1. Chem. 133 – 2/9 Lecture

2. Announcements Return Q1 + HW 1.1 This week’s seminar: on aerosol air pollution in Asia Lab –Sign up for term project instruments. I hope to have some better defined STORC term projects soon. Today’s Lecture –Electronic measurement digitization in digital voltmeters use of voltmeters in measurements

3. Electrical Measurements Analog to Digital Conversion Performance Measures: – Number of bits (more bits means analog signal is converted to more precisely known digital signal) –To ensure that digitization is NOT the limiting factor to sensitivity, noise should be seen following digitization –Speed (frequency): boards used in class could operate at up to ~100kHz. High speeds are needed for fast measurements. –Input range: the minimum voltage will correspond to all 0s and the maximum voltage will correspond to all 1s. Voltages greater than the maximum will be read as the maximum.

4. Electrical Measurements Analog to Digital Conversion Example of digitization of data (HPLC with fluorescence detector) Was digitization o.k.? digitization at 0.01 level shown

5. Electrical Measurements Analog to Digital Conversion Second Example: –A pH meter is used to monitor a process where a solution is acidified and then neutralized. The pH range that is desired to measure is 1 to 8. –The equation for the relationship between voltage and pH is found to be Voltage (in mV) = 231 – 60.1∙pH –The analog to digital convertor is a 12 bit convertor with the useful input range from -250 to 250 mV. –Answer the following questions: Before the solution is acidified, the binary # = 010 001 011 111. What is the voltage and the pH? After acidification, the voltage = 172 mV, what is the decimal # and pH corresponding to this? What is the maximum pH that can be read? Can a difference between pH = 7.00 and 7.05 be discerned?

6. Electronics Digital Volt Meter (DVM) Measurement Use of DVM for V, I, and R measurements voltage Thermocouple Pair (generates V) DVM Current transducer I out Multimeter Shunt resistor DVM -++- I = V meter /R shunt

7. Electronics Digital Volt Meter (DVM) Measurement Resistance Measurement thermistor multimeter Constant I source DVM example problem: The power source puts out 1.00 mA and the voltage read is 0.722 V, calculate the resistance

8. Electronics DVM Measurements Errors in Measurements –Errors in voltage measurements: can occur if a device also has "internal resistance" in combination with less than infinite resistance in DVM –Example: measurement of voltage from an ion selective electrode or pH electrode. Calculate the error in voltage if a pH electrode reads 0.721 V and has an internal resistance of 830 kΩ if the DVM has a meter resistance of 10.0 MΩ. –(go to blackboard) Cell DVM R(cell) R(meter) Cell = pH electrode

9. 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

10. 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 > 10 -15 A

11. 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

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

13. 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-

14. 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

15. 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.