2526
Anal. Chem. 1985, 57, 2526-2533
7439-95-4; Mn, 7439-96-5; Pb, 7439-92-1; Zn, 7440-66-6. LITERATURE CITED (1) Johnson, D. J.; Plankey, F. W.; Winefordner, J. D. Anal. Chem. 1975, 47, 1739-1743. (2) Ullman, A. H.; Pollard, B. D.; Boutilier, G. D.; Bateh, R. P.; Hanley, P.; Wlnefordner, J. D. Anal. Chem. 1979, 51,2382-2387. (3) McCaffrey, John T.; Mlchel, R. G.; Anal. Chem. 1983, 55,488-492. (4) McCaffrey, John T.; Wu, Man-Li Wang; Michel, R. G. Analyst (London) 1983, 106, 1195-1208. (5) Demers, D. R.; Allemand, C. D. Anal. Chem. 1981, 53, 1915-1921. (6) Lancione, R. L.; Drew, D. M. Paper No 8, presented at the 1l t h Annual Meeting of the Federation of Analytical Chemlstry and Spectroscopy Societies, Phlladelphia, PA, Sept 17, 1984. (7) Haarsma, J. P.; De Jong, G. J.; Agterdenbos, J. Spectrochim. Acta, Part B 1975, 298, 1-18. (8) Michel, R. G.; Coleman, Julla; Winefordner, J. D. Spectrochlm. Acta, Part B 1978, 338, 195-215. (9) Seltzer, M. D.; Michel, R. G. Anal. Chem. 1983, 55, 1817-1819. (10) Novak, John W., Jr.; Browner, Richard F. Anal. Chem. 1978, 50, 1453-1457. (11) Walters, P. E. Spectrochlm. Acta, Part B 1983, 368, 889-898. (12) Omenetto, N.; Nikdel, S.;Bradshaw, J. b.;Epstein, M. S.;Reeves, R. D.; Winefordner, J. D. Anal. Chem. 1979, 51, 1521-1525. (13) Epstein, M. S.; Nlkdel, S.;Omenetto, N.; Reeves, R.; Bradshaw, J.; Winefordner, J. D. Anal. Chem. 1979, 51, 2071-2077. (14) Kosinski, M. A.; Uchlda, H.; Wlnefordner, J. D. Anal. Chem. 1983, 55, 688-692. (15) Long, Gary; Wlnefordner, J. D. Appl. Spectrosc. 1984, 38,583-567. (16) Messman, Jerry D.; O’Haver, Thomas C.; Epstein, Michael S . Anal. Chem. 1985, 57, 416-420. (17) Messman, J. D.; Epsteln, M. S.;Ralns, T. C.; O’Haver, T. C. Anal. Chem. 1983, 55, 1055-1058. (18) Skogerboe, R. K.; Urasa, I. T. Appl. Spectrosc. 1978, 32,527-532.
(19) Mlchel, R. G.; Hall, M. L.; Ottaway, J. M.; Fell, G. S. Analyst (London) 1979, 104,491-504. (20) Coleman, G. N.; Allen, A. M. Appl. Spectrosc. 1982, 36, 116-120. (21) Wlnefordner, J. D.: Schulman, S. 0.; O’Haver, T. C. “Luminescence Spectroscopy in Analytical chemistry”; Wlley: New York, 1973. (22) Long, G. L.;Winefordner, J. D. Anal. Chem. 1983, 55,713A-724A. (23) Cavalli, Paolo; Rossi, Guglielmo; Omenetto, Nicolo Analyst (London) 1983, 108,297-304. (24) Omenetto, N.; Cavalli, P.; Rossi, G. Rev. Anal. Chem. 1981, 5 (3/4), 185-205. (25) Wu, Man-Li Wang, Ph.D. Dissertation, Unlversity of Connecticut, 1984. (26) Johnson, G. W.; Taylor, H. E.; Skogerboe, R. K. Appl. Spectrosc. 1980, 34, 19-24. (27) Nygaard, D. D.; Gilbert, T. H. Appl. Spectrosc. 1981, 35, 52-56. (28) Decker, R. J. Specfrochim. Acta, Part 8 1980, 358, 19-31. (29) Eastwood, D.; Hendrick, M. S.;Mlller, M. H. Spectrochlm.Acta, Part 8 1982, 378 293-302.
RECEIVED for review April 4, 1985. Accepted July 11, 1985. This work was presented in preliminary form a t the 13th Northeast Regional ACS Meeting, June 26-29, 1983, at the University of Hartford as paper 23 and at the 9th Annual FACSS meeting, Philadelphia, PA, Sept 19-24,1982, as paper 196. R.G.M. was supported by a Research Career Development Award from the National Institute of Environmental Health Sciences under Grant ES 00130. The work was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and Research Corporation.
Red and Near-Infrared Photodiode Array Atomic Emission Spectrograph for the Simultaneous Determination of Carbon, Hydrogen, Nitrogen, and Oxygen J. M. Keane, D. C. Brown,l and R. C. Fry* Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506
I n a single exposure, a small, crossed Crerny-Turner photodiode array spectrograph with a coarsely ruled gratlng covers an unusually large “wlndow” for atomlc spectra (650-950 nm). Slmultaneous monltorlng of C, H, N, and 0 atomlc emlsslons Is performed In mlcrowave or Inductively coupled plasmas. Detector coollng and Image lntenslflcatlon are unnecessary. Intense emlsslon spectra are obtalned In mllllsecond exposure times as the result of (1) enhanced red and near-Infrared (near-IR) sensltlvlty of photodlode arrays, (2) favorable fhumbers of short focal length spectrographs, and (3) large elemental concentrations occurrlng In purlfled Samples of Interest In synthetlc chemistry. Atomlc emlsslon spectra of organic compounds are relatlvely simple In the red and near-IR reglon. For compounds contalnlng C, H, N, and 0, Interference-free lines have been located In splte of the low resolutlon of this Instrument. Qualltatlve elemental analysls of organic compounds Is asslsted using Xerox transparent overlays of labeled master reference spectra.
Samples pertaining to applied disciplines such as geology or metallurgy often contain a large number of easily excited metals. The corresponding air-path ultraviolet and visible Present address: H a r r i s Corp., Rochester, NY.
atomic emission spectra in hot plasmas can be far too complex to sort out with photodiode arrays. A trade-off arises between (a) the need for extreme ultraviolet dispersion and resolving power to minimize spectral interference and (b) the conflicting desire to have a large spectral range or multielement UV “window” covered by the relatively small photodiode array chip. Unfortunately, these two needs cannot be simultaneously met with present day photodiode arrays which are only about 25 mm in length and typically have only 512 or 1024 channels for “dividing up” the entire spectrum. The use of self-scanned, linear photodiode arrays as detectors in ultraviolet atomic emission spectrometry has therefore been primarily limited to a few diagnostic studies where high resolving power was not a concern or where it was not necessary to cover a large spectral range simultaneously. Examples include (a) studies of one or two elements dissolved in distilled water (1-3) and (b) 90’ detector orientation for spatial source profiling involving a single UV wavelength (4-9). Another limitation of unity gain photodiode arrays in atomic spectrometry is poor sensitivity in the UV region where most metallic emissions occur. This often leads to the use of detector cooling and long exposure times or costly image intensifiers (accompanied by an additional 2- to 4-fold loss in resolution). Photodiode arrays are actually most sensitive to red and near-infrared (near-IR) radiation. A preliminary report from this lab included qualitative detection of inductively coupled
0003-2700/85/0357-2526$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
2527
Table I. ICP Apparatus and Experimental Conditions
inductively coupled plasma Plasma-Therm (Kresson, NJ) 27-MHz ICP 2500 with APCS-3 auto power control, AMNPS-1 auto matching network, and “stock”quartz torch located remotely (3 m) from the spectrograph 1.4 kW,incident; 35 W, reflected rf power plasma, 18; auxiliary, 2; sample carrier, 0.2 argon flows (L/min) butane, 0.001; compressed air, 0.025 sample flows (L/min) a 0.6 mm vertically isolated zone centered between the top two turns of the rf load coil observation zone a 3-m quartz fiber optic (0.6 mm diameter, single strand with black Teflon cladding) between the ICP and remote optical couple the spectrograph a 6.6 cm focal length, 6 cm diameter quartz lens doublet placed 21 cm from the ICP to focus a half sized external optics (1/2.2) image of the plasma onto one end of the fiber optic (located 9 cm from the lens system); the remote end of the 3 m fiber optic was positioned -1 mm from the entrance slit of the photodiode array spectrograph; no coupling optic was employed between the slit and this end of the fiber Hoya no. 25 (removes second-order blue and third-order UV emissions that otherwise severely complicate order filter the first-order red-near-IR spectrum) Jarrell-Ash Monospec 27 (0.275 m focal length, f/3.8 crossed Czerny-Turner triple grating spectrograph) spectrograph all 50 X 50 mm with 500 nm first-order blaze: no. 1,600 g/mm; no. 2, 300 g/mm (the one used in this gratings mounted study); no. 3, 150 g/mm entrance slit 10 pm none (replaced with photodiode array detector) exit slit Reticon 10245 photodiode array, (EG&GReticon, Sunnyvale, CA) 1024 channels, 2500 X 25 pm each; detector aspect ratio 1OO:l (slitlike channels) Reticon 1024SA with minor modifications (see ref 12) evaluation circuit 1. Analog Devices (Norwood, MA) ADC 51131 analog to digital converter (14 bit, 12 ps); the interface has data system been described by Hughes (12) 2. Digital Equipment Corp. (Maynard, MA) PDP11/34A minicomputer, 256K RAM, VT55 terminal, and Data Systems Design DSD880 (San Jose, CA) 30 Mbyte Winchester hard disk operating with this experiment at first level priority under TSX+ (S&H Computers, Inc., Nashville, TN) 3. Houston Instruments (Austin,TX) Hi-Plot DMP plotter and Epson (Epson America, Inc., Torrance, CA) MXlOO printer 67 ms array exposure time 185 kHz evaluation circuit clock spectrum repetition interval every other spectrum stored; this occurs once every 135 ms plasma (ICP) atomic emissions from the nonmetals carbon, hydrogen, nitrogen, and oxygen in the red and near-IR region by a 1024-channel photodiode array (IO). A remarkably simple spectrum was observed as the result of two important factors (IO): First, the number of elements simultaneously occurring in a purified organic compound of interest to a synthetic chemist is relatively small (e.g., just C and H, or perhaps C, H, N, and 0). Second, the upper states of nonmetallic atoms are unusually energetic. Only a few of these high energy excited states are appreciably populated at ICP and microwave induced plasma (MIP) temperatures, so only a few red and near-IR spectral lines appear. In ref 10, spectral simplicity permitted the use of low dispersion optics and should have minimized the dilemma of resolution vs. spectral range (window size). However, a vignetting problem characteristic of the Ebert design limited its useful range as a spectrograph. The 25-mm photodiode array was unavoidably masked to an 8-mm physical window corresponding to a wavelength range of only about 75 nm. Nearly two-thirds of the array length was blocked by vignetting. The spectra of C, N, and 0 could be recorded simultaneously from 800 to 875 nm, but a separate exposure had to be made in the region 640-715 nm for hydrogen. A third exposure would have to be made for sulfur in yet another region (875-950 nm) with the vignetted Ebert system of ref 10. An additional system limitation was imposed by using a storage oscilloscope as the only means of data acquisition. No provision was made €or subtraction of the fixed pattern dark current (10). The present paper is a report on the use of a short focal length, crossed Czerny-Turner spectrograph to eliminate vignetting and allow use of the full array length in the red and near-IR region of the atomic spectrum. A coarsely ruled grating is employed to lower the dispersion even further and compress the entire spectrum from 650 to 950 nm into the 25-mm linear format of a Reticon 1024s photodiode array. C, H, N, and 0 emissions are studied in a single exposure covering this 300-nm interval, and a computer data system
has been added. Labeled master reference spectra have been prepared in the form of Xerox transparent overlays for rapid line identification. Considerations for line selection and interference-free qualitative analysis involving these elements are explored in both microwave and inductively coupled plasmas. EXPERIMENTAL SECTION Apparatus and Procedure. The ICP source, photodiode array detector, Czerny-Turner spectrograph, external optics, and experimental operating conditions are listed in Table I. The gas sample mixture of butane and air was injected directly and continuously into the sample argon carrier stream. Dark current subtractions were performed followed by signal-averaging of ten corrected spectra. The signal-averaged spectra were stored on hard disk and plotted and each spectral line was labeled. Xerox transparencies of the labeled plots can serve as lowresolution “master plate” reference overlays for rapid line identification in subsequent unknown spectra. The 81/2X 11 in. overlays may be handheld and the spectra visually inspected without the aid of magnification. Rapid qualitative determinations of C, H, N, and 0 are thereby performed in purified unknown compounds of interest in synthetic chemistry. Upon request, the authors will mail labeled transparent master overlays to any interested reader. Although each spectrum was obtained with a 67-ms exposure, a similar spectrum can be obtained in less exposure time if the small-aperture external fiber optic is discarded in favor of a medium or large-aperture lens or mirror. The fiber optic was employed here simply as a convenience to facilitate study of the ICP from a remote location in the lab without dismantling other experiments normally coupled to this particular ICP unit. A helium microwave induced plasma (MIP) was also studied. The MIP system and experimental parameters are described in Table 11. Gas samples were injected into the “inner” helium flow of a threaded, tangential-flow, Bollo-Kamara-Codding MIP torch (11).
RESULTS AND DISCUSSION Argon Background. Figure 1 illustrates argon ICP background emissions in the region 650-950 nm. The plasma
2528
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Table 11. MIP Apparatus and Experimental Conditions (Same as Table I Except as Noted Below)
microwave generator microwave cavity
Model MPG 4M, Kiva Instruments, Inc. (Rockville, MD), 2450 MHz, 120 W cylindrical TMolo(Beenakker), dimensions: 93.5 mm diameter, 2 cm depth, lab-constructed from aluminum, water cooled tuner triple stub, Model S3-15N, Microlab/FXR (Livingston, NJ) torch Bollo-Kamara-Codding design (II), threaded tangential flow (obtained from J. M. Babbitt, Department of Chemistry, University of South Carolina, Charleston, SC) gas flows (L/min) helium: inner, 0.15; outer, 2.5 sample flows (L/min) propene, 0.001; compressed air, 0.005 external optic plano-convex (fi = 8 cm; fi = 6 cm) 6 cm diameter, quartz lens located 16 cm from the entrance slit to produce a 1:l image of the plasma on the slit 21
0-
25 26 27
2
32
33 34
47
m
0 0-
b
0
0J U
0
0 0In )-
Ed
~‘
v)O
8%-
I8
I-
z
37
LA 42 3-6 I
650.
6’h.
28
9,ll 13-16,17,19,20,22-24 7b0.
715.
740.
7$5
8bO.
I
825.
840.
I
875.
I
900.
-l----1
925.
950.
GAVELENGTH ( n m ) Flgure 1. Photodiode array spectrum of argon ICP background emissions in the region 650-950 nm. All lines are argon emissions and are numbered for Convenient reference. Wavelengths of these lines are listed in Table 111. The Yaxis has been arbitrarily terminated here at relative intensity 8000, whereas detector saturation occurs at 14 400. continuum background intensity in the figure is only about 2% of the detector saturation limit under the present experimental conditions. This is considerably weaker than ICP continuum background emissions occurring in the ultraviolet and visible region. For convenient reference, argon lines have been assigned numbers from 1to 51 in Figure 1. The numerical assignments are only for convenience and bear no relationship to intensity, rank, atomic number, atomic weight, or stoichiometry. Net relative intensities of the numbered Ar lines are given in Table I11 as a percentage of photodiode channel saturation. Wavelengths are from ref 13. When used in combination with Table 111, a Xerox transparency of Figure 1 is useful as a hand-held, low-resolution reference overlay for rapid identification of argon lines in any subsequent ICP emission spectrum plotted in this region. The only requirement is that the spectrum be plotted with the same X-axis “dispersion” as the overlay. Another useful overlay for line identification can be prepared by transparent reprography of just the labels, “pointers”, and axes of Figure
1 (omitting the actual spectral features).
Under the conditions of this experiment, nine argon lines are sufficiently intense to produce channel saturation, but many are weaker, and a total of only 51 argon background emission lines are found over the entire region 650-950 nm. Due to line broadening and optical resolution effects, these 51 argon lines account for about 350 (34%) of the original 1024 photodiode array channels. Even with the unusually poor dispersion of this grating and spectrograph, approximately 670 interference-free detector channels (representing 66% of the array length) remain available for red and near-IR spectrum analysis involving elements other than argon. Contamination Backkround. Special note should be taken of the absence of atomic nitrogen emissions at 746.8 and 821.6 nm and the absence of atomic oxygen emissions at 777.2 nm in the ICP background spectrum of Figure 1. The contamination level from these elements has been rendered insignificant in this ICP study as the overall result of three important parameters listed below: (1)viewing the plasma in the load coil zone where atmospheric entrainment is ex-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985 2529 C(IZ(FEAK)) Ar(47(STRONGI 1
I
I
. I
650.
I
675.
I
700.
I
725.
I
750.
I
775
I
800
I
825. GRVELENG~H (nm)
I
850.
I
875.
I
900.
I-
..
. I
925.
I
950.
Flgure 2. Photodiode array spectrum of ICP excited carbon, hydrogen, nitrogen, and oxygen atomic emissions in the region 650-950 nm. The emission lines are numbered for convenient reference, and wavelengths are listed in Table IV. Several interfering Ar emission lines are also labeled in this figure. The Y axis has been arbitrarily terminated here at relative intensity 8000, whereas detector saturation occurs at 14 400.
cluded by the torch wall; (2) the minimized erosion rate of torch walls inherent with an argon ICP (this minimizes elemental oxygen contamination from atomized quartz (SiOJ); and (3) the relatively large sample size and high analyte concentrations normally associated with purified samples of interest in synthetic chemistry. This overshadows residual N and 0 contamination in the welder’s grade argon supply and results in the use of sufficiently short exposure times to keep emission from the contaminants below the detection limit. Sample-Induced Background. Figure 2 shows the ICP excited spectrum (650-950 nm) of an atomized sample mixture of butane and air. A comparison of Figures 1 and 2 shows that the continuum background level increases when the sample mixture is introduced. For quantitative analysis, background correction is therefore a “must” and is readily performed by using the array to monitor the continuum emission beside each analytical line. Subtraction of the average continuum level from the total intensity a t the line center yields the net analytical line intensity. All line intensities given in the tables of this paper have been background corrected. Analyte and background channels are exposed simultaneously, so the correction is accurate even for transient sample introduction methods. In addition to the rise in continuum background upon introducing the sample mixture, a further comparison of Figures 1and 2 shows that argon line intensities increase. As an example, the line designated a t 922.4 nm approximately doubles in intensity upon adding the sample. This general trend is observed for most of the argon lines in the region under study. Preliminary attempts to employ spectral
“stripping” methods for subtracting the argon spectrum (Figure 1)from the sample spectrum (Figure 2) have therefore not been particularly successful for the sample sizes injected in this study. C, H, N, and 0 (ICP Emissions). Figure 2 shows the complete ICP excited, red and near-IR atomic emission spectrum of C, H, N, and 0 from 650 to 950 nm. The spectrum is from the 1:25 sample mixture of butane and air given earlier in Table I. This mixture was used for convenience in generating a master reference spectrum containing a full set of labeled C, H, N, and 0 lines in a single Xerox transparency overlay. In practice, the master overlay would be used to qualitatively evaluate the presence of C, H, N, and 0 in the formula of a single compound of interest to a synthetic chemist (rather than a mixture). Mixtures would first be separated by chromatography, distillation, or other means of purification. Upon request, the authors will mail labeled transparent master overlays (copied from enlargements of Figures 1 and 2) to any interested reader. Transparent overlays of “labels, ‘pointers’, and axes only” from Figures 1and 2 (without the actual spectra) will also be mailed upon request. The ink color of labels and spectra on the overlays will be black. In order to avoid confusion in line identification, it is recommended that unknown spectra be plotted in red or some ink color other than black. Unknown spectra should be plotted with exactly the same X-axis “dispersion” as the transparent master overlay. Some initial “trial and error” invested in matching the X-axis dispersion of a computer plotting routine to that of the transparent overlays will quickly result in a permanent set of plotting coordinates useful for all subsequent unknown spectra generated with a given photodiode array spectrograph
2530
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Table 111. Numbered Argon Emission Lines from 650to 950 nm in Ar ICP no.
wavelength, nm
1
2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21
1
z
1.1 1.8 1.6 1.5 1.0 1.2 12 12 4.4 97 14 81 2.6 1.8 9.8 2.3 5.6 24 4.2 6.6
738.398 737*212 741.221 742.524 743.533
25
751.465 750*387
t
100a
26
763.510 762*886
z
looa
27
772.421 772*3761
looa
28 29
789.108 794.818
1o o a
30
801.479 8oo.616
I
100a
31
811.531 810.369 826.452 840.82 842.467 852.144 860.578 862.047 866.794 876.172 879.913 884-03 9 884.082 884.997 887.484 906.677 907.334 907.542 912.296 919.468 922.450 929.158 935.422
z
100a
33
34 35 36 37 38 39 40-43
44-46 47 48 49 50 51 Detector saturated.
I
}
wavelength, symbol nm
3.5 2.3 4.9
2.4
88 100a 58 1.4 0.90 12 1.2 0.39 1.1
'(1*2)
67 2.3 28 0.90 2.9
5.6*
}
777.194
0(1+3)777.417
2.4* 12* 777.539 844.625 0(4-6) 844.636}
