Amlytica Chintica Acta, 252 (1991) 201-203 Elsevier Science Publishers B.V., Amsterdam

201

Sequential diffusion of ammonium and nitrate from soil extrzkts to a polytetraf!uoroethylene trap

for 15N determination

Peter Ssrensen * and Erik Steen Jensen

Plant Biology Section. Environmenfal Science and Technologv Department, Rise National Laboratory, DK-4000 Roskilde (Denmark)

(Received 21st May 1991)

Abstract

A novel diffusion method was used for preparation of NHf - and NO;-N samples from soil extracts far 15N determination. Ammonium, and nitrate following reduction to ammonia, are allowed to diffuse to an acid-wetted glass filter enclosed in polytetrafluoroethylene tape. The method was evaluated with simulated soil extracts obtained using 50 ml of 2 M potassium chloride solution containing 130 pg of NH:-N (2.3 atom% I5 N) and 120 b$g of NOT-N (natural “N abundance). No cross-over in the “N abundances of NH:-N and Nor-N was observed, indicating a quantitative diffusion process (72 h, 25O C). Owing to the presence of inorganic nitrogen lSN enrichments should be corrected for rhe blank nitrogen content.

impurities in the potassium chloride, the

Keywords: Mass spectrometry; Ammonium; Diffusion; Nitrate; Nitrogen; Polytetrafluoroethylene traps; Soils

The stable isotope 15N is wideiy used in investi- extract. Arnrrto~i~~~ is converted to ammnnia, gations of nitrogen transformations in soil [I]. In which diffuses through the PTFE membrane to this kind of ex

19 eriment it may be necessary to the acidified trap. The trap is removed after 72 h

determine the N abundance of nitrate and am- in the soil extract, dried and analysed for “N. monium separately. The preferred extractant for soil inorganic nitrogen is ~2 M potassium chloride solution [2]. However, the nitrogen has to be con- centrated before analysis on automated nitrogen analyser-mass spectrometer instruments.

EXFERIMENTAL

Steam distillation methods [3] and different diffusion methods 14-71 have been used for con- centrating samples prior to 15N determination. Steam distillation is more labour-intensive than diffusion methods, and it is subject to sample cross-contamination through glassware [3].

An easy and rapid diffusion method is pre- sented here by which nitrate and ammonium in a soil extract can be sequentially ‘and quantitatively concentrated for determination of “N. An acidified glass filter trap is enclosed in polytetra- fluoroethylene (PTFE) tape and placed in the soil

The diffusion cells used were 250~ml polyethyl- ene bottles with screw-caps. Simulated soil ex- tracts were prepared by adding 130 pg of NH:-N from a standard of ammonium sulphate (about 2.3 atom% 15N) and 1201fg of NO,-N from potas- sium nitrate (natural N abundance) to 50 ml of 2 M potassium chloride solution. Blanks were prepared. Glass filters (Whatman, GF/C, 25 mm) were acid washed and heated at 550 * C overnight. The filters were cut into strips of 5 mm and placed on a piece (6 cm X 12 mm X 0.076 mm) of PTFE tape (Eureka) normally used for sealing purposes. To the filter was added 15 ~1 of 2 M sulphuric

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202 P. SBRENSEN AND ES. JENSEN

acid. The tape was folded in the middle to sur- round the filter entirely. The double-layered tape was carefully compressed to make a perfect seal. A PTFE trap and 0.2 g of magnesium oxide were added to the simulated soil extract and the bottle was immediately closed tightly. The bottles were shaken for 72 h at 25°C on a horizontal shaker (100 t-pm). The PTFE trap was removed from the solution and a new trap and 0.4 g of Devarda’s alloy were added and the bottle was closed and shaken for another 72 h. After removal, the PTFE traps were opened with a pair of tweezers and the filters were dried in ammonia-free air. The filters were then packed in tin capsules and analysed as described [7,8] using an elemental analyser (Carlo Erba NA1500) coupled on-line to a mass spec- trometer (Finnigan MAT Delta}. Diffused sam- ples were made in 4-5 replicates.

Additionally, standard nitrogen solutions were directly pipetted into tin capsules, dried at 50- 80” C and analysed in 5 replicates (non-diffused samples). Results from diffused and non-diffused samples were compared by a t-test.

The recoveries of nitrogen were calculated by subtracting the blank nitrogen from the total amount of nitrogen found on the filters (Table 1). Recoveries were 96--99% and comparable to those reported by Burke et al. [S] and Brooks et al. f6]. Some sample nitrogen is probably left on the tape when the filter is removed for analysis. Therefore, the actual transfer of NH,-N to the trap by the diffusion is probably very close to 100%. The tape must be removed before analysis as the combus- tion of PTFE in the nitrogen analyser generates large amounts of silicon tetraf~uo~de.

Nitrogen was found in blanks of 2 M potas- sium chloride solution and as traces in the oxygen (99.998% 0,) used for combustion. The blank nitrogen, which is assumed to be close to natural abundance of “N, dilutes the percentage of 15N in an enriched sample, especially when the amount of sample nitrogen is small. Therefore, a correc- tion to the atom% “N measured was made, using the following equation:

atom% “N corrected

RESULTS AND DlSCUSS~ON

The r5N enrichment of the diffused NOT-N &owed that very small amounts of labelled am- mon~a~ammonium remained in the extract after the first 72 h of diffusion (Table 1). Additional experiments with 260 pg of NH:-N in the ex- tracts gave a similar recovery of NH:-N even after 48 h of diffusion (data not shown).

= (t pg N x atom% 15N measured)

- (pg N blank X 0.3663)}

X { pg N measured - pg N blank} -r

Using this correction, the difference in the atom% “N of diffused an non-diffused samples was not statistically significant (P c 0.05).

It is essential that the solution does not con- taminate the acidified filter, hence the tape must be perfectly sealed. The traps, which normally

TABLE 1

Reccvcry and atom% “N of sequentially diffused ammonium (130 gg N) and nitrate (120 pg N) from 50 ml of 2 M KC1 and of non-diffused standards, with standard deviations in parentheses (n = 4-5)

Sample

Non-diffused !Hz

Atom% “N

2.282 (0.010)

Atom% “N corrected

2.310

N content Recoverv t/e N) m - 133 (2.0)

Non-diffused NO; 0.368 (0.001) 0.368 Empty tin capsule Diffused NH$’ 2.256 (0.006) 2.298 Blank (2 M KCI) Diffused NO; p 0.375 (0.010) 0.375 Blank (2 M KCl} n

’ After diffusion of NH$-N and addition of Devarda’s alloy.

ND 1.9 (0.6)

128 (2.2) 96 2.8 (0.4)

124 (2.1) 99 5.7 (0.5)

DIFFUSION OF NH: AND NO: FROM SOIL EXTRACTS TO A PTFE TRAP 203

float on the solution surface, may stick to the container walls, which apparently results in a lower trapping efficiency of ammonia. In such an event, the recovery of ammonia may not be quantitative after 72 h. This may influence the measured r5N abundance of NH,-N if isotope fractionation takes place [3], and the “N abundance of the diffused NOT-N is especially sensitive to NH,-N left in the soiution from the first diffusion period. There- fore, the diffusion cells should be examined every day, and traps sticking to the walls should be returned to the solution.

Conclusions This sequential diffusion procedure, employing

an acidified trap enclosed in PTFE, provides an easy method for the concentration of NH:-N and NO;-N from soil extracts for “N determination. There are no cross-contaminations and the diffu- sion is quantitative after 72 h at room tempera- ture.

The authors thank Mr. H. Egsgaard for useful criticism of the manuscript and Mrs. M. Brink for skilled technical assistance.

REFERENCES

D.D. Myrold and J.M. Tiedje, Soil Biol. Biochem., 18 (1986) 559. D.R. Keeney and D.W. Nelson, in A.L. Page (Ed.), Methods of Soil Analysis, Part 2, American Society of Agronomy, Madison, WI, 1982, p. 643. R.D. Hauck, in A.L. Page (Ed.), Methods of Soil Analysis, Part 2, American Society of Agronomy, Madison, WI, 1982. p. 735. C.T. Ma&own, P.D. Brooks and M.S. Smith, Soil Sci. Sot. Am. J., 51 (1987) 87. I.C. Burke, A.R. Mosier, L.K. Porter and L.A. O’Deen, Soil Sci. Sot. Am. J., 54 (1990) 1190. P.D. Brooks, J.M. Stark, B.B. McInteer and T. Preston, Soil Sci. Sot. Am. J., 53 (1989) 1707. ES. Jensen, Plant Soil, 133 (1991) 83. H. Egsgaard, E. Larsen and E.S. Jensen, Anal. Chim. Acta. 224 (1989) 345.