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Anal. Chem. 1985,57,2397-2399
(7) Here, Xk is an 112 X q matrix and Yk is a n X q matrix, q (h = 1, ...,q ) is the rank of the matrix Nk. Equation 7 may also be written as Now we may select any one of the matrices on the right to reformulate the RAFA problem as
M = %,kYh,kT - AkD
(9)
By doing this we return to the rank one method. Another alternative to cope with this problem is to use all factors and define A as UT& and B as oqk. By use of the same reasoning as in ref 8, the resulting eigenvalue problem is
ABTS-lz = C Y Z
(10)
It may be easily proved that Ah is equal to = tr(ABTS-l)/q
(11)
here tr(*) denotes the sum of the diagonal elements. Standard Additions with Two-Dimensional Data. Traditionally, there have been two approaches to the quantitation of chemical species: calibration curve methods and the method of standard additions. The method of standard additions is the method of choice when the amount of information on the sample is minimal. The method was generalized by Saxberg and Kowalski (11) to the generalized standard addition method (GSAM) that uses one-dimensional data to correct for interferences. The GSAM is limited, as all one-dimensional methods, by the need to know prior to the analysis all the components in the sample. Thus, its applicability to complex samples is not general. The method of standard additions also suffers from an inherent disad-
vantage: the response of the analytical sensor must be “zeroed. This requirement is the major source for inaccuracy in the determined concentration. Quantitation by RAFA with standard addition not only permits determination of a target component without the knowledge of the other species present in the sample but also eliminates the need for “zeroing”. Therefore, the combination of two-dimensional data with calibration by standard addition provides a powerful tool for quantitation of complex samples. The quantitation procedure by standard additions consists of two steps: (a) the matrix, Nh, is obtained by subtracting the data of the sample to which a known amount, Cko, of the “component of interest” was added from the data of the original sample; and (b) application of RAFA by using eq 3 and eq 5 to determine the concentration in the sample.
LITERATURE CITED (1) Warner, I. M. I n “Contemporary Toplcs in Anaiytlcal and Cllnlcal Chemistry”; Hercules, D. M., HleftJe,G. M., Snyder, L. R., Evenson, M. A., Eds.; Plenum Press: New York, 1982; Voi. 4, p 75. (2) Sharaf, M. A,; Kowalski, B. R. Anal. Chem. 1982, 5 4 , 1291. (3) Ho, C.-N.; Christlan, G. D.; Davldson, E. R. Anal. Chem. 1978, 5 0 , 1108. (4) Lawton, W. H.; Sylvestre, E. A. Technometrlcs 1971, 13, 617. (5) Ho, C.-N.; Christian, 0. D.; Davidson, E. R. Anal. Chem. 1980, 5 2 , 1071. (6) Ho, C.-N.; Christian, G. D.; Davidson, E. R. Anal. Chern. 1981, 53, 92. (7) McCue, M.; Mallnowski, E. R. J. Chromatop. Sci. 1983, 2 1 , 229. (8) Lorber, A. Anal. Chlm. Acfa 1984, 164, 293. (9) Lawson, C. L.; Hanson, R. J. “Solving Least Squares Problems”; Prentlce Hall: Englewood Cllffs, NJ, 1974. (IO) Mallnowskl, E. R.; Howery, D. G. “Factor Analysis in Chemlstry”; Wiley: New York, 1980. (11) Saxberg, B. E. H.; Kowalskl, B. R. Anal. Chem. 1979, 5 1 , 1031.
Avraham Lorber Nuclear Research Centre-Negev P.O. Box 9001 Beer-Sheva 84190, Israel
RECEIVED for review March 4,1985. Accepted June 3,1985.
Double Resonance Multiphoton Ionization Determination of Mercury Vapor Sir: High-power lasers have been used for the multiphoton ionization of mercury vapor using a number of different excitation pathways. These have included nonresonant ionization using a ruby laser ( I , Z),four- and five-photon ionization with three- and four-photon resonances using a tunable dye laser (3, 4 ) , and three-photon ionization with a two-photon resonance using a frequency-doubled dye laser (5). Only one of these (5) has made an estimate of analytical detection limits and this was based on measurements at a single concentration some 3 X lo3times as large as the projected detection limits. In this correspondence a laser-based method for the photoionization of Hg is reported which uses two single-photon resonances and thus avoids the high-power densities required for the efficient excitation of multiquantum transitions. The excitation pathway used in these experiments is shown in Figure 1 and is similar to the previously reported double-resonance multiphoton ionization of rubidium (6). The sequential absorption process 6s2 l S 0
- - Xl
6p P1
XZ
7s 3S1
XZ
Hg+ + e-
is effected by overlapping two dye laser beams of wavelengths 253.73 nm (first step) and 435.95 nm (second and third steps). It should be noted that the final step leading to ionization is
enhanced by near resonance with the broad 6p’3P10autoionizing level (7). There are several features of this excitation pathway which make it attractive for routine analytical measurements: (1)As only one-photon transitions are used, relatively low power lasers may be used to achieve subpicogram detection limits. (2) The use of low laser power reduces background from nonresonant ionization processes. (3) The use of two atomic resonances gives a very high degree of selectivity (8, 9): with relatively modest laser bandwidths of 0.3 cm-l, an overall spectral selectivity of -5 X lo9 may be achieved, essentially eliminating the possibility of coincidental double resonances from other species. Actual analytical selectivity will depend on a variety of factors such as the ionization potentials and off-resonance cross sections for possible interfering species. (4) The low power lasers used are desirable for routine operation because of long-term reliability, stability, high repetition rate, and low consumption of dyes and electrical power. In addition to the development of the laser-induced ionization process, a procedure for the analysis of discrete air samples has been developed. This procedure is based upon
0003-2700/65/0357-2397$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
loo
r
axis and 3 mm away from the wall. This wire is held at a potential of +700 V by a regulated high-voltage power supply. Electrons
7s 'S"
3s1
\
4 3 5 9 5 nm
2
'
0
/
6~
3pc
253 73 nm
Flgure 1. Partial energy level diagram for mercury showing the double-resonance excitation pathway.
Sample &
Flgure 2. Apparatus for double-resonance ionization detection of atmospheric mercury. Scale is distorted to show details of sample preparation and ionization cell. Abbrevlatlons used are as follows: SHG, second harmonic generation crystal; A.C., activated charcoal; PSA, pulse shaping amplifiers; MPX ADC, multiplexed analog to digltal converter.
injecting an air sample into a He carrier gas stream which then passes through a small activated charcoal column. The Hg adsorbed on the column is released back into the carrier gas stream by rapid heating and is then transported into the laser interaction region. With this technique, known Hg samples as small as 15 pg have actually been measured and calculated detection limits are on the order of 0.2 pg.
EXPERIMENTAL SECTION A schematic of the experiment is shown in Figure 2. A nitrogen laser (8 ns fwhm pulses, 600 kW peak power, 30 Hz repetition rate) is used to simultaneously pump two Hansch configuration (IO)dye laser systems. The first dye laser is operated at 507.46 nm with coumarin 503 dye and is frequency doubled in a lithium formate monohydrate crystal to produce -1-wJ pulses at the 253.73-nm wavelength of the Hg resonance line. The second dye laser is tuned to a wavelength of 435.95 nm using coumarin 440 dye and has a pulse energy of 120 pJ per pulse. The bandwidth of both dye lasers is -0.3 cm-'. The two laser beams are directed toward the ionization cell from opposite directions and the apparatus is adjusted with an optical delay line such that the 435.95-nm pulse arrives at the cell approximately 2 ns after the 253.73-nm pulse. The ionization cell, which is open-ended without windows, is constructed from a 3 cm length of 1 cm i.d. brass tubing. The central conductor of a coaxial cable (RG-59U) is passed into the cell and bent at a right angle such that it is parallel to the tube
-
collected at the wire are detected with a capacitively coupled, charge-sensitivepreamplifier (ORTEC 142 PC) followed by a pulse shaping amplifier (ORTEC 572). The ionization and laser intensity signals are recorded with sample-and-holdmodules which are read with a 12-bitmultiplexed analog-to-digital converter and stored on floppy disk by a minicomputer (DEC PDP-11/03) for further data analysis. For each firing of the laser system, the ionization signal is normalized linearly to the pulse energy of the first dye laser and quadratically to that of the second dye laser. The helium carrier gas stream is introduced into the ionization cell through a 4 mm i.d. quartz tube on the side opposite the detection wire. The quartz tube is an extension of the column of activated charcoal (2 cm length) used to trap Hg vapor. The column is wrapped with nichrome resistive heating wire (0.4 mm diameter, 4 turns/cm) which is connected t o a variable transformer. At the front end of the column the quartz tube is expanded to 8 mm i.d. and fitted with a rubber septum for sample injections. The carrier gas which enters the injection region is prefiltered through a large (- 100 cm3) activated charcoal trap to remove any traces of Hg and water vapor. This section of the apparatus (activated charcoal column, injection port, and carrier gas trap) is constructed from a single piece of quartz to prevent Hg adsorption on metal surfaces. Ultrahigh-purity He gas is used and flow rate is controlled at 140 mL/min with a precision variable leak valve (Granville-Phillips 203). A t this flow rate the atoms reside in the ionization cell for a little over half a second, long enough to experience 20 shots from the laser system.
RESULTS AND DISCUSSION Tuning of both lasers to the correct Wavelength is relatively simple. The procedure involves loading the column with a sample of approximately 1 ng of Hg, which is then eluted by mild heating (130 O C ) to provide a continuous supply of Hg atoms. The first dye laser is tuned to approximately the correct wavelength (507.5 f 0.2 nm) using a reversion spectroscope (Ealing/Beck 3505). The second harmonic is focused into the ionization cell with a 5 cm focal length lens. Under these conditions, a three-photon ionization signal can be observed when only the first photon is in resonance at 253.73 nm. Once the first resonance is found, the focusing lens is removed (the ionization signal disappears) and the second laser beam is overlapped with the first within the cell. Similarly, the second laser is coarsely tuned with the reversion spectroscope and fine tuned by observing the reappearance of the ionization signal when the second resonance is reached. When both lasers are at resonance, other factors such as beam overlap, dye laser tuning and alignment, and frequency doubling crystal alignment may be optimized by maximizing the ionization signal. The procedure for the analysis of samples is as follows. Air samples are injected onto the activated charcoal column where the Hg is trapped. After 15 s the column is rapidly heated to 600 "C and held at this temperature for 30 s. During the heating cycle the Hg elutes within the first 5 s. The remaining portion of the heating cycle assures that the column is clean when the next sample is injected. After heating, the column is cooled for 2 min under forced (external) air flow and is then ready for the next sample injection. Overall sample cycle time is approximately 3 min. Figure 3 shows the results for a series of standard samples. These standards are obtained by injecting small quantities (1-10 pL) of air saturated with Hg vapor a t 22 "C. Since the vapor pressure of Hg is well-known ( I I ) , the absolute quantity injected may be easily calculated. While peak height response is not linear with sample size, the integrated peak area does exhibit a linear relationship. Linear least squares fitting of the integrated peak areas has a coefficient of determination of 0.996. However, there is a nonzero intercept corresponding to -10 pg of Hg which is attributed to Hg adsorbed on the
2399
Anal, Chem. 1985, 57, 2399-2403
-._
r
n
6
91
I-,",",4
TirneIonization signal as a functlon of time as a serles of standards were eluted from the column. The polnt labeled B is for a normal heatlng cycle with no sample injection. The peak labels are mlcroliters of saturated air (15.4 pg of HgIpL) injected. Data are recorded only durlng the heating portion of the sampling cycle.
Flgure 3.
exterior surface of the injection needle. This is confirmed by the fact that no peak is observed when the apparatus is run through the heating cycle without an injection (the point marked "B" in Figure 3), yet when the needle is inserted into the injection port, but no air volume is injected, a small and somewhat variable signal is observed. This situation might be improved if a nonmetallic injection needle were used. The method was used to measure Hg concentrations in the ambient laboratory air. The 100-mL air samples were found to contain from 40 to 70 pg of Hg (0.4-0.7 ng/L). While this is approximately 10 times the level found in what are considered "unpolluted" environments (12),it is still more than a factor of 100 below maximum acceptable levels (13). The ultimate detection limits may be estimated from the data shown in Figure 3. The integrated signal for the smallest standard used (1pL of air, 15.4 pg of Hg) is 3.98 (arbitrary units), while the standard deviation for integration of a series of blank cycles is f0.029. Thus, at a signal-to-background noise ratio of 2 the calculated detection limit is 220 fg of Hg. From the interaction volume and amplifier sensitivity, the overall ionization efficiency was estimated to be -5 X lo-*. This corresponds to the production of 10 ions per laser pulse
-
at the detection limit. To make actual measurements at levels near the detection limit, steps must be taken to ensure that the sample injection syringe and needle are free of residual Hg. Comparison of this method to the accepted procedures of flameless atomic absorption (12) shows approximately 3 orders of magnitude increase in sensitivity. To make a comparison with static three-photon ionization experiments (5), the Hg peak elution time was divided by the carrier gas flow rate, thus yielding an effective static concentration. Including considerations for signal integration periods, the detection limits reported in this work were roughly a factor of 2 better even though the total laser pulse energies were a factor of 6 less. Further, the three-photon ionization measurements were made under high vacuum conditions with a high gain (channeltron) detector. At atmospheric pressure, three-photon ionization might be expected to fare considerably worse due to the lower sensitivity of an ion detector that is capable of operating under these conditions and due to increased background ionization. Registry No. Hg, 7439-97-6.
LITERATURE CITED Chin, S. L.; Isenor, N. R.; Young, M. Phys. Rev. 1969, 788, 7-8. Nobata, K.; Saegusa, K. Jpn. J. Appl. Phys. 1978, 55, 1485-1489. Tai, C.; Dalby, F. W. Can. J . Phys. 1977, 55, 434-435. Dalby, F. W.; Sanders, J. H. Opt. Commun. 1981, 37,261-264. Miziolek, A. W. Anal. Chern. 1981, 53, 118-120. Whitaker, T. J.; Bushaw, B. A. Chem. Phys. Lett. 1981, 79, 506-508. Berkowitz. J.; Lifshitz, C. J. Phys. 6 1988, 1 , 438-440. Letokhov, V. S.; Mlshln, V. I.Opt. Commun. 1979, 29, 168-171. Miziolek, A. W.; Wlllis, R. J. Opt. Left. 1981, 6 , 528-530. Hansch, T. W. Appl. Opt. 1972, 7 7 , 895. Weast, R. C., Ed. "Handbook of Chemistry and Physics", 60th ed.; CRC Press: Cleveland, OH, 1979; p D-198. (12) Fitzgerald, W. F.; Gill, G. A. Anal. Chem. 1979, 51, 1714-1720. (13) Holden, A. V. IAEC Tech. Rep. Ser. 1972, 737, 143-165. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Bruce A. Bushaw Pacific Northwest Laboratory Richland, Washington 99352 RECEIVED for review April 11,1985. Accepted May 29,1985. This work was supported by the Office of Health and Environmental Research of the U S . Department of Energy under Contract DE-AC06-76RLO-1830.
Concomitant Effects in Analyses of Aqueous Solution Residues by Atomic Emission Spectrometry with Electrically Vaporized Thin Metal Films Sir: Thin metal films electrically vaporized by capacitive discharges can serve as vaporization and excitation sources for atomic emission spectrometry. In recent publications, their successful applications to analyses of refractory powders ( I ) , river sediments, coal, flour, and spinach ( Z ) , and acidic saline solutions and other waters (3) have been discussed. Samples of solid particles weighing up to 1 mg, suspended in 2propanol, can be analyzed with good accuracy and minimal concomitant effects ( 2 ) . Difficulties arise, however, in the analysis of aqueous solution residues weighing much more than 500 ng, probably because they are in the form of localized thick salt crusts that are difficult to vaporize completely ( 2 , 3 ) . The formation of thick crusts prohibits direct determination of trace metals in saline solutions without a separation step (3). In the study reported here, effects of added salts on analyte line intensities for a number of metals in aqueous solution were
investigated. The effects of three film materials, Al, Ag, and Au, three !"lm geometries, and two discharge types on line intensities and vaporization of salt residues were also studied in attempts to elucidate the reasons for the difficulties observed.
EXPERIMENTAL SECTION Discharge Circuit. The discharge circuit and film holding cassette have been described in detail ( I , 4 ) . Support gases were dry air at 700 torr and a flowing mixture of 40:60 02-Ar at atmospheric pressure. The discharge chamber and discharge characteristics have been described (1, 2, 4 ) . Thin Film Production. Duchane and Sacks ( 4 ) and Clark and Sacks (1)have reported on production of Al and Ag thin films. Brewer and Sacks (3) have treated thin Au film production. Film properties as well as discharge conditions are summarized in Table I.
0003-2700/85/0357-2399$01.50/00 1985 American Chemical Society
