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Multiple microflame quartz tube atomizer — further development towards the ideal hydride atomizer for atomic absorption spectrometry
Spectrochimica Acta Part B 57 (2002) 451–462
Multiple microflame quartz tube atomizer — further development towards the ideal hydride atomizer for atomic absorption spectrometry夞 ´ˇ Matousek*, ˇ ˇ´ Dedina, ˇ Tomas Jirı Anna Selecka´ Academy of Sciences of the Czech Republic, Institute of Analytical Chemistry, Laboratory of Trace Element Analysis, ´ ˇ ´ 1083, CZ-142 20 Prague, Czech Republic Vıdenska Received 21 August 2001; accepted 28 November 2001
Abstract The development of an improved type of hydride atomizer for atomic absorption spectrometry — multiple microflame quartz tube atomizer (MMQTA) — is presented. The main feature of this atomizer is recurrent analyte atomization proceeding over its whole optical tube length, which is achieved by production of H-radicals at multiple points within the tube by oxygen microflames burning in the hydrogen-containing atmosphere. The MMQTA design optimization leading to a complete filling of the observation volume with H-radicals is described. The influence of individual atomization parameters is discussed. Optimum H-radical producing oxygen intake into the MMQTA was found to correspond to a H2:O2 stoichiometric (3:1) ratio. The performance of the individual MMQTA tube designs is evaluated and compared to a typical externally heated quartz tube atomizer (EHQTA) — the linearity of calibration graphs for As, Se and Sb is significantly improved in all MMQTA tubes, without compromising the sensitivity, simplicity, low cost and easy operation. In fact, the free atom reactions within the tube causing calibration curvature are avoided up to an analyte concentration of at least 200 ng mly1 for Se and Sb and 100 ng mly1 for As. Tolerance limits of 0.7, 1.4, 0.2 and 0.2 mg mly1 are achieved for the atomization interferences of As on Se, Se on As, Sb on Se and Se on Sb, respectively, which is an improvement by 1–2 orders of magnitude in comparison to the conventional EHQTA with the same hydride generation system. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydride generation atomic absorption; Externally heated quartz tube atomizer; Reatomization; Multiple microflame quartz tube atomizer; Atomization interferences
夞 This paper was presented at Colloquium Spectroscopicum Internationale XXXII, held in Pretoria, South Africa, 8–13 July 2001 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Fax: q420-2-4106-2499. ˇ E-mail address: [email protected] (T. Matousek).
0584-8547/01/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 4 0 0 - 1
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1. Introduction Externally heated quartz tube atomizers (EHQTA) are presently by far the most common hydride atomizers for atomic absorption spectrometry (AAS) w1x. The typical EHQTA is a T-tube made of quartz with the optical tube approximately 100–150 mm long, 7–10 mm i.d. and a narrow inlet arm (1–3 mm i.d.). The L-design of EHQTA w1–3x did not achieve wide popularity. The accepted mechanism of free analyte atom formation in EHQTA via the hydrogen radical mechanism has been confirmed in a number of ˇ studies (see Dedina and Tsalev w1x and references therein). Briefly, hydride is atomized via consecutive reactions with hydrogen radicals. Hydrogen radicals are formed by reactions of oxygen in excess hydrogen. The H-radical cloud is confined to a small part of the atomizer w1,4–6x. Despite the undoubted advantages, there are also serious drawbacks of the EHQTA; poor linearity (or even a rollover) of the calibration graphs and low resistance towards atomization interferences. Calibration curvature was formerly explained by the formation of analyte dimers w7x or an insufficient amount of H-radicals for efficient hydride atomization at higher analyte concentrations w8,9x, whereas atomization interferences were ascribed to the competition for H-radicals between analyte and interferent, or to the alteration in the free atom decay rate by the interferent w1,10x. Strong indications were found recently, supporting the hypothesis that both these undesirable features do have a common source w11–13x. After leaving the H-radical cloud, the free atoms of the hydride-forming element (regardless if considered as analyte and interferent) do react forming dimers and polyatomic particles. These particles increase the surface available for the free atom decay reactions. In other words, the reacted analyte particles provide surface available for reactions not only on the tube walls but also in the atomizer free volume. The more analyteyinterferent present, the more particles and the more surface is available. This may lead to the interference or to the calibration roll-over (it can be considered as an ‘interference of the analyte on itself’) w11–13x.
The species originating from the reacted free atoms can be reatomized, but only upon another contact with hydrogen radicals w14,15x. This cannot be obtained without another introduction of oxygen into the tube. Obviously, the main goal is to fill the whole volume of the atomizer optical bar with hydrogen radicals, so that the free analyte atoms are recurrently atomized and reactions leading possibly to effects such as calibration curvature or roll-over cannot take place w13x. This was achieved in a quartz tube hydride atomizer of the next generation — a multiple microflame quartz tube atomizer (MMQTA) w16x. The optical tube of the MMQTA atomizer consists of two concentric tubes. The inner tube, the optical bar, has multiple tiny orifices over its length. The oxygen containing gas (either air or argonyoxygen mixture) is introduced inside the outer tube, from where it enters the inner tube through the orifices. At the orifices, oxygen reacts with hydrogen present inside the tube. A microflame producing Hradicals is thus formed at every orifice, which results in the presence of H-radicals over the whole observation volume. The analyte can be present in the form of free atoms only. Except the improvement in the linearity of calibration graphs, the sensitivity for selenium was improved by 10% and mainly the tolerance limit (the interferent concentration causing a 10% signal suppression) towards the atomization interference of arsenic was improved by one order of magnitude — from 0.04–0.05 to 0.5–0.55 mg mly1, compared to the conventional EHQTA w16x. However, the presented design of the MMQTA was still far from ideal. Namely the resistance to interferences, although one order of magnitude better than for the conventional EHQTA, was still two orders of magnitude worse compared with a miniature diffusion flame atomizer w11x. The extremely high resistance of the miniature diffusion flame atomizer, and also very good linearity of calibration graphs there, is due to the fact that its whole observed volume is filled with H radicals. The observed non-ideal performance of MMQTA is probably due to zones in the optical path, namely close to the ends of the tube, where no H radicals are present.
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Table 1 The inner dimensions of individual atomizer tubes Tube
Optical tube lengtha (mm)
Optical tube i.d.a (mm)
Inlet i.d. (mm)
Design
Sensitivity checkb
QT1 MM1 MM2 MM3 MM4 MM5
122 154 150 141 152 125
7 7.1 6 7.3 7.1 7
1 1.8 1.8 3.5d 1.8 1.8
w8 x Fig. Fig. Fig. Fig. Fig.
0.365 0.517 0.508 0.415 0.485 0.470
1a 1ac 1ac,d 1ae 1b
a
Narrow working part only — without wide end sections. Absorbance obtained for 10 ng mly1 Se at 196 nm at optimum conditions. c Gas outlets at the wide end sections; not shown in the figure. d Inlet with standard joint; not shown in the figure. e Additional orifices at the asterisk positions. b
The aim of this paper is to describe further development of the MMQTA. The performance of individual MMQTA designs is assessed and compared with an ordinary EHQTA as well as with the previous MMQTA. Selenium was chosen as the model analyte. The results are then verified for arsenic and antimony. Also mutual atomization interferences of these elements are studied. 2. Experimental 2.1. Instrumentation Instrument: Varian SpectrAA 30 atomic absorption spectrometer (Mulgrave, Victoria, Australia) without burner; 15-s integration mode. Steady state signals were recorded. Radiation source: Se and As hollow cathode lamps operated at 10 mA, Sb hollow cathode lamp operated at 8 mA. Wavelengths: Se 196.0 and 204.0 nm at 1-nm slit, As 193.7 nm at 0.2-nm slit, Sb 231.2 nm at 0.2-nm slit. 2.2. Atomizers Six atomizer tubes were used. Quartz Cell for FIAS 200 by Perkin-Elmer was chosen as the representative of conventional EHQTA (marked QT1 further in the text), and five MMQTA tubes were tested in this paper, marked here MM1– MM5. The detailed description and special features
of individual MMQTA tube designs of the tubes are discussed in Section 3.1. The outer dimensions of all the tubes are similar (outer diameter of the optical tube 14 mm to fit the furnace), inner dimensions are summarized in Table 1. All the atomizers feature wide ends of the optical tube preventing flame ignition on the tube ends. No end windows were used in this work. All the atomizers were heated electrically by an in-house made furnace (120-mm heated length, 15-mm inside diameter) controlled by a Rex C100 controller (SYSCON-RKC, IN, USA) with a K-type thermocouple sensor or by the commercial furnace of the same dimensions (AEHT 01 by ´ ˇ Bohdanec, ˇ Czech Republic). The RMI, Lazne temperature was always set so that at 30 mm from the tube center it was 900 8C (measured by Ktype thermocouple). Prior to the first use or if the original (reference) sensitivity could not be achieved, tubes were cleaned by leaching for 10 min in a mixture of conc. HNO3-conc. HF (7:3). The tube was then rinsed with distilled water and left to dry. The gas flow introduced to the inlet arm of the MMQTA atomizers (total inner gas flow) consists of the carrier Ar flow (termed here inner Ar flow) introduced to the hydride generator and of the hydrogen (inner H2 flow) formed from decomposed tetrahydroborate. If not stated otherwise the inner Ar flow rate was 50 ml miny1 and inner H2 flow rate was 15 ml miny1. This was also the
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gas entering the QT1; where mentioned, it was doped by O2 premixed upstream the hydride generator. The oxygen-containing gas entering the MMQTA outer shell (total outer gas flow) consists of an inert gas (outer Ar flow) and oxygen (outer O2 flow). Either argon mixed with oxygen or air was used throughout as the outer gas at the flow rates specified in the text. The total inner gas flow rate and composition entering the atomizer is similar to that of a commercial hydride generation system (Perkin-Elmer FIAS 200) w17x. A diffusion flame atomizer consisting of a short piece of quartz tube (4-mm i.d.) with the flame burning on its end w18x was used to check the efficiency of hydride generation in the presence of an interferent. Argon (650 ml miny1) and hydrogen (300 ml miny1) flows were in this case connected downstream the hydride generator. 2.3. Hydride generators A compact continuous flow hydride generator was described in detail elsewhere w12,16x. It consists of two T-pieces for reductant and carrier gas introduction, a 1-mm i.d.y750-mm long PTFE reaction coil and forced outlet quartz gas–liquid separator. A multichannel minicartridge peristaltic pump (Ismatec SA, Switzerland) was used for the transport of solutions. Blankystandard and reductant (0.5% myv NaBH4 in 0.4% KOH) solution flow rates were 4 ml miny1 and 1.2 ml miny1 (producing 15 ml miny1 of inner H2), respectively. Standards were prepared in 1 M HCl from a 1000mg mly1 stock solution. If not stated otherwise, a 50-ml miny1 carrier argon flow rate was used throughout this work, as lower flow rates are impractical concerning the hydride generator (too long time for establishing the steady state conditions and possibility of transport interferences). For the measurements of interferences, this sysˇ ˇ tem was modified as in Dedina and Matousek w16x. To avoid liquid phase interferences, analyte and interferent hydrides were generated separately in a semi-twin-channel system. In the analyte channel, a Se standard solution (2 ml miny1) was mixed with the reductant (0.57 ml miny1) in a Tpiece (PEEK, 0.8-mm inner bore). Analogous arrangement was used for the interferent channel.
Fig. 1. MMQTA tube designs. (a) Tube MM1 (similar to tubes MM2, MM3 — see Table 1 for the differences) and tube MM4. Asterisks mark orifices in tube MM4 only. (b) Tube MM5.
Both these channels joined in a cross-piece (PEEK, 1.5-mm inner bore), to where also the carrier gas (inner Ar gas) was introduced. The reaction mixture then continued to the reaction coil and gas– liquid separator (the same as for the single channel hydride generator above). The interferent channel was checked regularly to produce equal signals for selenium as the analyte channel. 3. Results and discussion 3.1. MMQTA tube designs An early MMQTA design is depicted in Fig. 1a, presented already in a communication w16x, corresponding to tube MM1. Tubes MM2 and MM3 differ from this design by details which do not influence principally the atomizer function, such
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as by outlets at the wide optical tube ends, and by wider inlet arm ending by a standard joint (tube MM3). The main disadvantage of this MMQTA design — tubes MM1, MM2 and MM3 — is the fact that it features the outer gas inlets into the outer cavity from the sides (Fig. 1a). Inevitably, there must be approximately 20 mm of the optical tube outside the furnace on both sides. In these cold parts, there are no oxygen inlets (as the temperature is too low for H-radical production), so that no H radicals are present in this part of the optical tube. This is underlined by the fact that low temperature in these regions is favorable to the formation of the analyte or interferent particles, which are probably responsible for the enhanced free analyte atom decay w11,13x. This impairs the analytical performance of the MMQTA (see Figs. 4 and 7) (although any MMQTA we tested still by far outperforms the QT1). Two approaches how to avoid free atoms in cold regions were tested. The first approach is the design of tube MM4. It has additional pairs of orifices also in these problematic cold regions (marked by asterisks in Fig. 1a). The detailed function of these orifices is discussed in Section 3.2.2. Molecular oxygen thus entered the inner tube outside the furnace, but does not produce H-radicals but rather reacts with free analyte atoms. Thus, free atoms are removed on purpose from these critical regions and cannot be subjected to interference. Although the calibration linearity and the interference tolerance limit were improved by this approach (Fig. 7), there are certain disadvantages. Namely, the oxygen intake to the hot and cold regions cannot be controlled (see Section 3.2.2); it depends on the minute orifice differences and the optimum oxygen intake may vary significantly from one tube to another. Separate control of oxygen intake to hot and cold regions would make the system even more complicated. Another approach is presented by tube MM5 shown in Fig. 1b, differing by a shorter optical tube and outer gas inlet in the central part. The outer gas is introduced to the cavity between the inner (optical) and the outer concentric tube of the horizontal arm via the cavity between the double wall of the inlet arm. The optical tube can be then ended at the edge of the furnace, without
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any troublesome cold regions. Shorter optical tube length results in the reduced photometric noise, which was the main noise source in our system (see Section 3.3.2). The inlet arm (usually rather narrow quartz tube) is also reinforced, so that it cannot be easily broken. This design yielded the best results both in the calibration curve linearity and in the resistance towards atomization interferences (see below) and was therefore chosen as the most prospective one, although the data are also presented for the other designs where available. Concerning the orifices, the initial intention as to the diameter was ‘as small and uniform as possible’ to provide even distribution of oxygen intake over the tube length. A small orifice diameter should provide the oxygen entrance inside the tube at high linear velocity, so that O2 and H2 reactions proceed inside the optical tube rather than within the orifice. In practice, the manual procedure of making the orifices resulted in their diameter variations between 0.1 and 1 mm, with the majority of approximately 0.5 mm. That is also the reason why the orifices are made in pairs — there is a lower chance of insufficient oxygen intake into any tube region due to a too small orifice. The distance of the orifice pairs was made 15 mm, based on the study of the longitudinal distribution and decay rate of free atoms in EHQTA w13x. This distance appeared sufficient with respect to the calibration linearity (see Section 3.3.1) and was kept the same in all tubes within this study. 3.2. Influence of individual atomization parameters 3.2.1. Inert gas flow rates Fig. 2 depicts the influence of the total gas flow rate on the signal. Either inner or outer gas flow rates were varied. Generally, with increasing flow rate of the carrier gas the signal decreases, due to dilution of free atoms in the observation volume. The signal decreases with increasing outer inert gas (circles in Fig. 2) is lower than when increasing the inner Ar gas, simply because the outer gas is not going through the whole observation volume but only through part of it. On the other hand, the use of pure oxygen as an outer gas (without addition of outer inert gas) does not impair the
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Fig. 2. The influence of total gas flow rate through the atomizer on the signal. Tube MM3, 10 ng mly1 Se. h, variable inner Ar flow rateq20 ml miny1 outer Ar and 3 ml miny1 outer O2 flow rate; n, variable inner 10% H2 in Ar flow rateq25 ml miny1 outer air flow rate; ●, 50 ml miny1 inner Ar flow rateq3 ml miny1 outer O2 and variable outer Ar flow rate.
atomizer performance. It means that outer inert gas has no function other than to avoid difficulties with control of minute outer oxygen flow rates. Air can be conveniently utilized as the outer gas. No other phenomena depending on the inert gas flow rate (or, in other words, depending on the analyte concentration in the gaseous phase) were identified for optimum oxygen intake (see Section 3.2.2) (such as, e.g. signal decrease at the lowest carrier gas rates as described for the EHQTA w1x, caused by the free atom decay within the tube w1,13x). This is in line with the assumed recurrent analyte atomization within the tube, so that the free atom reactions are virtually suppressed. 3.2.2. Oxygen intake by outer gas As oxygen is necessary for production of hydrogen radicals in quartz atomizers, the oxygen intake into the tube is the most critical parameter for the atomizer performance. In all the tested atomizer tubes (with the exception of tube MM4 as discussed below) for inner H2 flow rate of 15 ml miny1 (resulting from the NaBH4 decomposition), the oxygen intake for maximum sensitivity for Se was approximately 4.5–5 ml miny1 O2 or 20–25 ml miny1 air. The influence of outer O2 flow rate is much less pronounced in the case of arsenic and antimony (Fig. 3), however, an optimum can be
found in the same oxygen or air flow rate as for selenium. The optimum oxygen intake is dependent on H2 concentration inside the tube. Variable flow rates of H2 were introduced to the inner gas upstream the hydride generation system in addition to the 15 ml miny1 of H2 from the NaBH4 decomposition, comprising the total inner hydrogen flow rate of 15–60 ml miny1, and optimum outer O2 flow rate was tested for tube MM5. It appeared that the optimum outer O2 flow rate equals one-third of the total inner H2 flow rate, similarly as in the other tubes (see above). This should be compared with the conclusion that H radicals are produced in quantities corresponding to the total amount of oxygen, i.e. two radicals for each O2 molecule w1,19x. Provided that oxygen inlets are uniformly distributed over the length of the tube, this stoichiometric ratio leads to the most efficient production of the H-radicals over the whole observation volume. Hydrogen is gradually consumed on its way through the optical tube of MMQTA by reactions with O2 entering at individual orifices along the optical tube length. Too low O2 intake in the outer gas leads to sub-maximum production of H-radicals. On the other hand, at too high O2 intake the H2 is consumed prematurely — molecular oxygen entering through orifices near the ends remains unreacted, and it can interfere with free analyte atoms (as described for Se w11,20x) potentially present in this part, decreasing thus the signal.
Fig. 3. The influence of outer air flow rate, tube MM5. ●, Se 10 ng mly1; j, As 5 ng mly1; m, Sb 10 ng mly1.
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The outer oxygen flow demand of the tube MM4 is different. With the increasing outer air intake, the signal is increasing from 67% of maximum at 25 ml miny1 up to a maximum at 50 ml miny1 which is followed by a plateau with a slow signal decrease (to 90% of maximum at 120 ml miny1 of outer air); similar data are obtained if 20 ml miny1 of Arqcorresponding amount of O2 is used as the outer gas, just the decrease is faster (maximum at 11 ml miny1 O2, 90% at 15 ml miny1 O2) This is caused by the fact that oxygen has two functions here: it does not serve only for production of H-radicals, but also for depleting free atoms in the ‘cold zones’ (see Section 3.1). The difference between the optimum oxygen intake in tube MM4 and the ‘3:1 ratio’ is determined by the proportion of the outer gas entering the ‘hot’ and ‘cold’ zones, which is given by the permeability of orifices in these parts (thermal expansion of the permeating gas must be also taken into account). As the diameter of the orifices can vary from piece to piece, slightly different oxygen demand data could be obtained for other specimen of this design. 3.3. MMQTA performance 3.3.1. Calibration graphs It is a well-known fact that conventional EHQTA do suffer from poor linearity of calibration graphs or even calibration roll-over w1,7–9x. This was recently proven to be caused by the acceleration of the reaction rate of free atom decay inside the tube at increasing analyte concentration w13x, rather than by incomplete atomization as suggested earlier w9x. Since the analyte is recurrently atomized in MMQTA, the calibration graph linearity should be much improved there compared to EHQTA. Fig. 4 shows the calibration graphs for selenium in QT1 (under the conditions favorable to Hradical formation — the sufficient O2 supply and end-section reatomization possible w13x) and two MMQTAs: the best (MM5) and worst (MM2) specimen with respect to calibration linearity. The difference between both MMQTA is small. The initial sensitivity of tube MM2 is slightly higher than that of tube MM5 due to its longer optical
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Fig. 4. Calibration graph for selenium, 204-nm line. ●, QT1, 3 ml miny1 O2 added; h, MMQTA–MM2, 20 ml miny1 Ar and 5 ml miny1 O2 outer gas; n, MMQTA–MM5, 25 ml miny1 air outer gas.
tube, whereas the linearity is worse due to the cold end sections. Anyway, even the ‘bad’ MMQTA is performing much better than QT1. The sensitivity of QT1 and MMQTA tubes is not differing dramatically — it is given by the fact that the analyte can remain in the state of free atoms nearly in the whole observation volume in well-optimized QT1 at very low analyte supply (under sufficient O2 intake and with the possibility of analyte reatomization near the ends) as shown recently w13x. Fig. 4 was obtained at the 204-nm Se line which is approximately 20 times less sensitive than the commonly used 196-nm analytical line. Thus, lower absorbance values could be obtained, which are less influenced by the calibration curvature sources inherent to the optical and detection systems. To find the possible source of slight calibration curvature for tube MM5 at the 204-nm Se line (Fig. 4), the deviations from linearity in the same absorbance range for tube MM5 at 204 nm and at 196 nm (not shown) were compared. As the concentrations for the 196-nm line calibration are safely in the range which does not produce any deviations from linearity at 204 nm, and there are no spectral reasons for a difference in linearity at both these lines w21x, it can be concluded from the perfect match of both curves that even the slight curvature observed for tube MM5 in Fig. 4 is not originating within the MMQTA nor in the hydride
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Fig. 5. Calibration graph for antimony, 231.2-nm line. ●, QT1; n, MMQTA–MM5, 25 ml miny1 air outer gas.
generation system, but solely in the opticalydetection part. In other words, the linear response of the MMQTA is extended up to at least 200 ng mly1 Se (corresponding to the supply rate of 13 ng sy1) compared to approximately 10 ng mly1 (0.7 ng sy1) in the case of QT1. Fig. 5 shows the calibration graphs for Sb. In QT1, a sharp sensitivity drop was experienced at the concentrations over 80 ng mly1, similarly as observed and discussed recently w12x. No such phenomena were found in MMQTA (tube MM5) up to 200 ng mly1. Fig. 6 depicts the calibration graphs in MMQTA (tube MM5) and QT1 for arsenic. Even if the QT1 is not performing as bad as in the case of selenium and antimony (although the studied concentration range is lower), MMQTA yields significantly better results. 3.3.2. Atomizer noise The possible introduction of additional noise by the atomizer into the analytical system was tested. The procedure of the noise level evaluation is w13x. ˇ and Dedina ˇ described in detail in Matousek The dominant noise source in the present system was the photometric noise of the detector w13x. It is dependent on radiation attenuation by the atomizer (which is given by the length and i.d. of the optical tube of the atomizer). The heating and modeling of the atomization atmosphere in the atomizers (QT1 and all the MMQTA) does not increase the noise level. However, in the case of
QT1 and tube MM5 noise is increased by approximately 30% when the hydride generation is started. It is caused probably by the uneven gas flows from the generator rather than the atomizer itself; the noise source is probably the fluctuations of ingress of the ambient air into the hot ends of the tube (oxygen is absorbing radiation at elevated temperatures at wavelengths under 200 nm). In the other MMQTA tubes (MM1–MM4) featuring cold sections of the optical tube at the ends, noise is not increased with hydride generation, but this is outbalanced by higher photometric noise due to their increased length. In summary, there is no significant noise difference between the QT1 and any of the tested MMQTA designs. To sum up, the limit of detection is not going to be changed appreciably after replacement of an ordinary EHQTA by a MMQTA tube of similar dimensions. 3.3.3. Atomization interferences The tolerance limits (interferent concentration causing a 10% signal suppression) of the mutual interferences of Se–As and Se–Sb are summarized in Table 2. The results obtained in MMQTA (tube MM5) are compared with the results for QT1, which are not in contradiction to other previously published tolerance limit data in EHQTA. As there is a wide variety of hydride generation systems, atomizer dimensions and atomization conditions described in the literature resulting in tolerance
Fig. 6. Calibration graph for arsenic, 193.7-nm line. ● QT1, 3 ml miny1 O2 added; n, MMQTA–MM5, 25 ml miny1 air outer gas.
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limit data differing by several orders of magnitude, exclusively the data for experimental set-ups similar to our system were chosen. The detailed results of measurements of interference of arsenic on the selenium signal for the QT1 and the MMQTA tubes are presented in Fig. 7. Tolerance limits for the individual MMQTA tubes are at least one order of magnitude better than that of the QT1. It should be noted that this was achieved simply by replacing the QT1 by a MMQTA tube, without any modification of the hydride generation system. Even if the differences between the individual MMQTA tubes may be partially biased by the fact that the oxygen intake was optimized for maximum sensitivity and not for minimum interference extent, the initial design (tubes MM1 and MM2) does exhibit lowest tolerance to atomization interferences. As mentioned in Section 3.1, this design features the oxygen inlets into the outer shell from the sides (Fig. 1a). These colder parts do not contain the necessary H-radicals and free atoms are exposed to the reactions leading to the interference. The low temperature in these regions is favorable for the formation of the interferent particles, causing enhanced free analyte atom decay w11x. Both designs of tube MM4 and MM5 do overcome this problem (see Section 3.1) and yield lower interference magnitudes than tubes MM1 and MM2. Higher hydrogen and oxygen intake into the tube should produce higher density of H-radicals and thus reduce the interference. We studied the
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Fig. 7. Interference of arsenic on selenium. ●, QT1, 3 ml miny1 O2 added, Se 10 ng mly1; s, MMQTA–MM1, 22.5 ml miny1 air outer gas, Se 20 ng mly1; h, MMQTA–MM2, 20 ml miny1 Ar and 5 ml miny1 O2 outer gas, Se 20 ng mly1; e, MMQTA–MM4, 100 ml miny1 air outer gas, Se 20 ng mly1; n, MMQTA–MM5, 22.5 ml miny1 air outer gas, Se 20 ng mly1.
influence of oxygen intake on the As interference on Se under standard conditions (i.e. 15 ml miny1 H2 produced by NaBH4) and with the addition of 45 ml miny1 of H2 to the inner gas. The results are shown in Fig. 8, analogous results were obtained also for As interference on Se (not shown). With increasing oxygen intake, the interference extent is decreasing until it reaches a stable value. This ‘limiting’ interference value is lower at higher hydrogen intake, but at the cost of sensitivity. Higher production of hydrogen radicals, higher total gas flow rate diluting the interferent in the gaseous phase andyor probably slightly
Table 2 Tolerance limits for mutual As–Se and Sb–Se interferences Tolerance limits (mg mly1)
Reference
Interference As on Se
Interference Se on As
Interference Sb on Se
Interference Se on Sb
0.04 0.04 0.05 0.1 0.15 0.2 0.7
0.01
0.02 0.06
0.01
0.1 0.025 1.4
0.05 0.2
0.05 0.2
This work, QT1 w22x w23x w3 x w24x w25x This work, tube MM5
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Fig. 8. Interference of arsenic on selenium at various outer O2 and inner H2 intake. Tube MM5, 20 ng mly1 Se; outer inert gas 20 ml miny1 Ar. h, without interferent; j, with 5 mg mly1 As. Columns show interference influenced signals in relative scale (right axis — interference free signal is 100%).
higher temperature inside the tube may be the reasons for this lower interference level. It should be noted that this ‘limiting’ interference is an atomization interference — it is not originating in the hydride generation phase or during hydride transport w1,10x. This was checked using the same hydride generation system with the diffusion flame atomizer, which is very resistant against atomization interferences w11,20x. There was no interference of 5 mg mly1 of As and Se on the signal of 0.5 mg mly1 of Se and As, respectively, in the diffusion flame atomizer. The oxygen intake optimum to minimize interferences corresponds fairly well to that for maximum H-radical production; it is approximately 10% higher than the optimum oxygen intake for the maximum signal (oxygen-to-hydrogen ratio 1:3). This is probably caused by the slightly uneven oxygen distribution in the inner optical tube due to orifice diameter variations; the maximum production of H-radicals in all tube parts necessary for minimum interference may lead to O2 excess and free atom population decrease in some regions. The difference between the optimum oxygen intakes for maximum signal and minimum interference may be tube specimen-dependent — the smaller the more even distribution of oxygen in the tube. Although the interference extent of selenium on arsenic is slightly stronger than vice versa (Fig.
Fig. 9. Interference of selenium on arsenic. ●, QT1, 2.6 ml miny1 O2 added, As 10 ng mly1; n, MMQTA–MM5, 32.5 ml miny1 air outer gas, As 10 ng mly1.
9), the behavior for both combinations is analogous in both QT1 and MMQTA. Under an outer O2 flow rate optimized for minimum interference (6.5 ml miny1 O2 corresponding to 32.5 ml miny1 air for both analyteyinterferent combinations — see Fig. 8), the MMQTA yields two orders of magnitude better results than the QT1. Figs. 10 and 11 verify the superiority of the MMQTA over the QT1 also for the mutual Se–Sb interference pair. In both cases, the tolerance limit is one order of magnitude better in the MMQTA tube. It is interesting to compare the interference tolerance limits for individual interferents in the QT1 (Figs. 7, 9–11) with the corresponding cali-
Fig. 10. Interference of antimony on selenium. ●, QT1, Se 20 ng mly1; n, MMQTA–MM5, 25 ml miny1 air outer gas, Se 20 ng mly1.
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simple replacement of EHQTA by a MMQTA the improved accuracy of analysis may be achieved without compromising the limit of detection. Further improvement of the MMQTA design includes the optimization of the orifice pairs distance or of their pattern, which may further enhance the MMQTA resistance towards atomization interferences due to more uniform filling of the optical tube volume with H-radicals. The longterm stability also remains to be thoroughly evaluated; our to-date experience shows that MMQTA surpasses the EHQTA also in this respect. Fig. 11. Interference of selenium on antimony. ●, QT1, Sb 10 ng mly1; n, MMQTA–MM5, 25 ml miny1 air outer gas, Sb 10 ng mly1.
bration graphs (Figs. 4–6); please note that the actual element supply rates must be compared, as the data in Figs. 7–11 were obtained at a sample introduction flow rate of 2 ml miny1 whereas the data in Figs. 4–6 at 4 ml miny1 (see Section 2.3). It can be seen that this tolerance limit approximately corresponds to the upper limit of the linear range (or the point where the calibration graph for QT1 differs by 10% from that of MMQTA), i.e. supply rates of approximately 20–40 ng miny1 for selenium and 80 ng miny1 for arsenic, respectively. This fact gives evidence that the calibration curvature and atomization interferences do proceed via common mechanism. The above supply rates correspond approximately to the element supply at which the condensation or particle formation starts under given atomization conditions. In the case of antimony interference on selenium (Fig. 10) the tolerance limit is lower — supply rate of 40 ng miny1 compared to 200 ng miny1 (Fig. 5) — probably due to the synergic interference effect of both antimony and selenium. 4. Conclusions The new design of the MMQTA tube was shown to enhance the performance of the previously described design, by far superior to the EHQTA tubes in terms of linear range and atomization interference extent, while maintaining their excellent sensitivity and simplicity. In other words, by
Acknowledgments This work was supported by the grant of GACR no. 203y01y0453. References w1x J. Dedina, ˇ D.L. Tsalev, Hydride Generation Atomic Absorption Spectrometry, Wiley, Chichester, 1995. w2x M. Ikeda, Determination of arsenic at the picogram level by atomic absorption spectrometry with miniaturized suction — flow hydride generation, Anal. Chim. Acta 167 (1985) 289–292. w3x M. Yamamoto, M. Yasuda, Y. Yamamoto, Hydridegeneration AAS coupled with flow injection analysis, Anal. Chem. 57 (1985) 1382–1385. w4x P. Stauss, Untersuchungen zu den Reaktionen in der Quartzkuvette der Hydrid — Technik der Atomabsorp¨ fur ¨ Chemie tionsspektrometrie, Dissertation, Fakultat ¨ Konstanz, Germany, 1993. der Universitat w5x B. Welz, M. Sperling, Atomic Absorption Spectrometry, VCH Verlagsgesellschaft, Weinheim, 1997. w6x S. Tesfalidet, G. Wikander, K. Irgum, Determination of hydrogen radicals in analytical flames using electron spin resonance spectroscopy applied to direct investigations of flame-based atomization units for hydride generation atomic absorption spectrometry, Anal. Chem. 71 (1999) 1225–1231. w7x J. Agterdenbos, J.P. Van Noort, F.F. Peters, D. Bax, J.P. Ter Heege, The determination of selenium with hydride generation AAS — I. Description of the apparatus used and study of the reactions in the absorption cuvette, Spectrochim. Acta Part B 40 (1985) 501–515. w8x J. Dedina, ˇ B. Welz, Quartz tube atomizers for hydride generation atomic absorption spectrometry — mechanism for atomization of arsine — invited lecture, J. Anal. At. Spectrom. 7 (1992) 307–314. w9x B. Welz, T.Z. Guo, Formation and interpretation of double peaks in flow injection hydride generation atom-
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ˇ et al. / Spectrochimica Acta Part B 57 (2002) 451–462 T. Matousek ic absorption spectrometry, Spectrochim. Acta Part B 47 (1992) 645–658. ˇ J. Dedina, Interference of volatile hydride forming elements in selenium determination by atomic absorption spectrometry with hydride generation, Anal. Chem. 54 (1982) 2097–2102. ˇ A. D’Ulivo, J. Dedina, Interferences in hydride atomization studied by atomic absorption and atomic fluorescence spectrometry, Spectrochim. Acta Part B 51 (1996) 481–498. ˇ ˇ T. Matousek, M. Johansson, J. Dedina, W. Frech, Spatially resolved absorption measurement of antimony atom formation and dissipation in quartz tube atomizers following hydride generation, Spectrochim. Acta Part B 54 (1999) 631–643. ˇ ˇ T. Matousek, J. Dedina, Fate of free selenium atoms in externally heated quartz tube atomizers for hydride generation atomic absorption spectrometry and their reatomization at tube ends studied by means of the determination of longitudinal free atom distribution, Spectrochim. Acta Part B 55 (2000) 545–557. ˇ J. Dedina, Quartz tube atomizers for hydride generation atomic absorption spectrometry: mechanism of selenium hydride atomization and fate of free atoms, Spectrochim. Acta Part B 47 (1992) 689–700. B. Welz, M. Schubert-Jacobs, M. Sperling, D.L. Styris, D.A. Redfield, Investigation of reactions and atomization of arsine in a heated quartz tuber using atomic absorption and mass spectrometry, Spectrochim. Acta Part B 45 (1990) 1235–1256. ˇ ˇ J. Dedina, T. Matousek, Multiple microflame — a new approach to hydride atomization for atomic absorption spectrometry, J. Anal. At. Spectrom. 15 (2000) 301–304. FIAS: Setting Up and Performing Analyses. Commercial publication, Perkin-Elmer Corp., Analytical Instruments, Norwalk, CT USA, 1992.
w18x J. Dedina, ˇ ˇ A. D’Ulivo, L. Lampugnani, T. Matousek, R. Zamboni, Selenium hydride atomization, fate of free atoms and spectroscopic temperature in miniature diffusion flame atomizer, Spectrochim. Acta Part B 53 (1998) 1777–1790. w19x J. Dedina, ˇ I. Rubeska, ˇ Hydride atomization in a cool hydrogen–oxygen flame burning in a quartz tube atomizer, Spectrochim. Acta Part B 35 (1980) 119–128. w20x J. Dedina, ˇ A. D’Ulivo, Argon shielded, highly fuel-rich, hydrogen–oxygen diffusion microflame — a new hydride atomizer, Spectrochim. Acta Part B 52 (1997) 1737–1746. w21x J. Dedina, ˇ T. Matousek, ˇ W. Frech, On-line atomization of selenium hydride in graphite furnaces: estimate of atomic absorption coefficient and spectroscopic temperature, Spectrochim. Acta Part B 51 (1996) 1107–1119. w22x B. Welz, P. Stauss, Interferences from other hydride forming elements on selenium in hydride-generation atomic absorption spectrometry with a heated quartz tube atomizer, Spectrochim. Acta Part B 48 (1993) 951–976. w23x M. Walcerz, E. Bulska, A. Hulanicki, Study of some interfering processes in arsenic, antimony and selenium determination by hydride generation atomic absorption spectrometry, Fresenius J. Anal. Chem. 346 (1993) 622–626. w24x K.S. Subramanian, Rapid electrothermal atomic absorption method for arsenic and selenium in geological materials via hydride evolution, Fresenius J. Anal. Chem. 305 (1981) 382–386. w25x S. McIntosh, Z. Li, G.R. Carnrick, W. Slavin, The determination of tin in steel samples by flow injection hydride generation atomic absorption spectroscopy, Spectrochim. Acta Part B 47 (1992) 897–906.
Multiple microflame quartz tube atomizer — further development towards the ideal hydride atomizer for atomic absorption spectrometry夞 ´ˇ Matousek*, ˇ ˇ´ Dedina, ˇ Tomas Jirı Anna Selecka´ Academy of Sciences of the Czech Republic, Institute of Analytical Chemistry, Laboratory of Trace Element Analysis, ´ ˇ ´ 1083, CZ-142 20 Prague, Czech Republic Vıdenska Received 21 August 2001; accepted 28 November 2001
Abstract The development of an improved type of hydride atomizer for atomic absorption spectrometry — multiple microflame quartz tube atomizer (MMQTA) — is presented. The main feature of this atomizer is recurrent analyte atomization proceeding over its whole optical tube length, which is achieved by production of H-radicals at multiple points within the tube by oxygen microflames burning in the hydrogen-containing atmosphere. The MMQTA design optimization leading to a complete filling of the observation volume with H-radicals is described. The influence of individual atomization parameters is discussed. Optimum H-radical producing oxygen intake into the MMQTA was found to correspond to a H2:O2 stoichiometric (3:1) ratio. The performance of the individual MMQTA tube designs is evaluated and compared to a typical externally heated quartz tube atomizer (EHQTA) — the linearity of calibration graphs for As, Se and Sb is significantly improved in all MMQTA tubes, without compromising the sensitivity, simplicity, low cost and easy operation. In fact, the free atom reactions within the tube causing calibration curvature are avoided up to an analyte concentration of at least 200 ng mly1 for Se and Sb and 100 ng mly1 for As. Tolerance limits of 0.7, 1.4, 0.2 and 0.2 mg mly1 are achieved for the atomization interferences of As on Se, Se on As, Sb on Se and Se on Sb, respectively, which is an improvement by 1–2 orders of magnitude in comparison to the conventional EHQTA with the same hydride generation system. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydride generation atomic absorption; Externally heated quartz tube atomizer; Reatomization; Multiple microflame quartz tube atomizer; Atomization interferences
夞 This paper was presented at Colloquium Spectroscopicum Internationale XXXII, held in Pretoria, South Africa, 8–13 July 2001 and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Fax: q420-2-4106-2499. ˇ E-mail address: [email protected] (T. Matousek).
0584-8547/01/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 1 . 0 0 4 0 0 - 1
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1. Introduction Externally heated quartz tube atomizers (EHQTA) are presently by far the most common hydride atomizers for atomic absorption spectrometry (AAS) w1x. The typical EHQTA is a T-tube made of quartz with the optical tube approximately 100–150 mm long, 7–10 mm i.d. and a narrow inlet arm (1–3 mm i.d.). The L-design of EHQTA w1–3x did not achieve wide popularity. The accepted mechanism of free analyte atom formation in EHQTA via the hydrogen radical mechanism has been confirmed in a number of ˇ studies (see Dedina and Tsalev w1x and references therein). Briefly, hydride is atomized via consecutive reactions with hydrogen radicals. Hydrogen radicals are formed by reactions of oxygen in excess hydrogen. The H-radical cloud is confined to a small part of the atomizer w1,4–6x. Despite the undoubted advantages, there are also serious drawbacks of the EHQTA; poor linearity (or even a rollover) of the calibration graphs and low resistance towards atomization interferences. Calibration curvature was formerly explained by the formation of analyte dimers w7x or an insufficient amount of H-radicals for efficient hydride atomization at higher analyte concentrations w8,9x, whereas atomization interferences were ascribed to the competition for H-radicals between analyte and interferent, or to the alteration in the free atom decay rate by the interferent w1,10x. Strong indications were found recently, supporting the hypothesis that both these undesirable features do have a common source w11–13x. After leaving the H-radical cloud, the free atoms of the hydride-forming element (regardless if considered as analyte and interferent) do react forming dimers and polyatomic particles. These particles increase the surface available for the free atom decay reactions. In other words, the reacted analyte particles provide surface available for reactions not only on the tube walls but also in the atomizer free volume. The more analyteyinterferent present, the more particles and the more surface is available. This may lead to the interference or to the calibration roll-over (it can be considered as an ‘interference of the analyte on itself’) w11–13x.
The species originating from the reacted free atoms can be reatomized, but only upon another contact with hydrogen radicals w14,15x. This cannot be obtained without another introduction of oxygen into the tube. Obviously, the main goal is to fill the whole volume of the atomizer optical bar with hydrogen radicals, so that the free analyte atoms are recurrently atomized and reactions leading possibly to effects such as calibration curvature or roll-over cannot take place w13x. This was achieved in a quartz tube hydride atomizer of the next generation — a multiple microflame quartz tube atomizer (MMQTA) w16x. The optical tube of the MMQTA atomizer consists of two concentric tubes. The inner tube, the optical bar, has multiple tiny orifices over its length. The oxygen containing gas (either air or argonyoxygen mixture) is introduced inside the outer tube, from where it enters the inner tube through the orifices. At the orifices, oxygen reacts with hydrogen present inside the tube. A microflame producing Hradicals is thus formed at every orifice, which results in the presence of H-radicals over the whole observation volume. The analyte can be present in the form of free atoms only. Except the improvement in the linearity of calibration graphs, the sensitivity for selenium was improved by 10% and mainly the tolerance limit (the interferent concentration causing a 10% signal suppression) towards the atomization interference of arsenic was improved by one order of magnitude — from 0.04–0.05 to 0.5–0.55 mg mly1, compared to the conventional EHQTA w16x. However, the presented design of the MMQTA was still far from ideal. Namely the resistance to interferences, although one order of magnitude better than for the conventional EHQTA, was still two orders of magnitude worse compared with a miniature diffusion flame atomizer w11x. The extremely high resistance of the miniature diffusion flame atomizer, and also very good linearity of calibration graphs there, is due to the fact that its whole observed volume is filled with H radicals. The observed non-ideal performance of MMQTA is probably due to zones in the optical path, namely close to the ends of the tube, where no H radicals are present.
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Table 1 The inner dimensions of individual atomizer tubes Tube
Optical tube lengtha (mm)
Optical tube i.d.a (mm)
Inlet i.d. (mm)
Design
Sensitivity checkb
QT1 MM1 MM2 MM3 MM4 MM5
122 154 150 141 152 125
7 7.1 6 7.3 7.1 7
1 1.8 1.8 3.5d 1.8 1.8
w8 x Fig. Fig. Fig. Fig. Fig.
0.365 0.517 0.508 0.415 0.485 0.470
1a 1ac 1ac,d 1ae 1b
a
Narrow working part only — without wide end sections. Absorbance obtained for 10 ng mly1 Se at 196 nm at optimum conditions. c Gas outlets at the wide end sections; not shown in the figure. d Inlet with standard joint; not shown in the figure. e Additional orifices at the asterisk positions. b
The aim of this paper is to describe further development of the MMQTA. The performance of individual MMQTA designs is assessed and compared with an ordinary EHQTA as well as with the previous MMQTA. Selenium was chosen as the model analyte. The results are then verified for arsenic and antimony. Also mutual atomization interferences of these elements are studied. 2. Experimental 2.1. Instrumentation Instrument: Varian SpectrAA 30 atomic absorption spectrometer (Mulgrave, Victoria, Australia) without burner; 15-s integration mode. Steady state signals were recorded. Radiation source: Se and As hollow cathode lamps operated at 10 mA, Sb hollow cathode lamp operated at 8 mA. Wavelengths: Se 196.0 and 204.0 nm at 1-nm slit, As 193.7 nm at 0.2-nm slit, Sb 231.2 nm at 0.2-nm slit. 2.2. Atomizers Six atomizer tubes were used. Quartz Cell for FIAS 200 by Perkin-Elmer was chosen as the representative of conventional EHQTA (marked QT1 further in the text), and five MMQTA tubes were tested in this paper, marked here MM1– MM5. The detailed description and special features
of individual MMQTA tube designs of the tubes are discussed in Section 3.1. The outer dimensions of all the tubes are similar (outer diameter of the optical tube 14 mm to fit the furnace), inner dimensions are summarized in Table 1. All the atomizers feature wide ends of the optical tube preventing flame ignition on the tube ends. No end windows were used in this work. All the atomizers were heated electrically by an in-house made furnace (120-mm heated length, 15-mm inside diameter) controlled by a Rex C100 controller (SYSCON-RKC, IN, USA) with a K-type thermocouple sensor or by the commercial furnace of the same dimensions (AEHT 01 by ´ ˇ Bohdanec, ˇ Czech Republic). The RMI, Lazne temperature was always set so that at 30 mm from the tube center it was 900 8C (measured by Ktype thermocouple). Prior to the first use or if the original (reference) sensitivity could not be achieved, tubes were cleaned by leaching for 10 min in a mixture of conc. HNO3-conc. HF (7:3). The tube was then rinsed with distilled water and left to dry. The gas flow introduced to the inlet arm of the MMQTA atomizers (total inner gas flow) consists of the carrier Ar flow (termed here inner Ar flow) introduced to the hydride generator and of the hydrogen (inner H2 flow) formed from decomposed tetrahydroborate. If not stated otherwise the inner Ar flow rate was 50 ml miny1 and inner H2 flow rate was 15 ml miny1. This was also the
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gas entering the QT1; where mentioned, it was doped by O2 premixed upstream the hydride generator. The oxygen-containing gas entering the MMQTA outer shell (total outer gas flow) consists of an inert gas (outer Ar flow) and oxygen (outer O2 flow). Either argon mixed with oxygen or air was used throughout as the outer gas at the flow rates specified in the text. The total inner gas flow rate and composition entering the atomizer is similar to that of a commercial hydride generation system (Perkin-Elmer FIAS 200) w17x. A diffusion flame atomizer consisting of a short piece of quartz tube (4-mm i.d.) with the flame burning on its end w18x was used to check the efficiency of hydride generation in the presence of an interferent. Argon (650 ml miny1) and hydrogen (300 ml miny1) flows were in this case connected downstream the hydride generator. 2.3. Hydride generators A compact continuous flow hydride generator was described in detail elsewhere w12,16x. It consists of two T-pieces for reductant and carrier gas introduction, a 1-mm i.d.y750-mm long PTFE reaction coil and forced outlet quartz gas–liquid separator. A multichannel minicartridge peristaltic pump (Ismatec SA, Switzerland) was used for the transport of solutions. Blankystandard and reductant (0.5% myv NaBH4 in 0.4% KOH) solution flow rates were 4 ml miny1 and 1.2 ml miny1 (producing 15 ml miny1 of inner H2), respectively. Standards were prepared in 1 M HCl from a 1000mg mly1 stock solution. If not stated otherwise, a 50-ml miny1 carrier argon flow rate was used throughout this work, as lower flow rates are impractical concerning the hydride generator (too long time for establishing the steady state conditions and possibility of transport interferences). For the measurements of interferences, this sysˇ ˇ tem was modified as in Dedina and Matousek w16x. To avoid liquid phase interferences, analyte and interferent hydrides were generated separately in a semi-twin-channel system. In the analyte channel, a Se standard solution (2 ml miny1) was mixed with the reductant (0.57 ml miny1) in a Tpiece (PEEK, 0.8-mm inner bore). Analogous arrangement was used for the interferent channel.
Fig. 1. MMQTA tube designs. (a) Tube MM1 (similar to tubes MM2, MM3 — see Table 1 for the differences) and tube MM4. Asterisks mark orifices in tube MM4 only. (b) Tube MM5.
Both these channels joined in a cross-piece (PEEK, 1.5-mm inner bore), to where also the carrier gas (inner Ar gas) was introduced. The reaction mixture then continued to the reaction coil and gas– liquid separator (the same as for the single channel hydride generator above). The interferent channel was checked regularly to produce equal signals for selenium as the analyte channel. 3. Results and discussion 3.1. MMQTA tube designs An early MMQTA design is depicted in Fig. 1a, presented already in a communication w16x, corresponding to tube MM1. Tubes MM2 and MM3 differ from this design by details which do not influence principally the atomizer function, such
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as by outlets at the wide optical tube ends, and by wider inlet arm ending by a standard joint (tube MM3). The main disadvantage of this MMQTA design — tubes MM1, MM2 and MM3 — is the fact that it features the outer gas inlets into the outer cavity from the sides (Fig. 1a). Inevitably, there must be approximately 20 mm of the optical tube outside the furnace on both sides. In these cold parts, there are no oxygen inlets (as the temperature is too low for H-radical production), so that no H radicals are present in this part of the optical tube. This is underlined by the fact that low temperature in these regions is favorable to the formation of the analyte or interferent particles, which are probably responsible for the enhanced free analyte atom decay w11,13x. This impairs the analytical performance of the MMQTA (see Figs. 4 and 7) (although any MMQTA we tested still by far outperforms the QT1). Two approaches how to avoid free atoms in cold regions were tested. The first approach is the design of tube MM4. It has additional pairs of orifices also in these problematic cold regions (marked by asterisks in Fig. 1a). The detailed function of these orifices is discussed in Section 3.2.2. Molecular oxygen thus entered the inner tube outside the furnace, but does not produce H-radicals but rather reacts with free analyte atoms. Thus, free atoms are removed on purpose from these critical regions and cannot be subjected to interference. Although the calibration linearity and the interference tolerance limit were improved by this approach (Fig. 7), there are certain disadvantages. Namely, the oxygen intake to the hot and cold regions cannot be controlled (see Section 3.2.2); it depends on the minute orifice differences and the optimum oxygen intake may vary significantly from one tube to another. Separate control of oxygen intake to hot and cold regions would make the system even more complicated. Another approach is presented by tube MM5 shown in Fig. 1b, differing by a shorter optical tube and outer gas inlet in the central part. The outer gas is introduced to the cavity between the inner (optical) and the outer concentric tube of the horizontal arm via the cavity between the double wall of the inlet arm. The optical tube can be then ended at the edge of the furnace, without
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any troublesome cold regions. Shorter optical tube length results in the reduced photometric noise, which was the main noise source in our system (see Section 3.3.2). The inlet arm (usually rather narrow quartz tube) is also reinforced, so that it cannot be easily broken. This design yielded the best results both in the calibration curve linearity and in the resistance towards atomization interferences (see below) and was therefore chosen as the most prospective one, although the data are also presented for the other designs where available. Concerning the orifices, the initial intention as to the diameter was ‘as small and uniform as possible’ to provide even distribution of oxygen intake over the tube length. A small orifice diameter should provide the oxygen entrance inside the tube at high linear velocity, so that O2 and H2 reactions proceed inside the optical tube rather than within the orifice. In practice, the manual procedure of making the orifices resulted in their diameter variations between 0.1 and 1 mm, with the majority of approximately 0.5 mm. That is also the reason why the orifices are made in pairs — there is a lower chance of insufficient oxygen intake into any tube region due to a too small orifice. The distance of the orifice pairs was made 15 mm, based on the study of the longitudinal distribution and decay rate of free atoms in EHQTA w13x. This distance appeared sufficient with respect to the calibration linearity (see Section 3.3.1) and was kept the same in all tubes within this study. 3.2. Influence of individual atomization parameters 3.2.1. Inert gas flow rates Fig. 2 depicts the influence of the total gas flow rate on the signal. Either inner or outer gas flow rates were varied. Generally, with increasing flow rate of the carrier gas the signal decreases, due to dilution of free atoms in the observation volume. The signal decreases with increasing outer inert gas (circles in Fig. 2) is lower than when increasing the inner Ar gas, simply because the outer gas is not going through the whole observation volume but only through part of it. On the other hand, the use of pure oxygen as an outer gas (without addition of outer inert gas) does not impair the
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Fig. 2. The influence of total gas flow rate through the atomizer on the signal. Tube MM3, 10 ng mly1 Se. h, variable inner Ar flow rateq20 ml miny1 outer Ar and 3 ml miny1 outer O2 flow rate; n, variable inner 10% H2 in Ar flow rateq25 ml miny1 outer air flow rate; ●, 50 ml miny1 inner Ar flow rateq3 ml miny1 outer O2 and variable outer Ar flow rate.
atomizer performance. It means that outer inert gas has no function other than to avoid difficulties with control of minute outer oxygen flow rates. Air can be conveniently utilized as the outer gas. No other phenomena depending on the inert gas flow rate (or, in other words, depending on the analyte concentration in the gaseous phase) were identified for optimum oxygen intake (see Section 3.2.2) (such as, e.g. signal decrease at the lowest carrier gas rates as described for the EHQTA w1x, caused by the free atom decay within the tube w1,13x). This is in line with the assumed recurrent analyte atomization within the tube, so that the free atom reactions are virtually suppressed. 3.2.2. Oxygen intake by outer gas As oxygen is necessary for production of hydrogen radicals in quartz atomizers, the oxygen intake into the tube is the most critical parameter for the atomizer performance. In all the tested atomizer tubes (with the exception of tube MM4 as discussed below) for inner H2 flow rate of 15 ml miny1 (resulting from the NaBH4 decomposition), the oxygen intake for maximum sensitivity for Se was approximately 4.5–5 ml miny1 O2 or 20–25 ml miny1 air. The influence of outer O2 flow rate is much less pronounced in the case of arsenic and antimony (Fig. 3), however, an optimum can be
found in the same oxygen or air flow rate as for selenium. The optimum oxygen intake is dependent on H2 concentration inside the tube. Variable flow rates of H2 were introduced to the inner gas upstream the hydride generation system in addition to the 15 ml miny1 of H2 from the NaBH4 decomposition, comprising the total inner hydrogen flow rate of 15–60 ml miny1, and optimum outer O2 flow rate was tested for tube MM5. It appeared that the optimum outer O2 flow rate equals one-third of the total inner H2 flow rate, similarly as in the other tubes (see above). This should be compared with the conclusion that H radicals are produced in quantities corresponding to the total amount of oxygen, i.e. two radicals for each O2 molecule w1,19x. Provided that oxygen inlets are uniformly distributed over the length of the tube, this stoichiometric ratio leads to the most efficient production of the H-radicals over the whole observation volume. Hydrogen is gradually consumed on its way through the optical tube of MMQTA by reactions with O2 entering at individual orifices along the optical tube length. Too low O2 intake in the outer gas leads to sub-maximum production of H-radicals. On the other hand, at too high O2 intake the H2 is consumed prematurely — molecular oxygen entering through orifices near the ends remains unreacted, and it can interfere with free analyte atoms (as described for Se w11,20x) potentially present in this part, decreasing thus the signal.
Fig. 3. The influence of outer air flow rate, tube MM5. ●, Se 10 ng mly1; j, As 5 ng mly1; m, Sb 10 ng mly1.
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The outer oxygen flow demand of the tube MM4 is different. With the increasing outer air intake, the signal is increasing from 67% of maximum at 25 ml miny1 up to a maximum at 50 ml miny1 which is followed by a plateau with a slow signal decrease (to 90% of maximum at 120 ml miny1 of outer air); similar data are obtained if 20 ml miny1 of Arqcorresponding amount of O2 is used as the outer gas, just the decrease is faster (maximum at 11 ml miny1 O2, 90% at 15 ml miny1 O2) This is caused by the fact that oxygen has two functions here: it does not serve only for production of H-radicals, but also for depleting free atoms in the ‘cold zones’ (see Section 3.1). The difference between the optimum oxygen intake in tube MM4 and the ‘3:1 ratio’ is determined by the proportion of the outer gas entering the ‘hot’ and ‘cold’ zones, which is given by the permeability of orifices in these parts (thermal expansion of the permeating gas must be also taken into account). As the diameter of the orifices can vary from piece to piece, slightly different oxygen demand data could be obtained for other specimen of this design. 3.3. MMQTA performance 3.3.1. Calibration graphs It is a well-known fact that conventional EHQTA do suffer from poor linearity of calibration graphs or even calibration roll-over w1,7–9x. This was recently proven to be caused by the acceleration of the reaction rate of free atom decay inside the tube at increasing analyte concentration w13x, rather than by incomplete atomization as suggested earlier w9x. Since the analyte is recurrently atomized in MMQTA, the calibration graph linearity should be much improved there compared to EHQTA. Fig. 4 shows the calibration graphs for selenium in QT1 (under the conditions favorable to Hradical formation — the sufficient O2 supply and end-section reatomization possible w13x) and two MMQTAs: the best (MM5) and worst (MM2) specimen with respect to calibration linearity. The difference between both MMQTA is small. The initial sensitivity of tube MM2 is slightly higher than that of tube MM5 due to its longer optical
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Fig. 4. Calibration graph for selenium, 204-nm line. ●, QT1, 3 ml miny1 O2 added; h, MMQTA–MM2, 20 ml miny1 Ar and 5 ml miny1 O2 outer gas; n, MMQTA–MM5, 25 ml miny1 air outer gas.
tube, whereas the linearity is worse due to the cold end sections. Anyway, even the ‘bad’ MMQTA is performing much better than QT1. The sensitivity of QT1 and MMQTA tubes is not differing dramatically — it is given by the fact that the analyte can remain in the state of free atoms nearly in the whole observation volume in well-optimized QT1 at very low analyte supply (under sufficient O2 intake and with the possibility of analyte reatomization near the ends) as shown recently w13x. Fig. 4 was obtained at the 204-nm Se line which is approximately 20 times less sensitive than the commonly used 196-nm analytical line. Thus, lower absorbance values could be obtained, which are less influenced by the calibration curvature sources inherent to the optical and detection systems. To find the possible source of slight calibration curvature for tube MM5 at the 204-nm Se line (Fig. 4), the deviations from linearity in the same absorbance range for tube MM5 at 204 nm and at 196 nm (not shown) were compared. As the concentrations for the 196-nm line calibration are safely in the range which does not produce any deviations from linearity at 204 nm, and there are no spectral reasons for a difference in linearity at both these lines w21x, it can be concluded from the perfect match of both curves that even the slight curvature observed for tube MM5 in Fig. 4 is not originating within the MMQTA nor in the hydride
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Fig. 5. Calibration graph for antimony, 231.2-nm line. ●, QT1; n, MMQTA–MM5, 25 ml miny1 air outer gas.
generation system, but solely in the opticalydetection part. In other words, the linear response of the MMQTA is extended up to at least 200 ng mly1 Se (corresponding to the supply rate of 13 ng sy1) compared to approximately 10 ng mly1 (0.7 ng sy1) in the case of QT1. Fig. 5 shows the calibration graphs for Sb. In QT1, a sharp sensitivity drop was experienced at the concentrations over 80 ng mly1, similarly as observed and discussed recently w12x. No such phenomena were found in MMQTA (tube MM5) up to 200 ng mly1. Fig. 6 depicts the calibration graphs in MMQTA (tube MM5) and QT1 for arsenic. Even if the QT1 is not performing as bad as in the case of selenium and antimony (although the studied concentration range is lower), MMQTA yields significantly better results. 3.3.2. Atomizer noise The possible introduction of additional noise by the atomizer into the analytical system was tested. The procedure of the noise level evaluation is w13x. ˇ and Dedina ˇ described in detail in Matousek The dominant noise source in the present system was the photometric noise of the detector w13x. It is dependent on radiation attenuation by the atomizer (which is given by the length and i.d. of the optical tube of the atomizer). The heating and modeling of the atomization atmosphere in the atomizers (QT1 and all the MMQTA) does not increase the noise level. However, in the case of
QT1 and tube MM5 noise is increased by approximately 30% when the hydride generation is started. It is caused probably by the uneven gas flows from the generator rather than the atomizer itself; the noise source is probably the fluctuations of ingress of the ambient air into the hot ends of the tube (oxygen is absorbing radiation at elevated temperatures at wavelengths under 200 nm). In the other MMQTA tubes (MM1–MM4) featuring cold sections of the optical tube at the ends, noise is not increased with hydride generation, but this is outbalanced by higher photometric noise due to their increased length. In summary, there is no significant noise difference between the QT1 and any of the tested MMQTA designs. To sum up, the limit of detection is not going to be changed appreciably after replacement of an ordinary EHQTA by a MMQTA tube of similar dimensions. 3.3.3. Atomization interferences The tolerance limits (interferent concentration causing a 10% signal suppression) of the mutual interferences of Se–As and Se–Sb are summarized in Table 2. The results obtained in MMQTA (tube MM5) are compared with the results for QT1, which are not in contradiction to other previously published tolerance limit data in EHQTA. As there is a wide variety of hydride generation systems, atomizer dimensions and atomization conditions described in the literature resulting in tolerance
Fig. 6. Calibration graph for arsenic, 193.7-nm line. ● QT1, 3 ml miny1 O2 added; n, MMQTA–MM5, 25 ml miny1 air outer gas.
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limit data differing by several orders of magnitude, exclusively the data for experimental set-ups similar to our system were chosen. The detailed results of measurements of interference of arsenic on the selenium signal for the QT1 and the MMQTA tubes are presented in Fig. 7. Tolerance limits for the individual MMQTA tubes are at least one order of magnitude better than that of the QT1. It should be noted that this was achieved simply by replacing the QT1 by a MMQTA tube, without any modification of the hydride generation system. Even if the differences between the individual MMQTA tubes may be partially biased by the fact that the oxygen intake was optimized for maximum sensitivity and not for minimum interference extent, the initial design (tubes MM1 and MM2) does exhibit lowest tolerance to atomization interferences. As mentioned in Section 3.1, this design features the oxygen inlets into the outer shell from the sides (Fig. 1a). These colder parts do not contain the necessary H-radicals and free atoms are exposed to the reactions leading to the interference. The low temperature in these regions is favorable for the formation of the interferent particles, causing enhanced free analyte atom decay w11x. Both designs of tube MM4 and MM5 do overcome this problem (see Section 3.1) and yield lower interference magnitudes than tubes MM1 and MM2. Higher hydrogen and oxygen intake into the tube should produce higher density of H-radicals and thus reduce the interference. We studied the
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Fig. 7. Interference of arsenic on selenium. ●, QT1, 3 ml miny1 O2 added, Se 10 ng mly1; s, MMQTA–MM1, 22.5 ml miny1 air outer gas, Se 20 ng mly1; h, MMQTA–MM2, 20 ml miny1 Ar and 5 ml miny1 O2 outer gas, Se 20 ng mly1; e, MMQTA–MM4, 100 ml miny1 air outer gas, Se 20 ng mly1; n, MMQTA–MM5, 22.5 ml miny1 air outer gas, Se 20 ng mly1.
influence of oxygen intake on the As interference on Se under standard conditions (i.e. 15 ml miny1 H2 produced by NaBH4) and with the addition of 45 ml miny1 of H2 to the inner gas. The results are shown in Fig. 8, analogous results were obtained also for As interference on Se (not shown). With increasing oxygen intake, the interference extent is decreasing until it reaches a stable value. This ‘limiting’ interference value is lower at higher hydrogen intake, but at the cost of sensitivity. Higher production of hydrogen radicals, higher total gas flow rate diluting the interferent in the gaseous phase andyor probably slightly
Table 2 Tolerance limits for mutual As–Se and Sb–Se interferences Tolerance limits (mg mly1)
Reference
Interference As on Se
Interference Se on As
Interference Sb on Se
Interference Se on Sb
0.04 0.04 0.05 0.1 0.15 0.2 0.7
0.01
0.02 0.06
0.01
0.1 0.025 1.4
0.05 0.2
0.05 0.2
This work, QT1 w22x w23x w3 x w24x w25x This work, tube MM5
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Fig. 8. Interference of arsenic on selenium at various outer O2 and inner H2 intake. Tube MM5, 20 ng mly1 Se; outer inert gas 20 ml miny1 Ar. h, without interferent; j, with 5 mg mly1 As. Columns show interference influenced signals in relative scale (right axis — interference free signal is 100%).
higher temperature inside the tube may be the reasons for this lower interference level. It should be noted that this ‘limiting’ interference is an atomization interference — it is not originating in the hydride generation phase or during hydride transport w1,10x. This was checked using the same hydride generation system with the diffusion flame atomizer, which is very resistant against atomization interferences w11,20x. There was no interference of 5 mg mly1 of As and Se on the signal of 0.5 mg mly1 of Se and As, respectively, in the diffusion flame atomizer. The oxygen intake optimum to minimize interferences corresponds fairly well to that for maximum H-radical production; it is approximately 10% higher than the optimum oxygen intake for the maximum signal (oxygen-to-hydrogen ratio 1:3). This is probably caused by the slightly uneven oxygen distribution in the inner optical tube due to orifice diameter variations; the maximum production of H-radicals in all tube parts necessary for minimum interference may lead to O2 excess and free atom population decrease in some regions. The difference between the optimum oxygen intakes for maximum signal and minimum interference may be tube specimen-dependent — the smaller the more even distribution of oxygen in the tube. Although the interference extent of selenium on arsenic is slightly stronger than vice versa (Fig.
Fig. 9. Interference of selenium on arsenic. ●, QT1, 2.6 ml miny1 O2 added, As 10 ng mly1; n, MMQTA–MM5, 32.5 ml miny1 air outer gas, As 10 ng mly1.
9), the behavior for both combinations is analogous in both QT1 and MMQTA. Under an outer O2 flow rate optimized for minimum interference (6.5 ml miny1 O2 corresponding to 32.5 ml miny1 air for both analyteyinterferent combinations — see Fig. 8), the MMQTA yields two orders of magnitude better results than the QT1. Figs. 10 and 11 verify the superiority of the MMQTA over the QT1 also for the mutual Se–Sb interference pair. In both cases, the tolerance limit is one order of magnitude better in the MMQTA tube. It is interesting to compare the interference tolerance limits for individual interferents in the QT1 (Figs. 7, 9–11) with the corresponding cali-
Fig. 10. Interference of antimony on selenium. ●, QT1, Se 20 ng mly1; n, MMQTA–MM5, 25 ml miny1 air outer gas, Se 20 ng mly1.
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simple replacement of EHQTA by a MMQTA the improved accuracy of analysis may be achieved without compromising the limit of detection. Further improvement of the MMQTA design includes the optimization of the orifice pairs distance or of their pattern, which may further enhance the MMQTA resistance towards atomization interferences due to more uniform filling of the optical tube volume with H-radicals. The longterm stability also remains to be thoroughly evaluated; our to-date experience shows that MMQTA surpasses the EHQTA also in this respect. Fig. 11. Interference of selenium on antimony. ●, QT1, Sb 10 ng mly1; n, MMQTA–MM5, 25 ml miny1 air outer gas, Sb 10 ng mly1.
bration graphs (Figs. 4–6); please note that the actual element supply rates must be compared, as the data in Figs. 7–11 were obtained at a sample introduction flow rate of 2 ml miny1 whereas the data in Figs. 4–6 at 4 ml miny1 (see Section 2.3). It can be seen that this tolerance limit approximately corresponds to the upper limit of the linear range (or the point where the calibration graph for QT1 differs by 10% from that of MMQTA), i.e. supply rates of approximately 20–40 ng miny1 for selenium and 80 ng miny1 for arsenic, respectively. This fact gives evidence that the calibration curvature and atomization interferences do proceed via common mechanism. The above supply rates correspond approximately to the element supply at which the condensation or particle formation starts under given atomization conditions. In the case of antimony interference on selenium (Fig. 10) the tolerance limit is lower — supply rate of 40 ng miny1 compared to 200 ng miny1 (Fig. 5) — probably due to the synergic interference effect of both antimony and selenium. 4. Conclusions The new design of the MMQTA tube was shown to enhance the performance of the previously described design, by far superior to the EHQTA tubes in terms of linear range and atomization interference extent, while maintaining their excellent sensitivity and simplicity. In other words, by
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