ANALYST, MARCH 1987, VOL. 112 271

Flow Injection Atomic Absorption Spectrometry with Air Compensation

lgnacio Lopez Garcia, Manuel Hernandez Cordoba and Concepcion Sanchez-Pedreiio” Department of Analytical Chemistry, Faculty of Chemistry, University of Murcia, 30001 Murcia, Spain

Flow injection atomic absorption spectrometry (FI-AAS) with air compensation was studied and compared with FI-AAS using solvent compensation and with conventional AAS. It seems clear that air compensation offers important advantages, as the great increase in nebulisation efficiency improves the sensitivity and selectivity. The reproducibility obtained for both peak height and peak area is similar to or better than that obtained using other FI-AAS systems and comparable to that given by conventional AAS. The problem of using a pumping flow-rate lower than that of the nebuliser aspiration (necessary for chromatographic and solvent extraction procedures coupled with FI-AAS) is solved using the air compensation method with advantages over other procedures. Keywords: Flow injection atomic absorption spectrometry; air compensation; sensitivity; selectivity; reproducibility

When flow injection (FI) is used with flame atomic absorption spectrometry (AAS), it is important to design a suitable manifold for transport of the carrier stream towards the nebuliser of the spectrometer.lJ It has been pointed out that the flow-rate should not be less than the aspiration rate of the spectrometer under normal conditions (about 6 ml min-1). If the flow-rate is below the aspiration rate, erratic results are obtained, because of air entering the injection system through the connector.2

In most chromatographic and solvent extraction operations, the pumping rate is less than the nebulisation rate. Hence extensive use has been made of either a manifold based on a “compensation method,”>s or of an aspiration rate reduction system including a large tube prior to the nebuliser2 or a modified nebuliser position.6 In the above instances, a noticeable decrease in sensitivity is observed, owing to the dilution effect resulting from solvent compensation, disper- sion of the sample injected and very poor nebulisation.

On the other hand, it has been pointed out that higher peaks can be obtained for FI-AAS if the sample pumping rate exceeds the optimum aspiration rate of the nebuliser.7 This increase in the peak height, however, was achieved through an increase in the sample delivery and not in the nebulisation efficiency.8 A decrease in nebulisation efficiency can be seen from a report of a less than linear response of the sample signal to the sample flow-rate? To date, increased efficiency for conventional nebulisers has been achieved only by reducing the sample pumping rate.

The extensive work on atomic absorption response falls within two broad categories? aerosol generation and nebu- liser action. From these investigations it seems clear that the average droplet diameter is related to the efficiency of nebulisation. Thus, a decrease in droplet size can be obtained by reducing the sample pumping rate.

As fine droplets improve the sensitivity and linearity of response of an instrument whilst reducing interference effects,loJl the most important role of the nebuliser assembly is to enable a large mass of the sample, in the finest possible form, to reach the flame. Much effort has been directed towards attaining this, both in nebuliser design and in aerosol research. 11312 Our understanding of the processes involved is, however, still limited.

It has been demonstrated6 that without changing any other parameters, a reduction in the sample uptake rate reduces the signal but increases the atomisation efficiency. The net result

* To whom correspondence should be addressed.

is a 33% decrease in the peak height but a four-fold increase in the peak area.

It has been pointed out that ingress of air13J4 or de-gassing15 is troublesome in FI-AAS systems. However, Yoza et aZ.16 deliberately introduced an air stream through a T-connector prior to the nebuliser. As a result, an increase in sensitivity was obtained using air instead of water compensation. As far as we know, this idea has not been exploited by others.

The aim of this work was to demonstrate that air compensa- tion offers advantages over both solvent compensation and a reduction in aspiration rate. Indeed, with air compensation, both the dilution of the injected sample by solvent compensa- tion and the dispersion effect were minimal, owing to the T-piece acting as a pre-nebuliser. In this way, a substantial increase in sensitivity was obtained, mainly for low pumping flow-rates, as a result of more effective liquid fragmentation, enabling a signal to be obtained comparable to that of the conventional continuous aspiration method. Further, we have demonstrated that when the FI-AAS air compensation method is used, classical interferences that occur in the conventional AAS determination of calcium are minimised, thereby avoiding the need for a releasing agent.

Experimental All reagents were of analytical-reagent grade and doubly distilled water was used throughout. A stock solution of calcium (1000 mg 1-1) was prepared from calcium carbonate. Working solutions were prepared daily by appropriate dilu- tion.

D /RI n

c1

l!l W I ii U OR

Fig. 1. Single-line FI-AAS manifold in which the carrier solution is propelled by either a peristaltic pump or an air-pressurised reservoir, P. AA, atomic absorption spectrometer; R, recorder; V, sample injector; T, three-way connector; C, pre-coil (2 mm i.d., 5 m long); PS, pulse suppressor; C1 and G, tubes (0.5 mm i.d.); W, carrier solution reservoir; L, tube (0.5 mm i.d.) for compensation; OR, open reservoir for water compensation

Publ

ishe

d on

01

Janu

ary

1987

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 30

/10/

2014

18:

36:5

2.

View Article Online / Journal Homepage / Table of Contents for this issue

272 ANALYST, MARCH 1987, VOL. 112

Two atomic absorption spectrometers, a Pye Unicam SP1900 and a Perkin-Elmer 300, were used together with a Hewlett-Packard 7040A X - Y recorder and a Perkin-Elmer Sigma 15 chromatographic data station. For this study, data were acquired only for calcium at 422.7 nm in an air - acetylene flame and conventional hollow-cathode lamps were used. The spectrometers were operated in accordance with the Perkin- Elmer and Pye Unicam standard procedures for maximum sensitivity with air - acetylene flames.

The flow injection manifold consisted of either a reservoir with air at very low pressure (0.14.5 bar), maintained by a precision regulator or a peristaltic pump (Pharmacia P-3), an injection valve, end-fittings and connectors, PTFE tubing of 0.5 mm i.d. (Omnifit) and a three-way connector (Technicon PT2) as the T-piece. The pulse suppressor was laboratory- made from PTFE and silicone-rubber tubing in the usual form.17 Polyethylene tubing (0.5 mm i.d.) was used to connect the sample injector to the spectrometer. Flexible tubing (PVC, 2 mm i.d.) was used as a pre-coil.

The compensation method proposed is shown in Fig. 1. The experimental conditions of the atomic absorption spec- trometer were adjusted in order to obtain the maximum negative pressure at the nebuliser and these were kept constant in all the experiments. Both the pulse suppressor (PS) and pre-coil (C) in Fig. 1 damped the pulses of the peristaltic pump (P). The sample solution was injected at V via a loop valve into a stream of water pumped from W by the peristaltic pump. The tubes C1 and C2 had to be very short in order to avoid sample dispersion. By using the T-piece, the water in an open reservoir (OR) was aspirated through a branch tube (L) into the spectrometer at a flow-rate vb just sufficient to compensate for the starvation of the nebuliser; vb

varied with the flow-rate of the carrier stream, v,, but the total flow-rate, v, (= vb + v,), into the nebuliser was automatically kept constant. The length of tube L could be modified to obtain different flow-rates, vb, of either air or water for the

I (a)

o.61: 0.3

I f I

0 1 2 3 Pumping rate/ml min-’

Fig. 2. Effect of pumping rate on absorbance values for three different tube (L) lengths. Calcium solution (10 pg ml-1) as carrier stream. (a ) , (b) and (c) 0.5, 20 and 170 cm, respectively. In all instances A = air compensation and B = water compensation. Absorbance by conventional AAS, 0.632 A

same pumping flow-rate. In this way, lengths of tube L of 0.5, 20 and 170 cm were used in order to obtain different vb values.

Results and Discussion Our first aim was to compare the instrument response as a function of pumping rate for the situation without compensa- tion with the situation with air or water compensation. This comparison was made under steady-state conditions. The next interesting aspect was to examine the use of flow injection introduction and to compare peak heights with peak areas.

Compensation when the Analyte Solution is Propelled by a Propulsion System

Using the manifold shown in Fig. 1, we compared the absorbances obtained for a 10 pg ml-1 calcium solution as carrier with those obtained by the conventional AAS method. As can be seen in Fig. 2, the plots of absorbance at the

steady state with air and water compensation versus pumping flow-rate have very different shapes.

1

Scan - Fig. 3. Results of comparative study of the signal obtained when 10 pg ml-1 of Ca was measured with conventional AAS (F) and superimposed FI-AAS peaks (A-E). Pump flow-rates (A-E) were 2.3, 1.4, 0.75, 0.35 and 0.25 ml min-1, respectively. Sample volume, 235 pl. The carrier solution (re-distilled water) was propelled by gas maintained at different pressures in order to obtain the above v, values. Length (L), 0.5 cm

150 s - B

Scan - Fig. 4. Calibration and precision run for the FIA system. (A Concentrations: 1,2.5,5 and 10 pg ml-1 Ca2+ (from left to right). (BI 15 successive injections of 5 pg ml-1 calcium solution. Instrumental conditions: pumping rate, 1.8 ml min-1; sample volume, 235 pl (0.5 mm i.d. tubing); AA spectrometer (Pye Unicam) set at 422.7 nm

Publ

ishe

d on

01

Janu

ary

1987

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 30

/10/

2014

18:

36:5

2.

View Article Online

ANALYST, MARCH 1987, VOL. 112 273

Using water compensation, the absorbance decreased with decreasing pumping flow-rate (v,) as there was an increase in dilution (vb increased in order to keep the total flow-rate, v,, constant). At the same pumping flow-rate, this effect increased as vb increased (by reducing the length of tube L).

However, with air compensation, the absorbance values were high (equal to or slightly higher than those obtained using conventional AAS) and constant over a wide range of pumping rates. This was more noticeable at high vb values (very short lengths of tube L), owing both to the greater prior fragmentation of the calcium solution by the air stream in the T-piece and to the absence of dilution by the solvent. In spite of this, the sensitivity decreased with decreasing pumping rate as a result of a similar decrease in the number of calcium atoms in the flame per unit time.

From the results obtained it seems clear that the highest air flow-rate in tube L leads to maximum absorbance values for any pumping flow-rate. In order to extend this study, we injected air through tube L with the aid of an air compressor. The absorbance increased by about 5-10% for all pumping rates (0.4-3.8 ml min-1) at air pressures between 0.1 and 0.5 kg cm-2 and decreased with increasing pressure. Moreover, at 1 and 1.5 kg cm-2 the absorbance signal decreased by about 4 and lo%, respectively, and the pump pulse effect was more noticeable. To summarise, the forced introduction of air compressed through the T-piece is not recommended, and the best results were obtained when the T-piece was open to the atmosphere and the length of tube L was minimal.

Hence air compensation using a T-piece, when the analyte solution is propelled by an external propulsion system, offers a

signal similar to that obtained with conventional AAS, the possibility of working at a low pumping flow-rate and better nebulisation (because a smaller number of atoms per unit time than with conventional AAS reach the flame, giving a similar signal) and, as a result, the formation of very fine droplets that improve the selectivity.

Compensation when the Analyte Solution is Injected in a Flow Injection System

We applied the observed advantages of air compensation to flow injection atomic absorption spectrometry (FI-AAS) to obtain an increase in sensitivity and selectivity compared with conventional FI-AAS systems.

To address the questions arising from Brown and RGiiCka’s paper,7 viz., (1) is the reproducibility of FI-AAS worse than that of conventional AAS?, (2) does FI-AAS always allow signal reduction? and (3) are there any advantages of using external propulsion?, there now follows a discussion of the possibilities of air compensation with respect to peak heights and areas. We consider that the air-compensation method provides possible answers to the above questions.

0.6 a c m

0.4 2 a 0.2

0 4 8 12 Calcium concentration/pI ml-1

Fig. 5. Calibration graph for determination of Ca. A, Conventional AAS and B-F, using FI-AAS system with air compensation with sample sizes of 235, 135,85,60 and 35 1.11, respectively. Pumping rate, 1.8 ml min-1. Tube length (L), 0.5 cm

Fig. 3 shows the effect of the carrier flow-rate using air compensation on the signal when 235 pl of 10 pg ml-1 calcium solution were injected into the water carrier stream. As can be seen, a signal equivalent to conventional AAS (graph F) was obtained.

Fig. 4 shows a typical calibration run for calcium using FI-AAS air compensation in the range 1-10 pg ml-1, each sample being injected three times. The resulting linear calibration graph has a regression coefficient of 0.9989. The limit of quantification18 was calculated as 0.060 pg ml-1 of Ca. Fig. 4 (B) shows a recording for repeated injections of 5 pg ml-1 Ca standard solution.

Optimisation Variables

As has already been discussed in the literature,2 the signal obtained depends on the flow-rate of the carrier stream, the volume of sample injected and the aspiration flow-rate. These were therefore investigated in order to optimise sensitivity and precision.

The sensitivity of the system is indicated by the slope of the calibration graph and increases with increasing slope. We examined the slope of the calibration graph for a series of eight calcium standards in the range 0.5-12 pg ml-1 using different sample loops in the injection system. Fig. 5 shows the results for peak height in absorbance versus calcium concentration. As expected, the slope of the calibration graph increased with increasing size of the sample. For sample loops up to 100 p1, the signal obtained was almost equal to that given by conventional AAS.

Fig. 6 shows the great effect on peak heights of different pumping rates using air compensation. As can be seen, there is an optimum range of pumping rates for all sizes of sample where the absorbance value is a maximum. The lower is the

0.6 -

a 2 UJ

r“ 0.4 . .-

2 0

a

0.2 i 0 1 2 3

Pumping rate/ml min-1

Fig. 6. Effect of pumping rate on peak height (absorbance) using different sample sizes. Sample injected, 10 yg ml-1 calcium solution. Sample sizes A-E: 235, 135, 85, 60 and 35 yl, respectively. Tube length (L), 0.5 cm

1.8

* =::- Z - c t I I I ‘Lr

3 4 0 1 2 Pumping rate/ml min-1

Fig. 7. Effect of pumping rate on eak area using both air (A and B) and water compensation (A’ and if). Sample sizes: A, 235; and B, 35 pl. The injected samples were from a 10 yg ml-1 calcium solution. Tube length (L), 0.5 cm

Publ

ishe

d on

01

Janu

ary

1987

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 30

/10/

2014

18:

36:5

2.

View Article Online

274 ANALYST, MARCH 1987, VOL. 112

Table 1. Comparison of peak height and area using FI-AAS. Peak height in absorbance; peak area in absorbance seconds; conventional nebulisation (4.5 ml min-1) = 0.634 A. Each value was obtained from four successive injections of 10 pg ml-1 calcium solution (Pye Unicarn instrument)

Length of tube Wcm

Loop Com- size/ pensation pl method Signal 0.9* 35 Water Height 0.048

Area 0.200 Air Height 0.269

Area 1.221

235 Water Height 0.091 Area 1.221

Air Height 0.528 Area 7.794

* Pumping rate (ml min-1).

0.5 20 170

1.8* 2.8* 0.072 0.077 0.191 0.191 0.264 0.230 0.734 0.525

0.148 0.202 1.173 1.145 0.620 0.634 4.903 3.539

3.8* 0.086 0.181 0.182 0.372

0.200 1.106 0.620 2.690

0.9* 0.072 0.324 0.281 1.106

0.139 2.089 0.499 7.479

1.8* 2.8" 0.108 0.115 0.286 0.267 0.247 0.206 0.667 0.486

0.226 0.283 1.784 1.622 0.567 0.571 4.398 3.348

3.8* 0.113 0.248 0.175 0.381

0.341 1.517 0.562 2.442

0.9* 0.132 0.527 0.228 0.916

0.241 3.339 0.447 6.373

1.8* 2.8* 0.153 0.145 0.419 0.324 0.228 0.199 0.620 0.439

0.336 0.394 3.987 3.081 0.523 0.567 2.671 2.194

3.8* 0.134 0.267 0.163 0.334

0.432 2.315 0.547 1.890

t

0 50 100 150 Tube lengthkm

Fig. 8. Effect of length of tube C, on peak height using both air (A) and water compensation (B). Injected samples, 35 pi of 10 pg ml-1 calcium solution. Pumping rate, 1.8 ml min-l. Tubelength (L), 0.5 cm

pumping rate the lower are the absorbance measurements, owing to the flame being supplied with a smaller number of calcium atoms per unit time. Moreover, the dispersion effect increased with increasing sample size, and greater pumping flow-rates were therefore necessary in order to obtain the maximum absorbance when the size of the sample increased. At high pumping flow-rates, the peak heights (absorbance) decreased owing both to the atomisation rate and the detector response characteristics. The air flow entering the T-piece also decreased, causing poor pre-nebulisation. To summarise, for all sizes of sample there was an adequate range of pumping rates.

Fig. 7 shows the effect of the pumping rate on the peak area for two different sizes of sample. This effect was very important for air compensation. When the carrier flow-rate decreased there was a large increase in peak area. Moreover, the peak area was almost proportional to the sample size for different sizes of sample in the flow-rate range 0.2-4 ml min-1. As can be seen, using water compensation the peak area was almost independent of the pumping flow-rate.

Table 1 summarises the results obtained for peak height and area measured in absorbance and absorbance seconds, respec- tively, using air and water compensation for three different lengths of tube L and four pumping flow-rates. The signal obtained decreased only slightly when the length of tube L was modified to decrease vb. As Table 1 shows, the signal obtained with air compensation, measuring both the peak height and area, was always greater than that obtained with water Compensation as there is prior fragmentation of the sample injected in the T-piece and no dilution. Hence the sensitivity was the best when the air flow-rate through the T-piece was a maximum, that is, the length of tube L was a minimum.

0 1 2 3 Pumping ratelml rnin-1

Fig. 9. Nebulisation efficiency as a function of pumping rate using the FI-AAS system

On the other hand, the lengths of both tubes C1 and C2 had to be the shortest possible in order to avoid dispersion of the injected sample. Fig. 8 shows the effect on the signal of the length of tube C, with a 35-p1 sample and a 1.8 ml min-l pumping flow-rate. Similar results were obtained when the length of tube C1 was modified.

To summarise, as can be seen in Figs. 4-7 and Table 1, under adequate conditions the sensitivity is similar to that obtained by conventional AAS (this may be the answer to the second question above).

EfFciency of Nebulisation

The efficiency of nebulisation for air compensation was studied and determined in a similar manner to that described by Wolf and Stewart.6 A calibrated flask was placed in the waste of the spray chamber of an atomic absorption spec- trometer and, using an FI system, a specified number of samples containing 10 pg ml-1 calcium solution were run. The system was washed out with copious amounts of water before and after injections. The waste solution containing the non-nebulised fraction of the sample was collected in one calibrated flask (A) and an equal number of samples were injected directly into another calibrated flask (B), then the solutions in both flasks were made up to volume. Returning the system to the initial position, the concentration of metal in the solution in each flask was determined. The difference in results between flasks A and B was assumed to indicate the amount of metal nebulised in the flame. Results of these experiments, using several pumping flow-rates and a 235-pl sample, are given in Fig. 9. The nebulisation efficiency values are higher than those obtained by Wolf and Stewart,6 who adjusted the aspiration rate by modifying the position of the nebuliser. These differences were higher at low pumping rates

Publ

ishe

d on

01

Janu

ary

1987

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 30

/10/

2014

18:

36:5

2.

View Article Online

ANALYST, MARCH 1987, VOL. 112 275

because there was greater fragmentation of the sample in the T-piece. This explains the results obtained in Fig. 7.

Measurements of the peak area reflected the total concen- tration of analyte passing through the flame and not the instantaneous concentration. The nebulisation efficiency was greater at lower than at higher sample flow-rates, i.e., a large percentage of the sample was actually transported to the flame and consequently atomised.19 This effect has been reported previously.6>20

Limits of Precision and Detection

Using a calcium standard solution, studies on precision were made for measurements of peak heights and areas. The precision for the peak heights decreased with decreasing pumping rate, owing to the pulse effect of the peristaltic pump, despite a long pre-coil and a pulse suppressor being added to the manifold. The reproducibility for peak-area measurements, however, was in some instances independent of the pumping rate.

Table 2 shows the results obtained for air and water compensation using as the propulsion system both a peristaltic pump and air at low pressure. As can be seen, for both peak heights and areas, the reproducibility obtained was higher using air at low pressure than with the peristaltic pump. However, the air at low pressure was more sensitive to both the inner diameter of the loop and to changes in sizes, so the flow through the system had to be carefully checked for all sizes of loop.

Comparison of the variances calculated by converting peak heights (absorbance) into concentration values, using calibra- tion graphs for the air and water compensation methods, with the variance ratio test showed no significant differences between two methods, and therefore they are equally accurate and precise.

As Table 2 shows, FI-AAS does not always lead to poorer reproducibility. Half of the values obtained had coefficients of variation equal to or less than those calculated by conventional AAS (1 % under the same conditions). This answers the first question above.

Studies on the limits of detection18 were carried out by injecting 60-pl samples of 4 pg ml-1 calcium standard solution at a pumping rate of 1.8 ml min-1. The detection limits, calculated as the concentration corresponding to three times the standard deviation of both peak height and area for 30 injections of water, were 0.018 pg ml-1 for measurements of peak height and 0.019 pg ml-1 for measurements of peak area. The steady-state limit of detection on the same instrument (a Pye Unicam spectrometer) was 0.020 yg ml-1 under the working conditions.

Table 2. Coefficients of variation ("/o). Each value was obtained from thirty successive injections of 10 pg ml-1 calcium solution (Pye Unicam instrument)

Pumping rate/ml min-1

Loop Com- Peristaltic pump size/ pensation

p1 method Signal 0.4 1.8 3.2 35 Water Height 10.99 2.65 1.83

Area 6.56 2.56 1.86 Air Height 6.75 1.71 1.36

Area 2.96 1.66 1.24 235 Water Height 2.77 0.94 0.57

Area 2.00 1.34 1.52 Air Height 1.24 0.95 0.80

Area 1.61 1.04 1.39

Pressurised air

0.4 1.8 3.2 2.72 0.98 0.76 2.37 0.90 1.15 1.94 1.12 0.69 1.81 1.05 0.50 0.43 0.35 0.34 0.51 0.70 0.95 0.72 0.71 0.62 0.57 0.68 0.75

Improvements in Selectivity

As a result of the greater nebulisation efficiency, the diameter of an average droplet using air compensation is smaller than with normal nebulisation, the atomisation rate in the flame being higher. Hence, using- the FI-AAS system with air compensation, it is possible to minimise classical interferences in the conventional AAS determination of calcium.

We studied the effects of phosphate, sulphate and fluoride on the determination of calcium. Under suitable conditions of burner height and gas flow-rate, a 0 . 0 1 ~ concentration of phosphate, fluoride or sulphate was tolerated without the need for releasing agents. Fig. 10 shows the effect of phosphate. The results are compared with the interference of this species on the determination of calcium using conven- tional nebulisation.

It is important to note that the shapes of the relationships between signal and gas flow-rate and burner height appear to depend on the particular designs of the nebuliser, spray chamber and impact devices.21 The results in Fig. 10 were obtained using a Perkin-Elmer instrument, although very similar results were achieved when the Pye Unicam spec- trometer was used.

Taking the above into account, it is clear that, with respect to selectivity, the use of the FI-AAS air-compensation system can be more advantageous than conventional AAS. More- over, the handling of small samples is also possible, which could provide an answer to the third question above.

Conclusions The method proposed here is similar in some respects to discrete sample nebulisation ,** without changes in back- ground and flame geometry, memory effects or deterioration of precision, as there is both an adequate and continuous carrier flow washing the spray chamber.

0.4

al

C m f-' s n a 0.2

0.2

a 2

g 0.1

0,

al z Y

.-

Q

0 I I I I I

0 1 2 3 1 2 3 4 Burner heightkm Acetylene flow/l min-'

0.6

al

0.4 6 f-' s 2

0.2

0.3

Y m

0.1 a

0

Fig. 10. Effect of phosphate on the determination of 10 pg ml-l calcium using conventional AAS [(a) and ( b ) ] and an FI-AAS system with air com ensation [ ( c ) and ( d ) ] . Acetylene flow-rate = 3.5 1 min-1 in (a) and (cy. Burner height = 0.8 cm in ( b ) and (d ) . A, Calcium; B, calcium in the presence of 0 . 0 1 ~ phosphate. FI-AAS system: pumping rate = 1.8 ml min-1, loop size, 35 p1. Perkin-Elmer spectrometer

Publ

ishe

d on

01

Janu

ary

1987

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 30

/10/

2014

18:

36:5

2.

View Article Online

276 ANALYST, MARCH 1987, VOL. 112

The use of air compensation produces a noticeable increase in nebulisation efficiency when the sample is injected into the manifold instead of being aspirated as in conventional AAS. This was applied to the determination of calcium using FI-AAS and the following advantages were observed: (i) in comparison with other FI-AAS systems, air compensation avoids dilution and minimises the effect of dispersion; hence the peak height and area are greater, and, moreover, the precision is the same or better; (ii) higher levels of interfer- ences in the determination of calcium are tolerated and the use of releasing agents can be avoided.

1. 2. 3. 4.

5.

6. 7. 8.

9.

References Inglis, A. S., and Nichols, P. W., Mikrochim. Acta, 1975,553. Tyson, J. F., Analyst, 1985, 110, 419. Yoza, N., and Ohashi, S., Anal. Lett., 1973,6,595. Miyajima, T., Yoza, N., and Ohashi, S., “Liquid Chromato- graphy,” Bunseki-Kiki, Zokan, 1974, p. 151. Sherwood, R. A., Rocks, B. F., and Riley, C., Analyst, 1985, 110,493. Wolf, W. R., and Stewart, K. K., Anal. Chem., 1979,51, 1201. Brown, M. W., and R88iCka, J., Analyst, 1984, 109, 1091. Harnly, J. M., and Beecher, G. R., J. Anal. At. Spectrom., 1986, 1, 75. Appleton, J. M. H., and Tyson, J. F., J . Anal. At. Spectrom., 1986, 1, 63.

10. 11.

12. 13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

Stapar, J., and Dawson, J. B., Appl. Opt. , 1968, 7, 1351. Browner, R. F., and Borrn, A. W., Anal. Chem., 1984,56,780 and 875A. Cresser, M. S., Anal. Proc., 1985, 22, 65. Zagatto, E. A. G., Krug, F. J., Bergamin F, H., Jorgensen, S. S., and Reis, B. F., Anal. Chim. Acta, 1979, 104,279. Kamson, 0. F., and Townshend, A., Anal. Chim. Acta, 1983, 155, 252. Rocks, B. F., Sherwood, R. A., and Riley, C., Clin. Chern., 1983, 29, 568. Yoza, N., Aoyogi, Y., and Ohashi, S., Anal. Chim. A d a , 1979, 111, 163. Bergamin F“, H., Reis, B. F., and Zagatto, E. A. G., Anal. Chim. Acta, 1978,97,427. ACS Committee on Environmental Improvement, Anal. Chem., 1980,52,2242. Wolf, W. R., Greene, F. E., and Stewart, K. K., Pittsburgh Conference on Analytical Chemistry and Applied Spectro- scopy, Cleveland, OH, 1978, paper 68. Szivos, K., Polos, L., and Pungor, E., Spectrochim. Acta, Part B , 1976,31, 289. Tyson, J. F., Adeeyinwo, C. E., Appleton, J. M. H., Bysouth, S. R., Idris, A. B., and Sarkissian, L. L., Analyst, 1985, 110, 487. Cresser, M. S., Prog. Anal. At. Spectrosc., 1981, 4, 219.

Paper A61209 Received June 27th, 1986

Accepted October 29th, 1986

Publ

ishe

d on

01

Janu

ary

1987

. Dow

nloa

ded

by N

orth

east

ern

Uni

vers

ity o

n 30

/10/

2014

18:

36:5

2.

View Article Online