tration to assure maximum removal of calcium without excessive loss of strontium when using 2 separations. Since fuming or regular concentrated nitric acid is not supplied with an accurate concentration value, i t is rccommended that the concentration be determined before separations ~f this type are made This concentration does not agree with that used by Willard and Goodsreed (6); however, Sunderman and rownley (71 found that in usinc: Willard’s coricentration the prxipitate was contaminated with calcium. They recommended using 60 IIN03. Hcwever this redvcrs the yield of the strontium recovery. “Radioassay for Era ironmental Sxnple:” (8) recommends 70 FINO3.
The latest proposal standard method for the “Determination of Strontium-89 and -90 in Milk by Ion Exchange” (9) recornmeiids the use of 115-16N (68-71 %) HNOa as optimum concentratim. T h e later works appear to be confirmed b> cur findings. EDMONDJ. EARATTA FORREST E. KNOWLES, JR. Northeastern R adjo1ogical Hmlth Laboratory Environmental Frotectron Agency Analytical Quality Control Service Winchester, Msss. 01899
RECFIVED for review March 19, 1971. Accepted April 30, (7) D. N Suiderman 8nd C . W . I‘ownley, N P . S - N S ? 0 ? 0 f p p 4 1 2 ) (1970). (8) “Radioassay for Frivironmentel Samples,” Enviro::meiital Ilealtl: Series, PHS Piiblicatioi, No. PFY-RH-27 (1357)
OBTAWINGA DIGITAL RECORD of the output of an infrared Spectrophotometer can be 21: expensive undertaking, depending upon the construction of the instrument. ‘The Hilger-Watts M-1200 is an inexpensive grating infrared spectrophotometer (marketed by Wilks Scientific Corp., Norwaik, Conn., for about $3500), and commercially available digital data reduction systems for such an instrument would requjre more capital investment than the spectrophotometer. The interface designed by Gulf General Atomic for this instrument can be built at a cost of about $450 in components and about 70 hours of labor for construction, installation, and checkout. We have employed the system to record digital spectra using a cassette magnetic tape recorder. Fhe interface can also he used to transmit data directly to the computer memory. Data are reco~dedand stored os &bit binary characters representing the ordinate every 2.5 cm-’ from 4000 to 2000 cm-I, and every 0.5 cm-1 from 2000 to 650 cm-l (because of scale change at 2000 cm-l). The precision of the recorded spectra is thus better than the manufactwer’s specifications for the instrument (10 cm-1 from QOOO to 2000 cm-I; 2 cm-’ from 2000 to 650 cml; 1 transmittance photometric precision). The interface can be modified to expand or compress the presentation to as many as 33,000 points per spectrum, or as few as desired. OUI spectra have 3300 points from 4000 to 650 cm-I. There are zero and gain controls for scale expansion of the ordinate up to IOX. An analog output is provided for a separatt: recorder, INTERF K E DESCRIPTION
The electronic circuitry for the interface between the HilgerWatts H-1?00 and a Mohaik 305 cassette tape recorder is Present address, Quar.iiita Indu:!ries, Monroe, La. 71201
Inc., P.O. Box 6010,
1.971. (’3) “APHA-Standard Methods for the Examination of Dairy
Products,” 12th ed., (in press),
shown in Figure 1. Th: value of the ordinate of the HilgerWatts H-1200 (the pen position on a transmittance scale) is determined from the eervopot which is already connected to the pen servomotor. 1Jsing this servopot, a 0 to -10 V analog signal is presented to an %bit analog-to-digital converter (Burr-Brown No. ADC30-08N-USB, no buffer a n plifier). The strobe pulse to the ADC is obtained in a unique but reliable fashion. There is a small D C motor on the Hilger-Watts H-1200 which rotates the grating and drives the chart recorder. This motor operates at a maximum of about 2000 rprn, and is geared down 488:l at the working end. The rear end of the motor shaft is flush with the rear bearing surfax, and this end of the shaft rotates at from 0 to 2001) rpin. One ha!f ofthe rear end of the motor shaft was painted black, and a small light is directed onto the end of the shaft, with the reflected light directed into a small PIN photodiode. The light bulb and photodiode are held in an aluminum block, which is machined to fit on the end of the motor casing. A sine wme output is obtained from the PIN photodiode as the shafi rotates, and the positive-going signal is used to obtain a 5-V pulse from a Schmidt trigger. In the present arrangement. a divide-by-ten digital logic circuit is used to present this 5-V strobe pulse to the ADC once every 10 revolutions of “re chart drive motor. The strobe signal is used to initiate the digitizing step of the ADC, and a dataready pulse from the ADC is used to strobe the tape recorder for data acquisition. The ADC has a maximum 20-psec conversion time, yet the interface/spectrophotometer arrangenient we employ has a maximum data rate of only 3 points per second. Of course:, by bypassing the divide-byten circuit, this rate can bc expanded to 30 points per second. The cassette tape recorder presently being uszd has a maximum incremental speed of 120 &bit characters per second. (The tape units are made 19) hlohark Instruments, Sunnyvale, A N A L Y L ? A L CHEMISTRY, VOL. 43, NO. 8 , JULY 1971
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re
I
I
C?
10 t 8
Figure 1. Schematic of digital data acquisition interface between Hilger-Watts H-1200 infrared spectrophotometer and Mobark 305 incremental cassette tape recorder
Figure 2. Photograph of visual display of last 4K core lmations of XDS Sigma-2, showing digital benzene spectrum which has been inverted and converted from transmittance to absorbance 1140
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
compare spectra n ~ e r i c a l b ' , and generally take advantage of high-speed digital computing techniques to make the spectrophotometer a much more valuable laboratory tool. A transmission spectrum of benzene was run on the Hilger-
Watts H-1200 and recorded simultaneously on the cassette tape recorder. This taped spectrum was then read directly into an XDS-Sigma 2 computer using a 300 character-persecond reproducer. The resultant spectrum, after the transmittance values had been converted to absorbance values, is shown in Figure 2. A five-point smooth, after Savitzky and Golay ( I ) , was imposed on the raw data. A single peak (actually the second one from the left in Figure 2) from this smoothed spectrum is illustrated in Figure 3. Such peaks obviously can be readily integrated, either numerically or by curve-fitting approximations.
Figure 3. Portion of spectrum displayed in Figure 2, showing a single peak and illustrating the quality of the digital spectra obtained in this manner
RECEIVED for review January 15, 1971 Accepted March 22, lg71* (1) A. Savitzky and M. J. E. Golay, ANAL.CHEM., 36, 1627 (1964)
Versatile Injection System for Gas Chromatography Glenn E. Pollock, Angelo Margozzi, Ralph Donaldson, and Fritz Woeller Ames Research Center, NASA, Moffet Field, Calij 94035
IN CAPILLARY COLUMN gas chromatography, it is the usual practice to inject samples dissolved in a minute amount of solvent. Quantitation is difficult, especially in cases when all of the concentrate from an isolation technique is to be taken up and injected-e.g., in geochemical studies. We have developed an evaporative injector system which we feel will be of value in cases where solutes are contained in dilute solution. The system can also be used for conventional samples and packed columns. It has no heated valves or O-rings which can leak and cause inaccuracies. Figure 1 is a schematic of the system.
EVAPORATION MODE
SAMPLE INPUT
3-WAY VAL
\RESTRICTOR
INJECT MODE
COIL
GAS OFF 5 W A Y VALVE-,
EXPERIMENTAL
The sample is placed into the injector pot ( B ) through orifice ( A ) . The injector is cold and the carrier gas is off during this operation. After the sample is added to the pot, the carrier gas valve is turned on, as shown in Figure 1, so that the gas passes through line (C) which contains a restrictor to control gas flow. Line (C) leads to the injector pot and sweeps out the solvent through orifice ( A ) . After the solvent is evaporated, the gas is shut off and orifice ( A ) is capped with a l/18-in. Swagelok cap. The injector pot is then flash heated and valve (D)is switched to a position such that carrier gas flows through both lines (C) and ( E ) , sweeping the injector pot ( B ) and carrying the sample onto the column (F). The operations of flash heating and turning on gas flow can be carried out in three different orders. They can be carried out as described above-i.e., simultaneously, flash heated, and then carrier gas on: or in the reverse order. The best mode will frequently depend upon the type of samples being injected. For amino acid derivatives ( I ) , we inject with simultaneous flash heating and carrier gas flush. This type of injector eliminates two troublesome problems of gas chromatography. The sample can be added to the injector while it is cold and unpressurized. Under these conditions, a syringe will have little tendency to leak and the sample in the tip of the needle will not be volatilized. Al(1) G . E. Pollock and V. I. Oyama, J . Gas Chromatog., 3, 174
(1966).
Figure 1. Schematic of evaporative injector though this injector may be used in the normal manner with a septum, we recommend using the l/I6-in. Swagelok cap to obviate septal leaks and bleeding. Heating the injector can be carried out in several different ways: the injector can be flash heated by (a) resistance wire, (b) hot air from a heat gun, (c) raising a preheated block to encase the injector, (d) moving the injector into a preheated block, or (e) using focused high intensity lamps. We have used most of these techniques and have obtained good results with them; however, we prefer methods (b) and (e). Methods (a), (b), (c), and (d) are self-evident and easily designed. To our knowledge, method (e) is new; it is used with time control (Figure 2). The lamps (LI) shown in the figure are General Electric Axial Quartzline lamps, EJL 24-volt, 200 watts, 3400 OK color temperature and 25-hour life. The integral reflector is aluminum coated to reflect all wavelengths since it is supplied with a dichroic coating designed to reject heat. The timer (R1) is an Agastat solid state timing relay, Model 3732BllB, 10-ampere, 120-V ac adjustable delay from 0.5 to 15 seconds. The switch (S1) is a push button switch; 120-V ac, 10 amperes, and the adjustable autotransformer (TL) is 120-V ac, 10 amperes. The normal operating range is around 30 to 48-V ac. The temperature of the injector can be controlled by varying voltage and time of duration of the light flash. The temperature range of the ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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