Teledyne Tekmar Versa - Optimization Steps for Static Headspace Analysis; Sample Preparation; Salting out

Versa User Manual www.teledynetekmar.com
Method Development • Page 6-2
OPTIMIZATION STEPS FOR STATIC HEADSPACE ANALYSIS
It is imperative that a rugged analytical method be developed to achieve the best results from the Versa. To complete
this task, a thorough understanding of the headspace technique is critical. This section will review:
• How to determine the appropriate method parameters for an application.
• How to inuence the partition coecient (K).
• The eects of the Universal Gas Law on headspace analysis.
Sample Preparation
The partition coecient (K) has a drastic eect on the sensitivity of an analyte in a mixture. The K value represents
the eect the matrix has on the analyte’s ability to move from the matrix to the gas phase. Compounds with low K
values transfer from the matrix to the gas phase very easily. Compounds with high K values would rather remain in
the matrix versus the gas phase. In many cases the K value can be forced to be lower or higher depending on the
needs of the analysis. This modication of the K value is often referred to as Matrix Modication. Matrix Modication is
typically done to increase the partitioning of analytes of interest from the matrix to the gas phase, but it can be used
to increase the K value, thus helping to eliminate an interference compound.
Salting Out
The most commonly used Matrix Modication technique is the addition of salt to the vial. Adding salt to a vial
containing a liquid sample can signicantly reduce Hydrogen to Hydrogen bonding, making it easier for the analytes
to move from the liquid phase to the gas phase. An example of the “Salting Out” technique can be seen in the analysis
of alcohols from an aqueous matrix. A highly polar compound, such as methanol, exhibits a very high K value out
of water (~1600). When the aqueous sample is saturated with salt, the K value drops dramatically permitting more
methanol to transfer to the gas phase improving sensitivity. A list of commonly used salts is shown below.
Common salts used to
decrease matrix eects
Ammonium chloride
Ammonium sulfate
Sodium chloride
Sodium citrate
Sodium sulfate
Potassium carbonate
pH
The pH level of the sample is often adjusted prior to analysis. There are two primary reasons this step is performed in
headspace analysis:
• First is to eliminate sample degradation due to biological activity within the vial.
• Second is to control the volatility of specic compounds within the mixture.
An example of using pH to control the volatility of a compound can be observed in the analysis of Pyridine, where at
low pHs the compound is non-volatile while at high pHs the compound is volatile. The adjustment of pH can be used
in this case to either see Pyridine or suppress Pyridine.
Table 6-1: Common Salts Table