Basic tricks of Atomic Absorption Spectrophotometry AAS technique

Basic `tricks’ of Atomic Absorption Spectrophotometry AAS technique üutilizes 1 element SOURCE üAtomizes sample The overall advantages of AAS: üEliminates N 2 problem present in UV-VIS approach üMUCH more sensitive to low concentrations (to 0. 1 ppm) üWith standard addition, eliminates matrix effects üCan study elements in mixture as if independent of each other

Single Beam Atomic Absorption Spectrophotometer (AAS) schematic (Skoog pg 240 Fig 9 -13) : the most commonly used (least expensive) version 5)Computer readout 4) Transducer (PMT) 3) Wavelength selector (Ebert Monochromator) 1) source 2) `cell’=flame=sample

DOUBLE BEAM AAS DESIGN (ASC’s PE Aanalyst 200 is double beam) advantages 1)Chopper allows ratioing of flame vs no flame condition (instrument background removed) But…a bit more expensive ASC instrument design (Perkin-Elmer 3030 B AAS) 2) lock-in amp filters all but chopper frequency signals out (lower noise, more sensitivity)

AAS source =HCL= Hollow Cathode Lamp How it works: (Skoog 238) 300 volts (made of single element) e. Ar Anode (+) Cathode (-) 300 V Alternative `EDL’ (electrodeless discharge lamp) Higher light output (100 X) => can measure very low concentrations Low life time (100 h ? ) vs years w/HCL 1) Ar Ar+* + e - 2) Ar+* accelerates towards (-) cathode made of M(s) and collides Ar+* + M(s) + e- Ar + M(g)* `sputtering’ of hot M by Argon ion Skoog 239 3) M(g)* M(s) + h (specific to atomic M)

Getting atoms of sample (Skoog p 231) Want to sample free atoms Acetylene –air: 2100 -2400 o. C Acetylene-O 2 : 2600 -2800 o. C Acetylene-N 2 O: 3050 -3150 o. C

AAS `sample cell’ design that achieves atoms: laminar flow (slotted’) burner with nebulizer One BIG disadvantage…sucks up lots of sample

Graphite furnace or Electrothermal AAS (Skoog 234) Big advantage: need only microliter volumes vs m. L volumes in flame AAS syringe drops sample on platform thru hol in Lvov platform Light goes through graphite tube where sample is dried, ashed and `flashed’ LVOV platform

Typical electrothermal analysis (Skoog page 235) Flame vs furnace 1)Sensitivity compared Element flame furnace Cu 2 ppb 1 0. 05 Ni 3 0. 5 As 200 0. 5 Hg 500 5* 1 0. 001 ppm = 1 ppb units From table 9 -3 Skoog p 249 Furnace AAS more sensitive 2)Accuracy compared …the relative error …with flame…is. . 1 to 2% (and) can be lowered to a few tenths…. errors in electrothermal analysis exceeds flame by factors of 5 -10. ” (Skoog p. 248)

Limitations of AAS Method 1) can do only one (1) element at a time 2) Chemical interferences (see Skoog pp 244 -5) Why burner head design is the `Achilles heel’ of AAS ü Many metal carbonates, sulfates and phosphates are hard to dissolve and nebulize into small, easily atomized mists FIXES a) Add soluble release agents like La(NO 3)2 or Sr(NO 3)2 which tie up carbonates, sulfates, phosphates and `release’ analysis metals Example: Sr(NO 3)2 (aq) + Ca. CO 3 (s) Sr(CO 3)(s) + Ca(NO 3)2 (aq) b) Add protective (chelating) agents (e. g. EDTA ) to protect analysis metals in a volatile, soluble form Example: EDTA(aq)4 - + Ca 2+ Ca(EDTA)2 -(aq)

Limitations of AAS Method (continued) Flame reactions lead to metal hydroxides in flame=> broadening No sure fix except to go to higher temperatures… but AAS can’t get to requisite values

ALTERNATIVE TO AAS…Inductively Coupled Plasma Atomic Emission Spectroscopy = ICP AES (or just ICP) (Skoog pp 254 -264) § allows simultaneous element analysis §Removes chemical interferences §Often better sensitivity Main “trick” for ICP Get atoms hot enough that they start to produce significant fluorescence and observe these atomic emissions directly and simultaneously

2) Wavelength selector (stepper + concave grating* 3) Transducer array (many PMT detectors arranged at predetermined slots) *echelle grating is less flattened than concave echelette type and allows 2 D dispersion (p 261 fig 10 -7) Source focusing optics 1) Cell and source 4) readouts Classic (old style) Rowland circle design

1 Source and Cell: the plasma torch ~ outer corona of Sun’s T

1: Source (and cell) in the flesh

Perkin-Elmer Optima 200 ICP (latest/ greatest ~ 150 -200 k$) Venting for Ar and torched samples 0) Sample sucked into nebulizer through capillaries 1) torch 4) Data collection 2) Echelle monochromator, Nebulizer in here 3) 2 D PMT array and relevant optics all in here Power supply (1 k. W) for torch down here Receives rejected solution from nebulizer

Traditional flat echellete grating 2) Wavelength dispersion: the Echelle Monochromator 2 D dispersion See page 186 -187 (section 7 C-2 of Skoog) 2 Wavelength selector For ICP: flat `Echelle’ grating Echelle grating Always used in conjunction with a dispersing prism 2 D

2) Wavelength dispersion (continued) A graphical picture of how 2 D dispersion works

Overview of optical path in a modern ICP with Echelle -type grating optics and new school CCD detection (a lot more compact)

3 Transducers in ICP ‘Old school’ Arrangement (Rowland circle with many PMT on circle to measure individual …big footprint) `New School’ Charge-coupled 2 D detection (CCD) (see previous slide also)

A bit more on CCD (Charge Coupled Devices (see also, page 198 -199 of text) From Echelle + dispersing prism Families of related colors across CCD essentially keeps track of charge collected for defined period at every pixel. Charge ~ light intensity Boyles and Smith AT & T Bell Labs 1969

CCD requires several chips to time and keep track of the many pixel data points

Minimum detection limits (in ppb) for Three Atomic Spectral Methods element Al Cu AAS 30 HGA 0. 1 ICP 0. 2 2 0. 05 0. 04 Fe 6 0. 25 0. 09 Ni 3 0. 5 0. 2 Cr 4 0. 03 0. 08 Taken from Page 249 Table 9. 3 of text