REPORT FOR ANALYSTS
Figure 1. Bausch & Lomb's dual grating spectrograph may also be furnished with a single grating having twice the speed of the gratings normally supplied
Figure 2. Jarrell-Ash Co. Ebert Spectrograph with the order sorter on the optical bar and the Varisource excitation unit underneath. The order sorter separates various spectral orders simplifying line identification
N e w Horizons for Emission Spectrochemical Analysis The field of emission spectrochemical analysis is again on the move. Not content with their extensive role in industry and research, restless spectrographers are reaching out further into new frontiers of analytical chemistry. To appreciate these developments, the author is presenting a bird's-eye v i e w of the over-all field t o d a y , summarizing those shortcomings which are so r a p i d l y being overcome
T
H E emission spectrograph is at present capable of direct measurements down t o 1 p.p.m. of m a n y elements, especially metals which have simple spectra. For elements with more complex spectra, such as the rare earths, platinum, and trans-uranium groups, the sensitivity is not quite so good, approaching 0 . 5 % concentration for a few. Various preconcentration techniques m a y be coupled with the spectrographic technique, however, to improve sensitivity. Simply ashing an organic substance, for example, will permit a hundredfold improvement in the limit of detection. These detection thresholds are being pushed back further through advanced instrumentation and techniques. Two n e w spectrographs (Figures 1 and 2) with vastly higher dispersion and resolution t h a n available heretofore are .-said to offer much more sensitivity. Increased dispersion and resolution result in far greater line-to-background •or signal-to-noise ratios. One of these instruments m a y be ordered with an oversize grating measuring 4 X 4 inches, which increases the calculated speed of the instrument to / / 1 7 . I t is claimed t h a t the instrument so modified has an actual speed about twice t h a t of a medium quartz instrument rated a t J/12. Here then is a spectrograph combining speed and resolution, another big stride forward in trace-element .technology.
I n this trace-element safari, accessories not long ago considered laboratory relics are being revived. One is the hollow cathode source which, b y entrapping atoms in a closed container, permits t h e m to be excited over and over to enhance their limit of detection. Moreover, the hollow cathode operates a t a low pressure and temperature, increasing the signal-to-noise ratio still further. Spectral lines from the hollow cathode source are sharp, without background or self-reversal, and m a y be detected a t a concentration level some 100 times below t h a t with a d.c. arc. (see Figure 3). T h e d.c. arc is normally . considered the most sensitive source.
Techniques, too, are destined for revolutionary changes. G. H . Morrison, of the Sylvania Chemistry L a b oratory in Flushing, N.Y., has recently announced an analytical method which has already proved immensely useful in evaluating transistor-grade silicon. T h r o u g h the use of a n ion exchange membrane (see Figure 4) followed b y spectrographic analysis, the method is capable of detecting 0.001 p.p.m. of boron, slicing off three decimal points from existing methods. Of even greater significance, the system seems adaptable to m a n y other elements in other matrices. Differing from conventional concentration techniques, Morrison's converts the matrix to a crystalline form so t h a t
ARTHUR J. MITTELDORF, president of Spex Industries, Inc., manufacturer of spectrographic standards and accessories, graduated from Brooklyn College (1942) and did graduate work at the Polytechnic Institute of Brooklyn and Illinois Institute of Technology. He has worked for Bendix Aviation, National Lead Co., Armour Research Foundation, and Jarrell-Ash. His work has been primarily in the field of spectrochemistry. He has been active in promoting the interchange of spectrographic information among spectrographers. He is a member of the ACS, Optical Society of America, Society for Applied Spectroscopy, and an ASTM subcommittee on standardization of spectrographic equipment and tools. VOL. 29, NO. 6, JUNE 1957 ·
17 A
REPORT FOR
ANALYSTS
startling, improvement in sensitivity. Used together, however, their power and effects compounded, they represent real progress in determining trace elements. A trace element may soon be regarded as one in the parts per billion range or below rather than parts per 100,000. Figure 3 . C o m p a r i s o n o f h o l l o w c a t h o d e d i s c h a r g e a n d d.c. arc d i s c h a r g e o f the lithium lines a t 6 7 0 7 Α., using a JAco-Ebert s p e c t r o g r a p h in the 4 t h o r d e r a t 0 . 7 7 A . / m m . r e c i p r o c a l dis persion
Courtesy Esso Research & Engineering
Co.
THE IDEAL PRESS FOR MAKING KBR PELLETS FOR INFRARED
SPECTROSCOPIC ANALYSIS A l s o for Forming pellets for x - r a y a n d other types of spectroscopic analysis
the wanted element may be extracted with water. Extremely low blanks are claimed with this technique (see page 892). New photographic emulsions are also getting their share of attention. The same emulsions used to photograph a pretty young lady by the flickering light of her escort's cigarette lighter are beginning to aid the spectrographer in photographing the weak spectral lines produced by a few elusive atoms. Individually, techniques and instru mentation such as those discussed offer a significant, but perhaps not
Extension of Scope It is often stated that the spectrographic method is confined to the de termination of the metals. These, plus such determinable nonmetals as phosphorus and fluorine, number about 70 elements. The rare gases, carbon, oxygen, nitrogen, hydrogen, and the halogens remain aloof, unwilling to have their noses counted spectrographically. The hold-out elements are, however, gradually being brought into the fold. V. A. Fassel and associates at Iowa State College have recently developed a novel procedure for measuring the oxygen content of metals, a method which promises to outperform standard vacuum fusion techniques. The oxygen is released from the sample by using a d.c. arc in an enclosed chamber con taining a fixed pressure of argon. P-t. ANODE
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A multi-purpose unit with q u i c k l y a d j u s t e d vertical opening, by handwheel, per mits many different set-ups in minimum time. Used around the world for RESEARCH, C O N T R O L & TESTING. Available accessories include: Heat ing and Cooling Platens; Temp. Con trols, Auxiliary low pressure guages; Fast air closing; Extruding units; Testing units, etc.
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Figure 4 . Electrodialysis cell with ion exchange membrane. Silico sample dis solved in NaOH is placed in polyethylene anode chamber and sodium ions are removed by electrodialysis through the membrane. The resulting silicate matrix is extracted with water to concentrate impurities which are then determined spectrographically SECONDARY SLITS MULTIPLIERPHOTOTUBES
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ANALYTICAL CHEMISTRY
Figure graph g e n , it almost
5. A p p l i e d Research Laboratories' Q u a n t o v a c , a direct r e a d i n g spectro f o r d e t e r m i n i n g c a r b o n , p h o s p h o r u s , a n d sulfur in s t e e l . Flushed w i t h n i t r o m a y b e used d o w n to 1 7 0 0 A. w h e r e air a n d w a t e r v a p o r a b s o r b r a d i a t i o n completely
REPORT FOR ANALYSTS Intensity ratios of an oxygen line to an argon line are plotted as a function of the oxygen content. When methods like this find their way to production, higher-conductivity copper, less brittle steel, and titanium are certain to result. Another approach toward determining several elements is b y tracking t h e m down in the far, or vacuum, ultraviolet. Reserved for pure research until just a few years ago, this region is being opened u p through commercial instrumentation. One new spectrograph (Figure 5) skirts high vacuum problems b y using a spectrograph flushed with nitrogen which is transparent down to around 1700 A. This instrument is specifically designed to determine photoelectrically the carbon, phosphorus, and sulfur content of steels. A second, more general instrument (Figure 6), m a y be used either photoelectrically or photographically from 500 to 2000 A. where there are strong lines of nitrogen, the halogens, oxygen, selenium, and the rare gases. Improvement in Analytical Accuracy A t present, the best accuracy obtainable spectrographically is within ± 1 % , an order of magnitude poorer t h a n the best wet chemical accuracy. This degree of accuracy, while satisfactory for t h e spectrographic monitoring of thousands of ingots of ferrous and nonferrous alloys poured every day, is not adequate for deciding exactly how much the producer owes t h e mine for a carload of tungsten ore
TIME OF WAIT USBS t02 SILICA BRICK L i a Co- CARBON BUFFER
'Baker Analyzed' REAGENTS
12 AMPERES
QtAs-..Ηο
>-
NAUSEATING ODOR
ζ UJ
Ξ
SUSTAINING ARC
DC. ARC
TIME
HIGH ASSAY
SECS.
onlV
Figure 7. Comparison of typicai moving-plate curves of the d.c. arc with those of the new National Spectrographic Laboratories' sustaining arc. The similarly shaped curves for lithium, silicon, and iron in the latter are related to the improved reproducibility claimed for the sustaining arc sold on the basis of its metal content. N o r is it accurate enough for full understanding of physiological reactions which maintain b o d y temperature, pulse rate, and blood composition, to better t h a n ± 0 . 5 % . Progress in improving spectrochemical accuracy has been continual over the years. More stable circuitry, more efficient sampling, better analytical techniques, the availability of purer materials, and more reliable standards have all aided in upping accuracy to its present values. There is every reason t o believe t h a t such slow b u t steady improvements will continue and will result in a n extension of spectrochemical analysis to m a n y new applications. One interesting example of such an improvement is the sustaining arc source, a modified a.c. arc. This source is claimed to approach b o t h t h e high sensitivity of the d.c. arc and t h e high precision of the spark source. I n t h e past, high sensitivity a n d high precision were usually obtained a t the expense of one or the other. W i t h the sustaining arc, elements will often emit their spectral energy at t h e same rate (see Figure 7) where in contrast, t h e y distill selectively in using a d.c. arc. Development of Universal Analytical Methods
Figure 6. The McPherson 1 -meter scanning vacuum U.V. monochromator, a high-vacuum spectrograph capable of photoelectric or photographic detection, is being used both in emission and absorption down to around 500A
iiîiiiiiii
Paradoxically, the spectrograph has proved most effective in analyzing materials of known composition. When the spectrographer is told a particular sample is of 24S aluminum, he can quickly confirm this and, in b u t a few minutes, calculate the concentrations of t h e elements present. B u t hand a spectrographer a sample labeled merely " o r e " a n d his analytical t a s k y s far more
*8.80 1/4 U B .
A superior sulfide precipitant free from hazards of H 2 S, Baker Thioacetamide, reagent grade, is a crystalline product of high assay and high purity, containing over 40% available sulfide. It is very soluble in water, alcohol and benzene, hydrolyzing smoothly in either acidic or alkaline solution to provide a convenient source of sulfide ion for both the quantitative and qualitative separation of the ions of heavy metals. Thioacetamide is rapidly supplanting gaseous hydrogen sulfide as a precipitant because of its ease-of-use and freedom from noxious and corrosive properties. Sulfides precipitated by Thioacetamide —unlike those with H2S —are coarsely crystalline, dense, easily filterable, and the rate of precipitation can he more easily controlled. Baker Thioacetamide is ideal for classroom use, particularly when a hood is not employed. M pound may suffice for 50 students for the usual semester qualitative course. Baker Thioacetamide Reagent, Crystal, is packaged in one-ounce and one-quarter pound amber glass bottles. Like all 'Baker Analyzed' Reagents, each label bears the actual lot analysis of the contents.
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A NEW Analytical
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The CAHN ELECTROBALANCE from W i l l Measures M a s s
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The Cahn Electrobalance is an entirely new concept in micro weighing. It's as portable as a typewriter and as easy to set up and operate. I t is unaffected by vibration, variable temperature, air cur rents or even by leveling. Sample is weighed by balancing against an electromagnetic torque which is electrically controlled with high precision. Weighing chamber may be removed and operated by remote control. SWITCH SELECTS A N Y ONE OF THESE FOUR RANGES: Range mg. mg. mg, mg.
Precision 0 . 0 2 %
Sensitivity 1 2 5 8
difficult. Still lacking is a method whereby all their constituent elements can be rapidly and quantitatively de termined in such complete unknowns. I n this area, progress has been made through the recent introduction of standards claimed to yield semiquanti tative accuracy. Using a dilution tech nique whereby all samples and standards are essentially reduced to a common matrix, a method is specified with which the spectrographer can calculate concentrations accurately to ± 3 0 % of the amounts present. Improvement in Speed of Analysis
MORE PRECISELY
0- 5 0-10 0-20 0-50
REPORT FOR ANALYSTS
microgram micrograms micrograms micrograms
Accuracy 2.5 5.0 1 2.0 30.0
We hear much about the speed of spectrochemieal analysis: less t h a n 1 minute b y photoelectric and about 20 minutes b y photographic techniques. N o t m a n y years ago, management was satisfied when the same analyses were performed in a day or so. I n their relentless quest for reduced costs, management now talks about contin uous (and instantaneous) process con trol analysis. An example is an instru ment which would measure t h e nitrogen content of corn feed a n d thus provide automatic control of the protein con t e n t b y hooking u p t h e flow valves to a direct-reading spectrograph. An other is an instrument which would con trol the feed of nickel to an electric furnace and stop it just when the minimum concentration called for in the stainless steel specifications is reached. Equipment manufacturers have al ready found some of t h e answers to these demands b3^ management and, un doubtedly, have m a n y more on the drawing boards. One manufacturer, for example, claims a significant advance in analytical speed with his new combi nation photographic-photoelectric in
micrograms micrograms micrograms micrograms
Accuracy: About 0 . 0 5 % of full scale.
W i l l N o . 3 2 7 6 T Cahn Electrobalance complete with stirrups, supply of disposable aluminum pans, and set of 4 Class M calibrating weights $615.00
Will
CORPORATION and subsidiaries
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Figure 8 . Read-out console used with the Baird-Atomic, Inc. direct reading attachment converts their 3-meter spec trograph to a photoelectric instrument
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*This illustration ts an authentic engraving during the 1600's. For your copy (ll"xl5") letterhead stating your name and title.
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strument. While such instruments have been available in the past, the photo electric part has lacked stability. This new one (Figure 8) uses a servo system to keep the exit slits centered on the spectral lines. It also features a de vice for rapidly locating a line photoelectrically. This instrument will aid research spectrographers in handling routine problems. Ordinarily used with a photographic plate, the spectrograph is converted to a direct reader for routine analysis by switching the cassette to a photoelectric detector box. It is claimed that, once cali brated, the box may be set up in as little as 10 minutes. While photoelectric detection is un rivaled for speed, it seems that photo graphic detection will be necessary for nonroutine problems. Here a new de velopment is rapid processing. Using hot, concentrated sprays, a plate may be completely processed and dried in less than 30 seconds. Moreover, re producibility is said to be better than that using the best present-day spectro graphs processing methods. Any summary of the fundamental contributions of spectrographic analysis would encompass sciences as diverse as astronomy and zoology. Indeed, with regard to astronomy, our knowl edge of the temperature, composition, velocity, size, age, and other character istics of heavenly bodies is based largely on spectrographic data. In the field of medicine, the spectrograph has con tributed vital information concerning the essentiality of such trace elements as zinc and cobalt. In geology, spec trographic analysis is an invaluable tool for correlating formations and discovering new mineral deposits. In electricity, the spectrograph is aiding materially the rapid progress being made in semiconductor research. Another electrical area with great fu ture promise is superconductivity—the ability of certain metals such as lead, to conduct electricity at temperatures near absolute zero with no resistance whatsoever. To date, this phenomenon is mainly an unexplained laboratory curiosity, although an application in digital computers is currently being explored. An intriguing potentiality is the application to transmission of power. Already, electrical engineers have cal culated that, if a metal is found which exhibits superconductivity at around —200° C , it will be economically ad vantageous to use it in certain long distance power lines by cooling with a jacket containing liquid nitrogen. If such a metal or alloy is ever discovered, it is likely that the emission spectrograph will be credited with a tremendous assist.
