Basic Instrumentation

Transcription

1 Basic Instrumentation Joachim Mueller Principles of Fluorescence Spectroscopy Genova, Italy June 30 July 3, 2008 Figure and slide acknowledgements: Theodore Hazlett Fluorometer ISS PC1 (ISS Inc., Champaign, IL, USA) Fluorolog-3 (Jobin Yvon Inc, Edison, NJ, USA ) QuantaMaster (OBB Sales, London, Ontario N6E 2S8) Fluorometer Components Excitation Polarizer Sample Light Source Excitation Wavelength Selection Computer Emission Polarizer Emission Wavelength Selection Detector Fluorometer: The Basics Note: Both polarizers can be removed from the optical beam path 1

2 Fluorometer Components Light Source Detectors Wavelength Selection Polarizers The Laboratory Fluorometer Standard Light Source: Xenon Arc Lamp Exit Slit P ex P em P em ISS (Champaign, IL, USA) PC1 Fluorometer Light Sources 2

3 Lamp Light Sources: Arc Lamps 1. Xenon Arc Lamp (wide range of wavelengths) Lamp Profiles: Ozone Free UV Visible 15 kw Xenon arc lamp 2. High Pressure Mercury Lamps (High Intensities but concentrated in specific lines) Lamp Light Sources: Arc Lamps 3. Mercury-Xenon Arc Lamp (greater intensities in the UV) ARC LAMP ISSUES: Limited Lifetime Stability (flicker + drifts) Safety high internal gas pressures hot never stare into burning lamp do not touch with bare hands LAMP HOUSING + OPTICS : Conventional OR Compact Cermax lamp Lamp Light Sources: Incandescent 4. Tungsten-Halogen Lamps A Tungsten-Halogen lamp with a filter to remove UV light. The color temperature varies with the applied voltage (average values range from about 2200 K to 3400 K). 3

4 Lamp Light Sources: Semiconductor 5. Light Emitting Diodes (LEDs) Spectra for blue, yellow-green, and red LEDs. FWHM spectral bandwidth is approximately 25 nm for all three colors. White LED: typical emission spectrum Lamp Luminous Flux (Lumens) Spectral Irradiance (Milliwatt/Square Meter/Nanometer) HBO 100 Watts ( nm) XBO 75 Watts ( nm) Tungsten 100 Watts 2800 < 1 ( nm) Superbright LED LED (Blue, 450 nm) Lamp Light Sources: Semiconductor 5. Light Emitting Diodes (LEDs) Wavelengths from 260 nm to 2400 nm Deep UV LEDs λ 260 nm Laser Light Sources Argon Ion: Wavelength Rel Pwr Wavelength Rel Pwr Wavelength 528.7nm nm nm 514.5nm nm nm 501.7nm nm nm 496.5nm nm nm nm nm 295nm 266nm 325nm Doubled Ti:Sapphire 350 nm 500 nm 355nm 351 nm 364 nm 442nm 514nm 488nm 528nm 532nm 633nm 543nm 612nm 594nm Ti:Sapphire 690 nm 1050 nm Argon-ion 100 mw Helium-cadmium Nd-YAG He-Ne Red 633nm >10 mw Orange 612nm 10mW Yellow 594nm 4mW Green 543nm 3mW 4

5 Laser Diodes (DPSS) Diode pumped solid state laser Wavelengths (nm): 262, 266, 349, 351, 355, 375, 405, 415, 430, 440, 447, 473, 488, 523, 527, 532, 542, 555, 561, , 638, 655, 658, 671, 685, 785, 808, 852, 946, 980, 1047, 1053, 1064, 1080, , 1444, 1550 Supercontinuum Light Focus ultrashort pulsed light into photonic crystal fiber Photonic crystal fiber Detectors 5

6 Photon Multiplier Tube Microchannel Plate Detector (MCP) MCP & Electronics (ISS Inc. Champaign, IL USA) For fast modulation f > 500 MHz The Classic PMT Design Photocathode Vacuum Dynodes λ e - e - e - e - e - e - e - e - - e - - e - e - Anode Window Constant Voltage (use of a Zenor Diode) Current Output High Voltage Supply (-1000 to V) resister series (voltage divider) capacitor series (current source) Ground Hamamatsu R928 PMT Family R2949 Window with Photocathode Beneath 6

7 PMT Quantum Efficiencies Cathode Material Window Material Photon Counting (Digital) and Analog Detection time Signal Continuous Current Measurement Photon Counting: Constant High Voltage Supply PMT Analog: Variable Voltage Supply PMT Discriminator Sets Level level TTL Output (1 photon = 1 pulse) Anode Current = Pulse averaging Computer Primary Advantages: 1. Sensitivity (high signal/noise) 2. Increased measurement stability Primary Advantage: 1. Broad dynamic range 2. Adjustable range Side-On PMT PMT Geometries Head-On PMT Opaque photocathode Semitransparent Photocathode Side-on PMTs have slightly enhanced quantum efficiency over Head-on PMTs Side-on PMTs often have larger afterpulsing probabilities than Head-on PMTs Side-on PMTs count rate linearity less than for Head-on PMT Head-on PMTs provide better spatial uniformity than Side-on PMTs Side-on PMTs have faster response time than Head-on PMTs (compact design) Side-on PMTs are less affected by a magnetic field than Head-on PMTs 7

8 Avalanche Photodiode (APD) APD for analog detection APD for photon counting The silicon avalanche photodiode (Si APD) has a fast time response and high sensitivity in the near infrared region. APDs are available with active areas from 0.2 mm to 5.0 mm in diameter and low dark currents (selectable). Photo courtesy of Hamamatsu Single photon counting module (SPCM) from Perkin-Elmer Wavelength Selection Fixed Optical Filters Tunable Optical Filters Monochromators Optical Filter Channel P ex P em P em 8

9 Long Pass Optical Filters 100 Transmission (%) Spectral Shape Thickness Physical Shape Fluorescence (!?) Hoya O54 More Optical Filter Types 100 Broad Bandpass Filter (Hoya U330) Interference Filters (Chroma Technologies) 80 Transmission (%) Neutral Density (Coherent Lasers) Tunable Optical Filters Liquid Crystal Filters: An electrically controlled liquid crystal elements to select a specific visible wavelength of light for transmission through the filter at the exclusion of all others. AO Tunable Filters: The AOTF range of acousto-optic devices are solid state optical filters. The wavelength of the diffracted light is selected according to the frequency of the RF drive signal. 9

10 Monochromators Mirrors Czerny-Turner design 1. Slit Width (mm) is the dimension of the slits. Exit Slit 2. Bandpass is the FWHM of the selected wavelength. 3. The dispersion is the factor to convert slit width to bandpass. Entrance slit Rotating Diffraction Grating (Planar or Concaved) The Inside of a Monochromator Mirrors Grating Nth Order (spectral distribution) Zero Order (acts like a mirror) Order of diffraction 1-st order 0-th order (acts like a mirror) 2-nd order 10

11 Changing the Bandpass 1. Drop in intensity 2. Narrowing of the spectral selection Fixed Excitation Bandpass = 4.25 nm Changing the Emission Bandpass nm 8.5 nm 4.25 nm nm Fluorescence x10 6 (au) nm 8.5 nm 4.25 nm nm Collected on a SPEX Fluoromax - 2 Higher Order Light Diffraction Emission Scan: Excitation 300 nm Glycogen in PBS Fluorescence x10 3 (au) Excitation (Rayleigh) Scatter (300 nm) Water RAMAN (334 nm) 2 nd Order Scatter (600 nm) 2 nd Order RAMAN (668 nm) Fluorescent Contaminants Raman scatter of water Vibrational modes of water Resonant Stokes Raman scattering Energy for the OH stretch vibrational mode in water (expressed in inverse wavenumbers): 3400 cm -1 Simple formula to calculate the wavelength of the Raman peak: 7 (1) Take the excitation wavelength (say 10 = nm) and insert in the following equation: 490 (2) The result specifies the position of the raman peak in nanometers (i.e. the raman peak is at 587nm for an excitation wavelength of 490nm. 11

12 Monochromator Polarization Bias Tungsten Lamp Profile Collected on an SLM Fluorometer Wood s Anomaly Parallel Emission No Polarizer Fluorescence Fluorescence Perpendicular Emission Adapted from Jameson, D.M., Instrumental Refinements in Fluorescence Spectroscopy: Applications to Protein Systems., in Biochemistry, Champaign-Urbana, University of Illinois, Correction of Emission Spectra ISSPC1 Correction Factors vertical horizontal (nm) Wavelength ANS Emission Spectrum, no polarizer ANS Emission Spectrum, parallel polarizer Fluorescence Intensity (a.u.) B Fluorescence Intensity (a.u.) C uncorrected corrected Wavelength Wavelength from Jameson et. Al., Methods in Enzymology, 360:1 Excitation Correction Quantum Counter Exit Slit P ex P em P em 12

13 The Instrument Quantum Counter Common Quantum Counters (optimal range)* Rhodamine B Fluorescein Quinine Sulfate ( nm) ( nm) ( nm) Quantum Counter Optical Filter Eppley Thermopile/ QC Linearity of Rhodamine as a quantum counter Fluorescence Here we want the inner filter effect! Reference Detector * Melhuish (1962) J. Opt. Soc. Amer. 52:1256 Excitation Correction Absorption (dotted line) and Excitation Spectra (solid line) of ANS in Ethanol Fluorescence Uncorrected A Fluorescence B Ratio Corrected (nm) Wavelength 1.0 Fluorescence Lamp Corrected C (nm) Wavelength (nm) Wavelength from Jameson et. Al., Methods in Enzymology, 360:1 Polarizers Common Types: Glan Taylor (air gap) Glan Thompson Sheet Polarizers The Glan Taylor prism polarizer 0 Two Calcite Prisms 90 0 Sheet polarizer 90 Two UV selected calcite prisms are assembled with an intervening air space. The calcite prism is birefringent and cut so that only one polarization component continues straight through the prisms. The spectral range of this polarizer is from 250 to 2300 nm. At 250 nm there is approximately 50% transmittance. 13

14 Sample Issues Signal Attenuation of the Excitation Light PMT Saturation Excess Emission Fluorescence vs Signal Instrument Signal LINEAR REGION x x10 6 [Fluorophore] Reduced emission intensity 1. ND Filters 2. Narrow slit widths 3. Move off absorbance peak Attenuation of the Excitation Light through Absorbance Sample concentration & the inner filter effect Rhodamine B from Jameson et. al., Methods in Enzymology (2002), 360:1 The second half of the inner filter effect: attenuation of the emission signal Diluted Sample 4 3 x x x Absorbance Spectrum (1) Spectral Shift (2) Change in Spectral Shape 14

15 How do we handle highly absorbing solutions? Quartz/Optical Glass/Plastic Cells Excitation Emission Path Length Emission Detector 4 Position Turret SPEX Fluoromax-2, Jobin-Yvon Excitation Path Length Front Face Detection Triangular Cells Thin Cells & Special Compartments Excitation IBH, Glasgow G3 8JU United Kingdom Emission Mirror Excitation Detector Sample [1] Reflected Excitation & Emission Absorbance Measurements [1] Adapted from Gryczynski, Lubkowski, & Bucci Methods of Enz. 278: 538 Lifetime Instrumentation 15

16 Light Sources for Decay Acquisition: Frequency and Time Domain Measurements Pulsed Light Sources (frequency & pulse widths) Mode-Locked Lasers ND:YAG (76 MHz) (150 ps) Pumped Dye Lasers (4 MHz Cavity Dumped, ps) Ti:Sapphire lasers (80 MHz, 150 fs) Mode-locked Argon Ion lasers Directly Modulated Light Sources Diode Lasers (short pulses in ps range, & can be modulated by synthesizer) LEDs (directly modulated via synthesizer, 1 ns, 20 MHz) Flash Lamps Thyratron-gated nanosecond flash lamp (PTI), 25 KHz, 1.6 ns Coaxial nanosecond flashlamp (IBH), 10Hz-100kHz, 0.6 ns Modulation of CW Light Use of a Pockel s Cell Pulsed Emission 0 Polished on a side exit plane Pockel s Cell Polarizer Mirror Double Pass Pockel s Cell Polarizer Radio Frequency Input 90 CW Light Source The Pockel s Cell is an electro-optic device that uses the birefringment properties of calcite crystals to alter the beam path of polarized light. In applying power, the index of refraction is changed and the beam exiting the side emission port (0 polarized) is enhanced or attenuated. In applying RF the output becomes modulated. Time Correlated Single Photon Counting Timing Electronics or 2 nd PMT Pulsed Light Source Sample Compartment Filter or Monochromator Neutral density (reduce to one photon/pulse) Constant Fraction Discriminator Time-to-Amplitude Converter (TAC) Counts TAC Multichannel Analyzer Time PMT Photon Counting PMT Instrument Considerations Excitation pulse width Excitation pulse frequency Timing accuracy Detector response time (PMTs ns; MCP 0.15 to 0.03 ns) 16

17 Histograms built one photon count at a time Fluorescence Decay 2 Fluorescence Instrument Response Function Channels (50 ps) (1) The pulse width and instrument response times determine the time resolution. (2) The pulse frequency also influences the time window. An 80 MHz pulse frequency (Ti:Sapphire laser) would deliver a pulse every 12.5 ns and the pulses would interfere with photons arriving later than the 12.5 ns time. Polarization Correction There is still a polarization problem in the geometry of our excitation and collection (even without a monochromator)!! Will the corrections never end??? An intuitive argument: [6] [4] [1] = I 0 + I 90 [2] = I 0 + I 90 [3] = I 0 + I 90 [4] = I 0 + I 90 0 Polarized Excitation [1] [3] [5] = 2 x I 90 [6] = 2 x I 90 Total = 4 x I x I 90 [2] 0 [5] The total Intensity is proportional to: I x I Setting the excitation angle to 0 and the emission polarizer to 54.7 the proper weighting of the vectors is achieved.* *Spencer & Weber (1970) J. Chem Phys. 52:1654 Frequency Domain Fluorometry Pockel s Cell CW Light Source Sample Compartment Filter or Monochromator RF PMT PMT Analog PMTs (can also be done with photon counting) Synthesizers S1 and S2 S1 = n MHz S1 Reference Turret RF S2 Signal Locking Signal Signal Digital Acquisition Electronics S2 = n MHz Hz Computer Driven Controls Similar instrument considerations as With TCSPC 17

18 Lifetime Station #3, LDF, Champaign IL, USA 18