(8) Fischer, R. B., Rhinehammer, T. B., ANAL.CHEW26, 244 (1954). (9) Handley, T. H., private communication
to R. R. Rickard, 1958. (10) Hopkins, B. S., “Chapters in the Chemiapy of the Less Familiar Elements, Vol. 1, Chap. 6, p. 12, Stipes Publishing Co., Champaign, Ill., 1939. (11) Jaquith, R. H., Michigan State Univ. Microfilms (Ann Arbor, Mich.) L. C. Card No. Mic 58-5713, 72 pp., Dissertation Abstr. 19, 1554 (1959). (12) Ketelle, B. H., Boyd, G. E., J . Am. Chem. SOC.73, 1862 (1951). (13) Klein, D. H., Gordon, L., Talanta 1. 3.14 - - f1958). ~ - I
\ - - - - ,
(14) Klein, D. H., Gordon, L., Walnut, T. H., Ibid., 3, 177 (1959). (15) Ibid., 187.
(16) Kleinberg, J., “Collected Radiochemical Procedures,” Los Alamos Scientific Lab. Rept. No. LA-1721 (2nd ed.) (1958). 17) Kervik, W. E., J. Phys. Chem. 59, 690 (1955). 18) Noyes, A. A., Bray, W.C., “A System of Qualitative Analysis for the Rare Elements,” hlacmillan, New York, 1927. 2. physik. Chem. 22, 19) Ostwald, K., 289 (1897). (20) Petrow, H. G., AXAL. CHEM. 26, 1514 (1954). (21) Rickard, R. R., Wyatt, E. I., “Promethium and/or Yttrium Activity in daueous or Organic Solutions.” Method
Master Analytical Manual, TID-7015, Suppl. 3. (22) Shaver, K. J., ANAL.CHEM.28, 2015
(1956). (23) Stevenson, P. C., Nervik, W. E.,
Natl. Acad. Sci.-Natl. Research Council NAS-NS 3020 (1961). (24) S h e , C. R., Gordon, L., ANAL. CHEM.25, 1519 (1953). (25) Vickery, R. C., “Chemistry of the
Lanthanons,” Academic Press, New York, 1953. (26) von Weimarn, P. P., Chem. Revs. 2, 217 (1925).
RECEIVEDfor review August 14, 1961. Accepted December 11, 1961. Southeast-Southwest Meeting ACS, New Orleans, La., Fall of 1961.
Far-Ultraviolet Spectroscopic Detection of Gas Chromatograph Effluent WILBUR KAYE Beckman Instruments, Inc., 2500 Harbor Boulevard, Fullerton, Calif.
b A Beckman far-ultraviolet DK-2 spectrophotometer has been adapted to record the spectra of effluent from a Beckman GC-2 gas chromatograph. Heated flow-through cells of 1.0-, 5.0-, and 10.0-cm. optical path length have been used, and the effects of wavelength and cell hold-up have been evaluated. The absorptivities of almost all organic vapors are sufficiently high to permit their quantitative detection from the chromatograph and, in favorable cases, to allow their identification. M a n y materials possess such high absorptivity to allow the far-ultraviolet detector to exceed the sensitivity of the thermal conductivity detector and even rival the hydrogen flame detector. The optical absorption of effluent may b e obtained as a function of elution time or wavelength. Rapid repetitive wavelength scans are possible.
T
HE FAR-ULTRAVIOLET SpeCtrOphotometer and the gas chromatograph complement each other (6). The former allows the selective and sensitive detection of gases, while the latter provides a convenient purification of sample. The near-ultraviolet spectra of organic compounds are seldom as characteristic as infrared spectra; however, as shorter wavelengths are employed, greater utility is found. At a wavelength of 1700 A,, almost all organic compounds have high absorptivities which may be 1000-fold greater than maximum absorptivities in the infrared region.
Earlier investigations of the farultraviolet region revealed that it had greatest appeal for the study of gases (8). Because of the extreme broadening of absorption bands of condensed phases, the study of liquids and solids in this region is restricted. However, many low molecular weight organic and inorganic gases possess discrete and characteristic far-ultraviolet spectra. The samples emerging from a gas chromatograph are in an ideal condition for study by an ultraviolet spectrophotometer. The handling of hot gases is not difficult, and the isolation of sample components greatly facilitates the study of the spectra of impure samples. Fractions need not be delicately transferred or condensed and may be recovered undamaged from the cell exhaust. Comparison with Other Detectors. Detectors based on far-ultraviolet absorption, infrared absorption, nuclear magnetic resonance. a n d mass spectrometry are selective in t h a t their response t o different compounds may vary considerably. The far-ultraviolet and mass detectors are more sensitive than the other selective detectors and, for this reason, can be coupled directly to the output of the column. The response time of the far-ultraviolet detector can be very fast although timeof-flight mass spectrometers may scan faster. The far-ultraviolet detector can possess a higher quantitative accuracy than other methods, provided absorptivity values on pure compounds can be obtained. Far-ultraviolet spect r a can be partially predicted, although
group frequencies are not as characteristic as infrared spectra. Near-ultraviolet spectra have been used in the identification of fractions isolated by chromatography (8). Radiation a t 2536 A. has been used to monitor polynuclear aromatic compounds in the gas phase from a chromatograph ( 3 ) . Usually the near-ultraviolet absorptivities are lower, and the bands are broader than in far-ultraviolet spectra. INSTRUMENTATION
Figure 1 shows a block diagram of the equipment used. Two gas chromatographs] a Beckman GC-2 and a Beckman ThermotraC, were employed. The unbalance of the thermal conductivity bridge was recorded with a n adjustable recorder of 1-mv. full scale maximum sensitivity. The temperature of the detector block was always maintained equal to or above the temperature of the column. A current of 325 ma. was passed through the filaments of the thermal conductivity bridge. Samples emerging from this detector were conveyed to an absorption cell in the spectrophotometer through a heated ‘/*-inch copper tube. One section of this tube was maintained a t a known temperature which was colder than any other part of the equipment in contact with the sample. A bypass valve between the cold trap and the absorption cell permitted static isolation of any emerging fraction in the cell. A DK-2 monochromator modified for far-ultraviolet performance was used (5, 7). This instrument was purged with dry nitrogen. I n most cases, no compensator was placed in the reference path. The monochromator was equipped with a time drive accessory to VOL. 34, NO. 2, FEBRUARY 1962
287
-.7-' SOURCE
DK.2 MONO,
w
7200
SAMPLE INLET
HELIUM
BYPASS VALVE
UCOLD TRAP
TC DETECTOR
Figure 1.
Block diagram of equipment
permit the scanning of chromatograph effluent at any preset wavelength. Flow-through cells of 1.0-, 5.0-, and 10.0em. path length were used. The windows of these cells were of special farultraviolet transmitting silica,
It is necessary to offset the wavelength adjustment screw a t the left end of the monochromator below 1700 A, The regular wavelength dial is then offset and nonlinear. However, no difficulty 1%-asexperienced in calibrating the mwelength of light because the hydrogen lamp possesses a characteristic knoa n emission spectrum ( 7 ) . The stray light within the 1640- to 1670-A. region varies with slit width. The true zero is easily established by recording the absorption of a large sample of octane. The "cold trap" between the gas chromatograph and the absorption cell was a 19-inch coil of '/*-inch copper tubing immersed in a Dewar flask filled with water or acetone. The temperature in the Dewar was adjusted from -45' to +52" C. for this study, although a wider range could have been used. The temperature of the Dewar flask was determined with a mercury thermometer and a potentiometer with copper-constantan thermocouple. An accuracy of 1 0 . 2 " C . seemed adequate for this work. The absorption cell was maintained at a higher temperature than the cold trap. A special thermostated block was developed to hold the cells. Cell temperatures as high as 110' C. were used. The rapid flow of purge gas caused a heating of the entire sample compartment and prevented higher temperatures from being practical. With further thermal insulation, higher temperatures should be possible. In one experiment, a Beckman flame ionization detector was used. Because a moderately high pressure of sample gas was required and small leaks existed about the ground-glass stopper of the absorption cell, the ionization detector was not connected to the exit of the absorption cell. Rather, the flame unit was substituted for the absorption cell. Absorptivities of organic gases range from below 1 cm.-'to more than 5000 cm.-1 (112,000 liters per molecm.) in the ultraviolet region. B y con-
288
ANALYTICAL CHEMISTRY
trast, the absorptivities of gases seldom exceed 10 cm.-' in the infrared region. A few inorganic gases such as hydrogen, helium, nitrogen, and the rare gases have negligible absorption at wavelengths longer than 1400 A. and make ideal carrier gases. Of the common organic materials, the saturated hydrocarbons have the lowest absorptivities (about 3 cm.-l at 1700 A.), yet the far-ultraviolet detector may be employed to detect alkanes emerging from a gas chromatograph. CELL VOLUME
Optical absorption detectors can be sensitive to both volume and concentration of sample. Thermal and ionization detectors usually are concentration scnsitive only. As long as the volume of the absorption cell is appreciably less than the volume of carrier gas containing a particular eluted sample component, the optical detector n ill be concentration sensitive. If the electronic circuit does not limit performance, the speed and chromatographic resolution will be determined exclusively by the gas chromatograph. An increase in optical path length of the cell results in a n increased absorbance for the same concentration of sample component in the carrier gas. However, when the absorption cell volume approaches that of the carrier gas containing the sample, the absorbance will not be linear with concentration. Chromatographic resolution (separation of adjacent peaks) will suffer. The maximum path length that can be used for linear operation depends upon the effective column cross section of the gas chromatograph. Ultimate concentration sensitivity is determined by the capacity of the gas chromatograph for resolving large samples. Other factors influencing sensitivity are discussed below. The optimum cell length depends upon the volume of carrier gas containing the sample and the path-to-volume ratio of the cell. Figure 2 shows the transmittance and the simultaneously recorded thermal conductance of the chromatographic effluent from the GC-2 gas chromato-
graph for a mixture of three hydrocarbons-isopentane, n-heptane, and n-octane. The time scales for the spectrophotometer and the thermal conductivity recorders ll-ere not identical. The delay between response of the spectrophotometer and the thermal conductivity detector was a few seconds varying with flow rate of carrier gas. The volume of the 1-cm. cell used was 1.5 cc., while that of the 10-em. cell was 30 ce. A flow rate of carrier gas 189 cc. per minute was used. The band width of the first peak (isopentane) in Figure 2.A, at half maximum absorbance is 1.7 seconds, Jvhile the half band m-idth of the corresponding thermal conductivity peak (Figure 2,C) was 4.6 scconds. The volume of carrier gas emerging from the column during a period of 1.7 seconds is 5.3 cc. Since this is times the volume of the optical cell, little distortion of the chromatogram would be expected. I n the case of the 10-em. cell, the volume of carrier gas containing the isopontane fraction is 0.17 times (5.3'30) the volume of the absorption cell. Both resolution and linearity has been impaired, although concentration sensitivity is improved. The two wavelengths, Figures 2 4 , and 2,B, were selected to yield a reasonable absorption. The obvious variation in peak intensity among the three component peaks is attributable t o nonlinearity of response for the 10-em. cell. CARRIER FLOW RATE
An unexpected dependence of resolution on flow rate of carrier gas was observed. B e k e e n flon. rates of 23 and 100
80
YoT 60 40
A TRANSMITTANCE; 1640 A, 1.0 cm.
B TRANSMITTANCE, 1700A., 10 cm. C THERMAL CONDUCTIVITY 0.00154 g. ISOPENTANE 0.00170 g. n-HEPTANE
40
0.00174 g. n-OCTANE
m v. 20
0 2
rnin.
0
Figure 2. Effect of cell volume on the transmittance of column effluent
174
1 00
23ml./min. (STP)
ABSCRBANCE 1605A.
80
0 005ml. ISOPENTANE n-HEPTANE CELL 6 8 ' C . COLUMN 220°C.
60
/Ill TRANSMITTANCE OF CONDENSED SAMPLES A NAPHTHALENE 0.0004 f. 2108 4 1 cm 7.9'C. B n-DECANE 0.0073 g. 1700 A. IO cm, 29.5'C. C ISO-OCTANE 0.0138 g. 1780 A, IO cm,-2.5'C.
J'L.
PATH 5.0cm.
%T
I-
THERMAL CONDUCTIVITY
2o
I 2 min.
Figure 4. Transmittance column effluent
Figure 3. Effect of carrier flow rate on absorbance ofcolumn effluent
174 cc. per minute, the resolution obtained with the thermal conductivity detector decreased with increasing flow rate, presumably because of the diffusion necessary in this detector. Figure 3 s h o w that the resolution actually increases with carrier flow rate for the optical detector. This may be attributed to a more efficient exhaust of sample from the cell where flow rate improves turbulence in the cell. The sensitivity as indicated by peak height goes through a shallow maximum. A &em. optical path cell of 15-cc, volume was used in preparing Figure 3. The use of absorbance rather than transmittance ofI'r.rs the advantage of directly recording a pen displacement proportional to concentration. QUANTITATIVE CALIBRATION
The determination of absorptivities of vapors in the far-ultraviolet region is very difficult, and the discrepancies of literature values attest t o this (4). The major problems are absorption and adsorption of samples in the absorption cells, the purity of the sample, and the difficulties in measuring very low partial pressures. These problems can be largely circumvented b y using a continuous flow of vapor obtaincd from a pure liquid or solid sample. The cold t r a p shown in Figure 1 was valuable in calibrating the instrument for quantitative analysis. The concentration of sample could be controlled in the absorption cell by the temperature of the cold trap. Samples may condense here, and the vapor pressure downstream is determined by the temperature of the tube wall. Once the vapor pressure is known, the absorptivity of the gas can be computed using Beer's law. The identity of the sample and the temperature us. vapor pressure data for the sample must ordinarily be known. However, if the total weight of sample
is knonn or can be computed by a second method. the concentration and vapor pressure t's. temperature data can be determined simp the integrated area under the rlution spectrogram (absorbance us. time) is invariant with respect to tempcrature. Minor difficulties were experienced in applying this method. Figure 4 shows transmittance us. time curves for three compounds. As the temperature of the trap is reduced, the eluting peaks are broadened and reduced in "height." Figure 4,A, shows the behavior of naphthalene. i l n initial quantity of this vapor passes the trap a t a supersaturated concentration. Hov-ever, as soon as some of the naphthalene forms on the trap walls as crystals, the gas rapidly establishes a n equilibrium concentration. The sample in Figure 4,B, behaved as though it condensed in the trap, then migrated as a liquid along the trap, and finally emerged before volatilizing completely a t the designed temperature of the trap. However, it is possible that impurities cause this. The majority of samples gave elution curves like Figure 4,C. The flat peaks permitted an accurate determination of absorbance, hence concentration and absorptivity. Of course, the change in 1 apor pressure due to change in teniperature between the cold trap and the absorption cell must be taken into account. The absorptivity, a, in ern.-', is computed from the following version of Beer's lana=-
A X b X P
< T
X 760
where A is absorbance, b cell length in centimeters, and p vapor pressure of sample in the cold trap. To is the absolute temperature of the absorption cell and T o = 273.1' K. Assuming that 1-gram molecular weight of gas at STP occupies 22.4 liters, one can readily
vs.
time
of
0
condensed
compute absorptivities in liters per molecm. SENSITIVITY
The sensitivity of the optical method in terms of detcctable neight of sample is determined by the absorption cell path-to-volume ratio; the signal-tonoise ratio of the spectrophotometer; and the absorptivity of the sample. Thcse variables arc interrelated. A large path-to-volume ratio requires a small optical aperture which nil1 reduce the signal-to-noise ratio. Also, large n~echanicalslit n idths widm the effective band pass of radiation which will influence the effective absorptivity of the sample (7'). The sensitivity of the optical method in terms of concentration is determined by the capacity of the chromatograph, although the above factors are influential. Xaphthalene was selected to illustrate the sensitivity of the ultraviolet method for it has a very intense absorption band ( a = 125,000 liters per molecm.) a t a relatively long wavelength (2108 -4.)and a n appreciable vapor pressure a t room temperature. Retention time in a 6-foot Apiezon column a t 190' C. is about 20 minutes. Figure 5 shows the spectrum of naphthalene in the vapor state. The absorption band a t 2108 A. is broad enough so that its measured absorbance is not materially influenced b y mechanical slit width values less than 0.5 mm. By increasing the period of the spectrophotometer to its maximum value (12 seconds), reducing the gain until the slit width measured 0.5 n m . , and expanding the ordinate 10 times, t h e emergence of 4 X 10-8 gram of naphthalene from the column can be seen in Figure 6,A. The noise level (about 0.07Yo) is less than a fifth the signal received from this sample; hence the ultraviolet method could sense less than gram of naphthalene. Such VOL. 34, NO. 2, FEBRUARY 1962
289
Table
Gram
x 10-8 2 x lo-’ 2 x 10-5 4 x 10-6 6X 8 x 10-6 1 x 10-5 2 x 10-5 4 x 10-6 4
EFFLUENT
50
GAS P A T H IO E m S c a n 2 5 min
I I800
25
A.
Figure 5. vapor
I 2200
2000
Spectrum of naphthalene
a small sample could not be detected by the thermal conductivity detector, and comparison of performance with the flame ionization detector was made. Figure 6,C, is a flame ionization record of the same size sample as that used in curve A . The hydrogen flame detector has greater sensitivity; however, the strongly sloping background caused by tailing of the isopentane solvent introduces a greater error in the quantitative estimation of naphthalene than the “noise” in curve A. If a different solvent were used lvhich eluted closer in time to the naphthalene, the “tail” of the solvent would have precluded the use of the sensitive hydrogen flame detector. If the solvent 1w-e an alkane, it would have no effect on the ultraviolet method even if its elution time were identical to that of naphthalene. If one were to compute the sensitivity of the ultraviolet and hydrogen flame detectors on the basis of concentration
I.
Weight of Naphthalene vs. Absorbance
(10-cm. cell, 2108A.) Solution, hI1. %T 0.01 99.6 0.01 98.0 0.01 83.0 0.02 69,3 0.03 59.0 0.04 0.05
0.10
HBW, Seconds
79 i8 i2
r-
i l
48.4 42.0
80
3.0
79 79
16.1
0.20
..
rather than n eight of sample, the ultraviolet method might n-ell prove the more sensitive. A larger volume and correspondingly more dilute sample of naphthalmc in isopcntane could be placed in the gas chromatograph and be detected by the ultraviolet method. It is estimated that 0.5 cc. of isopcntane containing 20 p.p.b. of naphthalene could be analyzpd for naphthalcne in this manntsr. It is unlikely that the flame ionization dctector TJ ould be of any use with such a mixture because of solvcnt tailing. The argon ionization detertors and any other nonselective detector ~ o u l d suffer the same limitation. Another advantage of the ultraviolet method may be seen in Figure 6. One might suspect the small absorption
i8
A 0.0017 0.0088
0.0809 0.1593 0.2292 0.3152 0.3767 0.7799 1.52
peak in Figures 6 , d and B, seen only a few seconds after sample intrnduction to a column, to be solvent. However, a study of the spectrum of the material indicated that it was naphthalene. Apparently, the solvent displaces some naphthalene from the Ivalls of the connecting tubing so that the naphthalene concentration momentarily increases as the solvent passes out of the column. The intcnsities of the peak vary with the length of time since the last sample and the amount of naphthalene in the previous sample. This phenomenon has been observed nith othcr sensitive selcctive detectors and is a general, if little recognized. problem. Long connecting tubing aggravates the problem, but it is observable with very short connections.
1850 A. 0.005ml. 1. PENTENE - 1 ( 1 % ) 2. 2 - METHYL PENTENE - 1 ( 1 % ) 3 OCTENE 2 ( 1 % ) ~
.-loo 1670 A. 0 Olml. 1 2 METHYL BUTANE 2 NEOPENTANE 3 CYCLOPENTANE 4 METHYL CYCLOPENTANE 5 2 , 2, 4 - TRIMETHYL PENTANE
-
- 80 .60 %T
lo%T + .40
NORMAL ALKANES
A
4. Amp.
3.
. TRANSMITTANCE, h - 2108
2
.20
A.
PATH . IO cm, 4 x l o - * g.NAPHTHALENE B - DITTO 2 x 10.7 g.SAMPLE C HYDROGEN FLAME DETECTOR
4
x
10-8 g.SAMPLE
Figure 7. Effect of unsaturation, branching, and molecular weight on the transmittance of column effluent Figure 6. Comparison of sensitivity between the ultraviolet and hydrogen flame detectors
290
ANALYTICAL CHEMISTRY
Time scales of transmittance and thermal conductivity records are slightly different. - - Coincidence
-
FIXED COLUMN TEMPERATURE
F
PROGRAMMED TEMPERATURE
I
-eo
- 60 Zl ~
40
.20
39
3e
1640a 0 005 ml C A R R I E R FLOW 61 m l / m , n PAlHlO~m
’
E THERMAL CONDUCTIVITY
Figure 8.
Effect of wavelength on transmittance of column effluent from gasoline
LINEARITY
The optical method has a wide dynamic range. Weights of naphthalene greater than 4 X l o + gram could be measured using shorter absorption cells. Table I shows that the widths of the emerging bands a t half their maximum absorbance remain constant indicating that the chromatograph column was not overloaded. When an attempt n-as made to compute the absorptivity of naphthalene
at the 2108-A. absorption maximum, some difficulty was encountered. The transmittance us. time curves (Figure 4,C) of naphthalene had well developed plateaus after allowing for the supersaturation peak; however, the absorptivities varied with the temperature of the cold trap. At first this was believed due to the known fluorescence of naphthalene, but subsequent experiments in which the absorption cell (1 cm.) was placed a t different distances from
the detector failed to substantiate this. Evaluations of flow rate and adsorption effects failed to explain this. The trouble was finally located in the vapor pressure data used. The equation of Swan and Mack (11) which wm used did not agree with any feasible ClausiusClapyron extrapolation of handbook data (1). The equation log p =
- 35;’8 --I- 10.809
VOL. 34, NO. 2, FEBRUARY 1962
0
291
was derived from the handbook data and finally used. As the temperature of the cold trap was varied from 4.7’ to 29.5’ C., the measured absorptivity ranged from 4770 to 5600 cm.-1 (107,000 to 125,000 liters per mole-cm.). This absorptivity variation is believed attributable to insufficient knowledge of the vapor pressures a t the low pressures involved (0.0137 to 0.149 mm.). The upper value of 5600 cm.-l (125,000 liters per mole-cm.) is believed the more accurate since the vapor pressure and absorbance nere higher in this case. This value is nearly the same as that (122,000 liters per mole-em.) for naphthalene solution (4). In Figure 6 4 , the flow of carrier gas was 174 cc. per minute a t STP, and the measured half band width was 7.8 seconds. Hence, 226 cc. (STP) flowed through the cell in a half band width. The concentration of naphthalene a t the peak is (c = A / a b ) : 0017/10 X 125.000 = 1.35 X moles per liter = 1.74 X grams per ml. Assuming the area under curve 6 4 , to be given by the product of height times half band width, the total grams of naphthalene must be 1.74 X
X 226 or 3.8 X lo-* gram
Considering the number of parameters involved, the discrepancy between the known and measured sample m i g h t is surprisingly small. If one computes the concentration of naphthalene a t the peak of Figure 6,C (hydrogen flame detector), values from to 1.7 X grams per 1.3 X cc. are obtained, depending upon the estimation of the base line. APPLICATIONS
Figure 7 shows the transmittance and thermal conductance us. elution time records of a number of hydrocarbons. A Beckman ThermotraC programmed temperature gas chromatograph, programmed for a linear temperature rise of 10’ C. per minute, was used. The same time base applies to records of A, B, and C. The difference between the transmittance and thermal conductance time bases is indicated for the Cn peak. The 5.0-cm. cell was used. Equal volume mixtures of the “pure” hydrocarbons were prepared, and the volunie
Table II. Absorptivities of Hydrocarbons a (Cm.-1) a, Liters per Compound at 1700 A. Mole-Cm. Cyclohexane 8.5 190 %-Heptane 0.79 17.7 n-Octane 1.41 31.6 2,2,4-Trimet hylpentane 67 1500 n-Decane 2.4 54
292
ANALYTICAL CHEMISTRY
,
20 SEC. REPETITIVE S C A N S
- PROGRAMMED
TEMPERATURE
Figure 9. Repetitive absorbance scans of column effluent from gasoline
of liquid mixture indicated in the figure was introduced into the chromatograph. Small concentrations of impurities are indicated in curves of 7,B and C. The ultraviolet chromatograms often reveal small concentrations of impurities. Under similar conditions, thermal conductance records do not even hint of such impuritics. Figure 7,C and D, shows the elution of a mixture of normal alkanes from npentane to n-undecane. The absorptivities of these homologous compounds increase with molecular weight. Quantitativedata on a few of these compounds were determined using the cold trap and are given in Table 11. Because of their transparency in the far-ultraviolet, the alkanes have been studied as solvents (IO). Apparently the absorption edges of the alkanes move to shorter wavelengths on condensation from the vapor to the liquid state. The absorptivity of n-heptane vapor is 17.7 liters per mole-em., while the liquid (purified through a column of activated silica gel) has an absorptivity of 3.2 liters per mole-em. at 1700 A. coniputcd from (8). Figure 7,B, shows the transmittance of some branched and cyclic saturated hydrocarbons. While the quantity of sample was less than in the case of curve 7 , C , the absorption of light is greater. I n all cases studied, branching moves the absorption edge to longer wavelengths. Cyclic compounds have slightly greater absorptivities than the corresponding linear compounds. Xhile normal and branched alkanes shorn an increasing absorption with decreasing wavelength (the absorption peak occurs below the limit of this instrument), the olefins exhibit a mayimum absorptivity at about 1850 A. The absorptivities a t 1850 A. of the olefins is more than a thousand fold greater than that of the n-alkanes a t 1850 A. Figure 7 4 , shows some of these compounds. With increasing unsaturation, the absorption mayimum
moves to longer mavelengths and increases the absorptivity. Gasoline illustrates the behavior of the ultraviolet detector for complicated mixtures. Figure 8 shows the results of the sample (regular Shell). which likely contained hundreds of isomeric C4 to CI1 compounds (9). The same column and temperature program used for Figure 7 was used for the right-hand curves in Figure 8. The appearance of the ultraviolet chromatogram changes considerably with wivelength. While 26 components can be identified in the thermal conductance record (curves C, E , and L ) , 51 mere distinguished from the ultraviolet data, The readily identified peaks are indicated by number. I n some cases (such as peak 3). the ultraviolet rccord revealed a component eluting from the column a t a time n-hen nothing could be discerned on the thermal conductance record. Thermal conductance chromatograms were prepared simultaneously Ivith each ultraviolet record and gave no indication of any change in behavior of the column or any holdover of high molecular weight components during this experiment. Using a wavelength of 1640 or 1670 A., all components of the gasoline would be expected to absorb (unequally). At 1750 A., the n-alkanes should have such small absorptivities so as not to be visible. The mono-olefins should be most apparent at 1850 A., while all the saturated compounds would have negligible absorption. The diolefins and some aromatics should be responsible for most of the absorption a t 2100 A. Aromatics should contribute most of the absorption at 2200 and 2400 A. Repetitive scanning of wavelength during the elution of components should yield additional information that might permit the identification of fractions. Figure 9 shows such scans. A special reversible polarit? 110-volt, 200-ma. d.c. power supply was used to power the mrelength driver motor of
the DK-2. A 1.5-volt battery was connected across the ends of the absorbance slide-wire of the DK-2 pen, and the potential between one end of the absorbance slide-wire and the wiper was used to drive an auxiliary strip chart recorder. The fastest time constant of the DK-2 was used. The n al-elength interval between 1650 and 2200 A. was scanned in a period of 20 seconds. At the end of 20 seconds, the direction of wavelength scan \\as reversed. The bqinning of each scan was indicated on the simultaneously recordcd t h c r n d conductance record (bottom curve of Figure 9). Only one component of the gasoline was unambiguously determined in this n a y . Coniponcnt 45 (Figure 8) \vas readily identified in scan 44 of Figure 9 as naphthalene.. By repeating the chromatograph and turning the bypass valve (Figure 1) a t the time the thermal
conductance record reached the interval labeled 44 on Figure 9, the fraction was trapped in the absorption cell, and its spectrum could be recorded easily. The spectrum was almost identical to Figure 5. Other components of the gasoline could undoubtedly be identified if a longer column of greater resolution were used. A “library” of far-ultraviolet reference spectra would be needed. While many of the compounds found in gasoline do not possess spectra exhibiting sharp absorption bands, the exact wavelength of the absorption varies considerably and could assist identification. LITERATURE CITED
(1) “Handbook of Chemistry and Physics,” C. D. Hodgman, ed., 41st Ed., Chemical Rubber Pub. Co., Cleveland, 1959.
(2) Helm, R. V., Latham, D. R., Ferrin, C. R., Ball, J. S., ANAL. CHEM. 32,
1765-7 (1960).
( 3 ) Johnstone, R. A. W.,Douglas, A. G., Chein. & Ind. (London) 1959, 154.
(4) Jones, L. c.,Jr., Taylor, L. CHEM.27.228-37 (1955).
w., ANAL.
Kaye, W. I., Proceedings, Fifth International Instruments and Measurements Conf., 1960. ( 7 ) Kaye, W. I., A p p l . Spectroscopy 15, 89-95 f 1961’1. (8) Ibid.,‘p. 130-44. (9) Lichtenfels, D . H., Fleck, S. A., Burrow, F. H., Coggshall, N. D., ANAL. CHEM.28, 1376-9 (1956). (10) Potts, \T7. J.. Jr., J . Chem. Phys. 20, 809-10 (1952). (11) Swan, T. H., Mack, E., Jr., J . Am. Chem. Soc. 47, 2112-16 (1925). RECEIVEDfor review May 8, 1961. Accepted December 11, 1961. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1961.
Correlation of Microwave Absorption and Moisture in Polymers SIR: Some microwave transmission mensuremepts nere made at 3 cm. to determine the correlation with moisture in rubber bales (28 X 14 X 7 inches) consisting of a copolymer of butadiene and styrene (specifically GRS-1500 and GRS-1712-type rubber). The absorption of radio frequency power as a function of moisture concentration is a n order of magnitude stronger than can be accounted for on the assumption that the moisture exists in the form of small, roughly spherical pockets of water dispersed in the voids of the rubber bale. The strong absorption can be reconciled with a moisture distribution consisting of thin surfaces or filaments. The direct current conductivity of the bales was very low, showing t h a t the moisture paths were discontinuous. Typical data are s h o m in Figure 1. The slope varied with bale temperature and rubber type. Such variations are to be expected because the relaxation absorption spectrum for liquid water shifts not only with temperature changes but also with changes in the concentra-
Figure 1 . Plot of In transmission vs. per cent moisture a t room temperature for
GRS-1712
t
-b 0 -7
0.2
tion and type of polar and ionic compounds which were undoubtedly dissolved in the moisture. The microwave system included transmission and receiving horns plus a collimating device Of nonreflective material which directed the radio frequency
0.4
0.6
0.8
1.0
1.2
1.4
1.6
power through the %-inch dimension of the rubber bale. JACK MERRITT Shell Development Co. Emeryville, Calif. RECEIVED for review xovember 20, 1961. Accepted December 5, 1961. VOL. 34, NO. 2, FEBRUARY 1962
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