Volume 93, Number 3, May-June 1988 Journal of Research of the National Bureau of Standards Accuracy in Trace Analysis Chemometricsand Standards L. A. Currie Center for Analytical Chemistry National Bureau of Standards Gaithersburg, MD 20899 1. Introduction discussion will be placed in the framework of the Analytical System, or Chemical Measurement Pro- Standards are central to the achievement and cess (CMP), for such a perspective makes it possi- maintenance of accuracy in trace analysis. This fact ble to consider logically a "theory of analytical is well-known and well-accepted in the interna- chemistry"; and certainly chemometrics is a very tional analytical chemical community, where important part of such a theory [4,5]. To set the "standards" are generally considered to be Stan- stage, the next section will include a brief view of dard Reference Materials (SRMs) or Certified Refer- the current content of Chemometrics, together ence Materials (CRMs). The term, standards, with a summary of its history and literature. This however, is multivalued, as noted recently by a for- article will conclude with a glimpse at the future of mer Director of the National Bureau of Standards chemometrics, with special emphasis on means to [1]. That is, even in our more conventional view of achieve increased accuracy in our chemical mea- trace analysis, we must consider in addition to stan- surements and increased understanding of the ex- dard materials: standard procedures (protocols), ternal (physical, biological, geochemical) systems standard data (reference data), standard units (SI), which provide the driving forces for analytical standard nomenclature, standard (certified) instru- chemistry. ments, and standard tolerances (regulatory stan- dards, specifications, norms) [2]. It is interesting, in light of these several types of "standards" which 2. A Brief History have some bearing oil accuracy in trace analysis, to consider the possible significance of standards in The content of Chemometrics, as viewed by the and for Chemometrics. "Working Party on Chemometrics" of the Union To pursue this objective, we first must have a of Pure and Applied Chemistry (ILTPAC), is given common understanding of the meaning of the term, in table 1 [6]. Included in the second, major portion chemometrics, and what significance it may have of the table are titles for some 30 chapters which for accurate trace analysis. A concise definition is comprise an overview document being prepared given by the subtitle of the volume which resulted for IUPAC. Two points are evident from the list of from the first NATO Advanced Study Institute on titles: (1) the scope of chemometrics is very broad Chemometrics, i.e., "Mathematics and Statistics in indeed, encompassing significant portions of ap- Chemistry" [3]. Implications for accuracy, espe- plied mathematics; (2) as implied by the name, ma- cially accuracy in trace analysis, are immediately jor emphasis is given to measurement, specifically evident. That is, wherever mathematical or statisti- chemical measurement. In a narrower sense, cal operations contribute to the experimental de- chemometrics is sometimes viewed as the intersec- sign, data evaluation, assumption testing, or quality tion of statistics and analytical chemistry, as seen control for accurate chemical analysis, "chemomet- by the emphasis on experimental design, control, ric standards" are at least implicitly relevant. and the analysis of signals and analytical data. The The major part of this paper will be devoted to several chapters on signal and data analysis include an explicit discussion of such chemometric stan- such topics as filtering, deconvolution, time series dards, including case studies drawn from recent re- analysis, exploratory data analysis, clustering, pat- search at the National Bureau of Standards. The tern recognition, factor analysis, and (multivariate) 193 Volume 93, Number 3, May-June 1988 Journal of Research of the National Bureau of Standards Accuracy in Trace Analysis regression. Standards and analytical accuracy have Chemistry has long served chemometrics well, special relevance to the chapters on terminology, through its biennial fundamental reviews of the precision and accuracy, performance characteris- subject, starting well before the term was known. tics, calibration, analysis, and quality control. As indicated in table 2, the term "chemometrics" was conceived by Svante Wold, in January 1971. Table 1. What is chemometrics? The reader's attention is called to the interesting paragraph by Wold, in reference [7], which details 1. NATO Advanced Study Institute (1983) the beginnings of chemometrics, including the start "Chemometrics: Mathematics and Statistics in Chemistry" of the Chemometrics Society by Wold and Kowal- 2. IUPAC-Working Group on Chemometrics (1987) ski, in Seattle on 10 June 1974. The intervening Scope decade, culminating in the forementioned NATO Advanced Study Institute, saw rapid growth in Producing Chem. Information Notation & Terminology chemometrics education and research, much of it Precision & Accuracy: promulgated by the Chemometrics Society and intralab, interlab published in journals such as Analytical Chemistry and Analytical Chimica Acta. Also, there appeared Calibration: univariate, Relating Chemical & multivariate Non-Chemical Data several notable texts which were largely chemo- metric in content, if not in title [8-13]. Information Theory Performance Characteristics Optimization & Exptl. Design: Table 2. A brief history sequential, simultaneous Signal Analysis: 4 chapters Data Analysis: 8 chapters IUPAC (1987): Report on Chemometrics (D.L. Massart, M. Otto) Expert Systems: custom Two textbooks: "Chemometrics: a textbook" made, knowledge (1987, Elsevier) engineering tools (Massart, Vandeginste, Deming, Operations Research Graph Theory Michotte, Kaufman) "Chemometrics" (1986, Wiley) Robotics (Sharaf, Ilman, Kowalski) Computational Techniques Journal of Chemometrics (Jan. (future strategies) Two Journals: 1987) (Ed. Kowalski, Wiley) Chemical Image Analysis Sampling Strategies Chemometrics and Intelligent Quality Control Systems Theory Laboratory Systems (Nov. 1986) (Ed. Massart; Elsevier) Chemometrics Conference: (NBS, May 1985)-dedicated to W. J. Youden (Spiegleman, A brief, chronological history of chemometrics Sacks, Watters; NBS J. is presented in table 2. To convey information on Research 90 [6]) both the history and the literature of this discipline, NATO Advanced Study Institute on Chemometrics: we have indicated milestones in the form of se- (Cosenza, Sept. 1983) lected references, to the extent possible. Impres- (Kowalski) sive, recent growth is seen by the fact that the first "Chemometrics: Theory and Application" (1977) two textbooks and the first two journals, specifi- (Ed. Kowalski; ACS Sympos cally devoted to chemometrics, were published 52) within the last 2 years. Looking to the beginning of Chemometrics Society founded (Seattle, 1974) (S. Wold, this history (bottom of table 2), we find the name of B. Kowalski) Jack Youden, certainly one of the earliest and most CONCEPTION-S. Wold (1971) (J. Chemometrics, V.1, No. notable chemometricians, whose excellent guide to 1, p. 1, Jan. 1987) chemometrics was published some 20 years prior to Analytical Chemistry (ACS), the invention of the term. (Youden, incidentally, Reviews on statistics . mathematics ... chemometrics (even was a proper chemometrician, in that he began his years) career as a chemist, and then went on to become a W. J. Youden, "Statistical Methods for Chemists" (1951, Wiley) distinguished statistician.) The journal Analytical 194 Volume 93, Number 3, May-June 1988 Journal of Research of the National Bureau of Standards Accuracy in Trace Analysis To complete this brief look at the content, his- Table 3. Chemometric standards tory and literature of chemometrics, it is fitting to refer to the Chemometrics Conference held at NBS Nomenclature (terminology, concepts, formulation) just 3 years ago. It was a special meeting in many Standards for accuracy (entire chemical measurement process) respects, for it epitomized the interdisciplinary na- detection, identification, estimation, uncertainties, ture and increasing scope of chemometrics; and it assumptions was "probably the first (such meeting) in the evaluation United States by that title" [14]. The meeting was of chemometric techniques, software, algorithms jointly planned by an interdisciplinary team, con- validation through "standard" data; interlaboratory exercises sisting of a chemist and two statisticians. It was design to meet external needs for adequate, accurate chemical jointly sponsored by two national chemical and information two national mathematical societies. Finally, it con- Advance the state of the art; stimulate multidisciplinary tained an extremely effective and balanced blend of cooperation experts from the two disciplines: mathematicians (and statisticians) providing critiques of chemomet- rics presentations by chemists, and chemists Supporting standards for accuracy, for the entire providing critiques of the presentations by mathe- Chemical Measurement Process, is perhaps our maticians. The synergism resulting from this ap- most important task. The primary components are proach is evident from examining the proceedings indicated under the second heading in table 3. Most [14]. It is appropriate to conclude with reference to important is a rigorous approach to the specifica- this volume, for it was dedicated to W. J. Youden, tion and evaluation of the