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Biomedical subjects

C A Cramers

Publications and source records attributed to C A Cramers.

36 records · Page 2Linked to original sources
PubMed1998

Determination of phenylurea herbicides in water samples using online sorptive preconcentration and high-performance liquid chromatography with UV or electrospray mass spectrometric detection.

A recently developed method for the extraction of organic micropollutants from aqueous samples based on sorptive enrichment in columns packed with 100% polydimethylsiloxane (PDMS) particles was coupled on-line with HPLC analysis. The sorptive enrichment procedure originally developed for relatively nonpolar analytes was used to preconcentrate polar phenylurea herbicides from aqueous samples. PDMS extraction columns of 5, 10 and 25 cm were used to extract the herbicides from distilled, tap and river water samples. A model that allows prediction of retention and breakthrough volumes is presented. Despite the essentially apolar nature of the PDMS material, it is possible to concentrate sample volumes up to 10 ml on PDMS cartridges without losses of the most polar analyte under investigation, fenuron. For less polar analytes significantly larger sample volumes can be applied. Since standard UV detection does not provide adequate selectivity for river water samples, an electrospray (ES)-MS instrument was used to determine phenylurea herbicides in a water sample from the river Dommel. Methoxuron was present at a level of 80 ng/l. The detection limit of the current set-up, using 10 ml water samples and ES-MS detection is 10 ng/l in river water samples. Strategies for further improvement of the detection limits are identified.

Chromatography, High Pressure Liquid↗
PubMed1997

Novel preconcentration technique for on-line coupling to high-speed narrow-bore capillary gas chromatography: sample enrichment by equilibrium (ab)sorption. II. Coupling to a portable micro gas chromatograph.

The technique of equilibrium (ab)sorption has been proven to be a powerful method for preconcentration of gaseous samples for high-speed narrow-bore capillary gas chromatography (GC) in general and field-portable GC instruments, often referred as micro GCs, in particular. Using a simple experimental set-up equipped with an open-tubular enrichment column it is possible to produce a homogeneously enriched sample plug, allowing reproducible injections of an enriched sample into the micro GC. Using a non-polar trapping column enrichment factors found for n-alkanes in the range of C7 to C10 ranged from 15 to 150 and agree well with calculated values. Using a highly retentive Thermocap column, the enrichment factor observed for heptane was above 500. As the use of this new preconcentration method requires only minimum modification of the micro GC, the chromatographic performance of the instrument was not compromised by direct coupling to the preconcentration device. Examples of on-line enrichment with portable micro GC analysis of VOCs from air are shown. These examples clearly demonstrate the potentials of the new method in field analysis.

Absorption↗
PubMed1997

Novel preconcentration technique for on-line coupling to high-speed narrow-bore capillary gas chromatography: sample enrichment by equilibrium (ab)sorption. I. Principles and theoretical aspects.

In recent years, there has been substantial progress in the field of high-speed narrow-bore capillary gas chromatography (GC) in general, and in the development of (trans)portable gas chromatographs for fast and accurate analysis in field applications in particular. Due to the limited (concentration) sensitivity of instrumentation for high-speed (portable) GC, environmental applications of this technique frequently require a preconcentration step. The equilibrium (ab)sorption technique described in this article was found to be very promising for on-line coupling to high-speed narrow-bore capillary GC and field portable GC instruments. Enrichment factors up to at least 100 can be obtained rapidly without the use of complicated instrumentation. The new preconcentration technique is shown to have clear advantages over enrichment based on conventional adsorption, i.e. thermal desorption techniques. It can be carried out at ambient trapping temperatures, gives uniform desorption profiles, reduces water effects, uses inert adsorption materials and does not require a (cryogenic) refocusing step. Moreover, the new preconcentration method allows the enrichment factors to be predicted from tabulated chromatographic data, thereby facilitating calibration.

Absorption↗
PubMed1996

Automated on-line ionic detergent removal from minute protein/peptide samples prior to liquid chromatography-electrospray mass spectrometry.

An automated on-line ionic detergent removal pre-column system coupled to capillary liquid chromatography-electrospray mass spectrometry is described. The system involves two micro precolumns, composed of a specific ionic detergent trapping column and a preconcentration column, respectively, and a packed 300 microns I.D. analytical column. Sample loading to the micro precolumns and regeneration of the detergent trapping column were performed at a flow-rate of 50 microliters/min, while the flow-rate through the analytical column was set at 5.0 microliters/min. Ionic detergent-containing tryptic-digested protein samples were directly applied to the micro precolumns without sample pretreatment and were analysed by UV absorption detection and electrospray mass spectrometry. The presented system allows for the fully automated removal of SDS with virtually no loss in protein/peptides. Maximum SDS load and breakthrough have been determined. Excellent protein recovery and complete removal of SDS is found. The chromatographic separation after SDS removal was completely restored and equalled the reference chromatogram. Mass spectral data confirm these findings. Finally, this technique allows for SDS removal from minute protein samples without the need for any sample handling.

Automation↗
PubMed1989

Study on the analogy of gel permeation chromatography and cuprophan dialysis for the isolation of uremic 'middle molecules' using model compounds.

Selectivities of substituted nitrobenzenes in gel permeation chromatography and cuprophan membrane dialysis were compared. Glucuronide-, glucoside-, acetic acid- and lactoside-substituted p-nitrobenzenes were chosen as model compounds for so-called 'middle molecules' in uremia. It was found that there was not a single linear relationship between the substituent effects in both processes. This was because of the anomalous behaviour of the model compounds in gel permeation. A combination of adsorption and ionic rejection in the latter technique, for the various solutes, was not encountered in cuprophan membrane dialysis, which appeared to be governed only by the molecular mass or volume of the solute. Therefore, gel permeation seems to be inappropriate for the analysis or isolation for so-called middle molecules in sera of uremic patients. Dialysis or filtration on relatively inert membranes is less susceptible to anomalies.

Cellulose↗
PubMed1988

A new method for the determination of L-dopa and 3-O-methyldopa in plasma and cerebrospinal fluid using gas chromatography and electron capture negative ion mass spectrometry.

L-3-(3,4-Dihydroxyphenyl)alanine (DOPA) and its 3-O-methyl metabolite (OMD) were measured in plasma and cerebrospinal fluid by a new assay which combines N,O-acetylation of amino acids in aqueous media, preparation of pentafluorobenzyl esters under anhydrous conditions, and analysis by gas chromatography-electron capture negative ion mass spectrometry. The N,O-acetyl, carboxy-PFB derivatives gave abundant carboxylate anions ([M-CH2C6F5]-) which were suitable for sensitive analysis using selected ion monitoring. Plasma and CSF samples were sufficiently purified by a simple organic solvent extraction. Analytical recovery for DOPA was 100.2 +/- 3.7% at the level of 100 nmol/l. Analysis of DOPA in plasma was performed with a relative standard deviation of 5%. The limit of quantitation in plasma and CSF was at the sub-nmol/l level. In healthy adults, DOPA concentration in plasma was 9.0 +/- 2 nmol/l (n = 11) and in CSF 3.5 +/- 0.9 nmol/l (n = 9). The concentration of OMD in plasma was 99.1 nmol/l (pool of 24 samples) and 15.3 nmol/l in CSF (pool of 12 samples). Measurement of 5-[2H]DOPA and 5-[2H]OMD in plasma of a healthy individual who had been orally loaded with 3,5-[2H2]tyrosine (150 mg kg body wt) was possible for several hours after the load.

Acetylation↗
PubMed1988

Hexachlorobenzene and 1,1-di(4-chlorophenyl)-2,2-dichloroethene in serum of uremic patients and healthy persons: determination by capillary gas chromatography and electron capture detection.

The concentrations of organochlorine pesticides in serum have been determined by GC and electron capture detection. Studies were performed in 7 nondialyzed, 10 dialyzed and 6 healthy persons. Only hexachlorobenzene (HCB) and 1,1-di(4-chlorophenyl)-2,2-dichloroethene (p,p'-DDE) were consistently present. No interference by other pesticides or polychlorinated biphenyls was found by checking with GC-MS. For the nondialyzed uremic patients the average HCB concentration was 16.2 nmol/l (sigma = 6.7, n = 7), and the p,p'-DDE level 26.4 nmol/l (sigma = 31.4, n = 7). For the dialyzed uremic patients the average HCB level was 15.5 nmol/l (sigma = 11.2, n = 10) before dialysis and 17.2 nmol/l (sigma = 14.4, n = 8) after dialysis, the concentration of p,p'-DDE was 27.0 nmol/l (sigma = 38.4, n = 10) before dialysis and 28.0 nmol/l (sigma = 37.4, n = 8) after dialysis. For the healthy persons the average concentration of HCB was 7.7 nmol/l (sigma = 1.8, n = 6) and the concentration of p,p'-DDE was 20.1 nmol/l (sigma = 10.4, n = 6). HCB concentrations were significantly higher in serum of dialyzed and nondialyzed uremic patients than in controls (Wilcoxon's test).

Adult↗
PubMed1988

Liquid-chromatographic profiling of solutes in serum of uremic patients undergoing hemodialysis and chronic ambulatory peritoneal dialysis (CAPD); high concentrations of pseudouridine in CAPD patients.

Using "high performance" liquid chromatography, we studied non-protein-bound fractions and total concentrations of 18 solutes accumulating in sera from a group of 12 patients who were undergoing chronic ambulatory peritoneal dialysis (CAPD) and in predialysis sera from a group of 15 hemodialysis (HD) patients. We monitored longitudinal changes in solute concentrations for two patients with respect to change of therapy between HD and CAPD. The concentrations of pseudouridine (P less than 0.001), uric acid (P less than 0.001), and an unknown fluorescent solute, "UKF3" (P less than 0.01), differed in sera of HD and CAPD patients. When standardized with respect to serum creatinine concentrations, the concentration of the transfer-RNA catabolite, pseudouridine, was significantly (P less than 0.0001) higher in sera of CAPD patients than in HD patients, suggesting an increase in turnover of transfer RNA. In stepwise linear discriminant analysis, the combination of pseudouridine and the probably biochemically related fluorescent unknown, UKF3, contributed most to the differentiation between sera from CAPD and HD patients.

Adult↗
PubMed1988

Are the classical markers sufficient to describe uremic solute accumulation in dialyzed patients? Hippurates reconsidered.

Interdependencies of accumulated solutes, analyzed by liquid chromatography in dialyzed and non-dialyzed patients, were studied by multivariate statistical analysis. In principal component analysis, three principal components (PC1-PC3) were retained from the data on 22 accumulated compounds in dialyzed patients, whereas only one principal component was retained from analogous data of a non-dialyzed patient group. PC1 in the dialyzed patient group comprises concentrations of hippuric acid, p-hydroxyhippuric acid, tryptophan, and five unidentified fluorescent solutes in serum. Concentrations of the classical markers urea, uric acid, creatinine, and phosphate were closely related to PC2 in these patients. Indoleacetic acid and two unidentified fluorescent compounds constitute PC3. The compounds associated with the groups found by principal component analysis may be characterized by chemical structure and by the mechanism of their excretion via the remaining nephrons of dialyzed patients. In the non-dialyzed group, most of the solutes could be described by a single PC. This PC and PC1 from the dialyzed group correlated significantly with residual renal function, and with total ultraviolet absorbance and total fluorescence emission. The data suggest that it is of value to introduce a marker of uremic solute retention in addition to urea, to account for renal-function-related "organic-acid-like" compounds that are excreted by renal tubular secretion in dialyzed patients. The hippurates may serve this purpose.

Chromatography, High Pressure Liquid↗
PubMed1986

Determination of acidic catecholamine metabolites in plasma and cerebrospinal fluid using gas chromatography-negative-ion mass spectrometry.

A method for the assay of acidic catecholamine metabolites in biological fluids using capillary gas chromatography--electron-capture negative-ion mass spectrometry is described. The method combines acetylation of phenolic hydroxy groups in buffered aqueous solution followed by pentafluorobenzyl ester formation and acetylation of aliphatic hydroxy groups under anhydrous conditions. The resulting per-O-acetyl carboxypentafluorobenzyl esters provided excellent negative-ion mass spectra with intense and diagnostic anions. The sensitivity of the analysis using electron-capture negative-ion mass spectrometry exceeds that using electron-impact mass spectrometry by two to three orders of magnitude. Analysis of acidic catecholamine metabolites in human lumbar cerebrospinal fluid and plasma were performed with good precision (sigma rel less than 5%) at the low nanomoles per litre level.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1985

Screening of UV-absorbing solutes in uremic serum by reversed phase HPLC--change of blood levels in different therapies.

In order to screen UV-absorbing solutes in large numbers of uremic serum samples, an automated liquid chromatographic method was developed. The method proved to be reliable and reproducible in more than 500 analyses. HPLC separation was performed using gradient elution on a 25-cm Ultrasphere Octyl reversed phase column, with 5 microns particles. Characteristic profiles for the uremic state were obtained in the analyses of serum samples of 43 uremic patients before and just after artificial kidney treatment; hemodialysis (n = 14), hemodiafiltration (n = 13) and hemofiltration (n = 16). In these profiles 20-40 peaks were resolved of which nine were 'quantitated' by peak height relative to a standard. Of these solutes creatinine, uracil, uric acid, hypoxanthine, indoxylsulfate, tryptophan and hippuric acid were identified. The heterogeneity of the population of uremic patients, with respect to the UV-absorbing solutes, was estimated. Significant differences of solute blood level changes during hemodialysis, hemodiafiltration and hemofiltration, were observed.

Blood↗
PubMed1983

Derivatization of catecholamines in aqueous solution for quantitative analysis in biological fluids.

The reaction of methyl chloroformate with catecholamines at the nanogram level in aqueous solution has been estimated. Optimal pH conditions of the aqueous medium were established which provide a quantitative yield of the formate derivative of primary and secondary catecholamines. First the catechol function was blocked by the reaction with methyl chloroformate under mild alkaline conditions (pH 7.2) and subsequently a pH shift was introduced (delta pH = 2) to improve the reaction of the amine function. The formate derivatives were extracted effectively (greater than or equal to 99%) into ethyl acetate and subsequently converted to their O-tert.-butyldimethylsilyl, N-formate derivatives. These mixed derivatives appeared to be very suitable for quantitative determination of catecholamines and related compounds in biological fluids by gas chromatography-mass spectrometry. The coefficient of variation estimated in urine samples was 6% (n = 6). The minimal detectable concentration in biological samples was 50 pg ml-1 with a signal-to-noise ratio of 5.

Catecholamines↗
PubMed1979

Profiling of uremic serum by high-resolution gas chromatography-electron-impact, chemical ionization mass spectrometry.

A fast and reliable procedure for gas chromatographic profiling of components in ultrafiltrated uremic serum has been developed, using glass capillary columns. Sample pretreatment consists of ultrafiltration, evaporation and silylation. Some twenty components are identified by electron-impact and chemical ionization mass spectrometry. A comparison is made between profiles of sera from a series of uremic patients, before and after hemodialysis, and from non-uremic sera. Significant differences are found between these profiles. A "dialysis ratio" is introduced as a parameter for the removal of retained components by hemodialysis treatment.

Gas Chromatography-Mass Spectrometry↗
PubMed1976

Quantitative determination of underivatized anti-convulsant drugs by high resolution gas chromatography with support-coated open tubular columns.

A method is described to prepare routinely high resolution support-coated open tubular (SCOT) columns. A siliceous support material (Cab-O-Sil) is deactivated with benzyltriphenylphosphonium chloride and deposited on the inside wall of a glass capillary column. After additional coating with a polar stationary phase (OV-225) a number of anticonvulsant drugs can be analyzed without prior derivatization. The columns described show high plate numbers and do not deteriorate in use. The repeatability of the gas chromatographic analysis is better than 1.2% (Orel). The minimum detectable quantity is of the order of 10(-10) g. An improved procedure for isolating the drugs from serum is given, which results in an extraction recovery of better than 90% for the drugs of interest.

Anticonvulsants↗
PubMed1975

The uptake and elimination of halothane in dogs: A two or multicompartment system? II: Evaluation of wash-in and wash-out curves.

In 14 mongrel dogs wash-in and wash-out curves for halothane were obtained from measurements of arterial blood, mixed venous blood and end-tidal gas. From a certain point in the wash-in period the halothane concentration of arterial blood can be calculated from the halothane concentration in end-tidal gas and the blood-gas partition coefficient for halothane. Analysis of the curves shows that they are composed of two exponential components suggesting the presence of two body compartments. The question of a two-compartment system versus a multi-compartment system in discussed.

Animals↗