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Critical Reviews in Analytical Chemistry, 32(4):271–300 (2002)

Trace Analysis — Challenges and Problems

Jacek NamiesnikDepartment of Analytical Chemistry, Chemical Faculty, Gdansk Univ. of Technology, 11/12 G.Narutowicz Str., 80-952 Gdansk, Poland. tel.: (058) 374-10-10, fax: (058) 347-26-94. e-mail:chemanal@pg.gda.pl

ABSTRACT: Analytical procedures and techniques employed to determine trace componentsare objects of growing interest to analysts. The determination of those components in samples ofdifferent matrix composition is becoming ever more common. Hence, attention should be paidto problems involved in this type of activity.

The article presents specific problems connected with trace analysis. The problems andsolutions proposed are documented with data from the literature.

KEY WORDS: trace analysis, sources of error, prevention, wall memory effect, cross-contami-nation.

I. INTRODUCTION

Even a cursory perusal of any analyticaljournal must lead one to the conclusion thattrace and ultra-trace analysis is a domain ofchemical analysis that is gaining in impor-tance recently. This conclusion is corrobo-rated not only by the feelings and opinionsof analysts. By the current definition of theterm ‘trace component’ proposed by theIUPAC, the limit from which we can talkabout trace analysis is the concentration of100 ppm (100 µg/g). Naturally, this limit ispurely conventional and is not a constant. Asrecently as 30 years ago ‘trace analysis’ wasunderstood to denote activities aiming to de-termine components at a concentration levelone order of magnitude higher (i.e., below1000 ppm, or 0.1%).

Even today the determination of compo-nents at a concentration level of 100 ppm,even in samples with complex matrices, doesnot pose major problems and is done rou-tinely in many laboratories. This is mainlydue to the rapid development of instrumen-

tation, or the science of the construction anduse of monitoring and measuring devices.Hence, one may expect the definition of theterm ‘trace component’ to change again soon.Table 1 presents a classification of analyti-cal methods and techniques by analyte con-centration in the sample examined.

II. AREA OF APPLICATION OFTRACE ANALYSIS

It is possible to distinguish three areas ofscience and technology that spur the devel-opment of analytical methods and techniquesemployed in the determination of low andvery low analyte contents in samples of vari-ous kinds. They are

• technologies of the production of high-purity materials; to date, the purity of thecleanest man-made material is denoted by11 N, which means that the sum total ofall impurities it contains does not exceed10–9%, or 10 ppt;

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• genetic engineering and biotechnology;• environmental protection.

The determination of ever lower con-centrations of analytes has brought into com-mon use special ways of expressing suchconcentrations. Table 2 lists the units em-ployed to denote concentrations in traceanalysis.

Efforts to determine analytes at everlower concentration levels in samples withcomplex matrices that additionally exhibit ahigh degree of nonhomogeneity are the mostimportant tendency in the development ofmodern chemical analysis.1,2 Advances inthis field are connected primarily with threetypes of activity:

• the use in analytical practice of new kindsof detectors and sensors (includingbiosensors) characterised by:—a low threshold of detectability and

determinability

—high selectivity, and often even speci-ficity of response;

• the designing of techniques of sample prepa-ration for the stage of final determination.There is no doubt that this avenue of researchopens up especially many new possibilitiesof determination of trace and ultratrace com-ponents. Table 3 lists the main tasks of thestage of sample preparation for analysis;

• the use of hyphenated techniques in analy-sis. Their development is connected withthe growing significance of speciationanalysis. Complex analytical systems havebeen devised which are often composedof three elements connected on-line:3-6

—a system for preliminary sample prepa-ration, usually involving the techniqueof extraction to the solid phase (Figure1 shows a diagram of the basic stagesof sample preparation using the SPEtechnique);

—a separation system. Here a special roleis played by chromatographic methods

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(mainly GC and HPLC). They are usu-ally employed to separate mixtures intoindividual components;

—a suitable detection system. For ex-ample, Figure 2 presents combina-tions of separation techniques andspectroscopic detection systems thatare most widely used in analyticalpractice.6

III. MAIN SOURCES OF ERROR INTRACE ANALYSIS

Obviously enough, the determination ofcomponents that occur in a sample at verylow concentrations presents a completely dif-ferent challenge to analysts. The various op-erations performed on the sample during itspreparation for the stage of final determina-tions can be a source of many errors cru-cially affecting the final result of the analy-sis.

The chief sources of error are changesin the composition of the sample understudy brought about by a variety of fac-tors. In Table 4 three principal kinds oferror distorting the result of analysis arepresented.

The bias of the final determination resultcaused by the errors listed (Table 4) may bedue to various processes occurring in thesample and due to the influence of the exter-nal environment on the sample. By way ofexample, Figure 3 presents diagrammaticallythe sources of error that can distort the resultof determination of a trace component in aliquid sample. The intensity of the processesand phenomena shown in the figure can varydepending on:

• the temperature of the sample and itschanges,

• the presence and intensity of solar radia-tion, and

• the presence of living organisms in the sample.

The contact of analytes present in bothgas and liquid mixtures with the walls ofvessels, tubing and appliances crucially af-fects the concentration levels of trace andultratrace components. It is the result ofadsorption and desorption of analytes onthe surface of a solid. These phenomenadistorting the sample composition are de-scribed collectively as ‘the wall memoryeffect’ and ‘cross-contamination’. Table 5shows diagrammatically the magnitude ofpolar organic analyte-wall surface interac-tion.

What gives rise to many misunderstand-ings is the use in analysis of water of suitablepurity. Table 6 lists data concerning the ef-ficiency of some water-purifying methods,while Table 7 gives specific electrical con-ductance values for water with various de-grees of purity.

Obtaining ultrapure water, which is thebasic reagent in laboratories performing traceanalysis, is usually a multistage process. Inthe installation for the production of water ofthis level of purity the following processesare employed:

• preliminary filtering to remove most sus-pended matter;

• activated-carbon filtering to remove freechlorine (to a sub-1-ppm level), small par-ticles of suspended matter (1 to 5 µm) andmost organic matter (to a level below1 ppm of carbon);

• preliminary chemical processing, con-sisting in the neutralization of water withacid or alkalies, as required, and in thetransformation of dissolved CO2 into car-bonates and bicarbonates;

• ionic exchange, during which ions presentin water, including those introduced in theprocessing, are exchanged for hydrogenand hydroxyl ions on the cation and anionexchangers;

• membrane filtration — microfiltration,ultrafiltration, reverse osmosis;

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FIGURE 1. Diagram of the main stages of extraction to the solid phase.

1 - conditioning of sorbent bed, 2 - adsorption (passing stream of sample through sorbent bed), 3 - rinsing andcleaning of sorbent bed, 4 - elution of analytes (obtaining of solvent concentrate)

FIGURE 2. Combination of separation techniques and spectroscopic detection techniques most widely usedin analysis.

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FIGURE 3. Diagrammatic presentation of factors that can affect concentrationlevels of trace components in a liquid sample.

1 - contact of sample with laboratory air; 2 - residues of components of dishwashingmixtures; 3 - distilled water; 4 - reagents and solvents employed; 5 - contact withanalyst; 6 - vaporization of the most volatile components; 7-8 - processes ofadsorption-desorption (wall memory effect); 9 - adsorption of analytes on suspen-sion; 10 - precipitation of sediment; 11 - leaching of components from vessel; 12-13 - permeation of solution components through vessel walls; 14 - reaction ofanalyte with vessel material; 15 - chemical reactions among solution components.

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• water degassing to remove oxygen andcarbon dioxide, and to reduce the contentof volatile organic compounds to a levelbelow 1 ppb;

• UV sterilization, which ensures a re-duction of bacteria in water to a levelbelow 1 bacterium/100 cm3 of water,and the oxygenation of the remainingpart of organic matter to a level below 1ppb;

• water ozonation to further reduce the con-tent of organic matter.

For years the amount of organic matterin water (usually expressed by the total con-tent of organic carbon) has been consideredthe chief measure of quality of high-puritywater.

Analytic work in the field of traceanalysis should be carried out not onlywith the use of highest-purity reagents,but also in rooms conforming to the ‘cleanroom’ standards. Such a room is a spaceseparated from ambient air and only ac-cessible through a sluice. A stream of aircleaned by a system of filters is pumpedinto the room under a slight positive gaugepressure. The excess is removed through alattice floor. The HEPA filter that is partof the system stops dust particles of morethan 0.3 µm with an efficiency of up to

99.99%. With the air cleaned in this way,controlled humidity and temperature, andsuitable working rules of the personnel,the risk of sample contamination is re-duced to a minimum in such a laboratory.There has appeared a first mention of theso-called ultraclean chemical laboratory(UCCL) in the literature. In its case the airquality must meet even more stringent cri-teria. Unfortunately, a significant limita-tion to ‘clean rooms’ is the very high costof such an investment. That is why it ismuch more common for laboratories toutilize much cheaper ‘clean boxes’ with alaminar flow of an inert gas. They can beused successfully in sample processing.The introduction of the robotization andautomation of sample processing into ana-lytical practice may be a solution to manyproblems connected with the cleanness ofthe laboratory environment.

The quality of the air is defined throughthe number of dust particles in a unit vol-ume. The American standards set in the U.S.Federal Standard 209 distinguish six classesof laboratory air on the basis of the numberof particles from 0.5 to 5.0 µm in diametercontained in 28.32 dm3 of air. The classifica-tion is presented in Table 8.

In Table 9 some measures are presentedwhich can eliminate, or at least reduce the

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Table 9 (continued)

intensity of, factors likely to affect the con-centration levels of trace components in aliquid sample. In the case of gaseous or solidsamples, there will appear not only their spe-cific sources of error, but also those (or atleast some of those) characteristic of liquidsamples.

In the literature one can find plenty ofmore detailed information about sources oferror in trace analysis. There have appearedcomprehensive overviews devoted to theseissues7-10. They deal primarily with inorganictrace analysis.

The available literature also suppliesmuch information (of various levels of de-tail) concerning problems and sources of errorinvolved in the determination of trace com-ponents in samples with complex matrices.In Table 10 the relevant information is listedthat concerns trace analysis, while Table 11provides data about specific issues related totrace analysis in three basic types of sample(gaseous, liquid, and solid).

SUMMARY

Trace analysis is an important part ofchemical analysis and is becoming an objectof interest to ever growing ranks of analysts.In solving analytical tasks in this field, moreand more complicated monitoring and mea-suring equipment is used. Unfortunately, thisdoes not always go hand in hand with thelevel of knowledge of the personnel thatoperates such devices. There have been agrowing number of cases when an analyticaldevice is treated like a typical black box intowhich it suffices to put a sample to get therequired analytical information. This ap-proach is, of course, false and the determina-tion results obtained in this way are often asource of misinformation. The author hopesthat the present article will be useful in point-ing out several problems relevant in deter-mining components occurring at low andvery low concentrations in samples with com-plex compositions.

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