Septum bleed

One of the biggest problems with present-day septum inlet systems is that of septum bleed. Kolloff (13) was the first to note the bleed of monomers and short-chain polymers (used in the production of the synthetic elastomers from which septa are derived) from gas chromatographic septa. Another problem is sorption of solvents and sample components on the septa. A thorough study of this phenomena was made by Adler (14) related to the use of selfsealing elastomer septa for quantitative operations with volatile laboratory solvents. It was found that a silicone septum could absorb over twice its weight of carbon tetrachloride and chloroform and more than its weight of benzene at 25°C. 

SEPTUM-COOLED INJECTORS. Increasing the tension on the septum markedly raises the quantity of bleed. Temperature has a greater effect on septum bleed. This bleed can be decreased by heat preconditioning the septa or by cooling them. Callery (17) found that the bleed rate was less by a factor of about two at 200°C than at 350°C. Obviously the septum temperature should be as low as possible—just high enough to prevent condensation of the injected substance. 

Septum bleed is worse than in split mode because all the flow goes through the column. Most of the splitless inlet systems that are presently available use a vented septum technique to avoid this problem. 

Chronic inhalation exposure to hexavalent chromium may give rise to nasal septum bleeding and perforation with an accompanying loss of the sense of smell and taste. Bronchial asthma may result from chronic exposure to chromate dust or chromium trioxide. 

Common problems with GC analysis include septum leaks and adsorption of components from the sample onto the septum durmg injection. In addition, because the septum is heated, decomposition products often form and bleed into the column. This results in spurious peaks, termed ghost peaks, appearing in the chi omatogram. Septum bleed is greater at higher injection-port temperatures. To minimize this problem, a Teflon-coated, low-bleed septum is used. The inner surface of the septum is purged continuously with the carrier gas that is vented before... 

Leakage from a damaged septum can be detected by inaccuracies in quantitation, problems with the chromatography such as a change in retention time, or the deterioration of the column from the entry of air. The septa can bleed substances into the gas flow and also can become brittle with use. Shreds from the penetrated septa can break loose and block wide-bore columns and block gas flow. Tailing can be caused by septum bleed and can be checked for as follows the noise level increases, the early eluters show decreased response, and the baseline drifts if programmed temperature is used. 

Describe the sequence of events that would lead to septum bleed giving rise to spurious GC peaks... 

Liquid samples are introduced into the vaporization chamber by syringe through a septum or airlock. An auxiliary flow of gas is used to purge septum bleed products and contaminants away from the vaporization chamber. Appropriate column loads are usually achieved by injecting sample volumes of 0.2-2.0 pi with split ratios between 1 10 and 1 1000. 

Syringe injection is accomplished through a self-sealing septum, a polymeric silicone with high-temperature stability. Many types of septa are commercially available some are composed of layers and some have a film of Teflon on the column side. In selecting one, the properties that should be considered are high temperature stability, amount of septum bleed (decomposition), size, lifetime, and cost. 

Contamination sources in the GC could be related to column or septum bleed, dirty injection port, contaminated liner, poor quality carrier gas, air leaks, and so on and in the MS detector to air leak, cleaning solvents and... 

The sample is introduced to the column in an ideal state, i.e., uncontaminated by septum bleed or previous sample components, without modification due to distillation effects in the needle, and quantitatively, i.e., without hold-up or adsorption prior to the column. The instrument parameters that influence the chromatographic separation are precisely controlled. Sample components do not escape detection, i.e., highly sensitive and reproducible detection and subsequent data processing are essential. 

Septum Bleed. Refers to the detector signal created by the vaporization of small quantities of volatile materials trapped in the septum. It is greatly reduced by allowing a small quantity of carrier gas to constantly sweep by the septum to vent. 

The first problem with septa is leakage. Leaks in the system completely destroy the ability to quantify. Septa should be changed on a routine basis (daily) to avoid loss of valuable time and samples. Septnm bleed will cause noise and sometimes drift in isothermal operations. In addition to these, in programmed temperature, septum bleed will produce extraneous peaks not dissimilar to those in an impure solvent. Both sensitivity and quantification may suffer. When bleed is a problem, high-temperature, low-bleed septa should be used even though they are considerably more expensive. 

FIGURE 9.3 Chromatograms showing septum bleed from various septa. Each type of septum is designated by a different color. [Adapted from the Supelco Catalog (Supeleo, Bellfonte, PA), 2000]. 

Septum bleed can become a serious problem during trace analysis, as the leached compounds elute in the middle of the GC run with high intensity. Persistent bleed causes low sensitivity and poor quantification. Once a septum is punctured, a small amount of silicon can be transferred into the injector with each injection. This is quickly transferred to the top of the column, and if the column is at a low temperature, focused here. Once the column is heated, the bleed components elute. The longer the column is held at a low temperature, the more intense the presence of bleed peaks in the chromatogram. The major component of septum bleed is a series of siloxanes with increasing molecular weight (see Table 3.16 and Figure 3.168). Other observed bleed components are phthalates and also hydrocarbons (Warden, 2007). 

Peaks from compounds not present in the original sample. Ghost peaks can be caused by septum bleed, analyte decomposition or carrier gas contamination. 

If the sample inlet system is heated above 300 C (572T), a blank analysis must be made after a new septum is installed to ensure that no extraneous detector response is produced by septum bleed. At the sensitivity levels commonly employed in this method, conditioning of the septum at the operating temperature of the sample inlet system for several hours will minimize this problem. A recommended practice is to change the septum at the end of a series of analyses rather than at the b inning of the series. 

Baseline Compensation Analysis—A baseline compensation analysis, or baseline blank, is performed exactly like an analysis except no injection is made. A blank analysis must be performed at least once per day. The blank analysis is necessary due to the usual occurrence of chromatographic baseline instability and is subtracted from sample analyses to remove any nonsample slice area fijom the chromatographic data. The blank analysis is typically performed prior to sample analyses, but may be useftil if determined between samples or at Ae end of a sample sequence to provide additional data regarding instrument operation or residual sample carry-over from previous sample analyses. Attention must be given to all factors that influence ba%line stability, such as column bleed, septum bleed, detector temperature control, constancy of carrier gas flow, leaks, instrument drift, etc. Periodic baseline blank analyses should be made, following the analysis sequence protocol, to give an indication of baseline stability. 

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