REPORT
The Hierarchy and Relationships of Analytical Properties
Miguel Valcárcel and Angel Ríos Department of Analytical Chemistry University of Cordoba 14004 Cordoba, Spain
The quality of the analytical information produced in a laboratory depends on the quality of the information available to m a n a g e r s as they define the analytical problem, design the analytical process, and adjust its features so that the results produced meet t h e r e q u i r e d objectives (e.g., c o m p l i a n c e w i t h l a w s , n o r m s , or quality control of products and systems). It is important to have a clear understanding of analytical properties such as accuracy and representativeness as well as their relationship to q u a l i t y in t h e a n a l y t i c a l process and to the results. These two facets of a n a l y s i s a r e f r e q u e n t l y dealt with inconsistently, resulting in confusion in laboratory work and in communications w i t h clients or service users. The purpose of this REPORT is to 0003-2700/93/0365-781 A/$04.00/0 © 1993 American Chemical Society
establish a hierarchy for analytical properties, illustrate their mutual relationships, and describe how the social and economic considerations of a given analytical problem affect the dominance of some properties over others. As we will see below, dealing with analytical properties in isolation when designing strategies to address specific problems is often inadequate and produces erroneous results.
Types of analytical properties Quality in the analytical results, the primary goal of analytical chemistry, relies heavily on two capital analytical properties: accuracy and r e p r e sentativeness. Accuracy means consistency between the results obtained and the actual concentration of the analyte in a particular sample; representativeness refers to consistency between the results and the analyzed sample as well as between t h e r e sults and the definition of the a n a lytical problem. Capital properties rely on another
group of analytical properties t h a t we will call basic properties. They determine the quality of the analytical process both inside the laboratory and outside the l a b o r a t o r y d u r i n g sampling. Basic properties are sensitivity, selectivity, precision, and sampling; t h e i r definitions can be found in a number of analytical textbooks and monographs. The analytical process is also limited by seemingly less significant accessory properties. These properties, however, often have major practical implications. They include expedit i o u s n e s s , cost-effectiveness, a n d personnel-related considerations, all of which affect laboratory output. Figure 1 shows the two essential components of analytical quality and displays capital, basic, and accessory analytical properties in a hierarchical m a n n e r . It also outlines the generic objectives of present-day analytical chemistry: obtaining larger amounts of analytical information of a higher quality by expending less material, time, and h u m a n re-
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REPORT sources; minimizing risks; and incurring the smallest expenses possible (i). Whereas capital analytical prope r t i e s a r e associated with r e s u l t s , basic and accessory p r o p e r t i e s are related to the analytical methodology used. Analytical properties, whether viewed individually or collectively, are not independent of one another. In fact, they exhibit mutual—and occasionally opposing—relationships. Attempts to optimize a given analyt-
ical procedure should, in principle, be aimed a t t h e capital properties. The m a x i m u m possible sensitivity, selectivity, and precision should be achieved, and proper sampling should be carried out. However, emp h a s i s is often placed on saving or minimizing t i m e , costs, m a t e r i a l s , reagents, labor, and hazards, which l e a d s to g r e a t e r e x p e d i t i o u s n e s s , cost-effectiveness, automation, and p e r s o n n e l safety a n d comfort. The balance between the objectives as de-
Personnel safety/comfort
Figure 1. Goals of analytical chemistry and their relationships to analytical quality and analytical properties.
picted in Figure 1 d e t e r m i n e s to a g r e a t extent how analytical procedures are designed. Analytical tetrahedra The most straightforward and intuitive form of graphically displaying a n a l y t i c a l p r o p e r t i e s involves two t e t r a h e d r a t h a t s h a r e a vertex and can be of v a r y i n g sizes r e l a t i v e to each other (Figure 2). This illustration provides a useful means of distinguishing among the three principal groups of analytical properties, establishing a hierarchy, and determining symbiotic and opposing relationships among them. Figure 2 also includes sampling, which determines representativeness. The vertices of the m a i n t e t r a h e dron (left) depict sensitivity, selectivity, and precision; one of its faces represents accuracy, which relies on t h e s e t h r e e b a s i c p r o p e r t i e s . This tetrahedron, together with represent a t i v e n e s s of s a m p l i n g , h a s t h e greatest statistical weight on analytical results and is connected with the objectives of obtaining more and better information. The secondary tetrahedron (right) has vertices depicting the accessory analytical properties; one of its faces r e p r e s e n t s laboratory productivity. This tetrahedron is related to the objectives of minimizing time, costs, labor, and hazards. The balance among analytical properties illustrated in Figure 2 will be shifted by the specific elements of t h e a n a l y t i c a l problem a d d r e s s e d (e.g., result quality is always in conflict with productivity). The balance between two competing sides represents a compromise and defines the c o m p e t i t i v e n e s s of t h e a n a l y t i c a l laboratory concerned. For example, although a laboratory may be more competitive if it can obtain results in a short time, some accuracy may be sacrificed. Each l a b o r a t o r y should determine the most competitive position of the equilibrium to address a particular problem. Shifts in t h e balance depicted in Figure 2 can be classified by t h r e e g r o u p s according to w h e t h e r one, two, or t h r e e analytical properties a r e to be prioritized. The initially regular t e t r a h e d r a become irregular prisms when a vertex, edge, or side is pulled, l e n g t h e n e d , or expanded while others are pushed, shortened, or contracted. Accuracy and representativeness
Figure 2. Analytical tetrahedra. 782 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
As indicated previously, overall quality in analytical chemistry is determined by accuracy and r e p r é s e n t a -
Figure 3. Mechanisms for increasing accuracy by enhancing basic analytical properties. (a) Including separation techniques to preconcentrate analytes of interest and remove interferences enhances one or more of the three basic analytical properties, (b) Increasing analysis precision results in increased accuracy.
tiveness and depends on the quality of the work performed outside (sampling) and inside the laboratory (the analytical process). The influence of sampling on analytical quality cannot be overstated. P r o p e r s a m p l i n g is e s s e n t i a l for achieving the degree of representativeness appropriate to the analytical problem and t h u s for providing quality results. Accuracy in the analysis of a nonrepresentative sample is meaningless. For example, an accurate determination of the fat content of milk in only a few randomly selected packages by use of validated methodologies based on modern instrumentation is not useful analytical information if the objective is to determine the fat content of a batch of 10,000 packs. Appropriate statistical sampling must be carried out if the i n f o r m a t i o n produced is to be r e p r e s e n t a t i v e of t h e problem as a whole. Another example involves the blood potassium concentration deter-
m i n e d from a sample t a k e n from a patient more t h a n 1 h after a h e a r t attack. The sample cannot be considered representative of the patient's blood potassium concentration at the time of the heart attack. Representativeness is therefore an unavoidable prerequisite and the bottleneck t h a t controls the entire analytical process: No other analytical property can be justified in the absence of representativeness. Accuracy is the analytical property most closely r e l a t e d to l a b o r a t o r y work in which the chemical composit i o n of a k n o w n s a m p l e is d e t e r mined. It is complementary to repres e n t a t i v e n e s s because it allows specific analytical determinations to be made about a sample. It also valid a t e s r e p r e s e n t a t i v e n e s s because, however representative a sample may be, it is useless if the analytical information about it is inaccurate. Although it is r a t h e r difficult to assess r e p r e s e n t a t i v e n e s s , c e r t a i n statistical approximations can be applied to s a m p l i n g t e c h n i q u e s t h a t make them easier to implement and more reliable. In m a n y cases, it is imperative to talk with individuals both inside and outside the area of interest and to investigate a variety of t e c h n i q u e s to e n s u r e a properly defined analytical question t h a t will provide results representative of the scientific, economic, or social problem a d d r e s s e d . A c c u r a c y , on t h e other hand, can be evaluated much more easily by, for example, use of certified reference materials. Accuracy and basic analytical properties Accurate r e s u l t s rely on a d e q u a t e sensitivity, the absence of interferences, and reasonable repeatability/ reproducibility (i.e., low uncertainty) for the analytical method used. Thus in Figure 2 the vertices of the face of the main tetrahedron, which repres e n t s accuracy, coincide w i t h t h e three basic analytical properties. In fact, if the a n a l y t e concentrations in the sample are lower t h a n the detection or quantification limits achieved, the analytical results will not be consistent with these limits. Any perturbations arising from positive or negative deviations in the analytical signal, whether of an additive or m u l t i p l i c a t i v e n a t u r e , will also shift the results from the actual values. The tetrahedron edge bound by the sensitivity and selectivity vertices can be markedly lengthened by including s e p a r a t i o n techniques in the analytical process; through preconcentration a n d interference re-
moval, such techniques indirectly enhance one or more of the three basic a n a l y t i c a l p r o p e r t i e s ( F i g u r e 3a). Liquid-liquid extraction is a separation technique widely used for this purpose. Although a given analytical procedure may be highly accurate but not precise (i.e., the mean of a set of disperse results obtained for the same sample may coincide with the actual value), accuracy usually depends on t h e a b s e n c e of s y s t e m a t i c e r r o r s arising from low levels of the basic analytical properties a n d from the so-called h u m a n factor and indeterminate (random) errors. Highly u n certain results are also generally not accurate. T h u s increased precision results in increased accuracy (Figure 3b); the former can be achieved by simplifying or a u t o m a t i n g the analytical process (2). For example, the use of an automatic sample/standard introduction system in electrothermal vaporization atomic absorption spectroscopy ensures greater precision in r o u t i n e analyses compared with the level of precision achieved with manual introduction. Relationships among basic analytical properties Figure 4 depicts the opposing relationships among the three basic analytical p r o p e r t i e s . E n h a n c i n g one usually can be achieved only at the expense of the other two. The central equilateral triangle represents a balanced situation and can be distorted to a n isosceles or scalene t r i a n g l e , depending on which property is favored. For comparison, t r i a n g l e areas have been kept constant. The most salient relationships among the three properties are discussed below. P r e c i s i o n a n d s e n s i t i v i t y . It is well known t h a t precision decreases as analyte concentrations decrease.
Figure 4. Relationships among basic analytical properties.
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REPORT Figure 5 shows an example of the res u l t s from i n t e r c o m p a r i s o n food analysis e x p e r i m e n t s in which t h e coefficient of variation increases exponentially with decreasing analyte concentration (3). Consequently, a highly sensitive method will also be highly variable when applied to trace and u l t r a t r a c e d e t e r m i n a t i o n s . E r rors of ± 10% in the determination of a few n a n o g r a m s per milliliter of a drug in blood are quite acceptable; however, errors made in determining the same drug at the milligram-pergram level in a pharmaceutical preparation should not exceed ± 2%. Incorporating an analytical separ a t i o n t e c h n i q u e to i n d i r e c t l y e n hance sensitivity and to facilitate the determination of analyte concentrations below the detection or quantification limit of the analytical method used complicates preliminary operations and increases the time spent by the researcher. In addition, precision is d i m i n i s h e d . D e t e r m i n a t i o n of trace cadmium in seawater by direct application of a selective s p e c t r o scopic t e c h n i q u e such as g r a p h i t e furnace atomic absorption spectroscopy or inductively coupled plasma atomic emission spectroscopy results in e r r o r s s m a l l e r t h a n t h o s e obtained when a preconcentration procedure such as liquid-liquid extrac-
Figure 5. Relationship between the precision and the concentration level of the analytes to be determined in the form of a Horwitz graph. (Adapted with permission from Reference 3.)
tion or ion exchange is used before spectroscopic analysis. The standard deviation of the estim a t e for a calibration curve e s t a b lishes a statistical relationship between precision and sensitivity: The h i g h e r t h e s e n s i t i v i t y (i.e., t h e greater the curve slope, as defined by IUPAC), t h e h i g h e r t h e p r e c i s i o n achieved in the analyte determination. In fact, at a given signal level t h e more sensitive method (Figure 6a) provides a less uncertain result t h a n does the less sensitive method (Figure 6b), provided both calibrat i o n curves h a v e s i m i l a r s t a n d a r d deviations of the estimate. Also, for a given curve, t h e h i g h e s t precision lies in t h e c e n t r a l portion a n d decreases toward the ends (quantification and u p p e r limit, respectively). Detection a n d q u a n t i t a t i o n limits, two ways of defining sensitivity, are calculated via a p r e c i s i o n - r e l a t e d parameter: the standard deviation of the blank. The relationships between sensitivity and precision discussed above contribute to the uncertainty of the measuring analytical technique used. Thus weighings on an analytical balance are subject to fixed errors (e.g., ± 0.1 mg) t h a t will affect t h e p r e c i s i o n of t h e m e a s u r i n g method to a n extent inversely proportional to the weighed amount of sample, reagent, and reaction product. With other analytical techniques, t h e highest possible precision is achieved over a given analytical signal range; such is the case with U V - v i s photometry, where the zone of minimum error is bound by a b s o r b a n c e v a l u e s b e t w e e n 0.2 and 0.8 AU. S e n s i t i v i t y a n d selectivity. Notw i t h s t a n d i n g t h e i r seemingly d i s tinct n a t u r e s , sensitivity and selectivity are also related and not always in an opposite m a n n e r . In fact, the difference between additive and mult i p l i c a t i v e i n t e r f e r e n c e s lies in whether the perturbation concerned affects the slope (sensitivity) of the calibration curve. The more sensitive t h e m e t h o d u s e d to d e t e r m i n e a given analyte, the more a sample can be diluted to decrease the concentrations of interferences and hence minimize or e l i m i n a t e t h e i r p e r t u r b a tions. This approach is widely known and exploited in the analysis of clinical/biological samples; sensitive methods call for high initial dilutions (1:10-1:10,000) to minimize the typical perturbations of macromolecules. Sensitivity and selectivity can be enhanced by similar mechanisms (4): chemical reactivity (e.g., by using or-
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Figure 6. Relationship between sensitivity and precision in terms of the uncertainty involved at a given signal level in two calibration curves of different slope. (a) More sensitive method, (b) Less sensitive method.
ganic r a t h e r than inorganic r e agents); material separation (e.g., by combining preconcentration and int e r f e r e n c e r e m o v a l , a s in l i q u i d liquid extraction a n d anodic s t r i p p i n g v o l t a m m e t r y ) ; s e l e c t i o n or alteration of the instrumental technique used (e.g., atomic r a t h e r t h a n molecular optical techniques or t h e use of chemically modified electrodes for potentiometric m e a s u r e m e n t s ) ; b i n a r y or t e r n a r y combinations of the above choices (e.g., pre- and postcolumn derivatization in chromatography, where chemical reactions are used to enable t h e s e p a r a t i o n a n d detection of the solutes, respectively); and use of chemometric techniques in combination with one or more of the previous alternatives (e.g., multivariate calibration procedures). Newly emerging analytical laboratory instruments tend to incorporate these mechanisms to enhance basic a n a l y t i c a l p r o p e r t i e s . T h e u s e of nonchromatographic continuous separation techniques and multichannel detectors as well as the development of hyphenated techniques—one p a r t of which can be a type of chromatogr a p h y — a n d on-line combined techniques are additional good examples in this context.
S e l e c t i v i t y a n d p r e c i s i o n . The uncertainty inherent in the mechanisms described above for enhancing selectivity is related to the complexity of the analytical process. When several such mechanisms come into play, the indeterminate error arising from each is seen in the final result, and precision is diminished. Thus if the selectivity of the methodology in q u e s t i o n a r i s e s from t h e d e t e c t o r used (e.g., in single- and multichannel atomic spectroscopy), the analytical process is simpler a n d h i g h e r precision is achieved. However, the precision is decreased when one incorporates new chemical reactions or a separation procedure to increase the selectivity because it expands the traceability chain. U s i n g a s e p a r a t i o n t e c h n i q u e to remove interferences introduces further u n c e r t a i n t y in the final result because the analytical process is being expanded or complicated. By automating the preliminary operations of the analytical process (2) or by using h y p h e n a t e d techniques, h u m a n errors can be minimized. In fact, online combinations of separation techn i q u e s a r e effective m e a n s of i n creasing the overall precision a c h i e v e d by u s i n g t h e c o m b i n e d techniques in isolation. Thus the coefficient of variation associated with the use of a supercritical fluid extractor and a gas chromatograph combined on line is lower t h a n the summation of those obtained when the extraction and separation/determination steps are carried out separately. Appropriate chemometric or simple m a t h e m a t i c a l t r e a t m e n t s i n crease selectivity. U n i v a r i a t e calibration and differential kinetic methods (e.g., logarithmic extrapolation) (5, 6) are two good choices. As a rule, the use of these methods results in poor precision—even poorer t h a n that obtained from analytical separation techniques. However, one can use t h e s e m e t h o d s to avoid some stages of the analytical process. Accuracy and accessory analytical properties Designing an analytical method by which to achieve high accuracy means t h a t high levels of the basic a n a l y t i c a l p r o p e r t i e s m u s t be obtained a t t h e expense of t h e accessory p r o p e r t i e s . As can be seen in Figure 7a, the m a i n t e t r a h e d r o n is distorted when the area of the triang u l a r a c c u r a c y face is i n c r e a s e d , whereas the secondary tetrahedron is considerably s h r u n k by shifting t h o s e forces p o i n t i n g t o w a r d t h e
Figure 7. Distortion of the analytical tetrahedra for boosting (a) accuracy or (b) accessory analytical properties.
Figure 8. Relationships among the complementary analytical properties and the prevalence of one or two of them. Results of enhancing (a) cost-effectiveness, (b) cost-effectiveness and expeditiousness, (c) expeditiousness, (d) cost-effectiveness and personnel safety/comfort, (e) personnel safety/comfort, and (f) expeditiousness and personnel safety/comfort.
main tetrahedron. This situation reflects the priorities among the capital, basic, and accessory analytical properties in Figure 1. The opposite s i t u a t i o n (Figure 7b) occurs w h e n temporal, economic, and/or personal
aspects related to laboratory productivity prevail upon the quality of the results. Obtaining immediate analytical results in response to an urgent dem a n d (e.g., in t h e clinical field) or
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REPORT controlling a process in n e a r real time e n t a i l s sacrificing some accuracy. Thus some recent applications rely on FT-IR spectroscopy to obtain accurate, reproducible r e s u l t s in as short a t i m e as possible w i t h m i n i mal sample handling and m a n u a l involvement (e.g., on-line process cont r o l i n m a n u f a c t u r i n g w o r k [7]). Occasionally, a timely "yes or no" r e ply to indicate w h e t h e r a given par a m e t e r is above or below t h e tolera t e d level is m o r e i m p o r t a n t t h a n obtaining a n accurate analytical re T suit a few h o u r s l a t e r . S e n s o r s (8) and screening tests based on i m m u noassay (9) a r e additional good examples. When economic constraints affect laboratories or when available equipment is limited, the quality of the r e sults suffers. Acquiring and processing analytical signals manually with poor-quality reagents and r u d i m e n t a r y equipment t h a t lacks a d e q u a t e electronic control adversely influence t h e accuracy of t h e r e s u l t s . Nevertheless, the best technical means are no guarantee t h a t results will be accurate unless t h e analytical process is developed properly, in compliance
GERÄTEBAU
SÄULENTECHNIK
Examples of the effects of enhancing one or two accessory analytical properties Enhanced property/properties
Example
Cost-effectiveness
The first step usually taken to cut laboratory expenses is to reduce personnel. This results in a heavier workload for remaining personnel and in a slower delivery of results (Figure 8a).
Expeditiousness
Increasing throughput requires augmenting laboratory equipment and compelling staff to work harder (Figure 8c).
Personnel safety/comfort
If political or trade union pressure takes personnel claims to extremes, costs immediately rise and throughput falls (Figure 8e).
Cost-effectiveness and expeditiousness
If costs need to be reduced and throughput raised, the analytical process is frequently automated fully or partly. Automation results in staff dismissals and the assignment of greater responsibilities to the remaining analysts, who thus require retraining (Figure 8b).
Expeditiousness and personnel safety/comfort
If personnel safety/comfort is not critical to obtaining the analytical results, the staff will be under no pressure and none of the extraordinary and also costly means needed to maintain a high throughput will be required (Figure 8f).
Cost-effectiveness and personnel safety/comfort
The laboratory budget must be increased to maintain a high throughput without altering personnel status (Figure 8d).
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786 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
with established quality a s s u r a n c e norms (10). Accuracy is also affected by t h e human factor which, despite the high d e g r e e of a u t o m a t i o n c u r r e n t l y available at many laboratories, r e mains crucial. The ease with which results are obtained from automatic analytical methods and the increasing isolation of instrument operators from the process foster operator errors because of overconfidence in aut o m a t e d e q u i p m e n t . These factors also diminish the attention given to a critical e v a l u a t i o n of the r e s u l t s obtained. Other major human factors are ope r a t o r comfort and safety p r e c a u tions, the implementation of which occasionally entails sacrificing some accuracy to prevent hazards for the operators and those near them. Such is the case with routine analyses involving hazardous reagents or methods. Though the need for accuracy exists, methodology should be modified to avoid hazards and the possibility of accidents, and to minimize the need for the continuous attention of the operator. Accessory analytical properties The accessory properties at the vertices of the productivity t r i a n g u l a r face of the secondary analytical tetrahedron also exhibit opposing relationships. The imposed or advisable p r e v a l e n c e of one or two of t h e s e properties d i s t o r t s t h e e q u i l a t e r a l t r i a n g u l a r face and hence disturbs the balance among them. When an analytical procedure is designed to enhance one of the three properties—cost-effectiveness (Figure 8a), expeditiousness (Figure 8c), or personnel safety/comfort (Figure 8e)—the other two sides of the r e sulting productivity t r i a n g u l a r face are markedly shortened. If two properties rather t h a n one are favored— cost-effectiveness and expeditiousness (Figure 8b) or expeditiousness and personnel safety/comfort (Figure 8f) or cost-effectiveness and personnel safety/comfort (Figure 8d)—the side of t h e r e m a i n i n g p r o p e r t y is contracted. Examples of such situations appear in the box on p. 786 A. Quality compromises M e e t i n g t h e d e m a n d s imposed on the information produced by an analytical l a b o r a t o r y r e q u i r e s t h a t a compromise be m a d e b e t w e e n t h e two c o m p o n e n t s of t h e a n a l y t i c a l chemistry/quality binomial: external quality, which refers to the properties of the monitored product or service, a n d i n t e r n a l q u a l i t y , w h i c h
deals with the analytical process and the results produced. External quality may result from compliance with legislation or quality requisites for a production process. It directly affects a n a l y t i c a l quality, which depends on capital, basic, and accessory analytical properties. As we have seen, analytical quality can be approached in various ways, depending on the a n a l y t i c a l problem. From a strictly theoretical point of view, the situation in Figure 8a can be considered i d e a l — a t the least, one should reach the balanced situation shown in Figure 2. However, the facts may justify the situation depicted in Figure 8a for the ana l y t i c a l l a b o r a t o r y to m e e t t h e d e m a n d s posed by t h e a n a l y t i c a l problem derived from the economic or social problem addressed. The c u r r e n t requirement t h a t all l a b o r a t o r y activities be in w r i t t e n form necessitates managerial planning. In m a n y cases t h i s r e q u i r e ment is imposed by law, and often a company produces a quality manual whereby the specific priorities affecting the analytical properties graphically related by the two t e t r a h e d r a are to be established via s t a n d a r d o p e r a t i o n a l procedures (SOPs). In R&D and in pharmaceutical industry laboratories, priorities are imposed by o r g a n i z a t i o n s such as the Food and Drug Administration or the Organization for Economic Cooperation and Development in the form of Good Laboratory Practices. On the other h a n d , in analytical R&D l a b o r a t o ries, such as those a t u n i v e r s i t i e s where new analytical methods and technological innovations are developed, the goal is to characterize each of the analytical properties inherent in the new method or technique according to quality control norms. Final remarks In the foregoing discussion, we have strived to establish a h i e r a r c h y of analytical properties as well as their relationships to one another and the two analytical quality components. In addition, we have underscored the need to accurately define objectives w h e n d e s i g n i n g a n a l y t i c a l procedures. Although a number of the individual factors dealt with here are well known, they have rarely been considered together or in a systematic m a n n e r d e s p i t e t h e fact t h a t t h e y a r e m o s t likely t h e u l t i m a t e foundation of analytical chemistry. This view can be of use to basic and applied researchers in planning and managing laboratories as well as in teaching analytical science.
References (1) Valcârcel, M. Fresenius Z. Anal Chem. 1992, 343, 814. (2) Valcârcel, M.; Luque de Castro, M. D. Automatic Methods of Analysis; Elsevier: Amsterdam, 1989, pp. 13-23. (3) Horwitz, W.; Kamps, L. R.; Boyer, K. W. /. Assoc. Of. Anal. Chem. 1980, 63, 1344. (4) Valcârcel, M.; Rios, A. Analusis 1990, 18, 469. (5) Martens, H.; Naes, T. Multivariate Calibration; John Wiley & Sons: New York, 1991. (6) Perez-Bendito, D.; Silva, M. Kinetic Methods in Analytical Chemistry; Ellis Horwood: Chichester, England, 1988. (7) Pandey, G. C ; Kulshreshtha, A. K. Process Control and Quality 1993, 4, 109. (8) Murray, R. W.; Dessy, R. E.; Heineman, W. R.; Janata, J.; Seitz, W. R. Chemical Sensors and Microinstrumentation; ACS Symposium Series 403; American Chemical Society: Washington, DC, 1989, pp. 1-22. (9) Van Emon, J. M.; Lopez-Avila, V. Anal. Chem. 1992, 64, 79 A. (10) Taylor, J. K. Quality Assurance of Chemical Measurement; Lewis: Chelsea, MI, 1987.
Miguel Valcârcel (left) has been a professor at the University of Cordoba since 1976. He received his B.S. degree in chemistry and his Ph.D. in analytical chemistry (1971) from the University of Sevilla (Spain). Valcarcel's research interests include the automation of analytical processes, flow injection methods, (bio)chemical sensors, nonchromatographic continuous separation techniques, and the implementation of quality systems in the analytical laboratory. He is the author or co-author of about 400 research papers, six textbooks, and three monographs. Angel Rios (right) is an assistant professor in the Department of Analytical Chemistry at the University of Cordoba. He received his B.S. degree in chemistry (1980) and his Ph.D. in analytical chemistry (1983) from that university. His research interests include automated flow systems and the implementation and application of quality principles in control and R&D analytical laboratories.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993 · 787 A
