expedition. For this purpose, the average N20 value was determined from each pair of parallel measurements. The deviations of the corresponding single values from this average were used to calculate a quasi standard deviation, whose percentage from the average of all measurements of one expedition was *1.7% for the 1970 expedition and f1.45 for the 1971 expedition. As mentioned previously, for 15- to 20-liter air samples the standard deviation from laboratory experiments was ?C 1.4%. Accordingly, the ac ual accuracy aboard ship is comparable with that from laboratory experiments. The slightly increased value from the 1970 expedition may be due to unfavorable weather conditions. At some stations in the Atlantic Ocean, parallel samples were taken from the sea surface water. The N20 values from these parallel samples suggest an actual accuracy for N 2 0 measurements in sea water aboard ship better than + 3 %. In a recent paper, LaHue et al. (8) described an analytical method for measurements of the atmospheric NzO. This
method, a combination of the methods of Bock and Schuetz (3) and of Leithe and Hofer (4,also works without a Toepler pump. LaHue et ai. remove the N20 from the molecular sieves trap by water treatment. The released N2O is adsorbed once more on cooled silica gel and then transferred to the gas chromatographic column by heating the silica gel to 140 “C. A technique which works at relatively low temperatures and without a Toepler pump is certainly of advantage. However, in the method of LaHue et al., this advantage is almost lost by the comparatively complicated procedure. The standard deviation which results from the data given in the paper by LaHue et al. is *1.75%. In contrast to the method by LaHue et al., the procedure described above is uncomplicated and easy to handle, especially under field conditions, and its accuracy is at least comparable.
(8) M. D. LaHue, H. D. Axelrod, and J. P. Lodge, Jr., ANAL. CHEM., 43, 1113 (1971).
RECEIVED for review January 25, 1972. Accepted May 8, 1972.
Principles of Hot Plate Chromatography Sredko Turina Institute for Material Investigations, University of Zagreb, D. Saleja I , Zagreb, Yugoslavia
Vjera Jamnicki Pharmaceutical and Chemical Works “PLIVA,” Zagreb, Yugoslavia
WARMING A CHROMATOGRAPHIC PLATE during the TLC process causes the solvents to evaporate from the plate and the chromatograms obtained show the following advantages (1): (1) better resolution of the spots and (2) possibility of detection of trace components which are undetectable under usual TLC conditions (2). These effects, obtained under special conditions, can be explained by the discontinuous counter-current model of the chromatographic process given by S. W . Mayer and E. R. Tompkins (3). Heating of the plate during the chromatographic process results in evaporation of the solvents from the plate, as is shown in Figure 1. If the temperature of the plate is uniform, the evaporation of solvents over the whole area is uniform, too, and the following equation applies. mdx)
=
- Im,(O)/O X
(1)
The value of I is a function of temperature T,solvent volatility and viscosity, thickness of the layer, and particle size of the adsorbents. Under ideal conditions, without evaporation of solvents, the velocity ut of each component of a mixture during the chromatographic process is constant. The position X’ that component i would reach on the chromatographic plate in the given time is : X’(r)
= us
*
RF
7
(2)
where us = R F =
a constant a value deduced from the chromatogram obtained at normal conditions
If evaporation of solvents from the plate occurs, the velocity of the components decreases with distance Xfrom the start:
where m,(X> = the flow of the mobile phase per unit width of the plate at the distance Xfrom the start m,(O) = the flow of the mobile phase per unit width of the plate at the start 1 = the distance between the start and the line where the static front is established
(3) where ut = the velocity of the component i at the distance X from the start. The time dependence of the position X of a component i can be obtained from the differential Equation 3 by integration :
( 1 ) S . Turina, 2. ioljii, and V. MarjanoviE, J . Chromafogr.,39,
T(X>
81 (1969). (2) E. Stahl, “Thin-Layer Chromatography,” Springer-Verlag, Berlin, 1962, p 100. (3) S . W. Mayer and E. R . Tompkins, J. Amer. Cliem. Soc., 69, 2866 (1947). 1892
= (l/us
. RF)l
r(X) = ( - 1 h
*
L 1
R F ) In
- (X/I)I-’dX
(4)
- (X/Ol
(5)
By transforming the equation, it follows:
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
Figure 3. Dependence ofspot width on the hot platedeveloping time for various amounts of substancesin the spot
S 0
‘ ’ *
1
TIME [h]
Figure 2. Dependence of spot position on the hot plate developing time for various R , values
X(r) = 61 - exp(-r. u,
. RF/O
(6)
Figure 2 shows curves that represent the theoretical dependence of distance on the time of developing the chromatogram for several components with various RF values. It is reasonable to suppose that theoretically the resolving power of chromatograms R,, according to Giddings (4), depends on the develoving time in usual TLC and hot d a t e
TZC: R,
Az’ = the distance between the centers of neighboring spots in usual TCL b’ = the spot width in usual Tu3 Az = the distance between the centers of neighboring =
spots in hot plate chromatography the spot width in hot plate chromatography
(4) J. C. Giddings, ANAL.CHEM., 37, 60 (1965).
*
mparisons of chromatograms I ained by (a)hot Figure 5. plate TL.C and (b)usual TLC of a commercial dyestuff ~
The time of develovment of the hot olate chromatomam was 4 hours. The developing soivent was a butanh, methanol, and water mixture @:l: 1v/v)
:
where
b
U
In practice there are deviations from the theory. The spot width b in hot plate TLC does not narrow in proportion to the decreases of distance between the centers of neighboring spots. The curves in Figure 3 show the change of spot width with the developing time for various amounts of a component. The disagreement with the ideal curve increases parallel to amounts. The concentration of a substance in the spot center increases during the chromatographic process on the hot plate
ANALYTICAL CHEMISTRY, VOL. 44,NO. 11, SEPTEMBER 1972
1893
Figure 8. Chromatogram o b tained by means of me high resolution zone
Figure 6. Comparisons of chromatograms obtained by (a) hot plate TLC and ( b ) usual TLC of a saccharide mixture Developing time of the hot plate chromatogram was 24 hours. Development was carried out by a butanol, ethanol, and water solvent
Temperature of the hot plate was 38 "C and time of development was 5 hours
(5:3:2v/v) HOT PLATE METAL SHEET SILICA GEL
I:RONT I
r II
GLASS PLATE
START
ASBESTOS SHEET
Figure 7. Principle of the high resolution zone which arises by interposing the special plate behveen the hot chromatographic plate, and increasing the temperature of the hot plate
YC,,'C..YoV.I.,
8s reached. The value of C,, 'the plate: .,. ...-.L,L.,,L.a..xeTof
Cmax T
(8)
and is independent of the amount of substance present.
temperature of plate: 29 "C; and time of development: 4 hours. Figure 56 shows a chromatogram of same dyestuff obtained without heating with the same solvents. Figure 6a shows a chromatogram where a mixture of saccharides was applied at the start. The temperature of the plate during the chromatographic process was 28 OC,and the time of development was 24 hours. The chromatogram of the same mixture developed without heating is shown in Figure 66. Both chromatograms were obtained with the identical solvent: butanol, methanol, and water ( 5 : 3 :2 v/v). According to Equation 8, it is possible to detect extremely small quantities of substances in a mixture by increasing the temperature in the relevant zone of the chromatogram before development without heating. By interposing a special plate between the hot plate and the chromatographic plate, it is possible to achieve a relatively high temperature in the investigated zone, as shown in Figure 7. This special plate is made of aluminum and asbestos. A zone of intensive evaporation forms on the boundary between the aluminum and the asbestos. The width of the zone is proportional to the temperature of the hot plate. A chromatogram obtained by means of the high resolution zone is shown in Figure 8. The ratio of trace components to the other components in the chromatograni in Figure 8 is about 1:lo-'.
EXPERIMENTAL
CONCLUSIONS
The experiments were carried out on 20 X 20 cm precoated Silica gel plates. The chromatograms were developed in a chamber 25 X 25 X 10 cm. The chromatographic plates were attached to the hot plate by clamps. The hot plate, dimensions 20 X .20 x 2.5 cm, is heated by warm water flowing from a thermostat, as shown in Figure 4.
As shown in Figure 5a, it is possible to separate components which have nearly equal R p values in a relatively short time. For the detection of trace components, as shown in Figure 6a, a longer time of development is necessary. When the ratio between the trace components and the major components is below 1:lW, it is necessary to apply hot plate chromatography with the high resolution zone.
RESULTS AND DISCUSSION
-
The chromatoeram of a commercial dvestuff obtained by hot plate TLC: i is shown in Figure 5a. The conditions in cluded solvent Ibutanol, methanol, and water (8:l:l v/v)1:
1894
RECEIVED for review Jan 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
