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The Laser Diode as a Light Source for Atomic
Absorption Spectroscopy
Bryan Holmes
Chem 226
Spring 2003
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Laser Diode Atomic Absorption Spectroscopy
Introduction
The development of laser diodes has increased the variety of practical
applications for lasers as light sources in analytical spectroscopy. Lasers are excellent
radiation sources due to their monochromaticity, good beam focusing capabilities, and
small line-widths but have traditionally been expensive and difficult to operate, limiting
their applications to specialized fields. Because the stability of the light source for atomic
absorption spectroscopy is critical, tunable dye lasers and optical parametric oscillators
are not well suited despite their high power densities. Due to their low cost, tunability of
several nanometers, stable output power, compactness and ease of operation, diode lasers
are finding an increasing number of uses in analytical spectroscopy. Some of these
applications include laser-enhanced ionization spectroscopy, laser-excited atomic
fluorescence spectroscopy, resonance ionization mass spectrometry, and specifically,
atomic absorption spectroscopy.
Diode laser technology is based on the optical emission of semi-conducting
materials when population inversions are created with applied electrical potentials. The
emission wavelength is mainly dependent on the semi-conducting material (Table 1).
Diode lasers used in analytical spectroscopy are typically small (300 m x 300 m x 150
m), have low power (mW), and convert electrical power to produce optical radiation
with high (10-30%) efficiency. Currently, diode lasers used for analytical spectroscopy
are commercially available in the 630-1600 nm range. Second harmonic generation using
non-linear crystals can yield 0.1 W between 335 and 410 nm and as much as 1-3 W in
the 410-430 nm range. Due to poor conversion efficiencies, power losses are significant
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with harmonic generation. Elemental transitions with energies matching ultraviolet
photons energies will be accessible by sum frequency generation, a nonlinear photon
conversion technique that will be able to yield ultra-violet radiation in the 10-100 nW
range in the near future.[1] The limited wavelength ranges available with diode lasers,
along with the poor conversion efficiencies of nonlinear optics limits the elements
accessible by diode laser atomic absorption spectroscopy. Devices emitting in the yellow,
green and blue regions have been developed but have had problems with short lifetimes
(sometimes minutes or hours). Robust diode lasers emitting in the blue region are
becoming commercially available.[2] For example, Melles Griot has recently introduced
a violet diode laser with output at 405 nm and 440nm, producing 6 mW and 1 mW of
power, respectively.
Laser Diode Operation and Characteristics
When atoms combine to form a solid (in this case, the semi-conducting material
in a diode laser), the previously distinct energy levels of the free atoms overlap and
broaden due to neighboring interactions. These broadened energy levels are called
energy bands. The outermost electron's energy states broaden and combine to form the
valence band. The collection of the lowest, unoccupied states forms the conduction
band.The emission wavelengths () depend on the band gap of the semiconductor
material. The energy spacing between the top of the highest filled energy band (the
valence band) and the bottom of the lowest unfilled band (conduction band) is referred to
as the band gap. Laser light in a diode is created by sending a current (injection
current) through the active part of the diode containing the n- and p-type cladding layers
(Figure 1) producing electrons (n-type, negative) and holes (p-type, positive) which
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recombine to emit radiation. The most common type of diode laser confines this active
area to a stripe generally 1-10 m wide, known as a stripe-geometry type laser. A
narrow channel that confines the light in the active region controls the lasers spatial
mode. Two types of diode lasers, gain guided or index guided, arise from different
manipulations of the transverse laser mode. The confinement of the transverse laser mode
is obtained by spatial variation of injection current density by resistive materials (gain
guided) or the spatial confinement of light caused by changes in the materials index of
refraction used in the lasers construction (index guided).[3] In both cases, limiting laser
action to a narrow space forms a wave-guide where there is gain. Also, this manipulation
improves the beam quality by helping to limit oscillations to a single transverse mode and
often to a single longitudinal mode. [4]
Gain-guided lasers are simpler than index guided lasers to make but rarely operate
with stability in a single longitudinal mode due to a relatively weak wave-guiding effect.
Also, non-linearity occurs in the output-versus-current characteristics. The advantage of
gain-guided laser diodes is their ability to achieve higher powers. [4] For gain-guided
lasers, powers of 200 mW are common and, in isolated cases, lasers up to 10 watts are
commercially available.
Most commercial single-stripe geometry, index-guided laser diodes emit radiation
with a single spatial and longitudinal mode (Figure 2). However, side modes appear just
above the lasing threshold at low injection currents. The cavity-length of a diode laser is
several hundred micrometers and the longitudinal modes are separated by 0.1-1 nm,
making it possible to measure these modes with a simple, low-resolution monochromator.
Index-guided lasers are advantageous due to their narrow bandwidth, relatively stable
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operation, and single longitudinal mode operation. Due to their design, they operate at a
lower power than gain-guided lasers (typically 3-40 mW). [3]
The wavelength emitted by a diode laser is primarily controlled by the band gap
of the semi-conducting material. A modest tuning range is available by varying the
current or temperature of the device. Generally, the tuning range is wide (30 nm) but
tuning of the longitudinal modes is restricted to a few nanometers. [5] Modifying the
temperature changes the laser cavity length as well as affects the gain within the lasing
cavity. Gain and optical path-length have different dependencies on temperature. As seen
in figure 3, the temperature-tuning curve exists, in the ideal case, as a staircase with
sloping steps. The slope of the steps is caused by cavity mode tuning, where the jumps
between steps are due to transitions from one longitudinal mode to another, a process
known as mode-hopping (0.3 nm). Since diode lasers are susceptible to optical
feedback (e.g. reflections), methods utilizing this sensitivity have been developed which
use optical feedback to control unwanted wavelength changes, including mode-hopping.
[3] Wavelength tuning is also accomplished by altering the injection current. Figure 4
shows the dependence of laser power on current and temperature. Changing the injection
current affects the temperature in the cavity of the diode laser as well as the gain
properties within the cavity, both varying the emitted wavelength. Mode-hopping is
affected by both the injection current and temperature. [3] Compared to other tunable
laser systems, restricted wavelength range and mode-hopping are limitations to this diode
lasers as a light source. Many spectroscopists are optimistic that the fast growth of the
semi-conductor industry as well as improvements in frequency-doubling technology will
reduce these limitations. [6]
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Analysis Techniques using DL-AAS
Laser diodes have excellent spectroscopic and operational properties for
analytical spectroscopy, providing an almost ideal line source for atomic absorption
spectrometry. Diode lasers typically have line-widths of 20 MHz, which is about 30 times
less than the atomic absorption line-widths for low pressure, room temperature atomic
reservoirs. Compared to atomic line-widths in flames, diode lasers line-widths are about
100 times less. The stability, tunability and line characteristics of diode lasers increase
experimental freedoms and opportunities within the field of atomic absorption
spectrometry. [2]
Generally, lasers offer advantages to the hollow cathode lamp as a light source for
atomic absorption spectroscopy because they are monochromatic, directional, and have
relatively high powers and narrow line-widths. The use of lasers adds simplicity to the
experimental arrangement because monochromators are generally not needed for
wavelength selection and simple photodiode detectors can be used to discriminate
between the laser signal and background absorption. Hollow cathode lamps have a
limited dynamic range of two to three orders of magnitude due to optical thickness in the
centers of the absorption lines. For optically thin conditions, diode lasers tunability offers
the advantage of selecting wavelengths on the wings of the absorption lines, extending
the linear dynamic range many orders of magnitude for high concentrations of analyte. In
addition, the linear dynamic range can be increased to lower concentrations by passing
the laser radiation through a multi-pass cell. This is an absorption arrangement where
radiation passes through the sample multiple times, usually using prisms or partially
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reflecting mirrors. Also, tuning the diode laser far off the resonance transition,
background corrections can be made for isobaric interferences and light scattering. [7]
Experimental Setup
A basic setup for diode laser atomic absorption spectroscopy using flame
atomization is shown in figure 5.James Winefordner and coworkers [8] created this
arrangement to measure rubidium using the 780.0 nm transition with a single-mode diode
laser. Due to the diode lasers single mode operation, a wavelength-selection
monochromators is not needed, which simplifies the experimental arrangement. The
transmission signal was monitored using a diode array spectrometer and a
monochromator/photomultiplier tube detector. Due to the diode array spectrometers low
resolution and stray light problems, wavelength monitoring was performed with the
monochromator/photomultiplier tube detector. This allowed for observing experimental
figures of merit such as determining laser power parameters where 100% absorption
occurred for the system.
Background Corrections
One of the valuable features of the laser diode is the availability of different
wavelengths either by temperature/current tuning or through multi-mode operation.
Despite the good stability of diode lasers, absorption experiments using this source are
generally source noise limited. In order to improve sensitivity, tunable properties of the
diode laser are utilized in background correction methods, which are ubiquitous
techniques used in most modern diode laser atomic absorption spectroscopy.
One method for background correction uses an off-resonance longitudinal mode
of a multi-mode diode laser to monitor source noise at the detector. James Winefordner
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and coworkers report the detection of lithium at its 670.780 nanometer resonance line
using a diode-array spectrometer to monitor the multi-mode laser emission. Figure 6
shows the spectral lines of the multi-mode diode laser with the A line, representing the
lithium resonance line position. For this experiment, the peaks line positions are
generally stable, but the line intensities varied. As shown in figure 7, a peak-ratio
measurement was performed using the resonance line (A) and a nearby off-resonance line
(B), improving precision by correcting for source noise. Without this correction method,
the absolute measurement of the absorbance line was 7% relative standard deviations for
five trials. Precision was enhanced by the peak ratio method as seen in the improvements
of the relative standard deviation to less than 1%. The sensitivity (1% absorption) was 20
ng/mL (ppb) and the detection limit (2of blank) was 4 ppb. These values are similar to
those (16 and 1 ppb, respectively) of conventional flame atomic absorption using a
hollow cathode lamp. [6]
Using a multimode laser diode, background correction is performed through peak
absorption measurements similar to that of the Zeeman background correction using a
hollow cathode lamp. To perform this correction, the multimode laser diode is tuned so
that lines are positioned on both sides of the analyte resonance line. With multimode
diode lasers, these nearby non-resonance lines are always accessible, allowing real-time
background correction. This technique is especially beneficial to fast atom reservoirs
such as the graphite furnace. [6]
Due to its fast tuning capability, a background correction method known as
wavelength modulation (WM) has been applied to laser diode atomic absorption
spectroscopy. This method is most often used in atomic spectroscopy, where a small
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wavelength range () is scanned rapidly back and forth across a wavelength interval,
which is on and off the resonance line. Conventionally, measurements using wavelength
modulations are performed by oscillation of an optical component (e.g. grating,
interference filter, refractor plate) near the exit slit of a monochromator. The detector
measures a signal whose magnitude is related to the signal change over the . [9] The
signal to noise ratio depends on the spectral width of the source and on the spectral
radiance. Laser diodes are better sources for wavelength modulation absorption
spectroscopy compared to classical instrumentation because of their inherent simplicity,
relatively narrow line-widths and fast tunability.
Many different variations of wavelength modulation atomic absorption
spectroscopy have been applied to address background correction issues due to dark
detector noise, shot noise from the laser and photo-detection, laser excess noise, non-
specific absorption and optical excess noise. Liger and coworkers [10] report a double-
modulation and double-beam technique for the detection of gaseous meta-stable chlorine
atoms to suppress these noise sources. In the single-beam, double modulation technique,
the laser diode wavelength is altered sinusoidally at a specific frequency while the low-
pressure microwave induced plasma or d.c. plasma atomization source is modulated at 5
kHz (Figure 8). The modulation frequency was mixed with the second harmonic of the
laser modulation frequency and the signal for lock-in amplification. This is known as 2f
wavelength modulation. The detection limit measured with a time constant of =1 is
about 1x10-6
absorbance units. A double-beam arrangement with logarithmic differential
amplification of the signals from the absorption and reference channels realized a
detection limit of 2x10-7
absorbance units with a time constant of =1. Generally, hollow
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cathode lamps express detection limits of 10-4
absorbance units. Diode lasers offer
advantages to other sources due to their increased stability and fast tunability, reducing
the detection limit by 2-3 orders of magnitude compared with classical atomic absorption
spectroscopy.
Isotope Selective Analysis
When isotopic line shifts are sufficient to permit spectral separation, diode lasers
offer the capability for isotope selective analysis. Sufficiently large isotopic shifts are
only expressed for light (e.g, Li) and heavy (e.g., U) elements, when isotope shifts are
larger than the Doppler broadening of the spectral lines. Isotope analysis is usually
measured using mass spectrometry. The narrow line-widths emitted from single-mode
diode lasers (typically 0.4 pm in red and infrared) offers capability for high-resolution
absorption spectroscopy, making this a competitive technique with mass spectrometry for
isotope analysis.
Lead isotopes in aqueous samples were measured by Wizemann and Niemax[11]
using atomic absorption in a furnace atomic non-thermal excitation spectroscopy
atomizer (FANES) which is an electrothermally heated graphite tube atomizer with
integrated low-pressure d.c. discharge (Figure 9). A frequency-doubled diode laser
delivers radiation at 405.78 nm as the excitation source and calibration was made by
isotope dilution. In order to achieve isotopic spectral resolution, measurements were
made in a low-pressure atom reservoir to limit pressure broadening and the FANES
instrument populated the 6p23P2meta-stable electronic level. As the FANES is prone to
severe matrix effects, application of wavelength modulation and isotope dilution
provided a matrix independent analysis. Detection limits are measured to be 4.5x10-3
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absorbance units for206
Pb and208
Pb. These are relatively poor values. These
measurements are shot-noise limited due to the low laser power of the frequency-doubled
radiation but should theoretically increase by at least 3 orders of magnitude by using laser
powers of 2 milliwatts instead of 20 nanowatts. It is expected that new types of laser
diodes with higher powers (milliwatts) and operational wavelengths in the blue will help
realize these goals.
Isotope selectivity using diode laser absorption techniques has also been shown
with solid samples. Recently, the coupling of laser ablation with diode laser-based
technique has proved to be useful for probing gaseous atoms from solid samples in the
expanding plasma plume. Quentmeier and coworkers [12] have demonstrated this
technique for detection of the235
U and238
U isotopes by laser ablation diode laser atomic
absorption spectroscopy. The need for a field instrument that is compact and portable led
to the development of many different optical techniques for solids in order to minimize
instrumental contamination. Because of the high concentrations of uranium atoms in the
laser plume, absorption proved to be a superior technique, even compared to
fluorescence. Unfortunately, due to spatial variations in the laser plume, initial studies on
these methods suffered from poor precision.
Niemax and coworkers [13] improved the sensitivity and selectivity of laser
ablation diode laser atomic absorption spectroscopy by characterizing the spatial and
temporal characteristics of the laser plume. This was done by using two diode lasers as
shown in figure 10 to measure the absorbance of the Uranium isotopes simultaneously in
order to improve reproducibility and accuracy. A relative standard deviation of 2-3% is a
significant improvement using this double-beam technique. This method is a competitive
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to alternative techniques such as laser-induced plasma emission and optogalvanic
spectroscopy.
Multi-element Analysis
The laser diodes simplicity, small size and low-cost, along with its many other
virtues, makes it a good source for multi-elemental analysis. The well-collimated beam of
a single diode laser allows a setup where many diode lasers can be directed
simultaneously through an absorption pathlength. Hergenroder and Niemax [14]
demonstrated multi-element detection of rubidium and barium in a graphite furnace. Two
semi-conductor lasers were tuned to the rubidium resonance line 5s
2
S1/2-5p
2
P3/2 at 780.2
nm and the barium inter-combination line 6s21S0-6s6p
3P1at 791.1 nm, respectively. For
background correction, the wavelength modulation technique was used. As the power of
the diode laser is modulated sinusoidally with a frequency,f1, Fourier analysis is used to
analyze the signal measured with a simple photodiode (Figure 11). By applying two
separate diode lasers at different modulation frequencies, the information can be
converted from the time to the frequency domain, where the relevant components of the
Fourier spectrum can be associated with specific elements.
Groll and Niemax [15] extended this method to the simultaneous detection of
lithium, potassium, rubidium, and cesium in an air-acetylene flame and of lithium,
potassium, rubidium, cesium, strontium and barium in a commercial graphite-tube
furnace using six independent laser diodes. Fourier analysis of more than two elements is
possible but requires a higher quality and more expensive analyzer. Instead, the
arrangement works as a quasi-simultaneous time-multiplex technique whereby only
one of the diode lasers is tuned to the resonance position and all other wavelength are
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positioned off-resonance. These measurements are comparable with the best data of
conventional single-element atomic absorption spectroscopy using hollow cathode lamps
published in the literature. The authors feel that optimization of the flame and graphite
furnace will have a significant impact on lowering the detection limits for this technique
even further.
Present and Future
Diode Laser Absorption Sensors
Laser diode sensors are currently being developed for interrogation of high-
pressure, transient, and sometimes hostile environments. Many laser-based strategies use
inefficient, complex, large and expensive pulsed lasers that are not practical for real-time
control applications. Advanced, wavelength agile diode lasers, known as, vertical cavity
surface-emitting lasers (VCSEL), are now available that offer rapid, broad wavelength
scanning capabilities compared to traditional diode-laser absorption techniques. The
VCSEL's principle of operation is similar to those of conventional index-guided, edge-
emitting semiconductor lasers. The lasing cavity of the VCSEL is an electrically pumped
gain region, also called the active region, which emits light. Layers of varying
semiconductor materials above and below the gain region create mirrors. Each mirror
reflects a narrow range of wavelengths back into the cavity causing light emission at a
single wavelength.
Broad-wavelength scanning diode lasers are increasing the analytical possibilities
of absorption spectroscopy. Sanders and coworkers [16] are developing wavelength-agile
sensing strategies for in situdetection of species in combustion sources. In this work,
cesium seeding is used as a tracer species in order to characterize this strategy. A
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VCSEL is used to monitor the temperature and pressure in a pulse detonation engine. By
using aggressive injection current modulation, the VCSEL is scanned through a 10 cm-1
spectral range at MHz rates. This is about ten times the scanning range and 1000 times
the scanning rate of a conventional diode laser. The VCSEL scans 10 cm-1
every
microsecond to measure cesium absorption and thermal emission waveforms (Figure 12).
This is significant because absorption bands in the high pressure and high temperature
gases can increase the collisional widths up to 4 cm-1
. The fast, broad-scanning
capabilities of the VCSEL allows for measurements in these harsh conditions. The
advancement of diode laser technology will make shorter wavelengths available so that
real-time detection of native species in combustion gases is increasingly practical. The
fast timeframes of the relatively cheap and simple VCSEL allows for the rapid
acquisition of data in dynamic and hostile environments, such as the high temperature
and pressure flue gases. Improvements for diode-laser sensor systems will include faster
algorithms for reducing the computational time, incorporation of laser-diode multiplexing
techniques for simultaneous multi-species measurements, and refined digital filtering
techniques for detection.
Oscillator strength (fabs) and Atomic Absorption Spectroscopy
The oscillator strength is a fundamental quantity in analytical atomic
spectroscopy. It determines the sensitivity of a transition and is described by the number
of electrons per atom or molecule undergoing a radiative transition. This value must be
accurately known if one is to correlate an absorption signal to the elemental
concentration. Hannaford [17] describes a surprising result related to the oscillator
strength, namely an independence of the peak absorption coefficient on the absorption
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oscillator strength for atoms where the absorption line is broadened only by natural
broadening. The peak absorption only becomes dependant on the absorption oscillator
strength when broadening processes such as Doppler and collisional broadening occur.
Because of this relationship, a spectral line, which has a natural width of 10-4
will
have a peak absorption 100 times larger than an absorption line which has a collisional
width of 10-2
. As Doppler and collisional broadening become more significant,
absorbance becomes increasingly dependant on the oscillator strength,fabs, and the half-
widths of the Doppler broadened and collisional broadened absorption lines. [2]
In order to interrogate an atomic absorption line where the broadening is only due
to natural broadening, a radiation source would have to have an emission line half-width
less than the natural broadened absorption line. Diode lasers are a suitable source for an
application of this nature, in which atoms are cooled so that all broadening sources except
for natural broadening are minimized. The limitations and challenge to these
investigations is that the cooling of atoms is complex, expensive, and sometimes
impossible with many atomic absorption atomizers. [2]
Hannaford and McLean [18] have shown theoretically that the absorption signal
at the line center for ultra-cold, quasi-stationary absorbing atoms and a near-
monochromatic resonant radiation source is independent of the oscillator strength. The
narrow lines widths of a diode laser are used to measure these relationships with cesium
atoms. The goal is to perform oscillator strength-free atomic absorption where the atomic
absorption signal at the line center is equal in magnitude for both strong and weak
(closed) transitions of the same wavelength. Hannaford and McLean demonstrate that it
is possible to perform atomic absorption experiments based on ultra-cold, laser-cooled
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cesium atoms using a diode laser source. These experimental conditions approximate the
conditions necessary for oscillator-strength free atomic absorption but major technical
challenges must be addressed before the analysis of ultra-cold atoms with diode lasers
can be used in atomic analytical spectroscopy.
Conclusions
Diode lasers offer many benefits compared to conventional sources for atomic
absorption spectroscopy. Their stability, low-cost, simplicity, narrow line-widths, and
rapid tunability make diode lasers an advantageous source compared to the traditional,
hollow cathode lamp. Because of the limited wavelengths available with diode lasers,
roughly 45% of the elements measured by hollow cathode sources are accessible.
Detection limits for basic experimental arrangements using diode lasers are on the same
order as the detection limits for hollow cathode lamp atomic absorption spectroscopy. By
applying background correction techniques using the diode lasers rapid scanning abilities,
increased selectivity, a larger linear dynamic range and much lower detection limits can
be achieved. Due to market driven forces, diode lasers with shorter wavelengths and
higher powers are becoming more available, increasing the number of elements that are
measurable by diode laser atomic absorption spectroscopy.
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Literature Cited
1. Nakamura, S., Senoh, M., Nagahama, N.,InGaN/GaN/AlGaN-based laser diodes
with cleaved facets grown on GaN substrates.Applied Physical Letters, 1998. 72:
p. 211.
2. Winefordner, J.D., et al.,Novel uses of lasers in atomic spectroscopy.Journal of
Analytical Atomic Spectrometry, 2000. 15(9): p. 1161-1189.
3. Wieman, C.E., Hollberg, L., Using diode lasers for atomic physics.Review of
Scientific Instruments, 1991. 62: p. 1-20.
4. Hecht, J., The Laser Guidebook. 2 ed. 1992, New York: McGraw-Hill Inc.
5. Imasaka, T.,Diode lasers in analytical chemistry.Talanta, 1999. 48: p. 305-320.
6. Ng, K.C., et al., The applicability of a multiple-mode diode laser in flame atomic
absorption spectroscopy.Applied Spectroscopy, 1990. 44(5): p. 849-52.
7. Niemax, K., H. Groll, and C. Schnuerer-Patschan,Element analysis by diode laser
spectroscopy.Spectrochimica Acta Reviews, 1993. 15(5): p. 349-77.
8. Ng, K.C., et al.,Flame atomic absorption spectroscopy using a single-mode laser
diode as the line source.Applied Spectroscopy, 1990. 44(6): p. 1094-6.
9. Ingle, J.D., Crouch S. R., Spectrochemical Analysis. 1988, Englewood Cliffs:
Prentice Hall Inc. 445-446.
10. Liger, V., et al.,Diode-laser atomic-absorption spectrometry by the double-beam-
double-modulation technique.Spectrochimica Acta, Part B: Atomic
Spectroscopy, 1997. 52B(8): p. 1125-1138.
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11. Wizemann, H.D. and K. Niemax, Cancellation of Matrix Effects and Calibration
by Isotope Dilution in Isotope-Selective Diode Laser Atomic Absorption
Spectrometry.Analytical Chemistry, 1997. 69(20): p. 4291-4293.
12. Quentmeier, A., Bolshov, M, and Niemax, K.,Measurement of uranium isotope
ratios in solid samples using laser ablation and diode laser-excited atomic
absorption spectrometry.Spectrochimica Acta, Part B: Atomic Spectroscopy,
2001. 56: p. 45-55.
13. Liu, H., A. Quentmeier, and K. Niemax,Diode laser absorption measurement of
uranium isotope ratios in solid samples using laser ablation.Spectrochimica
Acta, Part B: Atomic Spectroscopy, 2002. 57B(10): p. 1611-1623.
14. Hergenroeder, R. and K. Niemax,Laser atomic absorption spectroscopy applying
semiconductor diode lasers.Spectrochimica Acta, Part B: Atomic Spectroscopy,
1988. 43B(12): p. 1443-9.
15. Groll, H., K. Niemax,Multielement diode laser atomic absorption spectrometry
in graphite tube furnaces and analytical flames.Spectrochimica Acta, 1993.
48B(5): p. 633-641.
16. Sanders, S.T., Daniel W. Mattison, Lin Ma, Jay B. Jeffries, and Ronald K.
Hanson, Wavelength-agile diode laser sensing strategies for monitoring gas
properties in optically harsh flows: application in cesium-seeded pulse detonation
engine.Optics Express, 2002. 10(12): p. 505-514.
17. Hannaford, P., The oscillator strength in atomic absorption spectroscopy.
Spectrochimica Acta, Part B: Atomic Spectroscopy, 1994. 49B(12-14): p. 1581-
1593.
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18. Hannaford, P. and R.J. McLean,Atomic absorption with ultracold atoms.
Spectrochimica Acta, Part B: Atomic Spectroscopy, 1999. 54B(14): p. 2183-2194.
Tables and Figures
Table 1
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
Figure 9
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Figure 10
Figure 11
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Figure 12
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