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G Hendriks

Publications and source records attributed to G Hendriks.

5 recordsLinked to original sources
PubMed1996

Rational experimental design for bioanalytical methods validation. Illustration using an assay method for total captopril in plasma.

Generally, bioanalytical chromographic methods are validated according to a predefined programme and distinguish a pre-validation phase, a main validation phase and a follow-up validation phase. In this paper, a rational, total performance evaluation programme for chromatographic methods is presented. The design was developed in particular for the pre-validation and main validation phases. The entire experimental design can be performed within six analytical runs. The first run (pre-validation phase) is used to assess the validity of the expected concentration-response relationship (lack of fit, goodness of fit), to assess specificity of the method and to assess the stability of processed samples in the autosampler for 30 h (benchtop stability). The latter experiment is performed to justify overnight analyses. Following approval of the method after the pre-validation phase, the next five runs (main validation phase) are performed to evaluate method precision and accuracy, recovery, freezing and thawing stability and over-curve control/dilution. The design is nested, i.e., many experimental results are used for the evaluation of several performance characteristics. Analysis of variance (ANOVA) is used for the evaluation of lack of fit and goodness of fit, precision and accuracy, freezing and thawing stability and over-curve control/dilution. Regression analysis is used to evaluate benchtop stability. For over-curve control/dilution, additional to ANOVA, also a paired comparison is applied. As a consequence, the recommended design combines the performance of as few independent validation experiments as possible with modern statistical methods, resulting in optimum use of information. A demonstration of the entire validation programme is given for an HPLC method for the determination of total captopril in human plasma.

Calibration↗
PubMed1994

An automated analytical method for the determination of felbamate in human plasma by robotic sample preparation and reversed-phase high performance liquid chromatography.

An automated analytical method for the determination of felbamate in human plasma is described. Sample cleanup and preparation was performed by means of a Zymate II laboratory robot and consisted of a liquid-liquid extraction of felbamate and the internal standard, primidone, from human plasma to dichloromethane. The dichloromethane was evaporated and reconstituted in a phosphate buffer. Separation was performed by reversed-phase high performance liquid chromatography using a 5 microns Hypersil ODS column (150 x 4.6 mm) and a mobile phase consisting of a mixture of phosphate buffer (pH = 6.5, 0.015 M) and acetonitrile (79:21, v/v). Quantitation was performed by measurement of the UV absorbance at a wavelength of 210 nm. The lower limit of quantitation was 0.100 micrograms ml-1 using 200 microliters of plasma. The mean absolute analytical recovery of felbamate was 75.2% (n = 28). The recovery of the internal standard, primidone was 74.7% (n = 10). The within-day precision was below 3.8% at all concentration levels, except at the lower limit of quantitation (18.3%). The within-day accuracy varied between -3.7 and +7.4%. The between-day precision was below 5.0% at all concentration levels. The between-day accuracy of the method varied between -5.7 and +1.6%. The selectivity of the method towards several other anti-epileptic drugs has been demonstrated.

Anticonvulsants↗
PubMed1988

Reproducibility of counting immunoglobulin-containing cells in colonic mucosal biopsies.

Counting immunoglobulin (Ig)-containing cells in colonic mucosal biopsies can help to objectively support the differential diagnosis of ulcerative colitis and Crohn's disease. Before a method for counting Ig-containing cells can be applied in a clinical setting, however, its reproducibility must be determined. This study investigated the reproducibility of two different methods for counting such cells. The use of a light microscope with an ocular grid resulted in a slightly better reproducibility than did the use of a projection microscope with a graphics tablet. Moreover, the ocular grid method had a higher efficiency. The counting of IgM- and IgG-containing cells had a considerably higher reproducibility than did the counting of IgA-containing cells. To determine the minimal number of cells to be counted in order to ascertain a stabilized mean number of Ig-containing cells, the running means of counts of Ig-containing cells were calculated for two observers. When at least 600 Ig-containing cells (i.e., two to four fields) were counted, the interobserver variation of the running means was less than 10% for IgA and IgG counts and less than 5% for IgM counts. Since earlier studies showed differences in the counts of IgA-, IgG- and IgM-containing cells between ulcerative colitis and Crohn's disease to be, respectively, 3%, 25% and 28%, the results of the present study suggest that the proposed counting method can be useful in the differentiation between these entities.

Cell Count↗
PubMed1975

The acetylcholine-dopamine balance in the basal ganglia of rhesus monkeys and its role in dynamic, dystonic, dyskinetic, and epileptoid motor activities.

Behavioural analyses have been made of effects brought about by both single and combined injections of dopamine, haloperidol, carbachol and atropine into the caput nuclei caudati of rhesus monkeys. High doses of dopamine produced the subsequent development of three types of behavioural changes: an increase in the number of skilled manipulation movements (the dynamic phase); the appearance of a dystonic torticollis (the dystonic phase); and, finally, the appearance of an oro-lingual-facial dyskinesia and a number of dyskinetic activities in the extremities (the dyskinetic phase); low doses of dopamine solely produced the dynamic phase. Haloperidol only inhibited the dopamine-induced dynamic and dystonic phase: it did not suppress the dyskinetic phase. High doses of carbachol produced the subsequent development of four phases: a dynamic, dystonic, dyskinetic and epileptoid phase; the last one was marked by the appearance of secondary generalized epileptic seizures. Low doses of carbachol solely produced the dynamic phase. Atropine inhibited the carbachol-induced dynamic, dystonic and epileptoid phase; it did not suppress the dyskinetic phase. High doses of dopamine strongly modified the carbachol-induced phases: dopamine intensified the dystonic phase on the one hand, but it abolished the generalized epileptic seizures on the other hand. Apart from the fact that the data presented have confirmed that both dopaminergic and cholinergic mechanisms within the basal ganglia of rhesus monkeys are involved in the elicitation and modulation of both normal and abnormal motor activities, they have also revealed that the simple concept of a stristal acetylcholine-dopamine "see-saw" has to be revised. Furthermore, the data have suggested that development of supersensitive dopamine-sensitive sites is not the only mechanism that underlies the elicitation of the oro-lingual-facial dyskinesia. And finally, the present experiments have given clearcut evidence that an intrastriatal acetylcholine-dopamine "see-saw" fulfils and essential role in the process involved in the generalization of epileptic seizures.

Acetylcholine↗