D.L. Massart

Standardisation of near-infrared spectrometric instruments: A review

Abstract Complex calibration models developed on a first near-infrared (NIR) spectrometer cannot be used with spectra collected on a second NIR instrument, because of the differences between the instrumental responses of both instruments. The same problem occurs when the instrumental response of a single NIR instrument is subject to variations over time or when physical modifications of the samples such as particle size occur. To keep those calibration models, it is essential to correct those differences. This can be achieved by standardisation procedures, which are based on two main steps, namely estimation of the differences by measuring standardisation samples on both instruments, and correction of those differences. The different approaches available for each step are presented, and particular attention is paid to practical considerations.

Ruggedness tests on the high-performance liquid chromatography assay of the United States Pharmacopeia XXII for tetracycline hydrochloride. A comparison of experimental designs and statistical interpretations

Ruggedness tests were performed for the high-performance liquid Chromatographic method for the determination of tetracycline hydrochloride from the United States Pharmacopeia XXII using Plackett-Burman and fractional factorial designs. Different statistical interpretation criteria for both kinds of designs were compared. The interpretation criteria for the Plackett-Burman designs were based on the use of dummy factors in the design and for the fractional factorial designs on two-factor interaction effects. The statistical interpretation criteria were compared with graphical ones (e.g., normal probability plots). The results of the designs (the ruggedness tests) were interpreted based on the above criteria. Other potential alternatives for statistical interpretation were also examined. Two different types of stationary phases (C8 and C18) were tested and compared.

The chemical composition of idiopathic nonarteriosclerotic cerebral calcifications

Calculi from a case of cerebral idiopathic nonarteriosclerotic calcification (Fahrs disease) were examined. The stone consists of hydroxyapatite and possesses a typical structure: the calcification process seems to be initiated by the formation of small round bodies that are cemented to each other to form the final stone. Calcified vessels are also present, but seem to be a secondary effect. From a comparison with other calcifications, it is concluded that no pathologic significance should be attached to the relatively high levels of trace metals such as zinc, iron, copper, magnesium, lead, and others, with the possible exception of manganese. The organic matrix of the stone contains large quantities of protein. On hydrolysis of this fraction, an important unidentified ninhydrin-positive peak was found. No mucopolysac-charides were found.

Ruggedness tests for a high-performance liquid Chromatographic assay: Comparison of an evaluation at two and three levels by using two-level Plackett-Burman designs

Ruggedness tests are performed on a high-performance liquid Chromatographic assay for a drug substance. For this purpose two-level Plackett-Burman designs are executed. The factors selected are tested at two and three levels. Significant effects of factors are determined graphically and statistically. For the latter purpose experimental error is estimated from dummy factor effects. The use of other error estimates, as for instance the variance from replicate nominal experiments, as an alternative for the dummies was also examined. The dummy factor effects were found suitable to estimate the experimental error in a Plackett-Burman design. Appropriate replicate injections at nominal levels could be an alternative. Testing at three levels gives more detailed information about the behaviour of a response in function of the factors. It was also found that not only the effects on qualitative responses should be considered but those on quantitative ones as well in order to be able to make a full interpretation concerning the ruggedness of the method.

The use of information theory for evaluating the quality of thin-layer chromatographic separations

Abstract The possibility of using information theory for characterizing thin-layer chromatographic separations is investigated. It appears that the information content, as defined by the Shannon equation, can be used to compare the merits of different solvents used for the separation of the same group of compounds

Robustness testing of a reversed-phase high-performance liquid chromatographic assay: comparison of fractional and asymmetrical factorial designs

Robustness tests were performed on a reversed-phase HPLC assay for triadimenol. Different experimental designs were compared. Two-level fractional factorial designs with different resolutions were used to study the influence of procedure-related factors. The factors chromatographic column manufacturer at four levels and instrument at three levels were stepwise included in the study using asymmetrical factorial designs. The significance of the factor effects was determined statistically, using two types of error estimates in the calculation of critical effects, and graphically, by means of half-normal plots. The asymmetrical designs turned out to be an efficient and economic method to examine the influence of factors at different numbers of levels in the robustness testing of analytical methods.

The determination of platinum in biological fluid by means of graphite furnace atomic absorption spectrometry

Methods are described for the determination of aluminium in serum and in diluted urine by means of graphite furnace atomic absorption spectrometry. For the analysis of serum, thermal decomposition of the sample in the furnace, using a ramp charring program, was found to be necessary in order to eliminate the organic material. The precision of the method was 10%. For urine the relative deviation was 9%. The precision was markedly improved by the use of an automatic sample injector (5.3%). Since large contamination problems make the determination of Al at the parts per billion (ppb) level very difficult, special attention was paid to the precautions to be taken in order to avoid external aluminum contamination.

Fast development of separation methods for the chiral analysis of amino acid derivatives using capillary electrophoresis and experimental designs

The use of experimental design in method development was studied for the chiral separation of several amino acid derivatives with capillary electrophoresis. The aim of this study was to define rapidly experimental conditions under which the enantiomers can be sufficiently separated for quantification and to derive a methodology for the separation of new compounds. Three modified cyclodextrins (CDs) were used as chiral selectors: hydroxypropyl-beta-cyclodextrin, carboxymethyl-beta-CD and sulfobutylether-beta-CD. The following factors were examined: the type of cyclodextrin, the CD concentration, the pH and the % of organic modifier (methanol) of the electrolyte. Two types of fractional factorial design were used depending on the type of analyte and on the number of factors selected: a 3(4-2) fractional factorial design (4 factors studied at 3 different levels) and a 2(3-1) fractional factorial design (3 factors at 2 different levels). From the 14 compounds investigated, 12 could be separated with one or another CD and not more than 9 experiments were required. No generalisation of the best analysis conditions was possible within this family of compounds. Specific analysis conditions must be defined for each analyte. Experimental designs have shown to be very useful to determine rapidly conditions under which each enantiomer can be separated with an acceptable resolution.

A comparison of the heuristic evoling latent projections and evolving factor analysis methods for peak purity control in liquid chromatography with photodiode array detection

Abstract The performance of the heuristic evoling latent projections (HELP) method is investigated for peak purity control in liquid chromatography with photodiode array detection. HELP and evolving factor analysis performed equally well; both techniques could detect less than 1% of a spectrally similar impurity even for small chromatographic separations. In certain cases, the two techniques may be limited by instrumental and experimental difficulties.

Solid-phase extraction of the preservatives sorbic acid and benzoic acid and the artificial sweeteners aspartame and saccharin

In connection with the acquisition of knowledge for an expert system for solid-phase extraction (SPE), a preliminary investigation was made of the kinds of sorbents that can be used for the SPE of small acid analytes. The development of a simultaneous solid-phase extraction of the preservatives benzoic acid and sorbic acid and the artificial sweeteners saccharin and aspartame is presented. Both preservatives and saccharin are retained on a silica-based quaternary ammonium anion exchanger. Elution is effected with a mixture of methanol and 1% sulphuric acid. Aspartame is not retained on the anion exchanger, but the collected adsorption and was solvents containing the aspartame can be extracted on the octadecyl sorbent. The extracts are chromatographed in the reversed-phase mode on a C18 column with a mobile phase consisting of phosphate buffer (pH 4.5, ionic strentht 0.1) and acetonitrile. Recoveries of at least 95% were observed and the relative standard deviation was < 3.2%. Comparison of an external callibration line for aqueous standard solutions, a calibration line for extracted aqueous samples and a standard addition line for soft drinks showed that the developed method is unbiased when applied to concentrations of up to 20 mg l−1 in soft drinks.