may be easily synthesized with shorter (and longer) side chains (6). The reported ability of a compound in this series of m.w. 5200 to be distilled a t 0.1 mm. (6) is particularly intriguing since it should also be amenable to mass spectrometry. ACKNOWLEDGMENT
The author is indebted to Ian Brown of the CSIRO in Melbourne, Australia, for acquainting him with the
perfluoroalkyl phospholonitrilates (I and 11) and providing the sample. LITERATURE CITED
(1) Barber, M., Merren, T. O., Kelley, W., Tetrahedron Lettters 18, 1063 (1964). (2) Biemann, K., “Mass Spectrometry,” p. 39, McGraw-Hill, New York, 1962. (3) Bradt, P., Mohler, F. L., ANAL. CHEM.27, 875 (1955). (4) Clark, S. J., Wotiz, H. H., Steroids 2 , 535 (1963).
(5) Ratz, R., Schroeder, H., Ulrich, H., Kober, E., Grundmann, C., J. Am. Chem. Soc. 84, 551 (1962). (6) Sweeley, C. C., Bentley, R., Makita, M., Wells, W. W., Ibid., 85, 2497 (1963). HENRY M. FALES Section on Chemistry Laboratory of Metabolism National Heart Institute Bethesda, Md. 20014 RECEIVEDfor review April 5, 1966. Accepted April 22, 1966.
N e a r UItravi olet-Visi ble Atomic Absorption Spectra of the Noble Gases
A
TOMIC ABSORPTION techniques have
proved to be very useful for the quantitative determination of a large number of elements. The spectral lines used for most of the atomic absorption work reported thus far are strong arc linea corresponding to transitions from the ground states to the lowest excited states, in the 2000- to 8500-A. spectral region. Little analytical work by atomic absorption methods has been reported for any of the noble gases. The obvious reason is that for the closed-shell configurations of these elements (helium to xenon) the lowest excited states are far removed from the ground states (19.8electron volts for helium to 8.31for xenon). This means that all transitions originating in absorption in the ground state for these atoms are observed in the experimentally inconvenient vacuum ultraviolet region. McGrath et al. ( 5 ) ,from such considerations, concluded that absorption techniques have no value for analytical work with noble gases. Recently, Robinson (9) concluded that all elements in the periodic table cannot be determined by atomic absorption techniques used routinely. Some of the elements offering the most resistance are the nonmetals since they have absorption lines in the vacuum ultraviolet. With the techniques recently used in this laboratory, however, where, the atoms have been sputtered into the absorbing path in a hollow-cathode discharge tube, excitations to higher energy levels are accomplished much more easily than in conventional flame absorption techniques. For the noble gases, excitation in a hollow-cathode discharge tube will result in appreciable population of certain (possibly metaIf stable) excited energy levels. that is so, then absorption may be
Table 1.
Atomic Absorption Spectrum of Helium
Wavelength”
Intensity“ Absorption, % 7281.35 30 5.3 2. 7065.19 70 6.7 3. 100 28.4 6678.15 4. 1000 5875.62 45.5 5. 100 5015.68 17.6 6. 50 4921.93 4.2 7. 100 4471 ,48 7.4 8. 70 4026.19 4.2 9. 50 3964.73 17.7 10. 1000 3888.65 51.5 11. 50 3819.61 3.1 12. 200 3187.74 21.1 a Wavelength and intensities taken from Ref. (11). b Energy ievels determined from Ref. (8).
(-4.1
1.
Energy levels* (cm.-1) 171129.148-184859.06 169081.111-183231.08 171129.148-186099.22 169081.111-186095.90 166271.70 -186203.62 171129.148-191438.83 169081.111-191438.83
159850.318-191211.42
Table II. Atomic Absorption Spectrum of Neon
Wavelenct,ha Intensity a
I
1.
2. 3. 4. 5. 6. 7.
8.
9. 10. 11.
12. 13. 14. 15. 16. 17.
8418.43 8377.61 8300.33 8136.41 7438.90 7245.16 7173.93 7032.41 7024.05 6929.47 6678.28
400
800
600 300 300 1000 1000 1000 500 1000 500 100
2000 1000 lo00 1000 1000 1000 1000 300 1000
Absorption, yo 3.2 11.2 6.5 6.1 7.3 15.3 4.1 34.7 2.6 26. i 25.0 18.2 73.5 45.4 38.5 35.5 13.3 24.2 52.3
6217.28 6163.59 19. 6143.06 20. 6096.16 38.0 6074 33 21. .. 26.7 22. 6029.99 1000 8.9 23. 5944.83 500 33.3 24. 5881.89 1000 23.0 25. 5852.48 2000 18.9 a Wavelength and intenskies taken from Ref. (11). b Energy levels determined from Ref. (8). 18.
VOL. 38,
Energy levels* (em. -1) 149826.181-161701.623 i49659.000-i6it5mm 149659.006-161703 I4i3 150123.551-162410.617 148259.746-161701.623 134461.237-148259.746 135890.670-149826. isi 134043.790-148259,746 135890,670-150123,551 135890,670-150317,821 135890,670-150860,468 135890,670-151040.413 134043,790-149659,000 134461,237-150123.5.51 134043.790-i49826 .. _ ~ . -i_iz_i_ 134820.591-150774.072 134043.790-150123.551 134820.591-151040.413 134043.790-150317.821 134461.237-150860.468 134461.237-150919.391 134461.237-151040.413 134043.790-150860.468 134043.790-151040.413 135890.670-152972.697
NO. 8, JULY 1966
~ _
1059
Table 111.
Wavelength" (-4.) 1. 8521.44 2. 8424 65 3. 8408.21 4. 8115.31 5. 8103.69 6. 8014.79 7. 8006.16 8. 7948 18 7948.18 _. 9. 7635.11 10. 7514.65 11. 7503.87 12. 7383.98 13. 7067.22 14 14. 6965 6965.43 .... 43 _1.5 - -. ,5659 - - - - . 13 -16. ,5373.49 17. 5221.27 18. 5054.18 19. 5048.81 20. 41.58 - -. . -59 .. 21. 3834.68 22. 3675.22 23. 3670.64 ~~
~
a
b
Intensity" 2000 2000 2000 5000 2000 800 600 400 500 200 700 400 400 400 500 500 500 300 500 1200 800
300 300
Wa,velengtha (A*) 8.508.87 1. 8298.ii 2. 8281.05 3. 8190.05 4. 8112.90 5. 8104.36 6. 8059.50 7. 7913.44 8. 7854.82 9. 7694,54 10. 7685.24 11. 7601.54 12. 7587.41 13. 7224.10 14. 6056.13 15. 5870.91 16. 5570.28 17. 5562,22 18. 4273.97 19. b
Atomic Absorption Spectrum of Krypton
Intensity" 3000 5000 io00 3000 5000 5000 1000 200 800 . .
~
1000 1000 5000 1000 100 60 3000 2000 500 1000
Table V.
b
Energy levels6 (cm.-l) 95399.870-107131.755 93750.639-105617.315 95399.870-107289.747 93143.800-105462.804 93750.639-106087.305 93143.800-105617.315 93750.639-106237.597 94553.707-107131 755 94553.707-107131.755 93143.800-106237.597 93750.639-107054.319 95399.870-108722.668 93750.639-107289.747 93143.800-107289.747 93143.800-107496.463 106237 597-123903 295 106087 305-124692 02 105462 804-124609 917 104102 144-123882 30 104102.144-123903.295 93143.800-117183.654 95399 870-121470.304 95399.870-122601.290 95399.870-122635.128
Absorption, % 3.3 3.6 2.1 55.5 47.0 43.7 60.0 6.6 17.9 37.7 11.5 52.6 48.9 31.5 0.0
9.4 7.1 3.0 7.2
Energy levelsb (cm. -1) 85847.501- 97596.718 80917.561- 92965.194 85847.501- 97919.951 80917.561- 93124.140 79972.535- 92295.199 79972.535- 92308.177 85192.414- 97596.718 91169.313-103802.598 85192.414- 97919.951 79972.53.5- 92965.194 85847 501- 98855 871 79972 535- 93124 140 80917 561- 94093 662 91169 313-105008 054 91169 313-107676 953 80917.561- 97945.970 79972.535- 97919.951 79972.535- 97945.970 79972.535-103363.425
Wavelength and intensities taken from Ref. (11), Energy levels determined from Ref. (8).
Wavelength" (A.) 1. 8409.19 2. 8346.82 3. 8280.12 4. 8266.52 5. 8231.63 6. 8206.34 7. 7967.34 8. 7887.39 9. 7386.00 10. 6498.72 11. 6198.26 12. 4829.71 13. 4807.02 14. 4671.23 15. 4624.28 a
Absorption, 70 2.0 51.1 4.3 73.7 3.2 59.1 31.6 38.7 72.6 53.1 28.5 48.3 30.6 34.8 5.3 16 6 15 8 23 5 31.6 2.2 5.9 11.8 11.8
Wavelength and intensities taken from Ref. (11). Energy levels determined from Ref. (8).
Table IV.
a
Atomic Absorption Spectrum of Argon
Atomic Absorption Spectrum of Xenon
Intensityb 2000 2000 5000 500 5000 700 500 300 100 100 100
400
500
2000 1000
Absorption, yo 30.9 8.8
47.1 8.6
51,l 31 . O
10.5 2.2 16.6 18.3 10.5 2.1 3.2 11.5 6.1
Wavelength and intensities taken from Ref. (11). Energy levels determined from Ref. (8).
1060
0
ANALYTICAL CHEMISTRY
Energy levelsa (cm. - I ) 67068.047-78956.538 77185.560-89162.880 68045.663-80119.474 77185.560-89279.233 67068.049-79212.970 76197.292-88379,647 76197.292-88745 ,081 77185.560-89860 ,538 77269.649-90805 ,045 78956.538-94339 ,949 77269.649-93398 ,758 68045.663-88745 ,081 68045.663-88842.781 67068.047-88469.732 67068.047-88687.020
significant for transitions that originate not in the ground state, but in these populated excited states. Other workers in the field of atomic absorption have found that good results were obtained by employing lines which start in absorption from metastable states, not the ground states. For example, Menzies (6) found the best lines, so far, for cobalt and nickel start absorbing from metastable levels. The reason for a significant population of atoms in the metastable states is the relatively long life times of electrons in these levels ( 7 ) . Since information indicating u-hich emission lines of the noble gases in the spectral region normally used for routine atomic absorption work would be strongly absorbed was not easily available, absorption measurements were obtained of all the intense (about or greater than 100) interference-free arc lines, some 300, listed in the book, Tables of Spectrum Lines (11), for the five stable noble gases. The possibility of using atomic emission for the determination of the noble gases was not considered at this time because atomic emission techniques are generally not as accurate as atomic absorption techniques. The reason for this is that the latter is essentially free of spectral line interferences. Consequently, instrumentation usually used for atomic emission is more expensive and sophisticated than equipment used for atomic absorption. Which method, atomic emission or atomic absorption, gives better sensitivity is not readily apparent at the present time since a limited amount of analytical work has been done with the noble gases employing discharge tubes. EXPERIMENTAL
The usefulness of hollow-cathode discharge tubes as absorption sources for atomic absorption work has been demonstrated by Walsh and coworkers (1, 10) and subsequent work done at this laboratory (2-4). Essentially the same apparatus that was used in the past at this laboratory (3) was employed to investigate the atomic absorption spectra of noble gases. Figure 1 shows a schematic diagram of the apparatus used. The Schuller-Gollnow watercooled hollow-cathode emission tube, containing aluminum electrodes, is described by Tolansky (1%'). It was oprated at 30 ma. using a gas pressure of 2 mm. of Hg for the various noble gases tested. The emitted light was modulated by a 60-cycle mechanical chopper and was focused on the hole of a copper cylindrical insert that was the cathode electrode of the absorption tube. The absorption tube was operated at, 75 ma. employing the noble gases a t the following gas pressures: helium, 1.0; neon, 0.8; argon, 0.5; krypton, 0.5; and xenon, 0.5 mm. of Hg. At these pressures, a uniform discharge filled the cathode cavity, and thus absorption of
Figure 1 . gases
-
-
Table VI. Precision Obtained for Per Cent Absorption of 7601.54-A. Krypton Spectral Line
Tracing no. 1
Schematic diagram of apparatus used for atomic absorption of noble
incident light was obtained. Approximately 1 x 10-6 mole of gas was in the absorption tube. The transmitted light coming from the absorption tube was focused on a 20-micron slit of the monochromator, Jarrell-Ash, Model 82-000, equipped with 1P21 and 1P28 photomultiplier tubes. The noble gas samples used were taken from liter bulbs of spectroscopically pure gases from Linde Air Products Co. The lowpressure samples taken into the emission and absorption tubes were circulated in the conventional fashion by independent mercury diffusion pumps. Independent d.c. constant power supplies were used with the two tubes. RESULTS
Of the approximately 300 spectral lines checked, about one third showed significant absorption. The highest absorption observed was 74%, and about 6% of the lines checked absorbed more than 40%. Tables I to V list the detailed results for all the lines that showed significant absorption. Each absorption value reported is an average of some three independent absorption tracings that were obtained. To test the precision of this atomic absorption technique, ten absorption tracings were obtained of the krypton 7601.54-A, line. For the results of this test, shown in Table VI, the emission tube was on all the time while the absorption tube was turned on and then off after each absorption tracing. Results indicate that the relative standard deviation is b0.017. DISCUSSION AND CONCLUSION
Absorption has been realized for many spectral lines of the noble gases in the 3000- to 8500-A. region employing a
conventional monochromator. All of these lines start absorption from excited energy levels, not from the ground level. Therefore, when noble gases are excited in a discharge tube, a significant number of excited atoms, capable of absorbing incident light, are present in the absorption path. From these foregoing observations, it is reasonable to conclude that atomic absorption shows much promise a t this time for the determination of noble gases without the use of sophisticated ultraviolet instrumentation as anticipated earlier. The primary purpose of this investigation was to identify spectral lines of the noble gases which might be useful for the determination of the noble gases by atomic absorption employing simple instrumentation that is presently used for routine atomic absorption work. The absorption values given here for micromoles of gas in the absorption tube apply to our particular experimental conditions. This sensitivity is comparable to sensitivities reported for common impurities employing routine atomic absorption techniques and can be increased by using a longer discharge. No attempt was made at this time to study mixtures of pure gases, determine lower limits of detection, or plot analytical curves. This investigation work is in progress at this laboratory and will be reported at a later date. ACKNOWLEDGMENT
The author is indebted to three of our staff members: Howard Claassen for helpful consultations concerning the work described and Geoffrey Smith and Athos Giacchetti for informative discussions about metastable energy levels.
Light absorbed,
5%
53.5 51.8 52.0 52.4 51.7 51.8 52.1 51.1 50.9 50.4 Mean 51.8 Std. dev.: f 0 . 9 Rel. std. dev.: A0.017 2 3 4 5 6 7 8 9 10
Deviation from mean +1.7 0.0 +0.2 $0.6 -0.1 0.0 +0.3 -0.7 -0.9 -1.4
LITERATURE CITED
(1) Gatehouse, G. M., Walsh, A., Spectrochim. Acta 16, 602 (1960). (2) . . Goleb, J. A., Anal. Chim. Acta 30, 213 (1964). ’ (3) Goleb, J. A., Ibid., 34, 135 (1966). (4) Goleb, J. A., Brody, J. K., Ibid., 28, 457 (1963). (5) McGrath, W. D., Pickering, W. F., Magee, R. J., Wilson, C. L., Talanta 9,227 (1962). f6) Menzies. A. C.. ANAL.CHEM.32. 898 (1960). (7) Mitchell, A. C.,, Zemansky, M. .W., “Resonance Radiation and Excited Atoms,” University Press, Cambridge, ~. 1934. (8) Moore, C. E., “Atomic Energy Levels.” Circular of the National Bureau of Standards 467, Washington, D. C., 1959. (9) Robinson, J. W., in “Developments in Applied Spectroscopy,” E. N. Davis, Ed., Vol. 4, p. 455, Plenum Press, New York, 1965. 10) Russell, B. J., Walsh, A., Spectrochim. Acta 15, 883 (1959). 11) Saidel, A. N., Prokofjew, W. K., Raisk?, S. M.,“Tables of Spectrum Lines, Veb Verlag- Technik, Berlin, 1961.’ 12) Tolansfry, S., “High Resolution Spectroscopy, Methuen, London, 1947. \ - ,
J. A. GOLEB Chemistry Division Argonne National Laboratory Argonne, Ill. RECEIVED for review November 26, 1965. Accepted May 4, 1966. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
VOL. 38, NO. 8, JULY 1966
1061
