8
✶
Always use pre-cleaned vials for
sample preparation and storage.
SampleVial
Sample vials should be selected tomatch the type and size of the sample being
analyzed.Always use pre-cleaned vials for sample preparation and storage.Vials
that are not properly cleaned prior to packaging or that absorb contaminants during
shipping can produce unknown chromatographic peaks, or “ghost peaks.”Ghost
peaks that are the result of vial contamination can be identified by runningmethod
blanks and zero standards during the system calibration sequence.
The septa used to seal the headspace vials also can be a source for contaminants,
which can bleed into the headspace of the vial during equilibration. These contami-
nants can appear as single peaks ormultiple peak patterns. Some septa are available
with aTeflon
®
face to eliminate bleed from the rubber portion of the septa. These
septa should not be re-used. Once theTeflon
®
face has been punctured by a syringe,
contaminants from the rubber portion of the septa canmigrate into the headspace
and show up as unidentified peaks.Again, the use ofmethod blanks can help to
determine the source of contaminants.
System Optimization (Troubleshooting)
Chromatographic performance inHeadspace/GC is greatly influenced by how the
sample is introduced into the analytical column.Variables that affect sample
preparation and transfer of the sample from the headspace unit to the analytical
columnmust be optimized to obtain reproducible and efficient separations. Key
issues to addresswhen setting up headspace/GC systems includeminimizing system
dead volume, maintaining inert sample flow paths, and achieving efficient sample
transfer. This sectionwill explain how to optimize areas that are critical in address-
ing these issues and providing good chromatographic performance.
Sample Preparation
Samples for headspace/GCmust be prepared in such amanner as tomaximize the
concentration of the volatile sample components in the headspacewhileminimizing
unwanted contamination from other compounds in the samplematrix. Sample
matrices such as biological samples, plastics, and cosmetics can contain high
molecular weight, volatilematerial that can be transferred to theGC system.Water
from the samplematrix also can cause problems by recondensing in the transfer line.
Incomplete or inefficient transfer of highmolecular weight compounds or water
vapor from samplematrices can produce adsorptive areas in the transfer line or
injection port that can lead to split peaks, tailing peaks, or irreproducible responses
or retention times. Tominimizematrix problems and prevent water condensation
from aqueous samples, use a higher transfer line temperature (~125°C–150°C).
High-concentration samples need to be prepared appropriately to obtain optimal
chromatography. High-concentration samples can produce ghost peaks in subse-
quent analyses due to carryover of sample from previous injections. Sample
carryover can beminimized by using higher transfer line and injection port tempera-
tures, but some samplesmay need to be diluted and re-analyzed to obtain reliable
results.Additionally, we recommend injecting standards and samples in order from
low to high concentrations to helpminimize carryover.When sample carryover or
ghost peaks are evident, youmay need to bake-out the column at itsmaximum
operating temperature and elevate the transfer line temperature in order to remove
all of the residual sample. If high-concentration samples are anticipated in a
sequence of samples, running a blank after the suspected sampleswill reduce
carryover contamination of following ones. It is good lab practice to handle stan-
dards andmethod blanks the sameway samples are handled tomake any vial or
sample preparation problems easier to identify.
