Bleed, column

Rising temperature of the GC column not only assists transport of less volatile components, it also causes the slow release of the liquid phase from the inner wall of the capillary. As a result of slow thermal degradation, even chemically bonded liquid phases show such column bleed at elevated temperature. It is a characteristic of column bleed that it continuously rises as the temperature of the GC oven is raised and it falls again upon cooling of the system. Of course, the peaks from column bleed observed in the mass spectmm depend on the liquid phase of the GC 

Example In the partial TIC obtained by GC-EI-MS of an unknown mixture on a HP-5 column the chromatographic peak at 32.6 min is rather weak (Fig. 12.7). Scan 2045 extracted from its maximum yields a spectrum that is mainly due to background ions from column bleed as demonstrated by comparison to the average of scans 2068 2082. Finally, manual background subtraction ((2035 2052)-(2013 2020)) delivers a spectmm of reasonable quality, although some background signals are not completely erased. 

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Column bleed gives a mass spectrum (a) that is mixed with an eluting component to give a complex spectrum (b). By subtracting (a) from (b), the true spectrum (c) of the eluting component is obtained. 

Because the vacuum in the mass spectrometer and the cleanliness of the ion source, transfer line, GC column, and so forth are not perfect, a mass spectrum will typically have several peaks that are due to background. All GC/MS spectra, if scanned to low enough mass values, will have peaks associated with air, water, and the carrier gas. Other ions that are observed in GC/MS are associated with column bleed and column contamination. 

GC column bleed is a frequently encountered contaminant of mass spectra when high column temperatures are employed. Modern data systems offer the best way to eliminate this type of contamination by subtracting a spectrum showing column bleed from all other spectra in the GC/MS run. 

Bleeding An appearance of a background signal from a chromatographic system, caused by the stationary phase or contamination of the inlet system. The column bleed usually increases with increasing column temperature. 

The maximum column temperatures used in GC/MS are usually 25-50° lower than those used in capillary GC with a flame ionization detector. Higher temperatures can be used in GC/MS but there will be more column bleed, which will require more frequent cleaning of the ion source of the mass spectrometer. 

Every column (including chemically bonded columns) will have some column bleed. The amount of column bleed will increase with increasing column temperature, film thickness, column diameter, and column length. The base line starts to rise approximately 25-50° below the upper temperature limit of the stationary phase. After a column is installed in a GC/MS system, a background spectrum should be obtained for future reference. 

Standardization of the measurement of column bleed from different columns presents several proble,.. The influence of... 

Other thermal zones which should be thermostated separately from the column oven include the Injector and detector ovens. These are generally insulted metal blocks heated by cartridge heaters controlled by sensors located in a feedback loop with the power supply. Detector blocks are usually maintained at a temperature selected to minimize detector contamination from condensation of column bleed or sample components and to optimize the response of the detector to the sample. The requirements for i injectors may be different depending on the injector design and may include provision for temperature program operation. 

The last several years have seen an enormous growth in the number and use of chiral stationary phases in liquid chromatography [742,780-791]. Some problems with the gas chromatographic approach are that the analyte must be volatile to be analyzed and larger-scale preparative separations are frequently difficult. For entropic reasons relatively high temperatures tend to minimize the stability differences between the diastereomeric complexes and racemization of the stationary phase over time may also occur. The upper temperature limit for phases such as Chirasil-Val is about 230 C and is established by the rate of racemization of the chiral centers and not by column bleed. Liquid chromatography should be s ior in the above... 

GSC does enjoy some advantages over gas-liguid chromatography and these have been sufficient to maintain some interest in the technique. Adsorbents are generally stable over a wide temperature range and are often insensitive to attack by oxygen. Column bleed is virtually nonexistent, so high sensitivity... 

Column bleed is normally not a serious problem since capillary columns contain little stationary phase. 

In high temperature (HT)-GC-MS and PyGC-MS experiments, special attention should be given to the stability of the column. GC columns can lose some of the stationary phase ( bleeding ) when heated up to the maximum operating temperature the thicker the stationary phase, the more column bleed may be expected. When coupled on-line to a mass spectrometer, the stationary phase may foul the ion source, which leads to rapid decay in sensitivity and detection of usually siloxane-related mass peaks at m/z 207, 281, 355, etc. HTGC-MS coupling was discussed by ref. [218]. 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

There are a number of limitations on the use of extremes of temperature in HPLC. Clicq et al. [91] note that instrumental issues become increasingly limiting as one goes to very high temperatures and flow rates. They suggest that most separations will occur below 90°C where there are less instrumental constraints. As detailed below, column bleed can limit the selection of columns. Highspeed separations require a faster detector response than many systems allow and constrain extra column volume. This is especially true for narrow bore columns and sub-2 jam particles. In many cases, the additional speed gained above the temperature limits of commercial HPLC ovens will not be worth the additional expense and complexity required. For macromolecules, the effect of extreme pressure can also impact retention time as noted by Szabelski et al. [92]. 

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