Introduction
Since its classification as an atmophile element by
, the fate and nature of N in rocks and minerals has
received little attention. Many early budgets suggested most of Earth's N was
in the atmosphere, with only minor amounts in the biosphere, crust, and
mantle e.g.,. And while concentrations in rocks and
minerals are low, typically one to tens of parts per million, the great mass of the bulk
silicate Earth (BSE) compared to the atmosphere allows for a substantial
amount of planetary N to be contained in the BSE. In fact, the BSE and core
likely contain the majority of N in the Earth
. In addition, enriched δ15N
values from mantle-derived rocks and the correlation of N2 with
40Ar indicate that N has cycled between the surface and the deeper
planet over geologic time .
In spite of the new-found richness of the geologic N cycle, the relative
paucity of sample analyses limits robust interpretation or modelling of N
cycling over Earth's history. This paucity is due in large part to the
difficulty of measuring low concentrations of N in rocks and minerals. Though
a variety of analytical techniques are now able to measure N at parts per million level
concentrations in rocks and minerals e.g.,, several of these are either
analytically expensive or only operational at a handful of labs around the
world. The development of techniques that are more easily accessible and able
to be performed with standard geochemical equipment would be a great benefit
to the community.
We first describe, briefly, existing techniques used to measure N in geologic
samples. Classic techniques typically involve either whole rock or mineral
dissolution in HF and/or H2SO4 e.g.,. These techniques often involve hot digestion at
temperatures greater than 90∘C and sometimes up to hundreds of degrees Celsius.
After extraction, samples are typically distilled using a Kjeldahl method,
then analyzed using colorimetry (Nessler's reagent; ). Alternatively, samples could be oxidized under high
temperatures (800–1000 ∘C) in sealed tubes with CuO and CaO
. Extracted N could be inserted into a mass
spectrometer for isotopic characterization. Extraction of N by thermal
heating and release e.g., has also
been applied to mineral separates and geologic samples. In addition,
in situ analyses such as Fourier transform infrared microscopy have
been used to determine N speciation . Existing techniques
can be well-suited for measuring mineral-bound NH4+. Many existing
techniques, however, are either set up at only a few laboratories around the
world , require high-temperature or aggressive chemical extractions
e.g.,, or otherwise require lengthy mineral
separations or dedicated extraction lines.
In this study, we adapt a fluorometry technique developed by that is commonly used in
biologic and aquatic chemistry studies and compare it with two other techniques: colorimetry
and elemental analyzer mass spectrometry e.g.,. Through analysis of a number of rock standards,
we demonstrate that, while this fluorometry technique has some associated uncertainty, it reproduces published values for
standards BCR-2, BHVO-2, and G-2, especially if a distillation step is undertaken. It also performs better than elemental
analyzer combustion mass spectrometry or colorimetry methods for quantifying N in crystalline rocks. The fluorometry
technique has the advantage over other techniques by being relatively fast, requiring few reagents, requiring more
accessible analytical equipment, as well as specifically targeting NH4+. There are three main benefits: the relative
ease of the method which may increase the number of analyses of N in geologic samples, its use as a screening
method that can be used to guide further isotopic investigation, and its specificity to NH4+. We emphasize,
however, that N isotopes cannot be measured with fluorometry, and both fluorometry and colorimetry techniques
require a period of rock digestion which other techniques may not require.
In addition, we also present a preliminary application of the method
vis-á-vis a N budget for the continental crust based on glacial tills and
crystalline crustal rocks from North America. Along with the atmosphere and
mantle, the continental crust is one of the main N reservoirs on the planet
, thus determining
its content is key in the evolution of the N cycle over time.
We also call for the development of international geologic N standards
after. Method development for the measurement of
geologic N suffers without such standards. We present the first
δ15N values of a number of rock standards (BCR-2, BHVO-2, SY-4,
LKSD-4, Till-4, G-2) and suggest they may be suited for geologic N
standards, given more thorough analysis.
Methods
Rock standards and samples
We analyzed a number of geochemical rock standards (Table ). Several standards (LKSD-4, Till-4, SY-4) have no
previous N concentration measurements, to our knowledge. Remaining standards
have published N concentrations (BCR-2, BHVO-2, G-2), with values reported
from neutron activation analysis (NAA). This technique works by irradiating
samples with neutrons to transform 14N into 14C, where the
resulting material can then be purified and assayed radiochemically as a
proxy for N concentration .
Rock standards from the United States Geological Survey (USGS) and
Geological Survey of Canada (GSC) analyzed with published values, if available, and
N analysis reference.
Standard
Description
N
Reference
(ppm)
BCR-1/2
USGS Columbia river basalt
34±12
1, 2, 3
BHVO-2
USGS Hawaiian basalt
22.6±3
1, 3
G-2
USGS Paleozoic granite
34±4
1, 3
Till-4
GSC Till from Scisson's Brook,
New Brunswick
LKSD-4
Big Gull Lake sediment, Ontario
SY-4
Diorite gneiss, Ontario
1 .
2 .
3 .
In addition to rock standards, we analyzed a number of other lithologies,
including glacial tills, basalts, granites, and carbonates. These samples
were chosen as a proof of concept for the adapted fluorometry technique,
namely, investigating the N budget of the continental crust. All tills are
from British Columbia, Canada, and have eroded a variety of Phanerozoic
crustal lithologies. Basalts and welded tuffs are from the Bonanza Arc and
Sicker Group on Vancouver Island, British Columbia. Granites come from a
variety of locations in North America. Sample descriptions are given in the
Supplement.
Rock sample preparation
Using a rock saw, rock samples were cut into small blocks and the weathered
edges were removed. Rock powders were prepared by crushing the blocks in a
steel jaw crusher and then powdered using a shatterbox with a tungsten
carbide puck. The shatterbox puck and chamber were cleaned in between each
crushing step using deionized water and ethanol. Clean quartz sand was run
between each sample to prevent cross-contamination.
Method 1: elemental analyzer mass spectrometry
We analyzed all rock standards at the University of Washington Isolab
facilities following . First, ∼1 g of each sample
was weighed into a clean (baked at 500 ∘C overnight) Pyrex test
tube. Then, ∼10 mL 6N HCl was added, stirred with a glass stir rod,
sonicated for 30 min, and left in an oven set to 60∘C over
night to remove carbonate. Tubes were then centrifuged to settle suspended
sediment, acid was decanted, fresh acid was added, and the samples were
sonicated and dried a second time as above. This decarbonation was done once
more. Subsequently, we rinsed samples three times in DI-H2O, and all were
dried for 3 days at 60∘C.
Between 12 and 150 mg of decarbonated sample in 9 × 5 mm Sn capsules
was flash combusted at 1000∘C in a Costech ECS 4010 Elemental
Analyzer with an excess of O2. Combustion products passed over a reduced
copper column at 650∘C to convert all N to N2 and absorb
excess O2 and a magnesium perchlorate trap to absorb water. Sample gas
then passed through a 3 m gas chromatography column to separate N2 from
CO2 before being analyzed on a Finnigan MAT253 continuous-flow
isotope-ratio mass spectrometer via a ThermoFinnigan Conflo III. All analyses
were quantified using IsoDat software. Errors reported are standard
deviations from repeated analyses. We used the following isotopic standards:
two glutamic acids (GA-1 and GA-2), dried salmon (SA), and an internal rock
standard (McRae Shale).
Method 2: colorimetry
We followed the procedure outlined in
to analyze samples using colorimetry.
Reagent list
KOH
A 25% mass : volume solution of KOH was used for
HF-neutralizing. We dissolved 250 g KOH in 1 L of water.
Phenol reagent
We weighed out 7.0 g of crystalline phenol
and 0.02 g sodium nitroprusside into a 200 mL beaker. To this, we added 20 mL
of KOH reagent and 60 mL of deionized water. Solids were stirred to dissolve,
and then the solution was topped up to 100 mL total volume with DI-H2O.
NaOCl
We diluted 20 mL of commercially available NaOCl
to 100 mL total volume with DI-H2O.
Stock solution and working standards
A 1 g NH4+ L-1
stock solution was prepared in a 250 mL volumetric flask by dissolving 0.7433 g of NH4Cl salt in 250 mL DI-H2O.
From this, a secondary ammonium standard solution of 0.2 g NH4+ L-1 was prepared in a 100 mL volumetric flask using
20 mL of the ammonium stock solution, topped with DI-H2O. The stock solution was diluted to make working standard
solutions of 0.005, 0.01, 0.05, 0.1, and 0.2 g L-1, which were used to construct standard curves for sample concentration determination.
Sample digestion
Working standards, blanks (DI-H2O), and rock powder were subjected to
HF treatment at room temperature in 25 mL Teflon vials. To these vials,
we added 2 mL of working standard, 2 mL of blank, or approximately 0.25 g of
rock powder. The samples were digested for 7 days in 2 mL of 50%
hydrofluoric acid in a fume hood. We swirled vials every 2 days to
facilitate digestion.
After sample digestion period, the solution was neutralized by adding 20 mL
of 25% KOH to each vial. Exploratory analysis of different aliquots
from the top, middle, and bottom of each vial gave different results; thus, the solution was stirred with a glass rod to ensure homogenization. The
stir rod was rinsed with DI-H2O in between samples to prevent
cross-contamination. Homogenized solutions sat for 15 min to allow
suspended rock powder to settle.
Colorimetric analysis
All liquid, plus undissolved solids, were placed in a 100 mL round bottom
flask, which was attached to a standard distillation setup. Samples were
boiled for ∼10 to 15 min, and 8 mL of distillate was collected into
8 mL of 0.01 N H2SO4. To this, 1 mL of phenol reagent, 1 mL of
NaOCl, and 5 mL DI-H2O were added and stirred. The colour-change
reaction was allowed to proceed for 2 h. Absorbance was measured at 630 nm in a plastic cuvette on an Ocean Optics spectrophotometer and quantified
using SpectraSuite software.
Standard solutions were processed the same way as the samples. We made a
standard curve of absorbance plotted against starting concentration (0.005,
0.01, 0.05, 0.1, and 0.2 g L-1) of standard solutions. This was used to
calculate sample concentrations, applying appropriate dilution corrections.
Method 3: fluorometry
The following is a detailed description of our adaptation of the
method. Key considerations and suggestions for
improvement are discussed in the following section.
Reagent list
Working reagent (WR)
We made a mixture of sodium sulfite, borate
buffer, and orthophthaldialdehyde (OPA) solutions. The procedure for preparing
the working reagent follows . To make the sodium
sulfite solution, 0.2 g of sodium sulfite was added to 25 mL of DI-H2O.
For the borate buffer, 80 g of sodium tetraborate was added to 2 L of
DI-H2O, which was then stirred for 4 h with a stir bar. To make the
OPA solution, 4 g of OPA was added to 100 mL of 95% ethanol and protected
from the light while stirred with a stir bar. The borate buffer, OPA solution
and 10 mL of the sodium sulfite were mixed in a >2 L brown polyethylene
bottle. The working reagent mixture sat for at least 1 day prior to use.
Stock solution and working standards
The same stock solution was used for fluorometry as for colorimetry. A range of working standards was made by
diluting stock solution (0.2 g NH4+ L-1) into reaction bottles, resulting in concentrations of 0.005, 0.01,
0.05, and 0.1 g NH4+ L-1. This range of working standards was used to construct standard curves.
Sample digestion
The same digestion and neutralization procedures were used for fluorometry as
for colorimetry.
Optional distillation
Some replicates of samples were distilled, as described in Sect. . As this step is the most time-intensive step aside
from digestion, we ran most analyses without distilling. As discussed later,
this step may be useful for samples with either low N or samples that are
difficult to digest completely (e.g., G2).
Fluorometric analysis
Brown 50 mL polyethylene bottles were used for the fluorescing reaction. The
reaction bottles were first emptied of their storage solution (i.e., clean
working reagent) and rinsed with 5 mL DI-H2O. Using a pipette, 10 mL of
DI-H2O was added to each reaction bottle. Subsequently, an aliquot of
the neutralized solution was added. About 0.1 mL of sample solution was added
to each bottle. Then, 2.5 mL of working reagent was added to each reaction
bottle. After adding the working reagent, the reaction bottle was inverted to
homogenize the sample.
Immediately after homogenization, an aliquot of solution from the reaction
bottle was transferred to a plastic cuvette, and fluorescence was measured in
a 1 cm plastic cuvette using a Turner Designs AquaFluor fluorometer. The
fluorometer has a 350 nm excitation filter with a 25 nm bandpass, a ≥420 nm emission filter, and a minimum detection limit of 0.1µM NH4+.
After this initial measurement, the aliquot of solution was transferred back
into its reaction bottle, the cuvette was rinsed, and the bottle was capped
and inverted several times to homogenize the sample. The sample then reacted for 3 h. After 3 h, fluorescence was measured in three sample aliquots.
Sample concentrations were calculated using a standard curve (Fig. ).
After measurement, the remaining solution in the reaction bottle was emptied
and a small amount (∼2 mL each) of working reagent and DI-H2O was
added to the bottles for storage.
Calculation of sample NH4 concentration
Net fluorescence (Fnet) for working standards and samples were
calculated by averaging the three final fluorescence (F‾) readings,
subtracting initial fluorescence (Fi), and then subtracting average
fluorescence minus initial from the blank (B‾; Eq. ).
Fnet=F‾-Fi-B‾
Then, using standard curves (either corrected to digestion vials or in
reaction bottles; slope = s), sample concentration in either digestion
vials or reaction bottles was calculated. Importantly, we chose to force
standard curves through the origin.
Final NH4+ concentrations ([NH4+]) were calculated by correcting
concentration to the KOH-neutralized vial, multiplying concentration by total
volume in the vial (D) to determine a mass of NH4+. Then, we divided this
NH4+ mass by starting sample rock powder mass (m) to get the
concentration of NH4+ in parts per million mass (Eq. ).
[NH4+]=FnetsDm
Results
Method 1: mass spectrometry
Elemental analyzer mass spectrometry was able to measure N concentration in
all rock standards (Table ). Analyses for
crystalline standards are lower than published values, likely due to the
incomplete liberation of N from silicate lattices during combustion
. Values for BCR-1/2 are 62% of published values,
BHVO-2 59% of published values, and G-2 15% of published values. Values reported
herein for Till-4, SY-4, and LKSD-4 are the first to our knowledge.
In addition, we report δ15N values for all samples (Table ). While more analyses would need to occur for
these to be used as isotopic standards, the fact that there is measurable N
in all standards suggests they may be suitable candidates for geologic
N-isotopic standards. Samples with high N (presumably some organic N),
LKSD-4, and Till-4, could be ideal.
Method 2: colorimetry
The colorimetric method was able to analyze all rock standards (Table ), with standard curves having r2 values above
0.99 (Fig. ). Measured concentrations are lower than
published values: at 37, 15, and 5 % of published values for
BCR-2 , BHVO-2, and G-2, respectively (Table ). We
suggest that this potential underestimate is due, in part, to difficulty in
quantitatively distilling all NH4+ from a dissolved sample. The
distillation apparatus is imprecise in nature, and controlling the final
volume of distillate is difficult.
Nitrogen and δ15N data from colorimetric and mass
spectrometry analyses. Both techniques appear to underestimate N
concentration, perhaps due to incomplete N extraction during combustion for
mass spectrometry or incomplete recovery of NH4+ during distillation.
Concentrations are in parts per million.
Standard
Published
Colorimetric
Mass
δ15N
spectrometry
BCR-1/2
34±12
12.6±8
21
1.05±0.4
BHVO-2
22.6±3
3.5±0.7
13.3±0.6
-0.3±0.2
G-2
34±4
1.6±0.9
5±0.7
1.23±1.32
SY-4
6.9±2.8
14.3±0.6
2.13±0.5
LKSD-4
487±401
16000±8
3.59±0.1
Till-4
82.2±40
440±2
6.33±0.1
Carb
66.1
48.5±1.3
Example standard curves for colorimetry and fluorometry analyses.
Absorbance is the fraction of incoming light that is absorbed by the
indophenol-blue solution. The value larger than 1 for 0.20 g L-1 is due to dilution
for absorbance reading and correction for this dilution. These standard
curves were used to calculate sample values for those analyses occurring
after distillation. In addition, fluorometry curves show that distillation
does not greatly affect working standards, though the same may not be true
for actual samples.
Method 3: fluorometric method
Rock standards
Results for rock standard analyses are shown in Table .
Values range from 5.2 to 5200 ppm by mass. Analyses without distillation
match published values within error for BCR-2 and BHVO-2, with values of
33±8.3 and 15±5.7 ppm, compared to 34±12 and 22.6±3,
respectively. Analyses of G-2 are lower than published, at 11±4.9 from
our analyses and 34±4 from literature values. Other standards have values
of 5.2±4.5 (SY-4), 5200±1000 (LKSD-4), and 71±25 ppm (Till-4). Analyses
after distillation are generally higher than those with no distillation step.
Continental crust
Measured NH4+ concentrations from a number of continental crust samples
are shown in Table . We have included analyses of BCR-2
in the “volcanics” category. In general, samples of all crystalline rock
types have between 10 and 40 ppm, and sedimentary rocks are higher, with
values between 100 and 1000 ppm.
(a) Comparison of N concentrations in rock standards from three
different methods. Colorimetry consistently yields lower values than
fluorometry, and values are typically lower than EA mass spectrometry.
Fluorometry and EA mass spectrometry give similar results for sedimentary
rocks (LKSD-4, Till-4, 5.1) but tend to differ for crystalline rocks (BCR-2,
BHVO-2, SY-4, G-2), with fluorometry giving higher concentrations. (b) Though
associated errors are larger, fluorometry does reproduce published N
concentration values from three standards (BCR-2, BHVO-2, G-2), especially
after distillation. The legend is the same in both (a) and (b).
Sensitivity test for samples with 0.010 g L-1 NH4+ and varying
concentrations of KOH in reaction bottles. KOH clearly has an effect on
fluorescence. Fluorescence values are consistent between 0 and 0.2% KOH, with a
large increase approaching 0.35% KOH and a steep drop-off at higher KOH
concentrations. We suggest that high concentrations of KOH overwhelm the
buffering capacity of the working reagent and inhibit the completion of the
fluorescing reaction. All reported runs herein had between 0.05 and 0.2%
KOH. Controlling KOH concentration is a key factor in the success of this
method.
Discussion
Fluorometry
Reproducibility and reproduction of published values
Reproducibility of repeated analysis was between 10 and 50 % of mean values for
all rock standards (Table ). Overall, error appears to be
lowest for samples which are most easily digested: BCR-2 and carbonate. More
felsic standards, such as G-2, have higher error but also show lower
concentrations than previous work. On runs with distillation, G-2 (36±4 ppm) matched published values (34±4 ppm), BCR-1/2 (38±1 ppm) also
matched published values (34±12 ppm), and BHVO-2 (35±1 ppm) had
higher than published values (22.6±3 ppm). Runs without distillation gave
concentrations below their published values, with 11±4.9 ppm for G-2 and
15±5.7 for BHVO-2. Therefore, distillation appears to improve agreement
between fluorometry and NAA, though this may be coincidence, as NAA analysis
has several unresolved issues with calculation accuracy.
We stress that although NAA is of appropriate sensitivity to measure
parts per million level N concentrations e.g.,, there are
complicating issues. Neutron bombardment can also create 14C via
reaction with 17O, with a theoretical apparent N contribution in a sample
with 40% O of 18 ppm, though analysis of a synthetic Al2O3 doped with
20 % 17O suggests the actual effect may only be 6 ppm
. In addition, 52 analyses of BCR-1 by
yielded a range in concentration from 15 to 62 ppm.
The authors suggest this is due to heterogeneous distribution of N in BCR-1.
Alternately, such a range in concentration could be due to adsorption of
atmospheric N2. Though sample powders are prepared for irradiation after
vacuum pumping (or heating), the possibility of atmospheric N2
contamination appears unresolved . Thus,
though NAA analyses of N in rock standards should be able to quantify total N
in a sample, several outstanding issues prevent concentrations reported in
the literature from being accepted as geochemical standards. A major
difficulty in the development of new techniques for measuring geologic N is a
lack of international standards.
Effects of KOH
The most significant parameter affecting the quality of the standard curves
was the concentration of KOH in the reaction bottles. For a given
concentration of NH4+, varying KOH concentration altered resulting
fluorometry readings (Fig. ). Since the fluorescing reaction
is pH dependent (operating between pH 8 and 10 due to borate buffer), the
addition of excess KOH causes the pH of the solutions to be too high and the
fluorescing reaction to be inhibited. We adjusted sample dilutions and
volumes to result in KOH concentrations in reaction bottles of between
0.05 to 0.2%.
Standard curves from three different days comparing preparations
made with KOH solutions to those with only water. Since curves from the same
day are indistinguishable with and without KOH, the fluorescing reaction is
not affected by the presence of KOH (see text for details). Some variability
exists between runs, especially at lower concentrations, but variations are
small compared to changes in fluorescence units with changing concentration.
We have verified that small KOH concentrations, as described above, do not
affect standard curves (Fig. ). Curves prepared on
multiple days with water were identical to those produced with KOH,
indicating that the fluorescing reaction had gone to completion. High
corrected fluorescence values were due to corrections for dilutions occurring
when KOH was added to the initial standard volume, as well as dilutions
occurring when preparing reaction bottles. Though there is some variability
at concentrations below 0.020 g L-1 from day to day, the similarity of KOH curves
and water-only curves implies that the standard curve technique is viable.
Digestion length test for BCR-2 by the fluorometry technique.
Samples were digested in 2 mL 50% HF for the number of days
indicated. Analyses reproduce published value (34 ppm, black line) within
error after only 2 days and, aside from one analysis 8 days after digestion
began, approximate this value afterwards. There is large error in some
samples compared to Table , likely due to the increased number
of analyses reported in Table , decreasing standard
deviation.
Nitrogen from all three techniques (mass spectrometry, colorimetry,
and fluorometry) compared to published values from NAA. Shown is the mean with
standard deviation and coefficient of variation in parentheses. The values
after the commas in the Fluorometry column represent the number of separate
measurements for non-distilled samples. All concentrations are in parts per million.
Standard
Published
Colorimetric
Mass spectrometry
Post-distillation
Fluorometry
BCR-1/2
34±12 (0.35)
12.6±8 (0.63)
21
38±1 (0.02)
33±8.3 (0.25), 36
BHVO-2
22.6±3 (0.13)
3.5±0.7 (0.2)
13.3±0.6 (0.05)
35±1 (0.03)
15±5.7 (0.38), 9
G-2
34±4 (0.12)
1.6±0.9 (0.56)
5±0.7 (0.14)
36±4 (0.11)
11±4.9 (0.45), 13
SY-4
6.9±2.8 (0.41)
14.3±0.6 (0.04)
11±1 (0.09)
5.2±4.5 (0.87), 9
LKSD-4
487±401 (0.82)
16000±8 (<0.01)
9300±2150(0.23)
5200±1000 (0.19), 9
Till-4
82.2±40 (0.49)
440±2 (<0.01)
455±36 (0.08)
71±25 (0.35), 15
Carb
66.1
48.5±1.3 (0.03)
37
93±18 (0.19), 12
Nitrogen concentration (ppm) in upper-crustal rocks using the
fluorometry method.
Rock type
Mean ± SD
Coefficient
No.
of variation
samples
Tills
81.2±36.4
0.45
8
Silt
1060±113
0.11
1
Volcanics
21.4±12.5
0.11
13
Carbonates
114.2±40.9
0.36
1
Granitic
15.8±14.6
0.92
5
Gabbro
11.3±12.6
1.12
5
Gneiss
29.8±0.8
0.03
1
Xenolith
34.4±16.1
0.47
8
Acasta
6.2±1.3
0.20
1
Measured concentration for rock standards normalized to mean
concentration from analysis day plotted against the mass rock powder. Symbols
represent standards, and all analyses shown are fluorometry with and without
distillation. Decreasing initial sample mass does not seem to affect the concentrations calculated, as measured concentrations show no significant
trend away from the mean value given the different amounts of rock powder
dissolved.
Digestion length
As sample digestion is only partial, it is possible that the length of digestion
has an effect on final concentration readings. We conducted a digestion
length test for BCR-2 (Fig. ) and found no clear
relationship between digestion length and calculated N concentration. It
appears as though there are other factors that have a greater effect on N
concentration. Determining the length of time needed for the extraction of
all NH4+ would be an important step, as this could increase sample
processing efficiency and sample throughput.
Comparison of different method requirements and performance. SPAD
stands for samples per analysis day and indicates approximate number of
samples that can be run in 1 day, after all prep work has been completed.
Accessibility is a qualitative measure of how common analytical equipment is
in the geochemical community. Reproducibility is the average of the
coefficient of variation from Table . Sensitivity indicates
minimum concentration able to be potentially measurable.
Colorimetric
EA mass spectrometry
Fluorometry
NAA
Prep time required
1 week digestion
1 week
1 week digestion
weeks to months
SPAD
<30
18
50, or <30
with distillation
Species measured
NH4+
easily combusted
NH4+
total
organic, some NH4+
Accessibility of equipment
high
medium
high
low
Reproducibility
0.52
0.05
0.38, 0.09-distilled
0.2
Sensitivity
∼5 ppm
∼ tens of ppm
ppb
ppm
Reagent toxicity
HF high
low
HF high
radioactivity
Nitrogen concentration in upper and lower-crustal rocks based on
. Proportions are of upper or lower-crust mass; N
concentration (ppm) is from this study (where error is shown) or from
(JandG). Nitrogen contribution is simply
concentration multiplied by the proportion of crust. We use gneisses as a proxy
for felsic granulites and xenoliths for mafic granulites. Values from
Johnson and Goldblatt (2015) are given for comparison, with error shown as
standard error of the mean.
Rock type
Proportion
N (ppm)
N contribution
JandG
of crust (%)
Upper crust
Shale/silt
6.16
1064±113
65.6±7.0
860±64
Sandstone
2.94
230
6.8
230±110
Volcanics
2.80
21.±12.54
0.6±0.3
50±60
Carbonates
1.96
114.2±40.9
2.2±0.8
130±17
Granitic
45
15.8±14.6
7.1±6.6
54±7
Tonalite
5
24
1.2
24±7
Gabbro
6
11.3±12.6
0.7±0.8
5±2
Gneisses
19.20
29.8±0.8
5.7±0.2
135±50
Mica schist
4.80
500
24.0
500±44
Amphibolites
5.40
22
1.2
22±10
Marble
1
1000
9.0
1000±500
Total average
124±6.7
150±12
Lower crust
Felsic granulites
62
29.8±0.8
18.4±0.5
17±6
Mafic granulites
38
27.3±16.6
13.1±6.1
17±6
Total average
31.5±3.1
17±6
Total continental crust N based on tills, rock proportions, and
xenolith concentrations. Our results are consistent with previous work that
suggests there is about 2×1018 kg N in the continental crust. All N
masses are 1018 kg.
Reservoir
Mass in kg (% of total)
N (ppm)
N mass
Upper crust (tills)
1.01×1022 (53%)
81.2±36.4
0.82±0.4
Upper crust (rock abundance)
1.01×1022 (53%)
124±6.7
1.25±0.06
Lower crust
8.9×1021 (47%)
28.9±6.8
0.26±0.06
Total crust (till + lower)
1.9×1022
57±22
1.1±0.4
Total crust (abundance + lower)
1.9×1022
78±7
1.48±0.1
Methods comparison: pros and cons
The main difference between methods discussed in this work is that two (mass
spectrometry and NAA) analyze total N, while colorimetry and fluorometry
specifically target NH4+ (Table ). While assaying total N may be advantageous in
samples with mixed N speciation (e.g., sedimentary rocks), total N analysis
has a more difficult time accounting for N2 contamination from the
atmosphere. As rock samples are commonly ground to a fine powder before
analysis, the possibility of adsorbing some N2 could affect the accuracy
of total N methods, especially where mineral-bound N concentrations are low.
Targeted analysis of NH4+ is likely more better suited to crystalline
rocks, where NH4+ is the primary N species.
Mass spectrometry has the major advantage over fluorometry or colorimetry by
being able to measure N isotopes in a given sample. Isotopic values are
crucial in determining N cycling, both biologically and in geologic
reservoirs. One application of the fluorometry technique is as a
“first-pass” analysis to determine N concentration. The concentration of N
in a sample dictates what type of mass spectrometric technique (e.g., EA,
offline combustion, etc.) is most appropriate for isotopic analysis.
Given the expense of installing a mass spectrometer, the relative
inaccessibility of neutron irradiation sources, and the time required for NAA
(weeks to months), fluorometry presents a relatively quick and
straightforward alternative. All equipment and reagents are easily
obtainable. Fluorometers are much more affordable
(USD 3000 to 10 000) than mass spectrometers (hundreds of thousands of US dollars) and do
not require any supporting infrastructure and maintenance is comparatively
low. And while the technique in its current state may not fully liberate
NH4+ in all samples, it performs with similar reproducibility to mass
spectrometry and NAA. Fluorometry also performs well without distillation,
required for colorimetry. Distillation takes 15–20 min per sample,
limiting throughput, and makes either fluorometry or colorimetry more on par
with mass spectrometry in terms of time needed for analyses. Additionally, it
is difficult to consistently distill the same volume for each sample, which
limits accuracy and reproducibility.
In regards to fluorometric reagents needed, while HF is extremely dangerous,
it is more commonly used in geochemistry than any of the colorimetric
reagents (especially phenol and sodium nitroprusside), and with appropriate
training and caution, it can be handled safely. Additional fluorometric reagents
are less hazardous than colorimetric reagents and are linked with fewer
long-term exposure issues.
The fluorometry technique also has the potential to measure very small
quantities of NH4+. Its original development was for measuring parts per billion level
concentrations in natural waters, so if extraction from silicates can be
complete, there is no reason to think a similar level of precision could not
be developed for geologic samples.
Suggestions for fluorometry improvement
The most difficult to quantify aspect of the fluorometry and colorimetric
methods are the efficiency of the extraction of NH4+ during HF
digestion. Since HF digestion is only partial, assessing the amount of
NH4+ that remains in undigested materials could prove valuable.
Undigested materials are likely predominantly organics and/or oxides. Oxides
should have low NH4+, as there are no crystallographic spaces in mineral
lattices to accommodate NH4+. Organic content is typically low in
crystalline rocks (e.g., granites, gneisses) but would contribute N to bulk
rock concentrations. Minimizing the amount of rock sample powder used may
increase the efficiency of extraction. Indeed, preliminary tests suggest that
measured concentrations are not affected down to the initial rock powder mass
of 0.1 g (Fig. ), though at very small sample sizes
homogeneity issues could become apparent.
Determining the concentration of N in non-dissolved material could be another
area for improvement. The OPA reagent is sensitive to amino acids in addition
to NH4+; the addition of sodium sulfite destroys the sensitivity to
amino acids, making OPA react with ammonium alone .
If residual material left after HF dissolution were dissolved with an acid
that dissolves organic matter (e.g., H2SO4), one could carry out a
fluorometric analysis using a working reagent without sodium sulfite to
constrain non-silicate N content.
There are other small areas for fluorometry improvement. One is to filter
samples after neutralizing with HF, as sediment may affect fluorescence
readings. The second is to attempt a neutralizing agent other than KOH, which
may contain trace levels of NH4+. KOH was used initially due to its
better performance during distillation .
As noted by , acquisition and development of a robust
international standard would be extremely helpful in this or any future
analytic technique development. Our work demonstrates that existing rock
standards may be suited to this charge.
Preliminary application – continental crust
As a demonstration of the potential of the method, we calculated a N budget
of the continental crust using analyses of a variety rock types. Most samples
have come from Canada , though several are from other areas of North
America. We present only averages here; full analyses are available in the
Supplement. In addition, we have measured the N concentration in
samples of the Acasta Gneiss and found it to be 6.2±1.3 ppm (Table ). The Acasta Gneiss is currently the oldest widely
accepted rock on Earth and may be a remnant of processing by plate
tectonics or at least crustal recycling . We do not
include it in our calculations of continental crust, however, since it may
not be representative of “typical” continental crust.
We calculate the N content of the upper continental crust using two
approaches. The first is to use glacial tills as an average of upper-crustal
rocks (Tables , ). Again, this should be
regarded as a proof of concept approach, as these tills are all from British
Columbia, Canada, and primarily erode Phanerozoic rocks. The second method
relies on rock abundance data after and measured N in
those rocks either from this study (Tables ,
) or as compiled in . We
stress that while tills are used as a representative of the upper crust,
analyses of other rocks are not meant to be representative and are instead
analyzed to fill in gaps in poorly characterized reservoirs (e.g., volcanic,
middle-crustal xenoliths).
The two upper-crust approaches yield results that are distinct from each
other (Tables , ). The rock abundance
approach is most similar to . A wider range
of till samples, which have eroded a greater variety of crustal ages and rock
types, could address this discrepancy.
In addition, we use xenolith data to approximate the mafic lower crust and
gneisses to approximate the felsic lower crust. These results are quite
exploratory, and further analysis of lower-crustal samples would assist
interpretation greatly. By combining our lower-crustal estimate with upper-crust estimates, we find total-crust N to be 1.1±0.4×1018 kg
using the till + xenolith approximation and 1.48±0.1×1018 kg using
rock proportion+xenolith. While these are the same within error, we note that
this is primarily due to “diluting” the upper-crust estimates with lower-crustal values.
As an exercise to assess the impact of uncertainty in fluorometry
measurements, we can perform the same budget calculations by using the
highest and lowest analyzed values of BCR-2 compared to the mean value to
bracket calculation accuracy. Given a maximum measured value of 42 ppm and a
minimum measured value of 21 ppm (Supplement), compared to a mean of 33 ppm
for BCR-2, we can assess the effect of multiplying continental budgets by 1.3
and 0.64. This leads to a till-based upper-crust range of 0.5×1018
to 1.1×1018 kg N, a rock-abundance-based upper-crust range of 0.8
to 1.7×1018 kg N, and a lower crust of 0.2 to 0.4×1018 kg N. These differences in continental N budget estimates do not change the
broad agreement of our proof of concept budget with previously published
work, i.e., that the continental crust contains ∼0.5 present atmospheric mass
of N (PAN = 4×1018 kg N). For large-scale questions, fluorometry is
an appropriate technique. For questions that require finer resolution, such
as biologic incorporation of rock-bound N , more
method development is required.
The fluorometry technique has the potential to increase the number of
analyses of under-sampled rock types such as volcanics and middle- to lower-crustal xenoliths. These poorly sampled reservoirs have the potential to
sequester large amounts of N, and the continental crust can be both a
long-term storage reservoir of N and an important source of biologically
available N . Thus, determining its abundance is of
interest to both geology and biology.
Conclusions
We have measured N concentration in a number of rock standards using three
different methods – EA mass spectrometry, colorimetry, and newly adapted
fluorometry – and compared them to previously published values using
neutron activation analysis. Our analysis shows that fluorometry reproduces
previously published values for BCR-2 and may also do so for BHVO-2 and G-2
given an additional distillation step. Fluorometry appears more suited to
measuring geologic NH4+ in silicate rocks than either colorimetry or
EA mass spectrometry, while mass spectrometry is more suited to high-N
samples with significant organic N. No one method appears to be a “gold
standard” for geologic N analysis, and we call for further development in
this area. There are several suggested avenues for improving fluorometry,
namely improving HF-digestion efficiency and fine-tuning of
HF neutralization. Minimizing the volume of liquid required for digestion
(HF) or neutralization would increase the sensitivity of the method, which
could work for very low concentrations, down to parts per billion levels.
To demonstrate a potential application of fluorometry, we calculated a
continental N budget. This budget is based on analysis of glacial tills
(proxy for upper crust), a number of Phanerozoic volcanics, and a variety of
mid-crustal xenoliths to augment existing literature analyses. Our approach
estimates that between 1.1±0.4×1018 kg N (till + xenolith approach) and
1.48±0.1×1018 kg N (rock abundance approach) is in the
continental crust, consistent with recent estimates
. The fluorometry
technique appears most appropriate for these large-scale questions, where
exact precision is not required. An additional application could be as an
initial analysis to determine approximate concentration, which is a key step
in further isotopic investigations.
All methods assessed herein have strengths and weaknesses, which are
amplified due to the ability of N to exist as multiple species in the same
sample. We call here again for the development of internationally accepted
geologic N standards. New method development is difficult without such
standards, and care should be taken to classify what species of N (NO3,
NH4+, organic N, N2) is being measured. We also report the first
δ15N values for a series of rock standards, as isotope values
should be part of any international standard development.