precise results, have been invaluable diagnostic tools. However, the fact that radioactive materials are injected into the body makes these methods contraindicated for certain patients such as children and pregnant women. By use of nonradioactive chromium solutions and the new gas chromatographic method for the determination of chromium in blood and plasma, this diagnostic tool would be available to all patients. Procedures using radioactive chromium result in chromium increases of less than 1 ng/ml of blood. However, because of the relatively low toxicity of soluble chromium compounds ( I ) , enough chromium can be used to give a 50 to 100 ng/ml of blood increase, which can easily be measured by the G C method. Also, this method could possibly be applied to other radioactive metals used for diagnosis so that
radioactivity would nJt have to be injected or ingested by man. ACKNOWLEDGMENT
The authors thank W. D. Ross, M. L. Taylor, and E. L. Arnold for their assistance and counsel. RECEIVED for review August 24,1970. Accepted December 7, 1970. Research was sponsored by the Aerospace Research Laboratories, Air Force Systems Command, United States Air Force, Contract No. F33615-69-C-1062. Research was performed at Monsanto Research Corporation, Dayton Laboratory, 1515 Nicholas Road, Dayton, Ohio 45407.
Rapid Scan Infrared Spectrometer for Operation with Support Coated Open Tubular or Packed Column Gas Chromatographs R. A. Brown, J. M. Kelliher, and J. J. Heigl Esso Research and Engineering Co., Linden, N . J . C. W. Warren1 Instruments & Communications, Znc., Wilton, Conn. An infrared spectrometer has been developed to record spectra of gas liquid chromatographic fractions from packed or support coated open tubular (SCOT) columns. The instrument employs a four filter Czerny-Turner monochromator with 90° phase null electronics. A Nernst source is chopped at 400 Hz in front of the sample and reference cells. Spectra are measured as sample flows through a windowless multiple internal reflectance cell of 1.0-ml volume. The cell is heated up to 250 OC. A helium cooled mercury doped germanium detector is used. Scan speeds of 5 to 20 seconds cover the 3700-750 cm-’region. Conventional double beam transmittance and frequency data are presented on a linear wave number chart. Good quality spectra are obtained for as little as 20 pg of sample. Functional group bands are observed at the 3-pg level.
SINCE1964, A NUMBER of infrared spectrometers have been modified or adapted to record spectra of gas chromatographic fractions (1-3). These instruments could be connected to packed column gas chromatographs and spectra observed of fractions “on the fly” or trapped in special sample cells. Moderate quality spectra were obtained at scan speeds of 5 to 40 seconds. Minimum sample requirements varied from 0.1-1.0 p1 (100-1000 pg) depending upon the type of compound involved. An advance in capability was achieved by Low and Freeman (4) who employed a multiple scan interference spectrometer to monitor effluents from a packed column gas chromato-
graph. This instrument can scan the 2500- to 250-cm-I region in 1 second. Good sensitivity was attainable only when based on multiple scans (100-300 in number). More recently, Krakow (5) discussed the application of a grating spectrometer that continually produced spectra at the rate of one spectrum in 0.7 second. This particular instrument was limited in its spectral range of 3700 cm-I to 1100 cm-I due to the cryogenic detectors that were used. Wilks (6) and Gilby (7) used standard spectrophotometers equipped with micro cells to operate in the 25-pg range. As Whetsel (8) notes, fractions from a gas chromatographic separation represent ideal samples for study by infrared. To more fully apply infrared to identification of chromatographic fractions, greater sensitivity is required. In a n effort to achieve this as well as other improvements, a project was initiated to build a spectrometer that would be an effective analytical tool. Most of the original design goals were achieved. These are : Scan rate of five seconds to cover the spectral range, 3700 cm-I to 650 cm-l. Double beam operation. Handle sample sizes at the microgram level. Resolution sufficient to identify component(s) in a gas chromatographic fraction.
Present address, American Cyanamid Co., Bound Brook, N. J.
This paper describes the instrument and its applicability to packed column and support coated open tubular (SCOT) gas chromatography.
(1) A. M. Bartz and H. D. Ruhl, ANAL.CHEM.,36, 1892 (1964). (2) P. A. Wilks, Jr., and R. A. Brown, ibid.,p, 1896. (3) G. T. Keahl, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., Feb. 25, 1966, Paper 209. (4) M. J. D. Low and S. K . Freeman, ANAL.CHEM.,39, 194 (1967).
( 5 ) Burton Krakow, ANAL.CHEM.,41,815 (1969). (6) P. A. Wilks, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 8, 1968. (7) A. C . Gilby, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1970. (8) Kermit Whetsel, Chern. Eng. News, 46,(6), 82 (Feb. 5 , 1968).
1
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
353
DETECTOR
HELIUM
n
SOURCE
FILTERS
Figure 3. IR-GC cell
spectrometer gas
Figure 1. IR-GC spectrometer optical system layout Reference and sample beams are chopped 90" out of phase, separated in the vertical plane and both beams are recombined by the detector
Phase Null .4mplifier. In the phase null technique, shown in block diagram in Figure 2, the reference and sample beams are chopped 90" out of phase by a common rotating shutter and, after traversing the sample cell and monochromator, are recombined a t the detector. The chopping frequency employed is 400 Hz per second. Synthesized reference and sample signals are synchronously produced by the chopping motor drive through a generator. The synthesized signals thus generated have the same relative phase separation as the optical signals produced by the detector. As shown in Figure 2, both sets of signals are treated electronically through amplification and comparison, before the combined signals are rectified, filtered, and recorded. A complete description of the phase null principle is included in Reference 9. The phase null method offers excellent response time characteristics and good photometric accuracy in regions of both high and low transmission. This permits scale expansion to be applied without drastically degrading signal-tonoise ratio. It is common practice in these laboratories to utilize this technique up to and beyond a 10-fold expansion factor. Infrared Cell. The infrared cell consists of two optically flat, polished plates assembled as shown in Figure 3. Rods placed in V-grooves along the length of the plates divide the unit into separate sample and reference cells. The volume of each is 1 cubic centimeter as defined by the dimensions 10 cm X 0.1 cm X 1 cm. Sample or helium gas flows into the respective cells through the small tubes at the top and bottom. Outflow of gas occurs a t both ends since no windows are used. Temperatures (up to a maximum of 250 "C) are maintained constant by a thermostatically controlled block in which the cell rests. Some investigations were made of a special multi-reflective cell to optimize the path length to sample volume ratio.
EXPERIMENTAL The Spectrometer. The phase null spectrophotometer ( 9 ) was selected for our application. In our initial assessment, it was felt that this system offered the best promise in meeting our design criteria set forth above. The phase null systrm achieves double beam operation by electronically ratioing the two beams. Fast response time is thus attained. The overall spectrometer and associated components, shown in Figure 1, include a Nernst glower source which is focused by a three-mirror arrangement onto the entrance of the parallel plate sample cell. The beam is chopped directly before the sample cell. After passing through the monochromator's entrance slit, the beam passes through one of four filters which are automatically positioned in a timed sequence. The grating monochromator follows the Czerny-Turner design, with its single grating and two parabolic mirrors for beam focusing. From the exit slit, the beam is focused on the surface of the cryogenic detector which is refrigerated with liquid helium. As described in an earlier presentation (IO), a two-grating system was originally constructed. Later this was replaced with a single grating. This e!iminated discontinuities in the recorded spectrum at the point of grating change. Scan time was also reduced. These advantages were achieved with an insignificant loss in resolution. (9) C. W. Warren. U. S. Patent 3,414,729(1968). (10) C. W. Warren, J. J. Heigl, R. A. Brown. and J. M. Kelliher,
Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 8, 1968, Paper 217.
?RE AMP.
Figure 2. Phase null-block diagram SYNTHESIZING GENERATOR
354
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
SAMPLE
POWER
3000
2500
2000
1500
1000
cm
Figure 4. Spectrum of polystyrene (20-sec scan)
c W
W
W
I 3000
2500
2000
I500
IO00
Perkin-Elmer Model 226 50 ft X 0.02 inch i.d. column-Squalane coated Flow rate 4 ml/min of helium Column Temperature -40°C Sample size, 3.0 ,d Sample splitter used Attenuation X256
Figure 5. Spectrum of polystyrene (5-sec scan)
(11) M. J. E. Golay, “Sixth International Symposium on Gas
Chromatography and Associated Techniques, Rome, Italy, Sept. 20-23, 1966,” A. B. Littlewood, Ed., Institute of Petroleum, London, 1967, p 333. (12) D. S. Erley and B. H. Blake, “Infrared Spectra of Gases and Vapors,” Vol. 11-Grating Spectra, Dow Chemical Company, Midland, Mich., March 1965.
4
Figure 6. Chromatogram of five-component blend
-I
cm
This cell has been described elsewhere (ZZ). It consists of a gold cylinder with highly polished reflective surfaces with dual chambers for sample and reference gases. Provision is made to heat the entire assembly to prevent condensation of the higher boiling constituents. Gas inlet tubes are provided at the ends of the cylinder. The gas effluxes through the optical entrance and exit slits. The data, which are preliminary in nature, show that multiple reflection operation is attained. Under the experimental conditions employed, approximately 20 reflective paths were achieved. This is somewhat short of the expected path numbers calculated from theoretical considerations. An additional publication is planned to describe these experimental results. Monochromator. The monochromator utilizes a Bausch and Lomb replica grating, 2540 lines per inch and blazed at 1790 cm-l. It is used in the first order. The wavelength cam was constructed in accordance with a computer-derived design to produce a recorded output which is linear in frequency. Based on a comparison of spectra with precise measurements by Erley and Blake (IZ), accuracy of the wavelength scale is estimated to be within 0-20 cm-I in the 1000 to 1500 cm-I region. An error of 100 cm-l may occur in the 3000 cm-I region. While some improvement in wavelength scale accuracy is desired, this is not a serious limitation in gas chromatographic applications. Slit construction is based on well established designs; the entrance slit is fixed and only the exit slit varied as the wave-
2 3 TIME IN MINUTES
length interval is scanned. High pass interference filters effectively prevent radiation of undesired orders from reaching the detector. These filters, four in number, are inserted sequentially at 2.4, 3.5, 6.4, and 9.0 microns. A rapidly operating solenoid-actuated rotary stepper is employed for filter insertion. A small recognizable signal may occur at filter change intervals. Detector. The detector, which is refrigerated by liquid helium, is an impurity-activated type (13) manufactured by Texas Instruments, Inc. The germanium-mercury detector selected had been developed for the 700 crn-’-1250 cm-1 region. No degradation in response is experienced at fast chopping frequencies. The order(s) of magnitude greater for D* of this detector as compared with a thermocouple provides a significantly improved signal-to-noise ratio. Low noise spectra are observed as shown in Figures 4-10. Initially, some problems were encountered with loss of liquid helium but these were shortly overcome after storage and transfer facilities were improved. The detector employed initially and with some success was a Golay unit (14) with its acoustic chamber filled with helium in order to decrease response time. The only problem encountered was the presence of occasional spurious signals attributable to microphonism of the detector. This problem was overcome by incorporating a pneumatic ballasting (13) Henry Levinstein, “Applied Optics and Optical Engineering,” Vol. 2 , R. Kingslake, Ed., Academic Press, New York, N. Y., p 311. (14) H. A. Zahl and M. J. E. Golay, Rev. Sci. Instrum. 17, 511
(1946). ANALYTICAL CHEMISTRY, VOL. 43,
NO. 3, MARCH 1971
355
"
1 -
Figure
'
'
I
'
'
'
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\ \
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2500
3000
I
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2000 -I cm
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2000
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I500
,
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,
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,
, l 1500
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l
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)
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Figure 9. Spectra of ethyl acetate
ETHYL ACETATE
_,
2000
I500
1000
crn
356
0
l
approximately 3 seconds after the G C peak reached its maximum height. Accordingly, each peak scan by IR would be started after this 3-second interval. The volume of the infrared sample cell (1 ml) fits it to handle SCOT column fractions in an ideal manner. Typical SCOT column conditions provide a separation which approaches capillary column in efficiency. Mixtures are resolved as fairly narrow peaks that are 5 to 30 seconds in width. A 15-second peak width may be considered as about average. Thus, at a typical carrier gas flow rate of 4 ml per second, most of an average peak can be collected in the 1-ml gas cell 15 sec x 4 ml = 1 ml . 60 sec A 50-foot squalane column of 0.02-inch inside diameter provided fractions for the infrared. One to three microliter samples were injected to the chromatograph and the 1OO:l splitter was used. The 1OO:l splitter allows ljl00 of the injected sample to enter the analytical column when that column has a diameter of 0.01-inch. A much larger proportion of a sample enters a 0.02-inch column such as employed in this work. Measurements of peak areas observed with and without the splitter indicated that approximately
z
2500
~
Figure 8. A . Spectra of acetone B. Spectra of 1-hexene
feature. This feature was not available until after our investigations were concluded. Polystyrene Spectra. The quality of the spectral data is indicated by the polystyrene spectra in Figures 4 and 5 . These spectra were measured at scan speeds of 20 and 5 seconds, respectively. It will be noted that the 20-second scan gives a well resolved spectrum of grating quality. By comparison, the 5-second scan spectrum is inferior in resolution and also shows slightly lowered band intensities. In practice, the 5second scan would usually be used in SCOT column work and a slower scan up to 20 seconds might be employed with packed column separations. Operation of the Spectrometer/Chromatograph. SCOT COLUMNS. In our work a Perkin-Elmer Model No. 226 chromatograph was used in SCOT column work. This was fitted with a Gow-Mac microthermal conductivity detector. The connection between chromatograph and spectrometer consisted of a short, heated tubing of 0.02-inch i.d. To measure the infrared spectrum of a G C peak, it is desirable to know at what time the infrared cell contains most of the G C peak. By trial and error, it was found that this occurs at
3000
l ' 1030
cm-'
1000
Figure 7. A . Spectra of acetaldehyde B. Spectra of furan
U
l
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
9 ETHYL ACETATE
I
I500
20'COLUMN, 1/8" 20% CARBOWAX 1540 CHROMOSORB
IO00
I
I
I
I
I
I500
I
l
l
l
IO00
I
1
4
1
1
I
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I
I
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I
I
IO00
I500
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1000
cm
Figure 10. Infrared spectra of ethyl acetate as GC fraction
one fifth of a sample enters the column. This fractional value was used in calculating the amount of sample observed in the infrared spectrometer. PACKEDCOLUMNS.A Perkin-Elmer Model F-11 chromatograph was connected to the spectrometer for packed column experiments. This connection was the s a n e as that used for tieing in the Model 226 chromatograph. Experiments with the packed column were carried out as described for the SCOT column. RESULTS AND DISCUSSION
Spectra of SCOT Column Fractions. The chromatogram in Figure 6 shows the mixture that was used to evaluate the spectrometer. It consists of a five-component blend containing twenty liquid volume per cent each of acetaldehyde, furan, acetone, I-hexene, and ethyl acetate. Spectra were measured for 1 and 3 p1 of injected sample. This gave levels of 30 and 90 pg for each component to be measured. Spectra are discussed in the order of elution from the chromatograph. ACETALDEHYDE. Two spectra are shown in Figure 7A. It will be noted that 93 pg of sample resulted in 0 % transmission for the carbonyl band at 1780 cm-' and near 0% in the 2900 cm-1 carbon-hydrogen region. A number of other bands are also observed. Most of these bands can be seen for the smaller sample of 31 pg. The 1780 cm-1 band is quite intense and it is apparent that this band would be measurable for a substantially smaller amount of material. FURAN.Spectra in Figure 7B show furan for 38 and 114 pg of sample. The weak carbon-hydrogen band at -3250 cm-' agrees with Erley and Blake (10) whose spectrum No. 50 indicates it to be a weak absorber. Numerous other bands are observed with the most intense one at 980 cm-I. It can be estimated that this band would be observed at a level of 10 pg. ACETONE.Spectra for 32 and 96 pg of acetone are shown
in Figure 8A. This spectrum shows three intense bands at 1750 cm-', 1330 cm-I, and 1200 cm-'. The two latter bands would probably be apparent for 10 pg of acetone, whereas the carbonyl band at 1750 cm-' would be noticeable for considerably less than 10 pg. 1-HEXENE.Figure 8B demonstrates the measurement of an olefin which is a weak absorber. The characteristic double bond band at 900 cm-1 is observed for 27 pg of sample and it would appear that a fairly intense band would occur for 15 pg of the compound. ETHYLACETATE. As shown in Figure 9, ethylacetate is readily observed at 36 and 108 pg. Even at 36 pg, the carbonyl band at 1770 cm-1 as well as the ester band at 1210 cm-1 are very intense. It is apparent that the bands would be observed at a level of 3 pg. Spectra of Packed Columns. Principal effort has been placed upon the role of SCOT column gas chromatography because the superior resolution offered by this technique provides an optimum capability for the infrared spectrometer to perform as an identification tool. The spectrometer also works well with a packed column chromatograph, however. In packed column chromatography peaks are relatively wide and there is ample time to obtain one or several spectra if desired. More material is also available than in SCOT column work. The manner in which the spectrometer functions is illustrated in Figure 10. One microliter of ethyl acetate was charged to a chromatograph having a 20foot, 0.125-inch 0.d. column packed with 20% K 1540 on chromosorb. As the peak eluted, eight successive fivesecond scans were taken. Alternately numbered scans are shown in Figure 10 as they were measured. It will be noted that intense spectra were recorded. Scan 8 is not shown but its spectrum was weak, indicating that sample was quickly swept from the cell as the end of the peak was reached. ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
357
CONCLUSION
ACKNOWLEDGMENT
The infrared spectrometer described herein can provide good quality spectra of gas chromatographic fractions at levels of 20 micrograms or more. Frequently, observable bands occur at the microgram level. Variable scan speeds of 5 to 20 seconds and the small sample cell allow the spectrometer to be used with either SCOT or packed columns.
H. D. Raymond contributed significantly to the mechanical design and construction. H. C. Tsien and J. A. Wilson also assisted on various Phases of the Project. Laboratory measurements were carried out by T. H. Sara.
RECEIVED for review September 8, 1970. Accepted December 18, 1970.
Simple and Inexpensive Electronic Conductivity Manometer for Monitoring Pressure Changes Application to Pressuremetric Titrations of Iodate and Ammonium Ions D. J. Curran and S . J. Swarin Department of Chemistry, Unicersity of Massachusetts, Amherst, Mass. 01002
A novel pressure transducer system based on the electronic monitoring of the electrolyte level in a manometer with linear conductance circuitry has been developed. The instrument has a multirange capability and a high level dc output. I t is simple in design and construction and can be built with readily available electronic units. Its response to the quantity of gas generated in a closed system is linear to within 2-4 parts per thousand. It has been applied to the pressuremetric titrations of iodate with hydrazine sulfate and ammonium ion with electrogenerated hypobromite. Accuracies and precisions of a few parts per thousand have been obtained, down to the concentration level where only 12 pmoles of gas are evolved.
a multirange capability and a high level dc output suitable for recording. It appears to be suitable for use in all of the applications cited previously. Modular construction has been utilized to take maximum advantage of the economy and versatility of commercially available electronic units. We have demonstrated the applicability of this instrument as an end-point detection device in pressuremetric titrations of 14.33and 1.433-mg samples of iodate with hydrazine sulfate and 4.737- and 0.4737-mg samples of ammonium ion with electrogenerated hypobromite. In the first case nitrogen is generated according to: 103-
INRECENTYEARS there has been renewed interest in the applications of pressure measurements in chemistry. This undoubtedly is due to the availability of accurate, sensitive, and precise modern pressure transducer systems (1). Typical applications of these instruments include: monitoring gas evolution or absorption in reaction kinetics (2-4), end point detection in pressuremetric titrations (5, 6),and monitoring analytical hydrogenations (7-9). Simple, inexpensive transducer systems are commercially available, but they lack multirange capability. Systems possessing the latter feature are also commercially available, but at a considerable increase in complexity and cost. The manometer has always been a common instrument for pressure measurements because of its simplicity in principle and construction, and its low cost. However, manometry requires numerous readings and constant operator attention. As part of our studies of applications of pressure transducers in chemical analysis, we have developed a simple and inexpensive pressure transducer system, based on a U-tube manometer, which is sensitive, precise, and compact. This instrument has ( I ) D. J. Curran, J. Chem. Educ., 41, A465 (1969). 36,2516 (1964). (2) L. R. Mahoney, ANAL.CHEM., (3) T. G . Traylor and C . A. Russel, J. Amer. Chem. SOC.,87, 3698 (1965). (4) W. K. Rohwedder,J . Cutul., 10,47 (1968). ( 5 ) D. J. Curran and J. L. Driscoll, ANAL.CHEM., 38,1746 (1966). (6) D. J. Curran and J . E. Curley, ibid., 42, 373 (1970). (7) A. Reuter,Z. Aiial. Chem., 231,356 (1967). (8) D. J. Curran and J. L. Driscoll, ANAL.CHEM., 42,1414 (1970). (9) D. J. Curran and J. E. Curley, ibid., 43, 118 (1971). 358
212
+ 51- + 6Hf
+ NZH4 .H?S04
+
Nz
t
+ 3Hz0 + so42-+ 6H+ + 41+
312
(1) (2)
And in the second case : Br-
+ 20H-
+ 3 BrO-
2NH4+
+ H 2 0 + 2e+ 3 Br- + 2H+ + 3H20
+ BrO-
(3)
f
(4)
+ N2
PRINCIPLE OF OPERATION
In 1926, I. B. Smith (10) described a liquid-level detector based on the conductivity bridge principle. By placing two pairs of identical platinum electrodes in the arms of a U-tube manometer containing an electrolyte, and placing the resistance between each pair of electrodes in a Wheatstone bridge circuit, it was found that the difference in the height of the liquid in the two arms could be determined. Smith’s work was based on the conductivity equation:
L = - 1= - - L,A
R
d
where L is the conductance, R is the resistance, L, is the specific conductance, A is the area of each electrode, and d is the distance between the electrodes. Since, for rectangular electrodes, the area is the product of length (I> and width ( w ) , Equation 5 becomes : 1 LS.Z.W L=-=--R d (10) I. B. Smith,J . Opr. SOC.Amer., 12,655 (1926).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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