times of lignocaine and etidocaine were. 2.0 and 2.8 mm, respectively. There was no interference from mexiletine, pro- cainamide, disopyramide, phenytoin,.
lli in man (2), we find that MMT has a pleasant odor and can be easily prepared. In addition to the advantageous criteria reported by Fullinfaw and Marty, MMT is resolved from the interference peaks found in therapeutic drug monitoring proficiency samples (3). The instrument we use is from GowMac Instrument Co., Bound Brook, NJ 08805, equipped with a 153cm X 2mm glass column packed with 20% FFAP on Chromosorb W AW/DMCS, 60/80 toluamide
should be typed double-spaced (including references) with conventional margins. The length of the text is limited to five manuscript pages. Letters
to the Editor
Interferences from Wood
ApplicatorSticks Usedin Serum To the Editor:
Our clinical chemistry
laboratory had been using wooden applicator sticks (Healthco no. 8401-792114) to mark and
to remove fibrin from specimens that continued to coagulate after centrifugation. Testing these sticks for possible interferences in chemistry analyte determinations
showed
that
potassium,
calcium, and glucose concentrations in serum increased after only 3 mm of contact with the applicator stick. Wooden
sticks
from two other
sources
(American Scientific Products no. A5000-1 and Puritan no. 807-12) produced similar results. A 1-mL pooled serum sample that contained analyte concentrations as found in healthy persons was used to test all three brands, with two applicator sticks per specimen. Analyte increases after 30 mm of contact averaged 0.4 mmol/L for potassium, 0.2 mmolfL for calcium, and 0.4 mmol/L for glucose. These values continued to increase with longer times of contact.
Laboratories using sticks to process and mark chemistry specimens should avoid leaving wooden products in contact with serum for any significant length of time.
mesh. The operating conditions include injector temperature at 200 #{176}C, column at 150 #{176}C, and detector at 225 #{176}C, with carrier gas (N2) at 60 mL/min. The sample volume injected is 1 zL. Methyl-m-toluate is prepared by mixing 1 g of m-toluic acid (Aldrich Chemical Co., Milwaukee, WI 53233) with 3 mL of methanol and two drops of concentrated sulfuric acid in a reaction vessel, which is allowed to stand at room temperature for 24 h. Excess methanol is removed under reduced pressure, and the ester is used as an internal standard without further purification. The method used for the extraction of VA in serum is essentially that of Dijkhuis and Vervloet (4). Briefly, to 0.5 mL of serum add 0.5 mL of a 100 g/L solution of HC1O4 and 0.5 mL of CC14 containing 50 mg of MMT per liter. Vortex-mix and centrifuge at 1040 rcf; the organic (bottom) layer is ready to be analyzed. Under the described conditions, the MMT peak elutes at 1.5 mm, VA at 2 mm, with almost baseline resolution between them (see Figure 1). This procedure is simple, rapid, and sensitive and involves an inexpensive
Thomas P. Joseph
1. Fullinfaw, R. 0., and Marty, J. J., Gas chromatography of valproic acid, with benzyl alcohol as internal standard. Clin. Chem. 27, 1776 (1981). Letter. 2. Wu, A.,Pearson,M. L.,Shekoski,D. L.,et al., High resolution gas chromatography mass spectrometric characterization of urinary metabolites of N,N-diethyl-m-toluamide (DEET) in man. J. High Res. Chromatogr. Chromatogr. Commun. 2,558-562 (1979). 3. Therapeutic Drug Monitoring Update, Am. Assoc. Clin. Chem., October 1979, p 2, and March 1980, p 2. 4. Dijkhuis, I. C., and Vervloet, E., Rapid determination of the antiepileptic drug din-propylactic acid in serum. Pharm. Weekbl. 109,42-45 (1974).
Anthony Wu M. L. Pearson S. K. Mertens D. D. Breti G. S. Woiffe Clin. Toxicol. Lab. Dept. of Pathol. Med. Coil, of Wisc. Milwaukee, WI 53226
To the Editor: As is well known,
measurements
total iron-bonding capacity (TIBC)
Gas Chromatographyof Serum VaiproicAcid, with Methyl-rntoluate as InternalStandard To the Editor:
544
References
QualityControlof Measurementsof Total Iron-BindingCapacity
Dept. Pat hol. St. Joseph Mercy Hosp. P.O. Box 995 Ann Arbor, MI 48106
The letter of Fullinfaw and Marty (1) prompts us to report our analysis for valproic acid (VA) in serum, for which we use a gas chromatograph equipped with a flame ionization detector, with methyl-m-toluate (MMT) as internal standard. In connection with our study of the metabolism of N,N-diethyl-m-
instrument. We have used it for almost four years. The serum volume mentioned is for adults but it can be halved for pediatric specimens without discernable change in sensitivity. We found that the peak height ratio (VA/MMT) is linearly related to concentration to 150 mg/L, which covers the range of therapeutic and toxic concentrations in serum (4).
024
Fig. 1. Gas ctwomatoam of MMT (1) and VA (2) in serum of a patient VA concentratioO shown is 101 mg/L
CLINICALCHEMISTRY, Vol. 28, No. 3, 1982
of can
lead to erroneous results if the pH is not regulated at each stage of the assay. Furthermore, inadequate concentrations of total protein or sample dilution can also introduce substantial errors (1). Therefore TIBC assays require careful monitoring, with use of both internal and external quality-control (QC) schemes. Considerable variations from method to method as well as bottleto-bottle can arise when commercially prepared lyophilized controls are used, and it has been suggested (2) that “Decision” liquid controls (Beckman Instruments, Inc.) are probably the best currently available QC materials for developing interlaboratory proficiency-testing programs for TIBC.
We examined two types of lyophilized material for internal QC monitoring and also participated in two external QC schemes in which lyophilized samples were used during one year. Internal
QC
sera were purchased from General Diagnostics (Versatol-Lo) and Wellcome Reagents Ltd. (Autoset-Hi). The external schemes were based nationally at Wellcome Research Laboratories, Kent, U.K., and internationally at the Central Reference Institute, Bonn, F.R.G. TIBC was determined with an AutoAnalyzer II (Technicon Instruments) by a method in which ferric chloride is used to saturate the binding sites on the serum transferrin and magnesium carbonate is used to remove excess iron (3). Results of our study on the internal QC controls gave a value of (mean ±SD) 33.4 ± 2.2 zmol/L for Versatol-Lo and 65.7 ± 4.7 zmol/L for Autoset-Hi; the target values were 31.0 ± 3.0 and 60.0 ± 6.0 Mmol/L, respectively. In the above method the lyophilized control material gave very good precision and accuracy; inter-batch, intra-batch, and bottleto-bottle variations were not statistically significant. At no stage did our results fall outside two standard deviations when we used either lyophilized sera that had been stored frozen for longer than six months or reconstituted sera that had been stored for two to four weeks. In the two external QC schemes, in which as many as 696 laboratories participated, various methods were used for determining TIBC; 229 of these laboratories used a method similar to the one we used. Results from our laboratory correlated well (r = 0.89) with the overall mean for all methods. Also, the correlation between the overall mean for all methods and the overall mean for those tests done by methods similar to ours was excellent (r = 0.99). No significant differences were seen between our results and those of the other laboratories using similar methods (p >0.25). However, there were significant differences between our results and those of laboratories using different methods (p
