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Thermal Desorption Technical Support

Note 2: Prediction of Uptake Rates for Diffusive Tubes

Acknowledgement

Markes International Limited would like to acknowledge the work and help of Nico van den Hoed

and co-workers (Shell BV, Amsterdam), Mike Wright and Richard Brown (UK Health and Safety

Executive) and Jan Kristensson and Erik Nordstrand (University of Stockholm) in preparing this

Application Note.

Introduction

Diffusive (passive) monitoring provides a cost effective and 'wearer friendly' approach to monitoring

atmospheric concentrations of volatile organic air pollutants. The standard sorbent tube-type

diffusive monitor, reported by Brown and co-workers in 19811, overcame the air speed restrictions

of early badge-type diffusive samples and is now widely used for both workplace and ambient air

measurements.

Diffusive monitoring can only be used for quantitative measurements however, if an uptake rate

constant (U) is known for the particular analyte-sorbent combination in use. Uptake rates have

already been determined for many compounds on standard tubes2 (See also TD Technical Support

Note 1) and international protocols3,4,5 now also exist for validating new methods. However, the

experimental procedures demanded by such official validation protocols can be time consuming and

expensive to carry out.

To overcome this limitation, several researchers have investigated techniques for accurately

predicting uptake rates.

Prediction of uptake rates

The ideal uptake rate (Uid) of an analyte on a diffusive tube is independent of the sorbent and can

be calculated using the appropriate diffusion coefficient. Uid may be used for quantitative

measurements in some cases, particularly for strongly retained analytes, however, the actual or

effective uptake rate (Ueff) which applies in practice will differ considerably from Uid for more volatile,

less well retained components. There is therefore a requirement for some simple method of

calculating Ueff.

Coutant et al 6 and Posner and Moore 7 proposed simple linear-isotherm models for thin-bed

samplers, but for tube-type (thick-bed) diffusive samplers, this model cannot be strictly applied.

Work carried out by van den Hoed et al8,9, on the application of Freundlich isotherms to the

prediction of diffusive tube sampler behaviour, led to the development of a FORTRAN computer

program for predicting Ueff from Uid using an iterative process. Further work by Kristensson and

Nordstrand10,11 facilitated translation of the original program into PC compatible software and

simplified several operational aspects. Using the PC program, Uid is calculated and combined with

isotherm data to derive Ueff . This work has advanced the cause of accurate prediction of Ueff, but

the method still requires Freundlich isotherms to be determined for the sorbent-sorbate combination

concerned. The determination of Freundlich isotherms requires the generation of standard

atmospheres at a range of concentrations. These atmospheres are then sampled to determine the

breakthrough volume at each concentration. This relatively complex procedure limits the

usefulness of the approach.

Recent work sponsored by the European Commission Measurement and Testing Program (MPT

formerly known as BCR12), has shown that, under certain conditions, Ueff for the standard tube can

be predicted to ± 10% from the specific retention volume (Vg) alone. Broadly, these conditons are:
TDTS02_2 5-11-02 2

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• Total atmospheric concentration less than 10ppm

• Exposure time between 2 and 8 hours

• When the specific retention volume (Vg) falls within the range 100L/g < Vg < 10,000+ L/g

The following empirical relationship (Equation 1) between Ueff and Vg was derived from sampling

intercomparisons with analytes of wide-ranging volatility and polarity on both TenaxTM and

Chromosorb 106TM. It should be independent of sorbent type.

Equation 1: Ueff = (0.154 log10 Vg +4) x Uid

Determining Vg and Uid

The factors Vg and Uid must be obtained before Equation 1 can be used to calculate Ueff . The

determination of Vg does require some limited experimental work and the following stepwise

procedure is recommended:

• Inject 0.5 mg of analyte into a stoppered ~1L volume glass flask fitted with a septum. Check

that all the analyte has evaporated.

• Buy or prepare a standard sample tube with the maximum 6 cm bed length of sorbent.

Condition the tube carefully.

• Connect the sample tube to the injection port of a GC. Use 530 mm, uncoated fused silica

capillary tubing, or other appropriate narrow bore tubing, to connect the other end of the sample

tube to a FID detector. Use 1⁄4-inch fittings with graphite ferrules to connect to the sample tube

itself. [N.B. Metal ferrules damage the tube and result in leak test failures on UNITYTM and other

thermal desorbers.]

• Set a nitrogen carrier gas flow of 50 mL/min through the tube.

• Inject a 0.1 mL sample of the vapor phase standard onto the tube using a gas syringe. Adjust

the GC oven temperature so that the analyte peak elutes on the FID between 1 and 20 minutes.

• Repeat the experiment 4 or 5 times using different GC oven temperatures within a 30 or 40°C

range. The retention time of the peak will vary considerably with temperature. Try to ensure

that at each of the GC temperatures selected, the peak elutes within 1-20 minutes. [Note:Use

the time from injection to peak crest as the retention time. This may have to be measured

manually, depending on the type of integrator available. Strictly speaking, the mass median or

'centre of gravity' of a peak is the more rigorous measure of retention time, but the error can be

usually ignored.]

• Inject a sample of methane to measure the delay time of the system and subtract this from the

analyte retention times determined in step 6.

• Use the flow of nitrogen carrier gas and corrected retention times to calculate the analyte

retention volumes at different sorbent temperatures.

• Plot a graph of log10 retention vol. v. 1/temp(k). It should produce a straight line plot which can

be readily extrapolated to ambient temperatures. Use this to obtain Vg.

• Check your technique, if possible, with a compound that has a known literature value.

This procedure should take no longer than 1⁄2 a day per analyte.

The ideal uptake rate (Uid) is calculated using the PC program described above. The software

required can be obtained from Dr. J. Kristensson, Chemik Lab, AB, PL. 10840, Gribby 76172

Norrtalje, SWEDEN. Fax No. (46) 2 27307. The program calculates Uid using a procedure related

to atomic volumes 13. The information required by the software includes the chemical formula of the

analyte of interest and whether or not the substance is aromatic. Once these details have been

entered Uid will be predicted.

Calculation of the effective uptake rate Ueff.

Once Vg and Uid have been determined as described above, Ueff can be calculated using Equation

1. For example:

• A Uid of 2.6 ng/ppm/min and a Vg of 1000 L/g analyte would give a Ueff of 2.24

• A Uid of 3.3 ng/ppm/min and a Vg of 250l L/g analyte would give a Ueff of 2.54
TDTS02_2 5-11-02 3

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Summary

Provided a relatively strong sorbent-sorbate combination is selected with Vg >100 L/g, this method

should apply to all routine industrial hygiene and ambient air monitoring applications i.e. to all

atmospheres with analyte concentrations below ~10ppm.

The Ueff values predicted using Equation 1 are reported12 to be accurate within 10%. This is well

within international standards for method performance criteria14. A simple, parallel pumped and

diffusive monitoring exercise may be used to confirm Ueff if required.

By facilitating accurate prediction of Ueff for tube-type diffusive samplers in almost every practical

monitoring situation, this procedure will allow the cost and labor saving benefits of diffusive

monitoring to be realized for an even wider range of air pollution applications.

Trademarks

UNITY is a trademark of Markes International Ltd., U.K.

References

1 The development of an improved diffusive sampler, Brown R.H., Charlton J and Saunders K.J.,

Am.Ind.Hyg.Assoc.J.(42) pp 865-869 12/81

2 Methods for the Determination of Hazardous Substances No. 80 - Volatile organic compounds

in air. Laboratory method using diffusive solid sorbent tubes, thermal desorption and GC. A UK

Health and Safety Executive Procedure. Publ.: HMSO Books, PO Box 276, London, SW8

5DT, UK.

3 Protocol for assessing the performance of a diffusive sampler. UK Health and Safety

Executive, Method for the Determination of Hazardous Substances No.27.

4 CEN Pr EN 838: Workplace atmospheres - requirements and test methods for diffusive

samplers for the determination of gases and vapours.

5 Protocol for the evaluation of passive monitors. Kennedy E.R. et al, US National Institute for

Occupational Safety and Health (NIOSH). Published in: "Diffusive sampling; An alternative

approach to workplace air monitoring" CEC Pub.No.: 10555EN (1986).

6 Passive sampling devices with reversible adsorption. Coutant R.W., Lewis R.G. and Mulik

J.,Anal. Chem. (57), 219-223, 1985

7 A thermodynamic treatment of passive monitors. Posner J.C. and Moore G., Am. Ind. Hyg.

Assoc. J. (46), 227-285, 1985

8 Sampling and thermal desorption efficiency of tube type diffusive samplers: Selection and

performance of adsorbents. Van den Hoed N. and Halmans M.T.H., Am.Ind.Hyg.Assoc.J.(48),

pp364-373, 1987

9 A computer model for calculating effective uptake rates of tube-type diffusive air samplers..

Van den Hoed N. and van Asselen O.L.J., Ann.Occup.Hyg. (35) pp 273-285, 1991.

10 A computer program for prediction of uptake rates for diffusive sampling. Nordstrand E,

Kristensson J., Clean Air At Work- New Trends in Assessment and Measurement for the 1990s.

Eds: Brown RH, Curtis M, Saunders KJ., Vandendriessche S, ISBN 0-85186- 217-9, 1992.

11 A computer program for simulating the function of thick bed diffusive samplers - ideal and

effective uptake rates and a theoretical model for reverse diffusion. Nordstrand E, Kristensson

J., Am.Ind. Hyg. Assoc. J., Vol. 55, No. 10, pp 935-941, (1994)

12 Diffusive uptake rates for the Perkin-Elmer tube - BCR air sampling intercomparisons at VITO

(Mol, Belguim) February 1991-April 1992. Wright MD., UK Health and Safety Executive Report

No. IR/L/IA/93/3.

13 A new method for prediction of binary gas phase diffusion coefficients. Fuller E. N., Schettler

P.D. and Giddings J.C. Ind. Eng. Chem., 58, pp 19-27, 1966

14 EN 482: General requirements for the performance of procedures for workplace measurements