Mixture quality, detectors

The overall performance of a separation method is intrinsically linked to that of the detector used as part of the system. A detector is a device that monitors, in the dimensions of space or time, the presence of the components of a mixture that has been subjected to a chromatographic process. The detection methods provide evidence concerning the quality of the separation and serve especially to increase sensitivity and selectivity. The quantitative aspects of chromatographic analyses are dependent upon the detector capabilities. 

There are many GC detectors available although the flame ionisation detector remains the most widely used and the most widely applicable to quality control of pharmaceutical products. However, newer detectors such as the plasma emission detector for analysis of trace impurities or the GC-FTIR detector for the structural characterisation of components in mixtures are becoming increasingly important. 

In its broadest terms the discussion of HPLC detection for chiral species must include the analysis of mixtures with achiral substances as well as the quality testing of, for example, the enantiomeric purity of a chemically pure drug form. The distinction between the definitions of chemical purity versus optical purity can not be overemphasized. In an efficient chiral HPLC system the latter problem is trivial, and if retention times are significantly different then any conventional detector such as RI, electrochemical, absorption, etc., could be used. Co-elutions are a major experimental concern in separations of mixtures and at this juncture it is not only prudent but absolutely necessary to involve a chiroptical detector to preferentially identify the chiral analyte. 

The sequence, position, and distribution of separated components contain a good deal of information on the mixture. If properly measured and interpreted, this can serve many analytical goals without further tests. The quality of this information naturally improves as the system is better understood, characterized, and controlled. Informational content is greatest when, through theory and/or calibration, one can identify zones or peaks located at defined positions in the sequence with specific molecular species At that point, using a suitable sensor (detector), both qualitative and quantitative analyses follow. One can, at the same time, often measure certain physicochemical constants for the components, such as partition coefficients and diffusion constants. 

Panderi and Parissi-Poulou developed a microbore liquid chromatographic method for the simultaneous determination of benazepril hydrochloride and hydrochlorothiazide in pharmaceutical dosage forms [30]. The use of a BDS C-18 microbore analytical column was found to result in substantial reduction in solvent consumption and in increased sensitivity. The mobile phase consisted of a mixture of 25 mM sodium dihydrogen phosphate buffer (pH 4.8) and acetonitrile (11 9 v/v), pumped at a flow rate of 0.4 mL/min. Detection was effected at 250 nm using an ultraviolet absorbance detector. The intra- and inter-day relative standard deviation values were less than 1.25% (n = 5), while the relative percentage error was less than 0.9% (n = 5). The detection limits obtained according to the IUPAC definition were 0.88 and 0.58 pg/mL for benazepril hydrochloride and hydrochlorothiazide, respectively. The method was applied to the quality control of commercial tablets and content uniformity test, and proved to be suitable for rapid and reliable analysis. 

FIGURE 10-1. Additivity of plates on a totally porous packing. Mobile phase 5% CH2C12 in isooctane (50% water saturated). Flow rate 2 mL/min. Detector UV, 254 nm, 0.5 AUFS. Sample Mixture of aromatics, 10 fiL. Column Porasil A, 2 mm ID x 122 cm. (Note Actual separation will depend upon quality of mobile phase and column packing.)... 

FIGURE 11-2. Separation of mixture of phthalates by GPC on a 100-A Ultrastyragel column. Analytical loadings are shown in (a) and preparative loadings are shown in (b). Mobile phase THF. Flow rate 1 mL/min. Detector UV at 254 nm, 0.5 AUFS in (a) and 2.0 AUFS in (fe) RI at X8 in (a) and X64 in (b). Sample dioctyl, dibutyl, and dimethyl phthalates with 50 /xL injection in (a) and 100-/xL injection in (b). (Note Actual separation will depend upon the quality of the mobile phase and column packing). 

Mixtures of several elements or substances in solvents are available from many suppliers in certified quality. Such materials are very useful to environmental monitoring as many represent excellent materials for the calibration of instruments. Several target contaminants like PCB, PAH, PCDD and PCDF, metals etc. are available from NIST, BCR etc. Such certified materials have purity figures with small uncertainties, they can be used without any particular precaution compared to the equivalent pure substance. They are valuable tools to detect bias in calibration and allow rapid correction. Uriano and Gravatt [8] have cited an example of the use of CRMs to correct for bias in SO measurements in air and in particular how CRMs can help to correct for additive or multiplicative bias in interlaboratory studies. Massart et al. have also discussed similar effects on signals of detectors [9] and the reader should refer to them. 

The injector temperature was 260°C, the detector temperature 300°C. Analyses were run isothermally at 180 or 200°C using helium as carrier gas at a flow rate of 25 ml/min. The hydrogen flow rate for the detector was 3 ml/min and the air flow rate 60 ml/min using an attenuator setting of 2. The limit of detection by this method—as statistically defined by the American Chemical Society s Committee on Environmental Improvement (14)—was below 1 yg/liter or ppb but values below 1 ppb printed out by the instrument processor were reported as non-detects (ND). For quality control, standards were analyzed following every tenth sample. Samples of pure aldicarb, aldicarb sulfoxide and aldicarb sulfone were donated by Union-Carbide Agricultural Products Company, Raleigh, NC. Recoveries of over 90% were consistently obtained with 1, 10, and 100 ppb levels of the individual compounds or mixtures of the pure... 

Four fundamental blocks comprise a chromatographic system (1) sample application system (injector) (2) mixture separation system (column) (3) detection system responsive to the analytes eluting from the column (detector), and (4) data system. Analytical quality rests on the combined performance of all basic blocks. For example, the injection system must transfer the sample into the chromatographic column in a quantitative manner, without discrimination due to molecular weight or component volatility and without chemical alteration of any constituent substance. This entrance door into the column must remain clean, contaminant-free, inert and without leaks. Component separation is achieved when the operational parameters (temperature, mobile phase velocity or polarity, pressure) and the column characteristics (length, internal diameter, stationary phase chemical nature and thickness) are such that the distribution constants (between stationary and mobile phases) of all mixture components are different. The detector response (signal) results from the... 

Obviously, the chromatographic principles are the same in process and laboratory GCs and they are built up in a very similar way. Standard detectors are in each case the thermal conductivity detector (TCD), which is a universal detector for all components, and the flame ionization detector (HD), which is a specific detector for hydrocarbons. To detect sulfur gases selective detectors like an electrochemical detector, chemiluminescence detector and, most important, flame photometric detector (FPD) are used. Gaseous fuels hke natural gas, synthetic gases, and blends are complex mixtures that cannot completely be separated in a single column. Two or more different columns must be combined. To monitor the fuel quality a quasi-continuous analysis is necessary this means that very short cycle times must be realized. To do so, high-boiling components are removed... 

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