Figure 2. Infrared spectrum of ammonium hexafluoroferrate(" using a Halocarbon mull
Halocarbon oil series 11-14 has a viscosity similar to that of Nujol, and is as easy to work with. It has been used successfully in this laboratory, not only for inorganic compounds, but for organic compounds as well (Salts] of l-1CIethylpiperidones, Lyle, R. E., Adel, R. E., Lyle, G. G., J . Org. Chem., submi ;ted for publication). The instrument uried for this work mas a Perkin-Elmer h4'odel 21 with sodium chloride optics. ACKNOWLEDGMENT
08 10
4000
BOOO
2CaO
le00
1400
iboo
FREQUENCY
, CM:'
I200
1000
800
Figure 3. Infrared spectra of Nujol and Halocarbon oil showing interferencefree regions
The authors are indebted to Charlotte Lutinski, Perkin-Elmer Corp., for a. literature search on the use of Halocarbon oil as a mulling agent. WORKsupported in part by the Atomic Energy Commission.
Improved Scanner for Radioactive Paper Strips Frank Eisenberg, Jr., and Irwin G. Leder, National Institute of Arthritis and Metabolic Diseases, National lnstituies of Health Public Health Service, United States Department of Health, Education, and Welfare, Bethesda, Md. HE use of gas flow counters [LorvenTstein, S. LI.,Cohen, P. P., Nucleonics 14, No. 5, 98 (1956)l in the scanning of radioactive paper strips has resulted in an improvement over the older, relatively insensitive G-AI scanners [Solomay, S., Rennie, E. J., Stetten, D., Jr., Ibid., 10, No. 4, 52 (1952)]. The structural simplicity and high sensitivity for weak p-emitters of the Forro (Volk Radiocheinical Co.. 5412 North Clark St., Chicago 40, Ill.) iiianual chromatogram scanner have suggested its application to automatic paper strip scanning. By combining the Forro tube with the rigid metal strip carrier and drive mechanism available from the Nuclear Instrument and Chemical Corp., a simple, easily constructed automatic paper strip scanner has been devised which exhibits a reproducible and strictly linear relationship between corresponding points on the chromatogram and the record.
Construction. A Nuclear paper strip G-ilI scanner, previously in use in this laboratory, has been modified by replacement of tlie Jfodel 3031B shield and G-RI tube by the Foryo scanner. The other components-the Xodel 1615B scaler, the Model C-100 strip feeder and metal carrier strip, and the Model AW 1.0-ma. EsterlineAngus recorder have all been retained and can be purchased separately. T o conserve space, the strip feeder is driven by the recorder through a short coupling instead of the long flexible cable originally supplied. I n combining the Forro scanner with
I I
Figure 1. Modified Forro scanner in cornbination with Nuclear strip feeder
,
/I
DRIVE R O L L E R S
the Wuclear drive apparatus certain modifications have been introduced into both components. The scanner, shaded in Figure 1, is inverted with respect to its position in manual scanning, so that the paper is drawn under the tube rather than over. The feature of attaching the paper to a metal carrier strip for scanning has been retained, but because the guides for the carrier strip are part of the discarded lead shield, a new guide system has been devised. Flexafranie posts, grooved inch deep), have been mounted on the strip feeder base plate, tlie grooves serving to keep the carrier strip level in its passage through the rollers. Both posts are threaded and screived into tapped holes in the base plate; the right-hand post is threaded up to the groove. The height of the grooves is adjustable by turning the posts in or out. Lock nuts a t the base hold each post a t the desired height. The right-hand post
also supports the scanner and is mounted so that the scanner is as close to the drive rollers as possible and with the center of the window (,entered on the rollerb. Sufficient clearaiice should be provided to enable the sc'anner t o be raised for cleaning or replacing the window. A hexagonal nut, held in place by :I sctscrew tapped ir t o one face, maintains the instrument a t its proper height. Both posts are located symmetrically about the centei- line of the base plate. To keep the carrier strip moving a t right angles to the scanner and in close approximation t o the window, a rabbct inch wide x 7/32 inch high) has been milled frori the scanner block. The paper strip is held snugly against the n indow by 1hree polished aluininuni pins (1,~'~-inch dirtmeter), mounted on tlic scailner block a 3 s1ion.11 in the diagram. Pins 1 and 2 (each 12 mni. from the center of the tube) maintain the curvature of the paper around the tube; the third pin (4 mm. behind and 7 mni. below the second pin) provides ad& VOL. 31, NO. 4, APRIL 1959
627
tional friction to keep the paper taut as it moves through the apparatus. To minimize handling of the paper strip and to facilitate threading it around the pins, the paper is attached to a tab (la/* X 43/4 inches) made of rubber-coated cloth (Aldan Rubber Co., Tioga and Salmon Sts., Philadelphia, Pa.) or any other soft pliable nonstretching fabric. The tab is fixed to the metal carrier strip a t one end by means of an aluminun~plate (1/2 X 11/4 inch) and screws which go through the fabric and thread into tapped holes in the carrier strip. Operation. The operation of the scaler and recorder and the provision for gassing the scanner are all explained in the literature accompanying these components. The discussion is limited here to the loading and scanning of the paper strip. The chromatogram (11/2 inches wide, maximum) is fastened with cellophane tape (1/2 inch wide) to the underside of the tab, which overlaps the paper a t least 1/4 inch. The carrier strip is inserted in the grooves and pushed forward until the strip just engages the rollers. With the scanner in place the tab is drawn aside and threaded around the pins and scanner tube, as shown. The tension developed in the paper by its passage around the pins makes it unnecessary to tape the free end of the paper to the carrier strip. Heavy papers such as Whatman No. 3 are not flexible enough to pass smoothly around all three pins, so that in scanning these papers the third pin is not used. To relate points on the record to corresponding points on the chromatogram the following procedure is satisfactory. The carrier strip is manually pushed through the rollers until a reference p o i n t - e g . , origin-is almost up to the second pin. The recorder drive is then started and the slack taken up. When the reference point is precisely over the second pin, the drive is stopped and a pen line is made on the recording strip to serve as a starting marker by momentarily turning the scale selector to
4z-’
5.0
.5 0 Figure 2. Comparison of sensitivities of G-M:and without window
Figure 3. narrow s i t
Resolution
with
“Calibration.” The drive mechanism is started again and the paper, strip scanned. The drive stops automatically when the far end of the carrier strip passes the niicroswitch (not shown) mounted cenltially on the base plate and just to the rear of pin 3. The distance from tlw reference point a t the second pin to the center of the scanner tube window measured on the paper strip is 18 mm. for this machine, so that the Corresponding reference point on the record is 18 Inm. from the starting marker. Bectme the paper strip and scanning record move a t the same rate, corresponding points on the record and chromatogram are then equidistant from their resI1t:ctive reference points.
A scanning record of a carbon-14-
Forro scanners, with and
labeled chromatogram is shown in Figure 2. The lower curve was recorded by the original G-M apparatus, the middle curve by the Forro scanner with the window, and the upper curve without the window. The ratio of sensitivities is about 1:3:5. The procedure for windowless scanning is the same as described above and is used mainly for measuring tritium radiation which cannot penetrate the window. The resolution obtainable with the narrow (l/ls-inch) slit of this scanner is illustrated in Figure 3, which is the scanning record‘- for paper strip moving a t the rate of 6 inches per hour and marked with six radioactive bands separated by distances, respectively, of 1, 2, 3, 4, and 5 mm. The peak a t the right end represents the two bands separated by 1 mm.; the remaining four peaks are clearly separated, showing that the scanner can resolve bands as close as 2 mm. The area under a peak can be used as an approximate quantitative measure of the amount of radioactivity on the paper strip. When three radioactive bands were scanned, the areas under the peaks were found to be proportional within 10% t o the radioactivity of the bands. This apparatus and a duplicate model have been in use in several laboratories a t these institutes, for over a year, with satisfactory results.
Window Materials for Use in Infrared Analyse!; Involving Nitrogen Dioxide Aubrey P. Altshuller and Israel R. Cohen, Community Air Pclllution Program, Robert A. Taft Sanitary Engineering Center, Public Health Service, U. S. Department o f Health, Education, and Welfare, Cincinnati 26, Ohio
dioxide has been reported N to attack crystals of sodium chloride (3, 5, 6 , Q), potassium bromide (9), ITROGEN
and cesium iodide (9). I n gas cells sodium nitrate is formed on sodium chloride cell rvindoas and nitrosyl chloride is present in the gas phase. The presence of 0.1% or more of nitrogen 628
ANALYTICAL CHEMISTRY
dioxide will daruage the sodium chloride windows in gas cells on a single esposure (6). Appreciable amounts of w t e r vapor accelerale the attack of nitrogen dioxide (5). Jeveral tenths of 1% of nitrogen dio cide in carbon tetrachloride after threv or four determinations caused pronounced fogging of sodium
chloride windom in a liquid cell. The stronger sodium nitrate absorption band a t 7.3 microns is readily observable. Silver chloride cell windows have been used successfully in gas cells (6, 9) containing nitrogen dioxide and in a low temperature liquid cell (9). Silver
