Detection of trace amounts of Cr by two laser-based spectroscopic techniques: laser-enhanced ionization in flames and laser-induced fluorescence in graphite furnace
Ove Axner and Halina Rubinsztein-Dunlop, "Detection of trace amounts of Cr by two laser-based spectroscopic techniques: laser-enhanced ionization in flames and laser-induced fluorescence in graphite furnace," Appl. Opt. 32, 867-884 (1993)
The detectability of Cr in water solutions by two-step laser-enhanced ionization (LEI) in flames and two-step excitation laser-induced fluorescence in graphite furnace (LIF–GF) with nonresonant detection is investigated for what is, to our knowledge, the first time. A thorough investigation of possible excitation and detection routes for Cr for both techniques is given. The detection limit of Cr in water by the LEI technique was found to be 2 ng/mL, while the LIF–GF technique showed a detection limit of 1.4 pg (which corresponds to 0.3 ng/mL, with a 5-μL sample volume), both of which are limited by contamination (from the burner head–nebulizer unit in the flame and from the graphite material in the furnace). A more sensitive two-step LEI excitation scheme than that used here is also proposed. A new technique for reducing fluctuations from blackbody radiation by using sequential detection of the blackbody radiation from one photomultiplier by two boxcar integrators is presented. A possible means of increasing the nonresonant signal in two-step excitation LIF–GF by adding small amounts of quenching enhancing N2 to the Ar atmosphere gives no positive results. The influence of large amounts of Na on the detectability of Cr by LEI is investigated.
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Possible Two-Step Transition Wavelengths from the Ground State(3d54s7S3) within the Septet System in Cra
First-Step Transition Wavelengths from the Ground State (nm)
Intermediate Level
Second-Step Transition Wavelengths from the Intermediate Level (nm)
Upper Level
Configuration and Term
Energy (cm−1)
Configuration and Term
Energy (cm−1)
425.4–429.0
3d54p7P2–4
23 305–23 499
735.6–746.2
3d55s7S3
36 896*
527.5–533.0
3d54d7D1–5
42 253–42 261*
447.5–451.4
3d56s7S3
45 643
426.1–432.1
3d44s5s7D1–5
46 449–46 959*
409.8–413.0
3d55d7D1–5
47 700–47 710
386.4–389.3
3d57s7S3
49 178
331.8–335.6
3d44s4d7D2–5
53 195–53 628
357.9–360.5
3d44s4p7P2–4
27 729–27 935
1090.6–1115.7
3d55s7S3
36896
688.2–698.1
3d54d7D1–5
42 253–42 261*
558.0–564.6
3d56s7S3
45 643
525.5–534.5
3d44s5s7D1–5
46 449–46 959*
505.6–505.8
3d55d7D1–5
47 700–47 710
466.1–470.6
3d57s7S3
49 178
389.1–394.4
3d44s4d7D2–5
53 195–53 628*
The transitions that are marked with an asterisk (*) have documented transition probabilities for the second-step excitation in the literature.30
Table 2
Possible Two-Step Transition Wavelengths from the 3d54s7S3 Ground State to the 3d54d7D1–5 States at 42 253–42 262 cm−1 in Cr
First-Step Transition Wavelengths from the Ground State (nm)
Second-Step Transition Wavelengths from the Intermediate Level (nm)
Intermediate Level
Upper Level
Configuration and Term
Energy (cm−1)
Configuration and Term
Energy (cm−1)
428.97
527.602
3d54p7P2
23 305.01
3d54p7D1
42 253.42
428.97
527.571
3d54p7P2
23 305.01
3d54p7D2
42 254.52
428.97
527.523
3d54p7P2
23 305.01
3d54p7D3
42 256.26
427.48
529.846
3d54p7P3
23 386.35
3d54p7D2
42 254.52
427.48
529.797
3d54p7P3
23 386.35
3d54p7D3
42 256.26
427.48
529.738
3d54p7P3
23 386.35
3d54p7D4
42 258.37
425.43
532.974
3d54p7P4
23 498.84
3d54p7D3
42 256.26
425.43
532.914
3d54p7P4
23 498.84
3d54p7D4
42 258.37
425.43
532.838
3d54p7P4
23 498.84
3d54p7D5
42 261.06
Table 3
Possible One-Step Transition Wavelengths from the 3d54S7S3 Ground State In Cr Together with the Maximum Wavelength Difference within Each Set of Transitions
Transition Wavelengths from the Ground State (nm)
λmax–λmin within Each Configuration (nm)
Upper State
Configuration and Term
Energy (cm−1)
428.97
3.54
3d54p7P2
23 305.010
427.48
3d54p7P3
23 386.350
425.43
3d54p7P4
23 498.840
360.53
2.66
3d44s4p 7P2
27 728.870
359.39
3d44s4p 7P3
27 820.230
357.87
3d44s4p 7P4
27 935.260
236.68
0.21
3d55p7P2
42 238.040
236.59
3d55p7P3
42 254.110
236.47
3d55p7P4
42 275.200
Table 4
Relative LEI Signal Strengthsa for Various Combinations of First-and Second-Step Excitations between the 3d54s7S Ground State through the 3d54p7P State to the 3d54d7D State in Cr
Signal strengths are in arbitrary units.
Wavelengths are in nanometers.
The underlined values indicates direct transitions while the boldface value in italics represents the most sensitive excitation scheme that was also used for the subsequent LIF–GF measurements.
Table 5
Wavelength Difference between some Cr Excitation Transitions and Various Na 3p–ns and 3p–nd Transitions, which give rise to Spectral Interferences
The measurement notations are found in Table 5; they indicate which transitions are being detected.
pe, number of photoelectrons.
Tables (7)
Table 1
Possible Two-Step Transition Wavelengths from the Ground State(3d54s7S3) within the Septet System in Cra
First-Step Transition Wavelengths from the Ground State (nm)
Intermediate Level
Second-Step Transition Wavelengths from the Intermediate Level (nm)
Upper Level
Configuration and Term
Energy (cm−1)
Configuration and Term
Energy (cm−1)
425.4–429.0
3d54p7P2–4
23 305–23 499
735.6–746.2
3d55s7S3
36 896*
527.5–533.0
3d54d7D1–5
42 253–42 261*
447.5–451.4
3d56s7S3
45 643
426.1–432.1
3d44s5s7D1–5
46 449–46 959*
409.8–413.0
3d55d7D1–5
47 700–47 710
386.4–389.3
3d57s7S3
49 178
331.8–335.6
3d44s4d7D2–5
53 195–53 628
357.9–360.5
3d44s4p7P2–4
27 729–27 935
1090.6–1115.7
3d55s7S3
36896
688.2–698.1
3d54d7D1–5
42 253–42 261*
558.0–564.6
3d56s7S3
45 643
525.5–534.5
3d44s5s7D1–5
46 449–46 959*
505.6–505.8
3d55d7D1–5
47 700–47 710
466.1–470.6
3d57s7S3
49 178
389.1–394.4
3d44s4d7D2–5
53 195–53 628*
The transitions that are marked with an asterisk (*) have documented transition probabilities for the second-step excitation in the literature.30
Table 2
Possible Two-Step Transition Wavelengths from the 3d54s7S3 Ground State to the 3d54d7D1–5 States at 42 253–42 262 cm−1 in Cr
First-Step Transition Wavelengths from the Ground State (nm)
Second-Step Transition Wavelengths from the Intermediate Level (nm)
Intermediate Level
Upper Level
Configuration and Term
Energy (cm−1)
Configuration and Term
Energy (cm−1)
428.97
527.602
3d54p7P2
23 305.01
3d54p7D1
42 253.42
428.97
527.571
3d54p7P2
23 305.01
3d54p7D2
42 254.52
428.97
527.523
3d54p7P2
23 305.01
3d54p7D3
42 256.26
427.48
529.846
3d54p7P3
23 386.35
3d54p7D2
42 254.52
427.48
529.797
3d54p7P3
23 386.35
3d54p7D3
42 256.26
427.48
529.738
3d54p7P3
23 386.35
3d54p7D4
42 258.37
425.43
532.974
3d54p7P4
23 498.84
3d54p7D3
42 256.26
425.43
532.914
3d54p7P4
23 498.84
3d54p7D4
42 258.37
425.43
532.838
3d54p7P4
23 498.84
3d54p7D5
42 261.06
Table 3
Possible One-Step Transition Wavelengths from the 3d54S7S3 Ground State In Cr Together with the Maximum Wavelength Difference within Each Set of Transitions
Transition Wavelengths from the Ground State (nm)
λmax–λmin within Each Configuration (nm)
Upper State
Configuration and Term
Energy (cm−1)
428.97
3.54
3d54p7P2
23 305.010
427.48
3d54p7P3
23 386.350
425.43
3d54p7P4
23 498.840
360.53
2.66
3d44s4p 7P2
27 728.870
359.39
3d44s4p 7P3
27 820.230
357.87
3d44s4p 7P4
27 935.260
236.68
0.21
3d55p7P2
42 238.040
236.59
3d55p7P3
42 254.110
236.47
3d55p7P4
42 275.200
Table 4
Relative LEI Signal Strengthsa for Various Combinations of First-and Second-Step Excitations between the 3d54s7S Ground State through the 3d54p7P State to the 3d54d7D State in Cr
Signal strengths are in arbitrary units.
Wavelengths are in nanometers.
The underlined values indicates direct transitions while the boldface value in italics represents the most sensitive excitation scheme that was also used for the subsequent LIF–GF measurements.
Table 5
Wavelength Difference between some Cr Excitation Transitions and Various Na 3p–ns and 3p–nd Transitions, which give rise to Spectral Interferences