Capillary injection, problems with

Typically splitless injection is used for trace analysis by capillary GC. Splitless injections can exhibit problems with carryover, poor repeatability, and labile analytes. Penton (1991) reports improved results with the temperature-programmable injector. With a temperature-programmable injector, samples are injected into a glass insert at an injector temperature below the boiling point of the analysis solvent the injector temperature is then rapidly programmed to a higher value. Penton reported this technique offered greater ease of optimization and improved precision. 

Only a small injection volume can be applied to a CE separation capillary. This seriously limits the achievable concentration detection limits. Although various approaches have been developed to circumvent this problem, it effectively limits the use of CE-MS in real-life applications, although the technique has demonstrated its potential in several specific areas, e.g., the study of protein glycosylation [97]. Online SPE-CE removes the problems with the small injection volume. In a direct comparison between SPE-CE-MS and SPE-LC-MS on the same ion-trap instmment [98], LC-MS was 5-fold more serrsitive, but CE-MS provided better sequence coverage, especially for larger tryptic peptides. 

In order to avoid problems with sample inhomogeneity, the entire oil sample from each sample of shale was dissolved in 1.5 to 2.5 mL of CS2 (about 1 g oil to 1.5 mL solvent). One pL of this solution was injected into a Hewlett-Packard Model 5880 Gas Chromatograph equipped with capillary inlet and a 50 m x 0.25 mm Quadrex "007" methyl silicone column. Injection on the column is made with a split ratio of approximately 1 to 100. The column temperature started at 60°C and increased at 4°C/min to 280°C where it remained for a total run time of 90 min. The carrier gas was helium at a pressure of 0.27 MPa flowing at a rate of 1 cm /min. The injector temperature was 325°C and the flame ionization detector (FID) temperature was 350°C. Data reduction was done using a Hewlett-Packard Model 3354 Laboratory Automation System with a standard loop interface. Identification of various components was based on GC/MS interpretation described previously (4). For multiple runs on the same shale, the relative standard deviations of the biomarker ratios were about 10%. 

Using capillary liquid chromatography with microelectrospray and quadra-pole mass spectrometry can result in fewer problems with dialysate salts [22]. Using a perfusion medium of a mock artificial cerebral spinal fluid (KCL, NaCL, MgCl2, CaCl2, and phosphate-buffered saline at pH 7.4) analysis of a peptide in the rat brain was performed with samples collected every 30 min. The perfusate was then direcdy loaded on-line from the injection valve sample loop to a separation capillary (Fig. 11). 

Several techniques have been used to overcome the problem of low column loadings on capillary columns. Capillary columns have been used after preconcentration of the alkylderivatives on a wide-bore fused-silica column or by solid-phase microextraction (SPME). " Large volume injection techniques have been applied on capillary columns coated with 0.25 /rm DB-5. Multicapillary GC (MCGC) (919 capillaries, 1 m X 40/rm i.d. coated with 0.2/rm SE 30 stationary phase (Alltech)) coupled to allows column loadings and carrier gas flow... 

Thermal desorption Method development was done with sample amounts of 50-100 mg. The use of larger sample amounts can overload the injection system and the capillary column. Drying of the sample prior to thermal extraction is necessary because problems with humidity content of the soil and other matrix effects can happen. Variations of the parameters desorption time and desorption temperature lead to an optimized method. The calibration curve in the range from 1-100 ppm shows a good linearity with relative standard deviations of about 5-7% (Fig. 11). These values are higher compared to liquid extraction due to the inhomogeneity of the soil samples. An additional homogenization of the samples reduced the relative standard deviation to about 2-3%. 

The state of the art in capillary SFC is the application of 50 pm i.d. capillaries. As shown above [equation (2.3)], a smaller i.d. would be required to compensate for the relatively slow diffusion compared with that obtained in gases. However, practical considerations presently restrict the use of such capillaries. There are three main factors involved here difficulties with injecting with the precision necessary for quantitative analysis, difficulties with preparing the separation capillaries and difficulties with detection. A general feature of miniaturization is that, unless one has very sensitive detection available, for example fluorescence detection, the linear range will decrease when going to smaller dimensions. This is a result of overloading problems. The consequence of the difficulties with the small i.d. columns is that open tubular SFC presently cannot be utilized to its full potential. 

Ideally such a formulation should be universally applicable. This was achieved by the invention of the drug nanocrystals. " With the nanocrystals one went one step beyond micronization in fact to "nanonization". Drug nanocrystals dissolve very fast and thus they can overcome oral bioavailability problems in which the dissolution velocity is the rate limiting step for absorption (e.g. drugs of class II of the Biopharmaceutical Classification System (BCS)). Meanwhile five products for oral administration are on the market. Because of their small size drug nanocrystals can also be injected intravenously. With a size below 1 fim, typically between 200-600 nm, they are much smaller than the smallest blood capillaries being in the range of 5-6 tm. Intravenous products are under development. 

Split injection is very simple and the most common of the capillary injection techniques. The highest resolution and system efficiencies are obtained with split injections (Fig. 2). Split injections are used for highly concentrated samples with typical per-component concentrations of 0.1-10 pg/pl. Injection volumes of 0.5-2 pi are normally used, but volumes up to 5 pi can be used without significant problems. Split injection is a vaporization... 

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