Flow Injection Analysis: Caffeine Basics

Caffeine Characteristics

• Caffeine is classified as an alkaloid, indicating a nitrogenous organic compound.
• It functions as a psychoactive stimulant, affecting the central nervous system and altering consciousness, behavior, and mood.
• The chemical formula of caffeine is C8H10N4O2.
• Caffeine has a molecular mass of 194.19 g/mol.
• Functional groups present in caffeine include alkene, amide, and amine.

Flow Injection Analysis (FIA) System

• A peristaltic pump generates a special flow, facilitating the movement of samples through liquid.
• The detector (such as a spectrometer) measures both the light absorbed and the light that passes through the sample to determine concentration.
• A mixing coil is a thin tube for transporting the carrier fluid and samples to measurement devices.
• The light source in a spectrophotometer emits light that passes through the solution for analysis.
• Samples are automatically loaded via an autosampler, ensuring precise amounts are injected into the carrier fluid.
• Optical fiber cables transmit light from the source to the flow cell for sample screening.
• The FIA system produces only small quantities of waste after analysis.

Functions and Operations

• The carrier liquid is essential for transporting samples to the detector for analysis.
• The peristaltic pump creates a discontinuous flow to mix the sample with the carrier liquid.
• Samples, after mixing with the carrier liquid, flow through the mixing coil before detection occurs.
• To ensure a consistent measurement, the pump must be started and allowed to stabilize before sampling.

Calculations and Concentration

• A brewed coffee cup (237 mL) containing 80 mg caffeine has a concentration of 1.738 mM.
• Preparing a 10 mL standard solution at 3 mM requires 0.00583 grams of caffeine.
• To create a 0.0015M standard solution from a caffeine stock of 0.075M, mix 0.2 mL with distilled water.
• The concentration in undiluted dark chocolate is approximately 0.00133M.
• The gradient (m) from the standard curve for absorbance readings is 466.

Measurement Insights

• The x-axis in absorption spectrometry denotes the time from sample injection to detection, while the y-axis reflects the amount of light absorbed by the sample.
• Actual sample concentration is determined by comparing the sample's absorbance units (AU) with a standard's AU.
• Solid samples should be ground and dissolved prior to measurement to ensure accurate readings.
• Duplicate or triplicate dilutions help ensure unknown samples fall within the detectable range.
• An N/A value indicates that the sample's absorbance is outside the detection capability of the analysis system.

Sample Limitations

• In some syrup samples, caffeine concentration may be undetectable due to preparation issues, absence, or it might be present beyond detection limits.