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Anal. Chem. 1980, 52, 1716-1722
Atomic Fluorine Spectra in the Argon Inductively Coupled Plasma R. C. Fry,* S. J. Northway, R. M. Brown, and S. K. Hughes Department of Chemistry, Kansas State University, Manhattan, Kansas 66506
using the neutral density step filter. Emulsion calibration was repeated for each exposure. The final relative intensities determined are not further corrected for overall system response (which varies with wavelength). A short region of spectrum in the vicinity of 6856 A was scanned using photoelectric readout and continuous (1 mL/min) SF6 introduction. A vertical profile of F(1) relative emission intensity was made using the gas sampling loop (SFs),fixed wavelength (6856.02 A), photoelectric procedure. For this experiment, the intermediate ICP image (produced in the arc stand) was masked to a 0.5-mm limited vertical zone. The sampling loop carrying SF6 was switched repetitively each time the observation zone was vertically incremented by 0.2 mm. This vertical profile was then repeated a t several horizontal positions (center, 3.4-mm offset, and 6.7-mm offset). The sampling loop was “recharged” for 2 s by an SF6 sample flow of 8 mL/min prior to each injection. Fluorine photoelectric response factors (relative to SFs) were determined at fixed F(1) wavelength (6856.02 A) by separate, direct gas sampling loop injections of SF6,CF4, CC12F2,and CLF2CCF2C1. The limit of fluorine detection, precision of analytical measurement, and linear dynamic range were also assessed at fixed wavelength using the photoelectric mode and 43-pL gas sampling loop injections of varying SFs concentrations in argon. Element selective (F) detection for gas chromatography by inductively coupled plasma atomic emission spectrometry was achieved for fluorinated compounds using the gas chromatograph-ICP system described in Table I and Reference 4. For this purpose, the photoelectric F(1) response at fixed wavelength (6856.02 A) was employed. Spectral background correction was performed as described in Reference 4. A word of caution is offered here concerning the avoidance of procedures that may lead to quartz (SiOz) torch erosion. All present work was done with fluorinated compounds which will not attack quartz at room temperature. However, any fluorinated compound produces atomic fluorine upon dissociation a t plasma temperatures. Severe top end torch erosion can then occur unless procedural details are properly attended. Proper experimental procedure can extend the useful torch life by 2-3 orders of magnitude. Foremost, the fluorinated compound must be introduced through the ICP sample gas port. Even small quantities of fluorinated compounds introduced via the auxiliary ICP port will quickly erode all three torch tubes. Special procedural attention should be given to cases where the fluorine content of a compound is high, the concentration of this compound in the sample is high, and the sample is continuously introduced. Examples of this condition are: (1) during extended photographic exposures used for the purpose of establishing listings of ICP excited fluorine lines, and (2) during spectrometer wavelength dial adjustment aimed at locating the F(1) line prior to GC-ICP or gas sampling loop injections using the photoelectric, fixed wavelength procedure. In such cases, the ICP argon sample gas stream should be elevated in flow rate in order to effect extensive sample penetration into the plasma. If the ICP argon sample gas flow rate is not sufficient, the fluorinated sample interacts too extensively with the torch tubes and leads to extreme erosion of the torch. Such elevated argon sample gas flow rates do not afford maximum F(1) spectral sensitivity and are necessary only in the special cases noted above. Reduced fluorine concentrations and/or smaller samples introduced on a transient basis (e.g., direct gas sampling loop injections or GC-ICP applications) do not require elevated argon sample gas flow rates to avoid torch erosion. In such applications, the more normal conditions of flow rate given in Table I will lead
A table of relative intensities and Wavelengths of 56 ICP excited, nonresonance atomic fluorine lines Is presented for the region 3500 to 8950 A. The quantitative fluorine response is found to be independent of molecular species, yet proportional to fluorine content for the series: SF,, CF,, CC12F,, and CiF2CCF2Ci. The present fluorine detection limits are 0.35 pg for direct gas sampling loop injection and 1 pg for GC-ICP determination. The linear dynamic range for quantitative analysis is presently 5 X lo3. The precision is presently 1 % RSD for individual direct injections by gas sampling loop and 0.2% RSD for the group mean of a series of multiple injections.
T h e present paper on ICP excited atomic fluorine spectra a n d element (F) selective gas chromatography is the third of a series of reports from our laboratory concerning the determination of those nonmetals which have long eluded ICP quantitation. T h e first two papers of the series served t o introduce the quantitative ICP determination of the “elusive” elements, nitrogen and oxygen, by near-infrared (NIR) nonresonance emission spectrometry ( 1 , 2 ) . Since the first excited state of atomic fluorine lies 12.7 eV above t h e ground state, the resonance lines of this element would all have wavelengths shorter than 1000 A. This region of t h e spectrum presents considerable difficulty in terms of detector sensitivity, opacity of optical materials, and atmospheric opacity. For this reason, the present paper involves exclusive use of nonresonance, near-infrared and red transitions originating from high energy states of atomic fluorine
(F(I)). Windsor and Denton have reported a l-mg ICP detection limit and a n unusably poor linear dynamic range for the 6856.02 A F(1) line ( 3 ) . T h e present paper includes a report of an approximate 3000-fold improvement in the fluorine detection limit a n d linear dynamic range of inductively coupled plasma spectrometry. A table of observed I C P excited fluorine lines and relative intensities is presented here for the region 3500 to 8950 A. Gases and volatile liquids were studied by direct sampling loop injection and gas-liquid chromatography (ICP detector), respectively. T h e improved limit of detection and linear dynamic range for quantitative analysis were estimated using direct gas sampling loop injection. The fluorine response is compared here for gas sampling loop injections of SF,, CF4, CC12F2,and C1F2CCF2C1.
EXPERIMENTAL Apparatus a n d Procedure. The plasma system, spectrograph, spectrometer, gas sampling loop, gas chromatograph, and experimental conditions are described in Table I. Unless otherwise noted, all data in this manuscript are corrected for spectral “continuum” background emission (where significant). This background was assessed a t a wavelength about 18, removed from the spectral line in question. For the listing of all observed ICP excited F(1) lines between 3500 and 8950 A, the photographic mode was used with continuous introduction of SF, (added to the ICP sample gas stream). Photographic emulsion calibration (H&D) curves were prepared 0003-2700/80/0352-1716$01 .OO/O
C
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
Table I.
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Experimental System and Conditions Used to Generate Table I1
A. ICP incident power reflected power operating frequency modified ICP exhaust stack B. Spectrometer
external optics
Plasma Therm ICP 2500 with: APCS-3 auto power control and AMNPS-I auto matching network 1.5 kW 1 will produce a group of several lines which are close in wavelength (e.g. 6708.28, 6773.97, 6795.52, 6834.26, 6856.02 (the strongest line), 6870.22, 6902.46, and 6909.82 A). In this case, all lines of the group will appear in Table 11, but a single nominal value corresponding to the first line in the group is given (e.g., 6708 A in Figure 3) with fewer significant digits in Figures 2 and 3. Table I1 should therefore be consulted in combination with Figures 2 and 3 to determine which indicated “solid line” transitions are really observed as a group of lines which are close in wavelength. Most of the major transitions (with upper states 518.5 eV) predicted from atomic diagrams (7) are observed with notable exception of the resonance lines ((3
u w z 14 W
13
12 Figure 3. Partial Grotrian diagram of quartet atomic fluorine transitions. Redrawn in part from Reference 7 with heavy solid lines indicating some of the more important observed transitions in Table I1 for the argon ICP (present work). Wavelengths are given in angstroms (A). The indicated electronic configuration ls22s2sp4nlis the core. The additional electron can then assume energies indicated by the diagram
,
Ar (1) 6079.59
F (11 6902.46
F(I) 6070.22%
4
F(1) 6795.52
i
*.
Ar (I) 6827.25
~~d *' 6034.26
L
*Ar(I) 6007 IO
Figure 4. Abbreviated region of the ICP excited F(1) emission spectrum in the vicinity of 6856
A.
i
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Table 11. F(1) Line Intensities from 3500-8925 A wavelength, A 6149.76 6210.87 6239.64 6348.50 6 4 13.66 6569.69 6580.39 665 0.4 0 6690.47 6708.28 6773.97 6795.52 6834.26 6856.02 6870.22 6902.46 6909.82 7037.45 7127.89 7202.37 7298.98 7309.03 7311.02 7313.77a 7314.30 7331.95 7398.68 7425.64 7482.72 7489.14 755 2.24 7573.41 7607.17 7 7 54.70 7800.22 7879.18 7 898.5 9 8040.93 8075.52 8077.52 8159.51 8179.34 8197.73 8208.63 8214.73 8230.77 8232.19 8737.27 8777.73 8785.63a 8792.50 8807.58 8831.23 8900.92 8910.27 8912.78
relative intensity 0.06 0.04 17. 18. 10. 0.51 0.16 0.34 1.7 1.2
11. 3.2 25.
100. 17. 57. 2 2. 28. 12. 10. 0.35 3.9 12. 0.06 2.4 19. 32. 14. 5.9 1.3 13. 12. 7.9 36. 15. 0.98 1.3 2.5 1.5 0.58 0.38 0.66 0.26 0.94 2.6 5.0 0.83 0.48 0.59 0.16 0.19 1.8 0.48 2.3 0.39 0.63
upper state energy, eV 14.75 14.75 14.68 14.68 14.68 14.58 14.61 14.61 14.58 14.54 14.53 14.55 14.54 14.50 14.55 14.53 14.54 14.75 14.7 6 14.75 14.68 17.06 14.68 17.06 17.06 14.39 14.37 14.40 14.39 14.68 14.37 14.39 14.61 14.58 14.61 15.94 15.96 14.53 15.93 15.93 14.54 15.88 15.91 15.90 15.88 15.88 15.89 15.94 15.96 18.47 18.47 15.93 15.96 15.90 15.94 15.93
a Wavelength from Reference 6; all other wavelengths from Reference 5.
Table 111. Fluorine Response Ratios theoretical experimental formula ratio ICP response (F(SF,)/F(X))
S*,
CF, ClF,CCF,Cl CCl,F,
1.000 1.500 1.500 3.000
(F(SF6)/F(X))
1.000 1.485 1.5005 2.996
tions in plasma impedance, etc. and “flutter” in the gas flow control devices (valves, etc.) of the sampling system resulting from the “pressure surge” conditions generated by loop switching.
F(I1
R E L A T I V E EMISSION INTENSITY
Figure 5. F(1) relative emission intensity (vertical profile) for gaseous samples. The symbols indicate the location of observation areas B, C, D, and E. For convenient reference, the symbols have been
+
+
translated laterally to correlate with the graph of F(I) relative emission intensity ( x axis) vs. vertical observation position (yaxis) in the argon ICP. Area A is the zone normally monitored in trace metal analysis involving aqueous solutions T o improve the situation, advantage was taken of the fact that many small volume gas sampling loop injections can be conveniently made in rapid succession on a single sample or standard simply by switching the gas sampling loop a t a rate of about 0.5 Hz. If the group mean of -15 such injections is considered to be as a single “reading”, then such “readings” may be repeated and a new standard deviation measured which indicates the precision of the group mean. Using this approach coupled with similar measurements of group standard injections taken immediately before and after the group sample injections, a considerably improved level of precision of 0.2% RSD was achieved. This overall approach is necessary to eliminate long and short term drift and was used to generate the data of Table 111. It should be noted that several additional benefits arise for the area of gas analysis when the repetitive sampling loop injection method is used in place of a continuous flow approach. First, much less sample is consumed with the repetitive sampling loop approach. Second, the loop ensures that identical volumes of sample and standard are introduced, thereby eliminating the poor precision and accuracy of measuring the flow rate of a continuously flowing sample or standard to determine the amount introduced into the plasma. An important factor in achieving the 0.2% RSD level of precision is to make sure that all detected radiation originates above the top of the quartz torch. Despite the reduced intensity in this vertical observation zone (see area B in Figure 51, this procedure eliminates the effect of increasing and variable torch opacity which is otherwise inherent with area D. Detection Limit a n d Linear Dynamic Range. The limit of detection was determined by gas sampling loop injection of 43-pL aliquots (injected as SF6/argon mixtures of varied per cent composition) under plasma noise limited conditions. A calibration curve was estabished, an estimate of noise made, and the limit of F detection was determined to be 0.35 pg on the basis of a signal-to-noise ratio of 2. T o our knowledge, this is the first published report of the ICP detection of sub-milligram and sub-microgram levels of fluorine.
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
/+ b p . 81
OC
IENZENE TRI FLUORIDE”
I
volume ratio mixture of cyclohexane, “benzene trifluoride”, and o-fluorotoluene were injected through the heated inlet port. (“Benzene Trifluoride” was obtained from Columbia organic chemicals. The fluorine substitution pattern is not given.) The nonselective uncorrected spectral background response (similar to what a thermal conductivity detector would produce) is measured at a wavelength 1 A removed from the F(1) line and is shown in Figure 6. T h e peaks produced by fluorinated compounds are not distinguishable from nonfluorinated compound peaks. Without spectral background correction, a F / C selectivity ratio of only 1.0 occurs. In Figure 7, response at the F(1) 6856.02 6, line is then shown corrected for spectral background. Here the peak due to cyclohexane has completely disappeared (eliminated by the background correction), and only the spectral response due to the fluorine containing compounds is apparent (“benzene trifluoride” and o-fluorotoluene peaks). This type of spectral selectivity is especially valuable when fluorinated compounds are not chromatographically resolved from compounds not containing fluorine. T o our knowledge, this is the first published report of an analytically successful, element selective GC-ICP system specific for compounds containing the element fluorine.
-
Note Added in Proof. The gas chromatograph used and referred to in Reference 4 was a Gow Mac which employed a in. X 6 ft column packed with Amine 220 (not necessarily an optimum choice for fluorinated compounds). The carrier gas was 0.025 L/min Ar. The method of spectral background correction referred to in Reference 4 involves graphical subtraction of the background response (measured 1 6, removed from the spectral line). Instead of this, the authors recommend the use of a simultaneous automatic instrumental background correction scheme using a dynamic spectrum shifter or second wavelength channel set adjacent to the F(1) spectral line. This technology is routinely available with appropriate processing electronics from several manufacturers. It should be noted t h a t several expected F(1) lines (see Reference 6) did not appear in the present ICP studies. These are 8298.58 and 8274.62 A for which upper state energies are 15.88 eV and the present relative intensities are
