Quantative Analysis_Lecture 11&12 Spectrophotometry

Front 1

What are components of a spectrophotometer?

Back 1

1. Source: continuous radiation over the wavelengths of interest

2. Monochromator: selects narrow band of wavelengths from source

3. Cell: contains the solution to be measured.

4. Detector: converting radiant energy into electrical energy

5. read-out: device that read-out the response of the detector.

Front 2

What are the components of a monochromator?

Back 2

1. An entrance slit to focus the light

2. A wave selector

3. An exit slit to focus the light

Front 3

What are two main types of spectrophotometer?

Back 3

- Single beam: the sample and blank are measured separately

- Double beam:the sample and blank are measured at the same time.

Front 4

What is the difference between double-beam "in space" and "in time"?

Back 4

1. Double-beam in space:
- mirror creates another beam from light source
- two detectors

2. Double-beam "in time":
- Single beam splits into 2 beams

- one goes to sample, the other goes to blank

- bring back to on one detector thru a chopper --> rotates which beam goes to the detector (but not at the same time)

- Detector and chopper needs to be synchronized.

Front 5

What are sources of radiant energy?

Back 5

1. Tungsten-filament incandescent lamp:
- 350 nm to 2000 nm; continuous
- commonly used for visible region.

2. Tungsten iodide lamp
- 240 to 2500 nm; continuous

3. Hydrogen and deuterium discharge lamps
- 160 nm to 380 nm; continuous
- used for UV region

4. Mercury-vapor lamps:
- discontinuous/ line spectrum --> 313, 365, 405, 436, and 546 nm.

5. Tuned lasers:
- 265 to 800 nm
- Tune it different wavelength
- Very specific and flexibility.

Front 6

What are different types of wavelength selectors?

Back 6

Filter:
- Block out wavelength, and selects for what you want to use

- Diminish light source

Monochromators: can give a much narrower range of wavelength than filters can and are easily adjusted over a wide spectral range.
- Prisms
(dispersion by prism is nonlinear. Shorter wavelengths are refracted more than longer wavelengths.

- Gratings
(consist of large numbers of parallel, equally spaced lines ruled on a highly polished surface such as aluminum.

(the grooves act as scattering centers --> equal dispersion of all wavelengths; linear dispersion.

Front 7

What are different types (material) of cells or cuvettes?

Back 7

- Glass:
o 320 to 950 nm
o Visible light range
o not good for UV (some glass)

- Quartz: below 340 nm (shorter wavelengths)

- Plastic:
o Cheap, disposable, recyclable
o good for visible and UV range
o Not good as glass
o Not suitable for critical measurements, high temp, and cleaning.

Front 8

What are different types of detectors?

Back 8

Photodetectors: Material that emit electric signal when hit with light.
- Phototube (or Photocell):
o commonly used in UV and visible regions
o photoemissive cathode and an anode.
o photon strikes cathode
--> emits electron that goes to the anode --> current flow

- Photomultiplier tubes
o Create stronger signals
o light strikes first plate and emits electron ---> goes to 2nd plate which emit more electrons and so on...

- Photovoltiac cells
o plate of copper or iron on which a semiconducting layer of cuprous oxide orselenium is placed.

o light passes thru semiconductive layer ---> createelectron flow that is sensed by ampmeter

- Diode arrays
o Array = “set” each responds to different wavelength
o Over large array of wavelengths
o Detect multiple wavelength of light at one time

Front 9

What are different types of readout devices?

Back 9

Electrical energy from a detector is displayed on some type of meter or readout system.

- Direct Reading system: electricity pushes needle
o inaccurate

- Null Point System:
o Comparing output from two different systems at the same time.

- Recorders
o synchronized to provide line traces of transmittance or absorbance as a function of either time or wavelength.

Front 10

What is bandpass?

Back 10

The range of wavelengths passed at one-half the intensity of the nominal wavelength.

Front 11

What is nominal wavelength?

Back 11

The wavelength of maximum intensity passed by the slit.

Front 12

What to improve spectral purity?

Back 12

The purity can be improved (the band-pass decreased) by decreasing the slit width.

Front 13

What is parameters for checking detector response?

Back 13

- compare with other sample --> does it agree with beer's law

- Use various liquid solutions of varying concentration known to follow Beer's law.

Front 14

What is stray light and how can it be removed?

Back 14

- light that isn' the selected light (e.g. from outside, light that does not come from the slit)

- use stray-light filters

- Changing light source, verifying wavelength calibration, seal light leaks, cleaning optical surfaces.

Front 15

What are parameters for determining wavelength accuracy?

Back 15

If they wavelength calibration of an instrument changes, the measured absorbance will change.

- Checking: use source that was strong emission lines at well defined wavelengths e.g. mercury vapor lamp.

- A solution of stable chromogen can be used as a secondary wavelength calibration standard.