lamps for self reversal

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Lamps for Self Reversal (Smith-Hieftje)

Background Correction Technique.

Background correction in atomic absorption spectroscopy can use a Deuterium continuum

method or the Zeeman method utilising magnetic field polarisation. However, both of these

methods have limitations regarding the correction of uniformly distributed background. An

alternative method of background correction is the self reversal (Smith-Hieftje) technique in

which a high current is momentarily passed through the cathode producing a dense cloud of

neutral atoms in front of the cathode which effectively cuts off the stream of photons produced

during normal lamp operation at low current. This momentarily stops absorption in the flame the

spectrophotometer now reading the background absorption only, whilst at normal low current

operation theinstrument observes the sum of the absorption of the element and the background.

The spectrophotometer can then electronically subtract the background from the sample signal to

solve the analytical problems that may be encountered with other methods of background

correction. Cathodeon have developed a range of lamps specifically designed to be used at the

currents recommended by the manufacturers of instruments where the self reversal or Smith -

Hieftje method of background correction is available. These lamps have enhanced insulation to

cope with the high voltage pulse used by this background correction method, but may also be

used in normal atomic absorption applications. Not all elements are suitable for use with this

technique, the available range is included in the lamp listing.

Background correction techniques in LS AAS

In LS AAS background absorption can only be corrected using instrumental techniques, and allof them are based on two sequential measurements, firstly, total absorption (atomic plus

background), secondly, background absorption only, and the difference of the two measurements

gives the net atomic absorption. Because of this, and because of the use of additional devices inthe spectrometer, the signal-to-noise ratio of background-corrected signals is always significantly

inferior compared to uncorrected signals. It should also be pointed out that in LS AAS there is no

way to correct for (the rare case of) a direct overlap of two atomic lines. In essence there are

three techniques used for background correction in LS AAS:

Deuterium background correction

This is the oldest and still most commonly used technique, particularly for flame AAS. In this

case, a separate source (a deuterium lamp) with broad emission is used to measure the

background absorption over the entire width of the exit slit of the spectrometer. The use of a


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separate lamp makes this technique the least accurate one, as it cannot correct for any structured

background. It also cannot be used at wavelengths above about 320 nm, as the emission intensity

of the deuterium lamp becomes very weak. The use of deuterium HCL is preferable compared toan arc lamp due to the better fit of the image of the former lamp with that of the analyte HCL.

Smith-Hieftje background correction

This technique (named after their inventors) is based on the line-broadening and self-reversal of

emission lines from HCL when high current is applied. Total absorption is measured with normallamp current, i.e., with a narrow emission line, and background absorption after application of a

high-current pulse with the profile of the self-reversed line, which has little emission at the

original wavelength, but strong emission on both sides of the analytical line. The advantage ofthis technique is that only one radiation source is used; among the disadvantages are that the

high-current pulses reduce lamp lifetime, and that the technique can only be used for relatively

volatile elements, as only those exhibit sufficient self-reversal to avoid dramatic loss of

sensitivity. Another problem is that background is not measured at the same wavelength as total

absorption, making the technique unsuitable for correcting structured background.

Zeeman-effect background correction

An alternating magnetic field is applied at the atomizer (graphite furnace) to split the absorptionline into three components, the component, which remains at the same position as the originalabsorption line, and two components, which are moved to higher and lower wavelengths,

respectively (see Zeeman Effect). Total absorption is measured without magnetic field and

background absorption with the magnetic field on. The component has to be removed in thiscase, e.g. using a polarizer, and the components do not overlap with the emission profile of the

lamp, so that only the background absorption is measured. The advantage of this technique is

that total and background absorption are measured with the same emission profile of the samelamp, so that any kind of background, including background with fine structure can be correctedaccurately, unless the molecule responsible for the background is also affected by the magnetic

field; the disadvantage is the increased complexity of the spectrometer.

Absorptivity: also known as the molar extinction coefficient in molecular spectroscopy,it is the wavelength-dependent absorption of an analyte as a function of concentration andpathlength and is expressed in units of concentration -1 * cm -1.

Analyte: a sample component whose concentration is being measured (i.e. analyzed). Inatomic absorption spectroscopy (AA), this is an element.

Ashing: also referred to as charring, this is the step in a graphite furnace AA programthat is designed to remove matrix constituents that might interference with the

measurement of the analyte. Ashing temperatures vary from 200 to 1800 degrees C,

depending on the matrix and analyte element.

Atomization: the process of producing atoms for the atomic absorption measurement.The atom-forming process usually requires a high temperature (except for cold-vapor Hg

methods) which is produced by a flame or by electrical current flowing through a

resistive medium.
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Beer-Lambert Law: the law that defines a linear relationship between concentration andabsorbance. It is often written as absorbance = absorptivity * pathlength * concentration

or A = a*b*c.

Calibration: a quantitative procedure performed in order to relate the knownconcentration of standard solutions of the analyte element to the detector signal which is

generated from the analyte in the unknown solutions. An extensive discussion of variousmethods of calibration including bracketing and standard addition can be found on thissite in HTML, PDF, or MathCad formats.

Calibration curve: also known as a working curve, the relationship of instrumentresponse (absorbance) as a function of concentration. Ideally, this should be a linearrelationship in AA, under conditions that obey Beer's Law, where absorbance = (slope x

concentration) + intercept. Minor curvature can be corrected by a curve-fitting

algorithms.

Cold-Vapor: the method by which a cloud of atoms is produced from a solutioncontaining Hg ions. Hg in the +2 state is reduced by the addition of stannous chloride and

then swept by a flow of inert gas into a quartz-ended absorption cell kept at ~ 200

degrees C. Detector: the part of the instrument that converts radiant energy from the light source to

electricity. Typically a photomultiplier tube, but may also be a solid-state detector in

more modern instrumentation.

Detection Limit: the minimum amount of an analyte that can be detected reliably. This ismost often defined as three times the standard deviation of the blank measurement.

Deuterium Arc Background Correction: the first successful method to correct forbackground absorption in furnace AA, this method employs a continuum radiation source(the deuterium arc, a *white* light source) that is passed through the atomic vapor cell

along with the HCL radiation. While the deuterium arc is not significantly absorbed by

atoms of the analyte, it behaves similar to the HCL radiation with respect to molecular

absorption and scatter, thus allowing an accurate background correction. However, it isnot as accurate as Zeeman orSmith-Hieftje methods at high background absorbances that

approach or exceed 2.0 absorbance units.

Double-beam optics: the optical design whereby a percentage of the radiation from thelight source of an AA is diverted before it reaches the atomization cell and monitored to

compensate for drift in light source intensity.

Drain trap: A hole at the bottom of the mixing chamberthat leads though a plastic tubeto a water filled trap, allowing waste sample solution to drain from the mixing chamber

but not allowing combustion gases to escape.

Dynamic Range: sometimes known as linear dynamic range or linear range, the analyteconcentration range over which response is a well defined (usually linear) function of theanalyte concentration. The dynamic range can be increased by varying instrumental

parameters, such as choice of analyte absorption line or decrease of absorption pathlength

and sample volume.

Electrodeless Discharge Lamp (EDL): a more intense radiation source for AA than theHCL, it consists of a sealed quartz tube containing a small amount of the element of

interest and an inert gas. The lamp is placed in a radiofrequency field, which exites the

atoms to emit intense line radiation. EDL sources are less stable than HCL sources, but
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are far more intense and thus produce much better detection limits for elements such as

As, Se, Hg, Sb, and Te.

Flame AA: The atomic absorption method that uses a flame as an atomization cell.Typical flames are air-acetylene (2400 degrees C) and nitrous oxide-acetylene (3000

degrees C).

Flowrate (solution): the volumetric flowrate (mL/min) of solution uptake into thenebulizer of a flame AA instrument. This is typically from 5 to 10 mL/min.

Flowrate (gas): the volumetric flowrate of combustible gases (L/min) into the mixingchamber of a flame AA instrument, or of inert gas used for graphite furnace and gas

generation methods.

Flow Spoiler: This is a plastic, fan-shaped device placed in the mixing chamber of aflame AA to improve the mixing of combustion gases and analyte solution droplets and

facilitate the removal of large droplets down the drain trap at the bottom of the mixing

chamber.

Graphite tube: The atomization cell used in an electrothermal atomizer for AA.Typically made of pyrolytic graphite and bathed in an inert gas such as Ar to prevent

decomposition. Can be heated up to 3000 degrees C. [photo] Hollow Cathode Lamp (HCL): the most common radiation source for AA, consisting of

a low-pressure inert-gas-filled tube containing an anode and a hollow cathode made from

the element for which the lamp is to produce atomic line radiation. Current flowing

through the lamp (3 to 30 mA) is carried by the inert gas and sputters atoms of the analyteelement from the cathode, which are subsequently collisionally excited to produce

radiation characteristic of the analyte element. [photo]

Hydride-generation: the method by which hydride-forming elements, such as As, Se,Sb, and Te are released from solutions of their ions using sodium borohydride. The

released hydrides are swept from solution by an inert gas and decomposed to atoms in an

absorption cell at a temperature of approximately 1000 degrees C.

Integration: a process for identifying and calculating the amount of a component bymeasuring the area greater than the baseline defined by the instrument blank over a

specific time period. In flame AA, integration times of three to ten seconds are most

commonly used, since continuous signals are measured. In furnace and gas-generationmethods that produce transient signals, the "peak" produced by the analyte is integrated

from baseline to baseline.

Matrix Modifier: an element or compound that is added to the sample in a graphitefurnace AA measurement in order to increase the volatility of the matrix (and thus

remove it during the ashing stage of the temperature program), or to decrease the

volatility of the analyte element so that it can be atomized at high temperatures. Some

typical matrix modifiers are palladium, nickel and ammonium phosphate.

Microwave Digestion: the preferred method for dissolving most samples in acid foranalysis by AA. The method uses a closed Teflon container into which 5 to 10 mL of acid

and approximately 0.5 grams of sample are subjected to an increasing microwave field

for periods up to an hour. The high pressure and temperature inside the container rapidlydissolves most samples and no volatile analytes are lost since the container is sealed.

Mixing chamber: the heart of the sample introduction system for a flame AA, this is aplastic chamber in which combustible gases are mixed with the solution droplets from the
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nebulizer and then transported to the flame. Larger droplets (approximately 95% of the

sample) are removed from the mixing chamber through thedrain trap.[photo]

Modulation: the periodic variation of the radiation from the light source, eitherelectronically or mechanically with a chopper, at frequencies between 30 and 200 Hz.

Modulation of the light source allows the instrument to discriminate against other sources

of radiation that might reach the detector and bias the absorbance measurement. Monochromator- a wavelength selection device used in AA spectrometers to isolate the

absorbable radiation from the light source from other extraneous radiation, both from the

source (non or weakly absorbing lines) and the atomizer (flame or furnace emission).

Nebulizer: the component of a flame AA sample introduction system that draws aqueoussolution into the mixing chamber and converts it to a fine mist of small droplets that are

swept into the flame. It is typically a "pneumatic" nebulizer that operates on the principle

of theBernoulli effect, where the low pressure produced by air flowing rapidly into the

mixing chamber through the nebulizer pulls the analyte-containing solution through acapillary tube. [photo]

Noise: the variation in the signal produced by the instrument. Noise is caused by shortand long-term variations in different instrument components.

Peak: the transient increase in atomic absorption whose area represents the concentrationof analyte element in a sample.

Peak Area: the area enclosed between the peak and the peak base. Photomultiplier: the most often used detector in an AA instrument. It consists of a

vacuum tube containing an alkali-element photocathode that produces electrons when

struck by photons of sufficient energy (the photoelectric effect). Each photoelectron is

then multiplied by collisions with a series of dynodes so that the electrical signalproduced by each photon is greatly amplified. [photo]

Platform Atomization: also known as the L'vov platform, it is a small platform ontowhich a sample is placed inside the graphite furnace tube rather than placing the sample

on the tube wall. This delays sample atomization until the gas temperature inside thefurnace is higher than it would be for wall atomization, which reduces some interference

effects. A good discussion of graphite furnace history and the design of modern

instruments can be found atthis site at the University of Umea.

Qualitative Analysis: the determination of the identity of the components in the sample. Quantitative Analysis: the determination of the amount or concentration of the

components in the sample.

Resolution: a measurement of how well two spectral lines are separated from each other.In AA, this is of significance primarily in the spectrum of the light source.

Smith-Hieftje Background Correction: a method to correct for background absorptionin furnace AA that pulses the HCL at low and then at high current. During the highcurrent pulse, a large cloud of atoms is formed in front of the lamp cathode. This cloud

essentially prevents absorbable radiation from reaching the analyte in the atomic vapor

cell and thus allows discrimination of atomic absorption from other sources of

absorption.

Standard Addition: a method of calibration that compensates for matrix-inducedenhancement or supression of analyte signals. A known concentration of analyte element

is added to the sample and the instrument response of the known concentration of addedelement is used to calibrate the instrument response for the sample.
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Sensitivity: the relationship of analyte concentration to instrument response.Mathematically, this is the slope of the linear plot of "instrument response vs. analyte

concentration". Traditionally, in AA, the sensitivity is defined as the concentration ofanalyte that produces an instrument response of 0.0044 absorbance units (1% absorption).

Zeeman Background Correction: a method to correct for background absorption infurnace AA that uses a magnetic field around the atomizer. The field splits the energylevels of the absorbing atoms and allows discrimination of atomic absorption from othersources of absorption.

Modern Methods for Trace Element DeterminationBy C. Vandecasteele, C. B. Block

Modern methods for trace element determination C. VANDECASTEELE C.

B. BLOCK Published by John Wiley & Sons, Chichester, 1993

In order to analyze a sample for its atomic constituents, it has to be atomized. The atomizers

most commonly used nowadays are flames and electrothermal (graphite tube) atomizers. Theatoms should then be irradiated by optical radiation, and the radiation source could be an

element-specific line radiation source or a continuum radiation source. The radiation then passesthrough amonochromatorin order to separate the element-specific radiation from any other

radiation emitted by the radiation source, which is finally measured by a detector.

Atomizers

The atomizers most commonly used nowadays are (spectroscopic) flames and electrothermal(graphite tube) atomizers. Although other atomizers, such as glow-discharge atomization,

hydride atomization, or cold-vapor atomization might be used for special purposes.

Flame atomizers

The oldest and most commonly used atomizers in AAS are flames, principally the air-acetyleneflame with a temperature of about 2300 C and the nitrous oxide (N2O)-acetylene flame with a

temperature of about 2700 C. The latter flame, in addition, offers a more reducing environment,

being ideally suited for analytes with high affinity to oxygen.

Liquid or dissolved samples are typically used with flame atomizers. The sample solution is

aspirated by a pneumatic nebulizer, transformed into anaerosol, which is introduced into a spraychamber, where it is mixed with the flame gases and conditioned in a way that only the finest

aerosol droplets (< 10 m) enter the flame. This conditioning process is responsible that only

about 5% of the aspirated sample solution reaches the flame, but it also guarantees a relatively

high freedom from interference.
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On top of the spray chamber is a burner head that produces a flame that is laterally long (usually

510 cm) and only a few mm deep. The radiation beam passes through this flame at its longest

axis, and the flame gas flow-rates may be adjusted to produce the highest concentration of freeatoms. The burner height may also be adjusted, so that the radiation beam passes through the

zone of highest atom cloud density in the flame, resulting in the highest sensitivity.

The processes in a flame include the following stages:

Desolvation (drying) the solvent is evaporated and the dry sample nano-particles remain; Vaporization(transfer to the gaseous phase) the solid particles are converted into gaseous

molecules;

Atomization the molecules are dissociated into free atoms; Ionization depending on the ionization potential of the analyte atoms and the energy available

in a particular flame, atoms might be in part converted to gaseous ions.

Each of these stages includes the risk of interference in case the degree of phase transfer is

different for the analyte in the calibration standard and in the sample. Ionization is generally

undesirable, as it reduces the number of atoms that is available for measurement, i.e., thesensitivity. In flame AAS a steady-state signal is generated during the time period when the

sample is aspirated. This technique is typically used for determinations in the mg L-1 range, and

may be extended down to a few g L-1 for some elements.

Electrothermal atomizers

Electrothermal AAS(ET AAS) using graphite tube atomizers was pioneered by Boris V. Lvov

at theSaint Petersburg Polytechnical Institute, Russia, since the late 1950s, and further

investigated by Hans Massmann at the Institute of Spectrochemistry and Applied Spectroscopy(ISAS) in Dortmund, Germany.

Although a wide variety of graphite tube designs have been used over the years, the dimensions

nowadays are typically 2025 mm in length and 56 mm inner diameter. With this technique

liquid/dissolved, solid and gaseous samples may be analyzed directly. A measured volume

(typically 1050 L) or a weighed mass (typically around 1 mg) of a solid sample are introducedinto the graphite tube and subject to a temperature program. This typically consists of stages,

such as:

Drying the solvent is evaporated Pyrolysis the majority of the matrix constituents is removed Atomization the analyte element is released to the gaseous phase Cleaning eventual residues in the graphite tube are removed at high temperature.
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The graphite tubes are heated via their ohmic resistance using a low-voltage high-current power

supply; the temperature in the individual stages can be controlled very closely, and temperature

ramps between the individual stages facilitate separation of sample components. Tubes may beheated transversely or longitudinally, where the former ones have the advantage of a more

homogeneous temperature distribution over their length. The so-calledStabilized Temperature

Platform Furnace(STPF) concept, proposed by Walter Slavin, based on research of Boris Lvov,makes ET AAS essentially free from interference. The major components of this concept are:

Atomization of the sample from a graphite platform inserted into the graphite tube (Lvovplatform) instead of from the tube wall in order to delay atomization until the gas phase in the

atomizer has reached a stable temperature;

Use of a chemical modifier in order to stabilize the analyte to a pyrolysis temperature that issufficient to remove the majority of the matrix components;

Integration of the absorbance over the time of the transient absorption signal instead of usingpeak height absorbance for quantification.

In ET AAS a transient signal is generated, the area of which is directly proportional to the mass

of analyte (not its concentration) introduced into the graphite tube. This technique has theadvantage that any kind of sample, solid, liquid or gaseous, can be analyzed directly. Its

sensitivity is 23 orders of magnitude higher than that of flame AAS, so that determinations in

the low g L-1 range (for a typical sample volume of 20L) and ng g-1 range (for a typicalsample mass of 1 mg) can be carried out. It shows a very high degree of freedom from

interferences, so that ET AAS might be considered the most robust technique available

nowadays for the determination of trace elements in complex matrices.

Specialized Atomization Techniques

While flame and electrothermal vaporizers are the most common atomization techniques, several

other atomization methods are utilized for specialized use.

Glow-Discharge Atomization

A glow-discharge (GD) device serves as a versatile source, as it can simultaneously introduce

and atomize the sample. Theglow dischargeoccurs in a low-pressure argon gas atmosphere

between 1 and 10 torr. In this atmosphere lies a pair of electrodes applying aDCvoltage of 250

to 1000 V to break down the argon gas into positively charged ions and electrons. These ions,under the influence of the electric field, are accelerated into the cathode surface containing the

sample, bombarding the sample and causing neutral sample atom ejection through the process

known assputtering. The atomic vapor produced by this discharge is composed of ions, ground

state atoms, and fraction of excited atoms. When the excited atoms relax back into their groundstate, a low-intensity glow is emitted, giving the technique its name.

The requirement for samples of glow discharge atomizers is that they are electrical conductors.

Consequently, atomizers are most commonly used in the analysis of metals and other conducting
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samples. However, with proper modifications, it can be utilized to analyze liquid samples as well

as nonconducting materials by mixing them with a conductor (e.g. graphite).

Hydride Atomization

Hydride generation techniques are specialized in solutions of specific elements. The techniqueprovides a means of introducing samples containing arsenic, antimony, tin, selenium, bismuth,

and lead into an atomizer in the gas phase. With these elements, hydride atomization enhances

detection limits by a factor of 10 to 100 compared to alternative methods. Hydride generationoccurs by adding an acidified aqueous solution of the sample to a 1% aqueous solution of sodium

borohydride, all of which is contained in a glass vessel. The volatile hydride generated by the

reaction that occurs is swept into the atomization chamber by an inert gas, where it undergoesdecomposition. This process forms an atomized form of the analyte, which can then be measured

by absorption or emission spectrometry.

Cold-Vapor Atomization

The cold-vapor technique an atomization method limited to only the determination of mercury,due to it being the only metallic element to have a large enough vapor pressure at ambient

temperature. Because of this, it has an important use in determining organic mercury compounds

in samples and their distribution in the environment. The method initiates by converting mercuryinto Hg

2+by oxidation from nitric and sulfuric acids, followed by a reduction of Hg

2+withtin(II)

chloride. The mercury, is then swept into a long-pass absorption tube by bubbling a stream of

inert gas through the reaction mixture. The concentration is determined by measuring the

absorbance of this gas at 253.7 nm. Detection limits for this technique are in the parts-per-billionrange making it an excellent mercury detection atomization method.

Radiation sources

We have to distinguish between line source AAS (LS AAS) and continuum source AAS (CS

AAS). In classical LS AAS, as it has been proposed by Alan Walsh, the high spectral resolution

required for AAS measurements is provided by the radiation source itself that emits the spectrumof the analyte in the form of lines that are narrower than the absorption lines. Continuum sources,

such as deuterium lamps, are only used for background correction purposes. The advantage of

this technique is that only a medium-resolution monochromator is necessary for measuring AAS;however, it has the disadvantage that usually a separate lamp is required for each element that

has to be determined. In CS AAS, in contrast, a single lamp, emitting a continuum spectrum over

the entire spectral range of interest is used for all elements. Obviously, a high-resolution

monochromator is required for this technique, as will be discussed later.
http://en.wikipedia.org/wiki/SnCl2http://en.wikipedia.org/wiki/SnCl2http://en.wikipedia.org/wiki/SnCl2http://en.wikipedia.org/wiki/SnCl2http://en.wikipedia.org/wiki/File:HKL.jpghttp://en.wikipedia.org/wiki/File:HKL.jpghttp://en.wikipedia.org/wiki/File:HKL.jpghttp://en.wikipedia.org/wiki/File:HKL.jpghttp://en.wikipedia.org/wiki/SnCl2http://en.wikipedia.org/wiki/SnCl2


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Hollow cathode lamp (HCL)

Hollow cathode lamps

Hollow cathode lamps(HCL) are the most common radiation source in LS AAS. Inside the

sealed lamp, filled with argon or neon gas at low pressure, is a cylindrical metal cathodecontaining the element of interest and an anode. A high voltage is applied across the anode andcathode, resulting in an ionization of the fill gas. The gas ions are accelerated towards the

cathode and, upon impact on the cathode, sputter cathode material that is excited in the glow

discharge to emit the radiation of the sputtered material, i.e., the element of interest. Most lampswill handle a handful of elements, i.e. 5-8. A typical machine will have two lamps, one will take

care of five elements and the other will handle four elements for a total of nine elements

analyzed.

Electrodeless discharge lamps

Electrodeless discharge lamps(EDL) contain a small quantity of the analyte as a metal or a saltin a quartz bulb together with an inert gas, typically argon, at low pressure. The bulb is inserted

into a coil that is generating an electromagnetic radio frequency field, resulting in a low-pressure

inductively coupled discharge in the lamp. The emission from an EDL is higher than that froman HCL, and the line width is generally narrower, but EDLs need a separate power supply and

might need a longer time to stabilize.

Deuterium lamps

Deuterium HCLor even hydrogen HCL and deuterium discharge lamps are used in LS AAS forbackground correction purposes. The radiation intensity emitted by these lamps is decreasing

significantly with increasing wavelength, so that they can be only used in the wavelength rangebetween 190 and about 320 nm.
http://en.wikipedia.org/wiki/Hollow_cathode_lampshttp://en.wikipedia.org/wiki/Hollow_cathode_lampshttp://en.wikipedia.org/wiki/Electrodeless_lamphttp://en.wikipedia.org/wiki/Electrodeless_lamphttp://en.wikipedia.org/wiki/Deuterium_arc_lamphttp://en.wikipedia.org/wiki/Deuterium_arc_lamphttp://en.wikipedia.org/wiki/Deuterium_arc_lamphttp://en.wikipedia.org/wiki/Electrodeless_lamphttp://en.wikipedia.org/wiki/Hollow_cathode_lamps


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Xenon lamp as a continuous radiation source

Continuum sources

When a continuum radiation source is used for AAS, it is necessary to use a high-resolutionmonochromator, as will be discussed later. In addition it is necessary that the lamp emits

radiation of intensity at least an order of magnitude above that of a typical HCL over the entire

wavelength range from 190 nm to 900 nm. A special high-pressurexenonshort arc lamp,

operating in a hot-spot mode has been developed to fulfill these requirements.

Spectrometer

As already pointed out above, we have to distinguish between medium-resolution spectrometers

that are used for LS AAS and high-resolution spectrometers that are designed for CS AAS. The

spectrometer includes the spectral sorting device (monochromator) and the detector.
http://en.wikipedia.org/wiki/Xenonhttp://en.wikipedia.org/wiki/Xenonhttp://en.wikipedia.org/wiki/Xenonhttp://en.wikipedia.org/wiki/File:Continuous_radiation_source.jpghttp://en.wikipedia.org/wiki/File:Continuous_radiation_source.jpghttp://en.wikipedia.org/wiki/File:Continuous_radiation_source.jpghttp://en.wikipedia.org/wiki/File:Continuous_radiation_source.jpghttp://en.wikipedia.org/wiki/Xenon

lamps for self reversal

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