Chem. 133 – 2/11 Lecture

Announcements Lab today –Will cover 4 (of 8) set 2 labs (remainder covered on Tuesday) –Period 1 will extend one day Website/Homework –Have posted text data for problem –Also posted solutions for homework problems not collected –Problem 17.3 doesn’t give enough information to solve, but if you look at Example 17.2 (and assume a range of 0 to 1 for transmittance), it is not too hard Today’s Lecture –Transducers (pretty brief) –Amplifiers (covering qualitatively) –Noise (if time)

Electronics - Overview Generic Instrument Block Diagram Analog Electronics Transducer Analog Signal Processing Digital Electronics Analog to Digital Conversion Board Memory Signal Display Long-term Storage (Disk) Digital Signal Processing Exciter sample Digital to Analog (control) Covering today Covered last time

Electronics Transducers Definition: A transducer is a device that converts a physical (or chemical) property into an electrical signal Classifications: By output measure (V, I, R, frequency) By phenomenon measured (charged particle flux, temperature, light intensity, surface modification) Internally vs. Externally Amplified

Transducers Charge Particle Detectors Measurement of electrons, molecular ions and charged aerosol particles Most common type for GC and MS detectors Charge Collector or Faraday Cup I e-e- Can detect currents > A

Transducers Charged Particle Detectors Detection Process 1.Charged particle hits cathode 2.Electrons emitted from collision 3.Amplificaion occurs with each stage 4.Current (electron flux) increases before anode Cathode Dynodes M-M- e-e- e-e- I Electron Multiplier (MS detector) Example: if each stage produces 6 useful electrons out per ion in, amplification in current would be x6 3 or x216. With greater amplification, single particle detection is possible

Transducers Measurement of Temperature Applications: –Temperature control (e.g. GC ovens) –Infrared light (IR spectrometers) Resistance based –Thermistors and platinum resistance thermometers (both have R = f(T)) Voltage based –Thermocouples (voltage generated by metal junction which depends on T)

Transducers Detection of Light 1.Vacuum tube types -Based on photoelectron effect -Current based detectors -Photocells (see diagram) -All have minimum energy (maximum wavelength) where electron ejection just occurs -Photomultiplier tube (combination of photocell and electron multiplier) -Photomultiplier tube allows detection of single photons I hνhν e-e-

Transducers Detection of Light 2.Solid state types -Typically less expensive than vacuum tube types -Tend to operate better at longer wavelengths -Based on promotion of electrons to conducting bands -Photodiodes (I proportional to intensity) -Photoconductivity cells (R dependent on intensity) -Photovoltaic cells (V dependent on intensity) -Advanced devices (discussed in spectroscopy section) -Arrays (1D or 2D sets of detectors) + - n p Reversed-bias photodiode: High impedance until photons arrive e-e- e-e- e-e- 1D Photodiode Array

Some Questions on Transducers 1.List a transducer with a (primary) current signal. 2.List a transducer with a (primary) resistance signal. 3.List a transducer that can be used to measure charged particles. 4.What is the main reason that a photomultiplier tube is more sensitive than a photocell? 5.Give an example of a transducer that is readily available in an array form.

Operational Amplifiers General Use: Analog Signal Processing Common Uses –voltage amplification –current amplification (removal of effect of internal resistance) –current to voltage conversion –differential amplifier to remove common noise This time – only covering qualitatively (no calculations problems)

Operational Amplifiers Function –Requires power (+15 V/ -15 V) –Has inverting and noninverting inputs –Output voltage is equal to (gain)x(V + – V - ) (“real” op amp) –Main thing to know about real op amp is you can not connect the two input wires +15 V -15 V + - inverting input output

Operational Amplifiers “Ideal” Op Amp –V + = V - (infinite gain) –I + = I - = 0 (infinite input resistance) Useful Circuits –All use feedback circuits –Example: voltage follower (current amplifier) –V(output) = V(electrode) + - output feedback circuit + - electrode with V electrode

Operational Amplifiers “Ideal” Op Amp –V + = V - (infinite gain) –I + = I - = 0 (infinite input resistance) Useful Circuits –All use feedback circuits –Example: voltage follower (current amplifier) –V(output) = -V(electrode) + - output feedback circuit + - electrode with V electrode

Operational Amplifiers Other Useful Circuits –Inverting amplifier in text V out = -R f V in /R in useful for amplifying voltage signals –Differential amplifier in text V out = (R f /R in )(V 1 - V 2 ) allows removal of noise common to V 1 /V 2 –Current to voltage convertor Calculate V out + - transducer with current I RfRf

Noise Introduction Why worry about noise? –Both noise and signal affect sensitivity (the ability to detect low concentrations –While it is easy to increase the signal, noise often will also increase (e.g. inverting op amp amplifier circuit) –It is possible to reduce noise without also reducing the signal (e.g. differential op amp amplifier circuit or transducers with internal amplification) –If we know the source of the noise we can make improvements more easily

Noise Definitions Noise 1)“variability in a measurement due to (random) errors” (textual) 2) the standard deviation in the values (σ) (mathematical) or the root mean square value (more common in electronics – based on assumption of sine wave form of noise) 3) peak to peak noise (graphical and roughly 6σ) Peak to peak