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
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Figure 1.
Polystyrene spectra at slow and fast rates
spectruin at a normal scan rate. The utility of a gas cell ha,. now been conibined with a rapid sc-an infrared spectrometer as will be described. Thih spectrometer covers the spectral range, 2.5 to 15 microns, in 45 seconds and any portion of .the range can be scanned in proportionately less time. ITseful spectra can be obtained for components present in a liquid volume of less than 0.3 111. and having boiling points above 300" (1. In practice, spectra are usually measured concurrent with the chromatographic separation. Valves are provided, however, so that fraction.: can be trapped if desired. -4 different rapid iscan spectrophotometer was recently d'escribed by Bartz and Ruhl ( 1 ) . This in,struinent employs two single beam monochromators, each of which scans a portion of the spectrum in 16 seconds. I t s sample system is similar t o that described in this paper. EXPERIMENTAL
The Spectrometer. Infrared spectrophotometers are designed, built, and marketed t'o fill innumerable needs. T h e cost to a purchaser is directlg- related to what he demands. One prominent demand continues to be the application t o quantitative and qualitative organic a.nal cialized form of this effort is the analysis of mixtures liy conihiried gas chromatography and infrared spectrometry. I n this work t:he spectrometer need not suppl>- quantitative informntion since this originates from the chromat,ogral)h. I n addition, the spec-tral
Figure 2.
Schematic of rapid scan infrared analyzer
requirement for qualitative anal! less stringent' because the chromatographic effluent 1imit s the number of component,s to one as a rule. 111essence, commercial spertromrters I'rovide hetter quality spectra than are needed to measure and ident,ify effluents. A study of the characteristics for commercial sl)ectroI)hotometers confirmed this. Scanning times of several minutes were used to record a complete spect,rum a t a high degree of photometric accuracy. Rat,her narrow slits were needed to provide adequate spectral resolution. Since these characteristics are achieved at a sacrifice in light energy, it was felt t'hat a commercial inst,rument might be modified for faster wans by making more light energy available for detection. Greater energy is attainable by using wider slits. This increases the energy by the square
available infrared slwtrometer such as the Perkin-Elmrr Infracord could be modified t q provide good quality sliectra over the 2- to 15-micron range in a scan time of 45 swonds. Figure 1 presents a caoinparison bctwecn 1)olystyrene spectra run at the rate.; of 3 minutps and 45 swonds over the ranpe 2.5 to 15 microns. An amlilifier gain I w t r u n i to Iroduce a Flight, overshoot and hence enhance t'he qualitative ay)ects of the >pc>ctruni a t the esl)ense of accuracy. This is an arbitrary adjustment and can he varied arcording to analytical demand. The rebohition shown in the 45->rc*ond spectrum is of the order of 0.1 micaron.
r . 1 he only changc made in the spc'c.trophotometer (other than those madr i n ) \m.s to incrra+cx thv servo motor Ijullcy by a facator of two and to incrpasr thc slit opening. The servo balancing niotor functioned satisfactorily with this change. 'The larger pulley derreascs reslionsc timr of the sl)ectrol,hotometer. M?th these changes the noise lrvrl of the inqtrument is still satisfactory tiecause of off-setting efferts of incrmsed responsc 1's. higher energy lrvcl. Flow-Through Gas Cells. 'The nest prolilem in making the gas cshromatograjh and infrared spectro~iliotonieter coniliatihle is to pro\-itie an absorption chamher which allows a masimum exposure of niol(~c~u1esin the G C fraction to the infrarrri heam and t o a1.o allow t h r c'arricr gah strcam t o flow with a minimum of tur1)ulencc and mixing. 3Iultil)ass cells of the Khite type ( 1 1 ) 1)rovide the required sensitivit. but sincae their (arosb section is new. sarily much greater than the colunin and ronnwting lines. thc>y cannot lw filled and emptied without ausi1int.y pumliing. Xinimum vohinie (.ell>which mat(-h the contours of tht. sl)ectromc+c~ beam still cause ronsidei~a1)lrtur1)iilciic~r and lark sensitivi1y. It n-as found that a long intcrnall~. reflecting tubf with a ( w x s wation 01' approsiinatclj. thc sanw area a- thr image of the source 1)rovidtd suffiri(\nt s;c,n;itivity with a niininiunl of tul.l)n1cnc.e. T h e infrared mcrgy f i m i the, s o u r c ~is focwsetl on the entrtincae cnd of the tulic and t h r enrrgy emc,rging froin the other end is ~ ~ c - i n i a p ~ ond t l i v entranre slits of the ~:l,t'c'tronirtc~l,,
VOL. 36, NO. 10, SEPTEMBER 1964
1897
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Somo of the energy is t,ransmitted straight through the t,ulw with the rcst of it being rc+lecstcd hack and forth from the polkhcd \valls. Reflection lossif~sarc r(,l:ttivel>- small because of the low angle of invidence. In the 30cam. tube used it is estimated that' more than so%, of thc. cncrgy I'assrs through it. 'I'hc Iriigt,h of' t h r tubc-which, with t h i s tylw of ol)ticd systcm can be any i ~ c a ~ n n l ) lIciigth--\vas (~ selected t'o best, iriat,cah the. rhara(~tcrist~ics of the rarricxr gas st,iwni. It, was desired that thc t,iil)c rontaiii as much of a given fract,ion as I)ossible to provide maximum sensitivity and hold it long enough so that a sl)cctlruin could be rerordrd, yet it should not be so long that, there arr extended periods of overlap as a rrsiilt of more than one fracation ocwil)ying t,he t,iil)e at a time. 'I'hc 30-c~rn. tu!)r having a cross s c ~ion t 01' 0.5 x 1 .5 r i n . has a volume of 22.5, nil., which is ahout, tquivalent t o 0.5 miniifr of a t,ylii(d GC flow rate. 'I'h t.iL1w \vas fornicd b y t,hc electrofoi.ininc of nirkc.1 over a I)olished I)last'ic
I
50
Infrared spectra of five gas chromatographic fractions
heated to a maximum temperature of 250" C. so as to minimize condensat,ion of fractions. Valves a t either end of the sample tube permit three alternative modes of operation: gas may flow steadily through the cell; fractions may be trapped for scanning while the cell is by-passed by t,he chromatograph gas flow; fractions may be trapped and the chromatograph gas flow stopped. These valves are manually operated so that a few seconds are required to change sample flows. Recording of Spectra. Spectral presentation is on a Varian strip chart recorder. T h e standard Perkin-Elmer scale expansion attachment provides t h e signal. T h e Infracord chart' drum has been replaced by a cam and microswitch t o select the starting and ending point' of t,he spectral region to be scanned Spectra are recorded in both direct,ions and the instrument will cycle cont'inuously back and forth through the selected region or make one complete cycle and stop. RESULTS
To date, experience with the rapid scan infrared spectrometer is limited. Its capahilitg is illustrated by the spectra in Figures 3, 4, 5 , and 6 which were obtained from two temperature1)rogrammed runs. The cursory results indicate resolution is satisfactory and that nominally intense bands can be readily detected. 1898
ANALYTICAL CHEMISTRY
.i problem in timing was encountered, however. Thus, the scan button was pushed three seconds after a maximum was observed in a chromat,ogral)hic peak. With this technique, the spectrum of benzonitrile was inferior and some other fractions were not scanned a t their maximal concentrations in the cell as is noted in the detailed discussion that follows. To remedy this trouble, a detector is to be installed at the cell entrance. Output from the detector will show on a meter and can also be used to trigger a scan. Spectra for Low Boiling Point Mixture. Spectra are shown for the five components, each present to 20 wt. %, in order of their elution. Each spectrum is representative of band intensities that occur for 1.5 pl. of material since 7 . 5 MI. of sample were charged to the chromatograph. The spectrum of each constituent was scanned forward and backward over the region of 2.5 to 11 microns. Only one scan is shown. The ability to quickly measure infrared spectra is emphasized when it is realized that the chromatogra1)hic ]leaks varied in half widths over the range of only 22 to 56 seconds. -411 spectra of the low boiling mixture are shown in Figure 3. ~-HEsI.:NE: (E.P., 63.6" C.). This spectrum shows measurable bands as would be desired. The intense band at
Figure 4. IR spectra (of benzene and toluene fractions from gas chromatograph
11.O microns shows 05% transmittance. Some wale esliansion \vas utilized to awount for this sort of iten deflection. ~ . ~ C ~ : ' r . ~ L l ) ~ ; H (f ~ O \ V I I in Figure 4 the forward w a n ha> fairly intenw bands at 3.3. 5.1, 5 . 5 , and 6.75 niicrons, The backward scan + h o w a comparablr 6.75-1ni(~on absoriition with 110 o t h n notictbable bands. Apimeiitly.
WALE 1 LNSTY
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Figure 6. IR spectra of nitrobenzene and benzophenone fractions from gas chromatograph
bands than to be expected. Most of the samllle had pabsed through the cell prior to the scan. XITROEF;KZI:NE(H.P., 210.9' C . ) . T h e sl)rctlum of Figure 6 has intense at)wrl)tion at, 6.6 niirrons. Shoulders a1.r c i l t s c n d at 6.3 and 6.8 which occur as seltarate bands in better resolving instruments. Thebe shoulders ni part, be attributed to t h t very intense band at 0.6 iiiicrons, however. 13I:szortfl.:xoNl: (13.1'. , 303.9' C.). 13enzol)henone s h u w noticwble bands at 3.8, 6.0, 6.3, and 6.9 microns. For\ ~ a r danti l)nck\vard scam are similar in a1)],earance. LITERATURE CITED
Figure 5. IR spectra of chlorobenzene and benzonitrile fractions from gas chroma tog ra ph
the saml)le had pashed through the cell by the time that the 5.5 and lower bands would havc h e n nieasured. (H.P., 110.6' C.). T h e spectrum of toluene shows intense bands at 3.3., 6.2, and 6.7 microns as would be anticipated. A s in the case of benzene, the lower lxind at 3.3 microns was essentially absent in the backward scan because toluene is no longer in the cc.11. CHLOROBI:KZI.;Kl: (f%.p.> 132.0" c). Sliectra from forward and backn.arti xanh hai-c relatively intense ltands at 6.3 and 6.8 iiiicronh a' sern in Figure 5 , These are the most 1)rominent one5 foi, chlorobcnzenc in this region. I3h:NZOKITRILE (13.P., 191.1" c.). I'lenzonitrile has weaker ahsorlition
(1) Barta, .4. AI., liuhl, H. I)., AXAL. CHEW 36, 18i11LiW:iy,IV. S.,.k5A1,. CHEM. 31, 1'207 (1!159).
I ~ E V E I ~ E Ifor ) review 5Iay 8, 1064. June 2!Ij 1964. Pittsburgh Conference on Analytical Chemistry and hpplied Specatrosc.op)-, Ylarch 3, 1064. Acc,epted
VOL. 36, NO. 10, SEPTEMBER 1964
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