Column temperature limit

An important problem with all liquid stationary phases is their tendency to bleed from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a column s useful lifetime is significantly shortened. Capillary columns with bonded or... 

The allowable stress to be used for tliis gray-cast-iron material at its upper temperature limit of 232 C (450 F) is the same as diat shown in the 204 C (400 F) column. 

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. 

There are two major factors that influehce retention volume measurement and they are temperature and solvent composition. In order to measure retention volume with adequate precision it is necessary to know the relationship between retention time and temperature so that the control limits of the column temperature can be specified. 

The mild-alkali-extract was digested to completion with excess EPG at room temperature for at least 48 hours. Figure 1 shows the separation of the products on a Dionex PAl ion-exchange column. The limit digest of pectic acid (nonesterified HG) by this enzyme is mono-. 

Both multi-residue methods are presented in several parts, which separate general considerations from procedures for extraction, cleanup and determination/ confirmation. Whereas in EN 12393 several extraction and cleanup steps cannot be combined arbitrarily, the modular concept is utilized to a greater extent in EN 1528. In the latter standard, there is no limitation to the combination of several extraction procedures, mostly designed for different commodities, e.g., milk, butter, cheese, meat or fish, with different cleanup steps. Both standards, EN 1528 and EN 12393, do not specify fixed GC conditions for the determination and confirmation. All types of GC instruments and columns, temperature programs and detectors can be used, if suitable. 

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... 

Since these liquid phases on the adsorbant are, eventually, liquids, you can boil them. And that s why there are temperature limits for columns. It is not the best to heat a column past the recommended temperature, boiling the liquid phase right off the adsorbant and right out of the instrument. 

Calculation of the pressure drop and flooding rate is particularly important for vacuum columns, in which the pressure may increase severalfold from the top to the bottom of the column. When a heat-sensitive liquid is distilled, the maximum temperature, and hence the pressure, at the bottom of the column is limited and hence the vapour rate must not exceed a certain value. In a vacuum column, the throughput is very low because of the high specific volume of the vapour, and the liquid reflux rate is generally so low that the liquid flow has little effect on the pressure drop. The pressure drop can be calculated by applying equation 4.15 over a differential height and integrating. Thus ... 

Do not exceed the temperature limits of most columns, usually 60-80 C, without consulting the instructions from the column vendor. 

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]. 

The temperature stability of monolithic stationary phases based on alkyl methacrylate monomers in capillary HPLC has also been reported [103]. These columns allowed the separation time to be reduced by over 10-fold at temperatures up to 80°C. The upper-temperature limit for these columns was not reported. 

The support can then be mechanically coated with a variety of liquid stationary phases. The mobile phase most commonly used in packed column GC is nitrogen with a flow rate of ca 20 ml/min. Packed column GC affords a relatively low degree of resolution compared to capillary GC typically 4000-6000 plates for a 2 m column compared to > 100 000 plates for a 25 m capillary column. The high temperature limit of packed columns is ca 280°C beyond this temperature the liquid stationary phase evaporates at a rate which creates a large background signal. However, for many routine quality control operations, they are quite adequate. 

The range of variables can be limited by controls and thus prevent the discovery of statistically significant effects. For instance, if the column temperature was suspected to be critical and hence always controlled within a range that caused no major effects. Analysis of this data after its collection could lead to the conclusion that changes in temperature are insignificant. 

Injector temperature, °C Column temperature, C Detector temperature, °C Lower limit of quantitation (ppm)b... 

The upper temperature limit of some columns may be lowered by the use of certain tail reducers because these additives are often less stable than the stationary phase itself. If columns are operated at too high a temperature there may not be any noticeable increase in bleed, but the effectiveness of the tail reducer may be lost. Because of this, tail reducers were used to a lesser degree as new column materials such as porous polymers became available. Increasing interest intrace analysis has necessitated development of columns in which adsorption is minimized, and tail reducers are used in many of these applications. 

The primary requirements of a stationary phase are to provide separation of the sample with reasonable column life. Therefore, in addition to having suitable selectivity, the phase should have reasonable chemical and thermal stability. Many catalogs list upper temperature limits for stationary phases, but these should be used only as approximations because the true limit depends upon the type of detector used and the amount of column bleed one can tolerate to get the job done. Even if a phase is stable to 250°C, the column will last much longer if the temperature is limited to 200°C. Excessive temperatures result not only in shorter column life, but also in more rapid fouling of the detector. 

Some of the present liner designs can be used for on-column injection by placing the head of the column about 1/8 inch from the septum. The syringe needle can thereby extend at least 2 inches into the column. The injection block temperature should be set at or below the temperature limit of the liquid phase in the column. Greater injector temperatures should be avoided because the liquid phase will be stripped off or decomposed at the front of the column and result in baseline drift or large, skewed peaks. 

A vapor jacket was first used to control the column temperature (1). It kept the column at the constant temperature of a condensing vapor (at the boiling point of an appropriate liquid). This technique was quickly abandoned because it was cumbersome to use and was limited to liquids that boiled up to about 150 -180°C. 

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