Instrumentation Walter Slavin The Perkin-Elmer Corporation 901 Ethan Allen Highway Ridgefield, Conn. 06877
FLAMES, FURNACES, PLASMAS
An analytical technique is rarely replaced or made obsolete by a newer one. Techniques usually complement each other and provide an arsenal for the attack on analytical problems. The analyst must choose from these weapons that which is most appropriate for his or her specific problem. The task of choosing among analytical techniques is a difficult one. It requires a clear understanding of the analytical problems in your laboratory and of the capabilities of the different techniques available. Those who are expert in several techniques do not always agree on which to use because the grounds for decision are often judgmental and subtle. Many of us are particularly experienced in one technique and tend to be partial to that technique. Those who have an extreme case of this disease of partiality may tell you that the best analytical technique is atomic absorption (AA), or inductively coupled plasma (ICP) emission, or electrochemistry . . . There is no "best" technique; the choice depends on your skills and on the analytical situation. Spectroscopic techniques Flame AA. Flame AA is very well established. It has few interferences, and these are well identified and often easily controlled. Standardization is usually simple. The equipment is, by now, relatively inexpensive and easy to use. Of the techniques being compared, it requires the least operator experience. 0003-2700/86/0358-589A$01.50/0 © 1986 American Chemical Society
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analytical instrumentation, but the most widely used by far is the ICP. The ICP provides detection limits for most elements that are within a factor of two or three times those of flame AA; for some metals a little better, for some a little poorer. Detection limit comparisons are considered below. However, the very refractory elements can be easily determined with the ICP, and those elements that are only partially dissociated in the flame are usually completely dissociated by the high temperatures achieved by the ICP. It is for this reason that the ICP has been spoken of as freer of chemical interference than flame AA. However, interferences of other sorts are present in the ICP. For instance, high concentrations of inorganic matrix can shift the location of the hot portion of the plasma and therefore alter the signal for specific analytes. Spectral interferences are an important consideration for the ICP, and different lines are preferred for a specific element in different matrices. This sometimes requires that spectroscopic skills be used to solve some difficult analytical problems. However, the ICP is inherently multielement, and modern instrumentation takes advantage of this by using many detectors arranged along the focal plane of the monochromator or by very rapid, sequential scanning of the monochromator. The automation aspects, one of the most valuable features of the ICP, will be discussed later.
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choose ? But the more refractory elements, for example, B, V, Ta, and W, are only partially dissociated in the flame and are therefore not readily accessible by flame AA. Samples containing metals such as Mo and the alkaline earths are not fully dissociated in the flame, which sometimes leads to troublesome interferences. Elements that have their resonance lines in the very far UV are not easily determined, and these include P, S, and the halogens. Although the technique is rapid, AA instruments are rarely automated to permit simultaneous determinations of several elements. Plasmas. There are several kinds of plasmas that have been considered for Presented at the Australian Spectroscopy Conference, Melbourne, April 1985
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 · 589 A
Furnace AA
Historic Textile and Paper Materials
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Conservation and Characterization
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CONTENTS Conservation Principles of Henry Francis du Pont · Age Determination of Single-Fiber Textiles · Degradation in Museum Textiles · Fiber Damage in Textile Materials by SEM · Fractography of Historic Silk Fibers · Degra dation of Silk by Heat and Light · P a i n t e d Printed Chinese and Western Silks · Identifi cation of Dyes in Historic Textile Materials · Analysis of Natural Dyes on Wool Substrates • Characterization of Hyacinthine Purple (Tekhelet) · Mordanted, Natural-Dyed Wool and Silk Fabrics · Effect of Aqueous and Nonaqueous Treatments · Characterization of Metallic Yarns · Prehistoric Fabrics of Southeastern N. America · Standards for Archival Materials · Kinetics of Cellulose Deterioration · Use of FTIR and ESCA · Estimating the Effect of Water Washing · Discoloration of Paper · Influence of Copper and Iron on Paper · Accelerated Aging of Cellulosic Textiles Based on a symposium sponsored by the Division of Cellulose, Paper, and Textile Chemistry of the American Chemical Society. Advances in Chemistry Series No. 212 452 pages (1985) Clothbound LC 85-20094 ISBN 0-8412-0900-6 US & Canada $94.95 Export $113.95 Order from: American Chemical Society Distribution Dept. 98 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your credit cardl
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California-Davis
Explores the conservation and char acterization of historic textile and paper materials, an active area of interest to conservators, chemists, and other physical scientists for several decades. Focuses o n the preservation of textiles with historic a n d artistic value. P r o m o t e s the sharing of k n o w l e d g e o n preserva tion and restoration techniques between the scientific and conserva tion communities.
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H o w a r d L. N e e d l e s a n d S . H a i g Z e r o n i a n , Editors University
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Figure 1 . C o m p a r i s o n of d e t e c t i o n limits for f l a m e A A , ICP, a n d f u r n a c e A A
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o n a log s c a l e o f c o n c e n t r a t i o n in m i c r o g r a m s p e r liter Because furnace detection limits are inherently in mass units (picograms), they have been converted to concentration by assuming a 2 0 - μ ί sample
Those who are attracted to the ICP for reasons other than automation are usually particularly interested in those metals that can't be determined by flame AA. An important advantage of the plasma is that no special sources are necessary. For instance, one can determine Rh the one time per year it is required without having to retain a lamp inventory. The concentration range of the ICP usually spans three or four orders of magnitude. This is an important requirement for automated analyses. Flame AA spans only two or ders of magnitude. Furnace. If the lowest detection limits that can be obtained with spec troscopic techniques are necessary for your problem, the graphite furnace is the technique of choice. On a relative or concentration basis, furnace detec tion limits are 10 to 100 times better than flame AA or ICP. On an absolute mass basis, furnace detection limits are often 1000 times more sensitive because only a very small sample is re quired for the furnace. In the recent past, the major limita
A N A L Y T I C A L CHEMISTRY, V O L . 58, NO. 4, APRIL
1986
tion of the graphite furnace was the many chemical and physical-chemical interferences reported extensively in the literature. Those problems are now largely controlled using a combi nation of platform technology, new high-quality graphite materials, mod ern, fast photometric instrumentation, and Zeeman background correction. Although not totally interference free, the level of interferences for the graphite furnace is now no greater than flame AA or ICP. Nevertheless, furnace determinations are slow—ty pically several minutes per element per sample. They are generally single element; multielement analyses often are not practical. The analytical range is not very large—typically a little less than two orders of magnitude. Thus, the furnace is presently used when the flame or ICP provides inadequate de tection limits. Detection limits. In Figure 1 (right side) flame AA and ICP are compared. In addition to indicating that the de tection limits of the two techniques are similar, it can also be seen that
they are rather complementary. Re fractory metals like Ti, V, and Ba tend to be poor on the flame AA chart, but better on the ICP. In contrast, the very volatile, heavy metals such as Cd and Zn are very good on the AA chart and slightly poorer on the ICP chart. The left side of Figure 1 shows the detection limits in picograms for the bulk of the elements determined with furnace AA. The scale on the right side of the figure indicates the concen tration detection limits with assump tion of a 20-^L sample. Most instru ment companies make these numbers look about five times better by assum ing a 100-μΙ_/ sample. In fact, the con centration detection limits that we show in the figure are achievable in real sample solutions, even in the presence of a massive amount of ma
trix, if the Zeeman technique is used for background correction. This is the reason for the increasing interest in Zeeman background correction. The comparison of detection limits can be confusing when techniques such as flame AA, ICP, and furnace AA are compared. ICP and flame AA are steady-state techniques. The sig nal is determined after the sample has been aspirated long enough for the signal to come to some final equilibri um value. Thus, flame or ICP detec tion limits are usually related to con centration, with the assumption that the amount of sample is unlimited. On the other hand, the furnace technique uses the entire sample and detects an absolute amount of the analyte element. To compare the three techniques on the basis of concentration ignores the advantage of the furnace, which can handle very small samples. But it is meaningful to the analyst who has unlimited sample and does not care about small sample volumes. To com pare the techniques on the basis of ab solute detection limit (say in pico grams) is meaningful for those ana lysts who are working with very small amounts of biological tissue, semicon ductor materials, or fragments of fo rensic or art materials. Major element determinations. Although both flame AA and ICP are
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DMA45 with optional sample changer. CIRCLE 142 ON READER SERVICE CARD 592 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
mainly used for trace analysis work, the simplicity and speed of both tech niques make them valuable for major element analysis. Probably, flame AA is still under-used for major element analysis in competition with X-ray fluorescence or gravimetric proce dures. At high concentrations of the analyte metal, flame AA signals are re markably stable, and it is easy to dem onstrate a precision of 0.2% relative standard deviation (RSD). Because in terferences are also typically negligi ble, this means that an accuracy close to the same level is available if accu rate standards can be prepared. Many mining companies use this advantage of flame AA for buying and selling precious metals, where very small changes in percentage points can equal large amounts of money. The standard reference method for Ca in serum uses flame AA, using standards that are carefully prepared to bracket the analytical result from the sample. Precision of 0.5% to 2% is easily ob tained with the ICP and, with recent improvements, ICP may become com petitive with flame AA for accurate major metal determinations. The ICP advantage of a very wide linear analyt ical range makes it less necessary to redilute a sample that may have ex ceeded the range. Automation. It is easy to misrepreMettler/Paar density meters provide reproducible, accurate density measure ments without pycnometers or hydrom eters—usually in less than three minutes. And Mettler/Paar density meters are so easy to use that no special training or skills are required. Results in units you prefer. Obtain results that can be used immedi ately, such as g/cm 3 , °Brix, specific gravity, API number, % alcohol, % con centration, or dimensionless engineering units. Results can be transmitted directly to your computer for storage or analytical calculations. Flexible and versatile. Check incoming and finished goods samples. Improve cost controls by ensur ing that components are mixed in exactly the right ratio, and even standardize the quality control procedures for all lab and plant locations. Choose exactly what you need. You can select from a complete line of Mettler/Paar density meters, including hand-held field units precise to Ι Ο 3 g/cm 3 , laboratory models with precisions of 10 4 to 10"6 g/cm 3 , and in-line process control models with a precision of 10 5 g/cm 3 . Let Mettler take the hassle out of density measurement for you. Call 1-800-257-9535; in NJ, (609) 448-3000. Or write to Mettler Instrument Corpora tion, Box 71, Hightstown, NJ 08520 for complete technical details and pricing.
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sent the different analytical tech niques and particularly different in struments with respect to their ability to handle large numbers of determina tions. The situations in which large numbers of samples must be analyzed for a limited number of elements, for example, in the clinical lab or the agri cultural test station, favor automated flame AA. The situations in which many elements must be determined in many samples, such as a water test lab or a geological survey situation, favor the ICP. This is because each flame AA determination is rapid, but a sig nificant amount of time is required to change the element being determined. In contrast, because separate light sources are not needed for the ICP, changing between elements is rapid. But it takes longer for the sample to come to equilibrium in the ICP, so that each sample takes a longer time than in flame AA. The automation of flame AA is highly developed. For example, a typi cal automated system can determine six elements in 50 samples in
