Building an Analytical System for Plant Alkaloids

Quote: Originally posted by Anthracene

If the goal is an open source device this might be accomplished with LEDs of specific wavelength and photoresistors.

So far I have successfully connected the Raspberry Pi A I brought to my Ubuntu laptop through an ethernet switch not connected to a local or external network, but internet access was provided by the Pi through the laptop. If anyone would like details on how this was done, please ask.

I have also started testing the 250 nm UV LEDs that I bought. One is 0.1 watt with no lens, with an unspecified wide angle, on a 16mm board, rated at 6-7 volts at 0.1 amps. This was attached to a heat sink using thermal glue. I found that it just barely started shining at 5 volts as shown here:



The meter is showing the current produced by an OSI Optoelectronics UV-015 Inversion Layer UV Photodiode, 0.5 microamps in ambient light. When I expose the photodiode to the faint 250 nm UV from the LED, I get 33.8 microamps:



So my initial plan is to place the LED and photodiode on either side of a quartz flow cell to create a detector for aromatic organic compounds flowing through in solution.

I also wanted to make a 250 nm UV lamp for my friend Affo to use to visualize TLC. The LED above isn't bright enough even at seven volts. However, a 1W LED with a lens giving a 60 degree beam is sufficient:



The blue voltage converter is raising the 5 volts of the battery bank to 6.2 volts for the LED, and is adjustable using the gold knob of the trimmer. I was happy with the cost of this setup, as the voltage converter cost $5.65, the LED was $13.69 and the heat sink was $0.35, for about $20 total.

I got around to testing the cuvettes and the flow cell that I bought for this project. What I purchased from eBay are four 1cm quartz cuvettes from chenting2018, one 1cm quartz cuvette from osag7667, one 1mm quartz cuvette from labsciware and one expensive 1mm quartz flow cell from quartzcrystal. I mention the vendors because of the problem I discovered with the four 1cm "quartz" cuvettes from chenting2018:



As you can see, the 250nm UV from the LED is not penetrating the cuvette, so that a shadow is cast on the TLC and only the UV going past the cuvette is causing it to fluoresce. In contrast, the 1cm quartz cuvette from osag7667 transmits UV properly:



Thankfully, the more expensive and useful 1mm cuvette and 1mm flow cell transmit UV like the single 1cm quartz cuvette that I have. My intention with the cuvettes, to make them into flow cells, it to cut off the bottoms with a Dremel carbide cutting disk and attach glass tubes to the ends using polyolefin heat shrink tubing, which I hope will seal, be inert, and not swell when exposed to solvent.

I tried cutting the ends off three of the four glass cuvettes with a carbide disk on a Dremel:



The first one, at the bottom, I started on one corner and the bottom shattered off uncleanly, but at least there are no cracks on the body of the cuvette. For the middle one I tried working my way repeatedly around the cuvette, with the thinking that when the bottom cracks off that the crack would follow the scoring line. This seemed to work, but there is a crack coming up from one corner that could eventually cause the cuvette to shatter. After reading and watching everything I could find online about cutting quartz tubing, I tried dipping the cuvette into water every few seconds as I worked by way around the cuvette. The cracks seem to follow the cut lines better, but maybe I was lucky, and the job is still ugly because I can't follow the same line repeatedly while holding the tool and the piece by hand.

Can anyone offer advice on a better way to do this? I have one more glass cuvette to practice on before I turn to the two quartz cuvettes. Scoring and thermally stressing, I am told, doesn't work well for quartz due to a lower coefficient of expansion. One thing I could try is grinding the bottom off the cuvette with an abrasive wheel. I don't mind if it takes an hour, but I am concerned that the long grinding would only increase the chance of cracking, and I couldn't find any online material aside from grinding quartz countertop or faceting quartz stones.

Thanks for the reply, bnull. Both quartz cuvettes have 1mm thick walls but the glass ones are 1.25mm according to the questionable seller, but that looks right. I have sanding disks I can install on rubber disks backed by the carbide wheel. I don't know what grit these disks are, as the contents listed on the rotary tool accessory kit doesn't even list the sanding disks. We are having our first power failure now since I arrived, so I will wait until after to test the sanding.

I hope to use polyolefin heat-shrink tubing to connect each cuvette to tubing to create the flow cell, and that I will find the polyolefin tubing to share some of the chemical resistance of polypropylene and polyethylene.

Update:

I tried the sanding disks but they weren't very effective. I also tried the side of the cutting disk as the sanding disk but it shattered along with breaking a few more chips off the end of the cuvette. So I tried the 1/2" sanding band (of unknown course grit) on the rubber mandrel and this worked amazingly well to grind down the uneven edges of the three glass cuvettes from yesterday, as well as leaving a clean smooth edge without introducing any cracks. I wasn't able to get the edge completely flat but I didn't really try and figured what I got was good enough.

So I confidently tried grinding down the bottom edges of the last glass cuvette, planning to continue until the bottom of the cuvette became detached from the sides. At this point I should mention my concern with the invisible cloud of silica I must be generating that I should not inhale. Not having a mask available, I worked outside with the wind at my back and got into a rhythm of dipping the piece in water, inhaling, then working one edge of the cuvette back and forth against the band, working my way down from top to bottom, at about a 30 degree angle vs. the bottom, for about half a minute until time for another breath. Unlike the cutting disk, this felt very gentle, and I used little force between the sander and the cuvette. It seemed to take about an hour until I figured I was nearly done, when I noticed cracking:



What seems to be happening is that as the glass along the edge of the base becomes very thin, cracks form along the thin edge that can continue into the bulk of the glass, and have propagated into the cuvette, making this the most unsound of the jobs so far. I think the best method, based on what I have learned so far, is to work my way around the base of the cuvette with the cutting disk to lead cracks along that path, take the uneven cut I get, then grind the bottom end as flat as possible with the sander, which will smooth it.

[Edited on 19-7-2024 by Jenks]

It was an idea.

Quote: Originally posted by bnull

It was an idea.

It was a good idea, and I appreciate it. It was useful to even out the end of the cuvette, so I will still use it.



This is an iboga farm I visited today on the outskirts of Accra, Ghana. It wasn't known until a few years ago, as far as I know, that iboga could be cultivated in Ghana, since it is usually found in Gabon and Cameroon. This farm is relevant to this thread because it is the alkaloids from the bark of these shrubs, and related Voacanga africana trees, that are to be analyzed with the proposed system.

I've made a little progress in the last four days.

The Arduino IDE was installed on the Ubuntu 24.04 workstation simply by installing the "arduino" package from the command line:

sudo aptitude install arduino

Right off I couldn't find it among the serial ports. This turned out to be due to a conflict with a braille e-reader, as explained in this StackExchange post, and was removed with the command:

sudo apt remove brltty

At this point, what I have done is to connect two of the analog inputs of the Arduino Uno to two of the UV photodiodes which will measure the UV passing through the finished quartz flow cell from a 250 nm LED and a 270 nm LED. The power for the LEDs is controlled by two of four relays on a relay module, which itself is controlled by two of the digital outputs on the Arduino. A digital input on the Arduino is connected to a pushbutton switch which is connected between a 100k and 1M resistor so that if that digital input should fail to be configured as an output, the output won't draw enough current to burn it out.

The power supply for the UV LEDs was a challenge. I didn't realize that the two kinds of small voltage converters I bought only step voltage down, not up, so that the five volts from the Arduino can't drive the LEDs. I built the IC 555 Voltage Doubler Circuit with High Current Output described as project #6 here, but although it converted five volts to 7-8 volts without any load, with the load of the UV LEDs the voltage dropped to five volts. I had to substitute S8080 and S8550 transistors for the BD139 and BD140 transistors proposed in the design.

Since that didn't work, and the shipping time to order any electronics exceeds the time left for the project, I was glad to be offered a broken TV to cannibalize for its main circuit board, which apparently includes a regulated 12 volt power supply. That was attached to a power converter I have, which I adjusted to get 6.63 volt output:



When I turn on the LEDs using the aforementioned button, the voltage only drops to 6.53 volts:



A couple nasty things about this TV board are that the big capacitor there charges to 330 volts to run the switching power supply, but retains its charge when the unit is unplugged. I soldered a 1M resistor across the capacitor so that the voltage drops about 2 volts/second after unplugging. I also discovered with only a minor shock that the 12 volt plug on this board is apparently not isolated from the mains, and that the ground pin on the power plug is made of plastic. The voltage step-down module also doesn't isolate input from output. So I have to be careful, and hopefully find a nice wall-wart to use instead in the final design.

So it looks like this system is ready to put in a box. I know it is minimal, but I have to get this thing working and people trained to use it before I can make it any nicer.

[Edited on 24-7-2024 by Jenks]

1) A usb supply + 'power bank' form a nice 5V dc ups for low power.

2) 12V dc at high power can be had from an automotive battery + charger

3) 19V at fairly high power can be obtained from a laptop charger

4) Many voltages at high power are available from desktop PC power supplies.

In order of safety, 3),1),4),2)
................
PS using solar/hydro/wind power may win some points as a 'green' agenda?
(B.S. but some folk fall for it

[Edited on 26-7-2024 by Sulaiman]

Thanks for the suggestions, Sulaiman. I will see if any of those can be found without much time invested.

I know progress has been slow but there has been a lot going on. Here is my friend Harrison cutting an aluminum pipe to make shims to space the UV LED boards properly from the flow cell they will be glued to.



The two UV LED heat sinks will be glued together as shown. Three stacks of three aluminum shims will be glued together to make spacers between the heat sinks and the flow cell. Shown here is the wider stack already drying, placed where the heat sinks are joined, between the LED modules. The stacks on the left and the right will be glued together next, then glued at the left and right edges of the LED assembly. The LED assembly will then be glued to the side of the flow cell facing us, and then the photodiodes will be glued by their edges to the back of the flow cell, centered above the LED diodes. The silicone cement shown is used for everything. Now hopefully I can remember which of those LEDs is 250 nm and which is 270 nm...

Things are coming along.

I decided not to go with OpenChrom because, despite the name, I wasn't even able to test the trial version because it requires a license just to download the documentation, and my credit card was declined while trying to purchase a license. Since my goal is to make this analytical system as accessible as possible, I don't want to depend on closed source proprietary software that might be orphaned sooner or later.

I also checked out UniChrom, which I was finally able to satisfy the dependencies to install on Ubuntu. This time I was pleased to be able to open the simple .csv file I had prepared and could see the appropriate chromatogram. This software might work, but I still wasn't completely happy. It is produced by a company based in Belarus, New Analytical Systems, Ltd, which could complicate support reliability or ethical licensing given the Russian invasion of Ukraine. Furthermore, their Linux version seems like it has been on the back burner for a long time, with almost all their energy going into their Windows version. The website says the Linux version is still in beta testing, and the software itself says it is in demonstration mode. The link to license the Linux version directs to the licensing page for their Windows version. Finally, because UniChrom attempts to apply to several forms of analysis and to be compatible with as wide a range of instruments as possible, it has a high degree of complexity compared with what is required for the system I have in mind. This complexity will be an impediment to end users who may lack training in the use of analytical software.

After more pondering, I decided to look to software which is free, open source, easily obtained and in long-term and widespread use. One project uses the R statistical software to provide chromatographic analysis. This is a lovely effort. I only balk because I have not learned to use R, and I don't want my target audience to have to either.

I found an Excel spreadsheet that presents a very attractive HPLC interface with a chromatogram which might have worked, and it even loads into OpenOffice Calc, minus the preloaded data. However, I notice that the spreadsheet is hosted on a Chinese website, which made me a little nervous to enable the macros. More importantly, the spreadsheet is password protected so that I can't easily get in to fix it or tailor it to my own project. It seems possible the authors might help me, but with this inspiration I decided I would develop my own spreadsheet in OpenOffice Calc to import the .csv data from my PHP program and present the chromatogram and peak data. So far this has gone smoothly, and I have been able to automatically import data which is automatically updated in a chromatogram.

On the hardware side, the detector is finally ready, having been painstakingly glued together using silicone cement. I just finished installing the photodiodes after carefully applying the silicone to the thin rims around the lenses.

Here is the current system:



A lot of my time has been spent on writing and debugging the software to detect the peaks, written in PHP on the laptop, and I have also been slowed by finding a suitable way to connect the flow cell using only poly-hydrocarbon tubing. Testing has been by using the shadow from my hand to produce "peaks", but today I finally got around to passing an actual sample through the flow cell. I finally figured out that the smallest diameter polyethylene tubing I was able to buy, which has an outer diameter of only 1/8", fits tightly, but not too tightly, inside the inlet and outlet connectors of the quartz flow cell, well enough to form a seal. I also like that this tubing, owing to its thinness, is flexible enough that it is unlikely to snap the quartz connectors off.

So I simply put the outlet tube into a flask for waste solvent and adapted the other end to a plastic pipette with the top cut off to form a funnel. The system filled easily by adding solvent (acetone for this test) to the dropper/funnel, keeping the waste flask at the same level to limit the flow rate. Bubbles purge easily from the system thanks to the 1/16" inside diameter of the tubing, and keeping the flow cell vertical, flowing from bottom to top, as recommended here.

Once the system was full of acetone and free of air, and I could cause the solvent to flow by simply adding more to the "funnel", I ran a test on 1 mL of 1% toluene, flushed through the system with acetone. I expected a strong peak, but seeing none I ran it on 0.5 mL of straight toluene. Here is the sorry result:



On the left is where I signal to the software that the sample has been "injected", which causes the Arduino to turn on the UV LEDs. What I expected to see was a strong peak, at least for the 250 nm detector, that should have blocked out all the UV while it passed. I interpret the complete absence of a peak as the detectors picking up almost entirely the not-very-bright visible light from the UV LEDs, so that the actual UV from the LEDs, which we know is there from the TLC fluorescence, was not detected. Given that the UV LEDs cause the TLC to fluoresce more brightly than they are illuminated by visible light from the LEDs, I didn't think the visible light would be such a problem.

I happened to bring a sheet of shortwave UV selective glass which I can hope to cut to make a filter for the photodiodes, if I can get them safely off the flow cell. I might use the occasion to connect the photodiodes to amplifiers. Yesterday I learned that the LM1458n dual op-amp ICs that I bought were not the best choice because they are intended for use with a dual power supply - one with positive and negative voltage and a ground in between. What I, and most people, have is a single pole power supply which has positive and ground. Using the LM1458n will complicate my circuit a little. I should have used LM358.

Update: Here is a little test with the filter glass and 1mm quartz cuvette containing 1% toluene in acetone between a 250 nm LED and silica TLC with 254 nm fluorescent indicator. As expected, the UV is blocked by the toluene but passes through everything else, including the cuvette above the level of the solution. At least my previously untested filter glass is legit.



Update:

That ZW33 UV filter glass doesn't like to break along the score lines! I got some very poorly shaped pieces when trying to cut 12mm off the end of the piece. I will try grinding them down to small pieces that will fit on the individual photodiodes.

I remembered the responsivity of the OSI UV-15 photodiode being several times higher for visible light, and even IR, than for shortwave UV according to the datasheet:



Update 8/4/24:

Things got better this morning, once I tried the Dremel on the UV filter glass. I had planned to use the carbide cutting blade to score the pointed protrusions to create clean rectangles, but after running past the score line just a couple times, the pieces broke spontaneously along the line. When I tried trimming the edge of the main piece it took much longer, probably because my unsteady hand made the score line wide, but it was still decent in the end because the sanding drum accessory smoothed the edges and ground down small protrusions to make them as straight as my hand would allow. More practice scoring by hand may have led to better results, as demonstrated here. I then used the sanding drum, switching to the smaller diameter drum, to make grooves in the ends of the main piece so it fit snugly against the face of the flow cell. The silicone I applied to the corners is now curing:



One site I found reported damage to "Hoya" glass, which is what I have according to that site, from the slightest exposure to water. Higher grade Hoya glass is said to have a layer of quartz to prevent this. My sample must have such a layer, as washing it or soaking it in water led to no visible change.

[Edited on 4-8-2024 by Jenks]

I think I've finally finalized the design of the amplifier for each of the photodiodes, using parts I have.



The design is based on a simple, common design depicted as the first stage in the first stage of the amplifier on the photodiode datasheet:



I did not want to build a bipolar power supply, which the LM1458N op-amp is designed for, and the inputs and output of that op-amp are not designed to go "rail to rail", meaning, in this case, that voltages close to ground or five volts will not be properly amplified, and that the range of the output will be something less than 0-5 volts. Testing the op-amp as a voltage follower with the variable output provided by a potentiometer, I found the linear input range to be 1.5 to 4.4 volts. Unfortunately, this means that only 58% of the input range of the arduino will be utilized, with a corresponding loss of resolution. This is necessary because I didn't pick a more suitable op-amp like the LM358.

To bring the input range above 1.5 volts I used a series of three 1 watt rectifier diodes, which maintain a voltage of 1.57 volts across themselves. A voltage regulator would have been a better choice, as this voltage will vary with temperature. Fortunately, the climate in Ghana has a very stable temperature, at least during this part of the year.

The 10k resistor on the left limits the current through the diodes to about 30 microamps. The linear 10k potentiometer limits the gain to being the output voltage below the 4.4 volt cutoff, which is essential because the baseline of the chromatography, and any small peaks appearing, will depend on accurately measuring small variations in the full-intensity UV reaching the photodiode. I found experimentally that a setting around 1k brings the top of the output range to about 4 volts.

The other parts, which are standard, are the 10k resistor on top (usually a higher value) which also determines the gain, the 10 picofarad capacitor across it to prevent oscillation, and the 0.1 microfarad capacitor across the power connections to the op-amp to filter noise from the power supply.

I hope to build this onto a circuit board attached directly onto the photodiodes in the detector to minimize noise.

I could use some help if anyone has insight. I built amplifiers for both photodiodes before I realized that the photodiode I drew in my schematic actually has to be oriented the other way, with the anode to inverting pin 2 of the op amp. This is reversed from the manufacturer's proposed design and other common designs, but it gives the desirable behavior described above, and the standard orientation just slightly decreases the output on exposure to UV. I don't know if the response is linear but I figured I would have to calibrate anyway. If anyone has insight why this works, or any warnings I should be aware of in using this circuit, please inform me. Thanks.

Update:

Here is the final design, which I built and tested:



Here is how it looks (I'm ashamed to show the bottom):



Red and black wires are power, brown and yellow wires are first and second potentiometer, yellow and purple are left and right output. I decided to add wires for the photodiodes so I could test the board before installing it in the detector.

[Edited on 6-8-2024 by Jenks]

Here is the "finished" detector:



Like many of my perfectionistic projects, it looks good, at least to me, but the problem is that it doesn't work. Worse than that, I'm not entirely sure why. I've been charging along, and as I approach the end of my five weeks in Ghana and find this main project not working out as I envisioned, I can only stop worrying about running out of time and return to prayer to ask for a better way.

I'm pretty sure the amplifiers are working correctly, exactly the way the prototype in the solderless breadboard has. But the maximum voltage it puts out is only 2.2 volts, for a range of only 1.57 - 2.2 volts, not the 1.57 - 4.3 volts put out by the prototype. I don't know how to account for this, particularly the maximum voltage being the same within 1% on both channels. The only difference between the prototype and the pictured detector is that the single flow cell available is installed between the UV LEDs and the photodiodes. If I insert the 1mm quartz cuvette between the UV LED and photodiode/UV filter glass on the prototype, the extra distance brings the maximum voltage down to 4.0 volts, which actually would be ideal because it has to be limited to that voltage anyway. Maybe the optics of the flow cell are different, as putting two photocells on it forces them near the ends near the fluid connectors. The photodiodes could be out of alignment, but for them to be out of alignment exactly the same amount is unlikely.

I proceeded to test the detector anyway. The signal is noisy, varying by +/- 50 millivolts when the photodiodes connected straight to the Arduino, across 100k resistors, had noise around 5 millivolts, which is the limit of the Arduino's A/D converter to measure. Since the plain photodiodes give out 0 - 0.40 volts without amplification, the amplifier's 0.63 volt range gives less resolution because of the noise.

I planned to start my tests by sending a sample of toluene through the detector, eluting with acetone since it is our cheapest solvent. I quickly discovered, to my surprise, that acetone absorbs 254 nm UV:



Fortunately, ethyl acetate doesn't, so I switched to the 15% ethyl acetate/naphtha mixture I had prepared for actual chromatographic tests. This gave a peak when the toluene sample was flushed through, but exposed new problems. One is that the UV LEDs overheat in only a few minutes despite the heat sinks, causing the baseline to rise as the UV output drops. But the most troubling was that the absorbance for the toluene wasn't nearly as high as expected. I could feed half a milliliter of straight toluene through the system and it only absorbed 30% of the UV on passing through the detector. I thought maybe the little bit of visible light making it through the UV filter might still be exciting the photodiode, but thanks to the experiment running straight acetone through the detector, which lowered the voltage almost as low as with the UV LEDs off, that was ruled out. But the fact that I see any visible light coming out of the UV LEDs bothers me. The specifications volunteered by the vendor of the 0.1 watt, 250 nm UV LED claim that all emmission is between 250 and 260 nm:



If there is visible light, enough to overwhelm the photodiodes, there may be radiation between visible and UV-C also, unless it is due to fluorescense of some component in the LED. Maybe the UV absorption of toluene, which is sharper than that of acetone, is leaving more UV-B to excite the photodiodes. I plan to run straight toluene through the detector to see if it is maxed out as with acetone. And it may be that the absorption of iboga alkaloids is broad enough to be detected owing to it being bicyclic aromatic. But I have another idea I have to try with my remaining little time.

I brought some cadmium sulfide (CdS) photocells, just because I was trying to bring anything photoelectric I could conceivably use in this system, and I thought in my fantasy that I might get around to designing drop counters. The peak sensitivity of CdS is 540 nm, green. We know how effective the fluorescent zinc silicate indicator is for visualizing aromatic compounds on TLC. Taking advantage of this, I plan to try a detector that shines the 254 nm UV through the sample onto a piece of TLC plate and measures its fluorescence with a CdS cell. Response time of CdS cells is only 100 ms, but since I was planning to collect data every second, that is not a problem. I also hope to use 4mm ID/6mm OD quartz tubing as the flow cell, as I brought two sizes, and I expect it to seal well to polypropylene tubing to connect it to the chromatography column. Maybe if the UV LEDs can be switched on only 100 milliseconds every second it will prevent overheating, or I can install the UV LEDs on a fan-cooled CPU heat sink.

Update:

Here is what I have so far:



Of course it will benefit from plenty of screening to block any UV not passing through the sample.

Update:

The detector is coming along, and I'm glad I stopped waiting 24 hours for the silicone to completely dry as it seems firm enough to continue working gently after an hour or two. Here is the detector with aluminum screens installed, adhered to the quartz tube and heat sink with silicone, of course, and made by scraping the silica off TLC. There is a clear view of the LED through the "flow cell":



I also dared to try switching the power to the UV LED through a transistor, even though, with a TO-92 case, it isn't meant to dissipate much power. Of the ten types in my kit I selected the BC337 based on its high 800 mA maximum collector current, high gain of 630 and it being NPN. Its base is simply connected to an Arduino digital output through a 4.7k resistor with the emitter grounded and the collector controlling power to the UV LED. Neither the resistor nor the transistor gets hot.

Here is the detector in action, just needing the TLC and CdS cell to be mounted:



The name for this style of detector is the "dog house". The CdS cell selected from my kit was the GL5528, based on this design.

I ran toluene through the original detector. Unlike acetone, it absorbs only about 65% of the 250 nm UV and about 15% of 270 nm UV:



I interpret this as toluene having a narrower, or slightly different, absorption spectrum than the emission spectrum of the UV LEDs.

So far the detector seems to be working, if the room lights are turned off. The voltage in darkness is 0.1 volts and illuminated it is 3 volts, but I'm sure that can be increased. I was relieved to see the TLC go dark when toluene was passed through the flow cell, meaning this UV isn't just going around the inside of the tube:



[Edited on 8-8-2024 by Jenks]

Now I'm not sure if that transistor amplifier was non-linear or if the toluene I tested it with was simply not detected by the detector. After my last post I got around to making the dual power supply that can be made from a 555 timer IC and a few diodes and capacitors:



This low-power supply is often proposed for use with a dual-rail op-amp, which is the kind (LM1458N) that I have. The advantage is that it removes the severe constraint from the output range so that it can go all the way, in this case, from ground to five volts. It can go beyond that too, which is why the Arduino input it is connected to should be protected by a 10k resistor.

This power supply worked, but as I was connecting it to the aforementioned op-amp I remembered why I hadn't done this earlier - the ground of this power supply is not compatible with the Arduino's ground because the 12 voltage power supply, which itself was not properly isolated from the mains, had been grounded to the Arduino to prevent damage to the Arduino.

But I do actually have an isolated power adapter to use, but it is 24 volts. Looking back on it, my life would have been easier if I had brought the voltage of this down instead of using the TV board. I scrapped the dual power supply (voltage doubler) circuit and built a power supply splitter for the 24 volt adapter instead.



I used the LM1458N instead of the LM741 in the diagram above. This circuit adds a ground in between power and ground of a single-pole power supply. By carefully selecting matching 10k resistors, I changed 0-24 volts to +/- 11.6 volts.

Connecting this to an LM1458N, the voltage follower worked, follow by this now simple amplifier:



Since it worked well, except for amplitude, to connect the photodide (in parallel with a 100k resistor) directly to an Arduino analog input, the photodiode was connected, in photovoltaic mode, to the input of the above amplifier. With the UV LED in the detector on, full range was obtained at resistance values of 1k for R1 and 14k for R2. Rather than build a 14k resistor, I put a 10k potentiometer in series with a 10k resistor to make a gain control. When tested with my meter, this circuit worked perfectly and finally gave the desired 0-5 volt output, utilizing the full input range for the Arduino, maximizing resolution.

This was connected, through a 10k resistor, to the Arduino. The range was still good, but I was seeing a lot of noise in the readings that didn't show up on the slow meter. Adding the typically recommended capacitors (47 pF in this case) across R2 and the op-amp inputs to prevent oscillation did not solve the noise. But not wanting to be distracted from a demonstration of feasibility, I ran a few drops of toluene through the detector - and got a barely discernable peak.

The problem could be that the amplifier is somehow still not linear, and response in the detector is not being amplified. Or maybe the photodiode is not responding primarily to 250-260 nm UV, despite the UV filter glass intended to block visible light. I had the bright idea to hook the detector output to the Arduino also, so that the correlation of the detector and amplifier outputs could be observed. This caused the output of the op-amp to rise to three volts with the photodiode in the dark. That turned out to be because the voltage across the photodiode was now 0.20 volts, which could not come from the photodiode. The only things across the photodiode were the resistor, the Arduino A1 input and the op-amp non-inverting input, none of which are supposed to provide power. Disconnecting the photodiode from either the Arduino or the op-amp solved the problem. Thinking I had damaged A1 I reconnected the photodiode to A3 but got the same result. I think this was noise, as touching the wire gave a similar result.

So I ran three drops of toluene while monitoring the amplifier with the Arduino and the photodiode with my meter. There was no change in the three-decimal-place meter reading, showing that the problem with this detector design is the UV LED producing UV at wavelengths not depicted on the datasheet.

I plan to leave my equipment in Ghana, where it is more valuable than in the United States, owing to the difficulty of obtaining it here, hoping it will be put to use someday by the right person. A new batch of supplies is on its way to my house to continue these experiments. I'm down to two ideas. Either use a diffraction grating to select the proper range of wavelengths to detect, or find a combination of fluorescent material and corresponding detector sensitive enough to properly measure the fluorescence. I'm learning toward the latter idea to try to keep the skill needed to build the detector more easily attainable.

But then it uses this graph as an illustration:



The graph shows the same increase in photocurrent resulting from the same increase in illumination regardless of the bias at low illumination. So how can increasing the bias be said to increase sensitivity, particularly if it increases the dark current?

[Edited on 15-8-2024 by Jenks]