Static Headspace Sampling

September 13, 2015

       

Static Headspace

Static Headspace sampling is typically used for the determination of volatile and semi-volatile  analytes in liquids and, more rarely, solid matrices. Application examples include the analysis of alcohols in blood, residual solvents in pharmaceuticals, flavours and taints in food and beverages as well as fragrances in perfumes and detergents.   Samples are heated and agitated at a set temperature for a set time, after which an aliquot of the headspace gas is analysed by gas chromatography to determine the concentration of the analyte of interest in the headspace, which can then be related to the concentration in the original sample.   

  Irrespective of the level of automation or the instrument operating principles, the underlying theory is based on a form of Raoults Law (or Henrys Law when analyte concentrations are low), which state that the vapour pressure of a compound above a solution is directly proportional to its mole fraction in that solution multiplied by an activity coefficient. The activity coefficient relates to the degree of intermolecular attraction between the analyte and the other species within the sample.   The equation most often used to describe the basis of headspace determination is;  

  where  C

G

is the analyte concentration in the gas phase, C

o

the analyte concentration in the original sample, K is partition coefficient (equation 2), V

G

the volume of headspace gas and V

L

the sample volume.  

  C

S

is the analyte concentration in the sample liquid C

G

is the analyte concentration in the headspace gas.   In order to determine K it is necessary to calibrate instrument response by analysing standards containing a known amount of analyte.  It is very important that the standard is ‘matrix matched’ to the analyte as the matrix components can significantly affect the activity coefficient of the analyte as described above.  One difficulty with headspace analysis is the ability to obtain ‘blank’ matrices and it is sometimes necessary to exhaustively extract (purge) analyte from a sample using multiple headspace extractions in order to obtain a suitable matrix blank.   When determining ethanol in water a partition coefficient (K) value of around 500 is not unusual, indicating that there is around 500 times more ethanol in the water at equilibrium than in the headspace, not unusual given the high solubility of ethanol in water due to the comprehensive hydrogen bonding between the analyte and matrix. When determining hexane in water, K values of 0.01 are not unusual, meaning there is 100 times (1/0.01) more hexane in the headspace. How would these figures be affected by changing the various experimental variables?  

Sample Volume

Increasing sample volume will not significantly affect the headspace concentration for analytes with high values of K.  For intermediate values of K (~10) the increase in sample volume is approximately linear and for analytes with low values of K then an increase in sample volume will give a large proportional increase in headspace concentration.    Low analyte headspace concentrations due to good analyte solubility in the matrix cannot be significantly improved by increasing sample volume.  Use around 10mL of sample (if available) in a 20mL headspace vial. This also makes the phase ratio (β = V

G

/ V

L

) equal to 1 and simplifies calculations.  

Temperature

Samples with a high value of K will be significantly affected by temperature, and increasing temperature is a good way to improve headspace concentration.  However, to obtain good precision, one needs to carefully and accurately control the equilibration temperature and for analytes with K values of 500, a temperature accuracy of ± 0.1

o

C is required to obtain a precision of 5%!
With analytes where K is low, then increasing temperature has a lesser effect and can even cause a reduction in analyte headspace concentration.
One special note here is that as temperature is increased when using aqueous samples, the overall headspace pressure can increase markedly and the sudden release of pressure on inserting the sampling needle may case loss of analyte or a significant dilution effect.  

Equilibration Time

Headspace equilibration time will depend upon analyte vapour pressure, concentration in the sample, phase ratio and temperature / agitation.  Do not be tempted to draw a correlation between equilibration time and Partition Coefficient value.  Each analyte / sample combination and sample to headspace ratio will need to be investigated in to order to determine the time required to reach equilibrium for each analyte.  

Salting Out

The Partition coefficient of polar analytes in polar matrices can be significantly reduced by adding a very high concentration of salt (potassium chloride is typical) to the sample matrix.  

Instrument Variables

When using autosampler devices, use the smallest volume sample loop which gives the required signal to noise.  The sample, loop, transfer line and inlet temperatures should be off-set by at least +20

o

C to avoid sample condensation.  If signal to noise ratio allows, applying a small split flow of 10:1 often improves analyse peak shape and makes peak area measurement more reproducible.  

 

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