Séptima problemática: Estudio del Consumo de Lácteos

Atomic emission spectroscopy (also called flame emission spectroscopy or flame pho-tometry) has found widespread application to elemental analysis. Its most important uses have been in the analysis of sodium, potas-sium, lithium, and calcium, particularly in biological fluids and tissues. For reasons of convenience, speed, and relative freedom from interferences, flame emission spectroscopy has become the method of choice for these other-wisedifficult-to~etermine elements. The meth-od has also been applied, with varying degrees of success, to the determination of perhaps half the elements in the periodic table. Thus, flame emission spectroscopy must be

con-sidered to be one of the important tools for analysis.'

into a separate photometric system consisting of an interference filter (which transmits an emission line of one of the elements while absorbing those of the other two

1

a photo-tube, and an amplifier~ The outputs can then be measured separately if desired. Ordinarily, however, lithium serves as an internal stand-ard for the analysis. For this purpose, a fixed amount of lithium is introduced into each standard and sample. The ratios of outputs of the sodium and lithium transducer and the potassium and lithium transducer then serve as analytical parameters. This system pro-vides improved accuracy because the intensi-ties of the three lines are affected in the same way by most analytical variables, such as flame temperature, fuel flow rates, and background radiation. Clearly, lithium must be absent in the sample.

Automated Flame Photometers. Fully au-tomated photometers now exist for the deter-mination of sodium and potassium in clinical samples. In one of these, the samples are withdrawn sequentially from a sample turn-table, dialyzed to remove protein and particu-lates, diluted with the lithium internal stand-ard, and aspirated into a flame. Sample and reagent transport is accomplished with a roller-type pump. Air bubbles serve to separate samples. Results are printed out on a paper tape. Calibration is performed automatically after every nine samples. The instrument costs about $9000.

Instrumentation

Instruments for flame emission work are simi-lar in construction to the flame absorption instruments except that the flame now acts as the radiation source; the hollow cathode lamp and chopper are, therefore, unnecessary. Many modern instruments are adaptable to either emission or absorption analysis.

Much ofthe early work in atomic emission analyses was accomplished with turbulent flow burners. Laminar flow burners, however, are becoming more and more widely used.

Speetrophotometers. For nonroutine analy-sis, a recording, ultraviolet-visible spectropho-tometer with a resolution of perhaps 0.5

A

is desirable. The recording feature provides a simple means for making background cor-rections (see Figure 11-(7).

Photometers. Simple filter photometers often suffice for routine analyses of the alkali and alkaline-earth metals. A low-temperature flame is employed to eliminate excitation of most other metals. As a consequence, the spectra are simple, and glass or interference filters can be used to isolate the desired emis-sion line.

Several instrument manufacturers supply flame photomllters designed specifically for the analysis of sodium, potassium, and lith-ium in blood serum and other biological samples. In these instruments, the radiation from the flame is split into three beams of approximately equal power. Each then passes

Instruments for

Simultaneous Multielement Analyses During the past decade, considerable effort has been made toward the development of instruments for rapid sequential or simultane-ous flame determination of several elements in a single sample.!! We have already

con-• Fo~ a. mOre complete discussion of the theory and applications of flame emission spectroscopy, see: Flame Emission tmd Atomic Absorption Spectroscopy, vol. I:

TMory; voL 2: CompoMnt& tmd Teclrniqua; vol. 3: EIe-- tmd Matrices, eels.J.A. Dean and T. C. Rains.

New York: Marcel Dekker, 1974; J. A. Dean, Flame Photometry. New York: McGraw-Hill, 1960; and B.L.

Vallee and R. E. Thien, in Treatiu on AnalytictU Chem-istry, eels. 1. M. Kolthofr and P.J. EJvins. New York'

Interscience, 1965, Part I, vol. 6, Chapter 65. . • For a review of this topic, see: K. W. Busch and G. H.

Morrison. Anal. Chern. 45 (8~ 712A (1973~

sidered one example, the simple photometer for the simultaneous determination of sodium and potassium. Some of these efforts have resulted in. the development of computer-controlled m?nochromators that permit rapid sequential measurement of radiant power at several wavelengths corresponding to peaks for various elements. With such instruments, two to· three seconds are required to move from one peak to the next. The detector photocurrent is then measured for one or two seconds. Thus, it is possible to determine the concentration of as many as ten elements per minute. Instruments of this type have been employed with all three types of flame methods, although the emission method has the distinct advantage of not requiring several sources.

Simultaneous,multielement, flame emission analyses have been made possible by the use of the optical multichannel analyzer that was described earlier (p. 142). As an example,tO a silicon diode vidicon tube was mounted on the optical plane originally occupied by the slit of an ordinary grating monochromator.

The diameter of the tube surface was such that a 2O-nm band of radiation was contin-uously monitored; by adjustment of the tube along the focal plane of the monochromator, various 2O-nm bands of the spectrum could be observed. The resl>lution within the 20-nm band was such that lines 0.14 nm apart could be resolved.

Figure 11-15 shows a spectrum obtained simultaneously for eight elements that have emission peaks in the wavelength range of 388.6 to 408.6 nm. A nitrous oxide/acetylene flame was employed for excitation. Slightly more than half a minute was required to accumulate data for the eight analyses. A rel-ative precision better than 5

%

was obtained.

lOK. W. Busch. N. G. Howell, and G. H. Morrison, Anal. Chnn.46, 575 (1974~

the analyte peak. For nonrccording instru-ments, a measurement on either side of the peak suffices. The average of the two measure-ments is then subtracted from.the total peak height.

Chemic:aI Interferences. Chemical interfer-ences in flame emission studies are essentially the same as those encountered in flame ·ab-sorption methods. They are dealt with by ju-dicious choice of flame temperature and the use of protective agents, releasing agents, and ionization suppressors.

Self-Absorption. The center of a flame is hotter than the exterior; thus, atoms that emit

<Ie

;

"

:<

«

§~ S~ ...

-c: "

:; :;

FIGURE 11-15 Multielement flame emission spectrum from 388.6 to 408.6 nm. [K. W. Busch, N. G. Howell, and G.H.Morrison,Anal.Chem.,46,578 (1974). With permission of the American Chemical Society.]

Interferences

The interferences encountered in flame emis-sion spectroscopy arise from the same sources as those in atomic absorption methods (see p. 320); the severity of any given interference will often differ for the two procedures, however.

Spectral Line Interference. Interference be-tween two overlapping atomic absorption peaks occurs only in the occasional situation where the peaks are within about 0.1

A

of one another. That is, the high degree of spectral specificity is more the result of the narrow line properties of the source than the high resolution of the monochromator. Atomic emission spectroscopy, in contrast, depends

entirely upon the monochromator for selectiv-ity; the probability of spectral interference due to line overlap is consequently greater.

Figure 11-16 shows an emission spectrum for three transition elements, iron, nickel, and chromium. Note that several unresolved peaks exist and that care would have to be taken to aVQid spectral inteiference in the analysis for anyone of these elements.

Ba", Interference; Background Correction.

Emission lines are often superimposed on bands emitted by oxides and other molecular species from the sample, the fuel, or the oxi-dant. An example appears in Figure 11-17. As shown in the figure, a background correction for band emission is readily made by scan-ning for a few angstrom units on either side of

in the center are surrounded by a cooler region which contains a higher concentration of unexcited atoms; self-absorption of the res-onance wavelengths by the atoms in the cooler layer will occur. Doppler broadening of the emission line is greater than the corresponding broadening of the resonarice absorption line, however, because the particles are moving more rapidly in the hotter-emission zone. Thus, self-absorption tends to alter the center of a line more than its edges. In the extreme, the center may become less intense than the edges, or it may even disappear; the result is division of the emission maximum into what appears

-'"

~;S ",,,,

MM

zz

Wavelength, nm

FIGURE 11-16 Partial oxyhydrogen flame-emission spectrum for a sample containing 600 ppm Fe, 600 ppm Ni, and 200 ppm Cr. (Taken from R. Herrmann and C. T. J. Alkemade, Chemical Analysis by Flame Photometry, 2d ed. New York: Interscience, 1963, p. 527. With permission.)

I

2851 2853

FIGURE 11-18 Curve A illustrates the self reversal that occurs with high concentration of Mg (2000 Jlg/ml).

Curve Bshows the norma) spectrum of 100 pg/ml of Mg.

FIGURE 11-17 Flame emission spectrum for a natura) brine showing the method used for correcting for background radiation.

(Taken from R. Herrmann and C. T.J.Alkemade, Chemical Analysis by Flome Photometry, 2d ed. New York: Interscience, 1963, p. 484.

With permission.)

to be two peaks byself-reversal.Figure 11-18 shows an example of severe self-absorption and self-reversal.

Self-absorption often becomes troublesome when the analyte is present in high concentra-tion. Under these circumstances, a nonreso-nance line, which cannot undergo self-absorp-.tion, may be preferable for an analysis.

Self-absorption and ionization sometimes result in S-shaped emission calibration curves with three distinct segments. At intermediate concentrations of potassium, for example, a

(p. 324). Both calibration curves and the stand-ard addition method are employed. In addi-tion, internal standards may be used to com-pensate for flame variables.

linear relationship between intensity and con-centration is observed (Figure 11-19). At low concentrations, curvature is due to the in-creased degree of ionization in the flame. Self-absorption, on the other hand, causes negative departures from a straight line at higher concentrations.

Comparison of Atomic Emission and Atomic Absorption Methods

For purposes of comparison, the main advan-tages and disadvanadvan-tages of the two widely used flame methods are listed in the para-graphs that follow.^II The comparisons apply to versatile spectrophotometers that are readily adapted to the determination of numerous elements .

Analytical Techniques

The analytical techniques for flame emission spectroscopy are similar to those described earlier for atomic absorption spectroscopy

IIFor an excellent comparison or the two methods, see:

E. E. Piclcett and S. R. Koirtyohann, Anal. Chern. 41 (14~ 28A (1969).

1.Instruments. A major advantage of the emission procedure is that the flame serves as the source. In contrast, absorption methods require an individual lamp for each element (or sometimes, for a limited group of ele-ments). On the other hand, the quality of the monochromator for an absorption instrument does not have to be so great. to achieve the same degree of selectivity because of the narrow lines emitted by the. hollow cathode lamp.

2. Operator Skill. Emission methods gen-erally require a higher degree of operator skill because of the critical nature of such adjust-ments as wavelength, flame zone sampled, and fuel-to-oxidant ratio.

3. Background Co"ection. Correction for band spectra arising from sample constituents is more easily, and often more exactly, carried out for emission methods.

4. Precision and Accuracy. In the hands of a skilled operator, uncertainties are about the same for the two procedures (±O.5 to ) ~:) relative). With less skilled personnel, atomic absorption methods have an advantage.

5. Interferences. The two methods suffer from similar chemical interferences. Atomic

100

75

'i ~

.E

..

_> 50

";

a;a:

25

FIGURE 11-19 Effects of ionization and self-absorption on a calibration curve for po-tassium.

absorption procedures are less subject to spec-tral line interferences, although such inter-ferences are usually easily recognized and avoided in emission methods. Spectral band interferences were considered under back-ground correction.

6. Detection Limits.The data in Table ll-S provide a comparison of detection limits and emphasizes the complementary nature of the two procedures.

ATOMIC

FLUORESCENCE SPECTROSCOPY Since 1966, considerable research has been carried out on the principles and applications of atomic fluorescence spectroscopy.12 It is evident from such work that this procedure is somewhat more sensitive for perhaps five to ten elements than either of the two atomic methods we have just considered, particularly

12For further information on atomic ftuorescence spec-troseopy, see: C. Veillon, in Trace Analysu. ed. J. D.

Winefotdner. New York: Wiley. 1976, Chapter VI; and J. D.Winefordner. J. Chem. Edue. 55, 72 (1978~

1. Describe the basic differences between atomic emission and atomic -fluorescence spectroscopy.

2. Define the following terms: (a) atomization. «b) pressure broadening, (c) Doppler broadening, (d) turbulent flow nebulizer, (e) laminar flow nebulizer, (f) hollow cathode lamp. (g) sputtering. (h) self-absorption.

(i) sensitivity. (j) detection limit. (k) spectral interference, (I)chemical interference, (m) radiation buffer. (n) releasing agent. (0) protective agent, and (p) ionization suppressor.

3. Why is the CaOH spectrum in Figure 11-S so much broader than the Ba resonance line?

4. Why is atomic emission more sensitive to flame instability than atomic absorption or fluorescence?

S. Describe the effectk which are responsible for the three different absorbance profiles'in Figure 11-7.

6. Why is a nonflame atomizer more sensitive than a flame atomizer?

7. Why is source modulation employed in atomic absorption spectros-copy?

8. Describe how a deuterium lamp can be employed to provide a back-ground correction for an atomic absorption spectrum.

when nonftame atomization is employed. On the other hand, for many elements. the method is less sensitive and appears to have a smaller useful concentration range. Further-more, for comparable pCrCormance, fluores-cence instruments appear to be more complex and potentially more expensive to purchase and maintain,u

as an atomic line width is so low as to restrict 'the sensitivity of the method.

Conventional hollow cathode lamps oper-ated continuously do not provide enough .radiant power for fluorescence analysis. How-ever, very intense bursts of radiant energy can be achieved by pulsing the lamps with large currents for brief periods. Provided the pulse width and period are chosen to give a low average current. destruction of the lamp is avoided. The detector must be gated to observe;

the fluorescence only during the pulse.

Gaseous discharge lamps with sufficient:

radiant power for fluorescence measurement:

are available for several of the more volatile elements such as the alkalis, mercury, cad-:

mium, zinc, thallium, aDd gallium.

Electrodeless discharge lamps provide high-intensity line sources for the various elements.

Typically, these lamps consist of a sealed quartz tube containing an inert gas at perhaps;

1torr and the element or a salt of the analytei element. Excitation is obtained by placing the:

Inatrumentation

An instrument for atomic fluorescence mea-surements contains a modulated source, an atomizer (flame or nonflame), a monochro-mator or an interference-filter system, a de-tector, and a signal-processing system. With the exception of the source, most of these components are similar to those discussed in earlier parts of this chapter. .

A continuous source would clearly be de-sirable for atomic fluorescence measurements.

Unfortunately, however, the output power of a continuous source over a region as narrow

USee: W. B. Barnett and H. L. Kahn, Anal. Chem ••••••

935 (1972~

TABLE 11-5 COMPARISON OF DETECTION

LIMITS FOR VARIOUS ELEMENTS BY

FLAME ABSORPTION AND FLAME

EMISSION METHODS·

Flame Absorption More Sensitive AI. Da, Ca. Eu, Ga, Ho. In. K.

La. Li. Lu. Na, Nd. Pr, Rb. Re.

Ru, Sm, Sr, Th, TI. Tm, W. Yb

Se•• itivity About the Same Cr, Cu, Dy. Er.

Gd. Ge, Mn, Mo, Nb, Pd. Rh. Se.

Ta, Ti, V, Y, Zr

Flame Emission More Se.-itive Ag, As, Au, B.

Be, Bi, Cd, Co, Fe, Hg, Ir, Mg, Ni, Pb, Pt, Sb.

Se, Si, So, Te, Zn

• Adapted from E. E. Pickett aDd S. R. Koirtyobann, Anal. Chem. 41 (141 42A (1969)and reprinted with permission. Copyright by the American Chemical Society.

tube in a microwave field generated by an antenna or a resonant cavity.

Tunable dye lasers are beginning to find use as sources for atomic fluorescence studies.

Interferences

Interferences encountered in atomic fluo-rescence spectroscopy appear to be of the same type and of about the same magnitude as those found in atomic absorption spectros-copy.t4

Applications

Atomic fluorescence methods have been ap-plied to the analysis of metals in such materials as lubricating oils, seawater biological mate-rial, graphite, aDd agricultural samples.15

" See:J. D.Winefordner and R. C. Elser, Anal. Chem.

43 (4~ 2SA (1971).

" For a summary of applications, see: J. D. Wineford-nero J. Chem. Educ .• 55. 12 (1978).

12. What is the equilibrium ratio of excited

n

atoms to those in the ground state when radiation of 3776

A

is absorbed and the tempera-ture is (see Figure 11-4)

(a) 2200⁰K?

(b) 2800⁰K?

13. For the flame shown in Figure 11-6, calculate the percent decrease in sodium emission intensity (5893 A) when the flame is observed at 4.0 em rather than 3.0 em above the orifice?

14. Calculate the equilibrium constant for the reaction Ca ~Ca +

+

e-at 2800oK.

IS. Calculate the equilibrium constant for the reaction Ca+

+

Rb ~ Ca

+

Rb+at 2800oK.

16. The Doppler effect is one of the sources of line broadening in atomic absorption spectroscopy. Atoms moving toward the light source see a higher frequency than do atoms moving away from the source. The difference in wavelength, lU, experienced by an atom moving at speed v (compared to one at rest) is !M/A.=v/e, where e is the speed of light.

Estimate the line width (inA)of the sodium 0 line (5893A) when the absorbing atoms are at a temperature of2()()()OK. The average speed of an atom is given by v=J8kT/mn, where kis Boltzmann's constant, T is temperature, and m is the mass.

9. Why do organic solvents enhance atomic absorption and emission peaks?

10. Explain why curve A in Figure 11-18 is split into a doublet.

11. The cobalt in an aqueous sample was determined by pipetting 10.0 m1 of the unknown into each of four SO.o-ml volumetric flasks. Various volumes of a standard containing 6.23 ppm Co were added to the flasks following which the solutions were diluted to volume. Calculate the parts per million of cobalt in the sample from the data that follow.

Sample Unknown, ml Standard, ml Absorbance

blank 0.0 0.0 0.042

A 10.0 0.0 0.201

B 10.0 10.0 0.292

C 10.0 20.0 0.378

0 10.0 30.0 0.467

E 10.0 40.0 0.5S4

The term emission spectroscopy usually refers to a type of atomic spectroscopy that employs more energetic excitation sources than the flames and ovens described in Chapter 11.

Until recently, two types of sources were employed in emission methods. the electric arc and the electric spark.^l During the last decade. however. argon plasma sources have been developed that combine many of the best features of flame sources with the attri-butes of the classical arc and spark. As a consequence. there has been a recent resur-gence of interest in emission methods, and several new instruments employing argon plasma sources have appeared on the market since 1977.

Arc and spark emission methods have several advantages that account for their widespread use since the 19305 or earlier.

Among these is their wide applicability (70 or more metal or metalloid elements ~ their high degree of specificity. and their sensitivity. which often permits analyses in the parts-per-million . or parts-per-billion range. Arc and spark

emis-sion methods offer certain advantages over flame methods. which account for their con-tinued use even after the remarkable growth ofthe latter beginning in the mid-1960s. One of these advantages is the minimal sample preparation required for emission methods. be-cause excitation can usually be carried out directly on liquids. powders. metals, and glass-es. Second. the higher energies employed tend to reduce interelement interference. Third, good spectra for most elements can be obtained

IFor a more complete discussion of arc and spark

IFor a more complete discussion of arc and spark

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