Combined liquid chromatograph/mass spectrometer for involatile biological samples

A new liquid chromatograph/mass spectrometer has been developed in our laboratory for application to analysis of biological molecules of extremely low volatility. Oxyhydrogen flames rapidly vaporize the total liquid-chromatographic effluent, and molecular and particle beam techniques are used to efficiently transfer the sample to the ionization source of the mass spectrometer. This new instrument is comparable in cost and complexity to a combined gas chromatograph/mass spectrometer, but extends the capabilities of combined chromatography/mass spectrometry to a broad range of compounds not previously accessible. We are currently testing biologically significant samples with this instrument, using reversed-phase liquid-chromatographic separation and both positive and negative ion chemical-ionization mass spectrometry. Results have been obtained from mixtures of nucleic acid components--bases, nucleosides, and nucleotides--and from amino acids, peptides, saccharides, fatty acids, vitamins, and antibiotics. In all cases investigated to date, ions indicative of molecular mass are obtained in at least one of the operating modes available. Detection limits are typically in the 1-10 ng range for full mass scans (about 80-600 amu); sub-nanogram quantities are usually detectable with single-ion monitoring.

Liquid chromatography with amperometric reaction detection involving electrogenerated reagents: applications with in-situ generated bromine

We describe the use of electrogenerated reactants for continuous on-line reaction detection with thin-layer hydrodynamic amperometry. The reagent is introduced into the liquid-chromatographic column effluent at a constant rate by using controlled-current electrochemistry. After the effluent passes through a short reaction coil, the reagent concentration is monitored at the detector. Reaction of eluted compounds with bromine is signalled by changes in the current detected. The direct electrochemical control of the reagent concentration allows changes to be made, even during the course of obtaining a chromatogram. Depending on the specific reagent or reaction, the reagent is supplied either by addition of a second stream or by direct generation in the mobile phase. The latter configuration provides sufficient baseline stability to permit detection of the uptake of as little as 10 pmol of reagent bromine. The technique has been used to detect nanograms of underivatized fatty acids, prostaglandins, and phenols after separation by reversed-phase liquid chromatography.

Positive identification and quantitation of isomeric cocaines by high-performance liquid chromatography

A high-performance liquid chromatographic procedure for separating and identifying the four isomeric cocaines has been developed. Use of this procedure with an internal standard allows for the determination of the quantity of any isomeric cocaine in an unknown sample. The pitfalls and problems encountered in the use of gas chromatography and mass spectrometry in the analysis of cocaines and ecgonine methyl esters are discussed.

Quantitative high-performance liquid chromatographic determination of delta 4-3-ketosteroids in adrenocortical extracts

A high-performance liquid chromatographic method is described for the determination of seven steroids in adrenocortical extracts showing a delta 4-3-ketonic conjugated system. The seven steroids (cortisol, cortisone, 11-dehydrocorticosterone, corticosterone, 11-deoxycortisol, aldosterone and 11-deoxycorticosterone) were separated with a chloroform-methanol gradient on a 5-micron silica column and with a water-acetonitrile gradient on a 10-micron RP-8 column. Effluents were monitored by UV absorption at 242 nm. Quantitative analysis was performed by comparing peak areas, which are proportional to the amounts of the individual substances (external standard method). The method is rapid, sensitive, easy to perform and reproducible.

Interferences appearing in fluorometrically measured liquid-chromatographic profiles of creatine kinase isoenzymes in serum

We observed nonenzymic peaks when serum isoenzymes of lactate dehydrogenase (EC 1.1.1.27; LD) and creatine kinase (EC 2.7.3.2.; CK) were separated by "high-performance" liquid chromatography and detected by continuously monitoring the column effluent for enzyme activity. Such background peaks were particularly apparent in CK isoenzyme profiles obtained from human sera. We observed two nonenzymic peaks with fluorescence detection, one in the CK-MB region, the other in the CK-BB region. Serum albumin was a major component in the artifactual CK-MB peak, with lipoprotein as a minor component. We present evidence that the material responsible for the other peak fluoresced quite strongly and is mostly pre-albumin.

High-pressure liquid chromatography for quantitation of antimicrobial agents

Several methods are currently available for quantitation of antimicrobial drugs in serum and body fluids, e.g., microbiological, radioenzymatic, and radioimmune assays. Recently, high-pressure liquid chromatography (HPLC) has been implemented for quantitation of a variety of drugs, including antimicrobial drugs. The general principles of chromatography, particularly in reference to HPLC, are reviewed. The methods and procedures for extraction, separation, detection, and quantitation of antimicrobial drugs by HPLC are discussed. Current HPLC techniques for measuring antimicrobial drugs in biological fluids are also summarized.

Application of an automated extractor/concentrator to the analysis of anticonvulsant drugs in blood serum by high-performance liquid chromatography

An automated extractor/concentrator has been used to extract a variety of anticonvulsant drugs from blood serum in a concentration range from 0.075 to 80 microgram/ml. The volume of serum required varies from 0.1 to 2.0 ml, depending on the concentration of drug. The average absolute extraction recovery is 90% larger; recoveries relative to an internal standard are greater than 95%. The serum extracts were analyzed by high-performance liquid chromatography on a reversed-phase column with an acetonitrile and phosphate buffer mobile phase. Comparison of results with gas chromatographic and immunoassay techniques was excellent.

Determination of morphine and its major metabolite, morphine-3-glucuronide, in blood by high-performance liquid chromatography with electrochemical detection

The construction of an electrochemical detector flow cell for high-performance liquid chromatography from stainless steel, which also serves as the auxiliary electrode, is described. Glassy carbon is used for the working electrode in a novel configuration based on a hybrid of the wall-jet and thin-layer arrangements. Application of the detector to the determination of morphine in small blood samples is described. The metabolite, morphine-3-glucuronide, is also determined by hydrolysis with beta-glucoronidase prior to extraction. The detector is capable of detecting less than 1 ng of morphine injected.

Determination of dopa, dopamine, dopac, epinephrine, norepinephrine, alpha-monofluoromethyldopa and alpha-difluoromethyldopa in various tissues of mice and rats using reversed-phase ion-pair liquid chromatography with electrochemical detection

A method for the determination of catecholic amino acids and amines by reversed-phase ion-pair high-performance liquid chromatography with electrochemical detection has been developed. B using octanesulfonic acid for ion pairing and by optimising ionic strength, pH and methanol concentration of the mobile phase, separation was achieved of 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxyphenylacetic acid (DOPAC), norepinephrine (NE), epinephrine (EPI), and dopamine (DA). Alpha-Difluoromethyldopa (DFMD) and alpha-monofluoromethyldopa (MFMD), two potent enzyme-activated irreversible inhibitors of aromatic amino acid decarboxylase were also separated from the natural catechols. Concentrations of catechols and inhibitors were measured in brains, hearts and kidneys of mice treated with small repeated doses of MFMD. The method has also been applied to the determination of catechols in other organs such as prostates and seminal vesicles of rats and in smaller tissues like mesenteric arteries. A semi-automated procedure making use of an automatic sample processor and a digital integrator permitted the analysis of as many as sixty samples per day.

New developments in analysis of isoenzymes separated by "high-performance" liquid chromatography

We have developed two enzyme analyzers for use in "high-performance" liquid chromatography. In both systems two detectors are used, placed after the column effluent has been combined with assay reagent. In one system, an absorbance detector is placed before and after a post-column reaction coil. Peaks observed at one detector are subtracted from those at the other, to produce a two-point measurement of enzyme activity. The linear dynamic range was 17--1700 U/L for lactate dehydrogenase (EC 1.1.1.27). In the other system, two reaction coils were used and a single fluorescence detector was placed at the end of each coil. These coils were kept at different temperatures, and an automated switching valve diverted equal amounts of column effluent and reagent into both coils. The fluorescence readings were then subtracted to produce a differential measurement of enzyme activity. The linear dynamic range was 20--1000 U/L. We used both systems to chromatographically analyze lactate dehydrogenase isoenzymes, and could separately determine both the distribution and activity of sample isoenzymes.

Reverse phase ion pair high pressure liquid chromatographic determination of ethylenediaminetetraacetic acid in crabmeat and mayonnaise

A method is described for the determination of ethylenediaminetetraacetic acid (EDTA) in crabmeat and mayonnaise. EDTA is extracted from the food sample with water and converted to its copper chelate, which is then quantitated by reverse phase ion pair high pressure liquid chromatography with ultraviolet detection. Maximum sensitivity is obtained with detection at about 254 nm; higher wavelengths may be used for enhanced specificity. Cleanup procedures for crabmeat and mayonnaise were improved by using a radiotracer method. Analyses of crabmeat and mayonnaise samples spiked at 3 different levels showed greater than 90% recovery of EDTA.

Dual-detector-post-column reactor system for the detection of isoenzymes separated by high-performance liquid chromatography. I. Description and theory

We describe a dual-detector-post-column chromatographic reaction detector system that corrects for substances present in biological samples that interfere with the measurement of isoenzymes separated on a chromatographic column. The response observed at the detector in front of the reaction coil is mathematically dispersed, time transformed and subtracted from the detector behind the coil to produce a blank corrected chromatogram. The same computer program calculates peak areas and other chromatographic parameters such as height equivalent to a theoretical plate and retention time. In addition, we have evaluated the dispersion effects caused by various changes in our experimental system.

Measurement of testosterone with a high-performance liquid chromatograph equipped with a flow-through ultraviolet spectrophotometer

A technique is described for measuring nanogram amounts of testosterone using high-performance liquid chromatography with detection by a flow-through spectrophotometer. The addition of a non-radioisotopic internal standard (4-androsten-11 beta-ol-3,17-dione) to the biological specimen automatically corrects for testosterone losses due to extraction and non-quantitative sample injection into the high-performance liquid chromatograph. This new method, which can be performed by inexperienced personnel, is shown to be rapid, precise, accurate and specific for testosterone.

Evaluation of the characteristics of the differential amperometric detector in combination with anion-exchange chromatography, using L-ascorbic acid as test compound

The hydrodynamic characteristics of the thin-layer flow cell of the differential amperometric detector were deduced theoretically and confirmed by experiment. The response of the differential amperometric detector was compared with that of a fixed-wavelength UV detector in the analysis of L-ascorbic acid in combination with anion-exchange chromatography.

The screening and quantitation of diazepam, flurazepam, chloridazepoxide, and their metabolites in blood and plasma by electron-capture gas chromatography and high pressure liquid chromatography

A combined GLC-ECD and HPLC procedure has been developed for the analysis of the most commonly encountered benzodiazepine drugs and has been applied to both plasma and postmortem blood samples. There is no doubt that since their introduction the use of these sensitive analytical methods have resulted in an increase in the incidence of detection of these drugs in both clinical and forensic toxicology cases.

Reverse-phase HPLC of DNA restriction fragments and ribooligonucleotides on uncoated Kel-F powder

Uncoated Kel-F powder offers some unique features as a support for reverse-phase HPLC of oligonucleotides and DNA restriction fragments. Compounds are eluted from the column by a gradient of acetonitrile (0 tto 18% v/v) in 0.1 M aqueous triethylammonium acetate. In contrast to RPC-5 chromatography, oligonucleotides are not eluted by aqueous salt solutions alone, and the separation of restriction fragments depends only on the chainlength. The packing material is cheap, easy to pack, chemically inert, and does not bleed, so that separations are highly reproducible. The DNA loading capacity for Kel-F is presently inferior to RPC-5, but recovery of microgram amounts of material is typically better than 50%.

Assay of antipyrine and its primary metabolites in plasma, saliva and urine by high-performance liquid chromatography and some preliminary results in man

A reversed phase system for the HPLC separation of antipyrine and its primary metabolites is described. Based on this system an assay procedure for antipyrine in plasma and saliva was developed with a lowest measurable concentration of 25 ng/ml and precision of +/- 3.6 and +/- 4.5%, respectively. Furthermore, assays for the parent compound, 3-hydroxymethyl-antipyrine, norantipyrine and 4-hydroxy-antipyrine in urine were developed. The lowest measurable concentration for these compounds is about 100 ng/ml except for 3-hydroxymethyl-antipyrine with a lowest measurable concentration of about 200 ng/ml. The precision was established at +/- 3.6 and +/- 5.0% for 3-hydroxymethyl-antipyrine, and antipyrine, respectively, and +/- 7.0 and +/- 3.6% for norantipyrine and 4-hydroxy-antipyrine, respectively. The method was applied to studies on antipyrine metabolism in humans. Following administration of a single dose of 500 mg antipyrine to 5 healthy volunteers, 3.3 +/- 1.2% of the dose was recovered from 48-hour hydrolyzed urine as unchanged drug, 39.7+/- 8.7% as 3-hydroxymethyl-antipyrine, 14.5 +/- 6.8% as norantipyrine and 28.5 +/- 2.2% as 4-hydroxy-antipyrine.

High-performance liquid chromatographic investigation of the amino acid, amino sugar and neutral sugar content in glycoproteins

A methods for the simultaneous separation and determination of amino acids, amino sugars and neutral carbohydrates is described. Stepwise elution systems with sodium citrate and borate buffers have developed for the ion-exchange liquid chromatographic separation of amino acids and sugars, using 8-micrometer particle size resins and the Stein and Moore and orcinol colorimetric method for detection. With the aid of this system, the direct quantitative comparison of sugars and amino acids by liquid chromatography becomes possible for the first time.

High-performance liquid-chromatographic assay of aminoglycoside antibiotics in serum

We describe procedures for the high-performance liquid-chromatographic assay of gentamicin, amikacin, and tobramycin in serum. The aminoglycoside antibiotic is extracted from serum by using a CM-Sephadex column and is measured by reversed-phase, ion-pair chromatography. Continuous-flow, post-column derivatization with o-phthalaldehyde is used to form fluorescent products for detection. An internal standard is used in each assay. Synthesis of 1-N-acetylgentamicin C1, the internal standard for the gentamicin assay, is described. For each procedure, concentration and instrument response are linearly related in the range of therapeutically important concentrations. The methods are precise and results correlate well with results obtained by bioassay. The between-day CV (n = 10) for each procedure was less than 4%. No substances have been detected in clinical specimens that interfere with these chromatographic procedures. The possible use of these methods for analysis for other aminoglycoside antibiotics is discussed.

Semiautomated system for high-pressure liquid chromatographic determination of dissolution rate of fludrocortisone acetate tablets

A new semiautomated high-pressure liquid chromatographic (HPLC) system is described to determine the dissolution rate of fludrocortisone acetate tablets. The system uses a miniaturized dissolution basket and shaft assembly having the same geometry as that given in USP XIX. This reduced size permits use of smaller volumes of dissolution medium, allowing most very low dose oral solid dosage forms to be handled. The USP dissolution kettle was also replaced with a new miniaturized vessel that continuously filters the sample solution before it enters the flow system. Volumes of dissolution medium as small as 15 ml can be accommodated, depending on the sensitivity of the assay employed and the solubility of the drug substance under study. The concentration of fludrocortisone acetate in solution was monitored by a new HPLC system employing a reversed-phase column compatible with the aqueous dissolution medium used. A comparative dissolution study of different lots was made using different basket rotation speeds.

Quantitative microanalysis of perbenzoylated neutral glycosphingolipids by high-performance liquid chromatography with detection at 230 nm

A method is described which permits the quantitative microanalysis of glycosphingolipids in the picomole range by HPLC of their perbenzoylated derivatives with detection at 230 nm. A linear gradient of 1 to 20% dioxane in hexane is used to elute from a Zipax column derivatives that contain up to four sugar residues. Residual absorption due to dioxane is negated by directing the solvent gradient through a pre-column, pre-injector high-pressure reference cell. This path generates a horizontal baseline with a negative and a positive deflection at the beginning and end of the gradient, respectively. Neutral glycosphingolipids can be quantitated either with N-acetyl-psychosine as internal standard or by comparison of peak areas to those of external standards.

Studies of microhigh-performance liquid chromatography. IV. Application of the micro pre-column method to the analysis of corticosteroids in serum

The micro pre-column method, which is a useful method for the pre-treatment of samples, was applied to the analysis of corticosteroids in serum. Corticosteroids were concentrated in a micro pre-column packed with porous styrene-divinylbenzene copolymer particles and separated in a micro separation column packed with porous silica particles. Concentrations of corticosterone, cortisone and cortisol in the 20-130 ppb range were determined individually by pre-treatment of only 0.2 ml of serum.

Development of an electrochemical detector for high-performance liquid chromatographic assay of brain catecholamines

An electrochemical detector that utilizes the reduction-oxidation phenomena of electrochemically active substances has been developed for high-performance liquid chromatography. The construction and application of the detector to brain catecholamines, particularly for norepinephrine and dopamine analysis, are described. The differences between this detector and those reported previously are discussed.

Simultaneous and specific determination of proxyphylline, theophylline and other xanthine derivatives in serum by high pressure liquid chromatography

A HPLC-method for rapid, simultaneous and specific determination of proxyphylline, theophylline, glyphylline, theobromine, caffeine and 3-methylxanthine using 8-chlorotheophylline as internal standard is presented. Using 20 microliter deproteinised serum samples for injection, the the detection limits for theophylline and proxyphylline are about 0.040 and 0.055 microgram per ml serum or plasma, respectively, and the corresponding sensitivities are about 0.033 and 0.048 microgram compound for 0.0044 absorbance units at 280 nm. In duplicate analyses the coefficient of variation is less than 3%. A number of frequently used drugs did not interfere in the analyses of serum samples from patients under treatment or in experiments with drug additions to serum samples.

Design and evaluation of an automated system for in vitro dissolution testing utilizing a high-pressure liquid chromatographic multiport switching valve

An automated system for the simultaneous dissolution testing of six samples was developed consisting of four basic units: dissolution vessels and stirring unit, a peristaltic pump, a rotary stream multiport switching valve and programmer, and a UV spectrophotometer with recorder. Among the major advantages of such a system are: (a) paddle or basket stirring with variable speed is used, (b) the tablet or capsule (wire coil required) locates reproducibly at the bottom of a round-bottom reaction flask when utilizing paddle-type stirring, (c) a USP basket for tablet or capsule dissolution testing can be used, (d) continuous or intermittent sampling is possible, (e) the flow system readily adapts to UV-visible detector or fluorescence spectroscopy, (f) the system readily adapts to automated determination of the intrinsic dissolution of a material, and (g) the cost is low because of the multiport switching valve and inexpensive UV monitor required. Studies were performed using this apparatus to demonstrate the response characteristics of the system, its reproducibility, potential problems, and precautions required. This dissolution system was used to determine the dissolution characteristics of a new steroid tablet formulation, including a formulation and lot demonstrated to be bioavailable.

Reverse phase high speed liquid chromatography of antibiotics. II. Use of high efficiency small particle columns

Improved methods for the separation and quantitation of cephalosporins, penicillins, tetracyclines and several miscellaneous antibiotics by reverse phase high speed liquid chromatography are presented. The methods have been improved significantly by the substitution of high efficiency, small particle (approximately 10 micrometer reverse phase columns in place of the previously used medium efficiency, pellicular columns. The conditions and procedures described here illustrate that considerable improvements in separation and sensitivity of detection of antibiotics are achieved. Pure compounds, complex mixtures of antibiotics in a variety of dosage forms and fermentation broths are routinely analyzed by the described procedures.

Recent developments in the clinical assessment of the metabolism of aromatics by high-performance, reversed-phase chromatography with amperometric detection

High-performance liquid chromatography with electrochemical detection has developed into a tool with excellent capability to monitor picomole amounts of individual metabolites in both tissue and body-fluid specimens. Significant technical advances in the area of microparticle chemically-bonded stationary phases have led to dramatic improvements in both sensitivity and resolution. Reversed-phase systems can be modified to include charged exchange sites by addition of detergents to the mobile phase. Adjustment of the surface charge in this manner permits capacity factors for ionic sample components to be increased or decreased. This concept is quite compatible with electrochemical detection and has provided the foundation for several clinical assays now in routine use. Examples of additional applications are presented for the determination of catecholamines in tissue, 3-methoxy-4-hydroxyphenylglycol in urine, and dopamine-beta-hydroxylase activity in human serum.

Detection of carbofuran and metabolites directly or as their heptafluorobutyryl derivatives using gas-liquid or high-pressure liquid chromatography with different detectors

The gas chromatography of the heptafluorobutyryl derivatives of carbofuran, 3-ketocarbofuran and 3-hydroxycarbofuran is examined with electrolytic conductivity detection (halogen mode). The reaction consists of heating the compounds with heptafluorobutyric anhydride in the presence of trimethylamine catalyst. Although as little as 200 pg of carbofuran, 400 pg of 3-ketocarbofuran and 100 pg of 3-hydroxycarbofuran can be detected by electrolytic conductivity, the minimum detectable quantities by electron capture are about 20-fold less for each derivative. Application to the analysis of carbofuran and 3-ketocarbofuran in corn, potato, turnip and wheat is carried out. This technique is compared to direct gas chromatography in the nitrogen mode and to direct high-pressure liquid chromatography with UV detection at 254 nm using extracts of field-treated turnips.

Sampling and extra-column effects in high-performance liquid chromatography; influence of peak skew on plate count calculations

Chromatographic resolution and analysis accuracy can be seriously affected by broadening of very sharp bands associated with both high-performance size-exclusion chromatography and other liquid chromatography methods using less than 5 micron particles. The effect on column plate count by extra-column band broadening from injection systems, detectors, connectors and "guard" columns is discussed and exemplified with experimental data. The special cases of tailing and doublet peaks for a single species are investigated. These effects are eliminated by a special sample injection technique. Computer simulation studies demonstrated that serious errors can result from calculating column plate count with non-Gaussian peaks. The maximum allowable peak tailing is suggested and a new method for quantitating peak skew is proposed.

The quantitative determination of metabolites of 6-mercaptopurine in biological materials. II. Advantages of a variable-wavelength HPLC spectrophotometric detector for the determination of 6-thiopurines

The advantages of a variable-wavelength spectrophotometric detector for use in high-performance liquid chromatography are demonstrated with the detection of 6-mercaptopurine metabolites in cell extracts. 6-Methylthiopurines, unsubstituted 6-thiopurines and 6-thioguanines are most sensitively detected at 291, 322, and 342 NM, respectively. Compared with detection at 254 nm, the sensitivity at these wavelengths is about one to two orders of magnitude greater. Furthermore, in the 291-355-nm range, common purines, which are normally contained in cell extracts as free bases, nucleosides and nucleotides exhibit only minute absorbances and so do not interfere in quantitative determinations of 6-thiopurine compounds.

Rapid separation of urinary acids by high-performance liquid chromatography

Over a hundred acidic urinary constituents were separated within 30 min by using 5-micron octadecyl-silica columns and gradient elution with increasing acetonitrile concentration in dilute aqueous phosphoric acid solution at 70 degrees. The column effluent was monitored with a UV detector at 280 nm or with a fluorescence detector at 260 nm excitation and 340 nm emission wavelengths. The high sensitivity and speed of analysis, the excellent reproducibility and adequate resolution obtained suggest that this technique may be useful to obtain metabolic profiles in routine clinical work.

Determination of urinary normetanephrine, metanephrine, and 3-methoxytyramine by liquid chromatography, with amperometric detection

We describe a sensitive procedure for measuring the basic catecholamine metabolites--normetanephrine, metanephrine, and 3-methoxytyramine--in urine by use of liquid chromatography, with electrochemical detection. These substances are isolated from hydrolyzed urine by passage through small ion-exchange columns and then pre-concentrated by a rapid set of solvent extractions before being injected onto the liquid-chromatographic column. Concentration and instrumental response are linearly related to 2.0 mg/liter for all three compounds, and practical lower detection limits are about 20 micrograms/liter for actual urine samples. The high selectivity that accrues from the combination of the extraction procedure, high-performance liquid chromatography, and electrochemical detection makes this procedure suitable for quantitative studies of catecholamine metabolism. A set of six samples can be taken through the extraction procedure in parallel in less than 2 h; once prepared, the extracts can be analyzed at a rate of three per hour.

Positive peak identification in liquid chromatography using absorbance ratioing with a variable-wavelength spectrophotometric detector

It is frequently necessary to confirm the identification of component peaks isolated in high-performance liquid chromatograms. The UV spectrum of the pure compound is usually a very powerful signature for most compounds. For liquid chromatographic instruments that permit stop-flow operation, a simple approximation is obtained with a variable wavelength UV detector by determining the absorbance ratios at several specific wavelengths to characterize the compound. If the wavelengths are well chosen, these absorbance ratios provide a very specific and a very reliable technique for peak identification. The absorbance ratios are independent of the concentration of the compounds in the detector flow cell. Identity or purity of individual peaks can be confirmed when absorbance ratios are taken from the peaks of individually injected pure standards. Flow can be stopped several times on the leading and trailing edges of broad peaks, and ratios calculated. If the ratios are identical, the peak can be considered to be pure; if ratios differ, the presence of unresolved components can be suspected. Stop-flow ratioing can be performed during gradient elution: standards are injected under rapid elution conditions and the resulting ratios are compared with those of the peaks from the gradient run. Ratios are found to be identical for similar components, regardless of the differences in retention time, thus providing rapid identification of the compounds in the gradient chromatogram. Factors that influence the overall precision of the method are: ability to stop and re-start eluent flow without loss of chromagraphic efficiency, ability to obtain numerical absorbance data from the detector, the wavelength repeatability of the detector, and the requirement that the detector cell design allow slow interruptions without baseline upset.

Enzyme-selective detector systems for high-pressure liquid chromatography

This paper describes the performance of absorbance and fluorescence detectors in two configurations of post-column reactors for selectively detecting and quantitating isonenzymes eluted form a high-pressure liquid chromatography column. Superb resolution of the isoenzymes of lactic dehydrogenase is illustrated by the use of new ion-exchange materials.

A continuous-flow enzyme detector for liquid chromatography

A detection system has been developed for the selective and sensitive detection of enzymes eluting from a liquid chromatographic column. This system monitors a reaction that the enzyme catalyzes and provides a chemical amplification ranging from 10(4) to 10(5). The detection system consists of a reagent or substrate pump, a post-column reactor packed with non-porous spherical glass beads, and a photometric detector. A linear and selective response to a series of enzymes of clinical importance is demonstrated.