Question

A certain concentration method works best when the analyte's concentration is approximately 10 ppb. (a) If the method requires a sample of \(0.5 \mathrm{~mL}\), about what mass of analyte is being measured? (b) If the analyte is present at \(10 \% \mathrm{w} / \mathrm{v}\), how would you prepare the sample for analysis? (c) Repeat for the case where the analyte is present at \(10 \% \mathrm{w} / \mathrm{w}\). (d) Based on your answers to parts (a)-(c), comment on the method's suitability for the determination of a major analyte.

Short answer

The method is suited for trace analysis, but requires large dilution steps for major analytes, making it impractical for such applications.

Step-by-step solution

Understanding Parts-per-billion (ppb)

Parts-per-billion (ppb) is a unit of concentration that measures the number of units of mass of a contaminant per 1 billion units of total mass. In this exercise, a ppb concentration is equivalent to micrograms per liter (µg/L) for our purposes.

Calculate Mass for Part (a)

Given a concentration of 10 ppb and a sample size of 0.5 mL, convert the sample volume to liters: 0.5 mL = 0.0005 L. Then calculate the mass in micrograms: Mass = concentration x volume = 10 µg/L x 0.0005 L = 0.005 µg.

Preparing Sample for Analysis - Part (b)

For 10% w/v concentration, this means 10 g of analyte in 100 mL of solution. To prepare a 0.5 mL sample, we need to dilute this solution to achieve 10 ppb. First calculate the concentration of the original solution in ppb: 1 g/mL = 1,000,000,000 ppb (since 1 g/mL = 1,000,000 µg/L). So 10% w/v = 100,000,000 ppb. Calculate the dilution factor needed to achieve 10 ppb: Dilution factor = original ppb / target ppb = 100,000,000 / 10 = 10,000,000. Therefore, diluting 1 mL of the 10% solution with 10,000,000 mL of solvent achieves the desired concentration.

Preparing Sample for Analysis - Part (c)

For a 10% w/w concentration, assume density of the solution is about 1 g/mL for simplicity. The calculation of 10% w/w in situations where density can't be ignored requires a different approach, but for simplicity, continue with the assumption. For 10% w/w, 10 g analyte in 100 g solution provides an analyte concentration akin to 100,000,000 ppb, just as before. The dilution factor remains 10,000,000 as calculated previously.

Evaluating Suitability - Part (d)

Given the calculations, this method is ideal for trace analysis rather than major analyte determination. For major components present at high concentrations, the need for significant dilution illustrates inefficiency. This is due to the method's high sensitivity, making it more suited for detection and measurement of trace concentrations.

Key concepts

Parts-per-billion (ppb)

Parts-per-billion (ppb) is a unit of measurement often used in analytical chemistry to denote very low concentrations of substances. This measurement is crucial when dealing with contaminants or analytes present in small quantities within a large volume.
At its core, 1 ppb implies one part of a substance in one billion parts of another. For liquid solutions, this is often converted into micrograms per liter (µg/L) to maintain consistency with other measurement forms.
Understanding ppb is essential when conducting tests that require precision at minute scales, as it allows accurate determination of trace amounts of a substance within a sample.

Analyte concentration

In analytical chemistry, analyte concentration refers to the amount of a specific substance, the analyte, present within a given volume or mass of a sample. This concentration is usually expressed in terms of weight/volume (w/v) or weight/weight (w/w).
In this context, the problem involves evaluating a 10 ppb concentration which translates into a known quantity of the analyte when its weight is compared to the volume of the solution. Understanding how to determine and calculate analyte concentration is key to preparing samples and calibrating analytical methods for accurate analyses.

Sample preparation

Sample preparation is a critical step in analytical chemistry processes such as the determination of an analyte's concentration. This step involves obtaining a representative part of a larger mixture and often includes none, one, or several steps to ensure the sample is suitable for analysis.
In exercises such as this, sample preparation might involve processes like dilution or adding specific reagents to bring an analyte to a suitable concentration for measurement.

• Samples may need careful handling to avoid contamination, which can skew results.
• The goal is to form a sample that reflects the conditions and composition of the larger system.

Good sample preparation practices help minimize errors and improve the reliability of analytical results.

Dilution factor

The dilution factor is a critical concept when adjusting sample concentration from a higher to a lower level, as required in many analytical methods. It involves the ratio between the initial and desired concentrations.
To calculate the dilution factor, one must divide the concentration of the original solution by the target concentration. For example, when reducing a strong solution, such as 10% w/v to a trace-level concentration like 10 ppb, specific calculations inform the volume of solvent necessary for dilution.
Understanding dilution factors is vital for ensuring that analytes fall within a method's optimal detection range, avoiding the pitfalls of both over- and under-dilution.

Trace analysis sensitivity

Trace analysis sensitivity refers to the ability of an analytical method to detect and measure minute amounts of a substance within a sample. This sensitivity is critical in fields requiring detection of pollutants, toxins, or other analytes present at very low concentrations.

• Methods with high sensitivity can detect exceedingly low concentrations, often necessitating advanced instrumentation and precise technique.
• This capability is essential for applications such as environmental testing, forensic analysis, and compliance with safety standards.

The overall efficiency of a method in accurately identifying low-level analytes dictates its suitability for specific types of analysis and influences the decision-making process regarding its use.