1890
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
Laser Enhanced Ionization Spectrometry in Analytical Flames G. C. Turk,” J. C. Travis,
and J. R. DeVoe
Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
T. C. O’Haver Chemistry Department, University of Maryland, College Park, Maryland 20742
A new variety of analytical atomic flame spectrometry called laser enhanced ionization (LEI) has been developed. The method relies on the enhanced rate of thermal ionization of the analyte element following photoexcitation with a dye laser tuned to an appropriate transition wavelength. This enhanced lonlzatlon rate can be electrically measured directly In the flame, and therefore no optical detection system is required. Detection limits have been measured for 18 elements, showing order-of-magnitude superiority over other flame based spectroscoplc methods in many cases. A variety of types of transltlons have been successfully utlllzed, Including ground state transltlons, thermally excited state transitions, low transition probability transitions, and a two-photon transltlon. The strong dependence of LEI sensitivity on the ionization potential of the analyte element and the energy of the laser populated excited state is discussed. Other topics discussed include interference problems encountered and the appllcatlon of LEI to the analysis of real samples.
Laser enhanced ionization (LEI) is a new method of analytical atomic flame spectrometry. The method employs a tunable dye laser to promote the analyte atom to a discrete excited level. Thermal ionization of this excited state atom will occur at an enhanced rate relative to that of the ground state. This enhanced rate of ionization can be electrically detected by applying a voltage across the flame and observing changes in the electrical current through the flame. The nonoptical nature of the signal detection makes LEI unique among the various flame spectrometric techniques. Sources of interference and noise associated with optical detection are avoided. Particularly important is the freedom from interference caused by scattered laser radiation, which is a severe problem associated with laser induced atomic fluorescence. However, sources of interference and noise associated with the electrical measurement in the flame are present instead. The method is based on what has been termed the optogalvanic effect (OGE), first observed in the discharge of a hollow cathode lamp ( 1 ) . The first observation of this OGE in a flame was reported shortly after ( 2 ) ,and preliminary results of the application of this phenomenon to analytical measurements of trace metals in flames have been reported (3, 4 ) . Extensive study of the mechanism of this OGE has taken place both in hollow cathode discharges (5, 6) and in flames (7, 8). The more descriptive term-laser enhanced ionization-is used here in reference to the OGE in flames. (In discharges, ionization is often reduced rather than “enhanced”.) This paper will describe the considerable progress which has been made in the understanding and characterization of LEI as a method of analytical atomic flame spectrometry. Mechanism. The LEI signal production mechanism is described in detail elsewhere (3, using a four-level model of the atom which includes the ground state (level 0), the lower level of the optical transition (level l),the upper level of the optical transition (level 2), and the ionization limit (level i). 0003-2700/79/0351-1890$01 .OO/O
A brief summary of the results of this model is useful as an aid to explaining many of the experimental results to be reported here. The LEI signal consists of pulses of increased current through the flame which are synchronous with the laser pulses. Under the influence of a sufficiently large electric field, as is the case for the LEI measurement, the current density is a direct measure of the rate of ion production per unit volume in the flame (9). This rate is equal to the product of the ionization rate constant and the population density of the atoms in the energy level from which ionization is occurring. For LEI, the ionization is occurring from the excited state which is being populated via the absorption of laser radiation of the appropriate wavelength. The ionization rate constant from this excited state ( k 2 J is related exponentially to the energy difference between this excited state and the ionization potential of the element as follows: k2i a
exp(-(Ei*/kT))
(1)
where Ei*is the energy gap between the excited state and the ionization potential, k is the Boltzmann constant, and T is the flame temperature. The population density of this excited state can be assumed to be due entirely to photoexcitation induced by the laser and is therefore related to the laser spectral irradiance (E”),the absorption coefficient ( B I Zfor ) the optical transition used, and the concentration of analyte being aspirated into the flame. Using this information, the following semi-empirical function for calculating the LEI sensitivity to be expected for a particular optical transition of any element can be developed:
where the designation (A) specifies a particular transition of a given element; STis the predicted sensitivity (nA ng-’ mL) at the peak of the LEI current pulse; C1 is an empirically determined proportionality constant; XBis the Boltzmann population factor for the lower level of the optical transition ( X , = 1 for ground state transitions); p is the atomization efficiency factor for the analyte element in the flame being employed; E,P is the laser peak spectral irradiance; and C2 is an empirically determined constant. The empirical constant C2 is a function of the time profile of the laser pulse. For a square wave pulse, C2 is unity. However, if the laser pulse is not square, a significant portion of the neutral atom population can be ionized before the laser reaches its peak power. This results in a diminished ionization rate, and thus C, becomes less than unity. As reported in ref. 7, the empirical constants C1 and C2 were evaluated by correlation of the theoretically predicted sensitivity with the experimentally determined sensitivity for 14 elements using a total of 23 different optical transitions, yielding log C1 = -3.3 f 0.3
*
Cz = 0.65 0.05 where the quoted uncertainties are the standard deviation of the coefficients obtained from a linear least-squares fit. Use 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
9
1891
with Model 164 gated integrator). Readout is with a teletype for the computer based system or a strip chart recorder for the boxcar averager.
I I
A C T I V E FILTER
I
TRIG OUT
CHART RECORDER
SIC BOXCAR AVERAGER
I
1
Figure 1. LEI measurement system
of these factors in Equation 2 will yield a predicted sensitivity for a particular atomic line of any element using an airacetylene flame with a laser equivalent to the one used here. This predicted sensitivity can be expected to agree within an order of magnitude of the experimental sensitivity.
EXPERIMENTAL The instrumental system used for this work is basically the same as that described previously ( 3 ) and is summarized in Figure 1. A linear flashlamp pumped tunable dye laser (Chromatix,CMX-4) with frequency doubling capability is used. For simplicity, the laser is operated in the wide bandwidth (0.05 nm) mode, that is, without the use of bandwidth narrowing etalons, unless the bandwidth needs to be narrowed to avoid a spectral line overlap. The laser beam is directed down the axis of a slot-type premix air-acetylene flame. The only flames which have been utilized for analytical LEI measurements are the premix air-acetylene and premix air-hydrogen flames. The air-hydrogen flame has the advantage of having a very low ion-electron background and has performed well for Sn, K, and Na. However, because of its higher temperature and better atomization properties, the airacetylene flame has been used as the primary flame for LEI. A fuel lean condition generally performs best although Cr yields a better signal with a slightly fuel rich flame. The ion-electron background of the flame, and the resultant noise, increase as the flame becomes more fuel rich. A voltage is applied across the flame using the burner head as the anode and an external split electrode as the cathode. Two types of split cathode electrodes have been used here. The first consists of a pair of tungsten rods, 1 mm in diameter, separated by 10 mm, aligned parallel t o the burner slot, 10 mm above the burner head. The cathode wires are visually just outside opposite sides of the flame. The second type of split cathode is a pair of plates, rather than rods, aligned in basically the same manner as the rods. Both tungsten and molybdenum sheet metal have been used as cathode plates with no apparent difference in performance. Most of the results presented here were obtained using molybdenum plates, 2.5 cm high, 0.6 mm thick, aligned 12 mm apart, with the lower end of the plates 6 mm above the 5-cm slot burner head. The LEI current signal is measured from the anode with the aid of a 2 X lo6V/A current-bvoltage pre-amplifier (3). A passive high-pass filter (R = 10 kQ, C = 0.001 pF) on the input to the pre-amplifier rejects the dc flame background current and low frequency flame noise. The burner anode is separated from ground potential by the 10-kQ resistor of this filter. This resistance is small in comparison t o that of the flame even when high concentrations of easily ionized species are aspirated into the flame; and, therefore, the potential at the burner head is very small in comparison t o the cathode potential. Further rejection of low frequency flame noise at the output of the pre-amplifier is accomplished with a variable active high-pass filter (Tenelec,TC200, First Differentiator stage). Additional filtering and amplification is then performed with a nuclear linear pulse amplifier (Tenelec, 203BLR). The filtered and amplified signal pulse is about 3 1s long. Gated detection of these pulses is done either with a computer based system described in ref. 3 which measures and averages the peak voltage of each pulse, or with a boxcar averager (PAR Model 162
RESULTS AND DISCUSSION Detection Limits. Detection limits for LEI have been measured for 18 elements a t 28 wavelengths and compiled in Table I. Included in this table are previously reported (3, 4) detection limits which were measured using tungsten rods for the cathode. These detection limits were measured with the laser operated a t 5 pulses per second (pps), and are based on the means and standard deviations of the peak heights of 150 signal pulses with the aid of the computer based gated detection system. Being reported for the first time are detection limits obtained using the molybdenum plate cathode. These detection limits were measured with the laser operated a t 20 pps and the boxcar averager used for gated detection. The boxcar averager was used with a 1-ps gate width, a 1 0 - k ~ time constant on the processor module, and a 1-stime constant on the main-frame of the instrument, yielding an effective time constant of 1.1 s a t 20 pps. All detection limits are calculated as the concentration of analyte which yields a net signal amplitude equal to three times the rms noise level of the blank. For the boxcar averaged signals, the rms noise level was estimated as one fifth of the peak-to-peak noise level. All detection limits were measured using aqueous standard solutions. Table I1 compares the LEI detection limits with those reported using other flame based atomic spectrometric methods, namely, atomic absorption, atomic emission, and atomic fluorescence using both conventional and laser sources. Detection limits for Ba, Ga, In, Li, Pb, Sn, and T1 are significantly better by LEI than any of the competing flame techniques. The best competition offers significant improvement over LEI for Cu and Ni. Line Selection. As shown in Table I, a wide variety of atomic lines have been utilized successfully with LEI, including lines which originate from the ground state, lines originating from thermally excited states, very low transition probability lines, and even a two-photon transition. The fact that many of these lines would not be familiar to the analytical chemist who makes atomic spectroscopic measurements via the traditional optical detection techniques, is evidence of the unique nature of the LEI method. The basis of this unique aspect of LEI can be seen in Equation 2, which predicts a strong dependence of LEI sensitivity on E,*, the energy difference between the upper level of the optical transition, and the ionization potential of the element. As the laser populated excited state approaches the ionization potential, sensitivity increases dramatically. The close proximity of the excited state to the ionization potential is the reason that low detection limits could be obtained for low transition probability lines such as the 294.3-nm K line or the 285.3-nm Na line. Figure 2 illustrates this effect as it relates to the principal series of Na lines. For sodium, and the other alkali metals, the Einstein absorption coefficient, BIZ,decreases with increasing transition energy (decreasing wavelength). However, the enhanced ionization rate constant increases with increasing transition energy since E,* decreases. The LEI sensitivity a t a constant laser intensity follows the product of these two offsetting parameters. As shown in Figure 2, this product increases with increasing transition energy. This is the opposite of the trend of sensitivity observed for absorption, emission, and fluorescence. The poor performance for Cu at 324.8 nm is a result of the high value of E,*, and an even poorer performance can be assumed for the very high ionization potential elements (Cd, Zn, As, Se, etc.) which have not been attempted using the present measurement system.
1892
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
Table I.
D e t e c t i o n Limits for Laser Enhanced Ionization
wavelength, nm 328.1 307.2 306.8 300.7 298.6 301.8 282.4 324.8 298.4 302.1 287.4 294.4 303.9 294.3 610.4 639.3‘ 670.8 285.2 279.5 280.0 285.3 589.0 300.2 280.2 283.3 284.0 286.3 291.8
element Ag Ba Bi Ca Cr Cr cu cu Fe Fe Ga Ga
energy levels, cm-’ 0-30473 0-32547 0-32588 15316-48564 8 3 08-4 17 8 2 8095-41 225 11203-4 6598 0-30784 0-33507 0-33096 0-34782 826-34787 0-3 2 89 2 0-33973 14904-31283 0-31283 0-14904 0-35051 0-35770 17 052-52 758 0-35042 0-16973 205-33501 10650-4 6 3 29 0-35287 34 28-3 8 6 2 9 0-34914 1793-42049
E i * , cm“
s-1
B , , x 10-17e ( W cm-2 ~ z - ’ l r l
detection limit, ngimL
30633 2.8 i >50% lQ 9488 0.90 i 50% 26202 2Q 0.51 i 50% 26 4.3 i >50% 13345 2b 15719 0.29 i >50% 1006 1.7 i >50% 31533 1006 0.22 i > 50% 30193 4b 0.42 i >50% 30604 26 1.8 i >50% 0.07b 13606 0.16 1.8 I>50% 13601 In 2.5 i >50% 13788 0.008b, 0.006a K 4.6 x 10-5 + 50% 20603 lob: 2bd Sn 1.1 ?. >50% 24318 T1 2.3 i >50% 0.09b 7218 Molybdenum plate cathode; laser run a t 20 pps; boxcar averaged with 1.1-s effective time constant. Previously reported; tungsten rod cathode; laser run at 5 pps; based on mean and standard deviation of 150 signal pulse amplitudes. Twophoton transition. Air-hydrogen flame. e Values and quoted uncertainties calculated from data compiled from the following sources: W. L. Wiese, H. W. Smith, and B. M. Glennan, “Atomic Transition Probabilities”, Volume I, NSRDS-NBS 4, US.Government Printing Office, Washington, D.C., 1966; W. L. Wiese, M. W. Smith and B. M. Miles, “Atomic Transition Probabilities”, Volume 11, NSRDS-NBS 22, U.S. Government Printing Office, Washington, D.C., 1969; B. M. Miles and W. L. Wiese, “Critically Evaluated Transition Probabilities for Ba I and 11”, NBS Technical Note 474, U.S. Government Printing Office, Washington, D.C., 1969; S. M. Younger, J. R. Fuhr, G. A. Martin, J. Phys. Chem. R e f . Data, 7 , 495 (1978); C. H. Corliss and W. R. Bozman, “Experimental Transition Probabilities for Spectral Lines of Seventy Elements”, NBS Monograph 53, U.S. Government Printing Office, Washington. D.C., 1961.
___
WAVELENGTH [nm)
812
:I 1/ , ,
-:.
p
1013
1012
10
13
16
19
22
25
28
31
34
37
40
43
TRANSITION ENERGY 11000 c m - l j
Flgure 2. Effect of optical transition energy of the principal series of Na lines on: (a) absor tion transition probability coefficient, 672,in units of s-‘ (W cm-’ Hz-’)- , (b) the exponential factor of the excited state ionization rate constant: (c) the product of 6,,and exp(-Ei+lkT), which predicts the expected relative LEI sensitivity per unit spectral irradiance
r:
The dependence of sensitivity on Ei* favors the use of shorter wavelength UV transitions. Experimentally, this factor may be offset by the higher intensity of the laser output in the visible region, where frequency doubling of the laser emission need not be performed. T h e excellent results obtained for Li and Na using visible photons bear this out. Eleven of the transitions listed in Table I originate from thermally excited states. Because of the very low population density of atoms in the flame in these excited states, such lines have very little usefulness in optically detected atomic flame spectrometry and are commonly referred to as nonresonance or nonabsorbing lines. For LEI, however, a line originating from an excited state will have nearly the same sensitivity as one of the same wavelength and transition probability which originates from the ground state. The loss of sensitivity resulting from the lower population density of the lower level of the transition is compensated by the decrease of energy needed to reach the ionization potential from the upper level of the transition. Complete compensation of sensitivity is not achieved if the exponent C2 in Equation 2 is less than unity, as is the case here. The LEI detection limit obtained for P b at the 280.2-nm excited state line is over three orders of magnitude better than a detection limit reported using the same line by AF in a flame (IO),and the LEI detection limit for the Li excited state line a t 610.4 nm is nearly five orders of magnitude superior to the reported laser induced flame atomic fluorescence detection limit (11). One interesting type of transition to which LEI seems uniquely well suited is the two-photon transition (12, 13). A
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
1893
Table 11. Comparison of Detection Limitsa (ng/mL) laser eleFAFe F A F ~ FAAC FAEd ment LEI^ Ag Ba Bi
1
Mg
0.2 2 0.1 2 100 2 0.07 0.006 1 0.001 0.1
Mn
0.3
Na Ni Pb
0.05
Ca
Cr
cu
Fe Ga In K
Li
Sn T1
1 20 50 1 2 1 4 50 30
3g 1 0.1 0.8 0.8 5
2
1 20000 0.1 2 0.1 5 10 0.4
0.05g
8 0.6 2
10
0.02 5 1 0.1 20 100
2O h
100g
0.09
20
20
_ 0.1 __ 5 20
5 0.5 8 10 100 _--
-- -
0.1 1 _-.
3
10 50g 8
4 8 3
0.08 1 1 30 0.9 0.2
___
0.5 0.2 0.4 0.1 2 13
-__ 4
Detection limits for comparison taken from S. J. Weeks, H. Haraguchi, and J. D. Winefordner, Anal. Chem., 5 0 , 360 (1978), unless otherwise noted. This work, Flame atomic ablaser enhanced ionization in flames. sorption. Flame atomic emission. e Flame atomic fluorescence, conventional light sources. f Laser induced flame atomic fluorescence. g Taken from J. D. Winefordner, J. J. Fitzgerald, and N. Omenetto, Appl. Spectrosc., 29, 369 (1975). Taken from V. A. Fassel and R. N . Knisely, Anal. Chern., 46, l l l O A (1974). a
-
two-photon transition populates an energy level at twice the energy of the absorbed photons via an intermediate virtual level a t the single photon energy. Such transitions are potentially quite useful since two visible photons could be used rather than a UV photon in a situation where UV is difficult to use, or two UV photons could perform the same task as a vacuum UV photon. Unlike single photon transitions, the absorption transition probability of a two-photon transition increases quadratically with light intensity. However, even with the use of high intensity laser sources, the transition probabilities for twophoton transitions are generally very low. For this reason, analytical use of two-photon transitions in atomic spectrometry has been rare (14) although there has been application in the field of molecular fluorimetry (15). For LEI, however, the loss of sensitivity incurred as a result of the low transition probability is again compensated for by the closer proximity of the excited state to the ionization potential. A detection limit of 0.4 ng/mL was achieved for Li using a two-photon transition at 639.3 nm. In order to observe this signal, it was necessary to increase the source spectral irradiance by focusing the laser beam. One-photon transitions generally give smaller LEI signals when the beam is focused owing to optical saturation and the smaller volume of flame irradiated. The flashlamp pumped tunable dye laser used here is not the optimum choice of laser system to use for achieving the high laser powers needed for two-photon spectroscopy. Significant improvement could be realized by utilizing a higher power laser such as the N2 pumped dye laser or the Nd:YAG pumped dye laser. It is obvious that for many elements LEI is capable of utilizing many more spectral lines than the traditional optical detection techniques of atomic flame spectrometry. This has proved to be a useful characteristic of the method because the incomplete tuning range of the laser employed (435-730 nm fundamental, 265-340 nm frequency doubled) does not include the traditionally used analysis lines for many important elements. It also allows a greater number of elements to be done
1
0
5
I 10
I
1
15
20
I
25 SODIUM CONCENTRATION i p g l m l l
I
1
30
35
Figure 3. Percent recovery of LEI signal for 10 ng/mL In at 303.9 nm as a function of matrix Na concentration using the tungsten rod cathode at -1000 V (0)and -1500 V ( 0 )
using a single laser dye, thus minimizing the need for time-consuming dye changes. The optimum line to use for a particular element by LEI is not necessarily the same as the line which would be used for absorption, emission, or fluorescence. The case for P b is a good example, where the 280.2-nm excited state line gives a lower detection limit than the commonly used 283.3-nm ground state line. The use of Equation 2 to predict the sensitivity to be expected for a particular line has proved to be very useful in deciding which atomic line to use for a particular element. Of course, other factors such as the performance of the laser at the wavelength in question, the properties of the dye to be used, and the presence of possible interfering lines must also be considered. Interferences. Interferences encountered in the LEI measurement include some which are identical to the other flame spectrometric methods and some which are unique. Some interferences encountered with the traditional flame methods are not present in the LET method. Those interferences associated with the processes of sample aspiration and atomization in the flame are identical for any spectrometric method using the flame as an atomization source. The most severe interference problem encountered for LEI is ionization interference. In LEI, ionization interference refers to an interference upon the LEI signal strength as a result of nonspecific thermal ionization of matrix constituents. This matrix ionization results in an increased population of charged species in the flame with a direct effect upon the electrical properties of the flame as well as on the various ionization equilibria in the flame. In order to study this interference, a series of measurements were made using 10 ng/mL of In as the analyte and various concentrations of Na as the matrix. The measurements were all made in the air-acetylene flame operated in a fuel lean condition and using the tungsten rods for the cathode. The laser was tuned to the 303.9-nm In line. Figure 3 depicts the percentage of analyte signal recovery in the presence of 0-30 l g / m L Na a t two different cathode potentials. With the cathode potential at -1000 V, the signal from the 10 ng/mL In is almost completely suppressed in the presence of 20 lg/mL Na. This drastic loss of signal is overcome to a certain degree by application of higher (more negative) potentials to the cathode probes. At -1500 V, the In signal in 20 l g / m L Na is completely recovered. Signal enhancement was observed for intermediate levels of Na. This is a true enhancement of the In signal, not an additive background interference. The cause of this enhancement is not yet understood. Figure 4 is a plot of In signal as a function of applied cathode potential with various levels of Na present in the matrix. The figure shows a definite threshold potential below which no LEI signal
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
1894
i
81
z
u
Ln Y
->
r
4,
=
40
0
0
12
8
4
16
CATHODE POTENTIAL I-kVI
Figure 4. Relative LEI signal for 10 ng/mL In at 303.9 nm as a function of cathode potential using tungsten rod cathodes with distilled water matrix (+), 10 pg/mL Na matrix (A),and 20 pg/mL Na matrix ( 0 )
Table 111. Ionization Interference recovery of 0.1 wg/mL Pb signal at 280.2 nm,
%
2
0
4
8
6
10
CATHOOE POTENTIAL I-kVI
Figure 5. Measured electrical potential of a tungsten rod inside the air-acetylene flame in the position of the laser beam as a function of potential applied to the tungsten rod cathode while aspirating distilled water (+), 10 pg/mL Na (A),and 30 yg/mL Na ( 0 )
tungsten rod cathode ionization potential matrix, poten10pglmL tial, cm-' -500 V -750 V -1000 V K Na
Li Ca cu
35010 41449 43487 49305 62317
0 45 100 82 100
0 90 110 100
100
180 110 110 100 100
is obtained. This threshold potential becomes more negative in the presence of increasing concentrations of Na in the matrix. Similar results were obtained using P b as the analyte element at 280.2 nm. Table I11 lists the percent signal recovery for 100 ng/mL P b analyte in the presence of 10 pg/mL of K, Na, Li, Ca, and Cu a t three different cathode potentials. The signal is perturbed by those matrix elements which undergo significant thermal ionization in the air-acetylene flame. No interference was detected from the Cu matrix, which is not significantly ionized, nor was any detected for Cu matrix concentrations as large as 1000 pg/mL. The ionization interference is particularly severe for K, which is about 50% thermally ionized in the air-acetylene flame. Aside from affecting the bulk conductivity of the flame, matrices of easily ionizable elements also affect the electrical potential contours in flame. A series of measurements were made of the electrical potential inside the flame in the region where the laser beam is aligned. These measurements were made by placing a 1-mm diameter thoriated tungsten rod inside the flame and measuring the potential with a very high input resistance voltmeter. Figure 5 shows how the potential of this probe rod varied with cathode potential for matrices of distilled water, 10 pg/mL Na, and 30 pg/mL Na. The rod potential was found to be significantly diminished by the presence of Na in the flame. A threshold of cathode potential below which no significant level of rod potential could be detected was found to exist. This cathode potential threshold shifted to increasingly negative voltages as more Na was added to the flame. This behavior can be explained by the formation of positive ion sheaths (space charge) around the cathodes. Such sheaths shield the potential from the center of the flame and increase in magnitude as a result of the increased ion population of the flame when Na is present. There is a strong correlation between the flame potential and the LEI signal behavior as a function of Na concentration and cathode potential. This is convincing evidence that positive ion sheath formation around the cathodes and the resultant decrease in magnitude of the potential in the la-
0 0
1
1
30
60
1
1
l
l
l
I20 150 180 210 SOOIUM C O N C E N T R A T I O N Ips m i ) 90
l
240
,
210
Figure 6. Percent recovery of 10 ng/mL In signal at 303.9 nm as a function of matrix Na concentration using the tungsten plate cathode at -1000 V (0),and -1500 V ( 0 )
ser-perturbed region of the flame act to inhibit the measurement of LEI. Using rods for the cathode, the highest electric field strength occurs at the surface of the rods, which is a favorable condition for the formation of a positive ion sheath. Using plates as cathodes, the electric field strength a t the cathode surface is diminished for a given applied potential, and thus the formation of a positive ion sheath should be likewise diminished. Figure 6 depicts the recovery of the 10 ng/mL In signal as a function of matrix Na concentration using tungsten plates rather than rods for the cathode. The cathode plates allow a t least an order of magnitude greater tolerance to Na matrix than the cathode rods. With -1500 V applied, the signal for 10 ng/mL In is actually enhanced in the presence of 250 pg/mL Na. Using this type of electrode configuration, LEI signals can be measured in Na matrices in the hundreds of pg/mL range. Depending on the level of Na involved and the potential being applied, the analyte signal may be enhanced or diminished; standard additions or matrix matched standards may be necessary. The results obtained in the Na matrix can be extended to other easily ionized matrix elements, with appropriate scaling of the magnitudes involved depending on the ionization potential of the matrix element. It is expected that further improvement in the LEI tolerance to ionizable matrix elements is obtainable by optimizing the shape and position of the cathode plates. The presence of an easily ionized species in the sample matrix also increases the dc background current through the flame. This is illustrated in Figure 7, which shows the effect of Na on the dc flame background current using the plate type
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
1895
Table IV. Na Spectral Interference at 280.2 nma background signal, Na concn, pg/mL nA 0
10 20 30
0 19 23 24
Signal from 0.1 pg/mL Pb is 2 3 nA; tungsten rod cathode at -800 V.
! 200 SODIUM C O N C E N T R A T I O N
Ips
I
8
I I 500
t
/
,
Ib3
mt!
Figure 7. The dc background current through the air-acetylene flame as a function of aspirated Na solution concentration using the tungsten plate cathode at -1500 V. The background current when no sodium is aspirated is 180 p A
cathode at -1500 V. The current increases from a normal level of 180 pA with no Na being aspirated to 4.4mA when 1000 pg/mL Na is aspirated. Although the dc current is filtered out, an associated noise component is observed, which ultimately limits the detectability of the analyte signal. This noise component increases with increasing background current and detection limits are degraded accordingly. Interferences arising from overlaps of spectral features of nonanalyte atoms and molecules with the analytical wavelength occur in all of the flame spectrometric methods including LEI. However, the vastly modified criteria for spectral line sensitivity alter the occurrence of such overlaps. For many elements, the number of usable transitions is much larger, and thus the possibility of an atomic spectral line overlap is increased. However, the increased number of usable transitions makes it easier to avoid such situations. In addition, the wavelength tunability of the laser and the very narrow spectral bandwidth which can be attained with a dye laser are of great value in dealing with such problems. The linewidth of spectral lines in the flame is the limiting factor in determining the separability of lines. For very high ionization potential species, the number of usable transitions is smaller. Molecular spectral interferences have not been encountered thus far and may be minimal for LEI. Naturally occurring flame molecules such as C2and OH, which have spectral features which are encountered by the other flame spectrometric methods, generally have very high ionization potentials. As a result, such species are not expected to be observable by LEI. Molecular LEI spectra of the oxides of alkaline earth and rare earth elements in a flame have been observed (16). Spectral interferences for LEI have been encountered for wavelengths near very sensitive lines of matrix elements. In Na matrix solutions, the wavelength region from 280 to 290 nm is subject to spectral background interference from the wings of the very sensitive Na resonance line a t 285.3 nm. Table IV shows the magnitude of the background interference observed at the 280.2-nm P b line for various Na concentrations. A similar observation was made for Li matrices in the region surrounding the Li resonance line at 272.1 nm. Background interference from the wings of atomic lines of major matrix elements is not unique to LEI although the specific examples of this interference may be quite different because of sensitivity differences. Of course a signal may still be measured on the wings of these intense lines because the wavelength tunability of the laser allows scanning across the analyte transition wavelength in order to measure the spectral background.
Light scattering is in most cases the most prominent source of background interference in flame atomic absorption and fluorescence (17-19) and is a particularly severe problem in laser induced resonance fluorescence (14). Since LEI requires no optical detection, this source of interference is not encountered. Dynamic Range. Laser enhanced ionization exhibits a linear signal response to analyte concentration with a linear dynamic range of four or more orders of magnitude in most cases, The LEI signal response to concentration is similar in many respects to that of atomic fluorescence. Like AF, linear response is achieved as long as the absorbance is low. This restraint is relaxed under conditions of optical saturation. Unlike fluorescence, however, nonlinearity caused by postfilter effects is not a concern since there is no optical detection. Nonlinearity of the analytical curve has been noted at higher concentrations of easily ionized analytes, where an ionization self-interference can occur. Real Samples. Laser enhanced ionization has been successfully applied to the analysis of certain real samples. Alloy samples are particularly well suited because the levels of elements such as Na and K are not high. One sample studied was a Ni-based high temperature alloy obtained from Pratt-Whitney Aircraft Corp. This sample (Pratt-Whitney Standard r4-E) contains 63% Ni, 10% Co, 8% Cr, and high levels of Al, Mo, Ta, Hf, and Ti. Trace levels of a variety of elements in this alloy have important metallurgical significance. The analysis of this alloy for trace metals using atomic absorption, without resorting to time consuming extraction procedures (20),is difficult because of a large amount of spectral background interference. This interference is caused by scattering, by molecular absorption, and by atomic absorption of matrix elements (21,22). Using LEI, a determination of In in this sample was performed. The very high sensitivity of LEI for In, and the freedom from scattering interference and perhaps molecular spectral interference, render LEI particularly well suited for this task. One gram of the sample was dissolved with HNOBand HF, and diluted to 200 mL with distilled-deionized water. Bracketing the sample with aqueous standards of In, a value of 35 pg/g of In in the alloy was obtained. This agrees with the value reported by Pratt-Whitney (23) of 37 pg/g obtained by graphite furnace AA within the experimental error of the two measurements. Figure 8 is an LEI wavelength scan of the Ni-based alloy solution in the vicinity of the 303.9-nm In line which was used. Nearby lines of the major elements Ni and Cr are present, but base-line resolution of the In line is obtained without the use of an etalon. For the typical atomic absorption measurement of indium, these nearby lines fall within the spectral bandpass of the monochromator. This has been shown to interfere with continuum source background correction for this sample (21). The scan was made after diluting the prepared solution by a factor of ten in order to conserve sample. The diluted sample contains 310 pg/mL Ni, 40 pg/mL Cr, and 0.018 pg/mL of In. A determination of Mn in the NBS Standard Reference Material No. 1261 steel sample was also performed. One gram
1896
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979 120
/: [I u
i
I
I
I
Nt
80
I" I
J
40
:_
I
Ct
303 5
303 7
3
n
e
,
303 9 WAVELENGTH (nml
Cr
I
304 1
Flgure 8. LEI spectrum of high temperature Ni-based alloy in the vicinity of the In analysis line. The sample solution contains 310 pg/mL Ni, 40 pg/mL Cr, and 0.018 kg/mL In
of the sample was dissolved using HC1, "OB, HF, and HC104 and diluted with distilled-deionized water to 100 mL. The laser was tuned to the Mn line at 304.457 nm. This is an excited state transition which originates a t 17052 cm-' above the ground state. Although not as sensitive as the ground state Mn line at 279.5 nm, this line was chosen because of the superiority of the dye required, Rhodamine 6-G, over the dye Fluoral 7-GA, needed at 279.5 nm, which yields a low laser output, has a short useful lifetime, and is difficult to prepare. The level of Mn in the sample was high enough that the loss of sensitivity incurred by going to the Rhodamine 6-G line caused no problem. The measurement was initially made without the use of an etalon in the laser, but the Mn value obtained was higher than the certified Mn value. A wavelength scan of the vicinity of the analytical line revealed a partial spectral overlap from a Ni line a t 304.501 nm. The measurement was then repeated using the low finesse etalon of the CMX-4 laser which narrows the bandwidth to 0.004 nm. Using a calibration curve constructed from aqueous Mn standards, a value of 0.67% f 0.02 Mn in the steel sample was obtained which agrees with the certified value of 0.66%.
CONCLUSIONS Laser enhanced ionization has been shown to be a viable new method of analytical atomic flame spectrometry. Considerable improvement in detection limits over those of the traditional methods of flame spectrometry has been demonstrated for a variety of elements. Sub-ng/mL detection limits have been achieved for many of elements without sacrificing the convenience of the analytical flame as an atom reservoir. Interference from thermal ionization is the biggest limitation associated with the analytical applicability of LEI. Some progress in understanding the mechanism of this interference has led to a reduction of the effect, so that the application of this method to many classes of samples is now possible without requiring separation techniques. Work is continuing to improve this situation. With the possible exception of laser induced AF, the optically detected flame techniques have reached fundamental limitations to sensitivity, and dramatic further improvement
is not expected to occur. Laser enhanced ionization, however, is in its infancy and dramatic improvement can be expected. The use of more powerful and more appropriate laser systems is just one promising possibility for improvement. The further use of multiphoton transitions is another. Laser enhanced ionization has a number of attributes that make it appear to be a major improvement in spectroscopic chemical analysis. Tunability of the laser improves the chances of detecting interferences and thereby increases accuracy. General applicability will await improvement in laser technology for producing convenient tunability in the ultraviolet.
ACKNOWLEDGMENT The authors express their gratitude to W. G. Mallard, P. K. Schenck, S. J. Weeks, M. B. Blackburn, and K. C. Smyth, for their helpful suggestions and assistance.
LITERATURE CITED R. B. Green, R. A. Keller, G. G. Luther, P. K. Schenck, and J. C. Travis, Appl. Phys. Lett., 29, 727 (1976). R. B. Green, R. A. Keller, G. G. Luther, P. K. Schenck, and J. C. Travis, J . Am. Chem. Soc., 98, 8517 (1976). G. C. Turk, J. C. Travls, J. R. DeVoe, and T. C. O'Haver, Anal. Chem., 50, 817 (1978). J. C. Travis, G. C. Turk, and R. B. Green, in "New Applications of Lasers in Chemistry", G. M. Hieftje, Ed., American Chemical Society, Washington, D.C., 1977, pp 91-101. K. C. Smyth and P. K. Schenck, Chem. Phys. Led., 5 5 , 466 (1978). E. F. Zalewski, R. A. Keller, and R. Engleman, Jr., J . Chem. Phys., 7 0 , 1015 (1979). J. C. Travis, P. K. Schenck, G. C. Turk, and W. G. Mallard, Anal. Chem., 51, 1516 (1979). C. A. van Dijk, Ph.D. Dissertation: Utrecht, 1978. J. Lawton and F. J. Weinburg, Electrical Aspects of Combustion", Clarendon Press, Oxford, 1969, pp 319-322. V. Sychra and J. Matousek, Talanta, 17, 363 (1970). 8. W. Smith, M. B. Blackburn, and J. D. Winefordner, Can. J. Spectrosc., 22, 57 (1977). P. K. Schenck, W. G. Mallard, and K. C. Smyth, unpublished results. C. A. van Dijk, P. J. Th. Zeegers, G. Nienhuis, and C. Th. J. Alkemade, J. Quant. Spectrosc. Radlat. Transfer, 2 0 , 55 (1978). L. M. Fraser and J. D. Winefordner, Anal. Chem., 44, 1444 (1972). M. J. Wirth and F. E. Lytle, Anal. Chem., 49, 2054 (1977). P. K. Schenck, W. G. Mallard, J. C. Travis, and K. C. Smyth, J . Chem. Phys., 8g, 5147 (1978). N. Omenetto, L. P. Hart, and J. D. Winefordner. A.m. / . Smctrosc., 28, . 612 (1972). C. Veillon, J. M. Mansfiekl, M. L. Parsons, and J. D. Winefordner, Anal. Chem., 38, 205 (1966). G. K. Billings, At. Absorp. News/., 4, 357 (1965). M. Kirk, E. G. Perry, and J. M. Arritt, Anal. Chim. Acta, 80, 163 (1975). J. Y. Marks, R. J. Spellman, and B. Wysocki, Anal. Chem., 40. 1474 (1976). G. G. Welcher, 0.H. Kriege, and J. Y. Marks, Anal. Chem., 46, 1227 (1974). J. Y. Marks, Pratt 8 Whltney Aircraft, East Hartford, Conn., personal communication.
RECEIVED for review April 23, 1979. Accepted July 13, 1979. This paper was taken in part from the dissertation written by G. C. Turk and accepted by the Graduate School, University of Maryland, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry. In order to adequately describe experimental procedures, it was occasionally necessary to identify commercial products by manufacturer's name or label. In no instance does such identification imply endorsement by the National Bureau of Standards nor does it imply that the particular products or equipment are necessarily the best available for that purpose.
