A miniaturized cryogenic trap design for collection of arsanes

Spectrochimica Acta Part B 111 (2015) 46–51

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Technical note

A miniaturized cryogenic trap design for collection of arsanes Milan Svoboda ⁎, Jan Kratzer, Miloslav Vobecký, Jiří Dědina Institute of Analytical Chemistry of the CAS, v. v. i.; Veveří 97, 602 00 Brno, Czech Republic

a r t i c l e

i n f o

Article history: Received 9 April 2015 Accepted 22 June 2015 Available online 26 June 2015 Keywords: Hydride generation Atomic absorption spectrometry Arsanes Cryotrapping Radioactive indicator

a b s t r a c t A new miniaturized design of a cryogenic trap for collection of arsanes generated from inorganic and methylsubstituted arsenic species was developed. The detection was performed by atomic absorption spectrometry. The miniaturization of the cryogenic trap was achieved by replacing the commonly used quartz U-tube (45 cm long, 2.5/4.3 mm i.d./o.d., packed with chromosorb) by a U-shaped quartz capillary (U-capillary, 20 cm long, 0.53/0.65 mm i.d./o.d.). The type and material of the gas phase dryer, an essential part of the cryotrapping system, were necessary to optimize to prevent blockage of the U-capillary by frozen water and to prevent loss of arsanes. A cartridge with solid NaOH was found as the best solution because of a higher absorbing efficiency of water compared to commonly used nafion membrane. The diameter of the NaOH beads was found as a crucial parameter influenced loss of arsane. Processes during the cryotrapping procedure with the U-tube and U-capillary were investigated by 73As radioactive indicator and arsane trapping and volatilization efficiency were quantified. Trapping and volatilization efficiency of 100% were found in the U-tube as well as in the U-capillary. Relevant experimental parameters for the collection in the U-capillary (carrier gas flow rate, column length and capacity) were studied. Finally, miniaturized, simple and commercially available design of the cryogenic trap based on non-polar fused silica capillary successfully performed the collection procedure of arsanes. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Arsenic determination plays an important role in the analysis of environmental samples [1–10]. Metabolism studies of arsenic are widely performed [11–16] as well as toxicity studies of arsenic species [17–19]. Since arsenic species, namely inorganic arsenic (iAs), methylarsonate (MAs), dimethylarsinate (DMAs) and trimethylarsine oxide (TMAsO) differ in toxicity, [17,20] appropriate arsenic speciation analysis method is highly desirable. Although separation by means of high pressure liquid chromatography (HPLC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) detector is the most common approach to arsenic speciation analysis, an alternative technique has also been explored and found its use in analytical applications. It combines selective generation of substituted hydrides (HG) coupled to a cryogenic trap (CT) employed for collection/separation of analytes with subsequent detection most often by atomic absorption spectrometry (AAS). The inherent advantage of this HG-CT approach compared to the common method for arsenic speciation (HPLC-ICP-MS) is a much lower risk of changes of speciation thanks to minimum sample pretreatment and primarily because separation on HPLC column is avoided. The detailed description and several applications to environmental and biological samples of HG-CT can be found elsewhere [21–26]. ⁎ Corresponding author. E-mail address: [email protected] (M. Svoboda).

http://dx.doi.org/10.1016/j.sab.2015.06.014 0584-8547/© 2015 Elsevier B.V. All rights reserved.

The general aim of this work was to develop the cryogenic trap with respect to (i) miniaturization, (ii) simplification and (iii) to reach 100% collection (trapping and volatilization) efficiencies. The developed miniaturized cryogenic trap will be used in the future with gas chromatograph and atomic absorption spectrometer for arsenic speciation analysis.

2. Experimental 2.1. Reagents All reagents were of analytical reagent grade or higher purity. Deionized water (b0.1 μS cm−1, Ultrapure, Watrex, USA) was used to prepare solutions. Working arsenic standards were prepared from 1000 mg l−1 As stock solution of the following compounds: iAs from As2O5 in 0.5 mol l−1 HNO3, MERCK, Darmstadt, Germany; MAs from Na2CH3AsO3·6H2O, Chem. Service, West Chester, PA, USA; DMAs from (CH3)H2AsO2, Strem Chemicals, Inc., Newburyport, MA, USA; TMAsO [15] was obtained by courtesy of Dr. William Cullen, University of British Columbia, Vancouver, Canada and Miroslav Stýblo, University of North Carolina at Chapel Hill, Chapel Hill, USA. The reductant was 1% (m/v) solution of NaBH4 (Sigma-Aldrich, Germany) in 0.1% (m/v) KOH (p.a., Merck, Darmstadt, Germany) filtered after preparation and stored frozen. The buffer was 0.75 mol l−1 Tris (hydroxymethyl) aminomethane (TRIS·HCl buffer was prepared

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47

from a reagent grade Trizma® hydrochloride (Sigma)) and adjusted by 10% NaOH (Lachema, Czech Republic) to pH 6. L-Cysteine hydrochloride monohydrate ((C3H8ClNO2S·H2O) Merck, Darmstadt, Germany) was used for pre-reduction of pentavalent arsenic species. 1 mol l− 1 HCl (p.a., Merck, Darmstadt, Germany) was used for cleaning the hydride generator. 38% (m/v) HF (p.a., Spolchemie, Ústí nad Labem, Czech Republic) and 65% HNO3 (p.a., Lach-Ner, Czech Republic) mixed in the ratio of 3:7 (v/v) were used to clean the multiatomizer. The sodium hydroxide beads (NaOH, purum, pearls, bead diameter of 2–3 mm usually with 3% (w) of particles under 2 mm, Lach-Ner, Czech Republic) and (NaOH, purum, micro pearls, bead diameter of 1–1.9 mm, Lach-Ner, Czech Republic) were used as a filling of a dryer cartridge for water vapor and aerosol removal. A filling of the U-tube was realized by CHROMOSORB, OV-3 WAW-DMCS 45/60, Supelco, Bellefonte, USA. The silanization agent was REJUV-8 (hexamethyldisilazan, N,O-bis (trimethylsilyl) acetamid, (n-trimethylsilylimidazole) Supelco, Bellefonte, USA).

2.2. AAS instrumentation A Perkin Elmer AAnalyst 800 (Norwalk, Mass, USA) atomic absorption spectrometer equipped with FIAS 400 flow injection accessory (FIAS) was used as a detector. An arsenic electrode discharge lamp (EDL) powered by Perkin Elmer EDL System II was operated at 376 mA using 193.7 nm arsenic line. Deuterium background correction was used. The slit width was set to 0.7 nm. A multiple microflame quartz tube atomizer (multiatomizer, model MM5 in Ref. [27]) heated resistively to 900 °C and supplied with 35 ml min−1 of air as outer gas was employed. The transversally heated graphite furnace atomizer (THGA, on AAnalyst800, Perkin Elmer with Zeeman correction, with end cap and platform modified with 40 μl 1000 mg/l iridium) was employed to check the concentration of standard solution of each arsenic species after its preparation.

Fig. 1. The U-capillary immersion in the Dewar flask with a polyethylene foam lid on the top; the input and output ends of the U-capillary are heat-sealed in the teflon tubes; the arrows show the input and output of the carrier gas.

capillary in the volatilization step of the collection procedure is performed by ambient temperature only. The U-capillary was immersed in a Dewar flask (2 l) with liquid nitrogen during the trapping step. A special lid on the top of the Dewar flask made from polyethylene foam was designed (see Fig. 1) to reduce the risk of U-capillary blockage by frozen water (see Section 3.2.1 for detailed explanation). In both CT designs the gaseous phase containing arsanes has to pass through a drying tube (dryer) to reduce the water vapor content to the tolerable extent before reaching the cryogenic trap. Otherwise blockage of the cryogenic trap by frozen water occurs which presents a serious problem.

2.3. Hydride generator and cryogenic traps

2.4. Procedure

The setup consists of a flow injection hydride generator (3 channels with the same flow rates of: 1% NaBH4 in 0.1% KOH; 0.75 mol l−1 TRIS buffer; deionized water with sample coil) described earlier [24] coupled to a cryogenic trap (CT) via a dryer. Two dryer set-ups are employed: (i) a nafion membrane (MD-110-12FP, Perma Pure, Toms River, N.J., USA, with 2 l min− 1 N2); (ii) a polypropylene cartridge filled with solid NaOH. The usability of these two types of dryers is described below (Section 3.1.1 Elimination of the non-specific absorption and selection of a dryer). Two designs of CT are used. The first CT design (see Ref. [24] for details), further termed here as the U-tube, is based on a U-shaped tube (borosilicate tube i.d./o.d. 2.5/4.3 mm, 45 cm long) partially filled with chromosorb and treated with REJUV (100 μl and flow of helium for 5 h). The U-shaped tube, wrapped with a resistance Ni–Cr wire (15 Ω), is fixed in a special glass flask which serves for automatic filling of liquid nitrogen and immersed in a Dewar flask (5 l). The new miniaturized CT design, further termed as U-capillary in the text, is realized by a U-shaped nonpolar fused silica capillary (20 cm, 0.53/0.65 mm i.d./o.d. covered by polyimide, nonpolar fused silica capillary, Supelco, USA) with 90% of the length inserted into a teflon tube of 0.75/1.56 mm i.d./o.d. to protect the capillary against mechanical and overpressure damage. A piece of polyethylene foam insulation serves as a holder for stable shape of the capillary. The input end of the capillary was sealed in a teflon tube (0.50/1.52 mm i.d./o.d., 10 cm length). The output end of the capillary is interfaced to the multiatomizer in the same way (see Fig. 1). The heating of the U-

Two solutions containing arsenic species are prepared: i) a mixture of iAs, MAs, DMAs and TMAsO and ii) only TMAsO. The first solution is mixed with solid L-cysteine hydrochloride monohydrate to the final concentration of 2% m/v at least 1 h prior to analysis. The second is not [24,28]. All arsenic species are converted to trivalent forms in order to reach 100% efficiency of hydride generation. TMAsO is converted by L-cysteine to (CH3)3As which is gradually released from the solution before hydride generation procedure can start [15,29]. That is the reason to prepare the second solution containing only TMAsO. The cryotrapping procedure was similar for both CT designs. It consisted of two steps, trapping and volatilization. In the first step, arsines released from the hydride generator are trapped in a CT. Subsequently, after finishing hydride generation, trapped arsines are released from a CT and transported to the multiatomizer in the volatilization step. The detailed description of the HG-CT-AAS procedure can be found in Ref. [24], only a brief description is thus presented here. During hydride generation in the trapping step, all the investigated arsenic species are converted to corresponding arsanes. Three reagent flows are pumped at the same flow rate into three respective channels of the hydride generator [24]: water as the carrier, the buffer and the reductant. 0.5 ml sample volumes were injected into the carrier water flow. Arsanes released from the gas–liquid separator of the hydride generator are dried and trapped in the CT cooled by liquid nitrogen. In the volatilization step the CT is heated to volatilize trapped arsanes, stepwise according to their boiling points (the first AsH3, the second CH3AsH2, the third (CH3)2AsH, the last (CH3)3As). and transport

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them by a carrier gas flow to the multiatomizer. Either resistive heating (U-tube) or heating by ambient temperature (U-capillary) was employed in the volatilization step. When using the U-tube as the CT, the whole procedure (trapping and volatilization) is automated as described in Ref. [24]. The flows of helium (75 ml min−1) and hydrogen (15 ml min−1) introduced to the hydride generator served as the carrier gas. The FIAS400 unit was employed for the automatic control of pumps and sample coil. Three reagent flows are pumped at the same rate of 2.0 ml min−1. The FIAS program (included in Winlab program of AAS) summarized in Table 1 also controls the switching on (steps 3, 5) or off (steps 1, 2, 4 ,6) the U-tube heating. Moreover by using this program a heating time is shortly interrupted (step 4) to obtain better resolution of dimethyland trimethyl arsane. In step 5 afterwards, the U-tube is further heated up to approximately 150 °C to remove all the moisture. When using the U-capillary as the CT, the FIAS program is changed. The hydride generation is prolonged to 120 s with two times lower flow rate (1 ml min−1) because of an unacceptable overpressure inside the hydride generator with higher flow rates than 1 ml min− 1. The volatilization step is without interruption (90 s) and the cooling for the next analysis is faster (60 s) than for U-tube (120 s). The flow rate of carrier gas was optimized to 35 ml min−1. Because of manual removal of the U-capillary from the liquid nitrogen the whole procedure is not fully automated. Several terms which correspond to efficiencies are used in the text. The trapping efficiency is defined as the fraction of generated hydride trapped in the CT. The volatilization efficiency is defined as the fraction of trapped analyte volatilized and atomized. The collection efficiency is thus the fraction of generated hydride volatilized and atomized. 2.5. Radioactive indicator experiments The working standard contained radionuclides 71As (2 d), 73As (80 d) and 74As (17 d) further termed as radioactive indicator was prepared carrier-free in 7 mol l−1 HCl. A preparation of the radioactive indicator by unpublished way is described in a supplement as well as the procedure of using the radioactive indicator for the measurement. There are two reasons for using the radioactive indicator i) to determine the collection and volatilization efficiencies for both cryogenic traps and ii) to study the spatial distribution of trapped arsane inside the U-tube. 3. Results and discussion The iAs was used as a model arsenic species to optimize collection in both CT designs.

reported [30–32]. However, nafion membrane dryers are not useful for arsenic speciation analysis by cryotrapping. Although it can remove water vapor and droplets from the gaseous phase so that cryogenic trap blockage is prevented it causes significant loss of methylated arsanes from gaseous phase. This extent of the loss depended on the design (length) of the nafion dryer and reached from 19 to 66% for dimethylarsane and 100% for trimethylarsane, respectively [33]. Apart from this inherent disadvantage this device cannot completely remove water from the gaseous phase. The remaining water vapor is thus retained in the CT during the trapping step of the procedure and subsequently released during the volatilization step and transported to the atomizer. As a consequence, the non-specific absorption of water is observed in a chromatogram at the retention time close to that of methylated arsanes (retention times between 24 and 45 s in the system employed) which significantly affects their peaks areas. A typical chromatogram with non-specific absorption of water is illustrated in Fig. 2. The absorption of water in the vicinity of the arsenic 193.7 nm line was recently studied by means of line source AAS as well as high resolution continuum source AAS. The non–specific absorption by water was found to be pseudo-continuum in the vicinity of the arsenic 193.7 nm line [34]. As presented in Fig. 2 it can be, under employed experimental conditions, compensated by deuterium background correction (DBC) [34]. Moreover, the volatile decomposition product of L-cysteine also causes the non-specific absorption [24]. This peak shaped signal overlaps with the peak of inorganic arsenic. This nonspecific absorption can also be eliminated by the DBC (Fig. 2). In summary, the non-specific absorption caused by both residual water as well as L-cysteine decomposition products can be compensated by the DBC. Due to the unsatisfactory function of the nafion membrane also other drying approaches were tested. The dryer based on a cartridge filled with solid sodium hydroxide (further termed as NaOH dryer) was recently found to be promising owing to its efficiency and simplicity [33]. In this work, the performance of the NaOH dryer was compared to that of the nafion dryer with respect to its unsatisfactory ability to remove water vapor as well as Lcysteine decomposition products. A replacement of the commonly used nafion dryer by the NaOH dryer provided a higher absorption efficiency of water vapor and aerosol. This evidence was proved after volatilization step because no water droplets inside output part of the CT were observed and the blockage of CT by frozen water was eliminated. Moreover, the background signal caused by non-specific absorption of water vapor was not observed with the NaOH dryer as well as the non-specific signal rising from L-cysteine decomposition products. As a consequence, the chromatogram can be recorded without employing

3.1. U-tube cryogenic trap design

0.4

0.3

Absorbance

3.1.1. Elimination of the non-specific absorption and selection of a dryer Several problems are related to the use of the U-tube cryogenic trap. The major one is the blockage of the cryogenic trap by frozen water since water vapor and a lot of aerosol are co-generated with arsanes and transported into the nitrogen-cooled cryogenic trap. For that reason the use of a dryer inserted between the gas liquid separator and the cryogenic trap is necessary. Dryers are a common part of the apparatus for cryotrapping. The nafion membrane dryer is the most often one to be

0.2

0.1

1 Table 1 FIAS program for the U-tube in automatic mode. Step

t (s)

Hydride generation

1 2 3 4 5 6

60 90 23 7 60 120

X

Heating on

0 Trapping step

X

2

0.0

Volatilization step

X Cooling for a next run

10

20

30

40

50

60

t (s) Fig. 2. The typical chromatogram of arsane generated from 2 μg l−1 iAs with the U-tube, red line — signal from deuterium background correction shows nonspecific absorption from: 1 — L-cysteine decomposition and 2 — water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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3.1.2. Investigation of collection efficiencies 73 As radioactive indicator was employed to visualize analyte spatial distribution in the U-tube as well as to estimate the trapping and volatilization efficiency. Arsane was selected as a model analyte. The trapping and volatilization efficiencies were quantified by 73As radioactive indicator experiments and gamma radiometry in the U-tube packed with Chromosorb. The results are summarized in Table S2. The lossless trapping followed by complete volatilization resulting in 100% collection efficiency is reached for arsane in the cryogenic trap based on the U-tube. The arsane is mostly trapped on a Chromosorb filling in a small spot (ca 3 cm) in the upstream part of the U-tube as found by image plate autoradiography (see Fig. S1) suggesting a strong affinity of arsane to Chromosorb filling. This observation suggests that the effective length of the U-tube, i.e., its part immersed into the liquid nitrogen, might be made shorter without impairing trapping efficiency. Shorter U-tube might have resulted in a miniature cryogenic trap design with lower consumption of liquid nitrogen. However, an undetected portion of the radioactive indicators could be trapped in a different part of the U-tube than suggested in Fig. S1. Therefore, the length of the U-tube which has to be immersed in the liquid nitrogen to achieve a complete trapping was specified by an experiment with a spectrometric AAS arsenic standard solution with AAS detection. As illustrated in Fig. 3, the required length of the U-tube is ≥20 cm. 3.2. Miniaturized cryogenic trap design 3.2.1. Optimization of the U-capillary design In order to miniaturize the CT, its capillary design was investigated. The advantage of the U-capillary which is unpacked is lower overpressure (1.24–1.44 bar with the U-tube, 1.03–1.17 bar with the U-capillary). Moreover, simple and fast replacement of the U-capillary after damage is also feasible. Additionally, the U-capillary does not require any treatment prior its use. Last but not least the consumption of liquid nitrogen within this system is much lower than for U-tube system. On the other hand the U-capillary is much more sensitive to blockage by frozen water because of its smaller inner diameter than U-tube. It has been found that this problem can be overcome using a combination of NaOH dryer and a polyethylene foam lid placed on the Dewar flask ca 2 cm above the level of liquid nitrogen. The U-capillary goes through this lid. A zone of cold air is formed above the level of liquid nitrogen Table 2 Volatilization efficiency (%) of individual species obtained by the U-tube with NaOH (bead diameter 2–3 mm) dryer (each form 2 μg l−1); n = 6; ±standard deviation. iAs

MAs

DMAs

TMAsO

100 ± 2

96 ± 2

106 ± 3

101 ± 3

1.1

1.0

Peak area (s)

the DBC. Nevertheless the detection limits for all arsanes were not significantly improved when DBC is off. Also the effect of solid NaOH beads size on arsane loss was studied in this work. The diameter of sodium hydroxide beads of 1-1.9 mm caused 26 ± 5% (relative peak area ± combined uncertainty) loss of arsane. The loss can be explained by greater surface area and thus increased adsorption of analyte. These results indicate that the size of NaOH filling had to be optimized. The NaOH beads (2–3 mm diameter usually with 3% (w) of particles smaller than 2 mm) were found to be an effective filling to prevent both water vapor and L-cysteine decomposition products to be transported to the atomizer without loss of arsane (see Table 2). The batches of the commercially available NaOH are different (the content of beads under 2 mm of diameter) thus the polyethylene sieve made from two connected bottles (500 ml) is needed to obtain the right fraction. Optimized NaOH dryer is a cheap and efficient solution for cryogenic trap systems which replacing unsatisfactory nafion dryers in arsenic speciation analysis methods.

49

0.9

0.8

0.7

0.6 0

5

10

15

20

25

30

35

40

U-tube length under N2 (cm) Fig. 3. Influence of the U–tube length (immersed into liquid nitrogen) on peak area of 2 μg l−1 iAs; peak area 1.00 corresponds approximately to 100% trapping efficiency; n = 5 of each point, error bars represent a standard deviation of the measurements.

in the free volume of the flask under the lid (see Fig. 1). Under these conditions there is a less steep temperature gradient along the U-capillary. As a consequence the water vapor is frozen on a larger section along the capillary and the risk of the blockage of the U-capillary is minimized. The influence of the U-capillary length immersed in liquid nitrogen (effective length) on the arsane trapping efficiency was studied in the same way as in the U-tube (see Section 3.1.2). The capillary effective length was varied from 50 cm to 3 cm with no significant effect on either the peak area of the volatilization signal or its profile. These results demonstrate that the 3 cm of the U-capillary immersed in liquid nitrogen is sufficient for trapping as well as the 20 cm of the U-tube. A significant miniaturization is thus achieved employing the U-capillary design instead of the U-tube. 3.2.2. Investigation of collection efficiencies A radioactive indicator study (similar to that described above in Section 3.1.2) was performed to determine trapping and volatilization efficiency. The effective length of the U-capillary immersed in liquid nitrogen was found to be 3 cm. The trapping and volatilization efficiency was 94% and 100% (Table S2) indicating promising potential of the Ucapillary. It should be highlighted that these encouraging results were obtained for the U-capillary under the experimental conditions found previously to be optimum for the U-tube with nafion dryer (75 ml min− 1 He) [33]. Therefore carrier gas flow rate for the U-capillary was subsequently optimized by investigating influence of flow rates between 35 and 95 ml min−1 on atomic absorption signals. Since the flow rate of 35 ml min−1 yielded optimum sensitivity and repeatability it was selected for further measurements as the optimum. The results (Table S3) indicate a complete trapping and volatilization of arsanes generated from all tested arsenic species with helium flow rate of 35 ml min−1. Therefore the flow rate 35 ml min−1 of helium is used for trapping of all arsanes in the U-capillary. Although the peak areas obtained for given analyte mass are comparable in both CT designs (peak area, 10 measurements, of arsane for the U-tube 1.042 ± 0.019 s and for the U-capillary 1.019 ± 0.041 s), the peak shapes differ significantly. Narrower and higher peaks were observed with the U-capillary compared to the U-tube (see Fig. 4). The full width at half maximum is approximately 1 s in the U-capillary (compare with 3 s for the U-tube) and peak height increased accordingly. It should be accounted to the faster volatilization from the U-capillary due to its smaller inner diameter (0.53 mm) and its shorter length under liquid nitrogen in the trapping step (3 cm). Also an absence of a filling inside the U-capillary makes arsane volatilization faster than with the U-tube.

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0.8

Absorbance

0.6

0.4

0.2

0.0 0

5

10

15

20

t(s) Fig. 4. The typical signal of arsane generated from 2 μg l−1 iAs with the U-capillary (black line) and with U-tube (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The collection of all arsanes (mixed standard solution with iAs, MAs, DMAs and TMAsO) with the U-capillary works as well as with the U-tube however the separation of arsanes in the volatilization step is unsatisfactory. All arsanes are released within 10 s. No significant improvement of the peak resolution was observed when an additional capillary (30 cm, kept at ambient temperature) was connected to the downstream end of the U-capillary. It indicated that the peak resolution was not controlled by a chromatographic contribution but mainly by the heating rate in the volatilization step. A resistive heating of the U-capillary in the volatilization step was also tested. The same wire (5 Ω) (winding on the teflon tube (0.75/ 1.56 mm i.d./o.d.) with capillary inside) as for the U-tube was used to achieve fast heating (I = 1 A) of the U-capillary. Nevertheless this arrangement caused arsane peak broadening. The resistive wire was placed on the Teflon tube where the capillary goes through. The Teflon tube and the cooling of a small part (3 cm) of the U-capillary caused uneven heating along the U-capillary. The parts of the U-capillary which were not under liquid nitrogen had higher temperature during the volatilization step than the part immersed in liquid nitrogen. Therefore the resistive heating was not further employed in this arrangement. 4. Conclusions The new design of the cryogenic trap based on the small, simple and commercial available non-polar fused silica capillary was developed. The material of the gas phase dryer, an essential part of the cryotrapping system, was optimized and solid NaOH particles (2–3 mm in diameter) were found as the best solution. The inherent advantages of the NaOH dryer with appropriate beads diameter compared to common nafion dryers are: i) no loss of any arsanes generated from iAs, MAs, DMAs and TMAsO ii) efficient removal of water vapor and L-cysteine decomposition products resulting in iii) no need of deuterium background correction system. It was proven, for the first time by the radioactive indicator approach, that the collection efficiency of arsane in the cryogenic collection system based on the U-tube filled with Chromosorb, described by Matoušek et al. [24], reaches 100%. Moreover the distribution in the U-tube indicated collection of arsane in a small volume of the U-tube input part which shows the miniaturization of the cryogenic system possible. Nevertheless the minimum length of the U-tube immersed in liquid nitrogen for 100% trapping efficiency is 20 cm thus no sufficient miniaturization of the system is possible. The novel design of the miniaturized cryogenic trap — U-capillary has the same collection performance for the cryotrapping as the much

bigger U-tube. The U-capillary cryogenic trap is capable to collect all relevant arsenic species with 100% trapping/volatilization efficiency as found by experiments with radioactive indicator and spectrometric AAS experiments. The U-capillary cryogenic trap has a potential to become a routinely employed device. Compared to the U-tube it is more economical since the consumption of liquid nitrogen is lower with the U-capillary and also its investment cost is substantially lower than that of the U-tube. Moreover, the sample throughput is higher with the U-capillary because no pretreatment of the cryogenic trap is necessary. The broken cryogenic trap can be easily and fast replaced by a new one without any delay. Although the U-tube is employed as both the collection device as well as primitive gas chromatographic column to separate directly the trapped arsenic species, also more sophisticated approach employing cryogenic trap (U-capillary) coupled to gas chromatograph and AAS detector will be used in future. The advantage of the approach is that both the cryogenic trap and the separation device can be controlled separately without any compromise regarding the operation conditions. The development of the connection between U-capillary and gas chromatograph as well as the interface between gas chromatograph and atomic absorption spectrometer is in progress. Whereas the U-tube serves as both the collection tool and the separation device, the U-capillary can be employed only as a collection tool to be coupled to the separation device. Therefore a development of a new cryogenic capillary trap operated as a trapping as well as a separation device (all-in-one), based on a precise control of thermal desorption of arsanes is also in progress. Acknowledgment This work was supported by GA CR (grant No. P206/14-23532S), by Institute of Analytical Chemistry of the CAS, v. v. i. (project no. RVO: 68081715) and by Charles University GA UK (project no. 133008). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.sab.2015.06.014. References [1] F.R.S. Bentlin, F.A. Duarte, V.L. Dressler, D. Pozebon, Arsenic determination in marine sediment using ultrasound for sample preparation, Anal. Sci. 23 (2007) 1097–1101. [2] J. Bian, F. Xu, L.H. Li, W. Wang, J.J. Han, L. Li, Determination of As(III) and As(V) in Sea water by hydride generation atomic fluorescence spectrometry, Spectrosc. Spectr. Anal. 30 (2010) 2834–2837. [3] G.G. Bortoleto, S. Cadore, Determination of total inorganic arsenic in water using on-line pre-concentration and hydride-generation atomic absorption spectrometry, Talanta 67 (2005) 169–174. [4] M. Deaker, W. Maher, Determination of arsenic in arsenic compounds and marine biological tissues using low volume microwave digestion and electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 14 (1999) 1193–1207. [5] J. Frank, M. Krachler, W. Shotyk, Direct determination of arsenic in acid digests of plant and peat samples using HG-AAS and ICP-SF-MS, Anal. Chim. Acta 530 (2005) 307–316. [6] M.A. Garcia-Sevillano, M. Gonzalez-Fernandez, R. Jara-Biedma, T. Garcia-Barrera, A. Vioque-Fernandez, J. Lopez-Barea, C. Pueyo, J.L. Gomez-Ariza, Speciation of arsenic metabolites in the free-living mouse Mus spretus from Doana National Park used as a bio-indicator for environmental pollution monitoring, Chem. Pap. 66 (2012) 914–924. [7] S. Musil, A.H. Petursdottir, A. Raab, H. Gunnlaugsdottir, E. Krupp, J. Feldmann, Speciation without chromatography using selective hydride generation: inorganic arsenic in rice and samples of marine origin, Anal. Chem. 86 (2014) 993–999. [8] A.H. Pétursdóttir, N. Friedrich, S. Musil, A. Raab, H. Gunnlaugsdóttir, E.M. Krupp, J. Feldmann, Hydride generation ICP-MS as a simple method for determination of inorganic arsenic in rice for routine biomonitoring, Anal. Methods 6 (2014) 5392–5396. [9] Q. Zhang, H. Minami, S. Inoue, I. Atsuya, Differential determination of trace amounts of arsenic(III) and arsenic(V) in seawater by solid sampling atomic absorption spectrometry after preconcentration by coprecipitation with a nickel-pyrrolidine dithiocarbamate complex, Anal. Chim. Acta 508 (2004) 99–105. [10] W.H. Zhang, Y. Cai, C. Tu, L.Q. Ma, Arsenic speciation and distribution in an arsenic hyperaccumulating plant, Sci. Total Environ. 300 (2002) 167–177.

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