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FLEXO Magazine : June 2009
TECHNOLOGIES & TECHNIQUES of samples need to be analyzed. It also is useful when the test is performed infrequently. At this point we should note that an alternative to using a laboratory oven to heat the sample vials is to use a specially designed headspace vial heater which also heats the sampling syringe. This provides a more stable temperature than a large laboratory oven. Careful heating of small 20ml headspace vials enables good quantitative results. Heating the gas-tight syringe also is necessary to prevent sample losses from condensation inside the glass syringe barrel and to prevent sample carryover into the next injection. However, continual heating of the gas-tight syringe causes these syringes to leak after a period of time, which means that they must be replaced on a regular basis. The small footprint of the headspace vial heater allows it to be placed next to the GC so that the operator can move the hot sample directly to the GC injection port. These heaters can heat up to fi ve headspace vials at one time. Sample heating time is determined using either a lab timer or a built-in timer in the vial heater itself. AUTOMATIC HEADSPACE SAMPLERS Today, the busy quality control laboratory may chose to use one of several automated headspace samplers available on the market. These samplers control the temperature, heating time and headspace injection size. They can hold from 16 to 110 headspace sample vials, making a large number of automated sample injections possible. The autosamplers control the heating and inject the hot headspace vapor sample; the operator never comes in contact with the hot sample vials or syringes. The headspace autosamplers on the market are stand-alone systems that can be used with most standard GCs. They are relatively simple to use but they do require careful initial setup by an experienced user or technician. FIGURE 4. Gas-tight syringe autosampler. These headspace autosamplers provide accurate and precise control of the sample heating temperature and time, both of which are important for good reproducibility and precision. The operator has only to place the sealed headspace vials into the autosampler turntable and the rest of the analysis is totally automated. The operator does not need to be present, allowing overnight operation of these systems. Three types of headspace autosamplers are available. Gas-tight syringe injection Pressure balanced Pressurized loop All of the systems have a heated zone to heat the sample vial and automation of sample vial heating time and headspace injection size. FIGURE 5. Pressure balanced autosampler. GAS-TIGHT SYRINGE The gas-tight syringe autosampler uses a robotic mechanism to duplicate the manual injection technique, but can perform them more reproducibly. This system automatically moves the samples into the sample heater and injects the sample using a heated gas-tight syringe directly into the GC injection port. No operator intervention is required after the headspace vials have been loaded into the unheated sample tray. PRESSURE BALANCED Unlike manual and automated gas-tight syringe samplers the pressure balanced systems do not use a syringe. Instead the sample vapor fl ows out of the heated sample vial under pressure through a heated needle and transfer line directly into the GC. The name pressure-balanced comes from the single pressure that controls both the headspace vial pressure and the fl ow through the GC column. After initial setup, the operator has simply to place the sample vials into the unheated turntable and the ensuing operation is totally automated. This autosampler type is highly reproducible (±1 percent) because of the accurate temperature and injection control and the inert sample fl ow path. 28 FLEXO JUNE 2009 www. f le xography. org
Sustainable Spring 2009