Question

The amount of camphor in an analgesic ointment is determined by GC using the method of internal standards. \({ }^{21}\) A standard sample is prepared by placing \(45.2 \mathrm{mg}\) of camphor and \(2.00 \mathrm{~mL}\) of a \(6.00 \mathrm{mg} / \mathrm{mL}\) internal standard solution of terpene hydrate in a \(25-\mathrm{mL}\) volumetric flask and diluting to volume with \(\mathrm{CCl}_{4}\). When approximately \(2-\mu \mathrm{L}\) sample of the standard is injected, the FID signals for the two components are measured (in arbitrary units) as 67.3 for camphor and 19.8 for terpene hydrate. A 53.6-mg sample of an analgesic ointment is prepared for analysis by placing it in a \(50-\mathrm{mL}\) Erlenmeyer flask along with \(10 \mathrm{~mL}\) of \(\mathrm{CCl}_{4}\). After heating to \(50^{\circ} \mathrm{C}\) in a water bath, the sample is cooled to below room temperature and filtered. The residue is washed with two \(5-\mathrm{mL}\) portions of \(\mathrm{CCl}_{4}\) and the combined filtrates are collected in a \(25-\mathrm{mL}\) volumetric flask. After adding \(2.00 \mathrm{~mL}\) of the internal standard solution, the contents of the flask are diluted to volume with \(\mathrm{CCl}_{4}\). Analysis of an approximately \(2-\mu \mathrm{L}\) sample gives FID signals of 13.5 for the terpene hydrate and 24.9 for the camphor. Report the \(\% \mathrm{w} / \mathrm{w}\) camphor in the analgesic ointment.

Short answer

Calculate RF from standard data, determine camphor concentration in sample, and convert to % w/w.

Step-by-step solution

Determine Standard Concentration Ratios

First, calculate the concentration of camphor in the standard solution. Convert the mass of camphor to mg/mL using the volumetric flask volume:\[ \text{Concentration of camphor} = \frac{45.2 \, \text{mg}}{25 \, \text{mL}} = 1.808 \, \text{mg/mL} \] The concentration of terpene hydrate (internal standard) is given as 6.00 mg/mL and is added to the same 25 mL volumetric flask, so its concentration remains unchanged.

Calculate Signal Ratios for Standard

Calculate the response factor (RF) based on the signals from the standard sample using the FID signals of camphor and terpene hydrate:\[ \text{RF} = \left( \frac{67.3}{1.808} \right) \times \left( \frac{6.00}{19.8} \right) \] Solve for RF to use it in the unknown sample calculations.

Calculate Analgesic Sample's Camphor Concentration

Given the signals for the analgesic sample, calculate the camphor concentration using the response factor:\[ \text{Concentration of camphor} = 24.9 \, \times \, \left( \frac{6.00 / 13.5}{\text{RF}} \right) \] Substitute the previously calculated RF into this formula to find the camphor concentration in mg/mL.

Convert Analgesic Sample's Concentration to % w/w

Using the concentration obtained in step 3, convert this to a weight percentage. Note the total mass of ointment (53.6 mg) and the volumetric procedure (25 mL flask):\[ \% w/w = \left( \frac{\text{Concentration in mg/mL} \times 25}{53.6} \right) \times 100 \] This will give the percentage weight/weight of camphor in the ointment.

Key concepts

Internal Standards

Internal standards are crucial in Gas Chromatography (GC) for achieving accurate quantitative analysis. Use of internal standards compensates for variations in injection volume and detector response that could otherwise lead to errors in measurement. Essentially, an internal standard is a known compound added in constant quantity to both the samples and standards. It helps in normalizing the variable responses that might occur during an experiment.

• The internal standard principle involves comparing the ratio of analyte signals to internal standard signals.
• This ratio helps in accounting for any inconsistencies in GC analyses.
• For example, in the discussed exercise, terpene hydrate acts as the internal standard.

This technique is particularly valuable when precise measurement devices are not available or the sample handling introduces potential errors. By maintaining a consistent internal standard, any fluctuations in response can be adjusted, leading to more reliable and accurate results.

Camphor Analysis

Analyzing camphor content in an ointment involves its extraction and measurement by GC. The preparation steps are pivotal in ensuring that camphor is accurately quantified. In this context, the sample of the ointment is dissolved and filtered to prepare it for the analysis process.

• The sample is usually dissolved in an appropriate solvent, like CCl extsubscript{4} (carbon tetrachloride), which is excellent for this purpose.
• It is then filtered and combined with an internal standard before injection into the GC.
• The Gas Chromatography analysis provides a chromatogram that shows peaks corresponding to camphor and the internal standard.

The peak area or height correlates with the concentration of the compound, allowing the quantification of camphor in the ointment. This comprehensive process involves both chemical preparation and precise measurement, ensuring that the camphor level is accurately determined.

Response Factor Calculation

In the context of Gas Chromatography, calculating the Response Factor (RF) is critical to quantify the amount of an analyte in a mixture. The response factor links the concentration of the analyte to the signal it gives in a detector. It adjusts the signal's response when an internal standard is used, resulting in a more precise quantification.

• The response factor is calculated from the known concentrations of analyte and internal standard, and their respective detector signals.
• In the exercise, the RF is monitored from the standard solution using FID signals for camphor and terpene hydrate.
• Formula applied, RF = \(\left( \frac{67.3}{1.808} \right) \times \left( \frac{6.00}{19.8} \right)\), where each term accounts for signal and concentration ratios respectively.

A properly calculated response factor allows for the analyte concentration in unknown samples to be accurately determined, serving essential for camphor quantification in the ointment. This underlines the importance of ensuring precise and consistent experimental conditions in GC analyses.