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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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