Module 7: Spectrophotometers

Question 1

6 parts of a spec

Answer 1

light source
wavelength selector (monochromator)
sample holder
photodetector
signal processor
display

Question 2

types of light source

Answer 2

Tungsten bulb
Tungsten-halogen (for visible light range)
Hydrogen lamp (for UV range)
Mercury and xenon Arc (very high intensity of UV light)
LED (for very specific wavelengths)
Laser (high intensity, nearly monochromatic)

Question 3

wavelength selector used to

Answer 3

isolate a characteristic wavelength that the analyte to be measured absorbs or emits

Question 4

desired wavelength

Answer 4

nominal wavelenth

represent peak intensity of the light leaving the wavelength selector

Question 5

Band pass or spectral bandwidth

Answer 5

the range of wavelengths on either side of the nominal wavelength that exit the monochromator
ex. nominal wl = 450
bandwidth of 10
actual wavelength would be 445-455

Question 6

the better the ability of device to select narrow wl

Answer 6

the better the spectral resolution

Question 7

wavelength selector in a photometer

Answer 7

called a filter

Question 8

wavelength selector in a spec

Answer 8

called a monochromator

Question 9

2 filters used in photometry

Answer 9

glass absorption (green filter allows green through but no other wavelengths)
Interference

Question 10

Monochromator used in spectrophotometers

Answer 10

prisms (refraction of white light, nonlinear spectrum)
Diffraction grating (diffraction of white light, linear spectrum)

Question 11

3 parts of a monochromator

Answer 11

entrance slit
dispersing device
exit slit

Question 12

2 types of gratings used

Answer 12

transmission (spectrum made as light passes through grating)

Reflection (spectrum made when light reflects off mirror surface with etched lines; more efficient; most common type)

Question 13

Sample holder (Cuvet)

Answer 13

usually a set width

affects beers law

Question 14

types of cuvets

Answer 14

borosilicate glass (visible range)
Quartz or silica (UV range)
Plastic (for visible and uv, but not good for repeated use)

Question 15

photoelectric effect

Answer 15

some metals when the surface atoms are hit by radiant energy become excited and cause a current to flow

Question 16

spectral response of a photodetector

Answer 16

the range of wavelengths over which it operates

Question 17

Dark current

Answer 17

current that flows from a photodetector even though no light is hitting it
compensated for setting 0%T

Question 18

Types of photodetectors

Answer 18

vacuum and gas photodiodes
photomultiplier tubes (PMTs)
Solid state photodiodes
Linear diode arrays

Question 19

advantages to diode array spectrophotometers

Answer 19

speed
measurements for multiple wl at once
fewer mechanical parts

Question 20

signal processor

Answer 20

amplifies and cleans electrical signal from photodetector and converts it into usable data relating to measurement

Question 21

3 error signals in photometry measurements

Answer 21

drift
noise
artifact

Question 22

Display

Answer 22

displays measurement data to operator

Question 23

types of output displays

Answer 23

panel peter (analog)
LED (digital)
Video display terminals (VDT)
Chart recorders
Printers

Question 24

Single beam photometer

Answer 24

single light path from light source

prone to variations of light intensity, drift in dark current

Question 25

Double beam photometer

Answer 25

beam of light from source is split using beam splitters

Question 26

2 types of beam splitters

Answer 26

dichroic mirror | photochopper

Question 27

wavelength accuracy

Answer 27

check that the wl of light striking the sample is the one selected on the wl dial of the instrument

Question 28

photometric linearity

Answer 28

check ensures that for a given change in concentration, there will be a proportional change in absorbance

Question 29

linearity is affected by (5)

Answer 29

``` the detector samples response to beers law wide band pass stray light molecular fluorescence of the sample ```

Question 30

Stray light

Answer 30

all radiant energy that does not pass through the sample but still reaches the photodetector

Question 31

sources of stray light

Answer 31

light scatter by dust smudges/scratches on optica surfaces scattering and diffraction within monochromator fluorescence of the solution reflected light from the light source room light leaking into the sample compartment

Question 32

Stray light produces

Answer 32

a decrease in sensitivity and an apparent deviation from beers law by falsely decreasing the A

Question 33

Photometric accuracy

Answer 33

check to verify that the A displayed by instrument is the true A of the sample

Question 34

sensitivity vs slope

Answer 34

steeper slope = greater sensitivity

Question 35

Instrument calibration

Answer 35

setting 0%T and 100%t on photometer Calibrate and slope on pH meter