a vidicon spectrometer system. The experimental results have shown how the ratio technique adequately compensates for matrix effects, thus improving the accuracy and precision of flame measurements. Trace elements can then be determined with minimal sample alteration or matrix duplication.. The system's versatility makes it amenable to a wide range of applications without lengthy instrumental modifications. Incorporating a computer interface with the vidicon system will reduce the additional time required for internal standard calculations and enable rapid experimental data interpretation. While the analyses presented in this report are not intended for adoption as routine clinical procedures, the impressive capabilities of the multichannel vidicon spectrometer in applying the internal standard method merit considerable attention.
(6) J. Pybus, F. J. Feldman, and G. N. Bowers, Jr.. Clin. Cbem., 18, 996 (1970). (7) W. B. Barnett, V. A. Fassel, and R . N. Kniseley, Spectrocbim. Acta. Part 8, 23, 643 (1968). (8) W. B. Barnett, V. A. Fassel, and R. N. Kniseley, Spectrocbim. Acta, Part B, 25, 139 (1970). (9) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (10) M. J. Milano, H. L. Pardue, T. E. Cook, R. E. Santini, D. W. Margerum, and J. M. T. Raycheba, Anal. Cbem., 46,374 (1974). (11) K. W. Jackson, K. M. Aldous, and D. G. Mitchell, Spectrosc. Lett., 6, 315 (1973). (12) G. Horlick and E. G. Codding, Anal. Cbem., 45, 1490 (1973). (13) D. 0. Knapp, N. Omenetto, L. P. Hart. F. W. Plankey. and J. D. Winefordner, Anal. Cbim. Acta, 89, 455 (1974). (14) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Cbem., 46, 1231 (1974). (15) K. W. Busch and G. H. Morrison, Anal. Cbem.. 45, 712A (1973). (16) J. A. Dean and T. C. Rains, "Standard Solutions for Flame Spectrometry", in "Flame Emission and Atomic Absorption Spectrometry", Vol. 2, J. A. Dean and T. C. Rains, Ed.. Marcel Dekker, New York, N.Y., 1971, p 327. (17) W. F. Meggers, "Tables of Spectral-Line Intensities, Part I", National Bureau of Standards Monograph 32, US. Government Printing Office, Washington, D.C.. 1961. (18) K. W. Bush, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 2074 (1974). (19) T. E. Cook, M. J. Milano, and H. L. Pardue, Clln. Cbem., 20, 1422 (1974). (20) J. D. Ganjei. N. G. Howell, J. R. Roth, and G. H. Morrison, Anal. Chem., in press. (21) E. Berman, Appl. Spectrosc., 29, 1 (1975). (22) R. L. Warren, Analyst(London), 90, 549 (1965).
ACKNOWLEDGMENT The authors thank J. H. Boutwell and D. D. Bayse of the Center for Disease Control, Atlanta, Ga., for supplying the samples of analyzed bovine serum, and K. W. Busch of Baylor University for his help and advice.
LITERATURE CITED (1) W. Gerlach. Z. Anorg. Allg. Cbem., 142, 363 (1925). (2) J. W. Berry, D. G. Chappell. and R. 8. Barnes, ind. Eng. Cbem., Anal. Ed., 18, 19 (1946). (3) H. M. Bauserman and R. R. Cerney, Jr., Anal. Cbem., 25, 1821 (1953). (4) R. J. Schlesinger. R. A. Lesonsky, and R. Lottritz. Clin. Chem., 18, 1005 (1972). (5) F. J. Feldman, Anal. Cbem.. 42, 719 (1970).
RECEIVEDfor review August 7 , 1975. Accepted November 12, 1975. This work was supported by the National Institutes of Health, Grant No. 5 R01 GM 19905-03.
Characteristic Noise Spectra of Some Common Analytical Spectrometric Sources Yair Talmi," Ronald Crosmun,2 and N. M. Larson Oak Ridge Nafional Laboratory, Oak Ridge, Tenn. 37830
This study deals with the noise characteristics of various analytical spectrometric systems, including primary plasma excitation and flame sources. The noise power spectra of these spectrometric sources have been obtained digitally using a dedicated Fourier processor which calculated the power density functions by direct Fourier analysis. in addition, a quantitative estimate of the magnitude of these noise fluctuations was also obtained. In the Discussion section, an attempt has been made to isolate the few experimental factors that are most dominant in the generation of noise in spectrometry sources and to determine their origin. The analytical implications of the noise phenomena are also dlscussed. In particular, the merits of signal modulation techniques and multiplex transform techniques, e.g., Hadamard transform spectrometry, are reevaluated, based on the experimental noise data obtained in this study.
The original purpose of this study, which deals with the magnitude and characteristics of the noise phenomena inCurrent address, Princeton Applied Research Corp., P.O. Box
2565, Princeton, N.J. 08540.
* Current address, E. I. du Pont, Wilmington, Del. 19898.
326 * ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
volved in the operation of various spectrometric systems, was to evaluate the applicability of Fourier and Hadamard Transform Spectrometers (HTS), to the uv-visible spectral range. The sensitivity of any spectrometric system is limited by noise originating in either the sampling device, the source, or the detection system. Most spectrometric sources are thermal; Le., they are characterized by a photon flux whose time variability obeys Poisson statistics. Similarly, the output signal of photomultiplier tubes (PMT), universally accepted as uv-visible detectors, also obeys this statistical mode. This behavior of PMTs is a direct result of their quantum efficiency being less than one. Thus, in uv-visible spectrometric studies, in which the P M T is operated in the dc mode (not saturated and not as a photon counter) the signal to noise ratio (S/N) is proportional to the square root of the signal regardless of whether the detector or the spectrometric source is the dominant noise source. Conversely, in infrared (ir) spectrometry, the detector is nearly always the dominant noise source. These detectors operate in an indirect mode; e.g., absorption of heat causes a change in the electrical conductivity of a crystal, a chnage in gas volume, etc. Their noise level is proportional to the square root of the detector area rather than the signal. These de-
using a dedicated Fourier processor to calculate the power spectral density function by direct Fourier analysis.
DATA COLLECTION
EXPERIMENTAL
r”l
I
I I
X-Y
I I
L
REC
_ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ J’ HP 54516 FOURIER ANALYZER
DATA
PROCESSING
Figure 1. Schematic diagram of the recording and processing system used in the noise analysis.
tectors are generally much “noisier” than PMTs, but their noise level is practically independent of the signal level. By studying the basic differences between the noise characteristics of ir and uv-visible detectors, we have theoretically demonstrated ( I ) that multiplex spectrometers based on Fourier and Hadamard mathematic transformations can improve the S/N of ir measurements but not that of uvvisible measurements. This conclusion has been also confirmed experimentally (2). Operation of H T S systems in the uv-visible regime will result in a very limited S/N(Fellgett or Multiplex) advantage even when the spectrometric sources have “white” noise spectra. This advantage deteriorates significantly when the noise spectrum contains some deterministic features, close to the stepping frequency range (sampling frequency) of the Hadamard mask. Thus, in order to evaluate the advantage of a multiplex detection system for a particular spectrometric source, a better understanding of its noise spectrum is required. I t has been postulated elsewhere (3) that a signal, a t the threshold of detection, would be interfered with by those noise components that fall within about two octaves of the sampling frequency. On the basis of this assumption and because the useful sampling frequency of an H T S mask was assumed to be in the 0.4-2 KHz range (for a 2047 slot mask), most noise measurements performed in this study were done a t the 0.1-8 KHz range. In most cases, noise spectra were taken also a t the 1-1000 Hz range, and a few noise spectra were taken a t the 0.01-50 Hz range. Because of the nature of the investigation described in this paper, the necessary data can be discerned most conveniently by means of a power spectrum. The power spectrum furnishes the intensity of each frequency component in the original random waveform. If a deterministic component is present in the random data (Le., if the spectrum is “colorful” rather than “white”), the power spectrum will reveal its presence. Both analog and digital methods are available for obtaining power spectra. Analog techniques were not used in this study and, hence, will not be discussed here. Digital methods exploit the Fourier transform properties of the power spectral density function ( 4 ) . The power spectrum is the Fourier transform of the autocorrelation function. The older “standard” approach to obtaining a power spectrum was to generate the autocorrelation function and then Fourier-transform. Since the advent of the “Fast Fourier Transform” algorithm in 1965 ( 5 ) , direct Fourier analysis has become the method of choice. All experimental power spectra presented in this paper were obtained digitally
Details of the instrumentation utilized in this study are given in Table I. A schematic diagram of the data collection and processing system is shown in Figure 1. Data Collection Procedure. An image of the light source studied was projected on the entrance slit of the monochromator. The resultant photomultiplier output current fluctuations were then converted to the corresponding voltage fluctuations by a fast response current-to-voltage converter and scaled to a level of f1.4V (by a decade voltage divider). Zero suppression was used in scaling the signal, so that the mean value of the waveform would be nearly zero. The photocurrent fluctuations produced by the source investigated were recorded for a length of time T , on a magnetic tape. Data Processing Procedure. The data recorded on the magnetic tape were then fed (played back) into the Fourier Analyzer. The input to the analyzer comprised a programmable active low pass filter (variable fc) and an analog to digital converter (ADC had 10 bits sign). The filter was to adjusted to a value 0.75 of the highest frequency F,,, = I/2At; be determined, in order to prevent aliasing (F,,, Nyquist limit, and At = Tl2048). A total of 2048 data points (data block) were sampled .in each record. The spectrum resolution, Af was thus related to the maximum frequency F,,, as follows: Af = 2Fm,,/2048. Sampling of data points was done a t equal time intervals, A t , which were related to the sample record (A sample function is a recording produced by a random phenomenon over an infinite time span; a sample record is a similar recording over a finite time interval) length, T (necessary for a single analysis) as t = T/2048. The recording and analysis parameters are summarized in Table 11. The computer display produced a power spectrum, i.e., a plot of the variance of the signal (V’Hz-’) vs. frequency. The standard deviation of the noise current was calculated by dividing ( V2)1/2 by the measurement resistance ( R ) of the converter. Thus, the final experimental noise spectra were displayed as plots of noise current (A X Hz-ll2) vs. frequency. Spectrometric Sources. A number of spectrometric sources were examined; emphasis was placed upon sources
+
__
~
~-
Table I. Instrumentation Monochromator A model 82000 0.5-meter EbertMount (Jarrell-Ash)with a 1180 groovesimm grating blazed at 600 nm Photomultiplier RCA 1-P28 operated at 600 V. Anode at ground potential produced a gain of ca. 10’. Anode currents used were at the 10-7-10-’o A range Current-to-voltage converter a . A model IX-C (ORNL Photometer) amplifier with.a gain of l o 8 V/A and a fixed resistance R = 10’ ohms b. A model 427 (Keithley Instruments) current amplifier with a variable gain of 104-10” V / A . The narrowest bandwidth (at 1 0 ” VIA) is 400 Hz Anti-aliasing filter A 76-L8MB (Burr Brown) 8-pole filter with a -48 dbioctave attenuation at 18 KHz (cut off frequency) FM tape recorder A model PS-204 (Precision Instrument) operating at variable speeds, ”Il6 to 60 inches per second (ips) Fourier analyzer A model 5451A, Hewlett-Packard
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
327
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3
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Figure 2. A white noise spectrum typical of primary spectrometric
I
-a
sources
most commonly used in molecular and atomic uv-visible spectrometry. Primary Spectrometric Sources. 1) A tungsten filament lamp operated by a stabilized, highly regulated, dc power supply a t 5 V and 120 mA. The near-ideal thermal behavior of this lamp made it an excellent reference source. 2) An LED, operated at 2.2 V dc with a 50 Hz (40 mV) sinusoidal modulation. 3) Two types of hollow cathodes (HC) were examined; Perkin-Elmer sealed lamp (operated at 5-8 mA) and homemade demountable lamps (helium pressure 6 Torr, at 50 mA). 4) A mercury low pressure pen lamp (Oriel C-73-16) that was modified to operate in the dc mode (10 mA). The discharge was initiated with a Tesla coil. 5) A 150-watt xenon arc used in the Aminco-Bowman spectrofluorimeter. For molecular fluorescence studies, the electronic detection system of the fluorimeter was replaced by the detection system described above. 6) A tungsten Halogen lamp from a Cary 14M spectrophotometer. Plasma Excitation Sources, 1) An argon microwave (2450 MHz) plasma (20-100 W) operated at atmospheric pressure. The plasma is used as a GC spectrometric detector (6, 7). Argon flow-rate was maintained at 75-100 ml/ min. 2) A radiofrequency furnace (RFF) spectrometric source (8). This system comprises a high temperature graphite atomizer and a helium plasma, both of which are operated by a 3.3-MHz, 7.5-kW R F generator (Lepel R F generator model T-7.5-3-MC-A-SW). Typical oscillator plate currents used were 0.6-0.9 A. 3) A dc arc (using an unfiltered DC power supply) struck between graphite electrodes was operated a t 3 A (without sample introduction). 4) AC-spark source (without sample introduction). Flame Spectrometric Sources. Three general types of burners were used; premixed slot, premixed Meker, and turbulent sprayer burners. A Perkin-Elmer 303 and a Varian AB-51 (both 10 cm) premixed slot burners were utilized in Atomic Absorption (AA) studies. The former also was used in the Atomic Emission (AE) mode. A Unicam-type Meker burner head with an inert gas sheath (9, I O ) was used in either the AE or AF modes. The Unicam head was a stainless steel cylinder whose top surface was perforated with a 1-cm array of 13 holes. The diameter of the holes was selected according to the fuel and oxidant utilized. Thus, for NzO-CzH2 flame, the diameter of the holes was 43 mils and for air-CzH2, it was 55 mils. For AF applications the flame was confined within a 70-mm quartz tube in which two longitudinal slots had been cut at angles of 90’ to each other. The quartz en-
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velope further reduced perturbations due to air currents. Flames were sheathed with either N2 or Ar. Measurements. The noise measurements performed in this study were intended to provide two different types of information. First, the distribution of current fluctuations in the frequency domain, which should provide some information concerning the source photon emission in the time domain. Second, a quantitative estimate of the level of these fluctuations in various analytical spectrometric sources. T o test and calibrate the performance of the noise analyzer system, it was applied to noise measurements of both a “white” (thermal) spectrometric source (tungsten filament lamp) and a “non-white” source with a distinct deterministic feature: a modulated LED. The “white” noise spectrum obtained for the tungsten reference lamp is shown in Figure 2. The gradual decrease in the noise amplitude (starting a t about half frequency scale) is due to the anti-aliasing filter in the analyzer. Preliminary studies with this lamp produced a noise spectrum with a deterministic component (12.5-13 Hz) caused by the sympathetic vibration of the lamp filament with the room vibrations. This phenomenon was eliminated when the electrical leads from the power supply were properly secured. T o verify the thermal behavior of the tungsten lamp (photon emission of thermal sources follows a Poisson distribution ( I I ) ) , its noise current was measured over three orders of magnitude of signal photocurrent; 10-10-10-7 A. Indeed, the logarithmic plot of the noise power (normalized in units of A X H Z - ~ ’ ~vs. ) signal (mean) photocurrent, Figure 3, was a straight line with a slope of 0.5, as expected for a Poisson distribution. The noise spectrum of the tungsten lamp was identical to that obtained from direct skylight illumination. Throughout the study, the tungsten filament lamp was used to provide a reference noise level against which all other sources could be compared.
Table 11. Recording and Processing Parameters Tape speed, inch per second
7.5 15 60
R e corder bandwidth, KH z
1.25 2.5 10.0
Resolution
0.75 Fmax,
Af, Hz
HZ
0.05
At,
ms
T,s
Analysis time, s
35 10 20 2000a 7 50 0.5 1 100a 7 50 0.5 1 loon 60 10.0 10 0.05 0.1 50b 7500 Ensembles o f 100 sample records were used with accuracies (approximation to true spectra) of at least 10%. b Ensembles of 500 sample records were used with accuracies of at least 5%. 1.o 1.o
Q
328
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
d Figure 4. ( a ) and ( b ) The raw voltage sample record from a modulated (50 Hz) LED. ( c ) The modulation waveform.(6The dark current of the detector
In order to obtain a qualitative estimate of the sensitivity of the Fourier analyzer to periodic waveforms hidden in random data, the noise spectrum of the 50-Hz-modulated LED was determined. The raw voltage sample record, the modulation waveform, and the dark current (PMT) are shown in Figure 4. As expected, the analyzer was able t o distinctly reconstruct the 50-Hz feature, Figure 5 , even though the signal intensity was very low. Primary Spectrometric Sources. The types of sources, the experimental conditions, a n d the noise figures are summarized in Table 111. All primary sources studied had essentially a noise spectrum identical to that of the tungsten reference source, Figure 2 (The low frequency component (I#) observed in the AA and AF modes, Figures 6 and 7, was caused by the flame itself and will be discussed later). Furthermore, the level of the noise increased as the square root of the average detector current. These observations were expected for the thermal sources, e.g., tungsten lamp Xenon LED but less so for the nonthermal sources, e+, hollow cathodes, arc, and discharge lamps. Consequently, it can be assumed that in the experiments carried out in this study, i t was impossible to distinctly detect any noise features which were related to the excitation mechanisms.
Plasma Excitation Sources. The noise spectrum of the argon microwave discharge was similar to that of the priA signal) was mary sources, hut the noise level (at 5 X approximately twice as high. When the signal level was lowered, by narrowing the slit width while maintaining the same microwave power output, the noise level was reduced to that of the primary sources. Thus, the higher noise level was truly a characteristic of the plasma and was not caused by the direct interference of the reflected microwave power with the PMT. It is interesting to note that, contrary to the vacuum operated primary sources, the microwave discharge studied was maintained a t atmospheric pressure and is believed to attain a local thermodynamic equilibrium state (11). The RFF plasma was sustained by a 3.3MHz-RF generator. The power was automatically switched a t a frequency of 180 Hz with a duty cycle which was determined by the output power requirements. This switching operation produced a dense "non-white" noise spectrum consisting of very intense lines (harmonics of the 60-Hz fundamental frequency) Figure 8. Because the intensity of these lines was a few orders of magnitude higher than that of the background noise level, the limited dynamic range of the ADC unit was exceeded and therefore any other noise features (of lower amplitude) existing in the source could not have been detected. A similar problem existed with the dc arc and the ac spark sources which had strong U f noise components. The dc arc spectrum, Figure 9, shows a very intense l l f component (below 2 KHz) with superimposed harmonics originating in the ripple of the power supply. The ac spark spectrum, Figure 10, contained a llf noise component with more dominant superimposed features of the 120-Hz fundamental frequency and its harmonics. Two interesting phenomena are confirmed by these observa tions. First, these plasma excitation sonrces (with the exception of the microwave discharge) d o not hehave as continuous sources, Le., excitation is initiated and extinguished by the on-off switching of the power. Second, the dc arc and ac spark are rather unstable sources, and their wandering across the optical path is reflected by the strong low frequency noise component. Table IV summarizes the noise characteristics of the discharge excitation sources. Flame Sources. Because of their versatility and widespread use, flame sources were studied in greater detail, Table V. Spectrometric observations were, generally, made a t flame regions which are most useful in analytical work (usually, just above the cone where signal-to-noise ratios were maximal). The two types of premixed slot burners 'used (operated a t slightly lean mixtures of air-C;Hd produced similar noise spectra. Na and Sr samples were used in the atomic emission (AE) studies because they produced adequate emission signals a t relatively low concentration levels (0.1-10 pgiml). This was necessary to minimize self-absorption and other perturbations typical of heavily salted flames. T h e noise spectra obtained from AE (Sr, Na) and molecular emission (ME) (CH and C2 flame species) measurements were identical. Also, the M E noise spectra were unaffected by the introduction of water or salt into the flame. T h e only significant noise feature was a low frequency component. Thus, it appears that the flame noise is caused in the gas flow rates rather than by processes which affect atom formation, Le., nebulization, desolvation, vaporization etc. The Meker burner was studied next. Noise spectra ohtained from AE and ME measurements of air-CzHg and N ~ O - C Z H ~and , NnO-Cz flames (with and without argon sheath) had a rather intense low frequency component (at 5 X A) and reference-source characteristics a t higher ANALYTiCAL CHEMISTRY, VOL. 48, NO. 2. FEBRUARY 1976
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Figure 5. The noise power spectrum of a (50 Hz) modulated LED
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Figure 7. A noise spectrum with a strong l / f c o m p o n e n t , typical of AF, AE, and ME measurements with the Meker burner using N20C2H2 flame ( 1 c m above burner head, red feather region)
Figure 12. A noise spectrum typical of AE and ME (Beckman burner) with 02-H2 flame (0.5 c m above burner head)
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Figure 8. A noise spectrum of the RFF
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frequencies. With air-C2H2 flame, there was also a low intensity double peak (5-6 KHz), Figure 11. This unusual component was most probably produced by vibration of the burner head, which could behave simultaneously as both an open (burner-holes) and a closed organ pipe. Thus, if the rather oversimplified open-(organ)-pipe vibration equation, f = V / 2 L ,(f is the first harmonic, L', the speed of sound in air and L , the length of the pipe, 3 cm) is used the expected vibration, of the burner head is f = (33 100 cm/ s)/6 cm = 5516 Hz. In the AF studies, a large section of the flame was directly viewed (flame was placed 5 cm from slit) without the use 330
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Figure 14. A noise spectrum typical of ME with (Beckman burner) 02-C2H2 flame (1.5 c m above burner head)
of a lens. Although the low-intensity (dc-operated) mercury lamp utilized, induced very weak Hg fluorescence signals, a distinct l/f noise component was observed in both airC2H2 and argon sheathed air-CzH2 flames. The unusual phenomenon of a broad minimum, a t high frequencies (3 KHz), in the noise spectra of sheathed and unsheathed flames (12) was not observed in any of these studies. This experimental disagreement was previously noted elsewhere (13). Finally, the Beckman burners were also examined. In the AE mode (H2-02 flame), the spectra of Na and OH were identical, Figure 12. The distinct noise feature in these spectra (500-1000 Hz) was therefore caused by the inefficient aspiration (droplet formation) process, characteristic of Beckman burners, or by flow rate fluctuations. On the other hand, with a C2H2-02 flame (burner model 40301, a different noise spectrum was obtained from Na emission, Figure 13 and C2 emission, Figure 14. Thus, the Na emis-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
Table 111. Noise Analysis of Primary Spectrometric Sources
Source
Wavelengt t i ,
Slit width ,
nm
m
Modea
Tungsten-fliament (reference source)
D
Afternoon skyb Perkin-Elmer HC
D D AA D AA D MFd
Demountable HC Xenon arcC
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525
405.8 (Pb) 283.3 (Pb)e 283.3 (Pb) 283.3 (Pb) 450 430f 345f
1
PhlT, Photocurrent
x
4
Noise current$ (A x HZ-’/’)
10-I”
x
500 100
... 10
24 100 100
5 1.5 500
500 500 500 500
...
500
1000 1000
5 500 50 50 50 50
i E;
2 1.3 2
Typical noise spectrum, Figure
2
1
2 2 5 2 5 2 2 2 2
2 2 6 6
6 2 2 2
AF (Unicam) 6 10 7 AF (Beckman) 253.6 S (Beckman) 10 7 RSd 1 2 D D 580 40 5 0.2 2 LED (modulated) D 580 10 1.5 0.1 2 600 ... 500 2 2 Tungsten halogen D UThe following abbreviations are used: D = Primary source is directly monitored, AA = Primary source is monitored through a n absorbing flame, MF = Molecular fluorescence induced by the xenon lamp, RS = Raleigh scattering of light from the xenon lamp, AF = Atomic fluorescence induced by the mercury lamp, S = Observed scattered source reduction. b Sky emission was measured by the PMT directly (without a monochromator). CThe molecular fluorescence intensity of a 0.1 N H,SO,, 1 pg/ml aqueous quinine sulfate solution was measured. d Excitation wavelength was set at 350 nm. e The hollow cathode discharge was focused approximately 0.5 cm above the slot burner head (above the luminous blue inner cone). A 50 pg/ml Pb solution was aspirated. fEmission (fluorescence) wavelength setting. R Noise current per ordinate division of the indicated Figure (given in next column). Mercury pen lamp
1 E;
-
-
-
Table IV. Noise Analyses for Plasma Excitation Sources0 Wavelength, Source
Mode
nm
Slit w i d t h , wm
PMT p h o t o current, A X IO-’’
Noise current, (A X Hz-%) X 10-1Ob
Typical noise spectrum, Figure
MES
AE 414.3 (Ar) 40 500 3 2 ME 320.8 (CH) 13 5 2 2 RFF ME 380.2 (N,) 82 160 22 8 DC arc ME 464.6 (CN) 116 500 31 9 AC spark ME 464.6 (CN) 100 500 100 10 0 The following abbreviations are used: MES = microwave emission spectrometric source, R F F = radiofrequency furnace source, AE = atomic emission, ME = molecular band emission. b Noise current per ordinate division of the indicated Figure (given in next column).
sion noise spectrum contained features which were related t o the inefficient sampling (aspiration) or desolvation-vaporization processes. The C2 spectrum lacked these features b u t instead had a distinct component peaked a t ca. 8 KHz. Again, this component is more than likely caused by the resonance of the capillary system in the burner (6 cm long). If the open pipe equation (an oversimplified model) is utilized, the third harmonic of the fundamental vibration frequency would be 3f = 3(33 100 cm/s)/l2 cm = 8275 Hz. For yet unknown reasons, the 1st and 2nd harmonics were very weak and could be observed in only a few spectra. When operating in the AF mode, the only noise feature obtained from a low-temperature, low-background H p A r (entrained air) flame was a strong l/f component, Figure 7. A major problem with the Beckman burner was the irreproducibility of gas flow rates that made i t impossible to reproduce the noise spectra with high precision. It should be realized that the noise spectra obtained in this study were dependent on fuel and oxidant ratios and rates, on fluctuations in these variables, and on the specific burner, nebulizer, and detection system used. Detectors. The spectra of a few other RCA 1-P28 detec-
tors and one EM1 9785QB detector were compared under illumination of both a “white” (LED) and a “non-white” (Na aspirated into a H2-02 Beckman burner flame, Figure 12) sources. T h e noise values of all five detectors, Table VI, were taken as that of 0.7 the maximum deviation of the broad line traced by a recorder pen with a bandwidth of ca. 100 Hz. All detectors produced similar spectra, although only the low noise 1P28(3) was able to detect the 500 Hz (in Figure 12) feature at a signal level of 1 X lou9 A.
CONCLUSIONS One of the earlier (although qualitative) attempts to determine the noise spectra of various common spectrometric systems was that of Belyaev e t al. (14). Generally, their results are in good agreement with the quantitative results, Tables I1 to IV obtained in this study, Thus, t h e two studies agree that arcs and sparks are rather “noisy” sources and so is the RF plasma, although to a lesser degree. Similarly, 02-H2 flames and all primary sources have, essentially, a noise spectrum identical to that of the reference source. On the other hand, Belyaev’s measurements (at a photocurrent level of 5 x IO-* A) show a distinct llf feaANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
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r'
2
1 2 0
m 0
m ?N
??r'
0 0 M 0 C W * N 0 0 0 rtri
I
E
+go u-
;
