Precision preparing volatile samples

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

HPLC analysis for pharmaceutical preparations of alkaloids started about 30 years ago. Gas chromatography (GC) is suitable for volatile compoxmds and requires laborious derivatization. Thus, GC is unsuitable to analyze the bisindole alkaloids due to their high boiling point. HPLC can provide better analytical precision and higher sample loading capacity. HPLC methods coupled to UV and MS detection have been applied for the determination of the active compounds in Catharanthus roseus. 

Y-Ray powder samples were prepared in 0-3 mm. quartz capillaries in the manner usual for volatile material and powder photographs were taken at various temperatures. Higher temperatures were achieved by blowing hot air over the capillary mounted on a standard 14-32 cm. diam. General Electric Precision camera. The temperature was controllable to 1°, and was recorded throughout the ex-... 

HRGC/PID/FID and GC/PI-IMS are methods that have been used to measure the volatile aromatic components of gasoline in soil (Eiceman et al. 1987 Roe et al. 1989). Sample preparation is simple because the static headspace method is used. The use of serial detectors (PID/FID) with HRGC enhanced selectivity. No recovery data or detection limits were reported for HRGC/PID/FID however, precision was good (2-8% RSD) (Roe et al. 1989). PID has a high selectivity to aromatic hydrocarbons (Eiceman et al. 1987). The combination of PI with IMS (PI-IMS) provided a second basis for resolution of chemical information and thus enhanced selectivity (Eiceman et al. 1987). Reproducibility for the GC/PI-IMS method ranged from 10 to 60% with a detection limit of 18 mg/kg (Eiceman et al. 1987). 

Dry ashing of cmde oils can cause serious loss of ash or elements through volatility of some metals, even in the presence of metal-retaining compounds. The methods using microwave acid digestion or bomb combustion are suitable for sample preparation for most trace metal analysis because they are retained in solution. This includes those that are volatile. Unfortunately, these methods are time-consuming and can be erroneous, and require experience skilled operators, but are necessary because they are precise, accurate and quantitative. 

One of the major advantage of HPLC is its abihty to handle compounds of limited thermal stability or volatility, therefore avoiding derivatisation procedures. The separation times are usually short, varying between 5 to 10 minutes for each run. Usually sample preparation is minimal. The technique is precise, normally quantitative, can be very sensitive and is usually nondestructive. HPLC columns can be used many times without regeneration, and the resolution achieved on such columns far exceeds that of the older... 

AAS is the most widely used analytical technique for the determination of lead in biological materials [57,58], The majority of AAS methods employ the electrothermal atomic absorption spectrometry (ETAAS) technique, using either Zeeman background correction or deuterium background correction for the determination of lead in biological fluids [55,59-65], Urine is less often employed as an indicator of exposure however, similar problems associated with AAS determination of lead exist for blood as well as urine (1) incomplete atomization (2) volatile lead salts (3) spectral interferences (4) buildup of carbonaceous residue reducing sensitivity and precision. These analytical problems are eliminated by optimal sample preparation, e,g., dilution, addition of matrix modifiers, deproteinization, and background correction and calibration by matrix-matched standards [66],... 

Membrane introduction mass spectrometry (MIMS) is a state-of-the-art technique that combines the quick separation of volatile analytes from complex matrices using selective membranes with the precision offered by mass spectrometry (MS) on chemical identification and quantification [1], Compared with gas or liquid chromatography (GC or LC) traditionally used in front of MS, the membrane separation technique has the advantages of being both simple, which minimizes sample preparation, and rapid, which makes real-time monitoring possible [2], The use of a mass spectrometer as the detector also makes MIMS less subject to analytical interference, a frequent limitation of non-MS-based techniques such as calorimetry [3,4],... 

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