XXIV. Dry Box
The manuals for the dry box are in the drawer next to the dry box.
(1)Check the manual "Model HE-63P" for installation, operation, and maintenance of HE-63P PEDATROL (pressure control system).
(2)Check the manual "Model HE-493" for installation, operation, and maintenance of HE-493 (regeneration system).
(3)Check the manual "DRI-LAB: GLOVE BOX: Operation Manual" for installation, operation, and maintenance of the HE-43 (dry box system).
A.Operating the Dry Box.
1.Important Operation Features.
1.Watch gloves during ante-chamber evacuation.
a.If gloves are pulled in, check to see it inside ante-chamber door and refill valve D (Figure 24-1) are closed.
2.Keep ante-chamber doors and valves tightly closed when not in use.
3.Never attempt to force ante-chamber doors open.
4.When opening ante-chamber doors, back door off firmly against bar before raising door.
2.Ante-chamber Procedures (from manual, Figure 24-1)
1.Evacuation.
a.Close both ante-chamber doors.
b.Vacuum pump should be running.
c.Close refill valve D.
d.Open evacuation valve C.
e.Watch action of gloves.
l.If gloves are drawn in close valve C.
a.Inside door is not sealed.
b.Refill valve is leaking.
c.This condition must be corrected
f.Evacuate to 50 microns minimum
l.Approximately 15 minutes
2.Refill
a.Close evacuation valve C.
b.Open refill valve D.
1.Introduce box gas into ante-chamber.
c.Let gauge reach 0 psig.
d.Close refill valve D.
e.Re-adjust box pressure.
3.Passing materials into dri-lab.
a.Close inside ante-chamber door.
b.Close valves C and D.
c.Open outside door.
d.Place material in ante-chamber.
e.Close outside door.
f.Open valve C.
g.Evacuate to 50 microns minimum.
h.Close valve C.
i.Open refill valve D.
j.Let pressure equalize.
1.Let gauge reach 0 psig.
k.Close valve D.
l.Open inside door.
m.Pass material into dri-lab.
n.Close inside door.
4.Passing materials out of dri-lab.
a.Ante-chamber to have pure atmosphere.
1.Evacuate and refill ante-chamber.
b.Close valves C and D.
c.Open inside door.
d.Place material inside ante-chamber.
e.Close inside door.
f.Open outside door.
g.Remove material from outside.
h.Close outside door.
3."Our Method"
The method we use in our lab for entering the dry box.
1.Place materials into the ante-chamber.
2.Close the door and place under vacuum.
3.After 5 min refill ante-chamber halfway using valve D.
4.Close valve D and then place under vacuum.
5.Repeat steps 3 and 4 two or three more times.
B.Regenerating the Dry Box
1.Dry Box Regenerating Set-ups
1.Old Green Dry Box
a.Turn off main dry box pump
b.Close valve A, open valves B and C (Figure 24-2)
c.Disconnect connections 1, 2, 3, and 4.
d.Connect a hose, which you can find in the bench close to the dry box, to connections 1 and 2. Connect hose ends 3 and 4 to a filter flask by the way shown in Figure 24-3.
e.After finishing steps a-d, set up vacuum trap D and turn on the south vacuum line by the window.
f.Follow regeneration procedure in section B.
2.New Tan Dry Box
a.Turn off main vacuum pump and remove the hose from the main vacuum pump at A, Figure 24-4.
b.Connect a large trap, B, onto a portable pump, Figure 5, do not use the pump connected to the dry box. A good portable pump to use is the portable vacuum line pump. Just remove the hose from the line and connect to the trap at B.
c.Connect A to C, Figure 24-5. Note: depending on the trap used, the hose, A may be too large to fit C. In this case, a filter flask can be used to reduce the bore-size of the hose, Figure 24-6.
d.Cool trap with liquid N2 and turn on pump
e.Follow regeneration procedure in section B.
2.Regeneration Procedure
Note: the circulator/blower runs continuously, including during regeneration cycle.
l.Establish an inert atmosphere in the glove box.
Caution:Never force the REGENERATION TIMER indicator in a counter-clockwise direction as this may damage the indicator.
2.REGENERATE/OFF switch is OFF (Figure 24-7) initially.
3.Check gas supply available to PURGE valve (J). 20 cubic feet per regeneration is required. Also, vacuum is set up as described in section A. The vacuum will be used to remove the H20 after the purge cycle.
4.Close Circulation Inlet and Circulation Outlet valves (A & B)
5.Turn REGENERATION TIMER indicator clockwise to START
6.Turn REGENERATE/OFF switch to REGENERATE. Timer will automatically cycle slowly to VACUUM.
a.Three hours to HEAT cycle.
b.One hour in PURGE cycle.
c.Leave timer in vacuum eight hours or overnight.
7.After the above eight hours (or more), turn REGENERATE/OFF switch to OFF. (This must be done manually as REGENERATION TIMER will remain in VACUUM mode until switch is turned off).
8.Open Circulation Outlet Valve (B) SLOWLY.
9.Open Circulation Inlet valve (A).
This is the end of the Regeneration Cycle.
C.Operation of Pressure Control System
A.Gage (Figure 24-8)
Photohelic
("Box Pressure")Reads positive (left) or negative (right) pressure inside the glove box. It also provides needles for presetting the pressure limits of the glove box atmosphere.
B.Electrical Switches
Automatic PressureSupplies power to the Photohelic. (Note: The
Control (ON, OFF)foot switch is not dependent upon this power switch.)
"L" (Foot Switch)Lowers pressure in glove box by applying vacuum pressure (suction).
"R" (Foot Switch)Raises pressure in the glove box by activating valve which allows inert gas to enter the glove box.
C.Knobs
Positive "+"
(Pressure Setting)Sets positive pressure limit for glove box (using red needle on the left side of the Photohelic as an indicator).
Negative "-"
(Pressure setting)Sets negative pressure limit for glove box (using red needle on the right side of the Photohelic as an indicator.
D.Important Features
-During normal operations, leave PEDATROL automatic pressure control switch ON.
-Let vacuum pump run continuously.
-Keep ample makeup gas supply at PEDATROL gas valve.
-To adjust Photohelic, turn appropriate black knob to set high pressure LEFT or low pressure RIGHT.
XXV. Lab Nitrogen
A. Dewars
Dewars are usually filled once a day (Mon-Fri) at a certain time. Care should be taken to maintain the dewars as full as possible, particularly before weekends and holidays.
B. Hook-Up
The dewars are equipped with quick release connections. Once a dewar is up to pressure, 120 psi, (This is accomplished by turning on the pressure builder.) the hose is connected. The hose is flushed clean with N2 through the vent valve. The valve to the lab is then turned on. The pressure builder of the recnetly filled dewar is turned off. (Only one dewar at a time should have the pressure builder valve on.)
XXVI. GC Operation
A.Carle GC Operation
1. Running a Sample
a.Check baseline zero.
1.Press LIST ZERO ENTER.
2.If not at zero, use coarse and fine zero adjust knobs to zero baseline.
b.Check controls
Readout: left column
Bridge Setting: 3
Output: 256
Column Temp.: 75
Inlet ΔT: On
Cycle/Slot Time: 20
V1, V2, V3: All Down
Light 8 should be the only one lit.
c.Select analysis method desired by pressing METH X ENTER. We generally use method 1 on most GC samples. See the following pages for methods. IF the power has gone out,you will need to type it in.
d.Inject a 0.5 mL of gas sample into the normal column using a gas syringe.
e.Immediately turn the mode knob on the GC control cabinet to repeat and puse the START button on the integrator simultaneously.
f.Sample is done after 20 minutes. Turn the mode knob back to single and make sure the timing cycle knob is lined up to after the last gray peg and before the first blue peg and check that the light is on #8.
2.Replacing Gas Tanks
If pressure in either tank drops below 300-400 psi replace tanks following procedure below. The Helium tanks are utilized by both CG's so only the changing of nitrogen tanks is discussed here. Helium tank changing is discussed with the HP GC. The nitrogen tank is located farthest from the Carle GC.
a.Close tank.
b.Close valve located under the drying tube all the way to the right.
c.Detach regulator without readjusting regulator.
d.Attach new tank; fasten well with teflon tape.
e.Turn on main valve of new tank.
f.Open tank.
g.Open purge valve and purge for 2-3 minutes.
h.Close purge valve.
i.Open valve located under the drying tube.
3.Changing the Injection Septa
a.Turn bridge setting to OFF.
b.Turn output to TEST.
c.Unscrew injection port and remove and replace septa with yellow side visible.
d.Screw injection part back in.
e.Turn bridge setting to 3.
f.Turn output to 256.
4.Reprogramming the Integrator
The integrator's functions which control plot appearance are defined through the ZERO, ATT 2↑ and CHT SP keys. PK WD, THRSH and AR REJ control data processing, integration and plot appearance. Either a default value or a value entered by the user is always associated with each key.
The default valves are listed below:
LIST: LIST
PEAK CAPACITY: 1159
ZERO = 0, 0.2
ATT 2↑ = 0
CHT SP = 1.0
PK WD = 0.04
THRSH = 0
AR REJ = 0
The methods we usually use are methods 1 and 2 which are listed in the following tables.
LIST: METH 1
LAST EDITED:
RUN PRMTRS
ZERO = 0
ATT 2↑ = 3
CHT SP = 0.3
PK WD = 0.26
THRSH = 2
AR REJ = 0
RPRT OPTNS
2. RF UNC PKS= 0.0000E+00
3. MUL FACTOR= 1.0000E+00
4. PK HEIGHT MODE NO
5. EXTEND RT NO
6. RPRT UNC PKS NO
TIME TBL
2.75 ATT 2↑ = 1
8.00 ATT 2↑ = 7
10.00 ATT 2↑ = 4
20.00 STOP
CALIB TBL
EMPTY
LIST: METH 2
LIST EDITED:
RUN PRMTRS
ZERO = 0
ATT 2↑ = 4
CHT SP = 0.3
PK WD = 0.26
THRSH = 2
AR REJ = 0
RPRT OPTHNS
2. RF UNC PKS= 0.0000E+00
3. MUL FACTOR= 1.0000E+00
4. PK HEIGHT MODE NO
5. EXTEND RT NO
6. RPRT NC PKS N0
TIME TBL
2.50 ATT 2↑ = 7
4.00 STOP
CALIB TBL
EMPTY
To program from default values to method values the following functional keys should be pressed in sequence:
ATT 2↑ integer value ENTER
CHT SP value ENTER
PK WD value ENTER
THRSH integer value ENTER
AR REJ integer value ENTER
The time table operates in the same way:
Functional key value time value (in minutes) ENTER
STOP TIME value ENTER
To store the method:
STORE METH n ENTER
To check the method:
LIST METH n ENTER
5.Proper Event Sequence
a. 8
b. 1 & 4
c. 2
d. 3 & 5
e. 1 & 4
f. 3 & 5
g. 6
h. 7
i. 8
6.Regenerating the Columns
a. Turn bridge setting to OFF.
b. Turn output to TEST.
c. Turn column temp to 145 oC.
d. Allow to heat for several hours (overnight).
e. Turn column temp to 75 oC.
f. Turn output to 256.
g. Turn bridge setting to 3.
XXVII. Rotary Evaporation
When solvents are to be removed from desired nonvolatile compounds, "flash evaporation" under reduced pressure is useful. Solvent can be removed more quickly and components which might be heat sensitive are protected. Rotary evaporators are most commonly used for reduced pressure evaporation, since the rotation of the flask agitates the mixture and prevents bumping and the solvent is spread on the walls of the flask and evaporates over a larger surface area.
There are several precautions to take in using any flash evaporator:
a)Make sure that all parts from which recondensed solvent can wash back into the flask are clean before attaching the flask.
b)Place some means of support only a few centimeters below the flask to catch it if it slips off.
c)Volume of solvent should be less than 1/2 of flask volume to prevent suddent agitating.
d)Attach the evaporator to the aspirator through a water trap. (Plastic clip is more useful to hold evaporating flask with aspirator.)
e)Do not take your hand away from the flask until you are sure it is held firm by the vacuum.
f)Start the evaporator turning as soon as you have pulled a vacuum.
g)Do not heat the mixture before evacuating the flask.
Our rotary evaporator is designed to fit any flask with a No. 24/40 Standard Taper Ground Joint. Flasks with larger or smaller joints can also be used if an appopriate reducing or enlarging glass joint adapter is used (Figure 27-1).
The vacuum seals are "O" Rings. USE HIGH VACUUM SILICONE STOPCOCK GREASE ONLY for lubricating and sealing the joints and bearings. The motor does not require any lubrication. The outer "O" Rings on the taper are neoprene; the internal "O" Ring seal is "Kel F".
The rotary evaporator can be used under a nitrogen atmosphere as indicated by Figure 19. Corrosive solvent systems may damage Gasket KD - 22, which must be routinely replaced.
CAUTION: Do not leave rotary evaporator unattended as complete evaporation of the water bath could damage the hot plate and bath. All running water connections to vacuum and condenser should be checked.
XXVIII. Computer
1.Turn on the power (POWER MASTER located on the right side of the computer), then quickly insert the disk labelled "WP 4.2" into disk drive A. Insert another formated disk into drive B to save the files you will be creating.
2.Follow the instruction on the screen to input date and time.
3.When the prompt "A >" appears on screen, type in wp to start using Word Perfect program. You should keep the WP disk in drive A as long as you use the WP program.
4.After a few messages the screen is clear and you can start to type in your text. Refer to the WP manual (located in the 3rd draw) for the usage of different function keys (labelled as F1 to F10 on the left side of the key board).
5.Always save your file on disk B. To save a file, push F10 and type b:xxxxxxxx.xxx. Filename can be eight symbols or less, followed by "." and three more symbols (optional).
6.There are two ways of retrieving a file from disk B:
(a)Push F5 then type b: followed by a RETURN. Move cursor to the filename you intend to retrieve and type 1.
(b)If you remember the filename, you can push both Alt and F10 keys at the same time, then type b: and the filename.
7.The principal printer for WP is the spinwriter. Turn its power on before you print a file. To select the EPSON LX-80 printer, push both Ctrl and PrtSc keys at the same time, and type 3, then type 2.
8.To print a file which is saved on disk B, push F5 then type b: followed by RETURN. Move the cursor to the filename you want to print out and type 4.
9. To print a file which you are working on, push both Ctrl and PrtSc keys the same time and select the option for printing.
10.To exit, push key F7. WP will ask "Save Document? (Y/N) Y". If you did not, you can do it now. When the question "Exit WP? (Y/N) N" appears, typing N will let you stay in WP but will exit (or delete if you didn't save) the previous file so that you can start to type a new text.
11.After you exit WP, remove both disks and turn off the power.
NOTE:Never turn off the power without first exiting the WP since this will leave some codes on the WP disk and it is impossible to remove these codes. Error messages will also appear when the computer is turned on again.
XXIX. Pump Maintenance
Pump maintenance involves routine oil changes and simple repairs. These operations are described below and can also be found in the Operator's Manual for the pump. In the described operations, the numbers refer to specific pump parts labeled on the exploded view of a pump (Figure 29-1).
A.Routine Oil Changes
Vacuum pumps which are used heavily (i.e. those on the Schlenk lines and dry boxes) should have their oil changed every 6 weeks.
To change oil, turn pump off and vent it to the atmosphere. Remove the oil filler plug (#2) and the drain plug (#3). Drain the used oil into the oil pan. Drainage may be accelerated by tipping the pump. When drainage is complete, replace the drain plug and add "Good Cheap Oil" (usually American Industrial Oil No. 32) via a funnel through the fill hole. Fill to the proper level indicatied on the oil level windows (#26) located on the end and side of the pump. Replace the oil filler plug and run the pump for 2-3 hr. Drain the used "Good Cheap Oil" as previously described and fill with Duo-Seal or "Good Expensive Oil". Oil filler plug and drain plug should be only finger tight.
Fresh oil may be obtained and waste oil disposed of in the Chemistry Shop. Waste oil must be deposited in 5 gallon drums.
B.Simple Repairs
a.Oil Leak -- Shaft Seal
If oil is leaking from around the base plate (#34) of the pump it generally means that the shaft seal (#44) is leaking and needs to be replaced.
Disconnect the pump from the system and drain the oil as described in part 1. To get to the shaft seal first remove the base plate by removing the four bolts (#37) which hold it in place. Next stand the pump on end with the motor (#67) pointing up. Loosen and remove the four nuts (#21) on the motor studs (#41). The handle assembly (#69 and #71) may be loosened and moved out of the way. The motor is now free and is lifted off. Remove the motor adapter (#28) and the coupling sleeve (#32). Remove the coupling flange (#30) by loosening the hex screws and pulling the coupling flange off with a gear puller. Do not lose the coupling key (#33). The shaft seal is now exposed. Remove the three screws (#45) on the seal flange. Pry the shaft seal loose. Remove the shaft seal and the seal gasket (#29). Wipe the pump facing clean. Place a new shaft seal gasket (lubricated with a film of oil) on the pump facing and lubricate the new shaft seal with a film of oil. Carefully slide the new shaft seal over the shaft. Align the screw holes and tighten the screws uniformly. Put the rest of the pump pieces back on in reverse order as they were taken off. Be sure to clean the base plate and the drip pad (#39).
Fill the pump with oil. Allow the pump to stand for 30 min before running.
b.Oil Leak -- Oil Case Gasket
If oil is leaking from around the oil case gasket (#5) the gasket should be replaced.
Disconnect the pump from the vacuum system and drain the oil as described above. Remove the screws (#4) holding the oil case assembly (#1). Remove the oil case assembly and gasket. Thoroughly clean the sealing surfaces of the case and pump. Lubricate the new gasket with thin film of oil. Apply the new gasket and position it into place. Mount the case and uniformly tighten the screws.
Fill pump with oil.
c.Coupling Sleeve
The repeated starting and stopping of the pump eventually wears out the coupling sleeve (#32). The coupling sleeve can be replaced by removing the motor as described under shaft seal replacement, part a.
XXX. Flourescence and the LS-100
A. Sample Prep
As almost all fluorescence is heavily quenched by the presence of oxygen, it is important that the solution to be tested be prepared and stored on an inert atmosphere, even if the substances in question is not particularly air sensitive. Use of the screw top fluorimitry cells with teflon/silica septa is recommended. These cells are not perfectly air-tight, so prepare sample as close to the test as feasible (no real change is seen over hours, but a week is likely too long). the cells hold about 3 ml.
B. Prior to Testing
Take an absorbance spectra under these conditions and note peaks. The lamp in the LS-100 can be used with hydrogen (deuterium) or nitrogen, and the selection will be based on what range the sample is to be stimulated at. One must evacuate the lamp and refill it at least fifteen times. Be sure the perssure is at 16.5 or near it.
Also, prior to testing lifetimes, an emission spectra should be obtained, and the LS-100 will do this under the Steady State menu. Take note of these peaks as well.
Before turning the nanosecond lamp on, allow twenty minutes for the machine to warm up. As this lamp is not used for steady state, one can obtain emission spectra while it warms up. There are two knobs that read steady state and nanosecond - these control mirrors, and it is important that they be set properly for the application being used.
C. Lifetimes
Patience is important here.
Choose the settings carefully, as this can make a big difference in the quality of data and how long it takes. Select as stimulation and dtections wavelengths the peask determined earlier. Choose the scatterer wavelength to be the stimulation wavelength. Set the number of runs to two and let it run. Make sure that the slits are open all the way for these tests. They are controlled by the unlabeled knobs on the top of the LS-100. If the machine says "scatterer too intense", dilute it with nanopure water. (It is rare that it will be too weak, as it evaporates with time and becomes more concentrated.) If no noticable signal shows up, it may be necessary to choose a different emission peak, especially as the sample is a mixture. (This does not make much sense, but seemed to work.)
If the signal seems to refine, return to the parameters and select the signal to noise option, choose the s/n desired and let it run. This will likely take several hours, perhaps overnight, so go do something else but check on it periodically.
If the machine says "lamp unstable" the electrodes probably need to be cleaned or the evacuation/refill procedure should be done again. the latter seemed to work a few times, but the former probably should be tried as well. This error message indicates a condition that generates a lot of noise and should be taken care of.
Other parameters to manipulate include number of channels, time to start, and time to end. Set the channels to maximum for the most data points. Time to start should be set to 50 nanoseconds or so as no signal is usally seen until then. Finally, for best data one should adjust the final time to reflect the lifetime - One should pick a value at least five times the lifetime of the sample. (A paradox here...) Generally one can take a reading and adjust to compensate. It is likely unadvisable to choose more than ten times the lifetime, as too much time will be spent measuring a noisy baseline, and this noise will be interpreted as data.
There is a setting to let the machine perform multiple runs and save the data for each as a file on a floppy disc. Do not use this. An error in the software makes it unable to save the first file.
D. Data Analysis
Generally it works best to let the machine auto-select all of the options in this part. Unless one is analyzing a mixture of fluorescent species, choose one exponential. (Choose as many exponents as there are species.) Depending on the data it may take several passes to converge on the lifetime. The plotter, like all of them, takes a long time to print.
If one saves the data as a series of names identical save for ending in an increasing sequence of numbers, V1, 2, 3, ..., then one can use the multifile option. This was designed to work in conjunction with the option mentioned earlier that does not work, but is useful on its own for files saved manually.
XXXI. Flash Photolysis
This section will describe how to obtain an oscilloscope trace of voltage vs time due to an absorption change related to the concentration of photogenerated transient species
The Nd-YAG laser is powered up in ‘program one’ and allowed to warm up for 15 min. Full details on start up procedures for the Continum Nd-YAG ND60 laser can be found in the user’s manual. In the meantime, the oscilloscope, Xe arc lamp and power meter can warm up and stabilize. After the 15 min. warm up is complete, and the laser parameters are set for the experiment. Using the select button on the control panel of the Nd-YAG laser, the desired program is selected and the activate button pressed. Next, the Q-switch button and internal shutter button must be pressed. This step activates the Q-switch and directs light to the oscillator.
At this point, the laser is producing either 532 nm light or 355 nm light, depending upon which set of harmonic crystals are present in the laser head. If light output from the dye laser is desired, 532 nm light is required. Converting the system from 532 nm light to 355 nm light is an operation that is conducted by the director of the Chemistry Department Laser Facility. Details on the operation of the dye laser, including optimizing dye concentration and optical alignments, can be found in the Continum Dye laser manual model NY60 or by contacting the director of the Chemistry Department Laser facility at Purdue University.
When the laser is ready to use, the current to the Xe arc lamp is raised to <8 A and the laser power is adjusted to the desired level, typically 30-50 mJ/pulse. Output power from the laser is attenuated by changing the Q-switch delay. The monochromator is set for the desired wavelength. With the mechanical shutter from the laser closed and the sample in place, the voltage generated across the terminator from the photomultiplier tube current is measured at 0% transmittance and 100% transmittance. The 100% transmittance voltage is maximized by adjusting the gimbal mount containing the uni-axis lens in the optical train and by proper alignment of the whole optical system. The voltage drop is measured as follows; the terminator connected to the photomultiplier is disconnected from the oscilloscope. A voltage meter is plugged into the BNC connector of the terminator and with the photomultiplier tube shutter closed, V0, corresponding to 0% transmittance is measured. The photomultiplier tube's shutter is opened, and with the sample in place and the laser head shutter closed, V100 corresponding to 100% transmittance is measured.
It is important not to saturate the photomultiplier tube. The photomultiplier tube is rated for a maximum current output of 1 mA. Determination of the current output from the photomultiplier tube is straightforward. With the terminator resistance measured in kilo ohms, the voltage drop across the terminator is directly related to current. For example, a 1 mA current passing through a 22 K resistor will produce a voltage drop of 22 V. For kinetic purposes, the photomultiplier tube response is optimal when the current is approximately 0.5 mA. Therefore, the light intensity from the Xe arc lamp should be set so that the value of V100 is half of the terminator resistance value. This can be accomplished by either tuning the Xe arc lamp current (not recommended) or by alignment of the uni-axis lens used to focus light into the monochromator. Light level stability from the Xe arc lamp decreases dramatically if the lamp current setting is below 6A. Often, a combination of tuning the lamp settings and alignment of the uni-axis lens are required to obtain optimal PMT response.
In the first generation design of this system, a mechanical shutter was placed in the optical train between the Xe arc lamp and the sample cell. The purpose of this shutter was to minimize sample heating while initial measurements and adjustments were being made. In the second generation design, the use of a flow cell eliminates most problems associated with sample heating. Once V100 and V0 are measured, the voltage meter is disconnected and the terminator is reconnected to the oscilloscope. The photomultiplier tube shutter is opened and the mechanical shutter of the laser is opened. At this time, the signal averaging function of the oscilloscope is reset to zero and successive photo-transients are collected until the desired signal to noise ratio is achieved, usually 100-500 laser shots. With the system in its present configuration, the DC current from the Xe arc lamp is nullified by use of AC coupled input to the oscilloscope. With this setup, signals with a half life longer than 10 ms will be distorted. If long lived phototransients are desired, a DC nulling circuit must be installed. Figure 31-1 shows the wiring schematic of the DC nulling circuit used for experiments requiring longer time base settings13.
The laser flash spectrophotometer can also be used to obtain electronic absorption spectra of excited state species as well as
luminescence lifetimes. In order to obtain electronic absorption spectra, the above procedure is repeated at varying monitoring wavelengths. Care must be taken to assure that the laser power is consistent at each observation wavelength, and the voltage applied to the photomultiplier tube must be the same at each wavelength observed. In order to use the laser flash photolysis spectrophotometer to determine luminescence lifetimes, the Xe arc lamp is turned off. Irradiation of the sample with the laser then produces a luminescence decay of the sample which is recorded by the photomultiplier and output to the oscilloscope.
A.Time Constants
The signal from the photomultiplier tube anode is passed through a one foot length 50Ω BNC cable which contains 50 picofarrads/foot capacitance. The signal is then passed through one of several terminators. The voltage drop across the terminator as a function of time is recorded on a LeCroy 9400A digital oscilloscope which has been externally triggered by the Q-switch synchronous output from the laser head. The terminators have all been shielded in pomona boxes and carefully wired to minimize stray capacitance. The signal then terminates at the 1MΩ AC coupled input of the LeCroy 9400A digital oscilloscope. The RC time constant of the system with each terminator is tabulated in table 1.1. The trade-offs between speed and signal gain are best expressed by comparing the system response using the 2.2 kilo-Ohm terminator to that with the 22 kilo-Ohm terminator. With the 2.2 kilo-Ohm terminator, there is that a 8.8 fold improvement in the RC time constant. However, this improvement in system speed was achieved at the expense of decreasing the signal gain by a factor of 10.
Table 1.1
RC time constants of the flash photolysis spectrophotometer as a function of term inator resistance
Resistor (KΩ) | System time constant: t½(ns) |
2.2 4.7 15 22 | 170 350 1000 1500 |
B. Data Conversion Spread Sheet for The LeCroy 9400A Oscilloscope
This appendix descries the spread sheet used to translate data from the oscilloscope into absorption vs. time traces. The data from the oscilloscope is transferred as two files into the spread sheet program. The first is a single column string representing the 8-byte value of the voltage obtained at each sampling. The second is a single column descriptor file that has encoded information regarding the settings on the front face of the oscilloscope during data acquisition. Both files are needed in order to properly analyze the data set. See Appendix B on how to transfer data files from the oscilloscope.
The spread sheet was written as a Quattro pro spread sheet and is saved under the name CONVERT.WQ1. A representation of the spread sheet is presented below. The user must import both data files into the spread sheet. Files are imported as ‘comma & delimited’ files. When importing the data file, the user places the cursor on the asterisk ( * ) in the column labeled ‘RAW DATA FROM SCOPE’. The user then selects import from the tools menu and then selects ‘comma & delimited’. If the user saved the data file with the suffix ‘.prn’ then the data files will appear in a menu tree ( example; DATA-A.PRN ). The user then selects the appropriate file and importation is complete. The same procedure is repeated for importation of the descriptor file with the exception of placing the cursor above the asterisk ( * ) in the column labeled ‘DESCRIPTOR FILE’. After the user has imported both data files, the user must input the skip interval used in the initial data transfer from the scope. See Appendix B for a complete description of the skip interval. The user must also input the voltage readings corresponding to zero % transmittance (V0) and one hundred % transmittance ( V100).
The 8-byte representation of the data is automatically converted to units of millivolts in the column labeled ‘DATA IN mV’. Each cell block in this column contains the following equation.
$I$17 * (((A15-128/32) - (($J$17-200)/25)) * (200/$K$17+80)) eq. A.1
Table A.1
Definition of descriptor file code for equation A.1
$I$17 | Gain code from descriptor file |
A15 | 8-byte data code |
$J$17 | Offset code fromdescriptor file |
$K$17 | Variable gain code from descriptor file |
The data code ‘A15’ scrolls as you go down the column. Once the data is converted into units of millivolts, the base line is automatically adjusted to zero by averaging the last 15 data points in the column and setting this value at zero. This data is stored in the column labeled ‘DC OFFSET CORRECTION mV’. Once the proper baseline has been established, data is automatically converted to absorbance unit in the column labeled ‘DATA IN ABS UNITS’. Each cell in this column contains the following formula.
Table A.2
Definition of terms for equation A.2
F15 | Voltage generated by transient signal |
$D$7 | Voltage corresponding to 0% transmittance |
$E$u | Voltage corresponding to 100% transmittance |
The time base is calculated in cell N23. The time base code is first read from the descriptor file and a sampling interval determined. The program then adjusts this sampling interval by the skip interval used during data transfer from the oscilloscope. The skip interval is input into cell block A7. The time base is displayed is cell C7 under the heading ‘USE /EF TO INPUT TIME BASE’. The time base array must be constructed by the user and is placed in column D under the heading of ‘ TIME IN ms’. The block fill command is the easiest way to construct this array. The command for a block fill is /EF. The spread sheet will then prompt the user to input the time base displayed in block C7.
At this point, the user now has two columns of data ( voltage vs. time) that represent the phototransient decay from the oscilloscope. Data can be kinetically analyzed in the spread sheet using standard kinetic models. However, if the user wishes to export the two column data array for analysis in a commercial kinetics package the following procedure is used to print the data file to a disk in ASCII format.
From the print menu, chose print block. The program will then prompt the user to select which block of the spread sheet is desired. Next, select destination from the print menu. Chose file from the menu displayed. The program will then prompt the user to name the file. Next chose print spreadsheet from the print menu. The file is now saved to disk as a two column voltage vs. time ASCII file.
2/4/91 RMGSPREAD SHEET FOR DATA CONVERSION
LECROY 9400A TO AUFS
*READ TEXT IN CELL H1
INPUT TRANSMITIONUSE /EF INPUT' ZERO INPUT 100 %
INTERVAL FROMTO INPUT% TRANS TRANS
SCOPE-FILE TIME BASE mV mV
+---------++-----------+------------------+-----------------+
| 20 || 0.0002 | 0 | -4100 |
+---------+ +-----------+------------------+-----------------+
RAW DATADESCRIPTION DATA IN TIME IN DATA IN DCOFFSET
FROM FILEmVms ABS. CORRECTION
SCOPE UNITS mV
-------------------------------------------------------------
* *
500150 = ARRAY LENGTH
----------+--------------------------------------------------
98 | 24 141.25 -0.021 -1.253E-03 -11.85
+---------
96 | 120 140 -0.0208 -1.385E-03 -13.1
+---------
95 | 0 139.375 -0.0206 -1.451E-03 -13.72
97 | 0 140.625 -0.0204 -1.319E-03 -12.47
+---------
97 | 0 140.625 -0.0202 -1.319E-03 -12.47
+---------
98 | 175 141.25 -0.02 -1.253E-03 -11.85
95 | 4 139.375 -0.0198 -1.451E-03 -13.72
93 | 0 138.125 -0.0196 -1.583E-03 -14.97
93 | 0 138.125 -0.0194 -1.583E-03 -14.97
Figure A.1
Layout of the QuattroPro spread sheet used to transform data form the
LeCroy 9400a Oscilloscope into Absorption vs. Time data.
C.Description of Data Transfer Program for The LeCroy 9400A Oscilloscope and The Source Code
This appendix is intended to aid the user in transferring data obtained on the LeCroy 9400a oscilloscope to an IBM/PC computer for further analysis in a remote data processing programs. All of the commands used are contained in the User’s Manual. The source code included below was obtained from the LeCroy User’s Bulletin board and was originally written by Lou Lombardi at LeCroy.
D. Data Transfer
The program is designed to transfer data from the oscilloscope to a disk drive of your computer as a single column data file suitable for analysis in a spread sheet such as Lotus 1-2-3 or QuattroPro. A voltage vs. time trace stored on the oscilloscope is transferred as a numerical representation of the 8-bit code of the oscilloscope. A descriptor file is also recorded with each voltage trace which contains encrypted information regarding all of the settings on the front panel of the oscilloscope during data acquisition. Both files must be transferred to the PC in order to properly analyze the data set. See Appendix A on how to convert the voltage file and the descriptor file into an absorbance vs. time, two column, data array.
When the program is activated, the program prompts the user to enter an option. The options available are listed below.
C = Command--This command allows you to send commands to the
oscilloscope.
Q = Query--This command allows you to obtain setting values from
the oscilloscope.
L = Local--This command returns local control to the oscilloscope.
F = File--This command allows you to obtain data files from the
oscilloscope.
E = Exit--This command exits the program and returns the user to “MS-DOS”.
In order to transfer a data file, option F must be entered at the prompt. The program will next ask you to enter the drive directory and file name. Drive pertains to the disk drive of your computer and is typically A, B, or C. Directory pertains to the name of the directory on you disk drive in which you want to store your file. If you are storing your file in the root directory, you can leave this blank. File name is the name you wish to call your file. For example, if you wished to store a file named Data-A.prn onto disk drive B in a directory named temp, you would enter the following command.
B:\Temp\Data-A.prn
After the carriage return, the program prompts you to enter a command. The command you wish to enter is ‘read’. The sequence is
‘READ SOURCE.TYP,SKP,LENGTH,START’.
The source can be either channel 1 (C1), channel 2 (C2), memory C (MC), memory D (MD), function E (FE), or function F (FF). TYP is the type of file you wish to transfer. If you are transferring a data file then ‘.da’ is typed at this location. When transferring a description file, then ‘.de’ is typed at this location. If you are transferring a descriptor file, the remainder of the read string is omitted. SKP is the skip interval. If you are transferring a large file and you wish to transmit only every 50th data point, then you will enter 50 at this location. LENGTH is the total number of data points you wish to transfer from the oscilloscope. If the total length of the data file stored on the oscilloscope is 25,000 bytes, and you are skipping every 50th data point, then you want to input 500 as the total array length (see page 5-5 of the 9400a users manual for array lengths as a function of time base settings). START is the number of data point you wish to skip before you start transferring data. For example, if you wish to start transferring from the beginning of the data file, you enter zero ‘0’ at this location. To summarize, the two read commands entered for the above example would resemble the following.
READ MC.DA,50,500,0
READ MC.DE