How can I subtract baseline values from my data automatically in Prism?

One of the most common data reduction tasks is to correct data by subtraction of "noise", variably termed "baseline", "background", "blanks", or "nonspecific" activity/binding. Prism's "Data manipulations" options allow you to do this automatically.

Prism performs automatic baseline correction. Click Analyze, then choose Remove Baseline and Column Math from the Data manipulations list.

Varying Baselines
If your baseline varies with the independent variable, such that it is necessary to measure the baseline at each X value, you can enter those values in columns adjacent to your "total" values, in alternating fashion.

Suppose you're making colorimetric measurements at varying times, and you've noted a tendency for the blank values to increase with time, so you make individual blank determinations at each time. Tabulate your blank and total measurements as separate data sets (the order of columns A and B here does not matter):

Click "Analyze", then choose the "Remove baseline" analysis. In the Parameters dialog, tell Prism which column(s) contain the baseline data (column A in this example) and that you want to subtract that data from the "Total" data. Here are the selections:

Prism displays the baseline-corrected data on a Results sheet:

The default graph shows both baseline and total data. If in the Parameters dialog, you choose "Create a new graph of the results", a new graph appears containing only the baseline-corrected data.

Constant Baselines
If you want to subtract the same value from all of your data, you have two options:

  • You can subtract a value contained in, or derived from, a data set itself. To do this, enter the baseline determinations in the first or last few rows of your data set. Then in the Parameters dialog for the "Remove baseline" analysis (see above), choose one of the last two options under "Where are the baseline values?". If you want to use just the first or last row, tell Prism to "average" the first (or last) 1 row.
  • You can subtract a constant value that is independent of your data set. Choose a "Transforms" analysis, then do the built-in analysis Y=Y-K. Note that you can specify one value of K for all data sets or a different K for each data set.

Remember that after you've done your baseline subtraction, you can perform additional analyses, such as curve fitting, on that result; there is no need to move that data to a new table. Just click "Analyze" and choose to "Analyze the results table you are looking at".

If you need more flexibility when reducing your data, remember that you can integrate an Excel spreadsheet into your Prism project.  

Keywords: correction signal baseline

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