Anal. Chem. 1980, 52, 1578-1582
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histidine, lactic acid, and aspartic acid respectively. The agreement between analyses using the different reagents is very good. An earlier publication (16) established the reliability of direct graphite furnace analysis using EDTA as a matrix modifier. The application of EDTA gave results in good agreement with APDC/ MIBK solvent extraction followed by graphite furnace atomic absorption and with ion exchange on Chelex-100 resin followed by isotope dilution spark source mass spectrometry. The reagents described here allow measurements of slightly higher sensitivity than did EDTA, and with improved background interference characteristics. Citric acid appears to be the most promising reagent we have used to date. The sensitivity possible with this reagent is higher than with the other reagents tested, and the background interferences are small and easily corrected by commercial deuterium arc background correctors. The major difficulty encountered during the use of this reagent was the short duration of the atomic absorption signal. Measurements were repeated at several atomization temperatures to reduce systematic errors introduced by the spectrometer and recorder. The lowest standard deviation in the measurement of cadmium in seawater was obtained using aspartic acid. In a seawater matrix, this reagent provided excellent sensitivity and low background absorbance. In deionized water, however, it did not promote low temperature atom production to the extent possible using citric acid or lactic acid. Nevertheless, with carefully chosen char temperatures, the analyses of the blank and seawater were particularly rapid and trouble-free.
The reproducibility for both blank and seawater analyses was excellent. ACKNOWLEDGMENT The author thanks R. E. Sturgeon, L. Ramaley, and F. G. Mason for helpful discussion and suggestions during the course of this work. LITERATURE CITED Brooks, R. R.; Presley, B. J.; Kaplan, I. R. Talanta 1967, 14, 809. Sturgeon, R. E.; Berman, S.S.;Desaulniers, A,; Russell, D. S . Talanta 1979, 2 6 . Kingston, H. M.; Barnes, I. L.; Brady, T. J.; Rains, T. C.; Champ, M. A. Anal. Chem. 1978, 50, 2064. Florence, T. M.; Batley, G. Talanta 1976, 2 3 , 179. Gomiscek, V. H. S.; Gorenc, B. Anal. Cbim. Acta 1978, 9 8 , 39. Batley, G. E.; Matousek, J. P. Anal. Cbem. 1977, 4 9 , 2031. Hirose A.; Ishii, D. J. Radioanal. Cbem. 1978, 4 6 , 211. Hirose, A.; Kobori, K.; Ishii, D. Anal. Cbim. Acta 1978, 9 7 , 303. Robinson, J. W.; W o b t l , D. K.; Slevin, P. J.; Hindman, G. D. Anal. Cbim. Acta 1973, 6 6 , 13. Campbell, W. C.; Ottaway, J. M. Analyst (London) 1877, 102, 495. Sturgeon, R. E.; Berman, S.S.; Russell, D. S.Anal. Cbem. 1979, 51, 2364. Ediger, R. D.; Peterson, G.E.; Kerber, J. D.At. Absorpf. News/. 1974, 13, 61. Sperling, K. R. Fresenius Z . Anal. Chem. 1977, 2 8 7 , 23. LeBihan, A.; Courtot-Coupez, J. Analusis 1875, 3 , 59. Hoenig, M.; Vanderstappen, R.; Van Hoeyweghen, P. Analusis 1978, 7 , 17. Guewemont, R.; Sturgeon, R. E.; Berman, S.S. Anal. Chim. Acta, 1980, 115, 163. Hydes, D. J. Anal. Cbem. 1980, 5 2 , 959-63.
RECEIVED for review February 11, 1980. Accepted June 2, 1980. NRCC Publication No. 18247.
Signal-to-Noise Ratio Performance Characteristics of an Inductively Coupled Plasma E.
D. Salin' and Gary Horlick"
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Analyte emlsslon signal-to-noise ratio characteristics of an Inductively coupled plasma were measured as a function of several ICP experimental variables. Above the detection ilmtt, the standard deviation of the analyte emlsslon signal Is llnearly related to signal level indlcating that analyte flicker is the dominant noise source. It Is shown that under such condttions, analyte signal-to-noise ratios are not strongly dependent on plasma power and plasma (coolant) gas flow rate. The emission intensities of Ca, Sr, and Ar were measured as a function of time to assess the potential utlllty of Internal standardization. The emission fluctuations In Ca and Sr are correlated while those of Ca and Ar are not. Signal-to-noise ratios of Ca/Sr and Ca/Ar ratloed intensltles Indicate that internal Standardization of Ca with Sr leads to an improvement in preclslon while use of Ar actually degrades precision.
The inductively coupled plasma is clearly established as one of the most effective sources for simultaneous multielement 'Present address: Department of Chemistry, McGill University, 801 Sherbrooke St., West, Montreal, Quebec, Canada H3A 2K6. 0003-2700/60/0352-1578$01 .OO/O
analysis by atomic emission spectrometry (1-5). One of the key characteristics of a spectrochemical analysis system is its basic signal-to-noise ratio performance. Few data have been published on the general signal-to-noise ratio characteristics of the ICP source. Most available data tend only to deal with detection limits and signal-to-background ratios (6-9). In this study, the signal-to-noise ratio characteristics of an analyte species have been evaluated from detection limit to high signal values. In addition, the effects of plasma power, plasma (coolant) gas flow rate, and internal standardization on analyte signal-to-noise ratio have been investigated. These measurements were carried out with a photodiode array spectrometer (10). Of necessity, certain signal-to-noise ratio characteristics of the photodiode array spectrometer were also evaluated. In particular, the array/measurement system background noise characteristics are discussed and presented in the main body of the text. The effects of slit width and number of diodes integrated across a peak on measured signal-to-noise ratios are discussed and presented in the Appendix. EXPERIMENTAL A commercially available radiofrequency inductively coupled plasma source was used in this investigation (Plasma-Therm Inc., 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Table I. Standard Deviation of Array Background Signal as a Function of Integration Time and Cooling integration time, s
dark current signal level
std. dev.
Room Temperature Array
0.04
60 231 0.64 790 2.5 1725 Cooled Array (-15 "C)
2.5 2.2 2.3 2.9
0.04
2.9 2.9 2.9 3.2
0.16
0.16
0.64 2.5
26 31 55 155
Kresson, N.J.). The source consisted of a Model HFP-2500D 2.5-kW RF generator (27.12 MHz), a Model ADC5-3 automatic power control, a Model AMN-2500E automatic matching network, and a Model PT-2500 plasma torch assembly. The plasma was imaged onto the entrance slit of a monochromator (Heath EU-700) with a single spherical quartz lens (F1 = 10 cm, diameter = 5 cm) at a magnification of unity. The entrance slit was 100 pm. The detection system was a 1024-element photodiode array. The computer coupled photodiode array spectrometer is capable of simultaneously measuring over 50 nm of continuous spectral information anywhere from about 190 nm to over loo0nm. The spectral signal covered by the 50-nm window can be integrated for frame times of anywhere from 40 ms to over 20 s. The spectra from successive frame times can be multichannel averaged or sequentially stored to provide time resolution. Complete details on the photodiode array spectrometer and its capability can be found in the literature (IO). All standard deviations and signal-to-noiseratios (l/relative standard deviation) reported in this paper are based on a signal integration period of 10 s and 32 replicate measurements.
RESULTS A N D D I S C U S S I O N Photodiode Array/Measurement System Background Noise Characteristics. Previous work has illustrated the signal integration capability of photodiode arrays and as well the necessity of cooling the array in order to effectively use this capability (IO). The standard deviations of the photodiode array background signal (Le., dark current and fixed pattern background) as a function of array integration time and cooling are reported in Table I. The results shown in Table I indicate that the standard deviation of the array/measurement system is essentially independent of integration time and cooling. This result has important consequences as to how a specific total measurement time should be achieved with this system. Since the sensitivity of the array is linearly related to integration time, two apparently equivalent ways of achieving a desired measurement time are possible if one thinks only in terms of total signal. One is to use an array integration time equal to the total desired measurement time and the second is to use a shorter array integration time and signal average sufficient repetitive scans to achieve the total desired measurement time. Since the standard deviation of the array background signal is independent of integration time, the contribution of detector background noise to any particular measurement will be a t least using the first approach. This important point is further illustrated in Figure 1. A solution containing 5 ppm of Zn and Cd was aspirated into the plasma. With such a solution, the intensities of the Zn 1213.8-nm and Cd 1228.5-nm lines are about equal. The signal-to-noise ratios of these lines were determined using a total measurement integration time of 10 s. However, the total measurement integration time was achieved using various combinations of array integration times and number of scans signal averaged. Thus, if the array integration time was 2.5
'256
64
16
4
No. of Scans (log scale)
Flgure 1. Signal-to-noise ratios vs. number of scans for a constant 10-s integration time. See text for discussion
100
200
300
SIGNAL
Flgure 2. Analyte (Ca I1 393.3nm) signal-to-noise ratio and standard deviation as a function of signal intensity
s, 4 repetitive scans of the array signal were summed; if it was
1.25 s, then 8 scans were summed. The data shown in Figure 1 clearly indicate that a more precise measurement of the signal is achieved with the combination of longer array integration times and fewer signal averaged scans, in agreement with the conclusion reached based on the data reported in Table I. The straight line in Figure 1 indicates the signal-to-noise ratio that could be expected for these analyte signal levels if array background noise was the limiting noise in the system. In other words, for these signals, this line represents the best signal-to-noise ratio that could be achieved in 10 s with a particular number of scans. It can be seen from Figure 1 that if more than 8 or 9 scans are used to achieve a total measurement integration time of 10 s, the array background noise becomes the limiting noise in the system. However, with less than about 8 scans, the array background noise ceases to be limiting and the actual noise associated with the ICP analyte signal can be observed. T h e characteristics of this analyte noise will now be discussed in more detail. Signal-to-Noise Ratios f o r Analyte Species in t h e I n ductively Coupled Plasma. Most of the signal-to-noise ratio data published with respect to ICP emission signals has centered on a discussion of detection limits, where plasma spectral background and/or detector noises are often limiting. However, many determinations are carried out well removed from the detection limit level. Plots of signal-to-noise ratio and standard deviation as a function of analyte signal level (Ca I1 393.3 nm) are shown in Figure 2. These data illustrate that at signal levels above the detection limit, the standard deviation of the analyte signal is linearly dependent on signal level and thus signal-to-noise ratio is a constant a t high signal levels. Thus, above the detection limit, the limiting noise in the ICP appears to be analyte flicker noise which has the characteristic of linear dependence on signal level. Similar conclusions have been reached by others (7, 11). A log-log plot of standard deviation vs. analyte signal level is shown in Figure 3 in order to provide a wide dynamic range overall view of the dependence of standard deviation on signal level. At the low signal end, the detector noise limit is reached
1580
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Tabel 111. Signal-to-Noise Ratios (S/N) and Signal-to-Background Ratios ( S / B ) as a Function of Power Level line, nm power, kW SIN Ca I1 (393.3)
Ca I1 (396.8)
1.5 2.0 2.5
420 380 330
39 25
1.5
400 370
14 9 5
2.0 2.5
1
Sr I1 (407.7) 26
30
34
3a
42
46
50
54
58
LOG SIGNAL
Flgure 3. Log-log plot of analyte signal-to-noise ratio as a function of signal intensity
Table 11. Typical Detection Limits (ppm, S / N = 2 ) with the ICP-Photodiode Array Spectrometer Be I1 (313.0) Ca I1 (393.3) Ni I1 (232.0) P I(214.9) V I1 (292.4) Zn I (213.8)
SIB
1.5 2.0 2.5
350 430 390 290
14
22 12
5
Table I V . Signal-to-Noise Ratios as a Function of Plasma Gas (Coolant) Flow Rate (Ca I1 393.3 nm, 1 ppm, 2 k W ) plasma gas flow rate, Limin
av. SIN over 4 detmns
range of SIN
0.001
14
0.3
16
420 400 470
130 150
460
120__
0.0002 0.8
0.03 0.05
and the standard deviation is independent of signal level. At the high signal end, the analyte flicker noise limit is reached as the slope of the log-log plot approaches unity. Note that there does not appear to be an abrupt transition from detector noise limit to analyte flicker noise limit but rather a slow transition between these two limiting noise situations seems to exist. The actual source and/or nature of the analyte flicker noise is not known at this time. The most probable sources lie in the nebulizer-sample delivery and gas flow systems of the ICP. In the course of making these signal-to-noise ratio measurements, a few representative detection limits for our ICP-photodiode array spectrometer were determined and are presented in Table 11. These data were obtained with single 10-s signal integrations. Detection limit is defined on the basis of a signal-to-noise ratio of two where noise is the measured standard deviation of a blank solution. In general, they are about one to two orders of magnitude poorer than some of the best published ICP detection limits determined with PMT detectors ( 5 ) . With P M T detectors, the limiting noise at the detection limit is typically plasma spectral background noise while in our system it is array/measurement system noise. New photodiode arrays are now available from Reticon (910 Benicia Ave., Sunnyvale, Calif. 94086) that should improve these detection limits by at least an order of magnitude. When working at signal levels well above the detection limit, analyte flicker noise becomes the limiting noise in the system. In this situation, equivalent analyte signal-to-noise ratios can be expected with either detection system. Signal-to-Noise Ratio and Signal-to-Background Ratio as a F u n c t i o n of P l a s m a Power. In many ICP studies, signal-to-background ratio is used as a figure of merit. However, this ratio becomes significant only when the limiting noise in the system is related to the plasma spectral background. In analytical practice, this often is not the case, and then the signal-to-background ratio may be a misleading figure of merit. This is shown in Table I11 where both signal-to-noise ratios and signal-to-background ratios (both analytes at the 1-ppm level) are tabulated for two calcium lines and one strontium line as a function of plasma power. It is seen that the signal-to-noise ratios decrease only about 10-20% as the power is increased but that the signal-to-background ratios (background measured adjacent to the line) decrease by a
18 20
values
80 -
factor of 3 to 4, which, if taken as a figure of merit, is misleading in terms of the actual precision performance as power is increased. This means that we need not be too apprehensive in raising plasma power when operating at signal levels above the detection limit because little loss in precision occurs and greater capability in handling difficult sample matrices can often result. Signal-to-Noise Ratio as a Function of P l a s m a (Cool a n t ) Gas Flow Rate. Another ICP variable is the plasma gas flow rate. The effect of plasma gas flow rate on analyte signal-to-noise ratio is shown in Table IV. As indicated by the data presented in Table IV, plasma gas flow rate has relatively little effect on analyte signal-to-noise ratios. Signal-to-Noise Ratio P e r f o r m a n c e w i t h I n t e r n a l Standardization. In contrast to the classic emission spectrometry techniques such as dc arc and spark, internal standardization has not been widely used in ICP measurements. Notable exceptions to this are studies by Dahlquist and Knoll (2) and Watters and Norris (12). In order to assess the potential utility of internal standardization of the intensities of Ca I1 393.3, Ca I1 396.8-, Sr I1 407.8-, and Ar I 415.9-nm lines were simultaneously monitored as a function of time using the photodiode array spectrometer. Previous work has shown that the Ca I1 and Sr I1 lines behave very similarly as plasma flow and power parameters are altered (13). An argon line was monitored to assess the utility of feedback control of plasma parameters based on argon lines intensities which has been proposed by others (11, 14). Plots of the fluctuations in intensities of the Ca I1 393.3and Ar 1415.9-nm lines as a function of time are shown in Figure 4a. Over this time frame, the fluctuations do not appear to be correlated. The analogous plots for the Ca IT 393.3- and Sr I1 407.7-nm lines are shown in Figure 4b. Fluctuations in intensities of these lines appear to be correlated. These data indicate that feedback control of plasma parameters (i.e., flows and power) based on Ar line intensities may not be effective in stabilizing analyte emission while a more traditional approach to internal standardization based on matching lines with similar characteristics should be effective. This is borne out by the data presented in Table V. Signal-to-noise ratios were evaluated for both individual peak intensities and ratioed peak intensities. The ratioed peak intensities for Ca I1 393.3 to Ca I1 396.8 (in effect an ideal internal standard) and to Sr I1 407.7 show a factor of two
Anal. Chem. 1980, 52, 1582-1585
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Boumans, P. W. J. M.; Bastings, L. C.; deBoer, F. J.; van Kollenburg, L. W. J. Fresenius' 2. Anal. Chem. 1978, 291, 10. (6) Boumans, P. W. J. M.; deBoer. F. J. Specfrochim. Acta. Part51977, (5)
32,365. (7) Ajhar. R. M.; Dalager. P. D.; Davlson, A. L. Am. Lab. 1976, 8 ( 3 ) ,71. (8) Winge, R. K.; Peterson, V . J.; Fassel. V. A. Appl. Specfrosc. 1979, 33, 206. (9) Boumans, P. W. J. M.; Bosveld, M. Specfrochim. Acta, Part 5 1979, 3 4 , 59.
(10) Horllck. Gary Appl. Spectrosc. 1978, 30, 113. (11) Greenfield,S. ICP Inform. New/. 1978, 4 , 199. (12) Watters, R. L.; Norris, J. A. FACSS V 1978, Paper No. 85. (13) Edmonds, T. E.; Horlick, Gary Appl. Specfrosc. 1977, 31, 5 3 6 . (14) Ohis, K. ICP Inform. New/. 1978, 4 , 83.
RECEIVED for review August 14,1979. Accepted May 28,1980.
Determination of Fluorine in Urine and Blood Serum by Aluminum Monofluoride Molecular Absorption Spectrometry and with a Fluoride Ion Selective Electrode Koichi Chiba, Kin-ichi Tsunoda, Hiroki Haraguchi, and Keiichiro Fuwa * Department of Chemistry, Faculty of Science, University of Tokyo, Bunkyo-Ku, Tokyo 7 73, Japan
Fluorine in blood serum and urine samples was determined by both AIF (alumlnum monofluoride) molecular absorption and an ion-selective electrode. In the AIF molecular absorption method, fluorine in the samples was determined by measuring at 227.45 nm the molecular absorption of AIF, which was produced in a graphtte furnace by mhtlng 5 pL of pure or dlluted sample solution and 20 pL of AI(N0,)9 aqueous solution (0.01 M). The ion-selective electrode was used to estlmate free fluoride ion. For urine samples, values for fluorine determlned by the AIF molecular absorption and Ion-selective electrode methods were consistent wlth each other. However, the values in blood serum obtafned by the AIF molecular absorption method were larger by about 2-10 times than those obtained by the ion-selective method. These results suggest the existence of some protein-bound fluorine in blood serum.
In recent years, the determination of fluorine in various biological samples has been extensively investigated because of clinical and environmental interest. Among the studies, increasing attention has been paid to the analysis of blood serum and urine since the study by Singer and Armstrong (1). In 1968, Taves reported the possibility that two forms of fluorine exist in human blood serum (2, 3); one is the free fluoride ion and the other, the protein-bound fluorine. The Taves study stimulated much work aimed at characterizing fluorine in serum (4-14). Recently, Ekstrand et al. (13)used gel chromatography and 18Fradioisotope analysis to investigate the binding of fluoride to macromolecules in human plasma. They found that fluoride was eluted together with low molecular weight fractions but that there was no indication of fluoride binding to macromolecules. They suggested that the discrepancy between their results and those obtained by Taves might be due to the methodological differences in sample pretreatment and fluorine analysis. Hence, more extensive study on the analysis of fluorine in blood samples as well as other biological samples is desirable to characterize chemical forms of fluorine. An ion-selective electrode (ISE) for the fluoride ion has been extensively used in recent papers (4,8, 10-12), while colorimetric analysis is also still applied ( 5 , 6). As is well-known, the ISE is sensitive only to free fluoride ion. T o estimate total fluorine, some sample pretreatment such as dry ashing or oxygen bomb digestion should usually be carried out. In 0003-2700/80/0352-1582$01 .OO/O
general, the difference between total fluorine and free fluoride ion is assigned to protein-bound fluorine. However, the procedures for sample pretreatments are tedious, and loss or contamination of fluorine may occur during the procedures. Therefore, more simple and convenient methods for sample treatment and fluorine determination are required to characterize fluorine in biological samples. Recently, the authors developed a new spectrochemical method for fluorine analysis (15, 16), where molecular absorption of aluminum monofluoride (AlF) produced in a high-temperature graphite furnace was measured a t 227.45 nm using a deuterium lamp (15) or a platinum hollow cathode lamp (16) as the light source. It has been shown that AlF molecular absorption spectrometry (MAS) can determine organic fluorine as well as the free fluoride ion. Therefore, this method may be suitable for the determination of total fluorine without any sample pretreatment, since biological samples can be digested in the graphite furnace during the analytical procedures for AIF molecular absorption measurement, as will be mentioned later. Hence, the AlF molecular absorption method will be applied to the determination of fluorine in blood serum and urine samples. In addition, blood serum and urine samples have been also analyzed by the ISE method for fluorine, and the results are compared to those obtained by the A1F molecular absorption method.
EXPERIMENTAL Apparatus. An atomic absorption spectrophotometer with a simultaneous background correction system (Model AA 170-50) from Hitachi Co., Ltd., was used for the measurement of A1F molecular absorption. A carbon rod furnace FLA-100 from Nippon Jarrell Ash Co., Ltd., was used as a high temperature cuvette. Argon gas (1.0 L/min) was used to purge the carbon rod furnace of air. A platinum hollow cathode lamp and a deuterium hollow cathode lamp from Hamamatsu T V Co., Ltd., were adopted as the light sources for molecular absorption and background absorption measurements, respectively. The spectral band-pass of the monochromator was usually set at 1.1nm. A fluoride ISE from Denki Kagaku Keiki Co., Ltd., was also used for the determination of fluoride ion in urine and blood serum samples, The electrode potential was measured via a potassium chloride reference electrode (Model HS305DP) from Towa Dempa Co., Ltd., together with a system for high precision on-line A/D conversion (17). Chemicals. All the reagents used, except for Na2CO3,were of analytical reagent grade purchased from Wako Pure Chemical Co., Ltd. Na2C03used for alkaline fusion of serum samples was of guaranteed reagent grade from Merck Co., Ltd. The fluoride 0 1980 American Chemical Society
