# I need to measure the impedance of the system, is it better to measure the resistance or capacitance?

The goal of this project is to measure Retinol levels in the blood. To do this I will be use two electrodes, one of which will be coated in nanowires. These nanowires will then have antibodies that will bind to the Retinol. To determine the levels of retinol, I have to determine the impedance of the system.

Now, I have a colleague that seems to think that it should use an ohmmeter and I am not too knowledgeable about EE but my understanding is that resistance wouldn't change. Rather, I should be trying to measure the change in voltage.

Does anyone have any thoughts or ideas about this? I apologize if this isn't clear enough, as I'm not sure what information is pertinent to this question. Let me know if there is anything I need to clarify.

• Since an ohmeter passes DC electricity through the electrodes, it would likely change the chemical nature of the solution through electrolysis. You probably should measure AC impedance, which is a combination of capacitance, inductance and resistance. See en.wikipedia.org/wiki/Electrical_impedance – DrMoishe Pippik Jul 21 at 0:31
• It is likely that devising an appropriate measuring technique is the crux of the matter, and makes your work novel and interesting. Find an assay, such as a reagent titration, and use it to obtain known concentrations of samples (with various allowed contaminants). Measure using all proposed techniques, and see which has best correlation to the actual ground-truth concentration. – nanofarad Jul 21 at 0:33
• you probably need to select such ac signal frequency that you would be able to detect whatever you need to detect better. You pass sine wave of some frequency thru whatever you test, then there is a resistor on the output. Resistor always has the same resistance (as long as you don't go to high megahertz or gigahertz where everything starts to act like inductors and capacitors). So yeah, resistance is constant, but your voltage divider will output higher or lower voltage depending on how much effective impedance it meets through the system – Ilya Jul 21 at 12:41
• You definitely need AC like the other guy said, and the equipment and circuitry to extract results are nontrivial, if you're trying to do it with great accuracy. Furthermore, the AC power may cause electrochemical effects (and yes, AC also does cause electrochemical effects) and you should find ways to mitigate this (perhaps a pulsed measurement?) and quantify the effect of the measurement through an alternate method. – hatsunearu Jul 22 at 6:44

What you need to measure depends on what you are doing and what is most useful for that task.

For example, this paper deals with Retinol detection. It mentions the "Single Frequency Impedance technique." If you were trying to get results comparable to those in the paper, then you would have to implement that technique - nothing we tell you about how to measure either resistance or impedance matters unless it applies to that technique.

"Resistance" is a purely real impedance. You can (normally) treat a regular resistor as a purely real impedance. At high frequencies (or with particular construction methods such as wire wound resistors,) resistors are less than purely real - a resistor at extremely high frequencies acts like a combination of a resistor, a capacitor, and an inductor.

Capacitors and inductors have an impedance that is not purely real. The complex values for the impedance have a frequency dependent effect on the signal passing through.

You can't say you want to measure the impedance of some unknown thing, then equate that with measuring the resistance, the capacitance, or the inductance.

You must measure the impedance. The resistance, the capacitance, and the inductance of what ever you are testing (together with the same properties of your test setup and equipment) will combine to form an impedance - and it will quite likely be frequency dependant.

You need to look into existing methods for measuring Retinol levels using impedance. See how they work. From there you can decide whether to implement a standard method or try some variation (or something new.)

Resistance is basically impedance at DC. If you want to measure the impedance, you also need to specify a frequency for the measurement since impedance varies as a function of frequency (assuming there is some capacitance or inductance in the circuit). As a starting point, you could try measuring the resistance and the capacitance, from which you can calculate the impedance.