An Overview of 

SW-846 Method 8261*

Chemistry

Michael H. Hiatt

U.S. Environmental Protection Agen

National Exposure Research Labora

Environmental Sciences Division

P.O. Box 93478, Las Vegas, Nevada 891

*Volatile Organic Compounds by Vacuum Distillatio

Gas Chromatography/Mass Spectrometry 
Method 8261

• Method 8261 is a RCRA analytical protoc

the concentration of volatile compounds in matrices.

• The protocol can be applied to many prog

matrices not amenable to other methods sbiota.

• The method incorporates the physical che

compounds in measuring matrix effects. assessment of matrix effects yields methodata by analyte and is the foundation for treporting of results with confidence interva

• The discussion that follows explains why 

chemistry of analytes is so important to th
Chemistry Behind Metho

• Method 8261 addresses compounds tha

gaseous in vacuum and which can be sa sample matrix.

• The vapor pressure of a compound indic

be made gaseous.

• The degree to which a compound disso

an affinity for sample matrix describes thseparating the compound from that mat8261 uses the term, “relative volatility”, tdegree of volatilizing a compound from w

• Diffusion of compounds through a samp

an important variable to address when twater. Note: Diffusion can be critical wmatrix impedes movement of compounddiscussed.
Major Points of Discu

• Compounds as gases can flow fro

container for isolation.

• Getting the compounds into gas fo

inhibited by a sample matrix.

• Once compounds are volatilized a

separated from the sample matrix recovery may be diminished by coin the apparatus. 

• Internal standards are used to me

behavior of compounds throughouanalysis.
Compounds as G

• Some compounds are gases 

temperature.

• These compounds can be pu

their container by flushing witstream of gas or pulling vacu

• Transfer of these gases by va

distillation is used for the detemeasurement by instrumentaas GC/MS.
Gases are Mobile and 

Removed from a Con

• A gas flow can flush gaseous compounds f

above container.

• With the gate valve closed, a vacuum at the 

side pulls the compounds from the containe
Only the Gas Fractio

Compounds are Readily 

from the Containe

• In the previous slide, gaseous 

compounds were purged from a container.

• However, some compounds are 

entirely gases but are liquids or with only a small fraction as a g

• The degree to which a compou

gas is described by its vapor prthe surrounding temperature anpressure.
Vapor Pressure

• Liquid or solid compounds may be slo

evaporated from a surface (sample or even when conditions are not sufficienthem gases. These compounds still epressure though much less than the psystem.

• The greater the vapor pressure of a co

room temperature, the greater percentbe gaseous.

• The vapor pressure of a compound is 

its boiling point1.

1 Antoine, C., “Tensions des Vapeurs: Nouvelle Relatio

Tensions et les Temperatures, Compt. Rend. 107, 681 (
Liquid or Solid Compounds 

Effectively Flushed from a 

A gas flow can only flush the gaseous fraction of gaseous fraction is relative to its vapor pressure aof the container.
Liquid or Solid Compoun

also Less Effectively Eva

from a Container

The introduction of a vacuum source only evacugaseous fraction of compound from the containethe vacuum also reduces the pressure in the cothus, increases the gaseous fraction of the com
Gases – in Conclus

• Compounds in a vapor ph

be moved from a containeeither pulling a vacuum owith gas.

• The fraction of a compou

present in the vapor phasdepends on its boiling po
What if a Compound is D

in a Sample?

• Introducing a sample matrix introdu

compartment that can have a stron

for compounds. For ease of discussample matrix being discussed will 

• This new compartment adds an att

must be overcome for a compound vapor phase.

• The tendency of a compound to se

water and become part of the gas the water can be described as watpartitioning.
Compounds Partition

between Water and 

• Just how a compound is distribute

the water and the air is directly relcompound’s affinity for the water aa gas. This is the water-to-air particoefficient, 

Kwa, and equals conce

compound in water compared to itconcentration in air. In this presenwater is the given matrix, we’ll just 

• The greater the K value, the more 

tends to stay dissolved in water th
More about Partition Coe

• Partition coefficients des

steady-state equilibrium condition.

• The partition coefficient v

dimensionless as the raticoncentration cancels ou

• The value of K is affected 

temperature and thereforpartition coefficients are specific temperature.
Partitioning Illustra

The concentration of red spheres in water is 20 spheresconcentration of red spheres in air is 5 spheres/volume volumes in the illustration). Therefore for the red spherpartition coefficient, K is 4. (20 spheres/volume 

÷ 5 sph

For the blue spheres K = 28 (28÷1)

Assume a 3-volume closed container with two volumes of air and one of water. There are 20 red and 28 blue spheres in the water and 10 red and 2 blue spheres in the air.What is K for the red and blue spheres?
Dynamic Partitionin

Relative Volatilit

• While K describes a steady state at equili

evacuating a container over a time intervadoes not reflect an equilibration.

• Vacuum distillation is a dynamic process 

to describe the relative ability of a compou

• The distillation of a compound still relates 

dynamic way. Chemical engineers use thvolatility to describe this dynamic partitiondistillation. This term describes the relativone compound to another compound as a respective partition coefficients. For instaprevious example of red and blue spheresvolatility of red to blue would be 4 ÷ 28 wh
Method 8261’s Relative V

• The chemical engineering use of relativ

comparison of partition coefficients and value. This is an awkward value when many compounds as does method 826

• In method 8261, relative volatility value

correspond to a compound’s partition cThese values are assigned and do not distillation.

• More discussion is available in the publ

of this webpage.

2

2Hiatt, M.H., Farr, C.M, “Volatile Organic Compound De

Using Surrogate-Based Correction for Method and Matrix Analytical Chemistry, Vol. 67, No. 2, p. 426-433, Januar
Relative Volatility Illus

• The partition coefficients for the red and blue spher

determined to be 4 and 28, respectively, and we us

their relative volatility values.

• Knowing the relative volatility values and the fractio

spheres collected in the cold trap provides us with tneeded to quantify the migration of spheres from thphase.

This time the 3-volume container from the previous partitioning illustration has a vacuum pump attached. A cold trap is added between the container and the vacuum pump. There were 30 red, blue and yellow spheres added to the water. A vacuum is applied for a short period of time. After the evacuation stopped, we find 10 red spheres and 2 blue spheres isolated in the cold trap. Later we will calculate how many yellow spheres also migrated.
Headspace-Equivalent V

• The first headspace volume that is removed f

container can be measured as the evacuation from the container.

• The first headspace, however, does not desc

volume removed from a container if we recovof a compound than could be in the initial hea

• After the air has been removed, measuremen

headspace volume is done by measuring the of compounds from the water. That is, if 10 compound is in the headspace before evacuaadditional “headspace-equivalent” volume muremoved from the container to volatilize 10% remaining compound from the water.

• A vacuum distillation can be thought as pullin

of headspace-equivalent volumes from a conThe total volume H, is the volume pumped thvacuum during the distillation time. 
The Vacuum Distillation V

• The vacuum distillation volume, H, is important because 

the vacuum distillation as creating a large headspace and distributed in this headspace and a water sample as K (oAnd as H passes through the cold trap, all the compoundheadspace are trapped in a cold trap. The mass of a com(M

H) divided by H and compared to the mass of the com

water (M

w) divided by water volume (V

w) equals K.

• The above equation can also be used to solve for H if we 

amount of a compound (with K), to the water before perfodistillation. Then by measuring the mass of the compounget

• Measuring the response of a reference compound, the re

previous relative volatility illustration, H can be determine

H = K red spheres in trap ÷ concentration in water H = 4 × 10 ÷ 20 = 2 volumes
Using H to Determine 

Vacuum Distilled

• After H is determined for a vacuum dis

can also predict how many of another csphere would be in the cold trap by knoand concentration in the water before dIf the K for yellow spheres in the relativillustration is 8, then there will be 6 sphcold trap (K = 8 = (30 –x) ÷ (x ÷ H)

• If a green sphere has K = 13, the solut

many spheres are in the cold trap is 4. all spheres would be in the cold trap an

there would be no spheres in the cold t
Relative Volatility vs. Rec

of the Example Data

• We can look at the number of spheres 

trap and determine the % recovery of sinstance, the recovery of blue spheres (10/30).

• Converting the number of spheres to p

K = 0; Recovery = 100 %K = 4; Recovery = 33.3 %K = 8; Recovery = 20 %K = 13; Recovery = 13.3 %K = 28; Recovery = 6.67 %K = 

∞; Recovery = 0 %

• The plot of recovery vs. relative volatilit

next slide.
Recovery vs. Relati

Volatility for Illustrat
Predicting Recover

• The use of spheres in the previous slides was a 

mass of a compound. From this point on, it is e

terms of recovery of compounds as the fraction has been volatilized from the sample. 

• It was shown that recovery of an analyte (as ma

distillation depends on the relative volatility of thwe added internal standards to every sample of distillation, by measuring their recovery we could 

function of relative volatility and interpolate the rother compound if we knew their relative volatilit

• If we have calibrated the GC/MS, a response of 

be translated into mass, M. Interpolating the recrecovery vs. relative volatility generated by the irecoveries, it is easy to identify how much comp

sample

Compound A in sample = M ÷ recovery from plot
Determination of Experim

Relative Volatility Val

The previous recovery vs. relative volatility plot can aexperimentally measure relative volatilities. The follobe used to experimentally derive relative volatility val

1. Compounds of unknown relative volatilities are a

sample containing internal standards of known revalues. 

2. The water sample is vacuum distilled and all reco

compounds and internal standards are measured

3. A plot of recovery vs. relative volatility is drawn u

standard recoveries and relative volatility values. 

4. Then experimental recoveries of compounds are 

relative volatility values.

Example: Using the slide 23 plot as a result of an ex

experimental recovery of a compound of 10% (y-in assigning the compound a relative volatility va
Water Interaction Sum

• The potential for a compound to b

from water into the gas phase durivacuum distillation is described as volatility.

• In method 8261, relative volatility i

physical property closely resemblito-air partition coefficients.

• The fraction of a compound separ

water as a gas relates to its recove

• Next we’ll look at the method 8261 

so we can put all of this informatio
Vacuum Distillatio

Components

• There are four fundamental parts to the vacuum d

chamber, condenser, cryotrap, and vacuum pump

• The sample container holds the sample and is of 

maintain low pressures.

• The condenser is a stainless steel cylinder whose 

temperature controlled and is used to separate w

pass through it.

• The cryotrap is similar to the condenser though s

interior surfaces can be made very cold (-196 oC) 

that have passed through the condenser. The cryrapidly heated to vaporize material frozen in it. Ntrapping material contained in the cryotrap.

• A mechanical vacuum pump provides the low pre

vacuum distillation. The vacuum in a vacuum dislower the pressure in the sample vessel so that w

temperature.
Apparatus During 

Vacuum Distillation

Multiport valve in vacuum distilling position allows the vacuum distillation of volatile compounds from vessel through condenser and subsequently condensed in the cryotrap. Light blue shows zones under vacuum, dark blue shows cooling zones (condenser = ~ 0 oC and cryotrap ~ -196 oC), and green shows helium carrier gas flow. 
Apparatus During 

Transfer of Distillate to 

GC/MS

Multiport valve in transfer position allows distillate in cryotrap (heated) to vaporize and transfer to GC/MS. Light blue shows zones under vacuum, red shows heating zones, and green shows helium carrier gas flow. 
Vapor Pressure

• Compounds take a long path from 

to the GC/MS. Losses of analyte thmigrated from the sample chambethe condenser. Because the condheld at ~ 0 oC, losses of compoundcondensation occur and these lossrelated to a compound’s vapor prenoted previously, vapor pressure focompound is a function of its boilinthus we use boiling point as the varelate these losses.
Condensation of Compo

Vacuum Distiller

• A compound must have a boiling point that refl

pressure giving the compound the ability to mig

sample chamber. A compound may not be sohave an elevated boiling point that makes the cunlikely to remain a gas. Such compounds cosides of the sample chamber.

• Some compounds may have lower boiling poin

to be partially condensed on the sample chameven on the lines connecting the components to 95 oC).

• The condenser column is a major site for cond

held at ~ 0 oC to condense water during the disbecause the condenser is also under vacuum, boil well above 0 oC are still partially gaseous.

• The cryotrap itself is cooled to a temperature th

condenses all analytes that have made their wsample chamber. 
Condenser

• As noted, the condenser serves to remo

of water that is volatilized from the sampdistillation. This is the major barrier for apass after they have become gaseous. is more thoroughly addressed in the traipresentation, “Tuning the Vacuum DistilAnalyte Response and Chromatography(http://www.epa.gov/nerlesd1/chemistry/vacuum/training/pd

• In order to describe how compounds be

distillation, losses in the condenser muscharacterized. 
Condensation of Compounds in Co

Function of Boiling Poin

2Hiatt, M.H., Farr, C.M, “Volatile Organic Compound Det

Surrogate-Based Correction for Method and Matrix EffeChemistry, Vol. 67, No. 2, p. 426-433, January 15, 199
Boiling Point Intern

Standards

• The previous plot is characteristic of ho

condensation, and therefore loss of ancondenser, is a function of a compoundpoint.

• Selection of internal standards that hav

that cover a range of boiling points alloa plot of recovery (total –loss) vs. boilin

• Adding these boiling point internal stan

sample before distillation and then calcrecovery, yields the data necessary to boiling point vs. recovery plot for every 

• After generating the plot, the prediction 

other compounds is interpolated.
Condenser Partition

• By trapping water, the condenser introd

additional water-to-air partitioning pote

• The water trapped in the condenser ca

dissolve a portion of the water-soluble that were volatilized from a water samp

• This trapping of compounds relates to 

water-air partitioning that describes thevolatilized from water. The recovery of by volatilization from a water sample aescaping the condenser water are treasame phenomenon using relative vola
Trapping in Condenser 

Function of Relative Vola

2Hiatt, M.H., Farr, C.M, “Volatile Organic Compound Deter

Surrogate-Based Correction for Method and Matrix Effects

Chemistry, Vol. 67, No. 2, p. 426-433, January 15, 1995. 

recovery vs. method 8261 relative volatility values.
Cryotrap

• The function of the cryotrap is to condense 

that have passed through the condenser ddistillation in a volume sufficiently small so carrier gas can quickly flush the trap and tanalytes to the GC.

• The analytes are condensed using liquid n

cryogenic temperatures (down to -196 oC)

• After the vacuum distillation is complete, th

heated to volatilize and transfer compoundNote: Heating and transfer time of the crymeet gas chromatograph requirements. Spresentation “Tuning the Vacuum Distiller Analyte Response and Chromatography” f(http://www.epa.gov/nerlesd1/chemistry/vacuum/training/pdf/tu
Apparatus Effect

• While the evacuation of a sample induce

to volatilize, we can still expect to lose cthe apparatus.

• Primarily, losses relate to a compound’s 

and these losses reflect condensation in condenser. These losses are measured of boiling point and are measured with binternal standards.

• Any loss of a compound in the condens

to its relative volatility is not distinguishefraction of the compound remaining in thvacuum distillation. Relative volatility is describes all forms of loss due to solubi
Diffusion-Warning

• Diffusion is another variable that must be considered 

organic compound determinations. While we have bwater samples, for organic samples this variable is m

variable can seriously inhibit the migration of composample matrix so that internal standards are not as h

matrix as would be analytes.

• Tissue and other organic solids could be expected t

compounds through them. Migration of volatile comorganic media can take hours. Therefore the introdustandards should allow ample time for the internal stthroughout the sample. 

• In organic solids, special precautions such as the “v

recommended to introduce the internal standards. T

removes air from a sample container while the sampfrozen and the internal standards are much more quwith the matrix. More thorough information is availapublications section of this website. 

3Hiatt, “Analyses of Fish Tissue by Vacuum Distillation/Gas 

Chromatography/Mass Spectrometry”, Analytical Chemistry1127-1134, March 15, 1997 .
Oil Samples

• Water was used as the sample matrix in the pre

discussions. However, these findings hold for odifferent characteristics.

• In the method 8261 protocol, water is added to o

analysis. We find that the internal standards (borelative volatility) still accurately describe recove

• The boiling point internal standards also measu

compounds for oil. In the publication referenced “Analyses of Fish Tissue by Vacuum DistillationChromatography/Mass Spectrometry”, Analytica

69, No. 6, p. 1127-1134, March 15, 1997, the re

compounds in an oil matrix were determined. Cbased relative volatility to boiling point we have of 0.92 (next slide).

• Because the boiling point internal standards me

from both boiling point condensation and volatilimethod 8261 also applies to oil.
Boiling Point vs. Rela

Volatility from Oil

Data from reference 3, Hiatt, “Analyses of Fish Tissue by VacDistillation/Gas Chromatography/Mass Spectrometry”, AnalytiVol. 69, No. 6, p. 1127-1134, March 15, 1997 .
Soil Samples

• In the method 8261 protocol, water is add

samples before analysis and therefore relinternal standards are needed to describe from the water phase. 

• Soil material is largely inert with the excep

organic matter. The affinity of compounds organic matter is also characterized by thinternal standards as seen in the discussimatrix.

• Again as with the oil matrix, the boiling po

standards measure recovery from both bocondensation and volatilization from orgamaking method 8261 applicable to soil.

4

4Hiatt, “Vacuum Distillation Coupled with Gas Chromatography/M

Spectrometry for the Analysis of Environmental Samples” Analyti

Vol. 67, No. 22, p. 4404-4052, 1997 .
Biota Samples

• The relative-volatility internal standards p

recovery of compounds from the water co(and any added water) while the boiling pstandards profile the recovery of compouorganic phase.

• The dimensions of biota samples may inh

of the internal standards and therefore amrequired to ensure the internal standards homogeneous in the biota. This is perforvacuum spike previously discussed.

• With ample equilibration of internal stand

8261 is applicable to biota samples.3,5,6,7

5Hiatt “Bioconcentration Factors for Volatile Organic Compounds in Vegetation” 

No. 5, p. 851-856, 1998.

6Hiatt “Leaves as an Indicator of Exposure to Airborne Volatile Organic Compou

1999, Vol. 33, p. 4126-4133 . 7Hiatt, Pia “Screening Processed Milk for Volatile Organic Compounds Using Va

Chromatography/Mass Spectrometry” Archives of Environ. Contam. Toxicol., Vo
Putting it All Togeth

• We have identified two primary che

properties, boiling point and relativthat control how much of a compoumake the trip from sample to detec

• The influence these properties hav

profiled using unique compounds tbeen introduced (“spiked”) to a sam

• These properties act independently 

their combined effects are multiplicproducts. (e.g., if only 1⁄2 of a compescapes water (relative volatility) alost in condenser (boiling point) the recovery will be 1⁄4).
Method 8261 Internal Sta

• In method 8261, internal standards are a

sample before a vacuum distillation.

• Their response measured by the GC/MS 

with a calibrated response to quantify rec

• The internal standard recoveries are use

recovery vs. relative volatility such as shand recovery vs. boiling point (slide 33) apredict recoveries of analytes.

• GC/MS responses of analytes are norma

predicted recoveries so that they can be concentrations in the sample.

• A stepwise discussion on the process of 

analytical results is addressed in anothe“Method 8261 Analytical Reporting”(http://www.epa.gov/nerlesd1/chemistry/vacuum/training/pdf
Summary

• Chemical properties affect the recovery 

during a vacuum distillation

• The volatilization of compounds from w

on relative volatility.

• The boiling point of compounds also af

of compounds during a vacuum distilla

• The addition of internal standards to m

impact relative volatility and boiling porecovery of compounds allows the meamethod performance.

• The boiling point internal standards als

volatility of compounds from an organimethod 8261 applicable to most matrice