330
Anal. Chem. 1981, 53, 330-336
(5) Regan, J. G. T.; Warren, J. At. Absorpt. News/. 1978, 17, 89-90, (6) Slavin, Walter; Mannlng, D. C. Anal. Chem. 1979, 51, 261-265. (7) Backman, Svenerik; Karlsson, Rune W. Analyst (London) 1979, 104, 1017- 1029. (8) Sturgeon, R. E.; Chakrabarti, ,C. L.; Malnes, I. S.;Bertels, P. C. Anal. Chem. 1975, 47, 1240-1249. (9) Sturgeon, R. E.; Chakrabarti, C. L.; Malnes, I. S. Anal. Chem. 1975, 47, 1250-1257. (IO) L'vov, B. V. Spectrochlm. Acta, Parf 8 1978, 338, 153-193. (11) Czobik, E. J.; MatouSek, J. P. Talanta 1977, 24, 573-577. (12) Goss, Robert G. Abstracts, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic Clty, NJ, March 1980, No. 591; ABC Press: Monroeville, PA. (13) Regan, J. G. T.; Warren, J. Analyst(London) 1976, 101, 220-221. (14) Fuller, C. W. At. Absorpt. Newsl. 1977, 16, 106-107. (15) Alder, J. F.; West, T. S. Anal. Chlm. Acta 1970, 51, 385-372. (16) Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1978, 50, 1714-1716. (17) Salmon, S. G.; Holcombe, J. A. Anal. Chem. 1979, 51, 648-650. (18) Savitzky, Abraham; Golay, Marcel J. E. Anal. Chem. 1964, 36, 1627-1638. (19) Hart, P. J.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1987, 5 , 363-371. (20) Lussow, R. 0.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1967, 5 , 591-602. (21) Vastola, F. J.; Hart, P. J.; Walker, P. L., Jr. Carbon 1964, 2 , 65-71.
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RECEIVED for review September 26,1980. Accepted November 18,1980. This work was supported in part by National Science Foundation Grant No. CHE78-15438 and the Robert A. Welch Foundation.
Noise Sources in Multielement Atomic Fluorescence Spectrometry B. D. Pollard,' A. H. Ullman,2 and J. D. Wlnefordner" Department of Chemistty, University of Florida, Gainesville, Florida 326 1 1
The multlelement atomic fluorescence-emission spectrometric system (continuum source of excitation) has been evaluated for indlvldual noise contributions as a function of flame type, modulation approach, and atom type. The flames studied included Ar-shielded airlacetylene, Ar-shielded N20/ acetylene, Ar-shlelded N,O/propane, and an air/acetylene flame with a liquid fuel component (Isooctane and jet engine oil). The modulatlon methods included AM (amplitude modulation) and WM (wavelengths modulation) as well as CW (contlnuous wave excltation with DC detection). The elements and wavelengths studied Include Cd (228.2 nm), Mg (285.2 nm), Cu (324.7 nm), Ca (422.7 nm), and Na (589.0 nm); Bi (306.8 nm) was studled in the air/acetylene flame only. Useful concluslons were that the N,O/propane flame was less useful than the two acetylene-based flames, that AM is superior for transitlons wlth wavelengths 5350 nm and WM for ones 5350 nm, that analyte emission/fluorescence flicker noise becomes slgnificant for analyte concentrations above 1OOX the limltlng detectable concentratlon, and that the air/acetylene flame should always be used instead of the N,O/acetylene flame unless atomizatlon is insufflcient to obtaln reasonable signal levels.
-
A signal-to-noise ratio approach is used to evaluate and optimize the multielement atomic fluorescence spectrometer (MEAFS) previously described (1). A review and tutorial discussion of noise and signal-to-noise ratios in analytical spectrometry have already been given (2-4), and so no attempt will be made here to review the basic concepts. In this manuscript, the total measured noise is partitioned between individual noise components; the influence of flame type, *Presentaddress: Chemistry Department, Marquette University, Milwaukee, WI 53233. Present address: Industrial Chemicals Division, Procter & Gamble Co., Sharon Woods Technical Center, Cincinnati, OH
45241.
0003-2700/81/0353-0330$01 .OO/O
Table I. Individual Noise Determinations Required for One Complete Noise Study expt no.
detector flame
conditions light analyte source concna
noise term estC
I on off off 0 N D S ,N D F IIb on on off o N B S ,N B F IIIb on on on 0 N S S , NSF IV on on off low NA,S? NA,F V on on off high NAeS> NA,F VI on on on low NAfS N A f F VI1 on on on high NAfS,NAfF a Low concentration is l o x LOD. High concentration is l O O O X LOD. If an interference exists, an off wavelength determination must be made (see text). The noise terms given are estimated from total noise measured, total shot noise measured, and total flicker noise measured and appropriate subtraction of the previous noise terms in earlier experiments. Also note that in no cases, were electronic measurement, concomitant emission, and concomitant flicker noises found to be significant. 9
wavelength, and modulation approach upon the total and individual noises is determined, and specific conclusions are made about the flame type and modulation method to be used for MEAFS. The total noise for any AFS individual measurements is given approximately (3) by
+
NT = ( N E s ~ Nl)s2 + Nss2 -k Nss2 + Nc,s2 + N ( y 2 + N&s2 + NA+J2 NEF2 + NDF2 + + (NBF
+
+ NC,F + NbF)' + (NSF + Nc, + NAp)2)1'2 (1)
where NEs and NEF= electronic measurement shot (S) and flicker (F) noises, N D S and NDF = detector shot (S) and flicker (F) noises, NBsand NBF= emission background shot (S) and flicker (F) noises, Nss and NSF= scatter shot (S)and flicker 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
Table 111. Elements, Wavelengths, and Fluorescence/Emission Characteristics of Spectral Lines Chosen
Table 11. Flame Conditions for Noise Studies
flame acetylenelair acetylene/N,O IOSAC (acetylenelair) propane/N,O
flow r a t e s , ~ nebu1ization L/min flow,b fuel oxidant mL/min 3.5 4.6 2.2 2.8
11.5 8.2 10.0 7.5
wavelength, element nm
5.0 5.0
%
5.0
All flames were Ar shielded (1, 5). Nebulizer system was standard Perkin-Elmer AA 303 pneumatic type (1, 5). IOSA = flame conditions: air, 1 0 L min-'; acetylene, 2.2 L min-'; N, sheath, 1 2 L min-'; isooctaneoil (19:1, v:v) nebulization rate, 1.0 mL min-'.
(F) noises, NC8 and Nca = concomitant emission shot (S) and flicker (F) noises, N ~ ,and s NQF = concomitant fluorescence shot (S) and flicker (F) noises, N ~ and s NQ = analyte emission shot (S) and flicker (F) noises, and NA,Sand NA~F = analyte fluorescence shot (S)and flicker (F) noises. In the above total noise expression, all shot noises add quadratically, whereas in the case of flicker noises, we assumed, based upon previous work by us, that some of the noises were dependent and added linearly. Independent flicker noises were assumed to add quadratically.
%
FL4aC FLbsC
notes
lowbkgregion Cd 228.2 90 best fluorescence Mg 285.2 LOD 95 line in OH bands Bi 306.8 90 10 has substantial Cu 324.7 fluorescence and emission Ca 422.7 70 5 has substantial fluorescence and emission 10 1 best emission LOD Na 589.0 In shielded In shielded acetylenelair flame. % FL = (fluorescence signal)/ acetylene/N,O flame. (fluorescence + emission signals) x 1000. 100
1.0
Table IV. AirIAcetylene Flame Noise" Study Results cadmium magnesium 285.213 nm 228.208 nm Cl, = 0.10 (/.Lg/mL)Cl, = 0.010 ( p g l m L ) C&h = 10 ( / . L g / d ) chi,&= 1.0 ( / . L g / d ) modulation CW/AM/WM method CW/AM/WM
331
92 50
For all analytical studies, paired measurements must be made, i.e., an analyte measurement which includes the blank (signal is SA+BI) and a blank measurement (signal is SBI). The where the noises corresponding noises would be NA+B1 and NB~, are given by expression 1,except that all analyk related terms are missing in the case of NB1 (it is assumed that the blank
copper 324.745 nm Cl, = 0.10 /Ig/mL chi&= 1.0 CW/AM/WM
sodium calcium 589.592 nm 422.673 nm C l , = 0.010 /.Lg/mL c,1, = 0.010 ILgImL Chi& = 1.0 Chj& = 1.0 /.Lg/mL CW/AM/WM
CW /AM /WM
Detector 4.14.14. 4.14.14. -I-/-/-I-/-/Source Scatter 9.110.19. 28./39./28. 41.l31.141. 29.133.129. 18./10./18. Nss -I-/-1-1-/-/-I-/-/-INSF Flame Background 35.153.135. 49.165.149. 43./68./43. 29./51./28. 9./15/10. NBS 55.1-1-I-/-1-1-1-l-I-/NBF Low Analyte Emission 87./2.232/59. -/-/-/-I16.1-113. -1-1NES 2.6E2/-/1.1E2 -/-I-/-I-/-/-/-INEF 2.8E211.2E211.4E2 45.144.145. 1.032/88./62. 66.198.172. 13./20./13. NT High Analyte Emission 5,9321-15.932 1.4E2/2.9E2/99. 24.155.11 7. 25.154.117. -/-INAeS 6.OE2/-/2.5E2 -/-I1O.l-l2.5321-188. -/-/NA,F 3.3E2/3.3E2/1.5E2 6.OE3/-/2.6E3 58.184 ./51. 1.132/91./67. 13./20./13. NT Low Analyte Emission and Luminescence 88./2.332/88. 12./13./8. 32.129.122. 36.14 2.125. 21.136.116. NAfS 96.1-11.332127.183. 1.8E2I-I-1-1-/-INA~F 1.4E2/1.2E3/1.2E3 35.126.129. 1.5E2/93./1.OE2 l.OE2/1.3E2/97. 90./1.132/84. NT 4.16.12. 55./
