Analytical Chemistry: Dynamic Range and Detection Limits

Spectrophotometric Analysis of Caffeine in Carbonated Drinks

• In the analysis of caffeine content in carbonated drinks using spectrophotometry, the matrix effect can be significant due to the complexity of the drink's matrix compared to the matrix of the caffeine standard.
• To minimize matrix effects, the Method of Standard Additions (MSA) is the most suitable method to use.

Method of Standard Additions (MSA)

• MSA involves the addition of one or more increments of standard solution to a fixed volume of sample aliquots, followed by dilution to identical volumes prior to measurement.
• There are two ways to perform MSA, depending on the amount of sample available:
  • Spiking: involves the addition of one or more increments of standard solution to fixed volume of sample aliquots.
  • Standard addition: involves the successive addition of increments of standard solutions to a single measured volume of the sample.

Calculating Concentration using MSA

• The instrument signal (S) can be plotted against the volume of standard solution added (Vs), resulting in a linear plot of the form y = mx + b.
• The concentration of the sample (Cx) can be calculated using the ratio of the instrument signal with and without the added standard.
• Cx can be calculated either by direct calculation method or the intercept method (also known as b/m extrapolation).

Dynamic Range

• The dynamic range, also known as the working range or calibration range, is the interval at which a certain method provides results with acceptable uncertainty.
• It ranges from the limit of quantitation (LOQ) to the point at which a certain calibration curve departs from linearity (also known as the Limit of Linearity or LOL).

Quantitation and Detection Limits

• The limit of quantitation (LOQ) is the lowest concentration of an analyte that can be quantitated with acceptable uncertainty.
• The detection limit is the lowest concentration of an analyte that can be detected with acceptable uncertainty.

Internal Standard Method (ISM)

• ISM involves the use of an internal standard to correct for variations in the signal of the chromatographic system and variations in the exact volume of sample or sample extract introduced into the chromatographic system.
• ISM can be performed in one-point and multiple-point calibration methods.

One-Point Internal Standard Method

• The concentration of the analyte in the unknown sample (Cx) can be calculated using the ratio of the analyte signal to the internal standard signal (Rx).
• The proportionality constant (F) can be calculated using the internal standard signal (SIS) and the concentration of the internal standard (CIS).

Word of Caution!

• In ESM, it is a common practice to correct the signal of standard solution with no added sample (or the blank).
• In MSA, NEVER correct for the signal obtained in the flask with no sample. That signal is due to the added standard solution!