This shortcut for the Shortcuts app allows me to save precious seconds and reduces the possibility of human error when I am aliquoting sensitive materials by quickly converting a measured absorbance of a stock solution to the volume that I need to add to each aliquot. It asks for the data it needs and then presents the output exactly as I want to copy it into my lab book. If this is something you need to do in your work, I hope it saves you some time! Even if this isn't something that applies to your work, think about what kind of repetitive calculations you could simplify using a similar shortcut. Take a look at how I built this shortcut to see how you could make this strategy work for you!
Get the Shortcut: Calculate Aliquot Volume
Use this shortcut to determine the concentration of a stock solution based on UV-Vis absorption of a diluted sample and the extinction coefficient of the compound. The shortcut will then calculate the volume of the stock solution that needs to be added per aliquot to achieve a given final concentration in a desired volume.
Set it up
Download the shortcut using the iCloud link. You will then be asked a series of input questions to help you customize the shortcut. The set-up process allows you to provide the names of two compounds you frequently use and their extinction coefficient so that you don't need to manually input these each time, as you will see in the next section, the shortcut also gives you the option to manually enter in another extinction coefficient.
The shortcut is now ready for use, but there are ways you can customize it to speed up the process of running the shortcut!
- Include more compounds in the list to choose from - add menu options with the names of the compounds, and then include the extinction coefficient in a text field for the menu output
- Add default values - if you will regularly be using the same volume to baseline your cuvette, amount added from the stock solution and aliquot concentrations and volumes, you can add these values in as Default Answers in the Ask For Input actions. The shortcut will still confirm these values with you each time it is run, so you can change them if necessary.
- Customize the output - update final text output by incorporating magic variables so that it reflects the way you want the data presented to you!
Run your sample
When it comes time to aliquot your stock solution, prep your UV-Vis by baselining with an appropriate volume of solvent in the cuvette. After baselining, add some of your stock solution to the cuvette and determine the absorbance, now you're ready for the shortcut!
The shortcut will ask you for the data it needs to carry out the calculation, starting with the extinction coefficient. You can choose from a menu of compounds you frequently use, or select "Other" to manually input the extinction coefficient of the compound.
Then you need to input the amount of solvent you used to baseline, followed by the amount of the stock solution you added. This allows the shortcut to calculate the dilution factor. The shortcut will then ask for the maximum absorbance of your compound which is used along with the extinction coefficient and the dilution factor to calculate absorbance based on Beer's Law (the shortcut assumes a path length of 1 cm).
Finally, the shortcut needs to know the desired concentration in a given volume for each aliquot, i.e. if you want your aliquots to be 20 μM when you add 40 μL of solvent to the aliquot, enter 20 when the shortcut asks for Aliquot concentration in μM and 40 when asked for Aliquot volume in μL.
The shortcut will now display a readout containing all the information you entered as well as the calculated concentration of the stock solution and the volume of the stock solution required for your aliquots. I copy this information down exactly as it appears into my lab book.
How it Works: Use my Example to Create Your Own
This shortcut uses the math functions built into shortcuts. Here are the equations I need to carry out and how I built them in the shortcut:
Calculate the dilution factor of the stock solution into the cuvette:
Dilution Factor = (Volume of stock injected (in μL)+Solvent Volume (in μL))/Volume of stock injected (in μL)
The shortcut uses an Ask for Input action to get the volume used to baseline (solvent volume) in mL. It then uses the Calculate action to multiply the number by 1000, which gives the volume in μL. The next Ask for Input value gets the volume of stock injected in μL and then adds the solvent volume in μL, which is retrieved as a magic variable from the output of the previous calculation. This sum is then divided by the volume injected in μL (called as a magic variable from the Ask for Input action) to give the dilution factor.
Calculate the concentration of the stock solution:
Stock Solution Concentration (in μM) = Absorbance / Extinction Coefficient x Dilution Factor x 1000000
The initial Choose from Menu action sets the value of the extinction coefficient, either by choosing a pre-programmed compound where the extinction coefficient is stored in a Text action, or by choosing other, which uses Ask for Input to obtain the extinction coefficient. Later, the shortcut uses an Ask for Input action to get the maximum absorbance of the compound, followed by a series of Calculate actions to first divide the absorbance by the extinction coefficient, multiply by the previously determined dilution factor and finally multiply by 100000 to convert the concentration to units of μM. The extinction coefficient and dilution factor are provided to the calculate actions as magic variables.
Calculate the volume of stock required for each aliquot:
Volume of stock = Desired Aliquot Volume (in μL) x Desired Aliquot Concentration (in uM)/Stock Solution Concentration (in μM)
The shortcut uses Ask for Input actions to get the desired aliquot volume and concentration, and then multiplies them together and divides by the concentration of the stock solution using Calculate actions and magic variables.
The final output that is displayed combines text with the variables used as above to give all the relevant data for lab bookkeeping purposes. In some cases I used rounding functions to get the number of decimal places I wanted for the purpose of the summary.
Hopefully you found this shortcut helpful, either through its direct use or because it inspired you to automate something about your own work. I would love to see examples of what you come up with!