Computers in Flavor and Fragrance Research - American Chemical

7 Instantaneous Analysis of Fragrances, Flavors, and Other Vapor-Phase Chemicals Atmospheric Pressure Chemical Ionization Tandem Triple Quadrupole Mass Spectrometry (APCI/MS/MS) BORI S H U S H A N

Downloaded by FUDAN UNIV on March 14, 2017 | http://pubs.acs.org Publication Date: August 22, 1984 | doi: 10.1021/bk-1984-0261.ch007

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Atmospheric Pressure Chemical Ionization (APCI) i s coupled to Tandem Mass Spectrometry (MS/MS) for the purpose of analyzing vapor phase perfume mixtures. Air-borne fragrances are analyzed d i r e c t l y by APCI/ MS/MS without the need for time consuming and potentially adulterating trapping and chromatography steps. V o l a t i l e fragrance chemicals have been rapidly identified by t h i s novel technique as they emanate from v i a l s or directly from skin. Recent analytical methodology in the i d e n t i f i c a t i o n of the chemical components of fragrances and flavors has relied heavily on gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). The success of these techniques are certainly well documented(l)and they have provided perfume and flavor chemists with a very good understanding of the chemical composition of complex formulations and mixtures. Despite the inherent s e n s i t i v i t y and wide a p p l i c a b i l i t y of conventional GC and GC/MS techniques they w i l l never replace the well trained nose as a means of identifying odiferous components. The obvious advantages that the nose has i s i t s great s e n s i t i v i t y and selectivi t y to those chemicals which give us smell. Another advantage of the olfactory organ i s i t s a b i l i t y to sample vapors directly at atmospheric pressure without the handling steps required for GC or GC/MS which may alter chemical composition. A major disadvantage of olfactory analysis, however, i s a lack of resolution where a l l the chemical components reach the "detector" at the same time causing masking of one odor by another. Ideally an analytical technique i s required which combines the resolution of GC/MS with the d i r e c t , "real-time" analytical capabilities of the nose. This paper w i l l describe a technique for the direct analysis of vapor-phase chemicals which uses an atmospheric pressure chemical ionization (APCI) ion source coupled to a tandem t r i p l e quadrupole mass spectrometer. This type of instrumental system 0097-6156/ 84/ 0261 -0075S06.00/ 0 © 1984 American Chemical Society

Warren and Walradt; Computers in Flavor and Fragrance Research ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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COMPUTERS IN FLAVOR AND F R A G R A N C E RESEARCH

(the TAGA® 6000 manufactured by SCIEX®) has been shown(2)to be particularly well suited to the detection of organic and inorgani c chemicals in ambient a i r . Vehicle mounted mobile TAGA® 6000 Laboratory Systems and similar fixed-site instruments have demonstrated the a b i l i t y to detect ppt-levels of a variety of organic compounds from gaseous, l i q u i d and s o l i d matrices using a number of i n l e t s including Direct Air Sample (DAS), Direct Insertion Probe (DIP), Liquid Chromatography (LC)(3), and GC.


In this a r t i c l e we w i l l present data obtained using the i n l e t , sampling perfume mixtures, emanating from open vials applied to skin, in order to demonstrate the principles APCI/MS/MS and i t s use in research and quality control within fragrance and flavor industry.

DAS or of the

EXPERIMENTAL The TAGA® 6000 i s a quadrupole-based mass spectrometer combining three quadrupole arrays aligned a x i a l l y (a schematic of the ion optics i s shown in Figure 1)(4). The instrument can be configured to accept a variety of inlets and ionization sources however, for the present work the APCI ion source was used. Ambient or purified a i r can be drawn through this ion source using the DAS i n l e t at controlled rates of up to 9 1/sec. Trace organics and inorganic vapours are ionized by reagent ions formed within a point-to-plane corona discharge, forming product ions which are pseudo-molecular (indicative of molecular weight). The ions pass from the ion source (at 760 torr) through a small o r i f i c e and are focussed into the analyser portion of the mass spectrometer which has a base operating pressure of ca. 10" Torr. This represents a transfer of ions through a pressure reduction of almost 9 orders of magnitude using a single pumping stage. The vacuum system efficiency i s due to a high capacity (60,000 1/sec) cryogenic pumping system consisting of a closed-loop helium-based refrigerator connected to strategically placed cryo-arrays. The pumping system requires only e l e c t r i c a l power to operate. The o r i f i c e which connects the ion source with the analyser i s protected by a stream of ultra-pure nitrogen. This gas curtain keeps unionized molecules, particulate matter and a i r away from the o r i f i c e and high vacuum analyser portion. Consequently the o r i f i c e never becomes plugged by particlates and the ion optics are always clean, being exposed only to ultra-pure nitrogen. Ionized molecules, on the other hand, proceed unimpeded through the gas curtain focussed along e l e c t r i c a l f i e l d s through the o r i f i c e and into the analyser. 6

The positive mode reagent ions consist of protons in various states of hydration H (H20) , while the negative mode reagent species are primarily 0",02" and CO3". In the positive mode proton transfer (1) dominates the CI chemistry i . e . +

n


Instantaneous Analysis of Vapor-Phase Chemicals


7. SHUSHAN


77

COMPUTERS IN FLAVOR AND FRAGRANCE RESEARCH

78

H (H 0) + M +

2

> MH (H20) . + mH 0 +

n

n

m

(1)

2

where M i s the trace species to be analyzed. The resulting product ion i s further dehydrated by exposure to the ultra-pure N curtain gas and by low energy c o l l i s i o n a l activation (CID on Figure 1) brought on by the application of slight e l e c t r i c a l f i e l d s during the free-jet expansion into the analyser portion. The result i s an APCI mass spectrum dominated by molecular or pseudo-molecular ions (usually MH ) with minimal associative or dissociative product ions. The negative mode reactions are dominated by proton abstraction (2) from Bronsted acids and electron capture (3) by Lewis acids again providing pseudo-molecular and molecular ions respectively. 2


+

R- + M

» (M-H)- + RH

(2)

R- + M

> M- + R

(3)

Because of the chemical complexity of gaseous phase perfume mixtures, i t i s sometimes necessary to exploit the s e l e c t i v i t y of the APCI process. In the positive mode, only those compounds which are more basic than the reagent ion H (H 0) species are ionized. Through the addition of a reagent with higher proton a f f i n i t i e s than water the s p e c i f i c i t y of the ion source i s increased. This also applies in the negative mode as w e l l , exploiting relative gas phase a c i d i t i e s and electron a f f i n i t i e s of the reagent to the analyte in order to alter ion source specif i c i t y . Alternatively, the APCI chemistry can be altered to provide charge transfer as opposed to proton transfer ion reactions. This i s accomplished through the addition of a charge transfer reagent (such as C5H6) to generate a reagent ion "plasma" which only ionizes those compounds which have an ionization potential (IP) lower than that of the reagent's. Thus, for example, C5H5 reagent ions would only ionize compounds with an IP

Computers in Flavor and Fragrance Research - American Chemical

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