ppbv, i.e., ambient air levels (in contrast to levels observed in
occupational settings). For detection of ambient HCHO concen-
trations, there are standard methods which use active air sampling
over solid adsorbent cartridges coated with dinitrophenyl hydra-
zine (DNPH) followed by analysis with HPLC.13 A method that
can be used to sample HCHO concentrations above 100 ppbv14
makes use of active air sampling over a solid sorbent coated with
(hydroxymethyl)piperidine (HMP) and analysis by GC. There
are a number of other methods that can be used to sample HCHO
at concentrations above 100 ppbv.15 In addition, it is clear there
is a requirement for better sampling and analysis methods for
HCHO for large concentration ranges as demonstrated by the fact
there are a number of recent methods that attempt to increase
method sensitivity while undertaking to significantly reduce the
time for sampling and analysis and simplifying the overall
method.16-19 Rapid and sensitive measures of HCHO levels during
the manufacture of materials such as foods and cosmetics can
provide enhanced process control.20 Therefore, a sampling and
analysis method for airborne or headspace levels of HCHO, which
is simple to use, highly sensitive and extremely cost-effective, is
required and would be of tremendous benefit.
Shown in this paper is a novel sampling method for gaseous
HCHO employing SPME and on-fiber derivatization with o-(2,3,4,5,6-
pentafluorobenzyl)hydroxylamine (PFBHA). It is not possible to
sample and analyze HCHO with SPME and GC/ FID without
derivatization. Derivatization sampling for HCHO realizes a
number of advantages over techniques not employing derivatiza-
tion: (1) provides analyte specificity based on key functional
groups, (2) allows for detection with conventional detectors (for
those compounds that do not yield a response), (3) can provide
for sampling based on first-order rate kinetics, and (4) can act as
a method for confirmation that the compound of interest is present
in the sample. This new sampling method can be used to sample
HCHO in air and the off-gassing of HCHO from materials. In
addition, it can be used to monitor ambient air levels of HCHO
while the flexibility to quantify high HCHO concentrations is
maintained. The sampling method is rapid and highly reproduc-
ible and can utilize an empirically obtained first-order rate constant
for quantitative analyses without the need for interpolation from
calibration curves; however, if required, the latter is possible. Data
are also included on the use of SPME to sample HCHO under a
number different conditions such as temperature, HCHO concen-
trations, and static versus dynamic gas sampling.
The use of solid-phase microextraction (SPME) has expanded
the range of analytical sampling tools available to scientists by
providing a cost-effective sampling system while significantly
enhancing overall analytical sensitivity and furnishing a large
range of analyte flexibility and selectivity.21 SPME has also been
used for the air sampling of a number of hydrocarbons.22
Sampling gaseous phases for target analytes with SPME provides
a significant advantage over traditional methods, both active
sampling and whole air sampling, since SPME requires no
sampling pumps and is easy to deploy, reusable, and amenable
to automation. At the heart of the SPME sampling device is a
fixed polymeric phase which is directly exposed to target analytes.
The analytes sorb, either via absorption for the fixed liquid films
such as with the poly(dimethylsiloxane) (PDMS) coating or
adsorption as with the PDMS/ divinylbenzene (PDMS/ DVB)
coating. The dimensions of the typical SPME fiber coatings are
1 cm long and less than or equal to 100 µm thick. Following an
appropriate sampling period, the fiber coatings are then directly
inserted into the injector interface of common gas chromato-
graphic equipment. SPME uses no solvents and is completely
reusable, extremely accurate, and reproducible. In addition, there
are a number of commercially available polymer phases and the
list of analytes that can be sampled with SPME is constantly
increasing; however, until now, no method employing SPME for
the specific sampling of HCHO has been successful.
THEORY
There are four steps to consider in describing the overall rate
of formation of oxime on solid sorbent SPME fiber coatings as a
function of the concentration of gaseous carbonyl. Depicted below
are the steps in the sampling system for PDMS/ DVB fiber
coatings loaded with PFBHA, followed by exposure to gaseous
carbonyl (S is the available binding surface of the sorbent),
k1
PFBHA + S
9
k-1
8 PFBHA*S
(adsorption)
(desorption)
(A)
(B)
PFBHA*S 8 PFBHA + S
k2
carbonyl + S
9
8 carbonyl*S
(adsorption)
(desorption)
k-2
carbonyl*S 8 carbonyl + S
carbonyl + PFBHA*S
9
K*8 oxime*S
(reaction) (C)
(D)
k3
9
oxime*S
8 oxime + S
(desorption)
and where KA ) k1/ k-1 and KB ) k2/ k-2 (see below). The first
step (A) is to load the sorbent with PFBHA. From experimental
data, it is understood that following loading the sorbent with
PFBHA, its rate of desorption is negligible, i.e., k1 . k-1. The
second step to consider is the possibility that an approaching
gaseous carbonyl molecule can bind to unoccupied surface sites
(B); however, in step B, the rate of carbonyl adsorption is expected
to be small, i.e., k2 ≈ O, because almost all sorption sites are
occupied by PFBHA. The third step to consider is C, where the
rate of reaction between sorbed PFBHA and gaseous carbonyl is
K*. It is assumed that the PFBHA aromatic moiety provides the
majority of binding affinity to the polymer while the hydroxylamine
moiety is free to react with an approaching carbonyl compound
(see Figure 1A for the structure of PFBHA). It is also assumed,
(13) Method TO-11, EPA-600/ 4-89-017. Compendium of Methods for the Determi-
nation of Toxic Organic Compounds in Air; U.S. Environmental Protection
Agency, Research Triangle Park, NC, 1988.
(14) NIOSH Method 2541. NIOSH Manual of Analytical Methods, Electronic
Version, 1994.
(15) Goelen, E.; Lambrechts, M.; Geyskens, F. Analyst 1 9 9 7 , 122, 411-419.
(16) Lange, J.; Eckhoff, S. Fresnius J. Anal. Chem. 1 9 9 6 , 356, 385-389.
(17) Luong, J.; Sieben, L.; Fairhurst, M.; de Zeeuw, J. J. High Resolut. Chromatogr.
1 9 9 6 , 19, 591-594.
(18) Shi, Y.; Johnson, B. J. Analyst 1 9 9 6 , 121, 1507-1510.
(19) Chan, W. H.; Huang, H. Analyst 1 9 9 6 , 121, 1727-1730.
(20) Tashkov, W. Chromatographia 1 9 9 6 , 43, 11-12.
(21) Pawliszyn, J. Solid-Phase Microextraction-Theory and Practice; Wiley-VCH:
New York, 1997.
(22) Martos, P.; Pawliszyn, J. Anal. Chem. 1 9 9 7 , 69, 206-215.
2312 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998