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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
the emitter by the syringe technique. High density sample loading is sometimes necessary to increase the detectable limit of samples of low ionization efficiency. However, electric breakdown sometimes occurs when samples are loaded with high surface density. The silicon emitter is advantageous in this point because it resists electric breakdown as described below. Strength of Silicon Emitter. The 60-pm silicon emitter is strong enough against the soft mechanical contact with the microsyringe a t the sample loading. The silicon emitter endures electric breakdown with the 60-pm wire never broken. A microscope inspection of the emitter surface shows that the whiskers were damaged only in an area of about 10-bm diameter. The sparking occurs once or twice in one run when high sample loading and high heating current are required. The same silicon emitter was used for several times with the same sample. However, normally a new emitter is used for a different sample because it is easily reproduced. Easy Production of Silicon Emitter. The production of silicon emitters is quite simple and easy as described in the previous section. Twenty-four emitters can be prepared in an hour or two. The 60-pm tungsten wire is easily spot-welded on the emitter holder and no special technique is necessary. The pressure of silane gas is not critical. The necessary stability of the electric power supply to heat the tungsten wire is about 1 .., 2 % and this is easily realized. The yield rate is more than 95%. I t should be noted that no high voltage supply is required during the producing process. It would be
a decisive advantage for any mass spectroscopist to be able to prepare the silicon emitters quite easily. We here propose the silicon emitter as a new emitter for field desorption mass spectrometry.
ACKNOWLEDGMENT The authors express their sincere thanks to Y. Izumi and Y. Shimonishi, Institute for Protein Research, Osaka University. They also thank JEOL for taking scanning electron microphotographs.
LITERATURE CITED (1) (21 (3) (4) (5) (6) I
/
(7) (8) (9) (10) ( 11) (12)
D. Beckey, Int. J . Mass Spectrum. Ion Phys., 2 , 500 (1969). R. Schulten. Methods Biochem. Anal.. 24. 313 (1977). R. Schulten and H. D. Beckey, Org. Mass Spectrom., 6: 885 (1972). R. Schuiten and H. D. Beckey, Messtechnik, 81, 121 (1973). H. D. Beckey, E. Hilt, and H. R. Schulten. J . Phys. E.,6 , 1043 (1973). R . M. Wightman, D. M. Hinton, M. C. Sammons, and M. M. Bursey, Int. J . Mass Spectrum. Ion Phys.. 17. 208 (1975). M. M. Bursey, C. E. Rechsteiner, M. C. Sammons, D. M. Hinton, T. S. Colpitts, and K. M. Tvaronas, J . Phys. E.,9 , 145 (1976) T. Matsuo, 1. Katakuse, Y . Tatsumi, M. Hirata, and H. Matsuda. Mass Spectrosc. (Tokyo), 26, 205 (1978). H. Matsuda, At. Masses Fundam. Constants, 5 , 185 (1976). H. D. Beckey, A. Hendrichs, and H. U. Winkler, Int. J . Mass Spectrum. Ion Phys., 3, App. 9 (1970). R. Gomer, "Field Emission and Field Ionization", Harvard University Press, Cambridge, Mass., 1961, p 156. H. D. Beckey, S. Bloching, M. D. Migahed. E. Ochterbeck, and H. R. Schulten, Int. J . Mass Specirum. Ion Phys., 8 , 169 (1972).
H. H. H. H.
RECEIVED for review February 10, 1978. Accepted October 11. 1978.
Precision of Flame Atomic Absorption Spectrometric Measurements of Aluminum, Chromium, Cobalt, Europium, Lead, Manganese, Nickel, Potassium, Selenium, Silicon, Titanium, and Vanadium N. W. Bower' and J. D. Ingle, Jr.* Depariment of Chemistry, Oregon State University, Corvallis, Oregon 9 733 1
The precision of flame atomic absorption measurements on a Varian AA-6 spectrophotometer has been evaluated for 12 elements. For some elements, the effect on precision of analysis line, type of flame, hollow cathode current, and spectral bandpass are determined. As previously shown for other elements, analyte absorption noise usually limits precision over most of the analytically useful absorbance range. Signal shot noise and amplifier readout noise are never limiting under normal conditions. At low absorbances, lamp and flame transmlssion noise usually dominate and, at high absorbances, analyte and background emission noise are usually most significant.
In a recently published paper ( I ) , data describing the precision characteristics for nine elements commonly determined by atomic absorption (AA) were presented. These characteristics must be established if instrumental parameters and analyte concentrations are to be adjusted for a maximum signal-to-noise ratio (S/N). With a knowledge of the nature Present address, Department of Chemistry, The Colorado College, Colorado Springs, Colo. 80903. 0003-2700/79/0351-0072$01.OO/O
and sources of imprecision of an analysis, the analyst can make systematic improvements in the performance of the instrument. Previously, the authors have presented the theoretical considerations (2) and the evaluation procedure (3-5) necessary to construct the precision curves (plots of the relative standard deviation in absorbance (gA/A) vs. absorbance ( A ) ) presented in this paper. The equations and procedure allow one to predict the experimental measurement precision at any absorbance with only a few measurements. The data presented in this paper extend these studies to 12 more commonly determined elements and demonstrate, for the first time, the effect of analysis wavelength, lamp current, and slit width on a few elements' precision curves. These studies demonstrate the applicability of the equations for exploring the results of optimization procedures over the whole calibration curve and provide typical values of noise parameters which should be applicable to most modern AA instruments. Random errors in sample acquisition or preparation or systematic errors are not considered.
EXPERIMENTAL A Varian AA-6 spectrophotometer was used with an external voltmeter and computer (PDP 11/20) to obtain all the data, as
0 1978 American
Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
73
Table I. Instrumental Variables elements
wavelength, nm
flame
spectral bandpass, nm
lamp current, mA
EPMTra
flow rate, Llmin, oxlfuel
v
0.1 10 303 N,O/C,H, 293 N,O/C,H, 0.1 10 4581462 co air/C,H , 0.1 5 349 air/C,H, 0.2 3 Cr 29 3 N,OIC,H, 0.1 10 Eu K air/C,H, 0.5 5 28 7 air/C,H, 358 Mnb 0.2 5 air/C,H, 406 Mn 0.05 5 Mn air/C,H, 1.0 5 485 Nib air/C,H, 0.2 5 425/440 Ni air/C,H, 0.2 5 360 Pb air/C,H, 1.0 6 3441359 air/C,H, 1.0 10 5571590 Seb 5541566 N,O/C,H, 1.0 10 Se 334 N,O/C,H, 0.2 15 Si 294 N,O/C,H, 0.2 20 Tib 411 N,O/C,H, 0.2 5 Ti V N,OIC,H, 0.05 20 417 For elements where the flame absorbs significantly, the voltage without and with the flame is given. conditions (manufacturer’s cookbook settings) for analysis.
A1 A1
309.3 396.1 240.7 357.9 459.4 766.5 279.5 279.5 279.5 232.0 341.5 217.0 196.0 196.0 251.6 364.3 364.3 318.5
previously described ( I ) . Instrumental conditions such as the hollow cathode current were generally those specified in the Varian manual and are summarized in Table I. Burner position was adjusted for maximum absorbance. The hollow cathode lamps were all manufactured by Westinghouse. Thirty 1-s measurements were made for Al, Co, Cr, Eu, K, Mn, Ni, Se, Si, Pb, Ti, and V. A1 and Ni were run at two different resonance wavelengths, Se was run in air/C2Hzand Nz0/CzH2, Ti was run at 5- and 20-mA lamp currents, and Mn was run with a variety of spectral bandpasses. Precision curves of u c / c vs. c are also presented for Mn at the various slitwidths to demonstrate the relationship between the absorbance precision plots and the concentration precision plots. These plots also clarify the effect of the slit width on the actual analysis precision at high absorbances. All noise calculations and construction of theoretical and experimental precision plots were carried out as previously described (1-5). All symbols are defined in the Appendix.
RESULTS AND DISCUSSION T h e precision plots for the elements tested are shown in Figures 1-7 and the pertinent calculated parameters and summary of experimental results are shown in Tables I1 and 111, respectively, in the format previously used ( I ) . Since good agreement between theory and experimental results was obtained, only the theoretical plots are reported. Figure 1 shows the precision curves for Al for both the 396.1 and 309.3 nm resonance lines. The data in Table I1 indicate that the intensity of both lines and all noises are about the same magnitude. The background emission noise (ob)is three times greater a t the 309.3 nm line but this has no effect on the precision as can be seen by comparing curves a (a’) and b (b’) in Figure 1. The analyte emission noise (a,) is greater for the 396.1-nm line which causes the precision curve to bend up slightly a t the highest absorbances. The precision at moderate and high absorbances appears to be better for the 309.3-nm line. However here analyte absorption noise is limiting and the steep dip in the precision curve is due to the greater nonlinearity of the calibration curve ( I ) a t this wavelength and provides no real improvement in determining concentrations. Since the AA sensitivities and precision characteristics are about the same, the less used 396.1-nm line should be preferred because of its more linear calibration curve. The precision curves in Figure 2 and the data in Table I1 for Ni at 341.5 and 232.0 nm indicate that the precision characteristics at a given absorbance and the magnitude of each type of noise (e.g., art, dot) are about the same a t both
9.515.5 9.515.5 1012.5 1013 10.515.8 1012.5 1012.5 1012.5 1012.5 1012.5 1012.5 1012.5 1012.5 2.515.1 9.515.8 9.515.5 9.515.5 9.515.8
Normal
I-
0.03
A
Figure 1. Recision plots for AI for different analysis lines. (a)309.3-nm line, all noise sources: (a’)396.1-nm line, all noise sources; (b) 309.3-nm line, all noise sources except those due to analyte; (b’) 396.1-nm line, all noise sources except those due to analyte; (c) 309.3-nm line, all noise sources independent of fhme; (c’)396.1-nm line, all noise sources independent of flame
wavelengths. In both cases, precision is limited a t low absorbances by source and flame transmission flicker and at moderate and high absorbances by analyte absorption noise. Since the AA sensitivity is about three times better for the 232.0-nm line, the precision a t a given concentration is better for this analysis line up to about 10 ppm; however above 10 ppm, either line gives equivalent precision. At moderate absorbances under analyte absorption noise limited conditions, the precision curve dips lower for the 232.0-nm line because of the greater nonlinearity in the calibration curve. Since the 232.0-nm line is directly on the molecular absorption maximum for NaC1, it is possible that one may wish to use the 341.5-nm line to avoid this spectral interference, but the reduced sensitivity, and therefore loss of precision when lamp and flame flicker noise (E1 and 6) are limited may not make
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.
-
-
-
.02
‘O5 0.01
t
’
‘I .02
I
I
1 1 0.0l
Figure 2. Precision plots for Ni for different analysis lines. (a) 232.0-nm line, all noise sources; (a’) 341.5nm line, all noise sources; (b) 232.0-nm line, all noise sources except those due to analyte; (b’) 341.5-nrn line, all noise sources except those due to analyte; (c) 232.0-nm line, all noise sources independent of flame; (c’) 341.5-nrn line, all noise sources independent of flame
the switch useful. A 3% NaCl solution gave an absorbance of 0.02 A a t 232.0 nm and background correction with the Hz lamp would compensate for this, but it may introduce additional limiting flicker noise (1). T i was run a t lamp currents of 5 and 20 mA as shown in Table I1 and Figure 3. The lamp intensity at 5 mA is about of that a t 20 mA which makes the signal shot noise ((uJq+J,background emission noise ( rh),and analyte emission noise (a,) relatively more important. However the precision characteristics at both lamp currents are about the same since none of these noise sources are limiting (Le,, lamp and flame transmission noise are limiting at low absorbances and analyte absorption noise at high absorbances). There is no decrease in lamp flicker noise (tl) at higher currents as has been shown for some elements ( 5 ) . Since the AA sensitivity is about the same a t both lamp currents, the lower current might be preferred to increase lamp lifetime even though the higher lamp current is recommended by the manufacturer. In Figure 4, the precision plots for Se in an air/C2H2and NzO/C2H2flame are presented. From Table 11,it is clear that background emission (obe) essentially limits precision a t all absorbances in both flames and analyte absorption noise is not observable. Since for the cooler air/C,H, flame, the AA sensitivity is about twice as good and the background emission noise is about three times less, it is preferred because it yields significantly better precision a t all absorbances and concentrations. Because the transmission of the air/C2H, flame is less, it would be expected to have a higher flame transmission flicker (&), as previously observed with As ( 1 ) ;however here, the higher q , e makes flame transmission noise not observable in the hotter flame. In Figure 5 the precision curves for Mn using spectral bandpasses of 0.05, 0.2, and 1.0 nm are presented. At high absorbances where measurements are limited by analyte absorption noise, u A / A a t a given A is much lower the larger the slit width as predicted because the negative deviation in the calibration curve increases ( I ) . This drastic improvement in precision of o A / A does not improve the measurement precision for the concentration over most of the calibration
I
I
A
I1
A
Figure 3. Precision plots for Ti for different hollow cathode currents. (a) 20 mA, ail noise sources; (a‘) 5 mA, all noise sources; (b) 20 mA, all noise sources except those due to analyte; (b’) 5 mA, all noise sources except those due to analyte; ( c ) 20 mA, all noise sources independent of flame: (c’) 5 mA, all noise sources independent of flame
1 0.01
A
Figure 4. Precision plots for Se for different fhmes. (a) N,O/C,H,
flame,
all noise sources; (a’) air/C,H, flame, all noise sources; (b) all noise sources independent of flame
curve, however, as can be seen from the three curves in Figure 6, where u,/c vs. c is plotted. The dip at 25 ppm is not truly significant, as the errors in determining the slopes of the calibration curves at that point are greater than 50%. Here u,/c is calculated from Equation 54 in reference 2. From Table 11, only signal shot changes significantly with slit width and accounts for curves b and b’ being higher than curve b” (Figure 6) as the slit width is reduced. For the smallest slit width, the lamp signal (i,) is reduced to the point that signal shot noise is the limiting noise at low absorbances and high absorbances (at moderate absorbance, analyte absorption noise is limiting) and causes a true decrease in
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
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I
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i
d
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Figure 5. Precision curves for Mn at different slit widths. (a) 05-nm spectral bandpass, all noise sources: (a') 0.2-nm spectral bandpass, all noise sources; (a") 1.O-nm spectral bandpass, all noise sources; (b) 0.05-nm spectral bandpass, all noise sources except due to analyte; (b') 0.2-nm spectral bandpass, all noise sources except due to analyte: (b") 1.0-nm spectral bandpass, all noise sources except due to analyte
I
I
t
0
ID0
C Figure 6. Precision curves in terms of concentration for Mn at different slit widths. (a) 0.05-nm spectral bandpass, (b) 0.2-nm spectral bandpass, (c) 1.0-nm spectral bandpass, C in ppm "(4
9
f r i
precision which is obvious in the u,/c vs. c plots of Figure 6. For Mn, a 0.2-nm spectral bandpass would probably be preferred because it yields precision as good as that a t the 1-nm bandpass but gives a more linear calibration curve. For the remaining elements, all of the precision curves are plotted as u A / Avs. A , since they are more useful for comparing the different sources of noise between elements as the scale is the same, and uA/A can be measured directly, while u c / c must be calculated. In Figure 7 , the precision plots obtained with the analyte solutions aspirating (all noise sources considered) are given for Co, Cr, Eu, K, Pb, Si, and V. As was noted previously ( I ) , for most of the elements, analyte absorption flicker noise (E3) is limiting from 0.1 to 1 or 1.5 absorbance units. Over this region u A / A will be relatively constant or dip depending upon the negative deviation in the calibration curve. Co, 'Mn, Ni, and P b exhibit this deviation particularly, with the Co and hln curves having a large dependence on the slit width, as discussed above. Eu and K both exhibit significant analyte emission noise ( u e ) at the higher absorbances (above 1.5 A). For all cases, the 0% T noise was dominated by background
76
ANALYTICAL CHEMISTRY, VOL. 51, NO. 1, JANUARY 1979
Table 111. Dominant Noise Sourcesa A at which
element Alb
co Cr
Eu K
*
best value of " * / A , % 0.09 0.14
0.27 0.70 0.36
best values occurs 1.1 1.1 1.8
limiting noises, A < 0.2
E 1
El
1.3 1.3 1.7 1.3 1.4
+ 52 + E*
limiting noises, 0.2 < A < 1.0
A > 1.0
t 3 E 3
E 2
E 3
E2
E 3
