Flame methods have become increasingly important in analytical chemistry. Flame emission as an adjunct to atomic absorption spectroscopy i s highly compatible and complementary in i t s usefulness
E. E. Pickett and S. R. Koirtyohann Agriculture Building University of Missouri Columbia, M o , 65201
ERHAPS THE MOST significant feaP t u r e of the field of flame emission spectroscopy in recent years is that it has fallen into a state of neglect. The obvious explanation of this is the overwhelming success of atomic absorption spectroscopy. The main purpose of this report is to survey some recent developments in flame emission spectroscopy (FE) which suggest that this neglect is unwarranted and that the method deserves a roughly equal place beside atomic absorption (AA) in the well-equipped analytical laboratory. A critical and, ih is hoped, unbiased discussion of the two methods is given. The third available mode of operation with flames, atomic fluorescence (AF) , will be discussed more briefly. It is assumed that the reader is familiar with the basic operations of AA, as presented, for example, in the two recent Reports for Analytical Chemists on this subject ( I , b). For many readers, the term “flame emission spectroscopy” will bring to mind only the very popular total-consumption turbulent flame and converted spectrophotometer (Beckman Instruments) which of course has been very useful in de-
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termining the alkali metal elements for many years, This equipment no longer represents the “state of the tart.” There has been no popular commercially available instrument which was designed primarily for doing high-quality FE analysis. In AA, on the other hand, following the pioneering work of Walsh (3,d) , the instruments have been designed to optimize the important parameters of the method, taking advantage of the latest developments in instrumentation available at the time. It is no accident that most of these parameters are the same in FE and AA. Several manufacturers of AA instruments now advertise their products as being usable in both ways. It is likely that many prospective purchasers are unaware of the significance of this fact and will fail to make the most of the FE capabilities of the more recent models. The modern grating spectrometer, equipped with suitable burners, especially the nitrous oxide-acetylene slot burner, and a good recorder, serves equally well for FE and AA. The features which are desipable in a modern FE instrument should be discussed. One needs a grating spectrometer capable of giving a band pass of about 0.5 A or less in the first order. Slits should be adjustable to give greater intensity in situations not requiring high resolution. Present-day spectrometers of about 0.5 m focal length with ad-
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14,DECEMBER 1969
justable or exchangeable slits meet the requirements quite well. These instruments are able to “dilute” the flame background emission and resolve atomic emission lines from nearby lines and molecular fine structure maxima much more effectively than the small prism monochromators used in moist earlier FE work. This background emission adds to the total measured signal. It is likely to be different for Samples and standards and must be corrected for whenever it is a significant fraction of the total signal. Thus it is often necessary to scan the spectrum for an angstrom or two in lthe vicinity of a line. For this purpose, the instrument should be equipped with a manning motor and a strip chart recorder having a pen speed of 1 sec or less. The requirements of the intensity measurements are not unusual and are met by many photomultiplier-amplifier-power supply combinations, either ac or dc. An important feature which often is not availlable in A,4 instruments is an accessible image of the flame. The need to produce compact and attractive instruments has caused manufacturers to hide most of the optical system, external to the spectrometer, under the cabinetry. The proper alignment of burners, lamps, lenses, and aperture stops, and above all the determination of the effective size, shape, and position of the flame, are almost impossible in many A-4 instruments. The need for proper positioning of the flame to ensure sampling of the optimum flame zone is inportant in AA and even more SO in FE. DETECTION LIMITS IN FE ANALYSIS
The publication of “detection limits” of analyt,ical methods is a hazardous business. They are not accurately reproducible and they change rapidly with new developments, Comparisons of detection limits between different methods are even more hazardous. Yet analysts inevitably are much interested in them. The comparison of FE and AA detection limits is especially cogent as it reveals some important
REPORT FOR ANAL'YTICAL CHEMISTS TABLE I.
Detection Limits by Flame Emission and Atomic Absorption
(In pg/ml in aqueous solution, measured in nitrous oxide-acetylene or air-acetylene premixed flames formed by 5- to 10-cm slot burners) Flame emission
Elernent
Wavelength,A
Ag AI
3280.7 3961.5 3092.8 1937.0 2676.0 2428.0 2496.8 5535 * 5 4708.6 (BeO) 2348.6 2230.6 4226.7 3261.1 2288.0 3453.5 2407.2 4254.4 3578.7 3274.0 3247.5 4046.0 4211.7 4008.0 4594.0 3719.9 2483.3 4033.0 2874.2 4401.9 3684.1 2651.2 2536.5 4053.9 4103.8 4511.3 3039.4 3800.1 2639.7 7664.9 5501.3 4418.2(LaO) 3927.6 6707.8 4518.6 3312.1 2852.1 4030.8 2794.8
As Au B Ba Be Bi Ca Cd co Cr cu DY
Er Eu Fe Ga Gd Ge Hg Ho
In Ir
K La Li Lu
Mg Mn
Flame emission
Atomic absorptiono
Elernent
Wavelength, A
0.005
Mo
0.2
Na Nb Nd
3903.0 3132.6 5890.0 4058.9 2924.5 4634.2 3414.8 2320.0 4057.8 2833.1 3634.7 2476 + 4 4951.4 2659.5 7800.2 3460.5 3692.4 3434.9 3728.0 3498.9 2175.8 4020.4 3911.8 1960.3 2516.1 4760.3 4296.7 2840.0 2246.0 4607.3 4812.8 2714.7 4318.9 4326.5 2142.8 3998.6 3642.7 5350.5 2767.9 3717.9 4105.8 4379.2 3184.0 4008.8 3620.9 4077.4 3988.0 2138.6 3601.2
NZO-
Air-
C3H3
CzHz Ref. CZHZ CZHZ
NzO-
0.02 0.005b
Air-
0.1 0.5 0.026
Ni
6 0.05
0. O O l b 0.2
Pb
0.002
0.05 0.002
O.UO01~ 2
0.005 0.05 0.005
Pd Pr Pt Rb Re
Rh
0.0056 0.005
Ru
0.01
0.005 Sb sc
0.070 0.2b 0.04c 0.0006c 0.05
0.16
0.005
Se Si Sm
0.07
Sn
0.086
0.016 2c Sr Ta
46 1
0.5
0.5 Tb
0.020 O.lb
0.0026 0.05
30d 2 0.005
0.00056 80
0.1 0.005
lc
facts bearing on the design and utilization of flame instruments in general. I n Table I we present a selected list of detection limits by AA and by FE taken from recent publications. As usual, the detection limit is the concentration of metal,
w
0.0003
Yb Zn
0 * 002
Zr
a All detection.limits in AA were taken from Ref. 10). b Determined in presence of high concentration o i a salt of an easil ionlzed metal generally potassium chloride. c determined'ln presence of easily ionized metal but would have shown ap reciably lower detection limit, probably two- or three-fold (8), ,Pit had.
dt
Tm
Y
50 0.005 0.005
TI
V
8 0.00003b
Te Ti
Atomic absorption"
NzO-
Air-
CzHn
CzH2 Ref. CaHz
NzO-
Air-
CzHz
0.1 0.03 0.002
1 0.20 0.03 0.005 0.2 0.03 0.05 0.02 10
0.1 0 * 005
2 0.2 0.02'
0.03 0.02, 0.3 0.1 0.03
0.5 0.20
0.5 0,068 0.0001~ 51
0.01
0.4c
0.3 0.2 0.02
0.2 0.020 0.01
0.5 0.40 0.002c
0.002 3
d Estimated from data of Fassel and Golightly for the premixed oxyacetylene flame [Ref. (9). eDetermination of detection was limited by contamination of the ionization suppressant. Data not previously published. f FE data not previously ublished. B Using an air-hydrogen i a m e .
in pg/ml, giving 1 signal twice as great as the rms noise level in the background signal. h definition of the detection limit based on the signal-to-noise ratio should be used in order t,o compare methods meaningfully. The commonly quoted
Imk
AA sensitivity, or concentration for 1% absorption, has no counterpart in BE or AF and is almost useless for making comparisons between methods. The basis for our selection of values needs to be explained.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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ANALYTICAL CHEMISTRY, VOL.
Report for Analytical Chemists
First, me have not tried to select all of the very lowest detection limit data to be found. Instead, we have tried to make comparisons which are as direct as possible. We present only data determined with the ordinary premixed flames burning on 6- or 10-cm slot burners for both methods, and using instruments which are fairly similar in their other characteristics. All coniponents conform, in general, to the required features mentioned above and are readily available. All data were measured on aqueous solutions using commercially available nebulizers which give solution uptake rates of about 3 ml/min. Other comparisons might be made, which would favor FE somewhat more heavily, if data obtained with other burner types or with organic solvent solutions were included, Thus we exclude the excellent results of lfossotti and Duggan (11) using the “PANT” burner and ethanolic solutions, of Fassel and coworkers with the fuel-rich premixed oxyacetylene flame and ethanolic solutions (9),of Kirkbright et al. using various “separated” flames (12, IS), and of Rains ( I d ) , using a total-consumption burner. These other burners give results in FE which may be superior to those in the table for certain elements. I n general, however, the slot burner, burning nitrous oxide and acetylene (or air and acetylene for the alkali metals), combines great sensitivity, convenience, versatility, and relative freedom from interference. Detection limits in AA are taken from recent literature supplied by an instrument manufacturer (IO). Both air-acetylene and nitrous oxide-acetylene flames were used in AA: The latter flame offers no special advantage in the AA determination of many elements. The frequently used practices of scale expansion, zero suppression, and adjustment of recorder time constant within reasonable limitsie., from one to several secondshave been used in each method whenever desirable. The longest recorder time constant used in the FE data was 3 sec. The time constants used in getting the AA data are not known, but vary from 1 up .I, NO, 14, DECEMBER 1969
to about 7 sec for the various elements ( 1 6 ) . Finally, not all possible elements have been included. We have tried to include all of those elements which have been shown to be detectable by one method or the other using slot burners and premixed flames a t less than 10 pg/ml in aqueous solutions. If it is agreed that detection limits for an element which are within a factor of three by the two methods really do not differ significantly, it is seen that there are 24 elements for which FE gives lower detection limits, 21 elements for which AA gives lower detection limits, and 17 elements for which the two give about the same detection limits. In the face of this information, many of the claims which have been made for the sensitivity of AA seem exaggerated. FE is distinctly more sensitive (or, better, gives lower detection limits) for the alkali metals, the alkaline earth elements Ca, Sr, and Ba, for the IIIA elements Al, Ga, and In, for most of the rare earth elements, and for several transition series elements. A F data were excluded from Table I because it was desired to make only the most direct comparisons possible. The burner designs and flames used in AF are considerably different from those conimonly used in AA. However, if AF is also considered, the score for FE remains unchanged, AF shows its greatest sensitivity for elements having their best lines a t short wavelengths and thus competes more strongly with AA. The detection limit depends greatly on the overall noise level and is in fact, by definition, a signal-to-noise ratio. Comparisons of absorption strengths with emission intensities, almost universally stated to favor absorption, are of little if any significance until they are related to the noise levels which can be achieved in practice in each method. Table I shows clearly that, in practice, the signal-to-noise ratios are such that AA in flames does not possess universal superiority. One safe generalization is that for elements which have their best emission and absorption lines a t
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hollow cathode source and in the flame, respectively, usually is considerably less than one. For sources in thermal equilibrium, J, < JpI. For nonthermal sources, J, G a y be much larger than Jpl,as in various electrical discharges such as hollow cathode lamps, permitting the great superiority of AA a t short wavelengths. Much hotter flames are required to improve FE in the deep ultraviolet. For lines a t longer wavelengths, evidently the radiance ratio does not outweigh the line profile ratio-ie., their product does not exceed unity, for many elements, and FE thus can be more sensitive than A,4. Note that the relation says nothing a t all about populations of energy levels. The reader should consult Alkemade's paper (17) for a more detailed discussion of this and other commonly accepted fallacies concerning AA. A systematic series of measurements of noise levels from the various sources, and of relative signal strengths, by the two methods and by AF, for a number of elements and flames, would help to put the field of flame photometry in better
TABLE II.
order. Several publications by Winefordner et al. have discussed the subject but without presenting much data ( 1 8 4 1 ) . The treatment of Alkemade ( 1 7 ) outlined above brings to mind this need for actual measurements of noise levels and of line strengths, independent of each other. No amount of measurements of detection limits-ie., signal-to-noise ratios-can serve the purpose. The treatment of the measurement of background, especially important in determining signal-to-noise ratios in FE, is taken up in the section on interferences below. ATOM FORMATION QUESTION
The efficiency with which the flame produces neutral atoms of the test element is of equal importance in FE and AA. This efficiency was not known, even approximately, for most elements until recently. Selected data from two studies (22, 23) are presented in Table 11. The table shows that many elements give free atom fractions not far below unity. This means that a rather large percentage of the metal
Free Atom Formation Fractions in Flames
(The ratio of the number of free atoms to the total number of atoms in all states of an element present in the flame at any instant) In the Air-CzH2flame
I
Ref. 123) Ref. ( 2 2 )
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