Analytical Chemistry Processes

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Questions and Answers

What is the primary purpose of sampling in analytical processes?

To determine the total amount of material in the lot

To collect data from the entire population

To minimize the time spent on analysis

To select a representative sample for analysis (correct)

Which step in the analytical process follows sample collection?

Sample preservation (correct)

Data processing

Analyte separation

Problem definition

What does the term 'analyte' refer to in an analytical sample?

The portion of the sample intended for analysis

The entire group from which the sample is drawn

The component of interest in the sample (correct)

The total amount of material taken from a lot

What is a major source of error in sampling known as?

Sampling error Signup and view all the answers

Which of the following correctly describes the relationship between a bulk sample and a laboratory sample?

A laboratory sample is a smaller portion derived from the bulk sample. Signup and view all the answers

What is the first step involved in sampling bulk material?

Identify the population from which the sample is to be obtained Signup and view all the answers

What can inappropriate sampling lead to?

Irreversible damage and invalid data Signup and view all the answers

What does a sampling plan specify?

The number of samples to be run Signup and view all the answers

What type of samples are referenced as homogeneous?

Well-stirred oil tank samples Signup and view all the answers

What is a primary requirement to obtain correct and informative results from sampling?

Ensuring sample contamination is minimized Signup and view all the answers

Why is representative sampling critical during collection?

To accurately reflect the target population Signup and view all the answers

What is sample error in the context of sampling?

Difference between sample composition and target population Signup and view all the answers

Which of the following is NOT a part of the sampling plan?

The properties of the analyte being studied Signup and view all the answers

What type of sample container is recommended for collecting surface water less than 5 meters?

Capped bottle Signup and view all the answers

Which of the following is NOT a common type of sample for municipal wastewater treatment plants?

Natural springs Signup and view all the answers

What is a primary purpose of sample preservation in water sampling?

To prevent contamination Signup and view all the answers

Which type of gas sample container is typically used for automotive exhaust emissions?

Stainless steel canister Signup and view all the answers

What is one disadvantage of using glass sample containers?

They can break easily and cause contamination. Signup and view all the answers

What type of sample is collected using a Niskin bottle?

Surface water deeper than 5 meters Signup and view all the answers

Which of the following options is commonly used for collecting compressed gases?

Tedlar bag Signup and view all the answers

What additional preparation might be necessary before analyzing a sample?

Dilution Signup and view all the answers

Which type of sorbent is an example of an inorganic sorbent?

silica gel Signup and view all the answers

What is a disadvantage of using inorganic sorbents?

Sensitive to humidity Signup and view all the answers

What is the preferred sample volume range for semi-volatile gases?

2–500 m3 Signup and view all the answers

What is a common application of glass fiber filters?

Filtration of non-volatile compounds Signup and view all the answers

Why is it necessary to collect solid samples carefully?

To prevent contamination and preserve composition Signup and view all the answers

Which tool is recommended for collecting soil samples deeper than 30 cm?

Trench or auger Signup and view all the answers

What sampling tool is typically used for obtaining small powders?

Sample thief Signup and view all the answers

What is one disadvantage of crushing and grinding solid samples?

It can lead to contamination of the sample. Signup and view all the answers

What may cause a solid sample to undergo a change in composition?

Lack of preservation methods Signup and view all the answers

Why is it important to prevent contact of air with anaerobic sediments?

To preserve the organic content unaffected. Signup and view all the answers

Which method is NOT used for bringing solid samples into solution?

Ball milling Signup and view all the answers

What is a common technique to reduce large particles in sample preparation?

Jaw crushing Signup and view all the answers

What is a potential consequence of increasing the surface area when processing samples?

Loss of volatile components Signup and view all the answers

Which method is appropriate for the digestion of difficult-to-dissolve samples?

Microwave digestion Signup and view all the answers

What is one disadvantage of using microwave digestion?

Not suitable for all types of samples Signup and view all the answers

Which process is commonly used for sample preparation before quartering?

Crushing and grinding Signup and view all the answers

Study Notes

Analytical Process Steps

An analysis provides chemical or physical information about a sample through carefully designed steps.
The process involves problem definition, method selection, sample collection, preservation, preparation, analyte separation, analysis, and data processing.
These steps apply to various disciplines such as food safety analysis.

Introduction

A sample is a small portion representing the characteristics of a larger population, whether it be a group of people or a batch of material.
Sampling aims to obtain a representative and homogeneous sample.
Sampling error arises from the difference between the true population value and the sample value.

Terminology

Types of analysis: Qualitative (identifies the presence or absence of a substance) and Quantitative (determines the amount of a substance).
Analyte refers to the substance being investigated, while the matrix comprises the rest of the sample.
A lot represents the total amount of material from which the sample is drawn, including the bulk sample (a large initial sample), the gross sample (several smaller portions of the bulk sample), the laboratory sample (a reduced portion of the gross sample), and finally, the analytical sample used for analysis.

Importance of Sampling and Sample Preparation

The accuracy of analytical results relies heavily on proper sampling and sample preparation.
These steps depend on the properties of the analyte and the nature of the sample.
Obtaining correct and informative results is the main purpose of sampling.
Improper sampling can lead to invalid data and compromise the entire analytical process.

Sampling Plan

A sampling plan is a detailed protocol outlining the number, location, and size of samples to be collected.
It also includes instructions for sample alteration, processing, reduction, and analysis methods, as well as reporting requirements.

Sample Types

Sample composition often deviates from the target population, contributing to sample error.
Homogeneous samples have a uniform composition throughout, such as well-stirred oil.
Heterogeneous samples exhibit variable composition, requiring careful selection of sampling points to ensure representativeness.

Sampling Methods

Random sampling (with or without replacement) ensures that each sample has an equal chance of being selected, minimizing bias.
Judgmental sampling relies on expert knowledge to choose samples from specific locations or regions believed to be representative.
Wastewater sampling usually focuses on specific points within the water treatment system to monitor effluent quality and pollution levels.

Sample Collection

The type of sample collected depends on the nature of the analyte and the sample itself.
Commercial solvents, suspensions like beverages, bodily fluids, and environmental waters require different collection methods.
Surface water is typically collected using capped bottles for shallow depths (< 5 meters) and Niskin bottles for greater depths (> 5 meters).
Purging is necessary for collecting samples from groundwater and municipal water systems.
Various types of containers are employed, such as glass (preferred for chemical stability and inertness, but breakage concerns) or plastic (ease of use and lower cost, but may leach chemicals into the sample).

Sample Preservation and Preparation

The primary goal of preservation is to prevent changes in the sample's analyte concentration or composition.
Commonly employed methods include refrigeration, freezing, addition of preservatives, filtration, or adjustment of pH.
Additional preparation may involve homogenization, dilution, or extraction depending on the analytical method.

Gaseous Samples

Gaseous samples are particularly challenging to collect and preserve due to their volatile nature.
Commonly encountered gaseous samples include automobile exhaust, industrial emissions, atmospheric gases, and aerosols.
Collection methods for gases include stainless steel canisters or Tedlar/Teflon bags, cryogenic cooling, trapping using solid sorbents, and filtration.
Solid sorbent types include inorganic sorbents (silica gel, alumina, molecular sieves), organic polymers (polyurethane foam), and carbon sorbents.
In-situ sampling (collecting samples directly at the source) is preferred when possible to minimize contamination or alteration.

Sample Collection for Solid Compounds

Solid samples require careful collection to minimize contamination and loss of volatile components.
Sediments are often collected with bottom grab samplers (provide a snapshot of the sediment layer) or corers (collect a vertical profile of the sediment).
Soil samples are obtained using scoops, shovels, soil punches for shallower depths (< 30 cm), and trenches or augers for deeper samples.
Particulate materials are collected based on size: large particles (ores) are collected using riffles (split the sample into two halves repeatedly until a small sample remains), and small particles (powders) are collected using sample thieves (a tube with a plunger that collects a portion of the powder).

Sample Preservation

Solid samples are susceptible to changes in composition without proper preservation.
Storing samples at lower temperatures (refrigeration or freezing) slows down chemical reactions and microbial activity.
Loss of volatiles can be minimized by sealing containers tightly and storing them under an inert gas atmosphere.
Contact of air with anaerobic sediments should be prevented to maintain their chemical composition.

Sample Preparation

Solid samples often require processing before analysis.
The first step involves reducing particle size to ensure homogeneity and improve analytical accuracy.
This is often achieved through crushing and grinding using various machines: jaw crushers, ball mills, disk mills, and mortars and pestles.
Coning and quartering is a common method for obtaining subsamples.
Sample dissolution is the next step, bringing the analyte into solution for analysis.
For easily soluble samples, water or organic solvents can be used directly.
Difficult samples are often dissolved through digestion, drying, or fusion.
Digestion with acid or base (wet ashing) is a common method, oxidizing the sample to break down the matrix and release the analyte.
Microwave digestion accelerates the process, but requires specialized equipment and careful handling.
Fusion is used for refractory materials by melting the sample with fluxes to form a soluble product.
Dry ashing involves heating the sample at high temperatures in air to remove organic matter and leave an inorganic residue containing the analyte of interest.

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Description

This quiz explores the analytical process steps involved in chemical analysis, focusing on sampling and types of analysis. It covers essential terminology and concepts such as sample collection, method selection, and data processing. Perfect for students of analytical chemistry.