the following equation for the critical pressure p , = 4pyqDmN/dp2
(12)
where S has replaced S,,, since we are now expressing limiting pressures rather than limiting plate numbers. This equation shows that if N plates are needed for a certain separation on a given kind of column (fixed d,, D,, q ) , an inlet pressure equal to or greater than p , must be available. Once again the demands on liquid chromatography are relatively small due to the slight magnitude of the OD, product. The nature of Equation 12 for gas chromatography is altered by the strong dependence of the gaseous diffusion coefficient, D , = D,, on pressure. This can be eupressed by
D,
=
D,'/p
(13)
where D,' is the value of the diffusion coefficient a t unit pressure. The mean value of D,, for use in Equation 12, can be obtained by using the lengthaverage pressure, p , in place of p . Under vacuum outlet conditions, with p , as the inlet pressure, I, is found to equal 2 p C / 3 . Consequently
D,
3D,'/2pc (14) Cpon substituting this back into Equation 12 we obtain p,' = ( 6 ~ y D,'S/dp2)'/2 q (15) =
;ilthough the gradients existing in gas chromatography have been rather loosely treated here, this equation differs from the earlier rigorous form only by the small numerical constant of d2j3. The development given above indicates the limits associated with single
chromatographic columns. One can imagine column segments joined together by pumps of low dead volume such that each segment experiences the maximum possible pressure drop. Under these circumstances the arguments given above would apply to the individual segments rather than to the column as a whole. LITERATURE CITED
(1) Giddings, J. C., ASAI,. CHEM.34, 314
(1962). (2) Giddings, J. C., Stewart, G. H., Ruoff, A4.L., J . Chromatog. 3 , 239 (1960). (3) Hamilton, P. B., ANAL. CHEM.32, 1779 (r960). ( 4 ) Knox, J. H . , J . Chem. SOC. 1961,~. 433.
RECEIVEDfor review April 1, 1964. Accepted June 1, 1964. JI'ork supported by a research grant from the National Science Foundation.
Rapid Scanning Infrared-Gas Chromatography lnstrument A. M. BARTZ and H. D. RUHL
The Dow Chemical Co., Chemical Physics Research laboratory, Midland, Mich.
b This instrument was designed to utilize the ability of gas chromatography (GC) to physically separate a multicomponent chemical sample into its individual components and the ability of infrared spectroscopy to specifically identify reasonably pure compounds. The effluent from a GC column (vaporized sample plus helium) i s passed through a heated light pipe which serves as an infrared absorption cell with a large optical path length-to-volume ratio. The infrared absorption spectrum of the vapor sample i s obtained by using two singlebeam grating spectrometers in parallel. One spectrometer covers the range 2.5 to 7 microns while the other scans from 6.5 to 16 microns. By using two spectrometers, a high chopping rate, and fast recorders, a complete spectrum may be obtained in 16 seconds comparable in quality to a normal 12-minute scan by commercial spectrometers. The high scanning speed i s necessary if an IR spectrum i s desired of each successive GC peak of a multicomponent sample.
total reflectance. Both methods are tedious, slow, and rather difficult since extremely small quantities of condensed sample are involved. An instrument was needed which would obtain an infrared spectrum of the
PREAMP I
1
GC sample directly, without condensation, and do it quickly so that closely spaced (in time) successive GC peaks could be readily identified. There were two problems to be solved: design a satisfactory sample cell and obtain a
'i COLUMN
P
of combining infrared and gas chromatography have required condensation of the GC sample (effluent) and obtaining an IR spectrum of the condensed sample by use of either a microcell or by attenuated REVIOUS TECHNIQUES
1892
ANALYTICAL CHEMISTRY
Figure 1.
Optical path of infrared radiation
good IR spectrum in 30 seconds or less preferably using a standard room temperature IR detector. The infrared absorption cell had to satisfy the following requirements: small sample volume (10 to 20 ml.. such that GC peaks of 10-second halfwidth a t a 1-nil.-per-second flow rate would fill the cell), large path length to volume ratio for maximum sensitivity, operable at 250' C., short sample flushing time, good optical transmittance, and good corrosion resistance. A cell satisfying these requirements is a rectangular light pipe. I n this cell the inside walls are made highly reflecting to propagate the light through the cell by multiple reflection. The cell used in
obtaining the spectra has inside dimensions of 1/8 X x 12 inches (volume of 12 ml.). Any metal with good corrosion resistance and capable of being polished to a highly reflecting finish may serve as the body material. For the particular optical arrangement we used, the 12-inch geometric length gave an effective optical path length of 16 inches. The transmittance is about 40y0 and is essentially constant with wavelength in the 2.5- to 16-micron range. The sample flushing time is 2 seconds using a nitrogen purge of 25 ml. per second (2 p.s.i.). The maximum scanning rate in an infrared spectrometer is ultimately limited by the detector response time.
The detector, which sees modulated or chopped radiation, must receive a certain minimum number of chops (bits of information) per wavelength interval (determined by the required resolution) to present a good profile of the spectrum being scanned. Rapid scanning therefore requires a high chopping rate; unfortunately room temperature IR detectors have decreasing sensitivity for increasing chopping rates thereby imposing a limit to scan rate. A performance goal in designing this instrument was that it should have resolution at least equal to that of a bench-top prism spectrometer and be able to obtain spectra of GC peaks 30 seconds apart. Since the spectra were
011 0.1 0.2
43 Q4 Qs
1.0 2.0
Figure 2. Top: standard spectrum of polystyrene film run on lab. prism spectrometer. Middle: instrument in 16 seconds. Bottom: polystyrene film run on this instrument in 80 seconds
polystyrene film run on this
VOL. 36, NO. 10, SEPTEMBER 1964
e
1893
WAVEWUYBLRS IN C k '
2 m
-3
S
4
5
WAMI(UYI)W IN CY.!
1500 1400
6
1300
1200
1100
7
IO
Figure 3. TOP: library standard spectrum of acetone vaDor run with a 1 0-meter cell. peak of a GC'separation of a 5% acetone in toluene mixtu're using a 1 0-PI.sample
to be used solely for qualitative purposes the wavy of single beam spectrometers was acceptable. The lxoblem of rducing the sican time to less than 30 seconds was .mlved in the follolving manner: the radiation from a single infrared w i r e e , after I)as;.ing through the light pil)r is flickrred with a reflecting c1ioi)l)er to two different single lieam grating sliectrometerh. One spec*ti~)ineter($over>the range 2.5 to 7 microns while the other cwnwrrently ( m ~ r sthe range 6.5 to 16 microns. Since chol)i)ing is necessary for each s1)ectrorneter normallj., this method halves the detector reqponse requirements a t no cost in energy. I t does, however, require single beam olwration The use of two spectrometer,*in parallel further reduces scan time in that each sl)ectrometer uses only one grating in first order--no time is spent switching gratings. EXPERIMENTAL
Apparatus. Figure 1 indicates the optical path of thf. infrnretl rntli:ition. The radiation froin the S c r n + t glower, S, is focused on the entrance end of t h e I R cell h y mcwv of spherical mirror, .I/ 1 . 'l'lic im:tge i* tr:in*fcrrctl through 1894
II
I
WAVELENGTH IN MICRONS
ANALYTICAL CHEMISTRY
the pipe b y multiple reflection and is dirertetl onto spheriml mirror, .113, by plane mirror, .112. T h e rotating chopper reflect.: the bram from .I13 onto sl)lierical mirror, .llS, one half of the time and allows the light t o 1)roceed t'o s1)herical mirror, J14, when it, is not reflccat'ing the beam. IIirrors, .118 and .1!4> fill out the entranre slits of the long wavelength and short wavelrngth spectrometer, reslwctivelj-. The light from the ent,rmc-c.slit is t3i;i)c'r.vd hy the grating of each sliectrometer arid focwaetl on the exit slit which permits radiation of a very narrow wavelength interval to pass and be focused onto thc thermocoul)le detect.or. Each spectrometer uses two transmission filters to remove higher order light. These are flil)ped in and out a t the apl)ropriate time fly an air piston-lever arrangement, the air ])istons being controlled by cam actuated valves. The signal from each detect,or is amplified, svnchronously demodulated, and recorded by a dual rhannel Sanborn recorder. scanning action is arc30mplished by rotating the gratings by i n ~ a i i sof cams and cam folloivor arms. The blits are 1 ) r o q w n n w d t o give ntwlF constant energy by using cams to rotate an arm attached to a al)indle which a r t s as the pivot ~iointof a lever. The slit jaws are fastenctl t o hnrs attavhcd (through
Bottom:
spectrum of the acetone
flexihle connections) to the lever arm. The wavelength sranning and slit program cams are mechanically coupled and driven by a six-speed motor. The sample handling arrangement' utilizes thrre solenoid valves which are (.ontrolled by a panel switrh. The valves have three conditions of operation; I)urge, fill, and seal, described as follows: Purge: GC efliuent passes through 1 to vent, N, passes through 2, through the 111 cell, past 3 to vent. Fill: GC effluent passes by 1, by 2 to the IR cell and by 3 to vent. N, is blocked at 2 . Seal: GC effluent passes through 1 to vent. N, is blocked at 2 , 3 is shut off, thereby sealing any gas in the IR cell. This arrangement allows the operator to select the desired GC peaks for spectra (by-passing others if he wishes) and keep the sample in the cell at a fixed concentration for the scan and for possible rescans under different spectrometer or recorder conditions. Figure 1 illustrates the 12-inch re11 in Idace with the top half of the magnesium heater block removed. The cell window are KaCI sealed with silicone rubher adhesive. The heater blork is
heat)ed by 12 symmetrically spaced, 100-watt cartridge heater3:. The ce!l temlwature is monitusred by a thermocoul~lewith a dial indicator. The overall instrumental arrangement is as indicated in Figure 1 with the infrared source compartment, IR cell heater block, spectro'meter case, and the GC equipment mounted on a table 46 X 60 inches. A 25- X 56-inch relay rack houses the IR amplifiers, power supl)ly, Sanborin recorder, and a control panel for t,he spectrometers and the sample cell. Operation of the instrument is simplified by using microswitches actuated by cams which are mechanically coupled t,o t,he scanning cams to automatically stop the recorder chart advance a t the end of the useful scan and stop the scan when the grating and islit program cams are in a ready (for rescan) position. During the reset portion of the scanning cam rotation, the transmission filters are also reset. The dry air used to drive the filter flippers also serves as a purge to remove atmospheric H20 from the spectrometer case. Scanning action is initiated by a push-button swit,ch. Scanning sl)ecd is selected by a six-position knob and spectral frequency is indicatrd b y a counter whose reading is linearly I)rol)ortiona! to the cm.-l position of earh spectrometer. RESULTlj
Each half of the spectrum is normally recorded on a chart, 50 mm. (the transmittance scale) by 80 nim. (the linear wavenumber scale). The charts are printed adjacent to each other on a single roll. Wavenumber identification of unknoa n absorption bands is made by ov~rlayinga traniparent calibration
_.
....
1.1.
~
..
-
--.:-
10% I S ~ C v y I AkOhOl
.......................
.............. - . - . .4 - . . ......................
~
~
Figure 4. IR vapor spectra of several alcohols which were separated chromatographically from toluene
1 p-Dichlorobenzene
.-.
....
.
2
670 1,2,3,5-Tetrachlorobenzene in acetone
o-Dichlorobenzene
. -..
. -
.
I
I
. .
........
ii W
. . .
-
!
. . . ... . . ... . . ... . . ... . . . . . . .
I
,
.
..,.
.I. ,..L-l..
...
v
... .....
. . .. . . .
. . . . . . . . .
.....
.............
. . . . . . . .
. . .
,,,.- ._.,..
Figure 5. Left: chromatogram showing two unresolved components. fication. Right: IR vapor spectrum of chromatographic cut of 246"
Middle:
c. boiler
IR spectra of each peak permitting identi-
VOL. 36, NO.
IO,
SEPTEMBER 1964
1895
chart, readable to about i0.1 nim. or =t3 cm.-l for the short wave spectronieter and i.1.2 cm.-I for the long wave spectrometer. Reproducibility error is small relative to the reading error. Figure 2 shows a scan of polystyrene film with the two halves of each spectrum rearranged to look more familiar for comparison with the standard spectrum on top. The spectrum on top was obtained with a laboratory prism spectrometer with a normal 12-minute scan time. The spectrum in the middle was obtained in 16 seconds and demonstrates comparable resolution to the priain instrument. Increasing scan time to 80 seconds noticeably improves the resolution as evidenced by the bands in the 3.5-micron region. There is no further improvement by increaving the scan time additionally. At a chopping rate of 24 c.p.s. the 16-second scan represents a spectral profile made from 384 bits of information or about 6 em.-' per bit for the short wavelength half of the spectrum. Figure 3 shows a spectrum (bottom) made from a 10-pl. sample of 5% acetone in toluene with the acetone chromatographically separated from the toluene, captured in the I R cell, and scanned in 16 seconds. The library reference standard spectruni (top) is a scan of acetone vapor obtained .with a 10meter path cell. .igain the two halves of
the spectrum have been rearranged to look more familiar to demonstrate that identification of the GC peak may readily be made by coinparing its IR spectrum with a standard. This instrument was designed for sizable components, 0.5 nig. or more, if increased senbitivity is needed, a longer cell would have to be used. For this cell, a t a carrier flow rate of 1 nil. per second, 12 seconds is required to fill the cell, 16 seconds to scan, 4 seconds for resetting and purge, for a total of 32 seconds. This is the minimum time required between adjacent GC peaks. Figure 4 shows spectra (rearranged) of three alcohols. In each case the GC sample was a 10-l~I.load of a 10% misture of alcohol in toluene and scanned in 16 seconds. Identification bv comparison with library standards possi ble. Figure 5 shows spectra (normal chart presentation) of some chlorobenzenes. This also indicates that complete GC separation of peaks is not required to obtain quite pure samples for IR identification, if the volume of the IR cell is small relative to the available sample volume. By filling the IR cell from the leading edge of peak 1 (taking a narrow slice out of 1) very little contamination from peak 2 is found since bandv clue to o-dichlorobenzene (peak 2) do not appear in the spectrum of peak 1 (identified as p-dichlorobenzene). The
p-dichlorobenzene is an 8% component again using a 10-pI. load and 16-second >can time. The spectrum on the right is included to demonstrate that relatively high-boilers may be easily separated chromatographically and identified by their I R vapor spectrum. 1,2,3,5-tetrachlorobenzene has a boiling point of 246' C. Our experience with this instrument haq indicated it requires some familiarization because the spectra are of vapor samples and are split into halves, but the advantage of obtaining an IR spectrum in 16 seconds to allow specific identification of each GC peak in a complicated sample makes it all worthwhile. ACKNOWLEDGMENT
The authors are grateful to L. Gould and his associates in the Mechanical Fabrication and Development Laboratory of The Dow Chemical Co. for valuable a'sistance in the design and construction of the light pipe cell. The contributions of C. Pratt, L. Westover, L. Herscher, W. Felmlee, and D . Erley (of this laboratory) are also very much aplireciated. RECEIVEDfor review -4pril 1, 1964. Accepted June 24, 1964. Pittsburgh Conference on Analytical Chemistry and hpplied Spectroscopy, Pittsburgh, Pa., 1964.
Construction and Performance of an Infrared Chromatog rap hic Fraction Analyzer P. A. WILKS, Jr. Wilks Scientific Corp., South Norwalk, Conn.
R. A. BROWN ESSO Research and Engineering Co., linden, N . 1. Because the gas chromatograph does not provide specific qualitative information, it is desirable to analyze chromatographic fractions as they appear in the carrier gas by some means such as infrared spectrometry. To accomplish this, a commercial infrared spectrophotometer has been modified for fast scanning ( 1 5 seconds for the 2.5- to 7-micron region) and equipped with a long, internally reflecting tube as an absorption cell. With a strip chart recorder and automatic control circuits, the instrument will produce a spectrum of any selected portion of the infrared spectrum approximately every 30 seconds. The cell and feeder lines can be heated to 250' C., and are designed so
1896
ANALYTICAL CHEMISTRY
that there is a minimum of mixing in the carrier gas stream as it flows through them. Spectra have been obtained on materials boiling over 300" C. Data indicate that strong bands will be readily measured in effluents of 0.3 mg.
R
mmTrox TIJII:S are rommonly used to identify gas chromatographic fractions. This is generally satisfactory for samples of known history. Gas chromntogral)hy i,5 widely used also in determining the conii)osition of mistures about which little or nothing may be known. In this tylle of alqdication, retention times are not satisfactory for component identific.ation.
To obtain more positive identification of effluents, a number of laboratories now use fast recording mass spectrometers to measure mass spectra of fractions as they leave a packed column chromatograph ( 2 , 5 , S). Infrared spectra of individual fractions can also serve to identify components in a gas chromatogram. Infrared measurements were initially used for this purpose whereby individual fractions were trapped and then examined on an infrared spectrometer (3, 7 ) . .in advancement of this technique was to employ an infrared spectrometer equipped with a long-path gas cell of small volume ( 4 , fi, ,9-11). The procedure with such equipment is to mani1)ulate valves to entrap a fraction and to record its
