Absorbance and emission measurement

[KirillM]

Hello, everyone, 

I am trying to use LabVIEW for managing the data acquisition in the absorbance and emission experiment. Generally, a sample is excited by a LED source and the time-dependent emission intensity and spectrum is collected, so the excitation and emission should be synchronized. It seems that the corresponding LabVIEW program is not trivial and at the same time I don't want to invent the wheel. So, it would help a lot if someone could let me know about the existing LabVIEW examples/solutions for a similar scheme. 

Here is the experimental setup layout:

[GerdW]

Hi Kirill,

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@KirillM wrote:

Generally, a sample is excited by a LED source and the time-dependent emission intensity and spectrum is collected, so the excitation and emission should be synchronized.

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Your experiment setup consists of (atleast) four main components (aka DAQ devices) external to your computer. When you want to sync all of them then there is not much you can do with LabVIEW: you have to find a way (by reading manuals) to sync all those devices using their hardware features!

[mcduff]

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@KirillM wrote:

Hello, everyone, 

I am trying to use LabVIEW for managing the data acquisition in the absorbance and emission experiment. Generally, a sample is excited by a LED source and the time-dependent emission intensity and spectrum is collected, so the excitation and emission should be synchronized.

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Are you doing a fluorescence or phosphorescence experiment? Fluorescence will basically end, within a fraction of a second, once your excitation beam is off.

As far a synchronization goes, I believe that is why you have the DDG (Digital delay generator).

1. It looks like all of your instruments are GPIB/USB/Serial etc. You cannot synchronize with software.
2. Each of your hardware devices needs to have an external trigger input.
3. Program your DDG to fire the appropriate pulses. For example, maybe you want a 2 us delay after triggering the excitation LED, etc.
4. Program your data collection devices to respond to an external trigger.
5. This can be the tricky part. If possible set up a RSQ Service request to occur after your devices trigger and receive data. If this doesn't work you will need to poll after you trigger to see if data collection is complete.

I did a similar setup for a LIBS apparatus using a DDG and Avantes USB Spectrometer. The DDG sent a trigger pulse to the laser, and after about 40 us, it sent a trigger to the USB Spectrometer to acquire data.

A state machine type architecture works well for this type of project.

mcduff

[KirillM]

Thanks a lot for a prompt reply! 

If you don't mind, I would like to ask about that a bit more. 

So, you said, "four components", meaning that I might synchronize less than four? If there will be three or two? (The thing is that technically I may simplify the experiment and the setup too. For example, collect the emission spectra and the intensity separately etc.)

Or, there is no way to make it synchronized in general?

[KirillM]

Thanks a lot for your comments! 

I want to make both long-term and short-term response measurements, but the focus is on the former one. 

You are right about the triggers, that is how it is basically done so far, yet manually. 

So, basically, what you are saying is that by setting up the delays for the triggers for the individual units and the data collection completion, I might initiate the measurement sequence by LabVIEW, right? 

The other thing here is that I want to make the whole set of measurements carried out by an algorithm. Namely, say, measure the intensity at different temperatures and trigger the measurement upon the cooling/annealing cycle is over, and the certain temperature level is reached. But I guess, it could be done just by adding a subprogram there. 

[mcduff]

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@KirillM wrote:

Thanks a lot for a prompt reply! 

If you don't mind, I would like to ask about that a bit more. 

So, you said, "four components", meaning that I might synchronize less than four? If there will be three or two? (The thing is that technically I may simplify the experiment and the setup too. For example, collect the emission spectra and the intensity separately etc.)

Or, there is no way to make it synchronized in general?

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From your diagram you need to control at least 3 instruments:

1. Digital Delay Generator
2. Multimeter
3. Spectrometer

When you want the emission LED ON you either use the DDG to open a shutter, or turn the LED on for a set amount of time.

YOUR SYNCHRONIZATION OCCURS IN HARDWARE.

Your setup obeys causality.

You trigger the excitation LED; there will be a delay, (probably extremely small), before emission occurs. Your second pulse from the DDG should trigger both the multimeter and spectrometer. You need to determine the time for the second pulse, it might be exactly the same time as the excitation trigger. Who knows, maybe you have weird hardware that takes a 1 ms to respond to a trigger, then you want that pulse before your excitation pulse.

You need to make a timing diagram for your specific case.

mcduff

[KirillM]

I see, what you mean. I think, it is not hard to make a test and figure out the hardware delays. Moreover, I might trigger both LED and the multimeter at the same time (with a relatively long exposure) and see the time zero of the luminescence by the start of the signal drop after the LED is off. So the actual issue would be to program the equipment running by following a certain algorithm. I guess, I would have to dig deeper into that to build a custom script, since there is no similar templates available (at least, I couldn't find any). 

Thanks a lot! 

[mcduff]

Use also need to keep in mind your setup.

For example, assume your PMTs are well shielded from the excitation light. At that point they are recording "0" current. You could record continuously and post process your non-zero data streams, because you know when its non zero you have a signal.

The USB spectrometer can be more tricky, especially if it doesn't have an external trigger. If it doesn't you will have to continuously acquire spectra until you have the proper emission spectra.

mcduff