Needle Trap Devices (NTD)
Needle Trap technology has been introduced in response to the demand for a more robust sampling system, which retains the advantages of SPME for extraction and injection. Stainless steel needles, sized similarly to gas chromatographic injection needles and packed with a sorbent bed, are used for extraction of gaseous samples, followed by thermal desorption into GC systems. All analytes, both freely dissolved in the gas and associated with particulate matter entrained in the sample, are extracted by the devices. The figures below show the designs of the current generation of Needle Trap Devices (NTD).
Different types of sorbents have been used in NTD depending on the sample type and analytes being targeted. Sorbents include quartz wool packing for sampling particulate matter and aerosols, and a variety of particulate sorbents for sampling volatiles (VOCs) and semi-volatiles (sVOCs). For VOCs and sVOCs, a single layer of PDMS coated particles, Tenax, divinylbenzene (DVB), a carbon-based sorbent, or a segmented bed composed of multiple sorbents may be selected depending on the nature of analytes in the sample.
Figure 1. Three different desorption options for needle trap devices. a) External gas assisted desorption; b) Expanded desorptive flow; c) side-hole needle with carrier gas diversion.
For the standard configuration of the device (Figures 1a and b) sorbent is packed near to the tip of the needle. After sampling, desorption in the GC injector is achieved either by applying an external flow of inert gas (Figure 1a) or by plugging the hub of the needle and allowing the rapidly expanding heated gas within the lumen of the needle to facilitate desorption. The latter configuration is particularly useful for breath analysis where the high water content of the sample further facilitates this 'expanded desorptive flow'.
In the side-hole configuration (Figure 1c) the sorbent is packed into the needle between the tip and a side hole. For desorption, the needle is inserted into the hot injector of gas chromatograph. The tip seals against the restriction in a narrow-neck liner (Figure 2), the carrier gas is diverted into the needle through the side hole, subsequently passing through the sorbent, and analytes are thermally desorbed and carried the into the GC column.
NTD may be used for either spot (grab) sampling or integrated (time-weighted-average) sampling. For spot sampling a gas tight syringe or gas sampling pump may be connected to the needle hub and used to draw a pre-defined sample volume through the needle. The gas concentration is determined by determining the amount desorbed and dividing by the sample volume.
For integrated sampling the hub and side-hole are sealed and the open needle tip is exposed to the sample for an extended period of time. The open tip of the needle provides a suitable diffusion restriction to provide for analyte uptake rates proportional to sample concentration for several hours. The amount desorbed is thus related to the average sample concentration during the entire exposure time.
Figure 2. Needle trap with side hole above sorbent. a) The needle tip, hub, and side-hole are sealed prior to use and during storage. b) This design of NTD, when used with a narrow neck GC injection liner, provides simplified, high-efficiency desorption. The side-hole is sealed during extraction and opened during desorption.
As for other gas sampling sorbent tubes, sampling rate and volume should be standardized and minimum breakthrough volume should be determined for the target sample during method development. Instrumentation to facilitate automated processing of NTD is commercially available for both desorption of multiple field-sampled NTD and automated extraction and desorption from sample vials. Automated processing also simplifies method development and the workstation is compatible with a variety of gas chromatographic instruments.
NTD
is
more
robust
than
SPME,
is
an
efficient
particle
filter
and
has
a
higher
sorbent
capacity,
which
makes
it
capable
of
performing
exhaustive
extraction.
Depending
on
the
degree
of
particle
loading
in
the
sample,
the
devices
may
be
re-used
from
a
few
to
dozens
of
times.
To
date
NTD
has
been
used
primarily
for
environmental
analysis
and
breath
analysis
but
is
amenable
to
application
for
additional
analytical
chemistry
applications.
Sampling
from
headspace
of
water
or
solid
samples
by
NTD
is
a
new
and
challenging
topic
in
this
area.
A
review
covering
the
development
of
the
technology
and
current
applications
is
available.
Lord, H. L., Zhan, W., Pawliszyn, J. Fundamentals and applications of needle trap devices, Anal. Chim. Acta, 677 (2010) 3-18.