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Atomic Absorption Spectroscopy
A summary
Gargee Sinha Roy
Abstract
Finding an efficient of analysis has always been a driving force in chemistry for exploring new ways of interaction with matter and specifically atoms and molecules. Atomic Absorption Spectroscopy and new ways of thinking about the properties of an atom surfaced through such a search. Atomic Absorption Spectroscopy is a very nuanced kind of spectroscopy, wherein the amount of radiation absorbed by the atoms in gas phase is used as an medium to indicate the presence of of certain element. Atomic absorption spectroscopy is praised for its analytical sensitivity
. Atomic Absorption Spectroscopy (AAS) is widely used for finding the concentration of metal in a sam- ple, there is used extensively in industries. The tech- niques of achieving gas phase itself is a broad topic of discussion and many mechanisms are deployed to find to achieve the gas phase. Irridation of radiation is also of a topic to be covered when discussing AAS. Scope of AAS is also a topic of discussion as it puts the viability of AAS in question.
1. Introduction
Many were aware of the excitation of atom , (our promotion or demotion of electron for higher en- ergy orbital to lower energy orbital) on absorbing Electromagnetic radiation at different frequency during the 1860s but it wasn’t until the 20th cen- tury that it will be used for chemical analysis by Alan Walsh, he used a Atomic Absorption Spec- trophotometer in 1953.Before we discuss what Spectrophotometer is, and how it works when we are to use it for Chemical Analysis using Atomic Absorption spectroscopy. This is the part where we should be discussing the principle revolving Atomic Absorption. The principle in this case is Beer Lambert’s law, which states that the con- centration is directly proportional to absorption
. This law holds weight here in this case as we are assesing the absorption spectrum of the sam- ple , wherein we make calibration with respect to the absorbance and and known concentrations , and find the unknown concentration of analyte of our interest. The role of calibration curve is this case is same except what actually differs is the procedure of nebulisation of sample for analysis,
which again is a broad topic of discussion in it’s own right ,as many factors like the particle sizes and sensitivity of the instrument comes into pic- ture. The ways of atomisation and instruments, will be discussed further.
2. Atomisation(Instrumentation)
Atomisation in general sense means reducing something into very minute form. In this case, the sample is atomised into free atoms and is ir- radiated with radiation (light source).This kind of atomisation involves breaking of some bonds, which is done when energy(heat energy) is pro- vided to the sample, these atoms will further ab- sorb the radiation. The oldest from of atomi- sation technique is using flames, and the most common way to witness is using flame test to know the composition of elements present in salts. In the later days of sophistication, the Flame atomic absorption spectroscopy became a thing , wherein flames where used to provide en- ergy for atomisation.
2.1. Flame Atomic Atomic Absorption Spectroscopy
Atomisation comes under instrumentations for spectrophotometers. Different atomisation tech- niques exist to provide accuracy and take in con- sideration the sensitivity of the device. Here we are going to discuss on the many steps of flame atomisation.
The solution to be analysed is first dissolved in suitable solvent(water,alcohols)
The solution is converted into aerosol( less than- 10μm) and passed to burner head(as we are using flames in this case) which uses the principle of venturi effect, which states the narrower the tube , the more is velocity and less is the pressure.
Before the aerosol is passed to flame , the larger droplets are discarded to waste , and fine aerosols are passed to flame chambers by dowstream impinger. 1
the burner converts the aerosol mixture cre- ated by the spray chamber and nebuliser, into free, ground state atoms to be irradiated by radiation.
For this procedure of atomization, the viscosity and surface tensions are one of the many impor- tant factors that is to be kept in mind to get a good amount of aerosol to be passed down for atomization process. The removal of bigger droplets give good signal to noise ratio, even if the amount of analyte for analysis reduces to 10 percent. Typical flame gases used in AAS include air-acetylene or nitrous oxide- acetylene, provid- ing maximum operating temperatures of 2400°C and 2800°C, respectively(basically increasing the flame). The air-acetylene flame typically uses a burner path length of 10 cm, whereas the nitrous- oxide acetylene flame requires a shorter path length.Some flame photometers also use propane operated flame atomisers. The temparature of the flame determines the number of atoms in ex- cited state. This relation is given in the form of formula as:
Ne Ng
ge gg
e
−E kT (^) (1)
where, Ne is the Number of atoms in the excited state Ng is the Number of atoms in ground state E is the excited state transition energy k is the Boltzmann constant T is the temperature gr and ge are statistical weights of atoms in ground state. In this maxwell-boltzmann relation the the num- ber of atoms in excited state is negligible com- pared to the atoms in ground state. Because of this we try to make sense of result by real- ising that that signals are produced only using from the when the atoms achieve an excited state. Anything that removes atom from ground state are called interference has a section devoted to it in this summary. FAAS gives us an oppurtunity to talk about flame chemistry as it is an impor- tant part of this discussion as it is due to this flame that processes like dissociation take place with the atom. Depending on the heat of vapori- sation , different elements might require different fuels to vaporise. The most preferable , however seems to be nitrious-oxide acytelene. To achieve maximum sensitivity, the radiation from the line source must pass through the area of the flame that contains the greatest number of free atoms. This condition is met by optimizing the height and alignment of the burner head relative to the radiation beam as it passes through the flame parallel to the major axis of the burner.
Figure 1: Schematic of an Electrothermal Atomiser
2.2. Graphite Furnace Atomic Absorption Spectroscopy
GFAAS is another way by which the sample can be atomised , it is also known as Electrothermal Atomic Absorption. This procedure for atomi- sation ensures overcoming of sensitivity problems and is an improvisation of FAAS.
Electrothermal atomizers include the graphite furnace which are heated using ohmic resis- tance with low voltage high current power sup- ply, carbon rod, and tungsten ribbon atomiz- ers.
In these atomizers, a 5 to 50 mL aliquot of the sample is placed in the resistively heated ele- ment and then heated stepwise to the temper- atures conducive to gaseous atom formation.
Electrothermal atomizers can be used to directly analyze solid materials without any form of modi- fication. A major limitation of electrothermal at- omizers, however, is the reduced precision of the results obtained, compared to FAAS. However, it is more preferred as they are not dependant on nebulisation efficiencies and the atomisation oc- curs in a single process. Citation is still absent for ways of reducing interference.
2.3. Glow Discharge Atomisation
This form of atomisation happens due to ionic breakdown of argon gas and ions running to the cathode which contains the sample those causing neutral sample excitation. It produces an atomic vapour which is composed of ions and ground state atoms which includes excited toms which relaxes back emitting a faint glowing light. The pair of elctrodes can are somewhere around 25V- 100V.
Figure 3: Demostration of aligment of line source and continuum source using
These non-atomic fragments are usually the un- wanted molecular fragments(-OH,-CN,-CH etc.).
4.1. Using Continuum Source
As we know for AAS , only line source is required to be absorbed by atomic concentartions. In a continuum source which in combination with line source provides coorection for non-atomic con- centrations. At the bandpass of monochroma- tor, the non-atomic species are responsible for ab- sorption for majority of continuum source. When the signal received at the detector is obtained , it is modulated between line source and contin- uum source so as to receive a background correc- tion signal. When using continuum sources, the line source and the continuum should be aligned properly so that the irradiation passes through the same spatial volume.
4.2. Pulsed Hollow Cathode Lamp
In the case of Pulsed HCL, we are pulsing the cur- rent from high(5 to 20mA) to low(100-500mA) currents for a small duration(15-40 microsec- onds). At low current , the absorption which is accounted for is from atomic and non-atomic species. In higher currents, the abosrption from only the non atomic species is taken into ac- count. If we substract both, we get the re- quired absorbance results. In this case, we should make sure the HCL is stable at higher currents. The non negligible amount of atomic absorption during high current signal reduces the sensitiv- ity of calibration curve as upon increase in cur- rent there was a decrease in light output. A steady flow of current to a peak current is usu- ally achieved by using a microprocessor.
4.3. Zeeman Effect
In Zeeman effect is an effect, the light of a spec- tral line is divided into two or more recurrences when it is under a strong magnetic field. the
Figure 4: A Jarrell-Ash Atomic Absorp- tion/Flame Emission 82-360 Spectrometer block diagram to show the working and components involved in PHCL,this spectrometer uses a mi- croprocessor
Figure 5: Zeeman Effect
atomic lines of the analyte can be split into de- generate spectral lines of differing polarization in an applied magnetic field. Since the spectrum is divided into diferent spectral line, the absorp- tion measured from the source radiation at the one wavelength(π)can be attributed to both the atomic and background absorbance, whereas ab- sorption measured at the (σ) wavelength can be attributed solely to the background absorbance. We can place the polarizer either before or after the atomiser for measuring independently. and can be subtracted to give better absorbance re- sults. Zeeman effect is better implemented in electrothermal atomisers than the falme atomiser as the atomising chamber is smaller in the former. And it can be used better in EDL than HCL as the HCL is not a magnetically stable lamp. This technique comes at risk with sensitivity.
5. Interference
Now we are going to talk about certain inter- ferences which can reduce the sensitivity of the devices. The interferences can occur in any form.
5.1. Chemical Interference
Chemical Intereferences are responsible for re- moving the atoms from ground state. The radi- cals present along with the vaporisation of metal atoms to produce metal hydroxides, metal hy- drides, or metal oxide species. In flame AAS, these species cannot be dissociated at cooler flame temperatures. This fact effectively reduces the number of free metal atoms produced in the flame, resulting in a loss in sensitivity. The method of reducing this type of interference are:
By adjusting the flame-gas stoichiometry. A fuel rich flame produces a reducing environ- ment with few reactive oxide species. By using nitrous-oxide acetylene fueled flame , to vaporise and dissasociate any refractory metal oxides formed By releasing agent that can compete with the metal-oxide formation equilibrium. In this case lanthanum with caalcium and phosphate By using EDTA which forms stable complexes with metal atoms.
5.2. Ionisation Interference
Being exposed to the flame, formation of ionisa- tion is not impossible due to temperature. The method reducing this kind of flame is:
By using cooler fire fuel in this case air acety- lene By adjusting flame gas stoichiometry by chemically alternating the ionisation equilibrium(ionisation buffer)
M 0 −→M^
n++ne−
5.3. Matrix Interference
This kind of interference occurs when a when other components of sample matrix other than analyte reacts to form molecular species and sam- ple background.
The blank sample is atomised and the instru- ment is zeroed such that the measured voltage is PMT is zero, this gives and idea that we are going to use help of a calibration curve. To follow Beer Lamberts Law, the atomisation efficiencies should be matched between the cal- ibration curve and unknown sample as t will help us know whether the nebulisation was in any way affected by any physical properties.
Figure 6: Calibration Curve to find concentra- tion of copper in the sample wherein the plot is extrapolated to x-axis to give analyte concetra- tion in sample
By adding standard solution as an increment we can in some way overcome , matrix interfer- ence. This helps in altering the bulk physical properties.
5.4. Spectral Interference
Spectral interferences rarely happens but when it happens it is usually because radiation of a wavelength different from that of the wavelength of the element of interest in the sample falls on the detector.That’s the usual case but in HCL it occurs due to lines of different element over- lapping with the spectral lines of the element of interest. We can overcome this by using a nar- rower slit such that transmission of only metal wavelength of interest is possible.
6. Conclusion
Atomic Absorption spectroscopy is widely used today after many corrections and improvisation for example for detection metal atoms in water, pharmaceuticals and other industries. It can be thought of as not only qualitative but also an quanttative way of analysis. It can be also used of purification purposes. A 1991 reaserch paper suggests that it can find it’s purpose as an ele- ment selective detector in various types of chro- matography(liquid ,gas and supercritical liquid).
References
ATOMIC ABSORPTION SPECTROSCOPY:
A SHORT REVIEW(2021)Journal DOI: 10.36713/epra Atomic Absorption Spectroscopy: A Tutorial Review(1991)APPLIED SPECTROSCOPY REVIEWS, 34(3), 173–189 (1999) A Microprocessor Controlled Pulsed Hollow
