Dear DSQAP participants, 

Thank you for your participation in Exercise D of the Dietary Supplement Laboratory 

Quality Assurance Program. The attached report contains four sections: an overview of 

the statistical analysis; your individual data table; graphs displaying the community 

performance; and general recommendations from the exercise. A participation certificate 

will be sent in a separate e-mail message. 

Please check your laboratory’s data in the table and make sure you agree with it. If you 

do not, please let us know so that we can correct it. 

We are planning to hold an analyst workshop at NIST in September 2010. Participants in 

exercises C, D, and E will receive further information about the workshop in the early 

summer. 

If you have any questions or suggestions, please let us know. 

Best regards, 

DSQAP Team 

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Dietary Supplement Quality Assurance Program 

Exercise D 

Final Data Report 

Your laboratory code for this exercise: NIST 

This report consists of several parts: 

• Overview: a general description of the statistical treatment of the data and how to 

read the plots. 

• Data Table: a table with your laboratory’s individual results, the community 

results, and the NIST results. 

• Graphs: a section that includes graphical representation of the data for the 

analytes tested in this exercise, points to consider when examining the data, and 

when appropriate, recommendations from Exercise D. 

Lead

Niacin

β-carotene

Organic acids

• Recommendations

As always, if you have any questions, please contact us. 

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OVERVIEW 

STATISTICS 

Your individual data table and graphs contain information about your performance 

relative to that of the other participants in this exercise and relative to a target around the 

expected result. 

INDIVIDUAL DATA TABLE 

Section 1 of the data table contains your results, including your mean, standard deviation, 

and Z-score. Please check these and make sure that you agree with the mean and 

standard deviation reported in the table. The significance of the Z-score is as follows: 

• |Z| < 2 indicates that your result is considered to be within the community 

consensus value 

• 2 < |Z| < 3 indicates that your result is considered to be marginally different from 

the community consensus value 

• |Z| > 3 indicates that your result is considered to be significantly different from 

the community consensus value 

Section 2 of the data table contains the community results, including the median value 

for each analyte, the MADe (a robust estimate of the standard deviation), and the 

minimum/maximum values reported for the analyte. 

Section 3 of the data table contains the NIST results. In most cases, the assigned value 

and the U95 confidence interval have been determined with two independent analytical 

methods. At least six samples have been tested with each of the methods and duplicate 

sample preparations from the sample package have been included, allowing the U95 to 

encompass homogeneity within and between packages. 

GRAPHS 

Two graphs are provided for each analyte in each sample, one which plots the results for 

the sample only (View 1) and one which plots the lab results for the sample vs. the lab 

results for the control (View 2). Both views include the consensus values and the target 

values. 

View 1 

In this view, individual laboratory data are plotted with the individual laboratory standard 

deviation. The black solid line represents the consensus median and the black dotted 

lines represent the consensus variability calculated as 2*MADe. The center of the region 

between the red lines represents the NIST view of the “correct answer;” it is bounded by 

two times the pooled standard deviation (2*Stotal, pooled) of the participants or the NIST 

uncertainty (U95) (whichever is larger), thus creating a target zone for “acceptable” 

performance. With this view, it is relatively easy to determine if a laboratory falls within 

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the target zone and to compare where the target zone lies relative to the consensus values. 

In most cases, the target zone falls within the consensus values, which is the expected 

result. One program goal is to bring the consensus values closer together and closer to 

the target value. 

View 2 

In this view, the results reported for the sample are plotted vs. the results for the control. 

The red box represents the target values for the control (x-axis) and the sample (y-axis) 

and the blue dotted lines represent the analogous information for the consensus values. 

This view provides additional information to complement view 1. For example, if your 

values are low (or high) for both the control and sample, you may have calibration issues. 

If your laboratory falls into this category, you may want to investigate how your 

calibrants are prepared as well as the purity of your calibrant material. 

Return to Introduction

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DATA TABLE 

Return to Introduction

5 
LEAD IN GINKGO-CONTAINING PRODUCTS 

The sample used for the determination of lead was comprised of ground and 

homogenized ginkgo-containing tablets. The control was SRM 3247 Ginkgo biloba

extract. The concentration of lead in the control was approximately 5 times higher than 

the concentration of lead in the sample. The NIST value was adjusted from the certified 

value to an “as-received” value. There was very good agreement among the results 

compared on an “as-received” basis. This is discussed further in the recommendation 

section.

• Twenty-five laboratories enrolled in this exercise and received samples, 17 

laboratories reported results (68 %). 

• View 1 shows that approximately half of the laboratories fall within the target 

zone with a roughly equal distribution of high and low results. 

• Most laboratories are making very precise measurements as indicated by the 

relatively small error bars. 

• View 2 shows that in many cases the laboratories that reported low values for the 

control also reported low values for the sample. This trend is typical of either 

incomplete sample digestion or a calibration error. Because these samples are 

relatively easy to digest, a calibration error is a possible source of error. A 

depression of signal can occur in samples read at the high end of the calibration 

curve giving low values. 

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7 
NIACIN AS NIACINAMIDE 

The milk powder used for this exercise was fortified with fat-soluble vitamins, watersoluble vitamins, elements, and fatty acids. The control was SRM 1849 Infant/Adult 

Nutritional Formula. The level of niacinamide in the control is very similar to the level 

of niacinamide in the fortified milk powder. This study was a repeat of a study from 

Exercise C where SRM 1849 was the unknown sample and SRM 3244 EphedraContaining Protein Powder was used a control. In Exercise C, the results were extremely 

scattered with values ranging over two orders of magnitude and approximately half of the 

results within the target range. In Exercise D, a vial of 500 mg of USP Niacinamide was 

sent to the participating laboratories for use as a calibrant. A comparison of the results 

from the two exercises is shown in the recommendation section. 

• Twenty-three laboratories enrolled in this exercise and received samples, 16 

laboratories reported results (69.5 %). 

• The results were significantly improved compared with Exercise C. The 

consensus values mirrored the target range and only a few laboratories were 

significant outliers. 

• This exercise highlights the importance of calibration materials. When all 

laboratories used the same calibration material there was good agreement among 

the data. This indicates that reference materials must be appropriate (niacinamide 

rather than nicotinic acid) and screened for purity for increased measurement 

comparability. 

Recommendations: 

• A new niacin study in a different matrix (breakfast cereal) without a common 

calibration material. 

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Return to Introduction

9 
β-CAROTENE IN OILS 

Two botanical extracts in oil were sent for this study. A carrot extract in oil was the 

sample with SRM 3251 Serenoa repens Extract as a control material. The level of βcarotene in the sample was approximately 30 % lower than the level in the control. Both 

samples required minimal sample preparation as they could be diluted in organic solvent 

with no additional extraction required. 

 

• Twenty-two laboratories enrolled in this exercise and received samples, 11 

laboratories reported results (50 %). 

• There is significant scatter in the results, with a low of ~10 μg/g and a high of 

~ 55 μg/g total β-carotene. 

• Only two laboratories provided specific details about calibrant preparation and 

only one laboratory provided information on the solvent and molar absorptivity 

used for the spectrophotometric determination of the calibrant concentration. 

Recommendations: 

• Check form of β-carotene reported. 

• Check molar absoptivity used for the determination of calibrant concentration. If 

different laboratories are using different values, there is the potential for wide 

variation in the results. 

• Repeat this study with different samples.

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11 
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13 
ORGANIC ACIDS IN BERRY PRODUCTS 

The berry sample was a spray-dried bilberry extract with SRM 3283 Vaccinium 

macrocarpon (Cranberry) Extract as a control material. The levels of quinic and malic 

acids in the sample were approximately 30 % lower than the levels in the control 

material. The level of citric acid is very similar in the sample and the control. 

Overall the precision was good for all of the organic acid measurements resulting in tight 

consensus values. Additionally there was reasonably good agreement with the target 

values for the unknown sample. Surprisingly there was more variability in the 

determination of organic acids in the control material than the unknown sample. 

• Seventeen laboratories enrolled in this exercise and received samples, 9 

laboratories reported results (53 %). 

• From information provided by some laboratories, it appears that there were 

chromatographic interferences in the control that effected results, particularly for 

quinic acid. 

• Laboratories that utilized solid-phase extraction (SPE) for sample cleanup tended 

to have low results for all of the organic acids. 

Recommendations: 

• Check filters and SPE cartridges with calibrants to determine recovery 

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RECOMMENDATIONS 

There were several global issues that we noticed while reviewing the data; please 

consider these recommendations as they may help improve measurement accuracy and 

precision. 

When a target range is provided for a control, the information should be used as a quality 

self check. The target value should NOT be used to determine a scaling factor. The 

matrices are often significantly different between the sample and control. Analytes may 

be extracted differently from the two materials, thus a mathematical adjustment for 

recovery is not valid. 

Sample cleanup with solid-phase extraction or filters may be useful prior to analysis. 

However, the cleanup method should always be tested with a calibrant during method 

development to ensure that the analytes of interest are not binding to the cleanup 

substrate. 

Evaluation of the purity and stability of calibration materials is very important; this point 

is highlighted by the measurement of niacin as niacinamide in Exercises C and D. In 

Exercise C, the results were extremely scattered with values ranging over two orders of 

magnitude and approximately half of the results within the target range. In Exercise D, a 

vial of 500 mg of USP Niacinamide was sent to the participating laboratories for use as a 

calibrant. The results from Exercise D are much improved, with nearly identical target 

and consensus zones, indicating that the variability in Exercise C is due to differences in 

calibration materials and not due to the extraction and instrumental methods. 

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When using Standard Reference Materials or other quality control materials it is 

important to determine whether or not the target range has been adjusted for moisture 

content, even when the material appears to be a “dry” powder. In Figure A below, 

laboratories have reported values for lead on an as-received basis and the target range is 

shown on a dry-mass basis. In Figure B, the target range has been adjusted to an asreceived value. The difference is significant! 

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