AMI, Chem. 1991, 69,1607-1611
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Single- and Double-Resonance Laser-Induced Ionization of Phosphorus Monoxide in an Air-Acetylene Flame for the Determination of Phosphorus Gregory C. Turk Center for Analytical Chemistry, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Lasor-lnducd lonlzatlon of phorphorw monoxlde in an alracdyl"Hann has beon hv.r5lgatd as a tecMque for the determlnatbn d In aspIratad soluths. Two a p proaches to lasorhducod lonhatlon were usod-resonantiy enhanced muItlphoton knlzatkn (REMPI) udng B-X trandtlons between 319 and 325 MI and doublaresonance laserenhancod M r a t l o n (LEI) udng tho BaE+-X2n, trcmdtkn togofher wlth W n , - B T or D*n,-Bq+ trandknr between 556 and 565 nm. The best ihnIts of detection for P are 200 ng/mL udng REMPI and 30 ng/mL urlng doubbresonanco LEI. SdectlvHy for P In varkur metal matrkes was studled, showing severe selectivity problems Wng the REMPI a p proach and rlgnnicant Improvements by LEI.
INTRODUCTION Flame spectrometrists have often found it necessary to utilize the band spectra of diatomic and triatomic molecules, rather than atomic line spectra, for the determination of non-metals. The higher population of the molecular forms of the non-metal analyte and the vacuum ultraviolet wavelengths of the atomic resonance lines are the primary reasons for this approach. In the case of phosphorus, the band spectra of the molecules HPO and PO (I, 2) have been utilized for flame emission spectrometry, and molecular fluorescence measurements of PO have been made (3).Atomic lines of P have ale0 been uwd for flame emission (2) and absorption (4) measurementa. In all casea however, sensitivity is inadequate for trace analysis. Limits of detection for aqueous standard solutions generally fall in the range of tens of mg/L. A 1000-fold improvement in this level of sensitivity for phosphorus is available by using the inductively coupled plasma or the direct current plasma for atomic emission. Consequently, there has been very little interest in recent years in the use of the flame for the determination of phosphorus, or non-metals in general, and little research in the area of molecular spectrometry in flames. The development of new laser-based spectroscopic techniques offers new possibilities for molecular flame spectrometry. Smyth and Mallard have reported on the detection of PO in a premixed air-acetylene flame using resonantly enhanced multiphoton ionization (REMPI) (5). In their measurement, the excited B22+ state of the PO molecule was populated by single photon absorption from the ground state, and photoionization of the excited molecules occurred by absorption of a second photon of the same wavelength. A total of 31 distinct band heads were observed in the 302-334-nm wavelength region, using the phosphine contamination in acetylene as the source of the PO and flame conductivity detection. Chou et al. also studied PO in an air-acetylene flame by using a two-frequency REMPI process in which a tunable laser was used to populate the B state and a fixed frequency laser at 266 nm was used to photoionize the excited PO molecules (6).
In this paper, the analytical utility of REMPI of PO is evaluated in terms of sensitivity and selectivity for the detection of P in aspirated solutions. In addition, the first use of double-resonancelaser excitation of PO is reported. This excitation scheme uses two lasers to excite the ground-state molecules first to the B state and then further to the D211, state or the D W , state. From the higher energy state, the molecule is ionized either by collision (laser-enhanced ionization) or by absorption of a second photon of either wavelength. The sensitivities and selectivities of the single- and double-resonance excitation procedures are compared. EXPERIMENTAL SECTION The instrumentation utilized in this work is identical with that used for laser-enhanced ionization (LEI) spectroscopy of metal atoms (6-9). The second-harmonic output of a Q-switched NkYAG laser with a pulse energy of 140 mJ and a pulse repetition rate of 10 Hz was split into two beams of equal energy in order to pump two dye lasers. Dye laser number 1was operated with the dye DCM and was frequency-doubled to provide tunable laser radiation between 319 and 325 nm,with typical pulse energies of 50 rJ. Dye h e r number 2,with rhodamine 6G dye, provided laser light from 557 to 565 nm,with typical energiea of 2 mJ/pulse. The laser beam diameter of dye laser number 1was varied by focusing the beam, in order to vary the laser irradiance. Four levels of focusing were utilized and are referred to throughout the paper as tight focus, mild focus, weak focus, or defocused. For tight focus, the beam was passed through a 25 cm focal length biconvex lens 25 cm from the center of the flame, yielding an average beam diameter of approximately 0.5 mm through the flame. For the mild focus, the same lens was moved to a position 36 cm from the center of the flame, and the average beam diameter through the tlame increasedto approximately 1.2 mm. Weak-focus conditions were achieved by moving the same lens to a position 46 cm before the center of the flame, yielding an average beam diameter of 2 mm. A beam-expanding telescope was used to produce the defocused beam,with a diameter of approximately 3 mm. The beam from dye laser number 2 was only used unfocused and was apertured to a beam diameter of approximately 3.5 mm. For the double-resonance experiments, the defocused beam of dye laser number 1was aligned collinearly with the beam of dye laser number 2 through the flame. The ltlame was a premixed air-acetylene flame supported on a 5cm dot burner head, with a pneumatic nebulizer used for liquid sample introduction at a rate of 3 mL/min. Phosphine-free acetylene was used (Atomic Absorption Grade, Air Products and Chemicals, Inc., Allentown, PA). Flame ionization was detected by monitoring the electrical current conducted through the flame between a water-cooled cathode (IO)and the burner head (the anode). The cathode was positioned 13 mm above the burner and held at a potential of -IO00 V. The laser beams were aligned approximately 3 mm below the cathode. A 108 V/A current-to-voltage converter (11)was used as a preamplifier for the laser-induced ionization pulses. The upper frequency reapom of thisdevice was approximately 1 MHz, which stretched the ionization signal pulse duration to approximately 1fis. A computer-interfacedgated integrator, triggered by the lae$er,was used for integration of the ionization pulses, analog-to-digitalconversion, and transfer to a microcomputer on a single laser shot basis. The microcomputer was programmed
Thlr artlck not rub)sct to U.S. Copyright. Publ)shed 1991 by ttw, American Chemical Society
ANALYTICAL CHEMISTRY, VOL.
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F e r 1. REMPI spectrum of the B-X i " of PO obtained while a sdutkn containing lOccg/mL of P wasaspiratedinto an &-acetylene flame. A tightly focused laser beam was used.
to perform signal averaging and collect spectra through motorized control of the grating drives of the two dye lasers. Typically, 10-shot (1-s) signal averaging was utilized. The laser pulse energy of dye laser number 1, after frequency doubling, was measured with a pyroelectric laser energy meter and that of dye laser number 2 was measured with a thermal joulemeter. Laser power studies were performed by varying the laser power over a range of approximately a factor of 10 with neutral density filters or a laser power attenuator while the laser-induced ionization signal and the laser pulse energy were simultaneously measured for 500 laser shots by using two channels of the computer-interfaced gated integrator. Phosphorus standard solutions were prepared by dilution of a stock solution of ammonium phosphate (reagent grade, 99.2% purity) dissolved in distilled water.
RESULTS AND DISCUSSION Figure 1 shows a portion of the REMPI spectrum of' PO for a P solution of 10 I.cg/mL using a tightly focused laser beam. In agreement with the observations of Smyth and Mallard (51, the most prominent feature in this portion of the spectrum is the band head at 320.42 nm, which is a 5-4 vibrational transition, while the 0-0 band head at 324.64 nm is not clearly observed. As Smyth and Mallard discussed, the 0-0 band would be expected to be more intense than the observed 5-4 band by 4 orders of magnitude on the basis of Franck-Condon factors and Boltzmann populations. However, the photon energetics are such that absorption of a second photon from the excited state does not provide enough energy for photoionization in the 0-0 case but doea in t i e case of the 5-4 transition. Chou et al. (6) avoided this situation by using a second laser at 266 nm for photoionization. For clarity,a small portion of the spectrum near 321.7 nm has been deleted from the figure due to interference from a potassium doublet. Potassium contamination in the ammonium phosphate reagent used to prepare the solutions was the source of this interference. Much of the structure observed in Figure 1 is not due to PO but rather to REMPI of other flame constituent molecules, most likely nitrogen monoxide (12, 13). This is seen more clearly in Figure 2, which shows a higher resolution scan around the 320.42-nm PO band head together with the background spectrum obtained while only distilled water was aspirated. This flame background has an obvious deleterious effect on the suitability of REMPI for trace analyriis. The weak cross section of the photoionization step of the REMPI process requires the use of high laser irradiances, but this also enhances the background REMPI, and such background
320
320.2
320.4
320.6
320.8
Wavelength (nm) FI(pm2. The!i-4bendheadofthea-xtransltknofpofor1occg/ml P (upper trace) and the flame background REMPI measured while distilled water was aspirated (bottom trace).
Table I. Effect of Focusing on the Laser Power
Dependence of REMPI of PO degree of focus"
exponent of laser power dependence
tight mild
1.17 1.53 1.73
weak a
As defined in the Experimental Section.
spectra are observed throughout the ultraviolet region in the flame. Background REMPI can be minimized by optimizing the level of laser irradiance through control of the degree of focusing. Adjusting the focus of the laser beam has two effects. A tighter focus gives a higher irradiance but a smaller beam diameter and thus a smaller interaction volume. In a 1 + 1 REMPI process (one photon electronic excitation one photon photoionization), REMPI increases with the square of laser irradiance. A tighter focus increases REMPI at a rate that overcomes the loss of signal due to the smaller interaction volume. If, however, the laser irradiance is high enough to saturate the fmt (and easier) step of the REMPI process, then only the photoionization step responds to further increases in irradiance. In this situation the effects of irradiance and interaction volume cancel, and REMPI signal intensity is insensitive to changes in the tightness of focus. It is, however, preferable to use a weaker focus in order to minimize background REMPI. The laser power dependence of PO REMPI at 320.42 nm was studied by using tight focusing, mild focusing, and weak focusing (see the Experimental Section for a more quantitative description). The results are shown in Table I, which shows the laser power dependence in terms of the exponent to which laser power is raised to describe the proportionality between laser power and REMPI signal. This exponent was determined from the slope of the lag (REMPI) vs log (laser power) relationship. The results range from a near-quadratic dependence with the weakly focused laser beam to a near-linear dependence under the tightly focused condition. This indicates that the B-X transition is b e i i saturated at the higher irradiances, and therefore it would be preferable to use milder focusing. This conclusion is borne out in Figure 3, which shows the 5-4 band head for the same three focusing conditions. Going from tight focus to mild focus has very little effect on the magnitude of the PO signal but significantly lowers the
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Flgurr 3. The 5-4 band head of the B-X transltbn using tlgM (top), mlld (middle), and weak (bottom) focusing. The P concentration Is 10 pg/mL.
Table 11. Phosphorus Selectivity Ratios" matrix tight focus mild focus weak focus
co Cr cu Fe
Mn Na Ni Pb
9 0.2 0.1
23
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42 14 3 6 94
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34 68 1800 1 100 29OOO
a Tight focus, mild focus, and weak focus measurements were all single-resonance REMPI at 320.42 nm. The double-resonance measurement was done at 324.64 and 557.96 nm. The selectivity ratio is the ratio of phosphorus sensitivity to that of the matrix element at the DhosDhorus wavelenath.
REMPI flame background. Further defocusing does lower the PO signal but also lowers the background interference. More important than flame background REMPI is the effect that high laser irradiance can have on spectral background interferences from matrix constituents in a real sample. Extreme broadening of nearby atomic lines of matrix elements (7, 14) and the increased possibility of interference from transitions excited by multiphoton processes cause selectivity problems under high-irradiance conditions. An evaluation of the extent of such matrix interference was performed for eight possible matrix elements: Co, Cr, Cu, Fe, Mn, Na, Ni, and Pb. The sensitivity (signal per pg/mL) for P and for each matrix element at the 320.42-nmPO transition was determined experimentally, and the ratio of the P sensitivity to that of each potential interferent was computed. This was done for tight, mild, and weak focusing, and the results are given in Table 11. The selectivity ratios for the mild- and weak-focus cases are comparable but are significantly superior to those using tight focusing. However, it is clear that in all the cases the selectivity is very poor, and the determination of P in most real samples would be impossible without prior chemical separations. Some of the notable culprit lines causing interferences include Cu at 320.823 and 324.755 nm (a strong resonance line), Fe at 320.540and 320.047 nm, Na a t 330.237 nm, and Ni at 320.214 nm. The fundamental reason behind the poor selectivity is the very low cross section of the photoionization step, which requires the use of high laser irradiance. This could be avoided
557
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564
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Wavelength (nm)
Flgum 4. Second-step spectrum of PO using double-resonance laser excitetion with the first-step laser flxed at the 0-0 band head of the B-X transition at 324.64 nm. The spectrum shows D-B and D'-B transitions. The P concentration is 10 pg/mL. Ticks below the spectrum mark rotatlonel line positions of the 0-0 bands of D-B and D'-B transitions according to Verma and Dixit (77). For the D-B transition the branches are marked as follows: (1) R,,, N = 0-15; (2) R,, N = 1-33 with head at 11; (3) a , N = 2-36; (4) P,,, N = 3 3 3 (5) R,,, N = 0-33 with head at 1 4 (6) a,,,N = 1-34 wlth head at 6; (7) PQ,,, N = 21-30 (8) P,,, N = 2-33; (9) OP,,, N = 3-27. For the D'-B transition the branches are marked as follows: (1) Qe, N = 30-34;(2) Pa, N = 25-35; (3) Q11, N = 2(4) Q12, N = 23-33 (5) Pi,, N = 23-35.
if collisional ionization of the excited molecule could be utilized instead of photoionization, as is the case for LEI of atoms, or molecules (15),in flames. The ionization potential of PO is 8.14 eV. By use of the 320.42-nmPO transition to populate the fifth vibrational level of the B state, an additional 3.6 eV of energy is required for collisional ionization. The rate of such ionization at a flame temperature of approximately 2400 K is too low to result in any significant LEI within the duration of the laser pulse. Double-resonancelaser excitation to an electronic state closer to the ionization limit can greatly enhance the rate of LEI. With dye laser 1 tuned to the 0-0 band head of the B-X transition at 324.64 nm, dye laser 2 was scanned through a wavelength region where &O bands of D-B transitions were expected (16). The lowest vibrational level of the D state lies 2.0 eV below the ionization limit, and collisional ionization from this level is possible. The beam from dye laser 1 was expanded, and that of dye laser 2 is unfocused. Thus irradianceswere much lower than for the singleresonance REMPI measurements, and the interaction volume was greater. Figure 4 shows the resultant spectrum,obtained by using a 10 a/mL P solution. The spectrum is very complex, with over 70 discernable peaks in the wavelength region between 556 and 565 nm. No PO spectrum is detectable in this wavelength range without the f i t step of excitation from dye laser 1. An extensive analysis of the PO emission spectrum in this region has been published by Verma and Dixit (17). Rotational lines from nine branches of the 0 4 band of the D-B transition were reported, as well as from five branches of the 0-0band of the D'-B transition. The line positions and branch identifications according to Verma and Dixit are shown at the bottom of Figure 4. The great degree of overlap between the various branches obscures any clear band heads. A good agreement between the line positions of the LEI spectrum and the emission spectrum exists. A copy of the spectra on a larger format reveals the agreement more clearly and is available on request. No detailed comparison of relative line intensities between the LEI and the emission spectra has been made, but
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
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with the 85co&step laser fixed at 557.96 nm. The largest peak is the 0-0 band of the 8-X transition. The concentratbn of P is 10 NglmL.
Fl~urr7. DouMe-resonance LEI spectrum of PO for 10 pg/mL P. 400
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Fbwr 8. Laser power dependence for the secondgtep laser in double-re9onanceLEI of PO wlth the Rrst-step iawr at 324.64 nm and a 50 pJ/pulse. The fitted Hne was determined by using a quadratic
Fbwr 6. The 0-0 band of the E X transition of PO using double-
llnear regression.
resonance LEI with the second-step laser fixed at 557.96 nm. The phosphorus concentratlon is 10 ccg/mL (top trace). The background was measured by using distlHed water (bottom).
A close-up view of the 0-0 band head is shown in Figure 6 for a P concentration of 10 crg/mL, together with the baseline signal obtained while distilled water was aspirated. The signal
the photographic plate spectra published by Verma and Dixit do show clear band heads, indicating that some differences do exist. These differences are to be expected sin& there are considerable temperature differences between the two experiments. Publications subsequent to that of Verma and Dixit have indicated that the D’ state is actually a higher vibrational level of the B’ state (18,19). The older terminology has been used here in order to more simply establish the agreement between the LEI spectrum and the Verma and Dixit spectrum. With dye laser 2 fiied on one of the prominent second-step lines at 557.96 nm, dye laser 1was scanned through the same wavelength range where the B-X REMPI spectrum was f i i t scanned, using the same 10 pg/mL P concentration. The spectrum, shown in Figure 5, differs greatly from the REMPI spectrum seen in Figure 1. The most prominent feature is the 0-0 line at 324-64 nm, as expected from the high Franck-Condon factor. As a result of the much lower laser irradiance, the 5-4 REMPI line at 320.42 nm is not observed. Aside from the 0 line at 324.64 nm, the other lines observed in Figure 5 do not correspond to reported B-X transitions by REMPI (5) or emission (20) and have not been identified.
to background ratio is greatly improved relative to that seen in Figure 2 for singleresonance REMPI. A three-dimensional representation of a small portion of the double-resonance PO spectrum, where both lasei wavlengths are scanned (21), is shown in Figure 7. At this point, not much can be said about the results of this measurement except that there are complex interactions between the two wavelengths. Looking at slices through the spectrum, where the wavelength of one laser is fixed, shows that the relative heights of peaks in the other wavelength plane differ. The relation between ionization current and the laser power of both lasers was studied. A linear relationship was observed for the first-step power, as expected for an LEI mechanism in the absence of saturation. The expanded laser beam widths keep the laser irradiance low enough to avoid saturation. Figure 8 shows that the laser power dependence for the second-step laser is dightly nonlinear. The slope of a log-log plot of the data is 1.2. This is an indication that photoionization as well ~8 collisional ionization is occurring from the upper state. A second photon from the second-step laser is energetic enough to photoionize. If both collisional ionization (LEI) and photoionization (REMPI) are occurring together, then
ANALYTICAL CHEMISTRY, VOL. 63, NO. 15, AUGUST 1, 1991
Table 111. Sensitivity and Limits of Detection sensitivity, nA/(&nL) single resonance tight focus mild focus weak focus double resonance
16 8 4 72
limit of detection, ng/mL 200 200 400
30
the total ionization signal, ST,would be related to laser power as follows:
ST = CI + p p where C and P are coefficients for the collisional and photoionization contributions and Z is the laser irradiance. It is assumed that no optical saturation is occurring. The coefficients were determined by an unweighted second-order polynomial regression analysis that was used to calculate the smooth curve in Figure 8. These coefficients can also be used to quantify the relative contribution of the LEI and REMPI components of the signal. At the highest laser pulse energy, 2.7 mJ, 74% of the total signal is due to LEI. Thus what is referred to as LEI of PO in this paper is in fact a mixture of LEI and REMPI but is predominantly LEI. In terms of selectivity, the doubleresonance measurement is clearly superior to single-resonance REMPI, as seen in the higher selectivity ratios listed in Table 11. The exception is for the sodium matrix, where interference from the wing of a sodium line at the wavelength of the second laser results in poor selectivity. This is a common problem for atomic double-resonance LEI (7).For some of the matrices, greater improvements in selectivity would have been achieved, except for closer near-line coincidences at 324.64 nm than at 320.42 nm. Notable in this regard are Cu at 324.75 and 324.32 nm, Fe at 324.70 nm, and Ni at 324.31 and 324.85 nm. Sensitivity. Sensitivities (given as nA of ionization current/(pg/mL P)) and detedion limits (ratio of signal to blank noise = 3) are listed in Table III. Measurements were made for single-resonance REMPI at 320.42 nm by using the three focus conditions and for double-resonance LEI at 324.64 + 557.96 nm. These measurements were made at fixed wavelengths, and d data were collected on a single day. Day-to-day variations in the performance (power, beam quality, etc.) of the dye lasers make relative comparisons of sensitivity difficult for data collected at different periods of time, and this accounts for some difference between relative sensitivities given in Table I11 and those which can be taken from the peak currents seen in the spectra presented earlier. The sensitivity of the LEI measurement is only approximately 4 times greater than that of the tight-focus REMPI result. Because of the larger beam diameter, the laser interaction volume for the LEI measurement is approximately 40 times greater than that of the tight-focus REMPI measurement. Therefore, the ionization efficiency of the LEI measurement, in terms of ionization per volume irradiated, must be less than that of the REMPI measurement. The reason for the poor efficiency may lie in the coupling efficiency between the first and second laser excitation steps. A rapid redistribution of population among the vibrational and rotational levels in the B state following the first step of laser excitation would make a fraction of such excited molecules unavailable to the second step of laser excitation to the D or D’ states. Also, the separation between the rotational lines
1611
in the D and D’ states is much greater than in the B state, and consequently, fewer rotational linea can be excited at any single laser wavelength setting. It is possible that a second-step laser with a larger bandwidth would be able to excite a greater fraction of the B population, while selectivity could still be preserved through the narrower bandwidth of the first step laser. The limit of detection of P using double-resonance LEI is 30 ng/mL-about 7-fold better than the REMPI results. The improvement is partly due to higher sensitivity and partly due to lower background REMPI noise. CONCLUSIONS
In termsof both sensitivity and selectivity,doubleresonance LEI is superior to single-resonance REMPI for the determination of P via the PO molecule. However, the limit of detection is approximately the same as that achievable by conventional plasma emission spectroscopy, and the selectivity in many important matrices is insufficient for trace determinations. Some hope for improved sensitivity, and thus selectivity also, lies in the fact that the ionization efficiency of the process is very low and that strategies for improvement in this area exist. Both LEI and REMPI may prove beneficial for the determination of other non-metal molecules. ACKNOWLEDGMENT Helpful discussions with W.Gary Mallard and Kermit Smyth are gratefully acknowledged. LITERATURE CITED (I) Syty,A.; Dean, J. A. Appl. a t . 1988, 7 , 1331-8. (2) Skoeerboe, R. K.; Olavatt. A. S.; Monlson, 0. H. Anal. Chem. 1987. 39,-1602-5. (3) Haraguchl, H.; Fowler, W. K.; JohnSon, D. J.; Wkretordm~,J. D. Spectroxhkn. Acta, PartA 1978, 32, 1539-44. (4) Kkkbrlght, G. F.; Marshall, M. AMI. Chem. 1973, 45, 1810-3. smyth, K. C.; Mauard. W. 0. J . Chem. p h y ~ .1982, 77, 1779-87. . , m u , J. S.; Sumlda, D.; WMg, C. Chem. phys. Lett. 1983, 100, 397-402. (7) Turk, 0. C.; Kingston, H. K. J . Anal. A t . Specbwn. 1990, 5 , 595-801. (8) Axner, 0.;RubhszteinDunbp, H. Specfrochhn. Acta, Part B 1989,
181
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44. . . 835-86. - - - - -. (9) Travis, J. C.; Turk, 0. C.; DeVoe, J. R.; Schen&, P. K.; Van Dip, C. A. Prog. AMI. A t . Specbpsc. 1984, 7 , 199-241. (IO) Twk, 0. C. AMI. Chem. 1981, 53, 1187-90. (11) Hawilk, G. J.; Oleen, R. B. (2”.B M . EnMLon. Instrum. 1981, 4 , 273-80. (12) Mallard, W. G.; Miller, J. H.; Smylh, K. C. J . Chem. phys. 1982, 76,
3483-92. (13) Rockney, B. H.; Cool, J. A.; Grant, E. R. (3”.phys. Lett. 1982, 87, 141-4. (14) Turk, 0. C.; Travls, J. C. Specirochkn. Acta, Part 8 1990, 45, 409-19. (15) Schenck, P. K.; Mallard, W. G.; Travls, J. C.; Smyth, K. C. J . Chem. PhyS. 1978, 69, 5147-50. (18) H u h . K. P.; Herzberg. 0. Mo&cu/ar Sbvchnr, I V ; Van Nostrand Relnhold: New York, 1979; pp 538-541. (17) Verma, R. D.; Dixit, M. N. Can. J . phys. 1988, 46, 2079-86. (18) Coquart, 8.; DaPaz, M.; Rudhomme, J. C. Can. J . phys. 1974, 52, 177-88. (19) ohoah, S.; Nagaraj, S.; Verma, R. D. Can. J . phys. 1978, 54, 895-708.
(20) Verma. R. D.; Singhal, S. R. Can. J . phys. 1975. 53, 411-9. (21) Turk, 0. C.; Ruegg, F. C.; Travls, J. C.; DeVoe, J. R. App/. Spectrosc. 1988, 4 0 , 1148-52.
RECEIVED for review March 18,1991. Accepted May 1,1991. In order to describe experimental procedures adequately, 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 Institute of Standards and Technology nor does it imply that the particular products or equipment are necessarily the best available for that purpose.
