Atomic Emission Spectroscopy

1. Atomic Emission Spectroscopy Lecture 21

2. Qualitative analysis is accomplished by comparison of the wavelengths of some emission lines to standards while the line blackness serves as the tool for semiquantitative analysis. Polychromators are also available as multichannel arc and spark instruments. However, these have fixed slits at certain wavelengths in order to do certain elements and thus they are not versatile.

3. Potential Source Detectors Grating

4. Recently, arc and spark instruments based on charge injection and charge coupled devices became available. These have extraordinarily high efficiency and performance in terms of easier calibration, short analysis time, as well as superior quantitative results.

5. CCD or CID Detector Potential Source Grating

7. Characteristics of Arc Sources 1. Typical temperatures between 4000-5000 oC are high enough to cause atomization and excitation of sample and electrode materials. 2. Usually, cyanogens compounds are formed due to reaction of graphite electrodes with atmospheric nitrogen. Emission bands from cyanogens compounds occur in the region from 350-420 nm. Unfortunately, several elements have their most sensitive lines in this same region which limits the technique. However, use of controlled atmosphere around the arc (using CO2, Helium, or argon) very much decreases the effect of cyanogens emission.

8. 3. The emission signal should be integrated over a minute or so since volatilization and excitation of atoms of different species differ widely. While some species give maximum signal, others may still be in the molecular state. 4. Arc sources are very good for qualitative analysis of elements while only semiquantitative analysis is possible. It is mandatory to compare the emission spectrum of a sample with the emission spectrum of a standard. In some cases, a few milligrams of a standard is added to the sample in order to locate the emission lines of the standard and thus identify the emission wavelengths of the different elements in the sample. A comparator densitometer can be used to exactly locate the wavelengths of the standard and the sample components.

9. Standard Sample The lines from the standard are projected on the lines of the combined sample/standard emission spectra in order to identify sample components. Only few lines are shown in the figure.

13. Why use Carbon in Atomic Spectroscopy? We have previously seen the use of graphite in electrothermal AAS as well as arc and spark AES, even though molecular spectra are real problems in both techniques due to cyanogens compounds absorption and emission. The reasons after graphite common use in atomic spectroscopy can be summarized below:

14. It is conductive. • It can be obtained in a very pure state. • Easily available and cheap. • Thermally stable and inert. • Carbon has few emission lines. • Easily shaped.

15. Spark Sources Most of the instruments in this category are arc based instruments. Spark based instruments are of the same idea except for a spark source substituting an arc source. The spark source is constructed as in the figure below where an AC potential in the order of 10-50 KV is discharged through a capacitor which is charged and discharged through the graphite electrodes about 120 times/s; resulting in a discharge current of about 1000 A.

16. Potential Source Transformer This very high current will suffer a great deal of resistance, which increase the temperature to an estimated 40000 oC. Therefore, ionic spectra are more pronounced.

17. An introduction to Ultraviolet/Visible Absorption Spectroscopy Chapter 13

18. In this chapter, absorption by molecules, rather than atoms, is considered. Absorption in the ultraviolet and visible regions occurs due to electronic transitions from the ground state to excited state. Broad band spectra are obtained since molecules have vibrational and rotational energy levels associated with electronic energy levels. The signal is either absorbance or percent transmittance of the analyte solution where:

19. Absorption measurements based upon ultraviolet and visible radiation find widespread application for the quantitative determination of a large variety species. Beer’s Law: A = -logT = logP0/P = bc A = absorbance  = molar absorptivity [M-1 cm-1] c = concentration [M] P0= incident power P = transmitted power (after passing through sample)

22. UV-Vis Absorption Spectroscopy Lecture 22

24. Measurement of Transmittance and Absorbance: The power of the beam transmitted by the analyte solution is usually compared with the power of the beam transmitted by an identical cell containing only solvent. An experimental transmittance and absorbance are then obtained with the equations. P0 and P refers to the power of radiation after it has passed through the solvent and the analyte.

25. Beer’s law and mixtures Each analyte present in the solution absorbs light! The magnitude of the absorption depends on its e A total = A1+A2+…+An A total = e1bc1+e2bc2+…+enbcn If e1 = e2 =en then simultaneous determination is impossible Need to measure A at nl’s (get n2e’s) to solve for the concentration of species in the mixture

27. Limitations to Beer’s Law • Real limitations • Chemical deviations • Instrumental deviations

28. 1.      Real Limitations a.       Beer’s law is good for dilute analyte solutions only. High concentrations (>0.01M) will cause a negative error since as the distance between molecules become smaller the charge distribution will be affected which alter the molecules ability to absorb a specific wavelength. The same phenomenon is also observed for solutions with high electrolyte concentration, even at low analyte concentration. The molar absorptivity is altered due to electrostatic interactions.

29. b.      In the derivation of Beer’s law we have introduced a constant (e). However, e is dependent on the refractive index and the refractive index is a function of concentration. Therefore, e will be concentration dependent. However, the refractive index changes very slightly for dilute solutions and thus we can practically assume that e is constant. c.       In rare cases, the molar absorptivity changes widely with concentration, even at dilute solutions. Therefore, Beer’s law is never a linear relation for such compounds, like methylene blue.

30. 2.      Chemical Deviations This factor is an important one which largely affects linearity in Beer’s law. It originates when an analyte dissociates, associates, or reacts in the solvent, or one of matrix constituents. For example, an acid base indicator when dissolved in water will partially dissociate according to its acid dissociation constant:

31. HIn D H+ + In- It can be easily appreciated that the amount of HIn present in solution is less than that originally dissolved where: CHIn = [HIn] + [In-] Assume an analytical concentration of 2x10-5 M indicator (ka = 1.42x10-5) was used, we may write:

32. 1.42x10-5 = x2/(2x10-5 – x) Solving the quadratic equation gives: X = 1.12x10-5 M which means: [In-] = 1.12x10-5 M [HIn] = 2x10-5 – 1.12x10-5 = 0.88x10-5 M Therefore, the absorbance measured will be the sum of that for HIn and In-. If a 1.00 cm cell was used and the e for both HIn and In- were 7.12x103 and 9.61x102 Lmol-1cm-1 at 570 nm, respectively, the absorbance of the solution can be calculated:

33. A = AHIn + AIn A = 7.12x103 * 1.00* 0.88x10-5 + 9.61x102 * 1.00 *1.12x10-5 = 0.073 However, if no dissociation takes place we may have: A = AHIn A = 7.12x103 * 1.00 * 2x10-5 = 0.142 If the two results are compared we can calculate the % decrease in anticipated signal as: % decrease in signal = {(0.142 – 0.073)/0.142}x100% = 49%

34. However, at 430 nm, the molar absorptivities of HIn and In- are 6.30*102 and 2.06*104, respectively. A = AHIn + AIn A = 6.30*102 * 1.00* 0.88x10-5 + 2.06*104 * 1.00 *1.12x10-5 = 0.236 Again, if no dissociation takes place we may have: A = AHIn A = 6.30*102 * 1.00 * 2x10-5 = 0.013 If the two results are compared we can calculate the % increase in anticipated signal as: % decrease in signal = {(0.236 – 0.013)/0.013}x100% = V. large

35. Comparison between results obtained at 570 nm and 430 nm show large dependence on the values of the molar absorptivities of HIn and In- at these wavelength. At 570 nm: A = AHIn + AIn A = 7.12x103 * 1.00* 0.88x10-5 + 9.61x102 * 1.00 *1.12x10-5 = 0.073 And at 430 nm: A = 6.30*102 * 1.00* 0.88x10-5 + 2.06*104 * 1.00 *1.12x10-5 = 0.236

36. Chemical deviations from Beer’s law for unbuffered solutions of the indicator Hln. Note that there are positive deviations at 430 nm and negative deviations at 570 nm. At 430 nm, the absorbance is primarily due to the ionized In- form of the indicator and is proportional to the fraction ionized, which varies nonlinearly with the total indicator concentration. At 570 nm, the absorbance is due principally to the undissociated acid Hln, which increases nonlinearly with the total concentration.

38. Calculated Absorbance Data for Various Indicator Concentrations

39. An example of association equilibria Association of chromate in acidic solution to form the dichromate according to the equation below: 2 CrO42- + 2 H+D Cr2O72- + H2O The absorbance of the chromate ions will change according to the mentioned equilibrium and will thus be nonlinearly related to concentration. A = eCrO4 *b*CCrO4 + eCr2O7 *b*CCr2O7

40. 3.      Instrumental Deviations a.       Beer’s law is good for monochromatic light only since e is wavelength dependent. It is enough to assume a dichromatic beam passing through a sample to appreciate the need for a monochromatic light. Assume that the radiant power of incident radiation is Po and Po’ while transmitted power is P and P’. The absorbance of solution can be written as:

41. A = log (Po + Po’)/(P + P’) P = Po10-ebc, substituting in the above equation: A = log (Po + Po’)/(Po10-ebc Po’10-e’bc) Assume e = e’ = e A = log (Po + Po’)/(Po + Po’) 10-ebc A = ebc However, since e’ # e, since e is wavelength dependent, then A # ebc

43. The effect of polychromatic radiation on Beer’s law. In the spectrum at the top, the molar absorptivity of the analyte is nearly constant over band A. Note that in Beer’s law plot at the bottom, using band A gives a linear relationship. In the spectrum, band B corresponds to a region where the absorptivity shows substantial changes. In the lower plot, note the dramatic deviation from Beer’s law that results.

45. Therefore, the linearity between absorbance and concentration breaks down if incident radiation was polychromatic. In most cases with UV-Vis spectroscopy, the effect is small especially at the wavelength maximum. The small changes in signal is insignificant since e differs only slightly.

46. UV-Vis Absorption Spectroscopy Lecture 23

47. b.      Stray Radiation Stray radiation resulting from scattering or various reflections in the instrument will reach the detector without passing through the sample. The problem can be severe in cases of high absorbance or when the wavelengths of stray radiation is in such a range where the detector is highly sensitive as well as at wavelengths extremes of an instrument. The absorbance recorded can be represented by the relation: A = log (Po + Ps)/(P + Ps) Where; Ps is the radiant power of stray radiation.

49. Instrumental Noise as a Function in Transmittance The uncertainty in concentration as a function of the uncertainty in transmittance can be statistically represented as: sc2 = (dc/dT)2 sT2 A = -log T = ebc = -0.434 ln T c = -(1/eb)*0.434 ln T (1) dc/dt = - 0.434/ebT sc2 = (-0.434/ebT)2 sT2 (2)

50. Dividing equation 2 by the square of equation 1 (sc/c)2 = (-0.434/ebT)2 sT2/{(-0.434 ln T)2/(eb)2} sc/c = (sT / T ln T) Therefore, it is clear that the uncertainty in concentration of a sample is nonlinearly related to the magnitude of the transmittance. Substitution for different values of transmittance and assuming sT is constant, we get: