b
a
Figure 1. Schematic diagram of back scattering geometry, diagonal, and diagonal holder
of a glass transmission plate as depicted in Figure IC. I n Figure 2, a-f, we compare spectra obtained using the conventional axial excitation, transverse viewing method with those obtained using the 180' back-scattering device. The line shown is the 992 cm-I line of benzene at 1 cm-1 resolution. The sample was contained in a 0.27 mm i.d. capillary cell (2). A 3-mm height of the sample was imaged o n the entrance slit of the spectrometer which was masked at 10 mm height. Figure 2a shows the spectrum obtained with axial excitation ( Y ) and transverse viewing (X). The laser polarization was along Z , and the direction of polarization of the viewed scattering is along Z (measured with Polaroid "-32). The signal at the top of the 992 cm-1 line was 28,000 counts/ sec. In Figure 2b, the analyzer was oriented along Y : Y ( Z Y ) X . Figure 2, c-fwere obtained with the same capillary in the same position, but using the 180" back-scattering device with slit-shaped beam (approximately 3 mm high at the sample). The excitation and viewing geometries are jndicated in the figure. The signal from the back-scattering arrangement should be less than one-sixth of that obtained from the axial/transverse method because of the decreased pathlength (3.0 mm:0.5 mm). The mask, diagonal, and lens
I
3 Y(ZZ)X
--TFII
Y(ZY)X
d
--pLII
X(ZY]X
e
f
A i X(YY)X
X(YZ1X
Figure 2. 992-cm-l benzene line at 1-cm-1 resolution approximately 28,000 counts full scale; axial/transverse method. c-fapproximately 2200 counts full scale; 180"back scattering
a, b
L are only about 50% efficient. The gain was held constant in Figure 2, c-f and the measured count rate at the peak in Figure 2c is approximately 2200 counts per second. Spectra in Figure 2, c-f demonstrate that good depolarization measurements can be made with this device. Adjustment of the device is extremely simple: an opaque object is placed at the sample position and the entrance slit is observed by means of a periscope microscope assembly. The diagonal is rotated (about axis X) until the image of the beam is observed between the slit jaws. The opaque object is replaced by a sample that may be scanned across the entrance slit (and focus of the laser beam) until a suitable scattering region is encountered. Once the laser beam focus has been adjusted no further adjustments are required between different sample positions. With this device, it is easy t o observe rotational Raman scattering of O2 and Nyin air, and below 100 Acm-l, the focal region should be purged if these lines interfere with the desired Raman scattering.
~
(2) G. F. Bailey, Saima Kint, and James R. Scherer, ANAL.CHEM., 39, 1040 (1967).
RECEIVED for review May 24, 1971. Accepted July 23, 1971.
Furnace Atomic Absorption with Reference Channel Ray Woodriff and Douglas Shrader Department of Chemistry, Montana State University, Bozeman, Mont. 59715 TRACE ELEMENT ANALYSIS is becoming increasingly important in medicine, geology, and in evaluating air and water quality. Dependable, sensitive, and convenient methods of analysis are necessary to satisfy the needs in these areas. Flame and furnace atomic absorption methods are finding widespread acceptance in these fields (1-13). Some matrix materials, (1) R. Woodriff and G. Ramelow, Abstracts, Society for Applied Spectroscopy, Chicago, Ill., June 1966, No. 83. (2) R. Woodriff and G. Ramelow, Speclrochim. Acta, 24B, 665
(1968). (3) R. Woodriff and R.Stone, Appf. Opt., 7, 1337 (1968). (4) R. Woodriff, R. W. Stone, and A. M. Held, Appf. Spectrosc., 22,408 (1968). 1918
(5)
R. Woodriff, 9. R. Culver, and K. W. Olsen, ibid.,24, 530
(1970).
(6) R. Woodriff, Abstracts, Society for Applied Spectroscopy, New Orleans, La., Oct. 1970, No. 207. (7) R. Woodriff, Abstracts Eastern Analytical Symposium, New York, N. Y.,Nov. 1970, No. 1. (8) H. Massman, Spectrockirn. Acta, 23B, 215 (1968). (9) 9. V. L'Vov, 1nzh.-Fiz. Zhu., Akad. Nauk Belorus. SSR, 2(2), 44 (1959). (10) B. V. L'Vov, Spectrochim. Acta, 17, 761 (1961). (11) 9. V. L'Vov. "Atomic Absorotion Analysis," . . Nauka, Moscow, USSR, 1966. ' (12) B. V. L'Vov. Soectrochim. Acta. 24B. 53 (1968). (13j D. C. Manning, Abstracts, Society for Applied Spectroscopy, New Orleans, La., Oct. 1970, No. 209. '
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
however, give broadband absorption which causes unexpected errors unless corrected for. Two systems have previously been published for doing this. One of these uses polarizers in the sample and reference compartments of a double-beam instrument through which the light alternately passes (5) and the other uses a rotation mirror-chopper (12, 14). Both of these systems employ alternating samplereference observation and give only the difference between the two signals. F o r routine work this is adequate but in working out new procedures it is often important t o know what the background absorption is. The present method gives a continuous, simultaneous record of both signals. The advantage of four o r five orders of magnitude greater sensitivity that furnaces have over flames is somewhat offset by the convenience and cost of operation. Progress is being made in the battle t o make furnaces competitive with flames in regard t o convenience and cost of operation. EXPERIMENTAL
Figure 1 shows the optical system which was used. The hollow cathode light enters the primary Glan-Taylor polarizer and is polarized parallel to the optical axis of the polarizer. The polarized hollow cathode light then passes down the optical path of the furnace. The hydrogen lamp light enters the primary polarizer through the side window and is divided into two perpendicularly polarized beams which are reflected in such a manner that the portion which is polarized perpendicular to the optical axis of the polarizer follows the same optical path as the beam from the hollow cathode. The other portion of the hydrogen lamp beam leaves the optical path and is absorbed. Before passage through the furnace, the two beams of interest are collimated by means of a quartz lens placed between the polarizer and the furnace. Both beams, the hollow cathode beam polarized horizontally and the hydrogen lamp beam polarized vertically, after passing through the furnace are focused o n the slit of the Beckman D U with a second lens placed o n the end of the furnace. After passing through the monochromator, the combined beams pass through the second slit and fall o n the secondary Glan-Taylor polarizer. The polarizer separates the two perpendicularly polarized portions of the beam. The hollow cathode portion is transmitted straight through the polarizer and falls on a photomultiplier tube. The hydrogen lamp portion is reflected through the side window of the polarizer and falls on a second photomultiplier tube. (14) S. R. Koirtyohann and E. E. Pickett, ANAL.CHEM., 37, 601 (1965); 38, 585 (1966).
PRISM I
\ ...
-HYDROGEN
, ^ lAMr
7 7
0
HOLLOW CATHODE
Figure 1. Optical system diagram
After the hollow cathode light and hydrogen lamp light fall o n their respective photomultiplier tubes, the signals are recorded individually. Two Heath Servo-recorders, Model EUW-2OA, were used to record the results. The two records are then available for comparison. A schematic drawing of the improved furnace design is shown in Figure 2. The heater tubes make contact in the center with the one-piece combination heat sink and shield tube. The outer ends are connected to a spiral copper tube which fulfills the dual purpose of electrode contact and cooling (5).
The shield tube prevents the graphite felt insulation from coming into contact with the heater tubes and also helps reduce heat loss from the heater tubes to the rest of the furnace. The one-piece heat sink and shield tube makes the optical path more stable and gives a better heat capacity for volatilization of the sample. This change in shield tube design from the previous three-piece construction also gives more uniform temperature by allowing more efficient heat conduction to the central part of the furnace. The side tube, through which samples are introduced, is very thin-walled next to the heat sink to reduce heat conduction away from the interior and has a thick lip approximately 4 cm from the outer end to hold a spring which provides constant tension on the side tube as it expands o r contracts with changing temperature. Argon gas enters the sample port and side tube through small aligned holes in both. A Vycor 18/9 socket is attached to the sample port through which samples are introduced. In addition to other improvements, this furnace is doublewalled to provide effective cooling (with water) of the entire furnace. The furnace is also made of stainless steel rather than iron. This design has proved very satisfactory. Previously, heater tubes needed to be replaced every week or two. The heater tubes in this furnace were used for ten months without replacement. Samples are placed into cups made of high density graphite and, after drying or ashing when needed, are inserted directly into the furnace. The cups, either for cleaning o r sample introduction, are screwed onto a threaded '/*-in. carbon rod
Figure 2. Drawing of the graphite tube furnace
SIDE TUBE-
TUBE CONIACT SAMPLE PORI-'
L C O O L I Y C JACKET
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1919
SAMPLE
REFERENCE
B
A and inserted through the Vycor socket and side tube into the furnace so that they rest against the heat sink. Any sample present in the cup vaporizes quickly, enters the optical path, and a reading is recorded. RESULTS AND DISCUSSION
The optical system described worked very well in eliminating errors due t o background absorption. If the sample exhibits broad-band absorption because of anions or carbonization of organic material, equal light fractions are absorbed from both the hollow cathode beam and the reference beam which can then be taken into account. Individual recorders permitted complete quantification of each beam, sample and reference. Many organic samples such as drugs and tissues pyrolyze and give broad-band absorption before the trace elements present are volatilized. An example of the two signals recorded in such a case is illustrated in A of Figure 3. The opposite behavior is shown in B. This type of curve is obtained with Hg in organic matrices under proper conditions. The Hg volatilizes and diffuses into the light path more rapidly than the pyrolyzed organic material. A very common behavior is one where the trace element and the broad-band absorbing material are simultaneously present in the light path as shown in C. If the peak separation in A and B is sufficiently large and one knows which peak is due to the element being analyzed, determinations may be made without background correction. Even in these cases, the broad-band absorption is ordinarily sufficiently broad to cause some error. The reproducibility of the blank, especially with solid samples, has been a problem with using the graphite tube furnace technique. This problem was encountered initially with Ag. We later found that the interior of the furnaceinsulation and side tube-were heavily contaminated. The same was true with the sample cup holders and desiccators. The contamination problem was reduced in several steps. First, graphite felt rather than graphite flake was used for
1920
C
insulation. The felt seemed to be much cleaner. Then the entire furnace was cleaned by prolonged heating, while flushing with large volumes of argon. Second, improved sample preparation was instituted. All cup holders, desiccators, and the Vycor socket are now cleaned regularly with a solution of sodium thiosulfate and/or a mixture of concentrated H N 0 3 and H2SO4, and rinsed with doubly-distilled water. The third step was standardizing the sample cups. We finally succeeded in obtaining very reproducible blanks. Fifteen blanks run on different days were obtained for Ag whose standard deviation was 0.51 % absorption. Defining the detection limit as the amount of element required to give a signal twice the standard deviation of the blank signal, 1.02% absorbance, one can see that the calculated detection limit for this procedure is approximately equal to the measured sensitivity. This continuous, direct current system is applicable t o any single-beam instrument. It can be applied to either flame or furnace atomic absorption. The greatly increased sensitivity of furnace atomic absorption over flames allows one to analyze very small samples, and with a minimum of sample preparation, provided any broad-band absorption is corrected for. The equipment involved in the system is comparable to flame atomic absorption with regard to complexity and cost of operation. It is comparable to neutron activation analysis with regard to sensitivity and its precision is much greater. ACKNOWLEDGMENT
We thank Dave Phelps for his part in constructing the furnace. RECEIVED for review April 5 , 1971. Accepted July 26, 1961. The support of this research by NSF and NASA is gratefully acknowledged.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
