Use a sample blank when the plant extract exhibits a persistent color that affects absorbance readings. This allows for accurate subtraction of its interference. A reagent blank is appropriate when the extract does not contribute to absorbance, ensuring accurate measurement of the reagents alone.

Abdelhak@ gave a satisfactory answer. Yet, I would add that you can measure the interference and decide for yourself the best route to follow. Reagent blanks should come first since you can also choose to buy less contaminated reagents based on their blanks. You can also compare using standard addition to using just the calibration curve, both methods in triplicates. If the results significantly differ in statistical terms - you can calculate standard deviations and apply statistical tests - the use of a sample blank is imperative. Or you can use standard addition curves to measure if the number of samples is not too large to use standard addition since standard addition implies one calibration curve for each sample.

It is crucial to understand the origin of the practice of ratioing the transmittance of a sample to that of a blank (or subtracting the "absorbance" of the blank). The underlying issue lies in the existence of two theoretical frameworks in spectroscopy: the Beer-Lambert approximation (BLA) and wave optics (WO). To illustrate, the BLA is akin to the geocentric model of the universe, while WO represents the heliocentric model. The geocentric model may appear accurate until closer scrutiny reveals its limitations.

With the BLA, some of these limitations can be partially addressed by ratioing the transmittance of the sample to that of the blank (typically, the transmittance of the pure solvent). This approach yields results that are more in line with those obtained through WO, especially when the solvent has a low refractive index (1.5 or lower) and lacks peaks in the relevant spectral range. Using a solvent (or reagents) with peaks in the same range, however, can lead to significant issues.

Why is this the case? The BLA originates from an era when the nature of light as electromagnetic waves capable of polarizing matter was not yet understood. While this distinction is negligible for gases at low pressure, where molecules are far apart, it becomes critical for liquids and solids. In condensed matter, a molecule polarized by a photon can, in turn, polarize neighboring molecules, affecting the original molecule as well. This interaction results in peak shifts and changes in intensity simply by exposure to light, a phenomenon that contributes to solvatochromism. In extreme cases, this means that a solution can appear a different color in darkness than under illumination.

Even if this effect is disregarded, WO reveals that it is not the absorption index (or the refractive index) that is directly proportional to molar concentration, but rather the real and imaginary components of the dielectric function. These factors cause absorbance to deviate from additivity, except when absorption is negligible.

If you want to understand the whole story, have a look at: Book Wave Optics in Infrared Spectroscopy - Theory, Simulation, a...

Article The Bouguer‐Beer‐Lambert Law: Shining Light on the Obscure

In analytical chemistry, understanding the difference between a sample blank and a reagent blank is essential for accurate measurement and data interpretation. Here’s a breakdown of each:

Sample Blank

• Definition: A sample blank is a control that contains all components of the sample except the analyte of interest. It is prepared using the same solvents, reagents, and sample matrix as the actual samples.
• Purpose: Its primary role is to account for any background signal or interference that may arise from the sample matrix itself. This helps in determining the baseline signal and subtracting it from the actual sample measurements.
• Usage: Used to identify and quantify matrix effects, ensuring that the results obtained from the actual samples reflect only the presence of the analyte.

Reagent Blank

• Definition: A reagent blank is a control that contains only the reagents used in the analytical procedure, without any sample or analyte. It often includes the solvents and reagents used in the sample preparation and analysis process.
• Purpose: Its main function is to identify any background signal or contamination introduced by the reagents themselves. This ensures that the reagents do not contribute to the measurement of the analyte.
• Usage: Used to assess the purity of reagents and their potential interference in the analysis, allowing for more accurate quantification of the analyte in samples.

Key Differences

• Composition: A sample blank mimics the actual sample but without the analyte, while a reagent blank consists only of reagents and solvents.
• Purpose: The sample blank accounts for the matrix effects, whereas the reagent blank helps identify any background noise or interference from the reagents themselves.
• Application: Both are used in calibration and validation of analytical methods but target different sources of potential error.

Summary

Using both blanks in an analytical procedure helps ensure accurate results by accounting for both sample matrix effects and any contributions from reagents. This practice enhances the reliability and credibility of the analytical data obtained.

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