Infrared Cell for Collecting Chromatographic Fractions Herman Szymanski, Richard Povinelli, Dennis Stamires, and Gregory Lynch, Canisius College, Buffalo, N. N
INFRARED
CELL
for collecting
,G
A chromatographic fractions is shown in Figure 1. It resembles a standard infrared gas cell, but has a heater coil, B, which is used to activate the surface material held on the salt plate, C. A thermocouple port, A , is used to measure the temperature of activation. L is the mlt window, held against a rubber gasket, E. G is connected to a vacuum line when the surface material is being activated. Originally, thin disks of Linde Molecular Sieve 13X were prepared and placed a t position D. I n this case C was a perforated salt plate. I n later work powdered sieve was used on a single salt plate a t C. This technique was adopted because the thinness of the disk required for reasonable transmissions in the 2- t o 16-micron regions resulted in extremely fragile disks. To cause the powder to adhere to the salt plate without support, it was ground into the plate with a 400-mesh diamond spatula. The resulting fine particle size and the slight roughing of the salt plate mere sufficient to cause the powder to adhere. Except for a peak near 10 microns, the sieve material is relatively transparent. Water peaks a t 3 and 6 microns interfere slightly if the material is not completely dry. To collect a fraction from the gas chromatographic apparatus, the exit capillary, F , is placed through a hole in the first salt plate of the cell, so that it nearly touches the sieve material. Any volatile material from the column of the chromatographic instrument thus sprays directly onto the sieve material. Polar materials mill adhere much more strongly t o these materials, but even volatile hydrocarbons are held sufficiently long to obtain a spectrum. Polar compounds such as ammonia and water give slight peak shifts when adsorbed, but these are not sufficiently large to distort the known spectrum. To test if the spectra obtained were primarily those of the materials on the sieve, a cell of a much smaller volume was designed. As the shorter path length required removal of the heater coil and vacuum line attachment, this cell was not adopted for general use. Spectra taken with it agreed with those taken in the larger volume cell, indicating that most of the compound was collected on the sieve. Visual inspection, as a fraction is collected, also shows the liquid condensing on the 21 10
ANALYTICAL CHEMISTRY
Y.
Figure 1. A. B.
C. D. E.
F.
Infrared cell for adsorbed phases
Thermocouple port Nichrome heater Salt plates KBr disks Rubber gasket Exit capillary of gas chromatographic apparatus
G.
Vacuum outlet Glass cell I. Salt window M. Metal end plate
H.
WAVE LENGTH IN MICRONS
Figure 2. Spectrum of acetone from 16 collections of a 0.02-ml. fraction from a gas chromatographic instrument
sieve. The cell is sealed during a run, so only an equilibrium amount of vapor would be present. The adsorbed material can be removed by allowing helium to flow over the sieve for a few minutes. Longer periods of flow can be used to remove water from the sieve, thus activating i t for the next run. Because the adsorbed material can be removed by helium flow, the cell should be attached to the chromatographic apparatus only when a fraction is going out. The method has several advantages over standard collecting techniques. The general procedure for the.collection of a 0.02-ml. or smaller sample from a chromatographic column is to trap the material in a small amount of solvent or condense it with a coolant, transfer it to a microcell, and determine its spectrum. These steps are eliminated by using a sieve to hold all the material to be collected, in a small area. No solvent or delicate handling techniques are required and a reasonable spectrum for as little as a 0.02-ml. sample is obtained. Samples as small as 0.001 ml. have been
analyzed and strong peaks can be seen; however, for accurate analysis, samples of a t least 0.02 ml. must be used. It is also possible to collect several times, thus increasing the intensity of the absorptions bands. A spectrum taken on a Baird infrared spectrophotometer Model AB-2 with sodium chloride prism and a 12-minute scan is shown in Figure 2. It shows 16 collections of acetone from the chromatographic apparatus, each 0.02 ml. A spectrum of acetone was obtained with the first collection of 0.02 ml. and all the major peaks were present. The intensity of the 5.9-micron band for this first collection m-as 20% compared to 60% for 16 collections. This larger number of collections probably represents a saturation amount for the sieve present. I n most cases a maximum of five collections is adequate. The break in the tracing a t 11.4 microns represents a reference beam adjustment to keep the curve on the paper. A number of other adsorbents could be used to replace the sieve material. This is being studied at present.
