Emission Flame Photometry— A New Look at an Old Method Flame methods have become increasingly important in analytical chemistry. Flame emission as an adjunct to atomic absorption spectroscopy is highly compatible and complementary in its usefulness
Ε. Ε. Pickett and S. R. Koirtyohann Agriculture Building University of Missouri Columbia, Mo. 65201
ERHAPS THE MOST significant fea Pture of the field of flame emis
sion spectroscopy in recent years is that it has fallen into a state of ne glect. 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 ne glect is unwarranted and that the method deserves a roughly equal place beside atomic absorption (AA) in the well-equipped analyti cal laboratory. A critical and, it 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. I t is assumed that the reader is fa miliar with the basic operations of AA, as presented, for example, in the two recent Reports for Analyti cal Chemists on, this subject (1, 2). 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 28 A ·
termining the alkali metal elements for many years. This equipment no longer represents the "state of the 'art." There has been no popu lar commercially available instru ment which was designed primarily for doing high-quality F E analysis. In AA, on the other hand, following the pioneering work of Walsh {3, 4), the instruments have been designed to optimize the important parame ters of the method, taking advan tage of the latest developments in instrumentation available at the time. It. is no accident that most of these parameters are the same in F E and AA. Several manufactur ers of AA instruments now adver tise their products as being usable in both ways. I t is likely that many prospective purchasers are unaware of the significance of this fact and will fail to make the most of the F E capabilities of the more recent models. The modern grating spectrometer, equipped with suit able burners, especially the nitrous oxide-acetylene slot burner, and a good recorder, serves equally well for F E and AA. The features which are desirable in a modern F E 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 ad justable to give greater intensity in situations not requiring high resolu tion. Present-day spectrometers of about 0.5 m focal length with ad
ANALYTICAL CHEMISTRY, VOL. 4 1 , NO. 14, DECEMBER 1969
justable or exchangeable slits meet the requirements quite well. These instruments are able to "dilute" the flame background emission and re solve atomic emission lines from nearby lines and molecular fine structure maxima much more effec tively than the small prism monochromators used in most earlier F E work. This background emission adds to the total measured signal. It is likely to be different for sam ples and standards and must be corrected for whenever it is a sig nificant fraction of the total signal. Thus it is often necessary to scan the spectrum for an angstrom or two in the vicinity of a line. For this purpose, the instrument should be equipped with a scanning motor and a strip chart recorder having a pen speed of 1 sec or less. The re quirements of the intensity mea surements are not unusual and are met by many photomultiplier-amplifier-power supply combinations, either ac or dc. An important feature which often is not available in AA 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 spec trometer, 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 AA instruments. The need for proper positioning of the flame to ensure sampling of the optimum flame zone is important in AA and even more so in FE. DETECTION LIMITS IN FE ANALYSIS
The publication of "detection limits" of analytical methods is a hazardous business. They are not accurately reproducible and they change rapidly with new develop ments. Comparisons of detection limits between different methods are even more hazardous. Yet ana lysts inevitably are much interested in them. The comparison of F E and AA detection limits is especially cogent as it reveals some important
REPORT FOR ANALYTICAL CHEMISTS TABLE I.
Detection Limits by Flame Emission and Atomic Absorption
(In Mg/ml in aqueous solution, measured in nitrous oxide-acetylene or air-acetylene premixed flames formed by 5- to 10-cm slot burners) F ame emission Ele ment Ag AI As Au Β Ba
Be Bi Ca Cd Co Cr Cu Dy El·
Eu Fe Ga Gd Ge Hg Ho In Ir Κ La
Li Lu Mg Mn
Wave- o length, Â
N20-
Air-
C2H2
C2H2
Ref.
3280.7 0.02 (5) 3961.5 0.005» (7) 3092.8 1937.0 2676.0 0.5 (5) 2428.0 2496.8 5535.5 0.001» (6) 4708.6 ( B e O ) 0.2 (5) 2348.6 2230.6 4226.7 0.0001» 0.005» (6,5) 3261.1 2 (5) 2288.0 3453.5 0.05 (5) 2407.2 4254.4 0.005» (5) 3578.7 3274.0 0.01 (5) 3247.5 4046.0 0.07" (8) 4211.7 4008.0 0.04" (8) 4594.0 0.0006* (8) 3719.9 0.05 (5) 2483.3 4033.0 0.01» (7) 2874.2 2" 4401.9 (8) 3684.1 2651.2 0.5 (7) 2536.5 4053.9 0.02" (8) 4103.8 4511.3 0.002» (7) 3039.4 30
