# satchelfrost

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4Instructables15,475Views19CommentsJoined March 5th, 2015

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• satchelfrost commented on satchelfrost's instructable Student Spectrophotometer8 months ago

Each wavelength is going to have a different amount of intensity on the detector. So for example lets say at 520 nm the intensity on the detector is some arbitrary number, say, 32 (but thats the max because theres no sample). Lets say you put a sample that absorbs light at 520 nm and when you measure again the detector at that wavelength says 16. Your transmittance is then 50% or 0.5. Now, at another wavelength lets say 700 nm your detector reads another arbitrary value 45, and when you measure a sample it then reads 15, this means your transmittance is 33% or 0.33. So in other words you need to find what the maximum intensity for a given wavelength is, and then measure your sample at that specific wavelength. Do this for all wavelengths and you have the transmittance across all wavelen...

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Each wavelength is going to have a different amount of intensity on the detector. So for example lets say at 520 nm the intensity on the detector is some arbitrary number, say, 32 (but thats the max because theres no sample). Lets say you put a sample that absorbs light at 520 nm and when you measure again the detector at that wavelength says 16. Your transmittance is then 50% or 0.5. Now, at another wavelength lets say 700 nm your detector reads another arbitrary value 45, and when you measure a sample it then reads 15, this means your transmittance is 33% or 0.33. So in other words you need to find what the maximum intensity for a given wavelength is, and then measure your sample at that specific wavelength. Do this for all wavelengths and you have the transmittance across all wavelengths. Now the purpose of the blank is less to do with the instrument and more to do with subtracting out your unwanted signal. For example, let's say I have a solution of gold nanorods and I measure the absorbance at 900 nm and get and absorbance of 0.1. The only problem is that water itself (i.e. the solvent) also absorbs at that wavelength, therefore you have to subtract out that signal by measuring water by itself first, then the water + AuNRs. In other words your blank should contain everything except your analyte.

• satchelfrost commented on satchelfrost's instructable Student Spectrophotometer1 year ago

The short answer is you don't. You would need to calibrate it somehow, either by using a well known sample that gives you a predefined spectrum and then match your points to the wavelengths. It's not worth trying to get the wavelengths unless you get the steps of the motor to be very very small.

• satchelfrost commented on satchelfrost's instructable Student Spectrophotometer1 year ago

First I measured a value for a blank (note: it came out as an integer value on the serial monitor in the range 0-1023), next I measured the value for my sample and got a similar but, of course, smaller value (because some of the light was absorbed by the sample). Since the transmittance is defined as the incoming intensity divided by the maximum (in this case the max is the blank) I used T = I(sample) / I(blank). Note once again, the values I was getting for I(sample) and I(blank) were integer values. So as an example, for a specific wavelength I might have gotten 600(sample)/1000(blank) or T = 0.6. From there I calculated the absorbance using the Beer-Lambert law. As you can see I never directly measured the value for the Intensity of the sample or the blank (at least not in SI units) ...

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First I measured a value for a blank (note: it came out as an integer value on the serial monitor in the range 0-1023), next I measured the value for my sample and got a similar but, of course, smaller value (because some of the light was absorbed by the sample). Since the transmittance is defined as the incoming intensity divided by the maximum (in this case the max is the blank) I used T = I(sample) / I(blank). Note once again, the values I was getting for I(sample) and I(blank) were integer values. So as an example, for a specific wavelength I might have gotten 600(sample)/1000(blank) or T = 0.6. From there I calculated the absorbance using the Beer-Lambert law. As you can see I never directly measured the value for the Intensity of the sample or the blank (at least not in SI units) instead I only needed the ratio of my sample to the blank to calculate the transmittance. Had I known the SI unit conversion based on my apparatus, the result still would have been the same, I still would have had to divide the incoming intensity by the maximum--only it would be in different units; but the ratio still would have been the same.

• satchelfrost commented on satchelfrost's instructable Student Spectrophotometer3 years ago

For this specific instrument: only methods using the visible spectrum will work. That means if you are trying to build a similar instrument to say detect ammonia then you should search for a methodology for doing this in the visible spectrum. Now if you are only ever going to use the instrument for one (or a few) types of molecules then you might simplify the design by finding wavelengths where these molecules have a high peak in the visible spectrum and use specific LEDs that only have most of their intensity around those wavelengths (also they make LEDs in many wavelengths, so you are not restricted to visible). If you use this latter method then you can omit the diffraction grating (CD), light bulb, and motor. I'm not sure if ammonia or oxygen exhibit wavelengths in the visible (def...

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For this specific instrument: only methods using the visible spectrum will work. That means if you are trying to build a similar instrument to say detect ammonia then you should search for a methodology for doing this in the visible spectrum. Now if you are only ever going to use the instrument for one (or a few) types of molecules then you might simplify the design by finding wavelengths where these molecules have a high peak in the visible spectrum and use specific LEDs that only have most of their intensity around those wavelengths (also they make LEDs in many wavelengths, so you are not restricted to visible). If you use this latter method then you can omit the diffraction grating (CD), light bulb, and motor. I'm not sure if ammonia or oxygen exhibit wavelengths in the visible (definitely check their IR and also what IR LEDs are available) but maybe you can find a paper where these molecules complex with a chemical to make a wavelength which can be detected by the instrument. Or who knows maybe you will find a better method. If you're trying to do quantitative analysis you need to take into consideration the limit of detection of your instrument (depending on how accurate you actually want to be). Finally, with pH you're better off buying a pH meter or making your own using arduino. If your knowledge of chemistry isn't very good I'd be happy to clear up anything I can, but your question is very broad so I can't really give you a yes or no.

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