Detectors compound detectability

The limit of detection for this instrument is about 10 pg/ ml for polystyrene in 2-butanone,163 which is close to two orders of magnitude higher than that of the deflection-type DRI. Moreover, the response of the ELSD is linear over only two decades in concentration.163 The ELSD is a useful backup detector when the DRI or UV detectors are not appropriate, e.g., when the UV absorbance or RI change is a function of copolymer composition as well as concentration or in gradient elution systems where changes in solvent composition cause drift in baselines of the UV and DRI detectors. Compounds about as volatile as the solvent are poorly detected by ELSD. 

A stream-splitter may be used at the end of the column to allow the simultaneous detection of eluted components by destructive GC detectors such as an FID. An alternative approach is to monitor the total ion current (TIC) in the mass spectrometer which will vary in the same manner as the response of an FID. The total ion current is the sum of the currents generated by all the fragment ions of a particular compound and is proportional to the instantaneous concentration of that compound in the ionizing chamber of the mass spectrometer. By monitoring the ion current for a selected mass fragment (m/z) value characteristic of a particular compound or group of compounds, detection can be made very selective and often specific. Selected ion monitoring (SIM) is more sensitive than TIC and is therefore particularly useful in trace analysis. 

Elimination of wet chemical sample preparation enables a complete analysis to be performed and data to be quickly analyzed. The detection limits are in the low part-per-million range using mass spectrometric detection. Alternatively, detection of compounds can be achieved by all common gas chromatography detectors (flame ionization detector, electron capture detector and flame photometric detector), and detection limits are determined by the method of detection employed. 

The current individual methods differ in procedure, compounds detected, extraction techniques, and extraction solvents used. Some methods may include a cleanup step to remove biogenic (bacterial or vegetation-derived) material, while others do not. The methods have in common a boiling point type of column and a flame ionization detector. Selection of a method depends on the type of hydrocarbon suspected to be in the sample. 

FIGURE 9 Moderately rapid gradient separation. Column XTerra MS C, IS, 4.6x 20mm 3.5p.m. Gradient 0 to 100% B over 4min,A 0.1% formic acid in water, B 0.1% formic acid in acetonitrile. Flow rate 3.0mL/min. Temperature 30°C. Detection UV at 254 nm. Instrument Alliance 2795 with 996 photodiode array detector. Compounds (I) acetanilide, (2) triamcinolone, (3) hydrocortisone, (4) 2-amino-7-chloro-5-oxo-5H-[l]benzopyrano[2,3-b]pyridine-3-carbonitrile, (5) 6a-methyl-17a-hydroxyprogesterone, (6) 3-aminofluoranthene, (7) 2-bromofluorene, (8) perylene, (9) naphtho(2,3-a)pyrene. 

Liquid chromatography with electrochemical detection (LCEC) is in widespread use for the trace determination of easily oxidizable and reducible organic compounds. Detection limits at the 0.1-pmol level have been achieved for a number of oxidizable compounds. Due to problems with dissolved oxygen and electrode stability, the practical limit of detection for easily reducible substances is currently about 10-fold less favorable. As with all detectors, such statements of the minimum detectable quantity must be considered only with the proverbial grain of salt. Detector performance varies widely with the analyte and the chromatographic conditions. For example, the use of 100- m-diameter flow systems can bring attomole detection limits within reach, but today this is not a practical reality. 

To be effective, the detector must be capable of responding to concentration changes in all of the compounds of interest, with sensitivity sufficient to measure the component present in the smallest concentration. There are a variety of HPLC detectors. Not all detectors will see every component separated by the column. The most commonly used detector is the variable ultraviolet (UV) absorption detector, which seems to have the best combination of compound detectability and sensitivity. Generally, the more sensitive the detector, the more specific it is and the more compounds it will miss. Detectors can be used in series to gain more information while maintaining sensitivity for detection of minor components. 

Most alkenone determinations are made with a flame ionization detector (FID). This detector is simple, reliable, and highly sensitive. However, the FID functions essentially as a carbon detector, and does not give diagnostic information on the structure of the compounds detected. Alkenones are identified by FID by their elution times and by... 

Direct, on-line coupling of an NMR spectrometer to an HPLC has required the development of special interfaces called flow probes. These systems can basically work in two ways. One is through an on-flow mode (the solute passes through the system as it passes out of the column and is analyzed), but this process has a low sensitivity even when the HPLC column is overloaded. In another way, which is much more used nowadays, the HPLC control unit is equipped with a valve that invokes a stop flow of the effluent when a compound (detected by a UV detector that is always used in connection with these systems) leaves the column in this case, the flow is stopped in order to have enough time for the acquisition of the NMR spectrum. 

Electrochemical detectors are a very interesting example of detectors used in liquid chromatography. These detectors can detect only compounds that are able to react by oxidation or reduction reactions. Usually, this technique functions by measuring the gain or loss of electrons from the flowing sample as it passes between two electrodes maintained at an electrical potential difference. 

The refractive index detector is applicable to all compounds, although it is not as sensitive as the uv detector. An RI detector can detect differences of about 10 RI units, which means that about 5x10 g/mL must pass through the detector for a favorable response. As a general rule, the sensitivity in mg of sample is almost equal to the reciprocal of the differences in refractive index between the solvent and the sample. Figures 19-43 and 19-44, show two basic methods of refractive index measurement. 

Transformation of non-detectable trace components into compounds detectable by highly sensitive detectors, through one-step [22] and two-step [23] conversion of carbon-free compounds (10) Using the chemical... 

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