Bài thực tập 4 phương pháp quang phổ nguyên tử (atomic spectrometry

Principles of the method -Atomic spectrometry is a method used to analyze; both identifying and quantifying trace amounts of eligible elements in the examining sample via the element’s s

TRƯỜNG ĐH KHOA HỌC TỰ NHIÊN,

KHOA HOA HOC

BAI BAO CAO

PGS.TS Nguyễn Văn Đông

CN Lê Văn Duy

21147029

21147060

21147074 21147083

I PRINCIPLES OF THE METHOD, BASIS OF THEORY AND EQUIPMENT

1 Principles of the method

2 Preparation of the standard solutions (prepared by the lab mentors)

3 Evaluating experiment data:

A Using the wavelength A = 279.5 nm

B Using the wavelength 4 = 403.1 nm

C Observations:

Ill OVERAL CONCLUSION

A Advantages of AAS in general and F-AAS in particular

B Some drawbacks regarding this method

C Characteristic Limit of detection (LOD) and sensitivity of each elements

D Affecting variables:

IV APPENDIX

11 lãi

1 Principles of the method

-Atomic spectrometry is a method used to analyze; both identifying and quantifying trace amounts of eligible elements in the examining sample via the element’s signature wavelength absorbed/emitted, in this case, absorbing spectrum (Atomic Absorption Spectrometry, AAS)

-AAS operates based on the phenomenon in which electromagnetic radiation at a certain wavelength carrying a certain amount of energy is absorbed by the well-separated neutral atoms An element’s signature absorbing wavelength is shown through how the covalent electrons changing position when excited, distributing into different energy levels unique

to the element

-The amount of energy absorbed can be calculated using Planck’s equation: E = hv = he, with % being the absorbing wavelength and serves as the identifying factor, whereas the measured Absorbance (A) serves as the quantifying factor

-For this reason, the atoms must be separated, severing the intermolecular bonds, yet remains at ground state This process is called “atomization”, and the sample is considered

“atomized”

+the analyzing sample are first nebulized, maximizing the atomizing efficiency Among the cloud of sample, the chosen solvent is also present, but its absorbance are to be accounted for using a blank sample of only solvent

+the light beam of the chosen wavelength depends on what the analyzing element is, is then directed at the nebulized sample, then difference in intensity between the light source and receiving end is the absorbance (A), reflecting the quantity of the analyte in the sample The chosen method of AAS is Flame- Atomic Absorption Spectrometry (F-AAS), thus the light beam emitted is at a single designated wavelength Furthermore, behind the atomizing flame lies a monochromator to eliminating any noise wavelengths that may come to existence due to various other reactions caused by the flame aside from the purposed atomization

+The detector measuring the incoming light intensity, thus “translating” that information into usable data regarding its quantity

2 Basis of theory

-While F-AAS can be used to identify up to 67 elements, the specific mixture of fuel being Acetylene C2H2 and oxidant being compressed open air (O2) gives out a lower burning temperature, and thus is more suitable for elements that can be atomized easier Manganese (Mn) having medium-low heat of atomization falls into this category, and thus this mixture and the temperature its flame provides is suitable to analyze manganese without being too energetic and excites the atoms

-Analyzing manganese using F-AAS can be done using a resonate wavelength at either 279.5, 279.8 or 280.1 nm, giving out the most sensitive results Among them, 279.5 nm is the most sensitive for analyzing manganese, and thus, chosen The wavelength of 403.1 nm

is also chosen as its listed operating concentration is roughly 9 times that of 279.5 nm, reflecting its sensitivity in relative to 279.5 nm This can be tested by comparing the slope coefficients between the results from the 2 wavelengths within their ranges of linearity (R?

> 0.995), collected from experiment data

3 Equipment

-The device used is a Shimadzu AA-6300 series

+atomizer- assembly shown in details in the diagram

+monochromator- redirect any noise back into the chosen wavelength going through; the most important part is its diffraction grating lens

+detector- translate receiving information into tangible data

A Hollow Cathode Lamp:

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-The lamp’s cathode is a hollow cylindrical shell made from either the analyzing metal at high purity or an alloy containing multiple metals, and thus can be used for any of the metals that made up the alloy (in this case, manganese)

-The lamp’s anode is made from a rod tungsten (W) or other heat resisting alloys Both the cathode and anode are placed in a glass tube, soldered shut, filled with an inert gas such as neon or argon (in this case, neon) at low pressure (~0.005 atm)

-The lamp’s inserting window is transparent, made from either glass (for wavelength in the visible light spectrum-near UV) or quarts (into UV) With one of the chosen wavelengths being at 279.5 nm, the lamp used in the experiment will be equipped with quartz window

+Sputtering: When supplied with electricity, the inert gas atom (Ne) got ionized (into Ne*) and liberate an electron The inert gas ion collides with the cathode wall and bombard the metal atom in this process

+Excitation: The bombarded metal atom became excited from the impact, moving onto a

higher energy state (M’)

+Emussion: When the excited metal atom returns to its ground state, the radiation it emits has a wavelength characteristic to said metal/analyte

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-Having limited life as the cathode wears out with use

-Depends on the desired stability, the lamp feeding current can be adjusted, the more intense the feeding current is, the faster the cathode wears out

-HCL using pure metal cathode (single-element lamp) have better durability, sensitivity and stability compare to one with alloy cathode (multi-element lamp)

-HCL for more volatile metals last significantly shorter than more stable metals

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-Avoid any physical contact with the window to minimize any noise from smudges -Current intensity directly correlates to how responsive and stable the analysis will be, lamp’s life must be taken into consideration to compensate (60-70% of recommended current intensity for new lamps, and 100% the recommended current intensity for worn out lamps) for any loss caused by old lamps

-HCL lamps need a preparation time by heating up and therefore stabilize the emission, it

is advised to prepare multiple lamps in the built-in tray for the upcoming analyses with different analyte, as each metal has different stability requirement, as the working lamp would heat up the surrounding lamps, saving time for the heating step

B Atomizer:

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(Scheme of the atomizer, Yonsei slide show)

-Sample injection port leads into the nebulizer, turn the sample into mist-like droplets of 10m in diameter or less Droplets larger than this are deemed unsuitable for atomization and thus are exhausted through the drain

-Glass bead, made out of glass or ceramic or a similar corrosive resistance material can further help aerosolize droplets, increase atomizing efficiency

-Baffle: filter of sort, ensuring the droplets of sample reaching the burner is consistent in size, giving stable readings

-The burner head is made from highly heat resistance and inert metal; titantum is one of the most frequently used metal for this part

-The atomizer operates similar to a pump:

+A vacuum is created by using airflow from the pipes through the drain, sucking the sample

in The oxidant (air) and fuel (air/C2H2 mixture) double as the vacuum creating gases +Injected sample got nebulized into mist and small droplets, the baffles serve as filter, only allowing drops with diameter less than 10m to go forward to the bumer

+Once the sample droplets reach the burner head, the solvent (water) got evaporated under intense heat (2100-2400°C), leaving behind fine powder-like particles containing the analyte The flame is hot enough to sever any chemical bonds existing in the salt, releasing the metal atoms

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-Limit of detection range mostly in ppb+ppm, sensitivity is relatively low

-Low sample usage efficiency (4+6 mL/min), low total dissolved solid rate (<2%), high sample consumption (5-10 mL per analysis)

-Slow and inefficient when analyzing multiple elements (reasoning included in TI.1.Procedures)

-Easy to operate, low skill celling, analysis of a single element fairly quickly

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-Ratio of fuel mixture must be taken into consideration as it reflects flame temperature and burn rate Improper selection may even cause back flowing combustion due to low burn rate (thus for the mixture of air/C2H2 with lower burn rate, a longer burner head of 10cm must be used to prevent this)

-Position of the burner head affect absorbance per Lambert-Beer’s law, A = ¢.1.C, with 1 being the length of light path, which in this case is the length of the flame Reading absorbances would be at its highest when the pathway from light beam emitter to the detector overlap with the flame completely This can also be used when analyzing highly volatile metals such as sodium or potassium, by intentionally lower the length of light path

by setting the burner head at a different angle

C Monochromator:

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— Spherical mirrors _

cntrance sïÏÊ plane diffraction grating exit slit

(Figure of monochromator’s mechanism, AZOOptics)

-Entrance and exit slits, letting in the light beam containing noises, and return a light beam focused back to the desired wavelength (based on the principle of slit diffraction), while other light beams of unwanted wavelengths are blocked at the exit slit

-Two conclave mirrors and an Echelle diffraction grate, with the latter’s receiving angle adjustable

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-The collimating (entrance) and focusing (exit) mirrors are both concave in nature, focusing the light dispersed after going through the slit into parallel light, which would then be diffracted into different beams of different wavelengths, and re-focused back to the exit

Normai to Surtace

1* Order incident 2" Order (Diffracted)

(How a diffraction grating operates, Andor Technology)

-Needless to say, the process of calculating the exact angle of the grate require machine- precision to receive accurate data Adjustment of the diffraction grate are usually done by the device

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-Ease of isolating the targeted wavelength from the noise depends on the resolution of the monochromator, or to be exact, the diffraction grate ability to diffract

-The data read has already been accounted for the inevitable loss of intensity going through the monochromator, regardless how small it is (transparency/reflective efficiency)

Radiation id PAY? secondary

hy Tne ae” | eee LAIN electrons

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-Convert electromagnetic radiation (emitted photon) into electricity (electron), thus the signal can be read based on the electricity’s intensity

-Amplify the signal until it’s strong enough to actually be readable

“chopper”

-A chopper is a rapidly rotating disk with intervals in / In half of the time, the light coming from the HCL is blocked, while in the other half, the light from HCL is allowed to travel through unimpeded This creates mtervals in reading of the signal, one is entirely coming from the flame, while the is a combination of both the HCL and the flame By constantly comparing the intensity, the noise from the flame can be compensated from the detecting signal

-Start the ignition using the built-in buttons on the device Should it fail to ignite, quickly using an external fire starter Do not let fuel accumulate inside the chamber

-Inject distilled water for roughly 3 minutes

Establishing the calibration curve

-Press F3 once the signal for distilled water is stabilized This effectively provide a blank signal that can be used to calibrate the true absorbance of the standard solutions by compensating the solvent’s background noise

-Start with standard solution 1 (lowest analyte concentration) In the Calibration menu shown on the monitor, choose Standard 1 Click save to save the measured signal Repeat this step for the remaining standard solutions, by order of increasing in concentration of analyte This is to prevent cross contamination when transitioning from a higher analyte concentration solution to one that’s lower Choosing to do so not in that order will need to repeat the blank sample injection step

-Once the calibration curve is established, start the purging process Inject distilled water and let the device run for at least 5 minutes Purge any remaining air and turn off the air

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compressor, the valve on the compressed C2H2 tank Press “Purge” to purge any remaining gases in the device, then close the leading pipes into the device

2 Preparation of the standard solutions (prepared by the lab mentors)

-In each of the solution, HNO3 must be added (~1%) to acidify the solution, preventing the formation of inert Mn(OH)2, changing the actual concentration of analyte in the solution Furthermore, adding HNO3 forms its and Mn?*’s salt Mn(NO3)2 having low melting point, making it easier to be atomized

-HNO3 at 1% have density ~1 g/mL making it convenient to prepare, as HNO3 concentration doesn’t have to be exact

3 Evaluating experiment data:

A Using the wavelength A = 279.5 nm

Measured range of concentration 0.5+50.0 ppm