While preparing a measuring setup for a project, I've realized that the multimeter our lab can provide only supports resistance measurements up to 10^8 Ohm. Since the materials we want to measure go way beyond that, I intended to connect the material in parallel to another resistance and simply add one calculation step to get the desired value, but what if the samples cannot be prepared that way, are there other alternatives to appropriating the measurement for the multimeters range?
If the things you want to measure have a resistance that goes way beyond what you can measure with a DMM, then paralleling a shunt resistor to bring it into range will rapidly turn small measurement errors into big device under test errors.
You would do better to go back to first principles and devise your own measurement system. For modest number of GΩ you can buy GΩ level resistors and use a very low bias current CMOS opamp as a buffer. Measuring the voltage Wheatstone style will also work.
For even higher resistances, measuring the discharge rate of a good PTFE capacitor (ie sample the voltage, disconnect, then sample again a minute later) shunted by the sample will allow you to observe very small currents.
Lets do the math. A 1 GΩ resistor in parallel to a 100 MΩ resistor, the result is 90.90 MΩ. A 2 GΩ resistor instead is 95.238 MΩ.
A large change of the GΩ resistor will result in a small change of the parallel connection. A precise measurement of the GΩ resistors is impossible this way.
Here's a way, using a DC voltage source (< 50 V), a DMM having an input impedance around 10 MΩ (to measure current using its voltage function), and a calculator to calculate the resistance of the specimen.
Measuring gigaohms with a simple multimeter