Laboratory oxidation studies have identified a large number of oxygenated
organic compounds that can be used as tracers to understand sources and
oxidation chemistry of atmospheric particulate matter. Quantification of
these compounds in ambient environments has traditionally relied on low-time-resolution
collection of filter samples followed by offline sample
treatment with a derivatizing agent to allow analysis by gas chromatography
of otherwise non-elutable organic chemicals with hydroxyl groups. We present
here an automated in situ instrument for the measurement of highly polar
organic semi-volatile and low-volatility compounds in both the gas- and
particle-phase with hourly quantification of mass concentrations and
gas–particle partitioning. The dual-cell semi-volatile thermal desorption
aerosol gas chromatograph (SV-TAG) with derivatization collects
particle-only and combined particle-plus-vapor samples on two parallel
sampling cells that are analyzed in series by thermal desorption into helium
saturated with derivatizing agent. Introduction of MSTFA
(N-methyl-N-(trimethylsilyl)trifluoroacetamide), a silylating
agent, yields complete derivatization of all tested compounds, including
alkanoic acids, polyols, diacids, sugars, and multifunctional compounds. In
laboratory tests, derivatization is found to be highly reproducible
(
In both urban and rural environments, the majority of submicron particulate matter is organic (Jimenez et al., 2009; Zhang et al., 2007), mostly formed through the oxidative conversion of gas-phase anthropogenic and biogenic emissions into secondary organic aerosols (SOA) (Goldstein and Galbally, 2007; Hallquist et al., 2009; Kroll and Seinfeld, 2008). One of the primary tools to understand oxidation pathways and source apportionment is the measurement of organic tracers: compounds known to come from specific emission sources and/or form through oxidation under specific conditions. Tracers are typically quantified using samples collected on traps or filters and extracted with solvents or thermally desorbed to be analyzed by gas or liquid chromatography (GC, LC) coupled to a mass spectrometer (MS) (Cass, 1998; Hamilton, 2010; Mazurek, 2002; Schauer et al., 1996; Simoneit, 2005). These tracers include non-polar compounds such as hydrocarbons that are conducive to analysis by GC–MS without extensive sample preparation, as well as polar compounds that require analysis by LC–MS or treatment with a derivatizing agent prior to GC analysis.
For GC analysis, filter samples are typically reacted with a derivatizing
agent following solvent extraction to form esters and ethers from hydroxyl
groups, which suffer from poor transfer through GC columns due to hydrogen
bonding interactions. Diazomethane, a common derivatizing agent, selectively
converts acid groups into methyl esters and has been used to quantify acids
and diacids in petroleum sources (Fraser et al., 1998;
Schauer et al., 1999b, 2002), meat cooking (Schauer et al.,
1999a), and biological processes (Simoneit, 2005). Source
profiles containing these derivatized acids in combination with hydrocarbons
(including alkanes, hopanes, steranes, and polycyclic aromatic
hydrocarbons), and biomass-burning tracers (Fraser and Lakshmanan,
2000; Ramdahl, 1983) have been used extensively in the analysis of
particulate matter in urban environments (Fraser
et al., 1999; Hildemann et al., 1991; Kubátová et al., 2002; Lough
et al., 2006). Alternately, derivatization can be performed using silylating
agents, which exhibit broader reactivity, converting all OH groups into
silyl esters and ethers. A large number of such reagents are available with
minor variations in volatility and reactivity (Fluka Chemie AG, 1995),
but the most commonly used in atmospheric applications is BSTFA
(N,O-bis(trimethylsilyl)trifluoroacetamide), which yields trimethylsilyl
(TMS) protecting groups as a derivatized product. Using BSTFA
derivatization, many compounds have been identified in the
laboratory-controlled oxidation of common biogenically emitted compounds,
including isoprene (Chan
et al., 2010; Claeys et al., 2004; Kleindienst et al., 2009; Szmigielski et
al., 2007), some monoterpenes
(Claeys et al., 2007,
2009; Jaoui et al., 2005), and some sesquiterpenes (
Derivatization can be performed in situ without the need for solvent extraction and offline reaction. Docherty and Ziemann (2001) demonstrated that co-injecting BSTFA into a hot GC inlet alongside the sample efficiently derivatizes organic acids, which is thought to proceed by fast reactions occurring in the gas phase after volatilization of the derivatizing agent and the analytes. Desorption of filters into a stream of derivatizing agent has been used in the analysis of atmospheric samples and laboratory oxidation experiments using both diazomethane (Sheesley et al., 2010) and silylating agents (Orasche et al., 2011; Ruehl et al., 2013; Zhang et al., 2013), but it is not yet a widely used technique.
Since most previous particle-phase tracer measurements have relied upon offline analysis, the time resolution of sample collection has typically been on the order of several hours to several days. Through development of the thermal desorption aerosol gas chromatograph (TAG), Williams and co-workers (Kreisberg et al., 2009; Williams et al., 2006) were able to measure non-polar and low-polarity tracers with hourly time resolution, resulting in insights into particle formation pathways and source apportionment (Williams et al., 2010a, b; Worton et al., 2011). A newer version of this instrument uses a high-surface-area filter to collect gas- and particle-phase low-volatility and semi-volatile compounds, considered here as any compounds expected to partition between the gas and particle phase under typical atmospheric conditions (SV-TAG; described by Zhao et al., 2013b). This has expanded the utility of this instrument into measurements of gas-to-particle conversion pathways (Zhao et al., 2013a). However, while this instrument provides the highest available time resolution for the measurement of organic tracer compounds, it has thus far been limited to hydrocarbons and less-polar oxygenates (oxygen-to-carbon ratio lower than 0.3 in most cases). Most known tracers for the oxidation of biogenic compounds are too polar to be measured by previous TAG instruments, as they often exhibit an oxygen-to-carbon ratio between 0.5 and 1.
Here we describe modifications to SV-TAG (semi-volatile thermal desorption aerosol gas chromatograph) to enable measurements of highly polar oxygenates with hourly time resolution through the inclusion of in situ derivatization. To validate the implementation, we demonstrate here that derivatization occurs to completion, yields linear calibrations, and is reproducible. We also present a rigorous quantification of error for calibrated data.
Schematic of dual-cell SV-TAG with in situ derivatization. Two
parallel collection and thermal desorption cells (CTD) simultaneously
sample, one directly collecting gas-plus-particle-phase compounds and one
through a denuder that removes all gas-phase compounds, thus collecting only
particles. Each sample is transferred in series to the GC–MS through a
two-stage purge-and-trap thermal desorption process. Purge helium is
saturated with derivatization agent by passing through a reagent reservoir
(inset). Derivatized and underivatized desorption flows are controlled
independently using MFC
The instrument described in this work is shown in Fig. 1. Briefly, it is a modified SV-TAG, a custom in situ instrument for quantifying gas- and particle-phase semi- and low-volatility organic compounds in the atmosphere (Kreisberg et al., 2009; Williams et al., 2006; Zhao et al., 2013b). SV-TAG measures organic compounds in both the gas and particle phase through collection onto a custom collection cell containing a reusable, high-surface-area metal fiber filter, the implementation and operation of which is described in detail by Zhao et al. (2013b). Two identical cells are used in parallel to collect simultaneous measurements with and without passing through a denuder to directly measure gas–particle partitioning. Thermal desorption of these compounds includes in situ derivatization and subsequent analysis by gas chromatography–mass spectrometry. Quantification of mass concentration and particle fraction is achieved with hourly time resolution. The implementation of two important novel instrument components is described and validated here: parallel dual-cell sampling system (Sect. 2.1.2) and in situ derivatization (Sect. 2.1.3).
Sample is collected at 10 L min
Samples are transferred from the CTD to the GC–MS in a two-step thermal
desorption process. First, compounds are desorbed from the cell into a
stream of helium saturated with a variable quantity of derivatizing agent,
ramping between 30 and 315
Calibration and correction for run-to-run variability is performed through
regular injection of liquid standards into the CTD using an automatic
injection system (“AutoInject”). A two-position valve containing a
4
Outfitting SV-TAG with a parallel dual-cell system provides flexibility for improved measurements and detailed method comparisons. Parallel cells are sampled simultaneously and then analyzed in series; samples can be collected or analyzed in two different ways. In a typical field deployment, gas–particle partitioning is targeted by collecting samples with different treatments: one cell collects an undenuded sample (gas-plus-particle), while the other collects a denuded sample (particle-only, all gas-phase species removed). The roles of the cells are swapped with each sample to avoid cell-to-cell bias. The dual-cell SV-TAG instrument can therefore directly measure gas–particle partitioning without any need for interpolation between points, a source of error in previous measurements of partitioning with TAG (Williams et al., 2010a; Worton et al., 2011; Zhao et al., 2013a). Alternately, when comparing methods of derivatization, each cell can be subjected to different treatments (i.e., derivatized and non-derivatized), allowing direct comparison between methods on ambient samples.
To ensure the identical sample size of each cell, sample flow is regulated
independently through two matching mass flow controllers (e.g., MFC
A typical duty cycle consists of 22 min of sampling on both cells, injection of standards (2 min), two-step desorption (12 min) and chromatographic analysis (14 min) of CTD1, and then desorption and analysis of CTD2, while collection of the subsequent sample begins after exactly 60 min. The valveless interface decouples the collection cells from the GC, allowing hourly time resolution by performing GC–MS analysis during CTD desorption and sampling.
Derivatization is implemented by saturating helium with derivatizing agent,
and using that saturated helium as a portion of the total CTD purge flow
during thermal desorption. Desorption flow is controlled as the sum of
helium flow through two mass flow controllers: MFC
The quantity of derivatizing agent introduced to the sample can be optimized
by controlling the fraction of total purge flow that is routed through the
reagent reservoir,
Dozens of derivatizing agents exist in the literature to improve GC analysis
for a wide variety of functional classes (Blau and Halket, 1993), but
the most commonly used reagents for atmospheric organic tracer analysis are
silylating agents, primarily BSTFA, due to their efficiency and broad
reactivity (all OH groups). As previous research has demonstrated the
utility of this agent for efficient, fast, gas-phase derivatization
(Docherty and Ziemann, 2001), silylating agents in the chemical family
of BSTFA are used in this work. MSTFA has been used for the dual-cell SV-TAG presented here
due to its similar reactivity but higher reagent and byproduct volatility than
BSTFA (Fluka Chemie AG, 1995), allowing all non-analytes to be
efficiently purged across the focusing trap. MSTFA has an estimated vapor
pressure approximately equal to decane (C
Derivatization efficiency was tested through repeated automatic injection of
a complex mixture of polar and non-polar compounds containing most
functional groups present in known atmospheric tracer compounds and spanning
a wide-range of volatilities. Response of 43 oxygenated compounds introduced
by an automated liquid standard injector is examined under varying
derivatization conditions. To provide a conservative test, injected
quantities of most constituents were larger than expected from atmospheric
sampling. Total injected mass was 2.0
Under laboratory conditions, reproducibility was measured through repeated
injection of an atmospheric aerosol sample: a filter extraction of a
high-volume (67 m
Responses of internal standards over 1 month of field data during two
separate field campaigns are used to estimate error in correcting for
run-to-run variability and reproducibility in real-world data. Dual-cell
SV-TAG with derivatization was first deployed as part of the Southern
Oxidant and Aerosol Study (SOAS) in the summer of 2013 in rural Alabama
(32.903
Effective, quantitative derivatization must be shown to be complete and reproducible, with linear or predictable response. Dual-cell SV-TAG with derivatization was shown to meet these requirements through a series of laboratory tests, as well as field deployment to the southeastern US and rural Brazil.
Response of 43 oxygenated compounds dependent on quantity of MSTFA
introduced, normalized to the average response of each compound at
Derivatization is shown in Fig. 2 as a function of quantity of
derivatization agent introduced, and it is found to occur to completion during
normal instrument operation. In a scenario of incomplete derivatization, a
small reduction in reagent would be expected to reduce the response of all
derivatized compounds in proportion to the number of OH groups on each
compound. For a large number of oxygenated compounds injected into dual-cell
SV-TAG, a reduction in signal is not observed when the fraction of derivatizing
agent,
With severely reduced quantities of derivatizing agent, detector response
was greatly diminished due to incomplete reaction because these compounds
cannot be measured by traditional gas chromatography in the absence of
derivatization. Compounds containing more hydroxyl groups are more sensitive
to a dramatic reduction in
Comparison of reproducibility of injection and derivatization of
50 consecutive injections of a methanol extraction of a filter collected in
the Sierra Nevada of California. Responses of 1410 peaks in each
injection were compared to the average response of each peak. This
comparison is shown for
Derivatization was found to be highly reproducible in a laboratory setting
through the repeated injection of a complex mixture. A methanol extraction
of a filter collected in the Sierra Nevada of California in the
summer of 2009 was injected 50 consecutive times into an I-CTD to quantify
the reproducibility of derivatization of an atmospherically relevant complex
mixture (Fig. 3). Figure 3a shows a comparison of all peaks in one of these
injections to the average areas, which is found to deviate from unity by
less than 1 %, with a correlation coefficient of 0.998, across
approximately 4 orders of magnitude in peak area. These correlation
metrics are consistent across all 50 injections (Fig. 3b), with a slope
almost always within 2 % of unity and very high correlation coefficients
(
Satisfied with
The ratio of two internal standards relative to their average
ratio, with relative standard deviations shown. Distributions of error from
two periods of field data collection (dashed: Study 1; solid: Study 2) are
shown for scenarios most closely representing operating conditions of TAG:
Figure 4 highlights the correction scenarios that apply to operational
conditions typically used in deployment of SV-TAG with derivatization. Given
the wide range of alkanes present in the internal standard, perdeuterated
alkanes that are close in volatility can be used to correct variability in
response to hydrocarbon analytes. The ratio of one perdeuterated alkane
(representing a hydrocarbon analyte) to the next closest in volatility has a
relative standard deviation of 8.4 % (Fig. 4a) during Study 1 and 11.2 %
during Study 2. More than 80 % of all points fall within these standard
deviations. Transfer efficiency can change dramatically between two carbon
numbers for species of high molecular weight, so a large fraction of this
error is due to real differences in response caused by variations in sample
loading; when considering only hydrocarbons not affected strongly by
transfer efficiency changes (smaller than C
Error in correction for run-to-run changes in derivatization efficiency
depends strongly on available internal standards due to its dependence on
functionality. In typical field deployment of TAG, most atmospherically
relevant oxygenated compound classes that can be detected by thermal
desorption, derivatized GC are represented by at least one internal
(isotopically labeled) standard: mono-carboxylic acids, di-carboxylic
acids, polyols, sugars, and hydroxy acids. Most of these compound classes
are limited to only one internal standard due to cost and availability of
isotopically labeled standards, but several perdeuterated
Standard deviation in error for correction of run-to-run variability of oxygenates using a variety of possible scenarios for pairing analytes to internal standards. Full distribution of error for scenarios in bold are shown in Fig. 4. In all cases, “nearest” refers to closest in volatility.
The scenarios for error in Fig. 4 are those that apply to most compounds under typical operation of TAG. However, error depends on the availability of chemically similar standards, so estimates of error have been provided in Table 1 for various scenarios representing possible selections of internal standards (scenarios highlighted in Fig. 4 are shown in bold).
In the absence of a wide array of internal standards, a small range of
oxygenates can be used with similar functionality (scenario 4) or volatility
(scenarios 3, 5) with an error of 20–30 %. Note that, based on scenario 3,
an oxygenated compound can be measured with a precision of
One month of multi-point calibrations of sample compounds during
Study 1 (both cells shown as circles and squares). Response is corrected
using a structurally similar deuterated internal standard injected into
every run, as is standard operating procedure to correct for run-to-run
variability (line color represents the scenario of correction from Fig. 4).
The average response of the instrument to one loop of concentrated standard
is used to normalize all data to demonstrate
Linear response of derivatized compounds within the range of expected
analyte abundances is necessary for accurate quantification of oxygenated
tracer compounds. A suite of over 100 authentic standards was injected at
various levels two to three times per day throughout Study 1 during the SOAS field
campaign. Daily schedules typically included injection of a single
AutoInject loop of “concentrated” standard (12–32 ng of each compound) and
one injection of one to three loops of “dilute” standard (1.2–3.2 ng of each
compound per loop), with occasional injections of two loops of concentrated
standard. Once corrected for variability using internal standards (per Sect. 3.2), calibration curves were found to be linear in nearly all cases.
Examples of four calibration curves spanning several atmospherically relevant
functional classes are shown in Fig. 5, with quantities injected that are
approximately in the range expected in the atmosphere for typical sample
sizes, and instrument response normalized to the response of one loop of
concentrated standard. Several correction scenarios shown in Fig. 4 are
represented: correction of a hydrocarbon using a hydrocarbon of similar
volatility (Fig. 5a: hexadecane corrected using hexadecane–d
Calibrations are in all cases linear, and spread in the data is typically limited to 10–20 %, within the precision error for run-to-run variability correction estimated by detailed error analysis in Sect. 3.2. Following correction by internal standards, both cells fall on the same calibration curves because any difference in sensitivity to a compound between the cells is mirrored in the sensitivity to a chemically similar internal standard. Consequently, uncertainty in mass concentrations is dominated by the correction for instrument and derivatization variability. As the ratio of signal from both cells, uncertainties in variability correction combine to yield approximately 20–25 % uncertainty in particle fraction. The Supplement contains a discussion of propagation of error in mass calibration, equalization between the two cells, and calculation of particle fraction, as well as the equations necessary to calculate uncertainty on a point-by-point basis.
Sensitivity to compounds is dependent on functionality, such that limits of
detection and quantitation are compound specific. Calibrations shown in Fig. 5
extend down to approximately 1–2 ng injected mass, though in most cases
analyte signal at this level is still 10–100 times chromatographic
background signals. As an example, erythritol as shown in Fig. 5d during
Study 1 is 20 times larger than background at 1.6 ng, and during Period 2 it is
10 times larger than background at 0.6 ng. Consequently, though limits of detection must
be calculated individually for each analyte of interest, a reasonable
average estimate is on the order of tenths of a nanogram, though in some
cases it is lower than 0.1 ng. When operating with hourly time resolution,
sample is collected for approximately 20 min at 10 Lpm, yielding
conservative limits of detection of 1–2 ng m
Sample final data of pinic acid, a known tracer for the oxidation
of
A sample of the data available using dual-cell SV-TAG with derivatization is
shown in Fig. 6, a short sample timeline from the SOAS field campaign. Three
days of hourly concentrations of gas- and particle-phase pinic acid, a
product of the atmospheric oxidation of
We present here an instrument for hourly measurement of both gas- and particle-phase oxygenated organic compounds in the ambient atmosphere. Though quantification of organic tracers has provided significant insight into atmospheric oxidation chemistry and aerosol formation pathways, traditional measurement techniques require sample times of several hours to several days, as well as offline sample processing. By modifying SV-TAG, a field-deployable instrument, to include online derivatization, known and novel organic tracers can be quantified hourly in situ. Inclusion of a dual-cell, parallel sampling system allows simultaneous measurement of both gas- and particle-phase components through a denuder-difference method. Automation of the dual-cell SV-TAG with derivatization provides improved time resolution over traditional measurement techniques with decreased operator interaction and offline sample preparation, minimizing exposure to derivatizing agent and solvents.
Derivatization is found to be reproducible, complete, and linear in all
laboratory and field tests performed. Field-deployable, in situ
derivatization of polar compounds is found to be robustly quantifiable
within approximately 10–20 % by using internal standards to correct for
variability in detector sensitivity and derivatization efficiency (range
represents similarity of internal standard to analyte). Non-polar compounds
can also be measured by this instrument with an error of
Atmospheric phenomena often occur on the scale of hours (i.e., break-up of inversion layers, rain events), so the time resolution provided by dual-cell SV-TAG with derivatization will enable a far more detailed analysis of atmospheric chemical pathways with similar accuracy to lower-time-resolution techniques. Furthermore, direct, hourly measurement of gas–particle partitioning will provide observational constraints to assess the relative importance of a wide variety of partitioning pathways that have been studied in great detail in laboratory experiments. Future exploration of alternative derivatization agents is also expected to expand the utility of this instrument.
Instrument development work was supported by the US Department of Energy SBIR/STTR under grant DESC0004698. G. Isaacman is supported by the National Science Foundation (NSF) Graduate Research Fellowship (NSF grant: DGE 1106400). Data collection as part of the SOAS field campaign was funded by NSF Atmospheric Chemistry Program grant no. 1250569. Internal standard reproducibility data collection in Brazil was thanks to the NSF Atmospheric Chemistry Program grant no. 1332998. Filter collection for the BEARPEX campaign was funded by NSF Atmospheric Chemistry Program grant no. 0922562.Edited by: P. Herckes