Sampling and Laboratory Analysis for Solvent Stabilizers - Environmental Investigation and Remediation - page 213

The major components of the MS system include a vacuum pump to maintain a vacuum in the
mass spectrometer, an ionization chamber, a focusing lens to direct the stream of ions, a mass
analyzer, a detector, and a data control system. Following the ionization process already described,
the charged-ion stream is focused through a pinhole and a series of electrical focusing lenses to
narrow and intensify the ion stream before it enters the mass analyzer. A quadrupole analyzer is
a set of four cylindrical metal rods clamped together in a specii c coni guration spaced so that
their electromagnetic i elds interact in a precise manner. Direct current and radio-frequency sig-
nals are applied across the rods; adjacent rods have opposite charges. The effect of the combined
direct current and radio-frequency i elds is to force the ion stream entering the quadrupole into a
corkscrew-shaped three-dimensional sine wave. The combined electromagnetic i elds establish a
standing wave for just a single mass-to-charge ratio at a particular frequency, allowing it to be
directed to the detector at the end of the quadrupole, while other ion fragments follow unstable
paths and collide with the walls of the quadrupole rods. As the combined electromagnetic i elds
are varied, larger or smaller masses impinge upon the detector (Chapman, 1993; McMaster and
McMaster, 1998).
Ionized analytes and their fragments leave the quadrupole and are del ected from a linear path to
the ion detector. Fragment ions colliding with the conductive surface of the ion detector induce a
cascade of ions in the body of the ion detector, amplifying the signal from the single fragment and
producing a signal strong enough to be counted in the data system. The combination of ion detectors
and data acquisition systems can record about 25,000 data points per second. The range and scan
rate are instrument dependent; for example, mass spectra from 35 to 550 m / z can be scanned up to
10 times per second in some mass spectrometers (McMaster and McMaster, 1998).

4.4.2.2 Mass Spectroscopy and 1,4-Dioxane
To improve analytical sensitivity for detection of 1,4-dioxane and other compounds at low con-
centrations, MS systems can be run in selective ion mode (SIM). In SIM, the MS can focus on a
narrow m / z range, rei ning the ability to detect ion fragments in the target range. To ensure accu-
rate compound identii cation using SIM, multiple ions should be monitored for each compound
(USEPA, 2006a). For 1,4-dioxane, the mass-to-charge ratio ( m / z ) of the molecular ion is 88, and
the masses of fragment ions of 1,4-dioxane are 58, 43, and 57. For 1,4-dioxane's deuterated inter-
nal standard, 1,4-dioxane-d 8 , the mass-to-charge ratio ( m / z ) of the molecular ion is 96, while the
m / z ratios for its fragment ions are 64 and 46. Mass spectra for 1,4-dioxane and 1,4-dioxane-d 8
are shown in Figure 4.3 .

4.5 ENVIRONMENTAL PROTECTION AGENCY METHODS
FOR ANALYSIS OF 1,4-DIOXANE
Commercial laboratories typically offer analysis of 1,4-dioxane in water by four EPA methods:
524.2 for drinking water and 8260 (purgeable), 8260B (SIM), or 8270 (extractable). Because of the
high water solubility and poor purging efi ciency of 1,4-dioxane, GC-MS methods without modii -
cations produce high detection limits, typically greater than 100 g/L (ppb) (Strout et al., 2004b).

The development of analytical methods for 1,4-dioxane followed demand for quantifying 1,4-
dioxane in the chlorinated solvents, pharmaceutical, cosmetics, and petroleum industries, as well as
personnel air monitoring in these industries. Earlier test methods in these applications required
quantitation in the percent range, which could be achieved by using various combinations of PT and
GC with FIDs. The motivation to analyze water-borne 1,4-dioxane in the part-per-billion range
stemmed from risk analyses leading to low-level regulatory guidelines for drinking water.
Among the earlier EPA methods for 1,4-dioxane, a heated PT method was proposed for polar,
water-soluble VOCs (Lucas et al., 1988). The method involved purging samples at 90°C. To shift the
aqueous phase/vapor phase equilibrium constant toward the vapor phase, salt was added to the
sample or the heated PT method was used. Sodium chloride was the salt selected for heated PT

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