TECHNOLOGY
Atomic absorption gains more adherents Sensitivity, versatility, boost AA into forefront of analytical techniques By most measures, atomic absorption (AA) has become one of the fastest growing areas of spectroscopy. It only first emerged from the primordial broth of analytical principles in the early 1960's, but since then instruments based on the technique have proliferated. And A A has now become an established method—in some cases, the only method—for analyzing for trace metals. In worldwide production of instruments, AA is probably the best documented of analytical techniques, and the figures show it has far from reached its peak. AA spectrophotometers are
based on methods developed in Australia by the Commonwealth Scientific and Industrial Research Organization, and the instruments are manufactured under license to that organization. CSIRO's figures show that in 1967 some 2300 units were produced throughout the world, up from about 1900 in 1966 and 750 in 1965. Prior to 1963, only 40 to 80 units had been made. Further evidence of the technique's potential growth comes from a spectroscopy census made by Analytical Chemistry in 1967. Based on a survey of more than 800 spectroscopists, the census disclosed that AA accounted for just under 7% of spectroscopy units in use in 1967. If the census holds true, A A will account for 23.5% of spectroscopy units bought between 1967 and 1969. (Only infrared, at not quite 16%, comes close to being as popular in terms of sales.) The reasons for AA's fast growth
Among atomic a bsorption instruments are these | Model
Type
Atomic analyzer
Single-beam a.c. (grating)
Manufacturer Aztec Instruments, Inc. Westport, Conn. Bausch&Lomb Rochester, N.Y. Beckman Instrument, Inc., Fullerton, Calif.
1 1 1
Single-beam a.c. (grating)
6200
979
Single-beam (grating) single or multipass
5320
Hewlett-Packard Avondale, Pa.
H-P5960A
4400
Instrumentation Lab, Inc. Watertown, Mass. Jarrell-Ash Div. Fisher Scientific Co. Waltham, Mass.
153
Resonant wavelength filters Dual doublebeam (grating) Single-beam (grating) multipass Single-beam a.c. (grating) Single-beam a.c. (grating) single or multipass
SP90
Single-beam a.c. (prism)
3780
290B
Single-beam a.c. (grating)
2950
303
Double-beam (grating)
6560
AA4
Single-beam (grating) Single-beam (grating)
6495
Dial-atom
Maximum versatility
1 1 1
$6975
AC2-20
Atomsorb
1 1 1 1 1
Base price
Norelco/Unicam Philips Electronic 1 nstrument, Mt. Vernon, N.Y. Perkin-ElmerCorp. Norwalk, Conn.
VarianTechtron Cary Instruments Monrovia, Calif.
AA100 AR200
40 C&EN JULY 29, 1968
Resonance detectors
8650 3500
4695 6185
3350 2400
1
can be traced to a fortuitous blend of advantages. Perhaps foremost is the technique's specificity. Elements are readily determined at their characteristic wave lengths. Unlike many techniques, including flame emission, there are few interferences of one element with another. Problems caused by the few that do exist have by now been largely overcome, and the methods involved are well documented. The technique is broad. At its current stage of development, AA can be used for determining 65 metallic elements in solution. It can also be used indirectly for many nonmetallics by treating them with a metal (using chelation, for example) and then analyzing for the metal. Moreover, AA's sensitivity reaches down into parts per million for all of the 65 elements, parts per billion for many, and even parts per trillion for some. Despite what might be expected for such sensitivity, the technique is relatively simple to use. In addition, AA is widely applicable. Although it finds ready use for constituent analysis, where percentages can be quite large, it is particularly adapted to trace analysis because of its sensitivity. And trace determination is becoming a requirement in more and more areas. Agricultural uses, for example, include determination of trace metals in soils, plants, fertilizers, and food. Petroleum, glass, cement, and paint industries all have applications. There are needs for trace analyses in such disparate fields as geology and pollution control. One of the rapid areas of growth in trace analysis is biomedicine. This area provides one of the largest single applications for AA—determination of calcium in blood serum. Serum, urine, and tissues are all being analyzed for traces of metals such as calcium, lead, cadmium, magnesium, copper, manganese, zinc, iron, lithium, and gold. With all these advantages, AA is, at the same time, inexpensive. It's possible to get started in a limited way for as little as $3000. Going top drawer all the way, an analyst would find it difficult to add anything useful after he had spent $12,000. Atomic absorption is a close relative of flame emission spectroscopy, a technique that has been common for analysis of alkaline and alkaline earth elements for quite some time. When an element is atomized in a flame,
ANALYZER. Laboratory worker logs data resulting from AA analysis. limits of such instruments now sometimes extend into parts per trillion
some of the atoms are excited. When they drop back to ground state, they emit light at a characteristic wave length. Flame emission is based on measuring the amount of emitted light. Ground-state atoms, however, will absorb light of the characteristic wave length. AA is based on measuring the amount of light absorbed. What throws the balance most often in AA's favor is that far fewer than 1% of the atoms become excited at currently obtainable flame temperatures. AA, in deriving an analytical signal from more than 99% of the atoms, is therefore much easier to use and gives far greater precision and freedom from interferences. Putting the technique to work in an instrument requires light sources that emit light of the characteristic wave length distributions for the elements to be determined. This is provided by hollow cathode lamps. Each of these lamps has a cuplike cathode that contains the element being analyzed. Some multielement lamps are available and can be used for determinations of three or four different elements. Major U.S. manufacturers of lamps today are Westinghouse and PerkinElmer. In operation, a sample containing the trace element is aspirated into a flame. Light from the hollow cathode lamp beams through the flame into a monochromator, which separates the wave length desired and sends it to a photomultiplier detector. Current model instruments are designed for either single-beam operation or double-beam (and are easily switched from an AA mode to flame emission when desired). Units by Bausch & Lomb, Beckman, Norelco/ Unicam, Jarrell-Ash, Aztec, and Varian Techtron are typical examples of single-beam operation. Perkin-Elmer
Detection
uses double-beam and Instrumentation Laboratory uses what it calls dual double-beam. In single-beam operation, light from the lamp is chopped, either mechanically or electronically, to provide an a.c. signal at the detector. In this way, the detector can distinguish between emission from the lamp and emission from the flame. An aspect of single-beam operation, however, is that the lamp must be stable, which means a 10- to 20-minute warmup period. Double-beam operation avoids the effect of lamp instability. Part of the chopped beam is sent through the flame, the other part bypasses it. When the two beams recombine in the monochromator behind the flame, the detector senses the difference between the two. In this way, any lamp instability is canceled out, and no warmup period is necessary. With Instrumentation Laboratory's dual double-beam operation, provision is made for internal standardization with an independent element. This, the company says, often overcomes any effects of instability in the flame or contamination in the sample. Hewlett-Packard's unit doesn't have a monochromator; instead it uses resonant wave length filters. Burners and burner heads are perhaps the most important single element in an AA spectrophotometer. They must be quiet, without flicker. They must not corrode. They must be able to run high concentrations without clogging. And they must respond rapidly. Two basic burner types have evolved—the premix and the total consumption. Premix burners combine fuel gases and sample in a mixing chamber, then pass the mixture to the burner head with laminar flow. Total
consumption burners throw the whole sample into the flame. Both have their proponents, but most manufacturers now provide the premix type. As instruments and technique evolve, the capability of AA continues to grow. There is, for example, seemingly no absolute limit to sensitivity, although it varies from element to element and depends heavily on instrument design, burner stability, and the like. Just this month, PerkinElmer began shipping a boat kit for use with its Model 303 that extends detection limits of the instrument for 10 elements: arsenic, bismuth, cadmium, mercury, lead, selenium, silver, tellurium, thallium, and zinc. In use, a small amount of sample is placed in a tantalum boat, dried near the flame, and then inserted in the flame. Detection limits are greatly extended for some of the elements— from 0.1 p.p.m. to 0.02 p.p.m. for arsenic, for example, or from 2 p.p.b. to 30 parts per trillion for zinc. AA now seems to be butting up against some other limitations, however. In the past, limited development of hollow cathode lamps held back the number of elements detectable. More recently, when refractory metals proved a problem, new burners using nitrous oxide and acetylene were designed. Nitrous oxide/acetylene burns with a 3000° C. flame, compared to the 2300° C. flame for more commonly used air/acetylene. Development of burners using an argon/ hydrogen fuel mixture, which burns with a cool flame, increased sensitivity for arsenic, cadmium, and tin. But thorium and cerium remain undetectable, apparently because flames are not hot enough. Some investigators claim to have been able to detect osmium—but not all. At present, the practical lower spectrum limit of AA is just above 1900 A. At that level, oxygen in the flame absorbs practically all of the light. This puts arsenic (at 1937 A.) at the limit, and leaves sulfur, phosphorus, and gases undetectable in the ultraviolet beyond. Likewise, mercury is relatively insensitive at detectable wave lengths, but has a line 50 times more sensitive just out of reach. Normally, sampling is performed with solutions. It hasn't been possible to work with solids, although Perkin-Elmer has now made a stab at it. With the company's "SolidMix" sampling system, sample is mixed with a powder, pressed into a pellet, and ignited in the beam where the flame would ordinarily be. It burns quietly, the company says, for about 10 seconds. Perkin-Elmer says it has made successful determinations of copper, nickel, gold, silver, mercury, bismuth, lead, and others in this way. JULY 29, 1968 C&EN
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