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The scope of analytical

chemistry: ground rulesand fundamentals

Part I

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The scope of analytical

chemistry and the natureof analytical measurements

Skills and concepts

This chapter will help you to understand:

What is meant by analytical chemistry.

The scope of analytical chemistry and its applications.

The importance of high-quality data and the implications of poor

quality or erroneous data.

How to differentiate between a qualitative and quantitative test.

What is meant by replicate measurements.

The concepts of specificity, sensitivity, and accuracy of a test. What is meant by the terms analyte, interferent, and aliquot.

1.1 When and where analytical chemistry is used?

1.1.1 What is analytical chemistry?

We might not realize it, but many of us unwittingly carry out analyticalchemistry on a daily basis. It is a common scene: wandering into the

kitchen first thing in the morning, bleary-eyed, to make a cup of coffee to

kick-start the day. While the kettle is boiling, we go to the fridge and take

out the carton of milk. The carton has been sitting there for a few days

and so we are not sure whether it is still fit to use, so we open it up, lift it

to our nose, and gingerly sniff it. In so doing, and whether we realize it or

not, we are carrying out an analytical test. In this case, our nose is evaluating

the products of a variety of bacteria such as Salmonella typhimurium. If the

milk is too old, and the bacteria have had a chance to multiply, then

the action of the chemicals that they produce whilst multiplying will makethe milk unfit to drink. We will know whether the chemicals are there or not

by sniffingand aiming to detect that characteristic smell of gone-off milk.

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You might recognize this test as being a chemical analysis more readily if

you were to use a so called electronic nose. At the time of writing, elec-

tronic noses represent an emerging state-of-the-art technique in the fieldof electronic chemical sensors even though there is a long way to go before

they can sense as wide a range of smells as the human nose.

This example demonstrates thatAnalytical Chemistry encompasses

any type of test that provides information relating to the chemical

composition of a sample.

We all benefit from the activity of analytical chemists. We all eat food,

live in homes, wear clothes, and many of us drive motor cars. These are all

examples that rely on the modern manufacturing chemical industry. This,

in turn, is critically dependent on its quality control processes, the

responsibility for which lies largely with analytical chemists.Every one of us acts as a consumer and relies on the analytical chemist

to play a major role within the manufacturing process to ensure that the

food we eat, the clothes we wear, and the medicine we take, are of a suit-

able quality. The chemical industry has some input into almost every man-

ufacturing industry, and represents the largest manufacturing sector of

most major industrialized countries. Indeed, many economists say a good

indicator towards the economic health of a nation can be gained by look-

ing at its chemical industry. It follows that the role of the analytical

chemist is a truly fundamental one!

If you are reading this text as an undergraduate chemist, and intendusing your degree following your university studies, there is a greater than

50% chance that you will be employed in some analytically related role.

Many chemists perform analyses as one part of their job even if they do

not think of themselves as analytical chemists. For example, the first thing

synthetic organic chemists will frequently do having made a new com-

pound is to analyse what they have just produced.

It is clear that the population is becoming ever more demanding of

analytical chemistry for ensuring both the quality of the products we

consume and how we treat our environment.The safety of the food we eat is entwined with many issues relating to

modern farming methods, the use of agro-chemicals such as preservatives,

pesticides, and fertilizers. We are also concerned about issues such as our

cholesterol intake, how much fibre a food contains, its vitamin content,

and the strength of alcoholic drinks. We demand low or acceptable

benzene contents in the petrol we put into our cars and are then con-

cerned with the quantities of CO and CO2 cars pump into the atmo-

sphere. As the world population increases and our planet becomes ever

more crowded, we can be sure that analytical chemists will be called upon

to provide ever more information upon which future decisions can berationally based.

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1.1.2 So when and where is analytical chemistry used?

Many people will think of analytical chemistry as involving high-tech

instrumentation such as state-of-the-art mass spectrometry, highperformance liquid chromatography, and infrared techniques. Perhaps,

on the other hand, we remember our school laboratory where we learnt

the basics of titimetric analysis and spot chemical tests. While each of

these techniques play their role within the arsenal of the modern analytical

chemist, we should not forget simpler approaches using, for example,

pH meters, litmus paper, and analytical balances; these are often used

prior to the more elaborate approaches that sometimes come to mind more

readily. The important thing is to view the subject as a whole.

Sensors reflect the push towards developing highly simplified analytical

tests that may be performed by non-chemists. Every time we take our car

for a fuel emission test, the mechanic will place a CO gas sensor within the

exhaust outlet pipe to determine whether or not the levels of CO exceed a

legal threshold. In another context, diabetics may use an electrochemi-

cally based sensor to monitor their blood glucose levels, and by this read-

ing, determine the insulin dosage required prior to the next meal.

Automated instrumental techniques are also being ever more widely used

as more interest is being shown towards environmental chemistry and

pollution issues; for example, weather reports often contain some refer-

ence to air qualityanalytical chemistry is at work here too.This discussion makes two things clear; first, the use of analytical

chemistry touches upon almost every aspect of our livesand, if any-

thing, our reliance on analytical chemists is set to increase further. Second,

the subject is responding to changing needs and is therefore a truly

dynamic subject; this is reflected by the vast research effort that is being

channelled into this subject.

1.2 The nature of dataIn general terms, there are two stages to chemical analysis: data collection

and data analysisin other words, gathering information and determining

what that information is telling us. Broadly speaking, data will come in

two forms: qualitative and quantitative. Likewise, the analysis of this

data will either give a qualitative or a quantitative result.

Qualitative analyses are those that give negative/positive, or yes/no

types of data; in other words, they say whether or not some substance (the

analyte) is present in a sample but do not actually measure the quantity of

the substance(s) present. A home pregnancy test represents a good exampleof a qualitative analysis since the result will either indicate the presence

(positive result) or lack (negative result) of a pregnancy.

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Quantitative analyses determine how much of a particular substance

there is within a sample rather than just its presence or absence. An

example of a quantitative test is the measurement of the pH of an aqueoussolution; here the result can range from 0 to 14.

Quantitative data are inherently normally expressed in a numerical

format; the sign (negative or positive) and the magnitude both give

meaningful information. The accurate use of units when quoting numeri-

cal data is also of paramount importance, yet often overlooked by

students. Even the yes/no or negative/positive types of qualitative data can

be mathematically handled by statistics. So almost all data demand some

mathematical treatment, even if at a rudimentary level.

1.2.1 The limitations of data

Both qualitative and quantitative data analyses face limitations. In the case

of a qualitative test, there may be a thresholdbelow which the test may not

be able to identify the presence of the substance. For example, a pregnancy

test may give a false negative result if performed too early during the preg-

nancy if the level of the hormone human chrorionic gonadotrophin (hCG)

within the urine is at too low a concentration to cause a colour change

within the test strip. Therefore, even though the mother will be pregnant,

the test will fail to detect this, and will give a false negative result. The im-

portant point to be noted here is that even a qualitative test has a lower limitof detection below which it will fail to detect the presence of the analyte.

Absolute accuracy is also impossible with quantitative datathere will

always be a margin of error that must be accounted for. For example, no

two pH meters will give absolutely the same measurement of pH; no two

electronic balances will measure absolutely the same mass of substance

being weighed. The treatment of quantitative data demands that error

limits be determined. In this way a data point or set of data may be quoted

to within a known range of possible error. A tap water supply may be

quoted as containing 100 10 parts per million (ppm) Pb. It is therefore

possible to fix the concentration range to be between 90 ppm at the lowerlimit to 110 ppm at the upper limit. Information of this kind may often be

highly useful for ensuring that the correct and most appropriate informa-

tion may be derived from an analysis. More on the determination of error

limits is given in Chapter 2.

1.3 Should a qualitative or quantitative testbe chosen?

Analytical chemistry should always be performed for a purpose; this

may sound obvious yet this fact is often forgotten. It has been estimated

that up to 10% of tests performed each year world-wide are unnecessary.


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Apart from the waste of money (which at the time of writing is estimated to

run to as much as 5% of gross national product for most industrialised na-

tions), there is clearly a huge wastage in human effort and resourcing. Whatis often lacking is a clear focus as to why tests are being performedand in-

deed what useful information may be obtained from them.

One obvious point that should always be kept in mind is who the end

recipient of the information is and what information is actually required

that is, why the test is to be performed and what useful purpose it may

fulfil this. The most common reason for any project failing is due to a lack of

planning at the outset and this is just as true for chemical analyses. The im-

portant message here is toplan your analysis carefully and appropriately.

In many situations, you may only want a qualitative determination to

be performed. For example, you may wish to only know whether or not apollutant is present above a reasonable threshold but not need to know

the quantities at which it is present. In many other circumstances a qualit-

ative test may be performed as a first filtering processand if the result is

positive then a more complicated analysis may be performed in order to

quantify the measurement. The contamination of water samples with

lead is a good case study to illustrate this point. The lead iodide test (see

Chapter 3) provides a simple positive negative result for the presence of

lead above a concentration of approximately 0.2 g dm3. This technique

provides a simple wet chemical approach which may be carried out at, for

example, the side of a river bank using only rudimentary equipment. If asample proves positive then the analyst may wish to quantify how much

lead is actually present. Another approach such as the lead dithiazone test

might be chosen to perform a quantitative analysis. This test provides a

colour change which is proportional to the content of lead presentthe

more lead that is present, the deeper red in colour the solution will

become. The colour of the lead dithiazone solution can be measured using

a spectrophotometer to actually quantify the amount of lead present.

(Since the colour of the solution will be proportional to the amount of lead

present, measuring the intensity of the colour will give a direct indicationof the quantity of lead in the solution.) Analyses of this kind are described

in Chapters 5 and 6. Even in this situation, erroneous or incorrect results

may be caused by the presence of other heavy metal ions, and if further

specificity and or sensitivity is required, then the analyst may use atomic

absorption spectroscopy (see Chapter 7).

Above all else it should be remembered that the correct test for any par-

ticular situation is the one which best meets the requirements of the end

user. A prospective mother will be carrying out a pregnancy test to get a pos-

itive/negative resultshe will not require a test to give the exact concentra-

tion of hCG present. Using the same argument, many pollutants have limitsspecified by legislation, which are deemed to be acceptable or not. In

these situations, the water company (or regulator) is not just interested in a

qualitative result (i.e. whether the pollutant is present or not), but in the

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actual concentrations in which it is presentand it is this figure that will

determine whether or not the water is deemed to be of an acceptable quality.

1.4 Data handling and terminology

Data handling involves using a number of terms and it is important that

these are clearly defined to avoid confusion.

Before data may be handled it must be collected or collated. A sample

is often taken from a larger volume and this sample is often known as an

aliquotif in the form of a solution.

The test is often repeated with two or more samples to evaluate

reproducibility and measurements of this kind are known as replicatemeasurements. Precision is the term used to describe the reproducibility of

two or more replicate measurements that have been performed in the

same way. There are several ways in which precision can be expressed and

these are discussed in Chapter 2.

The substance to be analysed within the sample is known as the

analyte, and substances which may cause incorrect or erroneous results

are known as chemical interferents. (If an analysis monitors the concen-

tration of a heavy metal ion, a different metal ion may have very similar

chemical properties and therefore may interfere with the analysis and acts

as a chemical interferent.)The lowest concentration below which the test will fail to recognize the

presence of an analyte is known as the lower limit of detection.

The specificity of the test defines how the test may respond to the

presence of a particular analyte, so if a test is totally specific then it will

only respond to the analyte of interest and in this case no chemical

interferents will interfere with the analysis.

The sensitivity of a test meanwhile describes how close or similar in

magnitude two readings may be, and still be distinguished from each

other. If a particular technique has a sensitivity of 1 ppm for Pb

, then twodeterminations for 220 and 222 ppm may be taken as being two distin-

guishably different readings. By contrast two readings of 220.1 and

220.9 ppm Pb may notbe differentiated from each other. Data should

never be quoted beyond the level of sensitivity and/or accuracy which may

be appropriate for the test or instrument. Indeed, inappropriate numbers

after decimal points may imply a level of sensitivity that is in fact

meaningless and can in fact be totally misleading.

Accuracy describes how close the measured value is to the true value,

which may in reality be very hard to determine. Certified reference materi-

als (Chapter 2) are often used to help estimate the levels of experimentalerror which might be expected to be associated with a particular analytical

technique.


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Once collected, the data will normally contain replicate measurements

for each data point. Sufficient information should also be collected to allow

an estimation of the uncertainty or errors associated with the method. Onlyin this way may the experimental error of the technique be quantified.

The data may be collected by human observation and written down by

hand, or be collected by some form of automatic sampling technique. An

instrument may sometimes collect and process the data directly (e.g., by

an autotitratorChapter 3). It is becoming increasingly common to use

computers to assimilate and process the data. In each situation, however,

the data must be evaluated or processed for their quality and repro-

ducibility, and finally of course for its meaning. Statistical methods are

frequently used for data handling and processing, and these will be

described in Chapter 2.

1.5 The quality of analytical data

Reproducibility and accuracy are normally the most important criteria for

the end user of a test. If a blood sample is analysed for its alcohol level in

connection with a possible drink-driving conviction, it is crucially import-

ant that two differing laboratories would come to the sameand

correctconclusion. In a similar manner, the fuel emission testing equip-ment used by differing gaseous road-worthy testing stations should be able

to give concordant results to within specified limits. There will always

be experimental error associated with any test (see Chapter 2), however,

the uncertainty of the result should be clearly quantified, if any reliable

judgements are to be made from the data. The scrutiny and assessment of

the quality of the data may be carried out by some form of data validation

process; much effort, time, and money is expended in statistical analysis of

data and validation processes, both of which are discussed in Chapter 2.

Numerous studies have shown how hard it is to attain concordantanalytical information from different laboratories.Poor or unreliable data

are at best useless. If poor data cause the wrong decision to be made, the

result may be very costly indeed. A plant manager, for example, may dispose

of a batch of some product believing it to be contaminated when in fact it

was perfectly acceptable. The mistaken action may even be dangerous or

life threatening, if for example, a clinician administers an inappropriate

drug dosage, due to an erroneous pathology laboratory test.

1.5.1 You as the analytical chemist

The quality of data and their interpretation are of paramount importance

to any analytical chemist. Data are frequently numerical in nature and

their handling and interpretation involves some simple statistics. You may

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well find the mention of mathematics and statistics offputting, yet the

numerical handling of data is intended to add clarity to complex issues

and lies at the very heart of analytical chemistry. If approached slowly andgently, none of the maths required to become truly confident in any aspect

of data handling should be too problematic! These skills are required

from the point at which the subject is first studied; so Chapter 2 of this

book covers the statistical handling of numerical analytical data. This and

the following chapter have been written to gently aid learning in these

areas and to make the learning experience a non-traumatic and possibly

even a pleasant and enlightening one! It is hard to study or use chemistry

without resource to some form of analysis and so it is worth getting the

basics firmly established at an early stage. By the time you have worked

your way through Chapter 2, and the worked examples it contains, youshould have at your disposal all of the principal mathematical skills you

need to be able to fully grasp all of the material contained within every

other chapter of the book.


Exercises and problems

1.1. Discuss how modern society is dependent uponanalytical chemistry.

1.2. What is meant by: (i) a qualitative analysis;and (ii) a quantitative analysis? Give two examplesof each.

1.3. Explain the difference between what is meantby the specificity, accuracy, and sensitivity of atechnique.

1.4. What is meant by the term lower limit of detectionfor a technique and how does this differ from thesensitivity?

1.5. What is meant by the term replicate measure-ments? Why are replicate measurements desirable whenperforming analyses?

1.6. Explain what is meant by an interferent. How mightan interferent affect an analysis? Give two examples.

1.7. What is meant by a data validation process? Howmight a data validation process be performed?

1.8. What is meant by an aliquot?

1.9. What are meant by error limits?

Click here for Solutions
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Summary 11

Summary

Further reading

1. Analytical chemistry encompases any type of test thatprovides information relating to the chemical composi-tion of a sample.

2. Qualitative analyses are those that provide informa-tion relating to the presence of an analyte.

3. Quantitative analyses are those that allow theconcentration of an analyte to be determined.

4. All data contain errorsand these should be

estimatednormally by statistical means.

5. An interferent is a substance that may erroneouslyaffect analytical measurements.

6. Replicate measurements are multiple measurementsupon the same sample.

7. The specificity of an analytical test describes howselective the test is towards a given analyte.

8. The sensitivity of an analytical test describeshow close in magnitude two readings may be and still bedistinguished from each other.

9. The accuracy of a test describes how close a measuredvalue is to the true value.

10. Data validation processes are vital if confidence is tobe assigned to data.

11. Poor or unreliable data are at best useless and atworst may be dangerous or costly.

Anand, S. C. and Kumar, R. (2002). Dictionary of ana-lytical chemistry. Anmol Publications.

Kennedy, J. H. (1990). Analytical chemistry practice.Thomson Learning.