T a b l e IV. C o m p a r i s o n of R e s u l t s of AFS and AAS D e t e r m i n a t i o n of C d in SRM-1577 (Bovine Liver) Cd, r g / g
AFS
0.28 0.24 0.21 0.25
Average Std dev Relstddev, 70
AAS
0.24 0.25 0.26 0.29 0.253 0.024 = 9.8 = =
0.29 0.24 0.26 0.26 0.27
0.24 0.27 0.30 0.26 0.266 0.020 7.6
U S is given in Table IV. Again, by AAS, a separation was required, while the AFS values were obtained without prior separations. Interferences. The major constituents of SRM-1571 (Orchard Leaves) are Ca, 2.09%; K, 1.47%; Mg, 0.62%; and P, 0.21%. At these levels, they did not interfere with the AFS determination of cadmium. Standard solutions of cadmium, as well as the analyte, were prepared in 5% HC104. I n comparing the fluorescence intensity of cadmium in 5% HC1 and HClO4, a slight enhancement was observed in a perchlorate medium. The major constituents of SRM-1577 (Liver) are K , 0.97% and Na, 0.243%. Sodium produced a stray light interference as previously noted by Barnett and Kahn ( 2 1 ) . In a 0.05 kg/ml cadmium solution, sodium concentrations greater than 100 pg/ml caused a n enhancement. In the (21) W. B. Barnett and H. L Kahn, A n a / . Chem., 44,935 (1972).
presence of lo00 pg/ml of sodium, the apparent recovery of cadmium was greater than 170%; a t the 5000 pg/ml level, the apparent recovery was 370%. T o overcome stray light interference due to relative high concentrations of sodium, the (Hamamatsu) R-106 multiplier phototube was replaced with a n R-166 solar blind multiplier phototube.
CONCLUSION The results obtained by the automatic correction system indicate t h a t scatter of incident radiation in AFS is readily subtracted. The speed of analysis is increased with improved accuracy. This technique eliminates the need for exact matrix matching or measurement a t some nonfluorescent line to correct for scatter. The detection limit for cadmium is 0.05 ng/ml a t 2a above the mean obtained from a background or a matrix blank.
ACKNOWLEDGMENT T h e authors are indebted to T. C. O’Haver for his helpful suggestion in changing the frequency of chopping, to K . D. Mielenz for his suggestions in optics, and to J . I. Shultz for his assistance in the preparation of this manuscript. Received for review May 31, 1973. Accepted September 12, 1973. In order to adequately describe experimental procedures it is occasionally necessary to identify commercial products and equipment by the manufacturer’s name or label. In no instances does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the particular product or equipment is necessarily the best available for that purpose.
Inductively Coupled Plasma-Optical Emission Analytical Spectroscopy Tantalum Filament Vaporization of Microliter Samples David E. Nixon, Velmer A. Fassel, and Richard N. Kniseley Ames Laboratory-USAEC
and Department of Chemistry, lowa State University, Ames, lowa 50010
The adaptation of a tantalum filament vaporization system as a sample introduction device for the inductively coupled plasma is described. The potential advantages of this analytical system for simultaneous multielement determinations of elements at the ng/ml level are discussed and a comparison of the plasma system with the filament techniques utilized in atomic absorption or fluorescence spectroscopy is presented. For one set of operating conditions, detection limits for 16 elements were in the ng/ml to fractional ng/ml range for 100-pl samples. Typical precision data and an analytical curve for the determination of Be in the range of 0.001 to 10 pg/ml are included. 210
One of the most sensitive contemporary analytical techniques is based on the thermal atomization of microliter volumes of a sample in graphite furnaces (1-11) from graphite (12-20) or tantalum (21-24) filaments, and from platinum or tungsten wire loops ( 2 5 ) , followed by the observation of the free atoms formed in either atomic absorption or fluorescence. Although exceptional relative and absolute powers of detection and acceptable reproducibility have been achieved by a multitude of variations of these atomization systems, this technique is subject to a rather extensive list of experimental constraints. The most important of these are: ( a ) for many of the systems, critical experimental parameters must be optimized for
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 2, F E B R U A R Y 1974
each element ( 2 3 , 26-28, 22, 23, 24); (b) reproducibility may be negatively affected by variations in the carbon filament or furnace tube porosity ( 1 , 4 , 6, 9, 14, 15, 20); (c) analytical curves may be nonlinear and limited in range of concentration ( 2 , 6, 7, 20-26, 18, 20, 24); (d) background interferences may arise from nonspecific a b sorption and light scattering caused by incandescent particles produced in t h e furnace or above the filament ( 1 , 4 , 6-9, 2 2 , 14, 15, 28, 24, 2 5 ) ; (e) the observed signals may be affected by interelement interferences which originate from the recombination and nucleation of the sample after atomization or from incomplete analyte vaporization and dissociation (6-12, 14-17, 19, 22, 23, 24); (f) simultaneous multielement determinations cannot be performed if the free atoms are observed by conventional atomic a b sorption or fluorescence techniques. I t has recently been demonstrated that inductively coupled plasmas are exceptionally sensitive excitation sources (26-34). The combination of either furnace or filament vaporization of samples followed by plasma excitation of the vapor therefore offers the attractive possibility of performing ultratrace determinations on a multielement basis on microliter or microgram sized samples. Moreover, the plasma system also offers promise of overcoming the other limitations discussed above. First, since the free atoms are actually generated in the plasma, a single set of parameters should suffice for the vaporization of many types of (1) B. V . L'vov. Spectrochim. Acta, 17, 761 (1961). (2) H . Massmann, Spectrochim. Acta. 2 3 8 , 215 (1968). (3) R . Woodriff, R . W. Stone, and A. M . Held, Appl. Spectrosc., 22, 408 (1968), (4) B. V. L'vov, Spectrochim. Acta. 2 4 8 , 53 (1969). ( 5 ) B. V. L'vov. Pure Appl. Chem. 23, 11 (1970). (6) D. C. Manning and F . Fernandez, A t . Absorption Newslett., 9 (3), 65 (1970). (7) R. W . Marrow a n d R. J . McElhaney. Paper presented at Sixteenth
Conference on Analytical Chemistry in Nuclear Technology, Gat-
iinburg, T e n n . , 1972. (8) F. J . Fernandez and D C. Manning, At. Absorption Newslett.. 10 (3), 65 (1971). (9) H . L. K a h n , Amer. Lab., August 1971, p 35. (10) G . P. Sighinolfi, At. Absorption Newsleft., 11 ( 5 ) ,96 (1972). (11) G . Baudin, M . C h a p u t , and L . Feve. Spectrochim. Acfa, 268, 425 (1971), (12) T . S. West and X. K . Williams, Anal. Chim. Acta, 45, 27 (1969). (13) R . G . Anderson, I . S. Maines, and T. S. West. Anal. Chim. Acta, 51, 355 (1970). (14) M . D . Amos. P. A . Bennett, K. G . Brodie, P. W . Y . L u n g , and J . P. Matousek. Anal. Chem.. 43, 211 (1971). (15) J . P. Matousek,Amer. Lab.. J u n e 1971, p 4 5 . (16) J . Aggett a n d T. S . West. Anal. Chim. Acta, 55, 349 (1971). (17) D Alger, R. G . Anderson, I . S. Maines, and T. S . West, Anal. Chim. Acta, 57, 271 (1971). R. G . Anderson, H . N. Johnson, and T . S. West, Anal. Chim. Acta, 57,281 (1971). R. B. Baird, S . Pourian, and S. M . Gabrielion. Anal. Chem.. 44, 1887 (1972). 8. M . Patei, R. D. Reeves, R. F . Browner, C. J. Molnar, a n d J , D. Winefordner, Appl. Spectrosc., 27, 171 (1973) H . M . DonegaandT. E. Burgess, Anal. C h e m . , 42, 1521 (1970). T . Y . H w a n g , P. A. Ullucci, and S. B. S m i t h , Jr., Amer. Lab.. August 1971, p 4 1 . J . Y . Hwang. C. J. Mokeler. and P. A. Uilucci, Anal Chem., 44, 2019 (1972). T . Takeuchi, M . Yanagisawa, and M . S u z u k i , 7alanta. 19, 465 (1972). M . P. Bratzel, J r . , R. M . Dagnall, and J . D. Winefordner. Appl. Spectrosc.. 24, 518 (1970). S. Greenfield, P. 8. Smith, A . E. Breeze, and N . M . D . Chilton. Anal. Chim. Acta. 41, 385 (1968). S. Greenfield and P. B. S m i t h , Anal. Chim. Acta, 59, 341 (1972). J, C . Soutlliart a n d J . P. Robin, Analusis, 1, 427 (1972) P. W . J. M . Boumans and F . J. d e Boer, Specfrochim Acta, 278. 391 (1972). R. H . W e n d t and V . A . Fassel, Anal. Chem., 37, 920 (1965) V . A . Fassel and G . W. Dickinson. Anal. Chem., 40, 247 (1968) V. A. Fassei. "XVI Colloquium Spectroscopicurn Internationale," Heidelberg, Germany, 1971 G . W. Dickinson and V. A . Fassel. Anal. Chem.. 41, 1021 (1969). R . H. Scott, V A. Fassel, R. N . Kniseley. and D E. Nixon, Anal. Chem., 46, 75 (1974).
R . F.
Generator
- TuningUnit-Coupling
Spectrometer
a n d Plasma
I
ll-----l Cl C. Amplifier
Filament Vaparizaticm
X-Y Recorder
Figure 1.
Block diagram of experimental facilities
Table I. Operating Conditions for the Tantalum Filament Vaporization Apparatus
Filament power supply:
General Electric transformer Model 9T51Y6172; 120-V primary; 12-V secondary: 80 "C temperature rise; 0.5 K V A . Transformer input controlled by a Standard Electrical Products Co. variable transformer type F15OOBL; 1 1 5 - V input; 0-115-V, 2 K V A , 15 A maximum output.
Tantalum strips:
37 X 10 m m ; 0.13 m m thick. Depressions formed with a die: maximum sample volume, 200 PI.
Gas flows:
Syringe:
Solutions and sample size:
Argon at 14.7 I./min proportioned through the coolant and plasma tubes to keep the plasma from melting the plasma tube. Argon at 1 . 2 I./min passes though the filament enclosure to the plasma. Hamilton 700 series: Model 710; 100-p1 capacity with fixed standard needle. Teflon plunger substituted for the original plunger. Stock solutions were prepared by dissolving pure metals or reagent grade salts in dilute acid or conductivity water. Samples were conductivity water dilutions of the stock solutions: 1 00-pl samples were used throughout.
samples. Second, interelement interferences arising from recombination or nucleation of the vapor above the filament should be minimized because the plasma subsequently achieves atomization of the vapor cloud. Third, background interference from the filament or furnace tube does not exist. Fourth, analytical curves obtained from the toroidal shaped inductively-coupled plasma are commonly observed to be linear over a concentration range of 4 to 5 orders of magnitude ( 3 4 ) . In this communication, we present some preliminary results on the combination of the tantalum filament vaporization (TFV) of samples followed by the inductivelycoupled plasma (ICP) excitation.
EXPERIMENTAL Apparatus. A block diagram of the overall apparatus is shown in Figure 1. The plasma facility and the spectroscopic apparatus are described elsewhere ( 3 4 ) . The operating conditions for the tantalum filament vaporization apparatus are described in Table I. A sketch of the filament vaporization device is shown in Figure
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 2, F E B R U A R Y 1974
211
Table II. Detection Limits (pg/ml) Element
TFV-ICP, 1 00-p1 samples
As
0.0 1
Sb
0.001 0.006 0.007 0.002
Se Te Hg Cd P Pb
Sn Ag
0.0001
TI Mn
(A)O
1936.96 231 1 . 4 7 1960.26 2142.75 2536.52 2265.02 2136.18 4057.83 2348.61 5350.46 2576.10 3175.05 3280.68 2497.73 4554.03 3067.72
0.006 0.02 0.003 0.00002 0.003 0.00003 0.02
Be
Wavelength
A A S , 100-!~1samples
' h
Sample volume ( P I ) , AFS 2h 30
0.003 (23) 0.001 (2) 0.007 ( 2 3 ) 0 . 0 0 3 (23) 0.002 (2) 0.000003 ( 9 )
0.25 ( 1 4 ) 0.007 (2)
0.01 (25) 0.000008 ( 2 )
2 30
0.0001 (23)
0.001 ( 2 )
0.00002 (23) 0.0001 (9) 0.00002 (9) 0 . 0 2 5 (8) 0.000007 (9)
0 . 0 1 5 (14) 0.04 ( 2 0 )
30 26 1
...
0.0001 ... 0.0000003 0.0001 (23) 0.002 0.0004 (9) a Wavelengths used for TFV-ICP detection limits. Exact volume not stated.
B Ba Bi
AFS
... ...
...
...
...
...
... ...
0.00005 ( 2 )
30
0.01 (13)
1
ed on the filament and a low current (from 10 to 17 A) is used to vaporize the water and t o ash t h e deposit. At this point the Variac is adjusted to the vaporization setting ( - 100 A). a n d a toggle switch is used t o rapidly and reproducibly deliver this current to the filament. T h e filament temperature quickly increases to a p proximately 1800 "C and the analyte together with the matrix elements are vaporized into the Ar carrier stream. T h e photocurrent produced by the emission of the element of interest passing through the plasma is amplified and recorded on a n X-Y recorder. Approximately 20 t o 30 samples can be examined in one hour, a n d the filaments have a n average lifetime of 200 to 300 samples.
. b
RESULTS AND DISCUSSION
'
0
h i
s
k S Figure 2. Tantalum filament vaporization apparatus ( a ) quartz dome, ( b ) O-ring, (c) tantalum filament, (d-k) copper post assembly, ( I ) O-ring channel, ( m ) aluminum base, ( n ) argon gas inlet port, ( 0 ) aluminum tabs to seal the dome to the base, (p) sample injection port, (9) port from which argon flows to the torch sample introduction orifice
2. A 0.13-mm thick tantalum filament with dimensions of 37 m m by 10 m m ( c ) is positioned between two copper post assemhlies (d-k) isolated from the aluminum base ( m ) by means of fired lava spacers ( f ) . T h e copper posts a n d the argon gas inlet port ( n ) are covered with a quartz dome ( a ) , 55-mm o.d. and 35 m m high. Three aluminum t a b s ( 0 ) press the dome flange against the recessed O-ring ( b ) to provide a gas-tight seal. T h e quartz dome is fitted with two ports ( p a n d q ) one of which allows the argon gas to flow from the inlet in the base ( n ) to the sample introduction orifice ( q ) of the plasma torch (.'34).'The remaining port ( p ) , fitted with a rubber septum, is positioned to allow the delivery of u p to 200 pl of sample into a depression in the tantalum filament. T h e filament is heated by the flow of an electric current obtained from a low voltage, high current transformer controlled by a Variac. Procedure. After the plasma has been generated and stabilized ( 3 4 ) , the argon flowing through the filament chamber to the plasm a is adjusted t o a flow rate of 1.2 I./min and the spectrometer is set for the desired wavelength. A sample of 1 t o 200 p1 is deposit212 * ANALYTICAL
CHEMISTRY, VOL.
A summary of typical detection limits measured for some elements that are readily vaporized from the tantalum filament a t the operating temperatures used in this study are shown in Table 11. column 1. The reported values represent the concentration required to give a signal level t h a t is three times greater than the standard deviation of the background noise level. The experimental parameters (height of observation in the plasma, filament currents, argon flow rates, and plasma power settings) for all elements were identical. An overall comparison of the detection limits shows that, for a majority of the elements studied, the TFV-ICP technique provides relative powers of detection which overall are equivalent to the best values commonly reported for nonflame atomic absorption (AAS) or fluorescence (AFS) techniques. Absolute powers of detection were not observed to change significantly for different combinations of sample volume and analyte concentrations. Thus 1OO-pl volumes of a solution containing 0.0075 pg of Sb/ml and 15-pl volumes of a solution containing 0.05 p g of Sb/ml yielded comparable absolute detection limits within a factor of 1.4. The observations summarized in Table I1 suggest the combination,of the tantalum filament vaporization, inductively coupled plasma excitation system with a multichannel spectrometer. This combination would thus provide the capability of performing multielement determinations (up to 20 or 30 elements) a t the fractional nanogram level in microliter volumes or microgram samples in less than one minute. An illustration of the precision obtained with the TFVICP system is shown in Figure 3. This figure represents the recorder tracings obtained for 15 replicate determinations of S b a t 10 times the detection limit. The relative standard deviation is 3.6%. An analytical curve is presented in Figure 4 to demon-
46, NO. 2, F E B R U A R Y 1974
S b 2311
1
Figure 3. Reproductivility of the TFV-ICP system for 100-pl samples of a solution containing 0.01 pg of Sb/ml (1.0 X g Sb)
strate the applicability of the TFV-ICP system to the analysis of samples over a n extended concentration range. The curve covers the four-decade concentration range of 0.001 to 10 fig Be/ml or from 2.5 X 10-11 t o 2.5 x 10-7 gram of Be. Discussion. A comparison of the TFV-ICP detection limits reported in Table I1 with t h e best values so far reported for the introduction of nebulized solution into the plasma shows t h a t the TFV-ICP system is superior by 1 to 2 orders of magnitude. This appears to be the result of the increased concentration of the analyte, already desolvated and vaporized by the filament, passing through the axial channel of the plasma per unit time. The same mechanism is utilized in AAS and AFS when filament vaporization is performed, but for these techniques, the free atoms must be produced a t the filament surface whereas for the plasma system it is only necessary to vaporize the analyte elements; the dissociation and excitation occur in the plasma.
1
t
oooo1
/i' 0
U 0001
-
d
0 01 01 Concentration (+Q B c l m l )
10
10 0
Figure 4. Typical analytical curve for Be in conductivity water covering over 4 orders of magnitude in concentration
Although the results presented here only represent studies on the behavior of a number of elements in water solutions, preliminary observations on real samples, such as blood and urine, suggest t h a t little difficulty will be encountered in measuring ultratrace impurities in these matrices. Received for review April 17, 1973. Accepted August 20, 1973. We wish to thank the Salsbury Laboratories, Charles City, Iowa, for providing the financial support of D. E. Nixon's graduate research assistantship.
Mode-Locked Laser Raman Spectroscopy-A New Technique for the Rejection of Interfering Background Luminescence Signals Richard P. V a n Duyne, David L. Jeanmaire, and D. F. Shriver Department of Chemistry, Northwestern University, Evanston, 111. 6020 7
A new technique for the rejection of interfering luminescence background signals employing a mode-locked argon ion laser and single photon timing detection electronics is described. Advantage is taken of the disparity between the lifetime of Raman scattering and the lifetime of luminescence emission. Only those photons emitted or scattered by the sample during the mode-locked laser pulse are passed on to the recording circuitry by the single photon timing detection system. Essentially all of the Raman signal can be recorded and a large fraction of the luminescence background is rejected. A detailed discussion of several alternative schemes for implementing this concept is given, along with a theoretical treatment of the appropriate signal-to-noise considerations. The fluorescence rejection capabilities of one of these configurations has been tested on samples consisting of a nonfluorescent Raman scatterer doped with a highly fluorescent dye impurity. The spectra obtained via the mode-locked technique show a substantial background suppression. Background slope is also reduced and the signal-to-noise
ratio shows an improvement consistent with our theoretical calculations based on fluorescence lifetime, laser pulse shape, laser pulse repetition rate, and average mode-locked power.
The recent development of reliable continuous wave (CW) lasers operating in the visible region of the spectrum along with advances in grating manufacture and photon detection electronics have resulted in great improvements in Raman spectrometers. The wide availability of these improved spectrometers has allowed chemists to exploit the complementary nature of infrared and Raman spectroscopy in a variety of structural, dynamic, and analytical problems ( 1-9). However, Raman spectros(1) J. Loader, "Basic Laser Raman Spectroscopy," Heyden Sedtler, London 1970 (2) T. R . Gilson and P. J. Hendra, "Laser Raman Spectroscopy," WileyInterscience. New York. N.Y., 1970. (3) M . C. Tobin. "Laser Rarnan Spectroscopy." Wiley-lnterscience, New York. N.Y.. 1970.
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