Interactive comment on “ An enhanced procedure for measuring organic acids and methyl esters in PM 2 . 5 ”

Abstract. A solid-phase extraction (SPE) pretreatment procedure allowing organic acids to be separated from methyl esters in fine aerosol has been developed. The procedure first separates the organic acids from fatty acid methyl esters (FAMEs) and other nonacid organic compounds by aminopropyl-based SPE cartridge and then quantifies them by gas chromatography/mass spectrometry. The procedure prevents the fatty acids and dimethyl phthalate from being overestimated, and so allows us to accurately quantify the C4–C11 dicarboxylic acids (DCAs) and the C8–C30 monocarboxylic acids (MCAs). Results for the extraction of DCAs, MCAs, and AMAs in eluate and FAMEs in effluate by SAX and NH2 SPE cartridges exhibited that the NH2 SPE cartridge gave higher extraction efficiency than the SAX cartridge. The recoveries of analytes ranged from 67.5 to 111.3 %, and the RSD ranged from 0.7 to 10.9 %. The resulting correlations between the aliphatic acids and FAMEs suggest that the FAMEs had sources similar to those of the carboxylic acids, or were formed by esterifying carboxylic acids, or that aliphatic acids were formed by hydrolyzing FAMEs. Through extraction and cleanup using this procedure, 17 aromatic acids in eluate were identified and quantified by gas chromatography/tandem mass spectrometry, including five polycyclic aromatic hydrocarbon (PAH): acids 2-naphthoic, biphenyl-4-carboxylic, 9-oxo-9H-fluorene-1-carboxylic, biphenyl-4,4´-dicarboxylic, and phenanthrene-1-carboxylic acid, plus 1,8-naphthalic anhydride. Correlations between the PAH acids and the dicarboxylic and aromatic acids suggested that the first three acids and 1,8-naphthalic anhydride were secondary atmospheric photochemistry products and the last two mainly primary.

Generally, organic acids are extracted from aerosol samples along with other organics by liquid-liquid or solid-phase extraction (SPE).SPE is the most widely used preconcentration technique which mainly depends on the sort of the packing and properties of the solvent.Due to the difference of selectivity, affinity, and capacity of packing, efficient enrichment of target compounds is provided through adsorption or ion-exchange separation mechanism.Adsorption-type SPE, such as a polyurethane foam, silica, and silica-alumina mixture, has been used to concentrate organic analytes from organic extraction in fine aerosol (Fraser et al., 1997;Hou al., 2006;Duan et al., 2006).
SPE also can separate target analytes and clean up interference with good recoveries and enrichment (Rosenfeld, 1999;Ericsson and Colmsjö, 2003;Hou et al., 2006;Zhao et al., 2014).Adler and Siren studied the enrichment of α, ω-dicarboxylic acids (C 2 -C 10 ) by revised-phase polymeric Oasis HLB, Strata X, strong nonpolar Isolute 101 and strong anion-exchange SAX, and J. T. Baker NH 2 and Quaternary Amine SPE materials in the water-soluble fraction of fine aerosols (Adler and Siren, 2014).Results showed that the nonpolar sorbents possessed higher selectivity than ionexchange SPE for C 4 -C 10 DCAs under inorganic matrix.However, an aminopropyl imidazole-modified silica sorbent exhibited high extraction efficiency towards carboxylic acids and PAHs in river water samples based on electrostatic, π -π , and hydrophobic interactions (Wang et al., 2014).The anionexchange SPE cartridges have also been used to cleanup and/or concentrate organic acids in organic matrices.Wang et al. (2013) demonstrated a novel NH 2 /Carb SPE procedure to separate organic acids from indoxacarb/acetonitrile solution based on ion-exchange interaction and electrostatic (Wang et al., 2013).Although the interactions between organic compounds and silica bonded ion-exchange SPE have been investigated, application of anion-exchange SPE to separate polar compounds from complex organic matrices was relative few.Hence, the silica anion-exchanged SPE shows potential for the separation and enrichment of organic acid in fine aerosol.
Gas chromatography/mass spectrometry (GC-MS) as a specific ion detection method has been widely used to study the detailed composition of atmospheric aerosols.For GC-MS, organic acids need to be derivatized to more volatile esters with BF 3 -methanol, BF 3 -butanol, or N,O-bis-(trimethylsilyl)trifluoroacetamide (Kawamura and Kaplan, 1987;Kawamura, 1993;Fu et al., 2009).The butyl and trimethylsilyl (TMS) derivatives are often used for lowmolecular-weight organic acids, including C 2 -C 10 DCAs, C 2 -C 9 ketocarboxylic acids (Boucharat et al., 1998;Limbeck and Puxbaum, 1999;Nolte et al., 2002;Wang et al., 2006;Oliveira et al., 2007).The methyl esters of C 8 -C 32 monocarboxylic acids and AMAs are more volatile and convenient for GC analysis than butyl and TMS esters (Kawamura and Gagosian, 1987a, b;Plewka et al., 2003;Fraser et al., 2003).However, quantification of trace amounts of oxygenated PAHs (O-PAHs), including aromatic acids, identified in urban or diesel particles is still difficult because of the low levels and interference of the complex background compounds (Walgraeve et al., 2010).Moreover, during purification or fractionation which involves evaporation, volatile and/or semivolatile compounds can be lost.To deal with this problem, an advanced pretreatment technique which is capable of enriching and separating target analytes from others and high-resolution analytical methods deserve to be developed.
Tandem mass spectrometry (MS-MS) is an approach which reduces the background caused by the complex matrix by excluding all ions except the parent ion, which can then be fragmented under specific collision energy and generate a unique product ion mass spectrum.GC-MS-MS has been applied to detect and quantify O-PAHs in airborne particulate (Nicol et al., 2001).For this work, we developed a pretreatment technique to aerosol sampled in Beijing during January 2013, when haze was particularly heavy and had spread over much of eastern China.The air quality index in Beijing (Beijing Environment Protection Bureau, http://www.zhb.gov.cn/) had reached nearly 500, and visibility was sometimes below 100 m.The objective of this technique was to separate organic acids from methyl esters with anion-SPE cartridge in the raw extract of fine aerosol and to quantify them by GC-MS and GC-MS-MS.

PM 2.5 sample collection
Samples of PM 2.5 were collected between 1 June 2012 and 30 April 2013.The 30 samples discussed here came from 1 to 30 January, which contained the periods of heaviest haze.The samples were collected on quartz-fiber filters (20 cm × 25 cm Pallflex; Pall Corporation, Port Washington, NY, USA), which were placed into a high-volume sampler (VFC-PM 10 ; Thermo Fisher Scientific Inc., Waltham, MA, USA) at 1.13 m 3 min −1 from 21:00 local time for 24 h.The sampler was installed on the roof of a building at Tsinghua University (40 • 00 N, 116 • 32 E; 52 m a.s.l.).Before sampling, unexposed filters and foils were baked at 560 • C for 6 h to remove organic contaminants.Each sample was wrapped in aluminum foil and stored in a freezer (at −25 • C) until it was extracted.

Sample pretreatment
For sample extraction, an area of 40 cm 2 was punched out of the quartz-fiber filter and extracted successively with 20 mL hexane, dichloromethane (DCM), and acetonitrile in turn (all chromatographically pure; Sigma-Aldrich, St. Louis, MO, USA).Acetonitrile was used as a polar organic extraction solvent (rather than methanol) because it gives better recoveries of fatty acids and avoids the risk of esterification between organic acids and methanol (Polidori et al., 2008).Each extraction lasted for 10 min in an ultrasonic instrument.The extracts were combined and filtered through a 0.45 µm nylon syringe filter (Millex, Billerica, MA, USA) to remove quartz fiber filter particles and insoluble suspended particles.
For SPE, separation of organic acid from other species in solvent extract is conducted by use of two anion-exchange SPE materials: NH 2 cartridge (3 mL, containing 500 mg of a silica-based matrix with bonded aminopropyl active groups; Supelco, Bellefonte, PA, USA) and SAX cartridge (3 mL, containing 500 mg of a silica-based matrix with quaternary amine active groups and Cl − counterions; Supelco) after filtration.Before sample application, the cartridges were preconditioned with 3.0 mL hexane, dichloromethane, and acetonitrile in turn.The 60 mL extract was percolated through the preconditioned cartridge at a flow rate of 1-1.5 mL min −1 (controlled throughout the SPE procedure by adjusting the vacuum).After sample application, the cartridge was washed with 8 mL hexane and then 4 mL dichloromethane, and air was passed through the cartridge to dry it.After washing in this way, the GC-MS chromatogram of the eluate exhibited no noise at all (flat baseline).The retained organic acids were eluted with 2 mL of 5 % HCl-methanol into conical flasks.For enrichment, the eluate was evaporated using a rotary evaporator at reduced pressure then under ultra-pure nitrogen gas to about 100 µL, and then the organic acids in the eluate were derivatized with 14 % BF 3 -methanol (500 µL, Sigma-Aldrich) to obtain the corresponding methyl esters.The mixture was placed in a water bath at 55 • C for 40 min.The reaction mixture was washed with 3 mL of hexane and then 1 mL of a saturated Na 2 SO 4 (aq) solution.The hexane layer (containing the methyl derivatives) was transferred to a clean 2 mL vial (Millex, Billerica, MA, USA) and then reduced in volume to 1 mL under ultra-pure nitrogen gas flow before analysis.The effluate and wash solutions were merged and concentrated by rotary evaporation under reduced pressure until nearly dry, then washed with 2 mL of dichloromethane, and then reduced to 1 mL under ultra-pure nitrogen before being analyzed.

GC-MS and GC-MS-MS analysis
The concentrated organic extracts were analyzed with an Agilent 6890N GC system (Agilent Technologies, Santa Clara, CA, USA) equipped with an Agilent 7683 autosampler, a Quattro Micro GC triple quadrupole mass spectrometer (Waters, Milford, MA, USA), and a SPB ™ -1 fused silica capillary column (30 m long, 0.25 mm i.d., 0.25 µm film thickness; Supelco).Individual analytes were identified by comparing the mass spectra to the standard or searched by the National Institute of Standards and Technology (NIST08) reference library.A 1 µL sample was injected in splitless injection mode at 260 • C. The GC-MS-MS system was operated in multiple reaction monitoring (MRM) mode using the parameters shown in Table 1.The collision gas was argon (99.995 %), at 2.8 × 10 −3 mbar.High-purity helium was used as the GC carrier gas at a flow rate of 1 mL min −1 .The column oven was programmed as follows: the initial temperature of the oven was set at 60 • C, held for 1 min, increased at 9 • C min −1 to 160 • C, held for 1 min, increased at 3 • C min −1 to 250 • C, held for 1 min, increased at 15 • C min −1 to 280 • C, and held for 1 min.The ion source and interface transfer line temperature were 230 and 270 • C, respectively.The analytes were ionized by electron ionization (70 eV), and the emission current was 800 µA.The trap current was 200 mA, the repeller was set at 7.2 V, and the multiplier was set at 650.

Method validation
To investigate the recoveries of analytes and the repeatabilities, 240 cm 2 quartz fibers were punched from one real fiber filter sample and divided into six equal pieces.The quantitative standard mixture was spiked into five of the six pieces, and then the samples were analyzed following the above-mentioned procedures.The methyl esters of nalkanoic acid standard were used to determine the recoveries and to quantify both the n-alkanoic and branchedchain isomer acid and corresponding methyl esters.For 9-oxo-9H-fluorene-1-carboxylic acid, 9-Hydroxy-9-fluorene carboxylic acid methyl ester was used.Hexamethylbenzene/hexane (50 µL, 1.0 ng µL −1 ; Sigma-Aldrich) was used as an internal standard to correct losses due to evaporation and variations in injection volume before each GC injection.The recoveries of organic acids were determined by comparing the whole peak areas of the eluate (after derivatized) with the peak areas from corresponding standard mixtures, and the recoveries of fatty acid methyl esters (FAMES) were determined by comparing the peak areas of the combined effluate and washing solvent (without derivatization) with the corresponding standard mixtures.The recoveries and relative standard deviations (RSD) of analytes are given in Table 1  and 2.
The limits of detection (LODs) were calculated based on a signal-to-noise ratio of 3 (S/N = 3) by analyzing a series of  standard mixtures.The liquid-phase LODs were transformed into the corresponding atmospheric method detection limit (MDL) of the compounds.The MDL as atmospheric concentration furthermore depends on the sampling rate of the filter sampler (1.13 m 3 min −1 ), the sampling time (24 h), the extracted filter area (40 cm 2 ), and the solvent volume for filter extraction (60 mL), for elution (2 mL), and for injection (1 mL).These sampling and sample preparation conditions result in a theoretical sampled air volume of 130.17 m 3 being enriched, and the enrichment factor was 30 in each concentrated SPE extract.The LODs (ng µL −1 ) in solvent were translated to MDL in air concentrations by multiplying 7.68 [= (LOD × concentrated sample solution volume)/sampled air volume].Seven blank filters were spiked, extracted, and analyzed to monitor the extraction procedures and detect possible contamination.The few significant contaminants identified via GC-MS as solvent byproducts were excluded from the data set.

Efficiency of SPE
The extraction efficiency of SPE, expressed as percentage recovery, was calculated as ratio of the concentration obtained by subtracting the measured quantity before adding standard from after adding standard to the theoretical adding standard.The results are shown in Table 1 for DCAs, MCAs, and FAMEs and in Table 2 for AMAs.Extraction efficiencies for DCAs ranged from 80.3 to 93.5 %, for MCAs ranged from 72.4 to 102.4 %, and for FAMEs ranged from 73.6 to 111.3 %.The results were similar with the DCM/menthol extract of organic acids (70-110 %) by ultrasonic without SPE (Huang et al., 2006;He et al.,2006) and hexane/DCM extract of organic acids (75-96 %) by ultrasonic and silica-based or silica mixed alumina SPE (Duan, 2006;Hou et al.,2006), which suggested the adsorption properties were suitable for the extract of organic acids from methyl esters.Method precisions, expressed as the RSD, were calculated across replicate measurements (n = 5).As can be seen from Tables 1  and 2, the RSD of DCAs ranged from 0.7 to 6.2 %, MCAs ranged from 3.3 to 10.9 %, FAMES ranged from 2.2 to 9.2 %, and AMAs ranged from 2.7 to 7.6 %, the repeatability values of which we considered to be acceptable.Adsorption-type SPE, such as silica-based materials, should be preferred for concentration of organic acids in organic matrices but provides little cleanup efficiency of polar functionalized acids, which are likely to be produced in oxidation reactions in the atmosphere.One advantage capacity of anion-exchange SPE was the cleanup efficiency of organic acids from acetonitrile matrices.Although the fatty acids could be separated from their corresponding methyl esters through butyl and TMS derivatization instead of methylation, the low volatility makes the detection by GC of longchain products difficult.To improve the sensitivities, the extraction efficiency of the SAX and NH 2 SPE cartridges for organic acids were investigated with 1 January 2013 provided as an example.Results for the extraction of analytes (DCAs, MCAs, and AMAs in eluate and FAMEs in effluate) are shown in Fig. 1.The total concentrations with the NH 2 SPE cartridge were 430 ng m −3 for nine DCAs, 250 ng m −3 for 17 AMAs, and 6020 ng m −3 for 34 MCAs.They con-F.Liu et al.: An enhanced procedure for measuring organic acids and methyl esters in PM 2.5 trasted with 320 ng m −3 for eight DCAs, 160 ng m −3 for 13 AMAs, and 3950 ng m −3 for 32 MCAs with the SAX cartridge, which indicated that the NH 2 SPE cartridge gave higher extraction efficiency than the SAX cartridge.
Nevertheless, of the two silica-based anion-exchange materials, NH 2 SPE gave higher extraction efficiencies to organic acids with long hydrophobic alkyl chain and polycyclic compounds, especially the C 14 -C 30 MCAs,1,benzoic, 2naphthoic, phenanthrene-1-carboxylic, and undecanedioic acid.In addition, NH 2 and SAX extracted equal amount of 17 FAMEs (251 and 256 ng m −3 , respectively), which indicated that π-π interactions make an important contribution to the extraction.These results indicate that the active group bonded to the silica-based matrix in the SPE cartridge had a marked influence on the extraction efficiency of the analytes from the PM 2.5 .The active quaternary amine groups based on the SAX cartridges might have exclusively electrostatic interactions with weak acids, which will lead some analytes to be retained by the cartridge rather than eluted.The aminopropyl active groups in the NH 2 cartridges offer only weakly selective retention.The results showed that the NH 2 cartridges were the most suitable for isolating organic acids but that the SAX cartridges were most suitable for purifying nonpolar compounds.

Identification of organic acids and FAMEs
Chromatograms of the organic acids in a sample that had been pretreated with an NH 2 SPE cartridge are shown in Fig. 2. The GC-MS chromatogram (Fig. 2a) shows that the organic acids were isolated and concentrated from the extract and the polar and weakly polar compounds had been cleaned (meaning that the background noise level was low).It is clear that there is a complex suite of organic compounds in PM 2.5 extracts, but the selective separation of organic acids improves our ability to detect species that are difficult to determine and increases the sensitivity of the method.
Even separated from the extracts, the signals of the trace multi-substituted AMAs and PAH acids were still under the LODs of GC-MS in SIM mode.The selectivity and sensitivity of the assay was improved by using GC-MS-MS in MRM mode, because only the daughter ions of the selected precursors were detected (Fig. 2b), decreasing the disturbances in the signals and improving the signal strengths.

Ambient concentrations of organic acids in January
Table 1 presents the ambient concentrations of the aliphatic acids in PM 2.5 during January 2013, along with the GC-MS validation parameters.Table 2 presents the ambient concentrations of the aromatic acids and GC-MS-MS validation parameters.The results suggest that the method could also be used on samples from other seasons.Nonane-dioic acid and succinic acid were the most abundant of the DCAs that were measured, with concentrations of 21-398 ng m −3 (mean 124 ng m −3 ) and 6-225 ng m −3 (mean 78 ng m −3 ), respectively.The aromatic DCAs were dominated by 1,2-benzenedicarboxylic (phthalic) acid and 1,4benzenedicarboxylic (terephthalic) acid, with concentrations of 31-286 ng m −3 (mean 131 ng m −3 ) and 13-187 ng m −3 (mean 54 ng m −3 ), respectively.The mean concentrations of the n-alkanoic acids, nalkenoic acids, and FAMEs in the samples from January 2013 are shown in Fig. 3.The alkanoic acids were strongly dominated by those with even numbers of carbon atoms, and the pattern reached a maximum at stearic acid (C 18 : 0 ), concentration ranged from 270 to 3268 ng m −3 (mean of 1491 ng m −3 ), followed by palmitic acid (C 16 : 0 ), ranged from 124 to 1487 ng m −3 (mean of 743 ng m −3 ).The C 18 : 0 and C 16 : 0 acids contributed 20-65 % (mean of 43 %) by mass of the quantified organic acids.
Seventeen FAMEs in the combined effluate and washing solvent were measured, from which the acids had been removed by passing them through the SPE cartridges.Of those, methyl stearate had the highest concentrations, 19-332 ng m −3 (mean of 65 ng m −3 ), and contributed 2-23 % (mean of 6 %) of the total FAME.The next higher one was methyl palmitate, which ranged from 18 to 278 ng m −3 (mean of 63 ng m −3 ) and contributed 2-28 % (mean of 10 %) of the total FAME.The integrated area of the palmitic acid peak in the initial extract of the PM 2.5 sample from 22 January and the area of the methyl palmitate peak in the combined solution (effluate and washing solvent) are shown in Fig. 2d, the methyl palmitate represented 6 % of the palmitic acid, but the highest result can account for 13 % in this period.
Like n-alkanoic acids, the methyl esters of the C 20 -C 30 waxy acids also have even carbon preference.The pattern had a maximum at methyl lignocerate (C 24 ), with concentrations ranging from 26 to 148 ng m −3 (mean of 70 ng m −3 ), followed by methyl hexacosanoate (C 26 ).The methyl esters of the C 13 and C 19 acids were lower than the GC-MS (in SIM mode) detection limits, and they also could not be measured by GC-MS-MS-MRM because of their low molecularion signal strengths.The correlations between C 14 , C 16 , and C 18 methyl esters and among the other C 13 -C 30 methyl esters were high (R 2 > 0.85), which was quite similar to the fatty acids.From the above information and the molecular structures, we suggest that FAMEs could have similar sources to carboxylic acids, be formed by esterifying carboxylic acids, or that a fraction of the fatty acids were formed by hydrolyzing FAMEs.Also, the uncertainty in the measurements of phthalic acid and dimethyl phthalate have been avoided by the use of the NH 2 SPE technique, like fatty acids separated from their corresponding methyl esters.
The correlation between biphenyl-4-carboxylic acid and benzoic acid (0.73) is higher than for other acids.Benzoic acid may be primary from vehicular exhausts (Kawamura and Kaplan, 1987;Rogge et al., 1993a) and secondary from photochemical degradation of aromatic hydrocarbons, such as toluene, emitted by automobiles (Ho et al., 2006;Sun et al., 2006;Li et al., 2009).The aromatic biphenyl is a widely distributed pollutant (Selesi and Meckenstock, 2009), and it is found in coal tar at concentrations of 0.2 to 0.4 %.The oxidation of toluene in the atmosphere is more complicated than the breaking of the single bond that is found in biphenyl, so we suggest that biphenyl-4-carboxylic acid is mainly a secondary product of the photochemical degradation of biphenyl.The biphenyl-4,4 -dicarboxylic acid correlated relatively strongly with di-C 8 acids (0.80), di-C 10 acids (0.82), and terephthalic acid (0.81), which are all mainly primary, as noted above.In view of this, we suggest that biphenyl-4,4dicarboxylic acid is mainly primary.

Summary (for January 2013, Beijing)
Organic acids (alkanoic acids, alkenoic acids, and AMAs) and FAMEs were identified and measured with NH 2 -SPE pretreatment and GC-MS and GC-MS-MS, they contributed strongly PM 2.5 in January 2013 at Beijing.Seventeen FAMEs were separated from their corresponding n-alkanoic F. Liu et al.: An enhanced procedure for measuring organic acids and methyl esters in PM 2.5 acids and the interference of FAMEs on the corresponding fatty acids was eliminated by using the NH 2 -SPE cartridge.Taking into account the large amounts and possible formation mechanism of FAMEs, this procedure is of potential use for determining the relative importance of primary emissions and secondary processes of organic acid esters that are found in typical winter haze episodes in Beijing.
The correlations between the FAMEs and the aliphatic acids were statistically significant, indicating that FAMEs could come from the same sources as the waxy acids.Five PAH acids and 1,8-naphthalic anhydride were identified and measured in PM 2.5 .The correlations between DCA and AMA tracers suggested that 2-naphthoic, biphenyl-4carboxylic, and 9-oxo-9H-fluorene-1-carboxylic acid plus 1,8-naphthalic anhydride were mainly secondary products of photochemical degradation during January 2013.Phenanthrene-1-carboxylic acid could be primary from fossil fuel emission.Biphenyl-4,4 -dicarboxylic could be primary from coal burning.The C 18 : 0 / C 16 : 0 ratio was > 1, which indicated that, apart from the contribution of vehicular emissions, there were significant inputs from cooking emissions in January 2013 in Beijing.

Figure 1 .
Figure 1.Concentrations of analytes found when an extract of PM 2.5 collected on 1 January 2013 in Beijing was analyzed using the SAX and NH 2 SPE cartridges.The left-hand y axis (in black) is for the DCAs, AMAs, and fatty acid methyl esters.The right-hand y axis (in red) is for the MCAs.The T-shaped marks indicate the numbers of compounds (cpds.) that were found.

Figure 2 .
Figure 2. Chromatograms of the organic compounds of the PM 2.5 sample collected on 22 January 2013 in Beijing after pretreatment using an NH 2 -SPE cartridge.(a) GC-MS chromatograms of organic acids in SPE eluate.(b) GC-MS-MS chromatograms of PAH acids in SPE eluate.(c) GC-MS chromatogram of compounds from combined effluate and washing solution.(d) The integrated areas of palmitic acid in the elute (top) and methyl hexadecanoate in the combined solution (bottom).

Figure 3 .
Figure 3. Mean monocarboxylic acids (MCAs) and fatty acid methyl esters (FAMEs) concentrations in PM 2.5 samples collected in January 2013 in Beijing.

Table 1 .
Parameters used to determine organic acids, FAMEs, and the concentrations found in samples collected in January 2013, Beijing.
a Molecular weight.b Retention time.c Method detection limit.

Table 2 .
Parameters used to determine AMAs, dimethyl phthalate, and the concentrations found in samples collected in January 2013, Beijing.

Table 3 .
Correlation matrix of C 4 -C 11 DCAs and AMAs concentrations measured in January 2013, Beijing.C11 i AMA j AMA k AMA l AMA m AMA n AMA o AMA p AMA q AMA r AMA s AMA t AMA u AMA v AMA w AMA x AMA y AMA z h