used for dipole moments): GeC14 (0)) SnCll (0), PC13 (0.78) (Fluka), and ASC13 (1.59). When isotopic exchange of different chlorides was compared, the isotopic exchange per atom of chlorine was considered. When different solid supports were used, isotopic exchange was determined for a normalized specific activity of the support. Where not otherwise specified, the temperature of the experiments was 60" C., nitrogen was used as carrier gas a t a flow rate of 20 ml./minute, and the amount of solid support used was 5 grams. The length of the column was 55 em. + 1% (except when Alundum was used).
the solid support is obtained by decreasing its particle size, the number of theoretical plates and the corresponding isotopic exchange reach a maximum, after which they decrease. This maximum occurs because the decrease in particle size and the corresponding increase in packing and tortuosity factors of the column have opposing influences on the height of the equivalent theoretical plate (HETP),
Influence of Particle Size and Surface Area of Support. Figure 3 shows
a
the G C isotopic exchange between the different inorganic chlorides and the C136sorbed on the uncoated solid support, as a function of particle size and Eurface area. Comparison with Figure 4 shows t h a t the isotopic exchange iq approximately proportional to the numlicr of theoretical plates in the coliimn. W h m the increase in surface area of 1568
ANALYTICAL CHEMISTRY
Particle
150
-10+12 -30+50
w
sire (U.S.sieve) -70+100-100+140 I 1
el
a
c
0
- 100
.-0u c
E
0
a
f
r 0 L
a
50
s
z
0
IO
20
30
40
Surface area (m?I Number of theoretical plates as function of particle size and surface area Figure 4. of solid support Column:
5 grams of uncoated Sil-0-Cel
brick
Table II.
Isotopic Exchange between AsC13 and C136-Labeled Static Experiments
Isotopic Amount of Surface exchange solid area Volume of (arbitrary (grams) (meters2) pores (ml.) units)
Solid material Natural Sil-0-Cel - 10+ 12 mesh Synthetic Sil-0-Cel - 1 0 f 1 2 mesh Natural Sil-0-Cel -loo+ 140 mesh
Table 111.
Figure 5. Apparatus for static isotopic exchange experiments (1) (2) (3) (4) (5) (6)
Solvent stream Nitrogen stream To vacuum, and sample collector Manometer Injection port Thermometer
The whole apparatus up to the solvent stream point was heated to the operating temperature, which was kept constant to w thin = t 2 O . The experiments were done with various amounts of C136labelled Sil-0Cel of natural - 10+ 12 and - l00f 140 mesh size and synthetic - l o + 12 mesh size, of different porosity but equal total BET surface area. The material was spread on the bottom of the apparatus in a n approximately mono-particular layer. After filling the apparatus with dry nitrogen and heating it to 150" for 1 hour, 500 pl. of hSCl3 were injected through the injection port. Uniform volume samples, as measured by the container, .4, mere withdrawn after various periods of time (1, 2, 5 , 10, 30 minutes) by connecting the container A to vacuum and the sample collector. Volume corrections were made by measuring the pressure in the apparatus before each sample withdrawal. An ethyl alcohol solvent stream was used to \\ash out the IsC13 ahich might have been absorbed on the tubes. The isotopic exchange was determined by measuring the fl activity of the collected hsC13 samples, using a p liquid counter. The results of these static experiments showed that the isotopic exchange was complete one minute after injection of the AsC13 Table I1 gives the isotopic exchange as measured one minute after injection of hsCls. The results given in Table I1 show that the natural and synthetic labelled material give the same isotopic exchange per unit area of solid material, indicating that there is no influence from the
Sil-0-Cel in
2.5 1.72 1.0
5 5 5
2.8 0.86 0.4
3.7 3.6 3.5
Isotopic Exchange and Column Efficiency as a Function of Porosity of the Solid Support
Solid support Xatural Sil-0-Cel - 10+ 12 mesh Synthetic Sil-0-Cel - 10+ 12 mesh Natural Sil-0-Cel -10+12 mesh Synthetic Sil-0-Cel - 10+ 12 mesh
Weight of solid (grams)
Surface area (meters2)
Porosity (ml.)
Isotopic Iiumber exchange of theoretper unit ical area plates (arbitrary) per HETP units) meters2 (cm.)
5
10
5.5
2.8
3.3
1.7
5
14.5
2.5
3.7
4.5
0.9
2.2
2.8
2.0
3.0
3.9
1.0
10
20
10
29
crystal cement. They also indicate that in the static experiments the porosity of the solid material does not influence the isotopic exchange, the only important parameter being its surface area. The influence of the porosity of the solid support on column efficiency and isotopic exchange in GC, was studied in a series of experiments using equal amounts of solid support in equal lengths of column. The same particle size of solid was used and the isotopic
11
5.0
exchange and column efficiency were computed per unit solid area, so that the porosity of the solid is the only variable. The results given in Table I11 are the average of the values found in four experiments. For a given surface area of the solid support, both the number of theoretical plates and the isotopic exchange increase with decrease of porosity. These experiments indicate that in porous material only a part of the surface area of the solid, as measured by the B E T
A "Natural" Sit-0-Cel -10*12 mesh (bgrams)
a
0 "Synthetic"Sil-O-Cel -tot12 mesh (5groms)
P 5
0
0
1
0
5
IO
15
20
25
30
35
40
Carrier g a l flow velocity (mlhinute) Figure 6. GC isotopic exchange as a function of carrier gas flow rate and porosity of solid support Gaseous sample: 2 0 PI.of AsClg ( 1 ) surface area 14.5 meters2; volume of pores 2.5 cc. ( 2 ) surface area 10 meters*) volume of pores 5.5 cc.
VOL. 3 6 , NO. 8, JULY 1964
1569
500 A "Natural" Sit-0-Cel -lot12 mesh (5gramr)
o
"Synthetic"Sil-0-Cel -10 +I2 mesh (5groms)
3
450
-
400
-
350
-
300
-
T
0
Y
5 250 -0
0
200
-
150
-
I t
0
5
IO 15 20 25 30 35 Carrier gas flow velocity (ml/minute)
40
Figure 7. Number of theoretical plates as a function of carrier gas flow rate and porosity of solid support Gaseous sample:
2 0 pl. of AsCla ( 1 ) surface a r e a 14.5 meterr'i vol ume'of pores 2.5 cc. ( 2 ) surface a r e a 10 meterra; volume of Dares 5.5 cc.
method, participates in the GC process. The partial participation of the surface of porous materials in the GC process was assumed to be caused by the increase in the tortuosity factor, y , of the column, due to the pore channels, and by the consequent de-
Table IV. Ratios of Exchange and Number of Theoretical Plates in Different Solid Supports" as a Function of Carrier Flow Rate
1.19 1 13 1.30 1.34 1.40 1.37
5
15 20 35 40
5 5 grams Sil-0-Cell -10+12 mesh, a ) synthetic, surface area 14.5 metersz, volume of pores 2.5 cc., b) natural, surface area 10 meters%,volume of pores 5.5 cc.
20 30 4 0 50 v (ml./minutr) Figure 8. Experimental and calculated values of HETP
V Sit-0 -Cel
1.22 1.28 1.30 1.38 1.43 1.66
-4
t
o
crease in column efficiency, as shown in Van Deemter's relation (4). Since the hydraulic resistance is a function of the square of the flow rate of the gas, the validity of this assumption could be checked by experiments in which different carrier gas flow rates were used in columns of equal length, containing solid supports of different porosity. If the assumption is valid, the difference in isotopic exchange and number of theoretical plates in columns containing solids of different porosity should diminish at lower carrier gas
Gaseous sample: 20 pl. of AsC13 Carrier Ratio of: flow Number of rate (ml./ theoretical minute) Exchange plates 2
loo 50
Sit-0-Cel
*------S i l - 0 - C e l -
k.4 Si1 - 0 Cel
6-4 Atundum
IO
Column: Sil-0-Cel (- 1 O+ 1 2 mesh); gaseous sample 2 0 pl. of ASCIS; temperature: 60' The line represents the calculated values while the points, the experimental values
velocities. Figures 6 and 7 show that this is indeed the case. The values given in these figures are averages of six experiments. The data plotted in Figures 6 and 7 are also presented in Table IV which shows the ratios of exchange and number of theoretical plates in two different columns, as a function of carrier flow rate.
-10 +I2 (porosity - 30 t 50 (porosity -70 +IO0 (porosity -100+140 (porosity -100+I40(porosity
11 .1 cdgram)
0.91cdgmm) 0.51 cc./gmm) 0.42 cc./gram) 0.05 cc./gram)
x
Table V. Experimental and Calculated Values of HETP
v!ml./
minute) 5 in 15 20 25 30 35 40
1570
0
100H,
(em.)
100H, (cm.)
( H , - H,)
100
416 208 204 191 169 214 212 272
414 223 183 178 189 208 230 252
+2 -15 +21 13 -20 +6 - 18 20
ANALYTICAL CHEMISTRY
+
+
Amount of liquid phase (YOw./w.) Figure 9. GC isotopic exchange as a function of the amount of liquid phase and porosity of solid support liquid phose:
n-decane;
salute sample:
2 0 pl. of GeClr
I4O I20
t
x1
c
- 100 .6 80 0 0
c
W
f
% 60
k?
n
5
z
40 20
c I
0
Sil-0-Cel
-tot12 (porosity 1.11 cc./grom) -30+50(porosity 0.91 cc./grom)
e-------* Sil-0-Cel
-70+100(porosity 0.51cc./grom)
+4
I
2
-
A-.+
Si1 -0 Cel -100+140( porosity 0.42 cc./grom)
&---A
Alundum
1 4
I
6
-100+140(porosity 0.05 cc./gmm)
I
I
I
8
I I I I IO 12 14 16 Amount of liquid phose (% w./w)
18
20
Figure 10. Number of theoretical plates as a function of the amount of liquid phase and porosity of solid support Liquid phase:
20 pl. of GeCl,
n-decane; solute sample:
Further confirmation of the effect of lowering the carrier gas flow rate is provided by the static experiments, the results of which are given in Table 11. I n these zero carrier gas flow rate experiments, the ratio of exchange when using the synthesized and natural Sil-0Cel was found to be 0.97. Thus the influence of the porosity of the solid support on the isotopic exchange and column efficiency is caused by the change in the tortuosity factor, y, due to the channeling within the pores. These experiments also indicated that the tortuosity factor, y, is not constant as assumed in Van Deemter's relation, but is dependent on the flow rate of the carrier gas. The results shown in Table I11 and Figures 6 and 7 also indicate that for a given surface area of the solid support and in the range of optimal carrier gas flow rates, the use of a low porosity solid increases the column efficiency. Similar conclusions were also reached by De Wet, Haarhoff, and Pretorius (6) and Eggertsen and Knight ( 7 ) . I n the investigation of the kinetics of the reaction between carbon and COz ( I O ) , it was also found that the rate of reaction on larger particles of carbon is slower, as a result of the lower diffusion rate of CO, in the pores of the larger particles. Theoretical Evaluation of Results. An a t t e m p t was also made to fit the experimental d a t a found for the HETP in different experiments t o a simplified form of Van Deemter's equation relating the HETP to t h e carrier gas flow rate. The Van Deemter equation expressing this relationship for GSC can be written in the following simplified form:
where H is the height equivalent of a theoretical plate, v the carrier gas flow rate and ii, B , and C are terms characterizing, respectively, the eddy diffusion, molecular diffusion, and the rate of transfer of the volatile material between the gas and adsorbed phase. The values of the experimental H ( H J were calculated from the number of theoretical plates as found in the
experiments, taking into account the corresponding length of column which was 550 mm. for 5 grams of Sil-0-Cel (-10+12 mesh). Considering the experimental values of H and the corresponding carrier gas flow rate, the values of A , B, and C were computed by the method of least squares. The values found were: A = - 6.8 X 10-l; B = 2.2 X 10; and C = 6.7 X 10-2 With these values of A , B , and C , the values of the calculated H (H,) were computed using Equation 1. Table V and Figure 8 show the values of H, and H , for different carrier gas flow rates. Mean values of H were used for experiments performed a t the same carrier gas flow rates. Good agreement (standard deviation &8'%) is found between the experimental values of H and the values calculated from the simplified Van Deemter's Equation 1. The physical significance of the A , B , and C terms in this relation implies that they should have positive values. However, the value computed for the A term from our experimental H values was found to be negative. Similar results were also obtained by others ( I , 15). An explanation of the negative value of A might be found if the tortuosity factor, y, one of the parameters of Van Deemter's relation for H E T P , is not considered to be constant but dependent on the flow rate of the carrier gas, as assumed previously in the present work, and expressed as a function
L.
c
&
Q
a a
0
U b c 0
I
100
C
a
80 0
P
C
{
b
-
o------O n decane (v=O) o- 4 n - butanol ( p = 1.17 x IO-" e.su.) +.-A nitrobenzene (u = 4.2 IO-'* asu.)
60
Y
a3 'y.
0
$
40
0)
g 20
r
2
I
I
8 IO 12 14 16 Amount of liquid phase ( % w/m)
18
I
W
0
2
4
6
I
I 20
Figure 1 1 . GC isotopic exchange as a function of the amount and polarity of liquid phase Solid support:
SII-0-Cel
(-30+50 mesh)) solute sample: 20 pi. of ASCI* VOL. 36, NO. 8, JULY 1964
1571
of that rate-e.g., a linear or parabolic function. For example if y is assumed to be a linear function of the carrier gas flow rate, expressed by y=m+nv
150
(2) tn
then Equation 1 may be written in the following form:
0 +
-n yj 100 .0
0
+
t0!
or
0)
f y.
0
(4) where A I
=
A
+ Bn, and BI
=
L
$ 50
Bm.
3
If this assumption is valid, the values computed previously for d and B are actually the values of A I and B1. The negative value of .II might then be explained by the contribution of a possible negative value of n. The negative value of A might also be explained by assuming another function for y . Influence of Liquid Phase. The influence of the amount of different liquid layers on the intrraction between the solid support and the gaseous sample was investigated. Figure 9 shows the results of experiments in which GeCl, was chromatographed on columns containing cl38 labelled solid sup1)orts of different particle size and porosity, coated with various amounts of n-decane. The isotopic exchange in these experiments is expressed as a percentage of that obtained with the same solid support, not coated with the liquid layer. To diminish to a given degree the interaction between solid support and gaseous sample-i.e., the isotopic exchange-a larger amount of liquid phase is necessary when the solid is of higher porosity. For instance, in order to diminish the isotopic exchange to 10% of that found using an uncoated solid, 8% wv./w.of liquid phase was necessary for the solid of 1.11-ml./gram porosity,
Table VI.
( yc w./w.)
0 0.5 1 1.5 2 3 4
1572
I
I
2
4
n-decane
&-.-A
nitrobenzene (p.4.27 x
I
I
8
IO
e.s.u.1
I I I I 14 16 18 20 Amount of liquid phase ('70w./w.) Number of theoretical plates as a function of the amount and
I 6
I 12
Figure 12. polarity of liquid phase Solid support:
Sil-0-Cel
(-30+50 mesh); solute sample; 20 pl. of AsCla
while only 4y0 was necessary for the solid of 0.51-ml./gram porosity. The differences between the various Sil-0-Cel solid supports are accentuated when small amounts of liquid layer coatings are used. This accentuated difference might be explained by the assumption that the liquid first fills the capillary pores; therefore, with a solid of larger porosity coated with small amounts of liquid, some of the surface area may remain uncoated, and a stronger interaction between the solid and gaseous sample is found. Confirmat'on of this assumption can be found in the experiments investigating the reproducibility of the isotopic exchange using Sil-0-Cel of different porosity, and various amounts of liquid phase (Table VI). Table VI shows that with low
n-decane liquid phase. solute sample: 20 pl. of GeClr Isotopic exchange ( % of that found on uncoated solid) Porosity of Sil-0-Cel Porosity of Sil-0-Cel 0.42 ml./gram 1.11 ml./gram Experiment I Experiment I1 Experiment I Experiment I1 100 73.3 65.3 55.1 46.9 34.2
92.5 81.0 82.7 67.3 38.9 29.8
7.9
8.1
ANALYTICAL CHEMISTRY
(~"0)
n- butanol ( p = l . l 7 x 10-'ee.s.u.)
Reproducibility of Isotopic Exchange as a Function of Porosity of Sil-0Cel Solid Support and Amount of Liquid Phase
Amount of liquid phase
10
0
o----c7 o--o
100 57.3 40.0 40.8 28.5 16.7 12.6
94.2 72.2 58.5 34.6 32.8 16.1 12.0
amounts of liquid phase the reproducibility was unsatisfactory, whether for solid supports of higher or lowerporosity. However, the reproducibility improved when the amount of the liquid layer wag increased. Wi,th the solid of lower porosity, the improvement appeared at relatively lower amounts of liquid. The irreproducibility of the results indicates that t,he filling of the capillary pores is irreproducible. The conclusion m-hich may be drawn from the experiments shown in Figure 9 is similar to that drawn from the results given in Figures 6 and 7-i.e., that when low amounts of liquid layer are to be used, a low porosity solid support is to be preferred, in order to diminish the interact'ion between the solid and gaseous material. Figure 10 shows the number of theoret'ical plates in the column as a function of the porosity of the solid support and the amount of liquid phase. In the range investigated for Sil-0-Cel columns, the efficiency of the column increases with increasing amount of liquid layer. KO conclusion can be drawn from t,hese experiments with regard to the effect of both porosity and amount of liquid layer, on the efficiency of the column, since the columns used were of different solid surface area. However, the results obtained in experiments performed on an ;Ilundum column show that a t a given carrier gas flom- rate there is an optimal amount of liquid layer for maximum column efficiency. The results might, also indicate that when a lower porosit'y solid support is
used, the maximum in column efficiency is obtained with smaller amounts of liquid layer. A maximum in column efficiency was also found by other authors (3) who worked with a Sil-0-Cel solid support (- 100f120 mesh) coated with Apiezon oil liquid phase, and pentane, hexane, arid other solute samples. While the maximum column efficiency in these experiments was obtained for a coating of 20% w./w. liquid phase, t’he maximum in our experiments performed on the *\lundum columns was obtained with a coating of about 370 w./w. liquid phase. Figure 11 shows the change in isotopic exchange as a function of amount and polarity of liquid phase. The results show that a given decrease in isotopic exchange is obtained with a smaller amount of relatively highly polar liquid phase than wit’h a liquid of low or zero polarity; the interaction is reduced to lOy0 of that between the uncoated solid support and gaseous sample, using 2070 w . / w . n-decane (zero polarity) , as against 6.5% w./w. nitrobenzene phase (dipole moment 4.27 X e.s.u.). A certain interaction between the solid and gaseous phase is still observed, even with 6.5 w./w. of a highly polar liquid phase such as nitrobenzene. This is not in agreement with the results of Scholtz (f9), who found no interaction in columns with more than 0.25% w./w. polyethylene glycol-400. The difference is probably due to the sensitive met hod of measurement used in our experiments4etermination of effluent radioactivity caused by isotope exchange between the labelled solid
support and solute, as opposed to the measurements by Scholtz of changes in retention times and peak symmetries. The influence of the polarity of the liquid phase on the column efficiency is shown in Figure 12; a n increase in column efficiency is obtained with an increase of polarity of the liquid phase. The experiments described above show that the solid support in GLC interacts with the solute even through relatively thick liquid phase layers; a stronger interaction is obtained by using a more porous solid support and a less polar liquid stationary phase. The experiments also show that the efficiency of the GC column, for a given surface area of the solid support, decreases with the increase of its porosity. The present work indicates the advantages of the GC isotopic exchange method, which appears to merit further development and use. ACKNOWLEDGMENT
The author is grateful to Ytzhak Marcus for his interest in this work and for many helpful discussions. LITERATURE CITED
(1) Bohemen, J., Purnell, J. H. in “Gas
Chromatography,” D. H. Desty, ed., pp, 10-11, Academic Press, Yew York,
1958. (2) Bradford, B. W., Harvey, D., Chalklev. D. E.. J . Inst. Petrol. 41. 80 (1955). (3) Cheshire, J. D., Scott, R. P. W.‘, Ibid:, 44, 74 (1958). (4) Deemter, J. J. Van, Zuiderweg, F. J., Kleinkenbere, A ,., Chem. Ens. Sci. 5. 271 (1956). ( 5 ) De Wet; W. J., Pretorius, V., ANAL. CHEM.30,325 (1958). ( 6 ) De Wet, CY. J., Haarhoff, P. C., Pretor-I
ius, V., J . S.African Chem. Inst. 131 No. 1, 19 (1960). ( 7 ) Eggertsen, F. T., Knight, H. S., ANAL.CHEM.30,15 (19%). (8) Ellis, J. F., Forrest, C. W., Howe D. D.. “Estimation of Surface Areas bv a Gas Chromatographic Method,” L-K Atomic Energy Authority, 1I.E.G. Report 229 (CA) (1960). (9) Evans, J. B., Willard, J. E., J . Am. Chem. SOC.78, 2908 (1956). (10) Evropin, V. A,, Z h . Fiz. K h i m . S S S R 30, 348 (1956). (11) Haissinsky, hi., “La Chimie Nucleaire et Ses Applications,” p. 472, Masson et Cie, eds., 1957. (12) Innes, W. B., ANAL.CHEM.28, 332 i19561. 13) James, A. T., Martin, A. J. B., Biochem. J . 52,238 (1952). 14) Keulemans, A. J. M., “Gas Chromatography,” R. P. W. Scott, ed., p. 306, Butterworths. London. 1960. 15) Kieselbach, R., ANAL. CHEM. 33, 23 (1961). 16) Nelson, F. S., Eggertsen, F. T., Ibid., 30,1387 (1958). 17) Purnel, J. H., “Vapour Phase
Chromatography,’’ D. H. Desty, ed., p. 59, Butterworths, London, 1057. 18) Schmidt-Bleek, F., Stocklin, G. Herr, W., Ansew. Chenk. 72, i 7 8 (1960). 191 Scholtz.
R. G.. “Solid SuDDort Effects in Gas Chromatography,” Ph.1). Thesis, Purdue Vniversity, 1961. (20) Tadmor, J., Bull. Res. Council Israel
11A,144 (1962). T -UY.’.\,.) ”.,
1 91) T n A m n r ,-L,
P \ , ,h ” ,rVn . . ”m V ”nVtYn. n
Elsevier 5.223 11963). (22) Tadrnor, J., J . Inbrg.
Po*> od L,*.
Cy;. S i i c l . Chettr.
23, 158 (1961).
122’1 _ _ ,“Spnnmtion llr _. of. Z - lPtnl \--, Tnrlmnr_ , .T
Halides by Gas Chromatography,” Israel A E C Report IA-950(in press).
RECEIVED for review Februar). 3, 1!)64. Accepted April 30. 1964. Presented at 2nd International Symposium on Advances in Gas ChmmatoeraDhv. University of Houston. Houstoi;; ‘fecis, March 23-26, 1964.
END OF SYMPOSIUM
VOL. 36, NO. 8, JULY 1964
1573
