1915
Anal. Chem. 1981, 5 3 , 1915-1921
Atomic Fluorescence Spectrometry with an Inductively Coupleid Plasma as Atomization Cell and Pulsed Hollow Cathode Lamps for Excitation Donald R. Demers" Baird Corporation, 125 Middlesex Turnpike, Bedford, Massachusetts 0 1730
Charly D. Allemand Consultant, 1 1 Oakwood Road, Newtonville, Massachusetts 02 180
An atomic fluorescence spectrometer (AFS) comprlslng hollow cathode lamps (HCLs) for excltatlcan and an Inductively coupled plasma (ICP) as sample atomization cell was evaluated. Detection limits for 32 dlverse elements studied were comparable to flame atomic absorption (FAA) in general and to ICP-AES, except for ithe refractory elements where they were as much as 2 orders of magnltude inferlor. Linear ranges were 4 to 5 orders of magnitude. Experiments showed that spectral llne interferences should be as uncommon as with FAA and matrlx effects should be small In most cases, If experimental conditions are carefully selected. Particulate light scalterlng and molecular fluorescence should be absent for all practical purposes. Since synchronous detection was used, base llne shifts arid drifts due to nebulizer problems, recomblnatlon continua, molecular emission, stray light, and drifts in alignment, and rf power were circumvented. The HCL-ICP-AFS approach Is amenable to the analysis of several elements slmultaneously.
Inductively coupled plasma atomic emission spectrometry (ICP-AES) exhibits excellent detection limits for a wide range of elements, large dynamic ranges and small interlement effects. At the same time, ICP-AES is susceptible to any of a number of spectral interferences. An analysis of the ICP-AES technique shows that the former, desirable Characteristicsstem directly from the use of the ICP to excite the sample aerosol, while the latter, undesirable characteristics stem from the emission technique either inherently, such as from complex atomic and molecular spectra, or indirectly, from the hardware used to measure the atoimic emission of the sample (for example, stray light in the spectrometer and drift in the nebulizer). This important distinction implies that the potential of ICP-nonAES techniques should be explored. To date, studies of ICP-nonAES techniques for multielement analysis have been few and preliminary. One novel approach presently being pursued is inductively coupled plasma-mass spectrometry (ICP-MS) (I),but this technique will not prove viable unless the problems associated with sample transfer at the ICP and mass spectrometer interface are overcome. Another approach is inductively coupled plasma atomic absorption spectrometry (ICP-AAS). The limited results (2, 3) reported so far indicate this approach would yield detection limits inferior to XCP-AES, unless a double beam optical system is used (4). The latter requirement might severely restirict or complicate any ICP-AAS instrument for simultaneous multielement analysis. Also, the small linearity range shown by all AAS systems would be another serious drawback for multielement analyses. The combination of ICP and atomic fluorescence spectrometry (AFS)has also been neglected, but this combination deserves serious consideration, because AFS possesses, in 0003-2700/81/0353-1915$01.25/0
principle, the most desirable characteristics of AES and AAS while it circumvents the disadvantages of both. Specifically, AFS exhibits large dynamic ranges like AES,and is well-suited to the development of simple and low-cost instrumentation for the simultaneous analysis of several elements (up to a maximum of about 110). At the same time, AFS spectra are simple, like MS spectra. Moreover, if the excitation sourceb) and the detection electronics are synchronously modulated, the base line drift of an AFS system will be limited to that of the electronics. Finally, if the ICP is used as the atomization cell, rather than a cooler flame, the possibility arises that the interelement effects may be substantially reduced and, very important, scattered radiation interferences, a problem with some AFS systems in the past (5-7), may be eliminated for all practical purposes. The most thorough study to date using the ICP as the atomization cell for atomic fluorescence is that of Montaser and Fassel (8). The findings from this premier study carroborated some of the above mentioned characteristics of tm ICP-AFS system. However, these workers used electrodeless discharge lamps (EDL) as excitation sources, and these are not yet commercially available for a wide enough range of elements and are still unreliable in any case. The viability of any ICP-AFS combination, wherein the ICP serves as the atomization cell, hingles on whether acceptable detection limits are attainable with reliable and inexpensive excitation sourc~!s, which, at the present time, means hollow cathode lamps (HCLs) or xenon arc lamps. For this work HCLs were evaluated as excitation sources for ICP-AFS. This paper presents HCL-ICP-AFS spectrum characteristics, systeim operating conditions, preliminary detection limits, linearity ranges, and the results of interelement effects and particulate light scattering studies. Also, the susceptibility of HCL-ICP-AFS to background interferences from a variety of sources was evaluated.
EXPERIMENTAL SECTION Instrumentation. The major components of the singlechannel experimental iietup developed for this work are listed in Table I. The ICP served &s the atomization cell, and commercially available HCLs, at right angle to the detector, served as thhe excitationsource. The HLCs were operated in a square wave pulrie modulated mode at 20% duty cycle. The HCL modulation frequency and its duty cycle were set by the signal generator, which also drove digital logic circuits that produced signals which gated and synchronized the operation of the HCL with the detector electronics. Signals from the photomultiplier tube passed through the broad band (50 Hz to 10 kHz) ac amplifier section of the lock-in amplifier to a synchronous three-mode integrator (i.e., integrate,hold, and discharge) circuit adapted for this application. The integrated dc signals were then fiitered by the output dc signid processing circuits of the lock-in amplifier. The sample introduction system was typical of those used for ICP-AES. The torch used for the bulk of this work was a conventional 18-mm4.d. ICP-AES torch, as described by Fassel and 0 1981 American Chemlcal Society
1916
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table I. HCL-ICP-AFS Experimental Facilities item description and/or supplier spectrometer 0.5-m scanning monochromator, Model 1870, Spex Industries, Metuchen, N J photomultiplier tube S-19 response with lO5-nresistor at the output, Type R106,Hamamatsu Corp., Middlesex, N J photomultiplier power supply Model 312A, Baird Corp., Bedford, MA rf plasma source Plasma-Therm, Inc., Kresson, NJ. System comprises: rf generator, type HFS25OOD (2.5 kW, 27.12 MHz); automatic power control module, Al'CS-3; automatic matching network, AMN-2500E; two-turn load coil, 25-mm i.d. gas flow system single-stage pressure regulation from liquid argon tank followed by flow meters (FM-1120 series, tube no. 3BM for aerosol and auxiliary gas glows, and tube no. 5AM for coolant gas flow), Matheson Instruments, Horsham, PA nebulizer cross-flow pneumatic (as described in ref 9) but with Teflon capillaries (0.35 mm i.d., except where noted) aspirator chamber conical torch see text lock-in-amplifier Model 128A, Princeton Applied Research, Princeton, N J signal generator Model F34, Interstate Electronics Corp., Anaheim, CA strip chart recorder Servolriter I1 Model FLOWGD, Texas Instruments, Houston, TX optics between ICP and spectrometer 50-mm diameter, 10.5-cm focal length, planoconvex fused quartz lens between HCL and ICP 40-mm diameter, 38-mm focal length, double convex fused quartz lens ~
~~~
Table 11. HCL-ICP-AFS Operating Conditions 550-900 W rf power range 55-75 mm observation height range (above aerosol tube nozzle) 2 mm (3.2 nm) slit width (band-pass) 10 mm slit height plasma gases flow 7 L/min coolant none auxiliary 1.0 L/min aerosol sample aspiration rate 1.2 mL/min aqueous 0.4 mL/min organic 80-90% of rated max av HCL currents 20% HCL duty cycle 500 Hz HCL modulation freq overall system time constant 12.2 s Kniseley (IO),except its coolant tube extended 48 mm in length relative to the nozzle of the aerosol tube. The extended outer tube reduced ambient air entrainment and gave 3-10 times better detection limits with the refractory elements. Operating Conditions. The operating conditions used for HCL-ICP-AFS are listed in Table 11. Three of the operating conditions differ substantially from those customarily employed in ICP-AES. Specifically, for HCL-ICP-AFS: (1) the rf power is much lower, (2) the observation height in the plasma is much higher, and (3) the optical spectral band-pass is much greater. All other conditionswere similar to those used for ICP-AES work. Miscellaneous. Aqueous metal solutions were prepared from serial dilutions with doubly deionized water of stock solutions made from dissolutions of the pure metals or reagent grade salta. Low concentration solutions were prepared fresh in 0.5% nitric acid. Metalloorganic solutions were prepared by dilution of single-element 5000 ppm Conostan standards (Continental Oil Co., Conostan Div., Ponca City, OK) in number 2 home heating oil. ICP-AES spectrograms of single-element solutions were obtained with a 1-m optical emission spectrograph (Model FAS-BPL, Baird Corp., Bedford, MA). Eastman Kodak SA-1film and 10-s exposureswere used. An observation height of 20 mm above the aerosol tube nozzle and 1200 W were employed for the ICP-AES portions of this work. RESULTS AND DISCUSSION HCL-ICP-AFS Spectrum Characteristics. The top spectrum in Figure 1 is an ICP-AES spectrogram of 1000 pg/mL of vanadium in the 220-440 nm region in number 2 home heating oil. The spectrum exhibits a plethora of intense lines. In contrast, the bottom spectrum shows the corresponding HCL-ICP-AFS spectrum of 500 pg/& of vanadium
410
438
WAVELENGTH NANOMETERS
Figure 1. Comparison of the relative complexity of ICP-AES and HCL-ICP-AFS spectra of vanadium in number 2 home heating oil: (top) ICP-AES spectrogram of 1000 pg/mL vanadium; (bottom) recorder scan of HCL-ICP-AFS spectrum with 500 pg/mL vanadium.
obtained with a scanning monochromator and 50-pm slits. The spectrum is radically simpler-the only detectable fluorescence lines are those corresponding to strong atomic resonance transitions. In general, the HCL-ICP-AFS spectra of some 30 elements studied in this way consisted of only the principal atomic resonance transitions and, in a few cases, the direct line transitions-the same ones that are observed in flames (see ref 11 and 12 for tabulations of these). Ionic fluorescence transitions were not observed for any family of elements studied, except the alkaline earths. For these the ionic lines detected were Ba I1 455.4 nm, Ca I1 393.4 and 396.8 nm, Mg I1 279.5 and 280.3 nm, and Sr I1 407.8 and 421.5 nm. Except for barium, these lines were about 50 times less intense than the principal atomic transitions, under the conditions used herein. In ICP-AES, these lines are usually at least 1 order of magnitude more intense than the principal atomic transitions of these elements (13). The bottom spectrum also illustrates that the HCL-ICPAFS spectrum throughout the UV-visible region exhibits no base line shift, quite unlike ICP-AES spectra in general. The base line shows only noise, which becomes significant only above about 350 nm, in regions where intense emission bands from the organic solvent occur. With aqueous samples, there occurs considerable base line noise in regions of the hydroxyl emission bands (281 and 307 nm). Spectral Line Overlap Interferences in HCL-ICPAFS. To test the susceptibility of HCL-ICP-AFS to spectral line overlap interferences, we carried out two sets of experi-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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Table 111. Spectral Line Overlap Interferences in HCL-ICR-AFS 6 4 hinterferent,nm
5 hSparation, nm
min detectable interferent concn? pg/mL
228.802 228.812 252.13 7 252.136 253.652 253.649 213.856 213.859 231.147 231.097 216.999 217.023
0.01 0.01
14 >10,000 1500 > 10,000 20 40 30 > 10,000 (20)b 105 2500 ( 3000)c 2 50 >10,000(40)c
308.211 308.215
0.004 0.004
300 0.75
250.690 250.690
V A1 Si V
250.690 250.690
0.000 0.000
20 2000 (6)
213.856 213.853
cu Zn
213.853 213.856
0.003 0.003
1
2
3
HCL
~ H C L nm ,
As Cd
228.812 228.802 252.136 252.1 37 253.649 253.652 213.859 213.856 231.097 231.147 217.023 216.999
interferent Cd
co In
co
Hg Fe Zn Ni
Sb Sb
Pb AI V V Si
Zn cu
AS
In
co Hg
co Zn Fe Sb Ni
Pb Sb
308.215 308.21 1
0.001 0.001 0.003 0.003 0.003 0.003 0.05 0.05 0.024 0.024
75 (4)d 35
Values in parentheses in column 6 are the minimum detectable concentrations, in pg/mL, by furnace or flame AAS. From ref 15. From ref 16. From ref 17.
a
*
ments. In the first set, an iron HCL was installed, a 1000 pg/mL nickel solution was aspirated, and the HCL-ICP-AFS spectrum from 200 to 55Q nm was obtained. The iron HCL emits numerous intense lines throughout this region, while nickel exhibits a complex atomic fluorescence spectrum. This experiment was then repeated with a nickel HCL in place, while aspirating a 1000 ,ug/mL iron solution. The optical spectral band-pass used during each experiment was 3.2 nm. During both scans no base line shift was detected. This meant that over the spectral region investgated, no spectral line overlap interferences occurred between nickel and iron at concentrations about 5 orders of magnitude above their respective detection limits. In the second set of experiments, 9 of about 15 spectral line overlap interferences actually observed to date in AAS (14) were investigated for their severity in HCL-ICP-AFS. In column 6 of Table I11 is listed the minimum concentration of interferent (column 3) which was found to give a false detectable atomic fluorescence signal when a given HCL (column 1) was in place. The inverse interference is also listed. Where known, the degree of these interferences in AAS is also presented (values in parentheses, column 6). For a significant spectral line overlap interference to occur in HCL-ICP-AFS: (1) the degree of line overlap, (2) the absorption coefficient of the interferent in the plasma over the overlapping region, (3) the atom population of the interferent in the relevant energy level, and (4)the iintensity of the emission line from the excitation source must all be large. Thus, even though there is very strong overlap at 250.690 nm hetween silicon and vanadium, one or more of the other prerequisite conditions was (were) not met, and a3 a result, a detectable, false atomic fluorescence signal does not appear until the vanadium concentration reaches 2000 bg/mL. In AAS, vanadium begins to interfere at onlg about 6 hg/mL. On the other hand, it is apparent that all the prerequisite conditions are met between aluminum and vanadium at 308.215nm and a false vanadium signal occurs at only 0.75 pg/mL of aluminum. In those cases where a comparison with AAS was possible, the severity of the spectral overlap interference by IHCLICP-AFS was significantly less, except for the nickel interference on antimony where the two techniques were comparable. This indicates that the widths of the absorption lines
'
O
0
r
30 mm
40 rnm
65 mm
55 mm 47 mm
L---A-
0.1 0.5
~36
I
1
I
I
0.7
0.8
09
1.0
RF POWER, kW
Figure 2. Influence of rf power and observation height of the ICP oln zinc HCL-ICP-AFS detection limit.
in the wings in the ICP, under AFS measuring conditione,, range from comparable to somewhat narrower than in flameis and furnaces and, in turn, that the frequency and the severity of spectral line overlap interferences in HCL-ICP-AFS should be, at worst, comparable to AAS. Influence of rf P'ower and Observation Height on Detection Limits. The rf power and the observation height in the ICP were found to be the two most important parameters affecting atomic fluorescence detection limits. Tho plasma gases' flow rates and the ICP torch configurationwere
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table IV. Comparative Detection Limits (SIN = 2), in rg/L ICP- flame element h a p c . nm AESa AASb HCL-ICP-AFSC
Table V. Comparative Linearity Ranges
A. Nonrefractory Ag As
Au Be Bi Ca
Cd
co Cr cu
Fe Hg In
Mg
Mn Na Ni
Pb Sb Se Sr
Ti Zn
328.0 Z189.0-193.0d 267.6 234.7 306.8 422.7 228.8 240.7 357.9 324.8 248.3 253.6 451.1 285.2 279.5 589.0 232.0 283.3 231.1 196.0-206.3d 460.7 377.6 213.8
3 30 10 0.2 40 0.05 2 7 4 3 3
20 40 0.08 1
20 8
30 30 60
0.2 40 2
B Ba Mo Si
Sn Ti V
w
309.2 249.7 455.4 313.3 251.6 303.4 335.4 318.4 295.6
20 2 0.4 4 10 25 2 4
20
20
1000 20 10 60 80 50 20 500
a Reference 18. Reference 19. theses are in no. 2 home heating oil. optical interference filter used.
ICP-AESa
HCL-ICP-AFS
Ca Mg
2 x 105 5 x 105 3 x 105 2.5 x 104 1.25 x 105
1 x 105 6 X lo4 2.5 x 104
Mn
2 100 10 1 40 3 1 5 3 2 5 200 30 0.2 3 0.4 8 20 60 100 6 30 0.6
2 200 10 0.8
50 0.08 0.8 5 10 1 10 25 10 0.2 0.3 0.5 10 25 40 150 0.7 7 0.5 (2)
B. Refractory A1
element
20 (15) (400) 50 30 (200) (200) 60 (60) (150) (90) (3000)
Values in parenWide band-pass
also found to be important, though to a somewhat lesser degree; these were not rigorously optimized during these studies. Figure 2 shows the effect of rf power and observation height on the detection limit for zinc using a conventional ICP-AES torch. The detection limit improves gradually with decreasing rf power or with increasing observation height, up to about 50 mm. This trend was typical of all the easily dissociated, hard-to-ionize elements (e.g., As, Pb, Fe, Cu, Tl). With this category of elements the analyte signal intensities changed only slightly with observation height and were almost constant over a broad range of powers. The trend exemplified by zinc in Figure 2 thus reflects the observation that as the rf power decreased or, equivalently, as the observation height increased, the base line noise decreased. With sodium, an easily dissociated, easily ionized element, the signal intensity decreased asymptotically with increasing rf power. As a result, the degradation in detection limits with increasing rf power was steeper (about 25-fold from 500 to lo00 W). On the other hand, the detection limits with the alkaline earths and many of the refractory elements varied only a fewfold over rf power ranges of as much as 400 W, because the increase in base line noise with increasing rf power was offset by increases in analyte signal intensities, presumably resulting from the production of increasingly greater free atom populations at the higher rf powers. All subsequent studies used an ICP-AFS torch, and the optimum observation heights found were around 65-75 mm and 55-60 mm above the aerosol tube nozzle for the nonrefractory and refractory elements, respectively (this compares
Na Ni a
Zn Reference 21.
1x
los
5x
lo5
8 X lo4 5 x 104
to 20-30 mm in ICP-AES work). Also, the same detection limits were maintained if increases in the rf power were offset by increases in observation height. In no instance was an rf power about 900 W (nominal) necessary or advantageous for HCL-ICP-AFS work. Detection Limits. Table IV lists the detection limits of 32 diverse elements studied by HCL-ICP-AFS. The detection limits in water were obtained at an rf power of 600 W, except for calcium and aluminum, which required 750 and 850 W, respectively, for best results. The detection limits in the organic solvent were obtained at 675 W. In all cases, a 10-s measuring time and a 60-mm observation height above the aerosol tube nozzle were used. For comparison,the detection limits observed with a flame AAS instrument under conditions optimized for each element and with a commercial ICP-AES instrument under simultaneous multielement conditions at the most sensitive ICP-AES spectral lines are also given. To facilitate the comparison, the detection limits obtained with the refractory elements are separated from the others. With the caveat that detection limits should differ by a factor of more than 3 to be considered significantly different, it can be concluded from Table IV that (1)compared to flame AAS, the HCL-ICP-AFS detection limits are equal for the majority of elements, better in a few instances (Ca, Hg, T1, Mn, and Sr), and never worse for any element (when data in the same solvent were compared), and (2) compared to ICP-AES, the detection limits are much worse for most of the refractory elements studied (up to more than 2 orders of magnitude in some cases). With respect to almost all the other elements studied, the HCL-ICP-AFSdetection limits are equal to those of ICP-AES (T1 and Na were better by HCL-ICP-AFS). All the listed HCL-ICP-AFS detection limits can be improved further by simply increasing the sample integration time, because around the modulation frequency (600 Hz) the ICP background noise was “white”. This was shown by the observation that its magnitude decreased as the square root of the sample integration time, up to a t least 100 s (the maximum possible with our setup). The background in ICP-AES, on the other hand, is flicker noise limited (20),which means the magnitude of its noise remains more or less constant with integration time and, in turn, so do the ICP-AES detection limits. Linearity. Table V lists the linearity ranges obtained for several diverse elements with our HCL-ICP-AFS setup, along with the corresponding ranges reported with ICP-AES. The linearity ranges of the two techniques are seen to be comparable, within a factor of 2 for four of the six elements investigated. Thus, the linearity advantage of ICP-AES over AAS is retained with HCL-ICP-AFS. Above the linearity range, the shape of the AFS analytical curves were observed to reach a maximum at analyte concentrations about 1-1.5 orders of magnitude higher and to bend toward the concentration exis thereafter, in accord with theory for AFS when a line excitation source and right angle viewing are used (22). Interelement Effects. The presence or absence of interelement interferences was primarily dependent on the temperature experienced by the sample in the plasma and, in turn, on those system parameters having the strongest
ANALYTICAL CHEMISTRY, VOL. 53,NO. 12, OCTOBER 1981
: I 9
120
tL_Zn213.9nm
1
10
100
1000
10,000
Na CONCENTRATION, pg/ml
Figure 3. Effect of sodium on calcium and zinc atomic fluorescence
signals at various ti powers tinder HCL-ICP-AFS measuring conditions (Ca and Zn concentrations: 20 wg/mL).
60
II 10
100
1000
J
10000
I N T E A F E R E N T CONCENTRATION pg/ml
Figure 4. Effect of phosphorus and aluminum on calcium atomic fluorescence signal at varlous powers under HCL-ICP-AFS measuring conditions (Ca concentration, = 20 pg/mL).
influence on the plasma temperature. Accordingly, the most important parameter was the rf power, followed by tihe observation height and the coolant gas flow rate (the latter two were of about equal importance). The sample aspiration rate and the configurationof tlhe torch and the aspirator chamber, insofar as they affected the plasma temperature at a given observation height, also lhad observable effects. The classical solute vaporization (phosphorus and aluminum on calcium) and ionization (sodium on calcium and zinc) interferences (23)were investigated at an observation height of 73 mm above the aerosol tube nozzle. Figure 3 shows the influence of sodium on the atomic fluorescence signals of calcium (an easily ionized element) and zinc (an easy-to-dissociate but hard-to-ionize element) a t rf powers spanning those where the best detection limits are obtained for each. Zinc ici seen to be very insensitive to ionization interferences over wide ranges of concentration and rf power, while calcium exhibits either a small enhancement, or a depression, depending on the concentration or the rf power. Figure 4 shows the influence of phosphorus and aluminum on the calcium atomic fluorescence. The Ca-P system exhibits the classical “knee” in the interference curve (23) at low to medium (for HCL-ICP-AFS) rf powers. If the rf power is increased to 800 W and higher, this interference is absent, as it is in the ICP-AES (24). In contrast, the Ca-A1 system exhibited a more complicated behavior-a substantial depression at medium to low d powers (
