National Institute of Standards & Technology

Certificate of Analysis
Standard Reference Material 1640a
Trace Elements in Natural Water
This Standard Reference Material (SRM) is intended for use in evaluating methods used in the determination of
trace elements in fresh water. SRM 1640a consists of acidified spring water with mass fractions and mass
concentrations assigned for 29 elements, 22 of which were gravimetrically added. The solution contains nitric acid
at a volume fraction of approximately 2 %. A unit of SRM 1640a consists of 250 mL of solution in a high-density
polyethylene (HDPE) bottle sealed inside an aluminized Mylar pouch.

Certified Values and Uncertainties: The certified values for 22 elements in SRM 1640a are given expressed in
mass fraction units and mass concentration units in Tables 1 and 2, respectively. A NIST certified value is a value
in which NIST has the highest confidence in its accuracy, in that all known or suspected sources of bias have been
fully investigated or taken into account [1].

Each certified mass fraction value given in Table 1 is the average of the value calculated from the gravimetric
preparation and the value determined using either inductively-coupled plasma optical emission
spectroscopy (ICP-OES) or inductively-coupled plasma mass spectrometry (ICP-MS), adjusted upward for
transpiration that may occur over the certification period while the SRM bottle remains sealed inside the aluminized
Mylar pouch. (NOTE: No correction has been applied for transpiration that will occur after the pouch seal has
been broken. See “Instructions for Use” for more information regarding transpiration.) The magnitude of the
transpiration adjustment (0.11 %) is based upon the results of unpublished NIST studies of transpiration rates of
similar HDPE bottles sealed inside similar aluminized Mylar pouches, and is such that the actual mass fraction is
expected to be equal to the certified mass fraction value approximately halfway through the certification period.
Each expanded uncertainty, U, in Table 1 is calculated as U = kuc, where k is the coverage factor for the appropriate
degrees of freedom (df) and a 95 % level of confidence (k and df are also given in Table 1) and uc is the combined
standard uncertainty calculated according to the ISO Guide [2]. The value of uc is intended to represent, at the level
of one standard deviation, the combined effect of uncertainty components associated with the gravimetric
preparation, the ICP-OES or ICP-MS analysis, method bias [3], and the transpiration adjustment.

Each certified mass concentration value given in Table 2 is calculated from the corresponding certified mass fraction
value in Table 1 through multiplication by the density of the SRM 1640a solution. Each expanded uncertainty, U, in
Table 2 is calculated as U = kuc, where k is the coverage factor for the appropriate degrees of freedom (df) and a
95 % level of confidence (k and df are also given in Table 2) and uc is the combined standard uncertainty calculated
according to the ISO Guide [2]. The value of uc is intended to represent, at the level of one standard deviation, the
combined effect of uncertainty components associated with the certified mass fraction value and the solution
density.

Expiration of Certification: The certification of SRM 1640a is valid, within the measurement uncertainty
specified, until 05 August 2020, provided the SRM is handled and stored in accordance with instructions given in
this certificate (see “Instructions for Use”). The certification is nullified if the SRM is damaged, contaminated, or
otherwise modified.

Maintenance of SRM Certification: NIST will monitor this SRM over the period of its certification. If
substantive technical changes occur that affect the certification before the expiration of this certificate, NIST will
notify the purchaser. Registration (see attached sheet) will facilitate notification.

Coordination of the technical measurements leading to the certification of SRM 1640a was provided by
M.R. Winchester of the NIST Analytical Chemistry Division.

Stephen A. Wise, Chief

Analytical Chemistry Division

Gaithersburg, MD 20899 Robert L. Watters, Jr., Chief

Certificate Issue Date: 08 June 2010 Measurement Services Division
See Certificate Revision History on Last Page

SRM 1640a Page 1 of 6

This SRM was prepared by T.A. Butler, L.L. Yu, and M.R. Winchester of the NIST Analytical Chemistry Division.
The ICP-OES analyses were performed by T.A. Butler, J.L. Molloy, and M.R. Winchester of the NIST Analytical
Chemistry Division. The ICP-MS analyses were performed by J.L. Molloy, T.A. Butler, L.L Yu, and
M.R. Winchester of the NIST Analytical Chemistry Division.

Statistical consultation was provided by W.F. Guthrie of the NIST Statistical Engineering Division.

Support aspects involved in the issuance of this SRM were coordinated through the NIST Measurement Services
Division.

Reference Values and Uncertainties: The reference values for seven elements in SRM 1640a are given expressed
in mass fraction units and mass concentration units in Tables 3 and 4, respectively. Reference values are non-
certified values that are best estimates of the true values. However, the values do not meet NIST criteria for
certification and are provided with associated uncertainties that may not include all sources of uncertainty [1].

The reference mass fraction values and expanded uncertainties given in Table 3 are calculated using the same
approach employed for the certified mass fraction values (see explanation above), including the use of the
transpiration adjustment, except that each reference mass fraction value is based solely upon analysis using either
ICP-OES or ICP-MS and uncertainty components are limited to those associated with the analysis and the
transpiration adjustment. The reference mass concentration values and expanded uncertainties given in Table 4 are
calculated using the same approach employed for the certified mass concentration values (see explanation above),
except that they are calculated using the reference mass fraction values in place of the certified mass fraction values.

NOTICE AND WARNING TO USERS

CAUTION: This SRM is an acidic solution. All appropriate safety precautions, including use of gloves during
handling, should be taken.

INSTRUCTIONS FOR USE

The SRM should be shaken before use to remix water that may have condensed on the inner surfaces of the bottle.
To help prevent contamination, pipettes or other labware should NOT be inserted into the bottle. Instead, a portion
of the solution should be decanted into another clean, dry container for use. Unused portions should not be returned
to the SRM bottle.

The accuracy of trace element analysis is limited by contamination, especially at the microgram per kilogram (or
microgram per liter) level. All apparatuses should be scrupulously clean, and only high-purity reagents should be
employed. Sampling and manipulations, such as evaporations, should be done in a clean environment, such as a
Class-100 clean hood.

The mass concentration values and uncertainties given in Tables 2 and 4 were calculated from the mass fraction
values and uncertainties in Tables 1 and 3, respectively, taking into account the anticipated range of density values
of the SRM solution in the temperature range 17 C to 27 C. Therefore, the mass concentration values and
uncertainties given in the tables are valid when the SRM solution is used within a temperature range of
22 C  5 C. A more precise estimate of the mass concentration for a given temperature can be obtained by
multiplying the mass fraction value by the accurately measured density of the solution at that temperature. The
uncertainty associated with a mass concentration value calculated in this way can be estimated by combining the
uncertainty components for the mass fraction value and the measured density following the ISO Guide [2].

Transpiration: The certified and reference values given in Tables 1 through 4 account for the effects of
transpiration that may occur prior to the first opening of the sealed pouch by the SRM user. After the SRM bottle
has been removed from the pouch, the rate of transpiration will rise, resulting in gradual increases in the mass
fractions (and concentrations) of the elements. It is the responsibility of the user of this SRM to account for this
effect. One approach is to weigh the SRM bottle both before and after each use. Mass loss observed during storage
can be utilized to correct for transpiration. In order to minimize transpiration, the SRM bottle should be stored
tightly closed and sealed inside an airtight container. The user should set a maximum shelf-life for a partially used
SRM bottle commensurate with accuracy requirements.

SRM 1640a Page 2 of 6

PREPARATION OF MATERIAL

SRM 1640a was prepared at NIST using only high-purity reagents. An acid-cleaned HDPE tank of 2 kL capacity
was filled with a known mass of commercially available spring water and enough concentrated nitric acid to adjust
the acid volume fraction to approximately 2 %. After thorough mixing with a precleaned recirculating pump, a
preliminary ICP-MS analysis was performed to determine the levels of the 29 elements of interest. The levels of the
22 elements to be certified were gravimetrically adjusted to target values by additions of aliquots of known masses
of the SRMs in the SRM 3100 series of single-element standard solutions. For each of these elements, the target
value was approximately 80 % of the Maximum Contaminant Level (MCL) listed in either the National Primary
Drinking Water Regulations or the National Secondary Drinking Water Regulations maintained by the United States
Environmental Protection Agency (EPA) [4], or approximately the mass fraction that was present in SRM 1640
Trace Elements in Natural Water, whichever was less. After addition of the aliquots and thorough mixing, the SRM
solution was packaged in acid-cleaned HDPE bottles of 250 mL capacity and sealed inside aluminized Mylar
pouches.

Table 1. Certified Values for Elements in SRM 1640a Expressed in Mass Fraction Units(a)

Element Mass Fraction k df

(µg/kg)

Aluminum 52.6  1.8 2.069 23

Antimony 5.064  0.045 2.365 7
Arsenic 8.010  0.067 1.980 120
Barium 150.60  0.74 1.984 98
Beryllium 3.002  0.027 2.060 25
Boron 300.7  3.1 2.365 7
Cadmium 3.961  0.072 2.365 7
Chromium 40.22  0.28 2.021 40
Cobalt 20.08  0.24 2.447 6
Copper 85.07  0.48 2.228 10
Iron 36.5  1.7 2.447 6
Lead 12.005  0.040 1.970 227
Manganese 40.07  0.35 2.201 11
Molybdenum 45.24  0.59 2.017 43
Nickel 25.12  0.12 2.026 37
Selenium 19.97  0.16 2.228 10
Silver 8.017  0.042 2.086 20
Strontium 125.03  0.86 2.179 12
Thallium 1.606  0.015 2.365 7
Uranium 25.15  0.26 2.145 14
Vanadium 14.93  0.21 2.447 6
Zinc 55.20  0.32 2.010 49
(a)
Certified mass fraction values are the equally weighted means of results from gravimetry and ICP-
OES or ICP-MS, adjusted upward for transpiration that may occur over the certification period
while the SRM bottle remains sealed inside the aluminized Mylar pouch. (NOTE: No correction
has been applied for transpiration that will occur after the pouch seal has been broken. See
“Instructions for Use” for more information.) The magnitude of the transpiration adjustment
(0.11 %) was selected so that the actual mass fractions are expected to be equal to the
corresponding certified values approximately halfway through the certification period. The
uncertainty listed with each value is an expanded uncertainty about the mean. The expanded
uncertainty is calculated following the ISO Guide [2] as U = kuc, where uc is intended to represent,
at the level of one standard deviation, the combined effect of uncertainty components associated
with the gravimetric preparation, the ICP-OES or ICP-MS analysis, method bias [3], and the
transpiration adjustment. The coverage factor (k) for each analyte is determined from the Student's
t-distribution corresponding to the degrees of freedom (df) and a 95 % level of confidence.

SRM 1640a Page 3 of 6

 Table 2. Certified Values for Elements in SRM 1640a Expressed in Mass Concentration Units(a)

Element Mass Concentration (b) k df

(µg/L)

Aluminum 53.0  1.8 2.064 24

Antimony 5.105  0.046 2.262 9
Arsenic 8.075  0.070 1.977 142
Barium 151.80  0.83 1.976 151
Beryllium 3.026  0.028 2.045 29
Boron 303.1  3.1 2.306 8
Cadmium 3.992  0.074 2.365 7
Chromium 40.54  0.30 2.008 51
Cobalt 20.24  0.24 2.365 7
Copper 85.75  0.51 2.120 16
Iron 36.8  1.8 2.447 6
Lead 12.101  0.050 1.965 517
Manganese 40.39  0.36 2.160 13
Molybdenum 45.60  0.61 2.013 46
Nickel 25.32  0.14 2.001 59
Selenium 20.13  0.17 2.160 13
Silver 8.081  0.046 2.040 31
Strontium 126.03  0.91 2.120 16
Thallium 1.619  0.016 2.306 8
Uranium 25.35  0.27 2.120 16
Vanadium 15.05  0.25 2.365 7
Zinc 55.64  0.35 1.995 68
(a)
Certified mass concentration values are calculated from the certified mass fraction values in
Table 1 through multiplication by the density of the SRM 1640a solution. The uncertainty listed
with each value is an expanded uncertainty about the mean. The expanded uncertainty is calculated
following the ISO Guide [2] as U = kuc, where uc is intended to represent, at the level of one
standard deviation, the combined effect of uncertainty components associated with the certified
mass fraction value and the solution density. The coverage factor (k) for each analyte is determined
from the Student's t-distribution corresponding to the degrees of freedom (df) and a 95 % level of
confidence.
(b)
The certified mass concentration values and expanded uncertainties are valid when the SRM
solution is used within the temperature range (22 oC  5 oC).

SRM 1640a Page 4 of 6

 Table 3. Reference Values for Elements in SRM 1640a Expressed in Mass Fraction Units(a)

Element Mass Fraction k df

(mg/kg)

Calcium 5.570  0.016 2.120 16

Magnesium 1.0502  0.0034 2.262 9
Potassium 0.5753  0.0020 2.179 12
Silicon 5.169  0.017 2.074 22
Sodium 3.112  0.031 2.776 4

(µg/kg)
Lithium 0.4034  0.0092 2.776 4
Rubidium 1.188  0.011 1.961 3204
(a)
Reference mass fraction values are the ICP-OES or ICP-MS values, adjusted upward for
transpiration that may occur over the certification period while the SRM bottle remains sealed
inside the aluminized Mylar pouch. (NOTE: No correction has been applied for transpiration that
will occur after the pouch seal has been broken. See “Instructions for Use” for more information.)
The magnitude of the transpiration adjustment (0.11 %) was selected so that the actual mass
fractions are expected to be equal to the corresponding reference values approximately halfway
through the certification period. The uncertainty listed with each value is an expanded uncertainty
about the mean. The expanded uncertainty is calculated following the ISO Guide [2] as U = kuc,
where uc is intended to represent, at the level of one standard deviation, the combined effect of
uncertainty components associated with the ICP-OES or ICP-MS analysis and the transpiration
adjustment. The coverage factor (k) for each analyte is determined from the Student's t-distribution
corresponding to the degrees of freedom (df) and a 95 % level of confidence.

Table 4. Reference Values for Elements in SRM 1640a Expressed in Mass Concentration Units(a)

Element Mass Concentration (b) k df

(mg/L)

Calcium 5.615  0.021 2.005 54

Magnesium 1.0586  0.0041 2.045 29
Potassium 0.5799  0.0023 2.040 31
Silicon 5.210  0.021 2.005 54
Sodium 3.137  0.031 2.571 5

(µg/L)
Lithium 0.4066  0.0094 2.776 4
Rubidium 1.198  0.011 1.961 3657
(a)
Reference mass concentration values are calculated from the reference mass fraction values in
Table 3 through multiplication by the density of the SRM 1640a solution. The uncertainty listed
with each value is an expanded uncertainty about the mean. The expanded uncertainty is calculated
following the ISO Guide [2] as U = kuc, where uc is intended to represent, at the level of one
standard deviation, the combined effect of uncertainty components associated with the reference
mass fraction value and the solution density. The coverage factor (k) for each analyte is determined
from the Student's t-distribution corresponding to the degrees of freedom (df) and a 95 % level of
confidence.
(b)
The reference mass concentration values and expanded uncertainties are valid when the SRM
solution is used within the temperature range (22 oC  5 oC).

SRM 1640a Page 5 of 6

 REFERENCES

[1] May, W.; Parris, R.; Beck II, C.; Fassett, J.; Greenberg, R.; Guenther, F.; Kramer, G.; Wise, S.; Gills, T.;
Colbert, J.; Gettings, R.; MacDonald, B.; Definition of Terms and Modes Used at NIST for Value-Assignment of
Reference Materials for Chemical Measurements; NIST Special Publication 260-136 (2000); available at
http://ts.nist.gov/MeasurementServices/ReferenceMaterials/PUBLICATIONS.cfm (accessed Jun 2010).
[2] JCGM 100:2008; Evaluation of Measurement Data — Guide to the Expression of Uncertainty in Measurement
(ISO GUM 1995 with Minor Corrections); Joint Committee for Guides in Metrology (2008); available at
http://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf (accessed Jun 2010); see also
Taylor, B.N.; Kuyatt, C.E.; Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement
Results; NIST Technical Note 1297; U.S. Government Printing Office: Washington, DC (1994); available at
http://www.nist.gov/physlab/pubs/index.cfm (accessed Jun 2010).
[3] Levenson, M.S.; Banks, D.L.; Eberhardt, K.R.; Gill, L.M.; Guthrie, W.F.; Liu, H.K.; Vangel, M.G.; Yen, J.H.;
Zhang, N.F.; An Approach to Combining Results From Multiple Methods Motivated by the ISO GUM; J. Res.
Natl. Inst. Stand. Technol., Vol, 105; p. 571 (2000).
[4] Drinking Water Contaminants, United States Environmental Protection Agency, available at
http://www.epa.gov/safewater/contaminants/index.html#listsec (accessed Jun 2010).

Certificate Revision History: 08 June 2010 (This revision includes corrected lithium values in Tables 3 and 4 and minor editorial changes.);
03 December 2009 (Original certificate date).

Users of this SRM should ensure that the Certificate of Analysis in their possession is current. This can be
accomplished by contacting the SRM Program: telephone (301) 975-2200; fax (301) 926-4751;
e-mail srminfo@nist.gov; or via the Internet at http://www.nist.gov/srm.

SRM 1640a Page 6 of 6