Elemental Analysis

An Introduction to Modern Spectrometric Techniques

Gerhard Schlemmer, Lieve Balcaen, José Luis Todolí, Michael W. Hinds

• Publisher: De Gruyter
• Year: 2019
• ISBN:
  9783110498325
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Elemental Analysis is an excellent guide introducing cutting-edge methods for the qualitative and quantitative analysis of elements. Each chapter of the book gives an overview of a certain technique, such as AAS, AFS, ICP-OES, MIP-OES, ICP-MS and XRF. Readers will benefit from a balanced combination of theoretical basics, operational principles of instruments and their practical applications.

1 Introduction

This book discusses about the determination of element concentrations in matter by different atomic spectrometric techniques. These include the following:

• Atomic absorption spectrometry (AAS) and atomic fluorescence spectrometry (AFS) with its atomizers, flame, graphite furnace and chemical vapor generation.
• Inductively coupled plasma optical emission spectrometry (ICP–OES)
• Inductively coupled plasma mass spectrometry (ICP-MS)
• X-ray fluorescence spectrometry (XRF)

We trust this book will give a student sufficient information to get acquainted with the basic principles of these techniques. For a practicing analytical chemist, this book is designed to give an overview of these techniques and some insight on how these techniques might be used.

Why would you pick a book about atomic spectrometry? There may be a number of different reasons such as:

• Curiosity (always good)!
• You are assigned to a new spectrometer.
• The existing method has not been reviewed for “x” years and you want/need to have a look at it.
• A replacement spectrometer is needed and you are looking into your options.
• A new process and/or analysis is coming, and you are looking for a technique to use (new or existing).
• You are having a lot of trouble with an existing technique for one or more sample types and you keep thinking that there must be a better way to do these determinations.
• There is a need to do an analysis better: faster, more accurate, better precision, lower detection levels or with less labor (pick one or more).

If there is a need to change an analysis method (whether a small tweak or complete overhaul), the first step must be to delineate the analysis parameters of the current state and the expected (new state).

The next section will assist in this process.

1.1 Analytical parameters

1.1.1 Define what is to be measured?

Have you ever had someone come to your laboratory and ask/demand a complete elemental analysis of a material for the next day? Upon asking a few questions, you probably will learn that only three elements are important and the results are really needed in two days.

Setting up a new method is an important process. What elements need to be determined? Is there a product specification or legislation that defines what elements are required to be determined? If not, then meet with stakeholders to find out what elements are needed and why. The why is important because if they do not know then why are you spending time and effort finding out.

When replacing an existing method, reviewing what elements need to be determined is also important because in some cases the material/process may have changed or legislation may have changed. This might add elements to the list or remove some that are no longer needed.

1.1.2 How important is this analysis?

Most analyzes have some importance (otherwise why would someone conduct the analysis?). However, some analyzes are more important than others (monetary value, time sensitive, final product, etc.). The amount of resources (time, labor and money spent) generally are aligned with the importance of the results from the analysis. It may also dictate how fast the samples get to the lab for analysis and how fast the analysis must be done.

1.1.3 What is the sample, how is it sampled and how does it get to the lab?

The knowledge of the sample is essential. What is the sample composition (solid, liquid or gas)? This will have a bearing on the sample preparation. Who takes the sample and how, is of vital concern because this can be the single biggest source of error in the whole analysis. If at all possible, try to get input into the sampling. Although sampling is outside the scope of this book, there is literature that can be of some use. The other question that needs to be answered is: How the sample gets to the laboratory and who has the responsibility for this activity? Defining this is important for attaining a smooth flowing operation.

1.1.4 Accuracy

All analytical chemists want to give the correct reply to the enquiry. A real proof of “correct”, however, is often very difficult. First of all, the integrity of the sample coming to the lab may be questionable, second the suitable sample preparation for the selected technique will play an important role for the results. Third, the analytical instrument may be biased by the composition of the sample which is compared to a suitable standard, yet different from the sample. The techniques described in this book are more or less sensitive to these matrix effects. The term accuracy includes both, the trueness of the result as well as the range where the result can be expected. It must be straightened out that the demand on accuracy depends very strongly on the question. For the control of most results from a soil analysis it may be enough to state that the value determined is clearly and far below a threshold level. If the result is very close to the limit, the demand on accuracy becomes high. If the concentration of gold is determined in a bullion, the demand on accuracy will always be extremely high. Accuracy, trueness and interferences will be discussed deliberately in Chapter 2 of this book and covered for the different techniques individually in the relevant chapters.

1.1.5 Precision

Of course, we know that each measurement is burdened with a standard deviation. What we really mean by that may be entirely different. The standard deviation of results obtained from the same sample by different techniques, different types of sample preparation and different operators will usually be significantly higher than that obtained from one instrument within a very short time obtained from one sample prepared by the same operator. We are using the terms repeatability and precision to distinguish. It is important to mention that the techniques are technically capable to provide a “best” repeatability under optimal conditions. Precision may therefore be a selection criterion for the analytical technique. It must be emphasized that precision, of course, depends very strongly on the concentration range of the sample relative to the detection capability of the selected technique. Read the paragraph in Section 2.4.1.2 and look on the graph sketching the relation between precision and concentration (Figure 2.38).

1.1.6 Sensitivity

It is a little bit like speed: we want to be speedy, but we often do not know why. Sensitivity is an output response to an input number of atoms to be measured. This response may be blown up (electronically or physicochemically) or it may be attenuated. Sensitivity becomes meaningful once we compare it with the baseline noise at “zero” concentration. By then it will show how significant the result is with respect to repeatability and accuracy. The sensitivity of the techniques under discussion can differ by orders of magnitude. We often talk about arbitrary numbers (intensities), in the case of atomic absorption about a physically defined magnitude (absorbance). Yes, it is good to be speedy, but sometimes the speed is too high for the path we want to go, and sometimes higher speed will result in sideslipping because of bumps on our way.

1.1.7 Limit of detection

Are we able to detect our analyte qualitatively or quantitatively? How many individual results do we need to decide? How is our detection limit connected with precision and hence with accuracy? The limit of detection is clearly defined by international standards. If our zero measurement is zero and our sample does not distort trueness and precision, the determination of the detection limit (LOD) or the limit of quantitation (LOQ) is an easy task. We need to know the repeatability of the blank, the reading of a standard close to the LOQ, maybe the confidence bands of a 10-point calibration curve spanning a range from the LOQ to 10 times LOQ. These issues will be discussed in detail. You will find out that the detection limits of the techniques under consideration may be orders of magnitude apart. The limit of detection is certainly an important criterion to select an instrumental technique. However, always keep in mind that the technique with the lowest detection limit may not be the best one for the application!

1.1.8 Time of analysis

The time needed for analysis consists of many factors. The workflow in the laboratory is as important as the sample preparation. The pursued analytical figures of merit play as important a part as the time for preparing and servicing of the analytical instruments and the time required, once it is running automatically and unattended. Focused on instruments, a few main questions need to be answered (the following list is just a selection of the most important ones)

• How many elements per sample need to be determined?
• How many samples must be run in an attended day shift or in an unattended night shift?
• What are the requirements for each of the elements with respect to LOQ, reproducibility and control of trueness of the result.
• How much sample mass or volume is available for the total analysis?
• How long is the automated operation time before the interaction of an operator is required?
• Can all elements be determined with one instrumental technique out of one sample vessel?

Only very well-equipped laboratories will be able to distribute the samples between instruments to perfectly meet the optimized analysis time. In most cases trade-offs will be necessary. It must be kept in mind that

• instruments running without being productive are often generating cost (inductively coupled plasma, flame atomic absorption spectrometry).
• extensive automated quality control may extend time and cost of the analysis significantly.
• the time for controlling the results acquired automatically is usually substantial as well and must not be disregarded when planning the total analysis time.

1.1.9 Importance of the results

The importance of the results will assist in guiding the number of replicates analyzed for the sample, the number of...

Table of contents

• Title Page
• Copyright
• Contents
• Preface
• 1 Introduction
• 2 Atomic absorption spectrometry and atomic fluorescence spectrometry
• 3 Inductively coupled plasma and microwave-induced plasma optical emission spectroscopy
• 4 Inductively coupled plasma–mass spectrometry
• 5 X-ray fluorescence spectrometry
• Index