riff (8) has described the resistance of a discharge to incorporate a vapor. This was demonstrated with an arc enclosed in a chamber and the vapor introduced into the chamber. It is possible that by the aerosol coming up through the electrode and the discharge rotating there would be less resistance to incorporate the aerosol. Initially, a new electrode was used for each determination. The electrode does erode and the change in the physical dimensions causes changes in the intensity of the analytical lines when the same electrode is used repeatedly. These changes are not due to memory effect because alternate exposures of sample and deionized water indicated no carry-over of even the matrix element. The practice now is to use each end of the tube electrode for one determination. Limits of detection have not been investigated because our primary interest was alloy analysis. The level of detection that might be achieved, however, is indicated by adequate line intensities being obtained (8) R. W. Woodriff, Appl. Specfrosc., 22,207 (1968).
for alloys from 10 mgjml solutions with only 15 second exposures. In preliminary studies a minimum of matrix effects were noted (Table 11) and both precision and accuracy appear to be very promising, (in the range of 3 average coefficient of variation and 3 % average deviation from known values, Table 111). The technique is not intended to replace R F plasmas, dc plasmas, rotating disc, porous or vacuum cup methods, but because of its simplicity ahd utilization of existing components, it should be worthy of further study and evaluation. The purpose of this correspondence is to bring this technique to the attention of others, to stimulate interest, and to encourage further investigation. J. H. MUNTZ
Air Force Materials Laboratory (MAYA) Wright-Patterson Air Force Base Ohio 45433
RECEIVED for review April 14, 1969. Accepted May 16, 1969.
Fast Gel Permeation Chromatography of Single Species SIR: Gel permeation ( I ) or steric chromatography has been considered by many to be a slow technique, restricted to low linear velocities and subsequent long analysis times. Virtually all work reported in the literature has been done at 1 mljmin with a 0.308-inch column, which represents 0.03 cmjsec. Current work in affinity chromatography has reported data at solvent velocities greater than 0.3 cmjsec. (2-7). Gel permeation chromatography, however, has not been included-apparently, on the basis that as a diffusion controlled process it is basically slow. Recent work in GPC, however, indicates that this need not be the case (8, 9). The chromatogram in Figure 1, showing the separation of three components in 80 seconds, affirms that GPC is applicable to “high speed” separations of discrete species. The separation was made on a Waters Associates ALC 100 equipped with a high volume pump and a 3-foot Poragel 100 A column. The data shown in Table I, taken from the chromatogram, shows 12 plates per second for the component at K’ = 0 and 3.9 for the component at a Capacity Ratio K’ = 1.1. If one defines the GPC distribution coefficient as :
where
[SI
x
=
Classical partition coefficient = -
,fVi
=
V,
=
Fraction of V , available to molecule of a given size Volume of trapped solvent
[ml
Since the trapped solvent is the same as the moving solvent, x = 1, and it is evident that K can never be greater than 1,
K = x -.fVi Vi
(1) J. C. Moore, J . Polym. Sci., Part A-2 835 (1964). ( 2 ) J. F. K. Huber and J . A. R. J. Hulsman, Anal. Chim. Acta, 38, 305 (1967). (3) C. G. Horvath, B. A. Breiss, and S. R. Lipski, ANAL.CHEM., 39, 1422 (1967). (4) R . P. W. Scott, D. W. J. Blackburn, and T. Wilkins, “Advances
in Gas Chromatography,” A. Zalatkis, Ed., Preston Technical Abstracts Co. (1967). ( 5 ) T. W. Smuts, F. A. Von Niekert, and V. Pretorius, J . Gas Chromatog., 5, 190 (1967). (6) J. J. Kirkland, ANAL.CHEM.,40, 391 (1968). (7) L. R . Snyder and D. L. Saunders, J . Chromatogr. Sci., 7, 195 (1969). (8) J. N. Little, J. L. Waters, K . J. Bombaugh, and W. J. Pauplis, Polymer Pre-Prints, 10, 326 (1969). (9) J. L. Waters, J. N. Little, and D. F. Horgan, J . Chrornatogr. Si.7, , 293 (1969).
0
32.5
65
V, rnl
Figure 1. Fast GPC chromatogram Column: %foot X 3/8-in~h Poragel 100 Flow: 32.5 ml/min U = 2.82 cm/sec Solvent: Toluene Temperature-Ambient Components: 1. 7.5 mg Polypropylene glycol 4000 2. 7.5 mg Hexadecane 3. 15.0 mg Acetonitrile Conditions:
VOL. 41, NO. 10, AUGUST 1969
1357
Table I. GPC Separation Data Polypropylene glycol 4OOO 17.4 3.4 41.5 32.4 12.9
Unit Elution volume Width
ml
Time
ml Plates Seconds
Nlt
PPS
NTheore tical
V8
K‘
- vo
ml
Plates PPS
0 0 0 0
mg
7.5
Vo’s
nsffeotive nt
Amount injected
Flow rates (mljmin)
1 27.7 33.6 34.4 37.7 46.5 48.0 20
PPG Octylether Dodecane Decane Orthodichlorobenzene Acetonitrile Pressure PSIG
Acetonitrile 36.0 9.0 260.0 67.0 3.9 18.6 1.1 70.0 1 .o 15.0
Hexadecane 24.4 8.2 145.0 45.4 3.2 7.0 0.4 12.0 0.3 7.5
Table 11. Elution Volume (ml) a t Several Flow Rates 3.07 6.77 13 21.5 26.5 25.7 26.1 27.5 .,.
...
...
...
36.4 37.7 46.1 50.2 66
34.8 36.4 45.3 49.2 130
35.0 37.0 45.2 49.2 230
36.6 38.2 47.5 51.3 345
28.3 27.1
...
35.9 37.5 46.8 50.6 490
34.5 26.8 33.7 35.4 37.2 46.8 50.2 635
Table 111. Peak Widths (ml) a t Several Flow Rates Flow (ml/min)
1 5.2 4.3 3.6 3.4 3.7 3.6 4.1
Polystyrene 41 1,OOO Polypropylene glycol 4OOO Octylether Dodecane Decane Acetonitrile Ortho dichloro-benzene
3.07 5.2 4.8
6.77 4.1 3.6
13 3.8 3.0
21.5 3.7 3.0
28.3
...
...
4.9 4.8 4.9 5.4
5.7 5.3 5.9 6.8
6.6 6.6 6.5 8.4
8.0 7.9 8.3 9.9
8.5 8.5 9.2 11.3
...
Table IV.
...
...
Elution volume Width N Theoretical Time NII ve
-
Vo
K’
VO’S
nrffeotive
Plates PPS mg
nr
Amount injected Injection by time
since fVc - approaches 1 as a limit. When
vo
PPG-4OOO 26.8 3.27 1075 46.6 23.1 0 0 0 0
2.9
x
>
Dodecane 35.4 9.45 225 61.6 3.7 8.6 0.32 13.2 0.21 2.9
1, additional
forces are affecting the retardation process and the separation is not by size alone. With K values limited t o 1 and capacity ratios (K’) limited to perhaps 3, long columns or recycle are often required to make difficult separations (10, 11). When, however, K’ values in GPC provide relative retentions comparable to
(10) K. J. Bombaugh, W. A. Dark, and R. F. Levangie, Sepn. Sci., 3 (4) 375, (1968). (11) K. J. Bombaugh, W. A. Dark, and R. F. Levangie, J. Chromatogr. Sci., 7, 42 (1969). 1358
ANALYTICAL CHEMISTRY
3.1
34.5 4.0 3.3 9.7 9.5 8.9 10.4 13.1
GPC Separation Data
Flow = 34.6 ml/min (600-635 psi) Unit ml ml Plates Seconds PPS ml
...
Column-60& 4-ft Decane 37.2 8.92 278 64.7 4.3 10.4 0.39 21.7 0.34 2.3
Ort hodichlorobenzene 46.8 13.15 203 81.4 2.5 20.0 0.75 37.0 0.45 2.3
Acetonitrile 49.9 10.38 370 86.8 4.3 23.1 0.86 79.2 0.91 2.9
LLC and LSC, equal speed is attainable by GPC to those reported by LLC and LSC (2, 6, 7). I n an earlier work it was shown that peak width was equal for all K values by GPC, indicating no spreading in the stationary phase (10). The chromatogram in Figure 1, however, appeared to indicate that at the flow rate used permeation was diffusion-limited, or that the separation was not a steric exclusion phenomenon. To test this further, a series of experiments was performed with additional components at a range of flow rates, I n addition, nearly equal amounts of each component was used in the fractionation mixture to avoid the illusion of increasing peak wigth shown in Figure 1. A 4-fOOt x 3/8-in~h column of 60 A Poragel was used in the confirmatory experiment. (The 60 A designation
I
Figure 2. Fast chromatogram with equal amounts of each component Conditions:
Column: 4-foot X 3/8-inchPoragel 60 6 Flow: 34.5 ml/min U = 2.99 cm/sec Solvent: Toluene Temperature-Ambient
Components: 1. Polypropylene glycol 4000 2. Dodecane 3. Acetonitrile
b
I
1.0
0.5 TIME
is currently used terminology, which is the contour length of a n extended chain of polystyrene excluded from the gel pore.) A plot of V, us. flow rate in Table I1 for all components tested indicates that the mode of retardation of all species at distribution ratios ( K ) from 0 to 1 are essentially the same at all carrier velocities used. The fact of size separation is evident from a comparison of the retention of the CI2relative to the Clo. I n addition, sample injections of 3.3, 6.6, and 13.2 mg, produced n o change in V,’s for all components. The acetonitrile, known to experience some adsorption, is, therefore, retained primarily by permeation, since its size is significantly smaller than the hydrocarbons. The data in Table I1 shows peak widths (w)at a range of carrier velocities. Several materials used in this investigation were among those used by Hendrickson, who, at the normal flow rate of 1 ml/min first reported the size separation of small molecules by GPC and determined their effective size in solution (12, 13). (The peak widths of all permeating peaks are essentially equal to each other at each flow rate, with the exception of ortho dichlorobenzene (ODCB).) The ODCB peak is noticeably wider a t each flow rate than all other materials tested. This is attributed t o a slower diffusion rate of ODCB. Similarly, 411,000 molecular weight polystyrene is wider than polypropylene glycol, although as excluded materials both peaks are more narrow than any permeating peak. The effect of diffusion is seen o n the peak width of different species, not o n the appearance of the peak center ( V,). The depth of permeation appears to be virtually independent of carrier velocity. Once the molecule has entered the gel pore it is in static solvent, where the volume (12) J. G. Hendrickson, ANAL.CHEM., 40, 49 (1968). (13) J. G. Hendrickson and J. C . Moore, J. Polym. Sci., Part A-14, 167 (1966).
IN
2.0
I.5
MINUTES
fraction available is independent of the carrier velocity as described by the definition of K. The effect of diffusion can be seen in the data in Table I11 where at 1 ml/min all materials show similar peak widths (A,,, = 0.7 ml) while at 34.5 ml/ min they are very different (A,,, = 9.8 ml for all species and 3.6 ml of permeating species). The decrease in w for the excluded species may be attributed to improved radial mixing in the moving phase under energized flow. This supposition, however, warrants further investigation. The chromatogram in Figure 2, using a 4-foot column, accomplished the separation in 102 seconds. Here dodecane was used in place of the hexadecane used in Figure 1, since the particular gel used in the confirmatory work provided a slightly differently shaped calibration curve, which in turn afforded better resolution with dodecane. The data in Table IV, taken in part from Figure 2, substantially confirms the results from Figure 1. The unequal widths of the permeating species in Figure 1 are clearly not resistance to mass transfer in the stationary phase, but are due primarily to the large amount of acetonitrile injected.
K. J. BOMBAUGH R. F. LEVANGIE Waters Associates, Inc. 61 Fountain Street Framingham, Mass. 01701
RECEIVED for review March 28, 1969. Accepted June 11, 1969.
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