Anal. Chem. 1980, 52. 1354-1356
1354
also be verified for samples containing other elements as well. Of particular interest would be samples containing these three elements plus nitrogen, sulfur, and the halogens.
Table 111. Hydrogen Weight Fraction Results XI
WH
sample a
known
exptl
known
exptl
1 2 3 4 5 6 7 8 9 10
0.44458 0.44546 0.43521 0.45886 0.49048 0.44441 0.48425 0.46194 0.60738 0.55409
0.44571 0.44828 0.43360 0.46105 0.49111 0.44230 0.48083 0.46225 0.60800 0.55375
0.12582 0.13127 0.10412 0.13598 0.09152 0.13417 0.10839 0.13721 0.09743 0.11189
0.12834 0.13805 0.09941 0.14087 0.09243 0.12816 0.09984 0.13733 0.09821 0.11123 0.0047
standard error a See Table I for sample identities.
values as compared to the known values for each sample are given in Table 111. The standard error for hydrogen weight fraction is 0.0047. The proposed approach gives a standard error for hydrogen weight fraction of 0.0047 for 10 samples that each contain hydrogen, carbon, and oxygen. This indicates that the approach is valid for samples of this type. The approach should
LITERATURE CITED Gray, P. R.; Chrey, D. H.; Beamer. W. H. Anal. Chem. 1960, 32,582. Gardner, R. P.; Dunn, J. W . , 111. Anal. Chem. 1965, 37,528. Gardner, R. P.; Roberts, K . F . Anal. Chem. 1966, 3 8 , 923. Jacobs, R. 6.; Lewis, L. G.; Piehl, F. J. Anal. Chem. 1956, 28, 324. Rowan, R., J r . Anal. Chem. 1959, 3 1 , 558. Smith, V. N.; Otvos,J . W . Anal. Chem. 1954, 26,359. Gardner. R. P. Anal. Chem. 1967, 39,668. Muller, R. H. Anal. Chem. 1957, 29. 969. Muller, D. G. Anal. Chem. 1957, 29, 975. DeLigny, C.L.; Levering, R.; DeHoyer. H. R e d Trav. Chim. Pays-Bas 1965, 8 4 , 503. Saldick, J.; Allen, A. D. J. Chem. Phys. 1954, 22,438.
R. P.Gardner* H. M. Lee Center for Engineering Applications of Radioisotopes Box 5636 North Carolina State University Raleigh, North Carolina 27650 RECEIVED for review December 26, 1979. Accepted March 28, 1980.
Atomic Resonance-Line Lasers for Atomic Spectrometry Sir: Most fixed-frequency atomic lasers, such as HeNe, Cu, Pb, and Hg, produce laser light at atomic wavelengths of little or no interest to the analytical spectroscopist. In particular, for analytical measurements it is desirable to excite from a ground atomic state because of preferential population of this level. However, for a variety of kinetic reasons, in a free-atom laser it is generally impossible to produce laser action from transitions which terminate in the ground state (resonance-line transitions). A recently developed class of lasers (1-6) circumvents the difficulty by directly populating metal atom excited states through UV photodissociation of a metal halide ( r ) , thereby producing an initial population inversion in the atomic species. Eventual buildup of the ground state population terminates the resonance transition laser pulse, but recombination with the free halogen atoms completes the cycle and depletes the atomic ground state before the next photodissociation pulse. To date, lasing action has been achieved for the resonance transitions of four elements of group 1 (Na (2, 4 ) , K ( 4 ) , Rb ( 4 ) ,and Cs ( 4 ) )and four from group 3a (T1 (3), In ( I ) , Ga (5, 6),and A1 (6)). Extension of the technique to other atoms with volatile parent salts is probable. The appeal of these lasers for analytical spectrometry is heightened by their simplicity. The atomic resonance laser used here consists of a 10 cm X 2.2 cm diameter spectrophotometer cell, evacuated except for a small amount of the appropriate metal halide salt. The cell is heated in a tube furnace to establish the optimum molecular vapor pressure. The laser is generally pumped by an excimer laser, but flashlamp pumping has been demonstrated (8) and is under further investigation. The gain of the laser is so great that no optical cavity is required. No degradation with time and/or usage has yet been observed for atomic resonance lasers. The intent of the present paper is to alert analytical spectroscopists to the existence and potential of metal-atom resonance-line lasers. Laser enhanced ionization (LEI) (9-1I ) was chosen for its simplicity as the means of demonstrating the application of these lasers to atomic analysis. Although 0003-2700/80/0352-1354$01 .OO/O
Table I. Operating Characteristics and Analytical Results
f o r TlI and NaI Lasers
resonance wavelength(s), nm energy/pulse, pJ nonresonance wavelength(s), nm energy/pulse, FJ operating temperature, " C element analyzed limit of detection, ng/mL linear dynamic range, orders of magnitude
TlI
NaI
377.6 9 535.0 30 380
589.0, 589.6 0.1 1140.3, 1138.2 5 660
T1 3 -4
Na 0.3 54
the low ionization potentials of the group 1 and 3a elements are favorable to LEI (12),the short (5-10 ns) pulse length of the atomic resonance laser (8)is not optimal for LEI (13),and laser induced fluorescence (LIF) ( 1 4 ) might have produced superior limits of detection. On the other hand, the multiple wavelength characteristics of these lasers may well be of more advantage in LEI than in LIF, as demonstrated and discussed below. The apparatus used for the experiments is illustrated in Figure 1. The laser system (8) and LEI spectrometer (11) have been described in more detail elsewhere. The 193-nm wavelength light from a 10-mJ ArF excimer laser was weakly focused into the heated atomic resonance laser (ARL) cell. A second lens beyond the ARL was used to collimate the laser output. A glass beam splitter provided light for a fast photodiode used to synchronize the gated detection with the laser and also blocked the residual 193-nm light from the excimer pump laser. Analytical data were obtained for T1, using a T1I laser, and for Na, with a NaI laser. Table I gives the previously reported (8) optimum operating temperature for each of these lasers and the output wavelengths and approximate energy at each wavelength of the lasers as used. Precise adjustment of the temperature to the optimum setting was not attempted with 'C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
11-1
oi,"lG
CHART RECORDER
1355
BOXCAR AVERAGER
Figure 1. Apparatus for laser enhanced ionization in flames with atomic resonance line laser excitation
30,000
Na
P
S
Table 11. Results of Color Filter Experiments
P
S
I
I-
I I
i
element TI
filter0 none 7-54 3-74
Na
-i e
20,000 535.0 nm
m
P e
0
s Ba 21
'
3 ~ 1 1 23,
1 / 2 7 1
I I
377.6 nrn
1
10,000
W
,
6p3/2 '\
I
\
589.6 nrn
I
0
Figure 2. Partial energy level diagrams for TI1 and NaI lasers, showing origination of atomic laser lines
the tube furnace, and the laser performance was significantly below the best previously reported values (8). The origins of the laser lines are illustrated in the partial energy level diagrams of T1 and Na shown in Figure 2. Calibration standards of from 0.1 to 100 pg/mL T1 or Na in water were aspirated into a premixed H2/air flame supported on a 5-cm slot burner head. The laser enhanced ionization signal was monitored by applying -700 V to a pair of plates which bracket the flame above the burner head. Laser induced pulses in the current taken from the burner head (anode) were first processed by an ac-coupled, lo6 V/A preamplifier (IO),and then a high-speed storage oscilloscope. The y-axis output feature of the oscilloscope allowed it to be used as a calibrated amplifier and bandpass filter between the pre-amplifier and the boxcar averager. For a sufficiently high T1 solution concentration, the signal could be observed on the fast oscilloscope without the bandwidth-limited pre-amplifier, yielding an observed actual pulse width of -200 ns. The pulsewidth was stretched to 1 ps by the pre-amplifier, and was additionally stretched to about 3 p s with a 0.3-MHz low-pass filter on the oscilloscope, following a brief study of signal-to-noise ratio behavior. A 10-kHz high-pass filter was employed t o discriminate against flame noise.
-
100 50
-
%T(hNR)c
signald
100
=lo0 45 3 =lo0 6.5 0.1.
0
0 -90
none
100
100
1-69 7-56
-85 0
0 -70
energy, PJ
35 4.5 25 4.5 0.1 3.3
\
I
I
\
%T(hR)b
a Identified by Corning Glass Number. Percent transmission quoted by manufacturer at the resonance line wavelength. Resonance wavelengths: Na = 589.0, 5 8 9 . 6 nm; T1= 3 7 7 . 6 nm. Percent transmission quoted by manufacturer a t the nonresonance wavelength, Nonresonance wavelengths: Na = 1 1 3 8 . 2 , 1 1 4 0 . 3 nm; T l = 5 3 5 . 0 nm. Expressed as percentage of signal without filter.
Signal averaging employed a 5-ps boxcar gate width and a 100-ps time constant-averaging approximately 20 shotsfor limits of detection and curves of growth. With the excimer laser operating a t - 2 Hz, the experimental averaging time constant was -10 s, and data acquisition required -50 s per standard or blank. Acquisition time could obviously be shortened by using a higher repetition rate excimer laser, which is commercially available. Measurements of the signal response to variations in laser energy and wavelength utilized a 10-ps time constant, for an - 2 shot running average. Thallium was found to have a limit-of-detection (LOD) of 3 ng/mL (Table I) and an approximately linear response up to 100 pg/mL, for a dynamic range of a t least four orders of magnitude. A slight downward trend in the curve of growth was observed a t the 100 pg/mL level. The LO11 for sodium was determined to be 0.3 ng/mL (Table I), and the curve of growth was approximately linear through 1pg/mI, but showed a pronounced ( 6 2 % ) decrease from the linear extrapolation a t 10 pg/mL. A useful dynamic range of nearly four orders of magnitude was thus demonstrated. Limits of detection were taken as the concentration which would yield a signal amplitude three times greater than the rms noise level of the blank. Corning glass color filters were used to examine the dependence of signal on each of the two wavelengths of each laser. These results, along with the specified transmission of the filter and transmitted energy measurements, are summarized in Table 11. Here the different relationship between the resonant and nonresonant transitions in the group 3a (Tl) and group 1 (Na) elements causes a dramatic difference in signal behavior. From Figure 2 it may be seen that the two
1356
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
thallium lines share the same upper state, while the two sodium transitions are sequential, sharing an intermediate level. As a result, the two thallium lines yield essentially additive signal (Table IIj, while the sodium signal evidently depends heavily on an interaction between the two transitions. This enhancement of LEI by stepwise excitation results from the exponential increase in the collisional ionization rate as the energy required to ionize decreases, and has been examined in more detail elsewhere (13). Finally, comparison of the signal from the two wavelengths of a common upper state system-such as T1-can, in principle, be used to locally map the temperature of a flame (15)or reactor. In the present case, assuming a statistical population distribution, the T1 535.0/377.6 nm signal ratio (see Table 11) is consistent with a spatially-averaged temperature in the H2/air flame of -2000 K. An energy dependence study for T1, extending down three orders of magnitude from the energy reported in Table I, showed a linear response of signal to laser energy. Thus, neither optical saturation nor neutral atom depletion (12) was indicated a t maximum energy. The energy dependence for Na was complicated by the combined effect of the two wavelengths in producing the signal, as discussed above. An overall slope of 1.7 in the log-log plot confirmed the conclusion reached with the color filter experiment concerning the dual wavelength interaction. A slope of 2.0 is anticipated for small-signal sequential absorption of two photons. The departure from this value could indicate that partial saturation of the 1.14-pm transition is occuring a t the highest fluences. Since our purpose is to demonstrate the potential of the atomic resonance-line photodissociation laser, comparisons with the dye laser are in order. The optical properties of the excimer-laser-pumped photodissociation laser are similar to those of the familiar nitrogen-pumped-dye-laser, which has been used extensively in analytical studies of laser induced fluorescence (14) and laser enhanced ionization in flames. The principal differences of the atomic resonance laser from the dye laser are that for the former (1)no wavelength tuning is required and no wavelength drift is possible, (2) the spectral bandwidth under typical operating conditions is -0.0024.008 nm, somewhat less than the absorption linewidths for analytical atom reservoirs, and (3) transition wavelengths other than the resonance line are generally present. The ability to wavelength-scan a dye laser is, of course, a useful facility for identifying, and correcting for, spectral interferences. However, the multiline output of the ARL can also be utilized to enhance selectivity in many circumstances. Indeed, Na determination by LEI is just such a case. For a real sample with potential spectral interference, color filters could be used to distinguish the two-wavelength signal from interfering signals arising from either color alone. Similar techniques could be employed in many cases for absorption or fluorescence measurements as well. Moreover, the possibility of spectral interference is minimized by the narrow bandwidth of the ARL in the first place. The ARL also invites comparison with the hollow cathode lamp. Physically, the laser resembles the lamp in size and shape, and should be of comparable cost to manufacture commercially. The fact that the laser must be heated is of little consequence, except for the temperature equilibration time. Of course, the excimer laser pump source for the ARL is more complex and expensive than the power supply for the lamp, but even this drawback may be diminished by the use
-
of flashlamp pumping for the lasers (8). As to performance, the laser bandwidth (8) is comparable to that of the lamp (16),and there the similarity ends. The spectral irradiance, coherence, and spectral purity of the laser favor it for such applications as remote fluorescence or long-path absorption in large-scale practical combustors, atomic flame fluorescence, and laser enhanced ionization in analytical flames. The potential advantage of using the multipass capability of the laser for conventional flame atomic absorption is negated by the usual pulse-to-pulse variability ( - 1-570 j characteristic of pulsed lasers. The ultimate analytical impact of the atomic resonance line photodissociation laser will depend on the number of elements for which such lasers are eventually constructed, the further development of flashlamp pumping, and the demand for metal analyses in hostile environments or a t higher sensitivity than conventional methods provide. In the meantime, the lasers demonstrated here, and other available group 1and 3a lasers, may well be the source of choice for many specific dedicated applications.
ACKNOWLEDGMENT We thank D. J. Sullivan for technical assistance and T. F. Deutsch for several useful discussions. LITERATURE CITED (1) Burnham, R. Appl. f h y s . Lett. 1977, 30, 132-134. (2) White, J. C. Appl. fhys. Lett. 1978, 3 3 , 325-327. (3) Ehrlich, D. J.; Maya, J.; Osgood, R. M. Appl. fhys. Lett. 1978, 33, 931-933 -. - . (4) Ehrlich, D. J.; OsgoOd, R. M. Appl. Phys. Lett. 1979, 3 4 , 655-658. (5) Hemmati, H.; Collins, G. Appl. Phys. Left. 1979, 3 4 , 844-845. (6) Deutsch, 37R-RRO T. F.; Ehrlich, D. J.; Osgood, R. M., Jr. Opt. Lett. 1979, 4 ,
-
--
- . - ---.
(7) Zare, R. N.; Herschbach, D. R. I n "Applied Optics Supplement No. 2: Chemical Lasers", Shuler, K. E., Bennett, W. R., Jr., Eds.; American Institute of Physics: New York, 1965; pp 193-200. (8) Ehrlich, D. J.; Osgood, R. M. Jr. I€€€ J . Quant. Electron. 1980, Q€-16. 257-268. (9) Green, R. B.; Keller, R. A,; Schenck, P. K.; Travis, J. C.; Luther, G. G. J . Am. Chem. SOC. 1976, 98,8517-8518. (10) Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1978, 50. 817-820. (1 1) Turk, G. C.; Travis, J. C . ;Devoe. J. R.; O'Haver. T. C. Anal. Chem. 1979, 51, 1890-1896. (12) Travis, J. C.; Schenck, P. K.; Turk, G. C.; Mallard, W. G. Anal. Chem. 1979. 51. 1516-1520. (131 Turk.'G. C.: Mallard. W. G.: Schenck. P. K.: Smvth. K. C. Anal. Chem. 1979, 5 1 , 2408-2410. (14) Weeks, S. J.; Haraguchi, H.; Winefordner, J. D. Anal. Chem. 1978, 50, 360-368 - - - - .- .
(15) Haraguchi, H.; Smith, B.; Weeks, S.; Johnson, D. J.; Winefordner, J. D. Appl. Spectrosc. 1977, 3 1 , 156-163. (16) West, C. D.; Human, H. G. C. Spectrochim. Acta, fart 5 1978, 31, 81-92.
D. J. Ehrlich R. M. Osgood, Jr. Lincoln Laboratory Massachusetts Institute of Technology Lexington, Massachusetts 02173
G. C. Turk J. C. Travis* Center for Analytical Chemistry U S . National Bureau of Standards Washington, D.C. 20234
RECEIVED for review March 12,1980. Accepted April 11,1980. This work was sponsored in part by the Department of the Air Force. The specification of commercial products does not imply endorsement by the National Bureau of Standards.
